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Luminescence-Based Spectroelectrochemical Sensor for [Tc(dmpe)3]2þ/þ (dmpe = 1,2-bis(dimethylphosphino)ethane) within a Charge-Selective Polymer Film Sayandev Chatterjee, Andrew S. Del Negro, Matthew K. Edwards, and Samuel A. Bryan* Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
Necati Kaval, Nebojsa Pantelic, Laura K. Morris, William R. Heineman,* and Carl J. Seliskar* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States ABSTRACT: A spectroelectrochemical sensor consisting of an indium tin oxide (ITO) optically transparent electrode (OTE) coated with a thin film of partially sulfonated polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SSEBS) was developed for [Tc(dmpe)3]þ (dmpe = 1,2bis(dimethylphosphino)ethane). [Tc(dmpe)3]þ was preconcentrated by ion-exchange into the SSEBS film after a 20 min exposure to aqueous [Tc(dmpe)3]þ solution, resulting in a 14-fold increase in cathodic peak current compared to a bare OTE. Colorless [Tc(dmpe)3]þ was reversibly oxidized to colored [Tc(dmpe)3]2þ by cyclic voltammetry. Detection of [Tc(dmpe)3]2þ was accomplished through emission spectroscopy by electrochemically oxidizing the complex from nonemissive [Tc(dmpe)3]þ to emissive [Tc(dmpe)3]2þ. The working principle of the sensor consisted of electrochemically cycling between nonemissive [Tc(dmpe)3]þ and emissive [Tc(dmpe)3]2þ and monitoring the modulated emission (λexc = 532 nm; λem = 660 nm). The sensor gave a linear response over the concentration range of 0.16-340.0 μM of [Tc(dmpe)3]2þ/þ in aqueous phase with a detection limit of 24 nM.
T
he compounds of technetium of environmental interest quickly oxidize to pertechnetate (TcO4-) under oxic conditions, which is the end of the redox path for this element particularly in an aqueous environment.1,2 Furthermore, TcO4- forms as the thermodynamic sink under acidic physiological conditions.3 This has generated a significant amount of interest in the detection of TcO4-, which has led us to anion exchange films for pertechnetate4,5 that concentrate this aqueous ion, leading typically to micromolar limits of detection in bulk solution. However, pertechnetate has no characteristic optical signature for convenient optical detection. Thus, designing a spectroelectrochemical sensor capable of achieving the low limit of detection needed for environmental applications has led us to pursue lower oxidation states that readily form complexes with the electrochemical and spectral properties needed for the sensor. In both optical and electrochemical sensing, the partitioning of an analyte molecule from a solution-phase molecule into a thin porous solid film on a transducer is a process that is fundamental in chemical sensing.6 As a result, much attention has been paid to the formulation and preparation of such thin solid films,7-9 which are typically several to several hundred nanometers thick. In a recent application, we have focused on making thin chemically selective films for chemical sensing within radioactive wastes2,5,10-12 and sensing of radioactive technetium compounds in environmental samples.13 As a companion effort, we have also r 2011 American Chemical Society
developed a luminescence-based spectroelectrochemical sensor that can achieve very low limits of detection, even as low as picomolar.14 We have previously reported on thin sensing films for a variety of aqueous cations15,16 including [Ru(bpy)3]2þ14 and [Re(dmpe)3]þ9,17-19 (dmpe = 1,2-bis(dimethylphosphino)ethane) complexes. In the latter case, Nafion or composites of Nafion showed efficient concentration of the complex from aqueous solution. Since that time we have also discovered another polyanionic film material, namely, partially sulfonated polystyreneblock-poly(ethylene-ran-butylene)block-polystyrene (SSEBS), that has exchange properties similar to Nafion and has the additional advantages of lower cost and commercial availability in several sulfonated forms.6,20 An absorbance-based sensor for [Re(dmpe)3]þ has been reported using SSEBS.21 The goal of making a luminescence-based spectroelectrochemical sensor for technetium presents a unique challenge in that only three compounds of technetium have been reported to be emissive so far.22,23 Among these compounds, the cation [Tc(dmpe)3]2þ is highly emissive in aqueous solution at room temperature.23 The reversible electrochemistry of the Received: November 18, 2010 Accepted: January 8, 2011 Published: February 04, 2011 1766
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Figure 1. Structural formula of the partially sulfonated triblock copolymer SSEBS.
