pubs.acs.org/Langmuir © 2010 American Chemical Society
Chemically Programmed Nanomechanical Motion of Multiple Cantilever Arrays Moyu Watari,† Joseph W. Ndieyira,†,‡ and Rachel A. McKendry*,† †
London Centre for Nanotechnology and Division of Medicine, University College London, 17-19 Gordon Street, London WC1H 0AH United Kingdom, and ‡Jomo Kenyatta University of Science and Agriculture, Nairobi, Kenya Received January 29, 2010. Revised Manuscript Received March 1, 2010 Biologically inspired cantilever systems which transform biochemical reactions into nanomechanical motion have attracted attention for label-free biosensing and nanorobotic applications. Here, we take advantage of chemically programmable proton-driven reactions to actuate both the direction and amplitude of nanomechanical cantilever motion in aqueous environments, corresponding to femto-Newton single molecule surface stress. By altering the end groups of self-assembled coatings, we deconvolute the dominant role of surface charge over hydrophilic/hydrophobic interactions and attribute reference cantilever signals to the silicon underside of the cantilever. These findings and underlying concepts will lead to the next generation of massively parallel intelligent nanomechanical systems triggered by self-assembled reactions.
Introduction The exquisite efficiency by which living systems translate molecular events at the nanoscale into mechanical motion on macroscopic length scales through the delicate interplay of hydrophilic, hydrophobic, and electrostatic interactions has inspired the advancing field of cantilever BioMEMS/NEMS. Yet while the tremendous advances of the microelectronic revolution have fabricated massively parallel miniaturized cantilever systems,1 the potential applications of this technology are limited by the development of intelligent coatings to specifically drive cantilever motion due to the apparent complexity of surface stress transduction at organic/inorganic/aqueous interfaces. Recently, our group has made significant experimental2-6 and theoretical6 progress in our understanding of surface stress using cantilevers coated with model alkanethiol self-assembled monolayers (SAMs, HS(CH2)nX), showing that the magnitude of stress depends on the charge and chain length (Youngs modulus) of the SAM.6 Using two different end chemistries (X = COOH and X = CH3), we could unidirectionally actuate the cantilever bending triggered by buffer pH and ionic strength, and this may already find application as a smart environmental switch or drug delivery valve. However, the next generation of intelligent machines will require more sophisticated nanomechanical functions including multiple coatings to simultaneously actuate the upward/downward motion of mechanical devices in response to *Corresponding author. Telephone: 0044 207 679 9995. Fax: 0044 207 679 0055. E-mail: Rachel McKendry:
[email protected]. (1) Fritz, J.; Baller, M. K.; Lang, H. P.; Rothuizen, H.; Vettiger, P.; Meyer, E.; G€untherodt, H.-J.; Gerber, C.; Gimzewski, J. K. Science 2000, 288, 316. (2) Watari, M.; Galbraith, J.; Lang, H.-P.; Sousa, M.; Hegner, M.; Gerber, C.; Horton, M. A.; McKendry, R. A. J. Am. Chem. Soc. 2007, 129, 601. (3) Shu, W.; Liu, D.; Watari, M.; Riener, C. K.; Strunz, T.; Welland, M. E.; Balasubramanian, S.; McKendry, R. A. J. Am. Chem. Soc. 2005, 127, 17054. (4) McKendry, R. A.; Zhang, J.; Arntz, Y.; Strunz, T.; Hegner, M.; Lang, H.-P.; Baller, M. K.; Certa, U.; Meyer, E.; G€untherodt, H.-J.; Gerber, C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 9783. (5) Ndieyira, J. W.; Watari, M.; Donoso-Barrera, A.; Batchelor, M.; Zhou, D.; Vogtli, M.; Bactchelor, M.; Strunz, T.; Abell, C. A.; Rayment, T.; Aeppli, G.; McKendry, R. A. Nat. Nanotechnol. 2008, 3, 691. (6) Sushko, M. L.; Harding, J. H.; Shluger, A. L.; McKendry, R. A.; Watari, M. Adv. Mater. 2008, 20, 3848.
Langmuir 2010, 26(7), 4623–4626
one trigger, for example, to grab a cell with nanotweezers. This requires a fundamental understanding of chemical interactions to specifically and sensitively manipulate motion with nanometer accuracy. In this letter, we take inspiration from biological systems to investigate the actuation of multiple cantilever arrays using proton-driven anionic and cationic interactions in comparison to hydrophilic and hydrophobic interactions. In contrast to the inherent complexity of zwitterionic biological molecules, we utilize model SAMs (HS(CH2)nX) with different end groups, where X = amine (NH2), carboxylic acid (COOH), hydroxyl (OH), and methyl (CH3), maintaining the same number of 11 carbons for all SAMs. Since the ionization of polar end groups depends on the buffer pH, we use arrays of eight silicon cantilevers each coated with SAM to deconvolute the force generated by anionic, cationic, hydrophilic, and hydrophobic interactions, respectively. Though electrostatic and hydrophilic interactions have been extensively characterized, hydrophobic interactions are less well understood and the subject of much scientific interest and debate. Indeed, cantilevers offer a unique tool to study in-plane hydrophilic/hydrophobic and charge interactions at biochemical interfaces, overcoming the three-phase boundary problems associated with contact angle measurements7 and liquid confinement issues surrounding surface force apparatus and atomic force microscopy force measurements.8
Experimental Section Cantilever Functionalization. Our sample preparation, measurement procedure, and data analysis have been detailed previously.2 Briefly, freshly cleaned cantilever arrays (500 μm 100 μm 900 nm, IBM Rushlikon, supplied by Concentris GmBH) were evaporated on one side with 2 nm Ti and 20 nm Au, functionalized via 20 min incubation in microcapillaries filled with 4 mM ethanolic alkanthiol solutions and stored in ultrapure water until use. High purity (>99%) 11-mercaptoundecanoic (7) Kwok, D. Y.; Neumann, W. Ad. Colloid Interface Sci. 1999, 81, 167. (8) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381.
