Electron Spin Resonance Technique Research Articles

Radiation induced defects in timolol: temperature dependent electron paramagnetic resonance spectroscopy analysis

Ionizing radiation-induced antihypertensive drug timolol in powder form was studied by means of electron paramagnetic resonance (EPR) technique as a function of temperature. The effect of temperature on EPR linewidth, intensity and spectra was investigated. The room temperature EPR spectra of ionizing radiation-induced timolol were recorded in dimethyl sulfoxide solution at a frozen state. The first-order kinetic mechanism rate constant of timolol was calculated. The findings on all EPR experimental data indicated that ionizing radiation-induced stable-free radical species on timolol are characterized by g = 2.0027.

Spectroscopic Analysis of Irradiated Natural Quartz and ESR dating aspects

Abstract In this study, we conducted a spectroscopic characterization of natural milky quartz sample from Wadi Araba at the Egyptian Eastern Desert. This quartz material had been studied using scanning electron microscope (SEM), X-ray powder diffraction (XRD), UV–visible spectrophotometer (UV–vis), Fourier transform infrared spectrometer (FTIR), X-ray fluorescence spectrometer (XRF), inductively coupled plasma optical emission spectrometer (ICP-OES) and electron spin resonance (ESR) technique. In the current study, we have used raw samples as-received and gamma-irradiated conditions. The results reviled that gamma radiation had a slight influence on the color of milky quartz samples. XRF and inductively coupled plasma optical emission spectrometer were used to recognition the elemental composition of the raw sample. The ESR spectrum accomplished at room temperature display intrinsic defects such as Aluminium (Al) trap center. Furthermore, with increasing the additive doses the area under the dose response curve increases as a polynomial function, which applicable for ESR dating aspects.

Physicochemical, in-vitro therapeutic activity and biomolecular interaction studies of Mn(II), Ni(II) and Cu(II) complexes tethered with O2N2 ligand backbone.

Two major health crisis of today's world are antimicrobial drug resistance and type II diabetes. To tackle them, there is an immediate requirement for the development of new and safer drugs and the present work is one such quest for novel and efficient drug candidates. We have developed three trace metal coordination compounds tethered with a reduced salen ligand {H2(hpdbal)2-an} (L), namely, a manganese-salan complex, [MnII(H2O)2{(hpdbal)2-an}] (1), a nickel-salan complex, [NiII{(hpdbal)2-an}] (2) and a copper-salan complex, [CuII{(hpdbal)2-an}] (3). The compounds were characterized by elemental analysis, vibrational spectroscopy, electronic spectroscopy, thermogravimetric analysis, nuclear magnetic resonance and electron-paramagnetic resonance techniques. The compounds were evaluated for antimicrobial activity against seven pathogens (Escherichia coli, Klebsiella pneumonia, Acinetobacter baumannii, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans and Cryptococcus neoformans) and antidiabetic activity by mimicking diabetic environment on the immortal human liver cancer cells, HepG2. Complexes 1 and 2 were additionally tested for their reactivity and stability in biological media mimic conditions. The nickel(II) salan complex (2) exhibited noteworthy antifungal activity against Candida albicans and the manganese(II) salan complex (1) induced increased glucose uptake by the insulin resistant cells. Both compounds were found to be stable when solution pH conditions were varied from 3 to 9. They exhibited strong affinity of binding towards a carrier protein, bovine serum albumin which was evaluated with the aid of multi-spectroscopic techniques.

Direct Z-scheme red carbon nitride/rod-like lanthanum vanadate composites with enhanced photodegradation of antibiotic contaminants

Herein, a novel direct Z-scheme mechanism of red carbon nitride/rod-like LaVO4 (RCNL) photocatalyst was prepared by a simple hydrothermal method. Under visible light irradiation, the photocatalytic degradation experiments on tetracycline (TC) and naproxen (NPX) showed that 5% RCNL material presented the best photocatalytic performance, which was 3 times (TC) and 2.69 times (NPX) higher than pure LaVO4, respectively. The degradation process of tetracycline and naproxen was further analyzed by mass spectrometry, and their possible degradation routes were proposed. The antibacterial results (against Escherichia coli) showed that the degrading products have lower toxicity. The analysis of band structure, electron spin resonance technique and free radical trapping experiments showed that the h+ at valence band of RCN and e- at the conduction band of LaVO4 participated in the final photocatalytic redox process to achieve high redox ability for the rapid degradation of pollutants.

