Abstract: A colorimetric assay of lanthanide or actinide (f-block elements) samples and a sensing construct of functionalized gold nanoparticles are provided. The construct is a gold nanoparticle with hydroxypyridinone ligands on the surface. The colorimetric assay can be performed in two different ways: (1) Gold nanoparticles functionalized with hydroxypyridinone ligands are synthesized, and then added to the sample solution. The presence of f-block elements triggers the gold nanoparticle aggregation, which changes the solution color. (2) Exploiting the competition between Au3+ and cations with higher binding affinities, where the f-block elements quench the nanoparticle growth and affect the color of solution. In both methods, the change in color is used as a sensing principle to quantify the f-block element concentrations in aqueous solutions.