Fabrication of Binary Opal Lattices in Microfluidic Devices - Chemistry

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Fabrication of Binary Opal Lattices in Microfluidic Devices Ali Malekpourkoupaei,†,§ Larry W. Kostiuk,† and D. Jed Harrison*,‡,§ †

Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 2G8 Canada National Institute for Nanotechnology, National Research Council Canada, Edmonton, Alberta T6G 2M9, Canada § Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ‡

S Supporting Information *

ABSTRACT: We have studied growth and fabrication of opal lattices made from polystyrene and silica particles by an on-chip colloidal self-assembly (CSA) approach. An optical microscopy method was used to analyze the lattice growth behavior. A mathematical model was also adopted and modified to describe the growth behavior of the silica system. On the basis of these studies, silica and polystyrene systems demonstrate different growth dynamics that originate from a sedimentation process in the microfluidic chip reservoir fractionating large and small particles. Binary lattices of silica particles were fabricated by a periodic chip rotation method. The effect of particle number ratio on the opal stoichiometry was also studied for the polystyrene system. By increasing the number ratio from 2.5 to 11, different structural stoichiometries were achieved. KEYWORDS: microfluidics, nanopores, protein separation, colloidal self-assembly

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factors in Kumacheva et al.;49 and an epitaxial technique reported by Hoogenboom et al.50 In all of the above-mentioned methods, monomodal crystals, made from only one particle size, are fabricated on a microfluidic chip. Recently the microfluidic colloidal self-assembly (CSA) approach has been used to fabricate monomodal colloidal crystals (mCC) of silica particles for a systematic study of dynamics of DNA and protein separation under electrophoretic conditions,20,22−24,51 with the goal of achieving efficient on-chip bioseparation.52−54 To the best of our knowledge, no microfluidic approach has been reported to fabricate bCCs on a microfluidic chip. Here we investigate the feasibility of making bCCs of polystyrene and silica particles in a microfluidic channel for the purpose of rapid creation of dense, nanoporous media featuring spatially long-range order. Furthermore, a mathematical model to describe crystal growth during CSA is adapted from Dufresne et al.,55 who studied growth of colloidal structures and the emerging cracks. According to our optical microscopy observations, this model reproduced crystal growth of the polystyrene system but fell well short of predicting crystal growth of silica particles with qualitatively the same sizes. The Dufresne et al.55 model was then modified to effectively describe the colloidal crystal growth of the silica system. In this study the approach for the fabrication of binary structures of silica particles in a microfluidic device is introduced and a SDS-denatured protein separation by electrophoresis is demonstrated.

INTRODUCTION Colloidal crystals have been the focus of numerous studies for about 5 decades in many different fields of science, including photonic technologies,1−7 cancer studies,8−13 tissue engineering,14−19 and bioseparation.20−24 This interest is due to the unique and highly flexible structural properties colloidal crystals present at the nanoscale and, most importantly, the ease of the fabrication techniques invented to date, which can enable their mass production. Binary colloidal crystals (bCC) of submicrometer particles can be fabricated via a spontaneous process called self-assembly.25−28 These nanoporous structures offer different structural stoichiometries, i.e., LSx, where L and S stand for large and small, respectively, and x = 1,29−31 2,27,29,32,33 3,34 4,29 5,29 6,33,35,36 8,29 and 13,27,32,37 based on relative number ratio and size ratio of particles present in the initial colloidal dispersion.34,38,39 The bCC fabrication methods can be categorized into two major approaches, a layer-by-layer (LbL) growth strategy34 and a one-stage approach. In a typical LbL method, such as controlled drying,34 stepwise spincoating,38 and confined convective assembly,39 the crystal is made by layering large and small particles sequentially on top of each other, leading to a 3D nanostructure. However, in a typical one-stage method, such as contact printing,40 self-assembly at an interface,36 horizontal deposition,35 or vertical lifting,41−43 a mixture of dispersed large and small particles is applied, and the self-assembly process simultaneously packs large and small particles into a 3D structure. Microfluidic approaches to make these novel structures include the use of centrifugal forces as per Lee et al.;44 electrocapillary forces in the works of Shiu et al.,45 Zhang et al.,46 and Velev et al.;47 continuous sonication in Gates et al.;11 chemically induced factors in Park et al.;48 confinement induced © XXXX American Chemical Society

