chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane)

Chemical Force Titrations of Amine- and SulfonicAcid-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 chemicallymodified atomic force microscopy (AFM) tip and sample surfaces as a function of pHshave been carriedout for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on bothtip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surfacemodification was carried out using a plasma oxidation process to form silanol sites; further modificationwith amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriatetrialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases oftip/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 thesurface 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 acid-terminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversingthe tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearingat lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereasthe higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effectson surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two differentoxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshlyprepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.

1. Introduction

Recently, poly(dimethylsiloxane) (PDMS) has beenwidely investigated as a material for constructing mi-crofluidic devices.1,2 Unmodified PDMS, with its hydro-phobic -OSi(CH3)2O- backbone, is itself unsuitable formicrofluidic applications: the hydrophobic surface makesfilling micrometer-sized channels with aqueous solutiondifficult, while native PDMS also has minimal surfacecharge (and hence zeta potential), resulting in minimalelectroosmotic flow (EOF) generation.3,4 Various oxidationmethods have been used to form silanol groups (Si-OH)on the PDMS surface in order to increase the surfacecharge 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 ozoneshave been employed in our lab, although we found thatthe latter has proven to be the least effective. It is knownthat oxidized PDMS surfaces show an aging effect,7,8

resulting in the decrease in the density of silanol groupsat the surface and consequently in the extent of surface

hydrophilicity. This process takes place within 24 hfollowing oxidation and has been attributed to migrationof short chain oligomers of PDMS, formed by cleavage ofthe polymer chain during the oxidation process, to thesurface.5,9

Our previous work with PDMS has focused on chemicalderivatization methods which we developed to enhancethe surface stability and terminate the surface withdifferent functional groups for specific microfluidic-basedapplications.10,11 We have employed an oxidation stepfollowed by chemical derivatization with either (3-ami-nopropyl)triethoxysilane (APTES) or 2-(4-chlorosulfo-nylphenyl)ethyltrimethoxysilane (SUFTMS) to attachamine or sulfonic acid functional groups to PDMS surfaces,respectively.10-12 PDMS-based microfluidic devices whichhad undergone this process were improved both in theirability to support electroosmotic flow and in increasingtheir operational lifetimes.11,12

The ability to probe interfacial forces with nanometer-scale resolution is critical to develop a molecular-levelunderstanding of a variety of phenomena such as adhesionand fracture at interfaces. In the case of the modifiedPDMS surfaces studied here, interactions that may playa major role in determining microfluidic device perfor-mance include van der Waals forces, hydrogen bonding,and electrostatic charge interactions. In particular, it isimportant to determine the surface pKa of these systems,as this will determine under what conditions microfluidicdevices can be operated. Two variants of atomic forcemicroscopy (AFM) are fast becoming important tools for

* To whom correspondence should be addressed: tel (613) 533-2379/(613) 533-6704; fax (613) 533-6669; e-mail [email protected].

(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.

(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 Lab-

on-a-Chip, Sept. 2004.

1290 Langmuir 2005, 21, 1290-1298

10.1021/la048388p CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 01/15/2005

the characterization of interfacial forces. One class directlyor indirectly measures the compliance of materials. Thisincludes such techniques as interfacial force micros-copy,13,14 nanoindentation,14 and phase imaging methods.15

A second class of methods is typified by chemical forcemicroscopy (CFM),16,17 a variation of traditional AFM inwhich chemical specificity is added by deliberate deriva-tization of an AFM probe. When tip sample interactionsare measured at a single point on the surface, as opposedto imaging, this technique is more properly called chemicalforce spectrometry. By use of chemically functionalizedtips, this method can be used to probe forces betweendifferent molecular groups, measure surface energeticson a nanometer scale, and determine pK1/2 values (thesolution pH value at which half the surface sites areionized) of the surface acid and base groups locally.10-12,16

This latter approach, on which we will focus here, hasbeen termed chemical force titration.

Previous force-titration studies have focused on sys-tems in which tip and sample have been functionalizedusing thiol self-assembled monolayers on Au. This hasthe dual advantage of giving well-characterized surfaces,which are effectively the same substrate type, on both tipand sample. One drawback to this approach is that forsome systems, particularly amine-terminated thiols, therehas been evidence shown in the literature for contamina-tion with sulfonic acid groups due to oxidation of exposedthiol groups.18 As we wish to explore the amine/sulfonicacid interaction, we have chosen to examine PDMSsubstrates modified with the appropriate triethoxysilanein order to remove any possibility of such contamination.Oxide-sharpened Si3N4 tips hydrolyzed with APTES orSUFTMS may also be used to form amine- and sulfonicacid-modified tips, respectively. We have previouslymeasured the interfacial interactions between 16-thio-hexadecanoic acid-terminated gold-coated AFM tips anda native PDMS surface as well as oxidized (hence silanol-terminated) PDMS surfaces as a function of solutionpH;10,11 other experiments have used similar tips to studymetal oxide colloids.19,20 One problem with using such tipson PDMS is that tip and sample no longer represent exactlythe same chemical system; indeed, in the case of the silanol/carboxylic acid combination even the functional groupsare different, albeit of similar surface pKa. To moresystematically investigate the effects on chemical forcetitration curves in “mixed” tip-sample systems, here wereport on the interactions between various combinationsof sulfonic acid-, silanol-, carboxylic acid-, and amine-terminated tips and samples. These functional groups arecharacterized by exhibiting a wide range of pKa values inthe solution phase, ranging from 0.7 for benzenesulfonicacid out to about 10.5 for a primary amine.21,22

In addition to the force titration measurements, we alsoreport briefly on the surface characterization of thevariously modified PDMS substrates. AFM is used to mapthe surface morphology of the amine- and sulfonic acid-modified PDMS. Electroosmotic mobility measurementsare used to determine the zeta potential, and hence surfacecharge density, of amine- and sulfonic acid-modified PDMSat pH values of 3.0 and 8.0.

