1 Introduction

Nanomaterials have been extensively used in medicine, sensors, electronic devices, and energy storage devices because of their distinctive physical and chemical properties compared to their bulk materials. Therefore, various nanomaterials are designed to find their applications in many commercial products, such as electronics, drug delivery, cosmetics, protective clothing, and sporting goods [1, 2]. In this regard, numerous studies have been done on the nanostructured semiconductors, and many papers have been published on nanostructured ZnO and TiO2 semiconductors in the recent literature because of  their useful properties and technological importance [3,4,5,6]. Among these, II-IV semiconductors nanoparticles especially ZnS and ZnO are well-considered as nontoxic, bio-safe, and biocompatible to be used in biological fields [7,8,9]. For instance, the first discovered and widely studied semiconductor material ZnS with a band gap of 3.8 eV has emerged as a potential antibacterial material due to its deeper conduction and valence bands as compared to other semiconductors including ZnO and TiO2 [10,11,12,13]. Also, ZnS is of major interest in biomedical, electronics, and photovoltaic devices due to its luminescent and catalytic properties [14,15,16].

The phenomena of aggregation and photocorrosion are likely to occur in nanostructural materials, which lead to minimize their performance. Therefore, various efforts have been made to overcome these difficulties by combining ZnS nanoparticles with other semiconductors [17], incorporating in mesoporous materials [18, 19] or polymer matrix [20], forming composites with graphene [21] or CNT [22] and applying alternative preparation techniques [23, 24]. Studies have revealed that polymers, when applied as templates, can confine the nanoparticles precipitated in them. In addition, some polymers can maintain a certain superstructure and produce inorganic-polymer hybrids showing interesting morphologies [25, 26]. Regarding this, various reports have been published for synthesizing semiconductor-polymer nanocomposites and their applications in electronics, optics, and catalysts. Besides, electrospinning is a convenient method to fabricate polymer nanofibers encapsulated with nanostructured materials [27, 28].

Electrospinning is the widely accepted technique for the fabrication of nanofibers from polymer solutions or melts using an electrostatic field. The nanofibers obtained by this technique possess unique characteristics like, high surface area, porous structure, flexibility, and component controllability. Because of these characteristics, electrospun nanofibers are being used extensively in biomedical applications, protective cloths, medicines, filtration, and composite reinforcement. Nanofibers with diameter ranging from50 to 100 nm or greater can be produced from this process; however, the properties of nanofibers depend on various parameters including properties of solution (viscosity, surface tension, and conductivity), applied electric potential, distance between needle tip to collector and atmospheric conditions (temperature, pressure and humidity) [29,30,31]. Also, electrospinning has been proven as a promising technique to obtain uniform dispersion of nanoparticles inside the nanofibers [32,33,34]. Till date, different types of nanofibers have been fabricated using this technique, such as nanofibers of organic polymer materials including synthetic and natural polymers and polymer nanofibers loaded with chromophores, nanoparticles, or active agents. On the other hand, metal and ceramics nanofibers cannot be electrospun directly although it is possible to fabricate either from their melts at extremely high temperature or by calcination/chemical conversion of precursor nanofibers containing polymer and inorganic precursor [29].

Therefore, present study aims to fabricate in situ-synthesized ZnS nanoparticles doped poly(vinyl)acetate nanofibers via cost-effective and versatile electrospinning technique. Interestingly, ZnS nanoparticles synthesized by adding ammonium sulfide solution to poly(vinyl acetate)/ZnAc sol-gel were incorporated inside polymer electrospun nanofibers. Thus obtained composite nanofibers are characterized using FE-SEM along with EDX, XRD, TEM, and FT-IR. Also, the nanofibers were investigated for antibacterial activity under indoor light activation toward Gram-negative and Gram-positive bacteria.

2 Experimental

2.1 Materials

Chemicals used in this work are zinc acetate dehydrate (ZnAc, Showa Chemical Ltd., Japan), N,N-dimethylformamide (DMF, Showa Chemical Ltd., Japan), poly(vinyl acetate) (PVAc, MW 5500,000 g/mol, Sigma-Aldrich), and ammonium sulfide ((NH4)2S, 40–48 wt% solution in water, Sigma-Aldrich). All the chemicals were used as received without further purification.

