Langmuir 2008, 24, 12097-12101
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Toposelective Electrochemical Desorption of Thiol SAMs from Neighboring Polycrystalline Gold Surfaces Michal Tencer*,†,‡ and Pierre Berini*,†,§ UniVersity of Ottawa, School of Information Technology and Engineering, 161 Louis Pasteur Street, Ottawa, Ontario K1N 6N5, Canada, MST Consulting, Ottawa, Ontario, Canada, and Spectalis Corporation, P.O. Box 72029, Kanata North RPO, Ottawa, Ontario K2K 2P4, Canada ReceiVed May 9, 2008. ReVised Manuscript ReceiVed July 1, 2008 We describe a method for the selective desorption of thiol self-assembled monolayers from gold surfaces having micrometer-scale separations on a substrate. In an electrolyte solution, the electrical resistance between the adjacent areas can be much lower than the resistance between a surface and the counter electrode. Also, both reductive and oxidative thiol desorption may occur. Therefore, the potentials of the surfaces must be independently controlled with a multichannel potentiostat and operating windows for a given thiol/electrolyte system must be established. In this study operating windows were established for 1-dodecanethiol-based SAMs in phosphate buffer, phosphate-buffered saline, and sodium hydroxide solution, and selective SAM removal was successfully performed in a four-electrode configuration.
Introduction In many areas of microelectronics and photonics, especially for chemo- and biosensing applications, there is a need for chemical differentiation of small, closely spaced (a few micrometers in size) flat areas or structural features of various shapes (patterns) whose dimensions and separation may be in the micrometer range. Taking into account that, for example, a halfspherical sessile droplet with 0.1 µL volume (the smallest achievable with an Eppendorf pipet) has a footprint diameter of 726 µm, it is easy to appreciate the difficulties of performing chemistry selectively on some of the features while avoiding the contamination of others, especially considering that the substrates typically employed (Si, SiO2, Si3N4, Al2O3, ...) have high-energy surfaces (often enhanced by commonly employed wet and dry surface treatment, e.g., using piranha solution, UV-ozone, and/ or plasma cleaning) and so both organic and aqueous solutions tend to spread uncontrollably on them. Furthermore, any such process must be followed by rinsing off excess reagent, again without contaminating adjacent features. Such contamination may occur not only by direct contact with effluent but also through diffusion, whose characteristic time constant for micrometerscale separation between features ranges from milliseconds for small molecules such as alkanethiols to under a second for biopolymers (e.g., albumins1) which is much shorter than the processing time. The spreading and cross-contamination may still be issues with techniques such as microspotting or ink jetting,2-6 microcontact printing,7 and dip-pen nanolithography.8 * E-mail:
[email protected],
[email protected]. † University of Ottawa. ‡ MST Consulting. § Spectalis Corporation.
(1) Tencer, M.; Charbonneau, R.; Berini, P. Lab Chip 2007, 7, 483. (2) Schena, M.; Heller, R. A.; Theriault, T. P.; Konrad, K.; Lachenmeier, E.; Davis, R. W. Trends Biotechnol. 1998, 16, 301–306. (3) Venkatasubbarao, S. Trends Biotechnol. 2004, 22, 630–637. (4) Chen, S.; Luo, Y. Microarray Fabrication Techniques and Apparatus. U.S. Patent 6,953,551, 2005. (5) Brennan, T. M. Method and Apparatus for Conducting an Array of Chemical Reactions on a Support Surface. U.S. Patent 5,474,796, 1995. (6) Brown, P. O.; Shalon, T. D. Methods for Fabricating Microarrays of Biological Samples. U.S. Patent 5,807,522, 1995. (7) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.
