Theoretical and experimental studies of metallated phenanthroline derivatives as carriers for the optimization of the nitrate sensor Original Research Article

Experimental and theoretical investigations were carried out on a series of metallated phenanthroline derivatives in order to elucidate the role of both the metal center and the ligand employed, to fully optimize the analytical characteristics of the nitrate ISE. The organic ligands of the ionophores used are based on the phenanthroline moiety, metallated with metals in the 2+ oxidation state. A series of sensors were constructed based on these ionophores in combination with different plasticizers and ionic additives. The potentiometric data combined with the information obtained from ab initio theoretical calculations of the electron density and the electronegativity of the metals were used to optimize the overall analytical characteristics of the sensor towards the anion of nitrate. It is concluded that there is a direct coordination of the metal center with the nitrate and other basic anions, since both the electronegativity of the transition metal and the acidity of the phenanthroline derivative affect both the binding constant and the selectivity of the ionophore towards the nitrate anion. Finally, Ni(II) 5-phenyl-1,10-phenanthroline dissolved in the plasticizer tris(2-ethylhexyl) phosphate (TEHP) and adsorbed onto a porous carbon transducer produces a nitrate sensor with very good selectivity and exceptional signal stability.