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Langmuir Monolayer Flow across Hydrophobic Surfaces. 2. Sensor Development Using Langmuir Monolayer Flow Brady J. Cheek,† Adam B. Steel,*,† and Cary J. Miller‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Received July 25, 2000. In Final Form: October 25, 2000 Two sensing mechanisms utilizing Langmuir monolayer flow, the pressure-driven movement of a Langmuir film into a hydrophobic surface-liquid interface, have been investigated and involve analyte interaction with either the flowing monolayer or stationary film. The sensing aspect is achieved through modulation of the bilayer formation rate. A sensor using this novel transduction mechanism was developed for Cd2+, based on the interaction of divalent cations with the carboxylic headgroup of the flowing monolayer. Aqueous divalent cations associate with an oleic acid monolayer, increasing its viscosity. As a result, the flow rate of the associated monolayer is slower than for the unassociated monolayer. A rudimentary pH sensor was also developed to exemplify flow-based sensing, in which oleyl alcohol flows into the interface between a mixed methyl-carboxylic acid terminated self-assembled monolayer modified gold electrode and water. The flow rate for this system was sensitive to subphase pH. Attenuation of the monolayer flow rate is dependent upon the relative surface concentration of the carboxylic acid moiety in the multicomponent surface monolayer. The major contribution to the modulation of the monolayer flow rate in this system is attributed to solvation thermodynamics of the carboxylate anion macroscopically averaged across the modified electrode surface.
Introduction Inherent to all functional chemical sensors is the property that the presence of a chemical analyte invokes a physical change that can be measured by a device through a physical transduction mechanism. Current sensor development is focused on the discovery of chemically sensitive recognition elements. Sensor characteristics are usually determined by a chemically selective membrane, film, or layer at the sensor surface.1 The chemically sensitive film can be thick, up to several hundred microns, or as thin as a monolayer. A large body of work has been devoted to developing self-assembled monolayers (SAMs) that exhibit favorable sensing characteristics.2-5 Sensors based on SAMs of organosulfur compounds on gold have been demonstrated using gravimetric,6-10 optical,11-16 and electrical transduction mechanisms, including coulom* To whom correspondence should be addressed. † Current address: Gene Logic, Inc., 708 Quince Orchard Road, Gaithersburg, MD 20878. ‡ Current address: i-STAT Corp., 436 Hazeldean Road, Kanata, Ontario K2L 1T9. (1) Diamond, D. Principles of Chemical and Biological Sensors; John Wiley & Sons: New York, 1998; Vol. 150. (2) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991. (3) Cass, T.; Ligler, F. S. Immobilized Biomolecules in Analysis: A Practical Approach; Oxford University Press: New York, 1998. (4) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771-783. (5) Zhong, C. J.; Porter, M. D. Anal. Chem. 1995, 367, 709A-715A. (6) Nakano, K.; Sato, T.; Tazaki, M.; Takagi, M. Langmuir 2000, 16, 2225-2229. (7) Dermody, D. L.; Lee, Y.; Kim, T.; Crooks, R. M. Langmuir 1999, 15, 8435-8440. (8) Eun, H.; Umezawa, Y. Anal. Chim. Acta 1998, 375, 155-165. (9) Burgess, J. D.; Hawkridge, F. M. Langmuir 1997, 13, 37813786. (10) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191-3193. (11) Rao, J.; Yan, L.; Xu, B.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 2629-2630. (12) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749-2755. (13) Huisman, B. H.; Kooyman, R. P. H.; vanVeggel, R. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561-564.
