Anal. Chem. 2003, 75, 319-323
Application of Disorganized Monolayer Films on Gold Electrodes to the Prevention of Surfactant Inhibition of the Voltammetric Detection of Trace Metals via Anodic Stripping of Underpotential Deposits: Detection of Copper Gre´goire Herzog and Damien W. M. Arrigan*
Transducers Group, NMRC, University College, Lee Maltings, Prospect Row, Cork, Ireland
Development of an approach to prevention of electrode surface fouling by surfactants in samples is demonstrated. Spontaneously adsorbed monolayer systems employing short alkyl chains and bulky end groups are used to form porous disorganized monolayers on gold electrodes. Detection of copper by stripping of underpotential deposits formed at electrodes modified with disorganized films of mercaptoethanesulfonate (MES), mercaptopropanesulfonate, mercaptoacetic acid, and mercaptopropanoic acid was possible, and to a much lesser extent at aminoethanethiol and L-cysteine films. Use of short deposition times in conjunction with linear sweep anodic stripping voltammetry allowed detection of Cu2+ ions down to 1 × 10-6 M in sulfuric acid solution, using underpotential deposition as the deposition step of the procedure. Calibration graphs were linear in the concentration range (1-80) × 10-6 M Cu2+ using 15-s deposition at 0.00 V versus Ag|AgCl. The surfactants Tween 20, Tween 80, and Triton X-100 were found to have no affect on detection of Cu2+ ions in the calibration curve concentration range using MES-modified gold electrodes, whereas at unmodified gold electrodes very severe attenuation of the detection capability was manifested. The average slope for all calibration curves at the MES-modified electrode in the absence and presence of the surfactants at two different concentration levels was 0.0710 ( 0.0024 µA µM-1; in contrast, the slope of the calibration line at uncoated gold electrodes in the presence of surfactant was 0.0268 µA µM-1. These results indicate the excellent ability of a disorganized, porous monolayer for prevention of fouling of the electrode surface by the surfactants. Electroanalytical and sensor applications of chemisorbed organosulfur films on gold surfaces have received much attention in recent years. The benefits of such films include ultrathin film processes, rapid response, chemical functionalization of the film after preparation, immobilization of biorecognition reagents, and chemical specificity for sensor applications.1-3 * Corresponding author. Phone: +353-21-4904079. Fax: +353-21-4270271. E-mail:
[email protected]. 10.1021/ac026093f CCC: $25.00 Published on Web 12/17/2002
© 2003 American Chemical Society
Underpotential deposition (UPD) of metals has been used as a characterization method for various self-assembled monolayers (SAMs). UPD of metals such as copper and silver at alkanethiol monolayers of varying hydrocarbon chain length have been studied.4-8 Such studies have concluded that SAMs formed on UPD monolayers are more stable than on the substrate electrode and that UPD after SAM formation results in metal monolayer being formed between the organic film and the electrode substrate.4-8 Analytical applications of UPD by stripping of such deposits from solid electrodes have been explored recently by KirowaEisner and colleagues.9-14 They have used bare metal electrodes as the substrate and have demonstrated extremely sensitive analytical capabilities. Analytically oriented studies of metal deposition at SAMs, as opposed to metal ion complexation within organic monolayer films,15-20 have not been widely investigated. Heineman and co-workers21,22 found that Pb UPD was suppressed (1) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207-213. (2) Wink, T.; van Zuilen, S. J.; Bult, A.; van Bennekom, W. P. Analyst 1997, 122, 43R-50R. (3) Mirsky, V. M. Trends Anal. Chem. 2002, 21, 439-450. (4) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173-6175. (5) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 52155217. (6) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298-3302. (7) Oyamatsu, D.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1999, 473, 59-62. (8) Himmelhaus, M.; Buck, M.; Grunze, M. Appl. Phys. B 1999, 68, 595-598. (9) Brand, M.; Eshkenazi, I.; Kirowa-Eisner, E. Anal. Chem. 1997, 69, 46604664. (10) Kirowa-Eisner, E.; Brand, M.; Tzur, D. Anal. Chim. Acta 1999, 385, 325335. (11) Bonfil, Y.; Brand, M.; Kirowa-Eisner, E. Anal. Chim. Acta 2000, 424, 6576. (12) Bonfil, Y.; Brand, M.; Kirowa-Eisner, E. Anal. Chim. Acta 1999, 387, 8595. (13) Bonfil, Y.; Kirowa-Eisner, E. Anal. Chim. Acta 2002, 457, 285-296. (14) Bonfil, Y.; Brand, M.; Kirowa-Eisner, E. Anal. Chim. Acta 2002, 464, 99114. (15) Rubenstein, I.; Steinberg, S.; Yitzhak, T.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (16) Turyan, I.; Mandler, D. Anal. Chem. 1994, 66, 58-63. (17) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894-897. (18) Markovich, I.; Mandler, D. Analyst 2001, 126, 1850-1856. (19) Arrigan, D. W. M.; Le Bihan, L. Analyst 1999, 124, 1645-1649. (20) Yang, W. R.; Jaramillo, D.; Gooding, J. J.; Hibbert, D. B.; Zhang, R.; Willett, G. D.; Fisher, K. J. Chem. Commun. 2001, 1982-1983.
