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Anal. Chem. 1969, 67,2805-2809
Using these data, we can construct a Nernst plot showing the progress of the titration. The observed slope of 63 mV/decade is close to that expected for a reversible one-electron couple a t 25 O C (59.19 mV). We have routinely used the cell to study the electrochemistry of biological redox couples in the absence of oxygen, including studies of mediated oxidation of glucose oxidase and diaphorase. Most recently the cell has been used in rotating disk studies of the electrochemistry of glucose oxidase modified by the covalent attachment of ferrocene monocarboxylic acid or ferrocene acetic acid where the combination of low sample volume, exclusion of oxygen, and good rotating disk hydrodynamics is a great advantage ( 9 ) . This work will be reported in a subsequent publication.
CONCLUSIONS This design of electrochemical cell is convenient and suitable for use with rotating disk electrodes where a small working volume is required and where it is desirable to exclude oxygen. The cell can be used for the determination of diffusion coefficients, either by using the variation of limiting current with rotation speed or by following the decay of current with time at a fixed rotation speed, and for coulometric titrations.
ACKNOWLEDGMENT We are grateful to Professor W. J. Albery (Imperial College) for initial discussions on the cell shape and to M. Pritchard (Oxford Electrode) for helpful comments on the design and for making the finished cell. Registry No. 02,1182-44-1. LITERATURE CITED (1) Adams, R. N. Electrochemistfy at Solid Hectrdes; Marcel Dekker: New York. 1969; p 219. (2) Cass, A. E. G.;Davis, G.; Francis, G. D.; Hlll, H. A. 0.: Aston, W. J.; Higgins, I . J.; Plotkin, E. V.; Scott, L. D. L.; Turner, A. P. F. Anal. Chem. 1984, 56. 667-671. (3) Crumbliss, A. L.; Hill, H. A. 0.; Page, D. J. J . Electroanal. Chem. Interfacial Electrochem. 1986, 206, 327-33 1. (4) Taniguchi, I.; Miyamoto, S.; Tomimura, S.; Hawkridge, F. M. J. Electroanal. Chem. Interfacial Electrochem. 1988, 240, 333-339. (5) Miller, B.; Bruckenstein, S. Anal. Chem. 1974, 4 6 , 2033-2035. (6) Eggli, R. Anal. Chim. Acta 1977, 97, 129-138. ( 7 ) Levich, V. G. phvsicochemical Hydrodynamics ; Prentice-Hall, Englewood Cliffs, NJ, 1962; pp 60-72. (8) Hitchman, M. L.; Albery, W. J. Electrochim. Acta 1972, 77, 787-790. (9) Whitaker, R. G. Ph.D. Thesis, University of Warwick, 1989.
RECEIVED for review July 3, 1989. Accepted September 7, 1989. R.G.W. thanks MediSense (UK), Inc., for a research scholarship.
Thin-Layer Microcell for Transmittance Fourier Transform Infrared Spectroelectrochemistry Chao-Liang Yao, Franqoise J. Capdevielle, Karl M. Kadish,* and John L. Bear* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Various cells for external reflectance infrared spectroelectrochemistry have been designed to study electrochemical phenomena at the electrode/electrolyte interface (1-14). A number of cells have also been designed for monitoring IR spectra of products formed during electrode reactions (15-19). These latter thin-layer transmittance cells adopt a “sandwich” configuration originated by Heineman et al. (15)and may have two major problems. The first problem is leakage, which is minimized by the use of mechanical spacers and/or O-rings to hold the windows together under pressure. Adhesives may also be used, but these are susceptible to solvent attack. An additional problem with these cells is their fragility because of the weak mechanical strength of most IR window material. A recently reported Fourier transform infrared (FTIR) cell design eliminated the mechanical spacer by hand-cutting a thin-layer IR chamber directly into a rectangular NaCl window material, which was then attached with Teflon film pressure seal to the bottom of a Teflon compartment (20). Another design also eliminated the use of spacers by utilizing a transmittance IR cell with silicon windows, which were directly flame sealed into Pyrex glass (21). However, both of these cells, as well as other ”simple” cells described in the literature (15-19), are somewhat difficult to construct, and their use has therefore been limited mainly to the individual laboratory that designed them. This note describes the construction and characteristics of a new thin-layer IR transmittance spectroelectrochemical cell that is simple to construct, durable, and completely avoids the problem of leakage. The cell is constructed from a commercially available microcavity IR cell, which has a 34-pL total cell volume. Applications of the spectroelectrochemical microcell are given by monitoring CO or CECH frequencies of the species generated by electrooxidation or electroreduction of Rh,(dpf),(CO), Rh,(ap),(CO), and Rh,(ap),(C=CH) in 0003-2700/89/036 1-2805$01.50/0
CH2C12,where ap = 2-anilinopyridinate and dpf = N,N’-diphenylformamidinate ion. The thickness limit of the transmittance IR spectroelectrochemical cell is demonstrated by using measured spectra of CH2C12,0.1 M TBAP with and without the dirhodium complexes.