etric,17 impedimetric,18-21 amperometric,22,23 and voltammetric24-29 methods. In contrast, little work has been reported in discovering new physical transducers. In this report, we describe a new physical transducer, Langmuir monolayer flow, for sensors with chemically sensitive monolayers. Langmuir monolayer flow is a simple method to produce bilayer assemblies using SAMs on gold and fluid Langmuir films at the air-water interface.30 Monolayer flow rates are controlled by the surface pressure of the flowing monolayer, the hydrophobicity of the stationary monolayer, the interlayer coupling (friction) between the flowing and stationary monolayers, and the intralayer coupling (viscosity) of the flowing monolayer. Formation of sup(14) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490-497. (15) Spinke, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821-1825. (16) Haussling, L.; Ringsdorf, H.; Schmitt, F. J.; Knoll, W. Langmuir 1991, 7, 1837-1840. (17) Steel, A. B.; Herne, T. M.; Tarlov, M. J. Anal. Chem. 1998, 70, 4970-4677. (18) Miller, C. J.; Cuendet, P.; Gratzel, M. J. Electroanal. Chem. 1990, 278, 175-192. (19) Rickert, J.; Gopel, W.; Beck, W.; Jung, G.; Heiduschka, P. Biosens. Bioelectron. 1996, 11, 757-768. (20) Krause, C.; Mirsky, V. M.; Heckmann, K. D. Langmuir 1996, 12, 6059-6064. (21) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222-4228. (22) Kawaguchi, T.; Yamauchi, Y.; Maeda, H.; Ohmori, H. Chem. Pharm. Bull. 1993, 41, 1601-1603. (23) Pandey, P. C.; Aston, R. W.; Weetall, H. H. Biosens. Bioelectron. 1995, 10, 669-674. (24) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183-1187. (25) Flink, S.; Boukamp, B. A.; Berg, A. v. d.; Veggel, F. C. J. M. v.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652-4657. (26) Sun, X.; He, P.; Liu, S.; Ye, J.; Fang, Y. Talanta 1998, 47, 487495. (27) Maeda, M.; Mitsuhashi, Y.; Nakano, K.; Takagi, M. Anal. Sci. 1992, 8, 83-84. (28) Turyan, I.; Mandler, D. Anal. Chem. 1994, 66, 58-63. (29) Liu, Z.; Li, J.; Dong, S.; Wang, E. Anal. Chem. 1996, 68, 24322436. (30) Steel, A. B.; Cheek, B. J.; Miller, C. J. Langmuir 1998, 14, 54795486.
10.1021/la001056+ CCC: $19.00 © 2000 American Chemical Society Published on Web 11/23/2000
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Figure 1. Electrode geometry for measuring Langmuir monolayer flow. SAM-modified gold electrodes are positioned at the air-subphase interface in a Teflon trough with the smooth edge (area approximately 0.28 cm2) in contact with the solution surface. The flowing monolayer, oleic acid or oleyl alcohol, was applied in excess of monolayer coverage by placing a drop of neat material on the subphase surface using a glass rod. The Wilhelmy plate method was used to measure the surface pressure of the Langmuir monolayer. Platinum wire and saturated calomel electrodes were used for counter and reference electrodes, respectively. The extent of monolayer flow was monitored by ac admittance measurements.
ported bilayer structures by Langmuir monolayer flow can be understood by the experiment described by Figure 1. A SAM-modified electrode was placed at a clean solution interface. A fluid Langmuir film is applied to the surface of the subphase, and the fluid Langmuir monolayer flows into the interface between the SAM-modified electrode and the subphase. The rate of monolayer flow into the solid-solution interface was monitored by measuring the capacitance of the electrode. As the bilayer is formed, the electrode capacitance decreases, reflecting the thicker dielectric layer at the electrode surface. Previous work has shown that the capacitance decreases linearly with the square root of time, until stabilizing at a new value characteristic of a SAM-Langmuir film bilayer.30 Capacitance-time profiles for oleic acid and oleyl alcohol flow across decanethiol-modified gold electrode surfaces are shown in Figure 2. The initial capacitance of the sensor electrode is consistent with the characteristic value of the alkanethiol SAM.31 The change in electrode capacitance as a function of time is well described by a pressure-driven flow model. Assuming a linear pressure gradient within the flowing monolayer, the rate of bilayer formation is predicted to follow
dx Π )κ dt x
()
(1)
where κ is a proportionality constant which contains the intra- and interlayer coupling terms for the flowing monolayer, Π is the equilibrium spreading pressure (ESP) driving flow, and x is the distance traversed by the flowing monolayer into the SAM-solution interface. The flow rate is proportional to the film pressure at the air-water interface. The κ term contains an inverse proportionality to the monolayer viscosity and a direct relationship with (31) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
Figure 2. Capacitance-time profiles for oleyl alcohol (open circles) and oleic acid (solid line) flow across a dodecanethiolmodified surface. The electrode capacitance was measured by ac admittance at a frequency of 10 Hz using 10 mV excitation. Electrode capacitance was calculated from ac admittance using an equivalent circuit model with two capacitors in parallel.
