Electrochemical Detection of Chloride by Underpotentially Deposited

Polycrystalline Gold. Hyun-Goo Choi ... a primary stripping peak for the Ag UPD adlayer at 550 .... ment of the gold electrode surface before a measur...
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Anal. Chem. 2004, 76, 5911-5917

Electrochemical Detection of Chloride by Underpotentially Deposited Silver Films on Polycrystalline Gold Hyun-Goo Choi and Paul E. Laibinis*,†

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307

This paper describes an electrochemical method for measuring dilute levels of chloride using an underpotentially deposited (UPD) Ag adlayer on polycrystalline Au substrates as a sensing agent. Specifically, chloride ions adsorb onto the Ag UPD adlayer and effect changes in the electrochemical deposition and stripping characteristics of the silver film. Cyclic voltammograms (CVs) of the native Au/Ag(UPD) electrode in 0.1 M H2SO4(aq) exhibit a primary stripping peak for the Ag UPD adlayer at 550 mV vs Ag+/0, and chloride adsorption onto the Au/Ag(UPD) surface effects a peak shift to ∼600 mV vs Ag+/0, depending on the amount of adsorbed Cl-, as affected by the Cl- concentrations and contact times employed in the derivatization. The chloride-treated electrodes also exhibit a stripping peak at 275 mV that is not observed on the native substrate and increases in intensity with Clconcentration and derivatization time. The integrated charge density for this latter stripping peak relative to that for the primary stripping peak at 550-610 mV provides a useful metric for quantifying adsorbed Cl- levels, and these values allow measurement of Cl- concentrations in dilute aqueous solutions. For Cl- concentrations between 0.5 and 100 µM, the kinetics of Cl- adsorption followed a transient Langmuir adsorption model and allowed measured surface coverages to be used for determining Cl- solution concentrations. Using contact times of 1 min for Cl- adsorption, the electrodes showed a linear response across Cl- concentrations of 0.5-20 µM. Chloride sensing is an important need in clinical diagnosis,1,2 environmental monitoring,3 and industrial applications.4,5 This wide application base across these various diverse areas produces a need for sensors with different sets of requirements but common needs for increased sensitivity and selectivity in many of these * To whom all correspondence should be addressed. Phone (713) 348-3539, Fax. (713) 348-5478. E-mail: [email protected]. † Current address: Department of Chemical Engineering, Rice University, MS-362, P.O. Box 1892, Houston, TX 77251-1892. (1) Huber, C.; Werner, T.; Krause, C.; Klimant, I.; Wolfbeis, O. S. Anal. Chim. Acta 1998, 364, 143-151. (2) Krapf, R.; Berry, C. A.; Verkman, A. S. Biophys. J. 1988, 53, 955-962. (3) Martin, A.; Narayanaswamy, R. Sens. Actuators, B 1997, 38-39, 330-333. (4) Badr, I. H. A.; Diaz, M.; Hawthorne, M. F.; Bachas, L. G. Anal. Chem. 1999, 71, 1371-1377. (5) Geddes, C. D. Sens. Actuators, B 2001, 72, 188-195. 10.1021/ac0497555 CCC: $27.50 Published on Web 08/25/2004

