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number of R6G molecules/sphere. Also, it was assumed that during evaporation of the solvent from the test tubes containing the silica spheres, all of the R6G was adsorbed to the spheres. Loss of R6G by adsorption to the walls of the glassware would mean there is actually less rhodamine per sphere than is believed, and hence a lower limit of detection, LOD. Despite the difficulties presented in making a more accurate calculation of the LOD, we have shown through conservative estimates that the detection power of this technique is quite good. This particular instrumental setup may have future application for the ultratrace measurement of polycyclic aromatic hydrocarbons on air particulates.
ACKNOWLEDGMENT The authors express their appreciation to Michael Kosinski for the SEM analysis.
LITERATURE CITED (1) Mlyaishi, K.; Kunlake, M.; Imasaka, T.; Ogawa, T.; Ishibashi, N. Anal. Chlm. Acta 1981, 125, 161-164. (2) Bradley, A,; Zare, R. J. Am. Chem. SOC. 1976, 98, 620. (3) Hirshfeld, T. Appl. Opt. 1976, 15, 2965-2966. (4) Dovlchl, N.; Martin, J. C.; Jett, J. H.; Trkula, M.; Keller, R. A. Anal. Chem 1984, 5 6 , 346-354. (5) Masterton, W. L.;Slowinski, E. J. "Chemical Prlnciples"; W. B. Saunders Co.: Philadelphia PA, 1973;p 262.
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Barbara Kirsch Edward Voigtman James D. Winefordner* Department of Chemistry University of Florida Gainesville, Florida 32611 RECEIVED for review February 21,1985. Accepted May 6,1985. This research was supported by DOE-DE-AS05-780R06022.
Multiple-Use Polymer-Modified Electrodes for Electroanalysis of Metal Ions in Solution Sir: We recently demonstrated (1) the analytical utility of electrodes modified with functionalized polymer films for performing electroanalysis of metal ions in solution. The approach is based on the use of polymer films that bear both an electroactive group as well as a ligand. The former serves to immobilize (or deposit) the polymer onto the electrode surface via electroprecipitation. This is advantageous since it allows for the direct control and determination of the coverage of the polymer on the electrode surface by controlling the deposition conditions and by monitoring the electrochemical response of this electroactive center after deposition, respectively. It also serves in the determination of saturation (I). The ligand itself is chosen so as t o have a high affinity for the metal ion of interest and in addition show high selectivity. The ligand can be incorporated by being part of the polymer backbone or, alternatively, by ion exchange to a polycationic polymer film. We have demonstrated the effectiveness of this approach to the determination of low levels of iron and copper with high sensitivity and selectivity ( I ) . A drawback of this approach is that in many instances the electrodes can only be used for a single determination and that they need to be modified prior to each use. This procedure not only is tedious but also requires careful normalization of the data (with respect to coverage on the electrode surface) prior to comparison. Clearly the development of reagents suitable for multiple determinations is highly desirable. One way to accomplish this would be through the use of metal ligand complexes whose stability constant i s a very strong function of the oxidation state of the metal and whose electrochemical response is metal localized so that the ligand remains intact. This strategy would allow for the use of a single modified electrode in multiple analytical determinations. In essence, since the redox process will be metal localized, after the redox transformation (oxidation or reduction) the metal/ligand complex dissociates, but the immobilized ligand is left unaltered, so that the electrode is ready to be reused. We wish to demonstrate this with the use of 2,g-dimethyl sulfonated bathophenanthroline (also known as sulfonated bathocuproine) for the determination of copper. This reagent has a very high affinity for Cu(1) (2) but the complex dissociates when oxidized to Cu(I1) leaving the ligand intact. This is due to the fact that as Cu(1) the d10 metal center strongly favors a tetrahedral geometry. Upon oxidation to Cu(II),
however, a square planar geometry is preferred by the now d9 metal center. However, due to the steric constraints imposed by the 2,g-dimethyl substituents, such a geometry cannot be accommodated and the complex dissociates ( 2 , 3 ) .
