Introduction

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 [7]. 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 [33]. The literature reveals that the nanometer range of the particles enhances the properties of semiconducting metamaterial due to its quantum confinement effect [34]. 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.

Experimental section

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.

Reaction scheme

$$\begin{aligned} {\text{Zn}}({\text{NO}}_{3} )_{2} \cdot 6{\text{H}}_{2} {\text{O}} + {\text{Na}}_{2} {\text{S}} + {D}{\text{-Glucose}} & \xrightarrow[{{\text{reflux}}/1\,{\text{h}}}]{{{{{\text{H}}_{2} {\text{O}}}}}} {\text{ZnS}}\;{\text{QDs}} \\ {\text{Zn}}({\text{NO}}_{3} )_{2} \cdot 6{\text{H}}_{2} {\text{O}} + {D}{\text{-Glucose}} & \xrightarrow{{{{{\text{H}}_{2} {\text{O}}}}}}{\text{Zn}}^{ + 2} - {D}{\text{-Glucose}} \\ {\text{Na}}_{2} {\text{S}} & \to {\text{S}}^{ - 2} \\ \end{aligned}$$

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 [43] against human erythrocytes and MTT assay [44] [(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 [2]. 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

This powerful technique gives accurate information about the zinc blende crystal structure of ZnS nanoparticles (Fig. 1). The broader FWHM of the peaks informs us about the generation of nanoparticles. The most intense peak was found along the plane (111), with the d-spacing 3.1 Å, indicating the formation of the majority of the nano-crystallites along this axis. Other prominent peaks were found for the planes (220) and (311) at the d-spacing 1.9 Å and 1.6 Å, respectively.

Fig. 1
figure 1

Powder XRD pattern of ZnS nano-crystals revealing the corresponding Miller indices of diffraction planes

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The X-ray analysis reveals the identity of the cubic crystallites of ZnS quantum dots having FCC structure. The unit-cell edge parameter ‘a’ and the volume of each unit cell were calculated to be 539.96 pm and 1.57 × 10−22 cm3, respectively. Broadening of the XRD peaks indicates the smaller particle size of ZnS nanoparticles. The Scherrer formula (Eq. 1) reveals that β (FWHM) is inversely proportional to τ (crystallite size). The actual broadening due to crystallite size, βa, and strain are calculated after instrumental broadening, βi, is deducted from the overall, observed broadening, βo, considering Cauchy (Lorentzian) [47] (Eq. A) profile as follows:

$$\beta a = \beta_{o} - \beta_{i}$$
(A)
$$\tau = \kappa \lambda /\beta \cos \theta$$
(1)

The average grain (τ) size, 4.09 nm, was calculated using Eq. (1). The lattice strain of the nanoparticles was calculated using Williamson and Hall model (Eq. 2).

$$\beta_{hkl} \cos \theta = \kappa \lambda /L + 4\eta \sin \theta$$
(2)

where βhkl denotes line broadening at the half of the intensity, θ Bragg angle (in degree), κ shape factor, λ X-ray wavelength, L mean size of the particle, and ƞ lattice strain

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).

The specific surface area (S) was also calculated with the help of Brunauer, Emmett and Teller (BET) method [48] (Eq. 3) from the powder XRD spectrum. Since the surface-to-volume ratio of the nanoparticle is higher, its surface activity will enhance immensely.

$$S_{\text{BET}} = 6000/D_{\text{p}} *\rho$$
(3)

where SBET denotes specific surface area, Dp size of the particles, and ρ density of the material

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.

XRD data were further used for calculating the dislocation density (δ) of ZnS QDs per unit volume:

$$\delta = 15\beta \cos \theta /4aD$$
(4)

where δ denotes dislocation density, β line broadening at the half of the intensity (FWHM—full width half maximum), θ Bragg angle (in degree), a lattice edge parameter, and D particle size.