2. Experimental Section2.1. 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-chlorosulfo-nylphenyl)ethyltrimethoxysilane and pyridine (reagent ACS)were acquired from ACROS Organics (New Jersey, USA). Sodiumhydroxide was obtained from Fisher Scientific (Fair Lawn, NJ)while hydrochloric acid was obtained from Fisher Scientific(Nepean, ON, Canada). Ethyl alcohol, 95%, was purchased fromCommercial Alcohol (Brampton, ON, Canada) while toluene wasacquired from Anachemia Canada (Montreal, QC, Canada).

2.2. Fabrication of PDMS Substrates and Surface Modi-fication. PDMS substrates were prepared using a Sylgard 184PDMS formulation kit.10 Sylgard 184 PDMS prepolymer wasmixed thoroughly in a 10:1 mass ratio of silicone elastomer tocuring agent and polymerized at 65 °C for 4 h (manufacturerrecommended protocol) to maintain a relatively smooth surface.The PDMS was cast against a flat glass plate. The surface incontact with the glass plate during curing was used as the PDMSsubstrate material. Oxidation of the PDMS substrate took placein an air-plasma chamber (Harrick Scientific Corp., Ossining,NY) for 2 min (10 MHz rf level at 70 mTorr). Efficacy of surfaceoxidation could be monitored by wetting of the PDMS surface.

The amine-terminated substrates were produced by immersingthe freshly oxidized PDMS substrates into a 20 mM (3-aminopropyl)triethoxysilane (APTES) in toluene solution forapproximately 4 h. The substrates swelled during the surfacemodification but returned to original size following drying. ThePDMS substrates were then left to dry for approximately 1 h ina fume hood. All glassware used in process was also derivatizedwith an inert cross-linked alkylsilane layer by previouslyimmersing the glassware in a 1-10 mM toluene solution ofoctadecyltrichlorosilane (OTS) for 24 h. This served to minimizethe effects of competition of the silanol groups on the glass andoxidized PDMS surfaces.

The sulfonated-PDMS substrates were produced by immersingthe freshly oxidized PDMS substrates into a 20 mM 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (SUFTMS) in py-ridine solution for approximately 4 h. Pyridine (a weak base)was required to prevent damage to the PDMS from the stronglyacidic sulfonic acid groups presumably a result of acid etching.The PDMS substrates were then immersed in Milli-Q deionizedwater (resistivity 18.2 MΩ cm at 25 °C) for about half hour tohydrolyze the chlorosulfonate groups.23 After the modificationwas completed, the PDMS substrates were then dried in a streamof dry nitrogen gas. Again, all glassware used in this process wascoated with an inert cross-linked alkylsilane layer.

2.3. Surface Characterization Procedures. All AFM imagedata 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, usingintermittent contact mode, while chemical force measurementswere carried out directly in aqueous solution. The cantileversused for image acquisition were terminated with standard Si3N4tips (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. Imageswere recorded at scan rates of 1-2 Hz using a 30 µm × 30 µmscanner.

Chemical force titration data were obtained using the sameapparatus.16 Force-distance curves were acquired using freshlyprepared unbuffered NaOH or HCl solutions of pH ranging from

(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. Langmuir1997, 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. ColloidInterface Sci. 2002, 254, 205.

(21) Perrin, D. D. Dissociation Constants of Organic Bases in AqueousSolution; Butterworth: London, 1965; Supplement, 1972.

(22) Serjeant, E. P.; Dempsey, B. Ionization Constants of OrganicAcids in Aqueous Solution; Pergamon: Oxford, 1979. (23) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446.

Chemical Force Titrations Langmuir, Vol. 21, No. 4, 2005 1291

2 to 12. Unbuffered solutions were chosen in order to avoidpotential adsorption of buffer ions in solution on the probe-substrate interactions. Solutions were checked at the conclusionof each experiment to ensure that their pH had not changedsignificantly. Experiments were carried out at ionic strengthconditions of 10-3 mol L-1, i.e., the only ions in solution werethose introduced by pH adjustment with NaOH and HCl. Some300-500 force-distance curves were obtained for each data point;the data were obtained at different points on the sample as thetip drifted over the surface. The same tip was used to acquire allthe data points within a single force titration curve; the curveswere repeated at least twice with a different tip and sample tocheck on the reproducibility of the data. The only variation wasone of about 10-20% in the overall magnitude of the forceinteraction, presumably due to differences in tip radius and hencethe average number of tip-sample interactions. The reportedvalues of the adhesive interaction are an average of all the forcecurves obtained while the reported errors reflect the standarddeviation of the data, which followed a roughly Poisson distribu-tion. Typical examples of force histograms and of two curvesshowing the reproducibility of the data are shown in the resultssection, below. The AFM tips used in the chemical force titrationmeasurements were silicon oxide sharpened Si3N4 tips. The tipswere functionalized with carboxylate end groups by coating with200 nm of Au followed by immersion in a 1 mM ethanol solutionof 16-thiohexadecanoic acid for 24 h. Modification with amineend groups was accomplished by immersion in a 20 mM toluenesolution of APTES for 24 h. Sulfonic acid end groups were attachedby immersion in a 20 mM pyridine solution of 2-(4-chlorosul-fonylphenyl)ethyltrimethoxysilane for 24 h, then immersing inMilli-Q deionized water for about half hour, finally drying in astream of nitrogen gas. The force constants of the cantileverswere calibrated using the method of Hutter and Bechhoefer.24

The maximum applied force was controlled by ensuring that themaximum excursion of the z piezo into the repulsive region ofthe force curve was the same between runs. The nominal radiusof curvature of the AFM tips, as given by the manufacturer, is<30 nm. Assuming a typical H-bond strength of 16 kJ/mol, anda distance of 0.1 nm to move the tip out of the potential well, thiswould result in a single bond force interaction of about 0.25 nN,which would correspond to about 12 tip-sample interactions, onaverage, at the maximum adhesive forces observed in the resultssection below. This would be consistent with such a radius ofcurvature.

