Characterization of Partially Sulfonated Polystyrene-block-poly

Jul 23, 2009 - The film was allowed to fully air-dry before the spectrum was ... This should be considered an intermediate wavelength region of limite...
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Anal. Chem. 2009, 81, 6756–6764

Characterization of Partially Sulfonated Polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene Thin Films for Spectroelectrochemical Sensing Nebojsˇa Pantelic´, Sara E. Andria, William R. Heineman,* and Carl J. Seliskar* Department of Chemistry, University of Cincinnati, 301 Clifton Court, Cincinnati, Ohio 45221-0172 The spectroelectrochemical sensor uses thin, solid polyelectrolyte films as an essential element in its operation. In this work we explored the potential of partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene (SSEBS) thin polymer films for chemical sensing. Spectroscopic ellipsometry was used to measure optical and surface properties of the air-dried and hydrated material. SSEBS incorporates a relatively small amount of water (overall change of 25%) mainly determined by the complex dynamics of the film. The decrease in the refractive index after complete hydration of the film can be predicted based on the magnitude of swelling using effective medium approximation models. Adhesion of the material on various surfaces (glass, indium tin oxide, gold) was evaluated with the tape peeloff method. The ability of the SSEBS material to preconcentrate cations was evaluated by cyclic voltammetry, absorbance, and luminescence measurements using model analytes (Ru(bpy)32+, phenosafranine, and rhodamine 6G). The detection limits of the sensor for Ru(bpy)32+ under unoptimized conditions can be significantly improved if luminescence is used as the detection modality (DL ) 5 × 10-10 M) instead of absorbance (DL ) 5 × 10-7 M). Overall, the results demonstrate the effectiveness of the SSEBS material for spectroelectrochemical sensing. A thin, chemically selective polymer film coated on the surface of an optically transparent electrode (OTE) is typically designed to preconcentrate analytes to be measured in aqueous solution.1,2 The selective partitioning of the analyte from the aqueous phase into the solid, polymeric structure is an essential step for this chemical sensing technique. These films deliver increased sensitivity because of the preconcentration effect and provide an important barrier for excluding potentially interfering species. Within the film, analytes are electrochemically transformed giving rise to a change in optical signal (absorbance, luminescence). The * To whom correspondence should be addressed. E-mail: william.heineman@ uc.edu (W.R.H.), [email protected] (C.J.S.). Phone: 1-513-556-9210 (W.R.H.), 1-513-556-9213 (C.J.S.). Fax: 1-513-556-9239 (W.R.H.), 1-513-556-9239 (C.J.S.). (1) Shi, Y.; Slaterbeck, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679–3686. (2) Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4819– 4827.

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intrinsic properties of thin, polyelectrolyte materials such as transparency, rigidity, affinity for certain ions, and electrochemical inactivity under an applied potential, are all of vital and practical importance in achieving efficient spectroelectrochemical sensing. Previously we have developed two types of materials for chemical sensing, namely, sol-gel processed silica3 and crosslinked poly(vinyl alcohol) (PVA) based composites.4-7 These composites contain different polyelectrolytes as the active medium for ion exchange. Neat polymer films, such as those made with quaternized poly(4-vinylpyridine)8 and Nafion (perfluorosulfonated ionomer),9 provide suitable preconcentration media. Nafion can also be blended into PVA or silica matrixes to provide useful composites.4,10 Nafion has been studied extensively and was originally designed as a separator material in batteries or fuel-cells.11 In a continuing effort to develop cheaper and more efficient battery and fuel-cell materials, a novel tri-block co-polymer material, partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS), was developed. Weiss and coworkers published comprehensive studies on this material.12-16 Briefly, the material has a characteristic domain structure commonly observed in block co-polymers (hydrophilic styrenerich domain with ionic aggregates and hydrophobic ethylene/ butylene phase)13-15 and displays order-disorder and mesophase transitions at elevated temperatures.16 Several applications that use SSEBS in a composite form have been (3) Shi, Y.; Seliskar, C. J. Chem. Mater. 1997, 9, 821–829. (4) Gao, L.; Seliskar, C. J. Chem. Mater. 1998, 10, 2481–2489. (5) Gao, L.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1999, 71, 4601– 4068. (6) Gao, L.; Seliskar, C. J.; Milstein, L. Appl. Spectrosc. 1997, 51, 1745–1752. (7) Gao, L.; Seliskar, C. J.; Heineman, W. R. Electroanalysis 2001, 13, 613– 620. (8) Conklin, S.; Heineman, W. R.; Seliskar, C. J. Electroanalysis 2005, 17, 1433– 1440. (9) Richardson, J. N.; Dyer, A. L.; Stegemiller, M. L.; Zudans, I.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2002, 74, 3330–3335. (10) Hu, Z.; Slaterbeck, A. F.; Seliskar, C. J.; Ridgway, T. H.; Heineman, W. R. Langmuir 1999, 15, 767–773. (11) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1–33. (12) Weiss, R. A.; Sen, A.; Willis, C. L.; Pottick, L. A. Polymer 1991, 32, 1867– 1874. (13) Weiss, R. A.; Sen, A.; Pottick, L. A.; Willis, C. L. Polymer 1991, 32, 2785– 2792. (14) Lu, X.; Steckle, W. P.; Weiss, R. A. Macromolecules 1993, 26, 5876–5884. (15) Lu, X.; Steckle, W. P.; Weiss, R. A. Macromolecules 1993, 26, 6525–6530. (16) Lu, X.; Steckle, W. P.; Hsiao, B.; Weiss, R. A. Macromolecules 1995, 28, 2831–2839. 10.1021/ac900765t CCC: $40.75  2009 American Chemical Society Published on Web 07/23/2009