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at films of 3-mercaptopropionic acid. In our previous work on metal detection by combining UPD and stripping voltammetry at modified electrodes,23,24 we set out to form disorganized monolayer films on gold electrodes by use of thiol compounds possessing short hydrocarbon chains (two- or three-carbon chain) and a highly charged end group (sulfonate). Such films would be disorganized and porous and could be likened to a “molecular brush”.18,25,26 To-date we have developed studies of Pb23 and Cu24 UPD at mercaptoalkanesulfonate film electrodes. Our aim was the characterization of UPD systems at such disorganized film-coated electrodes with a view to developing ultrathin films that can inhibit surface fouling (e.g., by proteins, surfactants) while allowing movement of target metal ions through the layer for voltammetric detection at the underlying electrode. We chose to study highly charged organosulfur films with short alkyl chains because they should form disordered films due to the repulsive interactions of the negatively charged end groups and the low lateral interactions between the short hydrocarbon chains. Thus, the disordered films resulting from the spontaneous chemisorption onto gold should produce a porous film amenable to solvent and electrolyte partitioning. The UPD of metals at disordered charged monolayers systems, such as those containing sulfonate or other charged end groups, has otherwise not been reported. In this report, we present our results on the continued investigation of this line of enquiry and show that low-level detection of copper ions in the presence of large excess of surfactant compounds is possible. The results demonstrate clearly the benefits of using porous disorganized monolayer systems for the prevention of electrode surface fouling and inhibition of electrochemical reactions thereon. In addition, the results show that determination of copper ions via their underpotential deposition on the modified electrode is possible. EXPERIMENTAL SECTION All electrochemical measurements were performed with a CHI600A (CH Instruments, from IJ Cambria Scientific, Burry Port, Wales, U.K.) in conjunction with a standard three-electrode cell. The working electrode was a polycrystalline gold disk (CH Instruments), with a diameter of 2 mm. The reference electrode was an Ag|AgCl|KCl (3 M) electrode, and the counter electrode was a platinum wire (both CH Instruments). All potentials are reported with respect to this reference electrode. Before undertaking a series of experiments, the gold disk working electrode was polished on alumina powder (first with 1-µm, then 0.3-µm, and finally with 0.05-µm powder) aqueous suspension and then repetitively cycled in sulfuric acid (1 M) between 0 and 1.5 V until stable voltammograms for gold oxide formation and reduction were obtained. The electrodes were modified by immersing them for 5 min in a solution of the modification reagent (5 mM) in 0.1 M (21) Shen, H.; Mark, J. E.; Seliskar, C. J.; Mark, H. B., Jr.; Heineman, W. R. J. Solid State Electrochem. 1997, 1, 148-154. (22) Shen, H.; Mark, J. E.; Seliskar, C. J.; Mark, H. B., Jr.; Heineman, W. R. J. Solid State Electrochem. 1997, 1, 241-247. (23) Arrigan, D. W. M.; Le Bihan, L.; Pickup, M. J. Analyst 1999, 124, 17971802. (24) Arrigan, D. W. M.; Iqbal, T.; Pickup, M. J. Electroanalysis 2001, 13, 751754. (25) Turyan, I.; Mandler, D. Isr. J. Chem. 1997, 37, 225-233. (26) Anzai, J.; Guo, B.; Osa, T. J. Jpn. Oil Chem. Soc. 1996, 45, 285-287.
