Engineering Electromagnetic Hot-Spots in Nanoparticle Cluster Arrays on Reflective Substrates for Highly Sensitive Detection of (Bio)molecular Analytes

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.

Identifying Surface-Enhanced Raman Spectra with a Raman Library Using Machine Learning

“…The aligned Ag NR array has an average diameter of approximately 80 nm at the tips, an average rod length of 1000 nm, and a broad absorption peak in the NIR region . All of these three SERS-based substrate geometries provide similar electric field enhancement factors (EFs), essential for enhanced SERS sensitivity. ,,,− …”

Highly Sensitive, Robust, and Recyclable TiO2/AgNP Substrate for SERS Detection

“…This resulted in average sizes of hotspots of around 5.2 nm due to the self-assembly of AgNPs during deposition [ 34 ]. By leveraging the self-assembly of block copolymer colloids, Rastogi et al demonstrated gold nanoparticle cluster arrays (NCAs) with intercluster hotspots of 10 nm in size [ 37 ]. As shown in Figure 1 b, in the presence of the TiO 2 layer, the boundaries of AgNPs became vague, so it is difficult to measure the sizes of these particles.…”

High Sensitivity SERS Substrate of a Few Nanometers Single-Layer Silver Thickness Fabricated by DC Magnetron Sputtering Technology

“…Table 2 shows the average sizes of the hotspots maintained at about 5.3 nm because of the metallic self-assembly during deposition. As Rastogi et al, reported, the gold nanoparticle cluster arrays exhibited less than 10 nm wide inter-cluster hotspots produced on glass [43]. During the deposition, the hotspot physically adsorbed silver vapor atoms, which might move across the surface by hopping from one potential well to another silver adatom, assisted by the thermal activation from the surface energy and kinetic energy parallel to the surface.…”