Surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) use the intense localized electromagnetic fields generated near plasmonic nanoparticles to amplify optical signals from nearby molecules.
The type and magnitude of enhancement depend on nanoparticle composition and geometry, spectral overlap, and the distance between the molecule and metal surface. Molecules very near a plasmonic surface can experience strong Raman enhancement, while fluorophores generally require controlled separation from the metal to avoid quenching and achieve fluorescence enhancement. For more background on the underlying optical response, see The Science of Plasmonics.
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Surface-Enhanced Fluorescence (SEF)
Surface-enhanced fluorescence (SEF), also called metal-enhanced fluorescence, occurs when a fluorophore interacts with the enhanced electromagnetic field near a plasmonic nanoparticle. The nanoparticle can increase excitation and modify the fluorophore's radiative decay rate, producing stronger emission under appropriate conditions.
Distance is critical. Fluorophores positioned too close to a metal surface can undergo fluorescence quenching, while a controlled spacer can preserve fluorescence and enable enhancement. Particle geometry, fluorophore spacing, and spectral overlap between the plasmon resonance and fluorophore excitation and emission all influence the result.
Core-shell architectures provide one strategy for controlling this separation. A plasmonic core surrounded by a fluorophore-containing silica shell can combine near-field enhancement with controlled fluorophore spacing while physically separating the dye from the metal surface.
Spectral overlap is also important. Changing nanoparticle size and geometry shifts the plasmon resonance, allowing the optical response to be matched more closely to different fluorophores. In the example below, fluorescein emission varies with silver core size as the nanoparticle resonance shifts relative to the dye's excitation and emission spectra.
Surface-Enhanced Raman Scattering (SERS)
Raman spectroscopy identifies molecules through characteristic vibrational fingerprints, but intrinsic Raman scattering is weak. Surface-enhanced Raman scattering (SERS) uses the enhanced electromagnetic fields near plasmonic nanostructures to increase Raman signal intensity.
Enhancement can be particularly strong at electromagnetic “hot spots,” such as regions of high surface curvature or narrow gaps between neighboring particles. Enhancement factors can span many orders of magnitude, and optimized hot-spot configurations have enabled single-molecule SERS detection. Particle geometry, surface structure, interparticle spacing, analyte position, and spectral overlap all influence SERS performance.
Strong enhancement generally requires appropriate overlap between the excitation wavelength and the nanoparticle's plasmonic response. The example below compares the Raman signal of Malachite Green in solution with the signal obtained when the dye is associated with gold nanoparticles.
Nanoparticle-based SERS is used in applications including diagnostics, chemical and material identification, biological labeling, optical sensing, and security technologies.
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Selected Literature
- Qian, X. M. & Nie, S. M. “Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications.” Chemical Society Reviews, 37(5), 912–920 (2008).
- Cao, Y. C., Jin, R. & Mirkin, C. A. “Nanoparticles with Raman spectroscopic fingerprints for DNA and RNA detection.” Science, 297(5586), 1536–1540 (2002).
- Aslan, K., Wu, M., Lakowicz, J. R. & Geddes, C. D. “Fluorescent core-shell Ag@SiO2 nanocomposites for metal-enhanced fluorescence and single nanoparticle sensing platforms.” Journal of the American Chemical Society, 129(6), 1524–1525 (2007).
- Chen, Y., Munechika, K. & Ginger, D. S. “Dependence of fluorescence intensity on the spectral overlap between fluorophores and plasmon resonant single silver nanoparticles.” Nano Letters, 7(3), 690–696 (2007).
- Shin, H., Oh, S., Kang, D. & Choi, Y. “Protein quantification and imaging by surface-enhanced Raman spectroscopy and similarity analysis.” Advanced Science, 7(11), 1903638 (2020).
- Shin, H. et al. “Early-stage lung cancer diagnosis by deep learning-based spectroscopic analysis of circulating exosomes.” ACS Nano, 14(5), 5435–5444 (2020).