Mohamed A. Morsy

Understanding the kinetics and stability of manganese redox chemistry is central to its application in energy storage, catalysis, and environmental systems, yet direct operando interrogation of Mn²⁺/Mn³⁺ transformations remain challenging due to the transient nature of higher-valent species. Here, we employ an electrochemical–electron paramagnetic resonance (EC-EPR) methodology to quantitatively probe the kinetics of the Mn²⁺/Mn³⁺ redox process under controlled electrochemical conditions. By selectively monitoring the depletion of EPR-active Mn²⁺ during anodic polarization, CW-EPR measurements were converted into normalized concentration profiles using calibrated signal–concentration relationships. The resulting Mn²⁺ decay kinetics exhibit pseudo-first-order behavior in the initial regime, enabling extraction of apparent electron-transfer rates. Systematic variation of electrode material and geometry, electrolyte acidity and ionic strength, and counter-anion identity reveals pronounced dependencies of manganese redox behavior on electrode–solution coupling and electrolyte composition, distinguishing surface-associated from solution-phase reaction pathways. This combined electrochemical–spectroscopic platform directly links Faradaic driving forces to paramagnetic population dynamics, providing a versatile operando framework for benchmarking manganese redox reactivity and identifying conditions that promote detectable Mn(III/IV) chemistry.

A detailed investigation of the structural changes of lithium borate (LiB) glass 25Li₂O-(75 - x)B₂O₃ was conducted in the absence and presence of lead(II) oxide or aluminum oxide (x = 10 mol %) glass modifiers. X-ray diffraction (XRD), Fourier transform infrared (FTIR), and electron paramagnetic resonance (EPR) spectroscopy were used to explore the structural properties of LiB glass by incorporating trace amounts of manganese(III) oxide (0.00-0.25 mol %) as a probe. Differential thermal analysis and XRD results for the glasses and their ceramics confirmed the integration of aluminum atoms into the glass framework by forming a lithium aluminum boron oxide Li₂(AlB₅O₁₀) crystalline phase. Lead atoms were located interstitially, which disordered the borate glass structure and produced a lithium tetraborate crystalline phase. Semiempirical modeling of the glass structures was conducted to estimate the fundamental vibrational modes of the glass materials using a parametric method 3 (PM3MM) with molecular mechanics corrections to elucidate the geometry of the borate (BO₃) groups and their possible vibrational modes. Our analysis revised the conventional representation of the tetrahedral BO₄ units, which were not observed, to "distorted-trigonal" BO₃ groups and associated with nonbridging oxygen (NBO) atoms. EPR spectroscopy established a link between the NBO in oxides and the well-defined peak at g-factor ∼4.2 in glass materials, which had been assigned to iron(III) ions according to the literature.

The photocatalytic activity of TiO₂ nanoparticles in aqueous solutions is commonly evaluated by monitoring the rate of methylene blue bleaching and phenols degradation, but both substrates suffer from many drawbacks, e.g., the high capacity of dark adsorption, self-degradation, and photosensitization. Besides, filtration is always required to separate the particulate photocatalyst before the analysis. Herein, we investigated the potential use of electron paramagnetic resonance (EPR) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) to directly monitor the photocatalytic activity of TiO₂ suspensions without the need for filtration. The results showed that TEMPOL aqueous solution is in the dark and under UV-A illumination, does not absorb UV-A and visible light, and has negligible dark adsorption. The influence of TEMPOL concentration, light intensity, and TiO₂ loading on the photocatalytic deactivation rate has been investigated. The mechanisms of TEMPOL deactivation in the presence and absence of oxygen as well as in the presence of methanol ^•OH radicals’ scavenger have been discussed. The photocatalytic deactivation products have been analyzed using EPR, ¹H-NMR, and ¹³C-NMR spectroscopies. It is found that the deactivation of TEMPOL is initiated by ^•OH radicals and α-H abstraction from the 4-piperidine position followed by the formation of TEMPONE (4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-oxo-2,2,6,6-tetramethylpiperidine). In the presence of methanol, the formed α-hydroxyl radicals (^•CH₂OH) attack the nitroxide side of TEMPOL and produce 4-hydroxy-tetramethylpiperidine. Same activity trends have been observed for the photocatalytic methanol oxidation and TEMPOL deactivation over different types of TiO₂ photocatalysts evincing that the proposed method has a potential for direct monitoring of the activities of photocatalyst suspensions. Graphical abstract