Scheme 1. Structural Formula of the SSEBS Polymer, Partially Sulfonated Tri-Block Co-Polymer (Styrene/ Ethylene, Butylene/Styrene)

proposed.17-20 Some electrochemical properties have also been reported in the literature.21-24 We are aware of only one report of the use of SSEBS for chemical sensing;22 the SSEBS films were used as proton conducting solid electrolytes in an electrochemical sensor for carbon monoxide. The current literature lacks a detailed characterization of this polymer in thin film form. Transport properties have not been addressed to date except for those directly related to the originally proposed application (i.e., proton conductivity and methanol permeability25,26). Here we report the characterization of SSEBS in thin film form. Our main goal was to investigate those properties that are important for spectroelectrochemical sensing. EXPERIMENTAL SECTION Chemicals 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 % of styrene, 55-65% sulfonation degree) (Scheme 1), tris(2,2′-bypyridine)ruthenium(II) dichloride hexahydrate (Ru(bpy)32+), 3,7-diamino-5phenylphenazinium chloride (phenosafranine, PSF), and 3-aminopropyltriethoxysilane (APTS) were purchased from Aldrich. o-(6-ethylamino-3-ethylimino-2,7-dimethyl-3H-xanthen-9-yl)benzoic acid ethylester (R6G) was purchased from Exciton. All solutions were prepared using deionized water (D2798 Nanopure water purification system, Barnstead, Boston, MA) in a 0.1 M KNO3 supporting electrolyte solution. Specifically, PSF solution was prepared with addition of 0.01 M HCl (under this condition the dye is electroactive). Sodium acetate, sodium hydroxide, ethyl alcohol, and potassium nitrate were obtained from Fisher Scientific. Indium tin oxide (ITO) coated 1737F glass pieces (hereafter termed substrates) were obtained from Thin Film Devices (∼135 nm thick ITO layer, 11-50 Ω/square). Fine annealed 8 mm thick SF11 and 1 mm 1737F glass were obtained from Schott and Corning, respectively. (17) Barra, G. M. O.; Jacques, L. B.; Orefice, R. L.; Carneiro, J. R. G. Eur. Polym. J. 2004, 40, 2017–2023. (18) Chen, S.-L.; Krihnan, L.; Shrinivasan, S.; Benziger, J.; Bocarsly, A. B. J. Membr. Sci. 2004, 243, 327–333. (19) Lee, W.-J.; Jung, H.-R.; Lee, M. S.; Kim, J.-H.; Yang, K. S. Solid Stat Ionics 2003, 164, 65–72. (20) Mauritz, K. A.; Blackwell, R. I.; Beyer, F. L. Polymer 2004, 45, 3001–3016. (21) Edmondson, C. A.; Fontanella, J. J.; Chung, S. H.; Greenbaum, S. G.; Wnek, G. E. Electrochim. Acta 2001, 46, 1623–1628. (22) Mortimer, R. J.; Beech, A. J. Electrochem. Soc. 2000, 147, 780–786. (23) Yao, L.; Krause, S. Macromolecules 2003, 36, 2055–2065. (24) Barusa, V. I.; Chuy, C.; Beattie, P. D.; Holdcroft, S. J. J. Electrochem. Soc. 2001, 501, 77–88. (25) Kim, J.; Kim, B.; Jung, B. J. Membr. Sci. 2002, 207, 129–137. (26) Kim, B.; Kim, J.; Jung, B. J. Membr. Sci. 2005, 250, 175–182.

