Electrochimica Acta, Vol. 43, No. 23, pp. 3467±3473, 1998                                                              # 1...
3468                                            M. Gerlache et al.able interest as electrode substrate for self-assem-    ...
Electrochemical detection of H2O2 on gold                                 3469regarded as a result of a competitive adsorp...
3470                                          M. Gerlache et al.dation (at +1000 mV) in order to avoid that thebaseline cu...
Electrochemical detection of H2O2 on gold                                  3471         Table 1.         Characteristics o...
3472                                            M. Gerlache et al.Fig. 7. Calibration curve for Triton X100 at the beginni...
Electrochemical detection of H2O2 on gold                                   347323. A. Hamelin and A. M. Martins, J. Elect...
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Pulsed electrochemical detection of h2 o2 on gold

  1. 1. Electrochimica Acta, Vol. 43, No. 23, pp. 3467±3473, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0013-4686(98)00093-0 0013±4686/98 $19.00 + 0.00Pulsed electrochemical detection of H2O2 on gold M. Gerlache, S. Girousi*, G. Quarin and J.-M. Kau€mann{  Institut de Pharmacie, Universite Libre de Bruxelles, Campus Plaine 205/06, Bvd du Triomphe, 1050 Brussels, Belgium (Received 22 September 1997; in revised form 6 November 1997)AbstractÐThe voltammetric oxidation of hydrogen peroxide at a polycrystalline gold electrode, in neutraland alkaline media, shows two oxidation waves. At pH 7.4, one wave occurs at +490 mV and the secondone at +870 mV vs Ag/AgCl. The latter appears at the expense of the ®rst one. Formation of the ®rstwave is favoured by the presence of an adsorbed layer of hydroxyl-groups on gold but its magnitude ismarkedly a€ected by adsorbed interfering species and gold oxide structures. The quantitative analysis ofhydrogen peroxide, using the ®rst wave, is possible by preferably applying a pulsed electrochemical cleaningwith a detection at +550 mV. In ¯ow injection analysis the response of hydrogen peroxide is linear over awide concentration range i.e., 7 mM±0.1 M with a detection limit of 3 mM. The inhibition of H2O2 oxi-dation by an interfering species is suitable for the indirect quantitative analysis of species which are readilyadsorbed on gold. This is shown in ¯ow injection analysis by recording a stable oxidation current due tohydrogen peroxide in the carrier stream with detection of the inhibition of the signal due to the adsorbedspecies. Application to a surfactant such as Triton X100 at concentrations as low as 0.3 mM is reported.# 1998 Elsevier Science Ltd. All rights reservedKey words: hydrogen peroxide, gold electrode, pulsed amperometry, Triton X100. INTRODUCTION gold oxidation domain (Ep= + 870 mV) in pH 7.4 phosphate bu€er depending on the electrode surfaceHydrogen peroxide is a molecule which is often stu- state [4]. The experimental conditions to observedied in electrochemistry and especially in the devel- the oxidation wave at +490 mV require a veryopment of electrochemical biosensors. Hydrogen drastic control of impurities in the analysed sol-peroxide can be electrochemically detected at elec- ution, electrode surface cleanliness and oxidationtrodes such as gold [1±4], platinum [5±10], metal state. Actually, the electrode surface must be free ofallows such as Pd/Au, Bi/Au [1, 11±15], and carbon any adsorbed species which may depreciate orelectrodes modi®ed by metal particles or with the totally inhibit the oxidation at +490 mV in favorenzyme horseradish peroxidase [16, 17]. Platinum is of the more positive one (+870 mV). Yet, it wascommonly used because of its high reactivity observed that the ®rst wave of H2O2 oxidation wastowards H2O2 oxidation at a relatively low applied favoured by the presence of an adsorbed layer ofpotential (+600 mV vs Ag/AgCl). The use of plati- hydroxyl ions on the gold surface [4]. The exactnum o€ers also a better reproducibility of the