Article pubs.acs.org/Langmuir
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.
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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 electrophoresis5,6 or hydrodynamic flow7 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 forces9 and by deflection switches,10 which use hydrodynamic flow. They can direct particles into thin lines by hydrodynamic flow focusing7 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 © 2012 American Chemical Society
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 proteins18 or nanoparticles19 in response to a thermal stimulus, and to control the position of nanoparticles and proteins20 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 12955
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actuators,21,22 able to push and pull small objects around, and are able to control the permeability of pores in a gel.23 Polymer brushes are widely known to be nonfouling24 and have also been shown to reduce frictional forces between surfaces and objects sliding along them, making them ideal candidates for use in the manipulation of small objects. They have also been used in chromatographic separation of small objects dispersed in water such as proteins.25 In this Article, we will show that small silica particles can be manipulated through repulsive surface interactions with suitably patterned poly(methacrylic acid) surfaces. This manipulation allows the particles to be directed from one region to another, gates formed where the particles may only pass through if the pH is low, and allows the particles to be counted by the polymer brush surface.
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observed under a microscope (Nikon Ellipse ME2000) at 10× magnification using episcopic lighting. Movies of the motion were recorded using a Pixelink PL-A742 machine vision camera at 10 frames per second for 100 s. It was necessary to refocus the microscope several times during movie capture to maintain good focus on the sedimenting particle. The motion of particles was tracked after movie capture using a custom built LabView script, which determined the position of particles in each frame of the movie. Ellipsometric Measurements. The thickness of polymer brushes were determined on unpatterned substrates using a J.A. Woollam spectroscopic ellipsometer and fitting data with a Cauchy layer model when dry or an effective medium approximation consisting of a Cauchy material and water when wet.
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RESULTS AND DISCUSSION Patterned Polymer−Brush Surface Preparation. To create patterned surfaces of silicon/poly(methacrylic acid), we used a combination of photolithography and surface initiated atom transfer radical polymerization (ATRP).17,28 In this protocol, a silane monolayer of an ATRP initiator was deposited onto silicon surfaces, and then selected areas of this monolayer were removed by exposure to UV light through a photomask. This gave a surface patterned with the ATRP initiator monolayer, which was then amplified into a polymer brush by ATRP of t-butyl methacrylate. Photolithography was confirmed to have successfully removed selected areas of the initiator monolayer by visual examination of the surfaces after polymerization. This examination showed a pattern of light and dark areas identical to the pattern of photomask used in the photolithography. These produced polymer brushes of poly(t-butylmethacrylate) were then hydrolyzed to give brushes of poly(methacrylic acid) (PMAA). Hydrolysis was confirmed to have been successful by measuring the thickness of the brush layer by ellipsometry, which showed a reduction in height of around 60% after hydrolysis. Ellipsometric titration showed that the brush became pH responsive after hydrolysis, with a pKa of 6.9, consistent with literature sources.29 In this fashion, areas of the silicon surface were functionalized with a polymer brush, resulting in a surface that has a corrugated height profile due to the protruding polymer brush. Ellipsometry on unpatterned polymer brush surfaces showed that the polymer brush was typically over 100 nm thick in water at pH 10, meaning that areas of silicon surfaces functionalized with PMAA brushes are over 100 nm higher than areas that are unfunctionalized. Behavior of Silica Particles Driven along Patterned Surfaces. Following successful preparation of patterned surfaces, we investigated if these surfaces can control the position and motion of silica particles in aqueous dispersion. Using the patterned surface as the bottom surface, an observation chamber was created using double-sided adhesive as a gasket and a glass slide as the top surface. This chamber was filled with a small volume of the silica particle dispersion, and allowed the position and motion of silica particles to be observed by optical microscopy through the transparent glass top surface. As the silica particles are more dense than water, gravity causes them to sediment downward through the liquid toward the patterned surface. As the particles come into close proximity to the patterned surface, they start to experience repulsive surface forces due to the double-layer forces acting between two charged surfaces, or by steric repulsion acting between the polymer brush and the particle. The closer the
