142
Anal. Chem. 1985. 57. 142-149 APPROX.
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tional groups also can be monitored by using the ion microscope and correlated with quantitative XPS data.
ACKNOWLEDGMENT We thank Surface Science Laboratories, Inc. (R. D. Cormia, D. A. Kozak, and R. E. Linder) for performing the small-area XPS analysis. Registry No. Csf, 18459-37-5; PMMA (homopolymer), 9011-14-7; PFBA, 653-37-2.
LITERATURE CITED (1) Heinrich, K. F.. Newbury, D. E., Eds. "Secondary Ion Mass Spectrometry"; National Bureau of Standards: Washington, DC, 1975; Speclai Publication 427. (2) Morrison, 0. H.; Siodzian, G. Anal. Chem. 1975, 47, 932A. (3) Muller, 0. Appl. Phys. 1976, 10, 317-324.
(4) Werner, H. W. Microchlm. Acta, Suppl. 1977, 7 , 63-71. (5) Campana, J. E.; DeCorpo, J. J.; Coiton, R. J. Appl. Surf. Scl. 1981, 8, 337-342. (6) Gardeiia, J. A.; Hercules, D. M. Anal. Chem. 1981, 53, 1879-1884. (7) Gardeiia, J. A.; Hercules, D. M. Anal. Chem. 1980, 52, 226-232. (8) Briggs, D.; Wootton, A. B. Surf. Interface Anal. 1982, 4 , 109-115. (9) Briggs, D. Surf. Interface Anal. 1082, 4 , 151-155. (10) Briggs, D. Surf. Interface And. 1083, 5 , 113-118. (11) Storp, S.; Holm, R. J . Electron Spectrosc. Relat. Phenom. 1979, 16, 183-1 93. (12) Chan, C.-M. Appl. Surf. Sci. 1082, 10, 377-382. (13) Rabalais. J. W. I n "Photon, Electron, and Ion Probes of Polymer Structure and Properties"; Dwight, D. W., Fabish, T. J., Thomas, H. R., Eds.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 162, pp 237-246. (14) Williams, P.; Lewis, R. K.; Evans, C. A,, Jr.; Hanley, P. R. Anal. Chem. 1977, 4 9 , 1399. (15) Gardeiia, J. A.; Novak, F. P.; Hercules, D. M. Anal. Chem. 1084, 56, 1371-1375. (16) Everhart, D. S.; Reiiiey, C. N. Anal. Chem. 1981, 53, 665-676. (17) Slmko, S.J.; Linton, R. W.; Murray, R. W.; Griffis, D. P., paper presented at the 1984 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1984, paper 543. (18) Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M. J . Am. Chem. SOC. 1977, 99, 4736-4745. (19) Lepareur, M. Rev. Tech. Thomson-CSF 1980, 72, 225-265. (20) Linton, R. W.; Farmer, M. E.; Ingram, P.; Walker, S.R.; Shelburne, J. D. Scanning Electron Mlcrosc. 1982, 3 , 1191-1204. (21) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muiienburg, G. E., Eds. "Handbook of X-ray Photoelectron Spectroscopy"; PerkinElmer Co.: Eden Prairie, MN, 1978; p 188. (22) Evans, J. F.; Ullevig, D. M. Anal. Chem. 1980, 52, 1467-1473. (23) Batich, C. D.; Wendt, R. C. I n "Photon, Electron, and Ion Probes of Polymer Structure and Properties"; Dwlght, D. W., Fabish, T. J., Thomas, H. R., Eds.; Amerlcan Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 162, pp 221-235. (24) Wagner, C. D. Anal. Chem. 1977, 4 9 , 1282-1290.
RECEIVED for review November 14,1983. Resubmitted June 22, 1984. Accepted September 10, 1984. This work was supported by the National Science Foundation (XPS, SIMS Instrumentation Grants and Grant 7920114),Microelectronics Center of North Carolina (SIMS Facility), and Tennessee Eastman Company (S. J. Simko-Graduate Fellowship).
Electroanalysis with Chemically Modified Electrodes Ana R. Guadalupe and Hector D. Abruiia* Department of Chemistry, Cornell University, Ithaca, New York 14853 The feasibility of using electrodes modlfled with functionallzed polymer fllms for performing electroanalysls In solution Is demonstrated. The proposed approach not only takes advantage of the favorable aspects of chemlcaily modlfled electrodes (e.g., sensitivity) but also provides for very broad synthetic variations (and therefore selectivity) as well as ways to detect and overcome matrlx and saturation effects. The method is based on the use of copolymer flims that Incorporate both an electroactlve center (used for inducing precipitationof the polymer on the electrode) and a coordinating site chosen on the bask of the species of interest. Even though the method Is presented In the context of electroanalysis of metal Ions In solutlon, this approach could be extended to be determlnation of organlc functlonalitles through the approprlate choice of reagents.
