Paramagnetic Probe Research Articles

Breaking of the Cubic Symmetry in Millimeter-Size SrTiO3 Crystals: ESR Manifestations

The found effect of lowering the symmetry of the crystal structure of thin (less than 1 mm) plates and bars of strontium titanate (SrTiO3) single crystals is discussed. This symmetry lowering manifests itself in the electron spin resonance (ESR) spectra of impurity centers (Fe3+and Mn4+ions, used as paramagnetic probes). It is shown that symmetry lowering is observed at temperaturesT 105 K, which generally correspond to the cubic phase of SrTiO3; it leads to the formation of a tetragonal nonpolar structure, differing from the antiferrodistortive (AFD) phaseD4ℎ18, which is characteristic of strontium titanate atT 105 K and was not observed previously in SrTiO3. The factors determining the distortion value are found to be the geometry and ratio of sample sizes, surface finishing quality, and crystallographic orientation of plates.

Lipid- and substrate-induced conformational and dynamic changes in a glycosyltransferase involved in E. coli LPS synthesis revealed by 19F and 31P NMR

WaaG is a glycosyltransferase (GT) involved in the synthesis of the bacterial cell wall, and in Escherichia coli it catalyzes the transfer of a glucose moiety from the donor substrate UDP-glucose onto the nascent lipopolysaccharide (LPS) molecule which when completed constitutes the major component of the bacterium's outermost defenses. Similar to other GTs of the GT-B fold, having two Rossman-like domains connected by a short linker, WaaG is believed to undergo complex inter-domain motions as part of its function to accommodate the nascent LPS and UDP-glucose in the catalytic site located in the cleft between the two domains. As the nascent LPS is bulky and membrane-bound, WaaG is a peripheral membrane protein, adding to the complexity of studying the enzyme in a biologically relevant environment. Using specific 5-fluoro-Trp labelling of native and inserted tryptophans and 19F NMR we herein studied the dynamic interactions of WaaG with lipids using bicelles, and with the donor substrate. Line-shape changes when bicelles are added to WaaG show that the dynamic behavior is altered when binding to the model membrane, while a chemical shift change indicates an altered environment around a tryptophan located in the C-terminal domain of WaaG upon interaction with UDP-glucose or UDP. A lipid-bound paramagnetic probe was used to confirm that the membrane interaction is mediated by a loop region located in the N-terminal domain. Furthermore, the hydrolysis of the donor substrate by WaaG was quantified by 31P NMR.

Enhanced degradation of tetracycline hydrochloride with in situ prepared ferrate supernatant based on calcium hypochlorite: Compare with purified and commercial ferrate

Ferrate (Fe(VI)) salts are commonly regarded as green and high efficient oxidants to eliminate organic contaminants. Current studies mainly focused on develop homogeneous activation technology or heterogeneous catalysts to promote Fe(VI) performances in contaminants degradation. Interestingly, this study disclosed that the degradation performance of tetracycline hydrochloride (TC) was enhanced by using in situ prepared ferrate supernatant (Fe(VI)s), and the oxidation performance and degradation mechanism were systematically explored by comparing with the ferrate precipitation (Fe(VI)p) and the commercial ferrate (Fe(VI)c). Results showed that the degradation rate of TC by Fe(VI)s reached to 98.5 % within 5 min, while the degradation rate of Fe(VI)p and Fe(VI)c were obtained less than 20 % in 30 min. Although degradation process of TC in these three system were all conformed to the pseudo-first-order reaction kinetic, the rate of degradation (Kobs) obtained in Fe(VI)s was 5.30 and 37.2 times higher than that in Fe(VI)c and Fe(VI)p, respectively. Electron paramagnetic resonance (EPR), radical quenching and PMSO probe experiments proved that Fe(IV)/Fe(V) were the main active species for TC degradation with Fe(VI)s, while ∙OH and ⋅O2− were the main active substance of Fe(VI)c. On the basis of the intermediates detected by LC-MS/MS analysis, a pathway for TC degradation was proposed. In addition, the toxicity of TC and its degradation products was evaluated by using the toxicity evaluation software tool. This study demonstrated efficient organic contaminant oxidation with in situ prepared ferrate solution (Fe(VI)s) and provided a comprehensive understanding of degradation process for pollutants removal.

