Femtosecond Laser-Induced Nanostructures in Copper Film for UV-SERS

The paper reports on the femtosecond laser-induced nanostructuring of copper films to develop effective substrates for ultraviolet surface-enhanced Raman spectroscopy (UV-SERS). Optimal SERS performance, tested using adenine under 325, 442, and 532 nm excitation, was achieved on laser-patterned copper surfaces exhibiting intermediate roughness and well-defined ring-like nanostructure morphologies. UV excitation at 325 nm produced a pronounced, morphology-dependent enhancement of adenine signals. TD-DFT calculations of adenine-copper and adenine-copper-oxide clusters identified near-UV metal-to-molecule charge-transfer states and predicted enhancement factors up to 71 for ring stretching vibrations on copper-oxide clusters, supporting a dominant chemical-enhancement component. This contrasts with the predominantly electromagnetic enhancement observed at longer excitation wavelengths. Positive correlation between adenine band intensity and the Cu₂O Raman signal with varying scan period suggests that surface roughness modulates oxide coverage and adsorption geometry, aligning the local work function with adenine orbitals. The substrates exhibited reproducible performance across independent batches and maintained signal stability over at least 2 weeks; calibration curves showed a detection limit of 10 μM. X-ray photoelectron spectroscopic analysis confirmed that optimal fabrication conditions preserved the necessary cuprous oxide layer, while aggressive ablation (short periods) led to exposure of the underlying silicon substrate and the formation of Cu(II) species. These findings suggest that nanostructured copper is a viable, efficient, and economical substrate material for UV-based analysis of biomolecules.