There is today a great need for the development of analytical methodology for the selective and quantitative determination of metal ions and organic contaminants at trace level, particularly in light of the new challenges posed by environmental samples. One field that offers great potential in this respect is that comprised by chemically modified electrodes 0003-2700/85/0357-0142$01.50/0
(1-3). This field is one that is fast approaching a mature state and as such there have been quite a few applications explored (4-17). Chemically modified electrodes are in addition very well suited for electroanalytical applications since they offer an inherently high sensitivity, and by the judicious choice of modifier and control of electrode potential, high selectivity can in principle be achieved. There have been some reports on the use of chemically modified electrodes for electroanalysis. One of the first examples of the analytical utility of these modified interfaces (although the analytical aspects were not emphasized) was by Lane and Hubbard (18). In this work they complexed Fe(II1) ions from aqueous solution using a salicylate ligand that was chemisorbed to a platinum surface via an olefinic group. They furthermore hinted at the possibility of modulating the coordinative properties of the interface through the control of the electrode potential. Cheek and Nelson (19) reported on the determination of Ag(1) from solution using modified carbon paste electrodes. They report a truly remarkable detection limit of about M. Oyama and Anson (20,21) reported on the use of polymer modified electrodes capable of incorporating metal complexes either by coordination to pyridine or nitrile groups in the polymer or by electrostatic binding to polycationic (protonated poly(viny10 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
pyridine)) or polyanionic (deprotonated polyacrylic acid) polymer films. They report that they could incorporate ions from solutions as dilute as 5 X M. These studies point to the feasibility of using not only metal/ligand interactions but electrostatic effects as well for performing electroanalysis. Cox and Majda (22) incorporated Fe(I1) onto a platinum electrode modified with adsorbed adenosine 5'-monophosphate and subsequently determined the amount of incorporated iron via cyclic voltammetry. In perhaps one of the most directed efforts of showing the potential of chemically modified electrodes to the analysis of organic moieties, Price and Baldwin (23)reported on the use of ferrocene carboxaldehyde for the determination of aromatic amines adsorbed on the surface of a platinum electrode. They reported detection limits of the order of M. These authors also point out the different aspects (such as saturation) that must be kept in mind when attempting to use these modified interfaces for analytical purposes. Studies on the determination of Co, Cu, and traces of O2have also appeared (24-27). Although the potential utility of chemically modified electrodes for performing electroanalysis in solution has been clearly recognized and pursued, the utility of many of the approaches outlined above has been greatly hindered by the fact that they do not provide for a way to detect saturation and account for matrix effects. In addition, the systems are themselves often ill-defined. The work presented here puts forth an approach based on the use of functionalized polymer films for performing electroanalysis with chemically modified electrodes which in addition to taking advantage of the favorable aspects of chemically modified electrodes is also able to circumvent the difficulties associated with the determination of saturation and correction of matrix effects. It is based on the use of copolymer films that incorporate both a redox center and a ligand site where the latter is either covalently attached through the polymeric backbone or incorporated into a cationic polymer film through ion exchange. The work presented here is within the framework of the analysis of metal ions in solution, but the proposed approach could be extended to the determination of organic functionalities by the appropriate choice of reagents.
EXPERIMENTAL SECTION Reagents. Vinylferrocene (Strem) was used as received. Vinylpyridine (Aldrich) wad distilled at reduced pressure and stored under nitrogen at 0 "C. Vinylbipyridine (vinylbipyridine is 4-vinyl-4'-methyl-2,2'-bipyridine) was synthesized according to our recently developed procedure (28). Sulfonated bathophenanthroline and sulfonated bathocuproine were obtained from G. F. Smith and were used as received. Tetra-n-butylammonium perchlorate (TBAP) (G. F. Smith) was recrystallized three times from ethyl acetate, dried in vacuo at 90 "C for 72 h, and stored in a desiccator. Sodium perchlorate (G.F. Smith) was used as received. Acetonitrile and methylene chloride (Burdick and Jackson distilled in glass) and DMF (Eastman White label) were dried over 4-8* molecular sieves. Water was passed through a column of activated charcoal and two columns of mixed-bed ion exchange resin and then distilled from an all-glass system (Barnstead). AIBN (azobis(isobutyronitri1e)) (Aldrich) was recrystallized from ether and stored under nitrogen in a refrigerator. All other reagents were of at least reagent grade quality and were used without further purification. Synthesis. Copolymers. Poly(vinylpyridine)/vinylferocene. These copolymers were synthesized by mixing the appropriate molar ratios of the monomers (the ratio of monomers was varied over a range of the factor of 1O:l vinylpyridine/vinylferrocene) in benzene with AIBN used as the initiator. The materials were charged into a Pyrex ampule and the solutions were deaerated on a vacuum line through three freeze/pump/thaw cycles. The ampules were sealed under vacuum and placed in a water bath at 75 "C for 72 h. The ampule was opened and the polymer was