An ultra-sensitive sensor of dopamine is introduced. The sensor is constructed by encapsulating platinum nanoparticles (PtNPs) between reduced graphene oxide (GR) nanosheets. The sandwiched PtNPs between GR layers acted as a spacer to prevent aggregation and provided a fine connection between the GR nanosheets to provide fast charge transfer. This specific orientation of the GR nanosheets and PtNPs on the graphite pencil electrode (GPE) substantially improved the electrocatalytic activity of the sensor. The synthesized graphene oxide and the fabricated sensor were comprehensively characterized by Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission-scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and square wave voltammetry (SWV). The value of the charge transfer coefficient (α), apparent heterogeneous electron transfer rate constant (k_s), and electroactive surface area for dopamine were found to be about 0.57, 8.99 s⁻¹, and 0.81 cm², respectively. The developed sensor is highly sensitive towards dopamine, and the detection limit is 9.0 nM. The sensor response is linear for dopamine concentration from 0.06 to 20 μM (R² = 0.9991). The behavior of the sensor for dopamine in the presence of a high concentration of L(+) Ascorbic acid and other potential interferents was satisfactory. High recovery percentage between 90% and 105% in the human urine sample, good reproducibility, and facile fabrication of the electrode make it a good candidate for dopamine sensing.

This study aims to investigate the effect of replacing Ti with Zr in the SBU of MIL‐125‐NH₂. We were able to replace Ti with Zr in the mixed metal synthesis of MIL‐125‐NH₂, for the first time. After experimentally confirming the consistency in their framework structure and comparing their morphology, we related the femtosecond light dynamics with photocatalytic CO₂ visible light conversion yield of the different variants in order to establish the composition‐function relation in MIL‐125 vis a vis CO₂ reduction. Introducing Zr to the system was found to cause structure defects due to missing linkers. The lifetime of the charge carriers for the mixed metal samples were shorter than that of the MIL‐125‐NH₂. The study of CO₂ photocatalytic reduction under visible light indicated that the NH₂ group enhances the photocatalytic activity while the Zr incorporation inside the MIL framework introduces no significant improvements. In addition, the material systems were modelled and simulated through DFT calculations which concluded that the decrease of the photocatalytic activity is not related to the system electronic structure, insinuating that defects are the culprit.

The unique capability of electron paramagnetic resonance (EPR) to detect and characterize paramagnetic species has been applied to measure free radical intermediates from any oxidative processes. Recently, sensitive benchtop EPR spectrometers have been used to quantify several radical intermediates of drug and pharmaceutical radio sterilization such as ketoconazole and alanine, respectively. During the earlier days of the EPR (also called, electron spin resonance, ESR), simultaneous EC-EPR techniques were developed and accepted as a highly powerful means for identifying paramagnetic intermediates in EC-reactions by using a variety of EC-EPR cells. However, these cells are either expensive or not easy to use, and the electrodes in these cells are made of platinum or precious silver metals. None of these cells could be utilized for the EC-EPR redox processes on the surface of graphite pencil electrodes (GPE) that produced the highest radical intermediate content (see Figure 1). The present work uses a simple, inexpensive flow electrochemical-electron paramagnetic resonance (FEC-EPR) cell for the determination of redox intermediate of different kind of drugs and their derivatives’ solutions. A well-established method for instantaneous electrochemical and EPR measurements with a new EPR-instrument, called EPR-Lab that is equipped with a FEC-EPR cell, flow pump, and OEM-potentiostat, produces data for both electro-analysis and EPR spectroscopy. The new system permits early radicals’ formation detection and analysis. It will also enable the lifetime analysis of radicals’ formation and decay by alternating the applied potential between ‘ON’ and ‘OFF’ states. The method also supports radical detection at a much lower concentration than other electrochemical or chemical oxidation techniques of similar materials. The new system has been used to elucidate the kinetics of the acidified ketoconazole (KTZ) oxidation in vitro in its real forms; tablet, cream, and shampoo extracts as well as in urine model matrix, that could be linked to its delivery in acidic solutions in vivo. Figure 1

In this study, electron paramagnetic resonance (EPR) is used, for the first time, as an analytical tool for the quantitative assay of ketoconazole (KTZ) in drug formulations. The drug was successfully characterized by the prominent signals by two radical species produced as a result of its oxidation with 400 μg/mL cerium(IV) in 0.10 mol dm⁻³ sulfuric acid. The EPR signal of the reaction mixture was measured in eight capillary tubes housed in a 4 mm EPR sample tube. The radical stability was investigated by obtaining multi-EPR scans of each KTZ sample solution at time intervals of 2.5 min of the reaction mixing time. The plot of the disappearance of the radical species show that the disappearance is apparently of zero order. The zero-time intercept of the EPR signal amplitude, which should be proportional to the initial radical concentration, is linear in the sample concentration in the range between 100 and 400 μg/mL, with a correlation coefficient, r, of 0.999. The detection limit was determined to be 11.7 ± 2.5 μg/mL. The method newly adopted was fully validated following the United States Pharmacopeia (USP) monograph protocol in both the generic and the proprietary forms. The method is very accurate, such that we were able to measure the concentration at confidence levels of 99.9%. The method was also found to be suitable for the assay of KTZ in its tablet and cream pharmaceutical preparations, as no interferences were encountered from excipients of the proprietary drugs. High specificity, simplicity, and rapidity are the merits of the present method compared to the previously reported methods.