[Tc(dmpe)3]þ/[Tc(dmpe)3]2þ couple was known in propylene carbonate from previous work,24 but relatively little information was available for aqueous media.25 In a previous paper, we reported on the fundamental photophysical properties of [Tc(dmpe)3]2þ.23 The purpose of this paper is to report the spectroelectrochemical properties of the [Tc(dmpe)3]þ/ [Tc(dmpe)3]2þ couple concentrated into SSEBS films exposed to aqueous solution and to demonstrate a spectroelectrochemical sensor based on luminescence. The work presented here serves as a proof-of-concept to justify the efficiency of the spectroelectrochemistry technique within polymer thin-films for the detection and sensing of trace amounts of [Tc(dmpe)3]þ (or other similar precursors of luminescent Tc(II) complexes) that are the anticipated end products of the complete reduction of environmentally abundant TcO4- species.
’ EXPERIMENTAL SECTION Radiation safety disclaimer: Technetium-99 emits a low-energy (0.292 MeV) β particle with a half-life of 2.12 105 yr, and common laboratory materials provide adequate shielding. Normal radiation safety procedures must be used at all times to prevent contamination. Reagents and Materials. All chemical reagents were used without further purification. SSEBS (average MW 80 000, 5 wt % solution in 1-propanol and dichloroethane, 29 wt % styrene, 55-65% sulfonation degree; Figure 1) and 3-aminopropyltriethoxy silane (APTS) were purchased from Sigma-Aldrich. All solutions were prepared using deionized water (D2798 Nanopure water purification system, Barnstead, Boston, MA) in a 0.1 M KNO3 supporting electrolyte solution. Indium tin oxide (ITO)-coated 1737F glass pieces (∼135 nm thick ITO layer, 11-50 Ω/cm2; hereafter termed substrates) were obtained from Thin Film Devices. Fine annealed 8 mm-thick SF11 and 1 mm 1737F glass were obtained from Schott and Corning, respectively. The complexes of technetium used in this work were prepared as described previously.23 Optical Measurements. Absorbance and luminescence spectra were acquired using either an Acton Instruments-based or an Ocean Optics-based system. The Acton Research InSpectrum 150 with controlling Spectrasense software was equipped with a back-thinned, cooled charge coupled device (CCD) camera and fiber-optic input. Excitation was performed using a 532 nm diode-pumped solid-state (DPSS) laser (Melles Griot, 20 mW CW). A 532 nm holographic notch filter (Kaiser) was used to reduce laser light backscattered into the InSpectrum 150 spectrometer. Signal integration times were typically 500 ms using a 2 mm slit width for a 600 gr/mm grating blazed at 500 nm. Stepindex silica-on-silica optical fibers were purchased from Romack, Inc. The Ocean Optics system consisted of a USB-200FL spectrometer and Ocean Optics 00IBase32 Spectroscopy Software.
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Electrochemical Measurements. For all electrochemical experiments, a standard three-electrode configuration was used on a cell stand from Bioanalytical Systems using either an Epsilon potentiostat or a PAR model 273A potentiostat/galvanostat (EG&G Princeton Applied Research) computer-controlled by electrochemical software from Scribner Associates (CorrWare Electrochemical Research Software, Version 2.9c). All scans were recorded employing a 10 40 mm ITO-glass transparent working electrode, a platinum auxiliary electrode, and an Ag/ AgCl reference electrode (3 M NaCl, BAS). All reported potentials were referenced versus a platinum wire quasi reference. Peak currents (ip) were estimated with respect to the extrapolated baseline current as described by Kissinger and Heineman.26 Radiocounting Methods. The activity of technetium was measured by a liquid scintillation-counting technique. Typically, 5 mL of Ultima Gold XR liquid scintillation cocktail (Packard BioScience, Meriden, CT) was used for 99Tc beta counting. The relative beta activity of the samples was determined by liquid scintillation counting using a Packard Tri-Carb Model 2500TR Liquid Scintillation Analyzer (Packard Instrument Company, Meriden, CT 06450) with a 0.98 counting efficiency. In a typical measurement, beta counts were integrated over a 2 min collection time; all counts were corrected for background. When absolute concentrations of 99Tc were required, the conversion factor 3.767 1010 dpm/g was used (dpm = disintegrations per minute). Thin-Film Preparations. Ellipsometric measurements (film optical constants and thickness) were made using a J.A. Woollam, Inc., variable-angle spectroscopic ellipsometer (vertical configuration). This instrument was equipped with an adjustable retarder (AutoRetarder) that enabled measurements of ψ and Δ over the full angular range (0-90 and 0-360 degrees, respectively). The instrument also permitted depolarization of the light to be measured. Woollam WVASE32 software was used for optical modeling. A plasma cleaner (Harrick Scientific) was used for final cleaning of substrates and electrodes. A spin-coater (model 1 p.m.101DT-R485 Photo-Resist Spinner, Headway Research, Inc.) was used for preparation of films of different thicknesses. Glass or ITO-glass substrates were thoroughly cleaned with soap, rinsed with ethanol and deionized water, and argon-plasma cleaned for about 30 min prior to spin coating. Generally, a 5% stock SSEBS solution was used for film preparation. When making the films with varied thicknesses, the stock solution was further diluted to the desired concentration (2, 1, and 0.5%) using 2-propanol. A 50 μL aliquot of solution was pipetted onto the substrates, which were spun at different speeds (between 1000 and 6000 rpm) for 30 s. The resulting film thickness (measured with ellipsometry), at 3000 rpm, for example, ranged from 24 nm (0.5% solution) to 300 nm (5% solution) on ITO substrates. The variation of film thickness, at constant concentration of SSEBS, versus spin speed followed an exponential relationship. For spectroelectrochemical experiments a 1 cm portion of one end of the ITO substrate was masked with tape before spin coating. The area free of film was then used for electrical contact with the electrode. In cases where loss of SSEBS layer (presumably due to dissolution in the solution in contact) was observed, the substrates were functionalized with the bifunctional linker APTS (vide supra). For this specific procedure, clean substrates were soaked overnight in 2 M NaOH in order to activate the surface. 1767
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Figure 2. (A) Side view of the spectroelectrochemistry sensor, (B) Overhead view of the optics module and the electrochemistry module, and (C) Magnified side view of the electrochemistry module.
