AFM nanometer surface morphological study of in situ electropolymerized neutral red redox mediator oxysilane sol–gel encapsulated glucose oxidase electrochemical biosensors

Four different silica sol–gel films: methyltrimethoxysilane (MTMOS), tetraethoxysilane (TEOS), 3-aminopropyltriethoxysilane (APTOS) and 3-glycidoxypropyl-trimethoxysilane (GOPMOS) assembled onto highly oriented pyrolytic graphite (HOPG) were characterized using atomic force microscopy (AFM), due to their use in the development of glucose biosensors. The chemical structure of the oxysilane precursor and the composition of the sol–gel mixture both influenced the roughness, the size and the distribution of pores in the sol–gel films, which is relevant for enzyme encapsulation. The GOPMOS sol–gel film fulfils all the morphological characteristics required for good encapsulation of the enzyme, due to a smooth topography with very dense and uniform distribution of only small, 50 nm diameter, pores at the surface. APTOS and MTMOS sol–gel films developed small pores together with large ones of 300–400 nm that allow the leakage of enzymes, while the TEOS film formed a rough and incomplete network on the electrode, less suitable for enzyme immobilisation. GOPMOS sol–gel film with encapsulated glucose oxidase and poly(neutral red) redox mediator, prepared by in situ electropolymerization, were also morphologically characterized by AFM. The AFM results explain the variation of the stability in time, sensitivity and limit of detection obtained with different oxysilane sol–gel encapsulated glucose oxidase biosensors with redox mediator.