A Europium Chelate for Quantitative Point-of-Care Immunoassays Using Direct Surface Measurement

New labels and assay techniques are needed to improve the sensitivity and quantitativeness of point-of-care immunotesting while sustaining the rapidity and ease of use of the assays. We synthesized a novel, intrinsically fluorescent nonadentate europium chelate with two chromophores and hydrophilic α-galactose side groups. The chelate is highly fluorescent, soluble in water, and provides effective shielding of Eu from water. The performance of the nonadentate chelate was compared with a heptadentate chelate in a dry reagent immunoassay for human chorionic gonadotropin (hCG). After 15-min incubation and washing, time-resolved fluorescence was measured directly from a wet or dried well surface. Contrary to the heptadentate label, the effect of aqueous quenching on the nonadentate label was found to be insignificant, with calculated analytical detection limits (background + 3 SD) of 0.9 and 0.7 IU/L hCG for wet and dry measurements, respectively, and a linear range up to 5000 IU/L. The CVs for the new label ...