Instrumentation. Normal incidence transmission and luminescence measurements were recorded using Varian Analytical Instruments (models: Cary 50 and Cary Eclipse, respectively). Contact angles were measured using a contact angle meter (Tantec, IL). Ellipsometric measurements were made using a J.A. Woollam, Inc. variable angle spectroscopic ellipsometer (vertical configuration). This instrument was equipped with an adjustable retarder (Auto Retarder) that enabled measurements of Ψ and ∆ over the full angular range (0-90° and 0-360°, respectively). The instrument also permitted depolarization of the light to be measured. Woollam WVASE32 software was used for optical modeling. The details of the construction of the ellipsometer flow cell for back-side measurements can be found in our previous publication.27 A peristaltic pump (Cole-Parmer Instruments Co.) was used to circulate water (1.0 L volume, 20 mL/min) through the ellipsometer flow cell. Electrochemical measurements were performed using a potentiostat (Epsilon, Bioanalytical Systems) with Ag/AgCl reference electrode (3 M NaCl, BAS or 3 M KCl, Cypress Systems) and Pt auxiliary electrode. A plasma cleaner (Harrick Scientific) was used for final cleaning of substrates. A spin-coater (model 1-PM101DT-R485 PhotoResist-Spinner, Headway Research, Inc.) was used for preparation of films of different thicknesses. Preparation of SSEBS Films on Substrates. The 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 the stock solution was pipetted onto the substrates and spun at different speeds (between 1000 and 6000 rpm) for 30 s. For example, at 3000 rpm, the resulting film thickness (measured with ellipsometry) ranged from 24 nm (0.5% solution) to 340 nm (5% solution), which were prepared on ITO substrates. We were not able to indentify significant differences in optical and surface properties for these two film thicknesses. The variation of film thickness (d) at constant concentration of SSEBS versus spin speed (t, given in revolutions per minute) followed an exponential relationship. This functionality for 1% SSEBS solution was appropriately fitted (R2 ) 0.997) using the empirical equation: d ) C1 + C2 exp[-(t/C3)] where C1 (35.6 ± 7.5), C2 (100.5 ± 19.0), and C3 (2.8 ± 1.0) are fitted coefficients. This functionality may be different for other experimental setups. For the spectroelectrochemical experiments, a 1 cm portion of each end of ITO substrates was masked with tape before spin-coating. For the electrochemical-only experiments, the ITO was taped at one end so that the film area was ∼2 cm2. The areas free of film were then used for electrical contacts. In certain cases the substrates were functionalized with the bifunctional linker APTS (vide infra). For this specific procedure, clean substrates were soaked overnight in 2 M NaOH to activate the surface. After rinsing with water, the substrates were soaked in a 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. (27) Zudans, I.; Heineman, W. R.; Seliskar, C. J. J. Phys. Chem. B 2004, 108, 11521–11528.

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Figure 1. Experimental setup for the spectroelectrochemistry in the absorption and luminescence modes (objects are not to scale). See text for details.

Film-coated substrates were left overnight to further cure at ambient temperature. Spectroelectrochemistry and Instrumental Arrangement. The detailed scheme of the experimental setup and the construction of the spectroelectrochemical sample cell can be found in our previous publication.28 Briefly, the incident radiation was guided into the ITO glass substrate under multiple internal reflection (MIR) conditions with a polished 400 µm silica stepindex optical fiber (incidence fiber, NA ) 0.22, RoMack, Inc.). The MIR arrangements for absorbance and fluorescence detection are depicted in Figure 1. Efficient coupling and decoupling of the light was ensured with the high refractive index prisms (Schott SF6, Karl Lambrecht, Chicago, IL). Decoupled light was collected with the collection fiber (fiber optic bundle with six individual fibers) that was further connected to a monochromator and phototube (Acton Research Corp.) and personal computer. The mass transport of the model analytes (Ru(bpy)32+, PSF, R6G) was measured both by absorbance and luminescence. For the absorbance measurements, the light source was a high intensity xenon lamp (model ILC302UV, ILC Technology, Inc.). After passing through the MIR element, the attenuated light intensity was monitored at the wavelength of the maximum absorption. In luminescence mode, the experimental arrangement was similar. The only exception was in the position of the collection fiber. By introducing this fiber near the glass substrate and usually at the first reflection spot (Figure 1) the emitted light was collected. Luminescence was excited with the appropriate lasers: R6G and PSF with a 532 nm laser (model 85-GCB-020, Melles Griot), and Ru(bpy)32+ with a 441.6 nm laser (HeCd, model 1K4153RC, Kimmon Electric Co.). Laser power was attenuated to 0.2 mW at the launching prism face using a variable neutral density filter (Newport). When R6G luminescence was excited the incidence laser intensity was further attenuated to prevent overload of the detector (R6G quantum yield 0.95 in ethanol).29 The intensity of the emitted light was monitored at the wavelength of the maximum emission. All experiments were carried out with liquids in thermal equilibrium with the surrounding laboratory temperature (∼25 °C). Spectroelectrochemical Measurements. The optical response versus time plots were completed for each analyte in the (28) Kaval, N.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2003, 75, 6334– 6340. (29) Kubin, R. F.; Fletcher, A. N. J. Lumin. 1982, 27, 455–462.

