Public concern over water pollution caused by per- and polyfluoroalkyl substances (PFAS) is increasing, underscoring the practical challenge posed by the lack of efficient degradation technologies for these highly persistent contaminants. As a green and effective advanced oxidation process (AOP), electrochemical oxidation (EO) offers considerable advantages in treating refractory organic pollutants. In particular, boron-doped diamond (BDD) electrodes have demonstrated promising potential for degrading perfluorooctanoic acid (PFOA), but their underlying degradation mechanisms remain unclear, and key process parameters have yet to be systematically optimized. To address these issues, this study combined density functional theory (DFT) calculations with electrochemical experiments to systematically explore the behavior and mechanism of EO degradation of perfluorooctanoic acid by silicon-doped boron-doped diamond (Si/BDD) electrodes. The Si/BDD electrode paired with Na2SO4 electrolyte constitutes the optimal system, which attains nearly complete PFOA removal (≥ 99%) within 120 min under optimized operational parameters: an initial PFOA concentration of 5 ppm, current density of 15 mA cm−2, pH of 2.0, and temperature of 60 °C. Radical quenching experiments and DFT analyses confirmed that hydroxyl radicals (·OH) serve as the primary reactive species, and PFOA degradation proceeds via a pathway involving sequential defluorination and stepwise carbon-chain shortening, with the carbonyl oxygen acting as the preferred site for electrophilic attack. Comparative experiments further indicated higher degradation efficiency for long-chain perfluorocarboxylic acids. These findings elucidate the molecular mechanism underlying the EO of PFOA on boron-doped diamond electrodes, offering theoretical support and process guidance for the efficient electrochemical treatment of PFAS pollutants, while broadening the technical pathways for remediation of water contaminated by persistent organic compounds.

The effect of pH-induced electrostatic conditions on the molecular interaction force of a single lysozyme molecule with TiO2 is investigated using atomic force microscopy (AFM). The force between the charged or neutral lysozyme molecule and the TiO2 surface is measured at different pH from 3.6 to 10.8. It is found to be directly proportional to the contact area, given by an effective diameter of the lysozyme molecule, and is further qualitatively verified by the AFM-measured friction coefficients. The results of the Derjaguin–Landau–Verwey–Overbeek theory show that the pH can change the surface charge densities of both lysozyme and TiO2, but the molecular interaction force at different pH is only dependent on the pH-induced effective diameter of lysozyme. The molecular interaction forces, quantified at the nanoscale, can be directly used to design high-performance liquid chromatography measurements at macroscale by tuning the retention time of a protein under varied pH conditions. They can also be applied to develop a model for predicting and controlling the chromatographic separations of proteins.

Detecting the small amounts of proteins interacting effectively with the solid film electrodes surface still remains a challenge. To address this, in this work, a new approach was proposed by the combination of the adhesion forces and the molecular interaction measured with AFM. Cytochrome c (Cyt C) interacting effectively with TiO₂ nanotube arrays (TNAs) was chosen as a probe. The amounts of Cyt C molecules interacting effectively on TNAs surface (C_TNA) range from 5.5×10^-12 to 7.0×10^-12 mol/cm2 (68.2-86.8 ng/cm²) and they are comparable with the values obtained by the electrochemistry method in the literature, in evidence of the accuracy of this AFM-based approach. The reliability of the proposed approach was further verified by conducting Surface Enhanced Raman Scattering (SERS) measurements and estimating the enhancement factor (EF). This interaction-based AFM approach can be used to accurately obtain the small amounts of adsorbed substances on the solid film electrodes surface in the applications such as biosensors, biocatalysis, and drug delivery, etc.