• surface enhanced Raman scattering;
  • flocculation;
  • localized surface plasmon;
  • orientation of adsorbates;
  • nanogap structures

The adsorption of cationic and neutral R6G molecules on Au nanoparticles was elucidated by surface enhanced Raman scattering (SERS). The steric hindrance at hydroethyl amino (-N(H)Et) groups in R6G was evidenced by the observation that R6G+ adsorb on as-prepared gold nanoparticles (AuNPs) only with electrostatic forces, in contrast to the electrostatic and chemical adsorption of R123+ with dihydro amino (-NH2) groups on as-prepared AuNPs. Large steric hindrance at the amino groups in R6G yielded saturated coverage of 700 molecules/AuNP for R6G+ significantly fewer than 1000 molecules/AuNP for R123+. In addition, neutral R6G0 on AuNPs showed markedly enhanced peaks at 1200–1600 cm−1, which were not observed in Raman spectra of R6G0 in bulk solution, and also in SERS of R6G+ on AuNPs. These bands are attributed to vibrational modes of an outer phenyl ring and ethyl amino groups, which are vertical to a xanthene plane, on the basis of theoretical analysis of molecular vibrations. Thus, Raman scattering of these bands is enhanced under an inclined orientation of R6G0 molecules chemisorbed on AuNPs via lone pair electrons at amino groups. Copyright © 2013 John Wiley & Sons, Ltd.