Fluorescent water-soluble responsive polymers site-specifically labeled with FRET dyes possessing pH- and thermo-modulated multicolor fluorescence emissions as dual ratiometric probes

Abstract

We reported on the synthesis of well-defined thermoresponsive polymers labeled with fluorescence resonance energy transfer (FRET) pairs at chain middle and terminals, which can act as single chain-based dual ratiometric fluorescent probes for pH and temperature under extremely dilute conditions. Starting from difunctional initiator containing a 7-nitro-2,1,3-benzoxadiazole (NBD) moiety, the atom transfer radical polymerization (ATRP) of oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA) and di(ethylene glycol) monomethyl ether methacrylate (DEGMA), and the subsequent terminal group functionalization with Rhodamine B (RhB)-ethylenediamine derivative afforded thermoresponsive NBD-P(OEGMA-co-DEGMA)-RhB₂, which were labeled with FRET donor (NBD) and acceptor moieties (RhB) at the chain middle and terminals. The fluorescence emission of terminal RhB functionalities is highly pH-dependent, i.e, non-fluorescent in neutral or alkaline media (spirolactam form) and highly fluorescent in acidic media (ring-opened acyclic form), thus the off/on switching of FRET process can be facilely modulated by solution pH. Moreover, at acidic pH and highly dilute conditions, the thermo-induced chain collapse and extension of NBD-P(OEGMA-co-DEGMA)-RhB₂ can effectively modulate the spatial distance between FRET donor and acceptor moieties, leading to prominent changes in FRET efficiencies. The site-specific incorporation of one FRET donor and two pH-switchable acceptors at the chain middle and terminals of thermoresponsive polymers allows for the effective off/on switching and the modulation of efficiency of FRET processes by dually playing with solution pH and temperatures. This work represents the first report of single thermoresponsive polymer chains acting as dual ratiometric fluorescent probes under highly dilute conditions.