EXPERIMENTAL SECTION Reagents and Instrumentation. Rh,(ap),(CO) and Rh2(dpf),(CO)were generated by bubbling CO into a CH2C12solution containing Rh2(ap), or Rh2(dpf), (22,23). Rh2(ap),(C=CH) was prepared by reaction of Rh2(ap),C1 with NaCECH in tetrahydrofuran (24). Spectroscopic grade CH2C1, was distilled over CaH, under Ar. The supporting electrolyte was tetra-n-butylammonium perchlorate (TBAP) and was twice recrystallized from ethanol. An IBM Model 225 voltammetric analyzer was used for both thin-layer voltammetric measurements and controlled potential electrolysis. IR spectra were recorded with an IBM Model IR/32 FTIR spectrophotometer. Cell Design and Method. The design of the thin-layer spectroelectrochemicdcell is shown in Figure 1. The cell chamber is formed directly from a commercial microcavity IR cell, which was purchased from Aldrich Chemical Co. (No. 211,229-1).The cell consists of a single block KBr crystal of dimensions 10 X 15 X 25 mm with a snap-in cell holder and spring clip. The cell cavity is formed by ultrasonic machining. KBr and NaCl cells with path lengths of 0.1,0.2, and 1.0 mm are also available from the manufacturer and can also be utilized. The cell described in this present paper has a path length of 0.2 mm and a volume of 34 pL. The top section of the cell is enlarged slightly by a blade to give a total cell volume of 40 p L as compared to 6 p L for the working electrode compartment volume. The FTIR cell utilizes a three-electrode configuration. The working electrode is a 52-mesh platinum gauze (120 pm i.d.) (Johnson Matthey, Inc.), which is folded to add mechanical strength and has dimensions of 3 X 10 mm (cellswith a path length of 0.1 mm can also be used and in this case a 100-mesh platinum 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 24, DECEMBER 15, 1989
Top view (a)
Electrode p
1
+
0
.
2
0.8 v
Front view
-
T
16
-
[
;
I
0.8 0.6 0.4 0.2
0
Potential
-0.1 v
20
60
40
Bo
100 120
Timelsec)
Flgwe 2. (a)Cyclic voltammogram and (b) current-time curve during controlled potential oxidation and rereduction of 1.O mM ferrocene in CH,Ci,, 0.1 M TBAP in the 0.2 mm path length IR microcell. Figure 1. Schematic illustration of KBr cell chamber (top and front views) and the electrode (with the cell cap): (i) KBr cell body, (ii) Teflon cell cap, (iil) Tefzel film, (Iv) platinum working electrode gauze; W, working electrode; C, counter electrode; R, silver wire pseudoreference electrode. gauze would be utilized to reduce the electrode thickness). The transparency of this platinum electrode is about 50% as measured by the intensity of the IR interferogram. The electrode is inserted into the thin-layer chamber to form a "sandwich" configuration. The working electrode connection is made through a 0.25-mm platinum wire and is insulated from the solution by a welded Tefzel film (E. I. du Pont de Nemours & Co.). The reference electrode is a silver wire. A carefully machined Teflon plug is used as the cell cap and the connection to the copper electrode conducting wire is made through three holes in the cap. Transfer of deoxygenated solutions into the JR cell is done with a 100 KL syringe using Schlenk techniques. Generally, 64 acquisitions of IR interferograms are recorded for each spectrum and give a resolution of 2 cm-'. The empty cell containing the working electrode is used to obtain the background, and a solution of CH2C12,0.1 M TBAP is used as the reference spectrum. IR spectra are displayed in an absorbance or transmittance mode after subtraction of the reference spectrum. Spectroelectrochemical data can be recorded in either a time-resolved or potential-resolved mode. The former method is commonly used, but the latter method can also be utilized (16, 18). A difference FTIR spectrum is obtained by subtracting spectra at two different potentials in potential-resolved experiments or, alternatively, by subtracting spectra at two different time frames in time-resolved experiments. Both methods