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SSEBS films. For each trial, a baseline was collected by flowing 0.1 M KNO3 for 250 s before introducing the analyte into the sample cell. Both the KNO3 solution and the analyte solutions were continuously flowed through the sample cell (syringe pump, model 341B, Sage Instruments, flow speed 0.1 mL/min). Electrochemical Measurements. For the analyte uptakes monitored only by cyclic voltammetry, the standard threeelectrode configuration was used instead of the spectroelectrochemical flow cell. The film-coated ITO, a Ag/AgCl reference electrode (BAS) and a Pt mesh auxiliary electrode were submerged into a 4 dram vial containing the analyte solution. Voltammograms were obtained under quiescent conditions. Assembly of the electrochemical cell consisted of first filling the vial with solution, inserting the electrodes, and then attaching the cell lead cables to the electrodes. Therefore, approximately 30 s elapsed between the time when the film was first exposed to solution and the beginning of the experiment. The potential was scanned in the appropriate potential window at 5 mV/s with a 20 s delay between each cycle. RESULTS AND DISCUSSION General Material Properties. Adhesion Studies. A homemade tape peel-off apparatus was used to test the adhesion of SSEBS films on various surfaces. These included: glass (quartz, 1737F, and SF11), ITO, and gold substrates. Tape was peeled at ∼180° with a speed of 26 mm/min from surfaces coated with 24 and 340 nm thick SSEBS films. Contact angles were measured first on bare, argon plasma cleaned substrates, and then on film-coated substrates, before and after the tape peel test. The following contact angles were measured for the bare substrates: quartz (16 ± 3)°, SF11 (26 ± 2)°, 1737F (36 ± 4)°, ITO (7 ± 1)°, and gold (53 ± 8)°. These values clearly indicate the relative hydrophilic character of the surfaces. Upon immobilization of the films on the different substrates, the contact angles attained the same value (SSEBS (94 ± 3)°), indicating uniform coverage and the intermediate wetting character of the material. No difference was found between the 24 and 340 nm films. After the peel test, the contact angles were remeasured where the tape had been applied and found to be 94° for both thicknesses, indicating that no damage to the film had occurred and that the adhesion of SSEBS was relatively good. Thus, we conclude that dry SSEBS films are adhered relatively strongly to these substrates. Interestingly enough, the contact angles for the film-coated glass substrates had a “dynamic” character, that is, they changed in time. At the beginning of the measurement, the values were as reported above, but after several minutes the angles began to decrease. After the water droplet dried, visual inspection of the substrate surface indicated that the film had delaminated. For our applications it is essential to have stable films in aqueous media. As a result, we exposed freshly prepared films to a 0.1 M supporting electrolyte solution to further test the films’ stabilities. The results were as follows: the films on the quartz, 1737F and SF11 glass substrates immediately delaminated, whereas those on gold and ITO coated substrates were stable even after several days of exposure. Those weakly adhering formulations were (SSEBS films on quartz and glasses) stabilized using the bifunctional linker APTS as described earlier. The immobilized films were again tested in the electrolyte solution, yielding stable films where no delamination was observed. In summary, dry SSEBS

fully hydrated in an aqueous solution. It was assumed that the SSEBS material was practically non-absorbing (k(λ) ) 0) in the wavelength range used. The bare SF11 glass was optically characterized prior to being used for film preparation. Both ellipsometric parameters, Ψ and ∆, (sensitive to n) and transmission measurement (sensitive to k) were acquired. Transmission data allowed us to model k ) k(λ) profiles for the SF11 glass substrate below 450 nm using the Urbach equation.30 The materials (glass, dry film) were interrogated in the wavelength range from 400-1000 nm at every 10 nm for three different incidence angles. Light depolarization data was also collected. The refractive index of non-absorbing optical materials is characterized by the Cauchy equation (eq 1): n(λ) ) A +

Figure 2. (a) Absorption spectrum of the SSEBS material. (b) Experimentally measured refractive indices, n ) n(λ), for the air-dry SSEBS material (top curve) and for the film hydrated in water (lower solid curve). Theoretically calculated functions in accordance with Layered (upper dashed curve) and Bruggeman (lower dashed curve) models are also shown for comparison.

films adhere strongly to these substrates but several delaminate when exposed to water for any length of time. Those films delaminating are easily stabilized by linking them to the substrate using APTS. Optical Properties and Swelling Studies. It is imperative that the polymer material be optically transparent to be used as a selective film so it does not interfere with analyte detection. Figure 2a shows a near-UV/vis/near-IR spectrum of the material. For this measurement a relatively thick film was cast onto a 1737F glass substrate. The film was allowed to fully air-dry before the spectrum was acquired using the normal transmission configuration. A bare glass substrate was used as the blank. The SSEBS material has near-zero absorbance in the spectral range from 400-1000 nm. Practically, this means that the SSEBS material can be used in this region for optical-based sensors. From 300-400 nm SSEBS is weakly absorbing as evident from Figure 2a. This should be considered an intermediate wavelength region of limited use. In the near-UV region (below 300 nm), the SSEBS material becomes significantly absorbing, preventing the film from being functional in this wavelength region. The complex refractive index, n ˜ (λ) ) n(λ) + ik(λ), contains the real part, n(λ) and the extinction coefficient, k(λ). The refractive index of the SSEBS on an SF11 glass substrate was measured by spectroscopic ellipsometry in the air-dry state and

B C + 4 λ2 λ

(1)

where A, B, and C are fitted coefficients and the units of wavelength are micrometers. Generally, the optical models required for “back-side” interrogation27,31 consisted of the following optical layers and interfaces: glass substrate/polymer film/ water. Experimentally measured data were fitted to theoretical models by minimizing the differences between the two sets of data using an appropriate regression algorithm. The errors for these fittings, expressed as MSE values (Mean Squared Errors),30 ranged from 1 to 3. The air-dry SSEBS material has a refractive index of n550 ) 1.514, and its dispersion is described by n(λ) ) 1.498 + 0.003/ λ2 + 0.0005/λ4 in the wavelength region of 400-1000 nm (Figure 2b). Film thickness non-uniformity was typically 4%, indicating a relatively smooth surface structure. The associated surface roughness layer was about 3 nm. We would like to note here that the term “air-dry” actually means a film dried under ambient conditions. It is reasonable to expect that such material still contained a significant amount of absorbed atmospheric moisture. Therefore, for a vacuum-dried material one would expect the refractive index to be higher. Thickness and optical constant changes were further monitored for a film exposed to circulating deionized water. A detailed description of the methods and strategies used for these dynamic in situ ellipsometric measurements may be found elsewhere.31 The film thickness changed from an air-dry film value of 356 nm to an equilibrium value in water of 444 nm which is a swelling ratio Sw of only 25% (Sw = (d∞ - d0)/d0 where d∞ is equilibrated film thickness and d0 is dry film thickness). Previously, Kim et al. measured the equilibrium water content of thick SSEBS samples as a function of degree of styrene sulfonation.25 They found that the equilibrium swelling was increasing with the degree of sulfonation. Moreover, the polymer water content increased exponentially with this parameter. For example, their sample with a degree of sulfonation of approximately 47% (similar to our case) was found to have an equilibrium water content of approximately 100% relative to the vacuum-dried sample mass. Dynamic measurements performed on the thin (30) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry; John Wiley & Sons, Inc: New York, 1999. (31) Pantelic, N.; Wansapura, C. M.; Heineman, W. R.; Seliskar, C. J. J. Phys. Chem. B 2005, 109, 13971–13979.

