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Highly stable CdS quantum dots embedded in glasses and its application for inhibition of bacterial colonies
Optical Materials 99 (2020) 109590
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Highly stable CdS quantum dots embedded in glasses and its application for inhibition of bacterial colonies S.R. Munishwar a, P.P. Pawar b, S.Y. Janbandhu a, R.S. Gedam a, * a b
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, 440 010, India Department of Physics, Sanjay Ghodavat University, Kolhapur, 416118, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Glass HRTEM Optical study S. aureus E. coli Antimicrobial study
An inhibition of pathogenic bacteria using light source is an attractive phenomenon. The present study is based on the application of CdS quantum dots (QDs) grown in the glass matrix for growth inhibition of bacteria. Glass system 36SiO2–15B2O3–15Na2O–9K2O–25ZnO with 3 wt % CdS was prepared by the melt-quench technique. CdS QDs were grown in glass matrix with single-step heat treatment and characterized by XRD, HRTEM, XPS, and micro-Raman for phase identification. Their size related to quantum confinement properties were studied with optical absorption (UV–Vis), photoluminescence (PL) spectroscopy. The structural and optical studies confirm the growth of CdS QDs in a glass matrix which was then utilized for antimicrobial activity. The generation and recombination of electron-hole pairs by CdS QDs due to absorption of light were responsible for the formation of reactive oxygen species (ROS). The use of these ROS in bacterial colonies inhibition has been discussed.
1. Introduction Nowadays, building clean energy sources are essential to face energy crisis and environmental problems because of modern lifestyle and fastgrowing advanced technology. A large number of experiments and research have been carried out to protect our planet from this energy crisis by recycling natural resources (i.e. industrial water or air) into the hygienic form. The industries such as chemical synthesis, textiles, de tergents, pharmaceutics as well as pesticides and herbicides synthesis, etc. discharge effluent which contains organic compounds that can not be eliminated by simple filtration. These organic pollutants may reach soil or water and create major health issues for terrestrial life as well as an aquatic biota [1]. Therefore, water treatment becomes essential for the degradation of pollutants [2] and the inhibition of pathogens present in the industrial wastewater before discharge to the environment [3]. Present water treatment methods like coagulation and adsorption are not effective for complete eradication of these organic pollutants, whereas sedimentation, reverse osmosis, ultrafiltration, membrane technology, etc. are expensive and generate harmful secondary pollut ants to the aquatic biota [4–7]. These problems require an immediate technique that must be efficient at a lower cost. However, advanced oxidative processes (AOPs) such as semiconductor photocatalysis and photochemical methods are very popular because they are low-cost and
eco-friendly methods [5,8]. These processes generate reactive oxygen species (ROS) such as H2O2, O2�-, OH� radicals which promote miner alization of organic pollutants and also inactive pathogens [9,10]. Semiconductors catalysts like CdS, ZnS, ZnO, TiO2, Fe2O3, and GaP are excellent in greenest technology [5]. These semiconductors become more effective when electron-hole pairs generated in visible light and get transferred to the respective reaction sites before their recombina tion [11]. When the size of the semiconductor particles is at the nano scale, the surface area-to-volume ratio increases than their bulk counterparts which increases available surface active sites and charge carrier transfer. However, the reduction of particle size from bulk to nanometer-scale reduces electron-hole recombination. With decrease in particle size, the bandgap energy and photon absorption on the surface of photocatalyst increase. Thus, nano-particles or quantum dots (QDs) expected to be highly active for photocatalysis than bulk semiconductors [12,13]. Along with photocatalysis, advanced oxidation also helpful in antibacterial activities. The detailed study of quaternized silicon nano particles (SiNPs) which are prepared by one-step reaction has been done for selectivity imaging and the inhibition of Gram-positive bacteria. They claimed that this material will be good replacement for antibiotics [14]. Semiconductors like TiO2 and ZnO have been successfully studied already as an antimicrobial agent due to their photocatalytic activity under light [10,15,16]. Some researchers have reported photostable,
* Corresponding author. E-mail address: [email protected] (R.S. Gedam). https://doi.org/10.1016/j.optmat.2019.109590 Received 6 November 2019; Received in revised form 28 November 2019; Accepted 28 November 2019 Available online 5 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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high brighten and having good water soluble carbon dots (CDs) are useful for antibacterial activity, particularly for inactivation of Gram-positive bacteria [17,18]. These materials also have the ability to inhibit or kill the microbes present in the water along with degradation of pollutant which helps to build up the green technology. Using such types of materials one can provide clean and safe drinking water to society. Recently, we have observed that photonic absorption can be tuned with size of CdS QDs in glass matrix by controlled heat treatment [19, 20] and also it was noticed that PL intensity depends on size of CdS QDs which transfer their energy to nearby energy levels of other ions present in the glass matrix during recombination [21]. CdS is active material having bandgap 2.42 eV and have an excellent response for photo catalysis in the visible region. Unfortunately, the electron-hole pair separation efficiency of pure CdS is low and it can easily photocorrode in aqueous media containing oxygen when exposed to visible light which is not helpful for solving environmental problems. The performance of CdS can be improved by dispersing it on another material or embedding CdS particles in mesoporous materials and it is observed that by embedding CdS QDs in the glass matrix, their stability is improved, and photo corrosion rate has been suppressed [2,8,22,23]. In this paper, we have discussed the inhibition ability of CdS QDs grown in the glass matrix for Gram-negative and Gram-positive bacteria. This is the first study that reports the antimicrobial activity of CdS QDs grown in the glass matrix.
wavelength 456 nm. Specific surface area (BET) was obtained from the nitrogen adsorption/desorption isotherm at 77 K using Quantachrome Autosorb Automated Gas Sorption System. The glass sample with grown CdS QDs was finally ground into the fine powder to study the antimi crobial activity against certain bacteria.
2. Experimental
2.2.2. Minimum inhibitory concentration (MIC) of CdS QDs The antimicrobial activity of CdS QDs grown in a glass matrix was investigated against Gram-positive bacterium, S. aureus and the Gramnegative bacterium, E. coli. The media used for this study is nutrient broth. CdS QDs embedded in a glass matrix is insoluble in aqueous media, therefore, broth dissolution could be an effective method to study their antibacterial activity. The nutrient broth was prepared by dissolving the standard amount of dehydrated nutrient broth in distilled water. It was further distributed into 5 conical flasks (20 ml each) and sterilized by autoclaving at 15 lbs pressure and 121 � C temperature for 15 min. The media was further cooled and inoculated with respective bacterial cultures. The inoculated cultures were then incubated at 37 � C for 24 on orbital shaker incubator at 120 rpm for uniform growth. The glass powder containing CdS QDs added to these conical flasks in different concentrations (i.e. 5, 10 15, 20 μg/ml) in order to check their antimicrobial activity against present bacterial culture in light. Then these flasks were further incubated at 37 � C for 24 h in light conditions. After 24 h these cultures were subjected to serial dilution using sterile distilled water up to appropriate dilution. Serial dilutions are used to reduce a dense culture of cells to measurable format [15]. This dilution was spread over the surface of the petri dish consists of nutrient agar medium and incubated at 37 � C from 24 to 48 h in an incubator and growth of the bacterial colonies was recorded in terms of colony-forming units (CFU) and minimum inhibitory concentration (MIC) was deter mined. The minimum inhibitory concentration (MIC) of CdS QDs is the lowest concentration of an antimicrobial agent that inhibits the growth of microorganisms [24].
