Phase switching to enable highly selective activity-based assays.

Traditional activity-based (or reaction-based) detection schemes rely on homogeneous reactions between an analyte and a substrate to provide a signal that is proportional to the concentration of the analyte. Selectivity in these assays is governed primarily by the ability of the desired analyte to react faster than other analytes with the substrate. In this Article, we describe a conceptually different approach toward activity-based detection whereby a soluble analyte is converted intentionally into a heterogeneous catalyst. This catalyst then reacts selectively with a substrate to provide the readout for the assay. This concept is particularly relevant to heavy metal detection, as demonstrated by a rapid and highly selective assay for palladium in which a soluble metal ion is converted in situ into a colloidal catalyst.