320 Analytical Chemistry, Vol. 75, No. 2, January 15, 2003
Figure 1. Structures of the thiol reagents employed for electrode modification.
perchloric acid, at open circuit, according to the protocol employed previously.23,24 Six thiol-based modification reagents have been tested with different functional groups or chain length. All were purchased from Sigma-Aldrich. In all experiments, Cu UPD was carried out in 0.1 M sulfuric acid, as this is a standard process in electrochemistry and is well known and characterized. The UPD process was used as the first stage of the two-step stripping voltammetry procedure. We have used 0.00 V as the deposition potential, as this was determined to be a suitable potential for UPD of Cu by cyclic voltammetry experiments. This deposition potential did not destroy the goldthiol bond leading to destruction of the disorganized monolayer. Cu UPD stripping was carried out without deaeration of the electrolyte solutions in 0.1 M sulfuric acid at the chosen deposition times and copper sulfate concentrations. In all cases, deposition was carried out in unstirred solution. The experimental conditions employed for the linear sweep anodic stripping voltammetry (LSASV) experiments were as follows: deposition potential, 0.00 V; deposition time, 15 s; sweep rate, 100 mV s-1 (unless stated otherwise in the text/figure legends). RESULTS AND DISCUSSION Selection of the Modification Reagent. A clean bare gold electrode was modified as described in the Experimental Section. Six different thiols were selected for examination: 2-mercaptoethanesulfonate (MES), 3-mercapto-1-propanesulfonate (MPS), mercaptoacetic acid (MAA), 3-mercaptopropionic acid (MPA), L-cysteine (Cys), and 2-aminoethanethiol (AET). Their chemical structures are shown in Figure 1. The common features of these molecules include the presence of thiol functionality, the presence of short alkyl chains (typically two- or three-carbon chains), and the presence of bulky substituents of either negative (sulfonate groups), neutral (carboxylic acid groups), or cationic (protonated amine groups) charge in the experimental conditions used. The thiol functionality serves as the means of anchoring these molecules to the gold electrode surface via spontaneous formation of the stable gold-thiolate bonds upon immersion of the gold surface into a solution of the reagent. The short alkyl chains and the bulky substituent groups serve to prevent the formation of self-assembled monolayers by minimization of the attractive lateral interactions typical between long hydrocarbon chain SAMs and maximizing the intermolecular repulsion via the end chain groups. A choice of negative, neutral, or positive end group charge also allows the evaluation of the role of this charge on sensitivity to the cationic metal species of interest.
Figure 2. LSASV of copper (0.1 mM) in sulfuric acid (0.1 M) at different modified electrodes. The deposition potential was 0.00 V with a deposition time of 5 s and a potential scan rate of 200 mV s-1. Electrode modified with (a) MAA, (b) MES, (c) MPS, (d) MPA, (e) Cys, and (f) AET.
Identical experimental conditions were employed in the assessment of the six electrode modification reagents using stripping of the copper UPD formed at 0.00 V versus Ag|AgCl in 0.1 M sulfuric acid. By comparing the voltammograms (Figure 2), the analytical signal differs from one modified electrode to another according to the alkyl chain length of the thiol and the terminating group. Indeed, in the experimental conditions employed, 0.1 M sulfuric acid, both AET and Cys contain a net positive charge: the pKa of cystamine, the disulfide dimer of AET, is 8.65,27 which we assume is similar to that for AET, while the pKa of the amine group in Cys is 8.15 28 and that of the carboxylic acid group is 1.88.29 This charge effect manifests itself as a very poor sensitivity for copper detection, due to a repulsion of the Cu2+ ions from the modified electrode surface during both deposition and stripping steps. The stripping peak of copper at AET- and Cys-modified electrodes is very broad and shifted to more positive potentials with respect to the other reagents as well as being of much lower current. The repulsion of the Cu2+ ions by the positively charged surface layer means that only small amounts of Cu2+ ions get through for deposition at the gold surface. Similarly, the stripping of the UPD deposit requires a higher potential because the stripping product, Cu2+, must be produced into a film that is already net positively charged; thus, more energy is required for this process. In contrast, LSASV of the UPD deposits at electrodes modified with neutral (MAA and MPA) or negatively charged (MES and MPS) monolayers presents well-defined peaks with substantially greater sensitivity to copper. For molecules with the same terminating group (MAA and MPA, MES and MPS), the alkyl chain length of the thiol has an influence on the deposition and stripping of Cu2+. An increase of the chain length leads to a decrease of the stripping peak current, and to a broadening of the peak with a shift to a greater potential, which is in agreement with previously reported work on such modified electrodes7,24,30 and the fact that longer chains lead to more organized films. The alkyl chains present a barrier to both the deposition of copper and stripping of that deposit. Direct (27) Molinero, V.; Calvo, E. J. J. Electroanal. Chem. 1998, 445, 17-25. (28) Martell, A. E.; Smith, R. M. Critical Stability Constants; Vol. 1. Amino Acids; Plenum Press: New York, 1974; p 4. (29) Christian, G. D., Analytical Chemistry, 5th ed.; Wiley: New York, 1994; p 777.
