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Archives of Toxicology

, Volume 91, Issue 9, pp 2967–3010 | Cite as

Toxicology of silica nanoparticles: an update

  • Sivakumar Murugadoss
  • Dominique Lison
  • Lode Godderis
  • Sybille Van Den Brule
  • Jan Mast
  • Frederic Brassinne
  • Noham Sebaihi
  • Peter H. Hoet
Open Access
Review Article

Abstract

Large-scale production and use of amorphous silica nanoparticles (SiNPs) have increased the risk of human exposure to SiNPs, while their health effects remain unclear. In this review, scientific papers from 2010 to 2016 were systematically selected and sorted based on in vitro and in vivo studies: to provide an update on SiNPs toxicity and to address the knowledge gaps indicated in the review of Napierska (Part Fibre Toxicol 7:39, 2010). Toxicity of SiNPs in vitro is size, dose, and cell type dependent. SiNPs synthesized by wet route exhibited noticeably different biological effects compared to thermal route-based SiNPs. Amorphous SiNPs (particularly colloidal and stöber) induced toxicity via mechanisms similar to crystalline silica. In vivo, route of administration and physico-chemical properties of SiNPs influences the toxicokinetics. Adverse effects were mainly observed in acutely exposed animals, while no significant signs of toxicity were noted in chronically dosed animals. The correlation between in vitro and in vivo toxicity remains less well established mainly due to improper—unrealistic—dosing both in vitro and in vivo. In conclusion, notwithstanding the multiple studies published in recent years, unambiguous linking of physico-chemical properties of SiNPs types to toxicity, bioavailability, or human health effects is not yet possible.

Keywords

Amorphous silica nanoparticles Pyrogenic Colloidal Stöber Oxidative stress Toxicity 

Introduction

Nanosilica, also known as the nanoform (<100 nm) of silicon dioxide or silica nanoparticles (SiNPs), possesses distinct physico-chemical characteristics compared to its bulk form; smaller size materials have an increased surface-to-volume ratio and a higher surface reactivity (Oberdörster 2010; Napierska et al. 2010). Due to their appealing properties, SiNPs are now extensively used in agriculture, food, and consumer products including cosmetics (Napierska et al. 2010; Khot et al. 2012; Kasaai 2015; Brinch et al. 2016). Until 2012, nearly 1.5 million tons of SiNPs had already been placed in the global market (Liljenström et al. 2013) and SiNPs became one of the three most produced nanomaterials (NMs) worldwide in 2013. Among the 846 nano-based products listed in a consumer products inventory, approximately 100 claim to contain SiNPs (Vance et al. 2015). Moreover, amorphous SiNPs are being synthesized with highly tunable biocompatibility and stability, and considered as a very promising candidate for various bio-medical applications such as gene carrier, drug delivery, and molecular imaging (Tang and Cheng 2013; Bitar et al. 2012).

In recent years, large-scale industrial production and global commercialization of SiNPs have resulted in increased risk of human exposures at workplaces (Kim et al. 2014a; Oh et al. 2014). Food additive silica (E551) is also in the nano size range (Dekkers et al. 2011), indicating that the general population is probably more exposed than initially anticipated. Moreover, in view of the efforts to use NM in medical applications, SiNPs could also be intentionally introduced into the human body for disease diagnosis and treatments (Croissant et al. 2017). Such growing potentials for exposure raised a global concern regarding the safety and potential adverse health effects of SiNPs.

Human health effects associated with silica exposure, especially crystalline silica (0.5–10 µm), have widely been studied. Occupational exposure to crystalline silica induces silicosis in workers (a fibrotic lung disease) and is also associated with lung cancer, emphysema, and pulmonary tuberculosis (Leung et al. 2012). Conversely, natural amorphous silica is generally considered as less harmful, since the toxicological potential of silica has so far been linked to its crystallinity. Recent studies have revealed that amorphous SiNPs can be as reactive as crystalline particles (Turci et al. 2016). In vivo, amorphous SiNPs are, however, cleared more rapidly from the lung, which may contribute to explain their lower pathogenic potential (Arts et al. 2007). The human health effects of nanosilica remain to be clarified and toxicologists believe that exposure to SiNPs, due to their small size, may bring different adverse effects compared to micron-sized silica (Napierska et al. 2010).

The comprehensive review of Napierska et al. (2010) suggested that exposure to SiNPs (1–100 nm) induced toxic effects in vitro (immortalized mammalian cell lines) and in vivo (rats and mice). Physico-chemical properties such as size, surface area, and surface features were found to play a key role in the toxicity of SiNPs. Importantly, Napierska concluded that physico-chemical properties of SiNPs differ based on their production method and, therefore, may cause different biological effects. However, no definite conclusions were made due to insufficient or no data available for,
  • Detailed physico-chemical characterization of different types of SiNPs;

  • Comparison of the toxicity of different types of SiNPs (based on their production process);

  • Comparison of the toxicity mechanisms of amorphous SiNPs and crystalline silica;

  • Exposure via different routes and adverse effects of chronic exposure in vivo;

  • Correlation of in vitro and in vivo studies and

  • Physico-chemical properties for the safer design of SiNPs.

Therefore, the aim of this review is to summarize the toxicity studies of SiNPs published after the Napierska review (2010), critically discuss the outcomes, and to evaluate how these data gaps have been addressed (Fig. 1).
Fig. 1

Systematic selection of studies

Methodology

The selection criteria was similar to the method described in Vriens et al. (2017), which was used to construct the MOD-ENP-TOX nanotoxicity database.

Exclusion criteria

To specifically focus on adverse health effects of SiNPs, papers reporting on other interventions such as ecotoxicity, synergistic effects, SiNPs doped with other materials, and therapy-based outcomes were excluded.

Inclusion criteria

Papers reporting,
  1. 1.

    physico-chemical characteristics such as primary size, shape, composition, and crystallinity;

     
  2. 2.

    toxicological endpoints such as cytotoxicity, apoptosis/necrosis, genotoxicity, oxidative stress, immunotoxicity, and autophagy using immortalized cell lines or primary cells (experimental in vitro studies); and

     
  3. 3.

    toxic effects in laboratory-animals, more specifically in vivo experiments using rats and mice.

     

Literature search

We searched two databases for papers published from June 2010 (after Napierska et al. 2010) to December 2016. In the PubMed “http://www.ncbi.nlm.nih.gov/PubMed” and EMBASE “https://www.embase.com” databases, the following keyword combinations were used: “silica nanoparticle” OR “silica NP” AND “toxicity” NOT “review”. We retrieved a list of 859 and 405 articles in English, respectively. In a second step, duplicates were removed and the titles were screened to identify studies that best matched with our search terms, leaving 611 relevant papers. In a third step, we excluded papers that met the exclusion criteria and left 128 most relevant papers. Finally, 82 papers reporting a minimum set of physico-chemical characterization and toxic effects were selected for the main content of the review. Notably, only one study was found on the toxicity of crystalline nanosilica (Chu et al. 2012), but it was finally excluded due to insufficient data on the size of particles. As a result, the review is dealing only with amorphous SiNPs.

Induction of oxidative stress is considered as the major mechanism involved in SiNPs toxicity (Wang et al. 2009; Ye et al. 2010a, b) and, therefore, the in vitro section of the review was structured according to toxic endpoints and its association with oxidative stress. For in vivo, studies were sorted based on exposure route and modalities, since they can significantly influence the toxicokinetics of SiNPs. Throughout the review, the following abbreviations were used to indicate the different types of silica nanoparticles: SiNPs when it is not clear which type was used, C-SiNPs for colloidal silica; S-SiNP for stöber silica; M-SiNPs for mesoporous silica; Pr-SiNPs for precipitated silica; and Py-SiNPs for pyrogenic silica.

The table summarizing in vitro studies (Table 1) was sorted according to the type of SiNPs (colloidal, stöber, mesoporous, pyrogenic, precipitated, and not specified) and cell types. Table 2 (in vivo studies) was organized according to the type of SiNPs and exposure routes.
Table 1

In vitro studies on SiNPs toxicity

Type of SiNPs

Cell line

Cell type

Particle primary size

Source

Exposure dose

Exposure duration

Endpoint

Assay(s)/method(s)

Results

References

C-SiNPs

V79

Hamster lung fibroblast

9,15, 30, and 55 nm

AkzoNobel AB

10–600 µg/ml

24 h

Cell viability

Tryphan blue exclusion and colony formation assay

Reduction with 15 nm NPs

Maser et al. (2015)

A549

Human type II alveolar epithelial

Genotoxicity

Comet assay

Increase with 15 nm NPs

C-SiNPs

H441

Human distal lung epithelial

30 nm

Sigma-Aldrich

0.6–600 µg/ml

4–20 h

Cell viability

MTS assay

Reduction at 600 µg/ml in monocultures

Kasper et al. (2011)

ISO-HAS-1

Human endothelial

Cytotoxicity

LDH assay

Increase at 600 µg/ml in all cultures

Cellular barrier integrity

TEER measurement

Reduction at 600 µg/ml in all cultures

Co-culture: H441/ISO-HAS-1

Human distal lung epithelial + human endothelial

Pro-inflammatory response(s)

ELISA

Increase of siCAM-1, IL-6 and IL-8

Cell morphology

Hoechst staining

Cells rounded at 600 µg/ml

Apoptosis

Western blot

Increase of apoptotic proteins

C-SiNPs

A549

Human type II alveolar epithelial

70 nm—with or without lung surfactant

Micromod Partikeltechnologie

100 µg/ml

4–20 h

Cell viability

MTS and crystal violet assay

Reduction in monocultures

Kasper et al. (2015)

Cytotoxicity

LDH assay

Increase in monocultures

Pro-inflammatory response(s)

ELISA

Increase of IL-8 in monocultures and co-cultures

Co-culture: A549 and ISO-HAS-1

Human type II alveolar epithelial + human endothelial

Cellular uptake

Hoechst staining

Increase in monocultures and co-cultures

C-SiNPs

Caco2

Human colon epithelial

15 nm (Levasil 200/40%) and 55 nm (Levasil 50/50%)

H.C. Starck

0.03–156.3 µg/cm2

24, 48 and 72 h

Cell viability

XTT assay

Dose-dependent reduction with 15 nm NPs

Tarantini et al. (2015a, b)

Oxidative stress

DCFH-DA assay

Dose-dependent increase with 15 nm SiNPs

Pro-inflammatory response(s)

ELISA

Increase of IL-8 with 15 nm NPs only at 156.3 µg/cm2

Apoptosis

Caspase-3 assay

Dose-dependent increase with 15 nm NPs

Genotoxicity

Micronuclei induction

Dose-dependent increase with 15 nm NPs

Genotoxicity

ɣH2AX fluorescence

Increase with 15 nm NPs

NP internalization

TEM

NPs detected in lysosomes and in endocytic compartments

C-SiNPs

HepG2

Human liver epithelial

19, 43, 68, and 498 nm

Laboratory synthesis

12–200 µg/ml

24 h

Cell viability

CCK-8 assay

Size and dose-dependent reduction

Li et al. (2011)

Cytotoxicity

LDH assay

Size and dose-dependent increase

Oxidative stress

DCFH-DA assay

Size-dependent increase

Genotoxicity

Comet assay

Size-dependent increase

Apoptosis

Apoptosis assay

Size-dependent increase

Cell cycle arrest

Flow cytometry

Size-dependent increase

Cell morphology

H & E staining

Cellular shrinkage, chromatin condensation and vacuolar degeneration detected

C-SiNPs

RAW 264.7

Mouse blood macrophage

20 and 100 nm (uncoated or l-arginine coated)

E&B Nanotech

10–640 µg/ml

24 h

Cell viability

WST-8 assay

Size and surface charge-dependent reduction

Kim et al. (2014b)

C-SiNPs and M-SiNPs

J774A.1

Mouse macrophage

100 nm

Laboratory synthesis

0.1–1000 µg/ml

24 and 72 h

Cell viability

MTT assay

Reduction only for C-SiNPs SiNPs

Lee et al. (2011)

Apoptosis

Annexin V/PI staining

Increase of caspase-3 activation

Pro-inflammatory response(s)

RT PCR and western blot

Increase of TNF-α, IL-6 and IL-1β

Pathway analysis

RT PCR and western blot

Activation of MAPKs and NF-kB

C-SiNPs

J744A.1

Mouse macrophage

25, 46, 183,182, and 188 nm

Sigma-Aldrich

24 h

Cell viability

WST-1 assay

ED50: 6–9 µg/ml and 15–22 µg/ml in J774 and 3T3, ,respectively

Rabolli et al. (2011)

BALB/c3T3

Mouse fibroblast

In vitro dosimetry

ISDD simulation

Similar delivered doses for all particle sizes

C-SiNPs

L5178Y/Tk+/−

Mouse lymphoma

7 nm

Sigma-Aldrich

0.01–150 µg/ml

4 h

Genotoxicity

Lymphoma assay

Mutations detected at 100 and 150 µg/ml

Demir and Castranova (2016)

C-SiNPs

PBMCs

Lymphocytes, monocytes and dendritic cells

10 and 100 nm

Polysciences

50–2000 µg/ml

24 and 48 h

Cell viability

FACS

Dose, time and size-dependent reduction

Mendoza et al. (2014)

Oxidative stress

GSH depletion

Size and dose-dependent increase

Oxidative stress

Western-blot

Dose-dependent increase in proteins with free radicals only with 10 nm

Pro-inflammatory response(s)

Multiplex bead array

Size and dose-dependent increase in cytokines

Cell morphology

Immuno electron microscopy

Size-dependent increase in cell damage

C-SiNPs

HUVEC

Human vein endothelial

10, 50, 150, and 500 nm

Polysciences

10 nm–10 μg/ml

50, 150 nm and 500 nm–50 μg/mL

1 h (10 nm) and 3 h (50–500 nm)

Oxidative stress

Fluorescent microscopy

Free radical increase

Corbalan et al. (2011)

Pro-inflammatory response(s)

Cytometric bead array

Size-dependent increase of IL-6 and IL-8

Oxidative stress

Nanosensors

Imbalance in [NO]/[ONOO−]

Inflammatory factors

RT-PCR

Size-dependent increase of ICAM1, VCAM1, SELE, MMP9, COX2 and F3

NF-κB-binding activity

ELISA

Increase of NF-κB-binding activity

NP internalization

TEM

NPs detected in cytoplasm and vesicles

C-SiNPs

Platelets

Mouse

50 nm

Polysciences

1, 5 and 25 µg/ml

30 min

Platelet aggregation

Aggregation assay

Dose-dependent increase

Nemmar et al. (2015)

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Calcium concentration

Fluorimeter

Increase in calcium concentration at 25 µg/ml

C-SiNPs

Platelets

Human

10, 50, 150, and 500 nm

Polysciences

1–200 μg/ml

15 min

Platelet aggregation

Aggregation assay

Size-dependent increase

Jose Corbalan et al. (2012)

Oxidative stress

Nanosensors

Size-dependent increase of NO

SELP and GPIIb/IIIa expression

Flow cytometry

Increase with 10, 100 and 150 nm

Morphology

TEM

Platelets strongly aggregated

S-SiNPs

A549

Human type II alveolar epithelial

2.1, 16.4, 60.4, and 104 nm

Laboratory synthesis

5 µg/cm2 of plate surface

12–24 h

Cell viability

MTT assay

Size-dependent reduction

Napierska et al. (2012b)

THP-1

Human monocyte

Cytotoxicity

LDH assay

Size-dependent increase

Biculture: A549/THP-1

Lung co-cultures

Pro-inflammatory response(s)

Cytometric bead array

Increase of all cytokines in the presence of 2 nm and 60 nm (except TNF-α)

Triculture: A549/THP-1/EA.hy926

Lung co-cultures

S-SiNPs

Co-culture: NCI-H441/ISO-HAS-1 with or without THP-1 cells

Lung co-cultures

15, 35, and 80 nm

Laboratory synthesis

50 and 100 µg/ml

72 h

Oxidative stress

DCFH-DA assay

Increase in the presence of THP-1

Farcal et al. (2012)

Pro-inflammatory response(s)

ELISA

Increase of IL-8 and TNF-α

Surfactant protein expression

RT-PCR

Increase of surfactant proteins in the presence of THP-1

Cellular barrier integrity

TEER measurement

No effect

S-SiNPs

HepG2

Human liver epithelial

43 nm

Laboratory synthesis

25–200 µg/ml

3 and 24 h

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Sun et al. (2011)

Mitochondrial membrane potential

Probe measurements

Dose-dependent increase

Apoptosis

Annexin V/PI staining and western blot

Dose-dependent increase

Cell morphology

TEM

NPs detected in cytoplasm, mitochondria and lysosomes

S-SiNPs

HepG2

Human liver epithelial

62 nm

Laboratory synthesis

25–100 µg/ml

24 h

Cell viability

MTT assay

Dose-dependent reduction

Yu et al. (2014)

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Autophagy

MDC staining

Dose-dependent increase

Autophagy

Immunoblot

Dose-dependent increase of LC3-II/LC3-I

NP internalization

LSCM

NPs detected in cytoplasm and mitochondria

S-SiNPs

LC-02

Human liver epithelial

50 nm

Laboratory synthesis

50–200 µg/ml

24 h

Cell viability

CCK-8 assay

Dose-dependent reduction

Wang et al. (2013)

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Apoptosis

Annexin V/PI staining

Dose-dependent increase

Mitochondrial damage

Mitotracker/laser confocal microscopy

Dose-dependent increase

Cell morphology

Hoechst staining

Dose-dependent increase in cell damage

S-SiNPs

HaCaT

Human keratinocyte

50 nm

Laboratory synthesis

25–500 µg/ml

4 h

Cell viability

MTT assay

Dose-dependent reduction

Liang et al. (2014)

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

GSH depletion

Increase

Apoptosis

Hoechst/PI staining

Increase

NP internalization

TEM

Detected in cytoplasm

S-SiNPs

EA.hy926

Human endothelial

16 (pure or iron-doped) and 60 nm

Laboratory synthesis

25 and 50 µg/ml

24 h

Cell viability

MTT assay

Reduction at 50 µg/ml with 16 nm NPs

Napierska et al. (2012a)

Cytotoxicity

LDH assay

Increase at 50 µg/ml with 16 nm NPs

Oxidative stress

DCFH-DA assay

Dose-dependent increase only with Fe doped NPs

Oxidative stress

GSH depletion

Dose-dependent increase only with Fe doped NPs

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation with Fe doped NPs

Oxidative stress

RT-PCR

Increase with 16 nm NPs

NP internalization

TEM

NPs found in cytoplasm

S-SiNPs

HUVEC

Human vein endothelial

62 nm

Laboratory synthesis

25–100 µg/ml

6, 12 and 24 h

Cell viability

MTT assay

Dose-dependent reduction

Duan et al. (2013a)

