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Qualitative Adsorption Measurements with an Atomic Force Microscope† Ian Larson* and Robert J. Pugh Institute for Surface Chemistry, Box 5607 SE-114 86, Stockholm, Sweden Received March 11, 1998. In Final Form: August 6, 1998 A simple method for the qualitative measurement of adsorption of solution species onto silicon nitride atomic force microscope cantilevers is presented. In this method, the change in resonant frequency of the cantilever, resulting from the added mass of the adsorbate, is measured as a function of time during the adsorption process. Results from the adsorption of copper(II) species and CTAB from aqueous solutions are presented. Cu(II) was seen to attain maximum coverage in a matter of minutes, while the adsorption of CTAB was beyond the resolution of the technique. Force measurements taken between the cantilever tip and a glass substrate during the adsorption process provide evidence that the change in cantilever frequency is a result of the adsorbate mass and is not just the result of any small viscosity differences.
Introduction The ability of the atomic force microscope, AFM, to measure surface forces between colloidal particles and flat surfaces in aqueous solution is well documented.1-4 In these force experiments the deflection of a cantilever spring is recorded and converted to force through Hooke’s law using the spring constant of the cantilever spring. A number of methods have been developed to measure the spring constants of AFM cantilevers,5 the most commonly used is probably that of Cleveland et al.6 The spring constant, k, is derived from the relationship between added mass, M, and resonant frequency, υ (rearranged from ref 7):
M)
k - m* (2πυ)2
where m* is the effective mass of the cantilever. Typically, small tungsten spheres are added to the tip of the cantilever and the resulting change in resonant frequency is used to calculate the cantilever spring constant. Derived strictly for cantilevers in a vacuum, the above equation cannot be quantitatively applied to changes in cantilever vibration in liquids. The equation is also only applicable to changes in frequency of the fundamental vibration. However, the physical basis of the equation still holds, adsorption onto a cantilever will lower the cantilever’s natural resonant frequency and the frequency of the harmonic vibrations. This present study utilizes the change in frequency due to added mass to follow adsorption of solution species onto * To whom correspondence should be addressed. Present address: Ian Wark Research Institute, University of South Australia, The Levels, SA 5095, Australia. † Presented at Characterization of Adsorption & Interfacial Reactions II, Keauhou-Kona, Hawaii, Jan 11-16, 1998. (1) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (2) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (3) Butt, H.-J. Biophys. J. 1991, 60, 1438. (4) Butt, H.-J.; Jaschke, M.; Ducker, W. A. Biochem. Bioenerg. 1995, 38, 191. (5) Sader, J. E.; Larson, I.; Mulvaney, P.; White, L. R. Rev. Sci. Instrum. 1995, 66, 3789. (6) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (7) Stokey, W. F. Shock and Vibration Handbook; McGraw-Hill: New York, 1989; p 7.1-7.44.
the cantilever. This change in frequency can be used to qualitatively measure the adsorption process as a function of time. In this way, the AFM is being used qualitatively in a fashion similar to a quartz crystal microbalance, QCM. Of course, a QCM is far better suited to study adsorption from solution. The AFM does, however, permit force measurements to be made between the surfaces being studied during the adsorption process. Qualitative adsorption data and the effect the adsorption has on the interaction between the cantilever and another surface can be measured almost simultaneously. The two experimental systems chosen for this study were the adsorption of a hydrolyzable metal ion, copper, and the adsorption of surfactant onto commercially available silicon nitride cantilevers in aqueous solution. It is well-known that the adsorption of hydrolyzable metal ions onto mineral surfaces takes place over a critical pH range and can reverse the sign of the electrophoretic mobility of the mineral surface.8,9 Other workers have shown that this mobility reversal is due to at least a partial coating of the metal hydroxide on the mineral substrate.10-12 Adsorbed surfactant bilayers have been used in AFM force measurements to determine effective cantilever tip radii and also for comparative work with other experimental techniques.13-15 Hexadecyltrimethylammonium bromide (CTAB), 1.5 mM, is known to form structured bilayers on AFM cantilevers in a period of about 1 h.13 Experimental Section Materials. Millipore Milli-Q water was used straight from the Millipore apparatus. AR grade copper(II) nitrate and hexadecyltrimethylammonium bromide, CTAB, were used as received from the commercial suppliers. To be consistent with our earlier work,16 fresh solutions containing 5 × 10-5 M Cu(NO)3 and 5 × 10-4 M NaCl were made up with doubly distilled (8) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 42. (9) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 53. (10) Tewari, P. H.; Lee, W. J. Colloid Interface Sci. 1975, 52, 77. (11) Bleam, W. F.; McBride, M. B. J. Colloid Interface Sci. 1985, 103, 124. (12) Xia, K.; Mehadi, A.; Taylor, R. W.; Bleam, W. F. J. Colloid Interface Sci. 1997, 185, 252. (13) Drummond, C. J.; Senden, T. J. Colloid Surf., A 1994, 87, 217. (14) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367. (15) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Colloids Surf., A 1995, 103, 195. (16) Larson, I.; Pugh, R. Submitted J. Colloid Interface Sci.
