Photo-reduction of heavy metal ions and photo-disinfection of pathogenic bacteria under simulated solar light using photosensitized TiO₂ nanofibers

Abstract

We report the photosensitization of electrospun titania nanofibers, with a mean diameter of 195 nm, by low bandgap silver sulfide nanoparticles of 11–23 nm mean size with the aim of treating heavy metal ions and pathogenic bacteria simultaneously under simulated solar light irradiation. The 17 nm Ag₂S/TiO₂ nanofibers showed 90% photocatalytic reduction of Cr(VI) at pH of 3 with a pseudo-first order rate constant of 0.016 min⁻¹ which is significantly better than the previously reported for Ag–Ag₂S/TiO₂ composite particles. The antibacterial capability of the Ag₂S/TiO₂ nanofibers was evaluated via photo-disinfection of the Gram-positive and Gram-negative bacterial strains. The smallest sized 11 nm Ag₂S/TiO₂ nanofiber showed the best bactericidal efficiency of 100% and 90.6% against Gram-negative E. coli and Gram-positive S. aureus after 1 h of irradiation, respectively, whereas, only 50% E. coli and 41% S. aureus were found to be inactivated in dark. Furthermore, a UV–O₃ treatment of the 11 nm Ag₂S/TiO₂ nanofibers remarkably enhanced the antibacterial activity where 89% E. coli and 81% S. aureus were inactivated in just 10 min of the irradiation. Enhanced photocatalytic activity is attributed to the efficient charge separation and transfer and reduced electron–hole recombination induced by the effective heterojunction formation between TiO₂ and the optimally sized Ag₂S nanoparticles. The disinfection nature of the Ag₂S nanoparticles, role of the generated hydroxyl species under irradiation, and the cell wall damage mechanism is also discussed. This study demonstrates the potential use of these multifunctional composite TiO₂ nanofibers for water remediation.

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Introduction

Degradation of water quality by harmful organic and inorganic pollutants is a serious problem world-wide leading to millions of deaths annually.^1,2 Pathogenic micro-organisms cause water-borne diseases such as diarrhea, cholera, gastroenteritis, etc. which may be fatal.³ There are several disinfectants used commonly for the effective control of pathogens in drinking water such as free chlorine, chloramines and ozone, etc. However, these usually result in the formation of the harmful disinfection by-products (DBPs), many of which are carcinogenic.⁴ Thus, there is a dire need to develop new green technologies of disinfection which are efficient, low cost, sustainable and environmentally friendly.

Moreover, contamination of water by heavy metals is another health risk because they have a tendency to store and accumulate in the living organisms causing several diseases and disorders.⁵ For example, hexavalent chromium, Cr(VI), is one of the most common inorganic water pollutants originating from the industries such as electroplating, leather tanning, and textile, etc.^6,7 Most of the Cr(VI) compounds have proven to be toxic, carcinogenic and mutagenic.⁸ However, trivalent chromium, Cr(III), is less toxic and less mobile in nature as compared to the Cr(VI) and can be easily removed in the form of hydroxides. To this end, numerous catalysts are being reported for the conversion of Cr(VI) to Cr(III) state.^9–13

Recently, the increasing demand of industrial wastewater treatment and focus on solar energy conversion has renewed interest in developing novel heterogeneous photocatalysis based water remediation technologies. For example, photodegradation using suitable bandgap semiconductor materials has been extensively investigated as an effective and energy efficient strategy for the eradication of hazardous pollutants from the wastewater.^14–16 Since the discovery of the photocatalytic activity of titanium dioxide (TiO₂) by Fujishima and Honda in 1972,¹⁷ it has been widely studied due to its preferred band positions, high photocatalytic activity, high resistance to corrosion, low cost, and lack of toxicity.¹⁸ However, the limited light absorbance in the visible range of the solar spectrum and the fast electron–hole recombination kinetics are the two major hurdles for its practical applications.

To address these challenges, several attempts have been made such as coupling TiO₂ with narrow bandgap semiconductors,^19,20 metal and non-metal doping,^21–23 dye sensitization,^24,25 and surface modification with noble metals.²⁶ Metal oxides and sulfides have drawn great attention as both core photocatalyst as well as co-catalyst.^27,28 Among metal sulfides, silver sulfide (Ag₂S) is considered a potential candidate for the photosensitization of TiO₂ because of its low bandgap (E_g ∼ 1.0 eV) and desired band positions relative to TiO₂, making it active in the visible range of the solar spectrum.²⁹ Moreover, due to its photocatalytic activity and ability to develop strong oxidizing power when irradiated under UV light, TiO₂ shows noticeable antibacterial effect which can be further enhanced by doping or heterojunction formation.^30–34

Silver and silver-based nanomaterials are well-known potential antimicrobial agents owing to their high cytotoxicity for microorganisms and outstanding broad-spectrum antimicrobial activity against different strains of the bacteria.^35–38 Pang et al.³⁹ explored the antibacterial properties of Ag₂S/Ag heterodimers against E. coli. Zhang et al.⁴⁰ synthesized hollow spherical Ag–Ag₂S/TiO₂ nanocomposites for the photo-reduction of Cr(VI) under UV-visible irradiation. Size and shape of the nanoparticles (NPs) significantly affect their photocatalytic and antibacterial activity.⁴¹ A comparative study of the antibacterial properties of spherical, triangular and rod shape silver NPs clearly reveals that there is a shape dependent interaction of the NPs.⁴² Similarly, the relationship of the biological activity and size of the NPs is also noticeable. Chen et al. evaluated the antibacterial capability of different sized silver NPs deposited over graphene oxide and found that the smallest silver NPs exhibited higher antibacterial activity.⁴³