are reported in this present paper. A potential was not applied to the working electrode during collection of the FTIR interferograms. RESULTS AND DISCUSSION Electrochemical Characterization of the Cell. The thin-layer behavior of the IR cell is demonstrated by the oxidation of ferrocene to ferrocenium ion in CH2C12,0.1 M TBAP. A cyclic voltammogram of 1.0 mM ferrocene is shown in Figure 2a. At a scan rate of 7 mV/s, the peak to peak separation is 50 mV, indicating a small solution resistance across the thin-layer chamber. Some edge effects are present as evidenced by the fact that the current does not return to the base line after electrooxidation. The double potential step current-time curve for this solution is shown in Figure 2b. Complete electrolysis of the electroactive species is achieved in less than 40 s. Integration of the current-time curve gives an average 6 p L thin-layer solution volume for five replicate
Wavenumber.(cm-') Figure 3. Transmittance IR spectra of Rh,(dpf),(CO) in CH,CI,, 0.1 M TBAP during controlled potentlal oxidation at 0.90 V for (a) 0 s (---), 10 s (-), and 30 s and for (b) 30 s (. 60 s (-), and 120 s (---I. (6
-
e)
e),
experiments assuming a 100% current efficiency. Case I: Monitoring the CO Band of a Stable Electrooxidation Product. The oxidation of Rh,(dpf),(CO) in CH2C1, 0.1 M TBAP under 1 atm CO occurs as shown Rhz(dpf),(CO)
F!
[Rh2(dpf),(CO)I+
+ e-
(1)
The ElI2for the above reaction is 0.37 V and the generated [Rh2(dpfl4(C0)]+is relatively stable in solution (23). The IR spectrum of Rh,(dpf),(CO) in CH2C12,0.1 M TBAP is shown in Figure 3a. The bands between 1650 and 1300 cm-' correspond to the bridging dpf ligands, while the intense band at 2050 cm-' corresponds to the axially bound CO stretching frequency of Rh2(dpf)&0. This band decreases during electrooxidation a t 0.90 V while a new band of [Rhz(dpf),(CO)]+ appears at 2100 cm". As the controlled potential electrolysis proceeds, the intensity of this band decreases
ANALYTICAL CHEMISTRY, VOL. 61, NO. 24, DECEMBER 15, 1989
ishi
3
2807
1600
Wavenumber.(cm-') Figure 4. Transmittance IR spectra of RhAap),(CO) in CH,CI,, 0.1 M TBAP during controlled potential oxidation at 0.50 V for (a) 0 s (- - -), 10 s (-), and 30 s and for (b) 30 s 60 s (-), and 120 s (---). (..e)
qL -
(..a),
IRh,tap),(C-OI)I+
slightly from its maximum value (see Figure 3b), due to a diffusion of electrogenerated [Rh,(d~f)~(CO)l+ away from the working electrode chamber. A steady-state value is reached in about 3 min. Case 11: Monitoring the CO Band of an Unstable Electrooxidation Product. The 2,2-trans isomer of Rhz(ap), undergoes two reversible oxidations at Ellz = 0.82 and 0.08 V vs SCE in CH2ClZunder Nz, but under a CO atmosphere, the values of Ellz for these reactions are shifted to 0.80 and 0.25 V (22). The large (170 mV) potential shift for the first oxidation and the small (20 mV) shift for the second oxidation provide evidence that CO is lost after formation of the singly oxidized species as shown in eq 2. The ultimate product of RhdapMCO)
[Rh(aph(CO)I'
+
e-
(2) +l4"
+ co
eq 2 is [Rh2(apI4]+.However, [Rhz(ap)4(CO)]+is present in solution as a transient intermediate for short time periods and was monitored in this present study. Figure 4a illustrates the FTIR spectra obtained from 0 to 30 s during the controlled potential electrolysis of Rhz(ap),(CO) at 0.50 V. The bands at 1607,1589,1537,and 1510 cm-' are due to the bridging ap ligands and are similar for Rhz(ap),(CO) and Rhz(ap),. These bands decrease in intensity upon oxidation while the peak at 1481 cm-' remains unchanged. The formation of a transient intermediate in reaction 2 was ascertained by monitoring the CO bands of both the reactant and the products formed during electrooxidation. The initial CO band of Rh,(ap),(CO) at 2044 cm-I decreases in intensity, while a new IR band appears at 2085 cm-' (see Figure 4a). The 2044-cm-' band continues to decrease as the electrolysis proceeds. At electrolysis time longer than 30 s, the band at 2085 cm-' also begins to decrease (Figure 4b), and after 2 min of electrolysis, no bands corresponding to a CO adduct remain. The transient 2085-cm-' band corresponds to the uc0 frequency of [Rh2(ap),(C0)]+,which is generated as an intermediate during reaction 2. The concentration of this complex will depend upon both the rate of its formation and the rate
