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Single-Nanoparticle-Terminated Tips for Scanning Probe Microscopy Ivan U. Vakarelski and Ko Higashitani* Department of Chemical Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed October 19, 2005. In Final Form: February 7, 2006 We have developed a wet-chemistry procedure to attach a 10-40 nm colloidal gold nanoparticle to the top of a scanning probe microscopy (SPM) probe tip, making experiments of single nanoparticle interaction possible. This procedure of particle attachment is flexible and can be modified to attach nanoparticles of different kinds and sizes. The single-nanoparticle-terminated tips also have potential in various other applications, such as probes of enhanced sensitivity for optical and magnetic modes SPM.
SPM techniques initiated with scanning tunneling microscopy1 and followed by atomic force microscopy2 (AFM) have revolutionized surface science by allowing studies of the surface topography and a variety of other surface properties on an angstrom-to-micrometer scale. The common principle of SPM techniques involves the utilization of a very sharp tip as a probe to scan across and image a surface. In the case of AFM, the interaction force between the tip and the surface is determined from the bending of a microfabricated cantilever with the tip positioned at the free end. In addition to topographic imaging, AFM allows for the direct measurement of the surface and intermolecular forces with near-molecular-scale resolution. Ducker et al.3,4 introduced the so-called “colloidal probe technique” in 1991 by attaching a micrometer-sized spherical particle to the AFM cantilever. Since then, the technique has become a powerful tool for the quantitative measurement of colloidal forces, which can dictate the behavior of a wide range of micro- and nanoscale systems, such as particle dispersions and biological macromolecules. Here we introduce a wetchemistry method that extends the applicable size range of the particles used in the current colloidal probe technique from the micrometer range to the submicrometer and nanometer particle size ranges, allowing various single-nanoparticle force measurements to be made with AFM sensitivity. We also foresee additional applications of these nanoparticle-terminated tips that will take advantage of or be used to analyze nanoparticle-specific properties (e.g. optical, electronic, magnetic, etc.). The majority of surface forces between colloidal particles scale with the particle radius. However, when the size of the particles becomes comparable to the range of the interaction force, some deviations can be expected. Under such conditions, the Derjaguin approximation,5 which is commonly used to scale the interaction forces by the particle radius and the interaction energy between flat surfaces, might no longer be applicable.6,7 Additionally, the discrete nature of solvent media species might become pronounced at short length scales, violating the continuum approach. In the past, investigations into the scaling of nanoparticle interaction force measurements with particles size were performed using an * To whom correspondence should be addressed. E-mail: k_higa@ cheme.kyoto-u.ac.jp. Tel: +81-(0)75-383-2662. Fax: +81-(0)75-383-2652. (1) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. ReV. Lett. 1983, 50, 120. (2) Binnig, G.; Quate, C. F.; Gerber, C. Phys. ReV. Lett. 1986, 56, 930. (3) Ducker, W. A.; Senden, T. J.; Pasheley, R. M. Nature 1991, 353, 239. (4) Ducker, W. A.; Senden, T. J.; Pasheley, R. M. Langmuir 1992, 8, 1831. (5) Derjaguin, B. V. Kolloid Z. 1934, 69, 155. (6) Stankovich, J.; Carnie, S. L. Langmuir 1996, 12, 1453. (7) Bhattacharjee, S.; Elimelech, M. J. Colloid Interface Sci. 1997, 193, 273.