2.2 Fabrication of ZnS doped PVAc (ZnS-PVAc) elctrospun nanofibers

ZnS/PVAc colloidal solution was prepared in two steps. First, ZnAc/PVAc solution was prepared by adding ZnAc solution (ZnAc, 0.5 g in DMF, 2 ml) to 5 ml of 18 wt% PVAc/DMF solution under constant magnetic stirring. The resulting solution was continuously stirred for 2 h to ensure homogeneity. Secondly, 0.5 ml of ammonium sulfide was added carefully to ZnAc/PVAc solution dropwise under vigorous stirring. The final solution of ZnS/PVAc was kept under vigorous stirring for another 5 h to ensure fine dispersion of ZnS nanoparticles and electrospun by setting 15 kV applied voltage (high voltage power supply, CPS-60 K02 V1, Chungpa EMT, South Korea) and 15 cm tip to collector distance. The developing ZnS-PVAc electrospun nanofibers were collected on a drum collector rotating at constant speed. For the comparison of antibacterial activity, pristine PVAc nanofibers were also fabricated by the same process without ZnS nanoparticles. Electrospinning was carried out under atmospheric pressure, where temperature and relative humidity were maintained at about 25 °C and 60%, respectively. Afterwards, the nanofiber mats were vacuum-dried for 24 h to remove the residual solvent and used for further analysis. The schematic illustration for the fabrication of ZnS-PVAc electrospun nanofibers is shown in Fig. 1.

Fig. 1
figure 1

Schematic illustration for the fabrication of ZnS-PVAc electrospun nanofibers

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2.3 Characterization

Field emission scanning electron microscope (FE-SEM, S-7400, Hitachi, Japan) equipped with energy dispersive X-ray spectroscopy (EDS) was used to study the surface morphology of the nanofibers. Before characterization, samples were prepared by coating with platinum for 180 s using a Pt coater (E-1030, Hitachi). In the meantime, FE-SEM EDS spectrum of the nanofiber mats was also recorded. Information about the phase and crystallinity was obtained with a Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα (λ = 1.540 Å) radiation over Bragg angles ranging from 10o to 80o. The distribution and high-resolution image of ZnS nanoparticles inside the polymer nanofibers were observed via transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) with a 200 kV accelerating voltage. The samples were prepared by directly collecting the nanofibers on the TEM grid during electrospinning. The bonding configuration of the polymer with ZnS NPs was characterized using Fourier-transform infrared (FT-IR, FT/IR-4200, Jasco international Co., Ltd.).

2.4 Evaluation of antibacterial activity

The antibacterial susceptibility of two ATCC strains towards the ZnS-PVAc and pristine PVAc nanofibers was assessed using the disc diffusion method under indoor light conditions. For this investigation, both the nanofiber mats were cut into circular disc of diameter around 5 mm each. The bacteria, Escherichia coli (E. coli) ATCC 25922 and Staphylococcus aureus (S. aureus) ATCC 25923 used in this study were obtained from Sukraraj Tropical and Infectious Disease Hospital, Teku, Kathmandu. Broth culture of E. coli and S. aureus were prepared on nutrient broth with OD600 measured between 0.08 and 0.12. The lawn culture of the organism was prepared by swabbing bacterial broth on the sterile, surface-dried MHA (Muller Hinton Agar) plate. Then the plate was left until the broth was completely soaked by the media. ZnS-PVAc nanofiber disc and pristine PVAc nanofiber disc (as control) were placed aseptically over the dried agar surface and incubated for 16–24 h at 37 °C. Finally, the growth of bacteria was observed, and the diameters of inhibition zones were measured.

3 Results and discussion

The surface morphology along with elemental composition of pristine PVAc and ZnS-PVAc electrospun nanofibers was characterized by FE-SEM/EDS (Fig. 2). Compared to pristine PVAc nanofibers, no any morphological difference was observed in ZnS-PVAc nanofibers (Fig. 2a & b). As shown in FE-SEM images, both pristine and doped nanofibers were found smooth, bead free, continuous and randomly oriented with uniform diameter ranging from 200 to 300 nm. Additionally, FE-SEM image of ZnS-PVAc nanofibers (Fig. 2b) confirmed the complete absence of nanoparticles on the surface of nanofibers indicating all the nanoparticles were incorporated inside the nanofibers. Moreover, the EDS analysis of ZnS-PVAc electrospun nanofibers revealed the presence of considerable amount of Zn and S (Fig. 2c). The spatial distribution of elements was represented by elemental mapping of ZnS-PVAc electrospun nanofibers (Fig. 3). As depicted in the figure, higher densities of C, Zn, and S are incorporated in the polymer chain suggesting good distribution of ZnS nanoparticles inside PVAc nanofibers.