143
precipitated through the addition of heptane. The polymer was purified through three recrystalIizations from benzene/ heptane. The copolymers were dried under vacuum and stored in a desiccator. Quaternized Poly(viny1pyridine)/Vinylferrocene Copolymers. These copolymers were synthesized by dissolving the appropriate poly(vinylpyridine)/vinylferrocene copolymer in benzene and adding a 10-foldmolar excess of methyl iodide. The solution was allowed to react at reflux and under nitrogen for 3 h. The quaternized polymer is easily separated from the reaction mixture since it precipitates from solution. The polymer was collected, washed with ether, dried under vacuum, and stored in a desiccator. Poly(vinylbipyridine)/Vinylferocene. This copolymer was synthesized by mixing a 3:l molar ratio of vinylbipyridine to vinylferrocene in benzene with AIBN used as the initiator. The remainder of the procedure was the same as that used for the synthesis of the poly(viny1pyridine)/vinylferrocene copolymer. It should be mentioned that due to the very high affinity of the bipyridine ligand for iron, extreme care had to be exercised to eliminate all sources or iron. AS such, all glasware used in the synthesis was washed with dilute nitric acid and soaked in aqueous EDTA (EDTA is ethylenediaminetetraacdtic acid) solution and the materials were handled with plastic spatulas. Even so, some incorporation of iron took place as evidenced by the light pink color of the copolymer. To further purify the copolymer, it was dissolved in ether and extracted with a saturated aqueous solution of EDTA. The ether layer was collected and dried and the copolymer precipitated from benzene with heptane. It was afterward dried on a vacuum line and stored in a desiccator. Elemental microanalyses were performed by Galbraith Laboratories, Knoxville, TN. Metal Complexes. [Fe(sulfonated bath~phenanthroline)~]. This complex was synthesized by mixing aqueous solutions of ferrous sulfate and sulfonated bathophenanthroline in a 1:3 molar ratio. The formation of the complex was immediately apparent from the formation of a deep reddish purple solution. The reaction mixture was allowed to stir at room terifperature for 30 min. The complex was isolated by rotoevaporating the water, collected and dried with ether. [Cu(sulfonated bathocupr~ine)~]. This complex was prepared in a manner analogous to the iron/sulfonated bathophenanthroline complex except that copper chloride and the sulfonated bathocuproine were mixed in a 1:2 molar ratio. [Cu(diethyldithio~arbamate)~)]. This complex was synthesized by mixing aqueous solutions of copper chloride and the sodium salt of diethyldithiocarbamate in a 1:2 molar ratio. Formation of the complex is evidenced by the formation of a precipitate immediately after mixing. This complex was purified by recrystallization from acetonitrile/ether. Equipment. Electrochemical experiments were performed by using a Princeton Applied Research Model 173 potentiostat with a Princeton Applied Research Model 175 universal programmer. Data were recorded on a Soltec Model 6431 or a Hewlett-Packard Model 7045-B X-Y recorder. Electrochemical cells were of conventional design. Techniques. Electrode Modification. For the poly(viny1pyridine)/vinylferrocene and poly(vinylbipyridine)/vinylferrocene copolymers, the electrodes were modified by electrodeposition of the copolymer from methylene chloride solution (typical copolymer concentration was 1-2 mg/mL) by holding the potential of the electrode at +0.70 V for a prescribed amount of time depending on the desired coverage of polymer on the surface of the electrode. The electrode was removed from the cell and washed with acetone and water. For the quaternized poly(vinylpyridine)/vinylferrocene copolymer, the same procedure was used except that the polymer was dissolved in 3:2 methylene ch1oride:DMF. Incorporation of Ligands. For the incorporation of the anionically charged ligands, the electrodes modified with the quaternized poly(vinylpyridine)/vinylferrocene copolymer were M aqueous solutions of the ligands for contacted with 1 X a period of 15 min, during which the solution was magnetically stirred. For the sulfonated derivatives, the pH of the solution was adjusted so as to ensure that the sulfonate groups were deprotonated. After the 15-min period, the electrodeswere washed with water.
144
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Scheme I. Pictorial Representation for the Use of Chemically Modified Electrodes in Electroanalysis Based on Covalent Binding of the Ligand to the Polymer Backbone
:Q: nFAr
Coordination of Metal Ions. For the coordination of metal ions, the modified electrodes were placed in solutions of the desired ions at the desired concentration for a period of 15 min during which the solution was magnetically stirred.