Unpaired Electron-Induced Wide-Range Light Absorption within Zn (or Cu) MOFs Containing Electron-Withdrawing Ligands: A Theoretical and Experimental Study.

In photocatalysis, it is of general interest to understand and design wide-range light-absorbing inorganic/organic hybrid materials with an excellent photo-induced intramolecular charge-transfer (ICT) effect. To verify the role of unpaired electrons in enhancing ICT within electron-withdrawing ligand-based metal-organic frameworks (MOFs), the molecular structure, density of states (DOS), and electronic structure of strong electron-deficient pyridine-diketopyrrolopyrrole (P-DPP)-based Zn (or Cu) MOFs were calculated in Gaussian package to validate the unpaired electron ICT. The electron spin resonance technique has detected the unpaired electrons for the coordination systems containing Zn-O or Cu-O clusters and P-DPP ligand on photoexcitation. The estimated band gaps from the DOS calculation for P-DPP-Cu and P-DPP-Zn are 1.4 and 2.4 eV, respectively, showing a good agreement with the experimental UV-vis optical spectra. The partial DOS, dipole moment, and frontier orbital analysis prove that the ICT should happen from Zn-O or Cu-O clusters to P-DPP ligands. This research may contribute to a comprehensive understanding of electron-withdrawing ligand-induced ICT within MOFs and shed light on the design of light-absorbing MOFs with excellent ICT or conductivity.

Remediation of Cu-phenanthrene co-contaminated soil by soil washing and subsequent photoelectrochemical process in presence of persulfate

A promising technology was proposed for the remediation of Cu and phenanthrene (PHE) co-contaminated soil. Firstly, Cu and PHE were simultaneously removed from soil using an extraction agent containing ethylenediaminetetraacetic acid (EDTA) and Tween 80 (TW80). Specifically, Cu was mobilized by the chelation of EDTA while PHE was desorpted from soil by TW80 micelles. At 20 mL g−1 of liquid to soil ratio, 10 g L−1 TW80 and 1 mM EDTA, 73.5% Cu and 68.1% PHE were removed by soil washing. The extraction agent was then recovered from effluent by photoelectro-persulfate (PE/PS) process after soil washing. Under the conditions of 10 mM PS, 4 mA cm−2 current density, natural pH of 3.5 and 150 min treatment, 83.6% Cu was reduced to elemental copper but only 36.8% EDTA was recovered, while 99.6% PHE was eliminated and 94.0% TW80 was recovered. The result of quenching experiments indicated PHE was predominantly oxidized by sulfate radical, though hydroxyl radical and singlet oxygen were also identified by electron paramagnetic resonance technique. The recovered extracting agent could be reused for further three washing cycles. The results demonstrated that soil washing followed by PE/PS process could be a cost-effective remediation technology for heavy metal and organic co-contaminated soil.

Application of spherical harmonics for DEER data analysis in systems with a conformational distribution

Double electron–electron resonance (DEER) and other pulse electron paramagnetic resonance (EPR) techniques are valuable tools for determining distances between paramagnetic centres. DEER theory is well developed for a scenario where relative orientations of paramagnetic centres do not affect the DEER data. In particular, such theory enables a number of approaches for extracting distance distributions. However, in a more general case, when orientation selection effects become substantial, the analytical theory of DEER is less well developed, therefore quite commonly researchers rely on a comparison of some model-based simulations with experimental data. This work elaborates the theory of DEER with orientation selection effects, focusing on a scenario of a moderate conformational disorder, leading to an orientation distribution in a pair of paramagnetic centres. The analytical treatment based on expansions into spherical harmonics, provides important insights into the structure of DEER data. As follows from this treatment, DEER spectra with orientation selection can be represented as a linear combination of modified Pake pattern (MPP) components. The conformational disorder has a filtering effect on the weights of MPP components, specifically by significantly suppressing MPP components of higher degrees. The developed theory provides a pathway for model-based simulations of DEER data where orientation distribution is defined by analytical functions with parameters. The theory based on spherical harmonics expansions was also applied to develop an iterative processing algorithm based on Tikhonov regularization, which disentangles the distance and orientation information in a model-free manner. As an input, this procedure takes several DEER datasets measured at various positions of an EPR line, and outputs a distance distribution and orientation distribution information encoded in a set of coefficients related to the weights of MPP components. The model-based and model-free approaches based on the developed theory were validated for a nitroxide biradical and a spin-labelled protein.

