Ask an Expert: Why do gold nanoparticles aggregate? How do I prevent them from aggregating? What do I do if I see aggregation?

What can cause aggregation of gold nanoparticles?

• If the pH of the particles’ solution is too high or too low for their surface coating (e.g. placing silica coated gold nanorods in solutions below pH=7.0)
• Placing charge stabilized particles (e.g. citrate stabilized gold nanospheres) in buffer solutions with high ionic strength (e.g. PBS). This allows for charge shielding to occur, leading to aggregation.
• Freezing a solution of particles
• Over-concentrating particles
• Centrifuging nanoparticles beyond recommended speeds (this applies primarily to silica coated nanoparticles which should be centrifuged at speeds no greater than 1500 rcf)

How to prevent aggregation

• Store particles at the recommended temperatures (usually between 2°- 8°C)
• Maintain pH of particles’ solution within recommended range (dependent on particle type)
• In applications where charge shielding can occur, be sure to use a particle which is sterically stabilized (e.g. a PEGylated ) versus a charge stabilized particle (e.g. citrate or CTAB)

What to do if gold nanoparticles aggregate

It is important to note the somewhat subtle difference between flocculation versus aggregation. Flocculation can be described as the visible precipitation of the particles out of colloidal suspension. Aggregation is a nearly irreversible process in which the particles have begun to attach to one another. Flocculation can be induced by changes in pH or through charge shielding and therefore usually precedes aggregation.

If particles have flocculated but not aggregated there are a few things that can be done to re-suspend them:

• Adjust pH to recommended range
• Gentle sonication
• Alter surface chemistry through methods such as PEGylation which force steric stabilization

If particles have aggregated you can attempt to recover non-aggregated particles via filtration through a 0.2 μm filter. Please feel free to contact us if you have any additional questions about nanoparticle aggregation or would like help with troubleshooting. 

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