Anal. Chem. 2003, 75, 2054-2059
Voltammetric Characterization of a N,N′-Diphenyl-p-phenylenediamine-Loaded Screen-Printed Electrode: A Disposable Sensor for Hydrogen Sulfide Nathan S. Lawrence,† Li Jiang,‡ Timothy G. J. Jones,‡ and Richard G. Compton*,†
Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, U.K., and Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, U.K.
The voltammetric response of a 10% (by weight) N,N′diphenyl-p-phenylenediamine (DPPD) and 90% (by weight) carbon and binder screen-printed electrode has been examined in aqueous media over a range of pH using cyclic voltammetry both in the presence and in the absence of sulfide. In the absence, the screen-printed electrode undergoes an initial oxidative process on the surface of the solid organic particles to form an insoluble layer of the corresponding cation radical salt, DPPD•+X-, where X- is an anion present in the solution. The charge transfer is thought to occur at the three-phase boundary between solid DPPD, carbon, and the aqueous solution. At higher potentials, a second oxidative wave is observed that is attributed to the oxidation of the bulk DPPD with intercalation of the anion species present to form a solid phase of DPPD•+X-. The two voltammetric processes were found to stabilize after repetitive scanning, after which time, sulfide was added to the solution. The voltammetric response was found to respond to sulfide by showing a decrease in both the oxidative and reductive waves, which can be attributed to the sulfide effectively blocking the three-phase boundary. The response was found to be independent of the electrode used and at pH 4 produced a linear range from 20 to 165 µM, and a limit of detection of 7.5 µM for sulfide detection was achieved. The determination of both gaseous hydrogen sulfide and dissolved sulfide anions is of growing importance to the analytical chemist.1,2 This interest is primarily due to the high toxicity of liberated hydrogen sulfide, as it poses a major problem to those who handle and remove sulfide-contaminated products. Although a wide range of protocols is available for the detection of sulfide that encompass techniques such as chromatography, spectrometry, and solid-state devices,2 electrochemical procedures provide some of the more viable options due to their inexpensiveness, design simplicity, and sensitivity. Several electrochemical * To whom correspondence should be addressed. Tel: 01865 275413. Fax: 01865 275410. E-mail:
[email protected]. † University of Oxford. ‡ Schlumberger Cambridge Research. (1) Patnaik, P. A Comprehensive Guide to Hazardous Properties of Chemical Substances, 2nd ed.; Wiley: New York, 1999. (2) Lawrence, N. S.; Davis, J.; Compton, R. G. Talanta 2000, 52, 771-784.
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procedures have been realized in potentiometric,3-5 galvanostatic,6-9 and amperometric10-14 methodologies. A recent amperometric approach has been developed based on the traditional methylene blue test devised by Emil Fischer.15 This utilizes the aqueous oxidation of dimethyl-p-phenylenediamine by a homogeneous oxidation agent (usually Fe(III)). In the presence of hydrogen sulfide, a characteristic color is observed and is attributed to the formation of methylene blue. The amperometric approach involves the replacement of the homogeneous oxidizing agent by a heterogeneous electrode substrate. It was found that the oxidative current due to the oxidation of dimethylp-phenylenediamine was usefully enhanced in the presence of sulfide.13 The corresponding electrochemically initiated reaction has been studied extensively with a wide range of solution-based phenylenediamines, and a mechanism has been proposed (Scheme 1).13,14,16 This reaction has been shown to be generic in nature and is now being utilized for the determination of biologically relevant thiols17-19 and environmentally important amines.20 However, the fact that the indicator species are soluble means that (3) Garcia-Calzalda, M.; Marban, G.; Fuertes, A. B. Anal. Chim. Acta 1999, 380, 39-45. (4) Orion 9614, A Sulfide Ion Selective Electrode; Orion Research