© 2004 American Chemical Society

applications. Among electrochemical methods for analysis, ionselective electrodes (ISEs) have been most widely used and are available commercially for a variety of ions, including chloride. Although the performances of ISE-based chloride sensors are notable, their detection limits (typically 5 × 10-5 M) and selectivities (particularly against other halides) remain as drawbacks.6-10 As an alternative to ISE-based chloride sensors, optical sensors using fluorescence measurements have been reported;11-14 however, they are ineffective at chloride concentrations less than millimolar, and photobleaching of the dye molecules during operation introduces additional limitations. Previously, we reported that monolayer films of Ag on Au(111) prepared by underpotential deposition (UPD) provide a promising possibility for halide sensing.15 Characteristics of UPD films include that the adlayers form on more noble supports, the films have coverages of up to a monolayer, and the adatoms exhibit stripping and deposition potentials that are positive of their bulk counterparts.16 Numerous UPD systems have been examined and illustrate the broad ability of this method for altering the composition and chemical properties of electrode surfaces.16-20 The differences in the redox properties of the atoms comprising the UPD layer from those of their bulk counterparts reflect the (6) Zielinska, R.; Mulik, E.; Michalska, A.; Achmatowicz, S.; Maj-Zurawska, M. Anal. Chim. Acta 2002, 451, 243-249. (7) Chemical Sensors; Edmond, T. E., Ed.; Chapman and Hall: New York, 1988; Chapter 3. (8) Janata, J. Principles of Chemical Sensors; Plenum: New York, 1989; Chapter 4. (9) Spichiger-Keller, U. E. Chemical Sensors and Biosensors for Medical and Biological Applications; Wiley-VCH: New York, 1998; Chapter 5. (10) Chemical and Biological Sensors for Environmental Monitoring; Muchandani, A., Sadik, O. A., Eds.; ACS Symposium Series 762; American Chemical Society: Washington D. C., 2000; Chapter 2. (11) Cosentino, P.; Grossman, B.; Shieh, C.; Doi, S.; Xi, H.; Erbland, P. J. Geotech. Eng.-ASCE 1995, 121, 610-617. (12) Barker, S. L. R.; Thorsrud, B. A.; Kopelman, R. Anal. Chem. 1998, 70, 100-104. (13) Huber, C.; Klimant, I.; Krause, C.; Wolfbeis, O. S. Anal. Chem. 2001, 73, 2097-2103. (14) Langer, P.; Mu ¨ ller, R.; Drost, S.; Werner, T. Sens. Actuators, B 2002, 82, 1-6. (15) Michalitsch, R.; Laibinis, P. E. Angew. Chem., Int. Ed. 2001, 40, 941-944. (16) Herrero, E.; Buller, L. J.; Abrun ˜a, H. D. Chem. Rev. 2001, 101, 1897-1930. (17) Ogaki, K.; Itaya, K. Electrochim. Acta 1995, 40, 1249-1257. (18) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208-5214. (19) Whelan, C.; Smyth, M. R.; Barnes, C. J.; Attard, G. A.; Yang, X. J. Electroanal. Chem. 1999, 474, 138-146. (20) Rooryck, V.; Reniers, F.; Buess-Herman, C.; Attard, G. A.; Yang, X. J. Electroanal. Chem. 2000, 482, 93-101.

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dissimilar local environments for the affected atoms in theses two states, with factors such as the substrate, its crystallinity, and the electrolyte composition all affecting the electrochemical and structural properties of the UPD adlayer. Particularly acute redox changes can result in some cases by anion adsorption onto the UPD film because the local electronic environment of the UPD adatoms may be changed greatly between the native and modified forms.16,21-26 For example, the adsorption of chloride onto a Ag(UPD) adlayer onto Au(111) can shift the primary stripping and deposition peaks for the Ag(UPD) film by ∼80 mV, whereas the adsorption of iodide can shift these peaks negatively by ∼120 mV. In both of these cases, intermediate states of adsorption show stripping peaks for both the native and the halide-modified silver atoms, thereby allowing the relative compositions of silver atoms in these two states to be determined electrochemically. On the basis of these observations, we suggested that the changes in these peaks could be used as the basis for a detection scheme for halides.15 Such an approach provides several advantages over conventional ISE-based sensors for halide detection in which membranes are used for their selectivity. These benefits include direct contact between the solution and the electrode surface (i.e., no diffusional barrier), a high electrode affinity for halide ions, a lower detection limit (10-7 M),27 and an approach that affords straightforward microsensor fabrication. Although the Au(111)/Ag(UPD) system shows remarkable abilities as a platform for halide sensing, the specific use of a crystalline Au(111) as substrate imparts some practical limitations. For example, the Au(111) surface reconstructs and develops a polycrystalline structure upon storage, thereby requiring pretreatment of the gold electrode surface before a measurement by methods such as flame-annealing in order to regain the Au(111) structure and achieve reliable electrochemical signals. To extend the practicality of the Au(111)/Ag(UPD) system for sensing, we consider the replacement of the original Au(111) platform with a polycrystalline Au surface bearing a Ag UPD adlayer. This latter substrate would offer many practical operational advantages should it offer sensing abilities analogous to those possible with Ag-modified Au(111) electrodes. To accomplish this goal, we examined the electrochemical changes to a Ag UPD adlayer on a polycrystalline Au surface as effected by the adsorption of chloride. The polycrystalline Au system similarly exhibits signature electrochemical changes in the stripping characteristics of the Ag UPD adlayer by chloride adsorption; however, these changes differ in important ways from those on Au(111) and require the development of a different method for analysis. For the polycrystalline Au system, we present a method for quantifying chloride concentrations by these (21) Uchida, H.; Miura, M.; Watanabe, M. J. Electroanal. Chem. 1995, 386, 261-265. (22) Mrozek, P.; Sung, Y.-E.; Wieckowski, A. Surf. Sci. 1995, 335, 44-51. (23) Mrozek, P.; Sung, Y.-E.; Han, M.; Gamboa-Aldeco, M.; Wieckowski, A.; Chen, C.-H.; Gewirth, A. A. Electrochim. Acta 1995, 40, 17-28. (24) Wu, S.; Lipkowski, J.; Tyliszczak, T.; Hitchcock, A. P. Prog. Surf. Sci. 1995, 50, 227-236. (25) Herrero, E.; Glazier, S.; Abrun ˜a, H. D. J. Phys. Chem. B 1998, 102, 9825-9833. (26) Zei, M. S.; Wu, K.; Eiswirth, M.; Ertl, G. Electrochim. Acta 1999, 45, 809-817. (27) Detection limit of the ISE-based chloride sensors is typically 5 × 10-5 M. More recently developed commercial products (Accumet and Orion Electrodes) show a detection limit of 5 × 10-6 M.

