Plasmonic Au nanorods stabilized within anodic aluminum oxide pore channels against high-temperature treatment

Au nanorods (Au NRs) are promising candidates for sensing applications due to their tunable localized surface plasmon resonance wavelength. At temperatures above 250 °C, however, these structures are morphologically unstable and tend to evaporate. We herein report a novel refractory plasmonic nanocomposite system comprising Au NRs entrapped in anodized aluminum oxide (AAO) scaffolds that are stable up to 800 °C. Au NRs were synthesized in the cylindrical pores of sapphire-supported AAO via in situ electroless deposition on catalytic Au nanoparticles (Au NPs) anchored on the pore walls. The morphological characteristics and surface-enhanced Raman scattering (SERS) functionality of Au NRs before and after heat treatment were evaluated using SEM, XRD and Raman spectroscopy. Compared to unconfined Au NRs that evolved into spherical particles at temperatures below 250 °C and subsequently evaporated from the substrate surface, the morphology of Au NRs in AAO was preserved upon heat treatment at temperatures up to 800 °C. Furthermore, by tuning the AAO scaffolds thickness and pore diameter, the aspect ratio (AR) of the entrapped Au NRs was varied from 2.4 to 7.8. The SERS sensitivity of Au NRs in AAO was found to increase with decreasing AR when the incident light was parallel to the rod longitudinal axis, in close agreement with the calculated fourth power of the local electromagnetic field using the finite-difference time domain method.