the wetting properties of the stationary film, in that more hydrophobic surfaces tend to have faster bilayer formation rates. Given that film pressure and viscosity can be modulated by environmental factors, we propose that the rate of bilayer formation can be used as a signal transduction mechanism. In a flow-based sensor, the rate of bilayer formation provides a signal that changes in response to a specific interaction between an analyte and one of the components of the Langmuir monolayer flow system, the stationary or flowing monolayer. Three aspects must be considered in the design of Langmuir monolayer flow-based sensors: (1) the rate of bilayer formation increases linearly with the square root of monolayer pressure, (2) changes in film viscosity or compressibility result in modulation of the flow rate, and (3) the flow rate across a SAM surface is related to the SAM-subphase interfacial free energy. Controlling two variables allows investigation of individual sensing mechanisms using the Langmuir monolayer flow transduction scheme. Two types of sensors developed using this transduction mechanism will be described here, defined by the following: sensor type 1, analyte interaction with the flowing monolayer, and sensor type 2, analyte interaction with the stationary monolayer. As an example of a flow-based sensor type 1, a subphase cadmium ion sensor will be described. A rudimentary pH meter will be discussed as an example of a flow sensor type 2. Experimental Methods General Reagents. Subphase electrolyte solutions were made with deionized water (Milli-Q system, Millipore, Bedford, MA). Ethanol (95%, Pharmcoproducts, Inc., Brookfield, CT) was employed as the self-assembly deposition solvent for all thiols. Potassium chloride (Sigma, St. Louis, MO), cadmium chloride (J. T. Baker, Philipsburg, NJ), mono- and dibasic potassium phosphate (Sigma), 11-mercapto-1-undecanoic acid (Aldrich, Milwaukee, WI), 1-decanethiol (Aldrich), 1-octanethiol (Aldrich), and oleic acid (Aldrich) were used as received. Oleyl alcohol (Aldrich) was purified by flash chromatography (silica gel, chloroform). Film Pressure as a Function of pH. ESP measurements were performed using the Wilhelmy plate method. A glass
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electrode (Orion Research, Inc., Beverly, MA) and a micro stir bar (VWR Scientific Products, West Chester, PA) were positioned in a Teflon trough placed upon a stir plate (Corning, Inc., Corning, NY). Subphase pH was monitored using an Accumet pH meter 910 (Fisher Scientific, Hampton, NH), for which the glass electrode was calibrated just prior using pH buffers 4 and 10 (VWR Scientific). The Langmuir film forming material, that is, oleic acid or oleyl alcohol, was applied in excess of monolayer coverage by placing a drop of neat piston oil on the air-water interface using a glass rod. Introducing surfactants in this manner resulted in a monolayer on the entire surface and a lens of material at the glass rod. The lens provided a vast excess of film-forming material in the event of desorption of material from the surface at high pH. The glass rod with a reservoir of piston oil was left at the solution interface for the duration of the experiment. The revolution rate of the stir bar was sufficient to enhance mixing of the subphase but did not disrupt the fluid Langmuir monolayer as determined by invariance of the surface pressure. The Langmuir film was spread on a 50 mM phosphoric acid subphase. Injecting small aliquots of a 6 M sodium hydroxide solution into the subphase increased the pH of the subphase. The film pressure was recorded by computer, and the pH was recorded manually after a 30 s equilibration period. Control experiments were performed to confirm that the surface tension of a clean subphase surface was unaffected by the change in pH and the presence of the glass electrode and stir bar. Preparation and Characterization of SAM-Modified Gold Electrodes. Gold electrodes were prepared on glass substrates with freshly cleaved smooth edges. Plate glass was cleaned in a chromic acid bath at 50 °C. After being rinsed with copious amounts of water, the substrates were placed in a radio frequency sputtering chamber with the cleaved edges normal to the metal source. Prior to deposition of metal layers, the substrates were cleaned with a 