Near-Infrared Heptamethine Cyanine Dyes: A New Tracer for Solid-Phase Immunoassays

Near-infrared (near-IR) fluorescence has been used to develop a solid-phase immunoassay that detects trace amounts of human immunoglobulin (HuIgG). Various concentrations of HuIgG bound to a nitrocellulose surface were determined from the fluorescence generated by near-IR labeled goat anti-human antibody (GAHG) bound to the HuIgG. The GAHG was labeled with a heptamethine cyanine fluorophore that has spectral properties in the near-IR region (above 780 nm). These fluorophores are versatile because they can be modified for several bioanalytical applications. Fluorescence was detected with a near-IR fluorescence instrument previously developed in the laboratory. Two cyanine fluorophore labels were evaluated for the ability to selectively bind to GAHG on a nitrocellulose matrix with a minimal amount of background interference. After the most appropriate near-IR fluorophore was selected, the labeling of GAHG was optimized under aqueous conditions. The most effective GAHG–dye conjugates were used to develop an immunoassay to detect various concentrations of HuIgG. The results are presented, here. Solutions of HuIgG with concentrations as low as 10−10 molar have been detected with a minimum of interference.