Electron Paramagnetic Resonance Signal Research Articles

Date fruit melanin is primarily based on (−)-epicatechin proanthocyanidin oligomers

Plant-based melanin seems to be abundant, but it did not receive scientific attention despite its importance in plant biology and medicinal applications, e.g. photoprotection, radical scavenging, antimicrobial properties, etc. Date fruit melanin (DM) has complex, graphene-like, polymeric structure that needs characterization to understand its molecular properties and potential applications. This study provides the first investigation of the possible molecular composition of DM. High performance size-exclusion chromatography (HPSEC) suggested that DM contains oligomeric structures (569–3236 Da) and transmission electron microscopy (TEM) showed agglomeration of these structures in granules of low total porosity (10–1000 Å). Nuclear magnetic resonance (NMR) spectroscopy provided evidence for the presence of oligomeric proanthocyanidins and electron paramagnetic resonance (EPR) spectroscopy revealed a g-factor in the range 2.0034–2.005. Density functional theory (DFT) calculations suggested that the EPR signals can be associated with oligomeric proanthocyanidin structures having 4 and above molecular units of (−)-epicatechin. The discovery of edible melanin in date fruits and its characterization are expected to open a new area of research on its significance to nutritional and sensory characteristics of plant-based foods.

Towards liquid EPR dosimetry using nitroxides in aqueous solution.

Objective. Water-equivalent dosimeters are desirable for dosimetry in radiotherapy. The present work investigates basic characteristics of novel aqueous detector materials and presents a signal loss approach for electron paramagnetic resonance (EPR) dosimetry.Approach. The proposed principle is based on the radiation dose dependent annihilation of EPR active nitroxides (NO·) in aqueous solutions. Stable nitroxide radicals (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP), 3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in aqueous solutions containing dimethyl sulfoxide (DMSO) as an additive were filled in glass capillaries for irradiation and EPR readout. Radiation doses ranging from 1 to 64 Gy were applied with a clinical 6 MV flattening filter free photon beam. EPR readout was then performed with a X-band benchtop spectrometer. The dose response, temporal stability and reproducibility of the samples' EPR signal amplitudes as well as the influence of the nitroxide concentration between 10 and 160μM on the absolute signal loss were investigated using MmP. CmP was used to examine the dependence of the dose response on DMSO concentration between 0 and 10 vol%. An indirect effect model was fitted to the experimental data assuming irradiation induced radical reactions as the underlying mechanism.Main results. For an initial MmP concentration of 20μM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(-NO·) of approximately 0.4μmol J-1. Within five weeks upon sample irradiation to doses between 0 and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0-5 vol%. An indirect effect model is applicable to describe the reaction mechanism resulting in the observed dose response curve.Significance. Liquids consisting of nitroxides in aqueous solution and small amounts of DMSO (2 vol%) show promising basic characteristics for application as water-equivalent EPR dosimeter materials in radiotherapy. The EPR signal loss is based on an indirect effect mediated by diffusing radicals originating from the radiolysis of the water/DMSO mixture.

Identification of paramagnetic centers in irradiated Sn-doped silicon dioxide by first-principles.

We present a first-principles investigation of Sn paramagnetic centers in Sn-doped vitreous silica based on calculations of the electron paramagnetic resonance (EPR) parameters. The present investigation provides evidence of an extended analogy between the family of Ge paramagnetic centers in Ge-doped silica and the family of Sn paramagnetic centers in Sn-doped silica for SnO2concentrations below phase separation. We infer, also keeping into account the larger spin-orbit coupling of Sn atoms with respect to Ge atoms, that a peculiar and highly distorted three-fold coordinated Sn center (i.e. the Sn forward-oriented configuration) should give rise to an orthorhombic EPR signal of which we suggest a fingerprint in the EPR spectra recorded by Chiodiniet al(2001Phys. Rev.B64073102). Given its structural analogy with theEα'and Ge(2) centers, we here name it as the 'Sn(2) center'. Moreover, we show that the single trapped electron at a SnO4tetrahedron constitutes a paramagnetic center responsible for the orthorhombic EPR signal reported in Chiodiniet al(1998Phys. Rev.B589615), confuting the early assignment to a distorted variant of the Sn-E' center. We hence relabel the latter orthorhombic EPR signal as the 'Sn(1) center' due to its analogy to the Ge(1) center in Ge-doped silica.

