Three-dimensional hotspots in evaporating nanoparticle sols for ultrahigh Raman scattering: solid-liquid interface...

Area of Science:

• Nanotechnology
• Surface Chemistry
• Spectroscopy

Background:

• Surface-enhanced Raman scattering (SERS) relies on plasmonic nanoparticles to amplify weak Raman signals.
• Controlling the three-dimensional (3D) arrangement of nanoparticles is crucial for optimizing SERS hotspots.
• Surface properties, such as hydrophobicity, can influence nanoparticle assembly and SERS performance.

Purpose of the Study:

• To experimentally demonstrate 3D hotspots for ultrahigh SERS using evaporating silver nanoparticle (Ag NP) sols.
• To investigate the impact of hydrophobic versus hydrophilic surfaces on SERS enhancement and signal stability.
• To elucidate the mechanism behind surface-influenced nanoparticle self-assembly and hotspot formation.

Main Methods:

• Droplet evaporation of citrate-Ag sols on controlled hydrophobic and hydrophilic surfaces.
• Utilizing rhodamine 6G (R6G) as a SERS reporter molecule at extreme dilutions.
• In situ synchrotron-radiation small-angle X-ray scattering (SR-SAXS) to monitor nanoparticle evolution and 3D geometry.

Main Results:

• Hydrophobic surfaces yielded a two-order-of-magnitude increase in Raman enhancement and improved signal stability over hydrophilic surfaces.
• Evaporation-driven progressive 3D self-assembly of Ag NPs was favored over ideal collaborative assembly due to interface effects.
• Distinct 3D geometries of Ag NPs were observed on both surface types, with hydrophobic surfaces (fluorosilylated) promoting smaller interparticle distances and more numerous hotspots.

Conclusions:

• Hydrophobic surfaces are superior for creating stable, high-performance 3D SERS hotspots.
• Controlling nanoparticle-surface interactions via surface chemistry is key to optimizing hotspot density and SERS signal amplification.
• The findings provide a pathway for designing advanced SERS substrates with enhanced sensitivity and stability.