EXPERIMENTAL SECTION 1. Reagents. The synthesis of the quaternized vinylpyridine-vinylferrocene copolymer ( I ) and the electropolymeri2f (v-bpy is 4-vinyl-4'-methyl-2,2'-bipyridine) zation of [R~(v-bpy)~] ( 4 ) were as previously described. Homogeneous polymerization of [ R u ( ~ - b p y ) ~was ] ~ effected + in acetonitrile by free radical initiation using AIBN (azobis(isobutyronitri1e)). Sulfonated bathocuproine was obtained from G.F. Smith and was 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, dimethylformamide (DMJ?),and methylene chloride (Burdick and Jackson "distilled in glass") were dried over 3-a molecular sieves. Water was purified by passing through a Hydro Systems unit. All other reagents were of reagent grade quality and were used without further purification. Electrochemical experiments were performed with a Princeton Applied Research Model 173 potentiostat with a Princeton Applied Research Model 175 universal programmer or an IBM EC-225 voltammetric analyzer. Data were recorded on a Soltec Model 6423 or a Hewlett-Packard Model 7045-B X-Y recorder. Platinum disk electrodes (of area ranging from 0.01 to 0.03 cm2) were used and these were polished with 1-pm diamond paste (Buehler) prior to use. Electrochemical cells were of conventional design. Modification of the electrodes with the quaternized vinylpyridine vinylferrocene copolymer (q-vp/v-fc) was performed by electroprecipitationfrom methylene chloride/TBAP solutions as previously described ( I ) . The coverage of polymer on the surface was controlled so as to be in the (1-5) X 10-lo mol/cm2 range. Modification with poly[Ru(~-bpy)~]~+ was effected by immersion of a polished electrode into an acetonitrile solution of the polymer (5 mg/25 mL) for 1 min after which the electrode was air-dried and rinsed with acetone and water. This procedure gives a coverage of adsorbed polymer on the order of (0.3-3) X mol/cm2 as determined by integration of the cyclic voltammetric wave for the oxidation of the [ R u ( ~ - b p y ) ~centers ] ~ + in the polymer. The sulfonated bathocuproine was incorporated by ion exchange by contacting the electrode modified with either the (qvp/v-fc) or p~ly[Ru(v-bpy)~]'+polymers (both of which are
0003-2700/85/0357-2009$01.50/00 1985 American Chemical Society
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2ol 150
._ -2Q‘
1
2
3
4 5 5 pprn C u (I I
7
8
Figure 2. Plot of peak current for the oxidation of the surface immobilized sulfonated bathocuprolne/Cu(I)complex as a function of the Cu(1) concentratlon In aqueous solution.
Flgure 1. Cyclic voltammograms at 50 mV/s in acetonitrile/O.l M TBAP for (A) a platinum electrode modified with the (q-vp/v-fc) copolymer and (B) cyclic voltammograms (scans 1, 2, 4, and 7) at 50 mV/S for the electrode in A with sulfonated bathocuproine incorporated M solutlon via ion exchange, alter exposure to an aqueous 5 X of Cu(1). Inset shows structure of sulfonated bathocuprolne.
polycationic) with a 10 mM aqueous solution of the ligand for 15 min, after which the electrode was rinsed with water. The incorporation of Cu(1) was achieved by immersion, for 10 min (note: longer exposure times, e.g., 20 min gave identical results), of the modified electrode (containing the ligand) into stirred aqueous solutions of copper at the desired concentration with hydroxylamine sulfate (10 mM) being added to ensure that the copper remained in the +1 oxidation state. The electrode was rinsed with water and placed in acetonitrile/O.l M TBAP where the electrochemical response of the sulfonated bathocuproine/Cu(I) complex was examined. The peak current or peak area associated with the complex was used as the analytical signal.