Development of a Suite of Gadolinium-Free OATP1-Targeted Paramagnetic Probes for Liver MRI.

Five amphiphilic, anionic Mn(II) complexes were synthesized as contrast agents targeted to organic anion transporting polypeptide transporters (OATP) for liver magnetic resonance imaging (MRI). The Mn(II) complexes are synthesized in three steps, each from the commercially available trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA) chelator, with T1-relaxivity of complexes ranging between 2.3 and 3.0 mM-1 s-1 in phosphate buffered saline at an applied field strength of 3.0 T. Pharmacokinetics were assessed in female BALB/c mice by acquiring T1-weighted images dynamically for 70 min after agent administration and determining contrast enhancement and washout in various organs. Uptake of Mn(II) complexes in human OATPs was investigated through in vitro assays using MDA-MB-231 cells engineered to express either OATP1B1 or OATP1B3 isoforms. Our study introduces a new class of Mn-based OATP-targeted contrast that can be broadly tuned via simple synthetic protocols.

Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes.

Layered zinc hydroxynitrate (ZHN), with the chemical formula Zn5 (OH)8 (NO3)2·2H2O, exhibits a range of special properties such as anion-exchange and intercalation capacity, as well as biocompatibility, making it attractive for a large variety of applications in fields from nanotechnology to healthcare and agriculture. In this study nanocrystalline ZHN doped with 1,000ppm Mn2+ was prepared by two synthesis methods (coprecipitation and solid state reaction) using similar environment-friendly precursors. The complex morpho-structural [X-ray diffraction, scanning and transmission electron microscopy, textural analysis] and spectroscopic [Fourier transform infrared and electron paramagnetic resonance (EPR)] characterization of the two ZHN nanopowders showed similar crystalline structures with Mn2+ ions localized in the nanocrystals volume, but with differences in their morphological and textural characteristics, as well as in the doping efficiency. ZHN obtained by coprecipitation consists of larger nanoplatelets with more than two times larger specific surface area and pore volume, as well as a dopant concentration than in the ZHN sample obtained by solid state reaction. The thermal stability and the on-set of the structural phase transformation have been investigated at atomic scale with high accuracy by EPR, using Mn2+ as paramagnetic probes. The on-set of the ZHN structural phase transformation toward ZnO was observed by EPR to take place at 110°C and 130°C for the samples prepared by coprecipitation and solid state reaction, respectively, evidencing a manganese induced local decrease of the transformation temperature. Our results contribute to the selection of the most appropriate ZHN synthesis method for specific applications and in the development of new green, cost-effective synthesis routes for Mn2+ doped nano-ZnO.

PH-Sensitive Paramagnetic Probe 4-(Methylamino)-2-Ethyl-5,5-Dimethyl-4-Pyridine-2-Yl-2,5-Dihydro-1H-Imidazol-1-Oxyl for Controlling Microacidity Inside Poly-D,L-Lactide Films during Degradation In Vitro

pH-Sensitive Paramagnetic Probe 4-(Methylamino)-2-Ethyl-5,5-Dimethyl-4-Pyridine-2-Yl-2,5-Dihydro-1H-Imidazol-1-Oxyl for Controlling Microacidity Inside Poly-D,L-Lactide Films during Degradation In Vitro

Fe-Trimesic Acid/Melamine Gel-Derived Fe/N-Doped Carbon Nanotubes as Catalyst of Peroxymonosulfate to Remove Sulfamethazine

