In 2009, Weston et al. [14] synthesized a new solution for the detection of nitrite content in drinking water, which is a colloidal gold solution composed of two kinds of gold nanoparticles, aniline gold nanoparticles and naphthalene gold nanoparticles. The surface plasmon resonance absorption of the colloidal gold solution is very strong at 520 nm wavelength, so it appears red. Under acidic conditions, if there is nitrite, the two gold nanoparticles will be closely combined and precipitated quickly, at this time, the color of the colloidal gold solution will change from red to colorless. Once the mass concentration of nitrite in the solution exceeds 1 μ g / ml, the color of the solution can be observed to change, and the solution will be nearly transparent when the mass concentration is 1.86 μ g / ml<br>
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