1972 cm-'
2000
2100
1900
1800
Wavenumber.(cm-' Figure 5. IR spectra of [Rh,(ap),((FSH)]" (where n = +1, 0, or -1) in CH,CI,, 0.1 M TBAP: (a) initial FTIR spectrum of Rh,(ap),(=H), (b) difference FTIR spectrum of neutral and singly reduced Rh,(ap),( C S H ) , and (c) difference FTIR spectrum of neutral and singly oxidized Rh,(ap),(CSH). of CO dissociation from electrogenerated [Rh,(ap),(CO)]+. When the electrolysis potential was stepped to 0.0 V after complete oxidation, the IR spectrum then corresponded to neutral Rh,(ap),, which was formed upon reduction of [Rhz(ap),]+. The short lifetime of [Rhz(ap)4(CO)]+ compared to [RhZ(dpf),(CO)]+is consistent with a faster rate of CO dissociation for the former complex. Case 111: Monitoring the C-H Band of a Stable Species during Electrooxidation/Reduction. Rh,(ap),(CECH) undergoes two reversible oxidations at Ellz = 1.03 and 0.38 V and a single reversible reduction at -0.52 V. These H of neutral reactions are shown by eq 3-5 (22). The v ~ band Rh,(ap),(C=CH)
__
Rh,(ap),(C=CH) [ R h 2 ( a p ) 4 ( C ECH)]'
+ e- * [Rh,(ap),(C=CH)]2 [Rh2(ap)4(C=CH)]+ + e[Rhp(ap),(C_
CH)f'
+
e-
(3) (4) (5)
L decomposition products Rhz(ap),(C=CH) in CHZClz,0.1 M TBAP is located a t 1954 cm-I (see Figure 5a). The difference spectrum of this compound during reduction (reaction 3) is shown in Figure 5b. The positive band at 1954 cm-' is due to the neutral reactant while the negative one at 1922 cm-' is due to the [Rh,(ap),(C=CH)]- reduction product. This electroreduction is reversible and the original IR spectrum of Rh,(ap),(C=CH) could again be obtained when the applied potential was stepped back to 0.0 V. The difference IR spectrum obtained during the first oxidation of Rhz(ap),(C=CH) (reaction 4) at 0.70 V is shown in Figure 5c. In this spectrum, the IR absorption band is
2808
ANALYTICAL CHEMISTRY, VOL. 61, NO. 24, DECEMBER 15, 1989
2880
1580
llB8
511
Wavenumber,(cm-') Flgure 6. IR spectra (a) of CH,CI,, 0.05 mM TBAP in the cell with 0.1-mm path length, (b) of 2 mM Rh,(dpf),(CO) in CH,CI,, 0.1 M TBAP in a cell with 0.2 mm path length, and (c)difference IR spectrum of neutral and singly oxidized Rh,(dpf),(CO) in a cell with 0.2 mm path
length. shifted from 1954 to 1972 cm-' as Rhz(ap),(C=CH) is converted to [Rh2(ap),(C=CH)]+. Finally, a further oxidation of [Rh,(ap),(C=CH)]+ a t 1.2 V (reaction 5 ) results in the complete disappearance of the C=CH band as well as most other bands in the spectrum. This is consistent with other electrochemical data which show a multielectron transfer process on the time scale of controlled potential electrolysis where a decomposition of the complex occurs (24). The FTIR spectroelectrochemical cell can be used in this case to monitor the CECH stretching frequencies of [Rh2(ap),(C=CH)ln, where n = +1, 0, or -1. All three species are stable, and their IR spectra can be recorded from the same solution by applying appropriate electrolysis potentials. Thickness Limit of the Transmittance Spectroelectrochemical Cell. The scaled absorbance subtraction method is commonly used in FTIR spectroelectrochemical experiments (25). FTIR spectrophotometers usually have a maximum absorbance at A = 4.0 and above this value, data overflow w ill occur. On the other hand, it has been shown that the maximum absorbance for accurate difference spectrometry should be less than 0.7 unit (26). When the maximum absorbance is between 0.7 and 3.5 units, inaccurate subtraction may result in deviation from Beer's law behavior. This is illustrated for the IR spectra of Rh,(dpf),(CO) in CH2C1,, 0.1 M TBAP.
