Closely adjacent gold nanoparticles linked by chemisorption of neutral rhodamine 123 molecules providing enormous SERS intensity

Addition of neutral R123 molecules (10−7 M) to an as-prepared gold nanoparticles (AuNPs) suspension generated flocculates that are a small number of closely adjacent particles. Formation of AuNP flocculates was evidenced by the coupled localized plasmon peak at 720–750 nm. The AuNP flocculates provided pronounced SERS spectra of adsorbed neutral R123 molecules (SERS-A) as anticipated by FDTD (Finite Difference Time Domain) simulations. The observed SERS spectra are significantly different from those of cationic R123+ molecules (SERS-B), which electrostatically adsorbed on Cl−-treated AuNPs. The difference is not simply due to deprotonation but reflects a distinct difference in adsorption nature between neutral R123 and cationic R123+ molecules. Indeed neutral R123 molecules exclusively gave an Au–N stretching band at 202 cm−1, showing the chemisorption on Au surfaces through lone pair electrons at the amino groups. The different adsorption nature is further evidenced by the observation that cationic R123+ molecules adsorbed on as-prepared (without NaCl addition) AuNP flocculates gave both SERS-A and SERS-B spectra. Thus, the cationic R123+ molecules form the flocculates both by chemisorption and electrostatic adsorption owing to modest surface charge on as-prepared AuNPs.