Highly Selective and Sensitive Detection of Pb²⁺ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin-Functionalized Thermosensitive Ionic Microgels

Tetra­(4-pyridyl)­porphyrin (TPyP)-functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have a hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically respond to trace Pb²⁺ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb³⁺, Gd³⁺, Ce³⁺, La³⁺, Bi³⁺, Ba²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cr³⁺, K⁺, Na⁺, Li⁺, Al³⁺, Cu²⁺, Ag⁺, Cd²⁺, and Fe³⁺) by using UV–visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb²⁺ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb²⁺ in aqueous solution at 50 °C is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher order derivative spectrophotometry and is much lower than the U. S. EPA standard for the safety limit of Pb²⁺ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have a potential application in the detection of Pb²⁺ in real world samples, which give consistent results with those obtained by elemental analysis

Poly(<i>N</i>‑isopropylacrylamide-<i>co</i>-1-vinyl-3-alkylimidazolium bromide) Microgels with Internal Nanophase-Separated Structures

Microgels with internal nanophase-separated structures were fabricated by surfactant-free emulsion copolymerization of <i>N</i>-isopropylacrylamide (NIPAm) and ionic liquid comonomers, namely, 1-vinyl-3-alkylimidazolium bromide (VIM<i>n</i>Br) with various lengths <i>n</i> of long alkyl side chain, in an aqueous solution at 70 °C using <i>N</i>,<i>N</i>′-methylenebisacrylamide as the cross-linker. Combined techniques of transmission electron microscopy, dynamic and static light-scattering, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and polarized optical microscopy were employed to systematically investigate the sizes, morphologies, and properties of the obtained microgels as well as the microstructures and phase transition of nanophases inside the microgels. The obtained P­(NIPAm/VIM<i>n</i>Br) microgels are spherical with narrow size distributions, and the nanophases have a radius of about 8–12 nm and are randomly distributed inside the microgels. The cooperative competition of the hydrophilic quaternary vinylimidazole moieties and hydrophobic long alkyl side chains determines the thermal sensitive behavior of the P­(NIPAm/VIM<i>n</i>Br) microgels. DSC and WAXD results reveal that the nanophases consist of the ordered alkyl side chains with a layered crystalline structure at low temperature, which exhibit a low melting temperature and a broad melting transition. SAXS results further show that the nanophases form a layered liquid crystalline structure at high temperature for the microgel suspensions and freeze-dried microgels