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Langmuir 2008, 24, 8918-8922
Sequential Electrochemical Oxidation and Site-Selective Growth of Nanoparticles onto AFM Probes Haitao Wang, Tian Tian, Yong Zhang, Zhiqiang Pan, Yong Wang, and Zhongdang Xiao* Biomedical Engineering, Southeast UniVersity, SiPaiLou 2#, Nanjing 210096, China ReceiVed February 13, 2008. ReVised Manuscript ReceiVed June 9, 2008 In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH4 and further development (if desired) to give a metal nanoparticlemodified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).
Introduction In addition to topographic imaging, atomic force microscopy (AFM) can also be used to measure the surface and the intermolecular forces with near-molecular-scale resolution. Ducker et al.1 introduced the so-called colloidal probe technique in 1991 by directly attaching a micrometer-sized spherical particle to the AFM cantilever for surface force measurement. Since then, the technique has become a powerful tool to study the molecular forces and mechanics of biological cells.2–4 In a colloid probe preparation process, a silica glass bead is used as the colloid particle and is mounted near the apex of the cantilever using an extremely small amount of epoxy resin by means of a micromanipulator. Because of the small size of the glass beads, the adhesion process should be carefully controlled to make the glass bead just adhered to the right position on the probe. Such a method needs special equipment and experience. Large particles of several to tens of micrometers were conveniently to glued onto AFM cantilevers by this method and were used to study particle-particle adhesion.5 Recently, Sokolov et al.6 employed this gluing method to attach single ∼50 nm and larger ceria nanoparticles to standard Si3N4 pyramidal tips and investigated the interaction between the nanoparticle and the flat silica surface, and demonstrate that a nanoparticle has different adhesion and long-range forces from those of a larger particle. In fact, one characteristic of nanoparticles is their size effect, that is, a small change in size can result in huge alterations in property. Therefore, it is valuable to attach single nanoparticles of various sizes, even several nanometers, to AFM tips for studying the physical chemistry of nanoparticles. However, AFM tips can be coated with various metals (commercially available from AFM tips suppliers) and used in * Corresponding author. E-mail:
[email protected]. (1) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (2) Sokolov, I.; Iyer, S.; Woodworth, C. D. Nanomed. Nanotechnol. Biol. Med. 2006, 2, 31. (3) Berdyyeva, T. K.; Woodworth, C. D.; Sokolov, I. Phys. Med. Biol. 2005, 50, 8. (4) Park, S.; Koch, D.; Cardenas, R.; Kas, J.; Shih, C. K. Biophys. J. 2005, 89, 4330. (5) Hodges, C. S.; Cleaver, J. A. S.; Ghadiri, M.; Jones, R.; Pollock, H. M. Langmuir 2002, 18, 5741. (6) Ong, Q. K.; Sokolov, I. J. Colloid Interface Sci. 2007, 310, 385.
AFM based nanolithography.7–9 Although the gold- or platinumcoated tips are commercially available, they are usually very large, the tip radii ranging from 35 to 100 nm, and thus hinder the resolution of the pattern fabricated by AFM nanolithography. A possible method to improve the resolution is to attach one nanoparticle to the tip apex of the AFM probe and use it in nanolithography. For example, an AFM tip functionalized with a nanoparticle of specific catalytic activity can be used to prepare nanowire or carbon nanotube AFM probes. AFM tips functionalized with magnetic nanopatricles can be used to increase resolution in magnetic force microscopy.10 Metal nanoparticle-modified AFM tips are the perfect candidates for probes in apertureless scattering near-field optical microscopy (ANSOM) because of the high spatial resolution and strong nearfield effects,11–16 and therefore, it promises unprecedented optical resolution (