Hand-held electron spin resonance scanner for subcutaneous oximetry using OxyChip.

Electron spin resonance (ESR) is used to measure oxygen partial pressure (pO2) in biological media with many clinical applications. Traditional clinical ESR involves large magnets that encompass the subject of measurement. However, certain applications might benefit from a scanner operating within local static magnetic fields. Our group recently developed such a compact scanner for transcutaneous (surface) pO2 measurements of skin tissue. Here we extend this capability to subsurface (subcutaneous) pO2 measurements and verify it using an artificial tissue emulating (ATE) phantom. We introduce a new scanner, tailored for subcutaneous measurements up to 2 mm beneath the skin's surface. This scanner captures pulsed ESR signals from embedded approximate 1-mm oxygen-sensing solid paramagnetic implant, OxyChip. The scanner features a static magnetic field source, producing a uniform region outside its surface, and a compact microwave resonator, for exciting and receiving ESR signals. ESR readings derived from an OxyChip, positioned approximately 1.5 mm from the scanner's surface, embedded in ATE phantom, exhibited a linear relation of 1/T2 versus pO2 for pO2 levels at 0, 7.6, 30, and 160 mmHg, with relative reading accuracy of about 10%. The compact ESR scanner can report pO2 data in ATE phantom from an external position relative to the scanner. Implementing this scanner in preclinical and clinical applications for subcutaneous pO2 measurements is a feasible next phase for this development. This innovative design also has the potential to operate in conjunction with artificial skin graft for wound healing, combining therapeutic and pO2 diagnostic features.

Fabrication of multi-component heterojunction Ag/AgCl/g-C3N4/Sn-In3O2 for enhanced degradation of RhB under visible light

High-efficiency visible-light photocatalysts have been recognized as a cost-effective approach for the degradation of organic pollutants in wastewater. In this study, we constructed a novel multi-component heterojunction photocatalyst Ag/AgCl/g-C3N4/Sn-In2O3 through a simple thermal polymerization and photo-assisted reduction processes. The photocatalytic activity of Ag/AgCl/g-C3N4/Sn-In2O3 was remarkably improved compared to other samples in the degradation of Rhodamine B (RhB). The degradation of RhB by 5 % Ag/AgCl/g-C3N4/Sn-In2O3 reached 98.5 % within 60 min, while the degradation rates under g-C3N4, g-C3N4/Sn-In2O3, and Ag/AgCl/g-C3N4 were found to be 41.8 %, 73.6 %, and 85.9 %, respectively. Electron spin resonance (ESR) signals trapping and radical scavenging confirmed that the h+ and ·O2− were the two main reactive species in photodegradation RhB. The result provides innovative insights that could inform the future design and development of photocatalysts with exceptional efficiency.

Optically Electroactive Polymer Synthesized in a Liquid Crystal with Cyclosporin A─Circularly Polarized Electron Spin Resonance.

Cyclosporine A (CsA), a naturally derived biomaterial and physiologically active substance, is commonly used as an immunosuppressant. In this study, CsA was revealed to function as a chiral inducer of cholesteric liquid crystals (CLCs) with a high helical twisting power. CsA induced helical structures in 4-cyano-4'-pentylbiphenyl, a synthetic liquid crystal (LC) used for general purposes. Electrochemical polymerization in CLC with CsA was also performed. The polymer prepared in CLC showed electro-optical activity via chiral induction by CsA. Synchrotron X-ray diffraction measurements indicated that the polymer film prepared in the CLC formed in the manner of LC molecular arrangement through molecular form imprinting from the LC order, although the polymer exhibited no liquid crystallinity. The polymer showed structural color and laser light oscillation diffraction derived from its periodic structure. The anisotropy of the circularly polarized electron spin resonance signals for the resulting polymer with respect to the magnetic field was observed.

