Biofunctionalized indigo-nanoparticles as biolabels for the generation of precipitated visible signal in immunodipsticks

A novel class of organic nanoparticles as biolabels that can generate an instant visible signal was applied to immunodipsticks. A new principle for signal generation based on hydrolysis of colourless signal precursor molecules to produce coloured signal molecules followed by signal precipitation and localization was demonstrated. The nanoparticle biolabels were applied to sandwich immunoassays for the detection of mouse immunoglobulin G (M IgG). In the presence of M IgG, a nanoparticle-immunocomplex was formed and bound on the test zone immobilized with goat anti M IgG (Gt α M IgG). A blue line was developed on the test zone upon the addition of a signal developing reagent. An optical signal could be simply assessed using naked eyes or quantified using a reading device. The lowest visible signal that could be observed using naked eyes was found to be 1.25 μg L−1 M IgG. The nanoparticle biolabel also showed a better sensitivity (signal-to-noise ratio) compared with the conventional colloidal gold biolabel. This novel class of organic nanoparticles offers an alternative biolabel system for the development of point-of-care immunodipsticks.