Scheme 11. Examples of Copolymers Where the Ligand Is Part of the Polymer Backbone (Note: No Structural Inference Is Implied)
CONCEPT The concept is based on the use of bifunctional or multifunctional polymer films on electrodes where both electroactive centers as well as coordinating groups are present. The electroactive center would be used to induce precipitation of the polymer on the electrode surface and would also serve in determining the number of immobilized ligand centers (vide infra). The coordinating group would be chosen so as to bind strongly and selectively to the metal ion of interest. The analysis is based on the electrochemical determination of the amount of immobilized metal/ligand complex (Scheme I). This would serve as the analytical signal which would then be related to the concentration of the ion in solution. Synthesis. For this application we have focused our attention on the use of functionalized copolymer films capable of incorporating both an internal redox system (that will be used to induce precipitation of the polymer) as well as coordinating groups capable of binding to metal ions. As the internal redox center we have chosen ferrocene because of its favorable electrochemical properties and other reasons outlined below (vide infra). In terms of the ways that both a ferocene moiety and a coordinating site can be incorporated into a polymer matrix, we have identified two generalized approaches: covalent binding and ion exchange. Covalent Binding. By covalent binding we mean that both the ferrocene group and the ligand site form part of the copolymer backbone. The easiest way to accomplish this is through the polymerization of vinylferocene with a vinylfunctionalized ligand (Scheme 11). The vinylbipyridine/vinylferrocene copolymer is representative of this type: This approach is attractive because of the general applicability to vinyl-containing species and because of the large body of data on polymers and copolymes of vinyl ferrocene (29-31).
H2C = HC
€on Exchange. In this case we are interested in a copolymer of ferrocene with a species that is itself ionically charged or that can be made to carry a charge via simple chemical manipulations. A family of compounds well suited for this application is copolymers of vinylferrocene and vinylpyridine with the subsequent quaternization of the pyridine groups to give rise to a polycationic polymer film capable of incorporating potential ligands (that are negatively charged) for the selective determination of metal ions (Scheme 111). A polycationic polymer was chosen over a polyanionic polymer due to the fact that the number of potential ligands that are negatively charged is far greater than those that are positively charged and as such a cationic polymer film would be much
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
145
Scheme 111. Examples of Copolymers Where the Ligands Are Incorporated through Ion Exchange: a Is Sulfonated Bathocuproine, b Is Diethyldithiocarbamate, and c Is Sulfonated Bathophenanthroline
I I
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00
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W Flgure 1. Single sweep cyclic voltammogram in methylene chloride containing 0.1 M TBAP for a platinum electrode in contact with a solution of the 1:1 vinylferrocene//vinylpyridine copolymer.
E Table I Elemental Analysis of Vinylpyridine/Vinylferrocene Copolymers ratio of vinylpyridine to vinylferrocene in reaction mixture ratio of vinylpyridine to vinyl ferrocene in
1.001.00
5.00:l.OO
10.01.0 V:
0.96:l.OO
5.50:l.OO
8.9:l.O
copolvmer
more versatile in the incorporation of a wide variety of potential coordinating agents. Once synthesized, the copolymers would be characterized in terms of their composition (ratio of monomers in the copolymer) by elemental analysis and these ratios would be compared to those expected on the basis of the ratio of monomers used in the polymerization reactions. Results on the composition of vinylpyridine/vinylferrocenecopolymers where the ratio of monomers was varied by a factor of 10 are presented in Table I. A good correlation exists, indicating that for these types of copolymers, the ratio of monomers in the copolymer will bear a close relation to that used in the polymerization reaction and as such gives us a certain degree of predictability of the polymer's composition and properties. Deposition of Copolymer Films. The feasibility of performing electroanalysis with chemically modified electrodes according to the proposed procedure hinges in great measure on being able to modify the surface of an electrode with films of the synthesized copolymers. It is in this respect that the utility of having vinylferrocene as one of the components of the copolymer comes to light. Analogous to the resulta of Bard and Merz (32)on poly(vinylferr0cene) we find that deposition of the copolymers takes place upon electrooxidation at 0.7 V in methylene chloride solvent with TBAP. The deposition is quite reproducible (*lo%) and easily controllable over a wide range of coverage, especially for the uncharged polymers. For the polycationic polymers, the reproducibility is at present not as good. Illustrative examples are shown in Figures 1-3. Figure 1 shows a single sweep cyclic voltammogram at 200 mV/s for an electrode in contact with a methylene chloride solution containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte and 2 mg/mL of a copolymer of vinylferrocene and vinylpyridine in a 1:l ratio. Notice that the anodic wave looks typical of a diffusion controlled reaction whereas the cat,hodicwave is larger and much
v
E vs
SSCE
Flgure 2. Cyclic voltammograms at 20, 50, 100, and 200 mV/s in acetonitrle containing 0.1 M TBAP for an electrode modified with the 1:1 vinylferrocene/vinylpyridine copolymer. Inset: plot of peak current vs. sweep rate. POly-vp/vfc
OAkl K C I / H 2 0
100rnv/scc.