Here, we report an approach for designing multifunctional TiO₂ nanofibers (NFs), photosensitized with varying sizes of Ag₂S nanoparticles (Ag₂S/TiO₂), and their effectiveness for the photo-reduction of Cr(VI) as well as photo-disinfection of the pathogenic bacteria, both Gram-positive and Gram-negative, under simulated solar light illumination. Overall the aim of this study is to investigate the size dependence of the Ag₂S NPs on the photosensitization of TiO₂ NFs and demonstrate their potential use in developing water remediation technologies using solar light.

Experimental

Material synthesis

Precursor NFs for TiO₂ were obtained by electrospinning of a mixture of 1.5 g polyvinylpyrrolidone (PVP, M_w = 1 300 000 g mol⁻¹), 4 g titanium(IV) n-butoxide (TNBT, 97%), 10 g ethanol and 4 g acetic acid. An electric field of 18 kV with a tip-to-collector distance of 18 cm was used. The collected precursor NF mat was left overnight for complete hydrolysis and then calcined at 500 °C for 2 h to obtain pure anatase TiO₂ NFs as shown in the Fig. 1(a) with a mean diameter of 195 nm. Ag₂S NPs of varying sizes were grown over TiO₂ NFs by reduction of Ag⁺ to Ag followed by a sulfurization treatment. For this, 0.1 M NaOH was added to 0.01 M AgNO₃ solution (10 ml) until brown precipitates appeared. All brown precipitates were then dissolved by the addition of 1.0 M ammonia solution which is due to the formation of the complex, [Ag(NH₃)₂]⁺, also known as the Tollens reagent. In this solution, 0.05 M dextrose solution (10 ml) and 20 mg of the TiO₂ NFs were added under constant stirring resulting in the formation of the Ag NPs. The size of the Ag NPs was controlled by varying the dipping times from 0.5–2.0 min. These NPs were subsequently sulfurized to Ag₂S by dipping in 25 ml of 0.01 M sulfur/acetonitrile solution at 60 °C for 10 min.⁴⁴ The synthesized composite Ag₂S/TiO₂ NFs were washed with acetonitrile and distilled water followed by oven drying at 60 °C. Selected samples were also subjected to a ultraviolet–ozone (UV–O₃) treatment for 2 h to induced excess surface Ti³⁺ chemical states which is expected to enhance the photo-reduction and photo-disinfection activity as reported earlier by the authors for the photodegradation of methylene blue (MB) dye.^45,46

Fig. 1 SEM images of (a) electrospun TiO₂ nanofibers with mean diameter of 195 nm, (b) 11 nm Ag₂S/TiO₂, (c) 17 nm Ag₂S/TiO₂, and (d) 23 nm Ag₂S/TiO₂ nanofibers. TEM images of (e) 11 nm Ag₂S/TiO₂ and (f) 23 nm Ag₂S/TiO₂ nanofibers. The Ag₂S size distribution in the insets of (b–d) and HR-TEM images in the insets of (e and f) are reproduced from our previous work,⁴⁶ available under the Creative Commons Attribution 4.0 International Public License.

Photo-reduction of Cr(VI)

The photocatalytic activity of the Ag₂S/TiO₂ NFs was evaluated using carcinogenic Cr(VI) compound as a target pollutant in the aqueous media. The Cr(VI) stock solution (10 mg L⁻¹) was prepared by dissolving K₂Cr₂O₇ in deionized water. Then 20 mg of the Ag₂S/TiO₂ NFs was dispersed in 20 ml of the Cr(VI) solution and agitated in dark for 30 min to establish the adsorption–desorption equilibrium. The suspension was then irradiated with simulated solar light using 100 W xenon lamp. 1 ml of the suspension was taken out every 30 min and centrifuged. The Cr(VI) reduction was estimated spectrophotometrically (λ = 540 nm) by making their colored complex with diphenylcarbazide. Photo-reduction of Cr(VI) was evaluated at different pH values (3, 5, 7, 9) adjusted with 0.1 M HCl and 0.05 M NaOH solutions.

