Bulk-modified amperometric biosensors for hypoxanthine based on sol–gel technique

Graphite–ceramic amperometric biosensors for hypoxanthine (Hx) have been fabricated and applied to the measurement of fish meat quality. The fabrication process involves the immobilization of xanthine oxidase in the bulk of a silica-graphite matrix by sol–gel techniques and the assembly of the enzyme-modified matrix into a glass tube to form a disk electrode whose surface can be easily renewed by simple mechanical polishing. Michaelis–Menten constants (Kmapp) of 0.45, 0.64, 0.35 mM were determined from three different types of Hx sensors operating in the unmediated hydrogen peroxide oxidation mode, the unmediated and the benzyl viologen-mediated oxygen reduction modes, respectively. The sensors delivered good linear response over a wide range of concentrations of ca. 1×10−6–1×10−3 M, with respective detection limits of 1.3×10−6, 5.6×10−6and 3.8×10−7 M in 0.05 M phosphate buffer (pH 7.6). The use of bulk immobilization and the possibility of renewable surfaces have enabled the easy operation of these sensors with high stability. Satisfactory results were obtained from the determination of the freshness of carp tissues stored under different conditions.