Self-assembled monolayers of supramolecular compounds for chemical sensors

We have studied the application of self-assembled resorcin[4]arene monolayers as chemically sensitive coating of quartz microalance sensors. Resorcin[4]arenes are typical supramolecular compounds with a molecular cavity formed by an aromatic ring system. The latter provides lipophilic binding sites and acts therefore as a molecular recognition structure for organic molecules. Reosrcinarenes are of particular interest for the selective detection of organic molecules in the gas and liquid phase. A high level of molecular order can be achieved by using modified cage compounds: dialkylsulfide-substituted resorcin[4]arene derivatives (with 4 aromatic rings) form well-ordered monolayers at Au surfaces by a self-assembling process which occurs spontaneously in ethanol/chloroform solutions of thes resorcin[4]arene derivatives. The driving force is the formation of very stable AuS bonds and the van der Waals interaction between the alkyl chains. We present results on the structure of resorcin[4]arene monolayers and their interactions with organic molecules. These results were obtained from angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and thermal desorption spectroscopy (TDS). The latter techniques makes it possible to determine binding energies of the molecule/resorcin[4]arene ‘key-lock’ interaction. The binding energies are then compared with mass changes Δm which are obtained from quartz microbalance oscillators with their Au electrodes coated with resorcin[4]arene monolayers. We tested different organic molecules in the gas and liquid phase such as tetrachloroethane (C2Cl4), Trichloroethylene (HClCCCl2), tetrachloromethane (CCl4), chloroform (CHCl3) and toluene (H3C-C6H5). By far the most pronounced sensor response and hence highest selectivity was observed to tetrachloroethene (C2Cl4)