In the Laboratory
Spectroelectrochemical Sensing of Aqueous Iron: An Experiment for Analytical Chemistry
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Tanya Shtoyko,* O. Dean Stuart, and H. Neil Gray Department of Chemistry, University of Texas at Tyler, Tyler, TX 75799; *
[email protected] Selectivity and sensitivity are the two biggest challenges in modern chemical sensing. The combination of two different techniques, spectroscopy and electrochemistry, opens a new door in chemical sensing by improving the selectivity and sensitivity of measurements (1–5). As such, it is of great importance that students on the undergraduate level become aware of the applications of spectroelectrochemistry. A number of important concepts are illustrated within this experiment, including the preparation of thin films on a substrate, electrochemical and optical studies of a complex ion, preparation and use of an analytical standard curve, and exposure to instrumentation for cyclic voltammetry and to UV–vis
spectroscopy. The procedures followed in this experiment, as well as the basic concepts presented, are designed for the typical undergraduate student in analytical chemistry. In the years that we have been using this experiment, we have found that the experiment successfully helps undergraduate students to understand spectroelectrochemistry. The students performed noticeably better (improved scores statistically significant at the 90% confidence level) on the appropriate section of the postlab exam than on the prelab quiz. The application of optically transparent electrodes (OTEs) makes spectroelectrochemistry possible (3–6). A beam of light may be focused on the OTE during the electrochemical process and the transmittance change recorded simultaneously while measuring current. The spectroelectrochemical sensor used in this experiment applies four modes of selectivity as shown in Figure 1. To be detected the analyte must partition into an ion-selective film, be trapped by a selective ligand, be electrochemically oxidizable and reducible at the electrode, and have different molar absorptivities at a specific wavelength for different oxidation states. Experimental
Figure 1. The selectivity diagram for spectroelectrochemical sensing.
Figure 2. Diagram of the interfacial region of the spectroelectrochemical sensor in transmission mode: a sensor platform with (a) a glass layer, (b) a thin layer of ITO, and (c) a cation-selective Nafion film; P0 is irradiance of beam entering cell; P is irradiance of beam emerging from cell.
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The spectroelectrochemical sensor used in this experiment is a simplified version of a sensor that was recently reported (1–3). The sensor platform, illustrated in Figure 2, has three important components: (a) a glass slide and (b) a thin layer of indium tin oxide (ITO), which together make the OTE, and (c) a cation-selective Nafion film (7) coated onto the ITO slide. The Nafion polymer adheres well to the ITO surface and contains negatively charged regions owing to the presence of sulfonated groups. Although Nafion is somewhat expensive, the volume quantity used for this experiment is very small (diluted µL portions at a time). The analyte, Fe2+, from the sample solution preconcentrates into the cation-selective film to enhance the detection limit of the sensor (2). The preconcentrated analyte forms a strongly colored complex with 2,2´-bipyridine (bpy), a colorless ligand that has been previously loaded into the film. The formed octahedral complex, Fe(bpy)32+, (log K = 17.2) (8) absorbs light at 520 nm with ε = 77000 M᎑1 cm᎑1 in 0.1 M NaCl (9), as shown in Figure 3A and is thus detected optically. The sensor platform is inserted at an angle (usually at 90⬚ to the incident light beam) into a 1 cm cuvette, as illustrated in Figure 2. Incident radiation at a specific wavelength (520 nm) passes directly through the film containing Fe(bpy)32+. A regular spectrophotometric cuvette, shown in Figure 2, is used as an electrochemical cell. The sensor platform is used as a working electrode; platinum and silver wires are used as auxiliary and reference electrodes, respectively. The sensor absorbance is due to all absorbing species present in the cation-selective film including Fe(bpy)32+, as shown in Fig-
