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.