CH,Cl, has two strong absorbance bands a t 1264 ( t = 563) and 740 ( t > 600) cm-' and two weaker bands at 1424 (t = 12) and 896 (t = 14) cm-' (27,223). The C10, counterion of TBAP has a strong absorbance band at 1100 ( t = 4000) cm-'. Molar absorbtivities of the bands at 1264, 1424,896, and 1100 cm-' were calculated with a 0.1-mm IR cell for a CCl, solution containing various concentrations of CHzClz and 0.05 M TBAP. All of these bands will determine the thickness limit for an IR cell where one may obtain a satisfactory subtraction of the sample and reference spectra. The preferred cell thickness for A I0.7 at the above absorbance wavelengths would be 3 X (896 cm-'), 2 x loY2 (1100 cm-') and 8 x lo4 (1264 cm-') mm (1424 cm-I), 2 x for a solution containing CHzC12and 0.1 M TBAP. However, it is virtually impossible to design a transmittance cell with a path length of lo-, mm, and one therefore cannot obtain an accurate subtraction by using the absorbances at 1264 or 740 cm-' (bands with asterisks in Figure 6). There are also problems with the other bands a t 896,1100, and 1424 cm-'. A transmittance cell with a path length of mm is technically difficult to achieve and, if constructed, would give a large solution resistance. On the other extreme, one can calculate cell thickness values for A I3.5 with the same solvent system. These values are 0.2, 0.4,0.1, and 0.004 mm at the above four wavenumbers. Most transmittance IR cells reported in the literature have 0.1 to 0.5 mm path lengths. Consequently, the absorbance bands shown as cross-hatched bands in Figure 6a may be subtracted but the final spectrum will almost always contain residual positive or negative components. This is illustrated in Figure 6b for a solution containing 2 mM Rh,(dpf),(CO) in CHZClz,0.1 M TBAP after subtraction of the blank solution reference. The cross-hatched bands in this figure result from a failure in Beer's law behavior and are not real bands of the complex. The difference spectrum for neutral and oxidized Rh,(dpf),(CO) (Figure 6c) also shows similar residual components over wavelength ranges where either the solvent or the supporting electrolyte strongly absorb. Summary. The IR spectroelectrochemical cell reported in this work is constructed from a single block of material and requires neither adhesives, spacers, nor O-ring pressure seals. For this reason the cell is easily reproduced, requires little maintenance, and has no leakage problems. The cell thickness is also reproducible. However, care must be taken to avoid misinterpretation of the resulting IR data in those regions where there are strong absorbances from either the solvent or the supporting electrolyte.
LITERATURE CITED (1) Hansen, W. N. I n Advances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Ed.; Wiley; New York, 1973; Vol. 9, pp 1-60. (2) Neugebauer, H.; Nauer, G.; Neckel, A.; Tourillon, G.; Garnier, F.; Lange, P. J. Phys. Chem. 1984. 8 8 , 652. (3) Bewick. A,; Pons, S . I n Advances in Inhredand Raman Spectroscopy; Hester, R. J. H., Clark, R. E., Eds.; WHey-Hayden: London, 1985;
... IJ 1 .