Photo-disinfection of E. coli and S. aureus

The antibacterial activity of the Ag₂S/TiO₂ NFs was studied via photochemical interaction of the NFs with the bacterial strains of Gram-negative Escherichia coli [ACCN, KJ880039] and Gram-positive Staphylococcus aureus [ACCN, KY635411] using the plate-count method. These strains were maintained in the LB broth and agar. Bacterial cultures were prepared by incubating their strains inoculated on the LB nutrient agar at 37 °C for 16–24 h under shaking and aerobic conditions. After this, the single colony of the bacterial strain was incubated in LB broth under standard conditions. Before the antibacterial assay, LB broth, nutrient agar, and all other apparatus such as L-rods, Petri plates, micro-tips, etc., were sterilized by autoclaving at 120 °C for 30 min. Bacterial cultures were then centrifuged at 4000 rpm and washed with 0.9% saline solution. Bacterial pellets were then re-suspended and diluted with normal saline to prepare their suspension having 0.2 optical density (OD). The antibacterial activity was evaluated by mixing 5 mg of the NFs with 5 ml of the bacterial suspension in the sterilized flask. The initial bacterial concentration was 1 × 10⁸ CFU ml⁻¹ for E. coli and 3 × 10⁸ CFU ml⁻¹ for S. aureus (CFU: colony forming units). A 100 W xenon lamp positioned at 15 cm from the reaction mixture was used for illumination under constant shaking. The irradiance, measured at the center of the reaction mixture with a power meter and Si photodiode sensor, was estimated to be ∼10 mW cm⁻². The sterilized nutrient agar solution was poured in the Petri plates and allowed to solidify. At regular intervals of illumination, 100 μL of the treated bacterial suspension was withdrawn and diluted serially with normal saline to adjust the bacterial concentration (aliquots were diluted to 10⁻⁵ times in the present study) to ensure that the growing (reactivated) bacterial colonies were being counted accurately. Small quantity of this diluted bacterial suspension (100 μL) was spread onto the nutrient agar plates and the number of colonies were counted after incubating at 37 °C for 24 h. An average number of the colonies was obtained by repeating the above procedure three times for each NF sample. Bacterial suspension without NFs was also serially diluted and incubated on agar plates to count the number of colonies in the control sample. The bactericidal efficiency was calculated by the following equation.
where N_control and N_sample denotes to the number of colonies counted in the control and the sample agar plates, respectively. Exactly the same procedure was also carried out in dark to explore the disinfection nature of the Ag₂S NPs only.

Results & discussion

Structure, morphology and chemistry of the materials

The SEM images of the fabricated Ag₂S/TiO₂ NFs in the Fig. 1(b–d) indicates successful deposition of the Ag NPs and their subsequent sulfurization to Ag₂S which was also confirmed previously by the X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies.⁴⁶ The number-averaged size of the Ag₂S NPs is 11, 17, and 23 nm for the dipping times of 0.5, 1.0, and 2.0 min, respectively. These sizes were statistically measured from at least 100 NPs by measuring their areas and calculating the corresponding area-equivalent diameters assuming circular shape. The corresponding Ag₂S size distribution, shown in the insets of Fig. 1(b–d), characteristically shows log-normal distribution.⁴⁶ The weight percentages of the Ag₂S in the NFs was estimated by inductively coupled plasma optical emission spectroscopy (ICP-OES) by calibrating the emission intensity of Ag using a standard AgNO₃ salt solution of 1–10 ppm. The weight fractions were 4, 10, and 18% for 11, 17, and 23 nm Ag₂S/TiO₂ NFs, respectively. Fig. 1(e and f) shows the TEM images of the 11 nm and 23 nm Ag₂S/TiO₂ NFs, respectively, which also confirms the uniform distribution of the Ag₂S NPs over the TiO₂ NFs. The insets show the (031) lattice for the 11 nm Ag₂S/TiO₂ with d-spacing of 0.212 nm and (121) lattice for the 23 nm Ag₂S/TiO₂ with d-spacing of 0.196 nm, respectively.⁴⁶

Fig. 2(a) shows the XRD scan of the pure TiO₂ and Ag₂S/TiO₂ samples where the Ag₂S lattices identified in the insets of Fig. 1(e and f) were clearly observed along with all other characteristics peaks for the monoclinic Ag₂S. No Ag peak was observed for any sample. Moreover, the formation of anatase TiO₂ is also confirmed from the observation of (101) and other characteristic peaks. Fig. 2(b) shows the XPS survey scan which confirms the presence of Ti and O in the pristine TiO₂ and furthermore Ag and S in the Ag₂S/TiO₂ NFs. The surface chemical states were characterized in detail using XPS and reported in our earlier work.⁴⁶ Fig. S1† shows the deconvoluted peaks of Ti 2p, O 1s, Ag 3d, and S 2p. Pure TiO₂ NFs showed 94.3% Ti⁴⁺ and 5.7% Ti³⁺ surface chemical states. The surface Ti³⁺ increases to 6.7% after deposition of 17 nm Ag₂S NPs with a shift to the higher binding energy indicative of the loss of the valence electron charge. Moreover, the surface UV–O₃ treatment further increases the Ti³⁺ to 8.2% indicating the removal of the lattice oxygen. The effect of these surface chemical changes i.e. the increased surface Ti³⁺ states and oxygen vacancies on the photocatalytic activity will be discussed later.

Fig. 2 (a) XRD scans of pristine TiO₂ and Ag₂S/TiO₂ NFs. (b) Full XPS survey scan of pure TiO₂, 17 nm Ag₂S/TiO₂ and 17 nm-Ag₂S/TiO₂ treated with UV–O₃. The figure is reproduced from our previous work,⁴⁶ available under the Creative Commons Attribution 4.0 International Public License.

