Functional Microgel for Selective and Sensitive Colorimetric Detection of Fe2+ Ions in HEPES Buffer Aqueous Solutions

A functional monomer, namely, 1-hexene-4-(benzal-2,2′:6′,2″-terpyridines)pyridinium bromide (HTP) was synthesized and incorporated into the cross-linking network of microgels. The HTP-functionalized microgels (HTP-MGs) were synthesized by surfactant-free emulsion copolymerization with N-isopropylacrylamide as the main monomer, 1-vinylimidazole as a quaternization cross-linkable comonomer, HTP as the functional comonomer, and 1,6-dibromhexane as the quaternization agent. The obtained HTP-MGs were spherical with a hydrodynamic radius Rh of 205 ± 2 nm and uniform size distribution in HEPES buffer aqueous solutions at 25 °C. The HTP-MGs exhibited a characteristic UV–vis absorption peak at 315 nm and a characteristic fluorescent emission peak at 430 nm, respectively. The HTP-MGs can selectively, sensitively, and optically respond to Fe2+ ions in aqueous solution with a fast response time of 3 s in wide pH (2–10) and temperature (25–50 °C) ranges, resulting in a color change in HTP-MG HEPES buffer aqueous solutions from colorless to purplish red, which can be easily distinguished by naked eyes. A linear concentration range of 0–14 μM and a detection limit (DL) of 82 nM were achieved for detecting Fe2+ ions in HEPES buffer aqueous solutions (pH = 7) at 25 °C. The molar binding ratio of the HTP moiety within HTP-MGs to Fe2+ ions was 2:1, as revealed by Job’s plot. It was found that the coexistence of 15 species of other metal ions except of Cu2+ ions studied here did not affect the detection of Fe2+ ions. Furthermore, a fluorescent emission peak at 371 nm with strongly enhanced fluorescent intensity was observed for HTP-MG HEPES buffer aqueous solutions with the addition of Zn2+ or Cd2+ ions. By combining the optical response to Fe2+ ions and fluorescent response to Zn2+ ions, it was demonstrated that HTP-MGs could also have potential applications in the fields of encryption and anti-counterfeiting.