2.2. Antimicrobial study 2.2.1. Bacterial culture and structural representation The bacterial cultures Staphylococcus aureus (S. aureus) and Escher ichia coli (E. coli) were used in this study to check the antibacterial ac tivity of CdS QDs. Depending upon cell wall S. aureus and E. coli are classified as Gram-positive and Gram-negative respectively. The cell wall covers the cell membrane and provides rigidity, strength, and shape, and also protects the cell from rupture and mechanical damage. The cell structure of Gram-negative bacteria consists of three layers namely cytoplasmic membrane, a thin peptidoglycan layer, and outer membrane. The outer membrane of the cell wall consists of lipopoly saccharides, phospholipid, porin, and Protein. This outer membrane often shows resistance to hydrophobic compounds including detergents and lipopolysaccharides increase negative charge which is essential for the structural integrity of bacteria. In Gram-positive bacteria outer membrane is absent, instead of the outer membrane, it has thick peptidoglycan layers in the outer region. The systematic construction of Gram-negative (E. coli) and Gram-positive (S. aureus) is shown in Fig. 1.
2.1. Materials The glass sample with general formula 36SiO2–15B2O3–15Na2O–9K2O–25ZnO with 3 wt % CdS was prepared by the melt-quench technique. All chemicals used for this synthesis were of 99.99% purity. These chemicals were weighed and mixed in agate mortar and pestle for several hours in order to achieve homogeneity of chemicals. These mixed chemicals then kept in a platinum crucible and melted in a furnace at 1100 � C for 2 h. The melt formed in the platinum crucible was quenched into the meter-sized pieces at room temperature using an aluminum mould. To reduce the thermal stress these quenched glass pieces were transferred to the annealing furnace which was maintained at 350 � C. Theses glasses were annealed for 3 h which pre vent glass samples from cracking and then cooled to the room temper ature to get transparent crack free glasses. The thermal study of these glasses was carried out with Differential Thermal Analysis (DTA) using HITACHI STA 7200. Glass powder put in � a platinum pan then heated up to 700 � C with a heating rate of 5 C/min under continuous nitrogen flow and thermal transition in material with temperature were observed. Here, the empty platinum pan was used as reference material to study the thermal changes which encounter during heating. These glass samples then heat-treated for the growth of CdS QDs in the glass matrix. The growth of these semiconductor QDs was further confirmed with an X-ray diffraction (XRD) pattern measured with PAnalytical X’Pert Pro. Raman measurement of these glass samples was carried out using Jobin Yvon Horibra LABRAM-HR visible (400–1100 nm). Arþ laser operating at 488.0 nm was used as a laser source to excite the glass samples in powder form at room temperature. The size of CdS QDs, SAED pattern and EDAX pattern was determined by Field Emission Gun Transmission Electron Microscope (FEG-TEM JEOL JEM-2100F). X-ray photoelectron spectroscopy (XPS) data were ob tained from PHI 5000 Versa Probe II, FEI Inc. Optical absorption spectra and photoluminescence study of the sample were carried out using JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP8200 respectively. Fluorescence lifetime measurement of heat-treated glass sample was performed at room temperature with PTI TimeResolved Spectrofluorimeter Pico Master (PTI, HORIBA, USA) using a light-emitting diode (LED) as excitation source with excitation
3. Result and discussion 3.1. Characterization of CdS quantum dots Fig. 2 shows the DTA curve of transparent and colorless glass which were obtained after melt quench. For the given glass sample the glass transition temperature (Tg) was found to be 480 � C (inflection point). Next to this feature a sharp exothermic peak that arises around 615 � C which corresponds to glass crystallization temperature (Tc). These values from the DTA curve used to give proper heat treatment temper ature in order to grow quantum dots in a glass matrix. The glass samples were heat-treated at 500 � C for different time duration which is 2
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Fig. 1. Structural representation of Gram-negative and Gram-positive bacterial membrane.
reported in our published article [19]. The size of grown CdS QDs was further confirmed from HRTEM. The particle size of glass sample G3 was further confirmed from HRTEM. Fig. 4 shows a highly resolved image of grown CdS QDs in the glass matrix. These dots are almost spherical in shape (Fig. 4a) which indicates the isotropic growth of semiconductor quantum dots in the glassy environment. Fig. 4b shows a uniform distribution of these nanoparticles throughout the glass matrix and because of this distribu tion, the glass appears yellow in color (Fig. 3). The histogram inset of Fig. 4b shows CdS QDs grown in the glass matrix is in the range of 0.5–6.5 nm. The average size of these QDs confirmed from Fig. 4b and it is around 3.85 nm (i.e radius R � 1.92 nm) which is comparatively very small than the Bohr exciton radius (aB ¼ 2.8 nm) [25]. The ratio R/aB is very much less than one therefore high quantum confinement effect is possible. Fig. 4c shows the cluster of CdS QDs with well-resolved lattice fringes having a spacing of 0.245 nm which indicates (102) plane. SAED pattern (Fig. 4d) shows seven diffused rings which demonstrate the amorphous nature of the material. These rings are assigned to (101), (102), (110), (202), (203) and (215) diffraction planes of CdS. Planes of the SAED pattern shows consistency with the reference code of the XRD diffraction pattern [19]. Fig. 5 shows the EDAX pattern of glass sample which gives chemical composition present in the glass matrix. EDAX plot shows the presence of C, O, Na, Si, S, K, Zn and Cd. Peaks for all chemical compounds appear in EDAX pattern which has been used during synthesis except boron because of its low atomic mass. The extra peak for Cu appears in the EDAX pattern due to the use of the carbon-coated copper grid for mounting the sample during TEM/EDAX characterizations [26]. The stoichiometry of the glass matrix was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis. This is a very sensitive method to inspect the precursors used in the given matrix except for hydrogen (H) and helium (He). Fig. 6 depicts the XPS spectra of the glass sample which shows peak positions along with their binding energy. The peak position and their binding energy are depicted in Table 2. From this figure, it is observed, that the peaks of carbon (C) and oxygen (O) are also observed in the given spectra due to absorbed gaseous from the atmosphere which is also evident in EDX spectra [27–29]. From the XPS spectra, it is also observed that Cd2þ and S2 are present in different oxidation states in the glass matrix. The spectrum of pure CdS shows peaks at 405.1 and 411.8 eV corresponding to Cd 3d5/2 and Cd 3d3/2 spin-orbit components of Cd 3d respectively. These peak positions correspond to the 2þ oxidation state of Cd in CdS. Binding energy values of S 2p1/2 peak of CdS is 163.6 eV and correspond to the S2 the state of S in CdS [30–34]. Absorption spectra (Fig. 7a) shows a redshift in the absorption band edge position due to the increase in size of CdS QDs quantum confine ment effect. The effective bandgap of the glass samples has been
Fig. 2. Differential thermal analysis curve of glass sample.
