Electron Spin Research Articles

Probing skin photoallergens in reconstructed human epidermis: An EPR spin trapping investigation.

Photoallergic contact dermatitis is a skin disease caused by combined exposure to photoreactive chemicals and sunlight. Exposure to allergens and the risk of skin sensitization is an essential regulatory issue within the industry. Yet, only few non-validated assays for photoallergy assessment exist as the pathogenesis is not fully deciphered. Improving such assays and/or developing new ones require an understanding of the chemical mechanisms involved. The first key event in the photosensitization process, namely chemical binding of the photoallergen to endogenous proteins, is thought to proceed via photo-mediated radicals arising from the photoallergen. Moreover, the mechanism of action of these radicals if formed in the epidermis is not known and far from being unraveled. We present here an original proof-of-concept methodology to probe radical generation from allergens in contact with photoexposed skin, using electron paramagnetic resonance and spin trapping in a reconstructed human epidermis model mimicking real-life exposure scenarios.

Rapid Oxidative Dissolution of Zerovalent Iron Induced by Sulfite for Efficient Removal of Arsenate and Arsenite: Selective Formation of Scorodite.

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO4•-, •OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

Core-Shell Structured Cobalt Sulfide/C. I. Pigment Yellow 53 Photocatalysts with Abundant Sulfur Vacancies for Efficient Photocatalytic Co-Production of Xylonic Acid and CO.

Photocatalytic biorefinery has been gaining increasing attention as a promising method for utilizing biomass and solar energy, yet it still faces the key challenge of designing stable, efficient, and cost-effective photocatalysts. In this study, cobalt sulfide/ C. I. Pigment Yellow 53 composite photocatalysts (CoS/PY53-CSx) with a core-shell structure, which has abundant sulfur (S) vacancies, are developed using a simple hydrothermal method. The CoS nanocage with S vacancies not only offers numerous active sites but also enhances the light-trapping performance of PY53. Moreover, the internal electric field within the core-shell CoS/PY53-CSx further enhances charge separation/transfer efficiency while reducing electron transfer resistance, thereby boosting photocatalytic activity. Remarkably, 75.2% of xylonic acid and 22.8µmol of CO from xylose are obtained using CoS/PY53-CS0.1 in an air atmosphere. Recycling experiments demonstrate that CoS/PY53-CS0.1 exhibits excellent recyclability due to the strong bonding force between the core and shell. In addition, electron spin resonance characterization combined with poisoning experiments suggests that h+ and ·O2 - serve as the main oxidation active species during this system. This work presents a simple and cost-effective method for efficient photocatalytic biorefinery.

X-Band Single Chip Integrated Pulsed Electron Spin Resonance Microsystem.

We report on the design and characterization of a single chip integrated pulsed electron spin resonance detector operating at 9.1 GHz. The microsystem consists of an excitation microcoil, a detection microcoil, a low noise microwave preamplifier, a mixer, and an intermediate frequency (IF) amplifier. The chip area is about 0.7 mm2. To exemplify its possible applications, we report the results of single pulse, Rabi nutation, Hahn echo, two echoes, Carr-Purcell, and inversion recovery echo experiments performed on 0.02 and 0.05 nL samples of α, γ-bisdiphenylene-β-phenylallyl (BDPA) and 1% BDPA in polystyrene (BDPA:PS) at room temperature. The measured spin sensitivity is about 8 × 107 spins/Hz1/2 on a sensitive volume of about 0.1 nL. The microsystem power consumption is less than 100 mW, the radio frequency (RF) input bandwidth is 8.8 to 9.8 GHz, the IF output bandwidth is DC to 350 MHz, and the deadtime is less than 30 ns.

Photoexcitation and recombination processes of the neutral nitrogen-vacancy center in diamond from first principles

Nitrogen-vacancy (NV) complex in diamond is one of the most prominent solid state defects as the negatively charged NV defect (NV−) is a leading contender for quantum technologies. In quantum information processing applications, NV− is photoexcited that often leads to photoionization to neutral NV defect, NV0, and re-ionization back to NV− should occur to control the S=1 spin of NV−. As a consequence, understanding the photophysics of NV0 is crucial for controlling NV−. Furthermore, recent studies have shown that the S=1/2 electron spin of NV0 can also be initialized and read out at certain conditions that turns single NV0 a potential quantum bit. Quantum optics protocols rest on detailed knowledge on the electronic structure of the given system, which is obviously missing for NV0 in diamond. In this study, we combine the group theory and density functional theory calculations toward exploring the nature of the ground and excited states of NV0. We show that the effective three-electron system of NV0 leads to high correlation effects that make this system very challenging for ab initio simulations.

