Silica sol±gel immobilized amperometric biosensor for the determination of phenolic compounds J. Li, L.S. Chia, N.K. Goh, S.N. Tan * Division of Chemistry, National Institute of Education, Nanyang Technological University, 469 Bukit Timah Road, Singapore 259756, Singapore Received 1 September 1997; received in revised form 15 December 1997; accepted 17 January 1998 Abstract An amperometric enzyme electrode for phenolic compounds was developed via an easy and effective immobilization method using the sol±gel technique. The enzyme electrode comprises tyrosinase immobilized by the thin silica sol±gel layer on a carbon-paste electrode. The tyrosinase retains its bioactivity when being immobilized by the sol±gel ®lm. Phenolic compounds were determined by the direct reduction of biocatalytically liberated quinone species at 0 mV vs. Ag/AgCl (sat. KCl). The process parameters for the fabrication of the enzyme electrode were optimised. The in¯uence of various experimental variables was explored for optimum analytical performance of the enzyme electrode. The effect of oxygen on the response of the enzyme electrode was evaluated. The sensitivities of the enzyme electrode for catechol, phenol, p-cresol, m-cresol, o-cresol and 2-chlorophenol were 1.53, 1.28, 1.05, 0.687, 0 and 0 A M 1 , respectively. The enzyme electrode retained ca. 50% of its activity after 15 days of storage in a phosphate buffer solution at 48C. # 1998 Elsevier Science B.V. Keywords: Sol±gel technique; Tyrosinase; Phenolic compounds; Amperometry 1. Introduction The determination of phenolic compounds is of great importance owing to their applicability in a broad range of chemical manufacturing processes and their inherent toxicity. Colorimetric- and ultra- violet-spectrophotometric analyses are now com- monly used for the determination of phenols as standard methods [1,2]. However, these schemes may suffer from the time-consuming complicated sample pre-treatment, lacking sensitivity and may not be suitable for in situ sensing application. To address this problem, a simple, effective and fast alternative method for the determination of phenolic compounds is desirable. Amperometric biosensors for phenolic compounds based on tyrosinase have proven to be promising for this purpose [3±9]. Tyrosinase, also known as polyphenol oxidase, is a copper-containing mono-oxygenase that can catalyse the conversion of phenolic compounds to the corre- sponding quinones in the presence of oxygen. The liberated quinone species can be electrochemically reduced to phenolic substances at low potential without any mediator. Numerous immobilization methods have been developed to stabilise the tyrosi- nase in enzyme electrodes. Besides native ®xing Analytica Chimica Acta 362 (1998) 203±211 *Corresponding author. Tel.: 0065 460 5323; fax: 0065 469 8952; e-mail: tansn@nievax.nie.ac.sg 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00064-6