Inc., Beverley, MA. (5) Stanic, V.; Etsell, T. H.; Pierre, A. C.; Mikulka, R. J. Electrochim. Acta. 1998, 43, 2639-2647. (6) Jeroschewski, P.; Haase, K.; Trommer, A.; Gru ¨ ndler, P. Fresenius J. Anal. Chem. 1993, 346, 930-933. (7) Jeroschewski, P.; Haase, K.; Trommer, A.; Gru ¨ndler, P. Electroanalysis 1994, 6, 769-772. (8) Jeroschewski, P.; Braun, S. Fresenius J. Anal. Chem. 1996, 354, 169-172. (9) Jeroschewski, P.; Steukhart, C.; Kuhl, M. Anal. Chem. 1995, 68, 43514357. (10) Kuhl, M.; Steuckart, C.; Eickert, G.; Jeroschewski, P. Aquat. Microb. Ecol. 1998, 15, 201-209. (11) Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Anal. Chem. 1995, 67, 318-323. (12) Batina, N.; Ciglenecki, I.; Cosovic, B. Anal. Chim. Acta 1992, 267, 157164. (13) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G. Electroanalysis 2000, 12, 1453-1460. (14) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G. Electroanalysis 2001, 13, 432-436. (15) Fischer, E. Chem. Ber. 1883, 16, 2234. (16) Lawrence, N. S.; Thompson, M.; Davis, J.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Mikrochim. Acta 2001, 137, 105-110. (17) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G. Analyst 2000, 125, 661-663. (18) White, P. C.; Lawrence, N. S.; Davis, J.; Compton, R. G. Anal. Chim. Acta 2001, 447, 1-10. 10.1021/ac020728t CCC: $25.00
© 2003 American Chemical Society Published on Web 03/28/2003
Scheme 1. Electrochemically Initiated Reaction of Dimethyl-p-phenylenediamine with Sulfide
any practical sensing device would require a “pool” of the phenylenediamine species in contact with the electrode surface. Therefore, there is a need to develop a sensor in which an insoluble indicator species is immobilized into the electrode surface.21,22 In the present study, we develop this type of sensor; specifically, insoluble N,N′-diphenyl-p-phenylenediamine (DPPD) is incorporated into the carbon-based working electrode of a screen-printed electrode. The uses of screen-printed electrodes as single-shot disposable sensors for the determination of gaseous thiol species have been documented.23-25 In the present report, the voltammetric response of the DPPD electrode in the absence of sulfide is first studied with the voltammetry consistent with intercalation of anions and concomitant oxidation of both surface and bulk DPPD (DPPD(surface), DPPD(bulk)) to form the corresponding insoluble radical cation salt either as a surface layer or as a bulk phase in two separate voltammetric waves. The voltammetric response was found to stabilize after which sulfide was introduced to the system. In the presence of sulfide, a decrease is observed in both the oxidative and reductive waves. This reaction is analyzed and is believed to occur at the three-phase (solid DPPD-water-carbon) boundary.26-29 The modified carbon-epoxy electrode responds to sulfide in a linear manner from 20 to 165 µM and is independent of the electrode used. EXPERIMENTAL SECTION Reagents and Equipment. All reagents were obtained from Aldrich, were of the highest grade available, and were used (19) Nekrassova, O.; White, P. C.; Threlfell, S.; Hignett, G.; Wain, A. J.; Lawrence, N. S.; Davis, J.; Compton, R. G. Analyst 2002, 12, 797-802. (20) Seymour, E. H.; Lawrence, N. S.; Beckett, E. L.; Davis, J.; Compton, R. G. Talanta 2002, 37, 233-242. (21) Lawrence, N. S.; Thompson, M.; Davis, J.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Chem. Commun. 2002, 1028-1029. (22) Thompson, M.; Lawrence, N. S.; Davis J.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Sens. Actuators, B 2002, 87, 33-40. (23) Zen, J. M.; Chen, P. Y.; Kumar, A. S. Electroanalysis 2002, 14, 513-518. (24) Hart, J. P.; Abass, J. K. Anal. Chim. Acta 1997, 342, 199-206. (25) Prodromidis, M. I.; Veltsistas, P. G.; Karyannis, M. I. Anal. Chem. 2000, 72, 3995-4002. (26) Dostal, A.; Meyer, B.; Scholz, F.; Schroeder, U.; Bond, A. M.; Marken, F.; Shaw, S. J. J. Phys. Chem. 1995, 99, 2096-2103. (27) Schroder, U.; Scholz, F. J. Solid State Electrochem. 1997, 1, 62-67. (28) Schroder, U.; Scholz, F. Fresenius J. Anal. Chem. 1996, 356, 295-298. (29) Scholz, F.; Meyer, B. Voltammetry Microparticles 1998, 1 and references therein.
