Iridium Glowstick Assay
Detection of molecular biomarkers via colorimetric and fluorescent signaling is important for a wide range of applications from disease diagnosis to molecular tracking. Small organic dyes have conventionally been used to label proteins and nucleic acids for molecular recognition. While some of these dyes bind biomolecules in vitro by intercalating among the base pairs of nucleic acids, other methods of labeling, such as covalent crosslinking to protein side chains, requires separation and purification of the target protein for specific labeling. An alternative class of signaling probes, based on emissive metal complexes, has come to the forefront in recent years. The light emitting properties of cyclometalated Ir(III) have been studied for various applications, including OLEDs, oxygen sensing, catalysis and cell staining.
Cyclometalated Ir (III) complexes are characterized by thermal and chemical stability, large Stokes shifts, long lifetimes, high quantum efficiency and photostability. Recently, it has been shown that cyclometalated Ir(III) complexes of the form [Ir(C^N)2(solv)2]+ selectively bind histidine and histidine containing peptides. Interestingly, these complexes are non-emissive in their solvento state, but when introduced into a solution of histidine containing biomolecules, the coordinated solvent molecules are displaced. Upon irradiating the histidine bound Ir(III) complex with UV light, the triplet excited state is activated by metal ligand charge transfer (3MLCT) and ligand centered transfer (3LC). A strongly blue emitting phosphorescent signal is then released as the triplet excited state electron relaxes back down to the ground state. Only in the presence of histidine is this response triggered (Figure 1).
With recent advances in the area of low-resource diagnostic tools, disease biomarker detection must be efficient and selective in order to couple to such innovative tools as microfluidic devices, paper based assays and rapid purification devices. Metal-based phosphorescent probes are excellent candidates for these applications due to their selective, stabile and efficient emission properties.
David Wright (website)