Recent developments in lanthanide chelates as luminescent labels for biomedical analyses

Abstract Recent advances in the synthesis and luminescence properties of lanthanide chelate labels for biomedical applications are summarized, especially with the aim to design better chelating ligands and lanthanide labels to be useful for real applications in biomedical and diagnostic fields. For such a purpose, emphasis is often made on designing highly luminescent lanthanide chelates. However, for biomedical applications, many more factors are required such as water solubility, stability in various buffers, no interference to antigen-antibody binding in immunoassay, no non-specific binding to cell surface and components within cells for imaging, and many more. In this chapter, lanthanide labels are classified according to the ligand structural motif, i.e., β-diketonates, pyridine derivatives, azacrowns, and cryptands. Each ligand has specific properties due to its intrinsic structure, linkers to labeling groups, and labeling groups. In addition to usual UV excitation, importance of visible light excitation is described for future labels. Modification of ligands induces unexpected influence in microscopic imaging. The effect of ligand structure on the selectivity of the label staining in various components of cells is discussed for several examples. Different labels sometimes stain different components in cells. It is now well recognized that time-resolved immunoassays using lanthanide chelate labels have remarkably improved detection limits for many target proteins in serum, plasma, and urine. The method has been also applied to assays of small biomolecules like hormones, food additives, drugs and opioids. Recently lanthanide chelate labels in time-resolved immunoassays have been combined with lateral flow immunoassays, and compact time-resolved lateral flow immunoassay devices with remarkably high sensitivity have been devised that are suitable for point of care use.