Direct Detection of the α-Carbon Radical Intermediate Formed by OspD: Mechanistic Insights into Radical S-Adenosyl-l-methionine Peptide Epimerization.

OspD is a radical S-adenosyl-l-methionine (SAM) peptide epimerase that converts an isoleucine (Ile) and valine (Val) of the OspA substrate to d-amino acids during biosynthesis of the ribosomally synthesized and post-translationally modified peptide (RiPP) natural product landornamide A. OspD is proposed to carry out this reaction via α-carbon (Cα) H-atom abstraction to form a peptidyl Cα radical that is stereospecifically quenched by hydrogen atom transfer (HAT) from a conserved cysteine (Cys). Here we use site-directed mutagenesis, freeze-quench trapping, isotopic labeling, and electron paramagnetic resonance (EPR) spectroscopy to provide new insights into the OspD catalytic mechanism including the direct observation of the substrate peptide Cα radical intermediate. The putative quenching Cys334 was changed to serine to generate an OspD C334S variant impaired in HAT quenching. The reaction of reduced OspD C334S with SAM and OspA freeze-quenched at 15 s exhibits a doublet EPR signal characteristic of a Cα radical coupled to a single β-H. Using isotopologues of OspA deuterated at either Ile or Val, or both Ile and Val, reveals that the initial Cα radical intermediate forms exclusively on the Ile of OspA. Time-dependent freeze quench coupled with EPR spectroscopy provided evidence for loss of the Ile Cα radical concomitant with gain of a Val Cα radical, directly demonstrating the N-to-C directionality of epimerization by OspD. These results provide direct evidence for the aforementioned OspD-catalyzed peptide epimerization mechanism via a central Cα radical intermediate during RiPP maturation of OspA, a mechanism that may extend to other proteusin peptide epimerases.

Radiological injuries under low energy x-rays in mice depending on dose and protocol: comparative characterization of lesion severity and impact of the in vivo bone response on retrospective dose estimations

Objective. To improve our knowledge about the biological effects of over exposures involving low-energy x-rays, we developed and characterized a preclinical mouse model allowing to mimic different lesion severity degrees induced by 80 kV x-ray depending on the dose and protocol (single or repeated exposure). Approach. Mice were locally exposed (paw) to 80 kV x-rays in a single (15, 30 or 45 Gy in K air) or repeated exposition (2 × 15 or 3 × 15 Gy in K air) to assess different degrees of lesion severity. Six post-irradiation euthanasia time points (0, 7, 14, 21, 42, and 84 days) were determined to follow up the evolution of lesions based on the lesion score, weighing and cutaneous blood perfusion. The bone dose was estimated at the different time points by electron paramagnetic resonance (EPR) spectroscopy. Main results. The monitoring of the lesion severity allows to classify the exposure protocols according to their severity. EPR spectroscopy measurements allow to determine the bone dose on the day of irradiation which is 7 times higher than the initial dose for single protocols. However, the initial signal measured at the end of the repeated exposure was 27% lower than the signal measured for a single dose. The study of the kinetics of EPR signal showed a decrease of the EPR signal which is dependent on the exposure protocol but not on dose highlighting the impact of bone physiology on the bone dose estimation. Significance: the preclinical model developed allows to assess the impact of the dose and protocol on the lesion severity induced by low-energy x-ray. For the first time, the dynamics of free radicals have been quantified in an in vivo model, highlighting that the doses actually administered can be underestimated if samples are taken weeks or even months after exposure.

