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Chemical Force Titrations of Amine- and Sulfonic Acid-Modified Poly(dimethylsiloxane) Bin Wang, Richard D. Oleschuk, and J. Hugh Horton* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received June 29, 2004. In Final Form: November 2, 2004 Chemical force titrationssmeasurements of the adhesive interaction between a pair of suitably chemically modified atomic force microscopy (AFM) tip and sample surfaces as a function of pHshave been carried out for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on both tip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surface modification was carried out using a plasma oxidation process to form silanol sites; further modification with amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriate trialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases of tip/sample combinations with the same functional group on each, surface pK1/2 values could be determined. In several “mixed” tip/sample combinations, a peak appeared in the titration curve midway between the surface pK1/2 values of the tip and sample, consistent with an ionic H-bonding model for the interactions. The amine/sulfonic acid pair showed more complex behavior; the amine-terminated tip/sulfonic acidterminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversing the tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearing at lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereas the higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effects on surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two different oxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshly prepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.
1. Introduction Recently, poly(dimethylsiloxane) (PDMS) has been widely investigated as a material for constructing microfluidic devices.1,2 Unmodified PDMS, with its hydrophobic -OSi(CH3)2O- backbone, is itself unsuitable for microfluidic applications: the hydrophobic surface makes filling micrometer-sized channels with aqueous solution difficult, while native PDMS also has minimal surface charge (and hence zeta potential), resulting in minimal electroosmotic flow (EOF) generation.3,4 Various oxidation methods have been used to form silanol groups (Si-OH) on the PDMS surface in order to increase the surface charge and hence make the surface more hydrophilic.5,6 Three different methodssexposure to an air plasma, discharge from a Tesla coil, and direct exposure to ozones have been employed in our lab, although we found that the latter has proven to be the least effective. It is known that oxidized PDMS surfaces show an aging effect,7,8 resulting in the decrease in the density of silanol groups at the surface and consequently in the extent of surface * To whom correspondence should be addressed: tel (613) 5332379/(613) 533-6704; fax (613) 533-6669; e-mail hortonj@ chem.queensu.ca. (1) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27. (2) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974. (3) Ocvirk, G.; Munroe, M.; Tang, T.; Oleschuk, R.; Westra, K.; Harrison, D. J. Electrophoresis 2000, 21, 107. (4) Ren, X.; Bachman, M.; Sims, C.; Li, G. P.; Allbritton, N. J. Chromatogr., Sect. B 2001, 762, 117. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (6) Murakami, T.; Kuroda, S.; Osawa, Z. J. Colloid Interface Sci. 1998, 202, 37. (7) Kim, J.; Chaudhury, M. K.; Owen, M. J. J. Colloid Interface Sci. 2000, 226, 231. (8) Morra, M.; Occhiello, E.; Marola, R.; Garbassi, F.; Humphrey, P.; Johnson, D. J. Colloid Interface Sci. 1990, 137, 11.
hydrophilicity. This process takes place within 24 h following oxidation and has been attributed to migration of short chain oligomers of PDMS, formed by cleavage of the polymer chain during the oxidation process, to the surface.5,9 Our previous work with PDMS has focused on chemical derivatization methods which we developed to enhance the surface stability and terminate the surface with different functional groups for specific microfluidic-based applications.10,11 We have employed an oxidation step followed by chemical derivatization with either (3-aminopropyl)triethoxysilane (APTES) or 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (SUFTMS) to attach amine or sulfonic acid functional groups to PDMS surfaces, respectively.10-12 PDMS-based microfluidic devices which had undergone this process were improved both in their ability to support electroosmotic flow and in increasing their operational lifetimes.11,12 The ability to probe interfacial forces with nanometerscale resolution is critical to develop a molecular-level understanding of a variety of phenomena such as adhesion and fracture at interfaces. In the case of the modified PDMS surfaces studied here, interactions that may play a major role in determining microfluidic device performance include van der Waals forces, hydrogen bonding, and electrostatic charge interactions. In particular, it is important to determine the surface pKa of these systems, as this will determine under what conditions microfluidic devices can be operated. Two variants of atomic force microscopy (AFM) are fast becoming important tools for (9) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013. (10) Wang, B.; Abdulali-Kanji, Z.; Dodwell, E.; Horton, J. H.; Oleschuk, R. D. Electrophoresis 2003, 24, 1442. (11) Wang, B.; Chen, L.; Abdulali-Kanji, Z.; Horton, J. H.; Oleschuk, R. D. Langmuir 2003, 19, 9792. (12) Wang, B.; Horton, J. H.; Oleschuk, R. D. Submitted to Labon-a-Chip, Sept. 2004.