The conventional precursor preparation of metal–organic frameworks (MOFs) for nitrogen-doping carbon materials is divided into the preparation of MOFs and the mixing of the nitrogen source, which is a complex and time-consuming step. In this study, Fe-BTC gel/nitrogen source-derived carbon materials were synthesized using one or more of the following raw ingredients: Fe(NO3)3·9H2O, FeCl3·6H2O, Fe2(SO4)3, trimesic acid, melamine and dicyandiamide. The influence of different raw ingredients on the preparation and performance of catalysts was investigated. Fe(NO3)3·9H2O can react with trimesic acid to form a gel with ethanol as solvent, and the gel helped the homogeneous dispersion of the added melamine and did not precipitate. Fe-C-N(M), synthesized from the three materials mentioned, was identified as the optimal catalyst; the removal rate of 5 mg/L sulfadimethoxine (SMZ) reached 100% at 15 min when the Fe-C-N(M) dosage was 50 mg/L, PMS concentration was 0.5 mM, and the pH was 5.78 (initial pH of the solution). The removal of SMZ was not significantly inhibited by the pH (3–9) and 0–10 mM inorganic anions (Cl−, NO3−, HCO3− and H2PO42−). Through quenching tests, electron paramagnetic resonance and probe experiments, 1O2 and a small amount of free radicals (•OH and SO4•−) bound on the catalyst surface are discovered to be the primary active ingredients that activate PMS to degrade SMZ.

Formation of extramembrane β-strands controls dimerization of transmembrane helices in amyloid precursor protein C99

The 99-residue C-terminal domain of amyloid precursor protein (APP-C99), precursor to amyloid beta (Aβ), is a transmembrane (TM) protein containing intrinsically disordered N- and C-terminal extramembrane domains. Using molecular dynamics (MD) simulations, we show that the structural ensemble of the C99 monomer is best described in terms of thousands of states. The C99 monomer has a propensity to form β-strand in the C-terminal extramembrane domain, which explains the slow spin relaxation times observed in paramagnetic probe NMR experiments. Surprisingly, homodimerization of C99 not only narrows the conformational ensemble from thousands to a few states through the formation of metastable β-strands in extramembrane domains but also stabilizes extramembrane α-helices. The extramembrane domain structure is observed to dramatically impact the homodimerization motif, resulting in the modification of TM domain conformations. Our study provides an atomic-level structural basis for communication between the extramembrane domains of the C99 protein and TM homodimer formation. This finding could serve as a general model for understanding the influence of disordered extramembrane domains on TM protein structure.

Controlling Disassembly of Paramagnetic Prodrug and Photosensitizer Nanoassemblies for On-Demand Orthotopic Glioma Theranostics

Controlling delivery and release of therapeutic agents to accomplish on-demand synergistic therapy of orthotopic gliomas is desired but challenging. Here, we report a glioma targeting and redox activatable theranostic nanoprobe (Co-NP-RGD1/1) for magnetic resonance (MR) and fluorescence (FL) bimodal imaging-guided on-demand synergistic chemotherapy/photodynamic therapy (Chemo-PDT) of orthotopic gliomas. Co-NP-RGD1/1 is formed via molecular coassembly of two paramagnetic and fluorogenic small-molecule probes CPT-RGD and PPa-RGD at an optimized molar ratio of 1/1, which shows a high longitudinal relaxivity (r1 = 17.0 ± 0.6 mM-1 s-1, 0.5 T) but weak FL emissions and low Chemo-PDT activity. Upon reduction by endogenous glutathione (GSH), Co-NP-RGD1/1 disassemble and release small molecules 2-RGD, chemodrug camptothecin (CPT), and near-infrared (NIR) photosensitizer (PS) PPa-SH that further binds to endogenous albumin to form PPa-SH-albumin complex, allowing to turn on FL, chemotherapeutic efficacy, and PDT activity for synergistic Chemo-PDT of orthotopic U87MG or U251 gliomas in living mice. Moreover, Co-NP-RGD1/1 can also allow noninvasive detection and monitoring of orthotopic brain tumor growth via FL and MR imaging. Findings suggest the potential of cascade coassembly and stimuli-controlled intracellular disassembly strategy for constructing targeted and activatable nanoagents for improving combinational cancer theranostics.