Spin-dependent recombination affected by post-annealing of organic photovoltaic devices

Organic photovoltaic devices (OPVs) are attracting attention because of recent rapid enhancement of their power conversion efficiency. For further improvement, optimization of fabrication processes is one useful path to a solution. During OPV fabrication, particularly of the bulk heterojunction active layer, annealing treatments contribute to the device performance. Many studies have examined annealing-related properties. However, further research must clarify how paramagnetic species in the devices play their roles by annealing. Using well-known OPVs, we investigated the relation between spin-dependent recombination (SDR) current and the paramagnetic species, which vary the numbers by post-annealing with active layers consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). A simultaneous detection method of electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR), which we originally developed, was applied to OPVs for the first time ever reported. Results show that PC61BM anion radicals generated by post-annealing of P3HT:PC61BM OPVs with a lithium fluoride (LiF)/aluminum (Al) electrode do not contribute to the SDR current at the interface and that P3HT cation radicals enhance the SDR current. By contrast, devices with an Al electrode without LiF decrease the total SDR current, although the quantities of cation radical molecules do not vary. This finding suggests that changes of the hole blocking layer in the devices caused by the annealing treatment affect the size of capture cross sections of P3HT cation radicals. Our new method of quantitative observation of the EDMR changes through the ESR signals is expected to be useful for investigating the capture cross sections in OPVs.

Identification and quantification of electron-generated atomic hydrogen through in-situ electron spin resonance and density functional theory

Electro-generated atomic hydrogen (H*) plays a crucial role in the electrochemical reduction process, serving as the key species that mediates the electrocatalytic hydrogenation reduction of stubborn contaminants in wastewater treatment. However, precisely identifying and quantifying transient atomic H* presents a significant challenge due to its limited lifespan and its existence solely within the boundary layer at the electrode/solution interface. Herein, we developed the electrodeposition of palladium nanoparticles onto carbon cloth and assessed its effectiveness as a cathode for generating and stabilizing atomic H*. The environmental application of atomic H* was validated through the dechlorination of 2, 4-dichlorophenol wastewater and the reduction of antimonite wastewater. Additionally, the identification of atomic H* was verified by electrochemical measurements, high-resolution mass spectra, and density functional theory. Moreover, introducing an excess of a spin trapping agent (5,5-dimethyl-1-pyrroline-N-oxide) and fast in-situ spin trapping facilitated creating favorable conditions for efficient trapping of atomic H* and subsequent electron spin resonance (ESR) spectroscopy quantification analysis. Subsequently, the quantification of atomic H* was achieved by double integration of the ESR signal of spin adduct and comparison with the external standard agent (4-hydroxy-2,2,6,6-tetramethyl-1-piperidine 1-oxyl). This study introduces a novel method for in-situ spin trapping and quantification of atomic H*, facilitating the advancement of electrochemical reduction technology and its application in wastewater treatment.

Perplexing EPR Signals from 5f36d1 U(II) Complexes.

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band electron paramagnetic resonance (EPR) data were collected at <10 and 77 K on 10 U(II) complexes with 5f36d1 electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K. In contrast, U(II) complexes with 5f4 configurations are EPR-silent. Unlike U(II), the congenic 4f35d1 Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f15d1 configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. Since the coupling in U(II) systems is expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration, the results are viewed as unexplained phenomena. However, they do show that 5f36d1 U(II) samples can be differentiated from 5f4 U(II) complexes by EPR spectroscopy.

The Z-Scheme MIL-88B(Fe)/BiOBr Heterojunction Promotes Fe(III)/Fe(II) Cycling and Photocatalytic-Fenton-Like Synergistically Enhances the Degradation of Ciprofloxacin.

