Green chemistry synthesis of biocompatible ZnS quantum dots (QDs): their application as potential thin films and antibacterial agent
- 143 Downloads
We are presenting here the synthesis of quantum dots (QDs) of direct band gap semiconductor, cubic ZnS through modified green chemistry-mediated chemical precipitation reaction. Green chemistry-synthesized (GCS) ZnS QDs were characterized using powder X-ray diffraction and high-resolution transmission electron microscope techniques. Analysis of results, revealed by both the techniques for the synthesized QDs, is complementary as far as the size range (2–6 nm) of ZnS QDs is concerned. UV–Vis spectrophotometric spectrum (λmax = 314 nm) showed a conspicuous blue shift than the bulk. The Fourier-transformed infrared spectra convincingly reported a Zn–S bond stretching frequency at 649 cm−1. The characterized QDs were subjected to the preparation of thin films over SiO2 template (57 nm thickness) using photoresist spin coating technique at the ambient condition. The surface topology of nanoscale-thick films was studied by atomic force microscope (roughness parameter—33.28 nm, rms; for a scan area of 3.48 × 3.48 μm2). The symmetrical (skewness = 1.68) and random distribution (kurtosis = 2.93) of the peaks and valleys revealed the nanoscale-thick films of ZnS QDs. Zeta potential (− 9.2 mV) fairly proved stable existence of ZnS QDs. The GCS QDs were found to be non-toxic toward L929 mouse fibroblastic cells and human erythrocytes. However, they demonstrated significant inhibitory effects against seven bacterial pathogens with an average zone of inhibition of 1.5 cm at 100 μg/ml concentration. The minimum inhibitory concentrations determined were in the range of 75 to 125 μg/ml for gram-positive and 100 to 150 μg/ml for gram-negative bacterial pathogens.
KeywordsZinc sulfide quantum dots Green synthesis Thin films Biocompatibility Antibacterial effect
Today’s world is significantly pursuing the research that indulges in the design, development and implementation of chemical products and processes to reduce the use and generation of products that are hazardous to human health and environment [1, 2, 3]. The sustainable and renewable green energy, as an alternative to carbon-emitting fossil fuels, has become the main target of green energy researchers [4, 5, 6]. Greenhouse gases are the main by-products of carbon-based traditional energy sources. To eliminate hazardous environmental issues, photovoltaic solar cells (PVSCs) apart from nuclear, thermal and wind energy have gained the momentum . Presently, quantum dot-sensitized solar cell (QDSSC) is gaining the preference as the most effective PVSC [8, 9, 10], although it is still at the investigation level only.
The semiconducting quantum dots (QDs) have full access to the whole solar spectrum. They have the additional advantage of production of high quantum yield by the generation of multiple excitons [11, 12, 13, 14, 15, 16]. Semiconducting QD (SQD) metamaterial has potential applications in the field of smart LEDs (light emitting diodes) [17, 18, 19, 20], photocatalytic activity [21, 22, 23, 24], space science [25, 26], biological fields such as biosensors, biomarkers and as bio-tagging agents [27, 28, 29, 30]. It has been observed that SQDs of IIB–VIA groups are efficient metamaterial for PVSC applications due to their direct band gap, large excitons binding energy [31, 32] and a high index of refraction . The literature reveals that the nanometer range of the particles enhances the properties of semiconducting metamaterial due to its quantum confinement effect . Among the nano-materials of group IIB–VIA, zinc sulfide (ZnS) QDs have wide and direct band gap Eg = 3.58 eV at 300 K (bulk cubic crystals), which might make it suitably promising optical material toward the design of solar cell. Keeping in view the above advantage and multi-faceted applications, the present work has been taken into consideration. The present work reports the green synthesis of SQDs of ZnS at the ambient conditions. The eco-friendly synthesis involves the use of water-soluble precursors and capping agents, resulting in the formation of non-hazardous by-products which can further easily be removed by repeated washing with water. The green chemistry synthesis (GCS) process was found to be cost-effective, reproducible, energy-efficient and sustainable. It has been observed that the mechanized thin film solar cells (TFSCs) are much thinner and light weighted as compared to the traditional first-generation solar cells [35, 36]. This has led to the development of thin films using nano-materials proving better efficiency as compared to the previously available one. The thin films are considered to be third-generation [37, 38, 39, 40, 41, 42] engineered device that can hold the desired PVSC applications quite well. Keeping in view this, the colloidal solution of ZnS nanoparticles was used for the fabrication of variable nano-sized (< 100 nm) thin films in order to study their structural morphology and surface topology. In order to make the thin films eco-friendly, the QDs used for making thin films need to be analyzed for their biocompatibility and antibacterial activity. Thus, green-synthesized QDs of ZnS semiconducting metamaterial were further subjected to the studies on their biocompatibility and zone of inhibition (ZOI) over human erythrocytes, mouse fibroblast cells and seven (gram positive and gram negative) bacterial pathogens, respectively.
