Gold nanoparticles

1951

Reducing a gold salt with citrate in water is one of the most widely used ways to make gold nanoparticles. In the Turkevich method, from 1951, citrate acts as both the reducing and the stabilizing agent. In 1973, Frens showed that particle size can be controlled by varying the ratio of gold to citrate. ^1,2,3

500 to 550 nm

In water, spherical gold nanoparticles show colors such as brown, orange, red and purple as the core grows from 1 to 100 nm. They have an absorption band, the surface plasmon band, caused by the collective oscillation of conduction electrons excited by light, which generally lies between 500 and 550 nm depending on size. Particles smaller than 2 nm lack this band. When the particles aggregate, the band shifts from about 520 nm to about 650 nm and the solution turns from red to blue, an effect used in colorimetric sensors. ^1,2,4

5 to 100 nm

A 2007 study calculated how the optical properties of spherical gold nanoparticles vary with size and compared the calculation with particles of 5 to 100 nm, synthesized and characterized by transmission electron microscopy and UV-Vis spectroscopy. Theory and experiment agreed. The authors derived equations to determine particle size and concentration directly from the UV-Vis spectrum and discussed the precision of various methods. ⁵

Proper surface functionalization is a prerequisite for any application of colloidal inorganic nanoparticles, because it determines how they interact with their environment. On gold it is usually based on the gold-sulfur bond: thiols can displace citrate from the surface, and the particles can carry oligonucleotides, antibodies and proteins. During this exchange, citrate-stabilized particles can aggregate irreversibly, and strategies exist to prevent it. ^2,6,7,8

In lateral flow tests, whose best-known application is the pregnancy test, colloidal gold is the most used label material, according to a 2009 literature survey; colored latex particles come next. A 2016 review describes it as the most widely used label in commercial lateral flow immunoassays: it has an intense color and needs no development step to be seen. ^9,10

Two reviews, from 2012 and 2015, point to three properties that make gold nanoparticles useful for electrochemical detection: conductivity, a large surface area and catalytic properties. The gold surface allows biomolecules to be bound, and electrodes with gold nanoparticles can act as electron wires that facilitate electron transfer between redox proteins and the electrode without redox mediators. ^1,7