128
Anal. Chem. 1986, 58, 128-131
Poly(pyrro1e-N-carbodithioate)Electrode for Electroanalysis D. M. T. O'Riordan and G. G. Wallace*' Chemistry Department, University College Cork, Cork, Ireland
The dedgn of approprlate chemically modHled solid electrodes should allow development of new electroanalyticalmethods wlth Increased seiecthrlty, sensltlvlty, and mechanlcal stabliky. I n this work the synthesls of a thio-containing poiypyrroiebased electrode Is descrlbed. Cyclic voltammetry, Fourier transform Infrared spectrometry, scanning electron mlcroscopy, and microanalysis were used to characterize the modlfled electrode. The ablllty of thls electrode to uptake copper ions from solution and subsequent voltammetric analysis was Investlgated. Under specified uptake conditions and by use of cyclic voltammetry, a detectlon limit of 1 ppm was estimated.
Dithiocarbamates (I) are particularly interesting complexing R -alkyl
I
agents. They form stable complexes with a wide range of metal ions and consequently have been used widely in analytical chemistry (11). A particularly interesting property of the dithiocarbamate ligand is the ability to stabilize metals in unusual oxidation states (12). For example Cull(dtc)z + e- + [CuI(dtc),]Cul'(dtc)z + [Cu"I(dtc),]+
The area of chemically modified electrodes (CMEs) has matured to such an extent (1-3) that these devices are beginning to find applications in the area of trace metal analysis (4-9).
One way of using CMEs to enhance the sensitivity of a voltammetric method is to use a surface capable of preconcentrating the analyte(s) from solution. Such preconcentration on (or in) to the electrode surface has proven extremely successful in conventional stripping voltammetry for determination of metal ions (4). To date the use of mercury, glassy carbon, or mercury thin film substrates as working electrodes has been most popular. However, the use of properly designed CMEs should add a new dimension to trace metal voltammetry. With CMEs, metal ions may be preconcentrated on the electrode surface, in a chemical form, or in a chemical environment, which permits monitoring of well-defined electrode processes (5-9). For example Cheek and Nelson used various amines to modify carbon paste electrodes, which were subsequently suitable for trapping silver ions via complexation. Silver was then anodically stripped from the electrode surface and the well-defined oxidation process used for determinations down to the lo-'' M level. Other workers (6) have employed platinum electrodes modified with poly(4-vinylpyridine) to preconcentrate chromium(V1) ions from solution. The chromium(VI)/chromium(III)reduction response could then be monitored free from the complications that have been associated with this response at a bare platinum substrate. These two examples illustrate the two most popular methods used to trap metal species on electrode surfaces: complexation and electrostatic attraction, respectively. The use of complexation on the electrode surface is a particularly exciting prospect for electroanalytical chemistry. The attainment of selectivity via pH control or by using appropriate masking agents should be possible using complexing electrodes. Furthermore, it is well-known that the electrochemical redox processes displayed by metal ions are influenced by complexation (10) and so the electrode surface may be designed to provide an optimum electrochemical response. Present address: Chemistry Department, University of Wollongong, Wollongong, New South Wales, Australia.
+ e-
(1) (2)
Processes such as those described in eq 2 are attractive since they can be monitored at positive potentials where dissolved oxygen is not electrochemically active and, therefore, does not interfere. Previous workers have reported the binding of dithiocarbamate ligands onto various substrates (13-18). Hercules and co-workers (13) used sililyzation to bind dithiocarbamates to glass surfaces. Resins have also been derivatized by using silylization or by reacting amine sites on the resin with carbon disulfide in an appropriate media (14-16). Dithiocarbamate ligands have been incorporated onto electrode surfaces by electrostatic attraction in a cationic polymer (17) or by coprecipitation into a graphite rod (18). In the course of this work a novel synthesis of a dithiocarbamate containing electrode was investigated. By derivatization of a suitable amine-containing polymer, which adhered to an electrode substrate, a dithiocarbamate electrode was produced. Perusal of the literature revealed that polypyrrole (PP) coated electrodes have received considerable attention in recent years (19-29). These electrodes are prepared by oxidation of the pyrrole monomer from acetonitrile, or aqueous, solution containing an appropriate supporting electrolyte. The insoluble polymer, which is formed, deposits onto the working electrode surface. The films are easily prepared, display good stability, and are conducting in the potential range 0.00-0.60 V vs. SCE. At more negative potentials the films become insulating and at more positive potentials it appears the integrity of the film is lost. Previous workers have used electrostatic attraction to trap metal species in polypyrrole electrodes (30),for the purpose of reagent release studies. In this work the PP electrode was investigated with respect to producing a poly(pyrro1e-N-carbodithioate)(PPdtc) (Le., a dithiocarbamate containing) electrode. (Previous workers have synthesized and studied the pyrrole-N-carbodithioate monomer (31, 32).) Characteristics of the polymer, during derivatization, were monitored with cyclic voltammetry (CV). Fourier transform infrared (FTIR) spectrometry, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The ability of the PPdtc electrode to uptake copper ions from aqueous solution and the voltammetry of the subsequent metal complex on the surface were investigated.