EXPERIMENTAL SECTION
Materials. All chemicals were purchased from Sigma-Aldrich and used as received except the silane initiator [11-(2-bromo-2-methyl) propionyl] undecyl trichlorosilane, which was synthesized according to Matyjaszewski et al.26 Silica particles (20 μm) were purchased from Kisker biotech. Preparation of Patterned Silane Initiator Monolayers. Silicon (100) wafers were cut into small pieces and cleaned by submerging in piranha solution for several hours, rinsed with water and then ethanol, and dried in an oven overnight. The clean silicon was then functionalized with the silane initiator by submerging in 10 mL of 5 μM solution of the silane initiator in dry toluene with 100 μL of triethylamine added. After 6 h of immersion, the silicon substrates were removed from the solution, washed with dry toluene and then ethanol, and blown dry in a stream of nitrogen. The functionalized silicon substrates were then patterned by contact photolithography, which removed the silane monolayer in selected areas by exposure to UV light from a halogen light source using a TEM grid (Agar Scientific) weighed down with a quartz slide as a photolithography mask.27 Following photo-oxidation, the substrate was washed with toluene and then ethanol and blown dry in a nitrogen stream. Patterned substrates were used immediately. Atom Transfer Radical Polymerization of t-Butyl Methacrylate from Patterned Silane Monolayers. 10 mL of t-butyl methacrylate, 5 mL of 1,4-dioxane, and 100 μL of pentamethyldiethyltriame were added to a reaction tube (Radley’s) with a cross-hair stirrer bar. A patterned silicon substrate along with an unpatterned silicon substrate were suspended in the reaction solution using stainless steel wire. The solution was then purged with nitrogen while being stirred for 30 min to remove oxygen. After 30 min, 50 mg of copper(I) chloride was added while the nitrogen flow was maintained. The tube was resealed and purged with nitrogen for a further 5 min, then heated to 50 °C and left to react for 14 h. Polymerization was then quenched by opening the tube, allowing oxygen to enter. Following polymerization, substrates were rinsed with copious amounts of 1,4-dioxane and sonicated in several washings of dilute acetic acid to remove copper(II), followed by washing with water and ethanol. Hydrolysis of Poly(t-butyl methacrylate) to Poly(methacrylic acid). Substrates with grafted poly(t-butyl methacrylate) were suspended in a reaction tube using stainless steel wire, and 15 mL of 0.2 M p-toluene sulfonic acid in 1,4-dioxane was added. The mixture was then heated to reflux and left to react for 24 h, after which substrates were washed with excess 1,4-dioxane and then ethanol and dried in a nitrogen stream. Observation of Particles on Patterned Substrates. Silica particles 20 μm in diameter (Kisker Biotech) were dispersed in distilled water and adjusted to the appropriate pH using dilute sodium hydroxide or hydrochloric acid solution. A drop of this dispersion was then sandwiched between a patterned substrate and a clean microscope slide using 0.1 mm thick double-sided adhesive (Grace Bio-Laboratories) as a gasket. Particles on the substrate were then 12956
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particles come to the surface, the stronger these repulsive surface forces become. Therefore, they come to rest at a position above the surface where the gravitational force acting downward is balanced by repulsive surface forces acting upward. By inclining the surface to the angle θ from the horizontal (see Figure 1), particles are driven by the combination of the
Figure 1. Schematic of the experimental setup. Silica particles (in aqueous dispersion) sediment down the sloped surface patterned with PMAA (dark stripes) and bare silicon (light stripes). A movie clip of this motion is captured via optical microscopy and particle tracking employed to find each particle’s trajectory (red line). Figure 2. Trajectories of silica particles on patterned PMAA brush surfaces at different angles of incidence, as they sediment from their initial position shown on the right to their final position shown on the left. PMAA regions appear dark and silicon regions light. (A) α = 82°, (B) α = 35°, (C) α = 25°. The pH of the solution was 10, the brush height was 85 nm, and the width of the pattern stripes was 20 μm.
