Concentration and pH-modulated dual fluorescence in self-assembled nanoparticles of phototautomerizable biopolymeric amphiphile

A dual-emissive biopolymeric amphiphile (GC-HBO) has been prepared by densely conjugating hydrophilic glycol chitosan (GC) with a phototautomerizable hydrophobic dye (a derivative of 2-(2′-hydroxyphenyl)benzoxazole, HBO) showing excited-state intramolecular proton transfer (ESIPT). Ratiometric modulation of phototautomeric (enol-keto) dual emission, governed by the chromophoric aggregation state, was studied with a low-molecular-weight model compound (GA-HBO) and was well correlated with the nanoparticle-forming behavior of GC-HBO via amphiphilic self-assembly in water. It has been found that the promoted chromophoric aggregation at higher concentration or at basic-to-neutral pH assists the nanostructure formation to generate predominant keto emission while the enol emission is intensified at lower concentration or with decreasing pH by disintegration of self-assembled structure. The assembly-related advantageous potential of GC-HBO for probing self-concentration and surrounding pH has been demonstrated with the ratiometric determination of critical particulation concentration (0.04 kg m^-3) and near-physiological pH sensitivity (pK_a ∼ 5.1).