The fabricated TiO₂ and Ag₂S/TiO₂ NFs are expected to be non-porous materials since no pore generating agent was used. This was confirmed by the adsorption–desorption isotherm shown in the Fig. S2(a)† which is typical for the multilayer formation in non-porous materials. The specific surface area of pure TiO₂ NFs and 17 nm Ag₂S/TiO₂ NFs was 7.1 and 6.6 m² g⁻¹, respectively. Fig. S2(b)† shows the pore size distribution using the nonlocal density functional theory (NLDFT) method. The t-plot method was used to determine the micropores volume which was negligible. The collection of NFs as a mat during electrospinning does create the so-called macropores indicated by the high intake of nitrogen at higher relative pressures (P/P_o > 0.8).

Photo-reduction of Cr(VI)

The presence of heavy metal ions in water pose a serious threat to the living organisms. One such contamination is the Cr(VI) which is a common by-product of many industries and can be photocatalytically reduced to less toxic and less mobile Cr(III) state. Fig. 3(a) shows the influence of the pH on the photo-reduction of Cr(VI) over Ag₂S/TiO₂ NFs under simulated solar light irradiation. The amount of Cr(VI) reduced to Cr(III) at pH of 3, 5, 7, and 9 is 90, 67, 58, and 48%, respectively, for the 17 nm Ag₂S/TiO₂ NFs. In acidic medium, Cr(VI) exists as HCrO₄⁻, CrO₄²⁻, and Cr₂O₇²⁻ anionic species.⁴⁷ Therefore, at lower pH, the surface of the Ag₂S/TiO₂ NFs are highly protonated which can easily accumulate the anionic species and hence facilitate the transfer of the photo-generated electrons towards the Cr(VI). At higher pH (>6.0), the surfaces of the NFs become negatively charged which tend to repel the anionic species and consequently decreases the photo-reduction kinetics. Moreover, the reduced Cr(III) is deposited on the surface of the NFs as Cr(OH)₃ at pH above 5, which is also responsible for the decreased photo-reduction efficiency.⁴⁸

Fig. 3 (a) Photo-reduction of Cr(VI) at different pH values using 17 nm Ag₂S/TiO₂ nanofibers, (b) photo-reduction using variable sized Ag₂S/TiO₂ nanofibers at pH 3 compared to the pristine TiO₂ nanofibers, and (c) the corresponding pseudo-first order rate constants.

The size effect of the Ag₂S NPs on the photocatalytic reduction of Cr(VI) under simulated solar light at pH 3 was further investigated. The 17 nm Ag₂S/TiO₂ NFs showed 90% photocatalytic reduction of Cr(VI) with a pseudo-first order rate constant of 0.016 min⁻¹. In comparison, hollow spherical Ag–Ag₂S/TiO₂ composite particles under visible light irradiation were able to reduce only 36% Cr(VI) after 8 h irradiation.⁴⁰ This illustrates the significantly enhanced capacity of the TiO₂ NFs, photosensitized with Ag₂S, reported here to reduce Cr(VI). The smaller (11 nm) and larger (23 nm) sized Ag₂S/TiO₂ NFs exhibited slower kinetics with pseudo-first order rate constants of 0.014 and 0.010 min⁻¹, respectively. This is in agreement with our findings on the role of Ag₂S NP size on the photodegradation kinetics of methylene blue (MB) dye. Fig. S3† shows the enhanced visible-light harvesting capability, reduced bandgaps, and diminished photoluminescence signal when TiO₂ NFs are photosensitized by Ag₂S NPs. This is attributed to the efficient charge separation, charge transfer, and reduced electron–hole recombination. The UV–O₃ surface treatment of Ag₂S/TiO₂ NFs generates excess Ti³⁺ states which further facilitates the charge separation process⁴⁶ (Fig. S3†) but the photo-reduction of Cr(VI) was hindered as shown by the reduction of the rate constant to 0.013 min⁻¹ for the 17 nm Ag₂S/TiO₂ NFs. As reported in the XPS results previously,⁴⁶ some of the Ag₂S get oxidized after UV–O₃ treatment and produces Ag₂SO₄ which is partially soluble in water. Introduction of the sulfate ions in the solution probably interferes with the sorption of the Cr(VI) over the NFs and, thus, suppresses the overall photo-reduction process. Thus, excess Ti³⁺ was beneficial for photo-degradation of methylene blue (MB) dye,⁴⁶ but not for the photo-reduction of Cr(VI) here. However, as shown below, the excess Ti³⁺ chemical states enhances the antibacterial performance of these materials.

Photo-disinfection efficiency in dark

The photocatalytic disinfection efficiencies of the Ag₂S/TiO₂ NFs were investigated on Gram-negative and Gram-positive bacteria. Fig. 4(a and b) shows the bactericidal efficiencies of both strains for 3 h in dark conditions. TiO₂ NFs alone have negligible effect on the survival of both strains, however, Ag₂S loading on the TiO₂ NFs significantly enhances the bactericidal efficiencies. Furthermore, UV–O₃ treatment of the 11 nm Ag₂S/TiO₂ NFs effectively inactivates almost all the bacteria. Smallest sized and spherical shaped Ag₂S NPs loading showed superior antibacterial activities compared to the larger and relatively irregular shaped Ag₂S NPs. These results are in good agreement with the reported studies on the effect of size and shape of Ag NPs on the bacterial viabilities.^43,49

Fig. 4 (a and b) Bactericidal efficiency of E. coli and S. aureus in dark and (c and d) under simulated solar light irradiation, respectively. Comparison of the bacterial viabilities of (e) E. coli and (f) S. aureus after treatment with pristine TiO₂ and Ag₂S/TiO₂ nanofibers. Error bars represent standard deviations from triplicate experiments.

