DOI: 10.1021/la903753z 1501 Langmuir 2010, 26(3), 1501–1503 Published on Web 12/10/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Directed Nanoparticle Motion on an Interfacial Free Energy Gradient Robert Walder, Andrei Honciuc, and Daniel K. Schwartz* Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309 Received October 3, 2009. Revised Manuscript Received November 30, 2009 Using total internal reflection fluorescence microscopy (TIRFM), we have observed the directed motion of 20 nm probe particles on specific regions of surfaces that exhibited strong gradients of hydrophobicity. Patterned surfaces were prepared by selective photodegradation (using a contact photomask) of a hydrophobically modifed fused silica surface. The lateral distribution of hydrophobicity was characterized in situ using the selective affinity of amphiphilic probes (i.e., hydrophobic interaction microscopy). Probe particles were observed to move unidirectionally from regions of lower to higher to hydrophobicity over distances of ∼1 μm when the hydrophobicity gradient was greater than d(cos θ)/dx = 0.05 ( 0.02 μm -1 , where θ is the water contact angle on the bare surface. Only adsorption events were observed on energetically homogeneous surface regions. Since capillary forces are dominant at small length scales, it would be useful to employ these forces to direct the motion of objects in nanoscale devices. Indeed, there has been significant interest in the motion of macroscopic liquid droplets driven by surface gradients including thermocapillary motion, 1 reactive wetting, 2,3 gradients of topography/roughness, 4,5 or lateral gra- dients in the hydrophobic interaction with surfaces. 6-11 In the latter case, a hydrophobicity gradient results in different contact angles on the leading and trailing droplet edges, leading to a net horizontal force in the direction of the smaller contact angle. Surfaces with hydrophobic gradients have been prepared using methods such as vapor-phase diffusion, 6 photoresponsive sur- faces, 7,10,11 photodegradation, 9,12 modification of surface rough- ness, 4,5 and a multitude of other techniques. 13,14 The hydrophobic interaction gradients in these experiments, characterized by con- tact angle measurements, typically occur over several millimeters. Millimeter-sized droplets are then displaced over several milli- meters. Recently, Burgos et al., 15 using fluorescence correlation spectroscopy, observed anomalous dynamic behavior of polymer molecules in the vicinity of a chemical surface gradient that extended over smaller length scales. In this Letter, we show that capillary interactions can be used to move nanometer-scale particles over micrometer-scale distances. In previous work, 16 we demonstrated the ability to use total internal reflection fluorescence microscopy (TIRFM) to quanti- tatively identify hydrophobic and hydrophilic regions of self- assembled monolayers (SAMs) using the affinity of individual fluorescent probe molecules. This approach exploited the sensi- tivity of the probe-surface interactions 17,18 to changes in surface energy in the presence of a particular solvent. 19,20 In the present experiments, this hydrophobic interaction microscopy (HIM) provided an in situ map of the lateral variations in surface functionality and allowed real time correlation of surface func- tionality to other processes in our samples. In particular, we used HIM to directly correlate sub-micrometer unidirectional displa- cements of a 20 nm diameter particle at the solid-liquid interface to gradients in the surface hydrophobicity of photopatterned SAMs. The length scale of this phenomenon is approximately 3 orders of magnitude smaller than previously reported results of hydrophobic gradient driven motion. Patterned surfaces were prepared by selective photodegradation of hydrophobically modified fused silica surfaces. 16 A 50 mm diameter epi-polished fused silica (FS) wafer (MTI Corp.) was cleaned in hot piranha solution for ∼60 min followed by UV- ozone treatment for another ∼60 min. The clean hydrophilic substrate was placed into a sealed glass container containing hexamethyldisilazane (HMDS) (99.8% purity, Acros Organics) and positioned ∼15 cm above the liquid to expose its surface to HDMS vapor for ∼48 h. In contrast with solution deposition of SAMs, this vapor-deposition process ensured that the trimethylsilyl layer contained no fluorescent impurities as confirmed by control TIRFM experiments carried out with pure deionized water (Millipore, Milli-Q UV, 18.3 MΩ 3 cm). These trimethylsilane- modified (TMS-FS) surfaces were then exposed for 200 s to UV illumination from a Hg pen lamp (UVP 254 nm) held ∼5 mm from the TMS surface. The intensity was ∼0.3 mW/cm 2 at this distance. Patterning was accomplished by photodegradation using 500 mesh transmission electron microscopy (TEM) grids (Ted Pella) as contact photomasks. A number of effects combined to create a continuous gradient of photodegradation at the boundaries *To whom correspondence should be addressed. E-mail: Daniel.schwartz@ colorado.edu. (1) Chen, J. Z.; Troian, S. M.; Darhuber, A. A.; Wagner, S. J. Appl. Phys. 2005, 97(1), 9. (2) Bain, C. D.; Burnetthall, G. D.; Montgomerie, R. R. Nature 1994, 372(6505), 414–415. (3) Dossantos, D. F.; Ondarcuhu, T. Phys. Rev. Lett. 1995, 75(16), 2972–2975. (4) Sun, C.; Zhao, X. W.; Han, Y. H.; Gu, Z. Z. Thin Solid Films 2008, 516(12), 4059–4063. (5) Khoo, H. S.; Tseng, F. G. Appl. Phys. Lett. 2009, 95(6), xxx–xxx. (6) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256(5063), 1539–1541. (7) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288(5471), 1624–1626. (8) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291(5504), 633–636. 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