oxi- stoichiometry of the hydroxide is still not welldation of H2O2 compared to the gold electrode [4]. de®ned but it may correspond to an incipient oxi-Following a recent investigation, we have been able dation of the gold surface which has been exten-to show that the voltammetric oxidation of H2O2 sively described in the literature [18±29]. Theon gold may occur at two di€erent potentials i.e., possibility of using gold to detect the oxidation ofat a potential preceding the onset of gold oxide for- H2O2 is of interest because, at a potential of ap-mation (Ep= + 490 mV) and at a potential in the proximately +500 to +600 mV in pH 7.4 bu€er, the H2O2 wave and the metal oxidation wave are*On leave of absence from the Aristotle University of more clearly separated on gold compared to theThessaloniki, Greece. situation on platinum allowing greater sensitivity at{Author to whom correspondence should be addressed. the former. In addition, gold is attracting consider- 3467
  2. 2. 3468 M. Gerlache et al.able interest as electrode substrate for self-assem- electrode). Amperometric measurements at a con-bling of molecular assemblies and biosensor stant potential were realized with a CV 27 voltam-development [30±32]. mograph (BAS) using the same cell con®guration. Considering that the amperometric detection of Pulsed amperometric detection (PAD) was per-hydrogen peroxide on platinum and gold at a con- formed in a ¯ow system setup with an electrochem-stant potential is a€ected by accumulation of ical unit ED 40 from Dionex using the Dionexadsorbed species and by oxide formation [4, 33, 34], three electrode ¯ow cell con®guration. The carrierit was of interest to investigate pulsed electrochem- stream was assured by a LKB pump and injectionical sequences as a means to maintain a reproduci- of the samples was performed with a Gilson auto-ble surface state and detect hydrogen peroxide at a sampler. All the experiments were made at roompotential corresponding to the ®rst oxidation wave. temperature (222 28C) and in 0.1 M sodium sulfateThis work also reports the suitability of the indirect supporting electrolyte.quantitation of readily adsorbed species on gold by Smoothing of the disk working electrode wasinhibition of hydrogen peroxide oxidation in ¯ow made using ®rst 1 mm then 0.3 mm alumina powderinjection analysis (FIA). on a soft tissue for 2 min. Subsequently, the elec- trode was rinsed with milli-Q water but preferably without sonication [4]. EXPERIMENTAL All chemicals were of analytical grade: sodium RESULTS AND DISCUSSIONlaurylsulfate and cetylpyridinium chloride (Acros,Belgium), sodium hydroxide, sulfuric acid, hydro- As described previously [4], the voltammetric oxi-gen peroxide (30% m/m), boric acid, sodium dihy- dation of H2O2 on gold in neutral and alkalinedrogenophosphate, sodium sulfate, potassium media may give only one oxidation wave atthiocyanide and sodium chloride (Merck) and +490 mV (Fig. 1(a)) or at +870 (Fig. 1(b)) or bothcysteine, cystamine, cysteamine, Triton X100 and waves may be detected (Fig. 1(c)) depending on theTween 80 (Sigma). All bu€ers and solutions were gold surface state. The ®rst wave is very sensitive toprepared daily with milli-Q water. poisoning by readily adsorbed species such as chlor- Cyclic voltammetry was achieved with an electro- ide ions, surfactants and by oxide structures onchemical analyzer 100B from BAS using a three- gold. Actually, the second wave occurs at the ex-electrode cell (Ag/AgCl 3 M NaCl as reference, pense of the ®rst and it may eventually be the onlyplatinum as auxiliary and BAS polycrystalline plati- one observed because of complete surface ``foulingnum or gold disk, 1.8 mm diameter, as working or strong oxide coverage. Such a behaviour can beFig. 1. Cyclic voltammograms (CV) of H2O2 at a polycrystalline gold electrode. Phosphate bu€er pH 7.4; (a) goldpolished with alumina, rinsed with water without sonication, (b) polished with a diamond suspension, (c) gold polishedwith alumina (CV in phosphate bu€er pH 7.4 + NaCl 10 mM).
