Design, synthesis and photophysical studies of alkynylplatinum(II) complexes with tridentate N-donor ligands : from supramolecular self-assembly to luminescent probes for molecules of biological interest

Abstract

Various series of cationic and anionic alkynylplatinum(II) polypyridine complexes with water-solubilizing pendants were designed and synthesised. Their luminescence, spectroscopic and self-assembly properties were investigated. By manipulation and modulation of the supramolecular assembly of platinum(II) complexes, polymer–metal complex ensembles and Förster resonance energy transfer (FRET) from conjugated polyelectrolytes to platinum(II) complexes, sensitive and selective label-free detection of biologically important analytes was presented. A new class of anionic water-soluble alkynylplatinum(II) 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) complexes was synthesised to illustrate protamine-induced supramolecular self-assembly with the aid of electrostatic interaction and non-covalent Pt(II)∙∙∙Pt(II) and π−π stacking interactions. Owing to the strong binding affinity of protamine to heparin, the ensemble were further employed to probe heparin. Other than heparin, this ensemble could also monitor the activity of trypsin through its ability to hydrolyse protamine into fragments, leading to the disassembly of platinum(II) complexes. By modulation of the self-assembly properties of platinum(II) complexes via real-time emission studies, the assay was demonstrated to be a sensitive and selective detection method for trypsin, as well as trypsin inhibitor screening. A novel sensing strategy towards hydrogen peroxide based on metal coordination polyelectrolyte-driven self-assembly of alkynylplatinum(II) bzimpy complex was demonstrated. The cationic in situ-generated Ag(I)–thiocholine coordination polyelectrolytes were shown to induce the supramolecular self-assembly of anionic low-energy red-emissive alkynylplatinum(II) bzimpy complexes via non-covalent Pt(II)···Pt(II), electrostatic and π−π stacking interactions. The presence of hydrogen peroxide was shown to inhibit the formation of coordination polyelectrolytes and the coordination polyelectrolyte-induced self-assembly of platinum(II) complexes. The weakening of Pt(II)···Pt(II), electrostatic and π–π stacking interactions, was supported by UV-vis absorption, emission, and resonance light scattering (RLS) studies. The present assay was also applied to probe glucose indirectly based on the enzymatic reaction of glucose oxidase on the substrate. In addition, the H2O2-responsive boronate pinacol ester moiety was introduced to the NIR-emitting alkynylplatinum(II) terpyridine complex system. Hydrogen peroxide was found to cleave the H2O2-responsive moiety, causing planarization of the complex molecules to promote the supramolecular self-assembly process, giving rise to UV-vis absorption and emission spectral changes. The current study shed light on the development of activity-based sensing by alkynylplatinum(II) complexes. A series of pH-responsive alkynylplatinum(II) bzimpy complexes with charge-reversal properties was synthesised and their interactions with PFP-OSO3–, as well as the FRET behaviours, were studied by UV-vis, emission and RLS. The two-component PFP-OSO3––platinum(II) complex ensemble was employed as a “proof-of-principle” concept for pH sensing by monitoring the ratiometric emission spectral changes. With the aid of judicious molecular design on pH-driven charge-reversal property, the polyelectrolyte-induced self-assembly and the FRET from PFP-OSO3– to the aggregated platinum(II) complexes were modulated. Together with the excellent reversibility and photostability of the ensemble, the extra stability provided by the Pt(II)···Pt(II) and π−π stacking interactions on top of the electrostatic and hydrophobic interactions existing in polymer–complex assemblies led to a selective and sensitive pH sensing assay.