Heavy metal ions detection in solution, in sol-gel and with grafted porphyrin monolayers

The detection of heavy metal ions - Hg2+, Pb2+, Cd2+ - by a free base porph yrin, 5,10,15,20-tetra(4-N-methylpyridyl)porphyrin (TMPyP), was studied. Th e porphyrin was either adsorbed in sol-gel matrix (TMOS) or grafted on glas s surfaces. Detection limit was 10(-7) mol/l for mercury and cadmium, and 1 0(-5) mol/l for lead. Apparent association constants were calculated at pH = 7: pK (Cd2+) = -5.2; pK (Pb2+) = -4; and at pH = 4 pK (Hg2+) = -5.7. Both the shift of the Soret band and the change in fluorescence lifetime enable d the identification of the metal ions. In sol-gel, detection limits were found to be 10(-3) mol/l but 90% of the m etal salts were trapped by the matrix. In sol-gel we have shown that the io ns were pumped by the matrix at a concentration of 1.5 x 10(-5) mol/ml of s ol-gel. The sol-gel limits both the reaction time and the detection thresho ld. No such effect is observed for grafted monolayers. A reaction time of 3 0 min and a detection limit of 10(-6) mol/l were measured for Hg2+ salts. ( C) 1999 Elsevier Science S.A. All rights reserved.