Received: May 3, 2013 Revised: August 22, 2013

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EXPERIMENT SECTION

Chemicals and Materials. Aqueous suspensions of monodisperse polystyrene particles (750 nm diam, 10% v/v; 356 nm diam, 10% v/v; 140 nm diam, 10% v/v; and 60 nm diam, 1.25% v/v) were purchased from Polysciences (Warrington, PA). Colloidal suspensions of silica particles (900 nm diam, 5% v/v; 540 nm diam, 5% v/v; 690 nm diam, 5% v/v; 310 nm diam, 5% v/v; and 150 nm diam, 5% v/v) were obtained from Bangs Laboratories (Fishers, IN), and their stock concentrations were taken to be accurate. All colloidal dispersions were used as-received and then diluted with deionized water (Millipore Milli-QUV plus, 18.2 MΩ cm) to 1.25% v/v of their original concentrations. Binary colloidal dispersions were made by mixing aliquots of individual monodisperse colloidal solutions with specific volume fractions of particles to get the desired small to large particle number ratios. To prepare bimodal dispersions, each individual colloidal suspension was sonicated (Branson 1200, Triad Scientific, Manasquan, NJ) at room temperature for 30 min separately before and after mixing. Precleaned glass microscope slides (25 mm × 75 mm × 1 mm) were purchased from Fisherbrand and treated with piranha solution, containing concentrated sulfuric acid (98%, Caledon Laboratories Ltd.) and hydrogen peroxide (30%, Fisher Scientific) (H2SO4:H2O2 = 3:1 v/v), overnight to oxidize their surface, making them more hydrophilic. (Caution! Piranha solution reacts violently with organic materials and generates gas, so it should not be stored sealed. Handle with caution.). Crystal Growth. The device for the study of the fabrication of the binary structures (Figure 1) is adapted from a device (Figure S1b in

Figure 2. Top view of the microchannel in the proximity of the evaporation reservoir (Figure 1) during crystal growth of 690 nm silica particles. The difference in color of crystal and dispersion identifies the compaction front. The arrow shows the direction of the compaction front advancement. The black edge is the perimeter of the evaporation reservoir. dispersion with a pipet or by inverting the chip. A periodic chip rotator was designed to invert the device regularly. The colloid suspension is thus repeatedly mixed, based on a pattern determined by a function generator. The apparatus is able to perform timed 180° rotations based on a specified-by-user time period. Crystal and Particles Characterization. The microscopic quality of the colloidal crystals was investigated using scanning electron microscopy (SEM) (JEOL JSM-6010LA and LEO 1430) in secondary electron mode. Before taking SEM images, the PDMS molds were peeled off the glass substrate very carefully, to avoid any disturbance to the structures (the procedure is described in the Supporting Information). After successful PDMS mold removal, the crystal on the glass substrate was coated with a thin layer of gold (3−4 nm) via a sputtering system (Branson 1200, Triad Scientific) before SEM analysis. Images were acquired using an electron accelerating voltage of 10−20 kV. The particle size and polydispersity of the particles were examined by dynamic light scattering (DLS) measurements, using a Brookhaven BI-200SM Multiangle instrument. Visible−near-infrared transmission spectra of bCC (LS6) and mCC structures were measured with a Perkin-Elmer Lambda 900. Protein Separation. Protein separation in a bidisperse silica bed was demonstrated in a four-channel device design reported previously.21 The device layout and fabrication procedure is shown in the Supporting Information (Figure S1a). Aqueous suspensions of monodisperse silica particles, 150 nm diam, 5% v/v (Bangs Laboratories, Fishers, IN) and 30 nm diam, 2.5% v/v (Discovery Scientific, Kelowna, Canada), were used as-received. Each colloidal dispersion was sonicated (Branson 1200, Triad Scientific) for 30 min before mixing or use, and the chip was mounted in the periodic chip rotator during growth. The apparatus and separation protocol is described in further detail elsewhere.21 Briefly, a mixture of two fluoresceinisothiocyanate (FITC) labeled and sodium dodecyl sulfate (SDS) denatured proteins, including trypsin inhibitor (soybean, 20.1 kDa) and bovine serum albumin (BSA) (66 kDa) were analyzed. The device microchannels were filled with 4× TBE buffer (pH 8) solution to reduce the electroosmotic flow. The protein mixture was electrokinetically injected into the separation microchannel at 30 V/cm between sample and sample waste reservoirs for 15 s, followed by separation at 30.9 V/ cm between buffer and buffer waste reservoirs, using the binary structure as a nanosieve. Proteins were detected using epifluorescence microscopy, excited with a 488 nm argon ion laser, and collected with a 40× objective and a 515 nm long-pass filter. A StellaCam astronomy CCD camera collected images at 10 fps. The collected frames were then processed in ImageJ56 software to produce fluorescence intensity versus elution time plots.

