Photoinduced Reactive Oxygen Species Research Articles

Exploration of interfacial dynamics in squaraine based nanohybrids for potential photodynamic action

Photodynamic therapy (PDT) is a clinically approved, minimally invasive therapeutic procedure exhibiting cytotoxic effects toward malignant cells. Hydrophobic nature of most photosensitizers used in PDT with lower light absorption ability restricts practical use of PDT. Herein, we have employed a squaraine drug (SQ) with 665 nm absorbance peak maxima as the photosensitizing agent and evaluate its photo-physical properties. The tendency of aggregation formation in aqueous media limits its practical usefulness. Thus, we have synthesised wide band gap semiconductor zinc oxide (ZnO) nanoparticles and functionalized the surface using squaraine molecules. The molecular cross-talking was evaluated using excited state fluorescence lifetime decay profiles and by employing Fӧrster resonance energy transfer (FRET) technique. The nanohybrids show improvement in three aspects compared to bare SQ molecule such as lesser aggregate formation in aqueous media, pH responsive precipitation of the drug and improvement of photo-induced reactive oxygen species (ROS) generation. Ultrafast dynamical study at the inorganic (ZnO) - organic (SQ) interface depicts presence of photo-induced charge transfer process at the junction which indeed improves the ROS generation capability. Finally, the photodynamic action has been evaluated in human breast cancer cell line MCF-7. The nanohybrids depict enhanced photo toxicity in cancer cell with loss of adherence and typical morphology of cancerous cell depicting controlled cell death. The present study employ characterisation of nanohybrids for potential use in PDT for cancer treatment.

Photo-induced disinfection property and photocatalytic activity based on the synergistic catalytic technique of Ag doped TiO2 nanofibers

Various metal-doped TiO2 nanofibers were prepared by hydrothermal synthesis followed by the thermal treatment in air. An array of metal dopants including Ag, Au, Co, Cr, Cu, Fe, Ni, Pd, Pt, Y, and Zn, were respectively doped into TiO2 NFs to acquire the visible-light photocatalytic activity. Considering the photodegradation results under visible-light illumination, Ag doped TiO2 NFs was chosen to study further. Then, the optimal doping level and calcination process were studied systemically to obtain the highly visible light active Ag doped TiO2 NFs. As the Ag doping concentration increased up to 5.00 mol%, anatase to rutile phase transition was observed. Meanwhile, based on the consistent results of synchrotron X-ray spectra, element analysis, and morphological observation, Ag2O was detected and located on the surface of as-synthesized Ag doped TiO2 NFs. Thus, the photodegradation of methyl orange by using 5.00 mol%-Ag doped TiO2 NFs calcined at 600 °C performed the highest visible-light photodegradation activity. Moreover, it also showed the excellent disinfection against E. coli and S. aureus under visible light due to the synergistic effect of Ag2O and photo-induced reactive oxygen species (ROS) formed by TiO2 photocatalyst. The synthesized Ag doped TiO2 NFs makes large-scale and convenient fabrication possible, and it has tremendous practical potentials in the photodegradation of environmental pollution and disinfects bacteria behavior.

Nano MOF Entrapping Hydrophobic Photosensitizer for Dual-Stimuli-Responsive Unprecedented Therapeutic Action against Drug-Resistant Bacteria

Multidrug resistance (MDR) of bacteria is a major threat to public health globally and its unprecedented increase calls for immediate alternative medical strategies. Antimicrobial photodynamic therapy (aPDT) offers alternative modalities to combat the growing MDR typically by means of targeted cellular internalization of a photosensitizer (PS) capable of producing photoinduced reactive oxygen species (ROS). However, aPDT is severely limited by the self-aggregation behavior and hydrophobicity of PS molecules, which significantly curbs its viability for clinical application. The present study reports the use of modified nanoscale metal-organic frameworks (NMOFs) encapsulating a hydrophobic PS drug squaraine (SQ) to enhance aPDT efficacy against drug-resistant planktonic bacteria and its biofilm for the first time. Zeolitic imidazolate framework (ZIF-8) NMOF nanocrystals are attached postsynthetically with SQ (designated as ZIF8-SQ) and the resultant drug-doped NMOF is characterized by TEM, FESEM, PXRD, Raman spectroscopy, UV-vis spectroscopy, and steady-state and time-resolved fluorescence techniques. The microporous structures of ZIF-8 behave as molecular cages ceasing the self-aggregation of hydrophobic SQ. In addition, the formulated ZIF8-SQ produces cytotoxic ROS under red-light irradiation (650 nm) in a pH sensitive way primarily due to molecular level interaction and charge separation between ZIF-8 and SQ depicting a dual-stimuli-responsive nature. Most notably, ZIF8-SQ provides unparalleled aPDT action against methicillin-resistant Staphylococcus aureus (MRSA) and leads to complete loss of adherence of structurally robust bacterial biofilms. Finally, the nontoxic nature of the nanoconjugate toward human cells holds great promise for effective treatment of MRSA and other detrimental antibiotic-resistant microbes in clinical models.

