Colloidal gold is a certain-size gold particle synthesized by gold ion reduction and polymerization of chloroauric acid (HAuCl₄) under the action of reducing agents such as white phosphorus, ascorbic acid, sodium citrate, tannic acid, and so on. Due to electrostatic action, gold particles repel each other and suspend into a stable colloidal state, forming a negatively charged hydrophobic colloid solution, so it is called colloidal gold. Colloidal gold has high electron density and can conjugated with a variety of biological macromolecules (such as protein, SPA, PHA, and ConA). It has become a commonly used non-radioactive tracer in immunolabeling technology after fluorescein, radioisotope, and enzyme. The particle size of colloidal gold is generally between 1-100nm, which can be uniformly and stably dispersed in the liquid, showing small spherical particles or large oval particles.

Producing Quality Colloidal Gold

Colloidal gold is mainly prepared by chemical reduction method, that is, various reducing agents are added to the aqueous solution of chloroauric acid to polymerize gold ions into colloidal gold. The reduction method can be regarded as a crystallization process, and the particle size depends on the crystal nucleus formation rate and crystal growth rate caused by the reducing agent. The commonly used reducing agents are white phosphorus, ascorbic acid, citric acid, tannic acid, sodium borohydride, hydrogen peraminate, and ethanol.

Colloidal gold has high dynamic stability, and its own condensation is very slow when the stability factors are not changed, and it can be placed for several years without condensation. The main factors affecting stability are electrolyte, sol concentration, temperature, non-electrolyte, and so on. Colloidal gold solution must have a small amount of electrolyte as a stabilizer, but the concentration should not be too high. A high concentration of hydrophilic non-electrolyte can peel off the hydration film outside the colloidal particles and make it agglomerate. A small number of macromolecular substances can promote sol coagulation, but a certain number of macromolecular substances can increase the stability of sol, such as the addition of protein, glucose, PEG20000, etc. have a good stabilizing effect.

How to better characterize and analyze the particle size of micro-nanoparticles and the state in the solution is very important for the use of colloidal gold. Transmission electron microscope is suitable for characterizing gold nanoparticles below 50nm and quantum dots of 2-10nm; scanning electron microscope is suitable for surface morphology characterization of particles from 50nm to micron level; dynamic light scattering can measure and analyze the state of particles from 0.3nm to 10 μ m in solution. The light extinction liquid-borne particle counter can measure the state of particles more than 0.5 μ m in solution, as well as the particle size distribution and the number of particles, etc., and it can also analyze whether particles gather in a specific solution.

Safe Handling and Storage of Colloidal Gold

• The distilled water used to prepare colloidal gold should be double- or triple-distilled.
• Glassware for preparing colloidal gold must be clean, undergoing acid passivation, and rinsed with distilled water before use.
• Colloidal gold can be preserved for a long time in clean glassware and can be stored at 4°C for more than 6 months or at room temperature for 1-2 months. Adding a little preservative (such as 0.02% NaN₃) can be beneficial to preservation. Bacteria will grow or particles will agglutinate when they are not well preserved. A small number of agglutinated particles does not affect the labeling of colloidal gold in the future. To improve the labeling efficiency, agglutinated particles can be removed by low-speed centrifugation. However, in any case, microscopic examination should be carried out after it has been preserved for a long time. If many colloidal gold particles agglutinate, it means that it has expired.

Colloidal Gold Applications Across Fields

Gold nanoparticles play a wide role in biomedical and chemical industries because of their high catalytic activity, good biocompatibility, excellent electrical conductivity, and a series of nano-effects.

• Colloidal gold is a kind of nanomaterial that is widely used in immunolabeling. Gold nanoparticles have important value in biochemical analysis because their color can vary with different particle sizes, and they are easy to conjugate with biological macromolecules such as proteins and antibodies. Almost all the immunoblotting techniques used in clinical use colloidal gold as labels. At the same time, it can be used in cell imaging, flow cytometry, electron microscopes, and even biochips.
• Gold nanoparticles have high catalytic activity, especially at low temperatures. In recent years, as heterogeneous and homogeneous catalysts, gold nanoparticles have been widely used in catalytic degradation, oxidation, and hydrogenation catalytic processes.
• Gold nanoparticles have special properties such as good electrical conductivity and high surface chemical activity, which can greatly reduce the distance between electron donor and acceptor and increase the transfer rate between electron and electrode. Thus, they have important applications in improving the performance of biosensors.