  3. 3. Electrochemical detection of H2O2 on gold 3469regarded as a result of a competitive adsorption try i.e., the intensity of the catalytic wave decreasedbetween hydroxyl groups and the interfering mol- and the position of the maximum of the oxidationecules on gold. It is worth mentioning that these peak shifted to more positive potentials e.g.observations are similar to inhibition phenomena Ep= + 600 mV in the presence of 0.01 M NaCl. Inwhich may occur in polarography in the presence of the presence of a sulfur derivate, the inhibition ofa surface active compound. For instance, the ad- the H2O2 was more important because of the strongdition of a surfactant to a cadmium solution chemical interaction of sul®de groups withshowed a depression of the cadmium reduction gold [33, 36]. This was also observed for potassiumwave with concomitant formation of a new re- thiocyanide, cysteamine and cystamine. In the pre-duction wave at more negative potentials [35]. The sence of 1  10À2 M cystamine, the linearity of theformation of the ®rst wave was favoured in alkaline response was only obtained above a concen-media i.e., by the presence of adsorbed hydroxyl tration of 5  10À3 M H2O2 (I(mA) = 0.29 +ions on gold, and this wave was only poorly (0.017 20.002)  CH2 O2 (mM) with r = 0.988).detected in acidic media. Regarding the second Surfactants such as cetylpyridinium chloride orwave, it has been shown that, at an applied poten- Triton X100 showed physical adsorption e€ects oftial of +1000 mV in phosphate bu€er pH 7.4, a the same magnitude as the chemical inhibitiondirect proportionality between the oxidation current e€ects observed for the thiol species.and H2O2 was established over a concentrationrange comprised between 2  10À5 and 6  10À3 M Pulsed amperometric detection (PAD) of H2O2with a detection limit of 4  10À6 M [4]. The experiments have been carried out in pH 7.4 The determination of H2O2 by amperometry, at phosphate bu€er, in the absence of interferents,an applied potential corresponding to the ®rst wave with a ¯ow through system ®tted with a 20 ml injec-(+600 mV), was investigated here (the electrode tion loop. The ¯ow rate was adjusted to 1 ml minÀ1.was smoothed between each measurement). A line- Taking into account the cyclic voltammetric curvesarity between H2O2 concentration and the magni- (Fig. 1) and the hydrodynamic voltammogramtude of the oxidation current was also observed (Fig. 2), the applied potential pulse waveform(I(mA) = 1.44 + (1.322 0.06)  CH2 O2 (mM) with selected were: a delay time and integration atr = 0.990) in the range 2  10À4 to 1.4  10À2 M. +550 mV, oxidative pulse at +1000 mV and reduc-Contrary to the amperometric data obtained for the tive pulse at +100 mV (potential corresponding tosecond wave [4], quanti®cation at the ®rst wave gold oxide reduction). After approximately 45 mino€ered less reproducible results (RSD of the of stabilisation, this potential sequence allows oneslope = 2.9% for N = 6) because of the necessity to observe quasi no baseline drift. The H2O2 re-to repolish manually the electrode surface between sponse intensity was found to increase with theeach experiment and risks of impurity adsorption. applied potential where the current integrationYet, in order to obtain more reproducible data, an occurred. However, at a potential higher thanelectrochemical multi-scan sequence between nega- +570 mV, the ratio signal/background intensitytive (i.e. À400 mV) and positive potentials (i.e. decreased quickly because of the gold surface oxi-+1000 mV) was investigated. This electrochemical dation. A measuring potential of +550 mV wascleaning procedure should allow one to control and selected as it appeared to be a good compromise. Itclean the surface state of the electrode in a reprodu- was found also that the duration for the reductioncible manner through surface oxidation and re- (at +100 mV) of the formed gold oxide must beduction of the formed oxide. Pulsed electrochemical longer compared to the time devoted to the oxi-cleaning [33] was exploited below as it should allowthe use of the ®rst oxidative wave for quantitativeanalysis. The potentials and the time window of thewaveform used have been selected so that, at eachcycle, a new and oxide free electrode surface wasmade. Interferences on the ®rst oxidation wave of H2O2 As mentioned earlier, the presence of surfaceactive compounds can interfere with the ®rst oxi-dation wave of H2O2. Many compounds can beadsorbed or/and oxidized at a gold electrodesurface [33]. Cyclic voltammetry in phosphate bu€erpH 7.4 suggested that sodium sul®te and sodiumthiosulfate interfere since their oxidation (peak po-tential) occurred at +520 and +220 mV, respect-ively. The presence of sodium chloride in the Fig. 2. Hydrodynamic voltamogramme of H2O2 at a poly-solution had a double e€ect on the cyclic voltamme- crystalline gold electrode in phosphate bu€er pH 7.4.