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Oxidative stress

SOD assay

Dose-dependent decrease

Oxidative stress

GSH Px assay

Dose-dependent decrease

Apoptosis

Annexin V/PI staining

Dose-dependent increase

Mitochondrial membrane potential

Probe measurements

Dose-dependent increase

Genotoxicity

Comet assay

Dose-dependent increase

Cell cycle analysis

Western blot

Dose-dependent upregulation of chk 1 and down regulation Cdc25c, Cyclin B1, Cdc2

NP internalization

LSCM and TEM

NPs detected in cytoplasm

S-SiNPs

HUVEC

Human vein endothelial

62 nm

Laboratory synthesis

25–100 µg/ml

24 h

Autophagy

MDC staining

Dose-dependent increase of LC-3 II/LC3-I

Duan et al. (2013b)

Autophagy

LSCM and TEM

Dose-dependent increase in cellular uptake and autophagic vacuoles, autophagosomes and autolysosomes detected

Expression of inflammatory factors

ELISA

Dose-dependent increase in the expression of CRP, TNF-α, IL-1β and IL-6

Oxidative stress

NO, NOS, eNOS and iNOS measurements

Dose-dependent increase of iNOS and decrease of NO, NOS and eNOS

Pathway analysis

Western blot

Dose-dependent decrease of p-mTOR/mTOR, p-P13 K/P13 K and p- Akt/Akt

S-SiNPs

HUVEC

Human vein endothelial

58 nm

Laboratory synthesis

12.5–100 µg/ml

24 h

Cell viability

MTT assay

Dose-dependent reduction

Guo et al. (2015)

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

GSH depletion

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Oxidative stress

SOD assay

Dose-dependent decrease

Oxidative stress

GSH-Px assay

Dose-dependent decrease

Inflammatory factors

ELISA

Increase of IL-1β, IL-8, TNFα, ICAM-1, VCAM-1, and MCP-1

Oxidative stress

NO, NOS, eNOS and iNOS measurements

Differential expression of NO, iNOS and eNOS activity and downregulation of ET -1

Pathway analysis

RT-PCR and western blot

Dose-dependent increase of Nrf-2, p-ERK, p-JNK, p-p38 MAPK and NF-kB

S-SiNPs

HUVEC

Human vein endothelial

62 nm

Laboratory synthesis

25–100 µg/ml

24 h

Cellular uptake

LSCM

Dose-dependent increase

Duan et al. (2014b)

Cytoskeleton damage

Cell cytoskeleton staining

Weakening of F actin at 100 µg/ml

Mitochondrial membrane potential

Probe measurements

Dose-dependent increase

Autophagy

TEM

Autophagic ultrastructures detected

Autophagy

Western blot

Increase of LC3-II/LC3-I

Inflammatory factors

Western blot

Decrease of ICAM-1 and VCAM-1

Pathway analysis

Western blot

Dose-dependent decrease of p-mTOR/mTOR, p-P13K/P13K and p-Akt/Akt

S-SiNPs

HUVEC

Human vein endothelial

58 nm

Laboratory synthesis

12.5–100 µg/ml

24 h

Cell viability

MTT assay

Size and dose-dependent reduction

Guo et al. (2016)

Cytotoxicity

LDH assay

Size and dose-dependent increase

Oxidative stress

DCFH-DA assay

Increase

Oxidative stress

LPO, SOD and GSH assay

Increase

Redox relative factors

RT-PCR

Increase in Nrf2 activation

Apoptosis

AO/EB staining

Increase

Mitochondrial membrane potential

Probe measurements

Decrease

Autophagy

TEM

Autophagosomes and autophagic vacuoles detected

Cellular uptake

ICP-AES

Increase

Pathway analysis

Western blot

Increase of MAPK/Bcl-2 and PI3 K/Akt/mTOR signaling

S-SiNPs and M-SiNPs

MPMCs

Murine peritoneal mast cells

25 nm

Laboratory synthesis

100 µg/ml

24 h

Cell viability

MTT assay

Reduction with non-porous NPs

Maurer-jones et al. (2010)

RBCs

Human red blood cells

Hemolysis of RBCs

Hemolysis assay

Increase with non-porous NPs

Co-culture: MPMC/3T3

Lung co-cultures

Cellular uptake

TEM and ICP-AES

M-SiNPs NPs internalized more than non-porous

S-SiNPs and M-SiNPs

A549

Human type II alveolar epithelial

115 nm (with or without amine modification)

Laboratory synthesis

10, 50, 100, 250 and 500 μg/ml

24 h

Cell viability

WST-8 assay

Dose-dependent reduction only in RAW 264.7. Amine modified SiNPs were less toxic

Yu et al. (2011)

RBCs

Human red blood cells

RAW 264.7

Mouse macrophage

Hemolysis of RBCs

Hemolysis assay

Increase only with M-SiNPs

Cytotoxicity

PI staining

Increase with M-SiNPs

Cellular uptake

ICP-MS

S-SiNPs internalized more than M-SiNPs

M-SiNPs

HT-29

Human colon epithelial

25 and 100 nm

Laboratory synthesis

10–150 µg/ml

24 h

Cell viability

SRB

No reduction

Sergent et al. (2012)

Cell viability

Impedancemetry

No reduction with 25 nm NPs; higher toxicity at lower doses with 100 nm NPs

Cell viability

Flow cytometry

Limited toxicity with 25 nm; higher toxicity at lower doses with 100 nm NPs

Genotoxicity

ɣH2AX fluorescence

No effect with 25 nm; greater DNA damage at lower doses with 100 nm NPs

M-SiNPs

NRK-52E

Rat kidney epithelial

198 nm

Laboratory synthesis

25–1000 µg/ml

3 and 24 h

Cell viability

MTT assay

Dose-dependent reduction

Chen et al. (2015)

Cytotoxicity

LDH assay

Dose-dependent increase

Expression of inflammatory factors

Western blot

Dose-dependent increase of FN, TGF-β, and ICAM-1 (50–400 μg/ml)

Pathway analysis

Immuno-fluorescence staining

Dose-dependent increase in the expression of NF-κB p65 (50–400 μg/ml)

M-SiNPs

HEK293

Human kidney epithelial

100 nm, 2.3 nm pore size

Laboratory synthesis

100 µg/ml

24, 48 and 72 h

Cellular morphology

TEM

Cells shrunk and nucleus condensed

Zhang et al. (2015)

Oxidative stress

Fluorescent dihydroethi-dium

No effect

Oxidative stress

RT PCR

No effect

Chromosomal aberrations

FISH assay

No effect

Mutations

EGFR and KRAS

No effect

Genotoxicity

Human mRNA micro array

579 genes upregulated and 1263 genes downregulated

M-SiNPs

EA.hy926

Human endothelial

48 nm (surface functionalized with PET and TMS)

Laboratory synthesis

20–1000 µg/ml at a flow rate of 30 µl/min using a microfluidic device

Cell viability

Tryphan blue

No effect

Kim et al. (2014a)

Platelets (activated and non-activated)

Human

Platelet adhesion

Adhesion assay

Increase at 1000 µg/ml

Platelet aggregation

Aggregation assay

Increase at 1000 µg/ml

Morphology

Fluorescent fixed cell imaging

Large aggregate formation at 1000 µg/ml

Py-SiNPs and S-SiNPs

A549

Human type II alveolar epithelial

12 (Py-SiNPs) and 50 nm(S-SiNPs)

Py-SiNPs—Evonik; S-SiNPs—Landsberg am Lech

52 μg/cm2 and 117 μg/cm2 (ALI)

15.6 μg/cm2 (sub-merged)

ALI (5 and 7 h)

Sub-merged (24 h)

Cytotoxicity

LDH assay

Dose-dependent increase

Panas et al. (2014)

Pro-inflammatory response

ELISA

Increase in IL-8

Expression of inflammatory factors

Western blot

Increase in the expression of COX-2 and phosphorylated p38; stronger effects observed in sub-merged culture

Pr-SiNPs

A549

Human type II alveolar epithelial

15 nm

Nanoamor

25–200 µg/ml

72 h

Cell viability

MTT assay

Dose-dependent reduction

Ahamed (2013)

A431

Human skin epithelial

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Oxidative stress

GSH depletion

Dose-dependent increase

Apoptosis

RT PCR

Dose-dependent increase of caspase 3 and caspase 9

Pr-SiNPs (NM-200) and Py-SiNPs (NM-203); NRT-808, NRT-817, NRT-820 and NRT-944

Immortalized balb/3T3

Mouse fibroblast

10–25, 5–30, 15,35, 80, and 90 nm

NM-JRC repository; NRT-Laboratory synthesis

1–100 µg/ml

72 h

Cell viability

MTT assay

Reduction with 15 nm NPs

Uboldi et al. (2012)

Cell viability

Colony formation assay

No effect

Cell viability

Cell transformation assay

No effect

Genotoxicity

Micronuclei induction

No effect

NP internalization

Fluorescent microscopy

NPs detected in cytoplasm

Two Py-SiNPs samples (different sizes) one Pr-SiNPs and two C-SiNPs (different sizes)

V79

Hamster lung fibroblast

20–80 nm

SiNPs

12.5–100 µg/cm2

24 h

Cell viability

WST-1 assay

Dose-dependent reduction

Guichard et al. (2016)

Oxidative stress

DCFH-DA assay

No effect

Apoptosis

Caspase-3 assay

Dose-dependent increase

Genotoxicity

FPG modified comet assay

Dose-dependent increase

Genotoxicity

Micronuclei induction

No effect

Mutation

HPRT test

No effect

Py-SiNPs(12 and 40 nm) and S-SiNPs (200 nm)

HT-29

Human colon epithelial

12, 40, and 200 nm

Py-SiNPs—Evonik; S-SiNPs—Angström Sphere

0.03–156.3 µg/cm2

24 h

Cell viability

WST-1 assay

Size-dependent reduction

Gehrke et al. (2013)

Cell viability

SRB assay

Size-dependent reduction

Cytotoxicity

LDH assay

Size-dependent increase

Oxidative stress

DCFH-DA assay

No effect

Oxidative stress

GSH depletion

Dose-dependent increase with 12 nm NPs

Pathway analysis

Western blot

Dose-dependent increase of ERK1/2 phosphorylation with 12 nm

Py-SiNPs

C2BBe1

Human colon epithelial

12 nm

Sigma-Aldrich

10 µg/cm2

Acute—24 h

Chronic—29 passages

Cell viability

MTT assay

No effect

McCracken et al. (2013)

Cytotoxicity

LDH assay

No effect

Apoptosis

Annexin V/PI staining

No effect

Necrosis

Sytox red staining

No effect

Cellular uptake

TEM

Uptake only by a fraction of cells

Py-SiNPs and Pr-SiNPs

GES-1

Human gastric epithelial

10–50 nm

Evonik and Haihua

10–600 µg/ml

24, 48 and 72 h

Cell viability

CCK-8 assay

Size and dose-dependent reduction

Yang et al. (2014b)

Caco2

Human colon epithelial

Cytotoxicity

LDH assay

Size and dose-dependent increase

Oxidative stress

DCFH-DA assay

No effect

Apoptosis

Apoptosis assay

No effect

Cell cycle arrest

Cell cycle assay

Increase at S phase for GES cells and G2/M for caco2 cells

Cellular uptake

TEM

NPs detected in cytoplasm

Cellular morphology

Ultramicrotome and TEM

Increase in cell damage

Cellular barrier integrity

TEER measurement

No effect

Py-SiNPs (NM 203) and Pr-SiNPs (NM 202)

MH-S and RAW 264.7

Mouse macrophages

14 nm

JRC repository

5 or 10 µg/cm2

24 h

Cytotoxicity

HCSA

Py-SiNPs induced greater reduction than Py-SiNPs

Di Cristo et al. (2016)

Cellular uptake

He Ion microscopy and flow cytometry

Higher for Py-SiNPs

Macrophage activation

RT-PCR

Increase

Pro-inflammatory cytokines

ELISA

Increase of TNF-α, IL-6 and IL-1β

Oxidative stress

Fluorescence microscopy

No ROS induction

Pr-SiNPs

HePG2

Human liver epithelial

14 nm

Nanoamor

1–200 µg/ml

72 h

Cell viability

MTT and NRU assay

Dose-dependent reduction at 25–200 µg/ml

Ahmad et al. (2012)

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase MDA formation

Oxidative stress

GSH depletion

Dose-dependent increase

Apoptosis

RT PCR and western blot

Up regulation of p53, bax, and caspase 3 and down regulation of bcl 2

Apoptosis

Caspase-3 assay

Dose-dependent increase

Pr-SiNPs (NM 200 & 201), Py-SiNPs (NM 202 & 203)

Human peripheral lymphocytes

Human

14–16 nm

JRC repository

200–1250 µg/ml

24 h

Genotoxicity

Cytokinesis block micronuclei induction

No effect

Tavares et al. (2014)

S-SiNPs

Primary Microglial cells

Rat macrophage like cells

150–200 nm

Laboratory synthesis

0.0728–7.28 µg/ml

24 h

Cell viability

MTS assay

No effect

Choi et al. (2010)

Cytotoxicity

LDH assay

No effect

Oxidative stress

DCFH-DA assay

No effect

NO production

DAF-FM assay

No effect

Pro-inflammatory response(s)

Luminex

Reduction in TNF-α. A small amount of IL-β detected

Inflammatory factors

RT-PCR

Increase in COX-2

Reverse microemulsion

HK-2

Human kidney epithelial

20 and 100 nm

Laboratory synthesis

5–500 µg/ml

24,48 and 72 h

Cell viability

WST-1 and clonogenic assay

Size, dose and time-dependent reduction

Passagne et al. (2012)

LLC PK-1

Porcine kidney epithelial

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Oxidative stress

Fluorescent dihydroethi-dium

Dose-dependent increase

Oxidative stress

RT-PCR

Upregulation of anti-oxidant genes

Cellular uptake

TEM

Internalization of particles

SiNPs

A549

Human type II alveolar epithelial

20 nm (with or without amine or carboxyl coated)

Laboratory synthesis

200–1000 µg/ml

12–48 h

Cell viability

PI staining

Reduction above 250 µg/ml

Nowak et al. (2014)

Autophagy

MDC staining

Threefold and fivefold increase at 100 and 1000 µg/ml ,respectively

Autophagy

RT-PCR

ATG-12 and BECN genes upregulated at 1000 µg/ml

Cellular uptake

Immunostaining and fluorescent microscopy

Functionalized particles efficiently taken up than bare SiNPs

Cellular morphology

TEM

Cell blebbing detected

SiNPs

BEAS-2B

Human bronchial epithelial

20–40 nm

Sigma-Aldrich

1 µg/ml

24 h

Cell viability

MTT assay, flow cytometry

Reduction in cell viability

Eom and Choi (2011)

Oxidative stress

DCFH-DA assay

Increase in ROS production

Pathway analysis

Western blot

Increase in HO-1, Nrf-2 and ERK phosphorylation

SiNPs

HFL-1

Human lung fibroblast

20 and 80 nm

Laboratory synthesis

250–2000 µg/ml

48 h

Cell viability

MTT assay

Size and dose-dependent reduction

Xu et al. (2012)

Oxidative stress

DCFH-DA assay

Size and dose-dependent increase

Apoptosis

Flow cytometry

Size and dose-dependent increase

Apoptotic pathway analysis

Western blot

Increase of p53 and differential expression of cytochrome C, Bax, Bcl‐2, caspase, β‐actin and COX IV

SiNPs

MRC-5

Human lung fibroblast

4–13 nm

NaBond Technologies

12.5–62.5 µg/ml for 24, 48 and 72 h (cell viability)

Other assays—62.5 µg/ml

Cell viability—48 and 72 h

Other assays—24, 48 and 72 h

Cell viability

MTT assay

Time and dose-dependent reduction

Voicu et al. (2015)

Oxidative stress

DCFH-DA assay

Time-dependent increase

Oxidative stress

GSH depletion

Time-dependent increase

Advanced oxidation protein products

Western blot

Time-dependent increase

Apoptosis

PI staining

Time-dependent increase

Autophagy

MDC staining

Time-dependent increase of LC-3 II/LC3-I

Cellular morphology

H & E staining

Vacuolization of cytoplasm detected

SiNPs

A549

Human type II alveolar epithelial

30 nm

Nanoamor

0.0–100 µg/ml

24 h

Cell viability

MTT assay

Dose-dependent reduction in A549

Michael Berg et al. (2013)

MeT-5A

Human bronchial epithelial

Oxidative stress

DCFH-DA assay

Dose-dependent increase in A549

Oxidative stress

GSH depletion

Dose-dependent increase in A549

Oxidative stress

Western blot and RT PCR

Differential expression of anti-oxidant proteins

Oxidative stress

Nrf 2 Immunofluorescence

Increase of Nrf 2 in A549

Oxidative stress

Catalase assay

Dose-dependent increase

SiNPs

HFL-1

Human lung fibroblast

20 nm

Nanjing High Technology of Nano

250–1500 µg/ml

48 h

Cell viability

MTT assay

Dose-dependent reduction

Zhang et al. (2011)

Apoptosis

Fluorescence microscopy

Dose-dependent increase

Cellular morphology

Phase contrast microscopy

Detection of morphological changes and NPs accumulation

SiNPs

HaCaT

Human keratinocyte

15, 30, 100, and 5000 nm

Wan Jing New Material Co. Ltd and Sigma-Aldrich (5000 nm)

0.5–100 µg/ml

24 h

Cell viability

CCK-8 assay

Size and dose-dependent reduction

Gong et al. (2012)

Oxidative stress

DCFH-DA assay

Size and dose-dependent increase

Apoptosis

Annexin V/PI staining

Size and dose-dependent increase

Genotoxicity

8-OH-dG

Size and dose-dependent increase

Genotoxicity

ɣH2AX fluorescence

Size and dose-dependent increase

Genotoxicity

Comet assay

Size and dose-dependent increase

Cellular morphology

Microscopy

Morphology affected at 10 µg/ml

SiNPs

HaCaT

Human keratinocyte

70, 300, and 1000 nm

Micromod Partikeltechnologie

10–1250 µg/ml

24 h

Cytotoxicity

LDH assay

Dose-dependent increase

Nabeshi et al. (2011a)

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

Hydroxyl fluorescein assay

Dose-dependent increase

Genotoxicity

8-OH-dG

Increase in tail length and tail moment

Cellular uptake

Assessment using Cytochalasin D and apocynin

Internalization of NPs by NADPH oxidase independent endocytosis

SiNPs

HaCaT

Human keratinocyte

70, 300, and 1000 nm

Micromod Partikeltechnologie

100 µg/ml

24 h

NP internalization

TEM

70 nm NPs into the nucleus and 300 and 1000 nm SiNPs only in endosomes

Nabeshi et al. (2011b)