gravitational force and the repulsive surface forces across the surface patterning oriented at the angle α from the direction of sedimentation. From optical microscopy, movies of the silica particles sedimenting across patterned surfaces were captured and analyzed by a custom-made LabView script to find the trajectory and velocity of the silica particles. In all of the following experiments, an inclination angle of θ = 4.5° was chosen, which propelled particles at velocities of ∼2 μm s−1 across the surface. Particles were observed to sediment across regions of the polymer brush-functionalized surface at the same velocity as they do on the silicon surface unfunctionalized with polymer brush. This leads us to believe that either particles do not come into contact with the polymer brush and are electrostatically repelled above the brush surface or the friction encountered between the brush and particle30,31 is much smaller than the viscous drag experienced by the particle when close to the surface.32 Figure 2 shows the trajectories typical of silica particles driven across a patterned surface, in water at pH 10. While being driven across the surface, particles travel from areas of bare silicon to areas of the surface functionalized with poly(methacrylic acid) and vice versa. We define the angle α as the angle between the direction in which particles are driven along the surface due to sedimentation and the orientation of pattern interfaces (see Figure 1). Three different types of behavior of silica particles were observed depending on this angle, α. When the angle α is large as in Figure 2A (α = 82°), the particle travels over the interfaces between silicon and PMAA regions relatively unperturbed, both in the silicon-to-brush and in the brush-to-silicon direction. Analysis of the particles
positions in different frames of the movie showed that the particle velocity remains constant when passing over the interface in both the brush-to-silicon and the silicon-to-brush directions. We call this type of observed behavior noncompliant (N), as the particle does not comply with the direction of the brush interface and sediments over the interfaces unperturbed. When α is smaller as in Figure 2B (α = 35°), the behavior changes, and particles are now perturbed by the interfaces of the patterned surface. The particle sediments across the silicon surface in the direction of gravity until it reaches the brush interface, at which point its motion is deflected to sediment in a direction that follows the brush interface. After sliding along the interface for some distance, the particle passes over the interface and resumes its motion in a direction parallel to its original course of motion. Upon traveling over the region of PMAA, the particle travels over the interface in the brush-tosilicon surface unperturbed. We call this type of behavior partially compliant (P), as the particle sediments in the direction of the brush interface part of the time. When α is small as in Figure 2C (α = 25°), particle behavior changes once again. Now particles travel along the silicon surface in the direction of gravity until they reach the brush interface, at which point they change direction and sediment in the direction of the brush interface. Unlike at larger angles, they do not pass over the interface after a certain distance but 12957
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instead continue to sediment along the same interface. We call this type of behavior compliant (C), as the particle sediments in the direction of the brush interface. The behavior of silica particles sedimenting along patterned surfaces at different angles of incidence is summarized in Figure 3. At small angles, the behavior is wholly compliant and at large
Figure 4. Schematic illustration of the forces acting on silica particles viewed from the side (left) and above (right).
