Properties of Gold Nanoparticles
Also known as colloidal gold, gold nanoparticles have several unique properties, physical as well as optical, because of their extremely small size. These nanoparticles are extensively used for various applications in the field of biology (for instance, bio-imaging) and technology (for example, photonics) owing to the very same reason. Let's go over some of their physical properties in brief below:
1. Gold nanoparticles' properties related to their volume, surface area and diameter
2. Gold nanoparticles' properties, especially their physical ones, for example, their stability and solubility are determined mainly by the nature of their surface.
The surface area to volume ratio of these nanoparticles is extremely high, no matter what their chemical constituents may be. Although such high surface area to volume ratio is extremely important for applications of gold nanoparticles like catalysis, the gold's actual properties are distinct at the nano scale. For instance, the spherical gold nanoparticles' Plasmon resonance brings forth their exceptional ability of scattering visible light.
3. Gold nanoparticles' properties related to their shape and crystallinity
These particles can be produced in all sorts of shapes and sizes, depending on the exact fabrication method used. The presence of a stabilizing polymer leads to typically anisotropic shapes, as it binds itself to one crystal face, leading to faster growth in one crystal direction compared to others. The crystalline domains' size inside a nanoparticle is determined by the fabrication method.
4. Gold nanoparticles' properties related to their surface
The surface of most of the gold nanoparticles is dynamic in nature and gets strongly affected by the local environment where they are placed. Different conditions influence gold nanoparticles' properties in different ways, for instance, environments rich in proteins and other types of biomolecules normally lead to association of these molecules to the nanoparticles, making them more stabilized; high salt environments on the other hand collapse their double layer, leading to their aggregation.
Whenever you're working with gold nanoparticles, you must plan ahead for such changes to their surface. For instance, when the gold nanoparticles need to be functionalized with biomolecules, it's considered best practice to first expose these particles to the specific biomolecule first, inside a low-salt solution, and only later add some more salt for creating isotonic conditions in the solution. Doing so facilitates easy binding of protein to the nanoparticles' surface, thereby stabilizing them more, before the salt is added.
Direct addition of nanoparticles to a high salt environment may cause the nanoparticles to aggregate even before the protein stabilization can happen.
5. Gold nanoparticles' properties related to their stability
It can be pretty difficult to prevent the gold nanoparticles' aggregation depending on the applications they're used for. Gold nanoparticles' properties are such that these particles can be either sterically stabilized or charge stabilized.
In case of charge-stabilized nanoparticles, their stability is measured by zeta potential. Normally, the gold nanoparticles having zeta potential higher than 20 mV or lesser than -20 mV have ample electrostatic repulsion to maintain their stability inside a solution.
The stability of gold nanoparticles can be precisely monitored using dynamic light scattering, dark field microscopy or UV-visible spectroscopy methods.