Partially etched Bi2O2CO3 by metal chloride for enhanced reactive oxygen species generation: A tale of two strategies

Light-mediated reactive oxygen species generation with water and oxygen is generally regarded as a mild and efficient way for organic pollutants removal. However, it is highly difficult but desirable to construct a photochemical system with increased reactive oxygen species production. Herein, by using Bi2O2CO3 as a prototype, we devise a simple metal chloride-involved etching method to achieve better light absorption and charge carriers separation in a wide-band-gap semiconductor, thus giving rise to improved molecular oxygen activation. The improved photoinduced reactive oxygen species production is further verified by excellent photocatalytic degradation ability of RhB, TC and BPA under visible and ultraviolet light illumination. In addition, the metal chloride-induced strategies—heterojunction formation and cation doping—significantly affect the dynamics and transfer of carriers, which are advantageous to manipulate one-/two-electron pathway for producing reactive oxygen species.

Antimicrobial activity of ZnO nanoplates and its Ag nanocomposites: Insight into an ROS-mediated antibacterial mechanism under UV light

We have previously shown that the bactericidal effect of ZnO nanoparticles in the absence of a light source originates from the released Zn2+ ions. The purpose of this study was to explore antibacterial activity arising from photo-induced reactive oxygen species (ROS) of ZnO nanoparticles under UV-A light irradiation. To achieve this, S. aureus and K. pneumoniae bacteria were exposed to three different ZnO nanoparticles under UV-A light. The concentrations of the ZnO nanoparticles were low, such that the antibacterial effect of the dissolved Zn2+ ions was negligible. From various empirical evidence, we found that the oxygen defects of the ZnO crystals enhanced the photogeneration of ROS and consequently, the ZnO nanoplates (NPs) with the polar facets exhibited the most pronounced antibacterial activity under UV-A stimulation. To enhance the antimicrobial activity of the NPs, we successfully synthesized silver-nanoparticle-decorated ZnO NPs and explored their antibacterial activity compared to that of the NPs.

Site-selective photoinduced cleavage and profiling of DNA by chiral semiconductor nanoparticles.

Gene editing is an important genetic engineering technique that enables gene manipulation at the molecular level. It mainly relies on engineered nucleases of biological origin, whose precise functions cannot be replicated in any currently known abiotic artificial material. Here, we show that chiral cysteine-modified CdTe nanoparticles can specifically recognize and, following photonic excitation, cut at the restriction site GAT'ATC (' indicates the cut site) in double-stranded DNA exceeding 90 base pairs, mimicking a restriction endonuclease. Although photoinduced reactive oxygen species are found to be responsible for the cleavage activity, the sequence selectivity arises from the affinity between cysteine and the conformation of the specific DNA sequence, as confirmed by quantum-chemical calculations. In addition, we demonstrate non-enzymatic sequence-specific DNA incision in living cells and in vivo using these CdTe nanoparticles, which may help in the design of abiotic materials for gene editing and other biological applications.

Addition of Hydrogen Peroxide to Groundwater with Natural Iron Induces Water Disinfection by Photo-Fenton at Circumneutral pH and other Photochemical Events.