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electrodes that relies on changes to an integrated charge ratio between two well-separated stripping peaks. A kinetic analysis of the chloride adsorption process onto the Au/Ag(UPD) surface shows that the detection range for chloride by this method is roughly 10-7-10-4 M. EXPERIMENTAL SECTION Materials. Au (99.99%) shot, Cr-coated tungsten filaments, n-doped Si(100) wafers were obtained from Americana Precious Metals (East Rutherford, NJ), R.D. Mathis Co. (Long Beach, CA), and Silicon Sense Inc. (Nashua, NH), respectively. Sulfuric acid (double-distilled, 98%, Cl- ≈ 0.05 ppm), silver sulfate (99+%, AgCl and other insolubles e0.02%) and potassium chloride (99+%) were obtained from Mallinckrodt, Baker Inc. (Paris, KY) and Aldrich (St. Louis, MO), respectively, and were used without further purification. Preparation of Gold Films. Polycrystalline Au films were obtained by the sequential evaporation of Cr (2 nm) and Au (150 nm) onto glass slides or Si wafers in a diffusion-pumped vacuum chamber (pressure ≈ 3 × 10-6 Torr). Prior to use in the electrochemistry experiments, the polycrystalline Au films were cleaned by treatment with an O2 plasma in a Harrick plasma cleaner PDC-32G at 0.15 Torr and medium RF intensity for 2 min or by immersion in a fresh piranha solution (80:20 v/v mixture of concentrated sulfuric acid and 35% hydrogen peroxide) at 90 °C for 5 s. (Caution: “piranha” solution reacts violently with many organic materials and should be handled with extreme care.) Au(111) films were prepared by flame annealing28 of polycrystalline Au films formed on borosilicate glass slides (Chemglass, 1/16in. thickness). Because continuous heating during the flame annealing process frequently broke the gold-coated glass slides, the polycrystalline Au films were repeatedly placed onto the flame of a Bunsen burner for 5 s and then cooled in air for 5 s through 50 cycles. This process produce Au(111) films without any cracking of the gold-coated glass slides. Electrochemistry. Cyclic voltammetry (CV) was performed with a computer-controlled EG&G PAR model 263A potentiostat. The electrolyte consisted of 1 mM Ag2SO4 and 0.1 M H2SO4 in deionized water (Millipore, 18.2 MΩ) for Ag UPD. A typical threeelectrode system was used for all cyclic voltammograms, with a supported polycrystalline gold film serving as counter electrode. The effective area of the working electrode was 1 × 1 cm2, and all potentials are quoted relative to silver wire (Ag+/0) used as a quasi-reference electrode.20,24,29 Ag UPD adlayers were prepared by cycling the potential of a polycrystalline Au or Au(111) electrode between 50 and 650 mV in supporting electrolyte. Starting from the open-circuit potential of the electrode in the electrolyte (typically ∼520 mV for Au(111) and ∼560 mV for polycrystalline Au) with a negative sweep, the electrode was cycled once, and the potential was stopped at 300 mV vs Ag+/0. The electrode was removed from the electrolyte solution under potential control after reaching 300 mV, rinsed with deionized water, and transferred into a potassium chloride solution for modification. After a specified contact time, the electrode was removed from the chloride solution, rinsed with deionized water (28) Haiss, D.; Lackey, D.; Sass, J. K.; Besocke, K. H. J. Chem. Phys. 1991, 95, 2193-2196. (29) Michalitsch, R.; Palmer, B. J.; Laibinis, P. E. Langmuir 2000, 16, 6533-6540.

