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

Article abstract of DOI:10.1039/c8ra01237g

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.

Full text of DOI:10.1039/c8ra01237g

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Photo-reduction of heavy metal ions and photo-
disinfection of pathogenic bacteria under
Cite this: RSC Adv., 2018, 8, 20354
simulated solar light using photosensitized TiO2
nanofibers†
a
Samina Ghafoor,ab Syed Zajif Hussain,
Sadia Waseemb
a
*
and Salman Noshear Arshad
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 minꢀ1 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.
Received 8th February 2018
Accepted 29th May 2018
DOI: 10.1039/c8ra01237g
rsc.li/rsc-advances
Moreover, contamination of water by heavy metals is another
health risk because they have a tendency to store and accu-
Introduction
mulate 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 chro-
mium, 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, photo-
degradation 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 photo-
catalytic activity of titanium dioxide (TiO2) by Fujishima and
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 carcino-
genic.4 Thus, there is a dire need to develop new green tech-
nologies of disinfection which are efficient, low cost,
sustainable and environmentally friendly.
aDepartment of Chemistry and Chemical Engineering, Syed Babar Ali School of Science
and Engineering, Lahore University of Management Sciences, Lahore 54792, Pakistan.
E-mail: salman.arshad@lums.edu.pk
bInstitute of Chemistry, University of the Punjab, P. O. Box 54590, Lahore, Pakistan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01237g
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Honda in 1972,17 it has been widely studied due to its preferred calcined at 500 C for 2 h to obtain pure anatase TiO2 NFs as
band positions, high photocatalytic activity, high resistance to shown in the Fig. 1(a) with a mean diameter of 195 nm. Ag2S
corrosion, low cost, and lack of toxicity.18 However, the limited NPs of varying sizes were grown over TiO2 NFs by reduction of
light absorbance in the visible range of the solar spectrum and Ag+ to Ag followed by a sulfurization treatment. For this, 0.1 M
the fast electron–hole recombination kinetics are the two major NaOH was added to 0.01 M AgNO3 solution (10 ml) until brown
ꢂ
hurdles for its practical applications.
precipitates appeared. All brown precipitates were then dis-
To address these challenges, several attempts have been solved by the addition of 1.0 M ammonia solution which is due
made such as coupling TiO2 with narrow bandgap semi- to the formation of the complex, [Ag(NH3)2]+, also known as the
conductors,19,20 metal and non-metal doping,21–23 dye sensitiza- Tollens reagent. In this solution, 0.05 M dextrose solution (10
tion,24,25 and surface modication with noble metals.26 Metal ml) and 20 mg of the TiO2 NFs were added under constant
oxides and suldes have drawn great attention as both core stirring resulting in the formation of the Ag NPs. The size of the
photocatalyst as well as co-catalyst.27,28 Among metal suldes, Ag NPs was controlled by varying the dipping times from 0.5–
silver sulde (Ag2S) is considered a potential candidate for the 2.0 min. These NPs were subsequently sulfurized to Ag2S by
photosensitization of TiO2 because of its low bandgap (Eg ꢁ 1.0 dipping in 25 ml of 0.01 M sulfur/acetonitrile solution at 60 ꢂC
eV) and desired band positions relative to TiO2, making it active for 10 min.44 The synthesized composite Ag2S/TiO2 NFs were
in the visible range of the solar spectrum.29 Moreover, due to its washed with acetonitrile and distilled water followed by oven
photocatalytic activity and ability to develop strong oxidizing drying at 60 ꢂC. Selected samples were also subjected to
power when irradiated under UV light, TiO2 shows noticeable a ultraviolet–ozone (UV–O3) treatment for 2 h to induced excess
antibacterial effect which can be further enhanced by doping or surface Ti3+ chemical states which is expected to enhance the
heterojunction formation.30–34
photo-reduction and photo-disinfection activity as reported
Silver and silver-based nanomaterials are well-known earlier by the authors for the photodegradation of methylene
potential antimicrobial agents owing to their high cytotoxicity blue (MB) dye.45,46
for microorganisms and outstanding broad-spectrum antimi-
crobial activity against different strains of the bacteria.35–38 Pang
Photo-reduction of Cr(VI)