Genotoxicity

Comet assay

Increase in tail length with 70 nm NPs

SiNPs

HepG2

Human liver epithelial

7, 20, and 50 nm

Shanghai Cabot Chemical

20–640 µg/ml

24, 48 and 72 h

Cell viability

MTT assay

Dose and time-dependent reduction

Lu et al. (2011)

LC-02

Human hepatocyte

Oxidative stress

DCFH-DA assay

Increase in HepG2

Oxidative stress

GSH depletion

Increase in HepG2

Apoptosis

Annexin V/PI staining

Dose-dependent increase in HepG2

SiNPs;

Kupffer cells

Rat liver macrophage

15 nm

Sigma-Aldrich

50–800 µg/ml

Supernatant was incubated with BRL cells

24 h

Cell viability

CCK-8 assay

Dose-dependent reduction in BRL

Chen et al. (2013)

BRF

Rat liver cells

Cytotoxicity

LDH assay

Dose-dependent increase in BRL

Oxidative stress

DCFH-DA assay

Dose-dependent increase in KCs

Oxidative stress

GSH depletion

Dose-dependent increase in KCs

Pro-inflammatory response

ELISA

Dose-dependent increase of TNF-α in KCs

NO production

Griess reagent

Dose-dependent increase of NO in KCs

Cellular morphology

Phase contrast microscopy

Cell damage observed for BRL

SiNPs

RAW 264.7

Mouse macrophage

12 nm

Sigma-Aldrich

200 and 400 µg/ml

24 h

Cell viability

WST-8 assay

Dose-dependent reduction

Hashimoto and Imazato (2015)

Genotoxicity

Hoechst/PI staining

Deformation of nuclei at both concentrations

Genotoxicity

Comet assay

Dose-dependent increase

Genotoxicity

Micronuclei induction

Increase

Cellular uptake

SEM and TEM

NPs detected in vesicles and in nucleus

SiNPs

Human peripheral lymphocytes

Human

10–20 nm

Sigma-Aldrich

50–100 µg/ml

24 h

Cell viability

MTT assay

Dose-dependent reduction

Rajiv et al. (2015)

Cytotoxicity

LDH assay

Dose-dependent increase

Oxidative stress

DCFH-DA assay

Dose-dependent increase

Oxidative stress

LPO assay

Dose-dependent increase of MDA formation

Oxidative stress

GSH depletion

Dose-dependent increase

Oxidative stress

SOD assay

Dose-dependent decrease

Oxidative stress

Catalase assay

Dose-dependent increase

Genotoxicity

Comet assay

Increase

Chromosomal aberrations

Giemsa staining and microscopy

No effect

SiNPs

Primary microglial cells

Rat brain macrophage like cells

20 nm

Sigma-Aldrich

250 and 500 mg/ml

24 h

Cell viability

MTT assay

No reduction

Xue et al. (2012)

PC12

Rat neuron like cells

Pro-inflammatory response

ELISA

Mild increase of cytokines such as IL-6, TNF-α and IL-1β

NO production

Griess reagent assay

No effect

Nuclear binding activity

RT-PCR

No effect

Inflammatory factors

Western blot

No effect

SiNPs

PC12

Rat neuron like cells

25± nm

Saint Louis

25–200 µg/ml

24 h

α-Synuclein levels

Western blot

Dose-dependent increase

Xie and Wu (2016)

Cellular uptake

TEM

Increase

Autophagy

Western blot

Dose-dependent Increase of LC-II and Beclin 1

Pathway analysis

Western blot

Dose-dependent decrease of PI3K-Akt-mTOR signaling

SiNPs

RAW 264.7

Mouse macrophage

10, 50, 300, and 1000 nm (with and without amine modification)

Micromod Partikeltechnologie

6.25–100 µg/m (with LPS and PGN)

24 h

Cell viability

WST-8

Dose and size-dependent reduction (only for bare SiNPs)

Uemura et al. (2016)

Pro-inflammatory responses

ELISA

Dose-dependent increase of TNF-α and decrease of IL-6 (only for bare SiNPs). IL-6 decrease for micro particles

TEER Transepithelial electrical resistance, RT-PCR Reverse transcription polymerase chain reaction, ISDD In vitro sedimentation, diffusion and dosimetry model, FACS Fluorescence-activated cell sorting, LSCM Laser scanning confocal microscopy, AO/EB Acridine orange/ethidium bromide, FISH Fluorescence in situ hybridization, EGFR Epidermal growth factor receptor, KRAS Ki-ras2 Kirsten rat sarcoma viral oncogene homolog, HPRT Hypoxanthine phosphoribosyl transferase, HSCA High content screening and analysis, DAM-FM 4-amino-5-methylamino-2′,7′-difluorescein, PI Propidium iodide, Nrf 2 Nuclear factor erythroid 2-related factor 2

Table 2

In vivo studies on SiNPs toxicity

Type of SiNPs

Exposure route

Primary particle size

Source

Species and Strain

Exposure dose

Exposure duration

Organ(s)/sample(s)

Effect(s)/endpoint(s)

Assay (s)/method(s)

Results

References

C-SiNPs

Oral

20 and 100 nm

E&B Nanotech

Rat; SD (female and male)

500 and 1000 mg/kg bw

1 h–10 days

Animal

Clinical signs

Observation

No animal death

Lee et al. (2014)

Lungs, liver, brain, kidneys, testis and spleen

Tissue distribution

Molybdenum blue method

NPs detected in lungs, liver, spleen and kidneys

NP Localization

TEM

NPs detected in the nuclei of hepatocytes

Urine and feces

Excretion kinetics

Molybdenum blue method

Most NPs excreted via feces

C-SiNPs

Oral

20 and 100 nm (uncoated or l-arginine coated)

E&B Nanotech

Mice; C57BL/6

750 mg/kg bw

Daily for 14 days

Blood

WBCs count

Serum analyzer

Increase

Kim et al. (2014b)

Pro-inflammatory response(s)

Multiplex analysis

Differential expression of cytokines

Spleen

Oxidative stress

DCFH-DA assay

Increase

Oxidative stress

SOD and GPx assay

No effect

Oxidative stress

Griess reagent/NO production

Increase

C-SiNPs

Oral

12 nm

ABC Nanotech

Rat; SD

Acute-1959 and 2061; Sub-acute-489.8, 979.5, 1959; Sub-chronic: 244.9, 489.8 and 975.9 mg/kg bw

Acute—14 days

Sub-acute—daily for 14 days

Sub-chronic: daily for 13 w

Blood

Blood cell count

Hematoanalyzer

No effect

Yun et al. (2015)

Organ damage bio-markers

Serum analyzer

No effect

Urine

Urinalysis

Urine analyzer

No effect

Eye

Ophthalmology

Morphological examination

No effect

Major organs

Tissue distribution

ICP-MS

NPs not detected

Urine and feces

Excretion kinetics

ICP-MS

Most NPs excreted via feces

C-SiNPs

Oral gavage

20 and 80 nm, coated with l-arginine

E&B Nanotech

Rats; Crl:CD(SD)

500, 1000 and 2000 mg/kg bw

Daily for 90 days

Animal

Clinical signs

Observation

No animal death

Kim et al. (2014c)

Blood

Blood cell count

Hematoanalyzer

No effect

Organ damage bio-markers

Serum analyzer

No effect

Eye

Ophthalmology

Observation of ocular fundus

No effect

All major organs

Necropsy

Weighing organs

No effect

Histopathology

H & E staining

No effect

C-SiNPs

Intratracheal

9,15, 30 and 55 nm

AkzoNobel AB

Rat; Wistar (female)

360 µg in 500 ml of saline

3 days

Blood

Blood cell count

Hematoanalyzer

Increase of polymorphonulcear neutrophils and lymphocytes

Maser et al. (2015)

Lungs

Histopathology

H & E staining

Mild increase of granulomatous inflammation

Genotoxicity

Comet assay

No effect

Bone marrow

Genotoxicity

Micronucleus test

No effect

C-SiNPs

Dermal

20 nm, coated with l-arginine

E&B Nanotech

Rat; Sprague–Dawley (SD)

500 mg, 1000 mg, and 2000 mg/kg bw

Daily for 90 days

Animal

Clinical signs

Observation

No animal death

Ryu et al. (2014)

Blood

Blood cell count

Hematoanalyzer

No effect

Organ damage bio-markers

Serum analyzer

No effect

All major organs

Necropsy

Organ weighing

No effect

Histopathology

H & E staining

No effect

C-SiNPs

Dermal

20 and 100 nm (uncoated or l-arginine coated)

E&B Nanotech

Rat

2000 (coated) and 1000 mg/kg bw(uncoated)

Daily for 28 d

Brain

Blood–brain barrier damage

Evans blue staining

No effect

Shim et al. (2014)

Tissue distribution

TEM-EDX

NPs not detected

C-SiNPs

Intraperitoneal

50 nm

Polysciences

Mice; Male tuck ordinary

0.25 mg/kg bw

24 h

Blood

Blood cell count

Hematoanalyzer

Increase of leukocytes

Nemmar et al. (2016)

Organ damage bio-markers

Serum analyzer

Increase of CK, ALT and AST

Lungs, heart, liver, kidneys, and brain

Oxidative stress

LPO, SOD and catalase assay

Increase

Pro-inflammatory response(s)

ELISA

Differential expression of IL-6, TNF-α and IL-1β

Genotoxicity

Comet assay

Increase

C-SiNPs

Intravenous

64 nm

Laboratory synthesis

Mice; ICR (male and female)

0, 29.5, 103.5, and 177.5 mg/kg bw

14 days

Blood

Organ damage bio-markers

Serum analyzer

Increase of LDH, ALT and AST

Yu et al. (2013)

Liver, spleen, kidneys, heart, lungs, and brain

Histopathology

H & E staining

Tissue damages observed

CD-68 positive cells

CD-68 staining

Increase in liver and spleen

NP localization

TEM

NPs detected in liver macrophages and in the endothelial cells of lungs and kidneys

Tissue distribution

ICP-OES

Si detected in liver and lungs

S-SiNPs

Intratracheal

50 nm, with or without amine modification

Laboratory synthesis

Mice; C57BL/6 (male)

4 and 20 mg/kg bw

24 h

Lungs

Inflammation

BALF cell count and LDH assay

Dose-dependent increase of total cell number, macrophages, neutrophils and LDH release

Morris et al. (2016)

Oxidative stress

ROS/RNS production

Dose-dependent increase

S-SiNPs

Intratracheal

58 nm

Laboratory synthesis

Mice; C57 (male)

2 mg/kg bw

Once every 3 days for 45 days

Testis

Histopathology

H & E staining

Decrease in mature sperm and primary spermatocytes

Zhang et al. (2016)

Meiotic regulating factors

Western blot

Cell cycle arrest observed

Oxidative stress

DCFH-DA assay

Increase

Sperm

Quality evaluation

Microscopy

No effect

S-SiNPs

Intratracheal

43 nm

Laboratory synthesis

Mice; BALB/c(female)

0, 7, 21, and 35 mg/kg bw

Once every 3 days for 15 days

Lungs, liver and heart

Histopathology

H & E staining

Increase of Inflammation

Yang et al. (2016)

Organ damage bio-markers

Serum analyzer

Increase of BUN, CREA, uric acid, and AST

NP localization

TEM

NPs detected in cytoplasm and lysosomes

Macrophage activation

Immunohistochemistry

Increase

Inflammatory response

Multiplex flow cytometry

Increase of IL-8, TNF-α and IL-6

Duan et al. (2014a, b)

S-SiNPs

Intravenous

62 nm

Laboratory synthesis

Mice; ICR

29.5, 103.5, and 177.5 mg/kg bw

14 days

Heart

Autophagy

LC3 and VEGFR2 positive staining

Increase of LC3

TEM

Increase of autophagic ultrastructures

Cell cytoskeleton staining

Weakening of F actin

MMP measurements

Dose-dependent decrease

LC3-II/LC3-I ratio

Increase

M-SiNPs

Intragastrical

Spherical 83 nm, short rods (AR 1.75) and long rods (AR 5)

Laboratory synthesis

Mice; ICR (male)

40 mg/kg bw

14 days

Blood

Blood cell count

Hematoanalyzer

No effect

Li et al. (2015)

Organ damage bio-markers

Serum analyzer

Increase of LDH release and CREA

Liver, kidneys, spleen, lungs and small intestine

Histopathology

H & E staining

Gross tissue damage in kidneys

Tissue distribution and excretion kinetics

ICP-OES and TEM

shape-dependent distribution and clearance in organs

M-SiNPs

Intravenous

1.5 (short rods) and 5 (long rods) aspect ratio; standard or PEGylated

Laboratory synthesis

Mice

20 mg/kg bw

2 h, 24 h and 7 days

Blood

Blood cell count

Hematoanalyzer

No effect

Huang et al. (2011)

Organ damage bio-markers

Serum analyzer

Increase of TBIL, CREA and BUN

Liver, spleen, lungs and kidneys

Histopathology

DAPI

Gross tissue damage in kidneys

Tissue distribution

ICP-OES

PEGylation reduced distribution in liver and spleen

Urine and feces

Excretion kinetics

TEM/EDX

Short rods cleared rapidly than long rods

M-SiNPs

Intraperitoneal

~198 nm

Laboratory synthesis

Mice; BALB/C

150, 300, and 600 mg/kg bw

2 and 12 days

Blood

Organ damage bio-markers

Serum analyzer

Increase of AST, ALT BUN and CREA

Chen et al. (2015)

Kidneys

Histopathology

H & E staining

Detection of renal interstitial fibrosis

Tissue damage

Masson’s trichrome staining

Dose and time-dependent increase in kidney injury

Pr-SiNPs (NM 200 & 201), Py-SiNPs (NM 202 & 203)

Oral gavage

18–24 nm

JRC repository

Rat; SD (male)

5, 10, and 20 mg/kg bw

0, 24, and 45 h

Blood

Oxidative stress

LPO assay

No effect

Tarantini et al. (2015a, b)

Liver, kidneys, spleen, intestine, blood and bone marrow

Histopathology

H & E staining

No effect

Genotoxicity

Alkaline, FpG modified comet assay and micronucleus test

No effect

Pr-SiNPs (NM 200)

Oral gavage

10–15 nm

JRC repository

Rat; Wistar

100, 300, or 1000 mg/kg bw at a dose volume of 10 mL/kg bw

Daily for 14 days (from gestation period 6–19)

Animal

Clinical signs

Observation

No animal death

Hofmann et al. (2015)

Body weight

Observation

No effect

Gravid uterus

Necropsy

Cesarean

No effect

Fetus

Fetus gross damage

Morphological examinations

No effect

Py-SiNPs

Exposure via food

7 nm and NM 202 (10–25 nm)

JRC repository

Rat; SD

Sub-acute: 100, 1000,2500 mg/kg bw

Sub-chronic: 2500 mg/kg bw

Sub-acute: daily for 28 days

Sub-chronic: daily for 84 days

Blood

Organ damage bio-markers

Serum analyzer

No effect

Zande et al. (2014)

Plasma IgG and IgM and cytokine analysis

ELISA

No effect

Liver

Transcriptomic analysis

mRNA quantification kit

No effect

Liver, jejunum, kidneys and spleen

Histopathology

H & E staining

Presence of fibrosis in the liver of NM 202 treated rats

Liver, spleen, lungs, brain and testis

Tissue distribution

ICP-MS

NM 202 detected in lungs, kidneys and spleen; SAS only in the spleen

Py-SiNPs

Intravenous

13 ± 5 nm

Vekton Ltd

Rat; Wistar

7 mg/kg bw

7, 30 and 60 days

Blood

Hemodynamics

Blood pressure measurement and heart rate

No effect

Zhuravskii et al. (2016)

Blood cell count

Hematoanalyzer

No change

Organ damage bio-markers

Serum analyzer

Increase of ALP at 7 d

Liver, heart, spleen, lungs and kidneys

Tissue distribution

Atomic absorption spectrometry

Si detected in liver, lungs and spleen

NP localization

SEM

NPs detected in hepatocytes

Histopathology

H & E staining

Induction of fibrosis

SiNPs

Oral gavage

70 nm, 300 and 1000 nm with or without carboxyl or amine groups

Micromod Partikeltechnologie

Mice; BALB/c

2.5 mg/mouse

Daily for 28 days

Blood

Blood cell count

Hematoanalyzer

No effect

Yoshida et al. (2014)

Organ damage bio-markers

Serum analyzer

No effect

All major organs

Histopathology

H & E staining

No effect

Intestine

Intestinal absorption

Evented gut sac analysis

Absorption of coated 70 nm SiNPs

SiNPs

Oral gavage

10–15 nm

TECNAN

Rat; Wistar (male)

333.3 mg/kg bw

Daily for 5 days

Animal

Clinical signs

Observation

Symptoms of vomiting and severe lethargy

Hassankhani et al. (2014)

Blood

Organ damage bio-markers

Serum analyzer

Increase

Kidneys, lungs and testis

Histopathology

H & E staining

Tissue damage observed

SiNPs

Intratracheal

Three SiNPs (30, 60, and 90 nm) and one fine-sized silica (600 nm)

Laboratory synthesis

Rat; Wistar (male)

2,5 and 10 mg/kg bw

Daily for 16 days

Blood and Heart

Blood cell count

Hematoanalyzer

Increase of WBCs & platelets and decrease of hemoglobin &RBCs

Du et al. (2013)

Oxidative stress

LPO, GSH, SOD and GSH-Px assay

Increase of MDA formation

Oxidative stress

NO/NOS

Increase of NO and decrease of NOS

Pro-Inflammatory response(s)

ELISA

Increase of TNF-a, IL-1b and IL-6

Tissue distribution

ICP-OES

NPs detected in heart

SiNPs in paints

Oropharyngeal

19 nm

SiNPs

Mice; BALB/c mice

20 µg/aspiration

Once a week for 5 w

Lungs

Inflammation

BALF Cell count

Increase of macrophages and neutrophils

Smulders et al. (2014)

Pro-inflammatory response(s)

ELISA

No effect

Tissue distribution

ICP-MS

NPs detected in lungs

SiNPs

Intranasal

10 and 80 nm SiNPs

Nanoamor

Rat; Wistar (male)

150 µg/50 µl PBS/rat

Daily for 30 days

Brain

Oxidative stress

LPO assay

Increase of MDA formation

Parveen et al. (2015)

Xylenol orange assay

Increase of H2O2 levels

GSH depletion

Increase

SOD, CAT and GPx levels

Increase

Pro-Inflammatory response(s)

RT-PCR and ELISA

Increase TNF-α, IL-1β and MCP-1

Nuclear binding activity

Immuno blot analysis

Increase

Tissue distribution

ICP-OES

Si detected in frontal cortex, corpus striatum and hippocampus

SiNPs

Topical

70 nm, 300 and 1,000 nm

Micromod Partikeltechnologie

Mice; BALB/c

250 mg/ear

Daily for 28 days

Skin

Apoptosis

TUNEL staining

Increase

Nabeshi et al. (2011b)

Animal

Tissue distribution

In vivo Imaging

70 nm SiNPs detected around the liver

Intravenous

30 mg/kg bw

24 h

NP internalization

TEM

70 nm SiNPs detected in cytoplasm and nucleus of the parenchymal hepatocytes (liver)

SiNPs

Intravenous

15 nm

Sigma-Aldrich

Rat; SD (male)

50 mg/kg bw

48 h

Blood

Blood cell count

Hematoanalyzer

Increase of WBC, lymphocytes, monocytes and neutrophils

Chen et al. (2013)

Liver

CD-68 positive cells

CD-68 staining

Increase

Oxidative stress

GSH and SOD assay

Increase

Injury bio-markers

Proton-NMR spectroscopic analysis

Increase

JRC Joint research commission, TEM Transmission electron microscopy, WBCs White blood cells, ICP-MS Inductively coupled plasma mass spectrometry, H & E Hematoxylin and eosin, EDX Energy dispersive X-ray spectroscopy, ELISA Enzyme linked immuno sorbent Assay, CD Cluster of differentiation, ICP-OES Inductively coupled plasma optical emission spectroscopy, BALF Bronchoalveolar lavage fluid, VEGFR Vascular endothelial growth factor receptor, MMP Mitochondrial membrane potential, DAPI-4′,6-diamidino-2-phenylindole, dilactate, SEM Scanning electron microscopy, TUNEL Terminal deoxynucleotidyl transferase dUTP nick end labeling, NMR Nuclear magnetic resonance

In vitro studies

Cytotoxicity

Oxidative stress (over production of reactive oxygen species, i.e., ROS) induced by NPs could damage the cellular components and lead to cell death via apoptosis (Fu et al. 2013). Therefore, studies reporting on cytotoxicity and oxidative stress were summarized in this section.