Samples of natural groundwater (with low turbidity, neutral pH and 0.3 mg L-1 iron concentration) inoculated with Escherichia coli K-12 were exposed to simulated solar light both in the presence and in the absence 10 mg L-1 of H2 O2. Results demonstrated that the viability of E. coli (by DVC-FISH) was grounded to zero after 360 min of irradiation. This abatement could be caused by the oxidative stress induced by ·OH radicals or another photo-induced reactive oxygen species. Two 23 factorial experimental designs enabled the evaluation of the effects of chemical factors on the inactivation of E. coli. The first experimental design considered the pH, iron and H2 O2 , while the second evaluated the ions fluoride, carbonate and phosphate found in groundwater. pH was found to play a key role in the inactivation of E. coli. The best reduction in viability was obtained at the lower pH (6.75), while a nonsignificant effect was observed when iron or H2 O2 concentrations were raised. At higher concentrations, anions, such as carbonate and phosphate, negatively affected the E. coli abatement. However, a higher concentration of fluoride accelerated it. In all experiments, the pH was observed to rise to values higher than 8.0 units after 360 min of treatment.

Neuroprotective effect of bilberry extract in a murine model of photo-stressed retina.

Excessive exposure to light promotes degenerative and blinding retinal diseases such as age-related macular degeneration and retinitis pigmentosa. However, the underlying mechanisms of photo-induced retinal degeneration are not fully understood, and a generalizable preventive intervention has not been proposed. Bilberry extract is an antioxidant-rich supplement that ameliorates ocular symptoms. However, its effects on photo-stressed retinas have not been clarified. In this study, we examined the neuroprotective effects of bilberry extract against photo-stress in murine retinas. Light-induced visual function impairment recorded by scotopic and phototopic electroretinograms showing respective rod and cone photoreceptor function was attenuated by oral administration of bilberry extract through a stomach tube in Balb/c mice (750 mg/kg body weight). Bilberry extract also suppressed photo-induced apoptosis in the photoreceptor cell layer and shortening of the outer segments of rod and cone photoreceptors. Levels of photo-induced reactive oxygen species (ROS), oxidative and endoplasmic reticulum (ER) stress markers, as measured by real-time reverse transcriptase polymerase chain reaction, were reduced by bilberry extract treatment. Reduction of ROS by N-acetyl-L-cysteine, a well-known antioxidant also suppressed ER stress. Immunohistochemical analysis of activating transcription factor 4 expression showed the presence of ER stress in the retina, and at least in part, in Müller glial cells. The photo-induced disruption of tight junctions in the retinal pigment epithelium was also attenuated by bilberry extract, repressing an oxidative stress marker, although ER stress markers were not repressed. Our results suggest that bilberry extract attenuates photo-induced apoptosis and visual dysfunction most likely, and at least in part, through ROS reduction, and subsequent ER stress attenuation in the retina. This study can help understand the mechanisms of photo-stress and contribute to developing a new, potentially useful therapeutic approach using bilberry extract for preventing retinal photo-damage.

Self-assembly of DNA-porphyrin hybrid molecules for the creation of antimicrobial nanonetwork

DNA derived well-controlled arrangement of porphyrins has emerged as promising hybrid nanostructures. Exceptional biocompatibility and DNA-directed surface addressability coupled with rich symmetry features of the porphyrin have made these hybrid nanostructures attractive candidates for potential biomedical and biotechnological applications. However, the noteworthy photophysical properties of porphyrin and related molecules when present in DNA based nanostructures are yet to be explored fully and should be a matter of intense research that may unearth a plethora of interesting applications of these nanostructures. Herein, we demonstrate the construction of novel self-assembled DNA-porphyrin hybrid nanonetworks that utilize the porphyrin core for antibacterial applications. Porphyrin derivative with four pendant NH2 groups was synthesized and conjugated with the 5′-PO4 of ss-DNA by solution phase phosphoramidation coupling reaction. The conjugation was followed by DNA hybridization mediated self-assembly to form DNA-porphyrin hybrid nanonetwork. The enhanced antimicrobial property of the nanonetwork was envisioned following light irradiation at relevant wavelength. In line with this, comparative antimicrobial activities against gram-negative (Escherichia coli BL-21) and gram-positive bacteria (Staphylococcus aureus) have been studied. Interestingly, DNA-porphyrin nanonetwork afforded highly efficient and coherent photoinduced reactive oxygen species (ROS) generation to display antimicrobial activity against Staphylococcus aureus. The escalated and coherent ROS generation from the nanonetworks was attributed to the ordered placement of the porphyrins that inhibits self-quenching. Our work points out to a good alternative for antibiotic free strategies for preservation of biological materials and other applications.