Figure 1. A single channel device is shown for the growth of unary and binary colloidal structures. A four-channel device used for protein separation is shown in a later figure. the Supporting Information) used for molecular separation, (vide infra). The device was made via a soft-lithography procedure shown in the Supporting Information and includes a microchannel molded in PDMS with a height of 20 μm, a width of about 120 μm, and a length of 14 mm sealed with a glass slide. The microchannel is open to atmosphere through two reservoirs, punched into the PDMS. One is filled with a colloidal dispersion and capped with a lid to prevent evaporation, while the other reservoir is open to atmosphere, allowing evaporation. Colloidal crystal growth was initiated via injection of 7 μL of colloidal dispersion in the dispersion reservoir, with capillary action pulling the dispersion through the microfluidic channel. At the evaporation reservoir the dispersion stops due to surface tension, and water evaporates to the surrounding atmosphere, triggering colloidal self-assembly. The reservoirs had a diameter of 3 mm, and the height of the liquid colloidal dispersion in the dispersion reservoir was about 1 mm. These dimensions were the same for all experiments. The growth of the structure (advancement of the compaction front) was observed with two 10× microscope lens (Olympus), used as eyepiece and objective to focus the incident light into a digital camera (SONY DSC-W330, 14.1 megapixels) with a 4× zooming capability. The growing structure inside the microchannel is depicted in Figure 2, which provides a top view of the device near the evaporation reservoir. Images of the growing colloidal crystal were taken at different times and imported into ImageJ software56 for measuring the crystal length, yc. In some studies with silica particles, agitation was used to resuspend the dispersion in the reservoir, either by periodic mixing of the

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RESULTS AND DISCUSION Crystal Growth Studies: System of Polystyrene Particles. The advancement of the crystal growth, or compaction front, observed by optical microscopy is plotted B

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length y0. Using these parameters, Dufresne et al.55 conclude that the following equation describes the growth−time response curves

in Figure 3 for different polystyrene particle sizes. No evidence of crack formation or air invasion into the lattice was observed

yc y0

=

⎛t⎞ 1 + 2⎜ ⎟ − 1 ⎝ t0 ⎠

(1)

where yc and t are crystal length and time, respectively. As is evident in Figure 3a, the length of mCC grown after the same amount of time tends to decrease as particle size shrinks. The crystal growth trajectories can, however, be described by a global trend via scaling these data with y0 and t0 associated with each particle size. These values were obtained from fitting the data in Figure 3a to eq 1. Figure 3b replots the scaled data, with the solid curve showing the fit of all data sets to eq 1. The excellent correlation shown demonstrates that this mathematical model is a reliable tool for prediction of colloidal crystal growth within microfluidic structures. Crystal Growth Studies: System of Silica Particles. The analysis of crystal growth performed for the polystyrene system was repeated for dispersions of silica particles. Figure 4a shows the growth of mCCs for different particle sizes, demonstrating a trend that is quite different than seen for polystyrene. All silica mCCs studied show distinct increases in local growth rate with time, in contrast to their polystyrene counterparts.