50 W argon plasma for 30 s. A chromium underlayer of ca. 50 nm was deposited prior to the deposition of ca. 200 nm of gold. Gold-coated substrates were placed in SAM deposition solutions (20 mM thiol in ethanol) immediately upon removal from the sputtering chamber, and the SAM was allowed to form for at least 12 h before use. Equilibrium contact angle measurements were performed by the sessile drop method on a Rame-Hart model 100 goniometer at room temperature and ambient humidity (21-23 °C, 50-60% relative humidity). The samples were rinsed with 95% ethanol and deionized water and then dried under a nitrogen stream. A 2 µL drop was formed at the end of a blunt-tip needle attached to a 2 µm syringe. X-ray photoelectron spectra were obtained using a monochromatized Al KR source for oxygen 1s, sulfur 2p, and carbon 1s. Flow Rate Measurement. The measurement of bilayer formation via Langmuir monolayer flow has been described in detail in a previous report.30 Briefly, a hydrophobic SAM-modified gold electrode was positioned at the air-solution interface in a Teflon trough (36 cm2 surface area) with the smooth edge in contact with the solution surface. The admittance of the SAMcoated electrode is monitored continuously using a lock-in amplifier (model SR530, Stanford Research Systems, Sunnyvale, CA) under computer control at a frequency of 10 Hz with a 10 mV excitation. At this frequency, the admittance phase angle was typically in the range of 86-90°, so that the admittance magnitude was converted directly into an equivalent capacitance. The lock-in amplifier was connected to the cell using a potentiostat (model 360, EG&G Princeton Applied Research, Wellesley, MA) that holds the system at 0.0V versus a saturated calomel electrode reference. The subphase electrolyte was 10 mM potassium chloride, and the pH was adjusted using HCl or KOH. The flowing monolayer, that is, oleic acid or oleyl alcohol, was applied in excess of monolayer coverage by placing a drop of the neat piston oil on the surface using a small glass rod. Similar to the film pressure as a function of pH experiments, the monolayer covered the entire subphase surface and a lens of material formed at the glass rod, providing a vast excess of Langmuir film forming material. The surface pressure was measured by differential weight measurements using a filter paper (No.1, Whatman, Clifton, NJ) Wilhelmy plate suspended from an analytical balance (model 100A, Denver Instruments, Arvada, CO).
Cheek et al.
Results and Discussion Sensor Type 1. The mechanism of eliciting a change in the flow rate for sensor type 1 is the interaction of an analyte with the flowing monolayer. Association of analytes with the flowing monolayer can change the film viscosity or compressibility, which in turn alters the monolayer flow rate. More viscous films exhibit slower monolayer flow rates.30 Monolayer viscosity is largely dependent on the film phase state (e.g., liquid expanded, liquid condensed, etc.) and the extent of mechanical coupling to the subphase.32-34 Both the phase state and subphase coupling can be affected by interaction of a subphase species, an analyte, with the Langmuir film. As an example of a sensor type 1, the association of divalent cations with carboxylate-containing Langmuir films was investigated. The influence of divalent cations on carboxylic acid containing Langmuir film behavior is well documented.2,35,36 Under appropriate conditions, divalent cations such as Cd2+ and Ca2+ have a condensing effect on carboxylate-containing monolayers. Many researchers have taken advantage of the condensing effect of divalent cations to produce high-quality LangmuirBlodgett films of otherwise fluid Langmuir monolayers.37-44 Association of cations with the monolayer increases monolayer viscosity. It has been proposed that the viscosity change is due to a single cadmium ion associating with two carboxylates, effectively coupling the motion of the two chains.45 The pressure-driven flow model predicts a decrease in the monolayer flow rate as a result of the increase in monolayer viscosity. For the example of sensor type 1, the stationary SAM surface was dodecanethiol and the fluid flowing molecule was oleic acid. Subphase solutions included 10 mM KCl with 1 mM total concentration of phosphate buffer. Relative concentrations of mono- and