Designing Nanostructured Titanium Dioxide-Based Potentiometric Sensors for the Determination of Chromium (III) and Iron (III) Ions

The objective of the first part of this study was to introduce a novel solid-state Cr(III)-selective potentiometric sensor based on Schiff base glyoxal bis(2-hydroxyanil) (GBHA) ionophores adsorbed onto nanostructured TiO2 electrodes, with the aim of eliminating the drawbacks associated with the conventional polymer based Cr(III)-selective membrane potentiometric sensors. The assembled sensor showed the best response characteristics with Nernstian behavior for Cr3+ (Nernstian slope of 19.45 0.44 mV per decade of Cr3+ concentration) over a wide working concentration range 1.000 107 to 1.000 102 M, and a low detection limit of 3.000 108 M. Also this sensor displayed an improved selectivity towards Cr3+ with respect to all the other tested ions. In addition the objective of the second part of this work was to fabricate novel solid-state Fe(III)-selective potentiometric sensors based on desferal ionophores physisorbed and chemisorbed onto the surface of nanostructured TiO2 electrodes. For the sensors designed based on desferal-chemisorbed onto TiO2 electrodes, carboxyl-terminated alkyl phosphonic acids with different alkyl chain lengths (short chain (3-C), medium chain (6-C), and long chain (11-C)) were used as linkers to anchor to the surfaces of nanostructured TiO2 electrodes. The results of the designed sensors, based on ionophores physisorbed and chemisorbed on TiO2 electrodes, were then compared with each other and with some of the best ferric-selective sensors reported in other studies. It was concluded that chemically anchoring the desferal ligand onto the surface of functionalized TiO2 electrodes using the SAMs of medium alkyl chain length PAs (6-C), gave the best performance. In addition, the fabricated sensor based on desferal-chemisorbed 6-C PA-modified TiO2 electrode showed the best response characteristics with Nernstian behavior for Fe3+ (Nernstian slope of 19.44 0.46 mV per decade of Fe3+ concentration) over a wide working concentration range 1.000 107 to 4.500 101 M, and a low detection limit of 2.820 109 M. The fabricated Cr(III)-selective and Fe(III)-selective sensors showed superior behavior, compared to the previously reported sensors, which can be ascribed to the higher stability of the solid substrates. These sensors could successfully eliminate the weaknesses related to conventional membrane sensors.