Electron Transfer Induced by the Change of Spin States as a Catalytic Descriptor on C2N-TM Single-Atom Catalysts.

The catalytic activity and selectivity of metal single-atom catalysts strongly depend upon their spin states. However, their intrinsic connections are not yet clear. In this work, we evaluate the catalytic activity and selectivity of oxygen reduction reactions (ORRs) on C2N-supporting 3d transition metal (TM = Mn/Co/Ni/Cu) single-atom catalysts (SACs) using the density functional theory calculations. It is found that all of the SACs with different spin states tend to follow the 2e- H2O2 pathway, except for C2N-Mn (S = 1/2), which takes the 4e- OOH pathway. Interestingly, we found that the sum of the changes in the electron spin moments of the metal active centers and the reaction intermediate OOH affects the OOH electron transfer, and the electron transfer promotes the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs. Moreover, there is a strong linear relationship between the OOH electron transfer and the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs. These findings indicate that electron transfer induced by the change of spin states serves as a descriptor of the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs, which is very helpful for designing more powerful SACs.

Mechanisms of adaptive resistance in Phytobacter sp. X4 to antimony stress under anaerobic conditions

Sb(III) oxidation by microorganisms plays a key role in the geochemical cycling of antimony and is effective for bioremediation. A previously discovered novel Sb(III)-oxidizing bacteria, Phytobacter sp. X4, was used to elucidate the response patterns of extracellular polypeptides (EPS), antioxidant system, electron transfer and functional genes to Sb(III) under anaerobic conditions. The toxicity of Sb(III) was mitigated by increasing Sb(III) oxidation capacity, and the EPS regulated the content of each component by sensing the concentration of Sb(III). High Sb(III) concentrations induced significant secretion of proteins and polysaccharides of EPS, and polysaccharides were more important. Functional groups including hydroxyl, carboxyl and amino groups on the cell surface adsorbed Sb(III) to block its entry. Hydroxyl radicals and hydrogen peroxide were involved in anaerobic Sb(III) oxidation, as revealed by changes in the antioxidant system and electron spin resonance (EPR) techniques. qPCR confirmed that proteins concerning nitrate and antimony transfer, antimony resistance and antioxidant system were regulated by Sb(III) concentration, and the synergistic cooperation of multiple proteins conferred high antimony resistance to X4. The adaptive antimony resistance mechanism of Phytobacter sp. X4 under anaerobic conditions was revealed, which also provides a reference value for bioremediation method of antimony contamination in anaerobic environment.

ESR chronology of the Quaternary sedimentary sequence of the Shinaimiao borehole in Hebi City, Henan Province, China

It is very important to distinguish the strata of different periods in the Quaternary period, especially the Middle Pleistocene and Late Pleistocene strata, for stratigraphic division, depositional environmental analysis and climate evolution analysis. In this study, the Shinaimiao (SNM) borehole sequence in Hebi City, Henan Province (China) was established by both electron spin resonance (ESR) and magnetostratigraphy methods. Both Al (eighteen samples) and Ti-Li (eight out of the eighteen samples) centers were measured using the Multiple Aliquots Additive Dose (MAAD) method. In addition, hundreds of magnetostratigraphic samples were measured to determine the precise position of the Matuyama/Brunhes (M/B) boundary. The results show that: (1) The equivalent dose (DE) values of the Al and Ti-Li centers are consistent within the error range, demonstrating that the Multiple Center (MC) approach has worked. (2) The ESR dating results are consistent with the order of sedimentation (the ages get older with depths) and with magnetostratigraphic results of the B/M boundary within the error range, indicating that the ESR ages obtained from the Al and Ti-Li centers are reliable in this study. (3) By comprehensive analysis of the dating results and lithology characteristics, the stratigraphic ages of the SNM borehole with a depth of 126.93 m spans from Neogene to Holocene, but part of the early Pleistocene strata are missing.

