Asymmetric gold nanodimer arrays: electrostatic self-assembly and SERS activity

A simple, scalable, low-cost and high-throughput nanofabrication method is developed to produce discrete gold nanoparticle (AuNP) dimer arrays. This method involves a two-step electrostatic self-assembly: (1) electrostatic immobilization of negatively charged AuNPs onto a positively charged surface and (2) electrostatic adsorption of a positively charged AuNP onto each pre-assembled AuNP. The latter requires a careful control of the electrostatic energy barrier originating from the interactions between the charged AuNPs and surfaces. This can readily be achieved by tuning the ionic strength of the self-assembly media. We calculate the interaction energies for immobilizing a single positively charged AuNP onto each pre-assembled NP at different ionic strengths and present successful experimental results on the synthesis of high-yield symmetric and asymmetric AuNP dimers (dimer yield: similar to 85%). A theoretical and experimental investigation of their optical properties is conducted to correlate the spectral properties of these dimers with their structure. We also study the SERS activity of the as-synthesized AuNP dimers using benzenethiol as a model analyte. It is found that, with the increase of the size dissimilarity between the two NPs in the dimers, the Raman intensities of the analyte increase gradually. This trend is completely different from those of both single AuNPs and AuNP aggregates with identical particle size.