Gold nanoparticles interact strongly with light because their conduction electrons can oscillate collectively in response to incident electromagnetic radiation. For discrete nanoparticles, this phenomenon is known as a localized surface plasmon resonance (LSPR).
LSPR gives gold nanoparticles large optical cross sections and strong absorption and scattering. The wavelength and intensity of the optical response depend on particle size, shape, surrounding refractive index, and interactions with neighboring particles. These tunable properties make gold nanoparticles useful for sensing, imaging, diagnostics, spectroscopy, and optical engineering.
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Key Gold Nanoparticle Optical Terms
| Term | Meaning |
|---|---|
| LSPR | Localized surface plasmon resonance: the collective oscillation of conduction electrons in a nanoparticle driven by incident light. |
| Absorption | Light energy absorbed by the nanoparticle and converted into other forms of energy, including heat. |
| Scattering | Incident light redirected by the nanoparticle into other directions. |
| Extinction | The combined loss of light from the incident beam due to absorption and scattering. |
| Albedo | The fraction of total optical extinction contributed by scattering rather than absorption. |
How Particle Size Affects Gold Nanoparticle Optical Properties
The optical properties of spherical gold nanoparticles are strongly dependent on particle diameter. Smaller gold nanospheres primarily absorb visible light and typically exhibit an LSPR peak near 520 nm. As particle diameter increases, scattering contributes more strongly to total extinction, and the plasmon peak generally broadens and shifts toward longer wavelengths.
The spectra below show this size-dependent behavior for gold nanospheres at the same gold mass concentration of 0.05 mg/mL. Comparing particles at equal mass concentration is important because particle number concentration decreases dramatically as particle diameter increases.
Gold nanosphere extinction spectra as a function of particle size.
Scattering becomes increasingly important as gold nanosphere diameter increases.
Gold nanosphere albedo, the fraction of extinction attributable to scattering, increases with particle size.
Larger gold nanospheres can often be observed individually using dark-field microscopy because their strong scattering produces bright diffraction-limited spots. The minimum observable particle size depends on the particle environment, illumination, collection optics, and imaging system.
Particle size also determines whether absorption or scattering is more useful for a particular application. Smaller particles are often selected when strong absorption or compact dimensions are desired, while larger particles may be advantageous when scattering-based detection is important.
How Particle Shape Affects Optical Properties
Particle morphology provides another powerful way to tune gold nanoparticle optical properties. Spherical particles exhibit a dominant plasmon resonance whose position changes with particle size and dielectric environment, while anisotropic and core-shell structures introduce additional geometric control over the resonance.
Gold Nanorods
Gold nanorods exhibit both transverse and longitudinal plasmon resonances. The longitudinal resonance is particularly sensitive to nanorod aspect ratio, allowing the optical response to be shifted substantially by changing the relative rod length and width.
Gold Nanoshells
Gold nanoshells consist of a dielectric core surrounded by a thin gold shell. Changing the relative dimensions of the core and shell provides strong control over the plasmon resonance and can shift the optical response across the visible and near-infrared spectrum.
These shape-dependent properties allow gold nanoparticles to be selected or engineered for specific spectral regions, absorption-to-scattering ratios, and optical applications. Explore the current gold nanoparticle portfolio for available particle morphologies.
How Refractive Index Affects Gold Nanoparticle Optical Properties
The optical response of a gold nanoparticle depends not only on the particle itself but also on the dielectric environment surrounding its surface. Increasing the local refractive index generally shifts the LSPR toward longer wavelengths.
This sensitivity means the same nanoparticle can exhibit a different spectrum when dispersed in water, incorporated into a polymer or resin, coated with another material, or surrounded by adsorbed biomolecules.
Calculated extinction spectra of a 50 nm gold nanosphere in environments with different refractive indices.
In the calculated example above, increasing the surrounding refractive index from 1.00 to 1.60 shifts the extinction peak by more than 40 nm. The magnitude of this shift depends on particle dimensions, morphology, and the optical properties of the surrounding material.
This refractive-index sensitivity is important in plasmonic sensing because adsorption or binding near the gold surface can change the local optical environment. It also matters when integrating nanoparticles into coatings, polymers, biological media, and other matrices.
Surface coatings can produce the same type of effect. Silica shells, polymer layers, biomolecules, and other materials surrounding a gold nanoparticle alter the local dielectric environment and can shift its optical spectrum.
For a widely cited technical treatment of how size, shape, and dielectric environment influence metal nanoparticle optical properties, see Kelly et al., The Optical Properties of Metal Nanoparticles, J. Phys. Chem. B (2003) ↗.
How Aggregation Affects Gold Nanoparticle Optical Properties
When gold nanoparticles move close enough together, their localized plasmon modes can interact. This plasmon coupling changes the optical response of the particle system and can produce spectral broadening, reduced intensity at the original plasmon peak, and increased extinction at longer wavelengths.
The exact spectral change depends on factors such as particle size, aggregate geometry, interparticle spacing, and the surrounding medium. Aggregation therefore does not produce one universal spectral signature, but pronounced red-shifting and broadening are common for spherical gold colloids.
Example extinction spectra showing changes as a gold nanoparticle dispersion aggregates.
In the example above, carboxyl-functionalized gold nanoparticles are exposed to progressively lower pH. As the carboxyl groups become protonated, electrostatic stabilization is reduced and the nanoparticles aggregate. The resulting plasmon coupling produces a pronounced change in the extinction spectrum.
UV-Visible spectroscopy is therefore a sensitive screening method for monitoring changes in the dispersion state of plasmonic nanoparticles. A decrease in the original plasmon peak, spectral broadening, or the appearance of longer-wavelength extinction can indicate aggregation. When additional confirmation is needed, UV-Vis can be complemented by techniques such as dynamic light scattering or electron microscopy.
Many small spherical gold colloids appear ruby red when well dispersed. Aggregation can change the solution to purple or blue as the optical spectrum shifts and broadens. Extensive aggregation may eventually produce visible sediment or precipitate.
For more information about using UV-Vis, DLS, and other methods to evaluate nanoparticle dispersion state, see Nanoparticle Characterization Techniques.
Using Optical Properties to Select a Gold Nanoparticle
Gold nanoparticle optical behavior can be tailored by controlling several complementary variables:
- Size: Controls spectral position and the relative contributions of absorption and scattering.
- Shape: Introduces additional plasmon modes and provides broader control over resonance wavelength.
- Surrounding refractive index: Shifts the resonance as the local dielectric environment changes.
- Particle spacing: Close particle-particle interactions can produce plasmon coupling and substantially change the spectrum.
The appropriate combination depends on whether the application relies on absorption, scattering, refractive-index sensitivity, spectral position, or another optical property. The Mie Theory Calculator can help explore these relationships for spherical and core-shell particles, while Nanomaterials for Optical Engineering provides application-focused guidance.
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