10.1021/la048388p CCC: $30.25 © 2005 American Chemical Society Published on Web 01/15/2005
Chemical Force Titrations
the characterization of interfacial forces. One class directly or indirectly measures the compliance of materials. This includes such techniques as interfacial force microscopy,13,14 nanoindentation,14 and phase imaging methods.15 A second class of methods is typified by chemical force microscopy (CFM),16,17 a variation of traditional AFM in which chemical specificity is added by deliberate derivatization of an AFM probe. When tip sample interactions are measured at a single point on the surface, as opposed to imaging, this technique is more properly called chemical force spectrometry. By use of chemically functionalized tips, this method can be used to probe forces between different molecular groups, measure surface energetics on a nanometer scale, and determine pK1/2 values (the solution pH value at which half the surface sites are ionized) of the surface acid and base groups locally.10-12,16 This latter approach, on which we will focus here, has been termed chemical force titration. Previous force-titration studies have focused on systems in which tip and sample have been functionalized using thiol self-assembled monolayers on Au. This has the dual advantage of giving well-characterized surfaces, which are effectively the same substrate type, on both tip and sample. One drawback to this approach is that for some systems, particularly amine-terminated thiols, there has been evidence shown in the literature for contamination with sulfonic acid groups due to oxidation of exposed thiol groups.18 As we wish to explore the amine/sulfonic acid interaction, we have chosen to examine PDMS substrates modified with the appropriate triethoxysilane in order to remove any possibility of such contamination. Oxide-sharpened Si3N4 tips hydrolyzed with APTES or SUFTMS may also be used to form amine- and sulfonic acid-modified tips, respectively. We have previously measured the interfacial interactions between 16-thiohexadecanoic acid-terminated gold-coated AFM tips and a native PDMS surface as well as oxidized (hence silanolterminated) PDMS surfaces as a function of solution pH;10,11 other experiments have used similar tips to study metal oxide colloids.19,20 One problem with using such tips on PDMS is that tip and sample no longer represent exactly the same chemical system; indeed, in the case of the silanol/ carboxylic acid combination even the functional groups are different, albeit of similar surface pKa. To more systematically investigate the effects on chemical force titration curves in “mixed” tip-sample systems, here we report on the interactions between various combinations of sulfonic acid-, silanol-, carboxylic acid-, and amineterminated tips and samples. These functional groups are characterized by exhibiting a wide range of pKa values in the solution phase, ranging from 0.7 for benzenesulfonic acid out to about 10.5 for a primary amine.21,22 (13) Burns, A. R.; Houston, J. E.; Carpick, R. W.; Michalske, T. A. Phys. Rev. Lett. 1999, 82, 1181. (14) VanLandingham, M. R.; Villarrubia, J. S.; Guthrie, W. F.; Meyers, G. F. Macromol. Symp. 2001, 167, 15. (15) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J. Langmuir 1997, 13, 3807. (16) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (17) Finot, M. O.; McDermott, M. T. J. Am. Chem. Soc. 1997, 119, 8564. (18) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (19) Kreller, D. I.; Gibson, G.; Novak, W.; vanLoon, G. W.; Horton, J. H. Colloid Surf., A 2003, 212, 249. (20) Kreller, D. I.; Gibson, G.; vanLoon, G. W.; Horton, J. H. J. Colloid Interface Sci. 2002, 254, 205. (21) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution; Butterworth: London, 1965; Supplement, 1972. (22) Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution; Pergamon: Oxford, 1979.