Multifunctional Nanosystem for Synergistic Tumor Therapy Delivered by Two-Dimensional MoS2.

A multifunctional nanosystem based on two-dimensional molybdenum disulfide (MoS2) was developed for synergistic tumor therapy. MoS2 was stabilized with lipoic acid (LA)-modified poly(ethylene glycol) and modified with a pH-responsive charge-convertible peptide (LA-K11(DMA)). Then, a positively charged photosensitizer, toluidine blue O (TBO), was loaded on MoS2 via physical absorption. The negatively charged LA-K11(DMA) peptide was converted into a positively charged one under acidic conditions. Charge conversion of the peptide could reduce the binding force between positively charged TBO and MoS2, leading to TBO release. Furthermore, the positively charged nanosystem was easily endocytosed by cells. Photo-induced hyperthermia of MoS2 in the tumor areas could promote TBO release and exhibited photothermal therapy. In vitro and in vivo results demonstrated that fluorescence and photo-induced reactive oxygen species (ROS) generation of TBO were severely decreased by MoS2 under normal conditions. While in the acidic condition, the pH-responsive nanosystem exhibited a highly specific and efficient antitumor effect with TBO release and photo-induced ROS generation, suggesting to be a promising accessory for synergistic tumor therapy.

A New Approach to Sensitize Antitumor Monofunctional Platinum(II) Complexes via Short Time Photo-Irradiation.

Sensitizing the antitumor activity of monofunctional PtII complexes is a reliable approach to developing antitumor agents different from the classic Pt-based drugs. Considering the poor intracellular accumulation of monofunctional PtII complexes, in this study, the photosensitizing monofunctional PtII complex Pt-BA was derived from a weak BODIPY (boron-dipyrromethene)-derived photosensitizer BA, with the purpose to improve its antitumor cytotoxicity via enhancing its intracellular accumulation with a short time photo-irradiation. Photoinduced reactive oxygen species (ROS) determination indicated that the PtII center in Pt-BA is able to improve the photoinduced ROS production ability of BA, which makes Pt-BA a mild photosensitizer. Fluorescence imaging disclosed that dark incubation makes Pt-BA accumulate mainly on the surface of cell membrane, and the later short time photo-irradiation (5 min) promotes distinctly the intracellular accumulation of Pt-BA, which has been confirmed by inductively coupled plasma-mass spectrometry determination. Flow cytometric Annexin V-FITC assay indicated that the short time irradiation of Pt-BA induces in situ the cell membrane damage, which might finally enhance the intracellular accumulation of this monofunctional complex. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay confirmed that the short time photo-irradiation promotes distinctly the antitumor cytotoxicity of Pt-BA against MCF-7, SGC-7901, A549, and HeLa cell lines. The photopromoted antitumor activity of Pt-BA implies that modifying monofunctional PtII complex as a mild photosensitizer to promote its cell accumulation is a useful approach to sensitizing the antitumor activity of monofunctional PtII complex and renders the possibility of monofunctional PtII prodrugs for precise chemotherapy via only short time photoactivation.

Well-Defined Water-Soluble C60-PVP Conjugate

Based on their high photosensitivity, the potential medical application of fullerenes (C60 and C70) such as photodynamic therapy agents has been discussed for decades. Due to their high hydrophobicity, it is necessary to find a good method to solubilize fullerenes in water or water-miscible solvents for such application. We have been using a biocompatible and water-soluble polymer, poly(vinylpyrrolidone) (PVP) to prepare water-soluble C60/ or C70/PVP complex1 and C60- or C70-PVP conjugates2. 3 with efficient reactive oxygen species (ROSs) generation under visible light.4,5 However, in the clinical use, well-defined compounds with narrower molecular weight distribution are favored. In this study, we synthesized a RAFT reagent 1 for the preparation of well-defined PVP. The terminal moiety was converted to an amine 2 to connect to a C60 acid anhydride derivative 3 which was obtained through a Prato reaction.6 Obtained C60-PVP conjugate had a narrower molecular distribution, high water-solubility and photoinduced ROS generation.7 (1) Yamakoshi, Y.; Yagami, T.; Fukuhara, K.; Sueyoshi, S.; Miyata, N. J. Chem. Soc., Chem. Commun. 1994, 517. (2) Iwamoto, Y.; Yamakoshi, Y. Chem. Commun. 2006, 4805. (3) Oriana, S.; Aroua, S.; Söllner, J. O. B.; Ma, X.; Iwamoto, Y.; Yamakoshi, Y. Chem. Comm. 2013, 49, 9302. (4) Yamakoshi, Y.; Sueyoshi, S.; Fukuhara, K.; Miyata, N. J. Am. Chem. Soc. 1998, 120, 12367. (5) Yamakoshi, Y.; Umezawa, N.; Ryu, A.; Arakane, K.; Miyata, N.; Goda, Y.; Masumizu, T.; Nagano, T. J. Am. Chem. Soc. 2003, 125, 12803. (6) Aroua, S.; Schweizer, W. B.; Yamakoshi, Y. Org. Lett. 2014, 16, 1688. (7) Aroua, S.; Tiu, E. G. V.; Ayer, M.; Ishikawa, T.; Yamakoshi, Y. Polymer Chem. 2015, 6, 2616. Figure 1

