Production of Reactive Oxygen Species and Electrons from

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Production of Reactive Oxygen Species and Electrons from Photoexcited ZnO and ZnS Nanoparticles: A Comparative Study for Unraveling their Distinct Photocatalytic Activities Weiwei He, Huimin Jia, Junhui Cai, Xiangna Han, Zhi Zheng, Wayne G. Wamer, and Jun-Jie Yin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11456 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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The Journal of Physical Chemistry

Production of Reactive Oxygen Species and Electrons from Photoexcited ZnO and ZnS Nanoparticles: A Comparative Study for Unraveling their Distinct Photocatalytic Activities Weiwei He†*, Huimin Jia†, Junhui Cai†, Xiangna Han†, Zhi Zheng†, Wayne G. Wamer‡, Jun-Jie Yin‡* †Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nanomaterials, Xuchang University, Xuchang, Henan 461000, China ‡Division of Bioanalytical Chemistry and Division of Analytical Chemistry, Office of Regulatory Science, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 20740, United States.

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Abstract: The photo-activity of semiconductor nanostructures makes them potentially useful for environmental remediation and antibacterial applications. Understanding the mechanism underlying the photochemical and photobiological activities of photo-excited semiconductors is of great importance for developing applications and assessing associated risks. In the current work, using electron spin resonance spectroscopy coupled with spin trapping and spin labeling techniques, we comparatively and systematically investigate the abilities of ZnO and ZnS to generate hydroxyl radical, superoxide, singlet oxygen, photo-induced electrons and oxygen consumption during irradiation. It was found that although ZnO and ZnS, when photo-excited, can produce hydroxyl radical, superoxide and singlet oxygen, ZnO is more effective than ZnS in producing hydroxyl radical and singlet oxygen while ZnS is more effective than ZnO in generating superoxide. The characterization with ESR spin labeling and oximetry indicates ZnS is about 4 times more active than ZnO in production of photo-induced electrons and consumption of oxygen. We compared the photocatalytic and antibacterial activities of ZnO and ZnS and found that ZnO exhibits efficient and broad photocatalytic and antibacterial activity, conversely, ZnS is only effective in photodegradation of RhB and killing S. aureus. The distinct photocatalytic activities of ZnO and ZnS nanoparticles were attributable to their unique capability to facilitate the generation of reactive oxygen species and charge carriers during photo-irradiation. These results provide valuable information for understanding the photocatalytic mechanism of metal oxide and metal sulfides and for predicting their photocatalytic activities. KEYWORDS: ZnO, ZnS, reactive oxygen species, photocatalytic, electron spin resonance, antibacterial

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INTRODUCTION Reactive oxygen species (ROS), commonly including hydroxyl radical (•OH), superoxide (O2-•), singlet oxygen (1O2) and hydrogen peroxide (H2O2), are a group of chemically reactive molecules containing oxygen. There is a continuing interest in ROS because of their essential roles in many environmental and biologic effects.1,2 Recently, nanomaterial families, such as metal,3-5 metal oxide,6-8 metal sulfide,9,10 metal/semiconductor hybrid nanoparticles11-13 and carbon nanostructures,14,15 have been found to generate or scavenge ROS under a variety of experimental conditions. It is well understood that noble metal nanostructures (Au, Ag, Pt) can produce catalytically the hydroxyl radical and reduce superoxide under moderate, biologically relevant conditions.3-5 A number of metal oxide or metal sulfide nanoparticles photocatalyze the generation of various ROS.6-10 Fullerene derived nanomaterials have been shown to be efficient scavengers of superoxide and have been used for cancer treatments.16,17 These capabilities of nanomaterials for mediating the generation of ROS are of fundamental importance for understanding the mechanisms underlying the functions as well as the safety of nanomaterials in environmental and biological systems. Therefore, the ability to produce ROS should be considered as a critical physicochemical parameter of nanomaterials for predicting and assessing their ROS related biological/environmental effects. A definitive understanding the capability of a given nanomaterials to generate ROS requires knowledge of the material and environmental properties which affect the generation of ROS. For example, it is well established that ROS generation and catalytic activity of many semiconductors results from their photoexcitation. Basically, the photocatalytic reaction is initiated when semiconductor absorb light having a wavelength with energy equivalent to their 3



