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Stripping Voltammetry of Reverslble Redox Species by Self-Induced Redox Cycling Tsutomu Horiuchi,' Osamu Niwa, Masao Morita, and Hisao Tabei NTT Basic Research Laboratories, Nippon Telegraph and Telephone Corporation, Tokai, Ibaraki 319-11, Japan A. Self Induced Redox Cycling
INTRODUCTION Voltammetryhasbeenwidelyusedtoanal~electroactive species in solution because of ita simplicity. However, the typicaldetectionlimitaofcyclicandlinearsweepvoltammetry are in the micromolar range. At these concentrations it is difficult to detect the amounts of trace materials. Various pulse methods have been proposed to improve the sensitivity of voltammetry. Their use has a detection limit 1or 2 orders of magnitude lower than that of cyclic or linear sweep voltammetry.1.2 Electrode miniaturization also improves the sensitivity. This is because a microelectrode has high mam transport density, smalldouble layer capacitance, and low ohmic losses which result in a higher S/N ratio than that of a conventional size electrode? We have reported that redox cycling on two closely spaced potentiostated microelectrodes increases the sensitivity of the reversible redox species and has been used to obtain a detection limit of 10-8 mol/dm3.' However, the sensitivityofthesemethodsisnotas highaswiththestripping analysis. Stripping voltammetry has the lowest detection limit of the commonly used electroanalytical techniques.l.2 It has been applied for the detection of metal ions and halides in water, foods, biological fluid, and environmental ~amples.6~3 The detectionlimit of stripping voltammetry for heavy metal ionsisfrom 10-1"to10-1'moVdrn3. Despite ita highsensitivity, stripping voltammetry can only be used to detect a limited range of species, which includes some metal ions, halides, or adsorptiveorganic compounds, unless special techniques are employed such aa electrode modification or the addition of ligands to form adsorptive complexes."-'S However, the experimentalconditions for electrode modification or ligand formation must he optimized for individual samples in order to deposit them efficiently on the electrode surface. Recently, we enhanced the limiting current hy self-induced redox cyclingat a closely spaced micro-macro twin electrode. This waa achieved by applying potential only to the microelectrode.1a Figure 1A shows the mechanism of self-induced redox cycling. When sufficient potential to electrolyze the (1) Bard,A. J.; FaulLnsr. L. R.Electmehemica1 Methoda. John Wilev
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121 Klwinper. P. T.; Heineman. W. R. Laborofory Techniques in Ekctroam1,ticol Chemistry; Marcel Lkkker, he.: New York, 1984. (31 Flelahrnm. M.: Porn. S:R o l k n . D. R.: Schmidt. P. P. U h o -
micmelecfrodes; Datatech Syaterns, he.: Morganton, NC, 1987. (4) Niwa, 0.;Morita, M.; Tabei, H. Anal. Chem. 1990,62,447. (5) Florence. T. M. J. Electroanal. Chem. Interfaeiol Electrochem. I.*,
I C 01,
L..I,Y,*YI.
(6) Florence, T. M.Anal. Chim. Acto 1982,141,73. ('7) Anderaon, L.;Jsgner, D.; Jasefson, M. Anal. Chem. 1982,54,1371. (81 Sadana. R. S. Anal. Chem. 19R3.55. 304.
(IO)Hu, A.; Dssay, R. E.; Graneil. A. Anal. Chom. 1983.55.320. G.W.; Blaedel. W. J. Anal. Chem. 1978,50, 99. (12) Fipurs, P.; McDuffie, B. Anal. Chem. 1979,51, 120. I131 van den Bera C. M. G . Anal. Lett. 1984. 17. 2141.
(11) Schieffer.
(14) vandenBe~CC.M.G.;Jacinto,G.S.Anal.Chim.Aeto1988,211,
129. (15) Wang, J.; Varughene, K. Anal. Chim. Acta 1987,199, 185. (16) Wang, J.; Farim, P. A. M.; Mahmoud, J. S. Anal. Chim. Acta 1985,171, 215. (17) van den b r g , C. M. G.Analyst 1989,114, 1527. (18) Moreira. J. C.; Zhao, R.; F a , A. G.Analyst 1990,115,1561.