Degradation and debromination of bromophenols using a free-base porphyrin and metalloporphyrins as photosensitizers under conditions of visible light irradiation in the absence and presence of humic substances

The photodegradation and debromination of 2,4,6-tribromophenol (TrBP) and tetrabromobisphenol A (TBBPA) were investigated in the presence of a free-base porphyrin and metalloporphyrins as photosensitizers under conditions of visible light irradiation. Among the photosensitizers, 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) and Zn(II)-tetrakis(N-methylpyridinium-4-yl)porphyrin (ZnTMPyP) were effective in the photodegradation of bromophenols. In particular, 96% of the debromination was achieved in the case of the ZnTMPyP photosensitization system for TBBPA after 24h of irradiation. ESR spectra with a spin-trapping reagent indicated that singlet oxygen (1O2) was the major photo-induced reactive oxygen species. The presence of humic substances and natural organic matter, which are common components in landfill leachates, inhibited the photodegradation of TrBP via competitive oxidation by 1O2 when ZnTMPyP was used as a photosensitizer. The presence of humic acids, which contained higher levels of electron-rich moieties like aromatic carbons, strongly inhibited the photodegradation of TrBP. TrBP and TBBPA were ultimately decomposed to organic acids, such as maleic and fumaric acids, as evidenced by LC/TOF-MS analysis. Thus, the photoirradiation of H2TMPyP and ZnTMPyP can induce 1O2 generation, which is effective in the photodegradation and debromination of TrBP and TBBPA.

Ultrafast dynamics at the zinc phthalocyanine/zinc oxide nanohybrid interface for efficient solar light harvesting in the near red region

Phthalocyanine-based light harvesting nanomaterials are attractive due to their low cost, eco-friendly properties and sensitivity in the red region of the solar spectrum. However, for any practical application, phthalocyanines need to be chemically modified for anchoring groups with wide-band semiconducting nanomaterials. In this paper, zinc phthalocyanine (ZnPc) was functionalized with two carboxyl groups containing a biologically important ligand, tartrate, using a facile wet chemistry route and duly sensitized zinc oxide (ZnO) to form nanohybrids for application in photocatalytic devices. The nanohybrids have been characterized using a high-resolution transmission/scanning electron microscope (HRTEM, FEG-SEM), X-ray diffraction (XRD), steady-state infrared/UV–vis absorption and emission spectroscopy for their structural details and optical properties, whereas the ultrafast dynamical events, which are key for understanding the photocatalytic activities, were monitored using picosecond resolution fluorescence techniques. More specifically, vibrational spectroscopy (FTIR) revealed the covalent connection of ZnPc with the host ZnO nanoparticles via the tartrate ligand. The efficiency of the material for photocatalysis under red light irradiation was found to be significantly enhanced compared to bare ZnO. A mechanistic pathway for the formation of photo-induced reactive oxygen species (ROS) in an aqueous medium for the photocatalytic efficacy was investigated. To make a prototype for a potential application in a flow device for water decontamination, we have sensitized ZnO nanorods (ZnO NR) with tartrate-functionalized ZnPc. The molecular proximity of ZnPc to the ZnO surface has been confirmed by picosecond resolution Förster Resonance Energy Transfer (FRET) from the intrinsic emission of surface defects of ZnO NR to the attached ZnPc. The excited-state electron transfer dynamics, as revealed by a picosecond resolution fluorescence study (TCSPC), is in good agreement with the enhanced charge separation at the interface of the nanohybrid. The enhanced photoresponse, wavelength-dependent photocurrent of the sensitized nanorods and photodegradation of a model water pollutant in a prototype device format confirmed the potential use of the nanohybrids in water purification.