dibasic phosphate were varied to give the desired pH in the range of 3-8.5. Cadmium-containing subphases were prepared using cadmium chloride. In flow modulation experiments with the carboxylic acid headgroup, control of subphase pH is imperative for two reasons. First, the ESP and extent of oleic acid ionization are a function of subphase pH. At low pH, the carboxylic acid group remains protonated, thus preventing association with divalent cadmium ions. Second, the solubility of cadmium ions decreases as the solution becomes more basic, Ksp for Cd(OH)2 ) 4.5 × 10-15 M3. To determine an appropriate pH value for the sensor subphase solutions, the effect of subphase pH on the flow parameter for (32) Schwartz, D. K.; Knobler, C. M.; Bruinsma, R. Phys. Rev. Lett. 1994, 73, 2841-2844. (33) Huhnerfuss, H. J. Colloid Interface Sci. 1985, 107, 84-95. (34) Schulman, J. H.; Teorell, T. Trans. Faraday Soc. 1938, 34, 13371342. (35) Aveyard, R.; Binks, B. P.; Fletcher, P. D.; Ye, X. Thin Solid Films 1992, 210/211, 36-38. (36) Langmuir, I.; Schaefer, V. J. Am. Chem. Soc. 1937, 59, 2400. (37) Ouyang, J. M.; Zheng, W. J.; Tai, Z. H. Thin Solid Films 1999, 340, 257. (38) Bettarini, S.; Bonosi, F.; Gabrielli, G. Thin Solid Films 1992, 210/211, 42. (39) Petruska, M. A.; Talham, D. R. Chem. Mater. 1998, 10, 3672. (40) Takeuchi, S.; Nogami, Y.; Ishiguro, T. Thin Solid Films 1994, 250, 243. (41) Hassan, A. K.; Nabok, A. V.; Stirling, C. J. M. Thin Solid Films 1998, 327, 686. (42) Ando, Y.; Hiroike, T.; Miyazaki, T. Thin Solid Films 1996, 278, 144. (43) Liu, M.; Ushida, K.; Kira, A. Adv. Mater. 1997, 9, 1099. (44) Liu, M.; Ushida, K.; Nakahara, H. J. Phys. Chem. B 1997, 101, 1101. (45) Roberts, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990.
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Figure 3. Equilibrium spreading pressure of oleic acid (filled circles) and oleyl alcohol (open circles) as a function of subphase pH. The ESP was measured using the Wilhelmy method on a 50 mM phosphoric acid subphase. Aliquots of 6 M NaOH were used to increase subphase pH.
subphases containing 0.1 mM cadmium chloride was investigated. The optimal pH for carboxylic acid headgroups was determined from subphase titrations of an oleic acid film. Figure 3 depicts the ESP as a function of pH for oleic acid and oleyl alcohol films spread on phosphoric acid subphases. The ESP of oleic acid is dependent on pH. For oleic acid, the ESP increases dramatically from 30 to 42 mN/m as pH increases from 4 to 8, whereas the ESP of oleyl alcohol is invariant of the pH range from 2 to 12. The deprotonation of the carboxylic acid groups causes a sharp increase in the electrostatic repulsive forces between the oleic acid molecules, which results in the dramatic increase in the ESP. From the data in Figure 3, the pKa of the oleic acid film is 6.8. Optimal pH to ensure solubility of divalent cadmium was determined by the observed flow parameter as a function of subphase pH at a cadmium chloride concentration of 0.1 mM. The monolayer flow rate as function of pH is given in Figure 4A. The flow parameter is fastest on acidic subphases. The flow parameter decreases near the pKa for oleic acid. As oleic acid was converted to carboxylate form by increasing pH, the association of Cd2+ with the flowing monolayer increases. Association with Cd2+ decreases the monolayer flow rate. By pH 8, the flow parameter stabilizes at about 40% of the acidic subphase value. Considering nearly complete ionization of the oleic acid film and the solubility of cadmium, a buffer pH of 7.5 was selected for the sensor subphase solutions. Experiments using a flowing phase that should not associate with subphase cadmium were performed as a control. Oleyl alcohol was used, and the flow parameter was found to be independent of the cadmium ion concentration. Figure 4B depicts the flow parameter as a function of cadmium concentration using the pH 7.5 buffer subphase solution. Concentrations of cadmium greater than 0.01 mM are effective at lowering the flow rate of oleic acid across these surfaces. The sensor response is limited to concentrations between 0.05 and 0.2 mM. Although this is a rather limited dynamic range for sensor response, it does show that the flow parameter is sensitive to interactions between the flowing monolayer and a subphase analyte.
