Multiple Gold Nanoparticle Cores within a Single SiO₂ Shell for Preservable Solid-State Surface-Enhanced Raman Scattering and Catalytic Sensing

Metal@dielectric core@shell nanoparticles (NPs) have attracted significant attention due to their multifunctional properties and vast applications in different fields of catalysis, photonics, sensing, nanomedicine, etc. However, there is a dearth of reports about the synthesis of controlled aggregation of gold cores without using any cross-linkers and the effects of the presence of multiple metallic cores in one shell, particularly for surface-enhanced Raman scattering (SERS) and electrochemical sensing. Nanoaggregates, N ≥ 2 (where N is the number of nanoparticles in aggregation), can effectively be used for fine-tuning plasmon wavelength, whereas collective encapsulation of number-selected nanoaggregates by functionalized SiO₂ generates multiple capacitors which in turn enhance the field at the nanojunctions and nanogaps for improved SERS and catalytic sensing activity. We successfully prepared controlled AuNP aggregation, passivated them by the SiO₂ outer layer to make them suitable for preservation in the solid state and functionalization by 3-aminopropyl trimethoxysilane (APTMS), and separated the nanoaggregates based on the aggregation size (individual nanoaggregates having 2 to 100 nanoparticles in each). The thickness of the silica shell was engineered in such a way that shell thickness does not make any hindrance in optical measurements, and the effect of multiple nanoparticle cores on the surface plasmon resonance and SERS can be understood properly and also allows external molecules to reach active gold nanoparticle surface for electrocatalytic activity. Functionalization allows individual encapsulations to further form multi-junction capacitors by bridging them through a positively charged dye molecule, here Rhodamine 6G (Rh6G). By using these multiple gold nanoparticle cores within a single silica shell (multi-Au@SiO₂ core@shell nanoparticles) with improved SERS and electrocatalytic activity, we have also successfully ultrasensed (0.003 μA·μM^–1·cm^–2) glucose in a nonenzymatic electrochemical pathway.