Electroosmotic mobility (µeo) experiments were carried outusing a “twin T” configured microfluidic chip constructed fromPDMS. Surface modification of the channels within this chipwere carried out in exactly the same fashion as the PDMS samplesused in the force microscopy experiments. Full details on theconstructionandconfigurationof these chips have beenpreviouslypublished.10-12 The measurement of µeo in the microchannelswas performed using current monitoring.3 The microchannelswere first filled with a low ionic strength buffer (I ) (6.0-12.0)× 10-3 M, depending on the pH conditions studied; the individualbuffer compositions are found in Table 1). The buffer reservoirwas filled with a higher ionic strength buffer. Electrodes werethen placed in buffer reservoir and buffer waste reservoir of the

microfluidic chip, and the current was monitored as a functionof time. The µeo can then be determined from the current increaseas the microchannel fills with the higher ionic strength buffer.The current was monitored under either cathodic or anodicconditions, depending on the sign of the zeta potential on thePDMS surface.

3. ResultsWe have previously reported the electroosmotic mobility

(µeo) of native and surface-modified PDMS at various pH.Measurements of µeo at pH values of 3.0 and 8.0 are shownin Table 1 for native, plasma oxidized, and amine- andsulfonic acid-terminated PDMS. From these values, wemay directly obtain the zeta potential, ú, on the surfaceof the PDMS, through the relationship

where η is the viscosity of the solution and εo and εr havethe usual meanings. In particular, we might note thatunder pH ) 3 conditions, the amine surface is positivelycharged, while the oxidized and sulfonic acid-terminatedsurfaces are negatively charged. At pH ) 3.0, theunmodified PDMS surface shows no evidence of elec-troosmotic flow, suggesting that the magnitude of the zetapotential is less than 5 mV on this surface. At a pH of 8.0,all four surfaces exhibit negative zeta potentials. The zetapotential of the sulfonic acid and oxidized PDMS surfacesare both considerably larger than those of either theunmodified or amine-terminated surfaces.

Chemical force titration measurements of the adhesiveforce between tip and sample as a function of pH are shownin Figures 1-4. Titration of a silicon oxide sharpened AFMtip hydrolyzed with SUFTMS against an air plasmaoxidized PDMS substrate similarly hydrolyzed usingSUFTMS are shown in Figure 1. This experiment iseffectively a chemical force titration of sulfonic acid onitself. A maximum adhesive interaction of about 2.5 nNat a pH of 3.0 is observed. A second, equivalent experiment,acquired using a different tip and sample is also shownsuperimposed on this curve to show the reproducibility ofour data. Figure 1 also shows a similar experiment carriedout with both tip and substrate derivatized using APTESto form amine-terminated surfaces. In this case, themaximum of 2.0 nN is shifted to a pH of 6.0. Chemicalforce titration measurements were also carried out usinga combination of Au-coated AFM tip and mica substrateboth modified using 16-thiohexadecanoic acid to form acarboxylic acid-terminated surface. Such experimentshave been reported widely in the literature,16,25 so thecurve will not be shown here. Our results were consistent

(24) Hutter, J. L.; Bechhoefer, J. Rev. Sci. Instrum. 1993, 64, 1868.(25) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am.

Chem. Soc. 1997, 119, 2006.

Table 1. Zeta Potential of Surface-Modified PDMS Obtained from Electroosmotic Mobility Measurements

pH and sample modificationelectroosmotic mobility,10-12

µeo (10-4 cm2 V-1 s-1) ú (mV)

pH ) 3.0a

unmodified no flow observedplasma oxidized 1.4 ( 0.2 (cathodic) -21 ( 3APTES (amine-terminated) 2.6 ( 0.2 (anodic) +38 ( 3SUFTMS (sulfonic acid-terminated) 2.4 ( 0.2 (cathodic) -35 ( 3

pH ) 8.0b

unmodified 3.5 ( 0.2 (cathodic) -50 ( 3plasma oxidized 4.5 ( 0.2 (cathodic) -65 ( 3APTES (amine-terminated) 2.9 ( 0.2 (cathodic) -42 ( 3SUFTMS (sulfonic acid-terminated) 5.6 ( 0.2 (cathodic) -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, [H2PO4-] ) 6.0 × 10-3 M. b I )

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, [HPO4

2-] ) 4.3 × 10-3 M.

µeo ) εoεrú/η (1)

1292 Langmuir, Vol. 21, No. 4, 2005 Wang et al.

with previous experiments carried out at intermediateionic strength values (10-3-10-4 M) with a 5 nN highpeak appearing at pH 5.7. We should also note thatprevious experiments using unmodified Si3N4 on silicasurfaces show peak maxima near a pH of 5.5,26 consider-ably higher than the maximum observed with the SUFT-MS-coated tip, thus demonstrating that the tip has indeedbeen chemically modified.

