Sensitivity, selectivity and nano-dimensional effects in gold nanocluster vapor sensors

A chemiresistor vapor sensor based on electron transport through an ensemble of ligand-stabilized gold nanoclusters is made exceptionally sensitive and selective by terminal carboxylic acid functionalization of the alkanethiol ligand. The directionality of the response (conductance increase or decrease) is strongly dependent on the nanoscale dimensions of the gold core and ligand shell thickness. Films of gold nanoclusters composed of a 2 nm metal core with a 0.5 nm -S(CH₂)₅COOH shell are compared to those based on an 8 nm core and a 1.5 nm -S(CH₂)₁₅COOH shell. Very strong and selective responses to amine vapors are observed but with a reversal of response in the direction of the conductance transduction. This unexpected result cannot be accommodated by known vapor response transduction mechanisms. A speculative new mechanism based on an ionic capacitance generated by the ligand-vapor interaction and confined to the outer surface fraction of the ligand shell is proposed.