The development of anion-sensing systems is of considerable current interest because of biological, environmental and pharmaceutical concerns. Anions play important roles in biological systems and medicine, pollutant anions are responsible for environmental problems such as the eutrophication of rivers, and the food industry constantly verifies the tolerable level of anions in our food. Can voltammetric measurements be used to selectively detect specific anions in solution? To answer this we are examining the redox properties of known ion-binding Re complexes via cyclic voltammetry in the presence of different anions. If the anions can be detected by electrochemical means, we will attempt to produce ion-selective electrodes by adhering the Re complexes to polymers affixed to the electrode surface. The potential benefit is greater sensitivity, selectivity, and/or stability of the sensor.
Challenges related to developing ion sensing systems: Charge/size ratio pH sensitivity Size/geometry Current (Field) Techniques Colorimetric, fast, simple No complex equipment Drawbacks Metal not reusable (high formation constants) Color & turbidity dependent
Ion Selective Redox Chemistry: Polymer Modified Rhenium Complex Electrodes
Nigel Rambhujun1, Susan Young1, William Vining2 1Hartwick College, Oneonta, NY 2State University of New York at Oneonta, NY
Techniques Cyclic voltammetry Controlled potential (bulk)
electrolysis Spectroelectrochemistry Polymer modification of
Transition metal complex ions can effectively sense anions due to the combined properties of the metal-anion system: 1. Metals are usually positively charged and effectively bind anions
through both electrostatic and covalent interactions. 2. The color of the complex ion can vary with the nature of the
ligands. 3. Metal complexes exist in a wide variety of geometries, which can
accommodate anions of different shapes. 4. The molecular structure is tunable and can be chemically
modified to exhibit greater selectivity in binding a specific anion. 5. Complexes can undergo redox reactions due to multiple
accessible oxidation states, giving them measurable electrochemical properties.
Compound 2: rhenium(I) diimine tricarbonyl complex
Metal-ligand reaction Naked eye change
Spectroelectrochemistry Cell (Pine) PF6
Previous research1 has studied the anion recognition properties of dinuclear rhenium(I) diimine tricarbonyl complexes by luminescence and UV-Vis spectroscopic methods. In this study, using a mononuclear rhenium(I) complex, we hope to translate the spectroscopic changes observed to electrochemical changes that can be detected using cyclic voltammetry.
Department of Chemistry, Hartwick College & Department of Chemistry, State University of New York at Oneonta for providing the funding for my project
Dr. Maurice Odago, SUNY Oneonta, for providing the rhenium complex ions and free (uncomplexed) ligands, and for helpful advice.
1. M.O. Odago, A.E. Hoffman, R.L. Carpenter, D. Chi Tak Tse, S.S. Sun, A. J. Lees, Inorg. Chim. Acta. 374 (2011) 558
2. P.T. Kissinger, W.R. Heineman, Journ. Chem. Ed., 60:9 (1983) 702
3. A. Ramdass, V. Sathish, M. Ve;ayudham, P. Thanasekaran, K.L Lu, S. Rajagopal, Inorg. Chem. Comm. 35 (2013) 186
Identify the nature of the different reduc t i ve and ox ida t i ve waves observed: compound 2 showed four irreversible oxidative waves and four irreversible reductive waves. At least one of each appears to be ligand-based.
Produce ion selective electrodes by
adhering 2 or other similar compounds to polymers affixed to the electrode surface.
The color change observed with cyanide results from an interaction with the ligand in 2.
Cyanide binding to the complex was observed electrochemically: the cyanide ion passivates oxidation of 2.
Comparison with the acetate ion shows that the ion binding is not simply an acid-base reaction with 1.
The chemical changes following bulk reduction of 2 in the presence of cyanide appears to be a one-step process, whereas that fol lowing reduction of 2 in the absence of cyanide appears to be at least a two-step process.
Isosbestic point at 410nm
Anions Tested as N(t-Bu)4+ salts
Re compound with CN-
Free ligand with CN-
Re compound with CN-
Re compound with CH3COO-
Fig. 1: CV of 2, showing four irreversible oxidative waves and four irreversible reductive waves. (At least one of each appears to be ligand-based.)
Fig. 2: CV of 2 in the presence of ClO4 (The scan is very similar to the CV of 2.)
Fig. 3: CV of 2 in the presence of CN. (Cyanide appears to passivate the oxidation of the Re compound.)
Fig. 4: UV-Vis spectra of 1 and 2 in the presence of CN. (The free ligand in the presence of cyanide has the same band, ~450 nm, as the Re complex in the presence of cyanide.)
Fig. 5: UV-Vis spectra of 2 in the presence of CN and CH3COO. (The difference suggests that CN is interacting with 2 and not simply acting as a weak base.)
Fig. 6: UV-Vis spectra of a controlled potential electrolysis of 2 at 600 mV. (The process appears to occur in at least two steps.)
Fig. 7: UV-Vis spectra of a controlled potential electrolysis of 2 at 600 mV in the presence of cyanide. (This appears to be a one-step process.)
Compound 1: free ligand