A DIFFERENTIAL CAPACITIVE BIOSENSOR USING POLYETHYLENE-GLYCOL TO OVERLAY THE BIOLAYER

The feasibility of immunosensors based on capacitance measurements on semiconductor-immobilized antibody-electrolyte heterostructures is bei ng investigated. Capacitance measurements on biosensors succeed only i f the successive biomolecular layers grafted onto the heterostructures an sufficiently electrically insulating and retain their recognizing ability (Jaffrezic-Renault et al., Sensors and Actuators B 15-16 (1993 ) 458-462: Bergveld, Biosensors and Bioelectronics 6 (1991) 55-72, Sch asfoort et al., Anal. Chim. Acta 235 (1990) 323-329). However. as with other groups, our system showed no reproducible capacitance decrease on addition of antigen. The biolayer was not sufficiently insulating o r did not have a suitable dielectric character. Tolls were moving thro ugh or around this layer causing 'shorting' of the system. In all atte mpt to drive the system toward exclusion of ions from the biolayer, an overlay of non-conducting polymer, in the form of polyethylene glycol (PEG), was added. Our results indicate that PEG forms an insulating l ayer on a bare chip surface, causing the capacitance to drop. The resp onse was self-consistent and reproducible. On surfaces where there was protein pre-immobilized or pre-adsorbed, the PEG could not form a con tinous, integral layer and a smaller drop in capacitance was observed. There is a difference in the response of single layers of protein, wh ere only antibody is immobilized, and triple protein layers, where ant ibody was immobilized, antigen added and a secondary antibody bound. T hese results show the possibility of developing a differential capacit ive biosensor. (C) 1997 Elsevier Science S.A.