Bio-synthesized multi-metal oxides with varying Fe/Mn ratios and transitional metals (Ni, Ce, Al, Cu) for catalytic ozonation

Fe and Mn are frequently used to synthesize ozonation catalysts via physicochemical routes. In this study, we used a manganese oxidizing bacterium Pseudomonas sp. to produce biogenic Fe-Mn oxides (Bio-FeMnOx) and compared the effect of doping a third transitional metal (Ni, Ce, Al, Cu). It was found that Fe/Mn ratios can effectively regulate the catalytic activity of Bio-FeMnOx and Cu showed the best doping effect among the four metals. The degradation kinetic constant of the optimal Bio-FeMnOx-Cu was approx. 3 times that of its chemogenic peer. Electron paramagnetic resonance and probe tests revealed that the reactive oxygen species were •OH, •O2－ and 1O2. The generation of •OH was preferred at pH 2 over pH 7 while •O2－ was generated in the opposite preference. We also observed the multi-valence of all metals in Bio-FeMnOx-Cu whose good stability were confirmed with 5 cycles of usage. The catalytic performance of Bio-FeMnOx-Cu was also confirmed with real wastewater. This study demonstrated the importance of Fe/Mn ratios and doping a third transitional metal regarding the activity of catalyzing ozonation and thus would provide a valuable reference for producing biogenic metal oxides of high catalytic activity.

Electrochemical activation of persulfate by Al-doped blue TiO2 nanotubes for the multipath degradation of atrazine

The combination of electrolysis and persulfate activation (E/PDS) is a cost-effective method for the treatment of refractory organics. However, persulfate is difficult to be activated into radicals at the anode, resulting in insufficient electro-activation efficiency. Herein, Al doped blue TiO2 nanotube electrodes (Al-bTNT) were first employed as cost-effective anode materials to fully activate PDS to radicals. In E/PDS, the kinetic constant of atrazine removal by Al-bTNT (0.048 min−1) substantially outperformed the other anodes, including the blue TiO2 nanotube electrodes (bTNT) (0.024 min−1), Ti4O7 (0.02 min−1), and B doped diamond (BDD) anodes (0.023 min−1). The Al-bTNT-E/PDS exhibited a low energy consumption (EEO = 0.72 kWh m−3) and a high mineralization rate. Based on the results of electron paramagnetic resonance, quenching experiments, and probe experiments, we propose that atrazine degrades in the Al-bTNT-E/PDS system mainly via a novel radical pathway that involves both·OH and SO4·- and the generated SO4·- is responsible for the enhanced removal rate. The oxygen vacancies (VO) generated from interstitial Al may serve as the active sites to adsorb and dissociate the persulfate molecules based on extensive characterizations. The attempt at soil-washing wastewater disposal indicated the synergistic system possessed good potential for future practical application.

