Thin films based on dodecylamine stabilized gold nanoparticles interlinked with different organic molecules are prepared by automatic layer-by-layer self-assembly in a microfluidic quartz crystal microbalance (QCM) cell, to obtain an in situ insight on the film formation by ligand/linker exchange reactions. The influence of interlinking functional groups and the length of the organic linker molecule on the assembly behavior is investigated. Alkyldithiols with different lengths are compared to alkyldiamines and alkylbisdithiocarbamates with a C8 alkylic molecular backbone. The stepwise layer-by-layer assembly occurs independently of the linker molecule, while the largest frequency changes always correspond to the gold nanoparticle step. During the solvent rinsing and ligand/linker exchange reaction step, the frequency is almost constant with slight increases or decreases dependent on the molar mass of the linker compared to the exchanged ligand. The assembly efficiency is higher for shorter molecules and for molecules with stronger interacting functional groups. The densities of the composite films are calculated from QCM data and independent thickness measurements. They reflect the higher fraction of organic material in the films comprising longer organic linkers. The plasmon resonance band of the gold nanoparticles in the final assemblies is measured with UV/vis spectroscopy. Band positions in films prepared from dithiols and diamines of comparable lengths are very similar, while the spectrum of the bisdithiocarbamate film exhibits a distinct blue-shift. This observation is explained by the longer molecular structure of the linker due to a larger binding group, in conjunction with a delocalization of particle charge on the organic molecule. Obtained results play an essential role in the understanding of thin film layer-by-layer self-assembly processes, and enable the formation of new gold nanoparticle networks with organic diamine and bisdithiocarbamate molecules.

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The assembly, structure, composition and sensing properties of novel bisdithiocarbamate based gold nanoparticle networks have been investigated. The sensing properties have been studied with vapors of toluene, 1-propanol, water, and 4-methyl-2-pentanone. We demonstrated that bisdithiocarbamates based chemiresistive sensors show sensing properties versus volatile organic compounds (VOC) comparable to thiol based composites but that they are superior in their long term stability.

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Chemiresistive composites of gold (Au) nanoparticles interlinked with different types of organic molecules were prepared automatically by layer-by-layer self-assembly using a microfluidic cell. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 3.7 nm as well as alkyl dithiols, alkyl diamines, and alkyl bisdithiocarbamates with different alkyl chain length (C6 and C8) were used. X-ray photoelectron spectroscopy was applied on prepared nanoparticle composites to study the film composition and the degree of interlinkage. For the measurement of electrical and vapor-sensing properties, silicon dies equipped with gold interdigitated electrodes were used. All films show linear current-voltage characteristics and conductivities in the range of 10–2 and 10–4 Ω−1 cm−1 at room temperature. The sensitivity of the film is investigated by dosing them with vapors of toluene, 1-propanol, 4-methyl-2-pentanone, and water in the concentration range from 100 to 5,000 ppm at 0% relative humidity. All composite films respond with an increase in their electrical resistance to the analytes. The sensors show a high signal-to-noise ratio which indicates a detection limit below 100 ppm for all test vapors. The response dynamics demonstrate a high reversibility and a fast sensing mechanism especially for dithiols and diamines with response and recovery times from 2 to 10 s. The dithiol sensors exhibit a high selectivity to toluene and 4-methyl-2-pentanone whereas the bisdithiocarbamate composites are suitable for the detection of water and 1-propanol. All materials are stable for (at least) several months.