Therefore, F acts to push particles over the brush interfaces, whereas f acts in the antiparallel direction and resists particles passing over the interface in the silicon-to-brush direction. Comparison of these two forces rationalizes the observed behavior. At large angles of incidence, F is larger than f, so particles are forced from areas of unfunctionalised silicon over the brush interface and onto the polymer brush, resulting in noncompliant behavior. At α = 90°, F is 3.2 pN and 1.5× larger than f, which is 2.1 pN. At small angles, F is smaller than f, meaning that particles cannot pass over the brush interface and must sediment without crossing the interface, resulting in compliant behavior. Particle velocity should have no effect on the observed behavior due to the low Reynolds number, which, as stated before, means that particle velocity or inertia is negligible, so it is only the surface forces that are important in determining the particles behavior. It should be noted however that the only way to increase particle velocity would be to increase the angle θ, which would also reduce the magnitude of the force f resisting particles traveling over interfaces. Therefore, increasing particle velocity by increasing the angle θ may change the observed behavior at a particular angle, for example, from compliant to partially compliant behavior. Experimental results show a third type of behavior that is not accounted for in the theoretical treatment, partially compliant behavior, which occurs between α = 30° and 60°. This coincides well with the angle at which F is approximately equal to f and seems to be a crossover regime between the two expected types of behavior. At these angles, the forces are finely balanced, and a small reduction in f will allow particles to pass over the interface. As the force f is proportional to the brush thickness, a small reduction in brush thickness would allow a particle to cross over the interface, and change compliant behavior to noncompliant. If the polymer brush was highly regular and of uniform height all over the surface, this crossover regime would not exist; compliant behavior would change directly to Nnoncompliant as F becomes larger than f with increasing angle α. We therefore attribute the observed behavior to inhomogeneity in the polymer brush thickness over the surface. This variation in thickness can cause the strength of f to vary with position over the patterned surface and be smaller than the mean value calculated. This would allow particles to pass over the interfaces at certain positions giving partially compliant behavior, rather than the expected compliant behavior.
Figure 3. Behavior of silica particles at different angles of α. See text for explanation of theory. Green, compliant (C); yellow, partially compliant (P); red, noncompliant (N). pH = 10, brush thickness 85 nm.
angles wholly noncompliant, and between these two regimes it is partially compliant behavior. The crossover between these types of behavior is not well-defined, and we have observed two different types of behavior occurring at the same angle, which we attribute to differences between individual particles and differences in the brush height at different positions on the surface. While the difference between compliant and noncompliant behavior is instantly obvious, as in one behavior particles follow the interface and in the other they do not, this is not the case for partially compliant and compliant behavior. Particles may travel a large distance along the interface before passing over and resuming their original course, meaning that many particles that appear to be compliant in the 100 s movies may actually be partially compliant, explaining the overlapping types of behavior. The three different types of behavior of silica particles observed can be explained by calculating the forces exerted on a silica particle undergoing sedimentation across the patterned surface. Taking a classical mechanics approach, which treats the polymer brush as an incompressible slab, rather than invoking more complex polymer physics, provides useful results. We can ignore the inertia of particles due to their small dimensions and slow velocity; that is, the particles are at low Reynolds number (ca. 10−5) where viscous forces dominate over inertial forces,33 and concentrate on the forces acting on the particles due to repulsions from the silicon or polymer brush surfaces. Gravity and the electrostatic/steric repulsion from the inclined surface result in a force that pushes particles along the surface and over brush interfaces (labeled F′ in Figure 4). Resolving the force so that it is perpendicular to the brush interface in the horizontal plane gives a force, F (see Figure 4, right). Gravity and steric repulsion from the PMAA brush interfaces result in a force, which pushes particles away from the brush interfaces and acts to prevent silica particles from crossing over the interface. This force can also be resolved in the horizontal plane, perpendicular to the brush interface, to give the force f, as shown in Figure 4 (see the Supporting Information for full derivations). 12958
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Figure 5. Behavior of silica particles at pH 4 (left) and pH 10 (middle) with an angle of incidence of 63° in both cases, as they sediment from right to left. The brush thickness is 103 nm at pH 4 and 154 nm at pH 10, and the width of the brush stripes is 30 μm. Right: Behavior of particles at different angles of α; “□” indicate compliant behavior, and “○” indicate partially compliant behavior. Note that each type of behavior is shifted to larger angles of α due to the larger brush thickness as compared to Figure 3.