Figure 3. LSASV of Cu2+ (0.1 mM) in sulfuric acid (0.1 M) at MESmodified electrodes at different sweep rates: (a) 50, (b) 100, (c) 200, (d) 300, (e) 400, and (f) 500 mV s-1. The deposition potential was 0.00 V, and the deposition time was 30 s.
comparison of the capabilities of both MES- and MAA-modified electrodes showed that the former were more stable to repeated use and yielded better precision and more consistent peak shapes. Further studies as to why this is so are currently underway. The nature of the disorganized monolayer plays a key role in the detection of copper. The MES-modified electrode tended to allow a greater quantity of copper to be deposited and stripped than the MPS-modified electrode, as can be expected due to the shorter alkyl chain length of MES. Concerning the nature of the terminal functional group in the monolayer reagent, even though MAA has a shorter length than MES, more copper is deposited/ stripped on the MES-modified electrode than at the MAA-modified one (as measured by the voltammetric peak charge). MES, in our conditions, is anionic, since the sulfonic acid residue is a strong electrolyte and there is therefore a greater attraction for Cu2+ than with MAA, which is neutral (pKa ) 4.52 on a gold electrode surface27). The MES monolayer electrode allows the detection of copper by anodic stripping of the underpotential deposit with a sharp and symmetric peak for different sweep rates (Figure 3) and was therefore chosen for further investigations. These data show that the stripping peak current varies linearly with the sweep rate and also that the stripping peak charge is independent of the voltammetric sweep rate, indicating the nondiffusional-controlled stripping of Cu2+ ions from the modified electrode surface. This well-defined peak was chosen for further study because of its stability and reproducibility. Analytical Copper Response at MES-Modified Electrodes. Now that the most effective modification reagent for the detection of copper by stripping of its underpotential deposit was selected, definition of the useable concentration range was investigated along with its capacity to prevent the adsorption of surfactants. In the Absence of Surfactant. Figure 4 shows stripping voltammograms for the detection of copper in 0.1 M sulfuric acid in the concentration range (1-80) × 10-6 M. The chosen concentration range corresponds to the concentrations of copper that can be found in some target sample types. Each concentration point was measured three times and the average value plotted. All measurements were carried out at the same MES-modified electrode. The peak currents exhibit a linear dependence on the (30) Esplandiu´, M. J.; Hagenstrom, H.; Kolb, D. M. Langmuir 2001, 17, 828838.
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Table 1. Results for the LSASVa of 80 µM Cu2+ in the Presence of Surfactants type of surfactant none Tween 20 Tween 80 Triton X-100
Figure 4. LSASV of copper in sulfuric acid (0.1 M) at MES-modified gold electrodes. Voltammogram from bottom to top are for 0, 1, 3, 5, 10, 20, 40, 60, and 80 µM Cu2+ in 0.1 M sulfuric acid. Inset: calibration curve for the detection of Cu2+ in sulfuric acid (0.1 M) at MES-modified electrodes.
concn/ppm
peak currentb/µA
recoveryc/%
0 10 100 10 100 10 100
5.72 (0.22) 5.83 (0.12) 5.62 (0.17) 5.35 (0.06) 5.64 (0.20) 5.70 (0.49) 6.00 (0.28)
102 98 94 99 99 105
a Deposition for 15 s at 0.00 V applied potential in unstirred solution. Average, n ) 3, standard deviation in parentheses. c Percentage recovery is the ratio (peak current with surfactant/peak current without surfactant) TMEx 100.
b
Table 2. Influence of Surfactants on the Sensitivity of the MES-Modified Gold Electrode to Cu2+ Ions type of surfactant Tween 20 Tween 80 Triton X-100
concn of surfactant/ppm
slopea/µA µM-1
R
0 10 100 10 100 10 100
0.0712 0.0717 0.0699 0.0666 0.0707 0.0729 0.0742
0.9995 0.9973 0.9994 0.9998 0.9996 0.9964 0.9989
a Copper concentration range 0-80 µM, n ) 8; deposition for 15 s at 0.00 V applied potential in unstirred solution.
Figure 5. LSASV of Cu2+ (0.1 mM) in sulfuric acid (0.1 M) in the presence of Triton X-100 (100 ppm): (a) at a MES-modified electrode; (b) at a bare gold electrode. Deposition time, 15 s at 0.00 V; scan rate, 100 mV s-1.