After rinsing with water, the substrates were soaked in 5% APTS in acetate buffer, pH 5.5, at 90 °C for 5 h. The substrates were then rinsed with deionized water, spun dry for 30 s, and immediately used for film coating. The film-coated substrates were left overnight to further cure at ambient temperature. Spectroelectrochemistry Measurements. All spectroelectrochemistry measurements were performed in the custom-made spectroelectrochemical sensor cell shown in Figure 2A. The sensor consists of two complementary parts: the electrochemistry module and the optics module, as shown in Figure 2B. The electrochemistry module consists of a thin-film flow cell (fashioned from a BAS electrochemical flow cell, model CC-44, Bioanalytical Systems). The electrochemical part of the sensor cell consists of three components shown in Figure 2C. The analyte solution, injected into the solution compartment through the fluid transfer ports, remains in direct contact with the working ITO electrode and the auxiliary clip in the solution compartment and with the reference through one of the fluid transfer channels. For electrochemical measurements, a potential is applied with a CV-25 potentiostat manufactured by Bioanalytical Systems. The optical mounting is highly symmetrical, and the module incorporates single-reflection optics. High index prisms are used to in-couple and out-couple light, with an added third optical leg devoted to measuring emission from the single-excitation-emission spot on the appended film-ITO slide. Light is transported to and from the sensor by fiber-optic cables with SMA 905 connectors making it easy to disassemble, clean, and modify. Concentration of [Tc(dmpe)3]þ and [Tc(dmpe)3]2þ from Aqueous Solution by SSEBS. The partitioning of [Tc(dmpe)3]þ and [Tc(dmpe)3]2þ from aqueous solution into free-standing films was measured using radiocounting of 99Tc. Thick films (about 1 mm thickness) of SSEBS were cast by evaporation of the solvent from stock solution transferred into Petri dishes. Cast films were then air-dried for at least 24 h before use. Weighed samples of SSEBS of 0.099-0.11 g were contacted at room temperature for 24 h with 6.5-7.0 mL of [Tc(dmpe)3]þ or [Tc(dmpe)3]2þ solution (8 10-7-4 10-4 M) in 0.10 M NaNO3 stirred with a magnetic stirrer. Solution Tc concentrations of initial and equilibrated solutions were determined by radiocounting techniques. Duplicate measurements were performed for each concentration. The F factor, which is defined as the ratio of the dry SSEBS mass to the conditioned wet SSEBS mass, was determined to be 0.936.
The distribution ratio Kd (in mL g-1) of each technetium complex was calculated, assuming that a mass balance of 100% is sustained for each sample as described by Ashley and Ball:27 Kd ¼
½Cinitial - Cequilibrium =gdrySSEBS resin Cequilibrium =mLsolution
ð1Þ
where Cinitial and Cequilibrium are the initial technetium complex concentration in solution prior to contact and the equilibrium technetium concentration upon its equilibration with the film, respectively, and “resin” is the SSEBS film. The mass of dry SSEBS (g) used was that calculated using the F factor determined for SSEBS as described above.
