Thermosensitive Fluorescent Microgels for Selective and Sensitive Detection of Fe3+ and Mn2+ in Aqueous Solutions

Thermosensitive fluorescent microgels (HMQC-MGs) were developed as selective and sensitive sensing systems for the detection of trace Fe3+ and Mn2+ ions in aqueous solutions. HMQC-MGs, which were synthesized via in situ quaternization reaction and surfactant free emulsion copolymerization (SFEP) of N-isopropylacrylamide (NIPAm) and 1-vinylimidazole (VIM) in the presence of 1,6-dibromhexane and 3-hydroxy-2-methyl-4-quinolinecarboxylicacid (HMQC), were fluorescent and thermosensitive with a hydrodynamic radius of 153 ± 2 nm at 25 °C in aqueous solutions. Interestingly, HMQC-MGs exhibited different affinities and distinct fluorescent patterns toward various metal ions in aqueous solutions with different pH values (2, 7, and 12). Selective and fast fluorescent quenching of HMQC-MGs was observed in the presence of Fe3+ and Mn2+ ions at pH 2 and pH 12, respectively. The detection limits (DLs) of Fe3+ and Mn2+ ions were determined to be about 60 and 247 nM, respectively, which are far lower than the safety limits in drinking water regulated by the U.S. Environmental Protection Agency (EPA). The mechanisms of fluorescence quenching after the addition of Fe3+ and Mn2+ ions were attributed to the combined dynamic/static quenching and inner filter effect, respectively. HMQC-MG fluorescent microgels could be potentially applied as sensing systems for the detection of trace Fe3+ and Mn2+ ions in real water samples.