copper concentration in bulk solution, yielding the straight-line equation ip (µA) ) 0.0712[Cu2+] (µA µM-1) + 0.0813 (µA), (n ) 8, R ) 0.9995) (see inset, Figure 4). Thus, at the MES-modified electrode, the peak current is proportional to the concentration of Cu2+ in the solution with a good correlation coefficient. Thus, quantitative determination of Cu2+ is possible with such electrodes. In the Presence of Surfactant. To test the idea that disorganized monolayer films can provide effective protection of electrode surfaces from fouling by surfactants, three detergents, Tween 20, Tween 80, and Triton X-100, were selected for study of their effect on the detection of copper at the MES-modified electrode. Stripping of underpotential deposits of copper at surfactant concentrations of 10 and 100 ppm were investigated. By comparing the detection of copper in the presence of surfactants at a bare and at a MES-modified electrode, the efficiency of the disorganized monolayer in repelling the surfactants is demonstrated (Figure 5). Whereas the stripping peak at the modified electrode is similar in magnitude in the presence (Figure 5) and absence (Figure 4) of surfactant, the adsorption of surfactants on the surface of the bare electrode leads to a more difficult stripping of the small amount of copper underpotentially deposited on the electrode surface. Indeed, the stripping peak is shifted to a more positive potential; i.e., more energy is required to redissolve the copper, as well as it being lower in current and charge than that obtained at the corresponding modified electrode. Moreover, the stripping peak of copper at a bare electrode is broader than the peak at a MES-modified electrode. Similar results 322 Analytical Chemistry, Vol. 75, No. 2, January 15, 2003
were obtained in the presence of the other surfactants (Tween 20 and Tween 80), for the entire range of copper concentrations. Comparison of the peak current for different concentrations of the three surfactants shows that the detection of copper ions at a MES-modified electrode is negligibly affected by the presence of surfactants in the solution (Table 1). Quantitative Detection of Copper in the Presence of Surfactants. With the knowledge that the electrode surface is protected from the adsorption of surfactants, it is important to assess the linearity of the analytical response versus the concentration of copper present in the solution. Calibration curves for Cu2+ were constructed for the different surfactant concentrations using the MES-modified electrodes. The analytical response increases linearly with the concentration of Cu2+, regardless of the nature or the concentration of surfactant (Table 2). The slopes of the different calibration curves were of a similar magnitude, confirming that the disorganized monolayer fulfills the roles of electrode surface protection and permeability to Cu2+. The average slope (and standard deviation) for the calibration curves (Table 2) was 0.0710 ( 0.0024 µA µM-1, and the average relative standard deviation of the slopes was 3.4%. These results demonstrate that copper can be detected over a large range of concentrations using these disorganized monolayer electrodes to protect the surface from deleterious adsorption by surfactant matter present in natural samples. Copper at concentrations of 1 × 10-6 M could be detected in the presence of the surfactants using LSASV with a 15-s deposition time in unstirred solution. The operational concentration range achievable with these modi-
fied electrodes, 1-80 µM, is suitable for a range of sample applications, which is the subject of ongoing work in this laboratory. A recent study has demonstrated the detection of copper in the presence of surfactants by sono-ASV.31 Ultrasound during the deposition step increases the mass transport to the electrode and enhances the depassivation of the electrode surface leading to the detection of 0.3 µM copper in the presence of Triton X-100, thereby achieving a better limit of detection but using a 60-s deposition time. Nevertheless, LSASV at disorganized monolayermodified electrodes provides numerous advantages. Indeed, the low-cost and simple preparation of the modified electrodes combined with portable electronics and the development of microfabricated electrode systems will favor on site analysis applications. CONCLUSIONS These investigations at disorganized monolayer-modified gold electrodes have identified the most suitable modification reagent for the detection of copper by stripping of underpotential deposits. Among the short hydrocarbon chain, bulky end group thiols considered, MES provided the best analytical signal for stripping voltammetry of copper UPD, as assessed by virtue of current, (31) Hardcastle, J. L.; Hignett, G.; Melville, J. L.; Compton, R. G. Analyst 2002, 127, 518-524.
shape, and stripping charge. Moreover, at the MES-modified electrodes, the stripping peak current of copper UPD varies linearly with the concentration of copper present in the solution. Thus, determination of copper is possible at such electrodes. Additionally, the ability of the MES disorganized monolayer to protect the electrode surface from surfactant adsorption and inhibition of the electrochemical detection of copper has been proven. The three surfactants tested, Triton X-100. Tween 20, and Tween 80, were not able to penetrate the disorganized monolayer. However, the cupric ions can be underpotentially deposited on the electrode surface and therefore detected by subsequent stripping. Finally, quantitative detection of copper in the presence of the surfactants was possible at the MES-modified electrodes, achieving acceptable limits of detection for a range of possible applications. ACKNOWLEDGMENT The Royal Society of Chemistry is thanked for a studentship under the “RSC/EPSRC Analytical PhDs” scheme.
Received for review August 30, 2002. Accepted November 1, 2002. AC026093F
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