title: Closely adjacent gold nanoparticles linked by chemisorption of neutral rhodamine 123 molecules providing enormous SERS intensity.
abstract: 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.
authors.affiliation: Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan.
authors.forename: Toru
authors.initials: T
authors.lastname: Yajima
authors.forename: Yingying
authors.initials: Y
authors.lastname: Yu
authors.forename: Masayuki
authors.initials: M
authors.lastname: Futamata
mesh_terms: D000327:Adsorption; D006046:Gold / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry*; D020112:Rhodamine 123 / Q000737:chemistry*; D012834:Silver / Q000737:chemistry; D013059:Spectrum Analysis, Raman; D055672:Static Electricity
chemical_list: D020112:Rhodamine 123; D012834:Silver; D006046:Gold