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band gaps leading to the promotion of excited electrons from valence band to the empty conduction band, concomitantly generating electron/hole pairs. The resulting electrons/holes with sufficient reductive/oxidative power can react with surrounding oxygen containing species, such as dissolved oxygen or H2O (OH-), and produce reactive oxygen species. One may expect that any factors, such as particle size, composition, structures, chemical surface, and microenvironment that influence the energy band structures, charge carrier generation and separation efficiency, or intermediate reactions of ROS will affect the net production of ROS. It has been reported that composite nanomaterials, comprised of Au nanoparticles deposited on metal oxides (e.g., ZnO and TiO2), exhibit a remarkable increase in generation of hydroxyl radical, singlet oxygen and superoxide. This effect is attributed to thus improved charge carrier separation efficiency and consequent higher net yield of photogenerated electrons.11-13 Up to now, two aspects have been well established in photocatalytic semiconductors: 1) involvement of reactive oxygen species in their photocatalytic reactions occur for both metal oxide and sulfide, 2) metal oxides may be more suitable for photocatalytic oxidation than metal sulfides because metal oxide possess a relatively more positive valence band edge.18,19 Though a great number of metal oxide and metal sulfides nanomaterials have been extensively studied for their photocatalytic and photo-biological activities,6-10 the detailed mechanisms underlying the diverse activities of metal oxide and metal sulfide are not well understood. For example, researchers still do not know how metal oxides and metal sulfides differ in the types and reactivity of photogenerated ROS and charge carriers. What kinds of ROS contribute dominate the photoactivities for both metal oxide and metal sulfides? These questions are critical for full understanding photochemical and photobiological activities of metal oxide and metal sulfides.

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Answering these questions requires an effective method to accurately identify ROS and photogenerated electrons and describe their roles in the overall photoreaction. In the past couple years, we have developed an electron spin resonance (ESR) method together with spin trap and spin label techniques to investigate the ROS related activity of noble metal and carbon nanomaterials in biologically relevant systems,3-5, 14-17 and the events involving ROS generation triggered by photo-excited semiconductor nanomaterials.6-10 ESR is the most powerful tool for studying materials with unpaired electron, which can not only provide a specific spectrum with paramagnetic characteristics for each ROS spin adduct but also can provide direct information for photogenerated electrons and oxygen consumption via spin label and oximetry technique. This methodology is a powerful tool for identifying active intermediates induced by nanostructures. Here, we use ESR techniques to compare the abilities of metal oxides and metal sulfides to generate ROS and their photocatalytic and antibacterial activities. ZnO and ZnS were selected as representative metal oxide and metal sulfide, respectively, due to 1) ZnO and ZnS have a large difference in their conduction band and valence band edge positions, which is convenient for us to examine the effect of energy band edge positions; 2) ZnO and ZnS are the representative metal oxide and metal sulfides, the results may benefit for understanding the distinct mechanism and activities of other metal oxides and metal sulfides in photocatalytic applications; 3) their widespread use in commercial products such as cosmetics and sunscreens.20 Our primary objective in this work is use of electron spin resonance spectroscopy to obtain a detailed comparison of ZnO and ZnS in generation of reactive oxygen species and charge carriers, oxygen consumption, photocatalytic degradation and antibacterial properties. These results will provide deep insights into their photocatalytic mechanisms and potential applications. 5



