Fabrication And Electrochemistry Of Two-dimensional Ordered SiO₂ Cavities On Electrode Surfaces

Uniform polystyrene particles were synthesized by a dispersion polymerization of styrene in ethanol-water mixtures. With self-assembled monolayer (SAM) techniques, the two-dimensional array arrangement of the polystyrene (PS) spheres were formed and transferred onto the surface of ITO electrode. With close-packed PS microspheres as the template, silica (SiO₂) was deposited into the interstices of the PS particles via sol-gel methods or electrodeposition. After the removal of the templating PS particles, an array of two-dimensional reversed micro-cavities of SiO₂ was subsequently obtained. The depth and diameter of the SiO₂ cavities can be easily varied through the alteration of the templating particle size.Silver nanoparticles were electrochemically deposited into the bottom of the cavities, and there were no Ag particle grew on the internal wall or outer fringe of the cavities. It demonstrated that the electrochemical reaction occurs at the bottom of the silica cavities. This is due to the poor conductivity of the SiO₂ cavities structure. Thus, the SiO₂ cavities exhibit a confinement effect on the electrochemical reactions the electrode, making the electrode to act as an array of microelectrodes. The confinement effect was further confirmed by the cyclic voltammetric behavour of Fe(CN)₆^3- in KCl solution. Moreover, the cyclic voltammetric behavour of methylviolegen cation (MV²⁺) in KCl solution on the SiO₂ cavities modified electrode suggested that the negatively charged surfaces of SiO₂ cavities, probably resulting from the deprotonation of hydroxy, lead to selectively enrich the positively charged MV²⁺cations. This may consequently improve the detection sensitivity of the cation species in solution. Furthermore, the selective enrichment effect of SiO₂ cavities was strongly dependent on the pH value of the solution.A novel biosensor was constructed by immobilization of MP-11 inside 2-D arrays of micrometer hemispherical SiO₂ cavities on the surface of ITO electrodes. With SAMs techniques, the monolayer of 4-mercaptobenzoic acid (MBA) was attached on Ag nanoparticles deposited inside the SiO₂ cavities through S-Ag bond. With the MBA as a linker, microperoxidase-11 (MP-11) can be immobilized into the cavities through the covalent bond formed between the carboxyl of MBA and the residual amino group of MP-11.Preliminary experimental data showed that such MP-11 modified electrode displayed good electrocatalytical properties for the O₂ reduction.