Figure 1. Thirty repetitive cyclic voltammograms (100 mV s-1) of the DPPD-doped screen-printed electrode in aqueous solution containing 0.1 M HCl.
without further purification. All solutions and subsequent dilutions were carried out using deionized water from an Elgastat UHQ grade water system with a resistivity of not less than 18 MΩ cm. Stock sulfide solutions (0.05 M) were made up by dissolving the sodium salt in previously degassed deionized water and used within 1 h of preparation to minimize losses due to aerial oxidation. All results were obtained at a temperature of 22 ( 2 °C. All ultrasonic experiments were carried out using an ultrasonic bath (Decon, Ultrasonics Ltd., Sussex, U.K.) at a frequency of 40 kHz. Electrochemical measurements were recorded using an Autolab PGSTAT 30 computer-controlled potentiostat (Eco-Chemie). All experiments were carried out in a cell of volume 20 cm3. Manufacture of the Screen-Printed Electrode. The ceramic screen-printed electrodes were manufactured by Gwent Electronic Materials Ltd. (Monmouth House, Pontypool, Gwent, Wales). These consisted of a carbon working electrode (0.04 cm2) impregnated with 10% (by weight) of the indicator species N,N′diphenyl-1,4-phenylenediamine. A carbon counter electrode and a Ag/AgCl reference electrode (0.01 cm2) were used to complete the screen-printed electrode assembly. It was observed that the Ag/AgCl electrode was found to be stable in both the presence and absence of sulfide. RESULTS AND DISCUSSION The following details the voltammetric response of the screenprinted electrodes in the presence of sulfide in aqueous media. First, however, their behavior in the absence of sulfide was analyzed. DPPD Electrodes in the Absence of Sulfide. Figure 1 details 30 cyclic voltammograms measured using a scan rate of 100 mV s-1 of the DPPD-loaded screen-printed electrode in a aqueous solution containing 0.1 M HCl. The first scan shows an oxidative wave at +0.52 V (vs Ag/AgCl). When the potential is reversed, a slight hysteresis effect is observed. The reductive scan reveals a small wave at +0.32 V (vs Ag/AgCl) along with a larger reduction wave at +0.19 V (vs Ag/AgCl). A second scan shows the oxidative wave at +0.47 V increased in magnitude and the hysteresis effect more prominent. The second reduction scan Analytical Chemistry, Vol. 75, No. 9, May 1, 2003
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Scheme 2. (a) Proposed Morphology of the Screen-Printed Electrodea
Figure 2. Effect of varying the scan rate from 10 to 600 mV s-1 on the DPPD-doped screen-printed electrode in aquous solution containing 0.1 M HCl.
shows that both of the reductive waves observed in the first cycle have grown in magnitude. Further scanning reveals that the first oxidative wave at +0.47 V continues to grow while a second oxidative emerges at +0.70 V and the hysteresis effect diminishes. These two oxidative waves continue to grow and merge until a steady current is reached after 27 scans. Analysis of the reduction waves between scans 3-27 show that the first reduction process observed at +0.30 V has stabilized after the second scan and the second reduction wave at +0.15 V continues to grow until a steady peak current is observed after 27 scans. This “breaking in of the electrode” was observed for 30 different electrodes with consistent wave shapes observed irrespective of whether the bulk solution was agitated during scanning, suggesting the electrochemical process is confined to the screen-printed electrode. The effect of scan rate is detailed in Figure 2; as the scan rate is increased, the potential difference between the oxidative and reductive waves increase from 230 mV at 10 mV s-1 to 860 mV at 600 mV s-1. Plots of peak current against the square root of scan rate for the main oxidation and reduction waves were linear, consistent with diffusion and intercalation of the anion species (Cl-) to the surface or into the bulk of the DPPD solid (Scheme 2a). Analysis of the second minor reduction wave at +0.30 V via a plot of current against scan rate reveals a linear plot suggesting the species is confined to the electrode surface. The effect of replacing the solution after electrochemical pretreatment was next examined. Voltammetric scans recorded in fresh solution (0.1 M HCl) were similar to those observed for the last voltammetric scan taken in the solution in which it had undergone its pretreatment, showing that DPPD is not leached from the electrode surface. Finally, the extreme effect of sonication was examined. The stable voltammetric response of the screenprinted electrode was first recorded in 0.1 M HCl, after which it was sonicated in a sonic bath for 5 min, removed from the old buffer solution, and placed in a fresh 0.1 M HCl solution, and the voltammetric response was recorded. The corresponding voltammetric wave shapes obtained before and after sonication (not shown) were then compared and produced similar oxidative and reductive peak currents, suggesting that the DPPD is contained in the electrode surface and does not dissolve into the solution. 2056
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a(a) The electrode is made up of solid DPPD and carbon, which are either in contact with each other or suspended independently either at the electrode surface or in the bulk of the electrode. The schematic details the species present after some oxidation has occured. (b) Proposed setup the three-phase boundary: solid DPPD-water-carbon. (c) A schematic explanation of the general cyclic voltammetric response.