S0743-7463(98)00294-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/01/1998
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Figure 1. Electrophoretically determined mobilities of (a) quartz and (b) silicon nitride as a function of pH. Symbols refer to (O) 5 × 10-4 M NaCl only and (0) 5 × 10-4 M NaCl + 5 × 10-5 M Cu(NO3)2. Data for precipitated Cu(OH)2, b, are included for comparison: Pugh, R. J.; Tjus, K., J. Colloid Interface Sci. 1987, 117, 231. water each day. These copper solutions were equilibrated at the desired pH for 20 min before use in either frequency or force measurements. Even after this equilibration time, the pH of the solution could still drift slightly, a change of only 0.2 or 0.3 pH units could dramatically affect the degree of adsorption; see Figure 1. Pure finely ground quartz powder (batch number NCQ 325) was obtained from Ernstro¨m Mineral AB, O ¨ rebro, Sweden. The quartz was washed in dilute nitric acid and rinsed with doubly distilled water. The sample of silicon nitride powder (SN E-10 Lot No. A24406 from UBE Industries, Yamaguchi, Japan) was a gift from Lennart Bergstro¨m, Institute for Surface Chemistry, who had previously characterized the same powder.17 Concentrated suspensions of these powders were prepared by initially dispersing the powders in doubly distilled water and allowing the mixture to stand overnight for preliminary conditioning (hydration) of the material’s surface. A 1 cm × 1 cm × 1 mm flat glass substrate was soaked in 0.5 M HCl overnight, rinsed with Milli-Q water, and then washed in acetone for 30 min before use in force measurements. Cantilevers. Contact mode silicon nitride cantilevers were obtained from Digital Instruments, Inc. The long thin-legged cantilevers had a spring constant of 0.046 ( 0.005 N/m measured by the Cleveland method. The two-beam approximation18 applied to the measured spring constant of the short wide-legged cantilever19 also resulted in a value of 0.046 N/m for the longer, thinner cantilever. All cantilevers used in this study were soaked in 0.5 M HCl overnight and washed in acetone for 30 min before use. (17) Bergstro¨m, L.; Bostedt, E. Colloids Surf. 1990, 49, 183. (18) Albrecht, T. R.; Akamine, S.; Carver, T. E.; Quate, C. F. J. Vac. Sci. Technol., A 1990, 8, 3386. (19) Meurk, A. Personal communication, 1997.
Langmuir, Vol. 14, No. 20, 1998 5677 Electrokinetic Measurements. Electrokinetic measurements were performed with a ZetaSizer MK 4 (Malvern Instruments, Ltd., Malvern, U.K.). The instrument calibration was checked before each experiment using the supplied latex standard and was always within 6% of the quoted value. Procedure. Freshly prepared stock solution (approximately 200 mL) containing the predetermined concentration of copper nitrate and the background electrolyte was equilibrated at the desired pH for approximately 20 min. After this conditioning period, 0.5 mL of a concentrated quartz or silicon nitride dispersion was added using a syringe. Electrokinetic measurements were then taken at regular intervals until the particle mobility reached a steady value, normally 20 min. Frequency Measurements. All frequency measurements were taken with a Digital Instruments, Inc., Nanoscope IIIa atomic force microscope. The Nanoscope’s Auto Tune function was first used to find the exact peak of the resonant frequency. Measurements were then taken at specific time intervals using either the Auto Tune or Find Peak function, both functions were found to give identical results. Procedure. Fresh Milli-Q water was introduced first and then, after sufficient measurements had been taken to establish the reproducibility of the measurement, 20 mL of the equilibrated copper(II) solution or, in the surfactant experiments 10 mL of pH 9 water and then 20 mL of 1.5 mM CTAB solution, were flushed through the AFM liquid cell (cell volume 7 solution. We attribute the observed change in frequency of the AFM cantilevers to the mass of the adsorbed Cu(OH)2. It should be noted that these AFM measurements give no information about the way in which the species adsorb, i.e., whether they adsorb in small clusters distributed over the surface or as a continuous layer. At 1.5 × 10-3 M concentration, CTAB is known to adsorb with a bilayer structure on silicon nitride.13 Drummond and Senden used the adsorption of CTAB onto silicon nitride AFM tips as method for determining the effective interaction radius of the tip during force measurements, the buildup in bilayer structure was followed by testing the strength of the bilayer to rupture by a colloidal particle attached to an AFM cantilever.13 In this study a clear decrease in frequency as with copper(II) adsorption was not observed; see Figure 2b. Although there was a slight decrease in the median resonant frequency, 0.016 kHz, the error in a measurement is approximately 0.01 kHz. There were also a small number of points that show a positive frequency change. Improvements in the experimental apparatus would be needed before adsorption of surfactants such as this could be measured definitively.