Figure 1. Typical cyclic voltammograms of (a) Ag UPD on Au(111) and (b) Ag UPD on polycrystalline Au. The CVs were obtained in an electrolyte consisting of 1 mM Ag2SO4 and 0.1 M H2SO4 at a scan rate of 25 mV/s. After UPD, each sample was immersed in a 1 × 10-4 M Clsolution for 1 min, rinsed with deionized water, and reimmersed into the electrolyte solution for analysis. Solid line, native Ag(UPD); dotted line, Cl- adsorbed Ag(UPD).

to remove remaining chloride solution, and reimmersed into the supporting electrolyte for electrochemical analysis by CV. In analyzing the CVs, the integrated charge ratio between two stripping peaks (A1 at 275 mV)/(A2 at ∼605 mV) was determined and used for quantitation. Details are described in Supporting Information. For analysis of the chloride adsorption process and a determination of detection limits, a series of experiments were conducted that varied both chloride concentrations and contact times. X-ray Photoelectron Spectroscopy (XPS). XPS analysis was used for quantifying the amount of adsorbed chloride on Au/Ag(UPD) samples. Spectra were obtained with a Surface Science Laboratories SSX-100 spectrometer equipped with a monochromatized Al KR X-ray source and a concentric hemispherical analyzer. The detector was positioned at an angle of 55° with respect to the surface normal. Peaks were fitted with 80% Gaussian and 20% Lorentzian profiles and a Shirley background; peak positions were referenced to Au(4f7/2) ) 84.00 eV. RESULTS AND DISCUSSION Cyclic Voltammetry of Au/Ag(UPD). The electrochemical properties and structures of underpotentially deposited adlayers of Ag onto Au(111) surfaces have been broadly investigated in a variety of studies.16,17,19-23 In H2SO4 as electrolyte, the primary stripping peak for the Ag UPD layer on Au(111) is observed at ∼530 mV vs Ag+/0. Figure 1a shows a typical cyclic voltammogram for the UPD of Ag onto a Au(111) surface and the changes that are effected when a Au(111)/Ag(UPD) sample is exposed to Clto saturate the Au(111)/Ag(UPD) surface, where the primary stripping peak at 530 mV vs Ag+/0 is replaced by a new stripping peak at 610 mV vs Ag+/0 via transient variation in the peak height at 530 and 610 mV;15 i.e., the peak at 530 mV decreased, and the new peak at 610 mV increased without change of primary stripping peak position. Between these extremes, the electrode exhibits stripping peaks at both 530 and 610 mV vs Ag+/0, and the integrated areas of these peaks show a direct relationship to the amount of adsorbed chloride on the electrode surface. XPS revealed that the stripping peak at 530 mV corresponded to Ag(UPD) adatoms in their native state, and the peak at 610 mV corresponded to Ag(UPD) adatoms that had adsorbed chloride. The adsorption of chloride could be readily followed electrochemi-

cally and described by a simple kinetic process.15 We suggested that this electrochemical determination of adsorbed chloride amounts coupled with an understanding of the kinetics of the adsorption process provides a strategy for making measurements of Cl- concentrations in aqueous solutions. We demonstrated that the integrated charge ratio of these two peaks could be used for measurement of chloride concentrations ranging from 10-7 to 10-4 M.15 On polycrystalline Au, the CV for Ag UPD and its changes as effected by chloride treatment exhibit differences from those on Au(111). As shown in Figure 1b, the primary stripping peak for Ag on the native Au surface appears at 550 mV vs Ag+/0 and is replaced by a new stripping peak at ∼605 mV vs Ag+/0 upon Cladsorption; the peak replacement occurs with a gradual positive shift of the primary stripping peak from its native state at 550 mV to its Cl-adsorbed state at ∼605 mV vs Ag+/0. The peaks are less intense and broader than those on Au(111). The measured integrated charge density for the Ag stripping peak on the polycrystalline Au surface was lower (20 ( 2 µC/cm2 on native Au and 22 ( 2 µC/cm2 after chloride treatment) than on the Au(111) surface (74 ( 4 µC/cm2 on native Au(111) and 80 ( 5 µC/cm2 after chloride treatment). This difference in charge density may be due to the different surface structures for the substrates because the charge densities obtained from the voltammograms are strongly dependent upon the surface structure of the electrode and could be affected greatly by the pretreatment of the surface, such as by flame-annealing in our case.26 Generally, the Au(111) surface has a homogeneous and close-packed planar structure, whereas the polycrystalline Au surface is heterogeneous and defective. Thus, it is not surprising that the charge density for the Au(111)/Ag(UPD) adlayer is higher than that for the Au/Ag(UPD) adlayer. A notable difference in the CVs for the Au/Ag(UPD) and Au(111)/Ag(UPD) surfaces after their adsorption of chloride is the new stripping peak at 275 mV vs Ag+/0 that occurs for the polycrystalline substrate. We rarely observed this feature in the Au(111)/Ag(UPD) system and never at the relative magnitude present for the polycrystalline substrate. On the Au(111) substrate, the only change induced by Cl- adsorption in the primary stripping features for the Au(111)/Ag(UPD) system was the replacement Analytical Chemistry, Vol. 76, No. 19, October 1, 2004

