Reversible Assembly and Dynamic Plasmonic Tuning of Ag Nanoparticles Enabled by Limited Ligand Protection

Dynamic manipulation of optical properties through the reversible assembly of plasmonic nanoparticles offers great opportunities for practical applications in many fields. The previous success, however, has been limited to Au nanoparticles. Reversible assembly and plasmonic tuning of Ag nanoparticles (AgNPs) have remained a significant challenge due to difficulty in finding an appropriate surface agent that can effectively stabilize the particle surface and control their interactions. Here, we overcome the challenge by developing a limited-ligand-protection (LLP) strategy for introducing poly­(acrylic acid) with precisely controlled coverage to the AgNP surface to not only sufficiently stabilize the nanoparticles but also enable effective control over the surface charge and particle interaction through pH variation. The as-synthesized AgNPs can be reversibly assembled and disassembled and accordingly display broadly tunable coupling of plasmonic properties. Compared to the Au-based system, the success in the reversible assembly of AgNPs represents a significant step toward practical applications such as colorimetric pressure sensing because they offer many advantages, including broader spectral tuning range, higher color contrast, a one-pot process, and low materials and production cost. This work also highlights LLP as a new avenue for controlling the interparticle forces, their reversible assembly, and dynamic coupling of physical properties

Porous Au–Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis

Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the fabrication of porous Au–Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ∼1.3 × 10⁷ on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications

Porous Au–Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis

Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the fabrication of porous Au–Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ∼1.3 × 10⁷ on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications