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Optical Properties of Nanoparticles

Advanced Characterization Techniques

Plasmonic metal nanoparticles – including gold, silver, and platinum – are highly efficient at absorbing and scattering light.  By changing nanoparticle size, shape, and composition, the optical response can be tuned from the ultraviolet through the visible to the near-infrared regions of the electromagnetic spectrum.

The reason for the unique spectral response of silver and gold nanoparticles is that specific wavelengths of light can drive the conduction electrons in the metal to collectively oscillate, a phenomenon known as a surface plasmon resonance.  At the plasmon resonant wavelength, the nanoparticles act as tiny antennas, dramatically increasing the intensity of the local electromagnetic field on and near the particle surface.  This module describes how these high-intensity fields associated with the surface plasmon resonance can interact with molecules close to the nanoparticle surface to produce enhanced Raman or fluorescence signals. 

Depending on the arrangement between the nanoparticles and the molecules, either the fluorescence or the Raman signal may be enhanced, as shown below.  For example, attaching a dye molecule directly to a metal nanoparticle surface typically results in fluorescence quenching due to energy transfer between the fluorophore and the metal.  The Raman spectrum of the molecule can be strongly enhanced, however, due to the high electromagnetic field at the surface of the particle.  Conversely, spacing the fluorophore slightly away from the particle surface prevents fluorescence quenching but can lead to a significant increase in the emission from the molecule.

Surface-Enhanced Fluorescence (SEF)

While fluorescent molecules are among the most popular biosensing reagents, they have significant drawbacks, including low optical cross sections which make individual fluorophores difficult to detect, and a poor photostability which can degrade emission complicating detection and quantification.  Surface enhanced fluorescence (SEF) is a phenomenon first observed in the 1970’s that occurs when a fluorophore is placed near the high electromagnetic fields at the surface of a plasmonic metal nanoparticle, enhancing the fluorophore emission intensity by orders of magnitude. The enhancement can be attributed to two effects: 1) the focusing of the incoming light due to the large absorption and scattering cross sections of the plasmonic particle and 2) a decrease in the fluorescence lifetime of the fluorophore that allows the excited state to return to the ground state at a higher frequency.

We have made highly fluorescent nanotags using a core plasmonic particle surrounded by a silica shell containing a fluorescent molecule, shown below.  When viewed using a fluorescence microscope, each particle is bright and has good photostability, making them useful in fluorescent tagging and labelling, assays, and other applications.  

The SEF effect is most pronounced when the plasmon resonance of the metal nanoparticle is spectrally coincident with the absorbance/emission of the fluorophore near the surface. Changing the size and shape of the nanoparticle has a dramatic effect on the optical properties, allowing the plasmon resonance to be shifted across the visible and near-IR regions of the spectrum for enhancement of a variety of different fluorophores.  The figure below demonstrates how the emission from fluorescein dye molecules varies as the size of the silver core – and hence the plasmon resonant wavelength of the particle – is varied. The plasmon resonance of the 70 nm-diameter silver cores overlaps strongly with the absorbance and emission of the dye, resulting in the highest level of brightness enhancement.

Surface-Enhanced Raman Spectroscopy (SERS)

Raman spectroscopy can be used to identify molecules by their unique vibrational modes.   While intrinsic Raman scattering of photons from molecules is weak and requires long measurement times to obtain a Raman spectrum, Surface Enhanced Raman Scattering (SERS) from molecules near the surface of plasmonic metal nanoparticles offers the potential for intensities comparable to that of fluorescent tags.  The SERS effect can enhance the Raman scattering of bound molecules by as much as 14 orders of magnitude, allowing for the detection of even single molecules!  The enhancement is driven by the high electric field intensities (or “hot spots”) created at locations on the nanoparticle surface and is therefore highly dependent on the nanoparticle geometry, surface features and the specific position of the molecule.

Like the SEF effect, the highest Raman enhancement is usually obtained when there is good overlap between the excitation laser wavelength, the plasmon resonance wavelength of the nanoparticle, and the optical properties of the analyte.  Shown below is the Raman signal from Malachite Green dye in solution (red line) and adsorbed to gold nanoparticles (blue line).

Nanoparticle-based SERS applications include diagnostics, material identification, biological labelling, and security applications. 

Additional Resources

Recommended Literature

  1. Qian, X. M., & Nie, S. M. "Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications." Chemical Society Reviews37(5), 912-920 (2008).
  2. Cao, Y. C., Jin, R., & Mirkin, C. A. "Nanoparticles with Raman spectroscopic fingerprints for DNA and RNA detection." Science297(5586), 1536-1540 (2002).
  3. 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 Society129(6), 1524-1525 (2007).
  4. Chen, Y., Munechika, K., & Ginger, D. S. "Dependence of fluorescence intensity on the spectral overlap between fluorophores and plasmon resonant single silver nanoparticles." Nano Letters7(3), 690-696 (2007).
  5. Liu, N., Prall, B. S., & Klimov, V. I. "Hybrid gold/silica/nanocrystal-quantum-dot superstructures: synthesis and analysis of semiconductor− metal interactions." Journal of the American Chemical Society128(48), 15362-15363 (2006).

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