Colorimetric Assay Using Mesoporous Fe-Doped Graphitic Carbon Nitride as a Peroxidase Mimetic for the Determination of Hydrogen Peroxide and Glucose

Iron can enter the electron-rich cavities of graphitic carbon nitride (g-C3N4). On account of this phenomenon, Fe-doped g-C3N4 (Fe-g-C3N4) was prepared as a peroxidase mimetic by using one-step pyrolysis of urea and FeCl3·6H2O. Compared to g-C3N4, Fe-g-C3N4 has a large specific surface area due to the presence of mesopores and cracks, a smaller band gap, and a high loading of Fe in its structure. Thus, Fe-g-C3N4 exhibits greater peroxidase activity with a more obvious color change when using 3,3′,5,5′-tetramethylbenzidine (TMB) as a substrate in the presence of hydrogen peroxide (H2O2). The color of a mixture of TMB and Fe-g-C3N4 gradually deepens with increasing concentrations of H2O2. Accordingly, a rapid, sensitive, and low-cost colorimetric assay for the detection of H2O2 was developed. After optimization, this method boasts a wide linear dynamic range for H2O2 detection from 0.005 to 400 μM (r2 = 0.9971) with a detection limit of 0.005 μM. Because H2O2 is a main product of glucose oxidation by glucose oxidase (GOx), a colorimetric assay for glucose detection was also realized, with a linear dynamic range of 1–1000 μM (r2 = 0.9996) and a detection limit of 0.5 μM. These assays were applied to the quantitative detection of H2O2 in milk and glucose in human serum, respectively

Ag–O–Co Interface Modulation-Amplified Luminol Cathodic Electrogenerated Chemiluminescence

It remains a great challenge to develop effective strategies for improving the weak cathodic electrogenerated chemiluminescence (ECL) of the luminol-dissolved O2 system. Interface modulation between metal and supports is an attractive strategy to improve oxygen reduction reaction (ORR) activity. Therefore, the design of electrocatalysts via interface modulation would provide new opportunities for the ECL amplification involving reactive oxygen species (ROSs). Herein, we have fabricated an Ag single-atom catalyst with an oxygen-bridged interface (Ag–O–Co) through the electrodeposition of Ag on a CoAl layered double hydroxide (LDH) modified indium tin oxide (ITO) electrode (Ags/LDH/ITO). Interestingly, it was found that the cathodic ECL intensity of the luminol-dissolved O2 system at the Ags/LDH/ITO electrode was extraordinarily enhanced in comparison with those at bare ITO and other Ag nanoparticle-based electrodes. The enhanced ECL performances of the Ags/LDH/ITO electrode were attributed to the increasing amounts of ROSs by electrocatalytic ORR in the Ag–O–Co interface. The electron redistribution of Ag and Co bimetallic sites could accelerate electron transfer, promote the adsorption of O2, and sufficiently activate O2 through a four-electron reaction pathway. Finally, the luminol cathodic ECL intensity was greatly improved. Our findings can provide inspiration for revealing the interface effects between metal and supports, and open up a new avenue to improve the luminol cathodic ECL

Mesocrystalline Nanocomposites of TiO₂ Polymorphs: Topochemical Mesocrystal Conversion, Characterization, and Photocatalytic Response

Four kinds of platelike mesocrystalline nanocomposites of TiO₂ polymorphs were successfully synthesized for the first time based on a topochemical mesocrystal conversion mechanism. In this conversion process, a [010]-oriented titanate H_1.07Ti_1.73□_0.27O₄ (□: vacancy of Ti) single crystal with lepidocrocite-like structure and platelike morphology was successively transformed into [001]- and [102]-oriented TiO₂(B) phases including a {010}-faceted TiO₂(B) twinning, [010]-oriented anatase phase, and [110]-oriented rutile phase. The platelike particle morphology is retained in the topochemical conversion process. The platelike particles are constructed from nanocrystals which well-aligned in the same orientation for the same phase, resulting in the formations of HTO/TiO₂(B), HTO/TiO₂(B)/anatase, TiO₂(B)/anatase, and anatase/rutile mesocrystalline nanocomposites. The reaction mechanism and the crystallographic topological correspondences between the precursor, intermediates, and the final product were given on the basis of the nanostructural analysis results. The mesocrystalline nanocomposite of anatase/rutile polymorphs exhibits unexpectedly high surface photocatalytic activity, which can be explained by the superior electron–hole separation effect and the high activity of {010}-faceted anatase surface in the mesocrystalline nanocomposite. Such mesocrystalline anatase/rutile nanocomposite is an ideal photocatalytic system