Materials and methods
All the chemicals used during the synthesis that were of analytical grade are reported under Supplementary information. Series of green synthetic reactions were carried out by either changing the concentration of precursors or capping reagent. At first, the Zn(NO3)2·6H2O (1 M, 1.487 g) was added to 5 ml of double distilled water (A). The solution was stirred well on a magnetic stirrer at room temperature (pH = 6.1) for about 5 min in order to get a homogeneous solution. In the meantime, d-glucose (2 M, 1.9816 g) solution was prepared in 5 ml of double distilled water (B). Solution B was added dropwise to the solution A till a uniform homogeneous mixture was obtained (pH = 6.3). At this stage, the aqueous solution of Na2S (2 M, 0.7808 g) in 5 ml double distilled water was added dropwise to the precursor solution till the entire solution became white in color (pH = 5.4). The resultant solution was refluxed for an hour. A yolk-colored solution was observed.
The resultant solution was subjected to centrifugation at 14,000 rpm, and the precipitate formed was washed several times with double distilled water (pH = 7.0) and acetone, respectively. The purified end product so formed was dried overnight by keeping in a vacuum desiccator. The dried sample was used for further characterization.
Instruments used for characterization
The X-ray diffraction patterns of synthesized ZnS QDs are recorded with Bruker D8 Advance X-ray diffractometer. The primary X-ray wavelength Cu-Kα1 is at 1.5405 Å with a 2.2-kW power source. The sample was recorded in a wide range of Brag’s angle from 5° to 90° to get the best profile for the sample. The high-resolution transmission electron micrographs are obtained from JEOL JEM-2100 Transmission Electron Microscope with a resolution of 1.9 Å to 1.4 Å. The images are captured with a 2.672 × 2.672 k high-resolution CCD camera at 200 kV acceleration voltage. The absorption spectrum (λmax) is recorded with Ocean Optics DH-2000-BAL UV–visible spectrophotometer. The FTIR spectrum is recorded with PerkinElmer FTIR spectrophotometer (Spectrum 400) in the range of 400–4500 cm−1. The zeta potential was measured with Anton Paar Litesizer 500.
Preparation of thin films
The variable concentration of purified and characterized nano-crystals of ZnS was redispersed in double distilled water followed by sonication for about 10 min. Variable volumes of sonicated ZnS colloidal solution were used for making nano-sized thin films using photoresist spin coater (Ducom, PR-6-M2) at different rpm over well-cleaned SiO2 templates. The thin films were dried at 50 °C for 5 h. Dried thin films were subjected to analysis regarding their thickness and surface structure using film thickness measurement instrument (Filmetrics F20, thickness range 15 nm–70 µm; visible wavelength range from 380 to 1050 nm) and atomic force microscope (AFM) (Nanosurf Easyscan 2 in dynamic mode), respectively. To analyze the surface roughness, 70-µm scan head was used in AFM to record the inference in non-contact tapping mode using Tap190Al-G probe.
Scheme for the analysis of biocompatibility and antibacterial activity
Biocompatibility of synthesized of ZnS QDs was evaluated through hemolytic activity  against human erythrocytes and MTT assay  [(3-[4, 5-assay dimethylthiazole-2-yl]-2, 5-diphenyl tetrazolium bromide), a yellow tetrazole against L929 mouse fibroblast cell lines. The antibacterial activity of ZnS QDs was performed in terms of zone of inhibition, minimum inhibitory concentration, minimum bactericidal concentration and killing kinetic assay [45, 46]. The details of materials used and the description of all the assays are reported in Supplementary information.