It should be first noted that the stable wave shape obtained at the screen-printed electrode is comparable with those observed first when solid DPPD is immobilized onto a bare basal plane pyrolytic graphite and second when a carbon-epoxy electrode consisting of solid DPPD | carbon | epoxy is studied.21,22 This again suggests “solid” DPPD present in the screen-printed electrode is responsible for the observed voltammetry. The effect of exchanging the 0.1 M HCl solution for 0.1 M HNO3 on the voltammetric response was next examined. The voltammetric response was found to be analogous to that shown in Figure 1 when the electrode was placed in 0.1 M HCl, where the electrode has to undergo a “breaking stage” before a steady response is observed. A comparison of the magnitudes of the peak currents obtained after a steady response is observed shows they are similar with current of 0.49 and 0.47 mA obtained for the 0.1 M HCl and 0.1 M HNO3 solutions, respectively. However, a comparison of the oxidation potentials for the electrochemical processes reveals that the oxidation process occurs at higher potentials in the 0.1 M HCl solution compared to the 0.1 M HNO3 solution, suggesting the electrochemical process is anion sensitive
and that the NO3- ions insert (see below) more readily than the chloride ions. A schematic diagram of the makeup of the screen-printed electrode is detailed in Scheme 2a. This shows a matrix in which there is solid carbon with a binder species, along with solid DPPD particles at the surface of the electrode that are either in contact with a carbon particle or isolated and held within the binder matrix. It should be noted that, in the following discussion, DPPD(surface) refers to the surface of the DPPD solid that is in direct contact with the solution phase and DPPD(bulk) refers to the rest of the DPPD solid as depicted in Scheme 2a. It is envisaged that a there is a three-phase boundary, solid DPPD/carbon/electrolyte, as detailed in Scheme 2b. We consider the voltammetric response detailed in Figure 1 for a solution containing 0.1 M HCl. When an oxidizing potential is applied to the screen-printed electrode, the DPPD species close to the carbon particles are likely to be oxidized first at the threephase boundary. The initial voltammetric scan shows a single oxidative wave at +0.52 V, which can then be attributed to the concomitant oxidation of the DPPD(surface) to its radical cation species and insertion of chloride ions across into the surface of the solid to keep charge neutrality such that a thin film of the DPPD•+Cl- is formed on the surface of the DPPD particle. At higher oxidation potentials, a hysteresis is observed where the current passed is greater on the reduction scan than on the oxidation scan. This can be attributed to the oxidation of the bulk of the solid DPPD species (DPPD(bulk), Scheme 2a) and the simultaneous intercalation of chloride ions to form bulk DPPD•+Cl-. Once the intercalation into the bulk has been initiated, it is enhanced and therefore the intercalation mechanism is still occurring when the scan is reversed and a hysteresis occurs. Upon reversal of the voltammetric scan, two reduction waves are observed. The first at +0.30 V can be attributed to the surface DPPD•+Cl- being reduced back to DPPD. This is qualitatively consistent with the analysis of the scan rate data detailed above, which revealed a linear plot with scan rate consistent with surface-confined voltammetry. The charge passing in this reduction process (∼0.13 mC) was also found to be consistent with monolayer coverage of the electrode surface thereby again suggesting that a surface film is responsible for the voltammetry. The second reduction wave can be attributed to the reduction of the newly formed DPPD•+Cl-(bulk) to the DPPD(bulk) species. Analogous behavior is observed on the subsequent cyclic voltammogam, which reveals more DPPD(bulk) undergoing intercalation by chloride ions due to the large hysteresis effect observed. Further voltammetric scanning shows that a new oxidation peak emerges at +0.70 V; this can be attributed to the oxidation of the DPPD(bulk) species to the corresponding radical cation DPPD•+Cl-(bulk) species as outlined in Scheme 2c. The fact that both the oxidative wave at +0.47 V and the reductive wave at +0.15 V continue grow can be attributed to the stabilization of the electrochemical process occurring. A schematic explanation for a general voltammetric response is detailed in Scheme 2c. Finally, analysis of the stable voltammetric response as a function of scan rate detailed in Figure 2 showed a linear response with the square root of scan rate. This behavior can be explained by the fact that there is diffusion of the anion species (x-) inside the solid thereby producing the square root of scan rate dependence.