Published on Web 03/11/2010
DOI: 10.1021/la100448v
4623
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Watari et al.
Figure 1. Schematic to illustrate the principle of nanomechanical actuation on multiple cantilevers coated with alkanethiol monolayers with different end groups: COOH, NH2, OH, and CH3. Cantilevers measure 500 μm long, 100 μm wide, and 900 nm thick.
acid HS(CH2)10COOH, 11-amino-1-undecanethiol HS(CH2)11NH2, 11-hydroxy-1-undecanethiol HS(CH2)11OH, and 1-undecanethiol HS(CH2)10CH3 were purchased from Asemblon (Redmond, WA, U.S.A.). All other chemicals were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). Cantilever Measurements. The absolute bending of the eight cantilevers was monitored using the Scentris (Veeco Instruments) optical beam device.2 Cantilevers were exposed to alternate injections of sodium phosphate solutions at pH values at a constant ionic strength of I = 0.1 using a home-built gravity flow system. Our previous work has shown that the magnitude of stress at higher pH is also dependent on the presence of specific counter and co-ions in solution; therefore, experiments reported herein were restricted to the maximum buffering range of sodium phosphate mono and dibasic solution (pH 4.5-9.0) to avoid convolution of different ions in solution. The raw data from 14 different cantilevers on 2 arrays, exposed to 3 pH cycles were analyzed using software and statistical models developed in our group,2 which converted the absolute bending signals (nm) into the difference in surface stress between the upper and underside of a cantilever (mN/m) according to the Stoney’s equation9 Δσ abs ¼
1 t 2 E Δzabs 3 L 1-ν
ð1Þ
where L is the effective length of the cantilever up to 500 μm, t is the thickness ∼ 0.9 μm, and E/(1 - ν) = 181 GPa is the ratio between the Young’s modulus E and Poisson ratio ν of Si (100).10 Contact Angle Measurements. Contact angles were probed in air with a contact angle goniometer (Kruss DSA10, Germany). Each silicon wafer was functionalized with a SAM and characterized using sessile and advancing contact angles with MiliQ water using an initial volume of 15 μL and injection rate of 8 μL/ min.
Results and Discussion The absolute nanomechanical surface stress generated by proton-driven chemical transformations of COOH, NH2, OH, and CH3 functionalized cantilevers on a single array is shown in Figure 2a. Absolute bending signals are notoriously convoluted by nonspecific signals, including temperature, refractive index, nonspecific binding to the SAM or underlying gold, and reactions occurring on the “bare” silicon oxide underside of the cantilever. To overcome these artifacts, we acquired differential measurements using an in situ reference cantilever coated with the CH3 SAM. Figure 2b shows the differential signals for COOH/CH3 for (9) Stoney, G. G. Proc. R. Soc. London, Ser. A 1909, 82, 172. (10) Brantly, W. A. J. Appl. Phys. 1973, 44, 534.
4624 DOI: 10.1021/la100448v
the pH 4.8 to 8.4 switch is compressive (corresponding to downward bending) ∼ -8.7 mN/m; NH2/CH3 is tensile (corresponding to upward bending) þ9.0 mN/m and a small tensile differential stress change of þ1.5 mN/m on the OH/CH3 SAMs. When the pH was switched back to 4.8, all cantilevers converged back toward the zero-stress baseline, indicating reversible interfacial interactions. To further investigate the nanomechanical response of different end groups, we probed differential signals on 14 different cantilever over an extended pH range between 3.5 and 8.4, with 3 repeat cycles, and determined the average and standard error between chips using in-house semiautomated software to remove user bias. Our findings reveal a monotonic differential stress profile: the COOH/CH3 differential stress was found to render progressively compressive (downward bending of the cantilever) with pH and measured þ0.5 ( 3.2 mN/m at pH 3.5 and -7.7 ( 4.4 mN/m at pH 8.4; the NH2/CH3 differential stress became increasingly tensile (upward bending of the cantilever) -5.2 ( 0.1 mN/m at pH 3.5 and þ7.8 ( 1.5 mN/m at pH 8.4; conversely, the OH/CH3 mean differential stress was found to be nondistinguishable from zero over the full pH range, i.e., þ0.2 ( 0.1 mN/m at pH 3.5 and þ1.5 ( 1.5 mN/m at pH 8.4 (no significant bending of the cantilever). Scheme 1 illustrates our model to rationalize these findings by directly correlating the cantilever bending with the hydrophilicity, hydrophobicity and charge state of the SAMs. The COOH terminated SAM is hydrophilic (contact angle with water is
