![]() Therefore, it is desirable to engineer nanostructures with inherent EM hotspots, which can significantly enhance the EM field and enable the sensitive detection of analytes using SERS. Furthermore, the fabrication of SERS substrates based on the controlled assemblies of nanostructures involves either complex chemical methods or expensive lithographic techniques. Although the assemblies of nanostructures are highly SERS-active, the SERS response is very sensitive to the assembly state, thus making it challenging to realize uniform and reproducible SERS substrates with high density of EM hotspots based on such traditional assemblies. ![]() On the other hand, interstitial sites between assembled or lightly aggregated nanostructures, often termed as electromagnetic hotspots, offer large SERS signal enhancements, enabling the single molecule detection under ideal conditions. ![]() Most of the SERS substrates based on individual nanostructures offer modest SERS enhancement. SERS involves the large enhancement of the Raman scattering from analytes adsorbed on or in close proximity to a nanostructured metal surface. Based on the sensitivity of the surface plasmon resonance to the changes in the dielectric properties of the surrounding medium and the enhancement of the electromagnetic (EM) field in proximity of metal nanostructures, two important classes of plasmonic sensors have evolved: refractometric LSPR and surface enhanced Raman scattering (SERS) sensors. The LSPR wavelength of noble metal nanoparticles (such as gold, silver and copper), which falls into the visible and near infrared range of the electromagnetic spectrum, is sensitive to the composition, size, shape, dielectric properties of the surrounding medium, and proximity to other nanostructures (plasmon coupling). Localized surface plasmon resonance (LSPR) involves the collective and coherent oscillation of dielectrically confined conduction electrons.
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