On-line monolithic enzyme reactor fabricated by sol-gel process for elimination of ascorbic acid while monitoring dopamine

Catecholamine, a well-known neurotransmitter, is released to restore heart function after the occurrence of cardiac ischemia. Endogenous ascorbic acid interferes considerably with the monitoring of neurotransmitters such as catecholamine and glutamate. In this work we have successfully developed a nanoliter volume monolithic enzyme reactor and integrated it with a carbon film electrode for monitoring dopamine in order to scavenge such electroactive interferents as ascorbic acid. A monolithic silica support prepared by the sol-gel process has a large through-pore structure and a nanoporous surface. The low back pressure resulting from the large though-pore structure makes it possible to immobilize the enzyme and introduce fluid into the flow monitoring system by using a microsyringe pump. Ascorbate oxidase (AOx) was immobilized on the nanoporous surface of a monolithic matrix by physical adsorption and used for converting the ascorbic acid into its electrochemically inert form, dehydroascorbic acid, while monitoring dopamine. We showed that more than 99.8% of 100 muM of ascorbic acid could be converted to its oxidized form in the monolithic enzyme reactor. We succeeded in monitoring dopamine at a concentration of less than 100 nM in the presence of 50 muM of ascorbic acid. Therefore, this work demonstrated that a monolithic silica support offers excellent potential in regards to realizing a highly selective enzymatic reactor for biosensors.