Development of Thiocyanate-Selective Membrane Electrodes by the Sol–Gel Method

摘要

To prepare thiocyanate-selective chemical sensors, a sol–gel-based matrix was employed as a hostmembrane for the encapsulation of a sensing material. Tricaprylylmethylammonium thiocyanate(Aliquat336-SCN) was used as the dissociated ion-exchanger type sensing ionophore. Both 3-isocyanatopropyl-triethoxysilane and 1,4-butanediol were used for appropriate precursor preparation.Under acidic conditions, the sol precursor slowly gelled to yield a film in which the thiocyanateionophore, Aliquat336-SCN, was encapsulated. Electrochemical responses of the potentiometricsensors were compared to those of traditional poly(vinyl chloride) (PVC) membrane electrodesprepared using the same ionophore. On the 7th working day, the average response slope towardthiocyanate was approximately 56.3 mV decade-1 in the concentration range 10-4.5 M to 5 × 10-1 M at20°C. Selectivity toward thiocyanate over lipophilic anions such as perchlorate and salicylate wasconsiderably improved. The results exhibit a clear tendency that deviates from the Hofmeisterselectivity pattern which would be observed in a membrane electrode made of Aliquat336-SCN typesalt. The potentiometric selectivity coefficients, as measured in a 0.05 M 2-morpholinoethanesulfonicacid monohydrate buffer, pH 5.5, were log kpotthiocyanate, chloride = ? 2.3, log kpotthiocyanate, salicylate = ? 0.6,and log kpotthiocyanate, perchlorate = ? 0.1. The optimized membrane sensors were employed for the flowinjection measurement of thiocyanate levels in saliva samples from both smokers and nonsmokers. Themembrane sensors exhibited a fairly good correlation with the traditional colorimetric method (n = 23;sensors = 0.84 (colorimetric) + 0.13: R2 = 0.97).