Cytotoxicity associated with oxidative stress

Duan et al. (2013a) showed that S-SiNPs (62 nm) induced time- (6, 12, and 24 h) and dose-dependent (25–100 µg/ml) reduction in cell viability (assessed by 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide, i.e., MTT), loss of membrane integrity (lactate dehydrogenase (LDH) release) and apoptosis (Annexin V/PI staining) in human umbilical vein endothelial cells (HUVECs). Apoptosis was also induced in lung (A549) and skin epithelial cells (A431) treated with Pr-SiNPs (15 nm). A dose-dependent increase (25–200 µg/ml for 72 h) in cytotoxicity (MTT and LDH), ROS production (assessed by dichlorodihydrofluorescein assay, i.e., DCFH-DA), lipid peroxidation (measurement of malondialdehyde, i.e., MDA), and apoptosis (caspase 3 and 9 activity) was observed in both cell lines. The lung cells showed, in general, a slightly higher toxic response compared to skin cells (Ahamed 2013).

SiNPs (20 and 80 nm) induced P53-mediated apoptosis in human fetal lung fibroblasts (HFL-1). At the dose of 500 µg/ml, 20 nm SiNPs induced a threefold increase in DCF fluorescence compared to 80 nm. In addition, increased expression of P53, upregulation of cytochrome C (CytC) and caspase 9, and downregulation of anti-apoptotic protein B-cell lymphoma 2 (bcl2) was observed in cells treated with 1000 µg/ml for 48 h (Xu et al. 2012). Another study with lung fibroblasts also showed that SiNPs (20 nm) could reduce cell viability (MTT) by inducing apoptotic cell death (fluorescence microscopy) in a dose-dependent manner (250–1000 µg/ml for 48 h) (Zhang et al. 2011). Athinarayanan et al. (2014) isolated SiNPs (10–50 nm) from commercial food products processed with food additive silica (E551) and exposed human lung fibroblasts (WI-38 cell line) with increasing doses (25–400 µg/ml). After 24 h, they observed cytotoxicity (MTT) in a dose-dependent manner and ROS production (DCFH-DA) at 50 µg/ml.

Cytotoxicity not associated with oxidative stress

Py-SiNPs (12 and 40 nm) induced a significant size and dose- (31.3, 93.8, and 156.3 µg/cm2 culture well) dependent cytotoxicity (LDH, Sulphorodhamine B assay (SRB) and water-soluble tetrazolium-1(WST-1)) in human colon carcinoma cell line (HT29), while no induction of ROS (DCFH-DA) was observed (Gehrke et al. 2013). In the study by Napierska et al. (2012a), 50 μg/ml (24 h) of 16 nm iron-doped S-SiNPs and pure S-SiNPs induced strong cytotoxicity (MTT and LDH) in a human endothelial cell line (EA.hy926), but a significant increase in oxidative stress markers [GSH depletion, malondialdehyde (MDA formation), induction of heme oxygenase-1, glutathione reductase, and NADPH oxidase-1] was observed only for iron-doped SiNPs.

Conclusion: cytotoxicity

Cytotoxicity of SiNPs was investigated using different cell lines and incubation times, making the comparison between studies difficult. However, from Table 3, it is clear that all types of SiNPs induced cytotoxicity. Significant (compared to untreated cells) cytotoxic effects were observed only at or above the concentration of 25 µg/ml. Furthermore, it can be clearly seen that SiNPs induced oxidative stress and mediated apoptosis mainly via the intrinsic or mitochondrial pathway (caspase-dependent pathway) in a size- and dose-dependent manner. ROS-mediated toxicity is believed to be an important mechanism of NP toxicity including SiNPs (Manke et al. 2013). Nevertheless, Py- and S-SiNPs caused cytotoxicity without measurable levels of ROS production. It was demonstrated that the disturbance of membrane integrity due to direct cell-membrane interaction might be another possible mechanism of NP cytotoxicity (Fröhlich et al. 2009; Thomassen et al. 2011). However, neither of these studies did substantiate these observations and, therefore, SiNPs cytotoxic effects in the absence of oxidative stress remain poorly understood.
Table 3

Comparison of toxic effects induced by different types of SiNPs (in vitro)

SiNP type

Cell type

Cytotoxicity

Apoptosis

Genotoxicity

Oxidative stress

Pro-inflammation

References

Colloidal

Caco2

Tarantini et al. (2015a, b)

Colloidal

HepG2

n/a

Li et al. (2011)

Colloidal

V79 and A549

n/a

n/a

n/a

Maser et al. (2015)

Colloidal

J744.1

n/a

Lee et al. (2011)

Colloidal

PBMC

n/a

Mendoza et al. (2014)

Stöber

Huvecs

Duan et al. (2013a)

Stöber

HepG2

n/a

n/a

Sun et al. (2011)

Stöber

HepG2

n/a

n/a

Wang et al. (2013)

Stöber

HaCaT

n/a

n/a

Liang et al. (2014)

Stöber

EA.hy926

n/a

n/a

n/a

Napierska et al. (2012a, b)

Precipitated

V79

n/a

Guichard et al. (2016)

Precipitated

Mouse fibroblast

n/a

n/a

n/a

Uboldi et al. (2012)

Precipitated

GES-1 and caco2

n/a

n/a

Yang et al. (2014a, b)

Precipitated

HepG2

n/a

n/a

Ahmad et al. (2012)

Precipitated

A549 and A431

n/a

n/a

Ahamed (2013)

Precipitated

M-HS

n/a

n/a

Di Cristo et al. (2016)

Precipitated

RAW.264.7

n/a

n/a

Di Cristo et al. (2016)

Pyrogenic

V79

n/a

Guichard et al. (2016)

Pyrogenic

GES-1 and caco2

n/a

n/a

Yang et al. (2014a, b)

Pyrogenic

HT-9

n/a

n/a

n/a

Gehrke et al. (2013)

Pyrogenic

RAW.264.7

n/a

n/a

Di Cristo et al. (2016)

Pyrogenic

M-HS

n/a

n/a

Di Cristo et al. (2016)

n/a, not investigated; ✓, positive; ✘, negative

Furthermore, some authors used very high concentrations that may cause “overloading” of cells and modify the nature of NP–cell interactions (Wittmaack 2011). In these cases, it is difficult to evaluate whether the observed effects are physiologically relevant. Although it is challenging, we consider a dose of 384 µg/cm2 or higher as irrelevant to human inhalation exposure for amorphous silica, based on the estimation that can be derived from the occupational exposure levels (OELs) (Fig. 2).
Fig. 2

In vitro dose estimation from human tolerable levels (OELs)

Genotoxicity

In this section, we presented studies on genotoxic effects of SiNPs as it is used as another major endpoint to characterize hazard of NMs. Direct interaction with DNA, oxidative DNA damage, depletion of anti-oxidants, cell cycle arrest, and abnormal expression of genes have been identified as potential mechanisms of NP mediated (geno)toxicity (Donaldson et al. 2010).

DNA damage associated with oxidative stress

Exposure to SiNPs (15, 30, and 100 nm) resulted in a size- and dose- (2.5–10 µg/ml for 24 h) dependent increase in 8-hydroxy-2′-deoxyguanosine levels (8-OH-dG), phosphorylation of histone on serine-139 (ɣH2AX), and DNA strand breaks (comet) in human keratinocytes (HaCaT) (Gong et al. 2012). Nabeshi et al. (2011a) also demostrated that exposure to SiNPs (70 nm; 10–90 µg/ml for 24 h) resulted in the increase of oxidative DNA damage (8-OH-dG levels) in HaCaT cells. SiNPs were taken up via actin-mediated endocytosis. Micron-sized particles used in these studies showed no or little effects.

The viability of human Caucasian colon adenocarcinoma (Caco-2) cells dropped to 40% when exposed to 15 nm C-SiNPs (64 µg/ml for 24 h), and, at this same concentration, nearly a threefold increase in micronuclei formation, fivefold increase in histone phosphorylation (ɣH2AX), and a significant increase in DCF fluorescence were observed. The particles were localized within lysosomes and endocytic compartments, but not in the nucleus. 55 nm C-SiNPs did not induce any of these effects at the same concentration (Tarantini et al. 2015b).

A non-significant increase in % tail DNA (comet assay) and no chromosomal aberrations were induced by 17 nm SiNPs in human peripheral lymphocytes treated with 100 µg/ml, while a dose-dependent (50–100 µg/ml for 24 h) ROS production (DCFH-DA) and GSH depletion were observed (Rajiv et al. 2015).

Cell cycle arrest associated with oxidative stress

S-SiNPs (62 nm) induced increase in DCF fluorescence and decrease in superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in HUVECs in a dose-dependent manner (25–100 µg/ml for 24 h). Oxidative stress was linked to cell cycle arrest at G2/M checkpoint (upregulation of chk 1 and downregulation of Cdc25c, Cyclin B1, and Cdc2) and increase in apoptosis (Duan et al. 2013a). In the study by Li et al. (2011), a size-dependent (19, 43, and 68 nm) increase in oxidative stress (DCF fluorescence) and cell cycle arrest in S and G2/M was observed in HepG2 cells exposed to 100 µg/ml of C-SiNPs. Cell cycle arrest in G2/M phase along with the increase in ROS was also noticed in human hepatic cell line (LC-02) treated with S-SiNPs (50 nm) in a dose-dependent manner (50–200 µg/ml for 24 h) (Wang et al. 2013).

DNA damage not associated with oxidative stress

Genotoxicity of Py-SNP (20 and 25–70 nm), Pr-SNP (20 nm), and C-SNP (15 and 40–80 nm) SiNPs were studied in Chinese hamster lung fibroblasts. Py-SiNPs (20 nm) induced a significant increase in DNA strand breaks at 66 µg/ml (24 h), while C-SiNPs (15 nm) showed a similar effect only at 252 µg/ml. Neither of these SiNPs did induce ROS. SiNPs in the size range 25–80 nm exerted no or little genotoxicity (Guichard et al. 2016).

Genotoxicity reports without the assessment of oxidative stress

M-SiNPs (100 nm) induced a significant increase in phosphorylated-ɣH2AX-foci in HT-29 cells treated with a dose of 10 µg/ml for 24 h (Sergent et al. 2012). In the human embryonic kidney cell line (HEK293), 579 genes were upregulated and 1263 genes were downregulated after 24 h of exposure (100 µg/ml) to 100 nm M-SiNPs (Zhang et al. 2015). In another study, 15-nm C-SiNPs induced a significant increase in DNA strand breaks (comet assay) in chinese hamster cells (V79) and A549 cells at 100 µg/ml (24 h), but, for 55 nm, this effect was observed only in A549 cells (Maser et al. 2015). A significant increase in DNA tail length (comet assay) was observed in HaCaT cells treated with 30 µg/ml (24 h) of 70 nm SiNPs (Nabeshi et al. 2011b).

Exposure to C-SiNPs (~7 nm) resulted in positive genotoxic effects (Lymphoma assay) in mouse lymphoma cells treated with 100 and 150 µg/ml for 4 h (Demir and Castranova 2016). SiNPs (12 nm) induced DNA strand breaks in RAW 264.7 at 200 and 400 µg/ml, but the induction of micronuclei was noticed only at 400 µg/ml. The particles were internalized in vesicles and in the nucleus (Hashimoto and Imazato 2015).

At any tested concentrations (1–100 µg/ml for 72 h), SiNPs (10–25, 5–30, 35, 15, 80, and 90 nm) neither induced cytotoxicity nor micronuclei in immortalized Balb/3T3 fibroblasts (Uboldi et al. 2012). In another study, Pr-SiNPs (NM-200 and NM-201) and Py-SiNPs (NM-202 and NM-203) with primary size between 14.5 and 16 nm did not induce any micronuclei (cytokinesis block micronucleus assay) in human peripheral lymphocytes exposed to different concentrations (200–1250 µg/ml) over 24 h (Tavares et al. 2014). It is also worthy to note that, in the latter study, the positive control used did not differ from control conditions.

Conclusion: genotoxicity

C-SiNPs and S-SiNPs induced genotoxicity in human tumor cell lines (lung, kidney, skin, and gastro-intestinal systems) and the amplitude of the effect negatively correlated with the size of the NPs. DNA strand breaks were observed at low concentrations (2.5–10 µg/ml), particularly in skin-derived cell lines. The genotoxic effects of C- and S-SiNPs were mainly associated with the induction of oxidative stress, while such information is very limited for other types (Py- and Pr-SiNPs). One study indicated that Py-SiNPs induce DNA damage without the generation of ROS, suggesting that other mechanisms such as direct DNA damage might be involved (Magdolenova et al. 2014). However, it is very difficult to judge whether such genotoxic effect is direct or indirect, since the cellular uptake and subcellular localization of SiNPs are not often reported. Furthermore, several factors such as SiNP properties, cell type, and exposure scenarios (such as concentrations, assays, and endpoints) may influence the outcomes (Magdolenova et al. 2014), making the comparison difficult between studies and indicating an urgent need for the standardization of genotoxicity studies.

Immunotoxicity

NPs entering the body will most probably interact with immune cells, as they are the first line of defence in human body. In this section, we presented the immune responses induced by SiNPs in different cell lines.

Immunotoxicity associated with oxidative stress

Hara et al. (2014) exposed THP-1-derived macrophages to 100 µg/ml of SiNPs (30 nm) for 6 h and found a significant increase in interleukin-1-beta (IL-1β), ROS production, and SiNP uptake via phagocytosis. In the study of Choi et al. (2010), larger sized SiNPs (150–200 nm) were effectively phagocytosed by primary rat microglial cells after 24 h of exposure to different concentrations (0.0728–7.28 µg/ml). A significant increase in ROS, reactive nitrogen species (RNS) and IL-1β was detected at all concentrations.

Immunotoxicity not associated with oxidative stress

At 10 and 20 µg/ml, Di Cristo et al. (2016) found that Py-SiNPs (~14 nm) induced a stronger increase of tumor necrosis factor-alpha (TNF-α), interleukin(IL)-6, and IL-1β in RAW.264.7 macrophages compared to similar sized Pr-SiNPs; Notably, no SiNPs induced ROS in RAW.264.7 macrophages.

Immunotoxicity reports without the assessment of oxidative stress

A significant increase in TNF-α, IL-6, and IL-1α, mitogen activated protein kinases (MAPKs), and nuclear factor (NF)-κB were observed only for C-SiNPs (100 nm) in J774A.1 macrophages exposed to 100 µg/ml of same sized (100 nm) C-SiNPs or M-SiNPs (Lee et al. 2011).

Uemura et al. (2016) showed that SiNPs (10 and 50 nm) caused dose-dependent (6.25–100 µg/ml) increase in the production of TNF-α and decrease of IL-6 in RAW.264.7 macrophages, while their amine surface-modified counterparts did not. Furthermore, 300 and 1000 nm micron-sized particles (both bare and amine modified) also showed a dose-dependent decrease of IL-6. Notably, the effects were stronger for 50 nm compared to other particles. The same cell line was utilized by Yu et al. (2011) to investigate phagocytosis using inductively coupled plasma mass spectroscopy (ICP-MS), and they found that S-SiNPs (25 nm) were phagocytosed at least ten times more than M-SiNPs of same size and high aspect ratio SiNPs (AR 2, 4, and 8). In the study by Napierska et al. (2012b), THP-1 cells dosed with 5 µg/cm2 S-SiNPs (2 nm) showed a significant increase of IL-8, TNF-α, and macrophage inflammatory protein (MIP)-1α, while only a non-significant increase in MIP-1α expression was observed for 16 and 104 nm S-SiNPs.

Conclusion: immunotoxicity

The main cells used to study immune responses to SiNPs were ‘innate’ cells such as monocytes and macrophages. Therefore, the identified in vitro studies only address a very limited part of the immune system, essentially pro-inflammatory responses and potential phagocytosis. Furthermore, the data on immune responses and oxidative stress are very limited and, therefore, no firm conclusions can be made. SiNPs, not only induced stronger pro-inflammatory responses compared to sub-micron and micron sized particles but also size-specific effects within the nano-range in immune cells are observed. Besides size, shape and porosity seem to influence the phagocytosis of SiNPs.

Autophagy

Recently, a growing body of evidence identified autophagy as a cellular defence mechanism against NP toxicity, since it plays a key role in removing misfolded proteins and clearing damaged organelles (Glick et al. 2010). Hence, we present here studies that show induction of autophagy upon exposure to SiNPs.