Free radical quantification in chemical systems: Challenges and future perspectives

The importance of free radicals in several chemical and biological systems has been extensively documented in the literature. Free radical detection is possible through techniques such as electron spin resonance (ESR), chemically induced dynamic nuclear polarization (CIDNP), and biochemiluminescence. Herein, we provide a comprehensive review of free radical detection, with an emphasis on free radical quantification. The ability to control free radical reactions of various matrices necessitates measuring free radical concentration, which can be obtained by quantifying free radicals. In the current work, we provide a review of various methods and procedures employed for free radical quantification in chemical systems. Procedures were discussed in detail and then grouped based on the instrument used, the operating conditions, and the methodology employed to convert the ESR signal obtained to free radical spins per unit mass of sample. We also provide a comparison with free radical quantification in biological systems. It was found that there is a notable dearth of work focused on quantification in chemical systems despite its potential to enhance control over free radicals. In the last part of this manuscript, we provide a summary and suggested methodology for free radical quantification in chemical systems using ESR focusing on factors affecting quantification.

Three-photon electron spin resonances

Three-photon electron spin resonances

Undoped MoOX with oxygen-rich vacancies as hole transport material for efficient indoor/outdoor organic solar cells

Molybdenum oxide (MoOX) films are typically prepared using a thermally-evaporated MoO3 source (ev.MoO3) as the hole transport layer for inverted organic solar cells (OSCs). However, their low conductivity and interface element diffusion severely limit their efficiency and stability. Herein, a novel and effective thermal evaporation method of the MoO2 source to prepare undoped MoOX films with oxygen-rich vacancies and inhibitory atom diffusion properties electrode using ev.MoO2 as the source, is reported. Electron spin resonance analysis indicates that ev.MoO2 has a stronger oxygen vacancy signal peak than ev.MoO3, which contributes to its excellent conductivity. The inverted organic solar cells modified by ev.MoO2 yielded a significant improvement in short-circuit current density (JSC) and fill factor (FF). The power conversion efficiency (PCE) increased from 16.65 % in ev.MoO3 to 17.08 % in ev.MoO2 under AM 1.5 G, which is one of the highest values ever reported for inverted OSCs. Furthermore, owing to the high transmittance and low trap density, the PCE increased from 20.18 % with ev.MoO3 OSC to 22.13 % with ev.MoO2 OSC under 500 lux. In addition, scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed that ev.MoO2 can prevent the diffusion of Al from the electrode to the functional layers, resulting in a T80 exceeding 800 h under continuous illumination.

Creating high, portable proton polarization with photo-excited triplet DNP

Dynamic nuclear polarization (DNP) is a powerful tool to polarize nuclear spins and enhance the intensity of their magnetic resonance signal. For DNP a sample is doped with an agent providing unpaired electron spins. Then the sample is cooled in a strong magnetic field to polarize these electron spins and a microwave field is applied to transfer this polarization to the nuclear spins. While DNP is very efficient, it has two inherent issues: the electron spins needed to polarize the nuclear spins are also the main source of polarization decay. Furthermore, polarizing the electron spins requires strong magnets and powerful cryogenics, that may obstruct further use of the polarized nuclear spins.These issues can be addressed by using the electron spin of photo-excited triplet states for DNP. After DNP the light creating the electron spins can be shut off, thus eliminating the main source of decay of the nuclear polarization. Moreover, for some well-chosen molecules the photo-excitation process creates the triplet state in a highly polarized state, so magnets and cryogenics can be significantly simplified.The present article presents the state of the art of producing a high proton polarization – up to 0.80 – with a long lifetime – typically 50 h at liquid nitrogen temperature and in a field of 0.75 T – using the photo-excited triplet state of pentacene in a naphthalene host. It describes sample preparation, experimental equipment and procedures required to obtain this result, as well the theoretical background required to maximize the polarization transfer from the triplet spins to the proton spins and to optimize the photo-excitation process. It finishes with methods for long-distance transport and final application of polarized samples.

Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks

Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the FeII central ion with electron-responsive ligands provide an innovative strategy toward multifunctional metal-organic frameworks (MOFs). Here, we developed a 2D planar network consisting of metal-cyanide-metal sheets in an unusual coordination mode, brought about by infinitely π-stacked redox-active bipyridinium derivatives as axial ligands. The obtained family of materials show vivid thermochromism attributed to electron transfer and/or electronic spin state change processes that can occur either independently or concomitantly. Importantly, the redox activity of the ligands within the structure leads to the quasi-reversible electrochemical reduction reaction on a spin-crossover complex at solid state. These observations have been confirmed via temperature-dependent single-crystal X-ray diffraction, magnetic measurements, Mössbauer, EPR, optical and vibrational spectroscopies as well as quantum chemical calculations.