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In addition to the force titration measurements, we also report briefly on the surface characterization of the variously modified PDMS substrates. AFM is used to map the surface morphology of the amine- and sulfonic acidmodified PDMS. Electroosmotic mobility measurements are used to determine the zeta potential, and hence surface charge density, of amine- and sulfonic acid-modified PDMS at pH values of 3.0 and 8.0. 2. Experimental Section 2.1. Chemical Reagents. Sylgard 184 silicone elastomer and curing agent were purchased from Dow Corning Corp. (Midland, MI). 16-Mercaptohexadecanoic acid and octadecyltrichlorosilane (OTS) were obtained from Aldrich Chemicals (Milwaukee, WI). (3-Aminopropyl)triethoxysilane (APTES) and 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane and pyridine (reagent ACS) were acquired from ACROS Organics (New Jersey, USA). Sodium hydroxide was obtained from Fisher Scientific (Fair Lawn, NJ) while hydrochloric acid was obtained from Fisher Scientific (Nepean, ON, Canada). Ethyl alcohol, 95%, was purchased from Commercial Alcohol (Brampton, ON, Canada) while toluene was acquired from Anachemia Canada (Montre´al, QC, Canada). 2.2. Fabrication of PDMS Substrates and Surface Modification. PDMS substrates were prepared using a Sylgard 184 PDMS formulation kit.10 Sylgard 184 PDMS prepolymer was mixed thoroughly in a 10:1 mass ratio of silicone elastomer to curing agent and polymerized at 65 °C for 4 h (manufacturer recommended protocol) to maintain a relatively smooth surface. The PDMS was cast against a flat glass plate. The surface in contact with the glass plate during curing was used as the PDMS substrate material. Oxidation of the PDMS substrate took place in an air-plasma chamber (Harrick Scientific Corp., Ossining, NY) for 2 min (10 MHz rf level at 70 mTorr). Efficacy of surface oxidation could be monitored by wetting of the PDMS surface. The amine-terminated substrates were produced by immersing the freshly oxidized PDMS substrates into a 20 mM (3aminopropyl)triethoxysilane (APTES) in toluene solution for approximately 4 h. The substrates swelled during the surface modification but returned to original size following drying. The PDMS substrates were then left to dry for approximately 1 h in a fume hood. All glassware used in process was also derivatized with an inert cross-linked alkylsilane layer by previously immersing the glassware in a 1-10 mM toluene solution of octadecyltrichlorosilane (OTS) for 24 h. This served to minimize the effects of competition of the silanol groups on the glass and oxidized PDMS surfaces. The sulfonated-PDMS substrates were produced by immersing the freshly oxidized PDMS substrates into a 20 mM 2-(4chlorosulfonylphenyl)ethyltrimethoxysilane (SUFTMS) in pyridine solution for approximately 4 h. Pyridine (a weak base) was required to prevent damage to the PDMS from the strongly acidic sulfonic acid groups presumably a result of acid etching. The PDMS substrates were then immersed in Milli-Q deionized water (resistivity 18.2 MΩ cm at 25 °C) for about half hour to hydrolyze the chlorosulfonate groups.23 After the modification was completed, the PDMS substrates were then dried in a stream of dry nitrogen gas. Again, all glassware used in this process was coated with an inert cross-linked alkylsilane layer. 2.3. Surface Characterization Procedures. All AFM image data shown were acquired using a PicoSPM (Molecular Imaging, Tempe, AZ), and a Nanoscope IIE controller (Digital Instruments, Santa Barbara, CA). Images were acquired in air, using intermittent contact mode, while chemical force measurements were carried out directly in aqueous solution. The cantilevers used for image acquisition were terminated with standard Si3N4 tips (40-100 nm) and had a resonance frequency of ∼100 kHz. Height and phase shift data were recorded simultaneously, although only the height mode images are shown here. Images were recorded at scan rates of 1-2 Hz using a 30 µm × 30 µm scanner. Chemical force titration data were obtained using the same apparatus.16 Force-distance curves were acquired using freshly prepared unbuffered NaOH or HCl solutions of pH ranging from (23) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446.