  4. 4. 3470 M. Gerlache et al.dation (at +1000 mV) in order to avoid that thebaseline current increased regularly with time. Inorder to increase the gold oxide reduction eciency,the value of this potential could also be decreasedbut this was not investigated here. If the delay timebefore integration of the signal was lowered to50 ms, the baseline increased very quickly. The opti-mized waveform sequence selected is displayed inFig. 3, i.e., a delay time of 100 ms and a currentsampling duration of 50 ms at +550 mV. Usingthis PAD sequence, successive injections of a 10 mMH2O2 solution showed only a slight decrease of thesignal with time (0.3% minÀ1) (Fig. 4). A cali-bration curve established in such conditions gave alinear response from 7  10À6 till 0.1 M (Fig. 5).Note that all the values recorded corresponded tothe integration of the area of the peak given by theacquisition system. The detection limit was23  10À6 M (S/N = 3). In our previous work, we showed that the ®rstH2O2 oxidation wave was more readily observedand less a€ected by interfering species in alkalinemedia. Subsequent quantitative attempts were car-ried out in pH 8.0 and 9.0 phosphate bu€ers. At Fig. 4. Hydrogen peroxide response in PAD-FIA inpH 8.0 and 9.0, the maximum of the ®rst wave of 20 mM phosphate bu€er. Flow rate = 1 ml minÀ1,H2O2 oxidation shifted by approximatively 40 mV Eapp= + 550 mV.and 100 mV, respectively, towards less positive po-tentials compared to the value at pH 7.4 [4]. At pH8.0, the signal/blank ratio was 1.03 2 0.01 for a the intensity of the signal decreased by raising the10 mM H2O2 solution and was potential indepen- pH of the phosphate bu€er yet the reproducibilitydent between 440 and 600 mV. At pH 9.0, the sig- of the injections was better. At pH 9.0, the intensitynal was potential dependent, the value of the signal/blank ratio decreased by raising the potential of the of the signal decreased slightly with time, probablymeasurement likely due to a higher extent of gold due to the instability of the hydrogen peroxide atoxide formation than at pH 8.0. However, between high pH values. This diminution of the response390 and 450 mV, this ratio was more stable with time also appeared in borate bu€er whatever(1.62 2 0.04). The same experiment has been carried the applied potential.out in borate bu€er pH 9.0 and, here, the ratio From these results, it appeared that the ®rst oxi-(1.03 2 0.01) showed no potential dependence dation wave of H2O2 on gold was suitable for ana-between 300 and 500 mV. lytical purposes and that the amperometric pulsed The characteristics of the FIA response are sum- technique should be preferred to amperometry atmarized in Table 1 for all selected working poten- constant potential. Whatever the three pH valuestials and di€erent bu€ers. As can be seen in Table 1, tested (7.4, 8.0, 9.0), the concentration range where the response was proportional to H2O2 was ap- proximately the same, i.e. from 7  10À6 to 0.1 M.Fig. 3. PAD waveform sequence for the analysis of H2O2 Fig. 5. Calibration curve of H2O2 in PAD-FIA at the goldon gold. FIA, phosphate bu€er pH 7.4. electrode (same conditions as in Fig. 3).
