Colloidal gold method definition and classification ELISA kit

Colloidal gold is an innovative immunolabeling technique that uses gold nanoparticles as a tracer for antigen-antibody interactions. It offers unique advantages, such as high sensitivity, rapid detection, and visual readout, which have led to its widespread use in biological research and clinical diagnostics in recent years. Most immunoassays in clinical settings now rely on colloidal gold-based markers. The technique was first introduced into immunology by Faulk and Taylor in 1971, marking the beginning of immunocolloidal gold technology. Since then, it has become a vital tool in biomedical fields. Today, it is commonly used in medical testing, particularly in methods like immunochromatography and Dot-immunogold filtration assay (DIGFA), for detecting substances such as HBsAg, HCG, and anti-double-stranded DNA antibodies. These assays are fast, accurate, and environmentally friendly. The basic principle of colloidal gold involves the synthesis of gold nanoparticles from chloroauric acid (HAuCl4) using reducing agents like sodium citrate, ascorbic acid, or white phosphorus. The resulting particles are stabilized by electrostatic repulsion, forming a negatively charged colloidal solution. In a weakly alkaline environment, these particles can bind to positively charged groups on proteins through electrostatic interactions, without affecting the protein's biological activity. In addition to proteins, colloidal gold can also bind to other biomolecules such as SPA, PHA, and ConA. Its physical properties—such as high electron density, size, shape, and color change—combined with its immunological characteristics, make it widely applicable in immunology, histology, pathology, and cell biology. The labeling process typically involves adsorbing proteins onto the surface of colloidal gold particles. This occurs due to electrostatic attraction between the negative charge on the gold surface and the positive charges on the protein. Different sizes of gold particles produce varying colors, making them easy to prepare and use. These particles can non-covalently bind to various molecules, including staphylococcal protein A, immunoglobulins, toxins, enzymes, and hormones, making them valuable tools in both basic research and clinical applications. Immunogold labeling takes advantage of the high electron density of gold particles. When bound to proteins, they appear as black-brown particles under a microscope. When concentrated, they form visible red or pink spots, enabling quick qualitative or semi-quantitative results. This reaction can be further amplified by silver deposition, known as immunogold-silver staining, enhancing sensitivity. Common techniques include: 1. **Immunocolloidal gold mirror staining**: This method involves labeling cells or tissue sections with colloidal gold-labeled antibodies, followed by silver enhancement to increase sensitivity. 2. **Immunogold electron microscopy**: Colloidal gold-labeled antibodies are used to detect viruses or ultra-thin tissue sections under an electron microscope. 3. **Dot immunogold filtration assay**: Antigen or antibody is immobilized on a membrane, and after sample incubation, the presence of the target is detected using colloidal gold-labeled antibodies. 4. **Colloidal gold immunochromatography**: A strip format where antigens or antibodies are fixed on a membrane. The sample is applied to one end, and the test result is visually read after capillary movement and specific binding. This technology has evolved into user-friendly diagnostic strips, widely used in point-of-care testing.

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