One-pot solvothermal fabrication of S-scheme OVs-Bi2O3/Bi2SiO5 microsphere heterojunctions with enhanced photocatalytic performance toward decontamination of organic pollutants

Bi2O3/Bi2SiO5 microsphere heterojunctions self-assembled by nanoplatelets with rich oxygen vacancies (OVs) were fabricated through a mild one-step solvothermal approach. The existence of OVs was confirmed by XPS and low temperature electron spin resonance (ESR) technique. The crystal texture, morphology, and optical property of the 3D OVs-Bi2O3/Bi2SiO5 microsphere heterojunctions were analyzed by a series of characterization methods. The results reveal that OVs-Bi2O3/Bi2SiO5 heterojunctions were constructed and the diameter of microspheres is about 1 μm. The photocatalytic performance of OVs-Bi2O3/Bi2SiO5 composite microspheres was assessed using methyl orange (MO) and colorless phenol as model contaminant. The synergistic effect of Bi2O3 and Bi2SiO5 greatly enhances the removal rates of organic pollutants. O2– and h+ are predominant active free radicals in the photocatalytic process, supporting by ESR, nitroblue tetrazolium (NBT) and trapping experiments. The separation and transfer of the photoinduced charge pairs of OVs- Bi2O3/Bi2SiO5 heterojunctions follow a step-scheme mechanism. The step-scheme mechanism endows OVs-Bi2O3/Bi2SiO5 heterojunctions with higher photocatalytic performance than the bare Bi2SiO5.

Morphological, dielectric and transport properties of garnet-type Li6.25+yAl0.25La3Zr2-yMnyO12 (y = 0, 0.05, 0.1, and 0.2)

Garnet-type solid Li ion conductor Li7La3Zr2O12 (LLZO) has received considerable attention due to its high total (bulk + grain boundary) ionic conductivity at room temperature and its chemical stability with Li metal. To improve the ionic conductivity of LLZO, the hetrovalent doping at Zr-site and Al-doping at Li site was done for the optimization of Li concentration in LLZO. Thus, considering prominent role of various dopant in ionic and/or electronic conduction of LLZO, the effect of Mn-doping at Zr-site in the Al-doped Li6.25+yAl0.25La3Zr2-yMnyO12 (y = 0, 0.05, 0.1, 0.2) on the microstructure, dielectric and transport properties is studied. The Mn-doping at Zr-site in Al-LLZO is confirmed by electron paramagnetic resonance and X-ray photoelectron spectroscopy techniques. A correlation between structure, microstructure and the Li+ ion conduction is established. Amongst all investigated compounds, Li6.35Al0.25La3Zr1.9Mn0.1O12 exhibit highest total ionic conductivity of 0.34 × 10−4 Scm−1 at 25 °C. Further, the electrochemical window and stability of Mn doped-LLZO with Li metal is examined by cyclic voltammetry.

Trivalent gadolinium doped SrZrO3 perovskite ceramic: Sol-gel synthesis, paramagnetic centres, and luminescence studies

Trivalent gadolinium (Gd3+) doped SrZrO3 perovskite ceramic has been prepared and their X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and photoluminescence (PL) spectra are recorded. The existence of divalent and tetravalent cations in SrZrO3 along with a trivalent dopant ion provides an opportunity to study the effect of charge compensators on the system properties. The cubic phase is evident from XRD diffraction patterns of the product. The SEM analysis suggests the formation of voids and pores in the ceramics caused by gases those liberated during the process of sintering. The EPR technique has been used to investigate the un-doped and Gd3+-doped SrZrO3 ceramics. The un-doped SrZrO3 shows an axially symmetric EPR line with g∥ = 1.94 and g⊥ = 1.99, which is attributed to the Zr3+ ion. The Gd3+-doped SrZrO3 shows dominant EPR lines with effective g-values at 1.93, 2.04, and 2.35. Weak lines are observed at 2.72, 3.64, 6.08, and 15.5. At higher dopant concentrations, a broad line with geff ~2.1 is seen, together with low field lines. In the low field region of the EPR spectrum, lines appear because of immediate environment distortions. Under the excitation of 273 nm, the emission spectrum shows band 6P7/2 → 8S7/2 (313 nm). The emission at 313 nm suggests that the prepared sample can be used as a narrow band UV radiation source. Emission and EPR analysis of the sample confirm that Gd3+ is distributed in both Sr2+ and Zr4+ sites in the SrZrO3 matrix.