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Figure 2. (a) CVs of Au/Ag(UPD) through continuous cycling of UPD and stripping processes at a scan rate of 25 mV/s. Solid line, native Au/Ag(UPD) (1 cycle); dotted line, after 21 cycles; broken line, after 51 cycles; dashed line, after 81 cycles; thick broken line, after 151 cycles; thick solid line, after 211 cycles. (b) CVs of Au/Ag(UPD) after 1 min of contact time in Cl- solutions of various concentrations. Solid line, native Au/Ag(UPD); dotted line, Au/Ag(UPD)-Cl (1 × 10-6 M); broken line, Au/Ag(UPD)-Cl (2 × 10-6 M); dashed line, Au/Ag(UPD)-Cl (1 × 10-5 M); thick broken line, Au/Ag(UPD)-Cl (2 × 10-5 M); thick solid line, Au/Ag(UPD)-Cl (5 × 10-5 M).

of the primary stripping peak at 530 mV by a new peak at 610 mV vs Ag+/0. This transition occurred by concurrent loss in intensity for the peak at 530 mV and an equivalent increase in intensity for the peak at 610 mV. The primary stripping peak at 530 mV exhibited no shift in position during this transition. Electrochemical and XPS analysis on the UPD process for Ag on Au(111) in sulfuric acid containing a trace amount of chlorides revealed that the replacement of the primary stripping peak for Ag results from Cl- adsorption onto the UPD adlayer.29 To verify whether this new peak at 275 mV for the polycrystalline Au substrate (Figure 1b) is produced by Cl- adsorption, we examined the UPD of Ag onto polycrystalline Au samples during continuous cycling for 3 h in a supporting electrolyte consisting of 1 mM Ag2SO4 and 0.1 M H2SO4 that contained trace amounts of Cl-.30 We also examined Au samples that were cycled for 1 min and 3 h by XPS to determine compositional changes between the initial UPD layer and the transformed UPD layer. Figure 2a shows the results of continuously cycling a polycrystalline Au electrode between 50 and 650 mV vs Ag+/0 in the supporting electrolyte. In the first cycle, we observed a primary stripping peak at 550 mV vs Ag+/0. In subsequent cycles, we observed a gradual positive shift in the primary stripping peak and the development of a new stripping peak at 275 mV that continued to increase in intensity. After 3 h of cycling, the primary stripping peak at 550 mV was completely replaced by a peak at 605 mV, and the peak at 275 mV reached its maximum intensity. From the XPS analysis, the polycrystalline Au samples that were cycled once showed no chloride, whereas samples cycled for 3 h exhibited chloride levels corresponding to roughly one-half a monolayer of adsorbed chloride. From these results, we concluded that the shift in the primary stripping peak from 550 to 605 mV vs Ag+/0 and the development of a new peak at 275 mV result from Cl- adsorption, with the latter peak providing an electrochemical signature for this process. The shift in the primary UPD peak at 550 mV and the development of the peak at 275 mV were accelerated by exposure (30) The chloride levels in Ag2SO4 and H2SO4 were given as e0.02% (including other insoluble) and ≈0.05 ppm, respectively, by the suppliers. These levels would give maximum chloride concentration of 2.2 × 10-7 and 1.4 × 10-8 M, respectively, in the electrolyte for Ag UPD.