Figure 3. Microfluidic mCC growth over time for polystyrene particles of various diameters. (a) Data for each particle size; solid lines are fits to a y ∼ ta function. Each curve was also fit to eq 1, giving values of y0 and t0, for each particle size. (b) Data in part a were scaled by y0 and t0 and replotted; the line is the best fit to eq 1. In the labels, yc, y0, t, and t0 are crystal length, characteristic length, time, and characteristic time, correspondingly.

during the course of crystal growth. Following the analysis of Dufresne et al.,55 a curve-fit of the growth−time curve to a power law, yc ∼ ta, shows a decrease in the exponential power from 0.76 for an average pore size of 116 nm (∼15% of the particle diameter) to 0.66 for an average pore size of 9.6 nm. Dufresne et al.’s55 study of flow and fracture in nanoporous media shows larger exponential growth values of 0.87 for 52 ± 6 nm diameter particles compared to the lower value of 0.66 for 62 nm particles in this study. This is probably due to the presence of micrometer-sized cracks in their structures, which are evident from the bright-field images they report. Cracks in the structure of a lattice allow for channeling phenomenon, which renders the colloidal column less resistant to fluid flow and facilitates a higher crystal growth rate compared to the crack-free structures in this study. The results in Figure 3a show that as the length of the nanoporous media increases with time, a decrease in the rate of growth is observed. On the basis of the Dufresne et al.55 model, the dynamic growth of the structure can be correlated with two parameters, the characteristic length, y0, and characteristic time, t0. The characteristic length represents the crystal length at which the rate of transport of water through the compacted region now controls the rate of evaporation and hence the rate of crystal growth. This term depends on particle size. The characteristic time is essentially the time required to reach the

Figure 4. (a) Microfluidic growth of silica particle-based mCC’s. The solid line is the correlation with eq 2. (b) Log−log plot of time to grow a 14 mm crystal for different mCC and bCC silica and polystyrene particles with the same conditions as Figure 3a and part a. The larger particle size was used in the plot to characterize the binary particle structure growth. C

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A log−log plot of the time required to grow an m- or bCC crystal the 14 mm length of the microchannel is plotted in Figure 4b versus particle size. For the silica system these values were measured via optical microscopy. However, for the polystyrene system, the growth times were very long, so the total time was estimated using the early time data and eq 1. Figure 4b shows that the total time needed to grow a crystal decreases with increasing particle size, regardless of particle type, correlating with the increasing permeability of the nanoporous lattice. However, mCCs of the silica system feature much faster overall growth than polystyrene of similar particle size. (The bCC silica structures are discussed below.) The time-dependent growth rate predicted by the Dufresne et al.55 model does not describe the silica system; however, the growth can be described by introducing an exponential factor on the dimensionless time parameter yc y0

=

⎛ t ⎞υ 1 + 2⎜ ⎟ − 1 ⎝ t0 ⎠

(2)

The experimental data can be empirically modeled with eq 2, giving coefficient of determination values fairly close to 1 (i.e., R2 = 0.99) for the particle diameters of Figure 4a. The values of υ are listed in Table 1. It is apparent that the effect of the time constant, τ = t/t0, is more pronounced with Table 1. Values of υ in the Modified Dufresne et al. Model (eq 2) system dp (nm) υ

silica 900

690 2.5

540

310 1.4

750

polystyrene 356 140 1

62

silica, as compared to polystyrene, even for particle diameters that are within the same order of magnitude for the two different materials. Sedimentation of the silica is the most likely explanation for the differences in crystal growth rates between the materials. The expression for sedimentation rate, υs = [2rp2g(ρp − ρf)]/ 9μ, shows a linear dependence on the difference in density between particle, ρp, and fluid, ρf, with a strong dependence on particle radius, rp. (In the expression, μ is viscosity and g is the gravitational constant). The expression predicts far more rapid settling of silica versus polystyrene, and much faster settling of larger diameter particles. Modeling of the streamlines of the cylindrical chip reservoir with an opening near the bottom of the wall for the microchannel makes it clear that the flow primarily samples the bottom of the reservoir. Settling rates thus lead to greater concentrations of particles in the flow channels over time and thus faster compaction rates. This hypothesis was further tested when crystallizing binary suspensions of silica particles.