(4) Bewick, A.; Kunimatsu, K.: Pons, S . ; Russell, J. W. J. €lecrroanal. Chem. 1984. 160. 47. (5) Pons, S.; Davidson, T.; Bewick, A. J. Electroanal. Chem. 1984, 160, 63. (6) Kunimatsu, K.; Golden, W. G.; Seki, H.: Philpott. M. R. Langmuir 1985, 1 , 245. (7) Pons. S.; Davidson, T.; Bewick, A. J. Am. Chem. SOC. 1983, 705, 1802. (8) McDonald, R. S . Anal. Chem. 1986, 5 8 , 1906. (9) Habib, M. A.; Bockris, J. O'M. J. Electrochem. Soc. 1985, 132, 108. 10) Foley, J. K.; Pons, S. Anal. Chem. 1985, 57, 945A. 11) Foley, J. K.; Korzeniewski, C.; Daschbach, J. L.; Pons, S. I n Electroanalytical Chemistfy; Bard, A. J., Ed. Dekker: New York, 1988; Vol. 14, p 309. 12) Seki, H.: Kunimatsu. K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437. 13) Best, S . P.; Clark, R. J. H.; McQueen, R. C. S.; Cwney, R. P. Rev. Sci. Instrum. 1987, 5 8 , 2071. 14) Best, S. P.; Clark, R. J. H.; McQueen, R. C. S.; Joss, S . J . Am. Chem Soc. 1989. 1 1 1 , 548.
Anal. Chem. 1989, (15) Heineman, W. R.; Burnett, J. N.; Murray, R. W. Anal. Chem. 1968, 40. 1974. (16) Bullock, J. P.; Boyd, D. C.; Mann, K. R. Inwg. Chem. 1987, 26, 3084. (17) DuBois, D. L.; Turner, J. A. J . Am. Chem. Soc. 1982, 704, 4989. (18) Nevln, W. A.; Lever, A. B. P. Anal. Chem. 1988, 60, 727. (19) DuBois, D. L. Inorg. Chem. 1984, 23, 2047. (20) Kadish, K. M.; Mu, X. H.; Lin, X. 0.Electroenalyst 1989, I , 35. (21) Flowers, P. A.; Mamantov, G. Anal. Chem. 1989, 67, 190. (22) b a r , J. L.; Yao, C.-L.; Liu, L.-M.; Capdevielle, F. J.; Korp, J. D.; Albright, T. A.; Kang, S.-K.; Kadish, K. M. Inorg. Chem. 1989, 28, 1254. (23) Lifsey, R. S.Ph.D. Dissertatlon, University of Houston, 1987. (24) Yao, C. L.; Park, K. H.; Khokhar, A. R.; Bear, J. I. Book of Abstracts; 197th National Meeting of the American Chemical Society, Dallas, TX; American Chemical Society: Washington, DC, 1989. INOR 363, (25) Blmths, P. R.; De Haseth, J. A. Fourier Transform Infrared Spectrometry. I n Chemical Analysis; Eiving, P. J., Winefordner, J. D., Eds.;
67 , 2809-28 1 1
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Wiiey-Interscience: New York, 1986; Vol 83; Chapter 10. (26) Anderson, R. J.; Griffiths, P. R. Anal. Chem. 1975. 47, 2339. (27) Palma, F. E.; Piotrowki, E. A,; Sundaram, S.;Clevend, F. F. J . Mol. Spectrosc. 1964, 73, 119. (28) Newbound, T. D.; Colsman, M. R.; Miller, M. M.; Wulfsberg, G. P.; Anderson. 0. P.; Strauss, S . H. J. Am. Chem. SOC. 1989, 7 7 , 3762.
RECEIVED for review August 14, 1989. Accepted October 12, 1989. The authors thank the Robert A. Welch Foundation (Grants E-918, J.L.B., and E-680, K.M.K.), the National Science Foundation (Grant CHE-8822881, K.M.K.), and the National Institutes of Health (Grant GM25172, K.M.K.) for financial support.
