Useful Nanoparticle Terminology

Nanoparticle terminology can become confusing because measurements of particle size, dispersion state, and optical response often describe different aspects of the same material. Understanding these terms is important when comparing specifications, characterization data, and nanoparticle behavior in solution or after processing.

This guide defines several commonly used terms related to nanoparticle aggregation, size, and optical properties.

Jump to: Aggregation & Agglomeration • Spectral Shifts • Extinction • Primary Particle Size • Optical Cross-Sections • Optical Density • Optical Efficiency

Aggregation & Agglomeration

Aggregation and agglomeration both describe clusters containing two or more primary particles, and the terms are sometimes used inconsistently in nanoparticle literature. In standardized nanotechnology terminology, however, they have distinct meanings.

• Agglomerate: A collection of particles or aggregates held together by relatively weak interactions, such as van der Waals forces or physical entanglement
• Aggregate: A structure containing strongly bonded or fused particles that are substantially more difficult to separate

In either case, the state of association can strongly influence nanoparticle behavior. A dispersion containing individually separated 10 nm particles can have very different transport, settling, surface-area, and optical properties from a material containing the same primary particles clustered into much larger secondary structures.

Mechanical dispersion methods such as probe sonication, microfluidization, or milling can break some weakly associated structures into smaller clusters. However, drying can create strong particle-particle contacts that make complete recovery of the original individually dispersed nanoparticles difficult or impossible.

In historical nanoComposix dispersion work with several dried oxide nanomaterials, extensive dispersion frequently produced secondary particle sizes on the order of approximately 200 nm even when the underlying primary particles were substantially smaller. This observation should not be interpreted as a universal lower limit for redispersing nanoparticle powders; achievable dispersion depends on material, surface chemistry, drying history, and processing method.

Several characterization techniques can be used to evaluate particle association:

• TEM: Directly images primary particles and clusters, although sample drying can introduce artifacts
• Dynamic Light Scattering (DLS): Measures an apparent hydrodynamic size distribution in a liquid dispersion
• Centrifugal sedimentation: Can help resolve multimodal or relatively broad particle-size distributions
• Aerosol sizing: Can characterize agglomerate size after dry powders are aerosolized

Because each technique measures a different physical quantity, primary particle size and hydrodynamic or secondary particle size should not be treated as interchangeable measurements.

Learn more in Nanoparticle Characterization Techniques and Salt Stability of Nanoparticles.

Spectral Shifts: Blue Shift & Red Shift

Nanoparticle optical peaks can shift when particle size, shape, composition, coating, surrounding refractive index, or aggregation state changes.

• Blue shift: Movement of a spectral feature toward shorter wavelengths, corresponding to higher frequency and photon energy
• Red shift: Movement toward longer wavelengths, corresponding to lower frequency and photon energy

For example, decreasing the aspect ratio of a gold nanorod can shift its longitudinal plasmon resonance from longer toward shorter wavelengths. Adding a silica shell around a silver nanoparticle can shift its plasmon resonance toward longer wavelengths because the shell changes the refractive index surrounding the metal surface.

Aggregation of plasmonic nanoparticles also frequently produces broadening and increased extinction at longer wavelengths because electromagnetic interactions develop between neighboring particles. The exact spectral response depends on particle geometry and aggregate structure.

See The Science of Plasmonics for more information about these effects.

Extinction

When light interacts with nanoparticles, some incident light can be absorbed and some can be scattered. The combined loss of light from the directly transmitted beam is referred to as extinction.

For nanoparticle systems:

Extinction = Absorption + Scattering

The relative contributions of absorption and scattering depend on particle material, size, shape, wavelength, and surrounding environment. For example, small gold nanospheres such as 10 nm particles are strongly absorption-dominated, while scattering becomes much more important for larger gold nanospheres such as 100 nm particles.

A UV-Visible spectrophotometer commonly reports the attenuation of transmitted light as:

T = I / I₀

Optical Density = −log₁₀(T)

where T is fractional transmission, I is the measured transmitted intensity, and I₀ is the incident intensity.

For nanoparticle dispersions, the resulting spectrum is commonly called an extinction spectrum because both particle absorption and scattering can contribute to the measured attenuation.

Primary Particle Size

Primary particle size describes the dimensions of the smallest discrete particle or grain within an aggregate or agglomerate. It is commonly measured using transmission electron microscopy (TEM).

Primary particle size should be distinguished from the size of secondary structures measured in a dispersion. For example, a material may contain 50 nm primary particles while DLS reports a substantially larger hydrodynamic diameter because the particles are associated into clusters or because the measurement includes contributions from a surface coating and solvation layer.

Optical Cross-Sections

An optical cross-section describes how effectively an individual particle interacts with incident light. Separate cross-sections can be defined for absorption and scattering:

• Absorption cross-section: Describes the effectiveness of a particle at absorbing incident light
• Scattering cross-section: Describes the effectiveness of a particle at redirecting incident light
• Extinction cross-section: The sum of absorption and scattering cross-sections

Optical cross-sections have units of area, but they should not be interpreted simply as the physical area of the nanoparticle. Resonant nanoparticles can interact with light over an effective optical cross-section substantially larger than their geometric cross-section.

The figure above illustrates how the extinction cross-section of silver nanospheres changes with particle diameter. Particle size changes both the magnitude of the optical interaction and the relative contributions of scattering and absorption.

Explore these relationships for spherical nanoparticles with the Mie Theory Calculator.

Optical Density

Optical density (OD), also commonly called absorbance in spectrophotometry, expresses the attenuation of light passing through a sample on a logarithmic scale:

OD = −log₁₀(T)

where T is the fraction of incident light transmitted through the sample, ranging from 1 for 100% transmission toward 0 as transmission decreases.

Optical Density	% Transmission
0.0	100
0.1	80
0.2	63
0.3	50
0.4	40
0.5	32
1.0	10
2.0	1
3.0	0.1
4.0	0.01

Because optical density is logarithmic, an increase of 1 OD corresponds to a tenfold decrease in transmitted light.

Optical Efficiency

Optical efficiency compares a particle's optical cross-section with its geometric cross-section. For example, extinction efficiency can be expressed as:

Extinction Efficiency = Extinction Cross-Section / Geometric Cross-Section

Because both quantities have units of area, optical efficiency is dimensionless.

An efficiency greater than 1 means the nanoparticle interacts with incident light over an effective optical area larger than its physical projected area. Resonant plasmonic nanoparticles can therefore have very high optical efficiencies.

For a sphere, geometric cross-section is straightforward to define. For nonspherical particles such as rods, plates, and pyramids, the appropriate reference area depends on the convention being used.

One approach is to define an effective radius based on a sphere with the same volume as the nonspherical particle:

R_eff = (3V / 4π)^1/3

and then define the reference geometric cross-section as:

πR_eff²

where V is the total nanoparticle volume. When reporting optical efficiencies for nonspherical particles, the convention used to define geometric cross-section should therefore be stated.

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Related nanoparticle resources

• Nanoparticle Characterization Techniques
• Salt Stability of Nanoparticles
• The Science of Plasmonics
• Mie Theory Calculator
• Gold Nanoparticle Optical Properties
• Silver Nanoparticle Optical Properties