Anal. Chem. 1990, 62, 274-278
274
Determination of Copper at Electrodes Modified with Ligands of Varying Coordination Strength: A Preamble to Speciation Studies Seong K. Cha and H6ctor D. Abruea*
Department of Chemistry, Baker Laboratory, Cornel1 University, Ithaca, New York 14853-1301
Electrodes mod#led with seven dmerent Hgands (incorporated by ion exchange Into a pdycatlonic Hlm of ekcb-ed [Ru( v-bpy),]*+) (v-bpy, 4-vlnyl-4'-methyl-2,2'-blpyrldyi)) whose formattan constants for copper vary over a very broad range, have been employed In the determination d copper In solutlon In an effort to ascertaln the utlllty of chemlcally modlfled electrodesto carry out speclath studies. The redox response of the surface lmmoblllred copper/llgand complex was used as the analytical slgnal. I n all cases, calibration curves (log of the surface coveragenormallzed redox response vs log [Cu]) exhlblted an excellent correlatlon ( r 1 0.98) for copper concentratlons ranglng from 5 X lo-' to 1 X lo3 M. More importantly, when the SOkRlon concentration of copper Is kept constant, we flnd an excellent correlatlon between the log of the normallred current (currenthutface coverage) for the surface knmoMlized copper complex (employlng the various ligands) and the log of the formatlon constants, lndlcatlng that the relatlve strength of coordination exhlblted In solullon Is retained for the surface-immoblllred ligands. The effects of having present other competlng ligands lndudhrg chbrlde, bromkJe, oxalate, ammonla and hunlc ackl on the uptake d copper by the modilled electrodeg have also been studled. We flnd that the presence of competlng ligands causes a dhlnutlon In the analytlcal slgnal due to copper Incorporation and that the magnitude of thls effect Is dependent on the relatlve strength d cOOIcWnatkn of the other competlng ligands for copper Ions as well as on their concentration. The relevance of tMs work to speciatlon studies Is discussed.
INTRODUCTION Within the general context of chemically modified electrodes (CMEs)(l-S), the development of analytical strategies and sensors represent two of the most active areas to which these modified interfaces have been applied. This is due, in part, to the realization by many investigators that there are numberous advantages that accrue from the use of chemically modified electrodes when applied to analytical problems. These include the high specificity that can be achieved by the appropriate choice of modifier in addition to the excellent sensitivity that derives from the fact that many of the analytical applications are based on the preconcentration of the analyte at the surface modified electrode so that all of the advantages of preconcentration methods are applicable. In addition, the methodologies and instrumentation involved are relatively simple and, when coupled to microelectrodes, may allow for analytical studies on very small samples including single cell specimens. Thus, it is clear that the use of CME in analytical applications offers a wide range of advantages. As mentioned before, such advantages have not gone unnoticed, and a number of analytical applications of chemically modified electrodes have been reported (9-26). 0003-2700/90/0362-0274$02.50/0
We have sought to exploit the advantages of polymermodified electrodes for the determination of transition-metal ions (18-23)and organic functionalities (24-26).Our methods are based on the preconcentration of the analyte (metal ion or organic functionality) at the electrode surface by modifying the same with functionalizedpolymers that carry reagents for the selective and sensitive determination of the species of interest. For the determination of transition-metal ions we employ bifunctional or multifunctional polymer films containing both electroactive centers as well as coordinating groups. The internal redox center is used to induce precipitation of the polymer on the electrode surface and thus allows for the precise control of the coverage and also serves in the determination of the number of immobilized ligand sites. This latter point is important as it allows an a priori determination of a saturation response. The coordinating group is chosen so as to bind strongly and selectively to the metal ion of interest. In addition, we have also employed carbon paste electrodes where the polymer containing the ligand is mixed with the pasting material. A very attractive feature of this approach is that it allows for the rapid regeneration of the electrode surface. The analysis is based on the electrochemical