(19) Horiuchi. T.;Niwa. 0.;Morita, M.; Tabei, H. J. Electmehem. Soc. 1991.138.3549. 00062700/92/036u206$03.0010
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Flpm 1. Schanatic dingam of self-induced redox cycling and new smpplng voltammetry. (A) Mechanism of seifinducsd redox cycling. Micro- and macroBlectrodes were placed in the same cell. (B) Selfinduced redox cycling In a twocell system. A twin microelectrodewas placed in one cell and a maaoelectrode In the other cell. The two c~llswere connected by a sah b w (hatched area). One electrode of the twin microelectrode was electrically connected to the macroelectrode. The preelectrolysls mechanism of the new stripping voltammetry is In parentheses. preelectroiys!softhereverslbbspecles al me twin electrode induces the preconcentralbn of metal. (C) Metal stripping stage on the macroelectrode.
redox species is applied to a microelectrode placed very close to a macroelectrode, the electroactive species pmduced on the former diffuses to the neighborhood of the latter. The oxidative reaction R 0 is induced on the macroelectrode edge nearest to the microelectrode,and the reductive reaction 0 R is induced on the far edge. The oxidized species produced at the macroelectrode edge nearest to the microelectrode diffuse hack to the microelectrode and are reduced again;then self-induced redox cycling is established between the micro- and macroelectrodes. Since electrons produced in the electrochemical reaction on the nearest edge of the macroelectrode are consumed in the reverse electrochemical reaction on the far edge of the macroelectrode, the current flows from the far edge to the nearest edge. The self-induced redox cycling effect also occurred in a two cell-system as shown in Figure 1B. In order to construct a closely spaced microelectrode/macroelectrcde pair, twin microelectrodes and a macroelectrode were placed in respective cells. One of the twin microelectrodes was connected to themacroelectrodeby alead. With thiscellsystem, thecharge produced by the self-induced redox cycling in the left cell is
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ANALYTICAL CMMISTRY. VOL. 64, NO. 24, DECEMBER 15. 1882 5207
transmitted to the electrochemical reaction of a different speeiea in the right cell. We developed a new stripping method using this selfi n d u d redox cycling effed in two cells which contain different eledmctive species. The Lett cell is fded with a sample solution containing the redox species and the right cell is filledwith an electrolyte containing metal ions. A twin microelectrode at which the redox cycling o c m and an electrode on which the metal ions can be deposited are placed in the left and right cells, respectively. At the preconcentration stage, the oxidative current of the redox species at the twin microelectrode in the left cell is transmitted to deposit metal ions on the electrode in the right cell (in parentheses of Figure 1B). After a fixed period of preelectrolysis, the quantity of metal ions deposited on the electrode in the right cell can be evaluated by conventional stripping voltammetry, which is measured by sweeping the potential of the macrcelectrode (Figure 1C). Since the amount of metal deposited ontheelectrodeintherightcellisrelatedtotheconcentration of the redox species in the left cell, we can estimate the concentration from the large stripping peak of the metal. Highlysensitivedetectionoftheredoxspeciescanbe achieved by this combination of electrodes without directly preconcentrating the electroactive species on the electrode surface. In this paper, wepresentthefmt exampleofanew stripping analysisof reversible redox species based on self-induced redox cyclingwithaninterdigitatedarray(1DA)andaglassycarbon combination electrode. EXPERIMENTAL SECTION E l e c t d m In this experiment, the combination electrode at which the self-induced redox cycling occurred, consisted of an interdigitatedarray electrode (IDA)andaglaeay-carbonelectrode (BAS, West Lafayette, IN). The IDA was used for the redox cycling, and the glassy-carbon electrode was used for metal ion deposition. The IDA electrode consisted of two series of finger electrodes. The finger widths and gaps were all 2 pm, and each finger was 2 mm long. The IDA consisted 750 pairs and was fabricated on a thermally oxidized silicon wafer by photolithographic, lift off, and dry etching techniques, as described elsewhere.*'l The diameter of the glassy-carbon electrode was 3 mm. The reference and auxiliary electrodes were a Ag/AgCl electrode (BAS, West Lafayette, IN) and a platinum wire, respectively. Chemical8 and Apparatus. Potassium nitrate (WakoChemicals,Osaka,Japan),Ruthenium(III)hexaamminechloride ([Ru(NH&JCb, ruthenium hexaammine) (Johnson MattheyIAlfa Products, Ward Hill, MA), pH 4.0 standard buffer solution (0.05 mol/dm3 potassium hydrogen phthalate) (Nakaraj Chemicals LTD, Kyoto, Japan),andsilver nitrate (Kanto Chemicals,Kyoto, Japan) were used as purchased. Electrochemical measurements were performed wing a twin potentiostate HECS 990 (Huso, Kanagawa. Japan), a potential sweep unit 175 (Princeton Applied Research, Princeton, NJ), and an X-Y recorder 3025 (Yokogawa Denki, Tokyo, Japan). A magnetic stirrer PC-351 (Iwaki Glass, Tokyo, Japan) was uaed to stir the electrolyte. Cells. Figure2showsasebematicdisgramofthemeanurement system. Two small glaas cells were connected by a salt bridge. A 0.05 molldm3standard buffer solution containing 1 pmoVdmJ ruthenium hexaammine was used in the left cell. A 0.1 mol/dma potassium nitrate solution containing 1pmoUdm3 silver nitrate was used in the right cell. The salt bridge was made of a glass tube, whose interior diameter was 5 mm. A Vycor disk, GOO70 (Princeton Applied Research, Princeton, NJ), was connected to each end of the glass tube with Teflon heat-shrink tubing. The electrolyte in the salt bridge was saturated potassium nitrate solution. (20) Sandemon. D. C.; Anderson, L. B. A w l . Chem. 1986.67.2388. (211 Aoki. K.; M o n k M.; Nicus, 0.; Tsbei, H. J . Eleetrwml. Chem. Interfociol Electrochem. 1¶88. 216.269. (22) Niwa,O.; Morita,M.; T&i,H. Tabei,H. J.ElectmMI. Chem.Interfacio1 Electrochem. 1989,267, 291.
