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Register a new userDFT Investigation of Biocatalytic Mechanisms from pH-Driven, Multi-Enzyme, Biomimetic Behavior in CeO2
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There is considerable interest in the pH-dependent, switchable, biocatalytic properties of cerium oxide (CeO2) nanoparticles (CeNPs) in biomedicine, where these materials exhibit beneficial antioxidant activity against reactive oxygen species (ROS) at basic physiological pH but cytotoxic prooxidant activity in acidic cancer cell pH microenvironment. While the general characteristics of the role of oxygen vacancies are known, the mechanism of their action at the atomic scale under different pH conditions has yet to be elucidated. The present work applies density functional theory (DFT) calculations to interpret, at the atomic scale, the pH-induced behavior of the stable {111} surface of CeO2 containing oxygen vacancies. Analysis of the surface-adsorbed media species reveals the critical role of pH on the interaction between ROS and the defective CeO2 {111} surface. Under basic conditions, the superoxide dismutase (SOD) and catalase (CAT) biomimetic reactions can be performed cyclically, scavenging and decomposing ROS to harmless products, making CeO2 an excellent antioxidant. However, under acidic conditions, the CAT biomimetic reaction is hindered owing to the limited reversibility of Ce3+ and Ce4+ and formation and annihilation of oxygen vacancies. A Fenton biomimetic reaction is predicted to occur simultaneously with the SOD and CAT biomimetic reactions, resulting in the formation of hydroxyl radicals, making CeO2 a cytotoxic prooxidant.
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There is considerable interest in the pH-dependent switchable biocatalytic properties of cerium oxide nanoparticles (CeNPs) in biomedicine, where these materials exhibit beneficial antioxidant activity against reactive oxygen species at neutral and basic physiological pH but cytotoxic prooxidant activity at acidic pathological pH. Oxygen vacancies play a key role in such biocatalytic activities. While the general characteristics of the role of oxygen vacancies are known, the mechanism of their action at the atomic scale under different pH conditions has yet to be elucidated. The present work applies density functional theory (DFT) calculations to interpret the pH-induced behavior of the stable {111} surface of CeO2 at the atomic scale. Analysis of the surface-adsorbed media species reveals the critical role of pH on the reversibility of the Ce3+ and Ce4+ redox equilibria and the formation and annihilation of the oxygen vacancies. Under acidic conditions, this reversible switching is hindered owing to incomplete volumetric filling of the oxygen vacancies by the oxygen in the water molecules, hence effective retention of the oxygen vacancies, and consequent inhibition of redox biomimetic reactions. Under neutral and basic conditions, the capacity for this reversible switching is preserved due to complete filling of the oxygen vacancies by the OH ions owing to their ready size accommodation, thereby retaining the capacity for performing redox biomimetic reactions cyclically.
It has been demonstrated in previous experimental and computational work that doping CeO2 with transition metals is an effective way of tuning its properties. However, each previous study on CeO2 doping has been limited to a single or a few dopants. In this paper, we systematically study the formation energies, structural stability and electronic properties of CeO2 doped with the entire range of the ten 3d transition metals using density functional theory (DFT) calculations at the hybrid level. The formation energies of oxygen vacancies, and their effects on electronic properties, were also considered. It is found that most of the 3d transition metal dopants can lower the band gap of CeO2, with V and Co doping significantly reducing the band gap to less than 2.0 eV. Furthermore, all of the dopants can lower the formation energy of oxygen vacancies, and those with higher atomic numbers, particularly Cu and Zn, are most effective for this purpose. The electronic structures of doped CeO2 compensated by oxygen vacancies show that the presence of oxygen vacancies can further lower the band gap for most of the dopants, with V-, Cr-, Fe-, Co-, Ni-, and Cu-doped CeO2 all having band gaps of less than 2.0 eV. These results suggest that doping CeO2 with 3d transition metals could enhance the photocatalytic performance under visible light and increase the oxygen vacancy concentration, and they could provide a meaningful guide for the design of CeO2-based materials with improved photocatalytic and catalytic performance as well as enhanced ionic conductivity.
Doped transition-metal dichalcogenides monolayers exhibit exciting magnetic properties for the benefit of two-dimensional spintronic devices. Using density functional theory (DFT) incorporating Hubbard-type of correction (DFT$+U$) to account for the electronic correlation, we study the magnetocrystalline anisotropy energy (MAE) characterizing Mn-doped MS$_2$ (M=Mo, W) monolayers. A single isolated Mn dopant exhibits a large perpendicular magnetic anisotropy of 35 meV (8 meV) in the case of Mn-doped WS$_2$ (MoS$_2$) monolayer. This value originates from the Mn in-plane orbitals degeneracy lifting due to the spin-orbit coupling. In pairwise doping, the magnetization easy axis changes to the in-plane direction with a weak MAE compared to single Mn doping. Our results suggest that diluted Mn-doped MS$_2$ monolayers, where the Mn dopants are well separated, could potentially be a candidate for the realization of ultimate nanomagnet units.
Transformation induced plasticity (TRIP) behavior was studied in steel with composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP mechanisms. The initial microstructure consisted of both {epsilon}- and {alpha}-martensites with 27% retained austenite. TRIP behavior in the first 5% strain was predominately austenite transforming to {epsilon}-martensite (Stage I), but upon saturation of Stage I, the {epsilon}-martensite transformed to {alpha}-martensite (Stage II). Alloy segregation also affected the TRIP behavior with alloy rich regions producing TRIP just prior to necking. This behavior was explained by first principle calculations that revealed aluminum significantly affected the stacking fault energy in Fe-Mn-Al-C steels by decreasing the unstable stacking fault energy and promoting easy nucleation of {epsilon}-martensite. The addition of aluminum also raised the intrinsic stacking fault energy and caused the {epsilon}-martensite to be unstable and transform to {alpha}-martensite under further deformation. The two stage TRIP behavior produced a high strain hardening exponent of 1.4 and led to ultimate tensile strength of 1165 MPa and elongation to failure of 35%.
Nanodiamond (ND) hosting nitrogen-vacancy (NV) centers is a promising platform for quantum sensing applications. Sensitivity of the applications using NV centers in NDs is often limited due to presence of paramagnetic impurity contents near the ND surface. Here, we investigate near-surface paramagnetic impurities in NDs. Using high-frequency (HF) electron paramagnetic resonance spectroscopy, the near-surface paramagnetic impurity within the shell of NDs is probed and its g-value is determined to be 2.0028(3). Furthermore, HF electron-electron double resonance-detected nuclear magnetic resonance spectroscopy and a first principle calculation show that a possible structure of the near-surface impurity is the negatively charged vacancy V-. The identification of the near-surface impurity by the present investigation provides a promising pathway to improve the NV properties in NDs and the NV-based sensing techniques.
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