3D hybrid networks of gold nanoparticles: mechanoresponsive electrical humidity sensors with on-demand performances†
We have engineered macroscopic 3D porous networks of gold nanoparticles (AuNPs) chemically interconnected by di-thiolated ethylene glycol oligomers. The formation of such superstructures has been followed by means of UV-Vis spectroscopy by monitoring the aggregation-dependent plasmonic band of such nanomaterials. The controlled chemical tethering of the AuNPs with di-thiolated linkers possessing a well-defined contour length rules the interparticle distance. The use of ad-hoc linkers ensures charge transport via direct tunneling and the hygroscopic nature of the ethylene glycol backbone allows interaction with moisture. Upon interaction with water molecules from the atmosphere, our 3D networks undergo swelling reducing the tunnelling current passing through the system. By exploiting such a behavior, we have devised a new approach for the fabrication of electrical resistive humidity sensors. For the first time we have also introduced a new strategy to fabricate stable and robust devices by covalently attaching our 3D networks to gold electrodes. Devices comprising both 4 (TEG) or 6 (HEG) ethylene glycol repetitive units combined with AuNPs exhibited (i) unprecedentedly high response speed (∼26 ms), (ii) short recovery time (∼250 ms) in the absence of any hysteresis effect, and (iii) a linear response to humidity changes characterized by a highest sensitivity of 51 kΩ per RH(%) for HEG- and 500 Ω per RH(%) for TEG-based devices. The employed green solution processing in water and the extreme robustness of our 3D networks make them interesting candidates for the fabrication of sensors which can operate under extreme conditions and for countless cycles.