All About Surface Plasmon Resonance

Surface Plasmon Resonance and Its Function Regarding Shape and Size of Gold Nanoparticles

Introduction to Surface Plasmon Resonance (SPR)

Surface Plasmon Resonance (SPR) is a phenomenon that occurs when conduction electrons on the surface of metallic nanoparticles resonate with incident light at specific wavelengths. This resonance results in unique optical properties that are highly sensitive to changes in the particle's environment, making SPR a powerful tool in various applications, including sensing, imaging, and spectroscopy.

Influence of Shape and Size on SPR

The SPR characteristics of gold nanoparticles (AuNPs) are profoundly influenced by their shape and size. Different geometries such as spheres, rods, triangles, and stars each exhibit distinct SPR peaks due to variations in how electrons oscillate on their surfaces.

  1. Spherical Gold Nanoparticles:

    • Spherical AuNPs typically exhibit a single SPR peak in the visible region of the spectrum.
    • The SPR peak's position is highly dependent on the particle size. As the diameter increases, the SPR peak shifts to longer wavelengths (red-shift), which is due to the increased path length for electron oscillations.
  2. Gold Nanorods:
    • Nanorods show two SPR peaks: one in the visible region (transverse mode) and one in the near-infrared region (longitudinal mode).
    • The longitudinal SPR peak is highly sensitive to the aspect ratio (length/diameter) of the nanorods. Higher aspect ratios result in a red-shift of the longitudinal SPR peak.
    • Experimental data indicates that for nanorods with diameters greater than 50 nm, the longitudinal SPR is more influenced by diameter than by length​ (Nanopartz)​​ (Nanopartz)​.
  3. Gold Nanotriangles and Nanostars:
    • Non-spherical geometries like nanotriangles and nanostars exhibit more complex SPR spectra with multiple peaks.
    • These shapes can support multiple plasmon modes due to their sharp edges and tips, which enhance the local electromagnetic field.

Applications of SPR in Gold Nanoparticles

  1. Biosensing:

    • SPR is extensively used in biosensing applications due to its high sensitivity to changes in the refractive index near the nanoparticle surface.
    • Functionalized gold nanoparticles can detect biomolecular interactions in real-time, providing valuable data for diagnostic applications.
  2. Imaging and Therapeutics:
    • In vivo imaging benefits from the tunable SPR properties of gold nanoparticles. For example, gold nanorods with near-infrared SPR peaks are ideal for deep tissue imaging due to better tissue penetration and reduced scattering.
    • Photothermal therapy exploits the SPR-induced heating of gold nanoparticles to selectively destroy cancer cells.
  3. Environmental and Chemical Sensing:
    • The shift in SPR peaks can be used to detect chemical and environmental changes, making gold nanoparticles valuable in pollutant detection and monitoring systems.


Understanding the relationship between the size and shape of gold nanoparticles and their SPR properties is crucial for optimizing their performance in various applications. By selecting the appropriate nanoparticle geometry and size, researchers and engineers can tailor SPR characteristics to meet specific needs in biosensing, medical imaging, and environmental monitoring.

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