Flexible & transparent breath sensor and conducting electrodes based on a highly interconnected Au nanoparticle network†

In this study, transparent humidity sensors and conducting electrodes (TCEs) were fabricated using a highly interconnected gold nanoparticle network (Au nanonetwork) via a simple solution approach. Here, the direct reduction of metal anions at lower temperatures (5 °C) followed by the addition of a non-polar solvent facilitates the cold welding of nanoparticles at junctions and yields an interconnected Au nanonetwork at the liquid–liquid interface. The formed Au nanonetwork is cautiously transferred to the desired flexible, stretchable, and transparent substrates for various applications. As the synthesis involves no capping agents, the prepared nanonetworks offer high conductivity without further chemical or thermal treatments. The fabricated Au nanonetwork is highly crystalline and thermally stable, with excellent mechanical robustness towards various deformations. Using an Au-1L (1-layer) nanonetwork, a highly transparent (>85%) humidity sensor is fabricated with short response and recovery times (1.1 s and 1.3 s). The Au-1L sensor is studied systematically for various humidity changes (40% to 90%) and breath conditions (normal/deep, hydrated/dehydrated, breathing/blowing, etc.) and exhibits high selectivity toward humidity. Notably, the sensing device offered a stable response for more than a year demonstrating its robustness for prolonged use.