AFM tip as a nanometric probe.8,9 In these types of measurements, the force scaling was compared by assuming an effective tip radius. Because the tip shape was irregular and had a conicallike rather than a spherical shape, its effective radius could be different at different separations. Therefore, a quantitative comparison of these tip measurements with theory or the results of larger-sized particles may be inaccurate. This problem can be overcome if the measurements are directly performed using a nanoparticle of a well-defined spherical shape attached to the end of the tip. By attaching nanoparticles to the end of an SPM probe tip, measurements for different types and sizes of particles can be directly compared. In the colloidal probe technique, particle attachment is achieved using a micromanipulator and an optical microscope, thus limiting the minimum particle size to the optical resolution limit, ∼1 µm. The attachment of smaller particles with this technique is further hindered by the increasing possibility of contamination from the adhesive used, usually an epoxy resin. To address these issues, we took advantage of principles in self-assembly and surface chemistry. By using a molecular glue rather than a macroscopic one, we can successfully deposit a single particle at the end of an AFM tip without the limitations imposed by either micromanipulation or glue contamination. We initiate the process by depositing a monolayer of bifunctional organic molecules onto the end of a sharp probe tip. Through the appropriate selection of exposed functional groups and tip geometry, we can then assemble a single particle at the end of the probe by simply dipping the tip into a nanoparticle suspension. The same principles of chemical cross-linking and nanoparticle deposition from suspensions have been widely used to manufacture nanoparticlefunctionalized surfaces.10,11 In the procedure that we have developed, gold nanoparticles were covalently bound to the tip surface coated with a monolayer of (3-mercaptopropyl)trimethoxysilane (3-MPTS). Gold nanoparticles immobilized on thiol monolayers are one of the most thoroughly studied nanoparticle-ligand systems. Moreover, nanoparticles of relatively high monodispersity and sphericity in the size range of 10 to 40 nm can be easily prepared using a standard citrate reduction method (Supporting Information). Selectivity of the particle attachment on the tip end was achieved by initial silanization of (8) Drummond, C. J.; Senden, T. J. Colloids Surf., A 1994, 87, 217. (9) Todd, B. A.; Eppell, S. J. Langmuir 2004, 20, 4892. (10) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (11) Grabar, K. C.; Allison, K. A.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Dolan, C. M.; Freeman, R. G.; Fox, A. P.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353.
10.1021/la0528145 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006
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Figure 1. Schematic diagram showing the procedure to attach a nanoparticle to the cantilever tip. (A) The tips are silanized with a PETS passivation layer and then (B) scanned over a silica wafer to wear them and (C) functionalized with 3-MPTS. (D) The tips are then dipped in a gold nanoparticle suspension. If the procedure is shortened by skipping step A, then the tips are coated with the nanoparticles.
the tip surface with a passive organosilane monolayer. An alternative to the wet-chemistry approach that we proposed can be to scale down the colloidal probe technique using a combination of a scanning electron microscope (SEM) and a nanomanipulator.12 However, exposing the nanoparticle to the ion beam in the SEM vacuum chamber is likely to coat the particle with a layer of carbon contamination. Another factor is that nanomanipulators are expensive and available only in a few laboratories. The wet-chemistry procedure that we have developed, however, can be performed using commercially available reagents and basic laboratory equipment. The main steps of the nanoparticle attachment procedure are schematically presented in Figure 1. We used standard silicon nitrate tip cantilevers with a tip apex angle of less than 30° and a radius of curvature of less than 20 nm (Olympus). The surface of the tip was made chemically passive by silanization with a monolayer of phenethyltrichlorosilane (PETS), which was chosen because it was moderately hydrophobic compared to the usually employed alkysilane monolayers (Supporting Information). Next we removed the passivation layer from the end of the tip to expose a fresh oxide surface for 3-MPTS deposition by rastering the tip against a clean silica wafer surface in an AFM. The nanoparticles were subsequently attached by first silanizing the tips with 3-MPTS and finally dipping them in a gold nanoparticle suspension for a certain period of time. The probes were then washed by gradually diluting the suspension with pure water to mitigate nonspecific nanoparticle deposition. The cantilever/tip arrangements were subsequently removed from solution and used for force measurements or were stored in a clean vacuum chamber for future use. A shortened variant of the described procedure was initially used to optimize the process parameters. In essence, this attachment route is identical to the lengthier version except for the initial tip-passivation step (PETS silanization). This abbreviated version results in the deposition of multiple gold nanoparticles distributed on the tip, although a single nanoparticle is often found to reside at the tip end (Figure 2B). After optimizing the parameters in each step, we were able to achieve a success rate of as high as 30-50% for attaching a nanoparticle to the end of the probe. For a batch of six levers that was usually (12) Nishijima, H.; Akita, S.; Nakayama, Y.; Houmura, K. I.; Yoshimura, S. H.; Takeyasu, K. Appl. Phys. Lett. 1999, 74, 4061.