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SSEBS films revealed that the hydration of SSEBS was not instantaneous; instead the hydration proved to be a rather slow, complex process lasting almost 10 h. On initial exposure to water, the film thickness rapidly reached a maximum value, which was then followed by a slow decrease as it approached an equilibrium value. The shape of film thickness versus time resembled a parabola with a long tail at longer times. The film thickness was 842 nm at its maximum, which was determined to be a swelling ratio of 135%. As a result of hydration, the film’s refractive index proportionately decreased because of the incorporation of water (Figure 2b). The hydrated film’s refractive index had a value of n550 ) 1.477, n(λ) ) 1.455 + 0.006/λ2 - 5 × 10-5/λ4. Film thickness nonuniformity slightly increased; however, the surface roughness layer could not be identified. The latter might be a consequence of the lowered sensitivity for the in situ measurement. The accuracy of the experimentally determined refractive index measurements for the film equilibrated in water was further tested using the Effective Medium Approximations (EMA).30,32 According to this theory, the refractive index of a medium composed of two or more constituents (here water and dry polymer) can be calculated by interpolating between their optical properties. Two commonly used theoretical models were employed: the Layered and the Bruggeman EMA. The first model assumes a layered composite geometry, and the second composite grains randomly interspersed among each other. For the Layered approximation, the equation based on the electric field parallel to the optical layers was used in computing.32 The result of one such calculation is shown in Figure 2b. The upper and lower solid curves are for experimentally measured refractive indices of the air-dry and fully hydrated SSEBS film, respectively. Corresponding theoretical results in accordance with the EMAs (dashed curves) were calculated assuming an 80% airdry film structure (as experimentally determined) and the remainder water. The theoretical and experimental data agree very well. The small discrepancy is more pronounced in the medium wavelength region for both models and is slightly larger for the Bruggeman EMA. However, it is to be noted that the difference between these two sets of data is on the same order as experimental errors. Taken as a whole, these results clearly show that our optical modeling and model-generated data were essentially correct. Electrochemical Evaluation of Partitioned Species in the SSEBS Film. For the successful detection of an analyte using the spectroelectrochemical sensor, three steps must occur: (1) the analyte must partition into the film; (2) the analyte must undergo an electrochemical reaction; and (3) the electrolysis product must have different optical properties than the original specie, resulting in a change in the sensor’s optical response. Therefore, in addition to being stable and transparent, the polymer material must also be able to preconcentrate the analyte and be inert to the electrochemical reaction of partitioned species. This aspect was examined by monitoring the uptake of electroactive cationic chromophores into different SSEBS films by cyclic voltammetry, absorbance, and fluorescence. Ru(bpy)32+ and phenosafranine (PSF) were chosen as representative inorganic and organic compounds for this study because they possess (32) Gehr, R. J.; Boyd, R. W. Chem. Mater. 1996, 8, 1807–1819.