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of the Au/Ag(UPD) substrates to solutions of increasing Clconcentration. In our experiments, we immersed Au/Ag(UPD) samples into Cl- solutions of various concentrations for 1 min and characterized them electrochemically after a rinse with deionized water. Figure 2b shows the primary stripping peaks shifted by up to 55 mV upon Cl- adsorption and that its peak position varied depending on the Cl- concentration. We note that the integrated charge for these peaks in each case showed little variation (22 ( 2 µC/cm2). In essence, the primary stripping peak for each sample can be attributed to some average electrochemical state that reflects a combination of the Cl-adsorbed Au/Ag(UPD) sites and the native Au/Ag(UPD) sites. Thus, the integrated charges show little change through all experiments, although the peak position varie, which is induced by the Cl-adsorbed Au/Ag(UPD) sites. For the stripping peaks generated at 275 mV vs Ag+/0 by Cl- adsorption, these peaks gradually increased in intensity with Cl- concentration and did not change in their peak position. Because the primary stripping peak reflects a primary stripping event for both Au/Ag(UPD)-Cl and Au/Ag(UPD) species and the stripping peak at 275 mV mainly represents changes induced by Cl- adsorption, we examined whether the ratio of integrated charges for these two peaks provided a correlation with Clconcentration. Specifically, after subtraction of the nonfaradic components, we divided the integrated charge of the peak at 275 mV (denoted by A1) by that of the primary stripping peak (denoted by A2). To determine whether this ratio (A1/A2) could reflect the adsorbed amount of Cl-, we examined the relationship between their values and the Cl- compositions determined from XPS on these Ag UPD adlayer samples. As shown in Figure 3, we observed a linear relationship, suggesting that the integrated charge ratio (A1/A2) is directly proportional to the adsorbed amount of Cl- on the Au/Ag(UPD) surface. On the basis of these results, we used the integrated charge ratio as a signal for monitoring the adsorbed amount of Cl- in all experiments. Adsorption Behavior of Chloride onto Au/Ag(UPD). To further elucidate the effects of Cl- concentration on Au/Ag(UPD), we examined the kinetic changes to its electrochemistry through experiments that varied both Cl- concentration and contact

be used for Cl- concentrations above 10-5 M because the plateau values at these higher concentrations can be as much as twice that at the 10-6 M level. To capture the concentration dependence of the limiting value of A1/A2 at the higher Cl- concentrations, we applied a transient Langmuir adsorption kinetic model to our system. We selected this approach because the monolayer-level adsorption can be viewed as a surface-site filling process in which the adsorption and desorption processes counteract each other. Assuming that the diffusional mass transfer is negligible, we can describe the overall process using the following rate equation,31-33 Figure 3. Integrated charge ratio as a function of adsorbed Cllevels for Au/Ag(UPD). Au/Ag(UPD) samples were contacted in pairs, with Cl- solutions ranging in concentration from 2 × 10-6 to 1 × 10-4 M for 2 min and analyzed either by CV (to obtain the integrated charge ratio A1/A2) or XPS (to determine the Cl/Ag elemental ratios).

time. Figure 4a shows kinetic results for Cl- adsorption onto Au/Ag(UPD) electrodes for several concentrations, in which the adsorbed amount of Cl- generally increased with contact time and concentration. To interpret the Cl- adsorption process, a kinetic analysis was performed. For each concentration, we assumed that its plateau value indicated an equilibrium level of Cl- adsorption on the Au/Ag(UPD) surface and that the adsorption can be described by eq 1,

θ ) [1 - exp(-kaCt)]

(1)

where θ is the fraction of converted surface, ka is an adsorption rate constant, C is the bulk Cl- concentration, and t is the contact time. To apply eq 1, we normalized each data set to its maximum (plateau) value to determine the fractions present as Au/Ag(UPD) and as Au/Ag(UPD)-Cl. Using these values, we plotted the unconverted fraction of Au/Ag(UPD) against the contact time. Figure 4b shows these kinetic data for the conversion of Au/Ag(UPD) surfaces after their contact with Cl- solutions of various concentrations for different contact times. The fitted slopes to the early time data in these data sets yielded the apparent Cladsorption rate in each case. We plotted these slopes against Clconcentration (Figure 4b inset) and observed that the adsorption of Cl- appears to follow a simple kinetic process that is first-order in Cl- over the concentration range of 5 × 10-7 to 5 × 10-6 M with ka ) 9.3 × 102 L/mol‚s. Because the adsorbed amount of Cl- is linearly related to the integrated charge ratio, A1/A2, we can rewrite eq 1 to describe the data in Figure 4a by

