A piezoelectric immunosensor based on antibody entrapment within a non-totally rigid polymeric film

A piezoelectric immunosensor is described which consists of a gold filmed quartz crystal, whose surface is modified with a thin Nafion layer entrapping known amounts of the specific antibody. This layer was applied by droplet evaporation on the gold surface of a mixture prepared by mixing a stock ethanol solution of Nafion with a stock antibody suspension in aqueous phosphate buffer. The coatings thus obtained could be easily and quickly removed by simple soaking in ethanol in an ultrasonic bath and renewed by repeating the procedure above, which resulted in quite reproducible coatings. The sensor was optimised with regard to parameters involved in its preparation and the assay process, in order to minimize viscosity-induced effects and energy-dissipation processes caused by the viscoelastic properties displayed by the proteic–polymeric layer adopted, so that frequency decreases found can reflect primarily mass changes at the surface. It permitted a reliable determination of enterotoxin in a wide range (up to about 800 ng ml−1), but a linear dependence was shown only at low concentrations (up to about 100 ng ml−1). The responses were characterized by good reproducibility and a DL of about 0.6 ng ml−1 could be estimated for a signal-to-noise ratio of 3. The profitable permeation properties of Nafion made these responses scarcely affected by the presence in the analyzed samples of high amounts of proteins, thus suggesting the possibility of using well simplified procedures for the preparation of biological samples.