Figure 4. Langmuir monolayer flow cadmium ion sensor. (A) The monolayer flow parameter on 0.1 mM cadmium chloride solutions with different pH. Differing ratios of mono- and dibasic sodium phosphate (total phosphate concentration ) 1 mM) were used to control pH in a 10 mM KCl subphase. (B) The monolayer flow parameter on a subphase pH 7.5 (1 mM phosphate/10 mM KCl) with different cadmium chloride concentrations.
Sensor selectivity was investigated for this system using calcium as an interferent. A 0.33 mM calcium subphase solution produced the same flow parameter as a 0.075 mM cadmium solution. The selectivity coefficient, defined as the ratio of interferent to analyte concentration for which equivalent sensor responses are produced, was equal to 4.4. This value agrees well with the ratio of the formation constants for cadmium and calcium acetate,46 which is 4.9. Sensor Type 2. Sensor type 2 monitors the interaction of an analyte with the stationary film. Interactions between the stationary monolayer and a subphase analyte can be used to alter the flowing characteristics of a system. In Figure 3, a dramatic increase in ESP for oleic acid was seen upon deprotonation of the carboxylic acid headgroup. The increase in the ESP is a result of electrostatic-driven film expansion. The effects of pH on the flow characteristics of a stationary acid monolayer were investigated. A rudimentary pH sensor was designed as an example of a sensor type 2. Primary design issues for sensor type 2 include the development of a chemically sensitive surface and the identification of a flowing monolayer that does not respond to the presence of analyte. Chemically selective surfaces were made using mixed SAMs. Control of mixed monolayer formation is crucial to define sensor performance. Relative surface concentrations of mixed alkanethiol systems can vary dramatically with the monolayer deposition solvent,47 (46) Blixt, J.; Gyori, B.; Glaser, J. J. Am. Chem. Soc. 1989, 111, 7784. (47) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164.
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Cheek et al. Table 1. Contact Angle Measurements as a Function of pH and A11 Concentration acid mole fraction solutiona 0 0.025 0.050 0.075 0.100
surfaceb 0 0.01 0.02
contact angle (deg) pH ) 2
pH ) 6
pH ) 10
pH ) 12
112 108 105 103 102
112 107 104 102 101
111 105 103 99 91
112 102 98 96 90
a
11-Mercaptoundecanoic acid (A11) and 1-octanethiol (M8) were used at 20 mM total thiol concentration in 95% ethanol for the SAM deposition solutions. b The relative surface concentration of A11 was estimated using x-ray photoelectron spectroscopy. Table 2. Flow Parameter Measurements as a Function of pH and A11 Concentration acid mole fraction solutiona
Figure 5. Capacitance-time profiles of oleyl alcohol flow across A11/M8 surfaces as a function of subphase pH. The subphase electrolyte was 10 mM KCl, and the pH was adjusted using HCl or KOH. Monolayer flow experiments at pH 2 (solid line), pH 9 (dashed), pH 10 (dotted), and pH 12 (dash-dotted) are shown.