A1 ) Rθ ) R[1 - exp(-kaCt)] A2

(2)

where R is a proportional constant that can be determined from the maximum value of A1/A2 in Figure 4a (i.e., R ) 0.18). Using this simple equation, we were able to fit all the integrated charge ratio data included in Figure 4a. We note that eq 2 is limited in use to the concentration range presented in Figure 4 and cannot

dθ ) kaC(1 - θ) - kdθ dt

(3)

where θ is the fractional coverage of converted sites; t is the adsorption (contact) time; ka and kd are adsorption and desorption rate constants, respectively; and C is the solution concentration of Cl-. Integration of eq 3 with the initial condition of θ ) 0 at t ) 0 gives

θ)

kaC {1 - exp[-(kaC + kd)t]} kaC + kd

(4)

At equilibrium (i.e., as t f ∞), eq 4 reduces to give the Langmuir adsorption isotherm.

θeq )

kaC C ) kaC + kd C + κ

(5)

where κ ≡ kd/ka. Again, replacing the fractional coverage of converted sites by the directly measurable integrated charge ratio, A1/A2, gives the following equation,

A1 kaC ) βθ ) β {1 - exp[-(kaC + kd)t]} A2 kaC + kd

(6)

where β is a proportional constant that is determined from the maximum value of the integrated charge ratio, A1/A2 (0.37). Figure 5a shows the time-dependence of the integrated charge ratio, A1/A2, from kinetics experiments conducted using six different Cl- concentrations. At higher concentrations, the measured values of A1/A2 reached their equilibrium values much faster than in experiments conducted at lower concentrations. Specifically, equilibrium values of A1/A2 were achieved in ∼1 min for [Cl-] ∼ 5.0 × 10-5 M, whereas ∼10 min was required for [Cl-] ∼ 5 × 10-6 M. In Figure 5a, the plateau values in A1/A2 at the different concentrations correspond to an equilibrium state for that condition. Figure 5b plots these values of A1/A2 as a function of Cl- concentration and shows that they are reasonably described by a Langmuir adsorption profile (eq 5). A reciprocal plot of Figure 5b (not shown) yields a value for κ of 5.0 × 10-6 M. (31) Grow, D. T.; Shaeiwitz, J. A. J. Colloid Interface Sci. 1982, 86, 239-253. (32) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978-987. (33) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991; Chapter 3.

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Figure 4. (a) Kinetic data of Cl- adsorption on Au/Ag(UPD) with various Cl- concentrations. 1, 5 × 10-7 M; 2, 1 × 10-6 M; b, 2 × 10-6 M; 9, 5 × 10-6 M; solid lines, curve fitting by eq 2. (b) Kinetic analysis of Cl- adsorption onto Au/Ag(UPD). 1, 5 × 10-7 M; 2, 1 × 10-6 M; b, 2 × 10-6 M; 9, 5 × 10-6 M. The inset shows the linear relationship between the apparent rate of Cl- adsorption and Cl- concentration, i.e., ln kapp ) ln ka + n ln C. The adsorption rate constant, ka, was determined from the intercept.

Figure 5. Validation of the transient Langmuir adsorption kinetic model for Cl- adsorption onto Au/Ag(UPD). (a) Integrated charge ratio as a function of contact time and solution concentration. Data were fitted with eq 6, and two parameters (adsorption and desorption rate constant) were obtained from the slope and intercept of the line fit of Figure 5c, respectively. 4, 1 × 10-6 M; O, 5 × 10-6 M; 0, 1 × 10-5 M; 2, 2 × 10-5 M; b, 5 × 10-5 M; 9, 1 × 10-4 M; solid lines, fitting by eq 6. (b) Equilibrium value of the integrated charge ratio as a function of Cl- concentration. Data were fitted with a Langmuir isotherm (eq 5). (c) Concentration dependence of the apparent rate. The slope and intercept for the line are 1.64 × 103 L/mol‚s and 8.0 × 10-3 s-1, respectively.

