A method for controlling the aggregation of gold nanoparticles: tuning of optical and spectroscopic properties.

Gold nanoparticles (AuNPs) have many interesting optical properties, which are derived from their surface plasmon resonance (SPR). However, the SPR of single AuNPs occurs around 520 nm, which is a limitation for biomedical imaging applications, because the maximum falls outside the tissue transparency window (∼650-1000 nm). Here the aggregation of AuNPs is mediated by balancing aggregation and steric stabilization processes. This is achieved by varying the relative amounts of hydrophobic small molecules, which act as aggregating agents, and end functional hydrophilic polymers that serve as steric stabilizing agents. This approach allows the position of the SPR shifted into the tissue transparency window, while maintaining colloidal stability. Importantly, increased depolarized scattering and surface enhanced Raman scattering (SERS) cross sections in this region are achieved compared to the single nanoparticles. By varying the structure of the aggregating agent slightly, the SERS spectra exhibit significant changes, thus demonstrating the potential to encode different aggregates. The aggregates have potential applications in biomedical imaging, as an encoding strategy for combinatorial chemistry, and for use in flow cytometry applications.