responsible for nucleation and growth of CdS quantum dots in a matrix. The heat-treatment schedule along with the sample name is depicted in Table 1 and the images of glass samples can be observed in Fig. 3. It is clearly noticed from the glass images that, a prepared glass sample is transparent and then changed into yellow upon the heat treatment which indicates the growth of CdS QDs in a glass matrix. X-ray diffraction pattern (XRD) pattern of glass samples G1, G2, G3, G4, and G5 shows diffraction peaks corresponding to (101), (102), (110) and (103) planes (reference code 01-075-1545) at 2θ positions around 28.64� , 36.91� , 43.71� and 47.18� respectively. This data already dis cussed in our previous reported article [19]. The size of the grown crystallite (2.73–5.17 nm) of CdS was calculated using the Scherrer formula also discussed [19]. The observed increase in electron-phonon interaction with the growth of crystallinity in the glass matrix due to the heat-treatment process is studied using Raman measurement and Table 1 Heat treatment schedule for all glasses. Glass ID
Annealing schedule
G1 G2 G3 G4 G5
As made 500 � C for 500 � C for 500 � C for 500 � C for
10 h 35 h 60 h 80 h
3
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Fig. 3. Images of glass samples showing growth of CdS QDs in glass.
Fig. 4. HR-TEM images of glass samples (a) growth of CdS QDs (b) size distribution of CdS QDs (c) Lattice scale fringes of CdS QDs (d) Selected area electron diffraction (SAED) pattern of CdS QDs in glass matrix.
calculated by extrapolating line at the band edge and depicted in Table 3 [22]. It is observed from this Table that energy band gap decreases with an increase in size of QDs which is the result of heat treatment schedule. Photoluminescence (PL) spectra have been performed to reveal the charge transfer, migration and recombination processes of the CdS QDs. Photoluminescence spectra (Fig. 7b) shows the emission due to compositional impurities or lattice defects, band-edge transitions, and trap states when excited at 345 nm. It is observed that the emission peak observed at 413 nm is common for all glass samples which appear due to the compositional impurities or lattice defects and other emission peaks appear with heat treatment i.e. growth of CdS QDs in the glass matrix [35]. For glass G2, emission peaks appear at 436 and 556.5 nm because of band-edge transitions and trap state emission. The emission peaks arise due to band edge transition and trap states show redshift with an increase in the size of CdS QDs for other glasses (i.e. G3, G4, and G5) [20,
36]. It is observed from Fig. 7b and Table 3 that, the band edge and trap state emissions of all heat-treated glass samples shift towards higher wavelength due to an increase in the size of CdS QDs. The photo luminescence study gives an idea of photogenerated charge carriers in a semiconductor on excitation. It is reported that lower PL signal leads to high photocatalytic activity due to higher photo-induced electron-hole pairs separation efficiency [19,37,38]. It is also reported that particles with reduced size have a large surface area, which increases the available surface active sites. If the particle size is very less (i.e. in nanometer scale), electron-hole pairs generated are sufficiently close to the surface. These particles undergo rapid sur face recombination mainly due to abundant surface trapping sites and the lack of driving force for electron-hole pair separation. Therefore, QDs with optimal size were selected for photocatalytic activity [12]. 4
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Fig. 5. EDAX pattern of glass sample.
dots with low luminescence decay and high recombination rate are useful in photocatalytic as well as antimicrobial activity. The N2 adsorption isotherm and pore-size distribution of CdS/glass composites are shown in Fig. 9. As shown in Fig. 9a, the sample is mesoporous in nature which can adsorb sufficiently larger molecules on its surface. The desorption curve with a sufficiently large surface is beneficial for gas adsorption and storage. Thus, CdS QDs embedded in a glass matrix can effectively capture a molecule larger molecule from aqueous solution. Since the pore size distribution curve (Fig. 9b) shows a peak around 2.48 nm. The large surface area derived from the mono dispersed mesopore Glass/CdS nanospheres and unique Glass/CdS het erojunctions are considered to be major contributions to supplying abundant active sites and separating photogenerated carriers, respec tively [41,42]. Table 4 shows the parameters obtained from the BET study. 3.2. Antimicrobial activity Fig. 10 shows the antibacterial activities of the CdS QDs against S. aureus and E. coli in light. It is observed that the number of colonies was dropped with the increase in the concentration of CdS QDs. From the experimental observations, the obtained value of the antimicrobial activity of CdS QDs found to inhibit 50% of that of CdS QDs free control. This is called MIC 50% or MIC50. MIC50 of experimental bacteria was evaluated by taking the regression curve of bacterial inhibition with respect to CdS QDs free control solution. The ranges of minimum inhibitory concentration (MIC) values obtained in broth media for E. coli and S. aureus to inhibit 50% of isolates (MIC50) are 88.07 and 61.36 μg/ ml respectively. The results show less MIC50 values obtained for Grampositive bacteria than Gram-negative bacteria. Since it is difficult to target Gram-negative bacteria to most of the antimicrobial agent, it has more intrinsic resistance to most antibiotics [43]. In this process, CdS QDs attached to the membrane of bacteria by electrostatic interaction and disrupt the integrity of the bacterial cell membrane and generate nanotoxicity by introducing free radicals of oxygen due to their photocatalytic reaction [44]. The CdS QDs combined with bacterial culture absorb photons of higher energy than the bandgap of semiconductor to form an exciton. Exciton is formed due to the movement of an electron from the valence bond (VB) to the conduction band (CB) of CdS QDs. These excitons undergo redox reaction to form reactive oxygen species (ROS). However, CdS QDs have good efficiency to generate reactive species like hydroxyl (OH�), superoxide (O�2 ) rad icals and hydrogen peroxide (H2O2) which is responsible for killing or ganisms including bacteria endospores in water, in air and on surfaces of various materials [45]. It is reported that bacterial inhibition and the generation of H2O2 increased with a decrease in particle size [46]. The generation of ROS in surface CdS QDs is very similar to photocatalytic
Fig. 6. XPS spectra of heat treated glass sample. Table 2 The attribution of peaks in XPS spectra along with their peak positions in terms of binding energy and their respective oxidation states. Peak number
BE/eV
Element
Peak number
BE/eV
Element
1 2 3 4 5 6 7 8 9
11.5 64.0 91.4 99.8 109.4 163.6 188.0 230.9 284.7
Cd 4d3/2 Cd 4p Zn 3p1/2 Si 2p1/2 Cd 4s S 2p1/2 B 1s S 2s C 1s
10 11 12 13 14 15 16 17 18
378.6 405.1 411.8 532.4 618.2 652.3 1021.6 1044.9 1070.8
K 2s Cd 3d5/2 Cd 3d3/2 O 1s Cd 3p3/2 Cd 3p1/2 Zn 2p3/2 Zn 2p1/2 Na 1s
Glass sample G3 has minimum PL intensity (Fig. 7b) and has an optimal size which is used for photocatalytic activity. Time-resolved fluorescence decay measured in the nanosecond range shown in Fig. 8. The sample was excited with 456 nm and fluorescence decay was recorded at 570 nm. The lifetime values obtained by fitting the curve was done by convolving the instrument response function (IRF) is 0.34 ns The amorphous matrix environment used for the growth of semiconductor quantum dots has various types of defects and dislo cation which can act as charge carrier recombination center with small carrier lifetime and high surface recombination velocity [39,40]. Thus, QDs grown in a glass matrix have a large surface area to volume ratio which can absorb and emit light in the visible region. These quantum 5
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Fig. 7. (a) Absorption spectra and (b) PL spectra of all glasses.