The remaining force titration experiments were carriedout using dissimilar tip-sample pairs. Figure 2a showsthe adhesive interaction between an amine-terminatedtip and sulfonic acid-modified PDMS substrate. Weobserve a maximum peak centered at pH 4 with themagnitude of 1 nN and a smaller peak at pH 8 with themagnitude of about 0.5 nN. The inset to Figure 2a alsoshows a typical adhesion force histogram, in this caseobtained for this system at a pH of 2.1. The histogramshows the number of observations made of a particularrange of force values. The distribution shows a roughlyPoisson distribution shape, which would be expected ifduring each tip/sample approach a random number ofsingle bond interactions were taking place. The “inverse”experimentssulfonic tip on amine modified substratesisshown in Figure 2b. Two peaks of similar magnitude areseen as before, but both are shifted upward by about 1 pHunit as compared with Figure 2a.

Figure 3a shows the result of the chemical forcemeasurements between carboxylic acid-terminated gold-coated Si3N4 tip and sulfonic acid-modified PDMS sub-strate. We observed a maximum adhesive interaction of2 nN, at a pH of 4.0. Figure 3b shows the experiment withtip and sample reversed. A similar result is observed. Thefinal tip/sample pairing to be examined was a carboxylicacid/amine pair. The results from this experiment areshown in Figure 4. For both tip/sample configurations,the overall force-titration profiles appear similar, with apeak at 5.0 pH units with a maximum of 2 nN. In bothcases, the curves show a tailing to the high pH end,although this is not as distinct as in the case of the amine/sulfonic acid combinations seen above.

Further surface characterization of the modified PDMSwas carried out using AFM. In Figure 5a, we show an

AFM image of the PDMS surface that has undergoneoxidation using the air plasma method. These surfaceshave been studied using chemical force titration andelectroosmotic mobility measurements, which showedevidence for the presence of silanol groups on the surface.In the case of the work reported here, the main interestis that these surfaces were the starting point for formationof amine or sulfonic acid-terminated surfaces via graftingof the corresponding silane derivative to the oxidizedsubstrate. The image was acquired within 1 h of theoxidation (i.e., freshly prepared surface). The most ap-parent features are the series of approximately parallelwavy rows present on the surface. Panels b and c of Figure5 show AFM images of amine-terminated and sulfonicacid-terminated PDMS surfaces, respectively. In bothcases, the images were acquired 24 h following samplepreparation. Images obtained immediately followingsamplepreparationwerealsoacquiredand lookessentiallyidentical. Both modified surfaces show some similarfeatures, particularly large wavy features that are muchhigher and more randomly oriented as compared with theoxidized sample.

4. DiscussionA key feature of the surface chemistry of hydrogen

bonding is the difference in the acid-base behavior offunctional groups incorporated on the surface comparedwith that of the same functional group on a molecule insolution. One important point to note is that the solution(26) Marti, A.; Hahner, G.; Spencer, N. D. Langmuir 1995, 11, 4632.

Figure 1. Chemical force titration curves showing the tip-sample adhesive force as a function of pH between (i) an AFMtip and PDMS substrate both terminated with sulfonic acidgroups via hydrolysis of SUFTMS (a second curve is shownsuperimposed to indicate the reproducibility of our data) and(ii) an AFM tip and PDMS substrate both terminated withamine groups via hydrolysis of APTES. The error bars representthe standard deviation in the adhesion force as measured fromthe average of 300-500 force-distance curves. In both cases,the line segments joining the points are meant as a guide to theeye.

Figure 2. (a) Chemical force titration curves showing the tip-sample adhesive force as a function of pH between an amine-terminated AFM tip and a sulfonic acid-terminated PDMSsubstrate. The inset shows a typical adhesion force histogram,in this case obtained for the tip-sample system at a pH of 2.1.(b) The inverse of the experiment shown in part a. Chemicalforce titration curves showing the tip-sample adhesive forceas a function of pH between a sulfonic acid-terminated AFMtip and amine-terminated PDMS substrate. The error barsrepresent the standard deviation in the adhesion force asmeasured from the average of 300-500 force-distance curves.In both parts of the figure, the line segments joining the pointsare meant as a guide to the eye.


pH (measured here) and the surface pH are not the same:the latter is shifted relative to the bulk value by thepresence of a surface potential.27,28 Although this shift isgenerally small, less than 0.5 pH units, it does mean thatthe solution pH at which exactly half the surface groupsare ionized, pK1/2, does not represent the surface pKa. Inthe ensuing discussion, we will use our force titrationresults to measure the value of pK1/2 for various functionalgroups, with the understanding that the pK1/2 valuesdetermined may be considered an estimate of the surfacepKa. We should also note that the surface pKa itself shouldbe expected to shift, perhaps dramatically, relative to thesolution phase. In-plane H-bonding between moleculeson the surface and the limited ability of the solvent toshield charged species at the interface as compared tosolution should both increase the difficulty in ionizingsurface bound species. This should lead to an upward shiftin the surface pKa value relative to the solution phase foracidic groups and a downward shift for basic groups.

The published solution pKa value for benzenesulfonicacid is 0.7, and that for propylamine is 10.60.21,22 The zetapotential measurements are consistent with the expectedsurface modification of the PDMS. Under pH 3.0 condi-tions, we should be well below the amine pK1/2, regardless

of any shift due to surface effect and indeed the APTES-modified PDMS is the only case where a positive zetapotential is observed, consistent with a protonated surface.The sulfonic SUFTMS-modified surface has a negativezeta potential which is similar in magnitude to that of thepositively charged APTES-modified surface under pH )3 conditions. That of the oxidized surface is somewhatsmaller in magnitude. Using force titration experiments,we previously determined a surface pKa for oxidized PDMSof about 4.010 which suggests that under pH ) 3.0conditions the oxidized surface is only partially depro-tonated. The zeta potential is a direct measure of thecharge density at the surface of shear. It therefore containscontributions not only from any ionized groups on thesurface but also from any electrostatically fixed counter-ions within the Stern layer. As all three modified PDMSsurfaces have a similar morphology and a similar siloxanebackbone, we might expect any adsorption of counterionsfrom solution onto these surfaces to be similar in extent.Thus, the pH 3.0 results indicate that the SUFTMS andAPTES-modified surfaces contain relatively similar con-centrations of ionized sulfonic acid and amine groups,while the extent of ionization on the oxidized surface issomewhat less under these conditions.

