Enhancing detection sensitivity of responsive microgel-based Cu(II) chemosensors via thermo-induced volume phase transitions

We report on the fabrication of responsive microgel-based Cu²⁺ chemosensors possessing tunable detection sensitivity via thermo-induced microgel collapse/swelling. A novel phenanthroline-containing fluorescentmonomer capable ofCu²⁺-binding and fluorescence sensing, PhenUMA (4), was synthesized at first by reacting 5-amino-1,10-phenanthroline (2) with 2-(3-isocyanato-propionyloxy)ethyl methacrylate (3). Near-monodisperse Cu ²⁺ -sensing microgels were synthesized via emulsion polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N0-methylenebis(acrylamide) (BIS), an anionic surfactant, and PhenUMA(4) at around neutral pHand 70 °C. At 20 °C, PhenUMA-labeled microgels in their swollen state can selectively bindCu²⁺ over othermetal ions (Al³⁺ ,Mg²⁺, Zn²⁺, Fe³⁺,Mn²⁺, Ni²⁺,Ag ⁺,Cd²⁺,Hg²⁺, and Pb²⁺ ), leading to prominent quenching of fluorescence emission intensity. At a microgel concentration of 0.25 g/L, the Cu²⁺ detection limit can be down to ̃125 nM. When heated above 32 °C, fluorescence intensity of PhenUMA-labeled microgels in the absence of Cu²⁺ exhibits an approximately 33% increase due to their volume phase transition, which is reasonable considering that fluorescent PhenUMA moieties are now located in a nonpolar environment. Furthermore, Cu²⁺ detection sensitivity of PhenUMA-labeled microgels can be dramatically enhanced via thermo-induced microgel collapse at elevated temperatures. At a microgel concentration of 0.083 g/L, detection limits of Cu²⁺ ions can be drastically improved from̃28nMat 20 °C tõ8nMat 40 °C.Aplausiblemechanism the thermo-induced enhancement of Cu²⁺ detection sensitivity has been proposed. To the best of our owledge, this proof-of-concept work represents the first example of responsive microgel-based metal ion chemosensor with thermo-tunable detection sensitivity, which simultaneously combining advantageous properties of small molecule sensing moieties and stimuli-responsive soft matter entities.