Nanoparticle Selection for Nanotoxicology Studies

Nanotoxicology studies often aim to determine how specific nanoparticle properties influence biological or environmental responses. To make those relationships easier to interpret, researchers should begin with well-characterized materials and control as many particle variables as possible.

Particle composition, primary size, shape, surface chemistry, aggregation state, and dissolution can all influence nanoparticle behavior. Using matched particle sets that vary one property at a time can help distinguish the contribution of each parameter and improve comparability across experiments.

Selecting Nanoparticles for Toxicology Studies

A central challenge in nanotoxicology is separating the effect of an individual nanoparticle property from the many other variables that can influence biological response. Core composition, size, shape, crystallinity, surface chemistry, aggregation state, dissolution, and interactions with the surrounding medium can all affect particle behavior.

Studies using precisely engineered, well-characterized nanoparticles can reduce this complexity. When materials are closely matched except for one controlled variable, researchers can more directly evaluate relationships between nanoparticle physicochemical properties and biological or environmental responses.

Two complementary material strategies can be useful in nanotoxicology:

1. Precisely engineered nanoparticles for controlled studies designed to isolate the effects of size, shape, composition, or surface chemistry.
2. Application- or exposure-relevant nanoparticles that represent materials likely to be encountered in commercial, biological, occupational, or environmental settings.

Precisely engineered particles are particularly useful for establishing structure-activity relationships because size, morphology, surface chemistry, concentration, and other properties can be independently characterized before exposure.

Understanding the Effects of Nanoparticle Size

Particle size can influence surface area per unit mass, dissolution behavior, transport, cellular uptake, biodistribution, and other processes relevant to nanotoxicology. It is also important to distinguish primary particle size from particle size in the experimental medium.

Primary size describes the dimensions of an individual nanoparticle and is commonly measured by techniques such as transmission electron microscopy (TEM). Once nanoparticles enter a liquid environment, however, they may associate into larger clusters. Their resulting hydrodynamic size can influence settling, transport, and the effective dose delivered during an experiment.

Silver nanospheres across a controlled range of primary particle sizes.

When designing experiments to investigate size-dependent effects:

• Start with unagglomerated nanoparticle suspensions whenever possible.
• Use a consistent surface chemistry across the particle sizes being compared.
• Characterize primary particle size independently from hydrodynamic size.
• Evaluate particle stability in media representative of the intended experiment.
• Keep sample preparation and dosing times consistent if agglomeration occurs over time.

Nanoparticle storage, handling, and stability should also be considered when comparing materials across sizes.

Understanding the Effects of Nanoparticle Shape

Nanoparticle shape can influence cellular interactions, clearance, surface reactivity, and interactions with proteins and other biomolecules. High-aspect-ratio structures can behave differently from spherical nanoparticles, while different crystal facets exposed by anisotropic particles may also affect surface interactions.

Separating shape effects from other variables requires carefully matched materials. Ideally, particles being compared should have similar composition, surface chemistry, and relevant size metrics so that morphology remains the primary experimental variable.

Silver nanowires provide an example of a high-aspect-ratio nanoparticle morphology.

Understanding the Effects of Surface Chemistry

Nanoparticle surface chemistry can strongly influence colloidal stability, surface charge, protein adsorption, cellular interactions, and the formation of a biomolecular corona. Surface coatings can also affect how nanoparticles respond to salts, proteins, pH, and other components of an experimental medium.

Common nanoparticle surfaces include citrate, PVP, PEG, silica, and functionalized surfaces designed for subsequent conjugation. When investigating the effect of surface chemistry, the underlying particle composition and primary size should remain as consistent as possible.

Surface properties can also change after exposure to biological or environmental media. Adsorption of proteins, ions, lipids, and other molecules can alter the effective interface presented by the nanoparticle during an experiment. Measurements such as zeta potential, DLS, spectroscopy, or other surface-sensitive characterization methods can help track these changes.

Learn more about how surface coatings affect dispersion behavior in Salt Stability of Nanoparticles.

Accounting for Nanoparticle Dissolution

Dissolution can be an important contributor to the biological effects of some nanomaterials. Silver nanoparticles are a common example because dissolved silver species can contribute substantially to responses observed during nanoparticle exposure.

Dissolution depends on particle size, surface area, surface chemistry, crystallinity, temperature, dissolved oxygen, pH, and surrounding solution chemistry. Smaller particles generally provide greater surface area per unit mass, which can increase dissolution potential under otherwise comparable conditions.

Researchers can investigate dissolution by separating the nanoparticle fraction from the surrounding solution and quantifying dissolved metal in the particle-depleted fraction. ICP-MS can measure elemental silver or other metals, although it does not by itself distinguish free ions from soluble complexes.

For additional guidance on nanoparticle dissolution measurements, see Silver Nanoparticle Dissolution and Ion Release.

Controlling for Residual Reactants and Contaminants

Residual synthesis reagents, processing materials, endotoxin, and other contaminants can complicate interpretation of nanotoxicology results. Researchers should consider whether an observed response originates from the nanoparticle itself, dissolved species released by the nanoparticle, or other components present in the formulation.

A particle-depleted supernatant can sometimes provide a useful control for soluble components of a nanoparticle formulation. However, the supernatant may contain both residual manufacturing components and species released from the nanoparticle surface, so results should be interpreted accordingly.

For sensitive biological studies, nanoComposix offers select gold and silver nanoparticles as concentrated low-endotoxin formulations with batch-specific characterization and endotoxin specifications.

Endotoxin can itself produce strong biological responses and should be considered separately from other soluble components. See Endotoxin Analysis in Nanoparticle Research for guidance on nanoparticle-related assay interference and endotoxin measurement.

Characterization and Standardization for Nanotoxicology Studies

Well-designed nanotoxicology studies require more than nominal material specifications. Researchers should characterize properties that are relevant to the experimental endpoint and document how the nanoparticles are prepared, dispersed, stored, and administered.

Useful measurements may include primary particle size and morphology by TEM, hydrodynamic size by DLS, surface charge by zeta potential, optical properties by UV-Vis spectroscopy, elemental concentration by ICP-MS, and endotoxin concentration for biological studies.

The OECD provides guidance for the preparation and dosimetry of manufactured nanomaterials used in safety testing, including consideration of agglomeration, sedimentation, characterization, dose metrics, and reporting. See the OECD Guidance on Sample Preparation and Dosimetry for Manufactured Nanomaterials for current recommendations.

Selected References

1. Monteiro-Riviere, N.A., Oldenburg, S.J., and Inman, A.O. Interactions of aluminum nanoparticles with human epidermal keratinocytes. Journal of Applied Toxicology. 2010;30(3):276-285.
2. Samberg, M.E., Oldenburg, S.J., and Monteiro-Riviere, N.A. Evaluation of silver nanoparticle toxicity in skin in vivo and keratinocytes in vitro. Environmental Health Perspectives. 2010;118(3):407-413.
3. Lankveld, D.P.K., Oomen, A.G., Krystek, P., et al. The kinetics of the tissue distribution of silver nanoparticles of different sizes. Biomaterials. 2010;31(32):8350-8361.

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Related nanoparticle resources

• Nanotoxicology and Nanosafety
• Nanoparticle Storage, Handling, and Stability
• Silver Nanoparticle Dissolution and Ion Release
• Endotoxin Analysis in Nanoparticle Research
• Nanoparticle Characterization Techniques
• Salt Stability of Nanoparticles