et al.39 explored the antibacterial properties of Ag2S/Ag hetero-
The photocatalytic activity of the Ag2S/TiO2 NFs was evaluated
dimers against E. coli. Zhang et al.40 synthesized hollow spher-
using carcinogenic Cr(VI) compound as a target pollutant in the
ical Ag–Ag2S/TiO2 nanocomposites for the photo-reduction of
aqueous media. The Cr(VI) stock solution (10 mg Lꢀ1) was
Cr(VI) under UV-visible irradiation. Size and shape of the
prepared by dissolving K2Cr2O7 in deionized water. Then 20 mg
nanoparticles (NPs) signicantly affect their photocatalytic and
of the Ag2S/TiO2 NFs was dispersed in 20 ml of the Cr(VI) solu-
antibacterial activity.41 A comparative study of the antibacterial
tion and agitated in dark for 30 min to establish the adsorption–
properties of spherical, triangular and rod shape silver NPs
desorption equilibrium. The suspension was then irradiated
clearly reveals that there is a shape dependent interaction of the
with simulated solar light using 100 W xenon lamp. 1 ml of the
NPs.42 Similarly, the relationship of the biological activity and
suspension was taken out every 30 min and centrifuged. The
size of the NPs is also noticeable. Chen et al. evaluated the
Cr(VI) reduction was estimated spectrophotometrically (l ¼ 540
antibacterial capability of different sized silver NPs deposited
nm) by making their colored complex with diphenylcarbazide.
over graphene oxide and found that the smallest silver NPs
Photo-reduction of Cr(VI) was evaluated at different pH values
exhibited higher antibacterial activity.43
(3, 5, 7, 9) adjusted with 0.1 M HCl and 0.05 M NaOH solutions.
Here, we report an approach for designing multifunctional
TiO2 nanobers (NFs), photosensitized with varying sizes of
Ag2S nanoparticles (Ag2S/TiO2), and their effectiveness for the
Photo-disinfection of E. coli and S. aureus
photo-reduction of Cr(VI) as well as photo-disinfection of the The antibacterial activity of the Ag2S/TiO2 NFs was studied via
pathogenic bacteria, both Gram-positive and Gram-negative, photochemical interaction of the NFs with the bacterial strains of
under simulated solar light illumination. Overall the aim of Gram-negative Escherichia coli [ACCN, KJ880039] and Gram-
this study is to investigate the size dependence of the Ag2S NPs positive Staphylococcus aureus [ACCN, KY635411] using the plate-
on the photosensitization of TiO2 NFs and demonstrate their count method. These strains were maintained in the LB broth
potential use in developing water remediation technologies and agar. Bacterial cultures were prepared by incubating their
ꢂ
using solar light.
strains inoculated on the LB nutrient agar at 37 C for 16–24 h
under shaking and aerobic conditions. Aer 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.,
Experimental
Material synthesis
Precursor NFs for TiO2 were obtained by electrospinning of were sterilized by autoclaving at 120 ꢂC for 30 min. Bacterial
a mixture of 1.5 g polyvinylpyrrolidone (PVP, Mw ¼ 1 300 000 g cultures were then centrifuged at 4000 rpm and washed with 0.9%
molꢀ1), 4 g titanium(IV) n-butoxide (TNBT, 97%), 10 g ethanol saline solution. Bacterial pellets were then re-suspended and
and 4 g acetic acid. An electric eld of 18 kV with a tip-to- diluted with normal saline to prepare their suspension having 0.2
collector distance of 18 cm was used. The collected precursor optical density (OD). The antibacterial activity was evaluated by
NF mat was le overnight for complete hydrolysis and then mixing 5 mg of the NFs with 5 ml of the bacterial suspension in the
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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.
sterilized ask. The initial bacterial concentration was 1 ꢃ 108 CFU nutrient agar plates and the number of colonies were counted aer
mlꢀ1 for E. coli and 3 ꢃ 108 CFU mlꢀ1 for S. aureus (CFU: colony incubating at 37 C for 24 h. An average number of the colonies
ꢂ
forming units). A 100 W xenon lamp positioned at 15 cm from the was obtained by repeating the above procedure three times for
reaction mixture was used for illumination under constant each NF sample. Bacterial suspension without NFs was also serially
shaking. The irradiance, measured at the center of the reaction diluted and incubated on agar plates to count the number of
mixture with a power meter and Si photodiode sensor, was esti- colonies in the control sample. The bactericidal efficiency was
mated to be ꢁ10 mW cmꢀ2. The sterilized nutrient agar solution calculated by the following equation.
