Keywords

1 Introduction

Carbon quantum dots (CQDs) are relatively new carbon allotrope. The first mention dates from 2004 when Xu et al. described new fluorescent material after electrophoretic purification of carbon nanotubes [1]. It triggered an investigation of new CQD research of synthesis, properties CQDs, and applications. As can be seen from Fig. 1, the number of publications about CQDs is increasing exponentially during the last decade with 3419 papers in the last years according to Web of Science. Antibacterial properties of CQDs represent 3.2% of them; however, their citations are exponentially increasing exceeding the number over 2900 in last year. These data suggest a significant future of CQD as an antibacterial material.

Fig. 1
figure 1

Number of CQD publications (green bar), the number of antibacterial properties of CQDs publications (blue line) and their number of references (red line) according to Web of Science

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CQDs are quasispherical carbon particles with a size less than 10 nm with crystalline sp2 cores of graphite and quantum effects. A subclass of CQDs are graphene quantum dots (GQDs), and they have a structure of one or several graphene layers with diameter < 10 nm with higher crystallinity than CQDs. In both cases, however, CQDs are functionalized by functional groups on their surface, which can improve the optical properties, solubility, and chemical stability and generally increase the surface variability and complexity of CQDs.

CQDs have many hydrophilic functional groups at the edges or on the basal plane. Specific hydrophilic functional groups in the CQDs include epoxy and hydroxyl groups. Hydrophilic CQDs are very well soluble in water and other polar solvents due to their chemical composition; therefore, they differ from other carbon-based nanomaterials. Additional advantages of CQDs include their nontoxicity and biocompatibility. The second types of CQDs are hydrophobic CQDs containing carboxyl and carbonyl functional groups. In comparison to hydrophilic CQDs, hydrophobic CQDs are more effective in producing reactive oxygen species (ROS) responsible for the antibacterial activity of CQDs.

CQDs have extensive application usage from sensors through photoelectrochemical water splitting, chemiluminescence, LEDs, photovoltaic solar cells up to photocatalysis and readers can found several reviews about it [2,3,4,5,6,7]. CQDs also play an important role in medicine. CQDs are used in intracellular ion detection, toxin detection, pathogen, vitamin, enzyme, protein, nucleic acid, and biological pH value determination [8]. QDs also have great utility in bioimaging, biosensing (for example, QD modification with metal ions or biomolecules), fluorescence labelling of cellular proteins (biolabelling), genetic technologies, and cell motion tracking [9, 10]. Despite the broad range of biomedical applications, we would like to focus on antibacterial properties of pure CQDs and their polymer composites. The antibacterial effect of CQDs is based on noninvasive photodynamic therapy (PDT). PDT can cause a specific biological response on the cellular or subcellular level, such as apoptosis, programmed death, or necrosis, a nonprogrammed pathway [11]. During this process, CQDs absorb light (photons). One electron absorbs this energy and moves into a higher excited single state. This state is in nanoseconds, and it can emit light and lose its energy or dissipate as heat. The excited photosensitizer (PS) in the singlet state may also undergo the process known as an intersystem crossing. The spin of the excited electron inverts to form the relatively long-lived (microseconds) excited triplet-state that has electron spins parallel. The long lifetime of the CQD triplet state is explained by the fact that the loss of energy by emission of light (phosphorescence) is a spin-forbidden process as the CQDs would move directly from a triplet to a singlet-state [12]. In the presence of molecular oxygen, part of the energy can be transformed into an oxygen molecule which changes to ROS. One of the most important types of ROS is singlet oxygen (1O2). Singlet oxygen and other ROS react with a wide range of biological targets and are known to be involved in cellular signalling and cell damage [13]. Therefore CQDs act as indirect antibacterial materials when after the illumination with light generates singlet oxygen. This whole process is presented in Fig. 2.

Fig. 2
figure 2

Mechanism of photodynamic therapy, or so-called Jablonski diagram

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2 Antibacterial Effect of CQDs

2.1 Pure CQDs

Jhonsi et al. showed that antibacterial and antifungal activities of CQDs on Escherichia coli and Candida albicans increased linearly with an increase in CQDs’ concentration. It reveals that CQDs can effectively inhibit the growth of the bacteria in a concentration-dependent manner. The results revealed that CQDs inhibit the growth of E. coli and C. albicans more effectively compared to other tested microbes, Staphylococcus aureus, Klebsiella pneumoniae, and Pseudomonas aeruginosa [14]. Nie et al. demonstrated that CQDs are effective photosensitizers for in vitro PDT, and revealed detection limit inactivation (99.9999 + %) of E. coli and S. aureus upon visible light illumination (λ ≥ 420 nm, 65 ± 5 mW/cm2; 60 min) [15]. The antibacterial effect of photoexcited GQDs using blue light (465–475 nm) significantly affected the viability of E. coli or S. aureus [16]. Sun et al. designed an antibacterial system combining GQDs with a low level of H2O2, which could inhibit the growth of E. coli and S. aureus (106 CFU/mL) bacteria and assessed the antibacterial efficacy of patches from GQDs in vivo using Kumming mice as a model. Results of that study indicated that GQDs have potential use for wound disinfection [17]. The results of the study of Kuo et al. showed that a nitrogen-doped graphene quantum dot (N-GQD), serving as a photosensitizer, was capable of generating a higher amount of ROS than a nitrogen-free GQD in PDT when photoexcited for only 3 min of 670 nm laser exposure, indicating highly improved antimicrobial effects. The N-GQD (5.1%) efficiently exerted an antibacterial effect, resulting in 100% elimination after a 3-min exposure [18]. Sulfur-doped CQDs (S-CQDs) and N-CQDs were evaluated for bactericidal activity against E. coli and B. subtilis subsp. subtilis (6 × 106 cells/mL). Antibacterial activity was slightly higher against B. subtilis than against E. coli for both S-CQDs and N-CQDs with greater effectiveness of N-CQDs compared to S-CQDs [19].

