D.A. Kidwell, E. Wei, J.N. Lanese, S. Mulvaney
Naval Research Laboratory,
Keywords: catalysis, nanoparticles, latteral-flow immunoassay
Summary:Lateral-flow immunoassays (LFIAs) are a widely used point-of-care technology. They are most familiar as home-pregnancy tests, but are also employed for testing for a wide variety of diseases, biomarkers, and even small molecules Visually-read, LFIAs typical use colloidal gold or colored latex as the reporter elements for the recognition binding event. While simple to perform and easy to read, the sensitivity of LFIAs is ultimately limited by the extinction coefficients of the reporter elements as enough material must be localized to be visually observed. We have substituted specially-prepared, colloidal palladium for the colorimetric labels and use the palladium nanoparticles as a chemical catalyst to produce an easily-observable, stable, blue dye that localizes at the capture line. We term our test system cLFIA™ for catalytic LFIAs. Importantly, the catalyst is heat stable, functions at room temperature, under physical conditions, and results in up to a 500-fold increase in sensitivity with only five additional minutes of development (Figure 1). Although blue is an optimum color for most biological assays, the dye chemistry is derived from color photography and hair dying products so that different colors can be produced, spanning the visual spectrum (Figure 2). Some versions of the catalyst have on their surfaces functionalities that covalently bind amines such as the lysines on proteins for facile conjugation. More recent results have optimized the preparation of the catalyst and labeling of proteins even further as a one-step process in the presence of the antibody partner. Removal of excess reagents can be accomplished by simple centrifugation. The catalytic chemistry requires that three chemicals react with the palladium nanoparticle label, and we have designed the assay to incorporate all chemical elements onto the LFIA test strip thereby requiring no additional steps to be performed by the user (Figure 3). The preparation and characterization of the catalyst, the basis for the dye chemistry, and their use in a model cLFIA™ will be described.