The Z-scheme MIL-88B/BiOBr (referred to as MxBy, whereas x and y are the mass of MIL-88B(Fe) and BiOBr) heterojunction photocatalysts are successfully prepared by a facile ball milling method. By adding low concentration H2O2 under visible light irradiation, the Z-scheme heterojunction and photocatalytic-Fenton-like reaction synergistically enhance the degradation and mineralization of ciprofloxacin (CIP). Among them, M50B150 showed efficient photodegradation efficiency and excellent cycling stability, with 94.6% removal of CIP (10mgL-1) by M50B150 (0.2gL-1) under 90min of visible light. In the MxBy heterojunctions, the rapid transfer of photo-generated electrons not only directly decomposed H2O2 to generate ·OH, but also improved the cycle of Fe3+/Fe2+ pairs, which facilitated the reaction with H2O2 to generate ·OH and ·O2 - radicals. In addition, the effects of photocatalyst dosages, pH of CIP solution, and coexisting substances on CIP removal are systematically investigated. It is found that the photocatalytic- Fenton-like reaction can be carried out at a pH close to neutral conditions. Finally, the charge transfer mechanism of the Z-scheme is verified by electron spin resonance (ESR) signals. The ecotoxicity of CIP degradation products is estimated by the T.E.S.T tool, indicating that the constructed photocatalysis-Fenton-like system is a green wastewater treatment technology.

Radical detection and electron‐spin resonance (ESR) monitoring in polymer materials irradiated with gamma and X‐rays: Polyethylene and polypropylene

AbstractWhen ionizing radiations interact with polymers, free radicals are generated and are known to be the initiating species for the modification in materials. Monitoring and identifying free radicals after irradiation allow assessing the impact of X‐rays and gamma rays on materials. A comparative study has been performed on materials made from polypropylene and polyethylene after irradiation by the two irradiation technologies. Electron spin resonance (ESR) signal has been monitored up to 380 days when signal was still present. ESR signals, concentration profiles, and kinetics were observed after X‐rays and gamma rays.

Reversible Multimode Luminescence Modulations in Photochromic-Translucent Yb3+/Eu3+ Codoped K0.5Na0.5NbO3 Ceramics.

Luminescent tunable materials have promising application potential in optical switches, optical information storage, and so on. Although europium (Eu) is a good downconversion red luminescent rare earth element, there are few studies on the upconversion luminescence and photochromism of Eu-doped potassium sodium niobate (KNN) ferroelectrics. In this paper, Eu3+ and Yb3+ codoped KNN translucent ferroelectric ceramics were synthesized and the effect of Yb3+ content on the luminescence and photochromism is studied. Both the up- and downconversion luminescence intensity and decay rate before and after photochromism can be well controlled by Yb3+ content. That is, an upconversion luminescent translucent ceramic that can be completely discolored by 405 nm light illumination for 10 s was obtained. The luminescence modulations are closely related to the evolution of oxygen vacancy and crystal field around the luminescence center, which can be verified by the illumination-induced electron paramagnetic resonance (EPR) signal and local piezoresponse switching behavior variation as well as the discovery of energy level splitting and spectral line shift. We believe that this work shows a paradigm for designing high-performance reversible multimode luminescence modulation ferroelectric ceramics.

A radical containing micellar probe for assessing esterase enzymatic activity with ultra-low field Overhauser-enhanced magnetic resonance imaging.

The ability to track altered enzyme activity using a non-invasive imaging protocol is crucial for the early diagnosis of many diseases but is often challenging. Herein, we show that Overhauser magnetic resonance imaging (OMRI) can be used to monitor enzymatic conversion at an ultra-low field (206 μT) using a highly sensitive "off/on" probe with a nitroxide stable radical containing ester, named T2C12-T80. This TEMPO derivative containing probe forms stable electron paramagnetic resonance (EPR) silent micelles in water that are hydrolysed by esterases, thus yielding narrow EPR signals whose intensities correlate directly with specific enzymatic activity. The responsiveness of the probe to tumours, facilitated by increased esterase activity, was initially determined by comparing EPR signals measured upon incubation with 3T3 (healthy fibroblasts used as control), HepG2 (human hepatoma) and Hs766T (human pancreatic cancer cells) cell lysates and then with Hs766T and 3T3 living cells. Next, Overhauser MR images were detected on a phantom containing the probe and the esterases to show that the approach is well suited for being translated to the in vivo detection at the earth's magnetic field. Regarding detection sensitivity, ultra-low field OMRI (ULF-OMRI) is beneficial over OMRI at higher fields (e.g. 0.2 T) since Overhauser enhancements are significantly higher and the technique is safe in terms of the specific absorption rate.

