Re-crystallization of silver nanoparticles in a highly concentrated NaCl environment—a new substrate for surface enhanced IR-visible Raman spectroscopy

Abstract

The common approach of silver nanoparticles activation for surface enhanced Raman spectroscopy often exploits an addition of chloride ions, generally at low concentrations of about 0.1–10 mM in the final dispersion. For the first time, we report the applicability of a highly concentrated NaCl solution (final concentration of 400 mM) for the SERS activation of silver nanoparticles (∼30 nm). Microscopic, optical and particle size distribution measurements reveal the rapid and reproducible re-crystallization of the primary silver nanoparticles to one-order larger crystallites (∼400 nm) already after 15 min after NaCl addition. The crystal growth mechanism is discussed with respect to the proved essential role of oxygen in the reaction system. The specific action of chloride ions is demonstrated through a comparison with NaBr and NaI solutions of the identical concentrations, which do not induce the analogous crystallization process. The recrystallized silver particles are efficient in an enhancement of the Raman signal not only for visible (488 nm) but also for near infrared laser excitation (1064 nm) as illustrated with the representative spectra of adenine.