E vs SSCE
Cyclic voltammogram in water with 0.1 M KCI at 100 mV/s for an electrode modified with the 1:1 vinylferrocene/vinylpyridine copolymer. Flgure 3.
sharper than the anodic counterpart indicative of precipitation on the electrode surface. The electrochemistry of a modified electrode is shown in Figure 2. As can be seen, the cyclic voltammograms are very well defined and have the wave shape characteristic of a surface-confined redox center ( I ) as well as the expected linear relation between the peak current and the rate of potential sweep (1) (Figure 2 inset). By determination of the charge consumed, the surface coverage is estimated at 2.2 X mol/cm2. Well-developed voltammograms can also be obtained in aqueous solution as shown
146
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 V bpy/ V Fc
@ Me-Py/ VFC
M @ C N / TB A P 100 mv/sec.
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E vsSSCE
Cyclic voltammogram at 10G mV/s in acetonitrile containing 0.1 M TBAP for a platinum electrode modified with the quaternized vinylpyridlne/vinylferrocene copolymer.
1;
Flgure 4.
in Figure 3. Similar results are also obtained for the vinylbipyridine/vinylferrocene copolymer. In the determination of the surface coverage, we assume that all of the immobilized redox centers are electroactive on the voltammetric time scale. This, however, has often been shown not to be the case for modified electrodes especially when dealing with relatively thick polymeric films on electrodes. We have also found this to be the case for thick polymer films. However, for the very thin films that we will be concerned with here (in the monolayer regime) we find that the amount of charge consumed is essentially independent of sweep rate (implying a constant number of electroactive groups) and furthermore, the results of chronocoulometric experiments indicate that the amount of charge consumed after a long integration time is essentially identical with that obtained from cyclic voltammetric experiments. Deposition of the quaternized copolymer can also be effected by electrooxidation in methylene chloride/DMF (3:2) solyent; however, in this case the amount of material that can be deposited on the electrode is considerably smaller than that obtained with the unquaternized copolymer probably due to electrostatic repulsions. As such, typical coverages range from 1x to 5 X mol/cm2 (which corresponds approximately to about 10% of a monolayer to five monolayers, respectively). This turns out to be quite advantageous since as will be shown later it is this range of coverages that will be most useful in the present context. A representative cyclic voltammogram for such a modified electrode is presented in Figure 4. In terms of anionic ligands, Scheme I11 illustrates some of the materials that we have employed. These include the sodium salts of sulfonated bathophenanthroline (for the determination of iron), sulfonated bathocuproine (for the determination of copper, especially in the presence of iron), and the sodium salt of diethyldithiocarbamate (for the determination of copper and cobalt). Coordination of Metal Ions. At this point the modified electrode (modified with either a copolymer containing the ligand on its backbone or incorporated through ion exchange) is ready to undergo extraction of the appropriate metal ion from solution. One example of each modality is presented here, while other examples will be presented later on. For the case where the ligand is incorporated through covalent binding, we will use as an example the coordination of iron with the poly(vinylbipyridine)/vinylferrocene copolymer. In the second case we will focus our attention on the use of sulfonated bathophenanthroline for the determination of iron though other cases will be considered later on. Figure 5 shows a cyclic voltammogram for an electrode modified with poly(viny1bipyridine)vinylferrocene after having been contacted with a 5 X M solution of iroh for 15 min. I t can be seen that in addition to the wave associated with the ferrocene group (at E"' = +0.41 V) a second wave at E"' = +0.97 V is also observed. Since the tris(bipyridine) complex of iron has a voltammetric wave in acetonitrile/TBAP at E"'
Cyclic voltammogram at 100 mV/s in acetonitrile containing 0.1 M TBAP for an electrode modified with the vinylbipyridine/vinyIM solution ferrocene copolymer after being contacted with a 5 X Flgure 5.
of iron.
/i 1.0
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,
,
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,
,
, ,
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Cyclic voltammogram at 200 mV/s in acetonitrile containing 0.1 M TBAP for an electrode modified with the quaternized poly(vinylpyridihe)/vinylferrocenecopolymer with incorporated sulfonated M solution bathophenanthroline after being contacted with a 5 X Flgure 6.
of iron.
= $0.95 V then by the similarities in the formal potentials for oxidation for the dissolved and immobilized complexes, the additional surface wave present in the modified electrode can be ascribed to the presence of iron coordinated to the bipyridene ligands in the polymer backbone. In the case where the ligand was incorporated through ion exchange we used sulfonated bathophenanthroline incorporated into the quaternized poly(viny1pyridine)/vinylferrocene copolymer for the determination of iron. The results for this case (under conditions similar to those of the previous example) are very similar to those of the vinylbipyridine/vinylferrocene copolymer (Figure 6). Preliminary quantitative measurements furthermore indicate that the magnitude of the response bears a close relation to the concentration of iron in the analysis solution. In fact, a relatively good correlation can be established for the determination of iron in the range of 1-40 ppm. Though we do not at present have extensive quantitative results, the reproducibility is at present about f15% and we believe that this is due in part to variations in the coverage of deposited polymer. It should be noted that this is not intended to serve as a definite analytical method at this stage but rather to demonstrate that electroanalysis of low levels of metal ions can be performed at these modified interfaces.
