Universal Determination of Gold Concentration in Colloids with UV–vis Spectroscopy

Extinction at 400 nm is a convenient, cheap, and fast method for in situ determination of Au concentration in colloids with good accuracy even in the presence of Au(+3) ions and other interference factors. However, these possibilities have been validated only with common citrate gold nanospheres and, in part, with gold nanorods. Here, we demonstrate the universality of the UV–vis extinction method with six experimental and theoretical models: (1) CTAC-stabilized gold nanospheres; (2) small and large nanosphere clusters; (3) gold nanorods with plasmon resonances (PRs) ranging from 670 to 980 nm; (4) 2D nanotriangles and (5) 2D nanoplates with PRs from 600 to 775 nm; and (6) gold nanostars with PRs from 680 to 820 nm. In total, we fabricated 34 samples with different nanoparticle sizes, shapes, morphologies, and Au concentrations. From COMSOL, T-matrix, and generalized multiparticle Mie simulations, we derived a universal relation between the extinction cross section and the particle or cluster volume V: Cext (nm2) = 0.51 × V (nm3), 102 ≤ V (nm3) ≤ 105, which gives a universal relation between the gold concentration and extinction [Au0] (mM) = 0.44 × A400. The same relation is derived from atomic absorption spectroscopy and inductively coupled plasma mass spectroscopy experimental determination of [Au0] concentration correlated with A400. While the universality of the derived equation is demonstrated by an unprecedented set of gold nanoparticle sizes, shapes, morphologies, and particle clusters, its accuracy can be limited by 20–30%. This uncertainty results from the light scattering contribution that violates the proportionality between the extinction cross section and the particle or cluster volume. However, for a particular colloidal system, the application of the derived relation can be useful in monitoring reduction or aggregation processes.