Colloidal gold is a kind of nanoparticles of gold which is suspended in a fluid (water, usually) to form a sol. Except for its important roles in the material sciences or other related areas, colloidal gold is also developed to fulfill a lot applicaiton in the areas ranging from medicine to enviromental science. Colloid gold is used for the detection and visualising of certain molecules or markers, as a gene/drug delivery system and also an agent in photothermal/radio related therapy. In this blog, we will mainly focus on its application in the detection part.
Why Colloidal Gold
The peculiar features of AuNPs, in fact, make them interesting candidates for colorimetric sensing of target molecules. Indeed, AuNPs surface plasmon band falls within the visible region, it is shape and size-dependent, and thanks to its high extinction coefficient,produces intensely colored colloidal suspension, appreciable with the naked-eye even at rather low concentrations. Moreover, AuNPs exhibit distance-dependent colorimetric changes (i.e. absorption shifts) due to the coupling of the surface plasmon of different particles (typically when the inter-particle distance becomes much smaller than the particles’ diameter). This effect is the underlying principle of many colorimetric sensing schemes.
And the different sizes of the particles make the sol or colloidal suspension to show different colors: for those particles with diameters less than 100 nm, they are red; while the larger ones looks blue or purple.
Turkevich method, a classical methods, produces monodisperse spherical gold nanoparticles suspended in water of around 10–20 nm in diameter. Larger particles can be produced, but this comes at the cost of monodispersity and shape.
Brust method, produces gold nanoparticles in organic liquids with a diameter around 5-6 nm. However, TOAB does not bind to the gold nanoparticles particularly strongly, so the solution will aggregate gradually over the course of approximately two weeks.
Perrault method, produces 15 nm gold nanoparticle seeds in an aqueous solution. Combining with Fren’s method, is ideal for particles of 12–20 nm, while with the hydroquinone method can produce particles of at least 30–300 nm.
Navarro et al. method, produces the gold core up to 90 nm with a narrow size distribution.
Nanotech applications, produces gold nanoparticles with sizes between 10 and 100 nanometres, which are usually dissolved in organic solvents and bacteria Bacillus licheniformis are used to produce the particles.
Sonolysis, the first method of this type allows the creation of gold particles with a diameter of under 10 nm; while another one can obtain particles of nanoribbons with width 30–50 nm and length of several micrometers.
Martin method, generates nearly monodisperse "naked" gold nanoparticles in water and also enables reproducible diameter tuning between 3–6 nm.
And several other methods are not listed here.
Colloidal gold is widely used to detect or visualise molecules of interest. Some of the classical applications include electron microscopy, tumor detection, toxic gas detection and gold nanoparticle based biosensor.
Many techniques are used to develop, modify and optimise the colloidal gold based applications. For instance, techniques such as Surface Plasmon Resonance (SPR), based on the high sensitivity of the sensor to its micro-environment, directly exploit the plasmonic properties of nanometals. On the other hand, plasmonic effects of metal nanostructures can lead to signal enhancement of several orders of magnitude, which has been exploited in techniques such as Surface-Enhanced Raman Spectroscopy (SERS) and Metal-Enhanced Fluorescence (MEF), with consequent increased sensitivity. Besides optical signal enhancement, nanomaterials-based approaches may enable multiplexed, sensitive detection of different analytes in tiny sample volumes. By combining nanomaterials with probes pieces for target recognition, several hybrid systems can be created, which can merge optical transduction and sensing properties.
In our site, we provide series of Colloidal gold strips based lateral flow assay kits which cover the areas ranging from infectious disease tests to food safety tests.