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EXPERIMENTAL SECTION Chemical and Materials. An aqueous dispersion of zinc oxide nanoparticles (20 wt%, 30-40 nm) was purchased from US Research Nanomaterials, Inc (Houston, TX). Zinc acetate, cysteine, Rhodamine B (RhB), salicylic acid were analytically pure and purchased from Shanghai Chemical Reagent Co. Ltd. The spin-trap 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) was purchased from Applied Bioanalytical Labs (Sarasota, FL). 4-Oxo2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMP), NaN3, and superoxide dismutase (SOD) were purchased from Sigma Chemical Co. (St. Louis, MO). 2,2,6,6 -Tetramethylpiperidine-1oxyl (TEMPO) was purchased from Alexis, Enzo Life Sciences, Inc. (Farmingdale, NY). Milli-Q water (18 MΩ cm) was used for preparation of all solutions. Synthesis of ZnS nanoparticles. Typically, 1 mmol Zn(Ac)2 and 1 mmol cysteine were added into a 28 ml Teflon-lined stainless steel autoclave. Then 21 ml water was added and the mixture was vigorously stirred. The autoclave was sealed, heated at 160 ºC for 12 h and aircooled to room temperature. Finally, the precipitate was collected by centrifugation, washed several times with double distilled water and ethanol, and dried under vacuum at 50 ºC. Characterization. X-ray diffraction (XRD, Bruker D8 Advance diffractometer) with monochromatized Cu Kα radiation (λ = 1.5418 Å) was used to characterize the crystal structure of ZnO and ZnS nanoparticles. Transmission electron microscopy data (TEM) were captured on a JEM 2100 FEG (JEOL) transmission electron microscope (accelerating voltage of 200 kV) located at the NanoCenter, University of Maryland, College Park, MD. The photocatalytic activities of ZnO and ZnS were evaluated by measuring the degradation of Rhodamie B (RhB) and salicylic acid (SA) in aqueous solutions. 0.1 mg/ml of ZnO or ZnS photocatalyst was dispersed in a 20 mL aqueous solution containing 0.2 mg/l RhB or 0.5 mM SA. The solution was 6


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continuously stirred in the dark for 30 min to establish an adsorption-desorption equilibrium between the photocatalyst and substrates. Then, the suspension was irradiated using a 450 W Xenon lamp filtered to deliver simulated sunlight. During irradiation, the solution was stirred to maintain a suspension. At selected time intervals, aliquots of suspension were removed and centrifuged. The residual concentration of the RhB or SA in the supernatant was monitored using a Varian Cary 300 spectrometer. Growth of bacteria and test of antibacterial activity of ZnO and ZnS nanoparticles. The procedure for bacterial culture and antibacterial experiments treated by ZnO and ZnS is similar to a previous report with slight modification.11 Escherichia coli (ATCC 25922) and Staphylococcus aureus subsp. aureus (ATCC 29213) were chosen as gram negative and positive model organisms for antibacterial activity experiments. Both bacteria were grown overnight on BHIA (Brain Heart Infusion Agar, BD-Difco) plates in an incubator at 37°C. Bacterial growth was harvested using a sterilized swab and resuspended in the 0.01 × PBS buffer to achieve a viable cell concentration of about 1.5 to 2.0 × 109 cells/ml, which was used as a bacterial stock suspension. Antibacterial activities of nanomaterials were tested in 96-well plates. 180 µl of the bacterial stock suspension was transferred into each well and followed by added 20 µl of 0.5 mg/mL ZnO or ZnS to each well. The 96-well plates containing bacteria and nanomaterials were irradiated under a solar simulator (Model 91190-1000, Oriel, Stratford, CT) for 10 minutes and then incubated at 37°C for 1 hour. The bacteria and nanomaterial mixture in each well was then serially diluted and 10 µl from each dilution was spread onto BHIA agar plates. The plates were incubated at 37°C overnight, and the resulting bacterial growth was enumerated in colony forming units (CFU). The bacteria survival rate was calculated as follows: Survival rate (%) = 100 × CFU (cell + nano + rad)/CFUo (cell) 7



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where CFU is the number of colony forming units measured after plating cells irradiated in the presence of nanomaterials, and CFUo (cell) is the number of colony forming units measured after plating cells exposed to neither nanomaterials nor simulated sunlight. All tests were conducted in triplicate and repeated at least twice to confirm reproducibility. Electron spin resonance spectroscopy. All the ESR measurements were conducted at room temperature using an X-band Bruker EMX ESR spectrometer (Billerica, MA). A light source consisting of a 450 W Xenon lamp coupled with bandpass filters (FSQ-U340) was used to generate light (250 nm 350 nm) (Figure S3), Photocatalytic activity of ZnO and ZnS toward degradation of rhodamine B and salicylic acid during simulated sunlight irradiation (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (W.He), [email protected] (J.Yin) Notes These authors declare no competing financial interests. 19