RESULTS AND DISCUSSION Figure 1A shows a cyclic voltammogram in acetonitrile/ TBAP for an electrode modified with the (q-vp/v-fc) copolymer and a well-developed wave at Eo’= +0.37 V vs. SSCE, associated with the ferrocene centers in the copolymer, can be observed. Figure 1B shows a series of cyclic voltammograms (scans 1,2,4, and 7) for the same electrode having the sulfonated bathocuproine (see inset for the ligand structure) incorporated via ion exchange, after having been exposed to M in Cu(1). (In this case the an aqueous solution of 5 X vinylferrocene serves as the internal redox couple and the quaternized pyridine groups give the polymer its cationic character.) A wave associated with the copper/sulfonated bathocuproine complex can be clearly observed at about +0.50 V vs. SSCE (sodium saturated calomel electrode). (The potential of this surface wave is very similar to that of the complex in solution.) The wave centered a t about +0.37 V is due to the ferrocene groups in the copolymer. The peak current for the sulfonated bathocuproine/copper complex can be used to quantitatively determine the concentration of copper in solution and, as seen in Figure 2, a very good linear correlation exists. Figure 1B also shows that, on consecutive scans, the height of the wave due to the copper complex diminishes greatly, and after approximately 10 scans, there is no electrochemical wave for the copper/sulfonated bathocuproine. The only electrochemical response observed at this point is that associated
E vs SSCE Figure 3. Differential pulse voltammograms in DMFITBAP for an electrode modlfled with a layer of polymerized [Ru(v-bpy),] with sulfonated bathocuproine (curve A) after contactlng the electrode with M solution of Cu(1) for 10 min, (curve B) iman aqueous 2 X mediately after curve A, (curve C) after contacting the electrode with the Cu(1) solution for a second time, and (curve D) after contacting the Cu(1) solution for a third time.
*+
with the ferrocene in the copolymer, and the voltammetric response in essentially that observed in Figure 1A. This reflects that upon oxidation to Cu(II), the complex dissociated and, after about 10 scans, the copper ions are completely lost from the surface. At this stage, the electrode is ready to be reused. We have performed a series of experiments where a single electrode has been used in multiple determinations of Cu(1) from aqueous solution. From these experiments we find that at least six determinations can be performed with a single electrode and that the variation in the area under the (differential pulse) voltammetric wave is f14%. We believe that the variation can be ascribed, at least in part, to the proximity of the redox process of the ferrocene center. In essence, the ferrocene decomposes when present in the oxidized form (ferricenium) so that fluctuations in the background will arise. The use of a polymer, with an internal redox couple whose redox potential is further removed from that of the Cu/ sulfonated bathocuproine complex, would be expected to give better reproducibility. With this in mind, polymerized layers of [Ru(v-bpy)~]~+ were used for incorporation of the sulfonated bathocuproine. Since the electrochemical response of the ruthenium centers is quite removed from that of the copper complex (EO’for oxidation of p~ly[Ru(v-bpy)~]~+ is +1.23 V), and the fact that this is a very stable material in both oxidation states, the background variations encountered with the ferrocene co-
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Anal, Chem. 1985, 57, 2011-2015
polymer should be eliminated and in fact this was found to be the case. Figure 3, parts A and B, shows consecutive differential pulse voltammograms in DMF/TBAP (0.1 M) for an electrode modified with a layer of [Ru(v-bpy)#+ polymer (coverage of about 3.2 x 10-lomol/cm2) with sulfonated bathocuproine incorporated via ion exchange after having been contacted with a 2 x M solution of Cu(1). On the first scan (A) a well-defined wave can be observed with an Epvalue of +0.515 which correlates very well with the value for the copper sulfonated bathocuproine complex in solution. On a second sweep (B), there is essentially no current, consistent with the dissociation of the surface immobilized complex after oxidation to Cu(I1). (Recall that there is no copper in solution.) The background current is also quite small and there is no interfering redox process in this potential region so that quantification of the signal is easier. The electrode was reimmersed in the aqueous Cu(J.) solution and the voltammetric response was examined. As shown in Figure 3C the response of the electrode is very close to the one previously obtained, consistent with the fact that the ligand has remained on the surface of the electrode. A third determination (Figure 3D) yields essentially the same results. This type of electrode has been used for as many as nine consecutive determinations with a standard deviation of f l l %which is an improvement over the results for the ferrocene based copolymer. We are currently exploring the use of this approach to the determination of other metal ions. For example, we are investigating the use of catechol-based ligands since these exhibit an extremely high affinity for Fe(II1) but dissociate when the iron is reduced to Fe(I1) (5). We are also investigating the use of oxalate for the determination of cobalt.
