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

Abstract

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 HTPfunctionalized 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 R h 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 Fe 2+ 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 (D L ) of 82 nM were achieved for detecting Fe 2+ ions in HEPES buffer aqueous solutions (pH = 7) at 25 °C. The molar binding ratio of the HTP moiety within HTP-MGs to Fe 2+ 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 Cu 2+ ions studied here did not affect the detection of Fe 2+ 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 Zn 2+ or Cd 2+ ions. By combining the optical response to Fe 2+ ions and fluorescent response to Zn 2+ ions, it was demonstrated that HTP-MGs could also have potential applications in the fields of encryption and anti-counterfeiting.

Aqueous microgels with engineered hydrophobic nano-domains

“…24 Common suitable functionalities for an enhanced hydrophobicity of the microgel network include various methacrylates, 25 trifluoromethyl groups, 26 terminal alkyl chains (linear, [27][28][29][30] or branched 31,32 ), oligoethylene glycol (OEG) derivatives, 33 or aromatic heterocycles. 6,34,35 Challenges in the copolymerization of monomers with a significant difference in hydrophobicity can arise from the…”