0003-2700/86/0358-0128$01.50/00 1985 American Chemical Society
ANALYTICAL
EXPERIMENTAL SECTION Reagents and Standard Solutions, All chemicals were A.R. grade unless otherwise stated. L.R. grade pyrrole and carbon disulfide were obtained from BDH. The pyrrole was distilled before use. L.R. grade benzene and 2-propanol were obtained from the Munster Chemical Co. HPLC grade acetonitrile and methanol were obtained from Rathburn Chemicals. Metal stock solutions were prepared by dissolving appropriate salts in distilled, deionized water. Synthesis of Pyrrole-N-carbodithioate (Pdtc). A 2.4-g portion of sodium hydride (NaH) was reacted with an excess (50 mL) of dimethyl sulfoxide (Me2SO). Seven milliliters of freshly distilled pyrrole was then added. The resulting pale yellow SOlution was cooled to 0 "C in an ice bath and 6.2 mL of carbon disulfide was added slowly, which yields the deep orange-red solution (- 1 M) of the ligand. The product was stored under nitrogen at 5 "C. Synthesis of Bis[pyrrole-N-carbodithioato]copper(II) ( C ~ ( P d t c ) ~To ) . a 0.1 M solution of C U ( N O ~ ) ~ . ~ 20 H ~mL O ,of the solution of sodium pyrrole-N-carbodithioate was added dropwise, with stirring, which gave rise to a brown precipitate. This was filtered and dried over P205in a vacuum desiccator. Polypyrrole Film Formation. Polypyrrole was plated onto the electrode substrate from an acetonitrile (0.1 M tetraethylammonium perchlorate (TEAP)) solution that contained 1% (v/v) water and 0.1 M pyrrole. Plating was carried out galvanostatically by passing a current of 1 mA for the desired period of time (24 mC/cm2 yields a thickness of 0.1 pm (20)). Solutions were deoxygenated with nitrogen for 15 min prior to plating. Preparation of Poly(pyrro1e-N-carbodithioate) Electrode. The electrode was derivatized in a solution containing 50 mL of benzene, 10 mL of 2-propanol, 10 mL of carbon disulfide, and 10 mL of 0.3 M sodium hydroxide in methanol. Instrumentation. All electrochemical experiments were performed with a Princeton Applied Research Model 174A polarographic analyzer. This instrument was used in conjunction with a Metrohm Model E612 VA Scanner for cyclic voltammetry. Either platinum wire, or a mini glassy carbon electrode obtained from Metrohm were employed as working electrodes. A platinum wire auxiliary electrode and a Ag/Ag+ (0.1 M AgN03) reference for acetonitrile solution work, or a Ag/AgC1(3 M KC1) reference for aqueous solution work, were employed. EPMA data were collected with a Jeol Model JXA58 electron probe microanalyzer. An energy dispersive detector also from Jeoi, Model PGT 3, was employed. (Accelerating voltage was 20 kV. Beam density was 2 X lo-" A.) FTIR data were collected with a Nicolet Model 5DX FTIR spectrometer. Samples were prepared in KBr disk form. Microanalysis (C, H, and N) data were collected with a Perkin-Elmer CHN Model 240 elemental analyzer. Atomic absorption spectrometric (AAS) measurements were performed with a Pye Unicam Model SP191 instrument (X = 324.8 nm). RESULTS AND DISCUSSION Unless otherwise stated platinum wire was used as the electrode substrate. Polypyrrole Film Formation. Electrooxidation of pyrrole onto an electrode surface from either acetonitrile or aqueous media may be represented as r
