6New Technologies Dept., ENEA, Casaccia, Via Anguillarese 301, 00060 Rome, Italy

We have developed two kinds of electrochemical biosensors for the detection of residual triazine-, urea- and phenolic-type herbicides, using isolated photosystem II (PS II) particles from the thermophilic cyanobacterium, Synechococcus elongatus, as biosensing elements. The herbicide detection was based on the fact, that in the presence of artificial electron acceptors, the light-induced electron transfer through isolated PSII particles is accompanied by the release of oxygen, which is inhibited by the herbicide in a concentration-dependent manner. In the first biosensor, the PSII particles were entrapped between the dialysis and Teflon membrane of the Clark oxygen electrode. The decrease of their oxygen evolution was used as a measure of herbicide concentration. In the second biosensor the rate of electron acceptor reduction was measured using a screen-printed electrode. The PSII particles were immobilised on the working electrode using BSA and glutaraldehyde. The combination of highly active PSII particles with the flow system resulted in re-usable herbicide biosensors with good stability (half-life of first biosensor was about 35 h work at 25°C) and high sensitivity (detection limit for diuron was 5 x 10^-10 M).