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ure 3A. To increase the selectivity of the sensor, the potential at the ITO electrode is cycled using cyclic voltammetry shown in Figure 3B. The colored Fe(bpy)32+ oxidizes reversibly to colorless Fe(bpy)33+ (log K = 16.29) (8), as shown in Figure 3C, at a specific positive potential. The change in absorbance is measured concurrently with current while cycling the potential. Although the current is proportional to the concentration of all electroactive species at the chosen potential range, the absorbance change, ∆A, is selectively proportional to the concentration of Fe(bpy)32+ in the film. The absorbance change is also proportional to the reduction–oxidation time and the film thickness. It is also related to concentration of the hydrated Fe2+ in the solution and depends on the partition coefficient. Other metal cations besides iron can also partition into the charge-selective Nafion film and form stable complex ions with bpy. The most severe interference is expected from the metal ions that have similar or larger complex formation constants with bpy than iron, for example, Ni2+, Co2+, Hg2+ (8). If the interfering cations are present at low concentrations, the selectivity of iron sensing is still achieved owing to different electrochemical and optical properties of interfering complex ions (2). If the potentially interfering cations are present in the solution in high concentrations, false negative results for iron can occur owing to a limiting concentration of bpy ligand. The sensitivity of the determination of tris(2,2´bipyridyl)iron(III) is much larger than for the hydrated Fe2+ species because the change in molar absorptivities for the Fe(bpy)32+兾3+ couple (∆ε = 77000 M᎑1 cm᎑1 in 0.1 M NaCl at 520 nm) (9) is much greater than the change in molar absorptivities for hydrated iron species Fe(H2O)62+兾[Fe(OH)(H2O)5]2+ (∆ε ∼ 0 M᎑1 cm᎑1) (10). As shown in Figure 3A, Fe(bpy)32+ absorbs strongly in the visible range (ε = 77000 M᎑1 cm᎑1 at λmax = 520 nm), whereas the oxidized form, Fe(bpy)33+ (Figure 3C), does not absorb light at 520 nm. Owing to the tail of charge-transfer bands in UV (11), the various hydroxo species of iron, Fe(H2O)62+ and [Fe(OH)(H2O)5]2+, absorb slightly in the visible region with ε < 1 M᎑1 cm᎑1. Hazards
Figure 3. (A) Absorbance spectrum of the sensor at the negative potential limit of 0.3 V. (B) Cyclic voltammogram recorded with Nafion-coated ITO electrode previously soaked with bpy. The voltammogram was recorded by cycling the potential at the sensor inserted into the solution of 1.0 x 10᎑4 M Fe2+ in 0.1 M NaCl (ν = 20 mV/s, E/V versus silver wire). (C) Absorbance spectrum of the sensor at the positive potential limit of 1.3 V. The absorbance measurements were performed in a singlepass transmission geometry described in Figure 2.
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Nafion is highly flammable and is a skin irritant. Although the quantity of Nafion used is on the microscale, Nafion vapors may cause drowsiness and dizziness, therefore handling should be performed under a hood or in a well ventilated area. Ferrous ammonium sulfate pentahydrate [Fe(NH4)2(SO4)2⭈6H2O] may cause irritation to the skin, eyes, and respiratory tract. The ligand, 2,2´-dipyridyl (2,2´bipyridine) is toxic if swallowed. Sodium chloride (NaCl) has typical irritation hazards; refrain from ingestion. Discussion The typical student results of the Fe2+ partitioning into the Nafion film loaded previously with bpy and Fe(bpy)32+ complex formation are accomplished electrochemically as illustrated in Figure 4. The sensor platform loaded with bpy is exposed to an aqueous solution of 0.1 mM Fe2+ in 0.1 M NaCl. Absorbance spectra and voltammograms are obtained simultaneously as a function of time as shown in Figure 4A and 4B, respectively.
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In the Laboratory
Figure 4. (A) Cyclic voltammograms taken at ν = 20 mV/s during (1) soaking of the sensor platform in 0.1 M NaCl; (2) uptake of Fe2+ from a solution of 0.1 mM Fe2+ in 0.1 M NaCl by a Nafioncoated ITO slide soaked previously in 5.0 mM bpy for 4 h. (B) Optical modulation (absorbance at 520 nm versus time) during the uptake of Fe2+ by the sensor.