The higher pH 8.0 results were carried out undersomewhat higher ionic strength conditions, in the presenceof the doubly charged HPO4

2- anion. This might be

(27) Lide, D. R. CRC Handbook of Chemistry and Physics, 72nd ed.;Boca Raton, FL, 1991.

(28) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1.

Figure 3. (a) Chemical force titration curves showing the tip-sample adhesive force as a function of pH between a Au-coatedAFM tip modified with 16-thiohexadecanoic acid and a sulfonicacid-terminated PDMS substrate. (b) The inverse of theexperiment shown in part a. Chemical force titration curvesshowing the tip-sample adhesive force as a function of pHbetween a sulfonic acid-terminated AFM tip and a Au-coatedmica substrate modified with 16-thiohexadecanoic acid. Theerror bars represent the standard deviation in the adhesionforce as measured from the average of 300-500 force-distancecurves. In both cases, the curves through the data points shownwere calculated using the model described in the text, fit withsurface pK1/2 values of 3.0 and 5.0 for sulfonic acid and carboxylicacid, respectively, and a ratio of ionic to neutral H-bondinginteraction strength, fHB-:fn of 600 (part a) and 1000 (part b).

Figure 4. (a) Chemical force titration curves showing the tip-sample adhesive force as a function of pH between a Au-coatedAFM tip modified with 16-thiohexadecanoic acid and an amine-terminated PDMS substrate. (b) The inverse of the experimentshown in part a. Chemical force titration curves showing thetip-sample adhesive force as a function of pH between anamine-terminated AFM tip and a Au-coated mica substratemodified with 16-thiohexadecanoic acid. The error bars rep-resent the standard deviation in the adhesion force as measuredfrom the average of 300-500 force-distance curves. In bothcases, the curves through the data points shown were calculatedusing the model described in the text, using surface pK1/2 valuesof 6.0 and 5.0 for amine and carboxylic acid, respectively, anda ratio of 25:2:1 (part a) or 50:2:1 (part b) for the parametersf+-:fHB+:fHB-.


expected to lead to great adsorption of solution counterionsonto the PDMS surface. Indeed, even the unmodified, andpresumably neutraly charged, PDMS surface carries azeta potential of -50 mV under these conditions. Theamine surface, which now carries a negative zeta potentialof only slightly lower magnitude, would also appear to bemainly deprotonated under these conditions. The oxidizedand sulfonic acid-terminated surfaces carry negative

zeta potentials of much greater magnitude, consistent witha fully deprotonated surface. The zeta potentials measuredhere are also comparable in magnitude to those for self-assembled monolayers of mercaptohexadecanoic acid onAu, which ranged from -20 to -55 mV for ionic strengthsof (5-3) × 10-3 M under pH 8 conditions.29

Plasma oxidation of the substrate evidently leads tolarge morphological changes to the sample surface, asseen in Figure 5a. Chua et al. have also imaged plasmaoxidized PDMS and found that there were disordered wavypatterns formed spontaneously and homogeneously acrossthe entire substrates investigated.30 Other groups haveseen similar results, again with different orientation ofthe structures.31,32 The wavy patterns have been attributedto the formation of an oxidized, silica-like surface layeron the PDMS. The presence of an increased oxygenconcentration at the surface has been demonstrated usingX-ray photoelectron spectroscopy.32 Due to the consider-able mismatch of the thermal expansion coefficient ofPDMS and the silica-like layer, the compressive stress inthe surface layer increases as the local temperature dropsafter plasma treatment. The built-up stress is relieved bythe buckling phenomenon.

Parts b and c of Figure 5 demonstrate that modificationof the oxidized surface using either APTES or SUFTMSleads to a surface morphology that is qualitatively similar.The large increase in overall sample roughness (the peakto trough height of the starting oxidized surface is <50nm, while that of the ethoxysilane-modified surfaces issome 300 nm) suggests that a multilayer of material hasdeposited on the surface. This is consistent with hydrolysisand cross-linking of the trialkoxysilane groups on APTESor SUFTMS. The widths of the wave patterns on all threesurfaces, are similar, which suggests that the underlyingoxidized surface templates the growth of the trialkoxy-silane-modified surfaces, via increased nucleation of theoverlying material at regions of the surface which containa high distribution of oxidized sites. We may thus expectthe general distribution of functional groups on theAPTES- or SUFTMS-modified surfaces to be similar, asindeed is suggested by the lower pH zeta potentialmeasurements.

When force titrations using tip/sample pairs containingthe same functional group are performed, two types ofbehavior have been observed. Early workers25,33 found asigmoidal shape to the force titration curve. However, itwas later found that under much lower ionic strengthconditions, a distinct peak could be observed. These resultshave been explained as being due to two distinct types ofH-bonding taking place at the surface. Under relativelylow ionic strength conditions, both regular (between twoneutral species) and ionic (between a neutral and a chargedspecies) H-bonding interactions take place, with the latterhaving been reported as up to 30 times stronger.18,34 Themaximum adhesive interaction between tip and sampleis thus dominated by ionic H-bonding which itself ismaximized at the pK1/2 of the ionizable group at the surfaceas this is the condition, statistically, at which themaximum number of ionic H-bonds can form between tip

(29) Schweiss, R.; Pleul, D.; Simon, F.; Janke, A.; Welzel, P. B.; Voit,B.; Knoll, W.; Werner, C. J. Phys. Chem. B 2004, 108, 2910.

