Self-assembled monolayers for chemical sensors: molecular recognition by immobilized supramolecular structures

We have studied the applicability of two different self-assembling monolayer systems as chemically sensitive coatings for quartz microbalances. The first consists of a monolayer of substituted resorcin[4]arenes and serves as a gas sensing device. The second is based on a modified peptide antigen embedded in a hydroxyundecanethiol-monolayer and serves as a liquid sensing device to monitor antibodies. Both monolayer systems are coupled to gold surfaces via S-Au bonds. Quartz microbalances (QMB) are employed as mass sensitive transducers and allow for sensitive detection of the molecular recognition process. Information about the orientation and structure of resorcin[4]arene monolayers as well as their interactions with organic molecules were obtained from angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and thermal desorption spectroscopy (TDS). The latter technique makes it possible to determine experimentally binding energies of the molecule/resorcin[4]arene-'key-lock' interaction. Several different organic molecules were detected in the gas phase. By far the most pronounced sensor response and hence highest selectivity was observed for tetrachloroethene (C2Cl4). The modified synthetic peptide was deposited on the electrodes of a QMB by self-assembly. The quartz crystals were mounted into a flow-through cell with only one side exposed to the liquid. The successful immunological reaction between the immobilized antigen and the antibody in solution could be monitored on-line with a QMB. The sensitive coating of the QMB could be regenerated for its successive use as a mass sensitive biosensor.