Autophagy associated with oxidative stress

The same S-SiNPs (62 nm) were used in three studies to investigate the induction of autophagy. Along with the dose-dependent (25–100 µg/ml) increase in ROS production, increase in autophagy bio-marker-microtubule-associated protein 1A/1B-light chain 3 (LC3) and monodansylcadaverine (MDC) labelled autophagic vacuoles were detected in HepG2 cells treated with 62 nm S-SiNPs. In addition, transmission electron microscopy (TEM) images revealed that autophagosomes and autolysosomes induced in the presence of SiNPs (Yu et al. 2014). The same S-SiNPs (62 nm) induced increase of LC3-II/LC3-I ratio and decrease of p-mTOR/mTOR, p-P13 K/P13 K and p-Akt/Akt in HUVEC cells in a dose-dependent (25–100 µg/ml) manner (Duan et al. 2013b, 2014a, b). The results of Guo et al. (2016) also suggest that 50 µg/ml of S-SiNPs (58 nm) could induce autophagy via MAPK/Bcl-2 and PI3K/Akt/mTOR signaling in HUVECs.

After 4 h of exposure to 200 µg/ml of S-SiNPs (50 nm), autophagosomes and ROS production was observed in HaCaT cells. The TEM images revealed that SiNPs were in the cytoplasm and lysosomes, but not in the nucleus. (Liang et al. 2014).

SiNPs (4–13 nm, 62.5 µg/ml) induced a time-dependent (24, 48, and 72 h) reduction in cell viability and increase in oxidative stress (DCF fluorescence and GSH depletion) in the lung fibroblast cell line MRC-5. Compared to control, a significant increase of autophagic vacuoles and LC-3 II/LC3-I ratio was also observed in a time-dependent manner (Voicu et al. 2015). A549 cells, when exposed to 100 and 1000 µg/ml of 20-nm SiNPs, showed threefold and fivefold increase in MDC fluorescence, respectively. In addition, autophagy genes such as ATG-12 and BECN were significantly upregulated (30- and 50-fold, respectively) along with increased production of ROS in cells dosed with 1000 µg/ml (Nowak et al. 2014).

Conclusion: autophagy

SiNPs, particularly S-SiNPs induced autophagy mainly via oxidative stress-mediated upregulation of autophagy-related genes and differential regulation of Akt/mTOR signaling. Similar to cytotoxicity, 25 µg/ml appeared to be the lowest exposure concentration at which SiNPs exhibited significant effects. Furthermore, induced autophagy is correlated to cytotoxicity, suggesting that exposure to SiNPs caused irreversible (serious) cellular damage and resulted in autophagic cell death. Besides autophagy induction, lysosomal and autophagy dysfunction could be a potential mechanism of NPs toxicity (Stern et al. 2012), which has, however, not been investigated for SiNPs.

Toxic effects on blood cells and endothelial dysfunction

Several studies suggest that NPs, when inhaled or ingested, can translocate across barriers (such as air–blood) of the body, enter the circulation, and interact with the cardiovascular system. In this section, we summarized the studies that report the effects of SiNPs on blood and endothelial cells.

Toxic effects on blood cells

Nemmar et al. (2015) showed that mouse blood platelets could aggregate after 3 min of incubation with 5 or 25 µg/ml of 50-nm C-SiNPs, while in the study of Jose Corbalan et al. (2012), such aggregation was observed in 15 min (10 µg/ml of 10 nm C-SiNPs). The latter study also showed fourfold reduction of the nitric oxide (NO)/peroxynitrite (ONOO) ratio compared to non-treated platelets.

Maurer-jones et al. (2010) investigated the role of porosity of SiNPs on blood cell toxicity. M-SiNPs (25 nm) reduced the cell viability of red blood cells (RBCs) to 50% at the concentration of 270 µg/ml, while non-porous S-SiNPs of similar size required only 20 µg/ml to reach this level of cytotoxicity. In another study, 10% hemolysis (LC10) of RBCs was observed at 36 µg/ml for S-SiNPs (115 nm), while M-SiNPs required 154 µg/ml to induce the same effects. For amine-coated counterparts, LC10 were 97 and 30 µg/ml for S- and M-SiNPs, respectively (Yu et al. 2011).

Endothelium dysfunction

Exposure to different concentrations (12.5–100 µg/ml) of S-SiNPs (58 nm) resulted in a dose-dependent increase in inflammatory mediators such as IL-1β, IL-8, and TNFα, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule (VCAM-1), and monocyte chemoattractant protein-1 (MCP-1) in HUVEC cells (Guo et al. 2015). In the study of Corbalan et al. (2011), 10 µg/ml of 10-nm C-SiNPs induced extremely low NO/NOO ratio (~0.1) in HUVEC cells. Furthermore, free radical production, pro-inflammatory cytokines (IL-6 and IL-8), and NF-κB-binding activity were significantly increased in treated cells. In these two studies, increase in ROS was observed at all tested concentrations.

S-SiNPs (62 nm) induced an imbalance in the ratio NO/nitric oxide synthase (NOS) enzyme in HUVEC cells and such imbalance resulted in a significant increase of pro-inflammatory response (c-reactive protein CRP, IL-1β, IL-6, and TNFα) in a dose- (50–100 µg/ml) dependent manner (Duan et al. 2014b).

Conclusion: Toxic effects on blood cells and endothelial dysfunction

Endothelial cells and platelets together play a key role in maintaining the vascular homeostasis (Rajendran et al. 2013). C-SiNPs induced oxidative stress and disturbed NO/NOO ratio, which resulted in the aggregation of platelets and endothelial dysfunction. This information is not available for other types of SiNPs. Furthermore, C-SiNPs mediated endothelial dysfunction resulted in pro-inflammatory signals via the secretion of cytokines and adhesion molecules. Together, these results suggest the potential of SiNPs to cause vascular thrombosis and atherosclerosis (Radomski et al. 2005). Furthermore, SiNPs caused hemolysis of RBCs in a size-, charge-, and porosity-dependent manner.

Neurotoxicity

NPs of very small size are capable of translocating across the blood–brain barrier (Hu and Gao 2010). Therefore, studies investigating effects on cells relevant for neurotoxicity are presented here.

Rat medulla tumor cells (PC12 cell line) incubated with the supernatant of 20 nm SiNPs-treated-microglial cells (250 µg/ml and 500 µg/ml for 24 h) did not show any effects compared to the control. Earlier in this study, no secretion of bio-mediators was observed in SiNPs-treated-microglial cells (Xue et al. 2012). In contrast to this study, PC12 cells exposed directly to SiNPs (25 nm; 25–200 µg/ml for 24 h) showed increased uptake and a dose-dependent increase in the induction of autophagy (increase in LC-II and Beclin 1) and inhibition of PI3 K-Akt-mTOR signaling (Xie and Wu 2016). Yang et al. (2014a) showed that exposure to SiNPs (15 nm; 10 µg/ml for 24 h) induced pathological signs of Alzheimer’s disease such as altered expression of amyloid precursor protein (APP) and neprilysin, enhanced phosphorylation of tau at Ser262 and Ser396, and activation of glycogen syntheses kinase (GSK)-3β in human SK-N-SH and mouse neuro2a neuroblastoma cells.

Conclusion: neurotoxicity

In vitro studies used cell lines of CNS-based cells, mainly neuron like cells. Although the data on the neurotoxic effects of SiNPs are very limited, studies above suggest that SiNPs can induce adverse effects including the markers of Alzheimer’s disease, when in direct contact with neuroblastoma cells.

Miscellaneous issues

Influence of cell lines on SiNPs cytotoxicity

A dose- (80–640 µg/ml) dependent decrease in the viability and increase of apoptosis were observed in HepG2 cells in presence of SiNPs (7 and 20 nm), but a significant reduction was observed in normal human liver cells (LC-02) only at the unrealistic dose level of 640 µg/ml (Lu et al. 2011). In another study, SiNPs (10–50 nm) induced a dose-dependent (100–600 µg/ml) increase in LDH release in Caco-2 cells, but a significant release of LDH was observed only at the unrealistic dose of 600 µg/ml in human gastric epithelial cells (GES-1). Furthermore, exposure to these SiNPs (200 µg/ml for 48 h) also induced cell cycle arrest in S phase for GES cells and G2/M in Caco-2 cells (Yang et al. 2014b). In a porcine kidney cell line (LLC PK1) exposed to 20-nm SiNPs, a dose- (5–50 µg/ml) dependent increase in DCF fluorescence and MDA formation was observed, whereas human kidney cells (HK-2) showed little effects at 50 µg/ml (Passagne et al. 2012).

Conclusion: miscellaneous issues

No firm conclusions can be drawn from these cases; however, the cytotoxicity of SiNPs appears to vary with species and cell line.

Physiologically relevant cultures

Lung co-culture models

Co-cultures of lung cells are usually made with epithelial cells on the apical and endothelial cells on the basal compartment of a transwell membrane, with or without monocytes on top of the epithelial cells. In a co-culture (A 549 at the apical and ISO-HAS-1 at the basolateral compartment) exposed to 100-µg/ml 30-nm C-SiNPs (coated with or without surfactant), nearly a fivefold increase of IL-8 release for both forms of SiNPs was observed in both compartments (Kasper et al. 2015). When other epithelial cells were used—H441 cells—at the apical together with ISO-HAS-1 cells at the basolateral compartment, these C-SiNPs induced IL-8 were expressed in both compartments, while SiCAM-1 (6–600 µg/ml) and IL-6 (at 6 and 60 µg/ml) were observed only in the apical part (Kasper et al. 2011). The same co-culture model was exposed to 100 µg/ml of S-SiNPs (15, 35, and 80 nm) and the authors noticed an increase of IL-8, TNF-α, and surfactant protein (SP-A1 and SP-A2) expression compared to the control. In addition, less IL-8 and surfactant protein expression, and more TNF-α were observed in the co-culture added with THP-1, notably the effect was the highest for 35 nm (Farcal et al. 2012).

Napierska et al. (2012b) tested SiNPs with primary size 2, 16, 60, and 104 nm (dosed at 10 µg/cm2) in a co-culture (A549 at the apical and EA. hy926 at the basolateral compartment) and observed increase in cytokines such as IL-6, IL-8, TNF-α, and MIP-1α only for 60 and 2 nm (except IL-8). When THP-1 were added to the co-culture, a significant increase in IL-8 and decrease in TNF-α were observed only for 2 nm. The expression of cytokines was also differentially regulated for 16 and 104 nm before and after THP-1 added, but the effects were stronger for 60 and 2 nm, particularly 60-nm NPs.

Air–liquid interface

At the air–liquid interface (ALI), aerosolized and deposited 12-nm Py-SiNPs (52 µg/cm2) and 50-nm S-SiNPs (117 µg/cm2) induced significantly less biological effects (LDH leakage, IL-8 release, COX-2 expression, and p38 phosphorylation) in A549 cells compared to A549 exposed to 15.6 µg/cm2 under sub-merged conditions (Panas et al. 2014).

Conclusion: physiologically relevant cultures

Sub-merged (co) cultures and ALI systems (Lenz et al. 2013; Panas et al. 2014) have been claimed to more closely mimicking the in vivo exposure scenarios compared to monocultures. In these systems, the toxicity and pro-inflammatory responses are significantly modulated by SiNPs, which represent the complexity of in vivo systems and need for the establishment of physiologically relevant in vitro cultures. However, at this moment, it is difficult to know whether these biological responses were influenced by SiNPs physico-chemical properties.

Chronic in vitro studies

In vitro chronic Py-SiNPs (12 nm) exposure of intestinal epithelial cell line (C2BBe1) was examined in a recent study. The cells were exposed to 10-µg/cm2 SiNPs for 24 h. After 24 h, the medium was replaced (without NPs) and cells were allowed to grow for 4–6 days. At the end of incubation, cells were passaged and again exposed for 24 h and grown for 4–6 days; this cycle was repeated for 29 passages (total life span). The cells and supernatants were collected at the end of each passage for analysis. Though the particles were internalized (only in a fraction of cells), no significant induction of necrosis, apoptosis, and LDH release and decrease in cell viability was observed for any of these conditions (McCracken et al. 2013).

In vivo studies

Ingestion exposure

Single exposure

Lee et al. (2014) investigated the tissue distribution and excretion profiles in Sprague–Dawley (SD) rats orally administered with a very high dose of 500 or 1000-mg/kg bw of C-SiNPs (20 and 100 nm). Silicon (Si) levels were significantly elevated in liver, kidney, lung, and spleen at 6-h post-administration, while no such increase was noticed in brain, ovaries or testes, esophagus, stomach, and intestine even after 7 days of post-administration. Nearly 75–80% of administered SiNPs were excreted via urine and 7–8% via feces. The author also noticed that 20 nm SiNPs excreted faster than the 100 nm SiNPs. In another study, C-SiNPs (12 nm) did not induce any toxicity after 14 days in SD-rats administered with a single oral dose (1959 or 2061 mg/kg bw) (Yun et al. 2015).

In the study of Li et al. (2015), M-SiNPs with different aspect ratios (AR, spherical 83 nm with AR 1, short rods with AR 1.75 and long rods with AR 5) were administered in mice (40 mg/kg bw via gavage). After 7 days, a very high amount of Si was detected only in the liver of spherical SiNPs exposed mice. Urinary excretion, intestinal absorption, and organ distribution of SiNPs were decreased with increasing aspect ratio. Furthermore, increase in renal damage such as hemorrhage, vascular congestion, and renal tubular necrosis with increasing aspect ratio was observed after 14 days of administration.

Kim et al. (2014b) exposed C57BL/6 mice (oral gavage; 750 mg/kg bw) to C-SiNPs (20 and 100 nm) that had been modified with or without l-arginine to determine the influence of surface charge on immunotoxicity in vivo. After 14 days, the author noticed a size-dependent decrease in WBCs cell count and cytokines (in blood), and reduced proliferation of B cells and T cells (from spleen) only for uncoated SiNPs.

Repeated exposure

C-SiNPs (12 nm) neither induce abnormal changes in blood biochemical and hematological parameters nor accumulated in any organs of the acutely (489.8, 979.5, or 1959 mg/kg bw/day, during 14 days) or sub-chronically (244.9, 489.8, or 975.9 mg/kg bw/day, during 13 weeks) exposed (via gavage) SD rats (Yun et al. 2015). In the study by Kim et al. (2014c), C-SiNPs (20 and 100 nm) did not induce any significant changes (compared to control) in clinical signs, blood biochemical, hematological, and histopathological analysis in sub-chronically exposed Crl: CD (SD) specific pathogen-free rats (500, 1000, or 2000 mg/kg bw/day for 90 days).

SD rats were administered (via oral gavage) with low doses (5, 10 or 20 mg/kgbw) of Pr-SiNPs (NM 200 and 201) and Py-SiNPs (NM 202 and 203) at 0, 24 and 45 h and sacrificed at 3 h after the last administration. No significant genotoxicity was observed in cells extracted from different organs (duodenum, colon, blood, kidney, liver, and spleen), and no pathological conditions were recorded in any of these organs (Tarantini et al. 2015a).

Hofmann et al. (2015) investigated the prenatal toxicity by exposing pregnant Wistar rats to Pr-SiNPs (NM 200) from gestation day 6–19. At doses of 100, 300, or 1000 mg/kgbw/day, administered SiNPs did not induce malformations in fetuses or death of the rats.

Zande et al. (2014) investigated the sub-acute and sub-chronic toxicity of Py-SiNPs (7 nm) and NM-202 (10–25 nm). In the sub-acute part, male SD rats were orally (via food) exposed to 100, 1000, or 2500 mg Py-SiNPs/kg bw/day for 28 days, while in the sub-chronic part, the rats were exposed only to the highest dose (2500 mg/kg bw/day) repeatedly for 84 days. ICP-MS analysis of target organs showed that NM-202 was significantly distributed in the lung, kidney, and spleen. Although no Si was detected in the liver, histopathological analysis and gene expression studies revealed that fibrosis was induced in the liver after 84 days of exposure. Si was found in the spleen of Py-SiNPs exposed rats only at the end of chronic exposure. The examined parameters such as blood biochemistry, antibody levels (IgG and IgM), and cytokines indicated no systemic toxicity in any of the Py- or NM-202 treated rats.

Yoshida et al. (2014) found an increased intestinal absorption of carboxyl (twofold) and amine-coated SiNPs (1.5-fold) compared to uncoated SiNPs (70 nm) in orally exposed BALB/c mice (2.5 mg/mouse/day for 28 days). In this study, the whole small intestine was processed using the everted gut sac method and a significant amount of Si was detected by ICP-MS. However, no signs of systemic toxicity were observed for any of these SiNPs.

In male Wistar rats that exposed to 10–15 nm SiNPs (oral gavage; 333.3 mg/kg bw/day for 5 days), histopathological analysis revealed gross tissue damage in kidney (cell swelling and necrosis), lung (interstitial pneumonia and bronchopneumonia), and in the testis (congestion, reduction of spermatogenesis and edema). In addition, blood biochemical parameters such as albumin, cholesterol, triglycerides, total proteins, urea, high-density lipoprotein (HDL), and low-density lipoprotein (LDL), as well as alkaline phosphatase (ALP) and aspartate aminotransferase (AST) activities were significantly increased in treated mice (Hassankhani et al. 2014).

Inhalation exposure

Single exposure

In the study of Morris et al. (2016), C57BL/6 mice were intratracheally instilled with 4- or 20-mg S-SiNPs/kg bw and significant effects were observed only at the dose of 20 mg/kg. Twenty hours after instillation, approximately 20- and 10-fold higher cell number was observed in the bronchoalveolar lavage (BAL) of mice treated with the bare and amine-coated SiNPs, respectively, compared to control mice; neutrophils were also increased about 30- and 20-fold, respectively.

In another study, 15- and 55-nm C-SiNPs neither induced DNA damage nor micronuclei in lung and bone marrow (erythrocytes) cells of Wistar rats at 72 h of post-instillation (360 µg) (Maser et al. 2015). However, the distribution of administered SiNPs to these organs was not investigated.

Repeated exposure

A size- and dose-dependent increase in the distribution of SiNPs was observed in the serum and heart of male Wistar rats intratracheally instilled (2, 5, or 10 mg/kg bw/day for 16 days) with SiNPs (30, 60, 90, and 300 nm). Blood parameters (WBCs and platelets), inflammatory bio-markers (TNF-α, IL-1β, and IL-6), and oxidative stress bio-markers (ROS and MDA formation) were significant increased, while NO, NOS, and eNOS were significantly decreased in the serum of the treated mice (Du et al. 2013).

A study compared the toxicity and biodistribution of pristine SiNPs (19 nm) and aged paints containing SiNPs in BALB/c mice. The suspensions (20 µg/aspiration) were oropharyngeally aspirated once a week for 5 weeks, and mice were sacrificed either at 2- or at 28-day post-final aspiration treatment. Pristine SiNPs were significantly distributed in the lungs and liver, while only a low amount of Si was detected in the liver of paint-exposed mice. No signs of toxicity were observed, except a slight inflammation (a slight increase in macrophages and neutrophils together with an increase of IL-1β) with pristine SiNPs (Smulders et al. 2014).