МЕТАЛООРГАНІЧНІ МАГНЕТИКИ НА ОСНОВІ КОМПЛЕКСІВ ЗАЛІЗА З 1-НІТРОЗО-2-НАФТОЛОМ

Providing macroscopic magnetism in organic materials is a very complex but quite promising scientific problem. The need to create an organic magnet is due to a number of expected advantages, such as lightness, transparency, flexibility, ability to switch under the influence of light (magneto-optics), or chemical influences (sensors), creation of modern toners for digital printing, materials for chemical power sources etc. To understand the mechanism of biological processes, in particular, human thinking and DNA functioning may help to study the state of spin glass, biomagnetism, the mechanism of action of paramagnetic probes in living tissues and others. In the present paper the peculiarities of the structure and magnetic behavior of the iron complex with 1-nitroso-2-naphthol Na[Fe(C10H6(NO2)3] have been studied. The powder X-ray diffraction method determined that the crystal structure of the complex is monoclinic with the space group P2/1. According to cyclic voltammetry, the electrochemical behavior of the complex anion Fe(C10H6(NO2)3] is characteristic of reversible electrochemical systems with one electron transfer. Optical absorption bands are observed in the electronic spectra of the complex at 389, 690, and 763 nm. The dependences of the magnetic susceptibility of the complex on the temperature, frequency and magnetic field strength in the temperature range 1.5–200 K in the external magnetic field up to 90 kE and in the frequency range from 95 to 2000 Hz are obtained and analyzed. At low temperatures, the peculiarities of magnetic behavior characteristic of the state of spin glass are revealed. The EPR spectrum of the complex is a superposition of two lines, the behavior of which is opposite when the temperature changes in the range of 4–293 K, which indicates the unusual dynamics of the molecular surrounding the Fe3+ ion. Such features may be due to the presence of two structurally inhomogeneous magnetic centers that exhibit opposite spin dynamics with changing temperature. The presence of this dynamic can have a significant impact on the properties of the substance.

INFLUENCE OF MOISTURE ON THE SHORT-RANGE ORDERING OF CROSSLINKED GLYCOPOLYMERS BASED ON PLANT POLYSACCHARIDE KONJAC GLUCOMANNAN

Using X-ray diffraction data the short-range ordering was analyzed of the plant polysaccharide konjac glucomannan (KGM) and glycopolymers of different composition based on KGM and ε-caprolactam blocked isocyanates in the presence of saturated water vapor both for moistened and dried systems. The decrease in the values of the Breg’s period corresponding to the average distances between atoms and atomic groups in saturated water vapor is shown for glycopolymers placed under normal conditions. It was found that when the crosslinked KGM based glycopolymers are moistened, a secondary maximum is observed on the diffractograms, which corresponds to the ordering of the polysaccharide chains in the transverse direction. This secondary maximum was observed on the diffractograms of uncrosslinked KGM, but was absent on the initial diffractograms of crosslinked samples. This secondary maximum does not appear again on the diffractograms of dried glycopolymer samples. The influence of moisturizing-drying processes on the structuring and moisture content in polysaccharide and glycopolymers is analyzed. The results of WAXS correlate with thermogravimetry data on changes in the ability of KGM and KGM-based glicopolymers to retain moisture in humidified and dried systems. Using a nitroxyl paramagnetic probe, it has been demonstrated that the increase in molecular mobility of glycomolymers based on konjac glucomannan under the influence of low molecular weight plasticizer does not depend on isocyanate used. This effect allows correlating the irreversible effect of swelling and subsequent drying on the characteristics of these KGM-based systems with the achievement of macrochains of the polymer more equilibrium conformations due to increasing molecular mobility in the presence of moisture.

Anionic lipids induce a fold-unfold transition in the membrane-translocating Engrailed homeodomain

Homeoprotein transcription factors have the property of interacting with membranes through their DNA-binding homeodomain, which is involved in unconventional internalization and secretion. Both processes depend on membrane-translocating events but their detailed molecular mechanisms are still poorly understood. We have previously characterized the conformational properties of Engrailed 2 homeodomain (EnHD) in aqueous solution and in micelles as membrane-mimetic environments. In the present study, we used small isotropic lipid bicelles as a more relevant membrane-mimetic model to characterize the membrane-bound state of EnHD. We show that lipid bicelles, in contrast to micelles, adequately reproduce the requirement of anionic lipids in the membrane binding and conformational transition of EnHD. The fold-unfold transition of EnHD induced by anionic lipids was characterized by NMR using 1H, 13C, 15N chemical shifts, nuclear Overhauser effects, residual dipolar couplings, intramolecular and intermolecular paramagnetic relaxation enhancements induced by site-directed spin-label or paramagnetic lipid probe, respectively. A global unpacking of EnHD helices is observed leading to a loss of the native fold. However, near-native propensities of EnHD backbone conformation are maintained in membrane environment, including not only the three helices but also the turn connecting helices H2 and H3. NMR and coarse-grained molecular dynamics simulations reveal that the EnHD adopts a shallow insertion in the membrane, with the three helices oriented parallel to the membrane. EnHD explores extended conformations and closed U-shaped conformations, which are stabilized by anionic lipid recruitment.

