Metallo-tetraazaporphyrin based anion sensors: regulation of sensor characteristics through central metal ion coordination

The iron(Ill) and cobalt(III) complexes of 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin, (OBTAP) were synthesized and incorporated into PVC matrix as ionophores to fabricate anion selective membrane electrodes that exhibit selective potentiometric response to azide and nitrite ions, respectively. The membrane of [Fe(OBTAP)] 1 (111) with a composition of 6:190:200 (III:DBP:PVC (w/w), and of [Co(OBTAP)](+) (IV) with a composition of 10: 148:200 (IV:DOP:PVC) (w/w), i.e. 1a and 2b, respectively (where DBP:dibutylphthalate and DOP=dioctylphthalate) gave the best performance. The membrane la showed a slope of 29.2+/-0.2 mV per decade of activity for N-3(-) in the working concentration range of 8.9 x 10(-6) to 1.0 x 10(-1) M. The membrane 2b showed a slope of 30.0+/-0.2 mV per decade of activity for NO2- in the working concentration range of 1. 1 x 10(-5) to 1.0 x 10(-1) M. The membranes worked satisfactorily in the pH range of 4.3-10.5 (1a) and 2.8-6.4 (2b) and had fast response time of 12+/-2 and 13+/-2 s, respectively. Electrodes exhibited a high degree of selectivity for N-3(-) and NO2-, respectively, over several other monovalent and bivalent anions. Only SCN- and S2- (at >1.0 x 10(-4) M) Cause moderate interference for electrode 1a and Cl- and S2- (at >1.0 x 10(-5) M) for electrode 2b. They gave reproducible results with the relative standard deviation in the observed values of potentials (sigma) of 1.96 and 1.80 mV for electrodes 1a and 2b, respectively, from the least-squares fit line. The 90% confidence limit lies within +/-0.2 mV per decade of activity. Reproducible results were obtained over a period of 5 months. Their performance in non-aqueous solvent mixtures having up to 50% (v/v) methanol, ethanol and acetone were evaluated and were found satisfactory. The proposed sensors are superior in terms of detection limit and response time in comparison to the reported ones. (C) 2003 Elsevier B.V. All rights reserved.