Magnetic dimeric metal-porphyrin rings mechanically bonded around carbon nanotubes: the role of nanotube defects modulating magnetic properties.

Carbon nanotubes constitute an excellent option to connect molecular nanomagnets (MNMs) to solid-state devices, allowing the electrical control of the molecule spin state. The main challenge in this area is to obtain stable nanohybrid structures that preserve the magnetic properties and functionality of the molecule. One of the strategies developed to build these structures combining MNMs and carbon nanotubes involves the encapsulation of the nanotube within magnetic organic macrocycle(s), leading to magnetic mechanically interlocked derivatives of carbon nanotubes (mMINTs). Recently, mMINTs with dimeric metal-porphyrin rings containing Cu2+ or Co2+ attached to carbon nanotubes have been synthesized. Different structural and spectroscopic techniques confirm that the metal centers in both mMINTs preserve the coordination sphere and structure of the free macrocycles, with only small deviations from the square planar geometry. However, different magnetic behaviors are observed depending on the metal. The Cu derivative has well-preserved spin geometry, with quantum coherence times on the microsecond scale, as in the free molecule (τ = 25 μs). In contrast, the continuous-wave electron paramagnetic resonance (EPR) spectrum measured on the Co derivative does not show any clear contribution from Co, although EXAFS unambiguously shows the presence of Co2+. In order to understand this different magnetic behavior, we studied mMINTs by means of periodic DFT calculations. The analysis of the optimized geometries, the spin density of the free macrocycles and the mMINTs, the adsorption energies, the extension of the charge transfer between the macrocycle and the nanotube and the density of states reveals a stronger macrocycle-nanotube interaction for the Co-mMINT. Our results point to carbon vacancies in the nanotubes as responsible for the different macrocycle-nanotube interaction and the observed loss of the Co2+ EPR signal in the Co-mMINT.

A dual-mode sensing platform for electron spin resonance and UV-vis detection of alkaline phosphatase based on Cu-based metal-organic frameworks.

Alkaline phosphatase (ALP) is an indispensable hydrolase in living organisms and the abnormality of ALP activity is correlated with a variety of diseases. Exploring ALP activity is important for clinical diagnosis and biomedical research to understand its physiological function. In this study, a dual-mode biosensing platform was constructed based on Cu-based metal-organic frameworks (Cu-MOFs) for electron spin resonance (ESR) and ultraviolet-visible (UV-vis) sensing of ALP. Cu-MOFs, as peroxidase mimics, catalyzed the decomposition of hydrogen peroxide (H2O2) and the generation of reactive oxygen species (ROS) which could oxidize ABTS into ABTS˙+ with good ESR and UV-vis signals. Pyrophosphate ions (PPi) with high affinity to Cu2+ in Cu-MOFs could suppress the peroxidase-like activity of Cu-MOFs, and ALP could hydrolyze PPi, resulting in the recovery of Cu-MOF catalytic activity. Thus, a quantitative dual-mode method for detection of ALP activity was established with good linearity in the range of 0-42 U L-1 and limits of detection as low as 0.386 and 0.523 U L-1 respectively for ESR and UV-vis detection. Benefiting from its high sensitivity and excellent selectivity, this method was applied for ALP detection in human serum and satisfactory recoveries were achieved. The off-on dual-mode sensing platform is more reliable than the single-mode sensor and shows merits like simple operation and cost-friendliness, making it have great potential in the diagnosis of ALP-related diseases.

Impact of electron irradiation on semi-insulating and conductive β-Ga2O3 single crystals.

