Abstract
Intense electromagnetic (EM) hot-spots arising at the junctions or gaps in plasmonic nanoparticle assemblies can drive ultrahigh sensitivity in molecular detection by surface-enhanced spectroscopies. Harnessing this potential however requires access to the confined physical space at the EM hot-spots, which is a challenge for larger analytes such as biomolecules. Here, we demonstrate self-assembly derived gold nanoparticle cluster arrays (NCAs) on gold substrates exhibiting controlled interparticle (<1 nm wide) and intercluster (<10 nm wide) hot-spots as highly promising in this direction. Sensitivity of the NCAs toward detection of small (<1 nm) or large (protein-receptor interactions) analytes in surface-enhanced Raman and metal-enhanced fluorescence assays is found to be strongly impacted by the size of the cluster and the presence of reflective substrates. Experiments supported by numerical simulations attribute the higher sensitivity to higher EM field enhancements at the hot-spots, as well as greater analyte leverage over EM hot-spots. The best-performing arrays could push the sensitivity down to picomolar detection limits for sub-nanometric organic analytes as well as large protein analytes. The investigation paves the way for rational design of plasmonic biosensors and highlights the unique capabilities of a molecular self-assembly approach toward catering to this objective.
Highlights
Sensitive detection of molecular analytes is of strong importance to different sectors, including healthcare diagnostics, environmental monitoring, security, and space.[1−4]Nanoplasmonic sensors based on surface-enhanced spectroscopies, viz., surface-enhanced Raman spectroscopy (SERS) and metal-enhanced fluorescence (MEF), are promising in this respect due to their ultralow detection limits within microscopic detection footprints and quick response times.[5]Nanoplasmonic sensors gain from high electromagnetic (EM) field enhancements arising at close vicinity to nanostructured metal surfaces
We demonstrate gold nanoparticle cluster arrays on gold thin films, with cluster dimensions that are engineered to drive high EM field enhancements at intercluster hot-spots
Gold nanoparticle cluster arrays (NCAs) exhibiting interparticle (
Summary
Nanoplasmonic sensors gain from high electromagnetic (EM) field enhancements arising at close vicinity to nanostructured metal surfaces. EM enhancements at certain locations (hot-spots), such as sharp corners, or junctions in plasmonic nanoparticle (NP) assemblies. Advances in colloidal synthesis and nanolithography have shown to deliver a range of plasmonic nanoparticle assemblies exhibiting gap or curvature hot-spots.[8−13] Gap hot-spots provide a practical advantage over curvature hot-spots, as the interparticle gaps can be readily controlled by varying the density of nanoparticles on the surface.[14,15] Further, the sharp edges or corners are subject to instability due to erosion, breakage, or collapse. The EM enhancements at the gap hotspots increase nonlinearly as a function of closing gap distances, resulting in EM enhancement factors over 108 for gaps of the order of a nanometer.[16,17] Despite high enhancements, the usefulness of such gap hot-spots for sensing of molecular analytes is restricted to analytes with dimensions
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