To overcome these difficulties, we have recently devised a method of confining and depositing small volumes of solution onto micrometer-scale features utilizing the high surface energy of an appropriately shaped “guide” positioned above the feature.1 This technique was applied to deposit bovine serum albumin selectively onto one arm of a Au plasmonic Mach-Zehnder interferometer (MZI)1,9 The technique, however, is not easily scalable to multiple structures in a wafer-scale process. Closely spaced Au features such as MZI arms are of ongoing interest for use as optical affinity biosensors operating with longrange surface plasmon-polariton waves.10,11 For such application, the features should be chemically differentiated to render one of them prone to the specific adsorption of target analyte (e.g., a protein) whereas the other is protected from any (specific and nonspecific) adsorption. Both of these functions can be achieved with the help of appropriate thiol-based self-assembled monolayers (SAMs) formed on the Au surfaces. Thus, whereas SAMs based on unmodified alkanethiols CH3(CH2)nSH promote the adsorption of proteins, SAMs formed with polyethylene glycol (PEG)-terminated thiols HO(CH2O)m(CH2)nSH prevent this process.12,13 Proteins also tend to adsorb on bare Au,9 but the process is nonspecific, not well defined kinetically or thermodynamically,14 and usually leads to denaturation.9,15 Thus, the aim is to have one feature coated with an adsorption-resistant SAM and the other neighboring feature coated with an adsorption-promoting SAM. The latter can attain high specificity through antigen-antibody interaction where the antibody is immobilized on the SAM.16,17 The difference in the quantity of material adsorbed onto the features results in a difference in the effective (8) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661–663. (9) Tencer, M.; Charbonneau, R.; Lahoud, N.; Berini, P. Appl. Surf. Sci. 2007, 253, 9209. (10) Berini, P. Phys. ReV. B 2000, 61, 10484. (11) Berini, P.; Charbonneau, R.; Lahoud, N. Nano Lett. 2007, 7, 1376. (12) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (13) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (14) Ramsden, J. J. Chem. Soc. ReV. 1995, 24, 73. (15) Yang, M.; Chung, F. L.; Thompson, M. Anal. Chem. 1983, 65, 3713. (16) Love, J.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (17) Chen, S.; Liu, L.; Zhou, J.; Jiang, S. Langmuir 2003, 19, 2859.
10.1021/la801443y CCC: $40.75 2008 American Chemical Society Published on Web 09/30/2008
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Figure 1. Configuration for electrochemical desorption using (a) singleelectrode control, (b) opposite bias on electrodes, and (c) a four-electrode configuration. (A) Feature to undergo desorption, (B) feature to retain its SAM, (CE) counter electrode, (RE) reference electrode, (BP) bipotentiostats RAB, RBC, and RAC are the resistances between the features and the counter electrode through the electrolyte.
refractive index of the surface plasmon wave that can be picked up by interferometry. Electrochemical desorption of thiol SAMs from Au18-30 provides another, potentially more convenient and scalable method of selective chemical differentiation of Au microfeatures in general. Its mechanism is reasonably well known,18-21,23,26,28,30 and the technique has been used to control SAM-mediated protein adsorption and desorption on Au.22,24,25 However, the selective desorption of a SAM from microfeatures by applying a negative voltage thereon but not to others22 cannot be directly adopted because of the following two main challenges, also highlighted in Figure 1. Relative Voltage Drop. The distance between feature A, from which the monolayer is to be removed, and feature B, which is to retain its monolayer, is typically tens of micrometers and so is much shorter than the distance between both of these features and counter electrode C, which is typically centimeters, as sketched in Figure 1a. All three are immersed in a conductive medium (electrolyte), and the resistance between them thus depends on their separation. In this system, RAC = RBC . RAB. Therefore, both features have practically the same potential (VA = VB), so if the SAM is desorbed from A then it will also be desorbed from B. Reductive and Oxidative Desorption. In view of the above, one may assume that whereas A is negatively biased B should be positively biased, ensuring that the potential of B does not follow that of A. This, however, leads to another problem. SAMs may undergo both reductive desorption (AuSR + e- ) Au0 + RS-) and oxidative desorption (AuSR + xH2O ) Au0 + RSOx + ne-, with x and n depending on the operating conditions23). When applying a negative voltage to one Au feature and a positive (18) Walczak, M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (19) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (20) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570. (21) Kakiuchi, T.; Usui, H.; Hobara, D.; Yamamoto, M. Langmuir 2002, 18, 5231. (22) Jiang, X.; Ferrigno, R.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 2366. (23) Sheridan, A. K.; Ngamukot, P.; Bartlett, P. N.; Wilkinson, J. S. Sens. Actuators, B 2006, 117, 253. (24) Wilhelm, T.; Wittstock, G. Electrochim. Acta 2001, 47, 275. (25) Balasubramanian, S.; Revzin, A.; Simonian, A. Electroanalysis 2006, 18, 1885. (26) Lee, L. Y. S.; Lennox, R. B. Langmuir 2007, 23, 292. (27) Shepherd, J. L.; Kell, A.; Chung, E.; Sinclar, C. W.; Workentin, M. S.; Bizzotto, D. J. Am. Chem. Soc. 2004, 126, 8329. (28) Williams, J. A.; Gorman, C. B. J. Phys. Chem. C 2007, 111, 12804. (29) Nelson, J. B.; Schwartz, D. T. Langmuir 2007, 23, 9661. (30) Imabayashi, S. I.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502.