mechanism and explain it by following reactions:
Table 3 Optical parameters for all glasses.
a) CdS QDs þ hν (E � Eg) → CdS QDs (hþ) þ CdS QDs (e )
Glass ID
Band gap (eV)
PL peaks λe (nm)
G1 G2 G3 G4 G5
3.7 2.71 2.54 2.5 2.43
413 413, 436, 556.5 413, 470, 570 413, 481.5, 576 413, 496, 597
b) O2þ e → O�2 c) hþþH2O → OH� þ Hþ d) O�2 þ Hþ → HO�2 e) HO�2 þ Hþ þ e → H2O2 In this reaction, photo-generated electrons (e ) of CdS QDs reacts with O2 to give O�2 radicals and holes (hþ) react with H2O and separates them into OH� and Hþ. Then, the separated Hþ further reacts with O�2 to generate HO�2. Finally, HO�2 again collides with Hþ and electron to give hydrogen peroxide (H2O2). From the generated species, hydroxyl and superoxide radicals are negatively charged and unable to penetrate the cell membrane and stay on the outer surface of the bacteria while H2O2 can penetrate the cell which inhibits bacterial growth. For Gram-negative bacteria, all reactive species (i.e. hþ, OH�, O�2 , H2O2) produced by CdS QDs due to irradiation of light initially were blocked by the outer cell wall of bacteria. How ever, an increase in the concentration of CdS QDs increases the pro duction of H2O2 and therefore the inhibition rate. In Gram-positive bacteria, the outer cell membrane is totally absent and it is replaced by multilayered peptidoglycan layers which are directly affected by H2O2 generated by CdS QDs. Therefore, Gram-positive bacteria show good inhibition initially but it does not increase with an increase in CdS QDs concentration. In this study, CdS QDs embedded in the glass matrix take active participation in the formation of ROS and cannot react or got adsorbed with other compounds or species. Therefore, the formation of ROS due
Fig. 8. Fluorescence decay curve of heat treated glass.
Fig. 9. (a) BET adsorption isotherm curve of glass powder at 77 K (b) pore size distributions of glass powder containing CdS QDs. 6
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Table 4 Porosity characteristic of glass powder containing CdS QDs. Sample
Sample weight (g)
Desorption Surface Area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3/g)
Hysteresis type
Glass powder
0.023901
271.7
2.484
0.2412
IV
Fig. 10. Antibacterial activity of CdS QDs against (a) S. aureus and (b) E. coli.
to photocatalytic activity under light is helpful in the degradation of organic dye dissolved in water as well as in the inhibition of microorganisms.
publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder.
4. Conclusions The glasses synthesized by the melt-quench technique and heattreated for the growth of CdS QDs in the glass matrix. The growth of the CdS phases was confirmed with XRD and micro-Raman spectra. The average diameter of CdS QDs obtained from HRTEM was 2.47 nm. The stoichiometry of precursors present in the system was confirmed by EDAX and XPS. Blueshift in bandgap from the bulk of the given material confirms the quantum confinement effect in the sample. PL spectra emission peak at 455 nm and 561 nm arise because of band edge and trap assisted electron-hole recombination respectively. The average lifetime of electron-hole recombination was 0.34 ns H2O2 generated from CdS QDs under light also plays an important role in antimicrobial activity. CdS QDs show more inhibition to S. aureus than E. coli and inhibit 50% of isolates (MIC50) are 61.36 and 88.07 μg/ml respectively. The minimum inhibitory concentration (MIC) values obtained in broth media for E. coli (Gram-negative) and S. aureus (Gram-positive) to inhibit 50% of isolates (MIC50) are 88.07 and 61.36 μg/ml respectively. Thus, the synthesized glasses plays an important role as an antimi crobial agent to inhibit the growth of pathogens. These semiconductor QDs embedded in the glass matrix can be a good and safe replacement for the existing antimicrobial materials.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We are very much thankful to the Department of Science and Technology (SERB-DST) for financial support for scientific research. We would also like to express our sincere, hearty thanks to Dr. V. G. Sathe, UGC-DAE-CSR, Indore and Ms. A. U. Pant, SAIF, IIT Bombay for providing a characterization facility for this work. We are very much thankful to Dr. (Mrs.) A. S. Shanware, Rajiv Gandhi Biotechnology Centre, RTMNU, Nagpur for providing facility to study antimicrobial activity. References [1] D.P. Sahoo, D. Rath, B. Nanda, K.M. Parida, Transition metal/metal oxide modified MCM-41 for pollutant degradation and hydrogen energy production: a review Dipti, RSC Adv. 5 (2015) 83707–83724, https://doi.org/10.1039/C5RA14555D. [2] S.Y. Janbandhu, S.R. Munishwar, R.S. Gedam, Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses, Appl. Surf. Sci. 449 (2018), https://doi.org/10.1016/j. apsusc.2018.02.065. [3] S. Rana, J. Rawat, R.D.K. Misra, Anti-microbial active composite nanoparticles with magnetic core and photocatalytic shell: TiO2–NiFe2O4 biomaterial system, Acta Biomater. 1 (2005) 691–703. [4] R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Effect of morphology of zinc oxide in ZnO-CdS-Ag ternary nanocomposite towards photocatalytic inactivation of E. coli under UV and visible light, Chem. Eng. J. 307 (2017) 966–980. [5] S.K. Sharma, R. Sanghi, Advances in Water Treatment and Pollution Prevention, Springer Science & Business Media, 2012. [6] D. Beydoun, R. Amal, G. Low, S. McEvoy, Role of nanoparticles in photocatalysis, J. Nanoparticle Res. 1 (1999) 439–458. [7] S.Y. Janbandhu, A. Joshi, S.R. Munishwar, R.S. Gedam, CdS/TiO2 heterojunction in glass matrix: synthesis, characterization, and application as an improved photocatalyst, Appl. Surf. Sci. (2019), 143758, https://doi.org/10.1016/j. apsusc.2019.143758.