Correction to Shifting-Nitroxides to Investigate Enzymatic Hydrolysis of Fatty Acids by Lipases Using Electron Paramagnetic Resonance in Turbid Media.

While optical methods are not efficient enough for the easy, fast, and efficient detection of enzymatic activity in turbid media, the properties of the electron paramagnetic resonance (EPR) technique make it suitable for use in such media. Nitroxides which exhibit a change in their EPR hyperfine coupling constants upon enzymatic activity and are selective to lipases were developed under the name of shifting-nitroxides. Several fatty acids, exhibiting saturated and unsaturated chains of various lengths, were coupled with the shifting-nitroxide via an enol ester link and tested against several lipases. As the solubility of fatty acids is low in HEPES buffer, experiments were performed in turbid aqueous solution. Almost all labeled fatty acids were hydrolyzed by Candida rugosa lipase, and more selectivity is observed with Porcine Pancreas lipase type II. No activity was observed for lipase AK Amano 20, Candida antartica lipase B, and Mucor miehei lipase.

Electron Spin Resonance in Strongly Correlated Metals

Modern technique of electron spin resonance (ESR) measurements and data analysis in strongly correlated metals, which allows finding the full set of spectroscopic parameters including oscillating magnetization, relaxation parameter (line width), and hyromagnetic ratio (g-factor), is described. Application of the considered method is illustrated by such examples as metallic systems CeB6, Mn1−xFexSi, GdB6, HoxLu1−xB12, and metallic surface of topological Kondo insulator SmB6. It is shown that ESR in strongly correlated metals provides a unique tool for studying short-range correlations at the nanoscale of a spin polaron type, electron nematic effects, and spin fluctuation transitions. Application of ESR technique opens new opportunities for observation of quantum critical points, including hidden ones. In many cases, it is just ESR, which either indicates the necessity for clarification or deep revision of the prevailing paradigm, or constitutes the basis for a new concept of magnetic properties of various strongly correlated metals.

The efficient degradation of sulfisoxazole by singlet oxygen (1O2) derived from activated peroxymonosulfate (PMS) with Co3O4–SnO2/RSBC

Co3O4–SnO2/rice straw biochar (RSBC) was prepared for the first time via calcining oxalate precipitation precursor dispersed on the surface of RSBC and used as a catalyst for activating PMS to degrade sulfisoxazole (SIZ). The results demonstrated that Co3O4–SnO2/RSBC possessed much better catalytic performance than Co3O4, Co3O4–SnO2, Co3O4/RSBC, and SnO2/RSBC, which is ascribed to the synergy of Co3O4, SnO2 and RSBC. Approximately 98% of SIZ (50 mg/L) was decomposed by PMS (1 mmol/L) activated with Co3O4–SnO2/RSBC (0.1 g/L) within 5 min. The optimal degradation efficiency of SIZ was realized at the initial pH 9. Co3O4–SnO2/RSBC also displayed remarkable stability and reusability, and the degradation rate of SIZ maintained over 90% even after the fifth recycle run. The electron paramagnetic resonance (EPR) technique and quenching experiments proved singlet oxygen (1O2) to be the main reactive oxygen species (ROS) responsible for the SIZ decomposition in the Co3O4–SnO2/RSBC/PMS system. On the basis of the characterization analysis, the identification of the ROS and the SIZ degradation products, the possible mechanism and pathways of the SIZ degradation by a combination of PMS and Co3O4–SnO2/RSBC were further proposed. This study provides not only a new insight into non-radical mechanism for the heterogeneous activating PMS over Co3O4–SnO2/RSBC to degrade organic pollutants but also an eco-friendly synthetic route for exploring novel and efficient catalysts.

Electron Paramagnetic Resonance as a Tool for Studying Membrane Proteins.

