Nanoscale crack generation of Au/Ag nanopillars by in situ galvanic replacement for sensitive, label-free, and rapid SERS detection of toxic substances

Abstract

Molecular fingerprint spectra obtained by surface-enhanced Raman spectroscopy (SERS) enable the trace analysis of chemical and biomedical materials, including toxic substances. However, molecules that undergo random diffusion have a lower probability of being positioned at a conventional hotspot, adversely affecting the sensitivity and the limit of detection. In the present work, we demonstrated in situ galvanic replacement (GR) in the presence of analytes to simultaneously form interior hotspots (i.e., voids and interstitials) and drive molecular dynamic diffusion to the desired areas. For GR, Ag nanopillars (AgNPs) partially substituted with Au (Au/AgNPs) were prepared. Their stoichiometric ratio induced the formation of interior hotspots with sufficient field enhancement, which provided femtomolar sensitivity (i.e., 100 fM) for methylene blue (MB) Raman dye in 10 s. The activation of the developed platform was verified by comparison with the post-addition of MB into the Au/AgNPs (i.e., dyes adsorbed onto the surfaces). The reproducibility was investigated using multiple specimens, which demonstrated stable operation, with a relative standard deviation of ∼10% at the analytic peaks of MB at 509, 1395, and 1623 cm−1. A linearly proportional relationship (R2 ≥ 0.94) was also found in quantitative studies. The Au/AgNPs were used for the trace analysis of toxic thiabendazole molecules at concentrations as low as 100 pM. The results show that the developed Au/AgNPs SERS platform demonstrates strong potential for use in on-site detection of toxic substances in a sensitive, reproducible, and rapid manner.