determination of the amount of immobilized metal/ligand complex and can be either a metal- or ligand-based redox process, and we have employed both in our studies. This serves as the analytical signal which is then related to the concentration of the ion in solution. We have demonstrated the applicability of this approach to the determination of iron, copper, cobalt, nickel, and calcium (18-23). We have also extended this method to the determination of organic functionalities by exploiting partitioning effects as well as simple chemical transformations that exhibit high selectivity toward a particular functional group (24-26). As part of our continued interest in the analytical application of chemically modified electrodes, we are interested in assessing the utility of such modified interfaces in speciation studies. Speciation studies (that is, the determination of the concentrationof a given ion and the identification of the forms in which it is found) are of great importance in the analysis of environmental samples since a wide variety of metal ions are toxic at very low concentration levels and, in addition, their toxicity is often strongly dependent on the particular form in which they are found. Being able to perform such speciation studies, however, is perhaps one of the most demandingthings to ask of any analytical technique. Not only are the concentration levels low, but the given ion may be present in numerous forms and one must ensure that the method of analysis not only is sufficiently sensitive but also does not perturb the distribution of species. At a fundamental level, speciation involves competitive equilibria between the metal ion of interest and ligands present in solution. Since our approach for employing chemically modified electrodes in analytical applications depends on coordination trends, we sought to assess its utility in such 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 3, FEBRUARY 1, 1990 HO
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=
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Chromotrope 2B Log K = 9.8
Sulphosalicylic Acid Log K = 16.4
Table I. Formal Potentials for Copper Complexes of Selected Ligands ligand 1. Alizarine Red-S 2. Eriochrome Red-B
3. Chromotrope 2-B 4. Chrome Azurol-S 5. Sulfosalicylic Acid 6. Buthocuproine Sulfonate 7. Eriochrome Violet
formal potential" +0.43 +0.47 +0.52 +0.52 +0.57 +0.56 +0.57
Potentials in volts vs SSCE.
OH OH N &&-N
275
Bathocuproine Sulphonate LogK=19.1
Chrome Azurol S Log K = 13.2
Eriochrome Violet Log K = 21.8
Flgure 1 . Structure and formation constants for copper for ligands employed in this study.
speciation studies. As an initial step we have investigated the response, for the determination of copper ions in solution, of electrodes modified with seven different ligands whose affmity for copper varies over a very broad range. In addition, we have studied the effects, on the analytical signal, of other competing ligands of varying coordinating strength toward copper and a t various concentrations. The relevance of these investigations to speciation studies is discussed. EXPERIMENTAL SECTION Reagents. The ligands employed (see Figure 1 for the structures of the ligands and formation constants for copper) and their sources are as follows: 1, Alizarine Red4 (Aldrich); 2, Eriochrome Red-B (Aldrich); 3, Chromotrope 2-B (Aldrich); 4, Chrome Azurol-S (Aldrich); 5, Sulfosalicylic Acid (Aldrich); 6, Buthocuproine Sulfonate (G. F. Smith); 7 , Eriochrome Violet (Aldrich). In addition, they were all purified by recrystallization (3 times) from ethanol. Water was purified by passing through a Millipore Milli-Q system or a Hydro purification train. Acetonitrile (Burdick and Jackson distilled in glass) was dried over 4-A molecular sieves. Tetra-n-butylammonium perchlorate (TBAP) (G. F. Smith) was recrystallized (3 times) from ethyl acetate and dried under vacuum at 75 "C for 48 h. [Ru(vbpy),] (PF& (v-bpy,4-vinyl-4'-methyl-2,2'-bipyridyl)was prepared as previously described (27). All other reagents were of at least reagent grade quality and were used without further purification. Instrumentation. Platinum disk electrodes (sealed in glass) were used throughout. They were polished prior to use with 1-pm diamond paste (Buehler) and rinsed with water and acetone. Three-compartment electrochemical cells (separated by medium-porosity sintered glass disks) of conventionaldesign were used. Electrochemicalexperimentswere performed on either an IBM EC225 voltammetric analyzer or a BAS 100 electrochemicalanalyzer. Data were recorded on a Soltec X-Y recorder. Differential pulse voltammetric experiments were carried out with a 50-mV pulse amplitude and at a sweep rate of 10 mV/s. All potentials are reported against the sodium-saturatedcalomel electrode (SSCE)without regard for the liquid junction potential. Procedures. Electrodes were modified with poly-[Ru(vb p ~ ) ~by ] ~electroinitiated + polymerization (27) of the monomer complex (typicallyat 0.5 mM concentration)from acetonitrile/O.l
