Ion-/Thermo-Responsive fluorescent perylene-poly(ionic liquid) conjugates: One-pot microwave synthesis, self-aggregation and biological applications

Abstract

This manuscript describes the attachment of alkylvinylimidazolium-based poly(ionic liquid) (PIL) to the imide positions of perylene bisimide (PBI) to introduce stimuli-responsiveness and to make the conjugate soluble in water and water-based mixed solvents with the possibilities of using them as a probe for imaging and diagnostics in biomedical domain. The alkylvinylimidazolium ionic liquid monomers (ILMs) are tailor-made and a one-step microwave-assisted method is employed for synthesizing the PBI-(Cys-PIL)2 conjugates by thiol-mediated radical polymerization of the ILMs using l-cysteine (Cys) with AIBN as initiator and in the presence of perylene-3,4,9,10-tetracarboxylic dianhydride (PDA). The introduction of PIL segment induces amphiphilicityto the conjugate, and it undergoes self-aggregation into vesicles in water and methanol/water mixture. Owing to the presence of PIL segments, the conjugates are responsive to anions forming insoluble spherical aggregates through the multipoint ionic crosslinking between the cationic PIL segments of neighbouring conjugate molecules in water and methanol/water mixture. The turbid color dispersion, containing spherical conjugate’s aggregates, shows upper critical solution temperature (UCST)-type phase transition to transparent solution upon heating. The variation of cloud point of conjugate could be done by changing different parameters such as molecular weight, nature of anions, and the length of the alkyl chain present in the PIL segment of the conjugate. The UV–vis and fluorescence spectra of the conjugate’s aggregates in solution change upon addition of anions. The monitoring of the spectral change upon heating due to disruption of aggregates allows us to determine the cloud point, which exactly matches with that obtain from turbidimetry. The water-soluble fluorescent PBI-(Cys-PIL)2 conjugate having cationic PIL segments shows low cytotoxicity and can be used in binding with ctDNA/plasmid DNA and imaging HeLa cells.