2.2 Doped CQDs

Nanorods of CQDs–ZnO had strong antibacterial activity under visible light irradiation, and a concentration of 0.1 mg/L was able to kill more than 96% of bacteria E. coli and S. aureus [20]. Feng et al. realized bacterial inactivation on a CQD/TiNT film. Use of CQD/TiNTs led to almost complete inactivation of S. aureus and E. coli (2 × 107 CFU/mL) within 10 min using 365 nm UV irradiation [21]. Nitrogen and zinc doped CDs displayed good bactericidal activity against E. coli (107 CFU/mL) and S. aureus (108 CFU/mL) under visible-light radiations [22].

Wang et al. compare photodynamic properties of GQDs, hollow mesoporous silica nanoparticles (hMSN) and GQDs@hMSN hybrid antimicrobial system triggered by commonly available LED lamps. GQDs@hMSN can produce singlet oxygen under light exposure to destroy bacteria’s structure, thus achieving a highly efficient antimicrobial effect. However, GQDs@hMSN-erythromycin’s antimicrobial efficacy was significantly better than that of GQDs@hMSN or erythromycin alone [23]. Kholikov et al. used GQDs and GQDs combined with methylene blue (MB) to eradicate E. coli (106 CFU/mL), and G+ Micrococcus luteus (106 CFU/mL) using irradiation with red LED light. Using MB-GDQ improved the deactivation rate more than twice compared with MB [24]. Similarly, Dong et al. evaluated the antimicrobial effects of the CQDs with surface passivation molecules 2,2′-(ethylenedioxy)bis(ethylamine) (EDA) in combination with MB or toluidine blue (TB) against E. coli cells with 1-h visible light illumination and showed their synergistic interaction. The combination treatment with 5 μg/mL CQDs combined with 1 μg/mL MB completely inhibited bacteria growth, resulting in 6.2-log viable cell number reduction. Similar results were observed using TB/CQDs combination [25]. Galdiero et al. evaluated the antimicrobial activity on S. aureus, P. aeruginosa, E. coli and K. pneumoniae, and the ecotoxicity of CQDs alone and coated with indolicidin and showed improved germicidal action and low ecotoxicity for modified CQDs compared to CQDs alone. Modified CQDs demonstrated a percentage of bacteria reduction related to an initial inoculum of 35.1 ± 3.0, 29.3 ± 2.7, and 39.3 ± 4.1, respectively, for E. coli, P. aeruginosa, and K. pneumoniae. Only for S. aureus, was observed a low killing ability of 12.3 ± 1.0% for modified CQDs, but this was always more significant than that for indolicidin alone and CQDs alone [26]. The antimicrobial activity of the as-synthesized spermidine-capped fluorescent CQDs (Spd–CQDs) (size ~ 4.6 nm) has been tested by Li et al. against non-multidrug-resistant E. coli, S. aureus, B. subtilis, and P. aeruginosa bacteria and also multidrug-resistant bacteria, methicillin-resistant S. aureus (MRSA). The minimal inhibitory concentration value of Spd–CQDs is much lower (> 25,000-fold) than that of spermidine. Spd–CQDs had promising antibacterial effects causing significant damage to the bacterial membrane with high biocompatibility, especially to multidrug-resistant bacteria [27]. Multi-walled carbon nanotubes filters incorporated with CQDs are highly effective to remove bacteria (E. coli, B. subtilis) from water and to inhibit the activities of the captured bacteria on filter surfaces [28]. The bactericidal function of EDA–CQDs to B. subtilis and E. coli (~ 106 CFU/mL) was evaluated under different light conditions, the bacteria-killing effect of EDA–CQDs treatment was possible increased dramatically to approximately 4-logs (~ 99.99) [29]. EDA–CQDs exhibited much greater antibacterial activity to B. subtilis cells compared to 3-ethoxypropylamine modified CQDs, treatment with EDA–CQDs resulted in an about 5.8-log reduction in viable cell number upon treatments under light illumination [30]. The CQDs–TiO2 properties and their antimicrobial activity against E. coli and G+ S. aureus were evaluated by Yan et al. [31]. The antibacterial efficiency reached 90.9% and 92.8% toward E. coli and S. aureus, respectively, with 1 mg/mL CQDs–TiO2 under visible light. Zhang et al. (2018) [32] showed that the bracket modified with ZnO/CQDs coating exhibited excellent antibacterial performance than the unmodified bracket (Streptococcus mutans 96.13%, S. aureus 90.28% and E. coli 92.35%) under natural light. Composite of CQDs/Na2W4O13/WO3 exhibited excellent antimicrobial activity against G− E. coli (107 CFU/mL) [33]. After visible light irradiation for 100 min, ~ 68.3% of the E. coli cells treated with Na2W4O13 survived, which have no more than 1-log reduction. For treatment with the synthesized WO3/Na2W4O13 and WO3 materials, approximately 72.6% and 0.6%, respectively, of the E. coli cells were alive. The CQDs-decorated Na2W4O13 composite showed the best photocatalytic bactericidal activity, with approximately 2 × 107 CFU/mL of the E. coli cells completely inactivated within 100 min, which have 7-log reduction.