(30) Chua, D. B. H.; Ng, H. T.; Li, S. F. Y. Appl. Phys. Lett. 2000, 76,721.

(31) Felton, M. J. Mod. Drug Discovery 2004, February, 18.(32) Fateh-Alavi, K.; Nunez, M. E.; Karlsson, S.; Gedde, U. W. Polym.

Degrad. Stab. 2002, 78, 17.(33) Zhang, H.; He, H.-X.; Wang, J.; Mu, T.; Liu, Z.-F. Appl. Phys.

A 1998, 66, S269.(34) Wallwork, M. L.; Smith, D. A.; Zhang, J.; Kirkham, J.; Robinson,

C. Langmuir 2001, 17, 1126.

Figure 5. AFM images of PDMS surfaces that have undergonevarious surface modification treatments. All images are 10 µmsquare in area, and the z (height) range is indicated by thescale bar. (a) The oxidized PDMS surface within 1 h of theoxidation using an air plasma, z-range 50 nm. (b) The surfaceof an amine-modified PDMS formed by hydrolysis of APTESonto an oxidized PDMS substrate, z-range 300 nm. (c) Thesurface of a sulfonic acid-modified PDMS formed by hydrolysisof SUFTMS onto an oxidized PDMS substrate, z-range 250 nm.The two latter images were acquired 24 h after samplepreparation.


and sample. Thus, the peak in the force titration curvesat least for like tip/sample pairsscorresponds to the pK1/2of the surface.10-12

As the ionic strength of solution increases, the relativestrength of the ionic H-bonding contribution to the tip-sample adhesive interaction is decreased. This is due tothe fact that ionized groups at the surface may more readilyform ion pairs with counterions from solution, hinderingionic H-bonding between tip and sample. Under high ionicstrength conditions, a sigmoidal-shaped curve is observed.Under these conditions, neutral H-bonding forces domi-nate, and the step in the curve is associated with thesurface pKa of the system, midway between the fullyprotonated (maximum adhesion) and fully deprotonated(minimum adhesion) states. Smith et al. have proposeda simple model that treats the total adhesion force as thesum of these two types of H-bonding components that fitsthe data for carboxylic acid systems rather well. However,they also point out that not only the relative adhesiveinteraction is a function of ionic strength but also themagnitude of the surface pKa is as well.18 Indeed for thecarboxylic acid system, they found the surface pKa rangedfrom 5.0 to about 8.0 pH units, with the latter value beingobserved under extremely low (10-7 M) ionic strengthconditions. This has been attributed to an increased degreeof in-plane intermolecular H-bonding taking place underlow ionic strength conditions, leading to increased dif-ficulty in ionizing the sample and hence the shift to ahigher pKa value. In the experiments described here, wehave chosen to examine the systems under relatively low(10-3 M) ionic strength conditions.

Although we have not attempted a systematic study ofthe effects of ionic strength on the titration curves, it isapparent from Figure 1, which shows distinct peaks inthe force titration curves, that we are operating underconditions in which ionic H-bonding forces dominate. Thus,our data indicate a pK1/2 of 3.0 ( 0.5 for sulfonic acid anda pK1/2 value of 6.0 ( 0.5 for the amine under the ionicstrength conditions used here. While no previous deter-mination appears to have been made for the surface pK1/2of sulfonic acid, a pK1/2 value of 3.9 has been determinedfor an APTES-coated AFM tip titrated against an oxidizedSi surface hydrolyzed using APTES.25 By use of a Au-coated AFM tip and substrate terminated with 11-amino-1-undecanethiol SAMs, a surface pK1/2 of 7.0 has also beendetermined.35,36 In both cases, these experiments werecarried out under higher ionic strength conditions thanthose used here, and both showed a sigmoid-shapedtitration curve. Our value of 6.0, intermediate betweenthese two, seems to be reasonably consistent. We noteabove that the surface pK1/2 values may be expected todiffer from the solution pKa values of 0.7 and 10.60 forbenzenesulfonic acid and propylamine, respectively. Therelative shifts of 2.3 and 4.6 pH units for sulfonic acid andamine observed here are consistent with the magnitudesof surface pKa shifts observed by other workers forcarboxylic acid or phosphoric acid systems.19,20,25 Thesurface pK1/2 values are also fully consistent with the zetapotentials measured on the APTES- and SUFTMS-modified PDMS.

Aminoalkanethiol SAMs of varying chain length, self-assembled on Au surfaces, have also been examined byWallwork et al.34 These experiments took place under ionicstrength conditions more similar to those used by us. They

observed two peaks in the amine-amine force titrationcurve, one at a pH of 6.3 and the other at a pH of 9.8.However, they also found evidence using IR spectroscopyfor partial binding of the amine terminus of the alkyl chainto the Au surface, leaving a free thiol group at the surface.They suggested that this thiol group was oxidized tosulfonic acid in solution, and thus their system was acomplex mixture of amine and sulfonic acid groups onboth tip and sample. Furthermore, they found evidencefor disordering in the alkyl spacer chains, which appearedto lead to strong hydrophobic interactions between tipand sample at higher ionic strengths. The lower pH peakin the force titration curve was attributed to a hydrophobicinteraction between tip and sample, while the higher pHpeak was attributed to an electrostatic interaction,presumably between SO3

- and NH2 species on the tip andsample.34

Our amine/sulfonic acid force titration data, shown inFigure 2, have a number of similar features with theirdata, in particular the presence of two peaks that wereobserved, within experimental error, at similar pH values.The maximum adhesive interaction observed is on theorder of 1 nN, about 10 times smaller. It should be notedthat we have previously measured adhesive interactionson the order of 17-20 nN between a carboxylic acid SAMand an unmodified PDMS substrate. The interactionstrength was independent of pH, and we have attributedthis to hydrophobic tip sample interactions betweendisordered alkyl chains on the tip and methyl groups onthe PDMS surface. Such forces are much closer inmagnitude to those observed by Wallwork et al. in theiraminoalkanethiol work. This suggests then that hydro-phobic interactions are not a significant factor in ouramine/sulfonic acid experiments.