Q k
L
+c-
- 4
Jx
where C in a counterion, e.g., Clod- and x is discussed in the text. Some doubt still exists as to the exact value of x (19,25, 26,29). It appears that x depends critically on experimental
CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
129
Table I. Microanalysis Data element
theory, %
found, '70
carbon hydrogen nitrogen oxygen chlorine
41.46 3.45 12.09 27.64 15.46
42.30 3.83 11.60 26.51 15.76
]om*
-1
.b
0:o
-0.5
d.5
EW a f s ) Flgure 1. Cyclic voltammogram of R-PP in acetonitrile (0.1 M TEAP) (scan rate, 200 mV/s): (1) original R-PP electrode; (2) after 4.5 h in derivatizing solution containing CS,, see Experimental Section for de-
tails. conditions during plating. Parameters such as solvent media, applied potential or current, electrolysis time, and the concentration of pyrrole are likely t o have an effect. In this work plating was initially carried out in 100% acetonitrile (0.1 M TEAP). However, it was found that polymers that were much more adherent and stable on the electrode surface could be produced by adding 1% (v/v) water to the solvent. With this solvent and the plating procedure outlined in the Experimental Section, microanalysis data for the polymer were obtained (Table I). With x given a value of 2, theoretical values compared well with experimental data. Similar results were obtained with polymers ranging in thickness from 0.75 to 12.0 Km. Voltammetry of the PP electrode in acetonitrile (0.1 M TEAP) gave two oxidation responses a t 0.00 V and +1.40 V vs. Ag/Ag+ (0.1 M AgN03). Similar results have been reported by other workers (22, 26, 29). FTIR spectra for the plated polymer were similar to those previously obtained by other workers (33). Derivatization of the PP Electrode. The method outlined in the Experimental Section was suitable for derivatizing the amine-containing polymer on the electrode surface. Cyclic voltammograms of the PP electrode in acetonitrile (0.1 M T E N ) indicated that the first oxidation response (0.00 V vs. Ag/Ag+) decreased with time spent in the derivatizing solution (Figure 1). This is presumably due to the fact that the -NH group is being derivatized. After 72 h of derivatization the first oxidation response disappeared completely. Derivatization had no effect on the second oxidation process a t +1.40 V vs. Ag/Ag+ (0.1 M AgN03). A response for oxidation of the dithiocarbamate ligand on the electrode surface was not observed. Similarly a response for oxidation of the Pdtc monomer in solution was not observed at the PP electrode. This is possibly due to masking of the Pdtc oxidation response by the PP responses or due to the limited anodic range imposed by oxidation of the polymer backbone, itself. FTIR spectra for the derivatized polymer displayed at least one additional absorption band compared to the underivatized
130
I' 4
ANALYTICAL CHEMISTRY, VOL. 58,NO. 1, JANUARY 1986
50
50
1
CI
I
01:5 '
d.5
'1+1'5
'
l
W
.
't
5
ENERGY(KM Figure 2. EPMA of Ft-PP electrode surface during derivatization in CS
I
0
5
10
containing solution (acceleratingvoltage, 20 kV; beam density, lo-'' A): (A) after 24 h of derivatization; (E) after 48 h of derivatization; (C) after 72 h of derivatization.
