Preparation and characterization of urease-encapsulated biosensors in poly(vinyl alcohol)-modified silica sol–gel materials

The microenvironments of the sol–gel-derived urease biosensors in terms of elemental ratio, surface morphology, specific surface area and pore size were investigated to characterize the physicochemical properties of poly(vinyl alcohol) (PVA)-modified sol–gel materials. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and surface area analyzer were used to identify the surface species, topography and pore distribution of the organically doped sol–gel network. XPS results showed that stoichiometric ratios of oxygen-to-silicon in sol–gel materials were in the range 2.08–2.11. The sol–gel materials were partially dried and negatively charged, which retained 6–8% water content to maintain urease activity. The surface morphology of the sol–gel altered obviously when macromolecules were encapsulated, resulting in the increase in surface mean roughness from 0.207 to 2.636 nm. The specific surface area decreased dramatically after the immobilization of biomolecules and organic additives, which clearly depicts that PVA and urease were co-encapsulated into the sol–gel network. However, there still exist enough pore volumes for analytes to mass transport. The apparent Michaelis–Menten constant value (Km) of the encapsulated urease was similar to that in solution and the overall catalytic efficiency in PVA-doped sol–gel-derived glasses only decreased by a factor of 3.2 relative to the value in solution. In addition, the analytical performance of the entrapped urease in PVA-doped sol–gel materials was examined by determining the Cu(II) concentration in aqueous solution. The analytical range of Cu(II) was in the range 2 × 10−6 to 2 × 10−2 M with a detection limit of 1.5 μg L−1. Results obtained in this study demonstrate a strategy for maintaining urease activity for biomedical and environmental applications.