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the qualities of an ideal analyte: they are charged, have reversible electrochemistry, and change colors when electrochemically converted between two oxidation states. The uptake of each analyte was monitored using 24 and 340 nm thick SSEBS films, which were spin-coated onto an ITO electrode. Each film was first exposed to a 0.1 M KNO3 supporting electrolyte solution overnight (∼15 h) to achieve the potassium form of the film. On the basis of the dynamic in situ ellipsometric measurements, it took almost 10 h for the SSEBS films to become fully hydrated. Therefore, it was assumed that allowing the films to sit overnight in the supporting electrolyte solution would be long enough for the potassium ions in the solution to exchange with the protons in the film. Cyclic voltammograms of 0.1 M KNO3 at bare ITO, ITO coated with a 24 nm thick SSEBS film, and ITO coated with a 340 nm thick SSEBS film (shown in Figure 3a) show that the SSEBS film is electrochemically inactive over the same potential window as bare ITO. The uptake of each of the analytes into each of the SSEBS films is illustrated in Figure 3: (b) Ru(bpy)32+/24 nm; (c) PSF/ 24 nm; (d) Ru(bpy)32+/340 nm; (e) PSF/340 nm. Welldefined oxidation and reduction peaks were observed during the uptake of both analytes. For the Ru(bpy)32+, the relatively symmetrical peaks of the voltammogram in which the current returns to baseline after peaking and the small peak separation (∆Ep ) 16 mV for 24 nm SSEBS; 39 mV for 340 nm SSEBS for the last voltammogram recorded) indicate that thin-layer conditions were approximated, which is expected behavior for the essentially complete electrolysis of a reversible redox couple concentrated in a thin film. Following the oxidation peak, current drops to a “baseline” with a substantial anodic current for the 24 nm film, which is due to flux into the film from the bulk solution containing Ru(bpy)32+. By comparison, the cathodic baseline current following the cathodic peak is closer to zero since the bulk solution contains no Ru(bpy)33+ to diffuse into the film. The non-zero ∆Ep is attributed primarily to IR drop in ITO, which is also observed at bare electrodes; resistance values calculated using the experimental data in Figures 3b and 3d (i.e., ∆Ep and ∆ip) were consistent with the resistance measured across a bare ITO slide using a multimeter. For both of the Ru(bpy)32+ uptakes, the first scan is somewhat distorted; however, this behavior for the first scan of a cyclic voltammetric experiment is not unusual. The PSF voltammograms had peak shapes that are indicative of essentially complete electrolysis in the thin films, but show the large peak separation that is characteristic of an electrochemically irreversible system (i.e., slow heterogeneous electron transfer). Large ∆Ep values were also obtained at bare ITO, increasing from 105 mV at 5 mV/s to 225 mV at 100 mV/s, which confirms the electrochemical irreversibility of PSF at ITO. Interestingly, the ∆Ep values for PSF in the SSEBS films continuously increased throughout the course of an uptake experiment, even after steady-state was reached. For example, the ∆Ep values for the initial cycles of PSF uptake were 38 mV in the 24 nm film and 92 mV in the 340 nm film. This smaller ∆Ep in the thinner film is consistent with the behavior described above for Ru(bpy)32+. However, at the point where the uptakes

Figure 4. Normalized absorption spectra of each analyte prepared in 0.1 M KNO3 supporting electrolyte solution (PSF was made in acidic solution): Ru(bpy)32+ (solid line, λmax ) 455 nm, ε452 ) 1.45 × 104 M-1 cm-1 in water), PSF (dashed line, λmax ) 521 nm, ε521 ) 4.10 × 104 M-1 cm-1 in water), R6G (dotted line, λmax ) 525 nm, ε530 ) 1.16 × 105 M-1 cm-1 in ethanol). The corresponding emission spectra in the same solutions are also shown.

Figure 3. (a) Cyclic voltammograms of 0.1 M KNO3 at bare ITO (solid line), ITO coated with a 24 nm thick SSEBS film (dotted line), and ITO coated with a 340 nm thick SSEBS film (dashed line). The potential was scanned in the appropriate window at 25 mV/s for 3 cycles. The last cycle for each electrode is shown. (b-e) Cyclic voltammograms of the uptake of Ru(bpy)32+ and PSF into SSEBS films of different thicknesses: (b) Ru(bpy)32+/24 nm; (c) PSF/24 nm; (d) Ru(bpy)32+/340 nm; and (e) PSF/340 nm. Analyte solution concentration was 10-5 M, which was prepared in a 0.1 M KNO3 supporting electrolyte solution. Voltammograms were obtained by scanning within the appropriate potential window (ν ) 5 mV/s) with 20 s delay between each scan.

appeared to reach steady-state, the ∆Ep measured was 116 and 102 mV for the 24 and 340 nm films, respectively, with the thinner film now being greater. Continued cycling with both film thicknesses gave continuously increasing ∆Ep even though peak current was constant. We speculate that PSF is slowly fouling the electrode surface, which inteferes with heterogeneous electron transfer. Spectroscopic Evaluation of Partitioned Species in the SSEBS Film. The final study completed in the characterization of the SSEBS material evaluated the optical response of the cationic chromophores loaded into the film. Luminescence and absorbance were used as the detection methods in this study as the spectroelectrochemical sensor has the capability to detect analytes based on both methods. Ru(bpy)32+, PSF, and R6G were the three model analytes used in this study because of their good optical properties and, in the case of Ru(bpy)32+ and PSF,

their reversible electrochemistry. The absorbance and emission spectra of each analyte are shown in Figure 4. Absorbance Studies. Figure 5a shows the uptake of each of the analytes into different 340 nm SSEBS films. The uptake data is plotted as normalized absorbance versus time to show all of the analytes on the same scale. Approximately 2.5 min after the introduction of the analytes into the sample cell, a measurable signal was detected, corresponding to the uptake into the film. Compared to the uptakes measured electrochemically in quiescent solution (Figure 3), the time required for the analytes to reach equilibrium in this study was significantly reduced because the solution was continuously flowed across the film at 0.1 mL/min, which enhanced mass transport of the analyte to the film. The portion of the Ru(bpy)32+ curve at 80 min and greater (Figure 5a) illustrates the optical response that resulted from cycling an applied potential. In the case of Ru(bpy)32+, the applied potential was cycled from 0.7 to 1.3 V at 5 mV/s. As the applied potential approached 1.3 V, Ru(bpy)32+ began to oxidize to Ru(bpy)33+, which is colorless and does not absorb at 450 nm. This results in the dramatic drop in absorbance. As the applied potential returns to 0.7 V, the Ru(bpy)33+ is reduced back to its original form, Ru(bpy)32+, causing the absorbance to again increase. The change in absorbance (∆A), also referred to as optical modulation, is the difference in the absorbance from peak to peak. The ∆A value is used to determine the concentration of analyte in the sample. Implementing the electrochemistry step is valuable in that it provides another level of selectivity. Species present that may potentially interfere with the detection of the analyte at the monitored wavelength can be excluded if they are not electroactive, do not have reversible electrochemistry, or if they do not electrolyze in the same potential window as the analyte. The Ru(bpy)32+ modulations shown in Figure 5a are atypical compared to the response usually seen with the Ru(bpy)32+/ Nafion system. The absorbance values of the peaks of the modulations are greater than the initial plateau. One plausible explanation is that electrostatic cross-linking of the polymer increases upon electrochemical generation of the 3+ form, which causes the film to contract. Although, the amount of Ru(bpy)32+/3+ in the film has not changed, the concentration has increased because of the decrease in film volume. In a Analytical Chemistry, Vol. 81, No. 16, August 15, 2009