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

HRTEM micrographs allow us to achieve the shape and size of each and every individual nano-crystallite in the most precise way. The lattice alignments, as well as their directions and defects, can also be detected with the help of this tool. Nanoparticles of ZnS ranging from 3 to 6 nm in diameter with spherical shape (larger surface energy) were observed in the scale of 5 nm (Fig. 2a inset). A large number of clusters of ZnS QDs (homogeneously distributed) were also observed in the range of 50 nm (Fig. 3). The average particle size for around 20 nano-crystallites was recorded to be 4 nm, which is in good agreement with the results revealed by the XRD analysis also. SAED (selected area electron diffraction) pattern clearly reveals the polycrystallinity of the ZnS QDs with the d-spacing (Fig. 2b inset) of 3.1 Å, 1.9 Å and 1.6 Å, which is in confirmation with the XRD results. The imprinting polycrystallinity with its every bright spot, observed in the SAED pattern, confirms the presence of nano-crystallites feature of ZnS QDs. The zigzag alignment of atoms in the FCC ZnS nanoparticle reveals the presence of planar stacking fault along the (200) plane, which is evident from (Fig. 4). The FFT pattern of (200) plane is reported in Fig. 4 inset. The formation of zigzag alignment may be attributed to the occurrence of two partial dislocations of atoms in the respective planes. Using the ImageJ software, the angles of atoms in each alignment in both sides of the stacking fault were found to be 58.2° and 65.7°. The presence of (102) plane (FFT of Fig. 5 inset) also speaks about the occurrence of the hexagonal plane (d102 = 2.3 Å). TEM micrographs also reveal the existence of twin boundary growth (Fig. 5) in ZnS nanoparticles. An angle of 155° was calculated between the two differently aligned planes observed in the twin boundary area. The regular FCC (ccp) and zigzag (hcp) crystal alignments were observed in the area under investigation. The FFT patterns of the crystal alignments are reported in Fig. 5a, b, c inset.

Fig. 2
figure 2

HRTEM image of colloidal distribution of spherical-shaped ZnS QDs with a size distribution and b with SAED pattern

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Fig. 3
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TEM image exhibiting clustering of ZnS QDs on copper grid

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Fig. 4
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Stacking fault (shaded area)

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Fig. 5
figure 5

Two differently aligned (ccp and hcp) planner conformations within the same grain

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The loss in contrast through (200) plane is probably due to the presence of stacking fault, which usually has been observed to occur during the synthesis of QDs [49]. From Fig. 6, it is observed that the twin growths tend to merge each other resulting in the formation of mirror images of each other. Such types of micro-twin growths can be pointed out in quite a number of QD alignments of ZnS, where these localized defects try to form parallel planes at the junction of contact twin. Since the edge dislocation (Fig. 7) can be seen prominently in the alignments of ZnS nanoparticles, the grain boundary cannot be distinctly visualized.

Fig. 6
figure 6

Growth mirror twin planes

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Fig. 7
figure 7

Edge dislocation leading to indistinct grain boundary

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Absorption spectra

The λmax for the ZnS QDs was found to be at 314 nm (blue shift). Unlike conspicuous absorption peaks which can be observed for bulk materials, the formation of QDs of ZnS is signified relatively by broader peaks (Fig. 8).

Fig. 8
figure 8

UV–visible absorption spectra (314 nm) of colloidal ZnS QDs synthesized

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This is attributed to the generation of variation in the band gap for each nano-sized particle of ZnS. The optical band gap of the particles was found to be 3.95 eV. The higher value of the band gap of ZnS QDs by 0.37 eV than the bulk cubic ZnS confirms the formation of nanoparticles in the solution. The particle size was calculated with the Brus Eq. (5) and found to be 5.8 nm, which is in agreement with the absorption spectrum:

$$E_{{g({\text{QD}})}} = E_{\text{bulk}} + h^{2} /8R^{2} (1/m_{e}^{*} + 1/m_{h}^{*} ) - 1.786e^{2} /4\pi \varepsilon_{0} \varepsilon_{r} R$$
(5)

where Egap denotes band gap energy of the bulk ZnS, m * e effective mass of the excited electron [50], m * h effective mass of the electron–hole, r radius of the particle, h Planck’s constant, e charge of electron, \(\varepsilon_{0}\) vacuum permittivity, and \(\varepsilon_{r}\) dielectric constant of ZnS.