Figure 3. Initial 20 repetitive cyclic voltammogram (100 mV s-1) for the DPPD-doped screen-printed electrode when placed in (a) pH 4 phosphate and (b) pH 8 phosphate buffer solutions.
It should be noted that after 30 scans both the oxidative and the reductive waves begin to decrease slightly, which can be attributed to the mechanical stress asserted on the DPPD molecules as they undergo this reproducing intercalation mechanism. Next, an examination into the effects of pH on the voltammetric signal was undertaken. The initial 20 voltammetric scans obtained when the screen-printed electrodes were placed into pH 4 and pH 8 phosphate buffers are detailed in Figure 3a and b, respectively. The voltammetric responses can be seen to be similar for each pH with only a slight oxidative wave at +0.01 V for pH 4 and -0.17 V for pH 8 observed on the initial scan. Upon reversal of the first scan, a large reductive wave is observed at -0.06 V for pH 4 and -0.34 V for pH 8. Upon subsequent voltammetric scans, a new oxidative wave occurs both at +0.08 V for pH 4 and at +0.02 V for pH 8; however, only a single reduction wave is observed at each pH. These peaks continue to grow in magnitude until a single steady peak current is ascertained after seven scans, after which a slight decrease is observed in the reduction peak current. This type of voltammetric behavior is similar to that observed for the pH 1 solution, whereby the electrode has to undergo an initial voltammetric scan such that the anion present in the solution can under go intercalation across the three-phase boundary. Analytical Chemistry, Vol. 75, No. 9, May 1, 2003
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Figure 4. (a) Voltammetric response of a pretreated screen-printed electrode placed in a solution of 0.1 M HCl to additions of sulfide from 0.1 to 1 mM. (b) Corresponding standard addition plot of reductive peak current against concentration of sulfide.
In order examine the reproducibility of the screen-printed electrodes, the peak current data obtained at a scan rate of 100 mV s-1 in 0.1 M HCl from three different electrodes after each electrode had undergone its initiation process were compared. These produced values of 0.48 ( 0.07 mA, giving a 14% difference, thereby serving to show that the electrodes could be used for one-shot disposable sensors. The results and analysis detailed in the above section have provided a valuable insight into the chemistry of the DPPD-loaded screen-printed electrode and therefore provide a sound basis for the evaluation of the electrode as a sensor. It is to this that we next turn. Voltammetric Response of the DPPD Screen-Printed Electrode in the Presence of Sulfide. The corresponding voltammetric responses for a solution containing 0.1 M HCl to additions of 50 µM sulfide are detailed in Figure 4a. It should be noted that for all sulfide studies, before sulfide was added to the solution, the electrode underwent 10 repetitive cycles to ensure a steady current. A look at the voltammetric response upon the addition of sulfide to the solution shows that there is initially little change in the voltammetric response. However, upon subsequent additions, both the oxidative and reductive peak currents begin to decrease in magnitude analogous to that seen previously with the carbon-epoxy electrode.22 This decrease is shown in Figure 4b as a function of concentration of sulfide. As can be seen from the figure, the peak current was shown to decrease from 100 µM to 1 mM and was observed for 20 different electrodes used under these conditions. This type of behavior is consistent with the sulfide “blocking” the three-phase boundary thereby inhibiting the oxidation and reduction process and therefore causing a decrease in the magnitude of the oxidation and reduction peak currents. To provide evidence that disposable electrodes are required for the determination of sulfide, a “memory” experiment was carried out, whereby the electrode was placed in 0.1 M HCl, and a standard addition experiment was carried out analogous to the one detailed above. After which, the electrode was taken from the solution and placed into a fresh solution of 0.1 M HCl with no sulfide present. A cyclic voltammogram was then recorded and compared with those obtained for a freshly prepared electrode and an electrode that had undergone a standard addition experi2058 Analytical Chemistry, Vol. 75, No. 9, May 1, 2003
Figure 5. Initial 13 repetitive cyclic voltammograms of a DPPDdoped screen-printed electrode (after having undergone an initial 7 repetitive cyclic voltammgrams) after being introduced to a sulfide (122 µM) when placed in a pH 4 (0.1 M phosphate buffer) solution.
