Noble Metal Nanoparticle-Based Multicolor Immunoassays: An Approach toward Visual Quantification of the Analytes with the Naked Eye

Noble metal nanoparticle-based colorimetric sensors have become powerful tools for the detection of different targets with convenient readout. Among the many types of nanomaterials, noble metal nanoparticles exhibit extraordinary optical responses mainly due to their excellent localized surface plasmon resonance (LSPR) properties. The absorption spectrum of the noble metal nanoparticles was mostly in the visible range. This property enables the visual detection of various analytes with the naked eye. Among numerous color change modes, the way that different concentrations of targets represent vivid color changes has been brought to the forefront because the color distinction capability of normal human eyes is usually better than the intensity change capability. We review the state of the art in noble metal nanoparticle-based multicolor colorimetric strategies adopted for visual quantification by the naked eye. These multicolor strategies based on different means of morphology transformation are classified into two categories, namely, the etching of nanoparticles and the growth of nanoparticles. We highlight recent progress on the different means by which biocatalytic reactions mediated LSPR modulation signal generation and their applications in the construction of multicolor immunoassays. We also discuss the current challenges associated with multicolor colorimetric sensors during actual sample detection and propose the future development of next-generation multicolor qualification strategies.