M TBAP solution by scanning the potential between 0.0 and -1.60 V for a prescribed amount of time depending on the desired coverage. The exact coverage was determined by measuring the charge under the voltammetric wave for the Ru(III/II) process at about 1.25 V. Typical coverages were in the 2-3 equivalent .. monolayers range. The electrodesmodified with a polymeric film of [Ru(~-bpy)~]~+ were immersed in an aqueous solution of the desired ligand (typically 5-10 mM depending on solubility) for 15 min while stirring. The electrodes were subsequentlyrinsed with water and placed in an aqueous pH 3.85 acetate buffer solution of Cu(1) (obtained by the addition of hydroxylamine hydrochloride at &fold excess) at various concentrations (from 5.38 X lo4 to 1.07 X M) and in the presence (or absence) of other competing ligands for 10 min (with stirring) after which the electrode was rinsed with water and acetone. The electrochemical response of the copper ligand complex was used as the analytical signal and was determined by differential pulse voltammetry in acetonitrile/O.l M TBAP. The currents were normalized to the surface coverage, which was determined as described above. All experiments were carried out in at least five replicate determinations and the deviations were typically of the order of 8-10% relative standard deviation.
RESULTS A. Preliminary Voltammetric Characterization. The redox responses of the copper complexes for all of the ligands were determined in solution and values of the formal potentials are presented in Table I. As can be ascertained, all of the complexes have a redox response that is well removed from that for the poly-[R~(v-bpy)~]~+ film so that no interference effects would be anticipated. Similar responses were obtained for electrodes modified with the various ligands incorporated by ion exchange. Figure 2A shows a cyclic voltammogram for an electrode modified with a thin polymeric film of [Ru(v-bpy),12+and a very well behaved, reversible response is observed a t +1.25 V, and in addition, a very low, flat background is observed from 0.0 to about +1.1 V. By integrating the charge under this voltammetric wave, the polymer coverage for this electrode was determined to be 2.6 equivalent monolayers. Parts B-E of Figures 2 show differential pulse voltammogramsfor electrodes modified with Chrome Azurol-S (B, E) and Eriochrome Red-B (C, D)after having been contacted with 5.38 X lo4 (B, C) and 1.04 X M (D, E) copper solutions. Well-behaved and quantifiable voltammetric waves were observed. Similar behavior was observed for all the ligands employed in this work. B. Calibration Curves. By use of the current (or the area under the voltammetric wave) for the surface-immobilized ligand/copper complex (normalized by the surface coverage of polymer) calibration curves (Log i / r vs Log [Cu]) for the determination of copper were constructed. Figure 3 shows representative plots employing electrodes modified with (A) Chrome Azurol-S and (B) Eriochrome Red-B, respectively. As can be ascertained, excellent correlations are obtained over the range of copper concentrations from 5 X lo-* to 1 X lo-, M. This is illustrative not only of the sensitivity of the method
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 3, FEBRUARY 1, 1990
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Figure 4. plot of log of the normalized current vs log of the formation constant for copper for electrodes modlfii with the various ligands after contacting a solution containing copper at a concentration of 1 x 10-5 M.
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E vs SSCE Flgure 2. (A) Cyclic voltammogram at 100 mV/s for a latinum electrode modlfiedwlth a thin polymeric film of [Ru(v-bpyk] . ( R E ) Differentlal pulse voltammograms for electrodes, modified with a thin polymeric fhof [ R u ( v - b p ~ ) and ~ ] ~ wlth Chrome AzurocS (B, E) and Eriochrome Red-B (C, D) incoporated by ion exchange, after exposure to copper solutions at 5.38 X lod (B, C) and 1.04 X lo4 M (D, E). S is (A) 20 nA and (B-E) 10 nA.
