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

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The flocculates of gold nanoparticles (AuNPs), which are a few closely adjacent AuNPs in solution providing enormous surface enhanced Raman scattering (SERS) intensity, were successfully formed using oxazine dye molecules. We found that neutral cresyl violet (CV⁰) molecules with two dihydro amino groups (–NH₂) chemisorb on Au surfaces via lone pair electrons at amino groups, whereas cationic CV⁺ molecules physisorb using electrostatic interaction between their positively charged amino groups and negatively charged AuNPs. Neutralization of CV⁺ gave appreciable peak shifts of their SERS bands, which also showed considerable spectral changes upon the deuteration of the amino groups, and thus assigned to vibrational modes of the amino groups. We found that diethyl substituent in amino groups in nile blue (NB), which has an oxazine structure similar to CV, sterically hinders chemisorption on AuNPs. Accordingly, NB⁺ physisorbed on as-prepared AuNP exclusively with electrostatic forces, whereas NB⁰ did not chemisorb on Au surfaces. Thus, the AuNP flocculates were readily formed using electrostatic and chemical interaction between target molecules and metal nanoparticles, albeit chemisorption was quite sensitive to bulky structure of substituent. [DOI: 10.1380/ejssnt.2012.417]