At small angles of α, F is much smaller than f, and so the reduction in brush thickness needed to reduce f to be smaller F, allowing a particle to pass over an interface, is large, whereas at larger angles, F is comparable to f, meaning that the reduction in brush thickness needed to reduce f to be smaller than F is small. We expect surface defects that give a small reduction in brush height to be more common than large surface defects, which cause large reductions in the brush height. So therefore we expect particles to slide a large distance along the interface at small angles of α, before encountering an area of the brush interface sufficiently low enough to allow the particle to pass over the interface. Consistent with this hypothesis, we have observed that the distance particles travel along the interface before passing over the interface increases as the angle α decreases. Partially compliant behavior also appears to occur at larger angles than expected, which could be caused by a polymer brush height that is larger than anticipated. We have used ellipsometry to measure the brush height, but the true brush height may be slightly thicker, due to top of the polymer brush having a low segment density and therefore only a small difference in refractive index, which is undetected by ellipsometry. We have also considered that Brownian motion may cause particles to diffuse over the brush interface and be responsible for partially compliant behavior. This can be discounted by comparing this work with our earlier work on small iron particles, which diffuse over a patterned polymer brush surface.34 In that study, the particles encountered an energy barrier of a few thermal energies (kT) caused by the polymer brush interface. In this current work, the energy barrier encountered by the larger silica particles is much higher at 700 kT, and the larger particles diffuse much slower. This means that the probability of silica particles diffusing over brush interfaces is very small. Also, particles do not diffuse over the brush interface during compliant behavior, which they should if Brownian motion was responsible for this behavior. Poly(methacrylic acid) brushes are pH responsive, meaning that the brush thickness increases with the degree of ionization of the methacrylic acid groups. We have investigated the behavior of particles on patterned surfaces at different pH values. Figure 5 shows two silica particles that sediment across a patterned surface at pH 4 and 10; in both cases, the angle of incidence was 63°. At pH 4, methacrylic acid groups in PMAA are mostly protonated, giving only a small surface charge, and the polymer brush thickness was measured by ellipsometry to
be 103 nm. Under these conditions, silica particles show partially compliant behavior. At pH 10, methacrylic acid groups are mostly deprotonated, leading to ionization of the polymer chain and an adsorption of water to dilute the increased ionic strength within the polymer matrix. The brush layer increases in volume to accommodate this extra adsorbed water swelling to 154 nm in thickness and is now a highly charged surface. Under these conditions, particles show compliant behavior. Figure 5, right shows the behavior of particles at different angles, at pH 4 and 10. At high pH the brush is thickest, and calculation of the relevant forces shows that F is smaller than f at all angles, and so compliant behavior is observed at all angles. At low pH the brush is thinner, giving smaller values of f. Now F is approximately equal to f at large angles of α, giving partially compliant behavior, and F is smaller than f at small angles of α, giving compliant behavior. It should be noted that the brush thickness in Figure 5 is larger than that in Figures 2 and 3, meaning that the value of f is larger, explaining the different angles at which type of behavior is observed. This interesting pH-dependent behavior is potentially useful in creating “valves”, which divert the flow of particles depending on pH. It was also observed that silica particles are much less colloidally stable at pH 4 than at pH 10 and are prone to adhering to both the silicon and the PMAA surfaces. This seems reasonable, as it is the charge on the silica particle and silicon surface that prevents them from adhering together. Considering that the isoelectric point of silica is pH 3, the charge on both the particle and the surface will only be small at pH 4. The PMAA brush is also not ionized at pH 4, having a pKa of 6.9, nor can it selectively adsorb ions from solution as an insoluble polymer brush can to acquire a surface charge.35 Therefore, particles are not repelled from the brush surface and can adhere to the brush surface. We note that the study of the surface forces between a similar polymer covered surface and silica particles showed only repulsive interactions at all pH values,36 suggesting that the silica particles should not adhere to the brush surface at any pH value. Unlike in the recent work by Edington et al.,16 where five different states of particle−surface interaction were observed, we only observed two states. Roughly 80% of particles were repelled from the surface and could sediment across the surface, and 20% of particles were bound to the surface and immobile. We have also observed particles that slide along the silicon surface freely until they reach the polymer brush, at which point they then are immobilized by adhering to PMAA. This is also potentially useful for creating “gates” that decide if particles are 12959
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Figure 6. Three sequential micrographs showing particle behavior at the limited occupancy sites. pH = 10, polymer brush thickness is 116 nm, and the width of the pattern stripes is 20 μm.