  5. 5. Electrochemical detection of H2O2 on gold 3471 Table 1. Characteristics of the hydrogen peroxide response in FIA Eapplied Phosphate bu€er Borate bu€er pH 7.4 pH 8.0 pH 9.0 pH 9.0 550 mV 500 mV 450 mV 450 mV 340 mV Relative signal 100 35 17 80 66 RSD (%) 6.4 3.1 1.2 3.5 4.7The response in pH 7.4 phosphate bu€er was higher 3  10À6 M (Fig. 6). From the peak shaped responsebut less reproducible, for successive injections of the it was inferred that the EC cleaning allowed a rapidsame sample, compared to the results at pH 9.0. desorption of the surfactant. Yet, further optimiz-The baseline and the H2O2 signal decreased by rais- ation would be required since some peak distortioning the pH, so that the signal to blank ratio was observed especially at high surfactant concen-increased with pH. However, if the same calibration trations (Fig. 6). Note that here all the measuredcurve was reprocessed twice a day (at the beginning values corresponded to the current height given byand at the end of the day), the correlation between the integrator. The trends of the response was notthe two sets of experiments was less good for high linear but actually characteristic of an electrode sur-pH values. This may be due to the instability of the face saturation behavior (Fig. 7). This saturationH2O2 when the pH is more alkaline or it might be pro®le appeared also for pH 8.0 or 9.0. From aattributed to an evolution of the surface state of the calibration made at the beginning and at the end ofelectrode (oxide buildup). the day, a shift of the calibration curve was detected but the sign and the magnitude of this Application of the PAD to the indirect detection of shift was random from one experiment to the other. surfactants Successive injections of TX100 (3  10À5 M), were As shown previously, the ®rst oxidation wave of quite reproducible with a RSD of 1.3% (pH 8.0H2O2 was highly sensitive to adsorption of interfer- phosphate bu€er and at a measuring potential ating species on gold. If a permanent ¯ow of H2O2 500 mV). A detection limit of 3  10À7 M TX100was passed through the amperometric pulsed detec- was obtained. The response was relatively fast (lesstor, a constant oxidation current related to H2O2concentration was observed at +550 mV. In thepresence of an interfering species, when injectedinto the carrier stream, the adsorbed hydroxylgroups on the gold surface may be substituted bythe injected compound and the oxidative current ofH2O2 diminished. In the following series of experiments, the injec-tion valve was ®tted with a 100 ml loop and the re-duction potential, in the electrochemical cleaningstep, was set at À500 mV in order to obtain a stablebaseline in the presence of a 1  10À2 M H2O2 inthe ¯ow carrier (phosphate bu€er pH 7.4). Usingthese parameters, the baseline was stable over aworking day. Sodium lauryl sulfate (SLS), an anionic surfac-tant, was investigated as possible interfering species.For the three pH tested, SLS induced a decrease ofthe H2O2 baseline only for concentrations higherthan 1  10À4 M. By injection of thiocyanate orchloride ions, which are known to be stronglyadsorbed at the gold surface [24], the perturbationof the base line appeared already at 1  10À6 M ofinterferent. The indirect detection technique wasalso tested on a polyethoxylated surfactant which ischaracterized by strong surface active propertiessuch as Triton X100 (TX100). At pH 7.4, a signal Fig. 6. Indirect detection of Triton X100 by H2O2 inhi-depression by addition of TX100 was detected at bition. PAD-FIA, 10 mM H2O2, phosphate bu€er pH 7.4.