Figure 3. Force (in arbitary units) versus separation data between cantilever tip and glass flat surfaces in (a) pH 6.2 Milli-Q water, (b) pH 7.6, 5 × 10-5 M Cu(NO3)2, and (c) pH 8.6, 5 × 10-5 M Cu(NO3)2. In both experiments containing copper(II), 5 × 10-5 M NaCl was present as a background electrolyte. Data in parts a and b were collected with the same tip and can be compared quantitatively.
(iii) Force Measurements. In Figure 3 we show force versus separation data for the interaction between a bare silicon nitride tip and a flat glass substrate in pure water (Figure 3a) and in pH 7.6 ( 0.02 copper solution (Figure 3b). In fresh Milli-Q water, pH ≈ 6.2, a repulsion between the surfaces can clearly be seen, we attribute this to an electrostatic repulsion between the negatively charged glass flat and the less negatively charged silicon nitride tip; see Figure 1. In the presence of pH 7.6 copper(II) solution this repulsion was replaced by an attraction, Figure 3b. Under these solution conditions the silicon nitride cantilever surface carries a positive charge in accord with electrokinetic measurements, see Figure 1b, and the glass substrate has a less complete coverage of Cu(II) and carries a small negative charge, see Figure 1a. The observed attraction is therefore attributed to a weak electrostatic attraction. Between pH 8 and pH 9 it is expected that both silica and silicon nitride would have sufficient coverage of Cu-
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(II) to ensure both surfaces were positively charged; see Figure 1. Force measurements were conducted between a silicon nitride tip and a glass flat surface in pH 8.6 copper(II) solution and are shown in Figure 3c. The expected simple electrical double layer repulsion between two positively charged surfaces was not seen. The interaction data look similar to those recorded in force measurements between solid surfaces in the presence of bridging polymers.23-28 This interaction will be investigated in detail in a future publication. Force measurements taken during the CTAB absorption experiments, not shown, were similar to previously published data and showed little difference with time in agreement with the earlier CTAB study.13 (23) Almog, Y.; Klein, J. J. Colloid Surf. Sci. 1985, 106, 33. (24) Granick, S.; Patel, S.; Tirrell, M. J. Chem. Phys. 1986, 85, 5370. (25) Taunton, H. J.; Toprakioglu, C.; Fetters, L. J.; Klein, J. Nature 1988, 332, 712. (26) Costello, B. A. de L.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1988, 34, 301. (27) Biggs, S. Langmuir 1995, 11, 156. (28) Frank, B. P.; Belfort, G. Langmuir 1997, 13, 6234.
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Conclusion Monitoring the changes in resonant frequency of AFM cantilevers was seen to give a qualitative measurement of the adsorption of Cu(II) onto weak AFM cantilevers. Force measurements indicated that the cause of the decrease in frequency was the mass of the adsorbed species and not to any small viscosity differences between the initial pure water and the copper(II) containing solutions. Maximum coverage of Cu(II), as indicated by an unchanging resonant frequency, was seen to occur within minutes of the introduction of the copper(II) solution. This is in agreement with our earlier electrokinetic studies. The adsorption of CTAB was seen to be beyond the resolution of this technique with the current apparatus. Acknowledgment. I.L. gratefully acknowledges the receipt of a Wenner-Gren Centre Foundation Fellowship. Lennart Bergstro¨m is thanked for his advice about the silicon nitride electrokinetic measurements. LA980294Q
