Biomacromolecular-Assembled Nanoclusters: Key Aspects for Robust Colloidal SERS Sensing

Superstructures of gold nanospheres offer augmented surface-enhanced Raman scattering (SERS) activities beyond the limits of their individual building blocks. However, for application as reliable and quantitative colloidal SERS probes, some key aspects need to be considered to combine efficiency and robustness with respect to hotspot excitation, analyte adsorption, signal stability, and colloidal stability. For this purpose, we studied core/satellite superstructures with spherical cores as a simple optically isotropic model system. Superstructures of different core sizes were assembled using bovine serum albumin (BSA), which serves as a non-specific biomacromolecular linker and provides electrosteric stabilization. We show that the "noisy" spectral footprint of the protein coating may serve as an internal standard, which allows accurate monitoring of the adsorption kinetics of analytes. The SERS activity was quantified using 4-mercaptobenzoic acid (MBA) as an aromatic low-molecular-weight model analyte. The molar SERS efficiency was studied by variation of the particle (Au⁰) and analyte concentrations with a limit of detection of 10^-7 M MBA. The practical importance of colloidal stability for robust measurement conditions was demonstrated by comparing the superstructures with their citrate-stabilized or protein-coated building blocks. We explain the theoretical background of hotspot formation by a leader/follower relationship of asymmetric control between the core and the satellites and give practical guidelines for robust colloidal SERS sensing probes.