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

For a model system, we used gangliosides as capture agents on the gold elec trode surface of a quartz crystal microbalance (QCM) to detect cholera toxi n (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 wa s corroborated by positive signals from the binding of appropriate anti-imm unoglobulin antibody in a manner analogous to a solid-phase enzyme-linked i mmunoassay (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 biolo gical sample. To avoid this problem, we have devised a self-assembled monol ayer (SAM) on gold that can be modified with a functional group able to cov alently bind and orient immunoglobulins without loss of antibody binding ac tivity. We present preliminary results demonstrating the formation of the S AM linker terminating in a hydrazide functionality. The hydrazide was used to link active antibody against E. coli O157:H7 to the self-assembled monol ayer. The antibody-derivatized QCM detected E. coli O157:H7 at concentratio ns of 10(7) and 10(4) cells per mi through a flow-cell. Antibody capture ag ents provide exquisite specificity, sensitivity and an essentially unlimite d range of capture agents that can be coupled to the QCM for analytical pur poses. (C) 1999 Elsevier Science B.V. All rights reserved.