Nonvolatile Electric Control of Rashba Spin Splitting in Sb/In2Se3 Heterostructure.

Ferroelectric Rashba semiconductors (FRS) are highly demanded for their potential capability for nonvolatile electric control of electron spins. An ideal FRS is characterized by a combination of room temperature ferroelectricity and a strong Rashba effect, which has, however, been rarely reported. Herein, we designed a room-temperature FRS by vertically stacking a Sb monolayer on a room-temperature ferroelectric In2Se3 monolayer. Our first-principles calculations reveal that the Sb/In2Se3 heterostructure exhibits a clean Rashba splitting band near the Fermi level and a strong Rashba effect coupled to the ferroelectric order. Switching the electric polarization direction enhances the Rashba effect, and the flipping is feasible with a low energy barrier of 22 meV. This Rashba-ferroelectricity coupling effect is robust against changes of the heterostructure interfacial distance and external electric fields. Such a nonvolatile electrically tunable Rashba effect at room temperature enables potential applications in next-generation data storage and logic devices operated under small electrical currents.

Alwyn George Davies. 13 May 1926—1 September 2023

Alwyn Davies was one of the last surviving members of Christopher Ingold (FRS)'s research group, a set of chemists who together transformed our understanding of mechanistic organic chemistry and laid the foundation for modern teaching of the subject. His early independent research on organic peroxides led into organometallic peroxides and thence into organometallic chemistry. His monographs Organic peroxides (1961, Butterworths) and Organotin chemistry (1997, 2004, Wiley) helped to promote the rapid growth of these topics. One route to organometallic peroxides was the reaction of organometallic compounds with dioxygen, and Alwyn Davies showed that these reactions occurred by a free radical chain reaction. Perhaps his greatest contribution was to establish that radical reactions would often occur at the metal centres in main group organometallic compounds, sometimes a million times faster than at hydrogen. He used electron spin resonance (ESR) spectroscopy to monitor the products and rates of these reactions, and to study the properties of the organic radicals that were produced. The theme of metals behaving as hydrogen surrogates permeated much of the work. In his later career, he developed improved methods for generating organic radical cations, and he recorded and analysed their unusual ESR spectra.

The impact of climate on relief in the northern Japanese Alps within the past 1 Myr–The case of the Tateyama mountains

The impact of climate on mountain relief is often questioned, mainly due to the difficulties of measuring surface processes at the timescale of glacial-interglacial cycles. An appropriate setting for studying mountain erosion in response to Quaternary climate change is found in the Tateyama mountains in the Hida mountain range (northern Japanese Alps) due to distinct geomorphological features. The Japanese Alps uplifted within the past ∼1–3 Myr and experienced multiple glaciations during the late Quaternary. We use ultra-low temperature thermochronometers based on the luminescence of feldspar minerals from 19 rock samples and the electron spin resonance (ESR) of quartz minerals from 8 rock samples, in combination with inverse modelling to derive rock cooling rates and exhumation rate histories at 104–106 year timescales from three transects in the Tateyama region. While luminescence signals have already reached their upper dating limit, ESR signals (Al and Ti centres) yielded ESR ages of ∼0.3–1.1 Ma, implying surface processes active in the Pleistocene. Based on a negative age-elevation relationship, local relief reduction at a cirque-basin scale is identified over the past 1 Myr, whereas a positive age distribution with elevation for samples close to the mountain top does not follow this trend. Inverse modelling reveals rock cooling rates on the order of 20–70 °C/Myr, with slightly faster cooling for cirque-floor samples, which equate with erosion rates of 0.5–1 mm/yr that exceed rates from periglacial and slope processes in the same locality. Thus, our data suggest that Quaternary climate change coupled with distinct surface processes modified the slopes of the Tateyama mountains leading to a localised decrease in relief within an individual cirque basin over the second half of the Quaternary.

