Multiplexible Wash-Free Immunoassay Using Colloidal Assemblies of Magnetic and Photoluminescent Nanoparticles

Colloidal assemblies of nanoparticles possess both the intrinsic and collective properties of their constituent nanoparticles, which are useful in applications where ordinary nanoparticles are not well suited. Here, we report an immunoassay technique based on colloidal nanoparticle assemblies made of iron oxide nanoparticles (magnetic substrate) and manganese-doped zinc sulfide (ZnS:Mn) nanoparticles (photoluminescent substrate), both of which are functionalized with antibodies to capture target proteins in a sandwich assay format. After magnetic isolation of the iron oxide nanoparticle assemblies and their bound ZnS:Mn nanoparticle assemblies (MZSNAs), photoluminescence of the remaining MZSNAs is measured for the protein quantification, eliminating the need for washing steps and signal amplification. Using human C-reactive protein as a model biomarker, we achieve a detection limit of as low as 0.7 pg/mL, which is more than 1 order of magnitude lower than that of enzyme-linked immunosorbent assay (9.1 pg/mL) performed using the same pair of antibodies, while using only one-tenth of the antibodies. We also confirm the potential for multiplex detection by using two different types of photoluminescent colloidal nanoparticle assemblies simultaneously.