The bacterial decay in dark is an interesting observation and is due to Ag₂S only since TiO₂ has negligible effect. Recently, uncapped Ag₂S NPs showed antibacterial effect, calculated by the disc diffusion method, towards Gram-positive S. aureus and Gram-negative E. coli, with recorded inhibition zones of 15.81 and 10.34 mm, respectively.⁵⁰ Similarly, Ag₂S and Cu-doped Ag₂S NPs, prepared by the simple co-precipitation method, showed antibacterial activity both in dark and visible-light irradiation against E. faecalis and S. aureus.⁵¹ However, the mechanisms of the antibacterial activity for silver containing compounds especially Ag₂S are not yet clearly known due to very limited work. Ag₂S exhibits very low solubility and reactivity and, thus, the mechanism for their cytotoxicity might be different than the widely accepted mechanism reported for ionic silver (Ag⁺) and Ag NPs.⁵² Wang et al. reported for the first time the toxic effects of Ag₂S NPs in plant tissues of cowpea (Vigna unguiculata) and wheat (Triticum aestivum) by direct uptake.⁵³ Using K-edge X-ray absorption near edge structure (XANES) and k²-weighted extended X-ray absorption fine structure (EXAFS) they showed that almost all the Ag in the roots is stored as Ag₂S and is not sourced from the dissolved Ag in the solution. Pang et al. reported negligible inactivation efficiency against E. coli in dark using Ag₂S/Ag heterodimers probably due to much larger Ag size of 50–75 nm.³⁹ Ag₂S/Ag heteronanostructures and Ag NPs showed minimum inhibitory concentration (MIC) of 8 μg ml⁻¹ against E. coli but Ag₂S NPs alone showed MIC of 32 μg ml⁻¹ and it was concluded that the major contribution to the antibacterial activity in dark is coming from Ag.⁵⁴ In contrast, Ag₂S NPs over TiO₂ in the present study shows appreciable antibacterial activity which is promising but warrants further work to investigate the mechanism of the Ag₂S cytotoxicity.

Photo-disinfection efficiency under simulated sunlight

Next, the inactivation of bacteria by Ag₂S/TiO₂ NFs under simulated solar light illumination was studied. As shown in the Fig. 4(c and d), for 11 nm Ag₂S/TiO₂ NFs, the bactericidal efficiency increased rapidly to 100% and 90.6% against E. coli and S. aureus after 1 h of irradiation, respectively, whereas, only 50% E. coli and 41% S. aureus were found to be inactivated under dark. UV–O₃ treatment on the 11 nm Ag₂S/TiO₂ NFs significantly enhanced the antibacterial activity with 89% E. coli and 81% S. aureus being inactivated in just 10 min of the irradiation. This is probably due to the synergistic effect of Ag⁺ ions, originating from the dissolution of the Ag₂SO₄ formed by UV–O₃ treatment. The viabilities of the bacteria significantly decline under the solar light irradiation compared to the dark conditions. Moreover, the initial disinfection effectiveness against E. coli is higher than S. aureus and is discussed below.

The Ag₂S/TiO₂ NFs in the present study, when irradiated under simulated sunlight, produces hydroxyl radicals (˙OH) by the reaction of the positively charged hole (h⁺) with the hydroxide ion (OH⁻).⁴⁶ The hydroxyl radicals are known to destruct the bacterial cell of Gram-positive and Gram-negative bacteria by breaking the covalent bonds in the peptidoglycan layer.⁵⁵ Moreover, the cell wall in Gram-positive and Gram-negative is structurally distinct and can initially respond differently to the toxic effects as seen in the Fig. 4(c and d) for the bactericidal efficiency after 10 min. Cell wall in Gram-negative consists of a membrane of lipopolysaccharide covered by a thin layer of peptidoglycan (∼few nm). Cell wall in Gram-positive consists of much thicker peptidoglycan layer (∼10s of nm). Thus, the initial response at 10 min to the effect of hydroxyl radicals and photo-disinfection of Ag₂S/TiO₂ NFs is slow in Gram-positive S. aureus versus Gram-negative E. coli. However, after 60 min both bacterial strains show similar inactivation. Similar trend was observed in dark which suggests that the effectiveness of Ag₂S NPs is indeed controlled by the nature of the target bacterial cell wall. Another contributing factor is the surface charge on the TiO₂ and the bacterial strains. TiO₂ is positively charged in acidic conditions (<pH 6).⁵⁶ Cell walls of most bacterial strains are known to possess negative surface charge due to the teichoic acids in Gram-positive and lipopolysaccharides & lipoproteins in Gram-negative bacteria.³⁰ Thus, the electrostatic bacterial adhesion to TiO₂ NFs is promoted for both Gram-positive and Gram-negative strains. Surface charge on the S. aureus (0.086 coulomb/10⁹ CFU) is reported to be less than E. coli (0.129 coulomb/10⁹ CFU) with corresponding electrostatically adhered TiO₂ amount of 0.61 and 1.88 g/10⁹ CFU, respectively. These observations are in good agreement with the initially higher antibacterial activity against E. coli versus S. aureus reported here.

