A signal generation mechanism that enables a few molecules of disease biomarkers to be selectively detected with the naked eye is reported in a paper published online this week in Nature Nanotechnology. Such a biosensor could potentially be very useful in diagnosing diseases in resource-constrained countries.
A commonly used analytical tool for medical diagnostics is a conventional sandwich enzyme-linked immunosorbent assay (ELISA). In this assay, a target molecule to be detected binds to a capture antibody that is anchored on a substrate. The bound target molecule is subsequently recognized and bound by a primary antibody to form a 'sandwich'. Detection is performed by an enzyme-linked secondary antibody that binds to the primary antibody. The enzyme converts a substrate into a coloured molecule, the intensity of which tells us the concentration of the target molecule. Although it is possible to differentiate coloured solutions from non-coloured ones with the naked eye in a conventional ELISA, this can only be done with confidence at high concentrations of the target molecule.
To accomplish this at ultralow concentrations, Molly Stevens and Roberto de la Rica used the enzyme on the secondary antibody to control the growth of gold nanoparticles in the presence of hydrogen peroxide. In the absence of target molecules, reduction of gold ions by peroxide occurs at a fast rate and non-aggregated gold nanoparticles are formed to give a red solution. Conversely, when target molecules are present, the reduction is slower, resulting in aggregated nanoparticles that give a blue solution. Using this method, the authors were able to detect prostate-specific antigen and the HIV-1 antigen, p24, in whole human serum at an ultralow concentration of 1 attogram per ml, which were undetectable by a more commonly used test. However, Stevens and de la Rica caution that the limitation to the method is that it is inherently not suitable for quantifying concentrations of the target molecules.