15
25
20
30
Time (Hrs.) Figure 3. Uptake of Cu2+ from aqueous solution obtained using AAS
polypyrrole. This band at 1450 cm-l may be attributed to the v(C-N) vibration of the
Lsy r s sC
L
- N g ]
n ligand (34,35). The frequency of this vibrational is relatively low compared with the corresponding vibration for other dithiocarbamate ligands (35), which would suggest that the C-N bond does not have much double bond character. The vibration does, however, occur a t higher frequencies than observed for the Pdtc monomer (32). With EPMA a sulfur response, which increases with derivatization time, was observed on the PPdtc electrode (Figure 2). X-ray mapping of the electrode surface a t E = 2.3 keV indicated that the sulfur was distributed uniformily. Uptake of Metal Ions. The above evidence would suggest that the PP electrode has been derivatized to form a thiocontaining ligand on the surface. Monitoring of a 10 ppm Cu2+ solution, using atomic absorption spectrometry, in which a PPdtc electrode was immersed indicated that the metal ions were removed from solution. A response observed at 8.07 keV using EPMA verified the presence of copper on the electrode surface. Uptake data (Figure 3) indicated that the copper ions were more readily removed from solution using electrodes with thinner polymer films on the surface. The rate and extent of removal were markedly improved by using the PPdtc electrode compared to a PP electrode. Surface coverage with a polymer thickness of 1.5 pm was estimated to be 2.4 X lo4 mol of Cu2+cm-2, and the initial rate of uptake for the PPdtc electrode was lo-' mol of Cu2+ mi&. This could be improved by a factor of 4 by applying a negative potential (-0.60 V vs. Ag/AgCl) to the PPdtc electrode. Application of this potential did not affect the final surface coverage of copper on the electrode surface. Voltammetry of the PPdtc electrode, after metal uptake, was investigated in 1 M NaN03. With electrodes that were derivatized for 72 h no voltammetric responses were observed. This may be due to the fact that the conductivity of the PP backbone is decreased during derivatization. Therefore metal uptake and subsequent voltammetry using PPdtc electrodes produced by 48 h derivatizations were investigated. The PPdtc electrode before uptake still exhibited an -NH oxidation response (E, = 0.00 V, Figure 1) indicating that at least some conductivity was retained in the polymer film. This conductivity is necessary to facilitate oxidation/reduction of
(For details see text; Cu Conc (ppm) Cu concentration in aqueous solution): (A)uptake using R-PP (A)uptake using Pt-PPdtc, thickness = 15.7 pm; p)uptake using Pt-PPdtc, thickness = 7.9 pm; (0)uptake using Pt-PPdtc, thickness = 1.5 pm. All were derivatized in CS2 solution for 72 h.
/O.ZmA 1
-110
d.0
-0:s
0'.5
0.7
E (Voks)
Figure 4. Cyclic voitammogram (in 1 M NaNO,) of Pt-PPdtc electrode after copper uptake: polypyrrole thickness, 1.5 pm; derivatized in CS2 solution for 48 h; uptake, electrode left in 15 ppm Cu2+ solution for 48 h (no potentlal applied); scan rate, 50 mV/s; response (111')first polypyrrole oxidation process; response (2/2')due to copper complex oxidation/reduction.
copper sites in the polymer. Following, metal uptake two additional responses, presumably due to the metal complex on the electrode surface were observed (Figure 4). Both responses decreased with increasing number of potential scans indicating the metal complex or the products of oxidation/ reduction were unstable on the electrode surface. The metal complex response observed a t -0.80 V vs. Ag/ AgCl was also observed in acetonitrile (0.1 M TEAP) a t -0.45 vs. Ag/Ag+. This response corresponds with reduction of the Cull(Pdtc)zmonomer in solution at a PP electrode. However, for the metal complex attached to the electrode surface, the reduction response is only observed following the oxidation response a t +0.55 V vs. Ag/AgCl (Figure 4). This would suggest that the copper ions are trapped on the electrode surface in the (+1)oxidation state. The metal complex responses may be attributed to
+
[Cu'(Pdtc),]- + C ~ ' ~ ( P d t c )e-, ~ Cu11(Pdtc)2
+ e- + [Cu'(Pdt~)~]-,
E , = +0.55 V E , = -0.80 V
It is unlikely that the copper(II)/copper(III) oxidation process would be observed in the potential range available at the PP electrode. Previous workers have indicated that these metal
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
0*07
0.06
' 5
0.04
1
achieved by using pyrrole that contains a suitable substituent or by using a different amine-containing polymer.
1
ACKNOWLEDGMENT The authors have pleasure in acknowledging invaluable discussion with M. A. McKervey, Chemistry Department, UCC on the derivatization of polypyrrole. The preparation of polymer samples and their subsequent analysis by FTIR by P. Bonner, State Laboratory, Ireland, is greatly appreciated. The technical assistance of J. Hyenga, Institute for Industrial Research and Standards, Ireland, in the collection of EPMA data is gratefully acknowledged. Registry No. Cu(Pdt&, 99147-90-7;polypyrrole, 30604-81-0; poly(pyrro1e-N-carbodithioate), 99148-01-3; carbon disulfide, 75-15-0; copper, 7440-50-8.
i
//I
6 0935
131
1
5
3
LITERATURE CITED
1
Cu Conc (ppm) Figure 5. Calibration curve obtained with cyclic voltammetry (scan rate, 50 mV/s): worklng electrode as employed for Figure 4; uptake conditions, Eapp= -0.60 V for 30 mln (unstirred solution).