was poured in the Petri plates and allowed to solidify. At regular
Ncontrol ꢀ Nsample
Bactericidal Efficiency ð%Þ ¼
ꢃ 100
intervals of illumination, 100 mL of the treated bacterial suspen-
sion was withdrawn and diluted serially with normal saline to
adjust the bacterial concentration (aliquots were diluted to 10ꢀ5
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 mL) was spread onto the
Ncontrol
where Ncontrol and Nsample 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.
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TiO2 is also conrmed from the observation of (101) and other
characteristic peaks. Fig. 2(b) shows the XPS survey scan which
conrms 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 re-
ported 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% aer deposition of 17 nm Ag2S NPs with
a shi 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.
The fabricated TiO2 and Ag2S/TiO2 NFs are expected to be non-
porous materials since no pore generating agent was used. This
was conrmed by the adsorption–desorption isotherm shown in
the Fig. S2(a)† which is typical for the multilayer formation in
non-porous materials. The specic surface area of pure TiO2 NFs
and 17 nm Ag2S/TiO2 NFs was 7.1 and 6.6 m2 gꢀ1, 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 negli-
gible. 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/Po > 0.8).
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 conrmed
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 distri-
bution, 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, respec-
tively, which also conrms 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 identied in the insets of
Fig. 1(e and f) were clearly observed along with all other char-
acteristics peaks for the monoclinic Ag2S. No Ag peak was
observed for any sample. Moreover, the formation of anatase
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 inuence 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
,
CrO42ꢀ, and Cr2O7 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 conse-
quently 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
2ꢀ
The size effect of the Ag2S NPs on the photocatalytic reduc-
tion of Cr(VI) under simulated solar light at pH 3 was further
investigated. The 17 nm Ag2S/TiO2 NFs showed 90% photo-
catalytic reduction of Cr(VI) with a pseudo-rst order rate
constant of 0.016 minꢀ1. In comparison, hollow spherical Ag–
Ag2S/TiO2 composite particles under visible light irradiation
were able to reduce only 36% Cr(VI) aer 8 h irradiation.40 This
illustrates the signicantly enhanced capacity of the TiO2 NFs,
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 Inter-
national Public License.
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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.
photosensitized with Ag2S, reported here to reduce Cr(VI). The
The bacterial decay in dark is an interesting observation and
smaller (11 nm) and larger (23 nm) sized Ag2S/TiO2 NFs is due to Ag2S only since TiO2 has negligible effect. Recently,
exhibited slower kinetics with pseudo-rst order rate constants uncapped Ag2S NPs showed antibacterial effect, calculated by
of 0.014 and 0.010 minꢀ1, respectively. This is in agreement the disc diffusion method, towards Gram-positive S. aureus and
with our ndings on the role of Ag2S NP size on the photo- Gram-negative E. coli, with recorded inhibition zones of 15.81
degradation kinetics of methylene blue (MB) dye. Fig. S3† shows and 10.34 mm, respectively.50 Similarly, Ag2S and Cu-doped
the enhanced visible-light harvesting capability, reduced Ag2S NPs, prepared by the simple co-precipitation method,
bandgaps, and diminished photoluminescence signal when showed antibacterial activity both in dark and visible-light
TiO2 NFs are photosensitized by Ag2S NPs. This is attributed to irradiation against E. faecalis and S. aureus.51 However, the
the efficient charge separation, charge transfer, and reduced mechanisms of the antibacterial activity for silver containing
electron–hole recombination. The UV–O3 surface treatment of compounds especially Ag2S are not yet clearly known due to very
Ag2S/TiO2 NFs generates excess Ti3+ states which further facili- limited work. Ag2S exhibits very low solubility and reactivity
tates the charge separation process46 (Fig. S3†) but the photo- and, thus, the mechanism for their cytotoxicity might be
reduction of Cr(VI) was hindered as shown by the reduction of different than the widely accepted mechanism reported for
the rate constant to 0.013 minꢀ1 for the 17 nm Ag2S/TiO2 NFs. ionic silver (Ag+) and Ag NPs.52 Wang et al. reported for the rst
As reported in the XPS results previously,46 some of the Ag2S get time the toxic effects of Ag2S NPs in plant tissues of cowpea