Fig. 2
figure 2

FE-SEM images; a pristine PVAc electrospun nanofibers, b ZnS- PVAc electrospun nanofibers, and c EDS image of Fig. b

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Fig. 3
figure 3

Elemental mapping of ZnS- PVAc electrospun nanofibers; a FE-SEM image showing selected area for elemental mapping, b carbon, c, zinc, and d sulfur

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In order to study the crystalline phase of fabricated samples, XRD analysis was performed (Fig. 4). The XRD patterns of pristine PVAc and ZnS-PVAc electrospun nanofiber mats displayed broad peaks at 2θ of 20° associated with the low crystallinity of PVAc. Conversely, the diffraction patterns of ZnS-PVAc nanofiber mat displayed three peaks at 2θ of 28.9°, 48.0°, and 58.0° corresponding to the crystallographic planes of (111), (220) and (311) in the cubic crystalline structure of ZnS (JCPDS, 77–2100). No peaks belonging to any other phase were detected.

Fig. 4
figure 4

XRD patterns; a pristine PVAc and b ZnS-PVAc electrospun nanofiber mats

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The detailed study related to structure, morphological characteristics, and distribution of nanoparticles was provided by TEM and HR-TEM imaging (Fig. 5). Compared to the pristine PVAc nanofiber (Fig. 5a), ZnS-PVAc nanofiber contains uniformly distributed rice grain-shaped ZnS nanoparticles in it (Fig. 5b). The perfectly doped nanoparticles and the polymer fiber boundary were clearly visible in the HR-TEM image as shown in Fig. 5c. The particle diameter appeared to be around 5–10 nm in range. In addition, the HR-TEM image (Fig. 5c) expresses the good crystalline nature of ZnS nanoparticles; the lattice fringes with planar spacing of 0.32 nm represent the interplanar distance related to ZnS (111) plane, which is in good agreements with the standard value of the JCPDS, 77–2100. Here, we believe that this type of immobilization is due to the smaller size of nanoparticles than that of polymer nanofibers [28]. The homogenous distribution of ZnS nanoparticles inside the polymer nanofibers might be assigned to the coulombic repulsion between charged metallic nanoparticles (M2+) during electrospinning.

Fig. 5
figure 5

TEM images; a pristine PVAc and b ZnS-PVAc electrospun nanofiber. Panel c represents the HR-TEM image of marked area of b

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The influence on the structure of PVAc due to incorporation of ZnS nanoparticles was investigated using FT-IR spectroscopy. Figure 6 shows the FT-IR spectra of pristine PVAc and ZnS-PVAc electrospun nanofibers. The peak at 1739.4 cm−1 (νc = o) and characteristic peaks at 1239.9, 1020.9 cm−1 (νc-o), and 1375.7 cm−1 (δCH3) corresponding to PVAc are clearly seen. The FT-IR spectrum is similar to the standard IR spectrum curve of PVAc (Sprouse collection of IR, card no. 187–189) [35]. The ZnS-PVAc nanofibers also showed the similar patterns to that of pure PVAc nanofibers with some reduced intensity. This may have occurred due to the presence of ZnS nanoparticles in the PVAc. Therefore, FT-IR spectrum indicates and confirms that PVAc was not affected by the synthesizing procedure of ZnS. Furthermore, these results indicated that the proposed fabrication strategy could embed ZnS nanoparticles inside the PVAc nanofiber without influencing its structural changes.

Fig. 6
figure 6

FT-IR spectra; a pristine PVAc and b ZnS- PVAc electrospun nanofibers

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The bactericidal effect of ZnS is already well-known and has been studied for many years [36, 37]. Antibacterial activity of the ZnS-PVAc nanofibers is shown in Fig. 7. It is well-known that when an antibacterial material is in contact with bacterial strain, a clear area (zone of inhibition) around the antibacterial material is formed [38]. The strain susceptible to antibacterial agent exhibits a larger diameter, while resistant strain exhibits smaller diameter of inhibition zone. As shown in the Figure, pristine PVAc samples showed no zones of inhibition, suggesting the absence of antibacterial activity for both bacterial strains, mainly due to lack of ZnS nanoparticles. On the other hand, ZnS-PVAc nanofibers clearly showed distinct area of zone of inhibition for both bacterial strains. In spite of low content and indoor light activation of ZnS nanoparticles confined in polymer nanofibers, the results obtained were promising. The zone of inhibition were 9 mm and 8 mm for E. coli and S. aureus, respectively. The higher zone of inhibition for E. coli compared to that of S. aureus might be related to the cellular wall thickness and content differences between Gram-negative and Gram-positive bacteria. Gram-positive bacteria possess continuous thick cell wall (20–80 nm) consisting mainly of peptidoglycane, whereas Gram-negative bacteria possess thinner layer of peptidoglycane (5–10 nm) surrounded by an outer phospholipidic membrane [39]. Finally, bactericidal activity of ZnS along with PVAc nanofibers may offer great promise to its application in pharmaceutical industries.

