Fabrication of Au nanoparticles on poly(vinylpyrrolidone) nanowires exhibiting reversible frequency change of localized surface plasmon resonance

Au nanoparticles (NPs) are formed on gel nanowires (NWs) based on poly(vinylpyrrolidone) (PVP) via photoreduction in a HAuCl₄-containing MeOH solution. The particle size and number density of the Au NPs increase with the photoreduction time. At a photoreduction time of 15 min, the surfaces of the PVP NWs are almost completely covered by Au NPs. The hybrid material exhibited visible optical absorption based on the localized surface plasmon resonance (LSPR) of the Au NPs. The peak LSPR absorption wavelength under dry conditions red-shifted slightly as the particle size and number density increased owing to increased coupling of the plasmonic bands of each particle. In water, the LSPR wavelength is blue-shifted compared with under dry conditions because of an increase in the interparticle distance between the Au NPs owing to the swelling of the PVP gel NWs; this causes a decrease of the plasmonic coupling of the particles. The absorption peak wavelength shifts reversibly when the hybrid NWs is alternately exposed to either air or water because the distance between the Au NPs is altered in response to the volume change of PVP gel during swelling and drying.