Vapor sensing mechanism studies for monolayer protected gold nano-clusters on QCM and chemiresistor transducers

This research reports a series of monolayer-protected gold nano-clusters (MPCs) with various organo-thiolate groups capped and coated onto both chemiresistor (CR) and quartz crystal microbalance (QCM) transducers as chemical sensors for the detection of airborne volatile organic compounds (VOCs). The three thiolates that were used to encapsulate nano-gold were 1-octanethiol (Au-C8), 4-tert-butylbenzenethiol (Au-TBT), and a mixture of both (Au-C8TBT). The vapor responses using MPC sensing film on both QCM and CR transducers were rapid, reversible, and linear (R2 > 0.99). The Au-C8-coated sensors represented the well known sensing behavior, which holds that sorption mass and resistance changes are correlated. The Au-TBT sensor showed the opposite—a high sorption mass for a QCM signal but virtually no change in resistance for CR. This result contradicts the conventional belief that the degree of resistance change is proportional to the absorbed mass. The mixed-ligand Au-C8TBT film showed alternative vapor selectivity for both CR and QCM transducers that deviated from both Au-C8 and Au-TBT film. The chemical structures of the mechanisms for vapor selectivity for two different transducers were investigated and are discussed here.