Figure 2. Contact angles of Au surfaces coated with 1-dodecanethiol SAM as a function of potential applied for 300 s in 0.1 M phosphate buffer. (The solid curve interpolates the measured data points.)
voltage to another (where the features are each others’ counter electrodes), as shown in Figure 1b, we do not know what the actual potentials of the individual features are. Thus, there is a distinct possibility that such an uncontrolled procedure may lead again to the dislodging of thiol from both features, although by different mechanisms, or to Au damage. Therefore, the potentials applied to both features should be independently controlled to ensure that the desorption of thiol will occur selectively, with the potentials being referenced to a standard electrode. This can be achieved with a bipotentiostat or a multichannel potentiostat, as shown in Figure 1c. Before this can be done, one must establish the values of the potential below which and above which the thiol desorbs and the potential range where the SAM is stable as well as where the Au integrity is not compromised. In other words, the operating window for the technique must be established first. Operating windows will be different for different thiols and electrolytes. In some cases voltammetry may help guide this search provided desorption peaks are not obscured by other redox processes. However, the ultimate result must be based on the properties of the resulting surface. This article describes such a study performed with 1-dodedecanetiol SAMs in three electrolytes: phosphate buffer, phosphate-buffered saline, and sodium hydroxide. The two buffered electrolytes are benign, pH-neutral solutions that can also be used in the presence of proteins and other biological materials, and the third one can be related to available cycling voltammetry data19-21 thanks to the nonoverlap of current peaks with those due to solvent reduction. The surface changes were followed using water contact angle measurements utilizing the hydrophobic properties of SAMs formed with the thiol used. The operating windows were established on separate ∼0.5 cm2 Au-coated dies, and then the process was applied to a die having Au surfaces separated by a gap of width comparable to that of the proposed MZI biosensor design.
Experimental Section Materials. 1-Dodecanethiol CH3(CH2)11SH (g98%, Arkema Inc.), phosphate buffer (PB) solution (0.1 M, pH 7.5), and phosphatebuffered saline (PBS) solution (145 mM NaCl in 150 mM phosphate buffer, pH 7.2) were purchased from Sigma-Aldrich Canada Ltd. and used as received. Deionized water (0.22 µm, 10 MΩ cm, Protocol
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Figure 3. Stability of 1-dodecanethiol SAM on polycrystalline Au as a function of potential (vs the Ag/AgCl electrode) in 0.1 M phosphate buffer (pH 7.5), in phosphate-buffered saline (pH 7.2, [NaCl] ) 0.145 M), and in 0.5 M NaOH.
Figure 4. Cyclic voltammetry between 0 and -0.4 V (vs Ag/AgCl) in phosphate buffer (immersed area ∼0.5 cm2) of (a) 1 dodecanethiol SAM and (b) the same sample after electrolysis at -2.0 V in situ.
Figure 5. (a) Groove (45 ( 10 µm) inscribed into the Au layer on a Cytop-coated Si die. (b) Optical fiber (125 µm diameter) shown for comparison at the same microscope magnification.
brand) was received from Fisher Scientific and 2-propanol (semiconductor grade, Puranal) was obtained from Riedel-de Hae¨n. Sodium (31) Cytop was used for its insulating properties (and for other reasons unrelated to this study).
hydroxide (1.0 M, HPLC grade, Fluka) was diluted with DI water to 0.5 M concentration. Two types of Au-coated substrates were used: (a) vacuumevaporated 30-nm-thick Au on 4.5 nm Cr on p-type Si wafers and (b) vacuum-evaporated 35-nm-thick Au on 238-nm-thick Cytop fluoropolymer31 (Asahi) on a p-type Si wafer. The Au films were polycrystalline. The wafers were cleaved into dies of ∼0.5 to 1 cm2 for single potential experiments and dies of ∼1 to 2 cm2 for twopotential experiments. Sample Preparation. The Au surfaces were degreased with 2-propanol, rinsed with deionized water, and placed in a Novascan PSD-UV UV-ozone cleaner (5 min of UV irradiation followed by 20 min of ozone action). The dies were then immersed in a 2 mM solution of 1-dodecanethiol in isopropanol for 16 h, rinsed thoroughly with isopropanol, and allowed to dry. Electrochemistry. Electrolysis and voltammetry experiments were performed with a Pine Research AFCBP1 bipotentiostat using a three-electrode configuration for single-potential experiments (one reference, one working electrode, and one counter electrode) and a four-electrode configuration for two-potential experiments (one reference, two working electrodes, and one counter electrode). The counter electrode was a platinum wire. For experiments in PBS, a single-junction Ag/AgCl electrode was used as the reference, and for experiments in PB and in NaOH, a double-junction Ag/AgCl reference electrode was used. All potentials are reported with respect to these references. The dies were mounted using alligator clips (contact resistance