Membrane proteins possess a variety of functions essential to the survival of organisms. However, due to their inherent hydrophobic nature, it is extremely difficult to probe the structure and dynamic properties of membrane proteins using traditional biophysical techniques, particularly in their native environments. Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling (SDSL) is a very powerful and rapidly growing biophysical technique to study pertinent structural and dynamic properties of membrane proteins with no size restrictions. In this review, we will briefly discuss the most commonly used EPR techniques and their recent applications for answering structure and conformational dynamics related questions of important membrane protein systems.

Biospeciation of Potential Vanadium Drugs of Acetylacetonate in the Presence of Proteins.

Among vanadium compounds with potential medicinal applications, [VIVO(acac)2] is one of the most promising for its antidiabetic and anticancer activity. In the organism, however, interconversion of the oxidation state to +III and +V and binding to proteins are possible. In this report, the transformation of VIII(acac)3, VIVO(acac)2, and VVO2(acac) after the interaction with two model proteins, lysozyme (Lyz) and ubiquitin (Ub), was studied with ESI-MS (ElectroSpray Ionization-Mass Spectroscopy), EPR (Electron Paramagnetic Resonance), and computational (docking) techniques. It was shown that, in the metal concentration range close to that found in the organism (15–250 μM), VIII(acac)3 is oxidized to VIVO(acac)+ and VIVO(acac)2, which—in their turn—interact with proteins to give n[VIVO(acac)]–Protein and n[VIVO(acac)2]–Protein adducts. Similarly, the complex in the +IV oxidation state, VIVO(acac)2, dissociates to the mono-chelated species VIVO(acac)+ which binds to Lyz and Ub. Finally, VVO2(acac) undergoes complete dissociation to give the 'bare' VVO ion that forms adducts n[VVO2]–Protein with n = 1–3. Docking calculations allowed the prediction of the residues involved in the metal binding. The results suggest that only the VIVO complex of acetylacetonate survives in the presence of proteins and that its adducts could be the species responsible of the observed pharmacological activity, suggesting that in these systems VIVO2+ ion should be used in the design of potential vanadium drugs. If VIII or VVO2 potential active complexes had to be designed, the features of the organic ligand must be adequately modulated to obtain species with high redox and thermodynamic stability to prevent oxidation and dissociation.

A New Conducting Copolymer Bearing Electro-Active Nitroxide Groups as Organic Electrode Materials for Batteries.

To reduce the amount of conducting additives generally required for polynitroxide-based electrodes, a stable radical (TEMPO) is combined with a conductive copolymer backbone consisting of 2,7-bisthiophene carbazole (2,7-BTC), which is characterized by a high intrinsic electronic conductivity. This work deals with the synthesis of this new polymer functionalized by a redox nitroxide. Fine structural characterization using electron paramagnetic resonance (EPR) techniques established that: 1) the nitroxide radicals are properly attached to the radical chain (continuous wave EPR) and 2) the polymer chain has very rigid conformations leading to a set of well-defined distances between first neighboring pairs of nitroxides (pulsed EPR). The redox group combined with the electroactive polymer showed not only a very high electrochemical reversibility but also a perfect match of redox potentials between the de-/doping reaction of the bisthiophene carbazole backbone and the redox activity of the nitroxide radical. This new organic electrode shows a stable capacity (about 60 mAh g-1 ) and enables a strong reduction in the amount of carbon additive due to the conducting-polymer skeleton.

An inter-laboratory comparison to evaluate the suitability of EN 1787 standard to detect irradiation in plant-origin foods with health benefits

This paper reports the results of a study carried out to verify the applicability of the EN 1787 method, which uses the Electron Spin Resonance (ESR) technique for the identification of irradiated plant-origin foods with health benefits. The method was tested on samples of herbal ingredients of Plant Food Supplements (PFSs), nuts and fresh blueberries. Untreated and irradiated samples of Camellia sinensis (leaves) Ginkgo biloba (leaves), Glycine max (seeds), Silybum marianum (fruits), Vaccinium myrtillus (fruits), almonds, hazelnuts, peanuts, pistachios, walnuts and fresh blueberries were analysed. The work includes an inter-laboratory blind test involving five Italian laboratories that perform routine analyses for the official control of irradiated food. A total of 180 untreated and irradiated samples of PFS ingredients, nuts and fresh blueberries were analysed. The analyses on the irradiated samples were replicated even a long time after irradiation (up to two years depending on the matrix) to test the reliability of the method throughout the shelf life of the products.The results were matrix-dependent: all the 5 kGy irradiated nuts and the 1 kGy-irradiated blueberries were correctly classified, whereas herbal ingredients showed complex ESR spectra with spurious signals which often prevented the correct classification of the sample.