DISCUSSION We will now proceed to discuss and analyze those aspects relevant to the use of chemically modified electrodes in electroanalytical applications in light of the proposed approach and the preliminary results presented. The particular aspects
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
147
Bathoc$/MePF/VFc MeCNiTBAP 50 mvis
E vs SSCE
Figure 7. Cyclic voltammogram at 200 mV/s in acetonitrile containing
0.1 M TBAP for an electrode modified with the quaternized poly(vinylpyrldine)/vinylferrocene copolymer with incorporated sulfonated bathocuprolne after being contacted with a 5 X M solution of iron and copper.
to be discussed include sensitivity, selectivity, saturation, and matrix effects. Sensitivity. Sensitivity is perhaps one of the more attractive features of electroanalysis with chemically modified electrodes. One can estimate that a monolayer coverage of the types of complexes involved in here represents about 1 x 10-10 mol/cm2 (33). If one assumes a conservative lower limit of detection of 10% of a monolayer, then for a typical electrode size (0.1 cm2) this would represent about 1 X 10-l' total moles of material. If the typical solution volume to be analyzed is of the order of a few milliliters, one obtains that the detection limits will be in the nanomolar range, assuming that 100% of the sample will partition into the polymer film on the electrode. A more realistic figure is then perhaps or IO-' M. In fact, we have obtained measurable signals (albeit not very reproducible) for lo-' M solutions of Fe(II), consistent with these arguments. In addition, the use of small area electrodes (microelectrodes) should allow for considerably lower limits of detection. Thus, this is clearly a very sensitive method. Selectivity. Selectivity can in principle be achieved by a judicious choice of coordinating agent and control of electrode potential. In the former case one can make to bear on this application the extensive body of coordination chemistry available on the selective coordination of transition metal ions by particular ligands. As such one could make use of selectivity trends exhibited in solution to serve as a guide in determining the optimal reagent for a specific application. The use of sulfonated bathophenanthroline for the determination of iron represents an example where solution studies indicate that it would be a useful reagent for the analysis of iron, and as previously shown, the experiments bear this out. As mentioned previously, one can also tailor a reagent for a particular application. For instance, in the previous example it was shown that sulfonated bathophenanthroline is a particularly good reagent for the determination of iron. Let us however, assume that we are interested in the determination of copper in the presence of iron (a not uncommon situation). Even though the sulfonated bathophenanthroline ligand forms complexes with both copper and iron, the formation constant for the iron complex is considerably larger than that for copper (34,35)so that the determination of copper with this reagent in the presence of iron would be extremely difficult if at all possible. One could however modify the sulfonated bathophenanthroline ligand by placing methyl groups (or other substituents) at the 2 and 9 positions to yield sulfonated 2,9-dimethylbathophenanthroline(better known as sulfonated bathocuproine see a in Scheme 111). It is well-known that because of steric hindrance this reagent (and in general all 2,9-disubstituted phenanthrolines and related ligands) (34, 35) has greatly diminished affinity for iron whereas the affinity for copper remains relatively unaltered. Using this ligand,
MeCNITBA?
20Omvlsec
c
Figure 8. (a) Cyclic voltammogram at 200 mV/s in acetonitrile containing 0.1 M TBAP for an electrode modified with the quaternized poly(vinylpyridine)/vinylferrocenecopolymer. (b) Cyclic voltammogram at 200 mV/s in acetonitrile containing 0.1 M TBAP for an electrode modified with the quaternized poly(vinyipyridine)/vinyiferrocene copolymer after incorporating diethyldithiocarbamate and after contact with a 1 X M solution of copper. (c) Cyclic voltammogram at 200 mV/s for a platinum electrode in contact with an acetonitrile solution of [Cu[diethyldithIo~arbamate)~] in acetonitrile containing 0.1 M TBAP.