  6. 6. 3472 M. Gerlache et al.Fig. 7. Calibration curve for Triton X100 at the beginning (series 1) and at the end (series 2) of the day. PAD-FIA,10 mM H2O2, phosphate bu€er pH 7.4, ¯ow rate 1 ml minÀ1, Eapp= + 550 mV.than 2 min for a 3  10À5 M TX100 injected sol- centre (Milmort, Belgium) and BAS (Lafayette,ution) so that 20 injections could be performed in U.S.A.) for instrumentation support.1 h. Some attempts were also carried out with thenon ionic surfactant Tween 80. This polysorbate is REFERENCESextensively used in pharmaceutical formulations asemulsi®er. The system responded also to this mol- 1. Y. Zhang and G. S. Wilson, J. Electroanal. Chem.ecule but, the adsorption was so strong that ap- 345, 253 (1993). 2. J. P. Hoare, J. Electrochem. Soc. 112, 608 (1965).proximately 40 min were necessary to recover the 3. A. M. O. Brett, F. M. Matysik and M. T. Vieira,base line value. Further studies, dedicated to the Electroanalysis 9, 209 (1997).optimisation of the pulse potential sequence, are 4. M. Gerlache, Z. Senturk, G. Quarin, and J. M.under investigation for the possible quanti®cation Kau€mann, Electroanalysis, 1997, in press.of this surfactant. 5. J. J. Lingane and P. J. Lingane, J. Electroanal. Chem. 5, 411 (1963). 6. V. G. Prabhu, L. R. Zarapkar and R. G. Dhaneshwar, Electrochim. Acta 26, 725 (1981). CONCLUSIONS 7. M. Fujihira, H. Muraki and S. Aoyagui, Bull. Chem. Soc. Japan 59, 975 (1986). The pulsed amperometric detection of hydrogen 8. J. O. M. Bockris and L. F. Old®eld, Trans. Faradayperoxide at a polycrystalline gold electrode com- Soc. 51, 249 (1955). 9. M. Honda, T. Kodera and H. Kita, Electrochim. Actaprised in a ¯ow system e.g. in ¯ow injection analysis 8, 727 (1983).or in high performance liquid chromatography 10. D. Knittel, Q. Wei and E. Schollmeyer, Fresenius J.appeared to be a useful method for the quantitative Anal. Chem. 348, 820 (1994).determination of H2O2 down to the mM level and in 11. D. A. Johnston, M. F. Cardosi and D. H. Vaughan,a large linear domain range. The indirect detection Electroanalysis 7, 520 (1995). 12. L. Gorton, Anal. Chim. Acta 178, 247 (1985).of surfactants by inhibition of the H2O2 oxidation 13. Q. J. Chi and S. J. Dong, Anal. Chim. Acta 278,wave on the gold electrode was shown to be suit- 17 (1993).able for quantitative purposes. Applications to elec- 14. S. Dong, Q. Deng and G. Cheng, Anal. Chim. Actatroinactive compounds but readily adsorbed on 279, 235 (1993). 15. J. Wang, N. Naser, L. Agnes, H. Wu and L. Chen,gold may be foreseen, but additional experiments Anal. Chem. 64, 1285 (1992).consisting of optimization of the pulsed electro- 16. M. Shan Lin and B. Iuan Jan, Electroanalysis 9,chemical sequence are required for improvement of 340 (1997).the analytical procedure. 17. E. Csoregi, L. Gorton and G. Marko-Varga, È Electroanalysis 6, 925 (1994). 18. S. Strbac and R. R. Adzic, J. Electroanal. Chem. 403, 169 (1996). ACKNOWLEDGEMENTS 19. W. R. Fawcett, M. Fedurco, Z. Kovacova and Z. Borkowska, J. Electroanal. Chem. 368, 265 (1994).Thanks are expressed to the Ministry of Walloon 20. M. W. Hsiao, R. R. Adzic and E. B. Yeager, J.District in application of the FIRST programme Electrochem. Soc. 143, 759 (1996).(M. G.) and to the European Science Foundation 21. J. R. Gardner and R. Woods, J. Electroanal. Chem. 81, 285 (1977).for an ESF-ABI fellowship (S. G.). 22. A. Hamelin, M. J. Sottomayor, F. Silva, S. C. ChangAcknowledgments are also expressed to the analyti- and M. J. Weaver, J. Electroanal. Chem. 295,cal department of the Colgate-Palmolive R and D 291 (1990).
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