Gold nanoparticle superlattices: correlating chemiresistive responses with analyte sorption and swelling

Chemiresistive and in situ GISAXS (grazing-incidence small-angle X- ray scattering) measurements were performed simultaneously on superlattices self-assembled from 1-dodecanethiol (DDT)-stabilized gold nanoparticles (GNPs, 4 nm). When dosed with vapors (1000-10 000 ppm) of toluene, 4-methyl-2-pentanone and 1-propanol, the GNP superlattice films responded with reversible increases in both the interparticle distance and resistance. Additionally, the mass uptake due to analyte sorption was determined microgravimetrically. Comparing the partition coefficients, chemiresistive sensitivities and GISAXS-measured swelling for the three solvent vapors revealed the same trends, which were consistent with the solubility match between DDT and the analyte. GISAXS-measured swelling and swelling deduced from microgravimetry showed remarkable agreement for 4-methyl-2-pentanone and toluene, whereas only a fraction of sorbed 1-propanol induced swelling. This suggests that due to the poor affinity of 1-propanol to DDT a significant amount deposited unselectively on the films' surface or within voids, where it is ineffective for swelling. The experimentally obtained data for analyte sorption and swelling were used to calculate the responses according to the commonly used chemiresistor model based on thermally activated charge transport. The comparison between calculated and measured responses demonstrated that the model can predict the chemiresistive responses to the analytes only qualitatively, i.e. with a precision of one order of magnitude and for 1-propanol with reversed direction. To enable a deeper understanding of the sensing mechanism and more precise predictions of the sensor characteristics, investigations into the microporosity of the assemblies and permittivity changes of the organic medium during analyte sorption are recommended as next experimental steps.