Chemically Etched Nanoporous Copper and Galvanically Displaced Silver Nanoflowers for SERS Sensing

Solid-state surface-enhanced Raman spectroscopic (SERS) substrates surmount the limitations of colloidal nanoparticles for many practical chemical and biomolecular sensors. The main bottom-up approaches adopted for SERS substrate fabrication include preparing plasmonic nanoparticles and their deposition to selected surfaces via chemical self-assembly, inkjet printing, spray coating, dip-coating, etc. In contrast, the top-down strategy is to create nanostructures on surfaces via different lithographic techniques (nanoimprint lithography, electron beam lithography), laser ablation, or chemical etching and then depositing a nanolayer of coinage metals. The ease and reproducibility of production, high enhancement, and uniformity of performance are the requirements for a reliable SERS substrate. This work describes an easy process for preparing SERS substrates from a less expensive and readily available material, brass. The treatment of brass with hydrochloric acid caused preferential etching of Zn from the surface, leaving a uniform Cu nanoporous substrate (CuNPS). The subsequent galvanic displacement of CuNPS with AgNO3 resulted in silver nanocrystal overgrowth, as evidenced by microscopic, spectroscopic, and elemental studies. The corresponding SERS studies with 4-mercaptophenylboronic acid (4-MPBA) as the probe molecule revealed ∼30 and ∼300 times improved performance compared to CuNPS and the parent brass substrates, respectively. Further, we explored the possibility of sensing creatinine, the biomarker for kidney functioning. Creatinine followed a Freundlich type adsorption, and the SERS substrate developed herein exhibited a sensitivity down to 1.7 pg/mL.