The value of the third quantity in Eq. (5) is often neglected for semiconductors due to the high value of dielectric constant.

The comparison of the particle size of ZnS nanoparticles calculated by different techniques is reported in Table 1.

Table 1 Comparative size of ZnS QDs calculated using different characterization techniques
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Zeta potential

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.

FTIR spectra

The FTIR spectrum (Fig. 9) shows a strong peak at 649 cm−1, which is significant for Zn–S (metal sulfide) bond. The other strong peaks at 1635 cm−1 and 3411 cm−1 reveal the presence of H2O bending and –OH stretching. The presence of a later peak confirms the presence of capping of d-glucose molecule. Another peak at 1010 cm−1 stands for C–O stretching.

Fig. 9
figure 9

FTIR spectra showing characteristic metal–sulfur bond as well as the interaction of ZnS QDs with d-glucose molecule as a capping agent

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Atomic force microscopy

From the AFM topologies, large aggregated nanoparticles are observed. The aggregation may be due to the particle coalescing (Fig. 10). The darker and brighter portions in Figs. 10, 11 correspond to the valleys and peaks on the surface of the nano-sized thin film of ZnS QDs. The ratio of average roughness, root mean square (Rq) and average roughness (Ra) is found to be 1.33, which is in accordance with the height distribution predicted by Gaussian for most of the engineering surface (1.31) [51]. The height symmetry of the surface of thin films can be understood by skewness (Rsk = 1.68) (Table 2). The positive Rsk value indicates the presence of a bumpy surface. Since the value is not much higher than zero, it relates to a nearly symmetrical distribution of peaks and valleys about the mean line position. The peakedness parameter, kurtosis (Rku) (Table 2), shows a value of 2.93 for the nano-sized thin film of ZnS QDs, which corresponds to the random distribution of peaks and valleys, known as mesokurtic behavior. The positive value of Rsk and Rku reveals that the nano-sized ZnS thin films are suitable for tribological applications [52]. The maximum peak to valley height difference (Rt = 210.78 nm) has an important impact on the surface properties of thin films. It gives us the idea about the maximum roughness of the topology of the thin film of ZnS nanoparticles. Gwyddion free software was used to calculate the average thickness (50 ± 20 nm) using AFM data of thin films, which is in good agreement with the thickness (57 nm) measured with the help of Filmetrics (Supplementary information). Since QD thin films (QDTFs) can absorb and convert harmful UV lights to use visible light, they can be used for the future fabrication of QDTF solar cells. These cells can expand the photoresponse range and therefore can enhance the photoelectric conversion efficiently [53].

Fig. 10
figure 10

Top view morphology of ZnS QDs thin film observed under AFM in dynamic mode

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Fig. 11
figure 11

3D view of Fig. 10 ZnS QDs thin film observed under AFM

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Table 2 Characteristic roughness parameters of ZnS QDs thin film evaluated using AFM at 3.48 X 3.48 μm^2 of the scan area
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Biocompatibility analysis of green chemistry-synthesized ZnS nanoparticles (NPs)

Cytotoxicity against mammalian cell

The synthesized ZnS NPs demonstrated noticeable biocompatibility as is evident from cytotoxicity (MTT) assay results. The tested NPs (up to 200 μg/ml concentration) did not induce any significant cytotoxicity toward L929 mouse fibroblastic cells even after 72 h of post-treatment. The synthesized ZnS NPs exhibited time- and concentration-dependent decrease in cell viability which is in the range between 99.60 and 78.23%. This is also in agreement with the previous reports (Fig. 12) [54]. Even, with 200 µg/ml concentration and 72 h of incubation, the synthesized ZnS NPs did not induce any significant cytotoxicity to the tested mammalian cell. The higher percentage of mammalian cell viability demonstrated by the tested ZnS NPs may be attributed to repulsive interaction that mediated inefficient uptake of negatively charged ZnS NPs through negatively charged cell membrane [55]. Before further investigation of the synthesized ZnS NPs in preclinical and clinical settings, various biomedical applications and the high dose tolerance exhibited by the mammalian cell line for a prolonged duration (72 h or more hours) are very important.