Author contributions section 1. The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors. 2. Role of Authors: Writing - Review & Editing – Dr. S. R. Munishwar. Project administration – Dr. P. P. Pawar. Investigation – Mr. S. Y. Janbandhu. Supervision Dr. R. S. Gedam. Author agreement Work described in this paper has not been published previously, that it is not under consideration for publication elsewhere, that its 7
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Highly stable CdS quantum dots embedded in glasses and its application for inhibition of bacterial colonies S.R. Munishwar a, P.P. Pawar b, S.Y. Janbandhu a, R.S. Gedam a, * a b
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, 440 010, India Department of Physics, Sanjay Ghodavat University, Kolhapur, 416118, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Glass HRTEM Optical study S. aureus E. coli Antimicrobial study
An inhibition of pathogenic bacteria using light source is an attractive phenomenon. The present study is based on the application of CdS quantum dots (QDs) grown in the glass matrix for growth inhibition of bacteria. Glass system 36SiO2–15B2O3–15Na2O–9K2O–25ZnO with 3 wt % CdS was prepared by the melt-quench technique. CdS QDs were grown in glass matrix with single-step heat treatment and characterized by XRD, HRTEM, XPS, and micro-Raman for phase identification. Their size related to quantum confinement properties were studied with optical absorption (UV–Vis), photoluminescence (PL) spectroscopy. The structural and optical studies confirm the growth of CdS QDs in a glass matrix which was then utilized for antimicrobial activity. The generation and recombination of electron-hole pairs by CdS QDs due to absorption of light were responsible for the formation of reactive oxygen species (ROS). The use of these ROS in bacterial colonies inhibition has been discussed.
1. Introduction Nowadays, building clean energy sources are essential to face energy crisis and environmental problems because of modern lifestyle and fastgrowing advanced technology. A large number of experiments and research have been carried out to protect our planet from this energy crisis by recycling natural resources (i.e. industrial water or air) into the hygienic form. The industries such as chemical synthesis, textiles, de tergents, pharmaceutics as well as pesticides and herbicides synthesis, etc. discharge effluent which contains organic compounds that can not be eliminated by simple filtration. These organic pollutants may reach soil or water and create major health issues for terrestrial life as well as an aquatic biota [1]. Therefore, water treatment becomes essential for the degradation of pollutants [2] and the inhibition of pathogens present in the industrial wastewater before discharge to the environment [3]. Present water treatment methods like coagulation and adsorption are not effective for complete eradication of these organic pollutants, whereas sedimentation, reverse osmosis, ultrafiltration, membrane technology, etc. are expensive and generate harmful secondary pollut ants to the aquatic biota [4–7]. These problems require an immediate technique that must be efficient at a lower cost. However, advanced oxidative processes (AOPs) such as semiconductor photocatalysis and photochemical methods are very popular because they are low-cost and
eco-friendly methods [5,8]. These processes generate reactive oxygen species (ROS) such as H2O2, O2�-, OH� radicals which promote miner alization of organic pollutants and also inactive pathogens [9,10]. Semiconductors catalysts like CdS, ZnS, ZnO, TiO2, Fe2O3, and GaP are excellent in greenest technology [5]. These semiconductors become more effective when electron-hole pairs generated in visible light and get transferred to the respective reaction sites before their recombina tion [11]. When the size of the semiconductor particles is at the nano scale, the surface area-to-volume ratio increases than their bulk counterparts which increases available surface active sites and charge carrier transfer. However, the reduction of particle size from bulk to nanometer-scale reduces electron-hole recombination. With decrease in particle size, the bandgap energy and photon absorption on the surface of photocatalyst increase. Thus, nano-particles or quantum dots (QDs) expected to be highly active for photocatalysis than bulk semiconductors [12,13]. Along with photocatalysis, advanced oxidation also helpful in antibacterial activities. The detailed study of quaternized silicon nano particles (SiNPs) which are prepared by one-step reaction has been done for selectivity imaging and the inhibition of Gram-positive bacteria. They claimed that this material will be good replacement for antibiotics [14]. Semiconductors like TiO2 and ZnO have been successfully studied already as an antimicrobial agent due to their photocatalytic activity under light [10,15,16]. Some researchers have reported photostable,
* Corresponding author. E-mail address: [email protected] (R.S. Gedam). https://doi.org/10.1016/j.optmat.2019.109590 Received 6 November 2019; Received in revised form 28 November 2019; Accepted 28 November 2019 Available online 5 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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high brighten and having good water soluble carbon dots (CDs) are useful for antibacterial activity, particularly for inactivation of Gram-positive bacteria [17,18]. These materials also have the ability to inhibit or kill the microbes present in the water along with degradation of pollutant which helps to build up the green technology. Using such types of materials one can provide clean and safe drinking water to society. Recently, we have observed that photonic absorption can be tuned with size of CdS QDs in glass matrix by controlled heat treatment [19, 20] and also it was noticed that PL intensity depends on size of CdS QDs which transfer their energy to nearby energy levels of other ions present in the glass matrix during recombination [21]. CdS is active material having bandgap 2.42 eV and have an excellent response for photo catalysis in the visible region. Unfortunately, the electron-hole pair separation efficiency of pure CdS is low and it can easily photocorrode in aqueous media containing oxygen when exposed to visible light which is not helpful for solving environmental problems. The performance of CdS can be improved by dispersing it on another material or embedding CdS particles in mesoporous materials and it is observed that by embedding CdS QDs in the glass matrix, their stability is improved, and photo corrosion rate has been suppressed [2,8,22,23]. In this paper, we have discussed the inhibition ability of CdS QDs grown in the glass matrix for Gram-negative and Gram-positive bacteria. This is the first study that reports the antimicrobial activity of CdS QDs grown in the glass matrix.
wavelength 456 nm. Specific surface area (BET) was obtained from the nitrogen adsorption/desorption isotherm at 77 K using Quantachrome Autosorb Automated Gas Sorption System. The glass sample with grown CdS QDs was finally ground into the fine powder to study the antimi crobial activity against certain bacteria.
2. Experimental
2.2.2. Minimum inhibitory concentration (MIC) of CdS QDs The antimicrobial activity of CdS QDs grown in a glass matrix was investigated against Gram-positive bacterium, S. aureus and the Gramnegative bacterium, E. coli. The media used for this study is nutrient broth. CdS QDs embedded in a glass matrix is insoluble in aqueous media, therefore, broth dissolution could be an effective method to study their antibacterial activity. The nutrient broth was prepared by dissolving the standard amount of dehydrated nutrient broth in distilled water. It was further distributed into 5 conical flasks (20 ml each) and sterilized by autoclaving at 15 lbs pressure and 121 � C temperature for 15 min. The media was further cooled and inoculated with respective bacterial cultures. The inoculated cultures were then incubated at 37 � C for 24 on orbital shaker incubator at 120 rpm for uniform growth. The glass powder containing CdS QDs added to these conical flasks in different concentrations (i.e. 5, 10 15, 20 μg/ml) in order to check their antimicrobial activity against present bacterial culture in light. Then these flasks were further incubated at 37 � C for 24 h in light conditions. After 24 h these cultures were subjected to serial dilution using sterile distilled water up to appropriate dilution. Serial dilutions are used to reduce a dense culture of cells to measurable format [15]. This dilution was spread over the surface of the petri dish consists of nutrient agar medium and incubated at 37 � C from 24 to 48 h in an incubator and growth of the bacterial colonies was recorded in terms of colony-forming units (CFU) and minimum inhibitory concentration (MIC) was deter mined. The minimum inhibitory concentration (MIC) of CdS QDs is the lowest concentration of an antimicrobial agent that inhibits the growth of microorganisms [24].