Figure 2. SEM images showing (A) an unmodified silicon nitride tip (Olympus TR400PSA) and (B) a tip coated with gold nanoparticles, which have an average size of 40 nm. Images of the tips terminated with single gold nanoparticles with approximate sizes of 25 and 14 nm are shown in C and D, respectively. In all cases, the tips were spattered with a gold coating of about 2 nm to enhance the quality of the image.
prepared, we could obtain two to three nanoparticle-terminated tips. The experimental details of the particle attachment procedure are given in Supporting Information. Typical scanning electron microscope (SEM) images of single-nanoparticle-terminated tips are shown in Figure 2. The robustness of the particle attachment was confirmed by performing test engagements of the tips with a flat mica surface, prior to taking the SEM micrographs. The procedure developed here can be extended to nearly any kind of nanoparticle if the surface chemistry of the nanoparticle includes ligation to some surface-confined functional group. Metallic nanoparticles such as Au and Ag can be attached to a variety of thiol and amino organosilanes.11 Semiconducting CdSe nanoparticles have also been reported to covalently bind to thiolterminated monolayers,10 some varieties of latex nanoparticles bind to amine-terminated monolayers,13 and so on. New types of functional surfactants such as ω-alkene-1-thiols14 and dithiocarbamates15 might provide more robust attachment links, and schemes for nanoparticle attachment to silicon or metal tips can be developed as well. However, gold nanoparticles can be further functionalized with thiol monolayers containing different end groups in order to achieve particles with tunable physical and chemical properties or to have biological recognition elements such as those used in chemical force microscopy16 (CFM). The difference between CFM and other techniques is that the (13) Park, J.; Lee, H. Colloids Surf., A 2005, 257-258, 133. (14) Yamanoi, Y.; Yonezawa, T.; Shirahata, N.; Nishihara, H. Langmuir 2004, 20, 1054. (15) Zhao, Y.; Perez-Segarra, W.; Shi, Q.; Wei, A. J. Am. Chem. Soc. 2005, 127, 7328. (16) Frisbie, C. D.; Rozsnyai, A.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071.
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Figure 3. Force vs distance data for the approach interaction between a mica surface and an R ) 20 nm gold-nanoparticle-terminated tip (upper force curve), an R ) 10 nm gold-nanoparticle-terminated tip (middle force curve), and a PETS silanized tip without a particle (lower force curve) in a solution of 10-3 M NaCl of pH 5.6. The inset shows the raw force data thermal noise away from the surface. Rough noise filtering was performed by taking each point as the average of 10 neighboring raw data points. Raw approach and separation data examples are available in Supporting Information.
functionalization here is localized on the nanoparticle alone. Another promising option is to use the initially deposited particle as a core particle for core-shell nanoparticle deposition, for example, gold-core silica-shell nanoparticles.17 With respect to the size of the attached particles, there is in principle no limitation with respect to attaching particles that are smaller than the ones we used here, approximately 10-40 nm in diameter. However, for particles smaller than 10 nm, the radius of curvature of the standard tips might be rather high, and the signal-to-noise ratio for long-range force measurements may be unfavorable (Figure 3). An appropriate alternative to the attachment of a particle that is only a few nanometers in diameter could be to use carbon nanotube tips,18 whose ends can be covalently functionalized.19 Such nanoparticle-terminated carbon nanotube tips would be appropriate for measurements against deformable surfaces, such as cell membranes, where a high aspect ratio is desirable. Another attractive option is to deposit a single metal nanoparticle onto the end of a tip to act as a seed for direct nanotube growth. To demonstrate the ability to measure long-range force with a single-nanoparticle-terminated tip, we performed a series of measurements between gold nanoparticle-functionalized tips and a mica surface in an aqueous electrolyte solution of 10-3 M NaCl. The sizes of the nanoparticles used were between 40 and 20 nm (i.e., radius (R) values of 20 and 10 nm). Measuring the forces with the nanoparticle-functionalized tips requires special precautions so as to avoid damaging the probes (Supporting Information). The conversion of piezodisplacement versus cantilever deflection data to force versus separation distance data was performed using a standard procedure.4 Representative approaching force curves collected for gold nanoparticles of R ) 20 and 10 nm and a PETS-passivated bare (without an attached nanoparticle) tip are compared in Figure 3. The data show a clear difference between passivated tips and tips with gold nanoparticles of different sizes. For the case of the passivated tip, the surface (17) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (18) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147. (19) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52.
