Development of an aminocarboxylic acid-modified infrared chemical sensor for selective determination of copper ions in aqueous solutions

An evanescent wave infrared chemical sensor for the sensitive and selective detection of copper ions in aqueous solutions is described. Because copper ions have no vibrational features, a band-shifting technique was utilized to produce the analytical signal. To enhance the sensitivity of the detection process, a three-step procedure was employed to prepare acidified tris(2-aminoethyl)amine (ATAA) phase on an evanescent wave sensing element. This sensing phase has a chemical structure similar to that of ethylenediamine tetraacetic acid (EDTA), a common chelating agent for metal ions. After formation of complex with copper ions, the shifts in the absorption bands of the ATAA phase were used for quantitation. An additional four sensing phases having chemical structures related to that of EDTA were synthesized to compare their performances for detection of copper ions. The synthetic sensing phases are highly stable in water and insensitive to changes in solutions at pH greater than 4. ATAA was the most sensitive of the phases tested, probably because of the accessibility and flexibility of the functional groups in the ATAA phase. To explore these systems in greater detail and to optimize detection, the effects of parameters such as the buffer concentration, the pH of the sample solution, and the matrix effect on response time and linearity of detection were examined. The analytical signals for copper ions were similar – and highly selective – when the pH of the solution was between 5 and 6.5. For a detection time of 5 min, these signals were linear for concentrations up to 200 μM with a detection limit ca. 3 μM.