Standard acid-base chemistry suggests that there arefour types of interaction that can take place at the sulfonicacid/amine interface: an electrostatic interaction betweenan ionized pair of NH3

+ and SO3- surfaces; two ionic

H-bond configurations between NH3+ and SO3H or NH2

and SO3- pairs; and finally a neutral H-bond between a

NH2 and SO3H pair. The last of these we can discount asa significant contributor to the overall tip/sample interac-tion for two reasons. First, it has already been noted thatneutral H bonding is much weaker than an ionic H-bondunder our conditions, and presumably must be far weakerstill than an attractive electrostatic interaction. Second,due to the large difference in surface pK1/2 values, neutralspecies do not coexist on the two surfaces to any significantextent at any pH value. Thus, the overall interaction force,FT, at an amine/sulfonic acid interface should be the sumof three terms

where NNH3+, NNH2, NSO3

-, and NSO3H represent the numberof ionized and neutral amine and sulfonic acid sites on thetwo interfaces, while f+- fHB+, and fHB- represent themagnitude of the electrostatic interaction, the magnitudeof the ionic H-bonding between protonated amine andneutral sulfonic acid, and the magnitude of the ionicH-bonding between deprotonated sulfonic acid and theneutral amine, respectively. While the absolute numberof such sites cannot be determined, it is possible todetermine the relative number of protonated and depro-

(35) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997,101, 9563.

(36) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13,4357.

FT ) NNH3+NSO3

- f+- + NNH3+NSO3HfHB+ +

NNH2NSO3

- fHB- (2)


tonated sites for a particular species, i.e., the fractionalionization of the surface, â. This is given by

and similarly for the sulfonic or carboxylic acid sites. Thus

where Namine and Nsa represent to total number of ionizedand neutral amine or sulfonic acid sites on the respectiveinterfaces and rfHB+/f+- and rfHB-/f+- represent the relativemagnitudes of the interaction strength between the twotypes of ionic H-bond and the electrostatic interaction.

As we have independently determined pK1/2 for boththe amine and sulfonic acid tips using the data in Figure1, it is straightforward to determine the value of â at anypH: a plot of the relative populations of the four types ofsurface site as a function of pH is shown in Figure 6. Ifthe tip-sample interaction were dominated entirely bythe electrostatic interaction between the two, i.e., f+- .fHB+ or fHB-, then the adhesive force should be expected tomaximize at a pH intermediate to the pK1/2 of the SO3Hand NH2 groups. As can be seen in Figure 2 (or indeed inFigure 4 for the case of the NH2/CO2H combinations) thelargest peak in the force titration curve indeed appearsapproximately midway between the two pK1/2 values.However, the secondary peak at higher pH in Figure 2and the plateau region at low pH that can be seen in bothFigures 2 and 4 cannot be explained by an electrostaticinteraction alone. In Figure 6, we also plot a simulatedforce titration curve using eq 4 for the amine/sulfonic acidsystem in which the surface pK1/2 values for sulfonic acidand amine are set at 3 and 6, respectively, consistent withour experimentally determined values using the data inFigure 1. In addition the relative values of f+-:fHB-:fHB+

were set in the ratio 10:3:2. The curve obtained closelyapproximates the shape of the force titration curve in boththe electrostatic binding region and low pH region. In thehigher pH region, where the secondary peak occurs, themodel instead predicts a shoulder of constant force all theway out to a pH of 12, instead of the peak that we observed.

This inconsistency might be explained by the fact that itishardtomaintain lowionic strengthconditions insolutionbeyond a pH of 10, where the force interaction is observedto drop off. As we have already noted, ionic H-bonds arevery sensitive to changes in ionic strength, decreasingrapidly with increasing ionic strength. This may explainthe fall off in adhesive forces then at higher pH. Nonethe-less, the model demonstrates the continued importanceof ionic H-bonding in addition to the strong electrostaticinteraction in these mixed systems.

A final point to note is that with the amine/sulfonic acidsystem a shift in the peak maximum is observed when thetip and substrate terminal groups are reversed. This wasnot observed in the other mixed systems explored. Thepeak shifts suggest that the pK1/2 values on the tip andsample have shifted relative to one another. Since thepeak shift is on the order of 1 pH unit, a surface pKa shiftof as little as 0.5 pH units on both tip and sample couldaccount for these observations. Since the substrate mate-rial differs from that of the tip, it is reasonable to assumethat the reaction of APTES or SUFTMS with surface sitesled to somewhat different function group densities ordifferent degrees of ordering on the tip and samplesurfaces. This in turn could affect the degree in inter-molecular H-bonding and consequently shift the surfacepKa.

