Hierarchical Self-Assembly of Discrete Organoplatinum(II) Metallacycles with Polysaccharide via Electrostatic Interactions and Their Application for Heparin Detection

In recent past years, investigation of hierarchical self-assembly for constructing artificial functional materials has attracted considerable attention. Discrete metallacydes based on coordination bonds have proven to be valid scaffolds to fabricate various supra-molecular polymers or smart soft matter through hierarchical self-assembly. Here, we present the first example of the hierarchical self-assembly of discrete metallacydes by taking advantage of the positive charges of the organoplatinum(II) metallacyde skeleton through multiple electrostatic interactions. Heparin, a sulfated glycosaminoglycan polymer that has been widely used as an anticoagulant drug, was selected to induce hierarchical self-assembly because of the existence of multiple negative charges. To investigate the hierarchical self-assembly process, an aggregation-induced emission (AIE) active moiety, tetra-phenylethylene (TPE), was introduced onto the metallacyde via coordination-driven self-assembly. Photophysical studies revealed that the addition of heparin to the tris-TPE metallacydes solution resulted in dramatic fluorescence enhancement, which supported the aggregation between metallacycle and heparin driven by multiple electrostatic interactions. Moreover, the entangled pearl-necklace networks were obtained through hierarchical self-assembly as detected by SEM, TEM, and LSCM experiments. In particular, single bead-like chains were observed in the AFM and TEM images, which provided direct, visual evidence for the aggregation of positively charged metallacydes and negatively charged heparin. More interestingly, further optical study demonstrated that this TPE-decorated metallacyde could function as a turn-on fluorescent probe for heparin detection with high sensitivity and selectivity. Thus, this research presents the first example of counter polyanion-induced hierarchical self-assembly of discrete metallacydes and provides a proof-of-principle method for heparin sensing and binding.