Humidity-induced direct modification of optical response of plasmonic nanorod metamaterials

Plasmonic nanorod metamaterials are often used as a high-performance optical sensing platform for the detection of biochemical and gas species. Here, we investigate the effect of environmental humidity on the optical response of gold nanorod metamaterials. Due to the high refractive index sensitivity, a significant change in the transmission is observed with ΔT/T reaching values of more than 5% when the relative humidity is changed from 11% to 75%. This is the result of the condensation of water molecules on a rough surface of the nanorods. This finding reveals the importance of protecting plasmonic nanostructures from relative humidity variations in many practical applications. By coating the nanorods with a monolayer of poly-L-histidine, the fractional change of transmission is doubled due to the hydrophilic property of the monolayer, which can be used for the development of high-sensitivity relative humidity and dew condensation sensors.