Rational design of stable Cu and AuCu nanoparticles for investigations of size-enhanced SERS applications

While significant progress has been made to clarify the effects of Au and Ag nanoparticle size on SERS enhancement, research on the size effects of copper nanoparticles and copper-related nanoalloys on SERS enhancement remain scarce. Nanoscale copper (Cu) is important because of its unique sensing and catalytic properties; however, research on its size and compositional effects remains a significant challenge because of the intricate fabrication process and difficulty in preventing oxidation.

Our study elucidated the size-dependent, surface-enhanced Raman scattering (SERS) of Cu NPs, particularly the sensing capabilities of both electromagnetic (EM) SERS at 1.5 × 10³ and chemical enhancement (CE) SERS at 3.6 × 10⁴ of approximately 58 nm Cu NPs. Additionally, a solution aging examination revealed preservation of the metal-related core structure, surface plasmon resonance, and SERS features of the PSMA/ONPG-coated Cu NPs for up to 7 days. With the introduction of galvanic replacement reactions and laser ablation syntheses, the incorporation of Au atoms enabled the fabrication of 7-75 nm Au_xCu_y nanoparticles by using the remaining Cu core after aging in water, which offered precise control over the Cu/Au ratio from 5/95 to 29/71. SERS measurements of the large Au_xCu_y nanoparticles amplified up to 1.4 × 10⁴ of the EM-mediated vibrational signals from the adsorbed molecules. The strong Au-S chemical bonds of the Au-rich Au_xCu_y nanocrystals increased the CE SERS to 5.5 × 10⁴, whereas the Au₃Cu₁ crystals at the Au_xCu_y interface decreased the CE SERS but improved the electron transfer for catalysis via SERS detection.

Our research provides further insight into the structural and size effects of Cu and AuCu alloys used as SERS enhancers and offers avenues for designing cutting-edge SERS catalytic sensors tailored to Cu-related catalytic reactive structures. For the first time, we also manipulated the Cu atomic structure and surface composition to understand the significance of surface effects on SERS substrates of the Cu series from a nanoscale analytical perspective.