this difference in behavior to the flow-streamlines of the water over the corrugated surface, which are not parallel to the silicon surface at the brush interfaces. This perhaps gives a lift force that pushes particles over the brush interfaces even when F is smaller than f. This result is somewhat disappointing considering the rapidly expanding field of microfluidics and the need to manipulate small particles in these devices.
allowed to pass depending on the pH of the surrounding solution. We have observed that patterned poly(methacrylic acid) brushes are able to assemble silica particles into multiparticle shapes. Figure 6 shows the edge of a pattern on a silicon surface in which particles are unable to cross over brush interfaces and have sedimented along interfaces by compliant behavior. Upon reaching the edge of the pattern, they then encounter a corner in the pattern that traps them in a fixed position, as the particle is prevented from sliding either left or right (up or down in Figure 6) by the two orthogonal interfaces that form the corner. This trapping occurs as the force F is smaller than f for both of the brush interfaces, which form the corner. Other particles then sediment along the same interface and collide with the first trapped particle, transferring their force F through the line of particles formed, to the corner interface. In this fashion, the force pushing the first particle toward the brush interface can be multiples of F, whereas the force resisting particles traveling over corner f remains independent of the number of particles at the corner. In Figure 6 (left) two particles have sedimented into a corner where they remain trapped as their combined F forces are smaller than f. Two further particles then sediment into the corner, increasing F to become larger than f (Figure 6, middle). Following a rearrangement of the particles, one of them is ejected from the corner, which reduces the force F, acting on the corner particle to be less than f. Now the remaining three particles are trapped in the corner once again. This indicates that f is between 3 and 4F at this particular angle of α. In this way, patterned polymer brush surfaces can act as particle assemblers, directing particles to sediment to specific locations where they could be joined together to form multiparticle assemblies. The patterned surfaces are also able to act as mass balances, measuring forces exerted on the interfaces by particles. If the measured force is smaller than f, particles are not allowed to pass over the interface, whereas if it is larger than f they are allowed to pass over the brush interface. In the case shown in Figure 6, the patterned surface measures forces of the order of piconewtons. By allowing particles to pass or not pass, patterned surfaces are able to trap a fixed number of particles at specific points on the surface and maintain this number even if more particles sediment into the same location. Finally, we investigated how particles behave on patterned surfaces when propelled along by a flow of water rather than gravity. Under these conditions, the particles show noncompliant behavior and pass over the brush interfaces unperturbed at all angles of α and all flow rates. We attribute
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CONCLUSIONS Patterned polymer brush surfaces have been created, which can control the position and motion of silica particles through repulsive surface interactions. Depending on the angle α, different types of behavior are observed, which can be explained by calculating the relevant forces exerted on the silica particle using simple classical mechanics. The surfaces were shown to be pH responsive and influenced particles in different ways depending on the solution pH, which is potentially useful in creating “gates” or “valves” to control small particles. Further, it was demonstrated that the surfaces can measure forces exerted on the brush interfaces of the order of piconewtons and that the surfaces can direct particles to a specific location. It is perhaps surprising that good control of particle motion and position can be achieved using polymer brushes that have heights ∼0.5% of the silica particles diameter. These patterned surfaces could be of vital use in devices that separate, purify, or analyze small particles or a chemical species attached to them.
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ASSOCIATED CONTENT
S Supporting Information *
Full calculations of forces. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: g.dunderdale@sheffield.ac.uk. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank EPSRC and The University of Sheffield for funding a prize postdoctoral fellowship (G.D.) and Dr. Stephen Ebbens for creating the particle tracking LabView script.
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dx.doi.org/10.1021/la302384j | Langmuir 2012, 28, 12955−12961
