In this study, coatings of cross-linked gold nanoparticles (AuNPs) on flexible polyethylene (PE) substrates were prepared via layer-by-layer deposition and their application as strain gauges and chemiresistors was investigated. Special emphasis was placed on characterizing the influence of strain on the chemiresistive responses. The coatings were deposited using amine stabilized AuNPs (4 and 9 nm diameter) and 1,9-nonanedithiol (NDT) or pentaerythritol tetrakis(3-mercaptopropionate) (PTM) as cross-linkers. To prepare films with homogeneous optical appearance, it was necessary to treat the substrates with oxygen plasma directly before film assembly. SEM images revealed film thicknesses between ∼60 and ∼90 nm and a porous nanoscale morphology. All films showed ohmic I-V characteristics with conductivities ranging from 1 × 10⁻⁴ to 1 × 10⁻² Ω⁻¹ cm⁻¹, depending on the structure of the linker and the nanoparticle size. When up to 3% strain was induced their resistance increased linearly and reversibly (gauge factors: ∼20). A comparative SEM investigation indicated that the stress induced formation and extension of nanocracks are important components of the signal transduction mechanism. Further, all films responded with a reversible increase in resistance when dosed with toluene, 4-methyl-2-pentanone, 1-propanol or water vapor (concentrations: 50-10 000 ppm). Films deposited onto high density PE substrates showed much faster response-recovery dynamics than films deposited onto low density PE. The chemical selectivity of the coatings was controlled by the chemical nature of the cross-linkers, with the highest sensitivities (∼1 × 10⁻⁵ ppm⁻¹) measured with analytes of matching solubility. The response isotherms of all film/vapor pairs could be fitted using a Langmuir-Henry model suggesting selective and bulk sorption. Under tensile stress (1% strain) all chemiresistors showed a reversible increase in their response amplitudes (∼30%), regardless of the analytes' permittivity. Taking into consideration the thermally activated tunneling model for charge transport, this behavior was assigned to stress induced formation of nanocracks, which enhance the films' ability to swell in lateral direction during analyte sorption.

In this study the chemiresistive responses of gold nanoparticle superlattices are investigated by GISAXS and microgravimetry.

The performance of nanoparticle assemblies with respect to various applications (e.g. sensors, catalysts, filtration membranes) depends on their microporosity. Here, the microcavities within the ligand matrix of superlattice films comprised of 1-dodecanethiol-stabilized gold nanoparticles (GNPs, core diameter $4 and $5.5 nm) were studied by positron annihilation lifetime spectroscopy (PALS). The superlattice composition, the size and spatial arrangement of the GNP cores were characterized by thermogravimetric analysis, transmission electron microscopy and grazing-incidence small-angle X-rayscattering. From these data a structural model was derived to predict the sizes of the voids formed within the interstitial (tetrahedral and octahedral) sites of the superlattices. The comparison of the PALSmeasured cavity sizes (0.50 to 0.74 nm) with the predicted void sizes of the interstitial sites ($0.7 to $1.7 nm) and the free volume in solid dodecane (0.36 nm), previously measured by PALS, indicate that both types of cavities may contribute to the experimentally determined cavity sizes. However, the GNP core sizes had only a minor influence on the measured cavity size. Larger cavities with sizes corresponding to the voids ($1.7 nm) expected within the octahedral sites of the superlattices comprised of $5.5 nm-sized GNP cores could not be detected. Assuming the intensities arising from these voids are measurable, this finding suggests that the octahedral sites are occupied by excess ligands trapped during film preparation.Apart from the voids predicted for the interstitial sites, the larger cavity sizes measured for the GNP superlattices compared to crystalline dodecane may result from some degree of disorder in the ligand arrangement.