Optical humidity sensor based on m-nitroaniline doped gold/poly-vinyl alcohol nano-composite

Gold/polyvinyl alcohol nano-composites with two different concentrations of gold (~1.0 wt% and ~2.0 wt% respectively) were prepared via in-situ reduction of gold salt by employing hydrazine hydrate as a reducing agent. The optical absorption spectra of nanocomposites reveal the shift in the surface plasmon resonance band from 525 nm to 560 nm with increase in gold concentration. The obtained nanocomposites were doped with meta-nitroaniline (m-NA). The m-NA doping resulted into slight blue shift of the surface plasmon resonance band of gold along with appearance of a new band at 356 nm due to m-NA. The nanocomposites were further characterised using X-ray diffraction, Scanning electron microscopy and transmission electron microscopy were used to study particulate properties. It is observed that the particle size of gold varies with gold concentration. In this report for the first time we report a humidity sensor based on these nanocomposites, having response time of 50 s (20-95% RH) and recovery time of around 3 min (95%-20%RH) on fibre optical waveguide, in the range of 20-100% RH. The optimum concentration of gold is found to be 2.0 wt %, which showed higher performance for humidity sensing. The sensing performance was found to be enhanced after m-NA doping.