Chemical sensing poses a challenge because it requires both sensitivity and selectivity for success. For many measurement schemes, sensitivity occurs at the expense of selectivity. To overcome this limitation, a sensitive detection technique is usually coupled to a highly selective screening system. We use monomolecular films to induce selectivity because they afford high permeability and excellent control over chemical identity. Following an overview of interfacial binding chemistry we use, we will focus on two different applications of monomolecular films to underscore the versatility of this structural motif.
We have designed ultrathin polymer layers to achieve differential control over adsorption and desorption kinetics of selected vapor phase molecules. We synthesize and deposit maleimide-vinyl ether alternating copolymer layers, where each polymer layer contains different pendant side group functionalities, and the order of layer deposition is controlled. The adsorption isotherm behavior of these interfacial structures, when exposed to methanol and hexane vapor, shows that the identity of the adsorbates and the order of polymer adlayer deposition both influence the interfacial adsorption characteristics.
We will also cover the growth and characterization of amphiphilic biomimetic assemblies grown on gold and ITO substrates. One assembly possesses a moiety capable of forming an intermediate hydrophilic region with amides that can participate in a hydrogen-bonding network within the monolayer (C6-A-C12), and the other is a tetradecane structure (C14). These structures present hydrophobic aliphatic chains to an adsorbate and are useful for subsequent deposition of a lipid monolayer, to form a hybrid bilayer membrane (HBM). Examination of the capacitance, cyclic voltammetry and FTIR data for the C6-A-C12 and C14 systems demonstrates their stability and organization, while preserving some fluid-like behavior, essential for biomimetic applcations.