Morphological changes in E. coli cell wall

The morphological changes in the cell wall of E. coli treated with Ag₂S/TiO₂ NFs under simulated sunlight was observed in SEM (operated under STEM mode). Fig. 5(a) shows the characteristic rod-shaped morphology of the untreated E. coli (length ∼2 μm, diameter ∼0.5 μm) with an intact cell wall and interior. Fig. 5(b) shows the significant shape changes and rupturing of the cell wall after exposure to simulated solar light in the presence of 11 nm Ag₂S/TiO₂ NFs. The image contrast suggests that the outer membrane of the E. coli is disrupted at several locations in the same cell with leakage of the intracellular components. In a related study using Ag₂S/Ag heterodimers, tested for bacterial disinfection using similar methods, it was shown by inductively coupled plasma mass spectroscopy (ICP-MS) that the Ag⁺ ion concentration in the E. coli suspensions was extremely low (∼75 parts per billion) and energy dispersive X-ray spectroscopy (EDX) confirmed no change in the molar ratios of Ag : S for the catalysts before and after the tests. These findings confirmed that Ag⁺ ions played no part in the bactericidal processes.³⁹ The cytotoxic action is, thus, initiated by the cell wall damage in the presence of Ag₂S/TiO₂ NFs followed by intracellular leakage. This cell wall damage could be due to the presence of hydroxyl radicals generated by Ag₂S/TiO₂ NFs⁴⁶ as well as due to the disinfectant nature of the pristine Ag₂S as reported here in dark. This later effect needs to be investigated mechanistically in the follow-up studies.

Fig. 5 The STEM images of the (a) untreated rod-shaped E. coli bacteria with a length of ∼2 μm and width of ∼0.5 μm, and the corresponding (b) treated E. coli bacteria showing significant shape changes and rupturing of the cell walls after exposure to simulated solar light irradiation in the presence of 11 nm Ag₂S/TiO₂ NFs.

Stability and the proposed photocatalytic mechanism

The stability of the UV–O₃ treated Ag₂S/TiO₂ NFs for the photocatalytic inactivation of E. coli was investigated for five cycles and the results are shown in the Fig. 6(a). After each cycle, the sample was washed with ethanol and subsequently with normal saline to decompose and remove all the captured bacteria. As it can be seen, the bactericidal efficiency of the bacteria slightly decreased initially probably due to the initial material loss during handling. Nevertheless, it becomes relatively stable after the second cycle.

Fig. 6 (a) Bactericidal efficiency against E. coli (10⁸ CFU ml⁻¹) in the presence of 11 nm Ag₂S/TiO₂ NFs treated with UV–O₃ over five cycles under simulated solar light illumination. (b) Schematic of the proposed mechanisms for the photo-reduction of Cr(VI) and photo-disinfection of bacteria by Ag₂S/TiO₂ NFs.

The proposed photocatalytic mechanism in Ag₂S/TiO₂ NFs for the reduction of Cr(VI) can be explained in the Fig. 6(b). Owing to the small bandgap of Ag₂S (E_g ∼ 1 eV), the valence band (VB) electrons are easily excited to the conduction band (CB) under visible light illumination. The photo-generated electrons transfer from CB of Ag₂S to the CB of TiO₂ which effectively inhibits the e⁻–h⁺ recombination. This effective charge separation facilitate the transfer of photo-generated electrons to Cr(VI) to reduce it to Cr(III), meanwhile the holes oxidize water and produce oxygen.⁵⁷ The reactions can be presented as:

Ag₂S/TiO₂ + hv → Ag₂S/TiO₂ + e⁻ + h⁺

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

2H₂O + 4h⁺ → O₂ + 4H⁺

Similarly, the underlying mechanism for the photocatalytic inactivation of bacteria after treatment with Ag₂S/TiO₂ NFs under irradiation can be explained. In addition, the Ti³⁺ states introduced after UV–O₃ treatment further promotes the lifetime of the charges.⁴⁶ The photo-generated electrons and holes react with O₂ and H₂O to produce the reactive oxygen species (ROS) such as peroxide (O₂⁻˙) and hydroxide (˙OH) radicals. These ROS diffuse in the cell wall of bacteria and react with the cell membrane and DNA leading to death of the bacteria.⁵⁸ As stated above, Ag⁺ and sulfate SO₄²⁻ ions introduced after UV–O₃ treatment, though not beneficial for the reduction of Cr(VI), further accelerate the bactericidal activity due to the synergistic effect from Ag⁺.

