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 fluorescent monomer capable of Cu2+-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 Cu2+-sensing microgels were synthesized via emulsion polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N'-methylenebis(acrylamide) (BIS), ail anionic surfactant, and PhenUMA (4) at around neutral pH and 70 degrees C. At 20 degrees C, PhenUMA-labeled microgels in their swollen state can selectively bind Cu2+ over other metal 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 similar to 125 nM. When heated above 32 degrees 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 it microgel concentration of 0.083 g/L, detection limits of Cu2+ ions can be drastically improved from similar to 28 nM at 20 degrees C to similar to 8 nM at 40 degrees C. A plausible mechanism for the thermo-induced enhancement of Cu2+ detection sensitivity has been proposed. To the best of our knowledge, this proof-of-concept work represents the first example of responsive microgel-based metal ion chemosensor with then-no-tunable detection sensitivity, which simultaneously combining advantageous properties of small molecule sensing moieties and stimuli-responsive soft matter entities.