Investigation of the dose-response in multiple irradiated fingernails and the combined effect of drying time-temperature to the ESR signal

Free radicals are formed when human fingernails are exposed to ionizing radiation. These free radicals can easily be detected using an electron spin resonance (ESR) technique. Although recent works have made important contributions to the development of ESR technique with fingernails, the effects of multiple irradiations on sensitivity of the radiation-induced signal and the influence of heat in the drying process of the samples have not been explored adequately. The researchers of this study have investigated the potential effects of multiple irradiations and exposure to heat on the ESR signals of fingernails. As a result, the slope of the dose-response curves decreased which reflected in the decrease of the sensitivity after multiple irradiations of the same fingernails to doses 35 Gy or 70 Gy of 137Cs γ-rays. Furthermore, it was observed that heat exposure considerably affected the visual shape and peak-to-peak intensity of the ESR spectra. Heating to temperatures above 80°C resulted in a noticeable increase to the intensity of the ESR spectra – generating a new ESR signal that overlapped with the native background signal in unirradiated fingernail samples. The results of this study suggested that some potential confounding effects need to be accounted carefully if ESR dosimetry of fingernails is to be applied in real radiation accident situations.

Production of Reactive Oxygen Species by the Reaction of Periodate and Hydroxylamine for Rapid Removal of Organic Pollutants and Waterborne Bacteria.

Periodate (PI, IO4-) can be activated by hydroxylamine (HA), resulting in the rapid removal of organic pollutants within seconds. While the previous studies on PI-based advanced oxidation processes (AOPs) have proposed iodate radical (•IO3) as the major reactive species, no evidence of •IO3 production was found in the present PI/HA system. Reactive oxygen species (ROS) including •OH, HO2•, and 1O2 are proposed to be the main oxidants of the PI/HA system, which is supported by various tests employing the scavengers, chemical probes, and spin-trapping electron paramagnetic resonance (EPR) technique. To minimize the risk of toxic iodinated byproduct formation caused by reactive iodine species such as HOI and I2, the molar ratio of HA/PI was optimized at 0.6 to achieve the stoichiometric conversion of IO4- to iodate (IO3-), a preferred nontoxic sink of iodine species. The PI/HA system also efficiently inactivated both Gram-positive and -negative bacteria with producing 1O2 as the dominant disinfectant. The mechanism of ROS production was also investigated and is discussed in detail. This work offers a simple and highly efficient option for PI activation and ROS production which might find useful applications where urgent and rapid removal of toxic pollutants is needed.

A novel Au-SnO2-rGO ternary nanoheterojunction catalyst for UV-LED induced photocatalytic degradation of clothianidin: Identification of reactive intermediates, degradation pathway and in-depth mechanistic insight

Herein, we have developed Au-SnO2-rGO ternary nanoheterojunction catalyst for photocatalytic degradation of clothianidin under UV-LED irradiation. Au nanoparticles (Au NPs) were decorated with quantum sized SnO2 nanoparticles (SnO2 NPs) and embedded in the reduced graphene oxide (rGO) matrix. The synthesized photocatalyst was characterized using common instrumentation techniques to evaluate the morphological, structural and surface chemical properties of the catalyst. The photocatalytic property of the as-synthesized nanocomposite was investigated by studying the degradation of clothianidin under UV-LED irradiation. Approximately 97% of the degradation was achieved in 120 min with pseudo first-order reaction rate of 0.0309 min-1. The effect of various parameters like contact time, catalyst dose, initial concentration, pH, irradiation source and interfering ions on the degradation performance has also been investigated. Identification of various intermediates in the degradation process was done with high performance liquid chromatography- mass spectral (LC/HRMS) technique. The reactive intermediates responsible for the degradation have also been identified with electron spin resonance (ESR) technique and complete mechanism of the degradation process has been proposed with the help of photo luminescence (PL) quenching studies.