’ RESULTS AND DISCUSSION Concentration of [Tc(dmpe)3]þ and [Tc(dmpe)3]2þ from Aqueous Solution by SSEBS. Kd values for both complexes for
different solution concentrations are shown in Figure 3. For both complexes, the Kd values increase with solution concentration. Kd values for Tc(dmpe)3]þ into SSEBS are higher than for [Tc(dmpe)3]2þ. The higher Kd values for [Tc(dmpe)3]þ over [Tc(dmpe)3]2þ complexes translates to a ∼3-fold enhanced partitioning of the monocationic complex into SSEBS compared to the dicationic form. This higher partitioning for [Tc(dmpe)3]þ into the film is consistent with the behavior reported for the analogous system [Re(dmpe)3]þ and [Re(dmpe)3]2þ in Nafion films28 and is reinforced by the observed electrochemical differences in [Tc(dmpe)3]þ and [Tc(dmpe)3]2þ loaded in the film (see Voltammetry in Thin SSEBS Films section below). These results reveal insights into the forces involved in binding of the complexes within both Nafion and SSEBS polymers. It appears that the hydrophobic interactions involving the hydrophobic ligand dmpe with the hydrophobic backbone of these two polymers are more important than electrostatic forces in the binding of the two complexes, causing the more weakly hydrated [Tc(dmpe)3]þ structure to partition more strongly into the film. Voltammetry in Aqueous Solution. Cyclic voltammograms for [Tc(dmpe)3]þ at bare ITO for a range of scan rates are shown in Figure 4A. The voltammograms exhibit a well-defined, chemically reversible wave for the one-electron Tc(I)/Tc(II) couple. ½TcðdmpeÞ3 þ a ½TcðdmpeÞ3 2þ þ 1e 1768
ð2Þ
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Figure 3. Plot of equilibrium distribution (Kd) values for [Tc(dmpe)3]þ and [Tc(dmpe)3]2þ with SSEBS versus concentration in solution. Each data point is an average of two independent determinations.
Figure 4. (A) Cyclic voltammetry showing the reversible Tc(I)/Tc(II) couple for the [Tc(dmpe)3]þ/2þ triflate salt in aqueous 0.1 M KNO3 at ITO vs Ag/AgCl. This plot is a compilation of multiple scans at variable scan rates from 4 to 196 mV/s. (B) Plots of peak current versus (scan rate)1/2; (red 9) anodic process; ipa(μA) = -5.25 ν1/2 (mV s-1)1/2 3.63, R2 = 0.99; (blue () cathodic process; ipc(μA) = 4.99 ν1/2 (mV s-1)1/2 þ 3.04, R2 = 1.00.
The peak separation increases with increasing scan rate due to the resistance of the ITO. The formal reduction potential E°0 obtained from the midpoint between the anodic and cathodic peaks is ca. 50 mV vs Ag/AgCl. This value is to be compared with 93 mV vs Ag/AgCl, 3 M Cl- (reported as 290 mV vs NHE) in aqueous 0.1 M LiCl,25 and 329 mV in propylene carbonate solvent.24 Plots of peak current versus (scan rate)1/2 are linear for
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Figure 5. Preconcentration of [Tc(dmpe)3](OTf) into SSEBS film. (A) (black) Cyclic voltammograms at 50 mVs-1 during a 40 min preconcentration of 2.2 10-4 M [Tc(dmpe)3]þ in 0.1 M KNO3 (aq) into a ∼315 nm thick SSEBS film at 50 mVs-1. (red) Cyclic voltammogram for bare OTE is shown for comparison. (B) Plot of cathodic peak current vs time for [Tc(dmpe)3]þ incorporation into the SSEBS film. (Initial potential = -450 mV.)
both anodic and cathodic peaks as expected for semi-infinite diffusion of a reversible system (Figure 4B). From the ratio of the slopes, the ratio of the diffusion coefficients for [Tc(dmpe)3]2þ/ [Tc(dmpe)3]þ is 1.1 (nearly equal coefficients). Voltammetry in Thin SSEBS Films. Partitioning of [Tc(dmpe)3]þ into SSEBS films from aqueous solution could be observed by cyclic voltammetry at SSEBS-coated ITO that had been preconditioned by soaking for 30 min in supporting electrolyte solution (aqueous 0.1 M KNO3). The electrode was then placed in a cell containing an Ar-purged solution of [Tc(dmpe)3]þ (2.2 10-4 M in aqueous 0.1-M KNO3), and voltammograms were immediately recorded at 90 s intervals. The uptake of [Tc(dmpe)3]þ by the SSEBS-coated electrode is indicated by the cyclic voltammograms in Figure 5A. The increasing peak currents in the repeated voltammograms indicate a rapid, continuous uptake into the film over about the first 1000 s. This is further shown in the Figure 5B plot of cathodic peak current vs time. From the figure, it is clear that under these conditions the concentration in the film equilibrates with the solution after about 2000 s. The voltammograms in Figure 5A compare the response at a bare ITO electrode (red trace) recorded on the same solution prior to introducing the SSEBS-coated ITO against the initial (
