Chemiresistor Arrays Prepared by Simple and Fast Vapor-Phase Thiol Place-Exchange Functionalization of Gold Monolayer-Protected Cluster Films

Abstract

We describe a simple, versatile, and relatively fast method to alter the sensor selectivity of chemiresistors based on films of Au monolayer-protected clusters (MPCs) by varying their functionalization via vapor-phase thiol place-exchange reactions. Drop-cast deposited films of hexanethiolate (C6S)-coated Au MPCs are exposed to volatile thiol liquids, such as mercaptoethanol (HOC2S), mercaptopropionic acid (HOOCC2S), and mercaptopropyltrimethoxysilane (H3COSiC3S), which leads to thiol place-exchange and new functionalities in the film. Generally, the film is about 10–20 % exchanged during the first 4 to 5 h and 80–90 % exchanged after 24 h. The extent and the rate of exchange decreases as the thickness of the film of C6S Au MPCs increases, leading to irreproducibility issues if film thickness is not carefully controlled. Importantly, the chemiresistive sensing response ratio of 2-propanol relative to toluene vapor increased significantly from about 0.3 prior to exchange to 2.0 after 24 to 48 h of exchange with HOC2S, showing that the selectivity of the sensor could be significantly altered by this method, which is simpler, faster, and more amenable to a wide variety of ligands compared to solution-based functionalization methods. The chemiresistive response of an unexchanged C6S Au MPC films and those exchanged with HOC2S, HOOCC2S, and H3COSiC3S demonstrates potential electronic nose applications.