ment (not shown). Analysis of the voltammetric waves shows that there is indeed a memory effect occurring as the voltammetric response recorded is analogous to that observed for an electrode in a solution of 0.1 M HCl, 1 mM sulfide. This shows that there is a need for disposable electrodes to be used. To examine the nature of the detection procedure further, the voltammetric response of the screen-printed electrode was examined at pH 4. The corresponding 13 cyclic voltammograms (100 mV s-1) after the introduction of 122 µM sulfide are detailed in Figure 5. It should be noted that prior to the addition of sulfide the electrode had undergone seven voltammetric responses in the solution and the sulfide was introduced after that scan. As can be seen from the voltammetric responses shown, upon the addition of sulfide to the solution, a decrease in the reductive peak current is observed analogous to that observed in 0.1 M HCl. It can be seen by repetitive cycling of the electrode this decrease continues and as such can provide a sensitive means of determining low concentrations of sulfide. At first sight, it may appear that this decrease in reductive peak current occurs in the absence of sulfide, which is shown in Figure 3 but it is enhanced when sulfide is added to the solution. This enhancement in the decrease in peak current is confirmed with the results detailed in Figure 6, which depicts a plot of the decrease in the reductive peak current with respect to the starting current against number of scans as a function of increasing sulfide in which a new electrode was used for each sulfide concentration. This plot shows that as the sulfide concentration is increased the decrease in reduction peak current is enhanced. This enhancement in the decrease in the reductive peak current can be attributed to the sulfide inhibiting the electrochemical oxidation and reduction of the DPPD species. These results reveal that the electrodes respond to sulfide both in 0.1 M HCl and in pH 4 phosphate buffer solution in an analogous manner and thereby demonstrates the possible utility of the device in an authentic matrix. As with many test strips, the response needs to be examined in the presence of other anions. However, due to the specific nature of the reaction between sulfide and DPPD, it can be envisaged that only sulfide would produce this decrease in the analytical signal.
Figure 6. Electrode response for the reductive peak current (measured at -0.02 V) of the screen-printed electrode (pH 4 buffer) to varying sulfide concentrations (0-165 µM) as a function of the number of scans.
The analytical viability of using the DPPD-loaded screen-printed electrodes for the determination of sulfide was assessed by examination of Figure 6. A plot of the decrease in reductive peak current against concentration of sulfide after 11 scans had been recorded revealed a linear range from 20 to 165 µM and a limit of detection of 7.5 µM (based on 3sb). These results were obtained by using a different screen-printed electrode for each concentration and as such prove the efficacy of using these devices especially in dirty media where fouling of the electrode is a problem. CONCLUSION The voltammetric response of a 10% (by weight) loaded DPPD screen-printed electrode has been examined both in the presence
and in the absence of aqueous sulfide. In the absence of sulfide, the screen-printed electrode has been shown to undergo an initiation process. The corresponding voltammetric responses have been examined and a rationale has been formulated, whereby the initial oxidative process occurs on the surface of the solid organic particles to form an insoluble layer of the corresponding cation radical salt, DPPD•+X-. The second voltammetric wave observed at higher potentials is attributed to the oxidation of the bulk DPPD with intercalation of the anion species present to form a solid phase of DPPD•+X- with the charge transfer occurring at the three-phase boundary found between solid DPPD, the carbon, and the aqueous solution. The voltammetry of the DPPD electrode was found to respond to the presence of sulfide: a decrease in the oxidation and reduction peak currents was observed in both 0.1 M HCl and pH 4 phosphate buffer solutions. This decrease was found to increase with repetitive scans in the pH 4 solution, and as such, a sensitive protocol for the determination of sulfide has been established. The response was found to provide a linear range from 20 to 165 µM with a limit of detection of 7.5 µM obtainable. ACKNOWLEDGMENT N.S.L. thanks Schlumberger Cambridge Research for financial support of this project.
Received for review November 25, 2002. Accepted February 12, 2003. AC020728T
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