Photo-formation of gold nanoparticles: Photoacoustic studies on solid monoliths of Au(III)–chitosan–silica aerogels

Photo-formation of gold nanoparticles in the solid monoliths of Au(III)–chitosan–silica aerogels with different Au/NH2 molar ratios has been investigated using photoacoustic spectroscopy. Upon exposing to 320 nm UV light, a new absorption feature in the visible region around 525 nm could be seen due to the surface plasmon resonance of gold particles that are generated as a result of UV-induced reduction of Au(III) to Au(0). The plasmon band becomes stronger and shows saturation effects upon increasing the UV exposure time. A blue shift of about 7 nm is also noticed on exposing the sample (Au/NH2 = 1/5) for 6 h, indicating a slight decrease in the nanoparticle size due to light-induced annealing with increasing the UV exposure time. The PA signals monitored as a function of chopping frequency show ω−1 dependence, implying the thermally thin character of Au(III)–chitosan–silica aerogels.