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Table 1. Zeta Potential of Surface-Modified PDMS Obtained from Electroosmotic Mobility Measurements pH and sample modification pH ) 3.0a unmodified plasma oxidized APTES (amine-terminated) SUFTMS (sulfonic acid-terminated) pH ) 8.0b unmodified plasma oxidized APTES (amine-terminated) SUFTMS (sulfonic acid-terminated)
electroosmotic mobility,10-12 µeo (10-4 cm2 V-1 s-1)
ζ (mV)
no flow observed 1.4 ( 0.2 (cathodic) 2.6 ( 0.2 (anodic) 2.4 ( 0.2 (cathodic)
-21 ( 3 +38 ( 3 -35 ( 3
3.5 ( 0.2 (cathodic) 4.5 ( 0.2 (cathodic) 2.9 ( 0.2 (cathodic) 5.6 ( 0.2 (cathodic)
-50 ( 3 -65 ( 3 -42 ( 3 -81 ( 3
a Ionic strength of solution, I ) 6.0 × 10-3 M consisting of [K+] ) 5.0 × 10-3 M, [H+] ) 1.0 × 10-3 M, [H PO -] ) 6.0 × 10-3 M. b I ) 2 4 1.4 × 10-2 M consisting of [K+] ) 5.0 × 10-3 M, [Na+] ) 4.3 × 10-3 M, [H2PO4-] ) 6.9 × 10-4 M, [HPO42-] ) 4.3 × 10-3 M.
2 to 12. Unbuffered solutions were chosen in order to avoid potential adsorption of buffer ions in solution on the probesubstrate interactions. Solutions were checked at the conclusion of each experiment to ensure that their pH had not changed significantly. Experiments were carried out at ionic strength conditions of 10-3 mol L-1, i.e., the only ions in solution were those introduced by pH adjustment with NaOH and HCl. Some 300-500 force-distance curves were obtained for each data point; the data were obtained at different points on the sample as the tip drifted over the surface. The same tip was used to acquire all the data points within a single force titration curve; the curves were repeated at least twice with a different tip and sample to check on the reproducibility of the data. The only variation was one of about 10-20% in the overall magnitude of the force interaction, presumably due to differences in tip radius and hence the average number of tip-sample interactions. The reported values of the adhesive interaction are an average of all the force curves obtained while the reported errors reflect the standard deviation of the data, which followed a roughly Poisson distribution. Typical examples of force histograms and of two curves showing the reproducibility of the data are shown in the results section, below. The AFM tips used in the chemical force titration measurements were silicon oxide sharpened Si3N4 tips. The tips were functionalized with carboxylate end groups by coating with 200 nm of Au followed by immersion in a 1 mM ethanol solution of 16-thiohexadecanoic acid for 24 h. Modification with amine end groups was accomplished by immersion in a 20 mM toluene solution of APTES for 24 h. Sulfonic acid end groups were attached by immersion in a 20 mM pyridine solution of 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane for 24 h, then immersing in Milli-Q deionized water for about half hour, finally drying in a stream of nitrogen gas. The force constants of the cantilevers were calibrated using the method of Hutter and Bechhoefer.24 The maximum applied force was controlled by ensuring that the maximum excursion of the z piezo into the repulsive region of the force curve was the same between runs. The nominal radius of curvature of the AFM tips, as given by the manufacturer, is
