Chelating behavior between metal ions and EDTA in sol-gel matrix

Ethylenediamine tetraacetic acid (EDTA) was encapsulated in a sol-gel-derived optically transparent glass. The complexation of metal cations, Cu(II), Ni(II), Fe(III), Cr(III), Co(ll), Mn(II), Pd(II), and Ir(IV), with EDTA in solutions and in gels was evidenced by the formation of individual colors. A comparison of the absorption spectra of M(n+)/EDTA in solution, M(n+)/EDTA in gel, and M(n+)/EDTA complex in gel was performed The results show that (i) all six chelating sites in EDTA remain active in a sol-gel matrix as they bind readily with Cu2+, Ni2+, Fe3+, Pd2+, and Ir4+, (ii) EDTA is rigidly oriented in a sol-gel matrix that inhibits its heat- or catalyst-assisted complexation with Cr3+, Co2+, and Mn2+, (iii) no absorption spectral shift for the d-d metal transitions was observed compared to those in solution, yet a 10-30 nm shift for the MLCT transitions of Cu2+, Ni2+, Fe3+, Pd2+, Ir4+ was detected, and (iv) the level of sensitivity in detecting metal cations using EDTA in gel is at least an order of magnitude higher than that in solution. The response time of M(n+)/EDTA complexation in gel is quite short, ranging from 1/30 to 15 min. This response time and the mode of sensing device guarantee that no leaching of EDTA from the sol-gel matrix is possible.