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Figure 5. (a) Normalized absorbance for the uptake of each analyte. Analyte solution concentration was 10-5 M in 0.1 M KNO3. The absorbance of each analyte was monitored at the following wavelengths: Ru(bpy)32+ at 450 nm; PSF at 520 nm; R6G at 526 nm. (b) Normalized luminescence for the uptake of each analyte. Analyte solution concentration was 10-6 M in 0.1 M KNO3. The excitation/ emission wavelengths used for each analyte are as follows: Ru(bpy)32+ 441 nm/605 nm; PSF 532 nm/586 nm; R6G 532 nm/556 nm. The modulation of Ru(bpy)32+ is also shown for both detection methods. (c) Normalized luminescence spectra of the R6G dye during uptake. The spectra were recorded under MIR at each 10 min period. A 340 nm thick SSEBS film was used for all measurements.

normal transmission experiment, this behavior of the film would have no affect on the absorbance measurement. However, sensor absorbance is measured via ATR; therefore, an increase in concentration close to the electrode surface where the electromagnetic field of the evanescent wave is greater has a significant effect on the absorbance measurement. This apparent increase in concentration of Ru(bpy)32+ in the film would explain why the absorbance further increases after 6762

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beginning the modulation step. Additionally, the modulations did not return to baseline when oxidized. Slow diffusion in the film limits the mass transport of the analyte to the electrode surface in the time frame of the potential scan. A slower sweep rate can remedy this problem; however, the drawback is that analysis times will take much longer. A calibration curve compiled for the absorbance-based sensor using the 24 nm film and Ru(bpy)32+ is shown in Figure 6a. The thinner film was used for the calibration curve to reduce analysis time. For each run, the uptake was monitored until equilibrium was reached. At that point, the optical signal was modulated by cycling the potential between 0.7 and 1.3 V at 5 mV/s. The ∆A value reported was the average of seven optical modulations. The calibration curve has a near linear relationship between ∆A and concentration up to 5 µM: ∆A ) aC + b, where a is the slope of the line (a ) 0.015 ± 0.0015) and b is the intercept (b ) 0.007 ± 0.004), R2 ) 0.980. In the high concentration region, the signal levels-off as the polymer becomes saturated with the analyte. Each error bar represents one standard deviation on either side of the mean of three trials. For each trial, a different 24 nm film was used. The calculated detection limit based on the calibration curve is 5 × 10-7 M. An absorbance-based sensor using Nafion for the detection of Ru(bpy)32+ under similar conditions exhibited a detection limit of 5 × 10-8 M,33 which is 1 order of magnitude lower compared to the sensor using SSEBS. Luminescence Studies. The uptake of the analytes into different 340 nm SSEBS films as monitored by luminescence is shown in Figure 5b. The uptake data is plotted as normalized luminescence versus time to show all of the analytes on the same scale. All three analytes rapidly reached steady-state; however, the loading times for the uptake of PSF and R6G appear to be almost half compared to the uptakes monitored by absorbance. Moreover, the characteristics of PSF and R6G uptake were noticeably different from that of Ru(bpy)32+. The optical signals of both PSF and R6G reached a maximum value and then began to decrease whereas the Ru(bpy)32+ signal leveled off. Because this behavior was not observed with these analytes using the absorbance-based sensor, it is unlikely that this decrease is due to analyte leaching from the film. Yet, there are several possible reasons for this behavior that might act separately or simultaneously. Kubin and Fletcher29 studied the fluorescence of several rhodamine dyes, and in the case of R6G, they found that the solution Stokes shifts were concentration dependent. In our studies where we monitored the uptake at the wavelength of the maximum emission, a shift in the measured fluorescence band would certainly cause a decrease in the signal. However, further investigation ruled out this explanation based on the results shown in Figure 5c. The uptake of R6G into the 340 nm SSEBS film was monitored by obtaining an emission spectrum every 10 min. In the initial period (until the 30 min mark) the signal increased rapidly. Thereafter, the intensity began to decrease until it reached a steady state (60-90 min marks). These changes in the luminescent intensity were consistent with the R6G response in Figure 5b, although to some extent faster probably because of variation in film thickness. The (33) Andria, S. E.; Richardson, J. N.; Kaval, N.; Zudans, I.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2004, 76, 3139–3144.

Figure 6. Calibration curves for Ru(bpy)32+ in a 24 nm thick SSEBS film based on absorbance (a) and luminescence (b) as the detection method.