Figure 5. Top-down SEM images of ordered bCC of polystyrene particles of nominally 750 and 140 nm, formed with various small to large number ratios, n, in the dispersion: (a) LS2 (n = 2.5), (b) LS6 (n = 9.8), and (c) disordered (n = 11.0).

particles of 2.5. The crystal demonstrates long-range order, with only one small particle filling the 3-fold interstitial space between larger particles, though not all interstitial spaces are occupied. The SEM also reveals particle aggregations somewhat randomly distributed at the surface of the LS2 crystal. This is likely due to some free space between the surface of the crystal and the PDMS wall, which allows unorganized packing at the crystal edge. Others have shown that the same bCC structural stoichiometry is achieved at number ratios of 1.8,59 2,36 or 2.535 by other packing techniques. Our results demonstrate that the evaporative method within a microfluidic chip is similarly effective in forming binary structures. Deposition of bCCs of polystyrene particles in a microchannel was performed by mixing a specific number density of small and large particles, with a specific size ratio. Suspensions were prepared so that the volume fraction of the larger particles remained the same as in the monodisperse studies above. For polystyrene particles of 140 and 750 nm nominal diameters,

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ON-CHIP BINARY COLLOIDAL CRYSTAL System of Polystyrene Particles. For bimodal systems to achieve the face centered cubic or hexagonally closed-packed structure of the larger particles, geometrical considerations necessitate size ratios be kept within specific values.1,32,35,40,57,58 The lower and upper size ratio limits for smaller particles placed in tetrahedral and octahedral spaces without disturbing the larger particle’s skeleton are 0.224 and 0.4142, respectively.59 Figure 5a shows structural details of the top layer of an LS2 structure formed with a number ratio of small to large D

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observed, indicating that the mCC crystal structure is of high quality. The bCC structure is known to red shift the stop band relative to crystals of the larger particle alone63 and we observe a red shift to 1654 nm. Using the band stop equations,35,63 we predict a red shift of 15 nm for an LS6 bCC crystal. The 5 nm difference in shift from theory is likely due to variability in particle size and some inaccuracy in identifying the true minimum of the relatively broad stop band. The optical results provide strong evidence that the bCC structure observed on the surface by SEM is existent throughout the crystal. System of Silica Particles. Bidisperse silica suspensions were also crystallized. The selected small to large size ratios achieved are reported in Table 2, using size estimates from the

measuring diameters by dynamic light scattering (DLS) gave a size ratio of 0.180, while SEM results gave a ratio of 0.183. For the size ratio used here, there is enough space in octahedral and tetrahedral sites of the lattice for 8 and 1 smaller particles to be accommodated,59 respectively. An increase of number ratio from 2.5 to 9.8 was required to result in a bCC with a long-range, ordered LS6 structure, for which on average each individual large particle is surrounded by six small ones, as shown in Figure 5b. This number ratio is somewhat higher than that required by other growth techniques, for which smaller number density ratios (e.g., 4,36 3.9, 4.1,35 or 3.4360) gave LS6 structures, based on the top layer of the crystal. The image in Figure 5b, shows that while most interstitial spaces at the surface are occupied by three 140 nm particles, the occupancy is not always complete. By increasing the smaller particle number density ratio to 11.0, the resulting structure, shown in Figure 5c, is significantly disordered. The lack of sufficient interstitial space in the lattice of the larger particle results in disruption of the lattice, in order to accommodate the volume of smaller particles, and adventitious packing of the small particles in any space available. Hence, in addition to keeping the size ratio within a specified range, the number ratio, n, must also be kept within a certain range, in this case 2.5 < n < 11, to obtain a bimodal crystal with a long-range order when using microfluidic devices for bimodal structure formation. SEM images provide a view of the surface, and lattice defects often provide a view at least one layer below; however, the order within the bulk is not easily evaluated by SEM. Nevertheless, the high degree of order in Figure 5a,b would not be readily achieved if the underlying orders were significantly disordered. Optical transmission band-stop spectra have been shown to provide clear evidence of internal order.11,35,61,62 The spectrum in Figure 6 evidences significant

Table 2. Bidisperse Colloidal Mixtures as Used in This Study ds/dL binary particle system

nom.

