Anal. Chem. 2001, 73, 2883-2889
Bidimensional Spectroelectrochemistry Jesu´s Lo´pez-Palacios,* Alvaro Colina, Ara´nzazu Heras, Virginia Ruiz, and Luis Fuente
Area de Quı´mica Analı´tica, Universidad de Burgos, Pza. Misael Ban˜uelos, s/n. 09003 Burgos, Spain
A new methodology is presented to offer the possibility of simultaneously obtaining two different spectroscopic signals in a single spectroelectrochemical experiment. Taking the plane of the electrode surface as a spatial reference, normal-beam and parallel-beam UV-vis absorbance signals are jointly analyzed, revealing important experimental differences between the two kinds of signals. Two different chemical systems are selected to show the possibilities of the bidimensional spectroelectrochemistry: a simple diffusive process and an adsorptive electrode reaction. Comparative results show clearly that the two kinds of spectroscopic signals, both normal and parallel to the electrode surface, have to be used together in the study of any electrode reaction scheme. Spectroelectrochemical techniques have been extensively used in the determination of a number of parameters,1-3 such as standard potentials, diffusion coefficients, electron-transfer rate constants, number of electrons in an electrode reaction, etc. In addition, they have been widely used in the elucidation of reaction mechanisms in different organic,4,5 inorganic,6,7 and biochemical8-10 systems. In the past few years, a number of different spectroelectrochemical devices have been proposed, allowing the study of both thin-layer7,11-15 and semiinfinite diffusion16,17 processes and using not only absorption of UV-vis radiation, but also a variety of spectroscopic techniques, such as Raman,18,19 FT-IR,20,21 EPR,22 and X-ray spectroscopy.23,24 * Fax: 34 947 25 88 31. E-mail:
[email protected]. (1) Kuwana T.; Winograd, N. Electroanalytical Chemistry; Bard, A. J., Ed.; 1974, Vol. 7, Chapter 1, pp 1-78. (2) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. Electroanalytical Chemistry; Bard, A. J., Ed.; 1984, Vol. 13, Chapter 1, pp 1-113. (3) Niu, J.; Dong, S. Rev. Anal. Chem. 1996, 15, 1-2, 1-171. (4) Lapkowski, M.; Strojek, J. W. J. Electroanal. Chem. 1985, 182, 315-333. (5) Flowers, P. A.; Mamantov, G. Anal. Chem. 1989, 61, 190-192. (6) Lanc¸ on, D.; Kadish, K. M. J. Am. Chem. Soc. 1983, 105, 5610-5617. (7) Paulson, S. C.; Elliott, C. M. Anal. Chem. 1996, 68, 1711-1716. (8) Deputy, A.; Wu, H. P.; McCreery, R. L. J. Phys. Chem. 1990, 94, 36203624. (9) Niemz, A.; Imbriglio, J.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119, 887892. (10) Keesey, R. L.; Ryan, M. D. Anal. Chem. 1999, 71, 1744-1752. (11) Murray, R. W.; Heineman, W. R.; O’Dom, G. W. Anal. Chem. 1967, 39, 1666-1668. (12) Salbeck, J. Anal. Chem. 1993, 65, 2169-2173. (13) Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983, 55, 2219-2222. (14) Flowers, P. A.; Callender, S. A. Anal. Chem. 1996, 68, 199-202. (15) Niu, J.; Dong, S. Electroanalysis 1995, 7, 1059-1062. (16) Salbeck, J. J. Electroanal. Chem. 1992, 340, 169-195. (17) Xie, Q.; Wei, W.; Nie, L.; Yao, S. Anal. Chem. 1993, 65, 1888-1892. (18) Mosier-Boss, P. A.; Newbery, R.; Szpak, S.; Lieberman, S. H. Anal. Chem. 1996, 68, 3277-3282. 10.1021/ac0014459 CCC: $20.00 Published on Web 06/05/2001
© 2001 American Chemical Society
UV-vis absorptive spectroelectrochemistry gives us the possibility of obtaining some information on the concentration of electroactive species off the electrode surface. Two different kinds of spectroelectrochemical devices are commonly used for planar electrode experiments: (i) normal-beam, in which the light beam passes through the diffusion layer perpendicularly to the electrode surface,25,26 for example, optically transparent electrodes (OTE) or reflectance electrodes; (ii) parallel-beam, in which the electromagnetic beam follows a direction parallel to the electrode surface,17,27,28 as occurs in long-optical-pathway thin-layer cells (LOPTLC). The information contained in each kind of measurement is different, because the normal-beam measurements consider mainly the global quantity of absorbent transformed during the electrode process, but the parallel-beam measurements are strongly influenced by the spatial distribution of this absorbent with respect to the electrode surface, that is to say, by the diffusion of the species taking part in the reaction. This fact has been pointed out in the literature,29 but both of the configurations have usually been employed without distinction in the determination of the same parameters.25,28,30 Some researchers have found the parallel configuration to be more advantageous solely because of its longer optical path length in absorbance measurements.31,32 On the contrary, we propose that during an electrochemical experiment, the solution near the electrode surface must be considered as an anisotropic medium with respect to the absorbance, and so the information the light beam contains is strongly dependent on the direction chosen for the observation. In the present work, we prove that both kinds of spectrometric data must be considered together if a reliable explanation about a possible adsorption process is to be given. We have constructed (19) Gouveia, V. J. P.; Gutz, I. G.; Rubim, J. C. J. Electroanal. Chem. 1994, 371, 37-42. (20) Kulesza, P. J.; Malik, M. A.; Denca, A.; Strojek, J. Anal. Chem. 1996, 68, 2442-2446. (21) Pharr, C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4673-4679. (22) Frapart, Y. M.; Boussac, A.; Albach, R.; Anxolabe´he`re-Mallart, E.; Delroisse, M.; Verlhac, J. B.; Blondin, G.; Girerd, J. J.; Guilhem, J.; Cesario, M.; Rutherford, A. W.; Lexa, D. J. Am. Chem. Soc. 1996, 118, 2669-2678. (23) Dewald, H. D.; Watkins II, J. W.; Elder, R. C.; Heineman, W. R. Anal. Chem. 1986, 58, 2968-2975. (24) Yoshitake, H.; Yamazaki, O.; Ota, K. J. Electroanal. Chem. 1994, 371, 287290. (25) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981, 53, 13901394. (26) Zhao, M.; Scherson, D. A. Anal. Chem. 1992, 64, 3064-3067. (27) Rossi, P.; McCurdy, C. W.; McCreery, R. L. J. Am. Chem. Soc. 1981, 103, 2524-2529. (28) Wei, W.; Xie, Q.; Yao, S. Electrochim. Acta 1995, 40, 1057-1061. (29) Pruiksma, R.; McCreery, R. L. Anal. Chem. 1979, 51, 2253-2257. (30) Zhangyu, Y.; Tiande, G.; Mei, Q. Anal. Chem. 1994, 66, 497-502. (31) Nagy, T. R.; Anderson, J. L. Anal. Chem. 1991, 63, 2668-2672. (32) Gui, Y.; Soper, S. A.; Kuwana, T. Anal. Chem. 1988, 60, 1645-1648.
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a new device that allows one to obtain more complete information on the spectroelectrochemical behavior of a system. Besides the electrochemical information (voltammetric or chronoamperometric data), both the normal-beam and the parallel-beam signals were recorded and processed simultaneously. We have rejected the option of obtaining and comparing both spectroscopic signals sequentially because of the uncertainty about the reproducibility of the process in the very small time window that we have to use during the measurements (of the order of one millisecond). The whole set of spectroelectrochemical information (two-dimensional) reveals the presence of phenomena, such as adsorption, that cannot be completely explained by performing only one-dimensional spectroelectrochemical observations. THEORY An electrode reaction creates a spatial perturbation that breaks the homogeneity of the solution in the proximity of the electrode. During the electrode reaction, reactants and products are distributed in the solution in a nonuniform way for which diffusion tends to compensate. Purely electrochemical measurements give information only about the surface phenomena occurring on the electrode. Other kinds of information, such as concentration in the bulk solution or diffusive parameters, can be indirectly deduced from the relationships between current, potential, and time, assuming the behavior of the system follows a previously known model. Spectroelectrochemical techniques give us the possibility of knowing immediately what is happening in the vicinity of the electrode surface by measuring the absorbance of the solution around the electrode in some different ways. When a light beam goes perpendicularly through an optically transparent electrode, the absorbance measured, AN, can be expressed by33
AN )
∫
ω
0
c(x) dx
(1)
where is the molar absorptivity, c(x) is the absorbent concentration at a distance x from the electrode surface, and ω is the thickness of the diffusion space. Eq 1 shows that the measured signal is the average of the absorbance in each one of the elements crossed by the light beam. The measured absorbance, which we call AN because the light beam is normal to the electrode surface, is here proportional to the total mass of absorbent in the optical pathway. Measurements of absorbance in the normal-beam configuration contain the same information as faradaic charge measurements, which are also dependent on the quantity of mass transformed in the electrode reaction. Electrical charge can be evaluated from
AN )
Q nFS
(2)
where S is the area of the electrode surface on which the electrical charge Q is transferred during the faradaic process, n is the number of electrons involved in the electrode reaction, and F is the Faraday constant. From eq 2, charge Q can be evaluated and derived with respect to time, giving a signal corresponding to the (33) Strojek, J. W.; Kuwana, T. J. Electroanal. Chem. 1968, 16, 471-483.
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faradaic current as a function of the electrode potential. The difference between the original voltammogram and the current evaluated from absorbance allows us to obtain a measure of the current as a result of processes other than this observed spectroscopically. As is well-known, the derivative of the absorbance with respect to time shows the same morphology as the voltammogram (derivative of charge with respect to time). Many models for voltabsorptometry25,34,35 and chronoabsorptometry17,33 have been developed taking into account this similarity. On the other hand, if the light beam goes through the diffusion layer in a direction parallel to the electrode surface, the absorbance, AP, is17
AP ) log
(∫
ω
0
ω 10
-lc(x)
dx
)
(3)
where all of the symbols have the same meaning as in eq 1, and l is the optical path length. Now the absorbance is not proportional to the total mass of the absorbent but, rather, depends on its distribution along the diffusion layer. The fact that both configurations, normal- and parallel-beam, yield different signals can be foreseen from eqs 1 and 3, which are easily comparable only when the concentration of electroactive chromophore is constant along the diffusion space (flat concentration profile). When this is not the case, that is to say, when there is a concentration gradient, comparison of eqs 1 and 3 is not a trivial question. The problem can be faced by digital simulation. With this aim, we have developed a software package written in FORTRAN 90 and based on previous models.17 For instance, some differences between normal-beam and parallel-beam signals are shown in Figure 1 for a simple system, Ox/Red, with no adsorption or coupled chemical reaction involved in the process, that is to say, where only diffusion controls the movement of Ox and Red to and from the electrode surface. Figure 1a shows normal (circles) and parallel (solid line) simulated voltabsorptograms obtained under finite diffusion conditions. We suppose the parallel-beam samples the whole of the diffusion space. A scan rate of 0.001 V‚s-1 and a thickness of the diffusion space of 100 µm were chosen to attain a thin-layer behavior. The signals were normalized to allow an easier comparison. Under the mentioned conditions, both signals became identical, because of the homogeneity of the solution. The only difference is the higher value of actual (non-normalized) absorbance obtained in the parallel configuration as a result of the longer optical path length. Figure 1b shows the voltammograms together with the derivative voltabsorptograms, dA/dt. As can be expected, all three normalized signals coincide exactly. Maintaining the rest of the conditions, the scan rate was changed to 0.02 V‚s-1 in order to reach a nonhomogeneous concentration profile in the finite diffusion space. Figure 1c shows that the normalized absorbance becomes different in both configurations. To emphasize these differences, Figure 1d shows derivatives of the normalized spectroscopic signals together with the current-potential response. As we can see, for the parallel(34) Zamponi, S.; Czerwinski, A.; Gambini, G.; Marassi, R. J. Electroanal. Chem. 1992, 332, 63-71. (35) Xie, Q.; Wei, W.; Nie, L.; Yao, S. J. Electroanal. Chem. 1993, 348, 29-47.
Figure 1. Simulated spectroscopic and electrochemical responses of a reversible process, assuming that the oxidized form is the absorber: ω ) 100 µm; DO ) DR ) 5 × 10-6 cm2‚s-1; Ο ) 100 L‚mol-1‚cm-1; l ) 1 cm; CR* ) 0.01 mol‚L-1; E0 ) 0 V; (a) voltabsorptograms in both configurations at scan rate of 0.001 V‚s-1; (b) voltammogram and derivative voltabsorptograms at scan rate 0.001 V‚s-1; (c) voltabsorptograms at scan rate 0.02 V‚s-1; (d) voltammogram and derivative voltabsorptograms at scan rate 0.02 V‚s-1; s, parallel-beam signal; o, normal-beam signal; + voltammogram.
beam configuration, the anodic peak becomes lower than the cathodic peak. Moreover, the electrical signal coincides with the normal-beam spectroscopic signal but not with the parallel-beam signal, as we expected from eqs 1, 2, and 3. Thus, the parallel signal depends not only on the amount of absorbent but also on its spatial distribution, as has been pointed out before using diffracted light measurements.27 However, the most important differences between normal and parallel configurations cannot be deduced from the equations above. The main advantage of joint and simultaneous observation of both kinds of spectral signals (bidimensional spectroelectrochemistry) lies with the information about adsorption steps and chemical coupled reactions in the electrode process. For instance, if an adsorption process of a chromophore product of the reaction takes place in the electrode surface, normal-beam configuration becomes the most sensitive one, despite its shorter path length. This behavior is not easily observed and understood unless both kinds of measurements, normal and parallel, are jointly obtained. The following experiments state the capabilities of this new technique. EXPERIMENTAL SECTION A conventional three-electrode system controlled by a PGSTAT 20 (Eco Chemie B.V., The Netherlands) potentiostat was used in all experiments. A light source DH-2000 (Top Sensor Systems, The Netherlands) of double lamp, halogen and deuterium, supplied an electromagnetic beam that was taken to the sample cell
through a bifurcated optic fiber (Figure 2) that was fitted with suitable lenses at its ends. After passing through the solution, both light beams were separately conducted to their respective spectrometers and then taken to the detectors (S2000 Fiber Optic Spectrometer; Ocean Optics, Dunedin, FL), both of them made up of a diode array with 2048 elements. Measurements were carried out using a special cell, designed and constructed in our laboratory, based on a planar optically transparent gold electrode, allowing the light beams to cross the solution simultaneously in both normal and parallel directions to the electrode surface (Figure 3). Our main objective was to make a cell able to fit into a standard 45 × 10 × 10 mm spectrophotometric cuvette, thus allowing us to use commercial holders and accessories. The cell (Figure 3) combines simplicity of assembly and cleaning with versatility of use and low cost. It can be used both in semi-infinite and thin-layer diffusion measurements, with both optically transparent electrodes and long-pathway configuration. Different kinds of electrodes can be used, depending on the nature of the system to be studied. As can be seen in Figure 3, two plane parts (a and b) are placed face to face inside the cuvette. The distance between them is controlled by two pieces of a selected thickness spacer (c). The smallest spacer used in our experiments was 30 µm. To ensure that radiation collected by the detector in parallel configuration comes only from the slit, lateral sides of the plates a and b were painted with inert black ink. The part denoted a in the figure acts as an optically transparent working electrode (OTE). It was made of a thin layer of gold sputtered (Emitech K550, Emitech, U.K.) over a piece of glass or quartz (70 × 9.5 × 2 mm) adequately masked to yield the desired geometric dimensions of the electrode. Previous to the sputtering, the piece of glass or quartz was cleaned with nitric acid and then rinsed with deionized water and ethanol. The electrode obtained was placed on two brackets, d, made of cold epoxy resin (10 × 9.5 × 2.5 mm). Two pieces of Teflon were used at times for the same purpose. Such assembly simplifies the alignment with the optical system of the spectrophotometer. Part b is an epoxy resin inert wall (45 × 9.5 × 4 mm) in which a circular hole of 3 mm diameter (e) was pierced just in front of the active surface of the working electrode. The hole is limited by a quartz piece that allows the light beam to go through the working electrode in a normal direction and maintaining at the same time the conditions of thin-layer cell. In part b, a small gap was made for the reference electrode (f). The reference electrode was placed just above the active surface of the working electrode in order to minimize ohmic drop and to have no interference in the optic pathway. With this aim, a Ag/AgCl/KCl 3 M reference electrode was handmade in the laboratory using a micropipet plastic tip. The pipet tip was filled with a stopper made of agar with a suitable alkaline salt, acting as supporting electrolyte, and potassium chloride in the upper part. A thin wire of silver coated with silver chloride was introduced into the pipet tip through a Teflon stopper. The reference microelectrodes were always calibrated versus commercial reference electrodes before use. A platinum wire (0.5 mm ø) placed sideways with regard to the inert wall was taken as counter electrode (g). Before use, the electrode was polished mechanically with 1-µm alumina paste Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
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Figure 2. Scheme of the main parts in the experimental device (electrodes and electrical connections are not represented). Table 1. Uncompensated Resistance and Peak Potential Separation for o-Tolidine Solutions C (mol‚L-1)
v (V‚s-1)
R (Ω)
(∆Ep)M (V)
(∆Ep)C (V)
5‚10-4
0.01 0.02 0.05
28.0
0.045 0.050 0.056
0.044 0.049 0.053
2.5‚10-4
0.01 0.02 0.05
23.1
0.036 0.039 0.039
0.035 0.038 0.038
1‚10-4
0.01 0.02 0.05
27.0
0.032 0.032 0.038
0.029 0.028 0.036
4‚10-5
0.01 0.02 0.05
25.2
0.037 0.045 0.055
0.036 0.045 0.054
(∆Ep)M is the peak potential separation before correction. (∆Ep)C is the peak potential separation after correction.
Figure 3. Schematic diagrams (3D, front and side views) of the bidirectional thin-layer spectroelectrochemical cell constructed and used in the experiments: (a) gold-sputtered working electrode; (b) epoxy resin piece; (c) spacers; (d) epoxy resin brackets; (e) circular window, Ø ) 3 mm; (f) reference electrode; (g) counter electrode. (N) Normal and (P) parallel-beam referred to the working electrode surface.
(Struers, Denmark) and then electrochemically conditioned in 1 M sulfuric acid. The lenses at the end of the optical fibers were arranged in such a way that the collimated light beams never exceeded the geometric dimensions of the working electrode. Special attention was paid to the synchronization of potentiostat and optical detectors in voltammetric experiments. An external trigger ensured that spectroscopic measurements as fast as 5 ms/ point were exactly synchronized with the electrode potential in each moment. During a spectroelectrochemical experiment, a number of spectra ranging from 100 to 5000 were taken in each detector. Their exact number depends on the length of the 2886 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
experiment and on the sampling frequency. Taking into account the number of diodes in each detector (2048), very often an experiment generates files with more than 10 million points. This high number of data could not be treated with commercial software, and a package of programs was developed using MATLAB 4.2 (MathWorks Inc., Natick, MA) and OCTAVE 2.0.13 for a double-processor Pentium II/300 MHz. All of the chemicals in the experiments were analytical grade and used as received, except the dimers used in the electropolymerization reactions, which were obtained in the laboratory. Aqueous solutions were prepared using high-quality water (MilliQ gradient A10 system, Millipore, Bedford, MA). RESULTS AND DISCUSSION Cell Validation. The thin-layer cell described above was validated carrying out voltammetric measurements on o-tolidine, a well-known electrochemical system usually taken as standard. Four concentrations of o-tolidine (5‚10-4, 2.5‚10-4, 1‚10-4, and 4‚10-5 mol‚L-1) were tested in 0.5 M acetic acid + 1 M perchloric acid. Table 1 shows the results of cyclic voltammetry measurements obtained at scan speeds of 10, 20, and 50 mV‚s-1. Uncompensated resistance of the solution was measured and corrected by applying positive feedback, giving a very low ohmic drop in every case. After compensation, the differences between the potentials of anodic and cathodic peaks remained almost unchanged. In addition, some parameters of the electrode reaction were obtained for the same system. Number of electrons, n )
1.89 ( 0.01, was found by absorptometric measurements in steady state; the standard potential, E°, obtained was 0.684 ( 0.005 V vs Ag/AgCl. A mean value of molar absorptivity, 438 ) 60670 ( 130 L‚mol-1‚cm-1, was calculated from measurements in both normal and parallel configurations under conditions of total electrolysis. Taking this system as electrochemically reversible, values for diffusion coefficients were estimated by fitting experimental data to the model based on eqs 1 and 3, giving as results DR ) 6.2 ( 0.5‚10-6 cm2‚s-1 and DO ) 6.5 ( 0.8‚10-6 cm2‚s-1. All of these values agree with the bibliography25,33,34,36 and prove the good performance of the cell that was developed. Bidimensional Measurements on Different Electrode Reactions. As has been stated before, spectroelectrochemical measurements, either by normal or parallel incidence, have been successfully used in the estimation of electrode reaction parameters. Preliminary trials showed that absorbance in long pathway configuration does not yield the best sensitivity with all the systems tested, contrary to what we had expected. Accordingly, the choice of one or another configuration needs the previous knowledge of the steps of the global process. Bidimensional measurements are themselves explanatory enough to resolve what is the most advisable configuration. With the aim of showing this capability, two different chemical systems were considered: (i) o-tolidine, which is usually taken as an example of a simple system; and (ii) electropolymerization of bithiophenes, taken as representative of a process leading to the formation of a sticky conductive film on the electrode surface (adsorption process). Noncomplicated Diffusive System. o-Tolidine (4,4′-diamino3,3′-dimethylbiphenyl, [-C6H3(CH3)-4-NH2]2) has been spectroelectrochemically studied in a number of works30,34,37 using both normal and parallel-beam arrangements. Figure 4 shows comparative results by bidimensional spectroelectrochemistry (Figure 4a, normal-beam signal; Figure 4b, parallel-beam signal) for a 10-4 mol‚L-1 solution of o-tolidine in 1 M perchloric acid and 0.5 M acetic acid. The scan rate was 0.005 V‚s-1 and a spacer of 160 ( 5 µm was used to obtain a thin-layer cell with a geometrically long path length of 3.00 ( 0.05 mm. Both absorbance signals (AN ) absorbance in normal configuration; AP ) absorbance in parallel) rise from the starting potential (0.35 V, t ) 0 s) to the vertex potential (0.9 V, t ) 110 s) and decrease in the backward sweep to the initial value. In agreement with previous data in the literature, a maximum value of the absorbance for the oxidized form of o-tolidine was obtained around 438 nm. The reduced form does not absorb in the visible zone, and no other absorbent was found in the same spectral domain, thus allowing us to suppose a simple reaction scheme. The parameters mentioned in the cell validation paragraph were taken to simulate the derivative voltabsorptograms that are represented in Figure 5, together with experimental data. The good agreement between both sets of data confirms that normal and parallel spectra are related by eqs 1 and 3, both signals providing the same information but with a different appearance. Under experimental conditions leading to an almost homogeneous distribution of chromophore in the diffusion space, AN and AP values become proportional. A mean value of 19.76 for the constant of proportionality was found in our experiment, which agrees with the optical path length (parallel (36) Petek, M.; Neal, T. E.; Murray, R. W. Anal. Chem. 1971, 43, 1069-1074. (37) Hansen, W. N.; Kuwana, T.; Osteryoung, R. A. Anal. Chem. 1966, 38, 18101821.
Figure 4. 3D plot absorbance/potential (time)/wavelength for otolidine 10-4 mol‚L-1 in CH3COOH 0.5 M and HClO4 1 M: v ) 0.005 V‚s-1, ω ) 160 µm. (a) Normal-beam signal, (b) parallel-beam signal.
configuration) to the diffusion space thickness (normal) ratio, l/ω. In such a case, the most sensitive signal (parallel arrangement) is usually advisable, as has been pointed out in the literature3. Adsorption and Related Processes. Electropolymerization of 4-4′-bis(methylthio)-2,2′-bithiophene in acetonitrile was studied as a model of a nondiffusive system. The cyclic oxidation-reduction of bithiophenes leads to the formation of a conducting polymer film on the electrode surface, which grows in successive cycles.38-40 The oxidized form absorbs around 750 nm, but the reduced (neutral) form exhibits a maximum of absorbance at approximately 540 nm. Figure 6 shows the bidimensional absorption spectra obtained during a six-cycle experiment performed between 0.0 and 1.1 V vs Ag/AgCl at scan rate of 0.01 V‚s-1 (see the rest of the experimental conditions in Figure 6). The polymer growth and its oxidation and reduction on the electrode surface can be clearly observed in normal-beam configuration (Figure 6a). The absorbance peaks of the oxidized polymer form (λmax ) 750 nm at 1.1 V) and neutral form (λmax ) 540 nm at 0.0 V) grow with an increasing number of cycles, which indicates that a new amount of polymer is formed on the electrode during each potential cycle. On the other hand, the spectra obtained by parallel-beam configuration (Figure 6b) do not show these bands, because the polymer stuck to the electrode surface does not diffuse into the solution, but another remarkable absorption region around 400 nm appears, as a result of soluble oligomers with shorter chain lengths. That is to say, spectra (38) Roncali, J. Chem. Rev. 1992, 92, 711-738. (39) Olbrich-Stock, M.; Posdorfer, J.; Schindler, R. N. J. Electroanal. Chem. 1994, 368, 173-181. (40) Iarossi, D.; Mucci, A.; Schenetti, L.; Seeber, R.; Goldoni, F.; Affronte, M.; Nava, F. Macromolecules 1999, 32, 1390-1397.
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Figure 6. 3D plot absorbance/potential (time)/wavelength during electropolymerization of 0.005 mol L-1 4-4′-bis(methylthio)-2,2′bithiophene in acetonitrile with TBAPF6 0.1 M as the supporting electrolyte. Display of six scans at v ) 0.01 V‚s-1; ω ) 220 µm; l ) 3 mm. (a) Normal-beam signal, (b) parallel-beam signal.
Figure 5. Derivative voltabsorptograms simulated using the software developed in our laboratory (o) and experimental curves of o-tolidine (s) for λ ) 438 nm. Same experimental conditions as in Figure 4. (a) Normal-beam signal, (b) parallel-beam signal.
obtained in both arrangements are, unlike the o-tolidine, completely different. This can be more easily observed in a cross section presented in Figure 7, in which spectra obtained in the two arrangements at potentials of 0.0 and 1.0 V are shown. The different spectral behavior, directly related to the adsorption process, is the first qualitative information given by bidimensional observation. Neither solely electrochemical nor monodimensional spectroelectrochemical experiments allow us to obtain this information. If only the parallel-beam signal were taken into account, the observer would never know that an adsorption process has occurred. If only normal-beam signal were observed, differentiation between adsorbed and soluble species would not be achieved. Absorbance measurements in normal-beam arrangement, AN, can be used in order to study the polymer growing process. In this case, a wavelength of ∼400 nm could be selected as the best choice, because both forms, oxidized and neutral, display almost the same absorbance. If the parallel-beam spectrum were not known, the contribution of the oligomers to the total absorbance at this wavelength would be overlooked, thus adding a serious error to the estimation. From a quantitative point of view, the absorbance owing to adsorbed polymer alone can be obtained from the measurement in normal-beam mode but corrected by 2888
Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
Figure 7. Sections perpendicular to the time axis of Figure 6 corresponding to the absorbance spectra in the normal- (AN) and parallel-beam (AP) configurations at 0 V (s) and 1V (- - -) during the last cycle.
subtracting the value of the weighted absorbance in parallel-beam at the same wavelength. That is to say,
ACN ) AN - AP ×
ω l
where ACN is the corrected normal-beam absorbance.
(4)
Table 2. Evolution of Normal-Beam Absorbance, AN, at 400 and 540 nm during the Electropolymerization of 4-4′-Bis(methylthio)-2,2′-bithiophene Measured at the End of Each Cycle and Corrected Values for the Same Measures, ACNa cycle no.
AN 400
ACN 400
e400 (%)
AN 540
ACN 540
e540 (%)
1 2 3 4 5 6
0.049 0.079 0.146 0.268 0.370 0.436
0.011 0.018 0.069 0.182 0.278 0.347
345 338 111 47 33 25
0.014 0.029 0.237 0.605 0.849 1.064
0.010 0.023 0.229 0.597 0.839 1.053
40 26 3 1 1 1
a The difference between them is expressed as percentage of error, eλ. Data from Figure 6.
The term Ap × ω/l has to be understood as the contribution of the soluble oligomers to the measure of the absorbance given by the normal-beam experiment. When the concentration of soluble oligomers is high, this term can become several times greater than ACN, and a significant error would be introduced if AN were directly taken as representative of adsorbed polymer. This error varies, depending on the selected wavelength. As an example, the experiment in Figure 6 was selected in order to evaluate the error introduced into the estimation of polymer absorbance. Table 2 presents AN at 400 and 540 nm when a potential of zero volts was applied at the end of each cycle, together with the corrected values for the same measures, ACN, and the difference between them expressed as percentage of error.
eλ )
AN - ACN ACN
× 100
(5)
Considering the error values, the first choice of 400 nm as the wavelength for monitoring the film growth must be ruled out. Noncorrected normal-beam measurements at this wavelength, at which oligomers absorb strongly, could involve differences higher than 300% with regard to the true absorbance of the polymer. Even at wavelengths at which the polymer absorbance reaches a maximum, that is, 540 nm, errors can be important during the first two cycles, indicating that nonnegligible ratios (soluble oligomers/adsorbed polymer) were found at the beginning of the electropolymerization. From values in Table 2, it can easily be deduced that there is no proportionality between AN400 and AN540, because the absor-
bance at these wavelengths is not due to the same species. When both signals were corrected, the resulting absorbances, ACN 400 and ACN 540, became perfectly correlated, following the equation ACN 400 ) 0.321 ACN 540 + 0.003 (R2 ) 0.997). Once corrected, any wavelength, not only these, is able to give information about the surface process, although the precision and sensitivity of the estimation depend on the wavelength chosen. CONCLUSIONS Simultaneous acquisition of both normal and parallel spectroscopic signals in a spectroelectrochemical process opens up a wide range of possibilities in the interpretation of electrode reactions. From the results stated above, we conclude that such a sensitive and powerful technique as spectroelectrochemistry cannot be used without taking into account the close relation between the nature of the electrode process and the direction we use for obtaining the spectroscopic information. The apparent advantage given by the high sensitivity of the long pathway spectroelectrochemical measurements pointed out in the literature cannot be taken for granted. When purely diffusive processes take place, as in the case of o-tolidine, bidimensional spectroelectrochemical measurements provide unique information that turns out to be more sensitive in the parallel-beam configuration. The similarity in the information contents of both spectroelectrochemical responses can be taken as evidence of purely diffusive control of the reaction. On the other hand, if normal-beam and parallel-beam signals seem to be different, a more complex mechanism can be expected. Adsorptive processes, in which some reactant or product does not diffuse to or from the electrode surface show very different voltabsorptograms, depending on the direction in which the optical measurement is performed. The normal-beam configuration signal contains more important information about the global process, and the parallel-beam signal gives some information about diffusive species only, so the comparison of both signals is essential in the elucidation of adsorptive processes. Quantitative information on the adsorption step requires suitable mathematical processing of data provided by both spectroscopic observations. Monodimensional measurements alone are not adequate enough to extract reliable conclusions on that score. ACKNOWLEDGMENT Support of the DGICYT (PB93-0677) and Junta de Castilla y Leo´n (BU11/98) is gratefully acknowledged. We thank Prof. Renato Seeber of the Modena University (Italy) for the synthesis of the dimers mentioned above. Received for review December 7, 2000. Accepted April 4, 2001. AC0014459
Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
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