one could then anticipate the feasibility of determining copper, in general, and in the presence of iron in particular. In fact, Figure 7 shows a cyclic voltammogram in acetonitrile with TBAP for an electrode modified with quaternized poly(vinylpyridine)/vinylferrocene and having sulfonated bathocuproine incorporated through ion exchange after being contacted with a solution containing both copper and iron at about 1 X lo4 M. One can clearly see a wave at E"' = +0.53 V associated with the copper complex whereas no wave can be seen at about +1.0 where the wave for the iron complex would be expected to appear. This lends a great deal of support to our contention that selectivity trends exhibited in solution can be extrapolated to an electrode surface. This approach is also not limited to only sulfonated derivatives of ligands. For example ligands that by their very nature are anionically charged can also be used. This is demonstrated in Figure 8. Part A is a cyclic voltammogram in acetonitrile with TBAP for an electrode modified with quaternized poly(vinylpyridine)/vinylferrocene copolymer with diethyldithiocarbamate incorporated via ion exchange. The only wave present is that due to the ferrocene couple in the polymer. Part B shows a cyclic voltammogram after the electrode was contacted for 5 min with a M solution of copper ions. One can notice here that the surface wave ascribed to the ferrocene group has increased dramatically and in addition a new surface wave at E"' = -0.50 V is obtained. Part C shows a voltammogram for dissolved Cu(diethy1dithiocarbamate)2 (Cu(dedtc)2). By comparison of the different voltammograms it is immediately apparent that the voltammogram in part B is a composite of the waves for the im-
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mobilized Cu(dedtc)2 complex as well as the ferrocene group in the copolymer. Since one of the waves of the copper complex coincides with that of the ferrocene group, the enhancement in the ferrocene surface wave is observed. The area under the two voltammetric waves for the immobilized complex is essentially identical once the contribution of the ferrocene couple to the wave a t Eo'= +0.45 V is subtracted out. Given the large body of data concerning coordination selectivity patterns in solution as well as the number of ligands that are either themselves anionic by nature or that can be made anionic by incorporation of, for example, sulfonate or carboxylate groups, one can rapidly ascertain that the matter of selectivity can be reduced to the synthesis of an appropriately modified ligand. The same applies to the approach based on covalently bound ligands. In this case, the matter of selectivity would be simply reduced to the development of the appropriate synthetic procedures for the incorporation of vinyl groups (or other sites of unsaturation) onto the appropriate ligand. Saturation. As mentioned previously, many of the proposed analytical schemes based on chemically modified electrodes suffer from the lack of providing for a means of detecting saturation (that is, when all of the coordinating sites have been occupied). Saturation is a particularly severe problem because once it is reached, the analytical signal will no longer bear any relation to the concentration of the species of interest. In order to be able to determine whether saturation of the surface has taken place, one must be able to obtain an independent measurement of the number of sites available for binding to the species of interest and from this determine the maximum signal possible. It was mentioned previously that in the proposed method one could obtain the number of immobilized redox centers (ferrocene in this case) that are part of the copolymer, by determining the charge consumed during a cyclic voltammetric experiment in a solvent where both the oxidized and the reduced forms of the copolymer are insoluble. From elemental analysis one can determine the ratio of redox centers to ligand sites (or the ratio of redox centers to cationic sites) in the copolymer. One can therefore determine the number of ligand sites at the surface by multiplying the number of immobilized ferrocene groups by the appropriate factor. Now, if the coordination number of the metal for the particular ligand is known (say from related solution data) one could determine what the expected maximum response should be. Knowing this would allow the determination of saturation of the surface anchored ligands. For example, in the determination of copper with diethyldithiocarbamate incorporated through ion exchange we find that contacting a modified electrode with a millimolar solution of copper gives rise to a signal that corresponds very closely (95 f 5%) to the saturation coverage estimated from knowledge of the number of immobilized ligand sites. Matrix Effects. When matrix effects are considered in the present context, perhaps the most severe has to do with multiple coordination to a single metal ion. Since the ligands have limited mobility, multiple coordination to a given metal ion could be severely hindered due to steric and/or other factors. Since the redox potential of a metal complex is a strong function of its coordination environment, the redox potentials of immobilized complexes might be severely altered from solution values due to differences in coordination thus largely complicating the analysis. In addition, even if multiple coordination does take place, it would imply that the metal ion is serving as a cross-linking agent and as such may have the effect of impeding mass transport, thus making certain ligand sites physically inaccessible. Thus, this clearly rep-
resents a very serious problem in the proposed method. We believe that we can deal with this problem by keeping the coatings on the electrode surface sufficiently thin (in the monolayer regime), that there will be sufficient mobility of the ligand sites so that multiple coordination to a single metal ion will be feasible and in addition if the layers are thin, even in the presence of multiple coordination, the number of ligand sites rendered inaccessible due to the cross-linking effect mentioned previously would be negligible. In order to test this hypothesis, we used the results from the coordination of iron by both sulfonated bathophenanthroline and bipyridine as well as the results on the coordination of copper by diethyldithiocarbamate. The results on the coordination of iron will be used to illustrate the fact that multiple coordination to a single metal ion can be carried out. It is known that even though the preferred coordination for iron by bathophenanthroline and related ligands is that of a tris chelate (pseudooctahedral), bis and even mono coordination can take place. Furthermore, since the