Colloidal gold refers to a suspension of tiny gold particles in a liquid medium, typically water. These particles are usually in the nanometer range, making them highly stable and capable of exhibiting unique optical and electrical properties. The preparation of Colloidal gold involves reducing gold salts into smaller particles using various chemical methods.

Colloidal gold has been widely used in various fields due to its distinct properties. In biology and medicine, it has found applications in diagnostics, imaging, and therapeutic treatments. Its ability to absorb and scatter light gives it a vibrant color, making it useful for staining and visualizing biological samples in techniques like immunohistochemistry and electron microscopy.

Additionally, colloidal gold nanoparticles have shown potential in targeted drug delivery systems and cancer treatment, where they can be functionalized with specific molecules to selectively bind to cancer cells. They can also be used in photothermal therapy, where the particles absorb light energy and convert it into heat, thereby destroying cancer cells.

Outside of the medical field, colloidal gold is utilized in electronics, catalysis, and sensors. Its unique electrical properties make it suitable for applications such as conductive inks, printable electronics, and surface-enhanced Raman spectroscopy.