Male Wistar rats were intranasally instilled with 150 µg SiNPs (10 and 80 nm) repeatedly for 30 days. After 30 days, a size-dependent increase in the levels of hydrogen peroxide (H2O2), MDA formation, TNF-α, IL-1β, MCP-1, and NF-ĸβ was observed in the frontal cortex, corpus striatum, and hippocampus of the brain. A similar quantity of Si was detected in all these three regions. In addition, a significant decrease in GSH levels was observed in these tissues (Parveen et al. 2015). The latter study shows the potential translocation of (mainly small) SiNP from the nose to the brain, but it has to be noted that the mice received a high dose during 30 days.

Intratracheally administered (2 mg/kg bw; 15 times, once every 3 days) S-SiNPs (58 nm) resulted in reproductive toxicity in C57 mice via enhanced ROS production and cell cycle arrest (G0/G1 phase) in the testicular tissues, and by decreasing the number of mature sperms and primary spermatocytes (Zhang et al. 2016). In another study, repeated intracheal instillation of S-SiNPs (58 nm; 0, 7, 21, and 35 mg/kg bw; 5 times, once every 3 days) induced local (macrophage activation in lung, liver, and spleen) and systemic inflammation (Increase in serum IL-8, TNF-α, and IL-6) in BALB/c mice. In addition, SiNPs were also detected in the lysosomes of macrophages in lung, liver, and heart tissues (Yang et al. 2016).

Dermal exposure

Repeated exposure

The skin of SD rats was repeatedly exposed to different doses (500, 1000 mg, or 2000 mg/kg bw) of l-arginine coated C-SiNPs (20 nm) for 90 days (6-h exposure/day). The repeated exposure neither induced gross changes in the skin nor in any organs. In addition, hematological and blood biochemical parameters did not change in SiNPs-treated mice compared to controls (Ryu et al. 2014). In another study, Shim et al. (2014) showed that C-SINPS (20 and 100 nm) did not induce toxicity in the right and left brain or distribution in the cerebellum, hippocampus, or striatum of the dermally exposed rats (1000 or 2000 mg/kgbw daily for 90 days). In contrast to these studies, a significant increase in apoptosis (TUNEL positive cells) was observed in the skin of the BALB/c mice topically exposed to 70-nm SiNPs (250 mg/ear/day for 28 days). The author reported that the SiNPs were found not only in the skin but also in the regional lymph nodes, cerebral cortex, hippocampus, and in the liver (Nabeshi et al. 2011b).

Parenteral exposure

Single exposure

In a 48-h study, exposure to 15-nm SiNPs (dose of 50 mg/kg bw by intravenous injection) resulted in a significant increase of CD68-positive Kupffer cells (KCs), WBCs, lymphocytes, monocytes, neutrophils, and TNF-α in the serum of male SD rats. There was a decrease in GSH activity and elevation in MDA levels in the liver of treated mice. Furthermore, bio-markers of liver dysfunction such as lactate, phosphorylcholine, sn-glycero-3-phosphocholine, tyrosine, phenylalanine, and lysine were increased in blood, while the levels of succinate, glucose, and glycine were significantly decreased in mice exposed to SiNPs (Chen et al. 2013).

In another study, 50 nm C-SiNPs (0.25 mg/kg bw) induced a significant increase in blood parameters such as leukocyte number, creatinine kinase (CK), ALT, AST, and LDH release in male tuck ordinary mice at 24-h post-intraperitoneal injection. Oxidative stress bio-markers such as MDA formation, SOD, and catalase were significantly increased in the lungs, liver, kidney, and brain of SiNPs-treated mice. Cytokines such as IL-6, IL-1β, and TNF-α were significantly increased in the lung and differentially expressed in other organs. Furthermore, all organs showed a significant DNA damage compared to saline treated mice and the extent of damage was in the order heart > kidney > lung > liver > brain (Nemmar et al. 2016).

In the study of Duan et al. (2014a, b), ICR mice were intravenously injected with 29.5, 103.5, or 177.5 mg/kg bw of 62 nm S-SiNPs. After 14 days, induction of autophagic vacuoles and mitochondrial rupturing in the heart tissues was observed using TEM. In addition, increase in LC3 positive staining in the heart tissues was detected at 103.5 and 177.5 mg/kg bw, and the expression of ICAM-1 and VCAM-1 was significantly decreased at 177.5 mg/kg. Huang et al. (2011) injected the mice intravenously with mesoporous rods (pure or PEGylated) at a dose of 20 mg/kg bw and found a dysfunction in biliary excretion and glomerular filtration. Blood analysis showed that there was a significant increase in total bilirubin (TBIL), blood urea nitrogen (BUN), and creatinine (CREA). The mesoporous rods mainly distributed in the lung, liver, and spleen, but PEGylation significantly reduced their distribution in these organs. In addition, short rods cleared via urine and feces more rapidly than long rods.

Yu et al. (2013) estimated the i.v. median lethal dose (LD50) for 64 nm C-SiNPs as 262 mg/kg bw in ICR mice using the Dixon’s Up and Down method. After 14-day post-administration, intravenously injected C-SiNPs (64 nm, 177.5 mg/kg bw) induced a significant increase in liver injury bio-markers (LDH, AST, and ALT). Histopathological analysis of target organs showed that the number of megakaryocytes in the spleen was significantly increased, and pulmonary hyperemia and interstitial thickening were observed in the lungs. Nearly 35% of the injected SiNPs were distributed in the spleen, 12.5% in the liver, and 2% in the lung of SNP-treated mice.

At 24 h of post-intravenous administration (30 mg/kg bw), TEM imaging of the BALB/c mice tissues showed that 70-nm SiNPs were localized in the regional lymph nodes, cerebral cortex, hippocampus, and the cytoplasm, and nucleus of hepatocytes (liver). Using in vivo imaging, the authors also found that the SiNPs were distributed near the liver immediately after the administration (20 min) and moved near the intestinal tract over time (6 h). Micron-sized silica particles (300 and 1000 nm) used in these studies were accumulated mainly around the gall bladder (Nabeshi et al. 2011b).

A significant increase in the levels of kidney injury bio-markers such as BUN and CREA and morphological changes associated with renal interstitial fibrosis was noticed in BALB/c mice intraperitoneally treated with 198-nm M-SiNPs (150, 300, or 600 mg/kg bw for 2 or 12 days). There was also a significant increase in the levels of fibronectin (FN), TGF-β and ICAM-1, and nuclear translocation of p65 in 300- or 600-mg/kg treated mice (Chen et al. 2015).

Zhuravskii et al. (2016) demonstrated that intravenously injected (70 mg/kg bw) Py-SiNPs (13 ± 5 nm) distributed and persisted in the liver of Wistar rats at 60-day post-administration. The authors also indicated that the administered SiNPs induced fibrosis and liver tissue remodeling by noticing the increase in blood ALT, and presence of mast cells, connective tissues, and foreign body-type granulomas in the liver.

Conclusion: in vivo

The in vivo toxicity studies have been carried out using rats and mice, and exposure through various routes of administration. In general, short-term exposure to SiNPs induced adverse effects in the lungs, kidneys, liver, and brain. SiNPs administered mainly via inhalation, ingestion, and intravenous routes were majorly distributed in the liver, lungs, spleen, and kidneys, and in the brain of the intranasally exposed rats. Most of the administered SiNPs were excreted via feces and to a lesser extent via urine, in a size- and shape-dependent manner. However, in most cases, the administered doses were very high compared to relevant human inhalation (Barsan 2007) and ingestion (Winkler et al. 2016; Dekkers et al. 2011) exposures to amorphous silica. Some studies showed accumulation of SiNPs in organs such as the liver, but such accumulation was not associated with any major effects. However, long-term effects of accumulated SiNPs were not studied. It is surprising that in contrast to acute studies, no toxicity (local or systemic) was observed in chronic oral and dermal exposure studies, regardless of the size of SiNPs and the high doses used. Moreover, surface-modified SiNPs showed a significant increase in absorption by the GI tract compared to bare SiNPs. Dosing SiNPs via i.v. showed in all studies some sign of damage/toxicity. No clear type-or size-dependent effect can be identified from the set of reviewed studies. Furthermore, where genotoxicity was studied, it was not clear whether SiNPs were taken up by the target cells or were able to reach the tissue examined, which is obviously needed to exert a direct genotoxic effect.

Discussion

Growing production and use of SiNPs increase the risk of human exposures. Available toxicity studies mainly focused on effects after exposure via inhalation or skin (occupational exposure), ingestion (food additive), or nano-therapeutics (parenteral exposure). In vitro and in vivo studies demonstrated that SiNPs can induce adverse effects, but multiple ‘inconsistencies’ were found in the collected toxicity data set. As small differences in the physico-chemical properties of SiNPs could contribute to significant variation in the toxicity (Napierska et al. 2010), it is critical to discriminate how these variations influence toxicity. It is also worthy to note that the choice of cell type, culture system, assay conditions (Fede et al. 2012; Geys et al. 2010; Hayashi et al. 2017), and exposure route influence the toxic responses to SiNPs. However, the nature of these variations remains unclear, which continues to limit our understanding of SiNPs toxicity and hampers their hazard assessment.

Physico-chemical characterization

In this review, a basic set of physico-chemical characteristics of primary SiNPs such as primary size, shape, crystallinity, and chemical composition (or) purity was set as inclusion criteria. Most studies associated the toxic endpoints to SiNPs size, while only a few related the toxicity to SiNPs porosity, shape, surface charge, and surface chemistry. Furthermore, hydrodynamic diameter is another widely reported property in these studies and the results suggest that, in most cases, SiNPs are often aggregated/agglomerated (AA) in cell culture media. Conversely, AA formation is also very likely in real-world matrices such as in air (Kim et al. 2014a), water, and in commercial products such as food (Dekkers et al. 2011). Such AA formation not only substantially alters the overall characteristics (such as size, shape and surface topology) but also potentially influences the biological outcomes (Luyts et al. 2013; Drescher et al. 2011). Although some progress has been made in the characterization of SiNPs AA in recent years (De Temmerman et al. 2012), their biological effects remain poorly understood.

Toxicity of different types of SiNPs

Synthetic amorphous SiNPs are produced via different methods such as thermal (pyrogenic) or wet route (colloidal, precipitated and gel). The physico-chemical characteristics of SiNPs produced by these methods differ (Fruijtier-Pölloth 2012; Napierska et al. 2010) and may influence the biological outcomes. In this review, nearly 70% of the papers clearly reported the synthesis method. Among those 70%, about 80% reported the toxicity of wet method based SiNPs and only approximately 20% on the other types (such as pyrogenic and mesoporous). A general overview of toxic effects induced by different types of SiNPs is presented in Table 3. Cytotoxicity and genotoxicity induced by S- and C-SiNPs are strongly correlated with the induction of oxidative stress. For Pr-SiNPs, oxidative stress was associated with cytotoxicity but not genotoxicity. Interestingly, Py-SiNPs caused cytotoxicity, mostly without the generation of oxidative stress. In addition, recent studies showed that Py-SiNPs are biologically more reactive than C-SiNPs (Zhang et al. 2012) and Pr-SiNPs (Di Cristo et al. 2016) of same composition and size. It is known that C-, S-, and Pr-SiNPs are hydrophilic in nature, while Py-SiNPs are hydrophobic due to the de-hydroxylation of surface OH groups during the production process (Napierska et al. 2010), and such difference in surface chemistries might contribute to different biological activities. However, more systematic studies are required to verify these differences depending on production processes.

Toxicity mechanisms of amorphous SiNPs and crystalline silica

A fundamental question was raised by Napierska et al. (2010): do amorphous SiNPs induce biological responses similar to crystalline silica? Oxidative DNA and membrane damage have been reported as the major toxic mechanisms involved in the health effects of micron-sized crystalline silica, which is also observed for C- and Pr-SiNPs. The latter two possess silanols on the surface while Py-SiNPs mostly contain siloxanes (Napierska et al. 2010). In addition, recent studies showed that the presence of surface moieties such as silanols is more correlated with crystalline silica toxicity than crystallinity (Zhang et al. 2012; Turci et al. 2016). Thus, silanols appear to be a common surface feature in C-SiNPs, Pr-SiNPs, and crystalline silica, which might contribute to a similar toxic activity. Furthermore, toxicity elicited by Py-SiNPs appears to be oxidative stress-independent, indicating that Py-SiNPs induce adverse effects via other mechanisms (Gehrke et al. 2013; Napierska et al. 2012a). More studies are required to verify these hypotheses.

Influence of exposure routes in vivo

The exposure route obviously influences the in vivo absorption, biodistribution, and toxicity of SiNPs. For instance, after oropharyngeal aspiration or intratracheal instillation, the lungs are clearly the main target (Smulders et al. 2014), while after ingestion (Zande et al. 2014) or iv injection (Yu et al. 2013), the liver and/or the spleen were the targets. SiNPs accumulated in the brain of intranasally exposed rats (Parveen et al. 2015) but did not after oral exposure (Shim et al. 2014). In addition to the exposure route, physico-chemical properties such as size and shape clearly influence the clearance, distribution, and toxicity (Huang et al. 2011; Li et al. 2015).

Adverse effects of chronic exposure in vivo

In repeated (oral and dermal) dose studies (≥28 days), SiNPs did not induce any local or systemic toxicity even in (very) highly dosed rats. A recent study demonstrated that mild and highly dosed rats excreted most of the orally administered SiNPs via the feces (Yun et al. 2015). In addition, Van der Zande et al. (2014) demonstrated the in vitro gelation of SiNPs with increasing concentrations, which might reduce the gastro-intestinal absorption in highly dosed animals and increase excretion via feces. Therefore, the use of (low) realistic exposure doses appears more appropriate, since the toxicokinetics may depend on the level of exposure (Paek et al. 2014). Moreover, information on the physico-chemical properties of ingested and digested SiNPs is lacking, representing a huge knowledge gap in the risk assessment of SiNPs in food (Dekkers et al. 2012).

Correlation between in vitro and in vivo studies

Correlation between in vitro and in vivo effects is an indication that specific cells or tissues are potential targets for SiNPs toxicity. The results of in vitro and in vivo experiments (included within the same study) suggest that exposure to SiNPs could induce Kupffer cell mediated liver injury (Chen et al. 2013), kidney injury via the activation of NF-kB signaling pathways (Chen et al. 2015), and endothelial dysfunction via autophagy (Duan et al. 2014a). Several in vitro studies showed that SiNPs caused DNA double-strand breaks in a wide range of immortalized cell lines at low doses, but no such genotoxic effects were observed, even in animals at high doses. In addition, SiNPs did not induce micronuclei either in vitro or in vivo.

Dosimetry in vitro

Accurate in vitro dosimetry is an important yet complex aspect of nanotoxicology (Lison et al. 2014). In many studies reviewed here, authors often tested high in vitro exposure doses. Considering the exposure dose as nominal dose may be appropriate only for well dispersed and stable SiNP suspensions (Lison et al. 2008). However, Py-SiNPs, for instance, often re-agglomerate in culture media with an effective density lower than the material density (Deloid et al. 2014), thereby potentially affecting the dose reaching the cells (Cohen et al. 2014). Thus, a realistic estimation of the delivered dose is necessary to compare the biological effects between studies and to establish good in vitro and in vivo correlations (Pal et al. 2015).

Physico-chemical properties for the safer design of SiNPs

Immune responses after the administration of SiNPs are very crucial, as they can induce cascades of events by the secretion of cytokines, which may be harmful or beneficial. The data collected in this review show that the smaller the size, the stronger the pro-inflammatory effect. In addition to the size, the surface charge appears to play a role, since less negative charges seem to suppress the immune response. Porosity is another crucial factor influencing blood biocompatibility, i.e., the more porous the SiNPs, the less hemolysis of RBCs (Maurer-jones et al. 2010). Furthermore, SiNPs aspect ratio was shown to closely relate to in vivo organ retention and clearance (Huang et al. 2011). Therefore, the size, surface area, porosity, and geometry (shape) appear to be the key parameters for designing less-toxic and less-inflammagenic SiNPs, for bio-medical applications for instance.

In conclusion, SiNPs generally exhibit acute toxic effects in vitro and in vivo. The data on chronic effects of SiNPs exposure are rather conflicting with the acute effects and are still insufficient to draw firm conclusions. No concrete data were found to conclude whether amorphous SiNPs induce fibrosis like micrometric crystalline silica. Moreover, translation to human health effects is impossible at this moment due to the lack of realistic exposure and epidemiological data. Surface moieties (such as silanols, silanolates, and siloxanes) of SiNPs are found to be production process-specific and seem to be the key determinants of SiNPs toxicity. AA formation for some SiNPs is very dynamic in physiological media, but it is still unclear how it contributes to the hazard characterization. Therefore, the design of safe(r) SiNPs for food, medical, and other applications will only be possible when physico-chemical characteristics can be unambiguously linked to toxicity. Furthermore, detailed investigations on the SNP bioaccumulation/bioavailability and their long-term consequences in vivo are required for a safer use.

