Investigating the composition of the layer formed on Au NRs by NADH reduction and its application in SERS detection of Cu²⁺

Surface metal deposition is an important method for tuning the optical property of plasmonic nanoparticles. In this work, we tried to deposit Cu shell on the surface of gold nanorods (Au NRs) by dihydronicotinamide adenine dinucleotide (NADH)-mediated reaction. Cu²⁺ were reduced to Cu⁰ through the gold-catalyzed oxidization of NADH. The deposition of Cu shell onto the Au NRs surface resulted in the obvious red-shift of longitudinal localized surface plasmon resonance peak. And the red-shift of longitudinal peak increased with the concentrations of NADH and Cu²⁺. The TEM images showed that a transparent layer structure with a different contrast around Au NRs was clearly displayed. And this transparent layer structure with the low contrast was generally considered to be pure Cu shell. However, the results from energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS) indicated that the transparent layer was not just pure Cu composition, but contained NADH/NAD⁺ and Cu²⁺ components. This grown complex dielectric shell resulted in the reduced surface enhanced Raman scattering (SERS) activity of Au NRs. Based on the decreased SERS intensity, we demonstrated a SERS method for Cu²⁺ detection using R6G as Raman reporter. The decreased SERS intensity had a linear correlation with the concentration of Cu²⁺ in the range of 10⁻⁶–10⁻² M, with a detection limit of 3.89 × 10⁻⁶ M. This study provides new insights about Cu deposition on Au NRs, which will further promote the spectral sensing applications with gold-catalyzed Cu²⁺ reduction on plasmonic nanoparticles by NADH.