Surface Potential of Au Nanoparticles Affected by Layer-by-Layer Deposition of Polyelectrolytes: A Surface-Enhanced Raman Scattering Study

We illustrate by surface-enhanced Raman scattering (SERS) how the surface potential of Au nanoparticles is affected by the layer-by-layer (LbL) deposition of polyelectrolytes onto them. For that purpose, 2,6-dimethylphenylisocyanide (2,6-DMPI) was adsorbed on a poly(ethylenimine) (PEI)-capped Au nanoparticle film, and then its Raman spectrum was taken in various conditions. The NC stretching frequency of 2,6-DMPI is observed at 2123 cm −1 in its neat state, but at 2174 and 2157 cm −1 when 2,6-DMPI is adsorbed on pure Au and on PEI-capped Au nanoparticles, respectively. The initial 51 cm −1 blue shift is due to the transfer of antibonding lone-pair electrons from the isocyanide to gold, and the subsequent 17 cm −1 red shift is caused by the electron donation of PEI to the Au nanoparticles. When negatively charged poly(acrylic acid) (PAA) is deposited onto the PEI-capped Au film, the NC stretching frequency of 2,6-DMPI is then shifted to 2164 cm −1 . Consulting a separate potential-dependent SERS measurement, this corresponds to a variation of the surface potential of Au nanoparticles from −0.52 to −0.32 V. The surface potential of Au nanoparticles alternates upon further deposition of PEI and PAA, but the extent of variation gradually diminishes. The surface potential of Au nanoparticles finally attains a plateau value, that is, −0.41 V, after the deposition of five bilayers of PAA and PEI. In contrast to the surface potential of Au nanoparticles, the zeta potential of the outermost polyelectrolyte layer varies alternately irrespective of the number of LbL depositions.