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ligands. Although saturation behavior was generally observed for all of the ligands employed, the solution copper concentration at which it became manifest varied from ligand to ligand, becoming higher for ligands with lower formation constant. At the low concentration end, the response appears to level off at a solution copper concentration of about 5 X lo-* M. This, however, appears to be due not to the limit of detection of the technique but rather to background levels of copper in the reagents employed. Similar calibration curves were obtained with electrodes modified with each of the ligands and, again, excellent correlations were found for copper concentrations ranging from 5 x io-* to 1 x 10-3 M. We were also interested in determiningif, at a fixed solution concentrationof copper, the observed response was dependent on the value of the formation constant for copper of each of the various ligands employed. We carried out such a study employing a solution concentration of copper of 1 X M. The results are presented in Figure 4 where the log of the normalized current response for the various incorporated ligands is plotted against the log of the formation constant (in solution) for the corresponding ligands. As can be ascertained, an excellent correlation is obtained ( r = 0.98) indicating that the relative strength of coordination exhibited in solution is also maintained at the surface. However, it should also be noted that the slope of such a line is significantly different from one, pointing to the presence of other effects affecting coordination. This, however, does not alter the previous assertion. Although Cu(1) solutions were employed in the studies described above, similar results (albeit with somewhat lower sensitivity) were obtained with Cu(I1) solutions. C. Effects of Competitive Ligands. As part of our studies aimed at an understanding of the various aspects that can affect the analytical determination, we have studied the effects of competitive binding of other ligands for the copper ions for electrodes modified with the various ligands. These studies will also be helpful in trying to apply the approach described here to speciation studies since at a fundamental level speciation involves competitive equilibria. In these experiments the modified electrode was exposed to solutions of copper at a fixed concentration of 5 X M which in addition contained a competing ligand at various concentrations. In this manner, we have studied the effects of chloride, bromide, oxalate, ammonia, and humic acid as competing ligands. Figure 5 shows representative results obtained when the competing ligands were chloride, bromide, oxdate, and humic acid (in this case since the molecular weight of humic acid is unknown, its concentration is expressed in terms of percent by weight) at electrodes modified with Chrome Azurol-S. There are a number of salient features that are immediately apparent from these plots. First of all, we
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but also of its wide dynamic range. A t the higher concentrations, there appears to be some evidence of saturation as the observed response begins to level off. This was corroborated by the fact that the current for the immobilized ligand/copper complexes did not increase with further increases in the solution concentration of copper. In addition, the observed currents (at saturation) correlated very well with our estimates for a completely metalated film calculated from the experimentally determined surface coverage of the polymer on the electrode surface and assuming complete neutralization of the charge due to the pendant [Ru(v-bpy)J2+groups by the sulfonate side chains on the
ANALYTICAL CHEMISTRY, VOL. 62, NO. 3, FEBRUARY 1, 1990 L I
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observe that in all cases there is a diminution in the response for the surface immobilized Chromazurol-S/Cu complex and that the magnitude of this effect is proportional to the solution concentration of the competing ligand (recall that the copper concentration was kept constant at 5 X M). The wellbehaved nature and excellent correlation (r 1 0.97) of the log i/r vs -log [competitive ligand] plots strongly suggests that indeed the observed effects are due to competitive binding between the surface immobilized Chromazurol-S and the ligand in solution. In addition, for the cases where there are reliable values for the formation constant for copper with the various competitive ligands, we find that a plot of log i/r vs log Kcu is also linear. Similar results were obtained for electrodes modified with the various other ligands pointing to the generality of this observation. These are important observations in that they not only support our assertion that competitive binding effects are responsible for the observed diminution in the analytical signal but also, more importantly, establish that the relative strengths of coordination of the various ligands are maintained under the experimental conditions employed. This implies that one can systematically and deliberately control the coordinative properties of an interface (modified electrode in this case) by the choice of the immobilized ligand as well as the presence of other competitive ligands in solution.
