A β-cyclodextrin-containing polymeric salicylidene Schiff base: synthesis, zinc ion coordination and fluorescence resonance energy transfer with protein

Abstract

Living/controlled polymerization of a salicylaldehyde-functionalized vinyl monomer, 2-hydrox-5-vinylbenzaldehyde (HVB), was achieved by reversible addition–fragmentation chain transfer (RAFT) polymerization with S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as the RAFT agent in THF at 65 °C. The resulting well-defined polymer with pendant salicylaldehyde groups could react directly with mono-6-deoxy-6-aminoethyl-β-CD to yield a new β-CD-containing polymeric salicylidene Schiff base PHVB-graft-β-CD with precise structure and high solubility. PHVB-graft-β-CD was then coordinated with zinc ions to give the luminescent PHVB-graft-β-CD/Zn²⁺ complex which displayed intense blue fluorescence with a maximum emission peak around 452 nm in DMF. The obtained PHVB-graft-β-CD/Zn²⁺ tended to self-aggregate in aqueous media because of the hydrophobic nature of PHVB-graft-β-CD backbone, and therefore its fluorescence emission in water was weak as a result of the aggregation-induced fluorescence quenching. An obvious decrease of the aggregate size from 290 nm to 40 nm in aqueous media was observed after adding 1.0 equiv. (relative to β-CD moieties) of the guest molecule, sodium adamantanecarboxylate (Ad-COONa), and concomitantly a remarkable fluorescence enhancement (4.5-fold) was obtained due to a better water-dispersibility of PHVB-graft-β-CD/Zn²⁺. The incorporation of carboxylic groups on the PHVB-graft-β-CD/Zn²⁺ complex via the inclusion with Ad-COONa also endowed the resulting fluorescent nanoparticles with a further protein-binding function. Upon gradual addition of the fluorescent nanoparticle aqueous dispersion to the BSA buffer solution, the fluorescence intensity at ∼349 nm corresponding to BSA decreased, while the fluorescence intensity of the nanoparticle at ∼429 nm increased through an isoemissive point at 388 nm, indicating the occurrence of efficient fluorescence resonance energy transfer (FRET) between the fluorescent nanoparticles and protein. This novel protein sensing capability would give the fluorescent nanoparticles great potential in biotechnology.