Colorimetric and visual detection of tyrosine based on its copper-catalyzed enzymatic oxidation to dopamine and subsequent reductive formation of colored gold nanoparticles using copper ions

We describe a highly sensitive and selective colorimetric test for L-tyrosine (Tyr). It is based on the tyrosinase-catalyzed oxidation of Tyr to dopamine, and its subsequent reaction with tetrachloroaurate to form gold particles whose purple color can be detected even with bare eyes. We also show that the enzymatic oxidation of Tyr proceeds faster in the presence of Cu(II) ions. A 2:1 ratio of Tyr to Cu(II) was found to give the best results. Tryptophan and threonine do no interfere in this assay. The spectrophotometric detection limit for Tyr is 8 × 10−7 mol L−1. Compared to others, this assay is simple, rapid, and label-free but yet enables visual read-out.