Figure 4. Force scaled by the particle radius vs separation distance data for (A) R ) 20 nm and (B) R ) 10 nm particles fitted with the DLVO theory for the constant surface charge (upper line) and constant surface potential (lower line) modes, respectively. The fitting parameters, which assumed equal potentials (ψ) for the particle and the surface, were ψ ) -80 mV and κ-1 ) 11.5 nm for R ) 20 nm and ψ ) -75 mV and κ-1 ) 13.8 nm for R ) 10 nm. The Hamaker constant was 8.2 × 10-20 J as calculated from the optical properties of the mica/water/gold system.20
did not have a measurable charge, and the only interaction force detected was short-range van der Waals attraction, which acted from a distance of about 5 nm. In all cases, the attached gold nanoparticles protruded more than 10 nm from the tip end, which therefore excludes the possibility that the side of the tips affect the measurements. Force curves scaled by the particle radius for the R ) 20 and 10 nm cases are compared in Figure 4 and are formally fitted with the standard DLVO theory.20 The most important feature observed was an apparent increase in the electric double layer interaction decay length (i.e., the Debye length (κ-1)) from about 11.5 nm for the R ) 20 nm particle to 14.0 nm for the R ) 10 nm particle. These values are well above the theoretical value of κ-1 ) 9.6 nm for 10-3M NaCl and κ-1 , R.20 This increase in the Debye length with a decrease in particle size was confirmed using several other probes. We found the same trend in the experimental data by Todd and Eppell9 for a silicon nitrate tip of effective radius 7 nm, where we estimated κ-1 ) 18 nm for 10-3M NaCl. A second trend that was found from these experiments was a significant decrease in the scaled electric (20) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992.
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double layer force value of about 20-40% for R ) 10 nm particles compared to the value for R ) 20 nm particles at small separation distances. It is possible that the deviations in the force magnitude may reflect differences in the particle surface potentials or errors associated with the cantilever spring constant estimation; however, the observed disparity in the decay length provides reasonable evidence indicating deviations in the scaling law. These deviations from the scaling law are likely due to the breakdown of the Derjaguin approximation for small values of κR, leading to some overestimation of the real force.6,7 If this overestimation is stronger at smaller separations, then the result could be an apparent increase in the Debye length. Such trends showing an increase in deviations at closer separations can be followed in some of the theoretical estimations performed by Carnie and Stankovich for κR ) 1 under constant charge regulation.6 It should be noticed that the theoretical estimations of the magnitude of the scaled force decrease with particle size variation at large limits depending on the calculation approach. For example, Carnie and Stankovich6 estimate a minimal deviation for the constant surface potential regulation interaction mode, whereas the surface element integration method of Bhattacharjee and Elimelech7,9 predicts a significant difference. It is beyond the scope of this work to attempt to clarify the reasons for such discrepancies, but the need for experimental confirmation even for the “simplest” case of purely electric double layer interaction is apparent. Measurements on this scale are relevant for the accurate prediction of the behavior of polymers, biological macromolecules, and other nanoscale colloids. The nanoparticle-terminated tips described here are expected to be applied to numerous studies of both fundamental and applied nanoscience. By confining the nanoparticles between the tip and substrate, one can design experiments to investigate singlenanoparticles properties such as elasticity, conductivity, and optical and magnetic responses. This process could also be used
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to fabricate probes with improved sensitivity for other forms of SPM, especially those that utilize a combination of specific sensor properties such as light reflectance or the magnetic moment that may be focused onto the nanoparticle itself, thereby decreasing the background signal from the tip. For example, metalnanoparticle-terminated tips are the perfect candidate for probes in apertureless scattering near-field optical microscopy.21,22 Magnetic-nanoparticle-terminated tips can be used to increase the resolution in magnetic force microscopy,23 and with some modifications, the method might contribute to the development of higher-sensitivity levers for dynamically developing magnetic resonance force microscopy.24,25 Because the majority of the fabrication steps in the method that we have introduced here are wet-chemistry-based, we believe that they can be adopted for the development of wafer-scale production of nanoparticle-terminated tips, thereby providing a means for large-scale research applications. Acknowledgment. This work was partially supported by a postdoctoral research grant from the Japanese Society for the Promotion of Science (JSPS). Supporting Information Available: Experimental details and raw force curve data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0528145 (21) Zenhausern, F.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1994, 65, 1623. (22) Kawata, Y.; Urahama, S.; Murakami, M.; Iwata, F. Appl. Phys. Lett. 2003, 82, 1598. (23) Rugar, D.; Mamin, H. J.; Guethner, P.; Lambert, S. E.; Stern, J. E.; McFadyen, I.; Yogi, T. J. Appl. Phys. 1990, 68, 1169. (24) Mamin, H. J.; Budakian, R.; Chui, B. W.; Rugar, D. Phys. ReV. Lett. 2003, 91, 207604. (25) Rugar, D.; Budakian, R.; Mamin, H. J.; Chui, B. W. Nature 2004, 430, 329.