oxidized aer UV–O3 treatment and produces Ag2SO4 which is (Vigna unguiculata) and wheat (Triticum aestivum) by direct
partially soluble in water. Introduction of the sulfate ions in the uptake.53 Using K-edge X-ray absorption near edge structure
solution probably interferes with the sorption of the Cr(VI) over (XANES) and k2-weighted extended X-ray absorption ne struc-
the NFs and, thus, suppresses the overall photo-reduction ture (EXAFS) they showed that almost all the Ag in the roots is
process. Thus, excess Ti3+ was benecial for photo- stored as Ag2S and is not sourced from the dissolved Ag in the
degradation of methylene blue (MB) dye,46 but not for the solution. Pang et al. reported negligible inactivation efficiency
photo-reduction of Cr(VI) here. However, as shown below, the against E. coli in dark using Ag2S/Ag heterodimers probably due
excess Ti3+ chemical states enhances the antibacterial perfor- to much larger Ag size of 50–75 nm.39 Ag2S/Ag hetero-
mance of these materials.
nanostructures and Ag NPs showed minimum inhibitory
concentration (MIC) of 8 mg mlꢀ1 against E. coli but Ag2S NPs
alone showed MIC of 32 mg mlꢀ1 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 prom-
ising but warrants further work to investigate the mechanism of
the Ag2S cytotoxicity.
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
signicantly enhances the bactericidal efficiencies. Furthermore,
UV–O3 treatment of the 11 nm Ag2S/TiO2 NFs effectively inactivates
Photo-disinfection efficiency under simulated sunlight
almost all the bacteria. Smallest sized and spherical shaped Ag2S Next, the inactivation of bacteria by Ag2S/TiO2 NFs under
NPs loading showed superior antibacterial activities compared to simulated solar light illumination was studied. As shown in the
the larger and relatively irregular shaped Ag2S NPs. These results Fig. 4(c and d), for 11 nm Ag2S/TiO2 NFs, the bactericidal effi-
are in good agreement with the reported studies on the effect of ciency increased rapidly to 100% and 90.6% against E. coli and
size and shape of Ag NPs on the bacterial viabilities.43,49
S. aureus aer 1 h of irradiation, respectively, whereas, only 50%
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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.
E. coli and 41% S. aureus were found to be inactivated under 81% S. aureus being inactivated in just 10 min of the irradiation.
dark. UV–O3 treatment on the 11 nm Ag2S/TiO2 NFs signi- This is probably due to the synergistic effect of Ag+ ions, origi-
cantly enhanced the antibacterial activity with 89% E. coli and nating from the dissolution of the Ag2SO4 formed by UV–O3
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treatment. The viabilities of the bacteria signicantly decline diameter ꢁ0.5 mm) with an intact cell wall and interior. Fig. 5(b)
under the solar light irradiation compared to the dark condi- shows the signicant shape changes and rupturing of the cell
tions. Moreover, the initial disinfection effectiveness against E. wall aer exposure to simulated solar light in the presence of
coli is higher than S. aureus and is discussed below.
11 nm Ag2S/TiO2 NFs. The image contrast suggests that the
The Ag2S/TiO2 NFs in the present study, when irradiated outer membrane of the E. coli is disrupted at several locations in
under simulated sunlight, produces hydroxyl radicals (cOH) by the same cell with leakage of the intracellular components. In
the reaction of thꢀe positively charged hole (h+) with the a related study using Ag2S/Ag heterodimers, tested for bacterial
hydroxide ion (OH ).46 The hydroxyl radicals are known to disinfection using similar methods, it was shown by inductively
destruct the bacterial cell of Gram-positive and Gram-negative coupled plasma mass spectroscopy (ICP-MS) that the Ag+ ion
bacteria by breaking the covalent bonds in the peptidoglycan concentration in the E. coli suspensions was extremely low (ꢁ75
layer.55 Moreover, the cell wall in Gram-positive and Gram- parts per billion) and energy dispersive X-ray spectroscopy
negative is structurally distinct and can initially respond (EDX) conrmed no change in the molar ratios of Ag : S for the
differently to the toxic effects as seen in the Fig. 4(c and d) for catalysts before and aer the tests. These ndings conrmed
the bactericidal efficiency aer 10 min. Cell wall in Gram- that Ag+ ions played no part in the bactericidal processes.39 The
negative consists of
a membrane of lipopolysaccharide cytotoxic action is, thus, initiated by the cell wall damage in the
covered by a thin layer of peptidoglycan (ꢁfew nm). Cell wall in presence of Ag2S/TiO2 NFs followed by intracellular leakage.
