Capture agents for a quartz crystal microbalance-continuous flow biosensor: functionalized self-assembled monolayers on gold

Abstract

For a model system, we used gangliosides as capture agents on the gold electrode surface of a quartz crystal microbalance (QCM) to detect cholera toxin (CT) and the closely-related Escherichia coli heat-labile enterotoxin (LT) in a continuous-flow cell. Positive signals were verified by introduction of anti-CT and anti-LT antibodies. Antibody binding to captured analyte was corroborated by positive signals from the binding of appropriate anti-immunoglobulin antibody in a manner analogous to a solid-phase enzyme-linked immunoassay (ELISA). While ganglioside receptor analogs are stable, easy to apply and have high affinity for particular bacterial toxins, they have the disadvantage of binding endogenous materials likely to be found in a biological sample. To avoid this problem, we have devised a self-assembled monolayer (SAM) on gold that can be modified with a functional group able to covalently bind and orient immunoglobulins without loss of antibody binding activity. We present preliminary results demonstrating the formation of the SAM linker terminating in a hydrazide functionality. The hydrazide was used to link active antibody against E. coli O157:H7 to the self-assembled monolayer. The antibody-derivatized QCM detected E. coli O157:H7 at concentrations of 10 7 and 10 4 cells per ml through a flow-cell. Antibody capture agents provide exquisite specificity, sensitivity and an essentially unlimited range of capture agents that can be coupled to the QCM for analytical purposes.