Synthesis and Structural Optimization of Iridium(III) Solvent Complex for Electrochemiluminescence Labeling of Histidine-Rich Protein and Immunoassay Applications for CRP Detection

[Image: see text] The reaction between an iridium(III) solvent complex and the histidine site of biomolecules as one kind of novel bioconjugation approaches has received much attention during the past few years. To extend this novel bioconjugation approach into electrochemiluminescence (ECL) immunoassay and optimize the performances, three iridium(III) solvent complexes with different C(∧)N bidentate main ligands have been designed and synthesized in this work. Bovine serum albumin (BSA) as the standard histidine-rich protein is initially employed to evaluate the labeling performances by comparing the ECL intensity of the same amount of BSA labeled by different iridium(III) solvent complexes. Importantly, a magnetic beads-based sandwich immunoassay platform using Ir-dmpq (iridium(III) acetonitrile complex with 2-(3,5-dimethylphenyl)quinoline as the main ligand) as a structurally optimized labeling agent has been successfully constructed to detect C-reactive protein (CRP, an important biomarker of systemic inflammation in clinic), and the limit of detection based on this novel labeling agent could reach below 1 ng/mL, which may further pave the way for applications of the iridium(III) solvent complex in histidine-rich protein ECL labeling beyond fluorescence labeling.