Nanodimensionally Driven Analyte Response Reversal in Gold Nanocluster Chemiresistor Sensing

The modulation of electron transport through an ensemble of ligand-stabilized gold nanoclusters by the sorption of vapors is made exceptionally sensitive and selective by terminal carboxylic acid functionalization of the alkanethiol ligand. Of further importance, 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)(5)COOH shell are compared to those based on an 8 nm core and a 1.5 nm -S(CF12)(15)COOH shell with the finding of very strong and selective responses to amine vapors 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 based on a swelling expansion and a dielectric alteration of the ligand shell to modulate conductance in the ensemble. A speculative new mechanism is proposed on the basis of intercluster nanodomains of low and high dielectric character whose domain dimensions are determined by the ligand molecular structure and dielectric character that can range up to that associated with an ionic capacitance if generated by a vapor interaction.