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A reference electrode, auxiliary electrode, and IDA working electrode were placed in the left cell, and the glassy-carbon electrodewasplacedin theright cell. Therefereneeandauxiliary electrodes were directly connected to the potentiostat. The workingelectrodeterminalofthe potentiostate,the twoterminals of the IDA working electrode, and the glassy-carbon electrode were connected to a switch box. When the switch was in the A position, one working electrode of IDA was connected to the potentiostat and the other working electrode was connected to the glassy-carbon electrode. In position B, the glassy-carbon electrode was directly connected to the potentiostat. Procedures. Aa with theconventional stripping method,our new method requires preelectrolysisand then stripping. During preelectrolysis,the switch in Figure 2 was in position A and one working electrodeof the IDA was potentiostated at -0.4 V. The electrolyte solution was stirred with the magnetic stirrer during preelectrolysisin the right cell. After 10 min of preelectrolysis, stirring was stopped and the solution was left for 10 s. During stripping, the switch was moved to position B and immediately the potential of the glassy-carbon electrode began to be swept from 4 . 4 V to +0.5 V at a scan rate of 20 mV1s. In addition, cyclic voltammetry was performed on the IDA electrode by potentiostating both finger seta of the IDA without connecting the glassy-carbon electrode. One finger set of the IDA electrode was fixed at 0 V, and the other was swept at the same scan rate as that of the stripping voltammetry in order to compare this current with the peak current obtained by the new stripping method.
RESULTS AND DISCUSSION The redox potential of the ruthenium hexannunine was -0.2 V in a pH 4.0 standard buffer solution. The stripping
peak of silver on the glassy-carbon electrode in potassium nitrate solution was 0.35 V, which wan measured with the conventional stripping method. When one fmger set of the IDA electrode in the standard buffer containing oxidized ruthenium hexannunine wan connected to the glassy-carbon electrode in the silver nitrate solutionoftheothercell,norutheniumhexaammineoxidation or silver ion depmition took place in spite of the redox potentialdifferencebetween ruthenium hexaammineand the silver ions. This is because there waa no reduced species in either cell. When the electrode of the IDA which was not connected to thecarbonelectrodewas potentiostatedat below the redox potential of the ruthenium hexaammine, we observedasteady-statecurrentflowingfromtheglaeay-carbon electrode to the IDA. This current flow indicates that the reduced ruthenium hexaammine is oxidized at the IDA and the electrons generated in this reaction are consumed by the reductive reaction on the glassy-carbon electrode. It also
3208
ANALYTICAL CHEMISTRY, VOL. 64, NO. 24, DECEMBER 15, 1992 scan. The observedlimitingcurrent was 0.09 PA, which agreed tiFA
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Flguro 9. New stripping voltammogram of the 1 pmoi/dm3ruthenium hexeammlne at a sweep rate of 20 mV/s compared to the cycilc voltammogramof the same specles at an interdigitated array electrode (IDA): (a)resultof the new strlppingmethod. (b) IDA working electrode potentlostated at 0 V during preelectroiysls (other experimental condttlons were the same as those in (a));(c)no ruthenium hexaammlne in the ekctrolyte (other experimental conditions were the same as those In (a));(d) cycilc voltammograms In twin potentlostated mode.