Li-Doped ZnO Nanoparticles as Novel Direct Generator of Singlet Oxygen for Potential Photodynamic Therapy Applications

ABSTRACTPhotodynamic therapy (PDT) is an alternative to traditional cancer treatments. This approach involves the use of photosensitizer (PS) agents and their interaction with light. As a consequence, cytotoxic reactive oxygen species (ROS) are generated that, in turn will destroy tumors. On the other hand, ZnO is a biocompatible, nontoxic, and biodegradable material with the capability to generate ROS, specifically singlet oxygen (SO), which makes this material a promising candidate for 2-photon PDT. Doping ZnO with Li species is expected to induce defects in the host oxide structure that favors the formation of trap states that should affect the electronic transitions related to the generation of SO. The present work reports the effect of the level of Li-doping on the ZnO structure and its capability to generate SO. Li-doped ZnO nanoparticles were synthesized under size-controlled conditions using a modified version of the polyol method. XRD measurements confirmed the development of well-crystallized ZnO Wurtzite; the average crystallite sizes ranged between 13.3nm and 14.2 nm, with an increase in Li content. The corresponding band gap energy values, estimated from UV-vis measurements, decreased from 3.33 to 3.25 eV. Photoluminescence (PL) measurements of Li-ZnO revealed the presence of emission peaks centered on 363nm, 390nm, and 556 nm; these emission peaks correspond to the exciton emission, transition of shallow donor levels near of the conduction band to valence band such as interstitial Zn, and oxygen vacancies, respectively. The observed increase of the emission intensity of the 390 nm emission peak, relative to the intensity of the main emission peak at 363nm, was attributed to the promote of trap states due to interstitial Zn or Li-incorporation into the host oxide lattice. SO measurements evidenced the enhancing effect of the Li concentration on the capability of the doped ZnO to generate this species. This Li-dependence of SO generation can be attributed to the enhancement of the concentration of trap states in the host ZnO, as suggested by PL measurements. Accordingly, Li-ZnO would become cytotoxic to cancer cells via photo-induced ROS generation enabling this nanomaterial to be considered as a potential direct PS agent for the 2-photon PDT route.

Water-soluble fullerene materials for bioapplications: photoinduced reactive oxygen species generation.

The photoinduced reactive oxygen species (ROS) generation from several water-soluble fullerenes was examined. Macromolecular or small molecular water-soluble fullerene complexes/derivatives were prepared and their (1)O(2) and O(2)˙(-) generation abilities were evaluated by ESR spin-trapping methods. As a result, efficient (1)O(2) generation was detected from photoexcited C(60), not only in organic solvents, but also in aqueous media and especially from small molecule C(60)-carboxylic acid derivatives. Whereas efficient O(2)˙(-) generation was observed in the aqueous solution of the C(60)/γ-CD complex under photoirradiation.

ZnO Quantum Dots For Biomedical Applications

Quantum dots (QDs), highly luminescent semiconductor nanocrystals, have found extensive applications in different fields, ranging from optoelectronic to bio-imaging. Numerous applications are emerging daily. Among these, ZnO QDs have higher biological significance because of their relative non-toxicity. The primary aim of this review is to overview the literature based on the biological applications of ZnO QDs, including gene therapy, drug delivery, optical imaging, allergen and antigen detection, cancer cell sensing, antibacterial agents and DNA detection. The luminescent properties of ZnO nanoparticles have attracted considerable attention for numerous applications like ultraviolet light emitting devices, flat panel displays, as low voltage phosphors and biosensing devices. The review throws light on the developments in fabrication techniques of nanometer-sized, water-dispersible, bio-compatible and stable ZnO QDs in aqueous medium for biological applications, including employing organic ligands, coating nanoparticles with inorganic shells, doping with a suitable element and capping nanoparticles surfaces with polymers. The low toxicity of ZnO and its high natural abundance make it a good alternative to cadmium based II-VI semiconductors, which cause toxicity via photoinduced reactive oxygen species (ROS) generation.