3 Antibacterial Effect of CQD Polymer Composites

As mentioned above, CQDs can be used in a wide range of applications, and especially in biomedicine for their antimicrobial, in the narrower sense—antibacterial effects. However, due to real use (catheters, stents, coatings, dressings, patches, textiles, etc.), CQDs need a certain carrier [34, 35]. The most common carrier solution is a polymer matrix (various kinds), and CQDs are incorporated in different ways onto the material’s surfaces. Unfortunately, there is still a minimum of polymer composites with CQDs that would exhibit antibacterial activity without side effects.

For more comfortable mixing with polymers, pure hydrophobic CQDs (hCQDs) were invented. Their great advantage is also that, in contact with water or other biological fluids, they do not elute from the polymer matrix and do not degrade [36, 37]. The antibacterial effect of these hCQDs was tested against S. aureus, E. coli, and K. pneumoniae in combination with various polymers, as polyurethane (PU), polydimethylsiloxane (PDMS). PU and PDMS are frequently used in medicine as medical devices and tools for their biocompatibility and desired properties. Materials work on the principle of classic PDT and produce singlet oxygen—in Fig. 3. As the light source was used common blue LEDs at 470 nm, the power of 50 W, and the intensity of 700 μW/cm2 on the sample surface placed at a distance of 50 cm from the LEDs. In both cases, the desired effect has been achieved. The nanocomposite hCQDs/PU has 100% bactericidal effectivity after 1 h of irradiation. Second polymer hCQDs/PDMS eradicated 100% bacterial colonies (5-logs) after 15 min of irradiation, because of its excellent oxygen diffusion [38, 39]. The bacterial reduction could be improved using diffuse coplanar dielectric barrier discharge. The plasma generated in atmospheric air oxidizes the surface of hCQDs and therefore enhances the energy transfer between the hCQDs and molecular oxygen. It means that the irradiation time is decreased, and material is suitable for faster disinfection [40]. Moreover, there is a possibility to create novel antibacterial textiles by a lamination process, using commercially-available transparent PUs and modified them with hCQDs [34].

Fig. 3
figure 3

Schematic view of light-triggered polymer nanocomposite with hCQDs

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Also, there is another possibility to create antibacterial polymer by electrospinning nanofibers filled with hydrophilic CQDs. Nie et al. prepared such material with polyacrylonitrile (PAN) and CQDs (synthesized by a facile one-pot solvothermal method from citric acid and 1,5-diaminonaphthalene in ethanol), which works after visible light illumination. Antibacterial activity was performed against several types of G+ and G− bacteria (~ 6-log units inactivation). In all cases, they achieved a reduction in bacterial cultures, although only very weak in the case of G+ bacteria [41]. The same combination PAN/CQDs (but CQDs obtained from the hydrothermal method—citric acid and urea) could be useful as fluorescent scaffold reported in research Kanagasubbulakshmi et al. Scaffold has antimicrobial properties, and it was tested for reepithelialisation in albino Wistar rats [42]. CQDs could be used for modification of polymer membrane, for example, polysulfone polymer membranes embedded with CQDs (obtained from activated carbon) for antibacterial effect against E. coli and S. aureus (tested by disk diffusion method) with improved permeability, high hydrophilicity and porosity [43]. Moreover, polycaprolactone/CQDs electrospun nanofibers were used for improved wound healing with antibacterial properties [44].

Eco-friendly antimicrobial material was reported by Salimi et al. as nanocellulose sheets-CQDs. CQDs were prepared from white mulberry (Morus alba L.), and nanocellulose is a natural biopolymer. Antimicrobial effectiveness was performed against L. monocytogenes via disk-covering method [45].

Very recently, few antimicrobial polymer hybrids with CQDs are known. One example is Ag2S–CQDs–PEI–GO composite material with strong antibacterial activity against E. coli, S. aureus, P. aeruginosa, and E. faecalis, which was evaluated by disk diffusion method [46].

4 Conclusion

CQDs are a very promising new antibacterial nanoparticles. Their antibacterial effect against different G+ and G− bacteria was confirmed. These nanoparticles work mainly as a photosensitiser and their antibacterial effect can be amplified by doping or surface modification. CQDs are very suitable for incorporation into different polymer matrices what makes them the antibacterial material with a very universal usage. Therefore, they can be used in almost any area.