Bound sialic acid (b-SIA) occurs linked t o macromolecules in organisms. An electrochemical method for determining b-SIA, using a potentiometric four-channel thick-film sensor, has been developed. The sialidase sensor consists of a bilayer of a membrane containing sialidase isolated from Clostridium perfringens immobilized in a poly(viny1 acetate)-polyethylene copolymer, which is then placed on top of an H+-selective poly(viny1 chloride)-poly(viny1 acetate) indicator membrane. The enzymic cleavage of the linkage t o the macromolecular substrate leads t o the release of sialic acid, which is an a-ketocarboxylic acid with a pK, of 2.6. The decrease in the pK, value of the carboxyl group results in a local pH change inside the sialidase containing sensor membrane, which is monitored by the H+-selective indicator membrane. The determination of non-pathological (0.5-0.8 mg cm-3) and pathological levels (1.3-3.0 mg cm-3) of b-SIA in serum samples prepared from human test sera is shown. The detection limit was found t o be 0.3 m g cm-3 of b-SIA in these sera, and the linear range was from 1 .O m g cm-3 t o more than 10.0 mg cm-3 of b-SIA. The slope of the sensor response was found t o be dependent o n the relative molecular mass of the substrates, which is crucial for quantitative measurements. The pH optimum of the sialidase sensor was found to be pH 4.0 for the substrates sialyl-lactose, mucine and colominic acid. The serum samples were diluted with an equal volume of 100 m m o l dm-3 phosphate buffer in order to bring the final pH t o 4 before analysis. The rise time t o reach 90% of the final reading ranges from 2 t o 5 min for the different substrates. The slope of the sensor response decreases t o 55% of the initial value during 100 h of continuous operation at room temperature at pH 4.0. By averaging the readings from the four channels of the sensor the accuracy for the determination of b-SIA in human sera can be improved and a standard deviation of +2.5% is achieved.

A potentiometric thick-film biosensor for the determination of the neurotransmitter acetylcholine is presented. The selective membranes consist of two layers. The first is a choline- and H(+)-selective membrane made from a poly(vinyl chloride)-poly(vinyl acetate) copolymer which is plasticized with 2-nitrophenyl octyl ether. It contains 1% m/m of the ion-pair choline phosphotungstate as an electroactive compound. Both products of the enzymic cleavage, choline and H+, are detected. The second layer is placed above the first and is made from a poly(vinyl acetate)-polyethylene dispersion polymer. It is loaded with 2% m/m of acetylcholinesterase isolated from electric eel. The linear region of the response curve extends from 2 x 10(-5) to 0.01 mol dm-3 in buffer solution and undiluted horse serum. The detection limit is 5 x 10(-6) mol dm-3 and the response time is about 4 min.

Ion-selective poly(vinyl chloride) membrane electrodes for the opium alkaloids papaverine and ethaverine are presented. The electrode membranes contain ion pairs of the alkaloids with the anionic counter ion tetraphenylborate. The detection limits for all electrodes were approximately 2 x 10(-6) mol dm-3 at pH 5.0 in 100 mmol dm-3 buffered solutions and the measured slopes were close to the values theoretically expected. The selectivity coefficient observed for the ethaverine-tetraphenylborate electrode is 10(-1.1) with respect to papaverine. The suitability of the membranes for single-use electrodes is discussed.

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Ion-selective poly(viny1 chloride) membrane electrodes and thick-film sensors for the determination of the antiarrhythmic drug bretylium in human serum are described. Ion-pair complexes of bretylium with the anionic counter ions tungstosilicate, reineckate [diamminetetracyanatochromate(~~~)], tetraphenylborate and dipicrylaminate were investigated as electroactive compounds for the electrode membranes. The membranes containing the bretylium-tungstosilicate complex showed the best properties with both types of transducers. The detection limits were determined to be 1 x 10-7 mol dm-3 for the macroscopic electrodes and 8 x 10-7 mol dm-3 for the thick-film sensors in 0.1 mol dm-3 Tris buffer at pH 7.4; slopes of 60.7 and 40.0 mV decade-' respectively, were found. In pathological and non-pathological test sera, a detection limit of 5 x 10-6 mol dm-3 was obtained with the thick-film transducers. The potentiometric selectivity coefficients towards structurally similar compounds such as ephedrine, dopamine and epinephrine (adrenaline) ranged from 10-1.7 to 10-3.4. Response times of 5-10 s were observed in buffered aqueous solutions as well as in human serum samples.