An additional advantage of this approach is that since the ligand chosen has a very high affinity for a metal ion in a particular oxidation state, it allows for the use of these materials in speciation studies. We are currently pursuing this aspect and will report on it in a further publication. Registry No. (q-vp/v-Fc) copolymer, 55279-05-5; poly[RuCu, 7440-50-8;Pt, 7440-06-4;sulfonated ( ~ - b p y ) ~ 75931-32-7; ]~+, bathocuproine, 92957-36-3.
LITERATURE CITED (1) Guadalupe, A. R.; Abru‘ra, H. D. Anal. Chem. 1985, 57, 142 (2) Schilt, A. A.: McBride, L. ”The Copper Reagents: Cuprolne, Neocuproine, Bathocuproine”; G. F. Smlth Co.: Columbus, OH, 1972. (3) Cotton, F. A.; Wllkinson, (3. “Advanced Inorganlc Chemistry”, 3rd ed.; Interscience: New York, 1972. (4) Abruk, H. D.; Denisevich, P. D.; Uma’ra, M.: Meyer, T. J.; Murray, R. W. J. Am. Chem. SOC.1981, 103, 1. (5) Raymond, K. N. I n ”Biolnorganic Chemistry 11”; Raymond, K. N., Ed.; Amerlcan Chemical Society: Washington, DC, 1977; Advances In ChemistryBerles No. 162, p 33.
‘Visiting Professor: Department of Chemistry, Hobart & William Smith Colleges, Geneva, NY 14456.
Larry M. Wier’ Ana R. Guadalupe Hector D. Abruiia* Department of Chemistry Cornel1 University Ithaca, New York 14853 RECEIVED for review March 25,1985. Accepted May 6,1985. This work was supported in part by the Research Corporation and by the National Science Foundation through the Presidential Young Investigator Award Program (H.D.A.).
AIDS FOR ANALYTICAL CHEMISTS Automated Cell: A New Approach to Polarographic Analyzers Chaim N. Yarnitzky
Department of Chemistry, Technion-Israel Institute of Technology, Haifa, Israel 32000 Electroanalytical methods such as differential pulse polarography (DPP) and square wave voltammetry (SWV) have been left behind in terms of sample handling. These methods offer several unique advantages over other techniques. They exhibit high sensitivity and accuracy at moderate to low basic and maintenance costs. “Industrial Research” reports reveal that, in spite of all their promising features, the application of polarographic analyzers in analytical laboratories has not increased significantly in the past 5 years. The introduction of a fast scan square wave voltammetry instrument (EG&G Princeton Applied Research Co. Model 384A) in the market encouraged some potential customers. Sample handling, however, is still poor and will probably be the main drawback feature of this kind of instrument. The procedure prior to the electroanalysis is quite problematic, i.e., complicated, cumbersome, time-consuming, and rather inconvenient. Recalling the procedure, the chemist rejects the old sample along with some mercury into a waste reservoir, cleans the beaker, introduces the new sample, puts the beaker in place, deaerates the solution, and analyzes it. With the fast scan SWV apparatus, fortunately, analysis time has been reduced to a few seconds; all other steps may take up to 5 min. Flow systems employing microcells frequently provide a 0003-2700/85/0357-2011$01.50/0
good solution to the sample handling problem. The thin-layer cell used by De Angelis and Heineman (I) and the microelectrolytic cell suggested by Freiha and Wang (2) are typical examples. Selectivity can be achieved by using scanning techniques (3). Since the concentration profile changes with time, the three-dimensional flow polarogram is recorded by means of a computerized square wave polarographic system (4).This device is extremely useful in HPLC. In routine work, however, one would like to see a system, operating in the batch mode, allowing the performance of several experiments on the same sample (e.g., polarographic cyclic, DPP, and SWV). Alternatively for signal averaging, the same experiment can be repeated several times. In these systems the deaeration step as well as the other steps must be changed. The timeconsuming step of deaeration can be eliminated by using a nebulizer (5)or solved partially by using several cells working in sequence and driven by a master analyzer (384-4 by PARC). Either way, sample handling remains the same. The object of this paper is to show a new approach to polarographic cell design which completely solves the sample handling problem. The new type of the polarographic analyzer with the automated cell ensures: (A) small sample volumes not exceeding 8-10 mL; (B)convenient sample introduction; (C) automatic 0 1985 American Chemical Society