Paramagnetic probes in an organic semiconductor: μSR and DFT calculations of the Mu adducts of Alq3 and 8-hydroxyquinoline.

It has been claimed that longitudinal field muon spin relaxation (LF-μSR) experiments on the organic semiconductor (OSC) tris-(8-hydroxyquinoline)aluminum(III) (Alq3) have measured electron hopping rates of ∼1012s-1, while density functional theory (DFT) calculations suggest that electron hopping between a muoniated radical and a neighboring molecule is energetically unfavorable and that the LF-μSR experiments were probing muoniated radicals with localized spin density. We have performed avoided level crossing muon spin resonance (ALC-μSR) and transverse field muon spin rotation (TF-μSR) measurements on Alq3 and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq3 when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-μSR and TF-μSR spectra of 8hq and Alq3 are best explained by Mu adding to all six secondary carbons of the quinolate rings with roughly equal yields and localized spin density. There is no evidence in the TF-μSR spectrum of Alq3 for the formation of radicals with muon hyperfine coupling constants of 23 or 91MHz as reported earlier by others. Our measurements support the view that there is localized spin density on the molecule to which Mu is covalently bound and the muon is not a passive probe in organic systems as it can be incorporated into radicals that have different electronic structures to the parent compounds. The muoniated radicals in Alq3 are more short-lived than in 8hq, which could be due to interactions with mobile electrons in the OSC, but with electron spin flip rates on the order of ∼107s-1.

Molecular oxygen as a probe molecule in EPR spin-labeling studies of membrane structure and dynamics.

Molecular oxygen (O2) is the perfect probe molecule for membrane studies carried out using the saturation recovery EPR technique. O2 is a small, paramagnetic, hydrophobic enough molecule that easily partitions into a membrane's different phases and domains. In membrane studies, the saturation recovery EPR method requires two paramagnetic probes: a lipid-analog nitroxide spin label and an oxygen molecule. The experimentally derived parameters of this method are the spin-lattice relaxation times (T 1s) of spin labels and rates of bimolecular collisions between O2 and the nitroxide fragment. Thanks to the long T 1 of lipid spin labels (from 1 to 10 μs), the approach is very sensitive to changes of the local (around the nitroxide fragment) O2 diffusion-concentration product. Small variations in the lipid packing affect O2 solubility and O2 diffusion, which can be detected by the shortening of T 1 of spin labels. Using O2 as a probe molecule and a different lipid spin label inserted into specific phases of the membrane and membrane domains allows data about the lateral arrangement of lipid membranes to be obtained. Moreover, using a lipid spin label with the nitroxide fragment attached to its head group or a hydrocarbon chain at different positions also enables data about molecular dynamics and structure at different membrane depths to be obtained. Thus, the method can be used to investigate not only the lateral organization of the membrane (i.e., the presence of membrane domains and phases), but also the depth-dependent membrane structure and dynamics, and, hence, the membrane properties in three dimensions.