Stokes shifts were nearly identical at any point of time and clearly resembled the luminescence profile of the low-concentration R6G spectrum in solution (Figure 4). The same study was completed with the PSF, and analogous behavior was observed. Additional explanations are also based on concentration dependent spectral changes. At concentrations approaching 1 M, the planar PSF and R6G molecules tend to self-aggregate into dimers and higher order aggregates, some of which are not luminescent or have changed spectral properties.34-36 In fact, our recent report37 documents in situ self-aggregation of R6G molecules in SSEBS thin films using dynamic spectroscopic ellipsometry. In addition, self-quenching of the luminescence at such high concentrations also offers another plausible explanation. It is unlikely that a decrease in R6G could be attributed to photobleaching given the robustness of this dye under lasing conditions where excitation intensity is orders of magnitude higher. Therefore, it is reasonable to conclude that the decrease in luminescence seen with PSF and R6G is most likely due to self-aggregation or self-quenching, and the difference in loading times between the luminescence and absorbance studies for PSF and R6G appears to be an artifact of these concentration dependent spectral changes. (34) Arbeloa, I. L.; Rohatki-Mukherjee, K. K. Spectrochim. Acta 1988, 44A, 423. (35) Sasai, R.; Fujita, T.; Iyi, N.; Itoh, H.; Tagaki, K. Langmuir 2002, 18, 6578– 6583. (36) Gavrilenko, V. I.; Noginov, M. A. J. Chem. Phys. 2006, 124, 44301–44306. (37) Pantelic, N.; Seliskar, C. J. J. Phys. Chem. C 2007, 111, 18595–18604.

The Ru(bpy)32+ curve in Figure 5b illustrates the optical modulations that resulted from cycling the potential from 0.7 to 1.3 V at 5 mV/s. However, unlike the absorbance study, there was a significant decrease in signal once oxidation began, which continued to decrease throughout the rest of the modulation cycles. This phenomenon has been observed before in our luminescence studies of Ru(bpy)32+ in Nafion. According to Rubinstein and Bard,38 the excited form of the complex (Ru(bpy)32+*) can be quenched by its oxidized form (Ru(bpy)33+) via electron-transfer. Therefore, once the Ru(bpy)33+ is produced in the first cycle of the modulations, it is nearly impossible to regenerate all of the ruthenium complex back to its 2+ form. However, this effect is decreased if the potential is scanned more rapidly between the oxidative and reductive potentials.28 Figure 6b illustrates a calibration curve compiled for the luminescence based sensor using the 24 nm films and Ru(bpy)32+. The thinner film was used in the calibration curve to reduce analysis time. In each run, the uptake was monitored until it reached equilibrium. Because of the decrease in optical signal during electrochemical modulation, we used the first modulation as the change in luminescence intensity. Instead of cycling the potential, the change in luminescence was produced by stepping the potential to 1.3 V for 50 s. The calibration curve demonstrates a near linear relationship between the change in luminescence and concentration in the range from 10-9 to 10-6 M: log I ) a log C + b, where a is the slope of the line (a ) 0.74 ± 0.01) and b is the intercept (b ) 8.95 ± 0.11), R2 ) 0.998. The calculated detection limit of 5 × 10-10 M is lower than expected based on visual inspection of Figure 6b. The discrepancy is a result of the film-to-film irreproducibility and contamination of the system by Ru(bpy)32+, which is apparent at extremely low concentrations. As was expected, a lower concentration of the analyte can be detected by luminescence. The sensor response begins to saturate at concentrations higher than 10-6 M. The dynamic range was significantly extended compared to the absorbance calibration curve. A luminescence-based sensor using Nafion to detect Ru(bpy)32+ under similar conditions exhibited a detection limit of 10-9 M,28 which is ∼1.5 orders of magnitude higher compared to the SSEBS results. The SSEBS luminescence-based sensor exhibits a lower detection limit when compared to a Nafion luminescencebased sensor, but the opposite is observed for the absorbancebased sensors. We attribute this to some degree of quenching of the fluorescence in the Nafion film, which would degrade the sensitivity of the fluorescence-based sensor. CONCLUSIONS The effectiveness of the SSEBS block co-polymer material in thin film form for chemical sensing has been demonstrated. The material has properties that make it a suitable element for sensing based on spectroelectrochemistry. The main conclusions of this work are as follows: (1) Thin, uniform, and optically transparent films can be easily fabricated with the SSEBS polymer. (2) The robustness of the SSEBS air-dried film was demonstrated with the peel-test, and formulations that weakly adhered in aqueous media can be stabilized using appropriate surface chemistries. (3) (38) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007–5013.

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There was no measurable interference of the material with the electrochemical reaction of the model analytes studied under the conditions used. (4) Both absorbance and luminescence sensor operations were demonstrated with the appropriate model analytes. As was expected, lower concentrations of the analytes can be detected using luminescence. Overall, the SSEBS material proved to be a good preconcentrating film for chemical sensing, and its performance is comparable to that of Nafion under similar conditions.

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ACKNOWLEDGMENT This research was suppored by the Office of Science (BER), U.S. Deparment of Energy, Grant DE-FG02-07ER64353. The purchase of the spectroscopic ellipsometer was made possible by a grant from the Hayes Fund of the State of Ohio.

Received for review April 9, 2009. Accepted July 3, 2009. AC900765T