dithiocarbamates are relatively difficult to oxidize (31). The metal complex response observed at +0.55 V vs. Ag/AgCl was investigated for possible analytical applications. This response was monitored after uptake of copper ions from solution of various concentrations. Uptake was carried out while applying a negative potential (-0.60 V vs. Ag/AgCl) to the electrode and preconcentrating for 30 min. The resultant voltammetric response was monitored using cyclic voltammetry. A calibration curve is shown in Figure 5. From results obtained a detection limit of 1 ppm was estimated. However, this should be substantially improved by using different uptake conditions and/or differential pulse voltammetry. The biggest drawback of this electrode with respect to analytical applications is the fact that the response due to the presence of copper decreases rapidly on subsequent potential scans and disappears after two or three scans. CONCLUSIONS A polymeric, ligand-containing, modified electrode has been produced. The ability of this electrode to uptake metal ions from solution and the subsequent voltammetry of the complex on the electrode surface suggest that analytical applications may be feasible. Any improvement in the rate of metal ion uptake would enhance this electrodes capability. Consequently the effects of solution stirring/electrode rotation and pH control on the deposition process are currently under investigation. Quantitative data on the determination of other metal ions using this electrode is being collected. In order to enable monitoring of processes such as the copper(II)/copper(III) oxidation process, the anodic potential range of this electrode must be extended. This may be
Murray, R. W. Acc. Chem. Res. 1980, 73, 135. Bard, A. J. J. Chem. Educ. 1983, 6 0 , 302. Faulkner, L. R. Chem. Eng. News 1984, 62(9), 28. Vydra, F.; Stulik, K.; Julakova, E. "Electrochemical Stripping Analysls"; Ellls Horwood: New York, 1976. (5) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 7 7 , 393. (6) Cox, J. A.; Kulesza, P. J. J. Electroanal. Chem. 1983, 759, 337. (7) Larochelle, J. H.; Johnson, D. C. Anal. Chem. 1978, 50, 240. (8) Stewart, E. E.; Smart, R. B. Anal. Chem. 1984, 5 6 , 1131. (9) Koval, C. A.; Anson, F. C. Anal. Chem. 1978, 5 0 , 223. (10) Bailar, J. C. "The chemistry of the Coordination Compounds"; Relnhold: New York, 1956. (11) Hulanlcki, A. Talanta 1987, 74, 1371. (12) Concouvanis, D. frog. Inorg. Chem. 1978, 2 6 , 301. (13) Hercules, D. M.; Cox, L. E.; Onlsick, S.; Nichols, G. D.; Carver, J. C. Anal. Chem. 1973, 4 5 , 1973. (14) Leyden, D. E.; Luttrell, G. H. Anal. Chem. 1975, 4 7 , 1612. (15) Dlngman, J. F.; Gloss, K. M.; Milano, E. A.; Siggia, S. Anal. Chem. 1974, 46, 774. (16) Suzuki, T. M.; Yokoyama, T. Polyhedron 1983, 2 , 127. (17) Guadalupe, A. R.; Abruna, H. D. Anal. Chem. 1985, 5 7 , 142. (18) Hassan, S. S.M.; Hablb, M. M. Anal. Chem. 1981, 5 3 , 508. (19) Kanazawa. K. K.; Diaz, A. F.; Geiss, R. H.; Gill, W. D.; Kwak, J. F.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. SOC.,Chem. Commun. 1979, 854. (20) Diaz, A. F.; Castiilo, J. I. J. Chem. SOC., Chem. Commun. 1980, 397. (21) Kanazawa, K. K.; Diaz, A. F.; Gill, W. D.; Grant, P. M.; Street, G. B.; Gardinl, G. P. Synth. Met. 1979, 7 , 329. (22) Diaz, A. F.; Martinez, A.; Kanazawa, K. K. J. Electroanal. Chem. 1981, 730, 181. (23) Burgmayer, P.; Murray, R. W. J. Electroanal. Chem. 1983, 747, 339. (24) Noufi, R. J. Electrochem. SOC. 1983, 730, 2126. (25) Asavaprlyanont, S.;Chandler, G. K.; Gunawardena, G. A,; Pletcher, D. J . Electroanal. Chem. 1984, 777, 245. (26) Asavpirlyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 777, 229. (27) Santhanam, K. S. V.; O'Brien, R. N. J. Electroanal. Chem. 1984, (1) (2) (3) (4)
.- -
-. . .
1 6 A , 377
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RECEIVED for review May 30,1985. Accepted August 26,1985.