The damage behavior and defect evolution in Si-doped and Fe-doped β-Ga2O3 crystals were investigated using an electron irradiation of 1 MeV at a dose of 1 × 1016 cm-2 in conjunction with structural and optoelectronic characterizations. Distinct decline in electron spin resonance (ESR) signal with g = 1.96 and a UV luminesce of 375 nm were observed in Si-doped β-Ga2O3 due to the capture of free carriers by irradiation defects. As for the Fe-doped sample, both defect-related blue emission and Cr3+ impurity-related red luminescence underwent prominent suppression after electron irradiation, which can be correlated to the creation of VO and VGa defects and the formation of non-radiative recombination. Noticeably, neither VO- nor VGa-related ESR signals were detected in Fe-doped and Si-doped β-Ga2O3 irrespective of irradiation; g = 2.003 resonance was observed in Mg-doped β-Ga2O3 and it experienced remarkable augmentation after electron irradiation. We assigned the g = 2.003 peak to the VGa acceptor. Besides, although the Raman mode of 258 cm-1 in Si-doped β-Ga2O3 has been suggested to be electron concentration dependent, no obvious change in peak intensity was observed before and after electron irradiation.

Mechanism underlying ultraviolet-irradiation-induced discoloration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria

The present study aimed to analyze the mechanism underlying the color alteration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria (3YZ and 5YZ, respectively) caused by 254-nm ultraviolet (UV) irradiation. Fully sintered specimens of 3YZ (Zpex, Tosoh) and 5YZ (Zpex Smile, Tosoh) were irradiated with 254-nm UV at 4 mW/cm2 (up to 60 min). Color alteration (ΔE) was analyzed using the CIE L*a*b* colorimetric system. The UV-induced excitation of oxygen vacancies, which capture two electrons in zirconia (i.e., F center) was analyzed using photoluminescence (PL) and electron spin resonance (ESR) spectroscopy. Furthermore, the spontaneous change in ΔE after cessation of UV irradiation was evaluated. UV irradiation increased ΔE to 13.4 in 3YZ and 10.3 in 5YZ within the initial 10 min of irradiation, and then, ΔE reached a plateau. A PL peak was observed at 435 nm, which decreased with UV irradiation. In addition, UV irradiation generated an ESR signal attributed to the F+ centers formed via one-electron detachment from F centers. The signal intensities of both 3YZ and 5YZ increased with irradiation time; however, the signal intensity was higher in 3YZ than that in 5YZ. After cessation of UV irradiation, ΔE gradually decreased to approximately 2, which took a few weeks. The discoloration of 3YZ and 5YZ caused by UV irradiation was due to the excitation of F centers. Although the specimens spontaneously reverted to their original color over time, clinical procedures involving UV irradiation, such as photo-functionalization, should be performed with caution to avoid damaging the esthetics of dental zirconia restorations.

ESR and luminescence thermochronometry of the Rhône valley, Switzerland

The impact of Quaternary glaciations on rates of erosion remains disputed. This is mainly due to a lack of geochronological data that cover the timespan of 103−106 years. To fill this temporal gap, we develop electron spin resonance (ESR) thermochronometry using both the Al- and Ti-centres in quartz. The combination of ESR thermochronometry with numerical modelling allows rock cooling histories to be determined, enabling changes in erosion rates to be related to glacial advance. In this study, we conducted a series of laboratory experiments using samples from the upper Rhône valley (Switzerland) that is thought to have substantially deepened around 1 Ma ago. Ten quartz samples, from a transect collected near Sion, were used to (i) optimize the measurement protocol (i.e., preheat conditions), (ii) constrain ESR signal growth and thermal stability of the Al- and Ti-centres to estimate trap kinetic parameters; and (iii) invert the ESR data to constrain rock cooling histories. A preheat plateau experiment showed that 170 °C is the appropriate preheat temperature whilst sensitivity changes are not significant. For most samples the Ti-centre has higher thermal stability than the Al-centre, although the thermal stability of the different samples investigated is highly variable. Preliminary inversion results yield consistent cooling histories of around 30−60 °C/Myr during the Late Quaternary period, except for two valley bottom samples that suggest more rapid rock cooling.