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.

Flocculation and SERS Activation of Au Nanoparticles Using Cationic and Neutral Cresyl Violet Molecules

“…To lift these issues, we have investigated to generate flocculates of silver and gold nanoparticles (AgNPs, AuNPs) linked by various target molecules in solutions [8,12,13]. This flocculation provides much higher probability to have appropriate gap sizes because they are primarily determined by the size of molecules, which are located at the nanogap.…”

Distinct Adsorbed States of DNA and RNA Bases Investigated by Flocculation-Assisted Surface Enhanced Raman Scattering

“…Gold nanoparticles were prepared by a citrate reduction method [6,7]. Shortly, AuNPs were prepared using the citrate reduction method reported by Frens [23].…”

“…Single molecule detection by Raman scattering has been investigated using a coupled localized surface plasmon (LSP) of adjacent metal nanostructures, by controlling the nanogap size and the adsorption of target molecules [1,2]. For this purpose, we have studied flocculates of silver and gold nanoparticles (AgNPs, and AuNPs), which are a few closely adjacent nanoparticles, providing various molecules with enormous SERS intensity in solutions [3][4][5][6][7]. Theoretical calculations suggest that one of AgNPs or AuNPs in flocculates can be replaced with metal substrates [8], such as Pd, Fe, Ni, Zn, Al and Pt [9][10][11][12] having large damping factors of surface plasmon, while providing enormous enhancement factors in SERS equivalent to those obtained in flocculates of AgNPs and AuNPs.…”

“…Indeed, the surface of AgNPs or AuNPs prepared by chemical reduction using trisodium citrate is covered with residual citrate anions through sodium cations in close proximity to the MNP surfaces. Substitution of such surface residuals with suitable chemicals is neces-sary for us to suppress steric hindrance by citrate anions for efficient adsorption of cationic xanthene and triphenyl methane dyes [3,4,6,8]. A larger amount of surface residuals such as a layer of citrates and oxides on AgNPs does not allow the chemisorption of neutral molecules.…”

“…In contrast to AgNPs, neutralized R123 molecules adsorb on AuNPs via a coordination bond of Au-N in a tilted orientation in sufficiently low coverage of citrates, whereas cationic rhodamine 123 (R123) molecules adsorb on AuNPs through electrostatic interaction in parallel to the interparticle axis [6,7]. Similarly, AuNPs are incubated into a living cell to monitor pH distribution [1], and used to detect hybridization of DNA [17].…”