Fig. 12
figure 12

Cytotoxicity evaluation (MTT) of ZnS NPs (The data are presented as mean ± S.E. of five replicates)

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Hemolytic activity evaluation

RBC constitutes 40–50% (v/v) of human whole blood and is responsible for the transport of carbon dioxide and oxygen. Hemolysis refers to the release of hemoglobin due to the disintegration of the three-layered RBC membrane, which is considered one of the most crucial parameters for evaluation of biocompatibility of biomaterials. The tested ZnS NPs in a concentration ranging from 0.01 to 1 mg/ml demonstrated mild to moderate hemolysis up to incubation period of 24 h in a dose- and time-dependent manner. For all the tested concentration of ZnS NPs, as compared to control, the RBCs maintained its membrane integrity and did not lyse significantly up to even 4 h of time point, whereas from 8 h onward, all the tested concentrations of ZnS NPs started inducing, a mild to moderate level of lysis of erythrocytes, reaching 2.86%, 3.21% and 4.32% for 0.01 mg/ml, 0.1 mg/ml and 1 mg/ml concentrations, respectively (Fig. 13). The negative surface charge of the RBC repulsed the ZnS NPs and obstructed interaction, which may be attributed to the higher level of hemocompatibility of the tested ZnS NPs. The mild or moderate hemolytic activity exhibited by the ZnS NPs may be due to the hydrophobic interaction that mediated damage of RBC membrane and subsequent release of hemoglobin [56]. These preliminary data on hemocompatibility of the synthesized ZnS NPs will help to standardize, in future, the bio-safe concentration for its prospective biomedical applications.

Fig. 13
figure 13

Hemolysis of the RBCs incubated with different concentrations of the ZnS NPs at 37 °C temperature

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In vitro Antibacterial activity of ZnS nanoparticles (NPs)

To explore antibacterial effectiveness of the synthesized ZnS NPs, zone of inhibition (ZOI), minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and time-kill assay studies were performed against seven pathogenic bacterial strains. The tested ZnS NPs demonstrated noticeable zone of inhibition with a concentration-dependent increase in antibacterial activity against all the tested bacterial pathogens (Table 2). It was observed that in spite of possessing thicker peptidoglycan layer, gram-positive bacteria exhibited more susceptibility toward the tested ZnS NPs as compared to gram-negative bacteria; this may be due to the absence of an outer membrane. We conclude that the outer lipopolysaccharide membrane present in gram-negative bacteria offers a significant obstruction toward nanoparticle penetration or diffusion and consequently protects the cellular ultra-structures from ZnS NPs-mediated disturbances and subsequent dysfunction of the cell. S. aureus and P. aeruginosa were found to be most and least susceptible to the tested ZnS NPs. The pathogenic bacterial strains demonstrated variable MIC/MBC values ranging from 75 to 125 µg/ml for gram-positive bacteria and 100 to 150 µg/ml for gram-negative bacterial pathogens, respectively (Table 3). Since S. aureus and B. subtilis were found to be most susceptible bacterial strains, these were further evaluated for killing kinetics assay. The tested ZnS NPs demonstrated significant enhanced killing in terms of reduction in CFU count of S. aureus and B. subtilis in 4-, 8- and 12-h time points as compared to the non-treated strains (Fig. 14). All experiments were done in triplicate and results are expressed in mean, and standard deviations were negligible.

Table 3 Antibacterial activity profile of ZnS NPs in terms of zone of inhibition (ZOI), minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)
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Fig. 14
figure 14

Time-kill assay of S. aureus (MTCC 3160) and B. subtilis (MTCC 441) were incubated with ZnS NPs. Surviving CFU at selected time points is shown. All the results were expressed in mean ± S.D. (n = 3). $$$p < 0.001, comparison of medium S. aureus and medium B. subtilis with ZnS QDs S. aureus and ZnS B. subtilis at different time intervals

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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 [57], electrostatic forces [58], hydrophobic interactions [59] and ligand–receptor interactions [60]. 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 [61]. 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.

Conclusion

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.