Gram-positive consists of much thicker peptidoglycan layer This cell wall damage could be due to the presence of hydroxyl
(ꢁ10s of nm). Thus, the initial response at 10 min to the effect radicals generated by Ag2S/TiO2 NFs46 as well as due to the
of hydroxyl radicals and photo-disinfection of Ag2S/TiO2 NFs is disinfectant nature of the pristine Ag2S as reported here in dark.
slow in Gram-positive S. aureus versus Gram-negative E. coli. This later effect needs to be investigated mechanistically in the
However, aer 60 min both bacterial strains show similar follow-up studies.
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
Stability and the proposed photocatalytic mechanism
The stability of the UV–O3 treated Ag2S/TiO2 NFs for the pho-
tocatalytic inactivation of E. coli was investigated for ve cycles
and the results are shown in the Fig. 6(a). Aer each cycle, the
walls of most bacterial strains are known to possess negative
sample was washed with ethanol and subsequently with normal
surface charge due to the teichoic acids in Gram-positive and
saline to decompose and remove all the captured bacteria. As it
lipopolysaccharides & lipoproteins in Gram-negative bacteria.30
can be seen, the bactericidal efficiency of the bacteria slightly
decreased initially probably due to the initial material loss
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 re-
during handling. Nevertheless, it becomes relatively stable aer
ported to be less than E. coli (0.129 coulomb/109 CFU) with
the second cycle.
The proposed photocatalytic mechanism in Ag2S/TiO2 NFs for
corresponding electrostatically adhered TiO2 amount of 0.61
the reduction of Cr(VI) can be explained in the Fig. 6(b). Owing to
and 1.88 g/109 CFU, respectively. These observations are in good
the small bandgap of Ag2S (Eg ꢁ 1 eV), the valence band (VB)
agreement with the initially higher antibacterial activity against
E. coli versus S. aureus reported here.
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
eꢀ–h+ recombination. This effective charge separation facilitate
Morphological changes in E. coli cell wall
the transfer of photo-generated electrons to Cr(VI) to reduce it to
The morphological changes in the cell wall of E. coli treated with
Cr(III), meanwhile the holes oxidize water and produce oxygen.57
Ag2S/TiO2 NFs under simulated sunlight was observed in SEM
The reactions can be presented as:
(operated under STEM mode). Fig. 5(a) shows the characteristic
rod-shaped morphology of the untreated E. coli (length ꢁ2 mm,
Ag2S/TiO2 + hv / Ag2S/TiO2 + eꢀ + h+
2ꢀ
Cr2O7 + 14H+ + 6eꢀ / 2Cr3+ + 7H2O
2H2O + 4h+ / O2 + 4H+
Similarly, the underlying mechanism for the photocatalytic
inactivation of bacteria aer treatment with Ag2S/TiO2 NFs under
irradiation can be explained. In addition, the Ti3+ states intro-
duced aer 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 (O2ꢀc) and hydroxide (cOH) radicals. These ROS diffuse
Fig. 5 The STEM images of the (a) untreated rod-shaped E. coli
bacteria with a length of ꢁ2 mm and width of ꢁ0.5 mm, and the cor-
responding (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.
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Fig. 6 (a) Bactericidal efficiency against E. coli (108 CFU mlꢀ1) 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.
in the cell wall of bacteria and react with the cell membrane and
References
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benecial for the reduction of Cr(VI), further accelerate the
bactericidal activity due to the synergistic effect from Ag+.
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Acknowledgements
We acknowledge Dr Murtaza Saleem, Sr Research Scientist in
the Central Labs of the Syed Babar Ali School of Science and
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