Xanthine sensors based on anodic and cathodic detection of enzymatically generated hydrogen peroxide

A xanthine biosensor was fabricated by the covalent immobilization of xanthine oxidase (XO) onto a functionalized conducting polymer (Poly-5, 2': 5', 2-terthiophine-3-carboxylic acid), poly-TTCA through the formation of amide bond between carboxylic acid groups of poly-TTCA and amine groups of enzyme. The immobilization of XO onto the conducting polymer (XO/poly-TTCA) was characterized using cyclic voltammetry, quartz crystal microbalance (QCM), and X-ray photoelectron spectroscopy (XPS) techniques. The direct electron transfer of the immobilized XO at poly-TTCA was found to be quasireversible and the electron transfer rate constant was determined to be 0.73 s(-1). The biosensor efficiently detected xanthine through oxidation at + 0.35 V and reduction at - 0.25 V (versus Ag/AgCl) of enzymatically generated hydrogen peroxide. Various experimental parameters, such as pH, temperature, and applied potential were optimized. The linear dynamic ranges of anodic and cathodic detections of xanthine were between 5.0 x 10(-6) - 1.0 X 10(-4) M and 5.0 x 10(-7) to 1.0 x 10(-4) M, respectively. The detection limits were determined to be of 1.0 x 10(-6) M and 9.0 x 10(-8) M with anodic and cathodic processes, respectively. The applicability of the biosensor was tested by detecting xanthine in blood serum and urine real samples.