pH-Dependent Control of Particle Motion through Surface Interactions with Patterned Polymer Brush Surfaces Gary Dunderdale,* ,† Jonathan Howse, ‡ and Patrick Fairclough † † Department of Chemistry, and ‡ Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S3 7HF, United Kingdom * S Supporting Information ABSTRACT: In this Article, we show that inclined silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the position and movement of 20 μm silica particles, which are propelled across the patterned surface by sedimentation forces. Three different types of behavior were observed depending on the angle between the direction in which a particle sedimented and the orientation of the polymer-brush silicon interface. At small angles, particles were found to sediment to the brush interface and then sediment following the direction of the brush interface. At larger angles, particles sedimented to the interface and then followed the direction of the brush interface, but then after a certain distance changed direction to pass over the interface. At the largest angles where the brush interface was approximately perpendicular to the motion of the particle, particles were found to travel over the interface unperturbed. This behavior was also found to be pH dependent, allowing the formation of pH responsive “gates”, which allow particles to pass at low pH but not at high pH. It was also found that if patterned polymer brush surfaces were oriented in the correct way, they were able to control the number of particles present at specific locations. ■ INTRODUCTION The precise placement and control of small particles is of great interest both scientifically and commercially. Precise placement of particles can be achieved through the formation of particle arrays in which the particles are bound to surfaces through electrostatic forces, 1-3 hydrophobic forces, or a specific biointeraction. 4 Although highly successful at getting a particle to a precise location, these techniques attach particles to the surface, meaning that particles are at least temporarily immobilized. To move particles dispersed in liquids, externally applied forces such as electrophoresis 5,6 or hydrodynamic flow 7 can be used. These usually have the drawback of moving all particles dispersed in the liquid in an identical fashion, meaning that different particles cannot move in different directions simultaneously. Alternatively, optical tweezers can precisely control a particles position, 8 although they can only manipulate a small volume of particles due to the size of the highly focused laser beam. Particles have been precisely placed and their motion controlled in microfluidic devices, enabling them to sort, separate, purify, and analyze dispersions of particles. So far, microfluidic devices have directed particles into channels in a variety of ways including applied optical forces 9 and by deflection switches, 10 which use hydrodynamic flow. They can direct particles into thin lines by hydrodynamic flow focusing 7 and sort them from solution through gravitational separation. The precise placement and control of particle motion has been studied using computational modeling by the Balazs group over numerous years. 11-15 They have suggested that a microparticle’s position and motion can be controlled through its surface interactions with a microfluidic channel, which is patterned appropriately. This leads to great advantages over existing techniques, as particles do not have to be tethered to a surface, and in contrast to the use of external fields, particles do not have to show identical motion, and so different particles can move in different directions simultaneously. Unlike optical tweezers, large numbers of particles can be manipulated. Recently, this approach has been implemented by Edington et al. 16 to control the motion of leukocyte cells driven along a patterned surface by the flow of water. We have also taken this approach and have created patterned surfaces that can control the position and motion of small particles through repulsive surface interaction forces. To create these patterned surfaces, silicon surfaces are functionalized with a polymer brush. These polymer brushes have been widely studied and are easily prepared by surface initiated polymerizations. 17 They have also been used in microfluidic devices for a variety of purposes: for example, to selectively trap and then release proteins 18 or nanoparticles 19 in response to a thermal stimulus, and to control the position of nanoparticles and proteins 20 by adsorption onto a patterned surface, creating an array. Poly(methacrylic acid) (PMAA) brushes are particularly useful as they show a volume response to changes in pH. They have also been proposed as nanoscale Received: June 14, 2012 Revised: August 8, 2012 Published: August 14, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 12955 dx.doi.org/10.1021/la302384j | Langmuir 2012, 28, 12955-12961