Conclusions

In summary, TiO₂ NFs were photosensitized by Ag₂S NPs with mean sizes of 11, 17, and 23 nm, for potential applications in water remediation by reducing heavy metal ions and inactivating bacteria under simulated solar light irradiation. The 17 nm Ag₂S/TiO₂ NFs showed superior photocatalytic reduction of Cr(VI) at pH 3. Enhanced photocatalytic activity is attributed to the efficient charge separation and transfer and reduced electron–hole recombination. Furthermore, the 11 nm Ag₂S/TiO₂ NFs treated with UV–O₃ to induce excess Ti³⁺ chemical states showed excellent photocatalytic disinfection of both Gram-negative E. coli and Gram-positive S. aureus bacterial strains. The enhanced cytotoxicity is attributed to several factors such as disinfectant nature of the pristine Ag₂S, role of hydroxyl radicals, good adhesion between positively charged TiO₂ and negatively charged bacteria, and the synergistic effect of Ag⁺ (only for UV–O₃ treated sample). These multifunctional composite nanofibers can potentially be used for water remediation under solar light.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge Dr Murtaza Saleem, Sr Research Scientist in the Central Labs of the Syed Babar Ali School of Science and Engineering, LUMS, for help with the scanning electron microscopy (SEM) and Dr Nasir Mahmood, University of Wollongong, for assistance in the X-ray photoelectron spectroscopy (XPS). We are also thankful to Mr Muhammad Usman Ghafoor for helping with the simulated solar light irradiation setup.

