4-(2-Pyridylazo)-resorcinol Functionalized Thermosensitive Ionic Microgels for Optical Detection of Heavy Metal Ions at Nanomolar Level

4-(2-Pyridylazo)-resorcinol (PAR) functionalized thermo-sensitive ionic microgels (PAR-MG) were synthesized by a one-pot quaternization method. The PAR-MG microgels were spherical in shape with radius of ca. 166.0 nm and narrow size distribution and exhibited thermo-sensitivity in aqueous solution. The PAR-MG microgels could optically detect trace heavy metal ions, such as Cu2+, Mn2+, Pb2+, Zn2+, and Ni2+, in aqueous solutions with high selectivity and sensitivity. The PARMG microgel suspensions exhibited characteristic color with the presence of various trace heavy metal ions, which could be visually distinguished by naked eyes. The limit of colorimetric detection (D-L) was determined to be 38 nM for Cu2+ at pH 3, 12 nM for Cu2+ at pH 7, and 14, 79, 20, and 21 nM for Mn2+, Pb2+, Zn2+, and Ni2+, respectively, at pH 11, which was lower than (or close to) the United States Environmental Protection Agency standard for the safety limit of these heavy metal ions in drinking water. The mechanism of detection was attributed to the chelation between the nitrogen atoms and o-hydroxyl groups the microgels and heavy metal ions. of PAR within