(360g) Gold Nanoparticle Clusters Stabilized in a Hydrogel Film for Highly Sensitive SERS Substrates

Gold nanoparticles (AuNPs) are widely known as prominent functional nanomaterials due to their unique catalytic and electromagnetic properties. Notably, surface electrons of AuNPs resonate with incident light at specific wavelengths, resulting in the generation of a localized electric field, called localized surface plasmon resonance (LSPR). Beyond LSPR, clusters of AuNPs exhibit further enhancement of the local electric field at the nanogaps between adjacent particles, commonly referred to as “hotspots.” These intensified electromagnetic fields play a crucial role in enhancing the sensitivity of Raman spectroscopy (surface enhanced Raman scattering, SERS). Molecules adsorbed on the surface of AuNP clusters exhibit significantly stronger Raman signals compared to free molecules or those adsorbed on dispersed AuNPs. So far, some approaches have been proposed to control the clustering of AuNPs. For example, surface modification of AuNPs with DNA ligands is the most effective way although it is only applicable to small scales and requires less commonly available modifiers.
In this study, we propose a novel and facile method for controlling the clustering of gold nanoparticles (AuNPs) using hydrogels. Hydrogels, composed of a three-dimensionally cross-linked polymer network and water, are capable of immobilizing the spatial distribution of colloidal nanoparticles without disrupting interparticle interactions. In our approach, the ionic strength of aqueous AuNP dispersion was modulated by the addition of an electrolyte to induce nanoparticle aggregation. Subsequently, the aggregation process was kinetically arrested by introducing hydrogel monomers followed by in-situ polymerization, thereby forming stable AuNP clusters. The cluster size could be tuned by adjusting the waiting time between electrolyte addition and hydrogel polymerization. This process enables the fabrication of hydrogel films containing either dispersed AuNPs or AuNP clusters, which can be directly employed as substrates for surface-enhanced Raman scattering (SERS). Raman spectroscopy revealed that hydrogel films containing AuNP clusters exhibited higher SERS sensitivity compared to those with dispersed AuNPs. However, excessively prolonged waiting times led to severe aggregation of AuNPs, resulting in a decline in SERS performance.