The carboxylic acid/amine force titration curves inFigure 4 can also be interpreted using the above model.In this case, we have overlaid curves obtained using eq4 (with carboxylic acid substituted for sulfonic acid)directly onto the data. The pK1/2 values used were takendirectly from our own experiments and the literature data,while the relative values of f+-:fHB+:fHB- were set in theratio 25:2:1 for the carboxylic acid tip on amine substratecombination (Figure 4a) and 50:2:1 for the amine tip oncarboxylic acid substrate combination (Figure 4b). Whilewhen these parameters are used the model reproducesthe experimental data, we emphasize that a precise valuefor the ratio of the electrostatic force to ionic H-bondingforce cannot be determined using our limited data set;with five independent parameters (the two pK1/2 valuesand three f values) the number of data points we haveacquired precludes using an iterative fitting process. Itis, however, clear that the ratio must be about 1 order ofmagnitude under the experimental conditions used here.A previous amine/carboxylic acid force titration has been

Figure 6. Dashed curves showing the relative concentrations of protonated and deprotonated sulfonic acid and amine speciespresent on the AFM tip or substrate surfaces as a function of pH. Solid curve showing the relative magnitude of the aminetip/sulfonic acid substrate adhesive force interaction as a function of pH predicted by the model described in the text.

logâNH3

+

1 - âNH3+

) pH - pK1/2,NH3+ (3)

FT

NamineNsaf+-) âNH3

+âSO3- + âNH3

+(1 - âSO3-)rfHB+/f+-

+

(1 - âNH3+)âSO3

- rfHB-/f+-(4)


carried out using an APTES-modified silica sphere at-tached to an AFM tip titrated against a Au surface coatedwith 11-thioundecanoic acid.37 At an ionic strength of 10-3

M, the force-titration curve similar in shape to thatreported here was observed, although the maximum wasat a slightly higher pH of about 6.5. In addition, the overallmagnitude of the forces observed was larger, about 10 nNat the peak maximum. It is likely that the larger contactarea of the silica sphere would account for this differencein the magnitude of the forces observed.

In the case of the carboxylic/sulfonic acid interaction,eq 4 must be modified somewhat. There are two possiblecombinations in which significant tip/sample attractiveinteractions could arise: one in which a neutral H-bondinginteraction takes place between SO3H and CO2H inter-faces; and the other in which an ionic H-bond arisesbetween SO3

- and CO2H species. The other two possiblecombinations will not give significant interactions. TheSO3H/CO2

- pairing will not occur simply due to the factthat the relative pK1/2 of the two species preclude asignificant fraction of both species being present at anypH value, while the SO3

-/CO2- interaction will be repul-

sive. Thus, the following version of eq 4 will apply

where rfHB-/fn represents the relative strength of the ionicand neutral H-bond. As with eq 4, we expect a maximumin the force titration curve midway between the pK1/2

values of the two species, assuming that the ionic H-bondis stronger than the neutral H-bonding. This is indeedthe case. A curve generated using eq 5 has been overlaidon the sulfonic acid/carboxylic acid force titration datashown in Figure 3. Again, our previously measured valuesof pK1/2 have been used to determine the values of â. Therelative values of the ionic to neutral H-bond strengthhave been set to 600 for the carboxylic tip/sulfonic acidsubstrate combination (Figure 3a) and 1000 for the sulfonicacid tip/carboxlic acid substrate combination. This re-produces the experimental data, but again we note thatthe relative values of the f parameters should be consideredorder of magnitude estimates only. Two observations arenotable. First, the force titration curves in Figure 3 showlittle or no adhesive interaction at pH values above 7,consistent with electrostatic repulsion between negatively

charged tip and sample dominating the interaction.Second, the relative magnitude of the ionic to neutralH-bond strength is much higher here than the values of12-40 which have been observed for carboxylic groups onboth tip and sample.18 Shan et al. have found that neutralH-bonding becomes weaker the larger the difference inpKa between the interacting groups34 which would explainwhy the ionic to neutral H-bond strength ratio is largerhere with an unlike tip/sample pair.

5. ConclusionsChemical force titrations were employed to measure

the adhesive forces between tip and substrate terminatedwith various combinations of amine, sulfonic acid, andcarboxylic acid. The amine and sulfonic acid substratesconsisted of oxidized PDMS that had been derivatizedusing trialkoxysilane derivatives of propylamine andbenzenesulfonic acid, respectively. The carboxylic acidsurfaces consistedofa16-thiohexadecanoicacidmonolayerself-assembled on Au. Data acquired using amine orsulfonic acid on both tip and sample demonstrated thatthe surface pK1/2 of sulfonic acid is 3.0 ( 0.5, while thatof the amine surface is 6.0 ( 0.5. The latter value isconsistent with previous results, while both values areconsistent with large shifts in pKa which have beenobserved between the solution phase and surface boundversions of an ionizable functional group.

Chemical force titrations of amine/carboxylic acid orcarboxylic acid/sulfonic acid tip/substrate pairs werecharacterized by a peak which maximized at a pH valuemidway between the surface pK1/2 of the two species. Asimple model which takes into account three possible tipsample interactions: neutral H-bonding, ionic H-bonding,and electrostatic interactions (either attractive or repul-sive), was able to readily reproduce the force titrationcurves. In the case of the amine/sulfonic acid tip/substratecombinations, it was clear that the electrostatic interactionbetween -SO3

- and -NH3+ groups was the largest

interaction observed. However, the contribution of ionicH-bonding between -NH2 and -SO3

- species led to asecondary peak in the force titration curve at higher pH.We also observed shifts in the pH position of the peakmaximum in this system when the functional groups ontip and substrate are exchanged. We attribute thisobservation to shifts in the surface pK1/2 of the functionalgroup due to different degrees of ordering and density ontip and substrate.

Acknowledgment. We acknowledge the NaturalSciences and Engineering Research Council of Canada,Canada Foundation for Innovation, Ontario InnovationTrust, and Queen’s University for financial support.

LA048388P(37) Giesbers, M.; Kleijn, J. M.; Cohen Stuart, M. A. J. Colloid

Interface Sci. 2002, 252, 138.

FT

NcaNsafn) (1 - âCO2

-)(1 - âSO3-) +

(1 - âCO2-)âSO3

-rfHB-/fn(5)


chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane)

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