2.2. Antimicrobial study 2.2.1. Bacterial culture and structural representation The bacterial cultures Staphylococcus aureus (S. aureus) and Escher ichia coli (E. coli) were used in this study to check the antibacterial ac tivity of CdS QDs. Depending upon cell wall S. aureus and E. coli are classified as Gram-positive and Gram-negative respectively. The cell wall covers the cell membrane and provides rigidity, strength, and shape, and also protects the cell from rupture and mechanical damage. The cell structure of Gram-negative bacteria consists of three layers namely cytoplasmic membrane, a thin peptidoglycan layer, and outer membrane. The outer membrane of the cell wall consists of lipopoly saccharides, phospholipid, porin, and Protein. This outer membrane often shows resistance to hydrophobic compounds including detergents and lipopolysaccharides increase negative charge which is essential for the structural integrity of bacteria. In Gram-positive bacteria outer membrane is absent, instead of the outer membrane, it has thick peptidoglycan layers in the outer region. The systematic construction of Gram-negative (E. coli) and Gram-positive (S. aureus) is shown in Fig. 1.
2.1. Materials The glass sample with general formula 36SiO2–15B2O3–15Na2O–9K2O–25ZnO with 3 wt % CdS was prepared by the melt-quench technique. All chemicals used for this synthesis were of 99.99% purity. These chemicals were weighed and mixed in agate mortar and pestle for several hours in order to achieve homogeneity of chemicals. These mixed chemicals then kept in a platinum crucible and melted in a furnace at 1100 � C for 2 h. The melt formed in the platinum crucible was quenched into the meter-sized pieces at room temperature using an aluminum mould. To reduce the thermal stress these quenched glass pieces were transferred to the annealing furnace which was maintained at 350 � C. Theses glasses were annealed for 3 h which pre vent glass samples from cracking and then cooled to the room temper ature to get transparent crack free glasses. The thermal study of these glasses was carried out with Differential Thermal Analysis (DTA) using HITACHI STA 7200. Glass powder put in � a platinum pan then heated up to 700 � C with a heating rate of 5 C/min under continuous nitrogen flow and thermal transition in material with temperature were observed. Here, the empty platinum pan was used as reference material to study the thermal changes which encounter during heating. These glass samples then heat-treated for the growth of CdS QDs in the glass matrix. The growth of these semiconductor QDs was further confirmed with an X-ray diffraction (XRD) pattern measured with PAnalytical X’Pert Pro. Raman measurement of these glass samples was carried out using Jobin Yvon Horibra LABRAM-HR visible (400–1100 nm). Arþ laser operating at 488.0 nm was used as a laser source to excite the glass samples in powder form at room temperature. The size of CdS QDs, SAED pattern and EDAX pattern was determined by Field Emission Gun Transmission Electron Microscope (FEG-TEM JEOL JEM-2100F). X-ray photoelectron spectroscopy (XPS) data were ob tained from PHI 5000 Versa Probe II, FEI Inc. Optical absorption spectra and photoluminescence study of the sample were carried out using JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP8200 respectively. Fluorescence lifetime measurement of heat-treated glass sample was performed at room temperature with PTI TimeResolved Spectrofluorimeter Pico Master (PTI, HORIBA, USA) using a light-emitting diode (LED) as excitation source with excitation
3. Result and discussion 3.1. Characterization of CdS quantum dots Fig. 2 shows the DTA curve of transparent and colorless glass which were obtained after melt quench. For the given glass sample the glass transition temperature (Tg) was found to be 480 � C (inflection point). Next to this feature a sharp exothermic peak that arises around 615 � C which corresponds to glass crystallization temperature (Tc). These values from the DTA curve used to give proper heat treatment temper ature in order to grow quantum dots in a glass matrix. The glass samples were heat-treated at 500 � C for different time duration which is 2
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Fig. 1. Structural representation of Gram-negative and Gram-positive bacterial membrane.
reported in our published article [19]. The size of grown CdS QDs was further confirmed from HRTEM. The particle size of glass sample G3 was further confirmed from HRTEM. Fig. 4 shows a highly resolved image of grown CdS QDs in the glass matrix. These dots are almost spherical in shape (Fig. 4a) which indicates the isotropic growth of semiconductor quantum dots in the glassy environment. Fig. 4b shows a uniform distribution of these nanoparticles throughout the glass matrix and because of this distribu tion, the glass appears yellow in color (Fig. 3). The histogram inset of Fig. 4b shows CdS QDs grown in the glass matrix is in the range of 0.5–6.5 nm. The average size of these QDs confirmed from Fig. 4b and it is around 3.85 nm (i.e radius R � 1.92 nm) which is comparatively very small than the Bohr exciton radius (aB ¼ 2.8 nm) [25]. The ratio R/aB is very much less than one therefore high quantum confinement effect is possible. Fig. 4c shows the cluster of CdS QDs with well-resolved lattice fringes having a spacing of 0.245 nm which indicates (102) plane. SAED pattern (Fig. 4d) shows seven diffused rings which demonstrate the amorphous nature of the material. These rings are assigned to (101), (102), (110), (202), (203) and (215) diffraction planes of CdS. Planes of the SAED pattern shows consistency with the reference code of the XRD diffraction pattern [19]. Fig. 5 shows the EDAX pattern of glass sample which gives chemical composition present in the glass matrix. EDAX plot shows the presence of C, O, Na, Si, S, K, Zn and Cd. Peaks for all chemical compounds appear in EDAX pattern which has been used during synthesis except boron because of its low atomic mass. The extra peak for Cu appears in the EDAX pattern due to the use of the carbon-coated copper grid for mounting the sample during TEM/EDAX characterizations [26]. The stoichiometry of the glass matrix was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis. This is a very sensitive method to inspect the precursors used in the given matrix except for hydrogen (H) and helium (He). Fig. 6 depicts the XPS spectra of the glass sample which shows peak positions along with their binding energy. The peak position and their binding energy are depicted in Table 2. From this figure, it is observed, that the peaks of carbon (C) and oxygen (O) are also observed in the given spectra due to absorbed gaseous from the atmosphere which is also evident in EDX spectra [27–29]. From the XPS spectra, it is also observed that Cd2þ and S2 are present in different oxidation states in the glass matrix. The spectrum of pure CdS shows peaks at 405.1 and 411.8 eV corresponding to Cd 3d5/2 and Cd 3d3/2 spin-orbit components of Cd 3d respectively. These peak positions correspond to the 2þ oxidation state of Cd in CdS. Binding energy values of S 2p1/2 peak of CdS is 163.6 eV and correspond to the S2 the state of S in CdS [30–34]. Absorption spectra (Fig. 7a) shows a redshift in the absorption band edge position due to the increase in size of CdS QDs quantum confine ment effect. The effective bandgap of the glass samples has been
Fig. 2. Differential thermal analysis curve of glass sample.
