Investigation of Sulfur Doping in Mn–Co Oxide Nanotubes on Surface-Enhanced Raman Scattering Properties

Doping engineering is an efficient strategy to manipulate the optoelectronic properties of metal oxides for sensing, catalysis, and energy applications. Herein, we have demonstrated the fabrication of sulfur (S)-doped Mn–Co oxides to regulate their band and surface electronic structures, which is beneficial to enhancing the charge transfer (CT) between the metal oxides and their adsorbed molecules. As expected, significantly enhanced SERS signals are achieved on S-doped Mn–Co oxide nanotubes, and the minimum detection concentration can reach as low as 10–8 M. Furthermore, the change in the electronic structure caused by S-doping provides different microelectric fields to influence the orientation of the interaction between the probe molecules and the substrate. Additionally, the evaluation of the oxidase-like catalytic activity of the substrate proved that, with an increase in the ratio of Co2+/Co3+ content, the number of electrons on the substrate increases, which promotes the CT process and further increases the degree of CT. The nonmetallic doping route in semiconducting metal oxides can provide effective and stable SERS activity; moreover, it provides a new strategy for exploring the relationship between CT in catalysis and SERS performance of semiconductors

Revealing Adsorption Mechanism of <i>p</i>‑Mercaptobenzoic Acid with TiO₂ Surfaces Using Electric Field-Enhanced Semiconductor SERS

p-Mercaptobenzoic acid (4-MBA) is a typical molecular probe for a surface-enhanced Raman scattering (SERS) study of the enhancement performance of semiconductor nanoparticles. Understanding the molecular adsorption mechanism of 4-MBA on a semiconductor surface is crucial to reveal the enhancement mechanism of semiconductor SERS. Herein, two types of submicrometer-sized TiO2 particles with amorphous (denoted as a-TiO2) and anatase structures (denoted as c-TiO2) were fabricated, and their potential as SERS-active substrates with high electric-field enhancement was explored based on the near-field scattering theory and finite-element method simulation. The electric field-enhanced semiconductor SERS provide a better vision for us to study the adsorption modes of molecules on the TiO2 surface. On this basis, adsorption behaviors of 4-MBA on a-TiO2 and c-TiO2 particles were systematically studied by the semiconductor SERS and density functional theory. The results demonstrated that the adsorption mechanism of 4-MBA with TiO2 surfaces is highly dependent on the exposure of acid sites of TiO2 surfaces. 4-MBA adsorbs preferentially on Brønsted acid sites of a-TiO2 through a carboxyl group, in contrast on Lewis acid sites of c-TiO2 through a sulfhydryl group. Furthermore, 4-MBA molecules may form multilayer adsorption on TiO2 surfaces through the hydrogen bond and/or π–π stacking interaction. Research results not only provide a new insight to re-evaluate the chemical enhancement mechanism for TiO2–4-MBA systems but also provide a theoretical guidance for the modification of TiO2 surface with organic molecules containing carboxyl and sulfhydryl groups

SERS Tracking Oxidative Stress on a Metalloporphyrin Framework by Vitamin C

Accurate control of charge transfer is crucial to investigate the catalytic reaction mechanism of the biological oxidation process that biomedicine participates in. Herein, we have established an assembly model of metalloporphyrin framework (MPF) nanosheets as the active centers of biological enzymes. The introduction of Vitamin C (VC) into the MPF system can precisely modulate its content of charges. The surface-enhanced Raman scattering activity and peroxidase-like catalytic performance are enhanced simultaneously for the first time by manipulating the optimal molar ratio of an MPF to VC and the reaction sequence with target model molecules. We have confirmed that the formation of the intermediate of Fe(2+)-OOH species is specifically enhanced after VC modulation, which indicates that VC can regulate the oxidative stress of the active center of biological enzymes. This discovery not only accurately resolves the mechanism of VC-selective anticancer therapy but also has important significance for the precise treatment of VC synergistic targeting medicines