According to eq 6, the apparent rate in the exponential term, (kaC + kd), should be linearly related to Cl- concentration. To verify this concentration dependence, we estimated the apparent rates from the data sets in Figure 5a at low conversions. Figure 5c plots the apparent rates against Cl- concentration and shows a linear relationship between the two variables in accord with the suggestion of eq 6. From the slope and the intercept of the line in Figure 5c, the values of the rate constants for adsorption and desorption were obtained as 1.6 × 103 L/mol‚s and 8.0 × 10-3 s-1, respectively. We used these values to generate the curves in Figure 5a. These parameters were also used to determine a value of κ because κ is equal to the ratio of the desorption rate constant to the adsorption rate constant. This approach gave a value of κ of 4.9 × 10-6 M that was consistent with the value (5.0 × 10-6 M) obtained from the curve fit to eq 5 in Figure 5b. On the basis of the determined values for ka and kd, the relative importance of Cl- desorption is much less than that of Cl- adsorption during the initial stage of overall adsorption process at the investigated Cl- concentrations. We conclude that the Cl- adsorption process onto the Au/Ag(UPD) surface can be well-described by transient Langmuir adsorption kinetics over the concentration range of 10-6-10-4 M. Sensor Performance. For the application of the Au/Ag(UPD) system as a chloride sensor, we can define limits on its operation 5916 Analytical Chemistry, Vol. 76, No. 19, October 1, 2004

by examining the relationship between the analyte concentration and the measured signal used for analysis. We have selected 1 min of contact time for this discussion, where shorter contact times are expected to increase the range of applicable concentrations and longer contact times would reduce the applicable concentration range. Figure 6 shows the measured integrated charge ratio, A1/A2, as a function of Cl- concentration using 1 min of contact time. The integrated charge ratio increased with increasing Cl- concentration and reached a limiting maximum value for Clconcentrations of ∼3 × 10-5 M, setting an upper bound on Cldetection. At concentrations 100˜o, P.; fold preference for adsorbing I- over Cl- at equilibrium (Castan Michalitsch, R.; Laibinis, P. E. unpublished results). (35) The adsorption of Cl-, Br-, and I- produce distinguishable shifts in the primary stripping peak of a Ag(UPD) adlayer on polycrystalline Au that can be used for their identification. The changes in the CVs for Ag(UPD) films on polycrystalline Au substrates show some similarities to those reported in ref 15 for Au(111)/Ag(UPD) substrates. CVs of halide-treated polycrystalline Au/Ag(UPD) electrodes are included in the Supporting Information.

quantifying Cl- concentrations in aqueous solutions. The positive shift in a primary stripping peak from 550 to 605 mV vs Ag+/0 and the generation of a new stripping peak at 275 mV vs Ag+/0 provided an electrochemical signature for Cl- adsorption on the Au/Ag(UPD) electrodes. The integrated charge ratio of these two peaks was proportional to the adsorbed amount of Cl- and provided a reliable metric for quantifying Cl- concentration. The adsorption behavior of Cl- onto the Au/Ag(UPD) surface was welldescribed by a transient Langmuir adsorption kinetics that allowed construction of a theoretical framework that related solution concentrations of Cl- to measurable electrochemical signals. Using contact times of 1 min for detection, the sensor response of the Au/Ag(UPD) electrode was linear over the Cl- concentration range of 5 × 10-7 to 2 × 10-5 M. The low level of Cl- detection is particularly noteworthy. Further, the Au/Ag(UPD) system shows a high selectivity for Cl- and relatively low interference by other anions,34 thereby overcoming a common problem of conventional Cl- sensors based on ISEs. The active element in sensing is simply fabricated by the electrochemical deposition of a Ag adlayer onto a polycrystalline Au surface. The simplicity of the sensor fabrication and its anticipated scalability offer advantages for miniaturizing the present system into a microsensor format. Another remarkable feature of the Au/Ag(UPD) system is that its electrochemical signature provides distinguishable signals for each of Cl-, Br-, and I-,35 thereby providing direct information about the identity of a halide under investigation. Such differentiation by signal output is not possible using ISEs, often confounding the reliability of their measurements. We conclude that the Au/Ag(UPD) system offers a useful platform for the development of a highly sensitive chloride sensor. Current efforts target the fabrication of microsensors based on this approach and methods for the routine analysis of ultrapure water streams. ACKNOWLEDGMENT This work was supported by the Office of Naval Research. H.-G. Choi gratefully acknowledges the Korea Science and Engineering Foundation (KOSEF) for a postdoctoral fellowship. SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review February 12, 2004. Accepted July 4, 2004. AC0497555

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