Self-assembling and aggregation of colloids

Stable self assembles

Nanotructure by self-assembly of nanoparticles

When the gold nanoparticles are allowed to accumulate instead of gold nanoparticles alone, the localized plasmon resonance produces more dramatic features. Thus technology to set up the assemblies of gold nanoparticles is important.

Bottom-up technique has attracted attention in nanotechnology. Being the first to the technology trend of the world, the self-assembling methods of gold nanoparticles by chemical and physical technique have been established. As results, unique functional nanostructures including one-dimensional chain-like assemblies which stably disperse in the solution, two-dimensional rod-like array of which large area product can be supplied, and bulky three-dimensional honeycomb are developed. The obtained technology to set up the assemblies of gold nanoparticles and resulting nanostructures is the key technology to dominate the industry in the future.

Aqueous dispersion of gold nanoparticles, that is, colloidal gold begins to aggregate by addition of salt. Usually, colloidal gold has been charged by the molecules adsorbed to the surface. Energy barrier due to the electrostatic repulsion of charged nanoparticles stabilizes the dispersion of the colloid by preventing the approach and the resulting collision of the gold nanoparticles with each other. When ionic strength of the solution is increased by the addition of salt, the effective distance of the electrostatic repulsion (Debye length) begins to decrease, gold nanoparticles are allowed to be close to each other, and collisions to overcome the lowered energy barrier  occur eventually. Thus colloidal gold aggregates.

When gold nanoparticles aggregate, an attractive color change occurs. Color of colloidal gold of 40 nm particle size is usually wine red, but it changes from red purple to blue-violet in the process of aggregation. This drastic change in the color can be explained as the action of localized plasmon; since new mode of collective oscillation of free electrons, that is, the new localized plasmon resonance, is born in the aggregated gold nanoparticles, new large absorption appears at the corresponding resonance wavelength.

If you carefully control aggregation process, chain-like aggregations grow under self-assembling of nanoparticles. New plasmon resonance mode appears in the long axis direction (longitudinal) of the anisotropic aggregates, as a result, new absorption appears in the near-infrared region of the absorption spectrum. Localized plasmon resonance of the short axis direction (transverse) is not much different from that of single gold nanoparticle.

In the self-assembling process of aggregation, assembles of various size and morphology are generated, then localized plasmon resonance corresponding to those nanostructure is expressed.

However, when aggregation continues to progress, gold nanoparticles become very small and black soot-like mass, soon precipitate to the bottom of the container. Unique properties of localized plasmon resonance of the anisotropic assembles are lost. The technique that uses a surfactant or a water-soluble polymer for stabilization is well known. Another methods of adding a linker molecule to initiate aggregation is also known. However, although we try to make use of the localized surface plasmon resonance in the very surface of the gold nanoparticles with much effort, any sequestering of the active surface of gold nanoparticles is far from our desire.

In the long time of history, the contradiction of aggregation process is not avoidable. By novel leverage of original "aggregation terminator", the useful self-assembles can be stabilized successfully for the first time in the world. As a result, the path to the various applications of self-assembly has been opened.

Black precipitation of aggregated gold colloid

Stable dispersion of colloidal gold assembles

Stable assembles maintain the same spectrum after concentration.

Stable assembles in the aqueous medium.

Stable assembles keep color after coating and drying.

Controlled and stable gold assembles enhance Raman scattering.