Notes

Acknowledgements

This work was funded by Belgian Science Policy (BELSPO) program “Belgian Research Action through Interdisciplinary Network (BRAIN-be)” for the project “Towards a toxicologically relevant definition of nanomaterials (To2DeNano)”.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

References

  1. Ahamed M (2013) Silica nanoparticles-induced cytotoxicity, oxidative stress and apoptosis in cultured A431 and A549 cells. Hum Exp Toxicol 32:186–195. doi: 10.1177/0960327112459206 PubMedCrossRefGoogle Scholar
  2. Ahmad J, Ahamed M, Akhtar MJ et al (2012) Apoptosis induction by silica nanoparticles mediated through reactive oxygen species in human liver cell line HepG2. Toxicol Appl Pharmacol 259:160–168. doi: 10.1016/j.taap.2011.12.020 PubMedCrossRefGoogle Scholar
  3. Arts JHE, Muijser H, Duistermaat E et al (2007) Five-day inhalation toxicity study of three types of synthetic amorphous silicas in Wistar rats and post-exposure evaluations for up to 3 months. Food Chem Toxicol 45:1856–1867. doi: 10.1016/j.fct.2007.04.001 PubMedCrossRefGoogle Scholar
  4. Athinarayanan J, Periasamy VS, Alsaif MA et al (2014) Presence of nanosilica (E551) in commercial food products: TNF-mediated oxidative stress and altered cell cycle progression in human lung fibroblast cells. Cell Biol Toxicol 30:89–100. doi: 10.1007/s10565-014-9271-8 PubMedCrossRefGoogle Scholar
  5. Barsan ME (2007) NIOSH pocket guide to chemical hazards. Department of Health and Human Services, Center for Disease Control and Prevention, DHHS (NIOSH). Publication No. 2005-149. NIOSH Publications, USGoogle Scholar
  6. Bitar A, Ahmad NM, Fessi H, Elaissari A (2012) Silica-based nanoparticles for biomedical applications. Drug Discov Today 17:1147–1154. doi: 10.1016/j.drudis.2012.06.014 PubMedCrossRefGoogle Scholar
  7. Brinch A, Hansen S, Hartmann N et al (2016) EU regulation of nanobiocides: challenges in implementing the biocidal product regulation (BPR). Nanomaterials 6:33. doi: 10.3390/nano6020033 PubMedCentralCrossRefGoogle Scholar
  8. Chen Q, Xue Y, Sun J (2013) Kupffer cell-mediated hepatic injury induced by silica nanoparticles in vitro and in vivo. Int J Nanomed 8:1129–1140. doi: 10.2147/IJN.S42242 Google Scholar
  9. Chen X, Zhouhua W, Jie Z et al (2015) Renal interstitial fibrosis induced by high-dose mesoporous silica nanoparticles via the NF-kappaB signaling pathway. Int J Nanomedicine 10:1–22. doi: 10.2147/IJN.S73538 PubMedCrossRefGoogle Scholar
  10. Choi J, Zheng Q, Katz HE, Guilarte TR (2010) Silica-based nanoparticle uptake and cellular response by primary microglia. Environ Health Perspect 118:589–595. doi: 10.1289/ehp.0901534 PubMedCrossRefGoogle Scholar
  11. Chu Z, Huang Y, Li L et al (2012) Physiological pathway of human cell damage induced by genotoxic crystalline silica nanoparticles. Biomaterials 33:7540–7546. doi: 10.1016/j.biomaterials.2012.06.073 PubMedCrossRefGoogle Scholar
  12. Cohen JM, Teeguarden JG, Demokritou P (2014) An integrated approach for the in vitro dosimetry of engineered nanomaterials. Part Fibre Toxicol 11:20. doi: 10.1186/1743-8977-11-20 PubMedPubMedCentralCrossRefGoogle Scholar
  13. Corbalan JJ, Medina C, Jacoby A (2011) Amorphous silica nanoparticles trigger nitric oxide/peroxynitrite imbalance in human endothelial cells : inflammatory and cytotoxic effects. Int J Nanomed. doi: 10.2147/IJN.S25071 Google Scholar
  14. Croissant JG, Fatieiev Y, Khashab NM (2017) Degradability and clearance of silicon, organosilica, silsesquioxane, silica mixed oxide, and mesoporous silica nanoparticles. Adv Mater. doi: 10.1002/adma.201604634 PubMedGoogle Scholar
  15. De Temmerman P-J, Van Doren E, Verleysen E et al (2012) Quantitative characterization of agglomerates and aggregates of pyrogenic and precipitated amorphous silica nanomaterials by transmission electron microscopy. J Nanobiotechnol 10:24. doi: 10.1186/1477-3155-10-24 CrossRefGoogle Scholar
  16. Dekkers S, Krystek P, Peters RJB et al (2011) Presence and risks of nanosilica in food products. Nanotoxicology 5:393–405. doi: 10.3109/17435390.2010.519836 PubMedCrossRefGoogle Scholar
  17. Dekkers S, Bouwmeester H, Bos PMJ et al (2012) Knowledge gaps in risk assessment of nanosilica in food : evaluation of the dissolution and toxicity of different forms of silica. Nanotoxicology. doi: 10.3109/17435390.2012.662250 PubMedGoogle Scholar
  18. Deloid G, Cohen JM, Darrah T et al (2014) Estimating the effective density of engineered nanomaterials for in vitro dosimetry. Nat Commun 5:3514. doi: 10.1038/ncomms4514 PubMedPubMedCentralCrossRefGoogle Scholar
  19. Demir E, Castranova V (2016) Genotoxic effects of synthetic amorphous silica nanoparticles in the mouse lymphoma assay. Toxicol ReportsToxicol Rep 3:807–815. doi: 10.1016/j.toxrep.2016.10.006 CrossRefGoogle Scholar
  20. Di Cristo L, Movia D, Bianchi MG et al (2016) Proinflammatory effects of pyrogenic and precipitated amorphous silica nanoparticles in innate immunity cells. Toxicol Sci 150:40–53. doi: 10.1093/toxsci/kfv258 PubMedCrossRefGoogle Scholar
  21. Donaldson K, Poland CA, Schins RPF (2010) Possible genotoxic mechanisms of nanoparticles: criteria for improved test strategies. Nanotoxicology 4:414–420. doi: 10.3109/17435390.2010.482751 PubMedCrossRefGoogle Scholar
  22. Drescher D, Orts-Gil G, Laube G et al (2011) Toxicity of amorphous silica nanoparticles on eukaryotic cell model is determined by particle agglomeration and serum protein adsorption effects. Anal Bioanal Chem 400:1367–1373. doi: 10.1007/s00216-011-4893-7 PubMedCrossRefGoogle Scholar
  23. Du Z, Zhao D, Jing L et al (2013) Cardiovascular toxicity of different sizes amorphous silica nanoparticles in rats after intratracheal instillation. Cardiovasc Toxicol 13:194–207. doi: 10.1007/s12012-013-9198-y PubMedCrossRefGoogle Scholar
  24. Duan J, Yu Y, Li Y et al (2013a) Toxic effect of silica nanoparticles on endothelial cells through DNA damage response via Chk1-dependent G2/M checkpoint. PLoS One. doi: 10.1371/journal.pone.0062087 Google Scholar
  25. Duan J, Yu Y, Li Y et al (2013b) Cardiovascular toxicity evaluation of silica nanoparticles in endothelial cells and zebrafish model. Biomaterials 34:5853–5862. doi: 10.1016/j.biomaterials.2013.04.032 PubMedCrossRefGoogle Scholar
  26. Duan J, Yu Y, Yu Y et al (2014a) Silica nanoparticles enhance autophagic activity, disturb endothelial cell homeostasis and impair angiogenesis. Part Fibre Toxicol 11:50. doi: 10.1186/s12989-014-0050-8 PubMedPubMedCentralCrossRefGoogle Scholar
  27. Duan J, Yu Y, Yu Y et al (2014b) Silica nanoparticles induce autophagy and endothelial dysfunction via the PI3K/Akt/mTOR signaling pathway. Int J Nanomed. doi: 10.2147/IJN.S71074 Google Scholar
  28. Eom H-J, Choi J (2011) SiO2 Nanoparticles induced cytotoxicity by oxidative stress in human bronchial epithelial cell, Beas-2B. Environ Health Toxicol 26:e2011013. doi: 10.5620/eht.2011.26.e2011013 PubMedPubMedCentralCrossRefGoogle Scholar
  29. Farcal LR, Uboldi C, Mehn D et al (2012) Mechanisms of toxicity induced by SiO2 nanoparticles of in vitro human alveolar barrier: effects on cytokine production, oxidative stress induction, surfactant proteins A mRNA expression and nanoparticles uptake. Nanotoxicology 7:1095–1110. doi: 10.3109/17435390.2012.710658 PubMedCrossRefGoogle Scholar
  30. Fede C, Selvestrel F, Compagnin C et al (2012) The toxicity outcome of silica nanoparticles (Ludox??) is influenced by testing techniques and treatment modalities. Anal Bioanal Chem 404:1789–1802. doi: 10.1007/s00216-012-6246-6 PubMedPubMedCentralCrossRefGoogle Scholar
  31. Fröhlich E, Samberger C, Kueznik T et al (2009) Cytotoxicity of nanoparticles independent from oxidative stress. J Toxicol Sci 34:363–375. doi: 10.2131/jts.34.363 PubMedCrossRefGoogle Scholar
  32. Fruijtier-Pölloth C (2012) The toxicological mode of action and the safety of synthetic amorphous silica-A nanostructured material. Toxicology 294:61–79. doi: 10.1016/j.tox.2012.02.001 PubMedCrossRefGoogle Scholar
  33. Fu PP, Xia Q, Hwang H-M et al (2013) Mechanisms of nanotoxicity: generation of reactive oxygen species. J Food Drug Anal. doi: 10.1016/j.jfda.2014.01.005 Google Scholar
  34. Gehrke H, Frühmesser A, Pelka J et al (2013) In vitro toxicity of amorphous silica nanoparticles in human colon carcinoma cells. Nanotoxicology 7:274–293. doi: 10.3109/17435390.2011.652207 PubMedCrossRefGoogle Scholar
  35. Geys J, Nemery B, Hoet PHM (2010) Assay conditions can influence the outcome of cytotoxicity tests of nanomaterials: better assay characterization is needed to compare studies. Toxicol Vitr 24:620–629. doi: 10.1016/j.tiv.2009.10.007 CrossRefGoogle Scholar
  36. Glick D, Barth S, Macleod KF (2010) Autophagy: cellular and molecular mechanisms. J Pathol 221:3–12. doi: 10.1002/path.2697.Autophagy PubMedPubMedCentralCrossRefGoogle Scholar
  37. Gong C, Tao G, Yang L et al (2012) The role of reactive oxygen species in silicon dioxide nanoparticle-induced cytotoxicity and DNA damage in HaCaT cells. Mol Biol Rep 39:4915–4925. doi: 10.1007/s11033-011-1287-z PubMedCrossRefGoogle Scholar
  38. Guichard Y, Fontana C, Chavinier E et al (2016) Cytotoxic and genotoxic evaluation of different synthetic amorphous silica nanomaterials in the V79 cell line. Toxicol Ind Health. doi: 10.1177/0748233715572562 PubMedGoogle Scholar
  39. Guo C, Xia Y, Niu P et al (2015) Silica nanoparticles induce oxidative stress, inflammation, and endothelial dysfunction in vitro via activation of the MAPK/Nrf2 pathway and nuclear factor-κB signaling. Int J Nanomedicine 10:1463–1477. doi: 10.2147/IJN.S76114 PubMedPubMedCentralCrossRefGoogle Scholar
  40. Guo C, Yang M, Jing L et al (2016) Amorphous silica nanoparticles trigger vascular endothelial cell injury through apoptosis and autophagy via reactive oxygen species-mediated MAPK/Bcl-2 and PI3K/Akt/mTOR signaling. Int J Nanomed 11:5257–5276. doi: 10.2147/IJN.S112030 CrossRefGoogle Scholar
  41. Hara K, Shirasuna K, Usui F et al (2014) Interferon-tau attenuates uptake of nanoparticles and secretion of interleukin-1?? in macrophages. PLoS One 9:1–17. doi: 10.1371/journal.pone.0113974 Google Scholar
  42. Hashimoto M, Imazato S (2015) Cytotoxic and genotoxic characterization of aluminum and silicon oxide nanoparticles in macrophages. Dent Mater 31:556–564. doi: 10.1016/j.dental.2015.02.009 PubMedCrossRefGoogle Scholar
  43. Hassankhani R, Esmaeillou M, Tehrani AA et al (2014) In vivo toxicity of orally administrated silicon dioxide nanoparticles in healthy adult mice. Environ Sci Pollut Res. doi: 10.1007/s11356-014-3413-7 Google Scholar
  44. Hayashi Y, Miclaus T, Murugadoss S et al (2017) Female versus male biological identities of nanoparticles determine the interaction with immune cells in fish. Environ Sci Nano 4:895–906. doi: 10.1039/C7EN00071E PubMedCrossRefGoogle Scholar
  45. Hofmann T, Schneider S, Wolterbeek A et al (2015) Prenatal toxicity of synthetic amorphous silica nanomaterial in rats. Reprod Toxicol 56:141–146. doi: 10.1016/j.reprotox.2015.04.006 PubMedCrossRefGoogle Scholar
  46. Hu YL, Gao JQ (2010) Potential neurotoxicity of nanoparticles. Int J Pharm 394:115–121. doi: 10.1016/j.ijpharm.2010.04.026 PubMedCrossRefGoogle Scholar
  47. Huang X, Li L, Liu T et al (2011) The shape effect of mesoporous silica nanoparticles on biodistribution. ACS Nano 5:5390–5399. doi: 10.1021/nn200365a PubMedCrossRefGoogle Scholar
  48. Jose Corbalan J, Medina C, Jacoby A et al (2012) Amorphous silica nanoparticles aggregate human platelets: potential implications for vascular homeostasis. Int J Nanomed 7:631–639. doi: 10.2147/IJN.S28293 CrossRefGoogle Scholar
  49. Kasaai MR (2015) Nanosized particles of silica and its derivatives for applications in various branches of food and nutrition sectors. J Nanotechnol. doi: 10.1155/2015/852394 Google Scholar
  50. Kasper J, Hermanns MI, Bantz C et al (2011) Inflammatory and cytotoxic responses of an alveolar-capillary coculture model to silica nanoparticles: comparison with conventional monocultures. Part Fibre Toxicol 8:6. doi: 10.1186/1743-8977-8-6 PubMedPubMedCentralCrossRefGoogle Scholar
  51. Kasper JY, Feiden L, Hermanns MI et al (2015) Pulmonary surfactant augments cytotoxicity of silica nanoparticles: studies on an in vitro air-blood barrier model. Beilstein J Nanotechnol 6:517–528. doi: 10.3762/bjnano.6.54 PubMedPubMedCentralCrossRefGoogle Scholar
  52. Khot LR, Sankaran S, Maja JM et al (2012) Applications of nanomaterials in agricultural production and crop protection: a review. Crop Prot 35:64–70. doi: 10.1016/j.cropro.2012.01.007 CrossRefGoogle Scholar
  53. Kim B, Kim H, Yu IJ (2014a) Assessment of nanoparticle exposure in nanosilica handling process: including characteristics of nanoparticles leaking from a vacuum cleaner. Ind Heal 52:152–162. doi: 10.2486/indhealth.2013-0087 CrossRefGoogle Scholar
  54. Kim J-H, Kim C-S, Ignacio RMC et al (2014b) Immunotoxicity of silicon dioxide nanoparticles with different sizes and electrostatic charge. Int J Nanomed 9(Suppl 2):183–193. doi: 10.2147/IJN.S57934 CrossRefGoogle Scholar
  55. Kim YR, Lee SY, Lee EJ et al (2014c) Toxicity of colloidal silica nanoparticles administered orally for 90 days in rats. Int J Nanomed 9:67–78. doi: 10.2147/IJN.S57925 Google Scholar
  56. Lee S, Yun HS, Kim SH (2011) The comparative effects of mesoporous silica nanoparticles and colloidal silica on inflammation and apoptosis. Biomaterials 32:9434–9443. doi: 10.1016/j.biomaterials.2011.08.042 PubMedCrossRefGoogle Scholar
  57. Lee JA, Kim MK, Paek HJ et al (2014) Tissue distribution and excretion kinetics of orally administered silica nanoparticles in rats. Int J Nanomed 9:251–260. doi: 10.2147/IJN.S57939 Google Scholar
  58. Lenz A-G, Karg E, Brendel E et al (2013) Inflammatory and oxidative stress responses of an alveolar epithelial cell line to airborne zinc oxide nanoparticles at the air-liquid interface: a comparison with conventional, submerged cell-culture conditions. Biomed Res Int 1:12Google Scholar
  59. Leung CC, Yu ITS, Chen W (2012) Silicosis. Lancet 379:2008–2018. doi: 10.1016/S0140-6736(12)60235-9 PubMedCrossRefGoogle Scholar
  60. Li Y, Sun L, Jin M et al (2011) Size-dependent cytotoxicity of amorphous silica nanoparticles in human hepatoma HepG2 cells. Toxicol Vitr 25:1343–1352. doi: 10.1016/j.tiv.2011.05.003 CrossRefGoogle Scholar
  61. Li L, Liu T, Fu C et al (2015) Biodistribution, excretion, and toxicity of mesoporous silica nanoparticles after oral administration depend on their shape. Nanomed Nanotechnol Biol Med 11:1915–1924. doi: 10.1016/j.nano.2015.07.004 CrossRefGoogle Scholar
  62. Liang H, Jin C, Tang Y et al (2014) Cytotoxicity of silica nanoparticles on HaCaT cells. J Appl Toxicol 34:367–372. doi: 10.1002/jat.2953 PubMedCrossRefGoogle Scholar
  63. Liljenström C, Lazarevic D, Finnveden G (2013) Silicon-based nanomaterials in a life-cycle perspective, including a case study on self-cleaning coatings. ISBN 978-91-7501-942-0Google Scholar
  64. Lison D, Thomassen LCJ, Rabolli V et al (2008) Nominal and effective dosimetry of silica nanoparticles in cytotoxicity assays. Toxicol Sci 104:155–162. doi: 10.1093/toxsci/kfn072 PubMedCrossRefGoogle Scholar
  65. Lison D, Vietti G, Van Den Brule S (2014) Paracelsus in nanotoxicology. Part Fibre Toxicol. doi: 10.1186/s12989-014-0035-7 Google Scholar
  66. Lu X, Qian J, Zhou H et al (2011) In vitro cytotoxicity and induction of apoptosis by silica nanoparticles in human HepG2 hepatoma cells. Int J Nanomed 6:1889–1901. doi: 10.2147/IJN.S24005 Google Scholar
  67. Luyts K, Napierska D, Nemery B, Hoet PHM (2013) How physico-chemical characteristics of nanoparticles cause their toxicity: complex and unresolved interrelations. Environ Sci Impacts 15:23–38. doi: 10.1039/c2em30237c CrossRefGoogle Scholar
  68. Magdolenova Z, Collins A, Kumar A et al (2014) Mechanisms of genotoxicity. A review of in vitro and in vivo studies with engineered nanoparticles. Nanotoxicology 8:233–278. doi: 10.3109/17435390.2013.773464 PubMedCrossRefGoogle Scholar