Figure 5. (A) Calibration plot ∆A at 520 nm versus aqueous iron concentration after the Nafion films were equilibrated with 1.0 x 10᎑6 M, 2.5 x 10᎑6 M, 5.0 x 10᎑6 M, 7.5 x 10᎑6 M, 1.0 x 10᎑5 M solutions of Fe(NH4)2(SO4)2 in 0.1 M NaCl. (B) Calibration plot anodic current versus aqueous iron concentration after the Nafion films were equilibrated with the same solutions as in (A) and the formed Fe(bpy)32+ complex ion was oxidized to Fe(bpy)33+ at 5 mV/s.
The increase in peak current and absorbance change at 520 nm as seen in Figure 4A and 4B indicates that Fe2+ partitions into the film, forming Fe(bpy)32+. After some time the peak current and absorbance reach their maximal values because the equilibrium in Fe2+ is established between the film and the aqueous solution. It prevents further Fe(bpy)32+ complex formation within the film. Comparisons of change in absorbance (∆A) and anodic peak currents for analyte-saturated films of several sensors exposed to different solutions of Fe2+ (∆A of complex versus iron concentration in solution and peak currents versus iron concentration in solution) are shown in Figures 5A and 5B, respectively. This is essentially a calibration curve of known iron concentration standards that students will be expected to construct so as to determine an unknown concentration of aqueous iron. Different films were utilized for different concentration measurements, therefore some variability in signal may exist owing to small differences in film thickness (2). The shape of the curve is determined by the partition isotherm for Fe2+ ions into the Nafion coating, the bpy con-
centration in the film, and the thickness of the film (not shown). The calibration curves are linear at low concentrations (Figures 5A and 5B) and deviate negatively at higher iron concentrations (not shown). The negative deviation at higher concentrations can be explained by a limiting concentration of bpy in the film. Assuming that the film thickness is 160 nm, a value that is somewhat lower than film thicknesses reported as having been prepared with more viscous Nafion solution (5%) (2) and knowing that the molar absorptivity is 77000 M᎑1 cm᎑1 (9), the concentration of Fe(bpy)32+ within the film was estimated (0.18 M). The concentration of bpy ligand within the film was estimated (0.55 M) by multiplying the stock concentration of bpy by a preconcentration factor of 110 (9). The comparison of available ligand concentration before exposure to Fe2+ and the available ligand concentration after exposure to Fe2+ indicates that there is not an appreciable excess of ligand within the film. An inability to sense concentrations higher than what is reported is a direct result of this limiting concentration of ligand.
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The sensor performance is sensitive to film thickness. The quantity of analyte partitioning into the film increases with film thickness and leads to expected increases in absorbance. Conversely, thicker films require more time for the analyte to saturate the film and reach the electrode surface during the cyclic voltammetry. This implies a longer oxidation period for all Fe(bpy)32+ trapped in the film to observe the greater change in absorbance. Consequently, depending on the application, the optimal thickness of the film should be established experimentally. For film-based sensors such as the one described here, the transmission configuration suffers from lower sensitivity than attenuated total reflectance (ATR)-based sensors (2). In transmission measurements the optical path length depends on the film thickness and is comparatively smaller than the increased effective path length observed in ATR spectroscopy. However, much more complicated setups and specifically designed cells are required for ATR-based sensors (1–3). W
Supplemental Material
Instructions for the students, notes for the instructor, and three optional experiments are available in this issue of JCE Online.
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Literature Cited 1. Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4819. 2. Richardson, J. N.; Dyer, A. L.; Stegemiller, M. L.; Zudans, I.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2002, 74, 3330. 3. Shi, Y.; Slaterbeck, A.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 9, 3679. 4. Rosenthal, J. J. Chem. Educ. 1991, 68, 794. 5. DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594. 6. Heineman, W. R. J. Chem. Educ. 1983, 60, 305. 7. Shtoyko, T.; Zudans, I.; Richardson, J. N.; Seliskar, C. J.; Heineman W. R. J. Chem. Educ. 2004, 81, 1617. 8. Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1989. 9. Pantelic, N.; Wansapura, C. M.; Heineman, W. R.; Seliskar, C. J. J. Phys. Chem. B 2005, 109, 13971. 10. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Saunders: Philadelphia, 1998; p 338. 11. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley-Interscience: New York, 1972; pp 860–861.
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