redox process is largely metal based (that is the redox process is formally the oxidation of Fe(I1) to Fe(II1) the formal potential for oxidation is a very strong function of the coordination environment around the metal. Thus, one can infer the coordination environment of the species at the surface through knowledge of the formal potential for oxidation for the surface immobilized redox center. The iron tris(sulfonated bathophenanthroline) complex has a formal potential for oxidation of +0.90 V vs. SSCE in acetonitrile/NaC104. The fact that in the coordination of iron by surface immobilized sulfonated bathophenanthroline only one wave (other than the ferrocene wave) is observed with an E"' value of +0.96 V vs. SSCE immediately indicates that not only is the tris complex formed but also that it is the only complex of iron formed. This indicates that for these modified interfaces and at these thicknesses, there is sufficient mobility of the ligand sites to accommodate tris coordination. It could be argued that due to the fact that in this case the ligands are incorporated via ion exchange that one would expect a certain degree of mobility of the ligand sites. As such the results presented here may not necessarily reflect a general trend of behavior. In order to make a much more rigorous test of our hypothesis, the data for the coordination of iron to bipyridine incorporated into the polymer backbone (that is, the copolymer in this case was vinylbipyridine/vinylferrocene)was used. Similar to the case of the sulfonated bathophenanthroline, when iron is incorporated (through coordination) into an electrode modified with the vinylbipyridine/vinylferrocene copolymer, only one wave (in addition to the ferrocene wave) was observed at an Eo'value of +0.97 V vs. SSCE. By the same reasoning as for the case of the sulfonated bathophenanthroline, we can conclude that in this case as well, not only does the tris complex form, but also it is the only complex formed. Thus it can be concluded that for both modalities of analysis there is sufficient mobility of the ligand sites to allow for multiple coordination to a single metal ion. The case of tris coordination was chosen because it represents a very rigorous test of this aspect. Clearly if tris complexes can be formed, then for complexes where the preferred coordination is bis or mono should present no problem whatsoever. This is clearly borne out by the data on the analysis of copper with diethyldithiocarbamate where as shown in Figure 8 the redox potentials of the immobilized complex are essentially identical with those exhibited by the complex in solution again indicative of the same coordination (bis in this case) around the metal ion. The other potentially deleterious matrix effect is related to the fact that upon multiple coordination to a metal ion some of the ligand sites might be rendered inaccessible due to the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
cross-linkingeffect that multiple coordination to a single metal ion would have. We previously proposed that if the thickness of the deposited film on the surface of the electrode was kept thin, that we would not anticipate having problems of accessibility of the metal ions to the immobilized ligands even in the presence of multiple coordination. To show that this is also not a problem in the present context, data for the determination of copper with diethyldithiocarbamate will be considered. In this case an electrode was modified with the quaternized vinylpyridine/vinylferrocene copolymer and the number of ferrocene units and ligand sites immobilized was determined as previously described. The modified electrode was then contacted with an aqueous solution of copper(I1) (1 X M) for a period of 30 min. It was believed that using this relatively concentrated solution of copper ions and allowing for a substantial equilibrating time, that whatever ligands were accessible would be coordinated to the copper ions. The electrochemical response of the electrode was investigated in acetonitrile with TBAP. By integration of the area under the voltammetric wave centered at -0.50 V, it was determined that essentially all (97 f 10%) of the immobilized diethyldithiocarbamate ligands had been coordinated to the copper ions. Similar results were obtained for the determination of iron using either electrostatically bound sulfonated bathophenanthroline or covalently bound bipyridine. As such this demonstrates that this matrix effect can also be largely minimized through the use of thin polymeric films. (It should be mentioned here that this type of behavior was only observed when thin layers of polymer were deposited on the electrode surface. When thick layers were used, it was found that only a fraction of the immobilized ligand sites were accessible.) One could also consider the effect of electrolyte type and concentration on the formation constants for the surface complexes since these factors affect formation constants in solution. This is a point that we are presently pursuing, though at this point, we have no data to report. These studies thus show that problems associated with matrix effects can be largely eliminated by the use of thin films of the copolymer on the electrode surface.
CONCLUSIONS In conclusion, we have demonstrated the feasibility of using chemically modified electrodes for performing electroanalysis in solution via functionalized polymer films. The proposed approach not only takes advantage of the favorable aspects of chemically modified electrodes for electroanalysis (e.g., sensitivity) but also provides for very broad synthetic variations (and therefore selectivity) as well as ways to overcome the problems associated with the determination and correction of saturation and matrix effects. Even though the concept is presented in terms of the analysis of metal ions, this approach could be extended to the determination of organic functionalities through the appropriate choice of reagents.
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Registry No. [Fe(B),] (B = sulfonated bathophenanthroline), 92937-64-9; [Cu(B),] (B = sulfonated bathocuproine), 75716-21-1; [Cu(B),] (B = diethyldithiocarbamate),13681-87-3;Pt, 7440-06-4; Fe, 7439-89-6; Cu, 7440-50-8;poly(vinylpyridine)/vinylferrocene, 55279-05-5;quaternized vinylpyridine methyliodide/vinylferrocene copolymer, 92957-37-4; poly(vinylbipyridine)/vinylferrocene, 92937-65-0; sulfonated bathophenanthroline, 92957-35-2; sulfonated bathocuproine, 92957-36-3; diethyldithiocarbamate, 14784-2.
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RECEIVED for review July 11, 1984. Accepted September 12, 1984. This work was supported in part by the Research Corporation and by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