responsible for nucleation and growth of CdS quantum dots in a matrix. The heat-treatment schedule along with the sample name is depicted in Table 1 and the images of glass samples can be observed in Fig. 3. It is clearly noticed from the glass images that, a prepared glass sample is transparent and then changed into yellow upon the heat treatment which indicates the growth of CdS QDs in a glass matrix. X-ray diffraction pattern (XRD) pattern of glass samples G1, G2, G3, G4, and G5 shows diffraction peaks corresponding to (101), (102), (110) and (103) planes (reference code 01-075-1545) at 2θ positions around 28.64� , 36.91� , 43.71� and 47.18� respectively. This data already dis cussed in our previous reported article [19]. The size of the grown crystallite (2.73–5.17 nm) of CdS was calculated using the Scherrer formula also discussed [19]. The observed increase in electron-phonon interaction with the growth of crystallinity in the glass matrix due to the heat-treatment process is studied using Raman measurement and Table 1 Heat treatment schedule for all glasses. Glass ID
Annealing schedule
G1 G2 G3 G4 G5
As made 500 � C for 500 � C for 500 � C for 500 � C for
10 h 35 h 60 h 80 h
3
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Fig. 3. Images of glass samples showing growth of CdS QDs in glass.
Fig. 4. HR-TEM images of glass samples (a) growth of CdS QDs (b) size distribution of CdS QDs (c) Lattice scale fringes of CdS QDs (d) Selected area electron diffraction (SAED) pattern of CdS QDs in glass matrix.
calculated by extrapolating line at the band edge and depicted in Table 3 [22]. It is observed from this Table that energy band gap decreases with an increase in size of QDs which is the result of heat treatment schedule. Photoluminescence (PL) spectra have been performed to reveal the charge transfer, migration and recombination processes of the CdS QDs. Photoluminescence spectra (Fig. 7b) shows the emission due to compositional impurities or lattice defects, band-edge transitions, and trap states when excited at 345 nm. It is observed that the emission peak observed at 413 nm is common for all glass samples which appear due to the compositional impurities or lattice defects and other emission peaks appear with heat treatment i.e. growth of CdS QDs in the glass matrix [35]. For glass G2, emission peaks appear at 436 and 556.5 nm because of band-edge transitions and trap state emission. The emission peaks arise due to band edge transition and trap states show redshift with an increase in the size of CdS QDs for other glasses (i.e. G3, G4, and G5) [20,
36]. It is observed from Fig. 7b and Table 3 that, the band edge and trap state emissions of all heat-treated glass samples shift towards higher wavelength due to an increase in the size of CdS QDs. The photo luminescence study gives an idea of photogenerated charge carriers in a semiconductor on excitation. It is reported that lower PL signal leads to high photocatalytic activity due to higher photo-induced electron-hole pairs separation efficiency [19,37,38]. It is also reported that particles with reduced size have a large surface area, which increases the available surface active sites. If the particle size is very less (i.e. in nanometer scale), electron-hole pairs generated are sufficiently close to the surface. These particles undergo rapid sur face recombination mainly due to abundant surface trapping sites and the lack of driving force for electron-hole pair separation. Therefore, QDs with optimal size were selected for photocatalytic activity [12]. 4
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Fig. 5. EDAX pattern of glass sample.
dots with low luminescence decay and high recombination rate are useful in photocatalytic as well as antimicrobial activity. The N2 adsorption isotherm and pore-size distribution of CdS/glass composites are shown in Fig. 9. As shown in Fig. 9a, the sample is mesoporous in nature which can adsorb sufficiently larger molecules on its surface. The desorption curve with a sufficiently large surface is beneficial for gas adsorption and storage. Thus, CdS QDs embedded in a glass matrix can effectively capture a molecule larger molecule from aqueous solution. Since the pore size distribution curve (Fig. 9b) shows a peak around 2.48 nm. The large surface area derived from the mono dispersed mesopore Glass/CdS nanospheres and unique Glass/CdS het erojunctions are considered to be major contributions to supplying abundant active sites and separating photogenerated carriers, respec tively [41,42]. Table 4 shows the parameters obtained from the BET study. 3.2. Antimicrobial activity Fig. 10 shows the antibacterial activities of the CdS QDs against S. aureus and E. coli in light. It is observed that the number of colonies was dropped with the increase in the concentration of CdS QDs. From the experimental observations, the obtained value of the antimicrobial activity of CdS QDs found to inhibit 50% of that of CdS QDs free control. This is called MIC 50% or MIC50. MIC50 of experimental bacteria was evaluated by taking the regression curve of bacterial inhibition with respect to CdS QDs free control solution. The ranges of minimum inhibitory concentration (MIC) values obtained in broth media for E. coli and S. aureus to inhibit 50% of isolates (MIC50) are 88.07 and 61.36 μg/ ml respectively. The results show less MIC50 values obtained for Grampositive bacteria than Gram-negative bacteria. Since it is difficult to target Gram-negative bacteria to most of the antimicrobial agent, it has more intrinsic resistance to most antibiotics [43]. In this process, CdS QDs attached to the membrane of bacteria by electrostatic interaction and disrupt the integrity of the bacterial cell membrane and generate nanotoxicity by introducing free radicals of oxygen due to their photocatalytic reaction [44]. The CdS QDs combined with bacterial culture absorb photons of higher energy than the bandgap of semiconductor to form an exciton. Exciton is formed due to the movement of an electron from the valence bond (VB) to the conduction band (CB) of CdS QDs. These excitons undergo redox reaction to form reactive oxygen species (ROS). However, CdS QDs have good efficiency to generate reactive species like hydroxyl (OH�), superoxide (O�2 ) rad icals and hydrogen peroxide (H2O2) which is responsible for killing or ganisms including bacteria endospores in water, in air and on surfaces of various materials [45]. It is reported that bacterial inhibition and the generation of H2O2 increased with a decrease in particle size [46]. The generation of ROS in surface CdS QDs is very similar to photocatalytic
Fig. 6. XPS spectra of heat treated glass sample. Table 2 The attribution of peaks in XPS spectra along with their peak positions in terms of binding energy and their respective oxidation states. Peak number
BE/eV
Element
Peak number
BE/eV
Element
1 2 3 4 5 6 7 8 9
11.5 64.0 91.4 99.8 109.4 163.6 188.0 230.9 284.7
Cd 4d3/2 Cd 4p Zn 3p1/2 Si 2p1/2 Cd 4s S 2p1/2 B 1s S 2s C 1s
10 11 12 13 14 15 16 17 18
378.6 405.1 411.8 532.4 618.2 652.3 1021.6 1044.9 1070.8
K 2s Cd 3d5/2 Cd 3d3/2 O 1s Cd 3p3/2 Cd 3p1/2 Zn 2p3/2 Zn 2p1/2 Na 1s
Glass sample G3 has minimum PL intensity (Fig. 7b) and has an optimal size which is used for photocatalytic activity. Time-resolved fluorescence decay measured in the nanosecond range shown in Fig. 8. The sample was excited with 456 nm and fluorescence decay was recorded at 570 nm. The lifetime values obtained by fitting the curve was done by convolving the instrument response function (IRF) is 0.34 ns The amorphous matrix environment used for the growth of semiconductor quantum dots has various types of defects and dislo cation which can act as charge carrier recombination center with small carrier lifetime and high surface recombination velocity [39,40]. Thus, QDs grown in a glass matrix have a large surface area to volume ratio which can absorb and emit light in the visible region. These quantum 5
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Optical Materials 99 (2020) 109590
Fig. 7. (a) Absorption spectra and (b) PL spectra of all glasses.
