Development of high performance microbiosensors based on oxidases for in-vivo monitoring

Abstract

Oxidase-based biosensors for continuous in vivo measurements have been developed. A platinum/iridium (9/1) wire electrode (o.d. of 0.178 mm, sensing area of 1.12 square millimeters is modified with (3-mercaptopropyl)trimethoxysilane employing self-assembly and electropolymerization to achieve a permselectivity for hydrogen peroxide over endogenous electroactive interferants. Glucose oxidase is immobilized on the modified surface, (i) by using 3-maleimidopropionic acid as a covalent linker, or (ii) by cross-linking with bovine serum albumin using glutaraldehyde. Sensitivities of 9.97 nA/mM glucose are observed when the enzyme is immobilized by method ii at an applied potential of +600 mV vs Ag/AgCl in pH 7.4 PBS. Lower sensitivities (0.113 nA/mM glucose) are observed when immobilization method i is employed. The sensor immobilized by method ii and coated with polyurethane exhibits a linear range to 15 mM glucose with a sensitivity of 0.47 nA/mM glucose.

Ferrocenecarboxylic acid is covalently immobilized to shuttle the electron transfer between glucose oxidase and electrode surface. A gold disk electrode (o.d. of 0.5 mm, sensing area of 0.196 square millimeters) is self-assembled with (3-mercaptopropyl)trimethoxysilane to solve the problem of electrochemical interferences. Sensitivities of 0.65 nA/mM glucose are observed for the sensor coated with Eastman AQ-55D, poly- scL-lysine and polyurethane at an applied potential of +400 mV vs Ag/AgCl. The interference from ascorbate becomes zero offer coimmobilizing ascorbate oxidase along with glucose oxidase. However, interferences from urate (0.05 nA/0.1 mM) and acetaminophen (1.2 nA/0.1 mM) are still problematic.

Covalently immobilized thionine is employed as a mediator. A potential of +230 mV vs Ag/AgCl to Au disk electrodes (0.5 mm o.d.) is applied. The sensors with the mass transport limiting layer containing Eastman AQ-55D, poly- scL-lysine and polyurethane show good linearity up to 15 mM glucose with a sensitivity of 3.0 nA/mM and show no interference from uric acid or acetaminophen, although a signal from ascorbic acid is noticed (0.4 nA/0.1 mM). The sulfoxide formation through the oxidation of thionine seems to be a part of the cause of the sensitivity loss over time.

The TTF-TCNQ crystals are directly prepared on the cylindrical surface of a Pt/Ir (9/1) wire electrode (o.d. of 0.178 mm, sensing area of 0.56 square millimeters). A potential of +150 mV vs Ag/AgCl is chosen for glucose measurements. The pattern of the sensor performance as a function of glucose concentration in an air-saturated solution is like a sigmoidal curve, which explains the strong oxygen competition with the TTF-TCNQ for reoxidation of glucose oxidase at low glucose concentrations. There is no signal from the oxidizable interferants such as urate and acetaminophen. However, still a large current from ascorbate is observed (195 nA/0.1 mM ascorbate). The ascorbate signal is completely removed at the oxygen levels higher than 55 μM when ascorbate oxidase is employed. When the TTF-TCNQ-glucose oxidase layer is coated with poly- scL-lysine and Nafion, the ascorbate interference is remarkably reduced (1.6 nA/0.1mM ascorbate) mainly due to the charge repulsion effect from Nafion. The improvement in linearity is an additional advantage of the coating (10.7 nA/mM glucose, Michaelis Constant, 48 mM).