SEM

150 and 900 100 and 540 50 and 310

0.167 0.185 0.161

0.165 0.209 0.177

DLS 0.256 0.252

N

φL (%)

φS (%)

4

1.25

0.024 0.032 0.021

manufacturer, SEM, and DLS analysis. According to the manufacturer values and SEM, the ratios are well below the upper limits for inclusion in interstitial spaces; hence, no major defects, caused by space limitations, were expected. DLS analysis predicts larger ratios, as the measured sizes are larger than nominal, probably due to the nonsphericity of particles and also the presence of a solvation layer around them in the dispersion. However, the small particles would still fit within octahedral lattice spaces. Binary colloidal crystal growth of silica particles showed qualitatively the same trend over time as their monomodal counterparts discussed above. The growth duration results are plotted in Figure 4b, demonstrating modestly longer durations for bimodal dispersions compared to monomodal dispersions. The trend is consistent with the formation of smaller pore size, which should lead to longer duration growth. However, electron micrographs of these structures showed a major deficit of the smaller particles, everywhere except at the initial compaction point of the crystal. Preferential sedimentation of the larger particles presents a reasonable explanation, based on the sedimentation equation. Using the modified form of the Dufresne et al.55 model, the growth rate of silica bCCs can be reproduced within an acceptable error, as illustrated by the curve fits of eq 2 to the open circles in Figure 7. An exponential factor of 0.6 was observed for the largest particle pairing in Figure 7a, while values of 1.1 and 1.0 were seen for the smaller particles in parts b and c of Figure 7, respectively, for which sedimentation will be least significant. Silica particles of 100 and 540 nm diameters were mixed at a number ratio of 4 in order to grow crystals. An SEM image of a device packed in normal, non-agitated fashion, compared with one where the slurry reservoir was agitated by periodic rotation of the microfluidic chip, is shown in parts a and b of Figure 8, respectively. Without agitation the structure formed was largely monomodal, with very few small particles. In contract, bCC’s were successfully formed using the chip rotator described in the Experimental Section, with a 15 s rotation interval per cycle to counter hydrodynamically amplified sedimentation phenomenon.

Figure 6. IR spectra of the binary LS6 structure (Figure 5b) compared with monomodal structure of nominally 750 nm particles.

order within the bCC structure of Figure 5b, as well as the mCC structure of the same 750 nm particles (note that this is the manufacturer’s nominally stated size). The pseudo-stopband between 1600 and 1700 nm, with ∼5% transmittance, is evidence of a high-quality structure, with narrower bandwidth and stronger attenuation than seen for bidisperse structures formed by the horizontal deposition method.35 For the mCC structure, the band stop center of 1632 nm yields a volume fraction of 0.71, based on calculation35,63 with the SEMmeasured particle size of 690 ± 27 nm (SD, n = 30). The ideal volume fraction is 0.74, and values around 0.70 are typically E

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Figure 7. bCC growth behavior of silica particles at a small to large particle number ratio of 4, showing the effect of regular in-reservoir agitation of the dispersion by a pipet on growth. Triangles indicate bCC’s made while the reservoir was not agitated and circles show results with agitation. The lines are the correlations to eq 2. Diameter ratios: (a) 150/900 nm, (b) 100/540 nm, and (c) 50/310 nm.

binary dispersion of 150 and 30 nm particles with a number ratio of 4, using periodic rotation for agitation, giving the LS2 structure also shown in Figure 8b. Figure 9e illustrates the result of size-based separation of the SDS-denatured trypsin inhibitor and BSA. Three distinctive peaks are observed at the detection point, 4 mm along the separation channel from the injector. The first peak at 3.1 m is free unreacted FITC, remaining from the labeling reactions. The following two peaks correspond to trypsin inhibitor and then BSA, as confirmed by separate injections of each. The mobilities for labeled trypsin inhibitor and BSA were 15.43 and 7.34 mm2/V/s, respectively. The separation in the binary structure may be compared to previous separations in monodisperse crystalline lattices, reported by Zeng and Harrison21 at the same field strength, for which mobilities of 16.13 and 10.26 mm2/V/s were observed for the same two proteins. Those structures used 160 nm particles, corresponding to 24 nm pores (15% of the particle sizes), while the bidisperse structure here should produce 10.5 nm pores (7% of the size of the larger particles). These pore sizes are much larger than that of unreacted FITC, so the migration time should be essentially the same for the same field and separation distance, as was observed. However, the globular, random coil of the denatured proteins is similar in hydrodynamic size to these pores, consistent with the decreasing mobilities as the pore size is decreased from the mono- to the bidisperse structures. The separation results and mobility measurements confirm that our approach in fabricating binary structure made of 150 and 30 nm silica particles has been successful and the fabricated binary structure has been effective in separating protein species.