Results and discussion
Mechanism involved in the synthesis of ZnS QDs
It has been observed that most of the green chemistry-synthesized nanoparticle reactions are carried out in the aqueous medium. In the present study, the suitability of the reaction in the water is probably because of the reaction between Zn+2 (borderline soft acid) and S−2 (soft base). According to the hard and soft acid and base (HSAB) theory, the reactions carried between a soft acid and a soft base give rise to a product having low-solubility product in the aqueous medium . This helps in the separation of the required pure compounds easily from the solvent. The by-products formed during the reaction can be easily removed from the medium. Addition of d-glucose (capping agent) was done to restrain the size of the end product. The density functional theory (DFT) calculation (Supplementary section) showed that the attachment of the ZnS molecule is with the oxygen atom bonded to the fourth carbon of d-glucose molecule. The conformation manages to have the least energy associated with it (− 722,197.6382 kcal/mol). The attachment of the ZnS molecule to the oxygen atom bonded to the third carbon may also be possible due to comparable bond energy associated with it (less by 0.2181 kcal/mol). The plausible reaction mechanism is reported in Supplementary section. Prepared nano-material was subjected to various microscopic and spectroscopic characterization techniques.
Powder X-ray diffraction analysis
The average value of lattice strain (ƞ) along all the 3 prominent planes calculated is 0.0258. The highest strain of 0.0381 was found along the most intense peak in (111) plane. The increase in lattice strain is also attributed to the increased surface energy when the size of the particle is very small (QDs).
To analyze the surface activity, the density of ZnS QDs (ρ) is calculated as 4.11 g/cm3. Calculation reveals the value of SBET as 356.93 m2/g, and the higher value indicates its expected efficiency in terms of the extent of adsorption.
The dislocation density for ZnS QDs along the highest intensity peak along (111) plane was calculated to be 6.40 × 1014 m−2. The existence of dislocation density may be attributed to lattice strain which is observed more along (111) plane. The smaller the particle size, the higher will be the possibility of dislocations as they have more tendencies to stabilize their higher surface energy. Dislocations are basically the topological defects, which may be screw and edge, and have been visualized through HRTEM for ZnS QDs.
High-resolution transmission electron microscope analyses
The value of the third quantity in Eq. (5) is often neglected for semiconductors due to the high value of dielectric constant.
Comparative size of ZnS QDs calculated using different characterization techniques
Average size of ZnS nanoparticle
Optical band gap
E = hc/λ
The surface charge (zeta potential) of ZnS nanoparticles was found to be negative (− 9.2 mV), attributed to the fairly stable configuration of synthesized QDs.
Atomic force microscopy
Characteristic roughness parameters of ZnS QDs thin film evaluated using AFM at 3.48 X 3.48 μm^2 of the scan area
Average particle size
Average roughness (Ra)
Average roughness (Rq) (rms)
The maximum peak to valley height difference (Rt)
56 ± 10 nm
Biocompatibility analysis of green chemistry-synthesized ZnS nanoparticles (NPs)
Cytotoxicity against mammalian cell
Hemolytic activity evaluation
In vitro Antibacterial activity of ZnS nanoparticles (NPs)
Antibacterial activity profile of ZnS NPs in terms of zone of inhibition (ZOI), minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)
In order to exhibit antibacterial functions, NPs need to undergo interaction with the bacterial cell. The prominent accepted forms of interactions can be established by van der Waals forces , electrostatic forces , hydrophobic interactions  and ligand–receptor interactions . While the NPs, capped with biocompatible cappers, are capable of forming such interactions with the bacterial cell membrane, they are able to cross the cell membrane and get into the metabolic pathways, thereby influencing the function and the shape of the cell membrane. The high antibacterial performance of the synthesized ZnS QDs is due to their very small size (~ 5 nm), i.e., within 10 nm range as observed in the present study. It is an established fact that the lesser the size (< 10 nm) of the nanoparticles, the better is their diffusion efficacy to penetrate through the pores of the cell membrane, and hence, they can easily go inside a microbial cell . The synthesized ZnS QDs are expected to readily access the interior of the bacterial cell and can interact with the basic components of the cell (viz. DNA, ribosome, lysosomes, etc.). These may lead to the generation of oxidative stress, protein deactivation, electrolyte balance disorder, enzyme inhibition and changes in cell membrane permeability inside the cell and finally cause death to the cell [62, 63, 64]. Conclusively, it can easily be understood that it is the advantage of the size (~ 5 nm) of the synthesized ZnS QDs (< 10 nm) that makes these nanoparticles (QDs) very promising as antibacterial agent. The rapid killing efficiency of the tested ZnS NPs is thus helpful in impeding biofilm formation at a faster pace. Findings of these experiments unveil the positive prospective attribute of the synthesized ZnS NPs utility in biomedical applications as anti-infective agents and hold tremendous potential in the application in superficial antibacterial therapeutics, implant coating, as well as in antibacterial textile material development with various advantageous functionalities.