suggests that self-induced redox cycling occurs on the IDA. Since the electroactive species in the other cell consists only of silver ions which can easily be reduced by the redox potential of ruthenium hexaammine, silver ions should be deposited accompanied by ruthenium hennnmmine oxidation. In Figure 3, (a) showsthe result of the new stripping method applied to 1pmol/dm3 ruthenium hexaammine by sweeping the glassy-carbon electrode potential from -0.3 to +0.5 V after 10 min of preelectrolysis. The potential of one of the IDA electrodes was fixed at -0.4 V during preelectrolysis. A large silver stripping peak with a magnitude of 7.1 pA was obtained at 0.35 V, indicating that silver ion deposition actually took place accompanied by the electrolysis of ruthenium hexaammine. (b) in Figure 3 is also the result of the new stripping volammetry of ruthenium hexaammine while one electrode of the IDA was potentiostated at 0 V during the 10-minpreelectrolysisperiod. Other experimental conditions were the same as those for (a). No stripping peak was observed in this experiment. Since the preelectrolysis electrode of the IDA was potentiostated above the redox potential of ruthenium hexaammine, no reduced ruthenium hexaammine was produced in the left cell. As a result, no silver ions were deposited on the glassy-carbon electrode because there was no electron supply from the f i i e r set electrode of the IDA connected to the glassy-carbonelectrode. The voltammogram shown by (c) in Figure 3 was obtained by the same experiment as that shown by (a) without ruthenium hexaammine. There was also no stripping peak observed in this experiment, because no redox reaction took place on the IDA electrode due to the absence of ruthenium hexaammine. These results indicate that the deposition of silver on the glassy-carbon electrode occurred only as a result of the charge arising from the redox reaction of ruthenium hexaammine. (d) in Figure 3 shows the cyclic voltammogram of 1 pmol/ dm3ruthenium hexaammine in the twin potentiostated mode in the left cell when the glassy-carbon electrode was not connected. The IDA cathode was swept from -0.4 to +0.5 V at a sweep rate of 20 mV/s. The anode was fiied at 0 V. The oxidative limiting current of ruthenium hexaammine is much clearer than the reductive limiting current because of the absence of charging current and residual current from the dissolved oxygen or hydrogen ions caused by the potential
well with the calculated value.21 As with the oxidative current of the IDA in the twin mode ((d) in Figure 31, the oxidative current generated by selfinduced redox cycling on the combination electrode also contains much less interference or charging current than that observed at the potentiostated finger set of the IDA. As a result of this noise reduction effect at the IDA during preelectrolysis, this new stripping method has the potential to achieve high sensitivity with an excellent signal to noise ratio. It was also confirmed that this method can be used to determine reversible redox species which cannot be preconcentrated by conventional stripping analysis. The peak height obtained by this new stripping method was 7.1 PA, which is 79 times higher than that obtained from a cyclic voltammogram ((d) in Figure 3) at the IDA. Furthermore, the limiting current observed in the cyclic voltammogram at the IDA was already amplified by the redox cycling. The peak current of a cyclic voltammogram obtained at a single working electrode was reduced to about one-tenth that at a twin potentiostated IDA at a sweep rate of 20 mV/s. Therefore the detected signal with the new stripping method was about 790 times larger than with conventional electrochemical detection with a single electrode. The peak area of (a) in Figure 3 was calculated to be about 23.2 pC by converting the horizontal axis to the time dimension. On the other hand, the total charge in preelectrolysis was 54 pC, which is calculated by assuming that the steady-state current whose magnitude was 0.09 PA continued to flow for 10min. From these results, the Coulombefficiency for conversionfrom ruthenium hexaammineoxidationtosilver deposition was estimated to be more than 43 5%. The detection limit of this measurement system was 10 nm0Udm3. The linear relationshipbetween the stripping peak height and the sample concentration was maintained in the 10-100 nmol/dm3 range. A higher silver ion concentration than the redox species and a small glassy-carbon electrode will be effective for achieving higher sensitivity. The higher concentration will increase the Coulomb efficiency during the preelectrolysis stage and result in a large stripping peak, The small glassy-carbon electrode will reduce the charging current during the stripping stage and flatten the baseline of the voltammogram.
CONCLUSION A new stripping method for reversible specieswhich cannot be preconcentrated on the electrode as in the conventional stripping method was developed by combininga closely spaced twin microelectrode and an electrode capable of depositing metal ions. An interdigitated array (IDA) electrode in a solution of the reversible redox species was used as the twin microelectrode, one finger set of which was connected to a glassy-carbon electrode in another solution containing metal ions. The glassy-carbon electrode was used because metal ions can be deposited on it. The preelectrolysis of the reversible species at one fiiger set of the IDA induced a reversible reaction at the other fiiger set, and the charge produced by thiireverse reaction was transmitted to deposit metal ions on the glassy-carbon electrode. By measurement of the current from dissolution of the metal ions, a highly amplified signal was obtained which is related to the concentration of the reversible species. This method willbe very useful for analyzing trace amounts of reversible redox species.
RECEIVED for review May 27, 1992. Accepted August 24, 1992. Registry No. Carbon, 7440-44-0;ruthenium(II1) hexaamsilver, 7440-22-4. mine, 18943-33-4;