and the relative surface concentrations need not reflect the relative concentrations in the deposition solution.48 The spatial distribution of the mixed alkanethiol system also may have a significant influence on sensor response. Phase-segregated groups within the SAM are anticipated to behave much differently than a homogeneous distribution. Mixed SAMs that recognized pH changes were made by incorporating an alkanethiol with a terminal carboxylic acid group as the minor component in a two-component SAM. The major SAM component was a methyl-terminated alkanethiol. The mixed SAM consisted of 11mercapto-1-undecanoic acid (A11) and octanethiol (M8) coadsorbed on a gold electrode from an ethanol solution. On the basis of previous studies of monolayer flow under variable surface free energy, the difference in interfacial energy is predicted to change the monolayer flow characteristics of the system. The mixed methyl/carboxylic acid surface will exhibit a change in the interfacial free energy upon the deprotonation of the surface-bound acid groups. The decrease in the interfacial free energy is predicted to decrease flow rates for oleyl alcohol at the interface. A flowing monolayer with properties independent of the analyte simplifies the analysis of sensor response considerably. For the pH sensor, the data in Figure 3 indicate that oleic acid would make a poor choice of flowing monolayer because the film pressure, which has a direct impact on the flow rate, varies dramatically with subphase pH. Oleyl alcohol is an acceptable fluid monolayer, given the relative independence of ESP on pH observed in Figure 3. Capacitance-time profiles for pH-dependent flow experiments are shown in Figure 5. Oleyl alcohol flowed across mixed A11/M8 surfaces at varying subphase pH. At t ) 0, oleyl alcohol was introduced to the air-water interface. The capacitance decreases as oleyl alcohol flows into the A11/M8 surface-subphase interface. Two conclusions are drawn from these data: (1) the rate of capacitance change is much slower as the pH is increased and (2) the (48) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561.
0 0.025 0.050 0.075 0.100 0.200
surfaceb 0 0.01 0.02 0.05
flow parameter (× 10-3 cm s-1/2) pH ) 2
pH ) 6
pH ) 10
pH ) 12
7.5 4.8 4.9 3.7 3.1 NFc
7.5 5.0 4.4 3.4 2.4 NFc
7.5 4.5 2.6 1.8 NFc NFc
7.7 2.8 1.4 1.3 NFc NFc
a 11-Mercaptoundecanoic acid (A11) and 1-octanethiol (M8) were used at 20 mM total thiol concentration in 95% ethanol for the SAM deposition solutions. b The relative surface concentration of A11 was estimated using x-ray photoelectron spectroscopy. c NF refers to conditions where flow was not measurable.
final bilayer capacitance increases with pH. The decrease in flow rate was predicted based on the change in surface free energy of the interface; however, the change in the final bilayer capacitance at pH 2 versus pH 12 was not expected and will be discussed in later paragraphs. When the subphase is at pH 2, a complete bilayer formed within 10 min. For the same system at pH 12, complete bilayer formation required over 1 h. The pH effect exemplifies the sensitivity of monolayer flow to changes in surface free energy at the solid-solution interface. Control experiments were performed to ensure that this pH effect was due to a change in the stationary monolayer. No change in flow rate was observed when oleyl alcohol flowed at pure M8 surfaces using subphase solutions ranging from pH 2 to 12. We postulate that the solvation thermodynamics of the carboxylate anion, macroscopically averaged across the modified electrode surface, is responsible for the observed monolayer flow modulation. Unlike the observed behavior of oleic acid, which undergoes film expansion upon deprotonation, stress in the SAM cannot be relieved by expansion. Solvation of the carboxylate anions decreases the interfacial energy. As shown in Table 1, contact angle measurements of the A11/M8 surfaces indicate lower contact angles as the pH is increased, suggesting a decrease in the interfacial free energy at the monolayerliquid interface. Table 1 also shows an A11 deposition concentration dependence on the contact angle. The dependence of flow rate on interfacial free energy is also evident when we compare the flow data of the multicomponent SAM surface to that of the pure M8 surface at pH 2. Mixed carboxylic acid-methyl termini SAMs with as little as 1% carboxylate functionality, as estimated by X-ray photoelectron spectroscopy, displayed dramatic attenuation of the bilayer formation rate. Table 2 depicts how the relative surface concentrations of A11 and subphase pH affect the flow parameter. A monolayer with 1% A11 results in a 35% decrease in the flow parameter
Monolayer Flow across Hydrophobic Surfaces
Figure 6. Bilayer capacitance as a function of pH. The initial bilayer was formed by monolayer flow. The subphase pH was increased from 2 to 12 by addition of 6 M NaOH at approximately t ) 5 min. The subphase pH was returned to 2 using 6 M HCl at t ) 15 min. Arrows denote the times at which the subphase pH was altered. The surfaces examined contained approximately 2% (open circles), 1% (filled circles), and 0% (solid line) A11 relative to M8.