We have presented a simple, time economic, cost-effective, reproducible and therefore successful method of synthesizing ZnS QDs. The method is an environmentally benign process, fulfilling the principles of green chemistry. The quantum dot thin films (QDTF) with its specified surface morphologies can absorb and transform harmful UV light to usable visible light efficiently, making them suitable for solar cell applications. The synthesized 2–6-nm nanoparticles were specially found to be biocompatible and also proficient antibacterial agents over a wide range of antibacterial studies. The synthesized QDs did not exert any adverse effect on mammalian cells.
The research scholar is grateful to University Grants Commission, New Delhi, India, for providing the financial support. The authors are thankful to Centre for Advanced Studies in Chemistry, NEHU, Shillong.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 25.http://www.google.com/patents/US7916065. Accessed 29 Mar 2011
- 36.Form EIA-63B, Annual Photovoltaic Module/Cell Manufacturers Survey, Energy Information Administration, USA, 2006Google Scholar
- 41.Green, M.A.: Third Generation Photovoltaics: Advanced Solar Energy Conversion. Springer, Berlin (2006)Google Scholar
- 47.General Area Detector Diffraction System (GADDS) User Manual (2005). Bruker Advanced X-Ray Solutions. https://depts.washington.edu/moleng/wordpress/wp-content/uploads/2015/03/GADDS_Manual.pdf. Accessed Jan 2005
- 51.Thomas, T.R.: Rough Surfaces. Imperial College Press, London (1999)Google Scholar
- 52.Kumar, B.R., Rao, T.S.: AFM Studies on surface morphology, topography and texture of nanostructured zinc aluminum oxide thin films. Dig. J. Nanomater. Biostruct. 7, 1881–1889 (2012)Google Scholar
- 53.Chen, Y., Li, S., Huang, L., Pan, D.: Single-step direct fabrication of luminescent Cu-doped ZnxCd1−xS quantum dot thin films via a molecular precursor solution approach and their application in luminescent, transparent, and conductive thin films. Nanoscale 6, 9640–9645 (2014). https://doi.org/10.1039/c4nr02237h CrossRefGoogle Scholar
- 57.Armentano, I., Arciola, C.R., Fortunati, E., Ferrari, D., Mattioli, S., Amoroso, C.F., Rizzo, J., Kenny, J.M., Imbriani, M., Visai, L.: The interaction of bacteria with engineered nanostructured polymeric materials: a review. Sci. World J. 2014, 1–18 (2014). https://doi.org/10.1155/2014/410423 CrossRefGoogle Scholar
- 62.Xu, Y., Wei, M.-T., Ou-Yang, H.D., Walker, S.G., Wang, H.Z., Gordon, C.R., Guterman, S., Zawacki, E., Applebaum, E., Brink, P.R., Rafailovich, M., Mironava, T.: Exposure to TiO2 nanoparticles increases Staphylococcus aureus infection of HeLa cells. J. Nanobiotechnol. (2016). https://doi.org/10.1186/s12951-016-0184-y CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.