Figure 8. Top-down SEM images of ordered bCC of 540 and 100 nm silica particles, formed from a dispersion with a number ratio of 4. (a) Nonrotated chip; the structure shows a deficit in smaller particles. (b) Rotated chip; the structure shows substantial occupancy of smaller particles in the interstitial spaces.

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CONCLUSIONS Dynamic growth of monomodal and bimodal colloidal crystals of polystyrene and silica particles confined within microfluidic channels was examined and compared to the predictions of a model derived for growth of polystyrene particles in a nearly open geometry. The model effectively described the growth of polystyrene particles within microchannels, with a density similar to that of the dispersion medium of water. For silicabased crystals, the growth rate was faster than predicted. The model was then modified through an exponent imposed on the dimensionless time parameter, employing a power law to capture the effect of silica sedimentation, providing fairly accurate growth descriptions. Repeated agitation of the feed reservoir containing the particles largely removed the effect of sedimentation on silica-based crystal growth. A periodic chip rotation tool was then implemented to automatically agitate the dispersion in the reservoir, allowing fabrication of heterogeneous LS2 bimodal crystals of silica in a microfluidic channel.

PROTEIN SEPARATION IN BINARY STRUCTURE To demonstrate the applicability of the binary silica structures to separate proteins we utilized a four-reservoir chip with a separation channel and a standard, double-T injector channel21 for analysis of two FITC-labeled SDS-denatured proteins, trypsin inhibitor (Mw: 20.1 kDa) and bovine serum albumin (66 kDa). The device design is illustrated in Figure 9a, with fabrication details given in the Supporting Information. The loading, injection, and separation are outlined in Figure 9b−d. Electrophoresis-based SDS-denatured protein separation in a nanoparticle sieve is the equivalent to sieve-based separation in a gel. SDS gives the proteins a uniform negative charge per unit mass of ∼1.4 g SDS/g protein and denatures the native folded structure, giving flexible structures that take random coil conformations.64−67 These globular objects are then impeded by the sieve structure of a gel, or the crystalline area in this study. The binary nanoporous structure was made from a F

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Figure 9. Separation of SDS denatured FITC−trypsin inhibitor and FITC−BSA using a bCC of 150 and 30 nm silica particles. (a) Schematic of the microfluidic device used for separation, illustrating the location of the bCC and the detection point 4 mm from the double-T injector. (b) Electrokinetic loading, (c) injection, and (d) separation of the protein mixture. (e) The fluorescence intensity−elution time plot.

Notes

These results confirm the usefulness of models to describe crystal growth rates within microfluidic structures and offer a simple approach to combatting the effect of sedimentation in the reservoirs during crystal growth in the microchannels. Binary colloidal crystals of polystyrene and silica particles were deposited using the microfluidic approach. Ordered LS2 and LS6 and dense disordered structures were obtained for polystyrene by increasing the small particle number ratio from values between 2.5 and 11.0. Ordered LS2 structures in silica were obtained with a number ratio of 4, with periodic rotation of the chip for agitation. An LS2 bCC structure was used to effect size-based sieving of SDS-denatured proteins. The migration times were significantly reduced compared to an mCC structure fabricated with a similarly sized large particle, demonstrating that the pore sizes in the LS2 device were reduced throughout the structure by the incorporation of the smaller particles. These structures enable systematic exploration of the mechanism of separation and electromigration of proteins within different pore size regimes. The structural flexibility in architecture and pore size provided by these kinds of structures is either difficult or expensive to achieve by other materials and approaches.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the National Institute for Nanotechnology (NINT) for their financial support, and the University of Alberta for support of NanoFab.

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ABBREVIATIONS CSA, colloidal self-assembly; bCC, binary colloidal crystal; mCC, monomodal colloidal crystal; LbL, layer-by-layer; DLS, dynamic light scattering; BSA, bovin serum albumin; SDS, sodium dodecyl sulfate; FITC, fluorescein isothiocyanate.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org/

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REFERENCES

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dx.doi.org/10.1021/cm401472j | Chem. Mater. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/cm401472j | Chem. Mater. XXXX, XXX, XXX−XXX