Incorporation of residual chemical shift anisotropy into the treatment of 15N pseudocontact shifts for structural refinement

Paramagnetic NMR experiments, including the pseudocontact shift experiment, have seen increasing use due to recently developed probes and labeling strategies. The pseudocontact shift experiment can provide valuable intra- or inter-molecular distance and orientation information. However, the use of 1H/13C or 1H/15N PCS data in structure calculations is currently complicated by the contribution of residual chemical shift anisotropy to the 13C or 15N datasets. Here, we present a corrected PCS energy term for the software package Xplor-NIH with the appropriate residual chemical shift anisotropy correction and show its suitability for model refinements of ubiquitin labeled at residue 57 with a Tm-M8-SPy tag. For data taken at 800 MHz, the improvement with the corrected energy term is sufficient to make the quality of the fit for the 15N dataset comparable to that of the 1H dataset, for which no correction is needed. The corrected energy term is expected to become more relevant with increased use of higher field instruments and as new paramagnetic probes with larger magnetic susceptibility tensors continue to be developed.

Iron active sites encapsulated in N-doped graphite for efficiently selective degradation of emerging contaminants via peroxymonosulfate (PMS) activation: Inherent roles of adsorption and electron-transfer dominated nonradical mechanisms

In this work, Fe-based graphite catalyst (Fe-0.6@N-GC) was successfully synthesized to degrade sulfisoxazole (SIZ) via peroxymonosulfate (PMS) activation. Fe-0.6@N-GC has large specific surface area and abundant pyridinic N sites, leading to superior SIZ adsorption. Complete degradation of SIZ (5 mg/L) can be achieved within 20 min in the Fe-0.6@N-GC/PMS system. Electrochemical analysis, electron paramagnetic resonance, scavenging and probe experiments illustrated that SIZ can be efficiently degraded via mediated electron transfer pathway from SIZ to PMS, as well as the partial contribution of singlet oxygen (1O2). Meanwhile, stable encapsulation of scattered Fe sites on N-doped porous carbon greatly reduced the iron leaching (≤0.023 mg/L). High selectivity for different pollutants degradation and tolerance of coexisting ions were performed which is beneficial to practical application. Finally, the possible degradation pathways were also proposed by analyzing intermediates. Overall, this study provides a new understanding of nonradical pathways in organic pollutants oxidation by catalysis of iron-based catalysts.

Application of electron paramagnetic resonance in solution property

<p indent="0mm">Electron paramagnetic resonance (EPR) technology is a powerful spectroscopy tool which can be used to detect substances containing at least one unpaired electron. The properties of the solutions can be quickly measured by the EPR parameters of the paramagnetic probe. This review summarized the application of EPR technology in four aspects: polarity, hydrogen bonding, viscosity, and pH. Because this technology measures the local properties around the probe, it has specific advantages for samples that are difficult to evaluate by other technologies, such as opaque, heterogeneous samples, and samples with heterogeneity in the nanometer range.

Bicarbonate-enhanced generation of hydroxyl radical by visible light-induced photocatalysis of H2O2 over WO3: Alteration of electron transfer mechanism

The generation of hydroxyl radical (•OH) by visible light-illuminated tungsten oxide (WO3) was found to be significantly improved in the presence of hydrogen peroxide (H2O2) and bicarbonate ion (HCO3−). A ternary system of hν/WO3/H2O2/HCO3− showed synergistic enhancement in oxidation of benzoic acid (BA, a •OH probe compound) into hydroxybenzoic acids (HBAs), exhibiting even less consumption of H2O2 than hν/WO3/H2O2. Analyses of HBAs from BA oxidation (three HBA isomers and 18O-labelled HBA from H218O2) suggested that hν/WO3/H2O2/HCO3−, contrary to hν/WO3/H2O2, generated •OH mainly via one-electron transfer from the conduction band of WO3 to H2O2. The dominant one-electron reduction of H2O2 over HCO3−-treated WO3 was further evidenced by Koutecký–Levich plots obtained with a rotating disk electrode setup. Based on different experiments using electron paramagnetic resonance spectroscopy, radical scavengers and probes, (photo-)electrochemical measurements, and density functional theory calculations, the mechanisms underlying the enhanced generation of •OH by hν/WO3/H2O2/HCO3− were discussed.