A differential capacitive biosensor using polyethylene glycol to overlay the biolayer

Abstract The feasibility of immunosensors based on capacitance measurements on semiconductor-immobilized antibody-electrolyte heterostructures is being investigated. Capacitance measurements on biosensors succeed only if the successive biomolecular layers grafted onto the heterostructures are 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; Schasfoort et al., Anal. Chim. Acta 238 (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 or did not have a suitable dielectric character. Ions were moving through or around this layer causing `shorting' of the system. In an attempt 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 layer on a bare chip surface, causing the capacitance to drop. The response was self-consistent and reproducible. On surfaces where there was protein pre-immobilized or pre-adsorbed, the PEG could not form a continuous, integral layer and a smaller drop in capacitance was observed. There is a difference in the response of single layers of protein, where only antibody is immobilized, and triple protein layers, where antibody was immobilized, antigen added and a secondary antibody bound. These results show the possibility of developing a differential capacitive biosensor.