References

1. S. Pigeot-Rémy, F. Simonet, E. Errazuriz-Cerda, J. C. Lazzaroni, D. Atlan and C. Guillard, Appl. Catal., B, 2011, 104, 390–398.
2. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marĩas and A. M. Mayes, Nature, 2008, 452, 301–310.
3. G. Xiao, X. Zhang, W. Zhang, S. Zhang, H. Su and T. Tan, Appl. Catal., B, 2015, 170–171, 255–262.
4. S. W. Krasner, H. S. Weinberg, S. D. Richardson, S. J. Pastor, R. Chinn, M. J. Sclimenti, G. D. Onstad and A. D. Thruston, Environ. Sci. Technol., 2006, 40, 7175–7185.
5. S. E. Bailey, T. J. Olin, R. M. Bricka and D. D. Adrian, Water Res., 1999, 33, 2469–2479.
6. Q. Li, S. Mahendra, D. Y. Lyon, L. Brunet, M. V. Liga, D. Li and P. J. J. Alvarez, Water Res., 2008, 42, 4591–4602.
7. Y. C. Zhang, J. Li and H. Y. Xu, Appl. Catal., B, 2012, 123–124, 18–26.
8. N. Hsu, S. Wang, Y. Lin, G. Sheng and J. Lee, Environ. Sci. Technol., 2009, 43, 8801–8806.
9. M. Yadav and Q. Xu, Chem. Commun., 2013, 49, 3327.
10. G. T. Fu, X. Jiang, R. Wu, S. H. Wei, D. M. Sun, Y. W. Tang, T. H. Lu and Y. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 22790–22795.
11. Z. W. Wang, Z. Y. Wang, D. M. Wang and M. Chen, RSC Adv., 2016, 6, 12286–12289.
12. A. Kumar, C. Guo, G. Sharma, D. Pathania, M. Naushad, S. Kalia and P. Dhiman, RSC Adv., 2016, 6, 13251–13263.
13. B. Vellaichamy and P. Periakaruppan, RSC Adv., 2016, 6, 57380–57388.
14. X. Xin, J. Lang, T. Wang, Y. Su, Y. Zhao and X. Wang, Appl. Catal., B, 2016, 181, 197–209.
15. S. Y. Chai, Y. J. Kim, M. H. Jung, A. K. Chakraborty, D. Jung and W. I. Lee, J. Catal., 2009, 262, 144–149.
16. W. Liu, M. Wang, C. Xu and S. Chen, Chem. Eng. J., 2012, 209, 386–393.
17. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
18. M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Renewable Sustainable Energy Rev., 2007, 11, 401–425.
19. H. Abdullah, D. H. Kuo and Y. H. Chen, J. Mater. Sci., 2016, 51, 8209–8223.
20. Y. Li, L. Zhang, W. Wu, P. Dai, X. Yu, M. Wu and G. Li, Nanoscale Res. Lett., 2014, 9, 270–275.
21. W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 1994, 98, 13669–13679.
22. Y. Cong, J. Zhang, F. Chen and M. Anpo, J. Phys. Chem. C, 2007, 111, 6976–6982.
23. A. Yousef, R. M. Brooks, M. M. El-Halwany, N. A. M. Barakat, M. H. EL-Newehy and H. Y. Kim, Chem. Eng. Process. Process Intensif., 2015, 95, 202–207.
24. G. Marotta, M. G. Lobello, C. Anselmi, G. Barozzino Consiglio, M. Calamante, A. Mordini, M. Pastore and F. De Angelis, ChemPhysChem, 2014, 15, 1116–1125.
25. P. Chowdhury, J. Moreira, H. Gomaa and A. K. Ray, Ind. Eng. Chem. Res., 2012, 51, 4523–4532.
26. P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo, Y. Sun and Y. Liu, J. Mater. Chem., 2011, 21, 17746.
27. T. Zhu, C. K. Nuo Peh, M. Hong and G. W. Ho, Chem.–Eur. J., 2014, 20, 11505–11510.
28. L. Deng, H. Liu, X. Gao, X. Su and Z. Zhu, Ceram. Int., 2015, 42, 3808–3815.
29. M. Gholami, M. Qorbani, O. Moradlou, N. Naseri and A. Z. Moshfegh, RSC Adv., 2014, 4, 7838.
30. S. P. Tallósy, L. Janovák, E. Nagy, Á. Deák, Á. Juhász, E. Csapó, N. Buzás and I. Dékány, Appl. Surf. Sci., 2016, 371, 139–150.
31. B. Liu, L. Mu, B. Han, J. Zhang and H. Shi, Appl. Surf. Sci., 2017, 396, 1596–1603.
32. R. Michal, E. Dworniczek, M. Caplovicova, O. Monfort, P. Lianos, L. Caplovic and G. Plesch, Appl. Surf. Sci., 2016, 371, 538–546.
33. X. Zheng, Z. Shen, C. Cheng, L. Shi, R. Cheng and J. Dong, RSC Adv., 2017, 7, 52172–52179.
34. J. Li, D. Ren, Z. Wu, C. Huang, H. Yang, Y. Chen and H. Yu, RSC Adv., 2017, 7, 55131–55140.
35. J. S. Kim, E. Kuk, K. N. Yu, J. H. Kim, S. J. Park, H. J. Lee, S. H. Kim, Y. K. Park, Y. H. Park, C. Y. Hwang, Y. K. Kim, Y. S. Lee, D. H. Jeong and M. H. Cho, Nanomed. Nanotechnol. Biol. Med., 2007, 3, 95–101.
36. C. Tang, H. Bai, L. Liu, X. Zan, P. Gao, D. D. Sun and W. Yan, Appl. Catal., B, 2016, 196, 57–67.
37. E. Albert, P. A. Albouy, A. Ayral, P. Basa, G. Csík, N. Nagy, S. Roualdès, V. Rouessac, G. Sáfrán, Á. Suhajda, Z. Zolnai and Z. Hórvölgyi, RSC Adv., 2015, 5, 59070–59081.
38. D. Wei, F. Tian, Z. Lu, H. Yang and R. Chen, RSC Adv., 2016, 6, 52264–52270.
39. M. Pang, J. Hu and H. C. Zeng, J. Am. Chem. Soc., 2010, 132, 10771–10785.
40. D. Zhang, G. Xu and F. Chen, Appl. Surf. Sci., 2015, 351, 962–968.
41. S. Agnihotri, S. Mukherji and S. Mukherji, RSC Adv., 2014, 4, 3974–3983.
42. S. Pal, Y. K. Tak and J. M. Song, J. Biol. Chem., 2015, 290, 1712–1720.
43. X. Chen, X. Huang, C. Zheng, Y. Liu, T. Xu and J. Liu, J. Mater. Chem. B, 2015, 3, 7020–7029.
44. X. Liu, Z. Liu, J. Lu, X. Wu and W. Chu, J. Colloid Interface Sci., 2014, 413, 17–23.
45. S. Dilpazir, M. Usman, S. Rasul and S. N. Arshad, RSC Adv., 2016, 6, 14751–14755.
46. S. Ghafoor, S. Ata, N. Mahmood and S. N. Arshad, Sci. Rep., 2017, 7(255), 1–10.
47. J. Su, Y. Zhang, S. Xu, S. Wang, H. Ding, S. Pan, G. Wang, G. Li and H. Zhao, Nanoscale, 2014, 6, 5181.
48. M. Shirzad-Siboni, M. Farrokhi, R. Darvishi Cheshmeh Soltani, A. Khataee and S. Tajassosi, Ind. Eng. Chem. Res., 2014, 53, 1079–1087.
49. A. GhavamiNejad, A. Rajan Unnithan, A. Ramachandra Kurup Sasikala, M. Samarikhalaj, R. G. Thomas, Y. Y. Jeong, S. Nasseri, P. Murugesan, D. Wu, C. Hee Park and C. S. Kim, ACS Appl. Mater. Interfaces, 2015, 7, 12176–12183.
50. D. Ayodhya and G. Veerabhadram, J. Photochem. Photobiol., B, 2016, 157, 57–69.
51. A. Fakhri, M. Pourmand, R. Khakpour and S. Behrouz, J. Photochem. Photobiol., B, 2015, 149, 78–83.
52. J. Y. Maillard and P. Hartemann, Crit. Rev. Microbiol., 2013, 39, 373–383.
53. P. Wang, N. W. Menzies, E. Lombi, R. Sekine, F. P. C. Blamey, M. C. Hernandez-Soriano, M. Cheng, P. Kappen, W. J. G. M. Peijnenburg, C. Tang and P. M. Kopittke, Nanotoxicology, 2015, 9, 1041–1049.
54. X. Ma, Y. Zhao, X. Jiang, W. Liu, S. Liu and Z. Tang, ChemPhysChem, 2012, 13, 2531–2535.
55. L. Ge, G. Na, S. Zhang, K. Li, P. Zhang, H. Ren and Z. Yao, Sci. Total Environ., 2015, 527–528, 12–17.
56. T. Preocanin and N. Kallay, Croat. Chem. Acta, 2006, 79, 95–106.
57. H. Chu, W. Lei, X. Liu, J. Li, W. Zheng, G. Zhu, C. Li, L. Pan and C. Sun, Appl. Catal., A, 2016, 521, 19–25.
58. W. S. Lee, Y.-S. Park and Y.-K. Cho, Analyst, 2015, 140, 616–622.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra01237g

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