Figure 1: (A) The conventional sandwich ELISA involves the specific binding of target molecules to the capture antibody-bound microtiter plate. This is followed by the detection of bound target molecules by primary antibody followed by the binding of enzyme-bound secondary antibody. The enzyme substrate reaction leads to the formation of colored product. On the other hand, in plasmonic ELISA, the biocatalytic cycle of the enzyme generates colored nanoparticle solutions with characteristic tonality (S, substrate; P, product; NP, nanoparticle). (B) Generation of colored solutions for naked-eye detection. (a) In the presence of hydrogen peroxide, gold ions are reduced. (b) High concentration of hydrogen peroxide favours the formation of non-aggregated, spherical nanoparticles that give rise to a red solution. (c) When the concentration of hydrogen peroxide decreases, for example due to the biocatalytic action of the enzyme catalase, aggregates of nanoparticles are formed and this turns the solution blue. Reproduced with permission from Macmillan Publishers Limited [3].