Highly Stable Voltammetric Measurements of Phenolic Compounds at Poly(3-methylthiophene)-Coated Glassy Carbon Electrodes Joseph Wang* and Ruiliang Li Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 The determination of phenolic compounds is of great environmental, industrial, and clinical significance. Since most phenols are oxidized at easily accessible potentials, voltammetry and amperometry may serve as highly sensitive tools for their quantification. Unfortunately, the oxidation of phenolic compounds a t solid electrodes produces phenoxy radicals which couple to form a passivating polymeric film on the electrode. A decrease in the response is thus observed upon repetitive scans, the rate of which is concentration dependent (as expected for a dimerization reaction). Consequently, conventional electrodes are usually unsuitable for reliable voltammetric measurements of phenolic compounds. Various strategies have been proposed to address this fouling problem. Anodic polarization in an acidic solution of ferric chloride was employed by Koile and Johnson (1) to remove phenolic films from platinum surfaces. Wang and Lin (2) described a repetitive electrochemical treatment for renewal in situ of glassy carbon electrodes in the presence of phenolic compounds. Laser activation was explored by Poon and McCreery (3)as a means to repeatedly renew solid electrode surfaces in the presence of phenols. Treatment with a flame ( 4 ) was also suggested to restore the surface activity. Such reactivation schemes are often time-consuming and/or require additional (high cost) instrumentation. A more attractive avenue for voltammetric measurements of phenols is to eliminate the passivation problem in the first place (rather than exploring means for surface reactivation). A deliberate modification of electrode surfaces may be very advantageous for this purpose. The objective of the research described in this note is to illustrate the unusual stable response of phenolic compounds at poly(3-methylthiophene) (P3MT) coated electrodes. Electroactive conducting polymers have received a great attention in the modification of electrodes because of potential application for energy storage or electrocatalysis and as electrochromic displays or “ion gate” membranes. Among these, films prepared by electropolymerization of thiophene derivatives have attracted considerable interest (5). In the course of our work on permselective electropolymerized films, we found that P3MT electrodes exhibit excellent resistance to fouling in the presence of high concentrations of phenolic species. The high stability is accompanied by enhanced sensitivity and selectivity. We wish to report these observations in the following sections.
EXPERIMENTAL SECTION Apparatus. The 10-mL electrochemicalcell (Model MF 1052, BioanalyticalSystems (BAS))was joined to the working electrode, reference electrode (Ag/AgCl) (3 M NaCl) (Model RE-1, BAS), and the platinum wire auxiliary electrode through holes in its Teflon cover. The three electrodes were connected to an EG&G PAR Model 264A voltammetric analyzer, the output of which was displayed on a Houston Instruments X-Y recorder. Flow experiments employed a glassy carbon thin-layer amperometric detector (Model TL-5, BAS) and a 100-pL injection loop. The flow injection system was described previously (6). Reagents. All aqueous solutions were prepared in doubledistilled water. 3-Methylthiophene, acetonitrile (LC grade), m-nitrophenol, p-chlorophenol (Aldrich), acetaminophen, dopamine (Sigma), phenol (Fisher), p-cresol (Kodak), and ascorbic acid (Baker) were used without further purification. The supporting electrolyte was 0.05 M phosphate buffer (pH 7.4). Procedure. Prior to its coating, the glassy carbon electrode was polished with 0.05-pm alumina slurry, rinsed with doubledistilled water, and sonicated in a water bath for 2 min. The electrochemical polymerization was carried out in deaerated acetonitrile solution, containing 0.1 M sodium perchlorate and 0.05 M 3-methylthiophene. For this purpose the potential was cycled three times between 0.0 and 1.7 V (vs Ag/AgCl) at a rate of 20 mV/s; the polymerization was terminated during the third cycle by holding the potential at +0.7 V for 10 min. A film thickness of about 1 pm was estimated. Prior to phenol measurements the modified electrode was pretreated in the phosphate buffer blank solution by repetitively scanning the potential between 0.0 and +0.7 V (10 cycles) until a stable background was obtained. RESULTS AND DISCUSSION The unusual stability of P3MT-coated electrodes will be illustrated in the presence of several phenolic compounds that exhibit rapid surface fouling at conventional electrodes. Figure 1 compares repetive cycle voltammograms for 2 X M p-cresol obtained a t 50 mV/s a t the P3MT-coated (A) and bare (B) glassy carbon electrodes. An irreversible oxidation process is observed at both electrodes. The inhibitory layer formed at the bare electrode results in disappearance of the peak after the third scan. In contrast, no degraded response is observed for the entire series at the P3MT electrode. Voltammograms of 2 x IO4 M chlorophenol (Figure 2), phenol, or m-nitrophenol (not shown) exhibit similar observations, with complete fouling of the glassy carbon surface within four
0003-2700/89/0361-2809$01.50/0C 1989 American Chemical Society