DISCUSSION We believe that the results presented here serve as a preamble to the application of chemically modified electrodes to speciation studies and present here the basic aspects of such an approach. Conceptually, the approach would be based on the use of a family of ligands whose formation constants for the metal ion of interest vary over a broad range. These materials would be incorporated into the surface of an electrode and would be employed in the determination of the metal ion under study. If, in the immobilized state, the various ligands retain
M at
their relative strength of coordination for the metal ion, the measured analytical response (electrochemicalin the present context) for the immobilized metal/ligand complex would vary in relation to the magnitude of the formation constant. In addition, if the metal ion in solution is present in various chemical forms (i.e. coordinated to other ligands in the sample such as chloride, oxalate, humic acid, etc.) competitive equilibria, for the metal ion, will exist between the ligands present in solution and the surface immobilized ligand. Again, the larger the value of the formation constant of the surface immobilized ligand for the metal ion of interest, the more effectively it will compete, relative to the other ligands present in solution (competing ligands), for the metal ion. In other words, the response depends on the conditional formation constant for copper under the specified conditions (28). In order to account for such competitive equilibria, the effect of other competing ligands (as a function of their concentration and coordinating strength) on the analytical determination of the ion of interest would be determined for each of the surface immobilized ligands. Thus, with the electrochemical response of the immobilized metal/ligand complex monitored as a function of the coordinating strength of the surface immobilized ligand and from a knowledge of the concentration of the competing ligands present in solution, one could make an assessment as to the forms (chemical environment) in which the analyte ion is found. The fact that the proposed analytical approach is based on coordination chemistry allows for a very different and potentially superior approach to speciation studies. In addition, the vast body of literature on coordination chemistry provides a wide range of candidates for study. In order for this approach to be applicable to speciation studies, we need to first of all establish that the relative strength of coordination (as measured by the formation constant) of the various ligands toward the specific metal ion is retained when the ligands are immobilized on the surface of
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an electrode. This is a critical point since the analysis will depend on the ability to vary the strength of coordination at the surface so as to distinguish the presence of ions in chemically distinct forms. As shown in Figure 4, this is clearly the case for the family of ligands employed in this study. In addition, the calibration curves for the determination of copper with the various ligands exhibited an excellent correlation over a broad range of copper concentrations, typically from about 3 x 10-8 to 1 x 10-3 M. The other point that one needs to be concerned with is the effect of other competing ligands for the metal ion of interest. As shown above, the general trends observed are that competing ligands cause a diminution of the signal that is proportional to the concentration of the competing ligand as well as to the formation constant for copper with the corresponding competing ligand. These two findings are quite significant in that they demonstrate that surface immobilized ligands retain their relative coordination strength for copper and that, similarly, the effects of other competing ligands can be interpreted in terms of concentration and coordinating strength. From these measurements, along with a measurement of the total copper concentration (e.g. from anodic stripping voltammetry after irradiation of the analysis sample with ultraviolet light to decompose the organic matter present and release the copper ions into solution), one could establish a series of simultaneous equations from which one could, in priniple, extract the fraction of the copper ions present in a given coordination environment. The main advantage of this procedure is that it relies on a sequence of reagents of increasing and known affinity for the ion of interest and where the effects of other competing ligands are also known. By monitoring the current for the surface immobilized metal/ligand complex as a function of the coordination strength of the immobilized ligand, one could ascertain the fraction of the metal present within a coordination environment of a given strength and from this infer the form in which this fraction is present. Furthermore, since only very small amounts of analyte are required to perform the analysis (due to the high sensitivity of the method), the system will be minimally perturbed. Thus this approach appears to be capable of fulfilling all of the requirements for speciation studies.
M. The relative strength of coordination for copper exhibited by the various ligands for copper in solution is retained when the ligands are immobilized on the surface of an electrode. In addition, the presence of competing ligands causes a diminution in the analytical signal and this effect is dependent on the concentration as well as the coordination strength for copper of the competing ligand. These results point at the feasibility of employing chemically modified electrodes in metal speciation studies.