at pH 2 compared to a 0% A11 surface. A 2% A11 surface results in 60% decrease. Deprotonation of the A11 component of the monolayer results in even greater attenuation of the flow parameter. At pH 12, the decrease in the flow parameter is 80% and nearly 100% for the same two surfaces, respectively. For surfaces containing A11, both a concentration and a pH dependence on the flow parameter were observed. The flow parameter data from Table 2 show that the surface created with a deposition solution of 0.05 acid mole fraction exhibits the largest dynamic range. Surfaces created with deposition solutions having an acid mole fraction greater than 0.10 show no monolayer flow regardless of pH. Lack of flow is believed to result from the solvated SAM being thermodynamically favorable enough to inhibit bilayer formation. As seen in Figure 5, the final interfacial capacitance increases with pH, suggesting that areas of the SAM surface are not being covered by a bilayer when the carboxylate groups are deprotonated. The data suggest that a perforated bilayer is formed as a result of the localized solvation thermodynamics of the deprotonated carboxylic acid groups. At pH 2, the capacitance of a bilayer formed either by monolayer flow or by trapping a monolayer by positioning the electrode on the spread Langmuir exhibited no difference in capacitance, 0.915 µF/cm2. The same was observed at subphase pH 12, resulting in a capacitance of 1.55 µF/cm2. To investigate the perforated bilayer, the subphase pH was changed following bilayer formation via monolayer flow. A bilayer capacitance-time profile is given in Figure 6. Initially, a flow experiment was performed at a subphase pH 2 and the bilayer
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capacitance was established. Subphase pH was then increased to 12 by injecting small aliquots of 6 M NaOH. Upon addition of base, the bilayer capacitance changed from 0.96 to 1.3 µF/cm2. The capacitance change suggests that areas of the SAM surface are not being covered by a bilayer when the subphase was increased to pH 12. Reversal of subphase pH back to 2, using HCl, resulted in the capacitance returning to the level consistent with the SAM-Langmuir film bilayer. The results indicate that localized areas of the surface, presumably in the vicinity of carboxylate groups, are solvated strongly enough to inhibit flow at high pH. Distinguishing unambiguously between the perforated bilayer and the effect of a polar moiety within a dielectric is not possible from the data. Although the lateral proton diffusion rates along stearic acid monolayers have been studied,49 the experiment described by Figure 6 suggests proton transport through a bilayer membrane. The experiment represents a stimulant-controlled switching of a membrane channel, inducing an increase or a decrease in the membrane permeability. Supported bilayers created by Langmuir monolayer flow may be able to provide a selective and sensitive process for the study of molecular recognition and signal transduction in biological membrane systems. Conclusion A novel sensor transduction mechanism based on Langmuir monolayer flow was described. Monitoring bilayer formation under pressure-driven flow conditions provides insight into the dynamic properties of bilayer assemblies. Monolayer flow sensor type 1 relies on analyte interaction with the fluid Langmuir monolayer, resulting in an alteration in the film properties, such as film viscosity or compressibility. A modification in the experiment geometry to include incubation with the analyte prior to flow might increase the sensitivity of these sensors. The selectivity of this type of sensor depends on designing molecules that incorporate a host-guest complex within the headgroup of the flowing monolayer. For example, formation of inclusion complexes of organic molecules with cyclodextrins may be of interest for pharmaceutical and diagnostic applications using monolayer flow. The essential element of sensor type 2 depends on the design and formation of mixed SAM surfaces that incorporate chemical recognition sites. The mechanism for a sensor response is change of the interfacial free energy of the surface caused by an alteration in the lateral flow properties of the system. Langmuir monolayer flow across a chemically sensitive flow path incorporates a large number of individual chemical recognition events into a sensitive detection scheme. Acknowledgment. We thank Dr. Janice Reutt-Robey (University of Maryland, College Park) for overseeing the completion of this work, which was funded by the State of Maryland. LA001056+ (49) Selvin, C. J.; Unwin, P. R. J. Am. Chem. Soc. 2000, 122, 25972602.