  69. Manke A, Wang L, Rojanasakul Y (2013) Mechanisms of nanoparticle-induced oxidative stress and toxicity. HINDAWI 2013:1–14. doi: 10.1155/2013/942916 Google Scholar
  70. Maser E, Schulz M, Sauer UG et al (2015) In vitro and in vivo genotoxicity investigations of differently sized amorphous SiO2 nanomaterials. Mutat Res Genet Toxicol Environ Mutagen 794:57–74. doi: 10.1016/j.mrgentox.2015.10.005 PubMedCrossRefGoogle Scholar
  71. Maurer-jones MA, Lin Y, Haynes CL (2010) Functional assessment of metal oxide nanoparticle toxicity in immune cells. ACS Nano 4:3363–3373PubMedCrossRefGoogle Scholar
  72. McCracken C, Zane A, Knight DA et al (2013) Minimal intestinal epithelial cell toxicity in response to short- and long-term food-relevant inorganic nanoparticle exposure. Chem Res Toxicol 26:1514–1525. doi: 10.1021/tx400231u PubMedCrossRefGoogle Scholar
  73. Mendoza A, Torres-Hernandez JA, Ault JG et al (2014) Silica nanoparticles induce oxidative stress and inflammation of human peripheral blood mononuclear cells. Cell Stress Chaperones 19:777–790. doi: 10.1007/s12192-014-0502-y PubMedPubMedCentralCrossRefGoogle Scholar
  74. Michael Berg J, Romoser AA, Figueroa DE et al (2013) Comparative cytological responses of lung epithelial and pleural mesothelial cells following in vitro exposure to nanoscale SiO2. Toxicol Vitr 27:24–33. doi: 10.1016/j.tiv.2012.09.002 CrossRefGoogle Scholar
  75. Morris AS, Adamcakova-Dodd A, Lehman SE et al (2016) Amine modification of nonporous silica nanoparticles reduces inflammatory response following intratracheal instillation in murine lungs. Toxicol Lett 241:207–215. doi: 10.1016/j.toxlet.2015.11.006 PubMedCrossRefGoogle Scholar
  76. Nabeshi H, Yoshikawa T, Matsuyama K et al (2011a) Amorphous nanosilica induce endocytosis-dependent ROS generation and DNA damage in human keratinocytes. Part Fibre Toxicol 8:1. doi: 10.1186/1743-8977-8-1 PubMedPubMedCentralCrossRefGoogle Scholar
  77. Nabeshi H, Yoshikawa T, Matsuyama K et al (2011b) Systemic distribution, nuclear entry and cytotoxicity of amorphous nanosilica following topical application. Biomaterials 32:2713–2724. doi: 10.1016/j.biomaterials.2010.12.042 PubMedCrossRefGoogle Scholar
  78. Napierska D, Thomassen LCJ, Lison D et al (2010) The nanosilica hazard: another variable entity. Part Fibre Toxicol 7:39. doi: 10.1186/1743-8977-7-39 PubMedPubMedCentralCrossRefGoogle Scholar
  79. Napierska D, Rabolli V, Thomassen LCJ et al (2012a) Oxidative stress induced by pure and iron-doped amorphous silica nanoparticles in subtoxic conditions. Chem Res Toxicol 25:828–837. doi: 10.1021/tx200361v PubMedCrossRefGoogle Scholar
  80. Napierska D, Thomassen LCJ, Vanaudenaerde B et al (2012b) Cytokine production by co-cultures exposed to monodisperse amorphous silica nanoparticles: the role of size and surface area. Toxicol Lett 211:98–104. doi: 10.1016/j.toxlet.2012.03.002 PubMedCrossRefGoogle Scholar
  81. Nemmar A, Yuvaraju P, Beegam S et al (2015) In vitro platelet aggregation and oxidative stress caused by amorphous silica nanoparticles. Int J Physiol Pathophysiol Pharmacol 7:27–33PubMedPubMedCentralGoogle Scholar
  82. Nemmar A, Yuvaraju P, Beegam S et al (2016) Oxidative stress, inflammation, and DNA damage in multiple organs of mice acutely exposed to amorphous silica nanoparticles. Int J Nanomed. doi: 10.2147/IJN.S92278 Google Scholar
  83. Nowak JS, Mehn D, Nativo P et al (2014) Silica nanoparticle uptake induces survival mechanism in A549 cells by the activation of autophagy but not apoptosis. Toxicol Lett 224:84–92. doi: 10.1016/j.toxlet.2013.10.003 PubMedCrossRefGoogle Scholar
  84. Oberdörster G (2010) Safety assessment for nanotechnology and nanomedicine: concepts of nanotoxicology. J Intern Med 267:89–105. doi: 10.1111/j.1365-2796.2009.02187.x PubMedCrossRefGoogle Scholar
  85. Oh S, Kim B, Kim H (2014) Comparison of nanoparticle exposures between fumed and sol-gel nano-silica manufacturing facilities. Ind Health 52:190–198. doi: 10.2486/indhealth.2013-0117 PubMedPubMedCentralCrossRefGoogle Scholar
  86. Paek H-J, Chung H-E, Lee J-A et al (2014) Quantitative determination of silica nanoparticles in biological matrices and their pharmacokinetics and toxicokinetics in rats. Sci Adv Mater 6(6):1605–1610. doi: 10.1166/sam.2014.1817 CrossRefGoogle Scholar
  87. Pal AK, Bello D, Cohen J, Demokritou P (2015) Implications of in vitro dosimetry on toxicological ranking of low aspect ratio engineered nanomaterials. Nanotoxicology 9:871–885. doi: 10.3109/17435390.2014.986670 PubMedPubMedCentralCrossRefGoogle Scholar
  88. Panas A, Comouth A, Saathoff H et al (2014) Silica nanoparticles are less toxic to human lung cells when deposited at the air-liquid interface compared to conventional submerged exposure. Beilstein J Nanotechnol 5:1590–1602. doi: 10.3762/bjnano.5.171 PubMedPubMedCentralCrossRefGoogle Scholar
  89. Parveen A, Rizvi SHM, Sushma et al (2015) Intranasal exposure to silica nanoparticles induce alterations in pro-inflammatory environment of rat brain: involvement of oxidative stress. Toxicol Ind Health. doi: 10.1177/0748233715602985 Google Scholar
  90. Passagne I, Morille M, Rousset M et al (2012) Implication of oxidative stress in size-dependent toxicity of silica nanoparticles in kidney cells. Toxicology 299:112–124. doi: 10.1016/j.tox.2012.05.010 PubMedCrossRefGoogle Scholar
  91. Rabolli V, Thomassen LCJ, Uwambayinema F et al (2011) The cytotoxic activity of amorphous silica nanoparticles is mainly influenced by surface area and not by aggregation. Toxicol Lett 206:197–203. doi: 10.1016/j.toxlet.2011.07.013 PubMedCrossRefGoogle Scholar
  92. Radomski A, Jurasz P, Alonso-Escolano D et al (2005) Nanoparticle-induced platelet aggregation and vascular thrombosis. Br J Pharmacol 146:882–893. doi: 10.1038/sj.bjp.0706386 PubMedPubMedCentralCrossRefGoogle Scholar
  93. Rajendran P, Rengarajan T, Thangavel J et al (2013) The vascular endothelium and human diseases. Int J Biol Sci 9:1057–1069. doi: 10.7150/ijbs.7502 PubMedPubMedCentralCrossRefGoogle Scholar
  94. Rajiv S, Jerobin J, Saranya V et al (2015) Comparative cytotoxicity and genotoxicity of cobalt (II, III) oxide, iron (III) oxide, silicon dioxide, and aluminum oxide nanoparticles on human lymphocytes in vitro. Hum Exp Toxicol. doi: 10.1177/0960327115579208 PubMedGoogle Scholar
  95. Ryu HJ, Seong NW, So BJ et al (2014) Evaluation of silica nanoparticle toxicity after topical exposure for 90 days. Int J Nanomed 9:127–136. doi: 10.2147/IJN.S57929 Google Scholar
  96. Sergent JA, Paget V, Chevillard S (2012) Toxicity and genotoxicity of nano-SiO2 on human epithelial intestinal HT-29 cell line. Ann Occup Hyg 56:622–630. doi: 10.1093/annhyg/mes005 PubMedGoogle Scholar
  97. Shim KH, Jeong KH, Bae SO et al (2014) Assessment of ZnO and SiO2 nanoparticle permeability through and toxicity to the blood–brain barrier using evans blue and tem. Int J Nanomed 9:225–233. doi: 10.2147/IJN.S58205 Google Scholar
  98. Smulders S, Luyts K, Brabants G et al (2014) Toxicity of nanoparticles embedded in paints compared with pristine nanoparticles in mice. Toxicol Sci 141:132–140. doi: 10.1093/toxsci/kfu112 PubMedPubMedCentralCrossRefGoogle Scholar
  99. Stern ST, Adiseshaiah PP, Crist RM (2012) Autophagy and lysosomal dysfunction as emerging mechanisms of nanomaterial toxicity. Part Fibre Toxicol 9:20. doi: 10.1186/1743-8977-9-20 PubMedPubMedCentralCrossRefGoogle Scholar
  100. Sun L, Li Y, Liu X et al (2011) Cytotoxicity and mitochondrial damage caused by silica nanoparticles. Toxicol Vitr 25:1619–1629. doi: 10.1016/j.tiv.2011.06.012 CrossRefGoogle Scholar
  101. Tang L, Cheng J (2013) Nonporous silica nanoparticles for nanomedicine application. Nano Today 8:290–312. doi: 10.1016/j.nantod.2013.04.007 PubMedPubMedCentralCrossRefGoogle Scholar
  102. Tavares AM, Louro H, Antunes S et al (2014) Genotoxicity evaluation of nanosized titanium dioxide, synthetic amorphous silica and multi-walled carbon nanotubes in human lymphocytes. Toxicol Vitr 28:60–69. doi: 10.1016/j.tiv.2013.06.009 CrossRefGoogle Scholar
  103. Tarantini A, Huet S, Jarry G et al (2015a) Genotoxicity of synthetic amorphous silica nanoparticles in rats following short-term exposure. Part 1: Oral route. Environ Mol Mutagen 56:218–227. doi: 10.1002/em.21935 PubMedCrossRefGoogle Scholar
  104. Tarantini A, Lanceleur R, Mourot A et al (2015b) Toxicity, genotoxicity and proinflammatory effects of amorphous nanosilica in the human intestinal Caco-2 cell line. Toxicol Vitr 29:398–407. doi: 10.1016/j.tiv.2014.10.023 CrossRefGoogle Scholar
  105. Thomassen LCJ, Rabolli V, Masschaele K et al (2011) Model system to study the influence of aggregation on the hemolytic potential of silica nanoparticles. Chem Res Toxicol 24:1869–1875. doi: 10.1021/tx2002178 PubMedCrossRefGoogle Scholar
  106. Turci F, Pavan C, Leinardi R et al (2016) Revisiting the paradigm of silica pathogenicity with synthetic quartz crystals: the role of crystallinity and surface disorder. Part Fibre Toxicol 13:32. doi: 10.1186/s12989-016-0136-6 PubMedPubMedCentralCrossRefGoogle Scholar
  107. Uboldi C, Giudetti G, Broggi F et al (2012) Amorphous silica nanoparticles do not induce cytotoxicity, cell transformation or genotoxicity in Balb/3T3 mouse fibroblasts. Mutat Res Genet Toxicol Environ Mutagen 745:11–20. doi: 10.1016/j.mrgentox.2011.10.010 CrossRefGoogle Scholar
  108. Uemura E, Yoshioka Y, Hirai T et al (2016) Relationship between size and surface modification of silica particles and enhancement and suppression of inflammatory cytokine production by lipopolysaccharide- or peptidoglycan-stimulated RAW264.7 macrophages. J Nanoparticle Res 18:165. doi: 10.1007/s11051-016-3475-1 CrossRefGoogle Scholar
  109. Van Der Zande M, Vandebriel RJ, Groot MJ et al (2014) Sub-chronic toxicity study in rats orally exposed to nanostructured silica. Part Fibre Toxicol 11:1–19. doi: 10.1186/1743-8977-11-8 CrossRefGoogle Scholar
  110. Vance ME, Kuiken T, Vejerano EP et al (2015) Nanotechnology in the real world: redeveloping the nanomaterial consumer products inventory. Beilstein J Nanotechnol 6:1769–1780. doi: 10.3762/bjnano.6.181 PubMedPubMedCentralCrossRefGoogle Scholar
  111. Voicu SNP, Dinu D, Sima C et al (2015) Silica nanoparticles induce oxidative stress and autophagy but not apoptosis in the MRC-5 cell line. Int J Mol Sci 16:29398–29416. doi: 10.3390/ijms161226171 CrossRefGoogle Scholar
  112. Vriens H, Mertens D, Regret R et al (2017) Chapter 11 case study III: the construction of a nanotoxicity database—the MOD-ENP-TOX experience. Adv Exp Med Biol. doi: 10.1007/978-3-319-47754-1 PubMedGoogle Scholar
  113. Wang F, Gao F, Lan M et al (2009) Oxidative stress contributes to silica nanoparticle-induced cytotoxicity in human embryonic kidney cells. Toxicol Vitr 23:808–815. doi: 10.1016/j.tiv.2009.04.009 CrossRefGoogle Scholar
  114. Wang W, Li Y, Liu X et al (2013) Multinucleation and cell dysfunction induced by amorphous silica nanoparticles in an L-02 human hepatic cell line. Int J Nanomed 8:3533–3541. doi: 10.2147/IJN.S46732 Google Scholar
  115. Winkler HC, Suter M, Naegeli H (2016) Critical review of the safety assessment of nano-structured silica additives in food. J Nanobiotechnol 14:44. doi: 10.1186/s12951-016-0189-6 CrossRefGoogle Scholar
  116. Wittmaack K (2011) Novel dose metric for apparent cytotoxicity effects generated by in vitro cell exposure to silica nanoparticles. Chem Res Toxicol 24:150–158. doi: 10.1021/tx100331w PubMedCrossRefGoogle Scholar
  117. Xie H, Wu J (2016) Silica nanoparticles induce alpha-synuclein induction and aggregation in PC12-cells. Chem Biol Interact 258:197–204. doi: 10.1016/j.cbi.2016.09.006 PubMedCrossRefGoogle Scholar
  118. Xu Z, Chou L, Sun J (2012) Effects of SiO2 nanoparticles on HFL-I activating ROS-mediated apoptosis via p53 pathway. J Appl Toxicol 32:358–364. doi: 10.1002/jat.1710 PubMedCrossRefGoogle Scholar
  119. Xue Y, Wu J, Sun J (2012) Four types of inorganic nanoparticles stimulate the inflammatory reaction in brain microglia and damage neurons in vitro. Toxicol Lett 214:91–98. doi: 10.1016/j.toxlet.2012.08.009 PubMedCrossRefGoogle Scholar
  120. Yang X, He C, Li J et al (2014a) Uptake of silica nanoparticles: neurotoxicity and Alzheimer-like pathology in human SK-N-SH and mouse neuro2a neuroblastoma cells. Toxicol Lett 229:240–249. doi: 10.1016/j.toxlet.2014.05.009 PubMedCrossRefGoogle Scholar
  121. Yang YX, Song ZM, Cheng B et al (2014b) Evaluation of the toxicity of food additive silica nanoparticles on gastrointestinal cells. J Appl Toxicol 34:424–435. doi: 10.1002/jat.2962 PubMedCrossRefGoogle Scholar
  122. Yang M, Jing L, Wang J et al (2016) Macrophages participate in local and systemic inflammation induced by amorphous silica nanoparticles through intratracheal instillation. Int J Nanomed 11:6217–6228CrossRefGoogle Scholar
  123. Ye Y, Liu J, Chen M et al (2010a) In vitro toxicity of silica nanoparticles in myocardial cells. Environ Toxicol Pharmacol 29:131–137. doi: 10.1016/j.etap.2009.12.002 PubMedCrossRefGoogle Scholar
  124. Ye Y, Liu J, Xu J et al (2010b) Nano-SiO2 induces apoptosis via activation of p53 and Bax mediated by oxidative stress in human hepatic cell line. Toxicol Vitr 24:751–758. doi: 10.1016/j.tiv.2010.01.001 CrossRefGoogle Scholar
  125. Yoshida T, Yoshioka Y, Takahashi H et al (2014) Intestinal absorption and biological effects of orally administered amorphous silica particles. Nanoscale Res Lett. doi: 10.1186/1556-276X-9-532 Google Scholar
  126. Yu T, Malugin A, Ghandehari H (2011) Impact of silica nanoparticle design on cellular toxicity and hemolytic activity. ACS Nano 5:5717–5728. doi: 10.1021/nn2013904 PubMedPubMedCentralCrossRefGoogle Scholar
  127. Yu Y, Li Y, Wang W et al (2013) Acute toxicity of amorphous silica nanoparticles in intravenously exposed ICR mice. PLoS One. doi: 10.1371/journal.pone.0061346 Google Scholar
  128. Yu Y, Duan J, Yu Y et al (2014) Silica nanoparticles induce autophagy and autophagic cell death in HepG2 cells triggered by reactive oxygen species. J Hazard Mater 270:176–186. doi: 10.1016/j.jhazmat.2014.01.028 PubMedCrossRefGoogle Scholar
  129. Yun J-W, Kim S-H, You J-R et al (2015) Comparative toxicity of silicon dioxide, silver and iron oxide nanoparticles after repeated oral administration to rats. J Appl Toxicol 35:681–693. doi: 10.1002/jat.3125 PubMedCrossRefGoogle Scholar
  130. Zhang XQ, Yin LH, Tang M, Pu YP (2011) ZnO, TiO(2), SiO(2,) and Al(2)O(3) nanoparticles-induced toxic effects on human fetal lung fibroblasts. Biomed Env Sci 24:661–669. doi: 10.3967/0895-3988.2011.06.011 Google Scholar
  131. Zhang H, Dunphy DR, Jiang X et al (2012) Processing pathway dependence of amorphous silica nanoparticle toxicity: colloidal vs pyrolytic. J Am Chem Soc 134:15790–15804. doi: 10.1021/ja304907c PubMedPubMedCentralCrossRefGoogle Scholar
  132. Zhang Q, Xu H, Zheng S et al (2015) Genotoxicity of mesoporous silica nanoparticles in human embryonic kidney 293 cells. Drug Test Anal 7:787–796. doi: 10.1002/dta.1773 PubMedCrossRefGoogle Scholar
  133. Zhang J, Ren L, Zou Y et al (2016) Silica nanoparticles induce start inhibition of meiosis and cell cycle arrest via down-regulating meiotic relevant factors. Toxicol Res 5:1453–1464. doi: 10.1039/C6TX00236F CrossRefGoogle Scholar
  134. Zhuravskii S, Yukina G, Kulikova O et al (2016) Mast cell accumulation precedes tissue fibrosis induced by intravenously administered amorphous silica nanoparticles. Toxicol Mech Methods 26:260–269. doi: 10.3109/15376516.2016.1169341 PubMedCrossRefGoogle Scholar

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Authors and Affiliations

  1. 1.Unit for Lung ToxicologyKatholieke Universiteit LeuvenLouvainBelgium
  2. 2.Louvain Centre for Toxicology and Applied Pharmacology (LTAP)Université Catholique de LouvainBrusselsBelgium
  3. 3.Department of Occupational, Environmental and Insurance MedicineKatholieke Universiteit LeuvenLouvainBelgium
  4. 4.EM-unitCenter for Veterinary and Agrochemical Studies and Research (CODA-CERVA)BrusselsBelgium
  5. 5.General Quality and Safety, Metrology DepartmentNational StandardsBrusselsBelgium

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