Elucidation of Electrostatic Interaction between Cationic Dyes and Ag Nanoparticles Generating Enormous SERS Enhancement in Aqueous Solution

Addition of a trace quantity of cationic triphenylmethane (TPM+) dyes to isolated Ag nanoparticles in an aqueous solution yielded Ag flocculates composed of a few closely adjacent suspended nanoparticles. This change was evidenced by coupled LSP peaks emerged at 600-800 nm. However, neutral para-rosaniline (p-RA) molecules as well as neutral rhodamine 123 did not cause the Ag flocculation in contrast to their cations, indicating a crucial role of electrostatic interaction between cationic dyes and negatively charged Ag surfaces for the flocculation. Accordingly, cationic dye molecules are located in the nanogap between closely adjacent Ag nanoparticles which evoked enormous SERS intensity. The formation of the Ag flocculates was insensitive to steric hindrance by different amino groups -NH2, -N(CH3)(2), and -N(C2H5)(2) in TPM dyes. This observation is consistent with dominant role of the electrostatic interaction. Nevertheless, distinct red shifts of fluorescence peaks were observed depending on the molecular structures such as coplanar and propeller phenyl rings, suggesting perturbed electronic states of TPM dyes upon adsorption through the amino groups.