Improved stability and altered selectivity of tyrosinase based graphite electrodes for detection of phenolic compounds

The operational and storage stability of tyrosinase biosensors were investi gated for different tyrosinase modified electrodes, i.e., plain bulk modifi ed carbon paste electrodes (CPEs), surface modified by simple adsorption to solid graphite electrodes (SGEs), and surface modified by the immobilisati on in Eastman AQ, a poly ester-sulphonic acid cation exchanger, and Nafion, a perfluorinated-sulphonated ionomer, on the surface of both CP and SGEs, Factors such as the pH of immobilisation, the enzyme loading, and the polym er concentration were investigated in regards to the influence on the sensi tivity, the limit of detection (LOD), the sample throughput (STP), and the operational and storage stability. Both storage and operational stability w ere improved by immobilisation of tyrosinase in either of the two polymer m atrices. For 50 consecutive injections of 50 mu M catechol, the response st ayed the same for the optimal Eastman and Nafion modified CP and SGEs. Afte r about 42 days 80% and 75% of the original response for the Eastman and Na fion modified tyrosinase electrodes remained, respectively, whereas the tyr osinase bulk-modified CPE and adsorbed tyrosinase SGEs had lost virtually 1 00% of the original response. The selectivity for nine phenolic compounds w ere investigated and found to change with the introduction of the polymer m embrane. The detection of especially monophenols was improved probably due to their preconcentration into the polymer membranes. The best performing b iosensor in terms of sensitivity, LOD, STP, operational and storage stabili ty was reached for one where tyrosinase was immobilised in Nafion, i.e., se nsitivity: 11.51 nA/mu M, LOD: 0.015 mu M catechol, and STP: 36 samples/h. The phenolic content, expressed as catechol equivalents was evaluated in si x waste water effluents from tannery industries in Spain and Sweden. The op erational stability after 90 consecutive injections of extremely contaminat ed waste waters showed that the Nafion sensor retained 70% of its initial r esponse. (C) 1999 Elsevier Science B.V. All rights reserved.