mechanism and explain it by following reactions:
Table 3 Optical parameters for all glasses.
a) CdS QDs þ hν (E � Eg) → CdS QDs (hþ) þ CdS QDs (e )
Glass ID
Band gap (eV)
PL peaks λe (nm)
G1 G2 G3 G4 G5
3.7 2.71 2.54 2.5 2.43
413 413, 436, 556.5 413, 470, 570 413, 481.5, 576 413, 496, 597
b) O2þ e → O�2 c) hþþH2O → OH� þ Hþ d) O�2 þ Hþ → HO�2 e) HO�2 þ Hþ þ e → H2O2 In this reaction, photo-generated electrons (e ) of CdS QDs reacts with O2 to give O�2 radicals and holes (hþ) react with H2O and separates them into OH� and Hþ. Then, the separated Hþ further reacts with O�2 to generate HO�2. Finally, HO�2 again collides with Hþ and electron to give hydrogen peroxide (H2O2). From the generated species, hydroxyl and superoxide radicals are negatively charged and unable to penetrate the cell membrane and stay on the outer surface of the bacteria while H2O2 can penetrate the cell which inhibits bacterial growth. For Gram-negative bacteria, all reactive species (i.e. hþ, OH�, O�2 , H2O2) produced by CdS QDs due to irradiation of light initially were blocked by the outer cell wall of bacteria. How ever, an increase in the concentration of CdS QDs increases the pro duction of H2O2 and therefore the inhibition rate. In Gram-positive bacteria, the outer cell membrane is totally absent and it is replaced by multilayered peptidoglycan layers which are directly affected by H2O2 generated by CdS QDs. Therefore, Gram-positive bacteria show good inhibition initially but it does not increase with an increase in CdS QDs concentration. In this study, CdS QDs embedded in the glass matrix take active participation in the formation of ROS and cannot react or got adsorbed with other compounds or species. Therefore, the formation of ROS due
Fig. 8. Fluorescence decay curve of heat treated glass.
Fig. 9. (a) BET adsorption isotherm curve of glass powder at 77 K (b) pore size distributions of glass powder containing CdS QDs. 6
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Optical Materials 99 (2020) 109590
Table 4 Porosity characteristic of glass powder containing CdS QDs. Sample
Sample weight (g)
Desorption Surface Area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3/g)
Hysteresis type
Glass powder
0.023901
271.7
2.484
0.2412
IV
Fig. 10. Antibacterial activity of CdS QDs against (a) S. aureus and (b) E. coli.
to photocatalytic activity under light is helpful in the degradation of organic dye dissolved in water as well as in the inhibition of microorganisms.
publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder.
4. Conclusions The glasses synthesized by the melt-quench technique and heattreated for the growth of CdS QDs in the glass matrix. The growth of the CdS phases was confirmed with XRD and micro-Raman spectra. The average diameter of CdS QDs obtained from HRTEM was 2.47 nm. The stoichiometry of precursors present in the system was confirmed by EDAX and XPS. Blueshift in bandgap from the bulk of the given material confirms the quantum confinement effect in the sample. PL spectra emission peak at 455 nm and 561 nm arise because of band edge and trap assisted electron-hole recombination respectively. The average lifetime of electron-hole recombination was 0.34 ns H2O2 generated from CdS QDs under light also plays an important role in antimicrobial activity. CdS QDs show more inhibition to S. aureus than E. coli and inhibit 50% of isolates (MIC50) are 61.36 and 88.07 μg/ml respectively. The minimum inhibitory concentration (MIC) values obtained in broth media for E. coli (Gram-negative) and S. aureus (Gram-positive) to inhibit 50% of isolates (MIC50) are 88.07 and 61.36 μg/ml respectively. Thus, the synthesized glasses plays an important role as an antimi crobial agent to inhibit the growth of pathogens. These semiconductor QDs embedded in the glass matrix can be a good and safe replacement for the existing antimicrobial materials.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We are very much thankful to the Department of Science and Technology (SERB-DST) for financial support for scientific research. We would also like to express our sincere, hearty thanks to Dr. V. G. Sathe, UGC-DAE-CSR, Indore and Ms. A. U. Pant, SAIF, IIT Bombay for providing a characterization facility for this work. We are very much thankful to Dr. (Mrs.) A. S. Shanware, Rajiv Gandhi Biotechnology Centre, RTMNU, Nagpur for providing facility to study antimicrobial activity. References [1] D.P. Sahoo, D. Rath, B. Nanda, K.M. Parida, Transition metal/metal oxide modified MCM-41 for pollutant degradation and hydrogen energy production: a review Dipti, RSC Adv. 5 (2015) 83707–83724, https://doi.org/10.1039/C5RA14555D. [2] S.Y. Janbandhu, S.R. Munishwar, R.S. Gedam, Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses, Appl. Surf. Sci. 449 (2018), https://doi.org/10.1016/j. apsusc.2018.02.065. [3] S. Rana, J. Rawat, R.D.K. Misra, Anti-microbial active composite nanoparticles with magnetic core and photocatalytic shell: TiO2–NiFe2O4 biomaterial system, Acta Biomater. 1 (2005) 691–703. [4] R. Gupta, N.K. Eswar, J.M. Modak, G. Madras, Effect of morphology of zinc oxide in ZnO-CdS-Ag ternary nanocomposite towards photocatalytic inactivation of E. coli under UV and visible light, Chem. Eng. J. 307 (2017) 966–980. [5] S.K. Sharma, R. Sanghi, Advances in Water Treatment and Pollution Prevention, Springer Science & Business Media, 2012. [6] D. Beydoun, R. Amal, G. Low, S. McEvoy, Role of nanoparticles in photocatalysis, J. Nanoparticle Res. 1 (1999) 439–458. [7] S.Y. Janbandhu, A. Joshi, S.R. Munishwar, R.S. Gedam, CdS/TiO2 heterojunction in glass matrix: synthesis, characterization, and application as an improved photocatalyst, Appl. Surf. Sci. (2019), 143758, https://doi.org/10.1016/j. apsusc.2019.143758.
Author contributions section 1. The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors. 2. Role of Authors: Writing - Review & Editing – Dr. S. R. Munishwar. Project administration – Dr. P. P. Pawar. Investigation – Mr. S. Y. Janbandhu. Supervision Dr. R. S. Gedam. Author agreement Work described in this paper has not been published previously, that it is not under consideration for publication elsewhere, that its 7
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