A new diagnostic test enables visual detection of biomarkers at significantly lower concentrations than other methods. Molly M. Stevens and Roberto de la Rica of Imperial College London have developed “plasmonic ELISAs” (enzyme-linked immunosorbent assays) in which results are determined by the color imparted to the test solution by gold nanoparticles (Nat. Nanotechnol., DOI: 10.1038/nnano.2012.186). Plasmonic ELISAs work like conventional ELISAs, except that the enzyme used to generate the signal, catalase, decreases the concentration of hydrogen peroxide in the solution instead of generating a colored molecule. Above 120 μM of H2O2, gold forms nonaggregating, spherical nanoparticles, resulting in a red solution. Below that concentration, the gold forms ill-defined, aggregating nanoparticles that turn the solution blue. The catalase acts on H2O2 only when the target analyte is present. Thus, any blue solution represents a positive result. Stevens and de la Rica used the plasmonic ELISAs to detect prostate-specific antigen and an HIV-1 antigen at concentrations as low as 1 x 10−18 g/mL in blood.