DNA bases in the gap: correlation between relative SERS spectra and gap separation in gold nanoparticle dimers formed by nucleobase-induced aggregation

The gold nanoparticle (AuNP) dimer represents an interesting structure that combines the chemical properties of AuNPs and its ability to generate a powerful plasmonic hotspot in the interparticle gap, which can enable the detection of single molecules via surface-enhanced Raman spectroscopy (SERS). This attribute can be furthermore used to study the orientation and binding mechanism of molecules on surfaces due to the selective enhancement properties of SERS. This work investigates the characteristics of AuNP dimers formed after adsorption of DNA bases on the surface which modify the Zeta-potential of the particles and enable controlled aggregation. We observe variations in the gap separation for different nucleobase concentrations and pH values that correspond well with changes in the relative SERS spectra and can be quantified by UV-vis spectroscopy of AuNP dimer solutions. The observed dependency of the gap separation on concentration and pH is speculated to originate from reorientations of the adsorbed molecular species. This method offers a way to estimate the vertical extension of surface monolayers on colloids, representing an additional tool for elucidating the orientation of various molecules on AuNPs. By applying this approach to the case of adenine, we suggest a new binding model for this nucleobase, thus making a unique contribution to the vast amount of literature on this complex interaction.