Fig. 7
figure 7

Bactericidal activity; pristine PVAc and ZnS-PVAc electrospun nanofiber mat with E. coli and S. aureus bacteria

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The possible mechanism for the antibacterial action of ZnS-PVAc nanofibers can be explained on the basis of published report [40]. ZnS being a semiconductor material can generate biologically reactive oxygen species (ROS) inducing hydroxyl radical (OH•) under UV-light irradiation due to its high band gap. However, this work attempts the indoor light activation of ZnS to produce ROS. During activation process, holes (h+) and electrons (e ̶) are generated in valence band (VB) and conduction band (CB), respectively. Low recombination of photogenerated holes and electrons is an important characteristic. So it can be occurred significantly with the decrease in particle size of semiconductor to nanoscale level. During indoor light activation, holes at VB get trapped by hydroxyl groups (OH─) or water (H2O) on ZnS surface to produce hydroxide radicals (OH•). Meanwhile, the good conductivity of PVAc polymer (due to the presence of active CH3 groups) plays an important role to transfer excited electrons from CB of ZnS to the surface of nanofibers, where they react with dissolved oxygen molecules to yield superoxide radical ions (O2• ─) [41]. The O2• ─, after protonation generate hydroperoxide radicals (HO2•) leading to the formation of OH•. Thus, produced OH• are responsible to assist the degradation of cell membrane of bacteria leading to the loss of cellular components and cell death [42, 43]. The conceptual mechanism for degradation of bacterial cell membrane is illustrated in Fig. 8. The reactions, which take place during the formation of ROS are summarized as follows [44]:

$$ \mathrm{ZnS}+\mathrm{hv}\to {\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{-}\ \left(\mathrm{CB}\right) $$
Fig. 8
figure 8

Schematic illustration showing the degradation of cell membrane of bacteria by biologically reactive oxygen species

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At valence band:

$$ {\displaystyle \begin{array}{c}{\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{+}+{\mathrm{OH}}^{\bullet}\\ {}{\mathrm{h}}^{+}+{\mathrm{OH}}^{\hbox{--}}\to {\mathrm{OH}}^{\bullet}\end{array}} $$

At conduction band:

$$ {\displaystyle \begin{array}{c}{\mathrm{e}}^{-}+{\mathrm{O}}_2\to {{\mathrm{O}}_2}^{\bullet -}\\ {}{{\mathrm{O}}_2}^{\bullet -}+{\mathrm{H}}^{+}\to {{\mathrm{H}\mathrm{O}}_2}^{\bullet}\\ {}2{{\mathrm{H}\mathrm{O}}_2}^{\bullet}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2\\ {}{\mathrm{H}}_2{\mathrm{O}}_2+{{\mathrm{O}}_2}^{\bullet -}\to {\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}\mathrm{H}}^{\bullet }+{\mathrm{O}}_2\\ {}{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}\mathrm{H}}^{\bullet}\\ {}{\mathrm{O}\mathrm{H}}^{\bullet }+\mathrm{Bacteria}\to \mathrm{degradation}\ \mathrm{of}\ \mathrm{cell}\ \mathrm{membrane}\end{array}} $$

4 Conclusion

Rice grain-shaped ZnS nanoparticles doped PVAc (ZnS-PVAc) nanofibers were fabricated by electrospinning of ZnS/PVAc colloid. FE-SEM and TEM analysis revealed that the ZnS nanoparticles were completely embedded inside the polymer nanofibers. The formed ZnS-PVAc nanofibers exhibited promising antibacterial activity over E. coli and S. aureus under indoor light activation. Experimental results showed that encapsulation of ZnS nanoparticles did not affect the antibacterial activity of ZnS-PVAc nanofibers. Moreover, the introduced strategy could overcome the photocorrosion and recovery problems of nanoparticles after their use. Overall, this work can provide a new path to design low-cost, reusable, and healthily safe Zn-based nanoparticles doped polymer nanofibers via electrospinning.