Construction of 2D/2D S-scheme Bi2MoO6/Zn-TCPP heterojunction via in-situ self-assembly growth strategy to enhance interface effect for efficient photocatalytic hydrogen production

Two-dimensional/two-dimensional (2D/2D) heterojunctions are considered to be an effective strategy for forming strong interface effects and facilitating photogenerated carrier separation. However, it is usually limited by the size mismatch of the materials, even at the expense of its redox capability. Herein, 2D/2D S-scheme heterojunction photocatalyst Bi2MoO6/Zn-TCPP (BMO/ZTP) composed of 2D Bi2MoO6 and 2D Zn-TCPP (TCPP: tetrakis (4-carboxyphenyl) porphyrin) (MOFs) was constructed by in-situ self-assembly growth strategy. The size-compatible 2D/2D composites had abundant surface active sites and strong interactions. In addition, band bending and interfacial electric field (IEF) effect based on S-scheme heterojunction could accelerate the separation and migration of photogenerated carriers in BMO/ZTP. The best hydrogen precipitation rate of the BMO/ZTP was 10900.94 umol·g−1·h−1, which was 38.90 and 3.24 times higher than that of Bi2MoO6 (280.26 umol·g−1·h−1) and Zn-TCPP (3360.34 umol·g−1·h−1), respectively. The results indicated that 2D/2D BMO/ZTP S-scheme heterojunction could enhance the interface effect and retain strong reducing electrons to achieve efficient photocatalytic hydrogen production, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), Tafel curve, electron spin resonance (ESR) and time-resolved photoluminescence (TRPL) characterization and density functional theory (DFT) calculations. This work provided a general strategy for constructing 2D Bi2MoO6 and 2D MOFs S-scheme heterojunctions to enhance interface effects for achieving efficient photocatalytic hydrogen production.

Sorbitol-Based Glass Matrices Enable Dynamic Nuclear Polarization beyond 200 K.

In magic angle spinning dynamic nuclear polarization (MAS-DNP) experiments, paramagnetic species are often dispersed in rigid glass-forming matrices such as glycerol/water mixtures, but their modest glass-transition temperature (Tg) restricts the viable temperature range for MAS-DNP. To expand applications of DNP at higher temperatures, new matrices and physical insights are required. Here we demonstrate that sorbitol, Tg ≈ 267 K, advantageously replaces glycerol, Tg ≈ 190 K, to carry out DNP at higher temperature while maintaining an identical 13C NMR spectrum footprint and thus minimizing spectral overlap. DNP stops being effective in glycerol/water at ∼180 K, but sorbitol/DMSO gives a significant enhancement at 230 K with AsymPol-POK biradicals at 600 MHz/395 GHz. For the first time, a simple analytical model is proposed that provides physical insights and explains the effect of biradical concentration, the temperature dependence of the enhancement, the signal buildup times, and the enhanced signal-to-noise ratio. The model reveals that electron spin relaxation is the limiting factor for high-temperature DNP in the case of AsymPol-POK. We showcase the efficacy of this new DNP formulation on an intriguing chitin sample extracted from cicada exoskeleton which allowed for the recording of rapid heteronuclear correlation spectra at 100 and 225 K.

Real-Space Imaging of Intrinsic Symmetry-Breaking Spin Textures in a Kagome Lattice.

The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.

Co-doped bismuth vanadate/zinc tungstate heterojunction with dual internal electric fields for efficient photocatalytic reduction of carbon dioxide

Enhanced carriers separation on photocatalysts is crucial for improving photocatalytic activity. In this paper, the Co-doped BiVO4/ZnWO4 S-scheme heterojunctions were constructed to induce double internal electric fields (IEFs) for enhancing charges separation and transfer for efficient photocatalytic reduction of CO2. The photocatalytic CO2 reduction efficiencies of the heterojunctions were significantly enhanced as compared with the counterparts. The optimized Co-doped BiVO4/ZnWO4 exhibited the highest CO yield of 138.4 μmol·g−1·h−1, which were 86.5 and 1.4 folds of the BiVO4 and Co-doped BiVO4. Results of X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and work function demonstrated that charge transfer path of Co-doped BiVO4/ZnWO4 conformed to S-scheme heterojunction mechanism. The kelvin probe force microscopy (KPFM) and density functional theory (DFT) calculations of the differential charge distributions confirmed the existence of double IEFs, which accelerated carrier separation and improved CO2 adsorption and activation. In addition, in-situ Fourier transform infrared spectroscopy (ISFT-IR) revealed that HCOO− was the major intermediate during the CO2 reaction. This study provides a feasible means to develop composite photocatalysts with dual IEFs for effective photocatalytic CO2 reduction.