Watch AuNP aggregation June 09 2017 by Drew Haser
A common stability problem with unprotected, non stabilized gold nanoparticles is aggregation. This stems from the inherent properties of atomic gold. Gold atoms keep rearranging. They are not static, they are not inherently stable, they are dynamic and moving all the time.
As seen here with one of the most powerful electron microscopes in the world, 3 gold nanospheres with individual atoms visible come together and aggregate into a new non-spherical shape and structure.
Gold Nanoparticle aggregation is often irreversible since gold has a strong preference to bind other gold atoms over other elements. Aggregation reduces colloidal stability and diminishes the optical properties of the original nanoparticles.
To improve colloidal stability gold nanoparticles can be stabilized with coatings such as citrate, PEG, or silica (each with its own benefit depending on application).