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Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 21303153), Program for Science & Technology Innovation Talents in Universities of Henan Province (14HASTIT008), and Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 144200510014). This article is not an official US Food and Drug Administration (FDA) guidance or policy statement. No official support or endorsement by the US FDA is intended or should be inferred. REFERENCE [1] Alfadda, A. A.; Sallam, R. M. Reactive Oxygen Species in Health and Disease. J. Biomed. Biotechnol. 2012, 2012, 936486. [2] Lee, S. H.; Gupta, M. K.; Bang, J. B.; Bae, H.; Sung, H. J. Current Progress in Reactive Oxygen Species (ROS)-Responsive Materials for Biomedical Applications. Adv. Healthcare Mater. 2013, 2, 908−915. [3] He, W. W.; Zhou, Y. T.; Wamer, W. G.; Boudreau, M. D.; Yin, J. J. Mechanisms of the pH Dependent Generation of Hydroxyl Radicals and Oxygen Induced by Ag Nanoparticles. Biomaterials 2012, 33, 7547-7555. [4] He, W. W.; Zhou, Y. T.; Wamer, W. G.; Hu, X.; Wu, X.; Zheng, Z.; Boudreau, M. D.; Yin, J. J. Intrinsic Catalytic Activity of Au Nanoparticles with Respect to Hydrogen Peroxide Decomposition and Superoxide Scavenging. Biomaterials 2013, 34, 765-773.

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Figure 1. TEM images of ZnO (a) and ZnS (b) nanoparticles, and XRD patterns (c) of ZnS and ZnO nanoparticles.

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Scheme 1. Band edge positions of ZnO and ZnS compared with standard redox potentials of superoxide, singlet oxygen and hydroxyl radical.

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Figure 2. Generation of hydroxyl radicals and superoxide from ZnO (left panel) and ZnS (right panel) nanoparticles during irradiation (250 nm < λ < 350 nm). ESR spectra obtained from 25 mM BMPO only (pink), 25 mM BMPO + 0.1 mg/ml ZnO or ZnS (blue), 25 mM BMPO + 0.1 mg/ml ZnO or ZnS + 10% DMSO (red), and 25 mM BMPO + 0.1 mg/ml ZnO or ZnS + 10 U/ml SOD (black). All the spectra were recorded at 3 min after irradiation.

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Figure 3. Generation of singlet oxygen from ZnO and ZnS nanoparticles during irradiation (250 nm < λ < 350 nm). ESR spectra obtained from samples containing 5 mM 4-oxo-TEMP and NPs without irradiation (a), 5 mM 4-oxo-TEMP in the absence (b) and presence of 0.1 mg/ml ZnO (c) or ZnS (d) during irradiation. The spectra for irradiated samples were recorded at 1 min after irradiation.

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Figure 4. Demonstration of electron generation during irradiation of ZnO and ZnS nanoparticles with light (250 nm < λ < 350 nm). ESR spectra obtained from samples containing 0.02 mM TEMPO before irradiation (a), 0.02 mM TEMPO in the absence (b) and presence of 0.1 mg/ml ZnO (c) or ZnS nanoparticles (d). The spectra for b, c and d were recorded at 10 min during irradiation.

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Figure 5. Oxygen consumption during the photoexcitation of ZnO and ZnS nanoparticles with light (250 nm < λ < 350 nm). (a) ESR spectrum of 0.1 mM CTPO in the absence (control) and presence of 0.2 mg/ml ZnO or ZnS after irradiation for 15 min; (b) The change of dissolved oxygen remaining in the solutions containing 0.2 mg/ml ZnO or ZnS versus irradiation time.

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Figure 6. Photocatalytic activity (plots of ln(C/C0) vs time) of ZnO and ZnS nanoparticles on degradation of salicylic acid and rhodamine B exposed to simulated sunlight. The concentration of catalysts was fixed at 0.1 mg/ml.

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Figure 7. Ability of ZnO and ZnS nanoparticles in killing E. coli (left) and S. aureus (right) during exposure to simulated sunlight for 10 min. Neither represents bacteria exposed to neither nanoparticles nor light. Light only represents bacteria exposed to simulated sunlight for 10 min but without nanoparticles. Nano Only represents bacteria that were exposed to 0.5 mg/ml ZnO or ZnS alone without light. Grouped under Nano + Light, bacteria were exposed to 0.5 mg/ml ZnO or ZnS and 10 min of solar simulated light.

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Production of Reactive Oxygen Species and Electrons from

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