CONCLUSIONS
RECEIVED for review August 17,1989. Accepted November
We have shown that electrodes modified (by ion exchange into an electropolymerized thin film of [ R u ( ~ - b p y ) ~ ]with ~+) various ligands whose coordination strength for copper varies over a very broad range can be employed in the determination of copper in solution over the range of 5 x to 1 x
LITERATURE CITED (1) Murray, R. W. I n E l e C t r o a n a ~ l C h e r n k t r yBard, ; A. J., Ed.; Marcel Dekker: New York. 1984 Vol. 13, p 191. (2) Murray, R. W. Anno. Rev. Mater. Scl. 1984, 14, 145. (3)Murray, R. W. Acc. Chem. Res. 1980, 13, 135. (4) Faukner, L. R. Chem. Eng. News 1982, February 27. 28. (5) Abruk, H. D. cwrd.Chem. Rev. 1988, 88, 135. (6)Abruk, H. D. I n Elecfroresponslve Molecuiar a n d m Systems; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1988; p 92. (7)Fujihka, M. I n Toplcs h Orpnlc Chemktry; Fry, A. J., Britton, W. R., Eds.; Plenum: New York, 1986;p 255. (8)Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A. (9)Baldwin, R. P.;Christensen, J.-K.; Kryger, L. Anal. Chem. 1988, 58, 1790. (10)Prabhu, S.V.; Baldwin, R. P.; Kryger, L. Anal. Chem. 1987, 59,1074. (1 1) Cox, J. A.; Kulesza. P. J. J . Ektroenal. Chem. 1985, 159,337. (12)hrdea-TONesdey, J.; Darnall. D.; Wang, J. J . Electroanal. Chem. 1988,252, 197. (13) Ikaniyama, Y.; Heineman, W. R. Anal. Chem. 1988. 58,1803. (14)Cox, J. A.; Das, B. K. Anal. Chem. 1985, 57, 2739. (15)Oyama, N.; Anson, F. C. J . Ehwirochem. Soc.1980, 127,247. (16)Espencheld, M. W.; Ghatak-Rog, A. R.; Moore, R. B., 111; Penner, R. M.; Szentirmay. M. N.; Martln, C. R. J. Chem. Soc., Faraday Trans. 1 1988, 82,1051. (17)k g y , G.;G e h r d t , G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B., 111; Szentkmay, M. N.; Martin, C. R. J . Ektroanal. Chem. 1985, 188, 85. (18)Guadelupe, A. R.; Abruk, H .D. Anal. Chem. 1985, 57. 142. (19)Wier, L. M.; Guadalupe, A. R.; Abruk, H. D. Anal. Chem. 1985,57, 2009. (20)Guadalupe, A. R.;Wier, L. M.; Abruk, H. D. Am. Lab. 1986, 18(8), 102. (21) McCracken, L.; Wler, L.; Abruk, H. D. Anal. Lett. 1987, 20 (IO), 1521. (22) Hwrell, H. C.; Abruk, H. D. Anal. Chem. 1988,60,254. (23)Kasem, K. K.; Abruk, H. D. J . Ektroenal. chem.1988,242. 87. (24) Guadalupe, A. R.; Abrufia. H. D. Anal. Lett. 1988, 19(15&16), 1613. (25)Guadalupe, A. R.; Jhaverl, S.; Llu, K. E.; Abrufia, H. D. Anal. chem. 1987, 59,2436. (26)Llu, K. E.; Abruk, H. D., Anal. Chem. 1989, 61,2599. (27) Abrufia, H. D.; Denlsevich, P.; Umab, M.; Meyer, T. J.; Murray, R. W. J . Am. Chem. Soc. 1981, 103,1. (28)Laitinen, H. A.; Harris, W. E. Chemical A n a m . 2nd.ed.;McGrawHlll: New York, 1975.
1, 1989. This work was supported in part by the National Science Foundation. H.D.A. is a recipient of a Presidential Young Investigator Award (1984-1989) and an A.P. Sloan Fellowship (1987-1991). S.K.C. acknowledges support by the Korean Ministry of Education.
