Letter pubs.acs.org/Langmuir
Photonic Crystal Beads from Gravity-Driven Microfluidics Hongcheng Gu, Fei Rong, Baocheng Tang, Yuanjin Zhao,* Degang Fu, and Zhongze Gu* State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China, and Laboratory of Environment and Biosafety, Research Institute of Southeast University in Suzhou, Suzhou 215123, China ABSTRACT: This Letter reports a simple method for the mass production of 3D colloidal photonic crystal beads (PCBs) by using a gravity-driven microfluidic device and online droplet drying method. Compared to traditional methods, the droplet templates of the PCBs are generated by using the ultrastable gravity as the driving force for the microfluidics, thus the PCBs are formed with minimal polydispersity. Moreover, drying of the droplet templates is integrated into the production process, and the nanoparticles in the droplets self-assemble online. Overall, this process results in PCBs with good morphology, low polydispersity, brilliant structural colors, and narrow stop bands. PCBs could be bulk generated by this process for many practical applications, such as multiplex-encoded assays and the construction of novel optical materials.
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INTRODUCTION Periodic dielectric structures, or photonic crystal (PhC) structures, that can control or manipulate light propagation in a manner similar to that of semiconductors modulate electrons,1,2 offer many useful features for materials, such as brilliant structural colors and photonic stop bands.3 PhC materials with lower-dimensional (1D or 2D) structures have been widely used in optical physics, such as thin-film optics and fibers.4 In 3D, PhC materials have demonstrated applications in catalysis, sensing, detection, and display technologies.5,6 Although the lower-dimensional PhC materials could be obtained with a high yield by adapting microelectronic techniques,7 it is still a challenge to bulk generate 3D PhC materials via a top-down routine.8 In contrast, nature has created numerous desired materials by using simple selfassembly methods.3,9 Inspired by nature, researchers have developed several kinds of 3D PhC materials with colloidal crystal structures.10−14 In particular, spherical PhC materials (photonic crystal beads, PCBs) have drawn a great amount of attention because of their potential application in barcode multiplex detection15−18 and in the construction of angleindependent optical devices.19 PCBs are typically formed by evaporating solvent droplets that contain colloidal nanoparticles. Many techniques have been employed to generate the droplet templates, such as micropipet devices,20 electrospray devices,21−23 mechanical mixing,24 ultrasound dispersal,25 and microfluidic technology. Among these methods, microfluidic technology provides fine control over droplet size, structure, and chemical components and has recently emerged as a very promising route to obtaining droplets for the generation of PCBs.26−33 However, the fluids commonly used in microfluidics are usually unsustainable and cannot run continuously, restricting the © XXXX American Chemical Society
bulk generation of droplets. Moreover, conventional driving forces of the microfluidic devices, from either mechanical methods or airflow, result in vibrations that likely increase the polydispersity of droplets.25 These factors, together with the slow solidification process of the PCBs, have limited their practical applications. Thus, novel approaches with the capability of high yield, stability, and precision for generating PCBs are still needed. In this Letter, we report a simple gravity-driven microfluidic device with the desired features for the generation of PCBs. The device takes advantage of (i) the continuous support of fluids, (ii) ultrastable gravity as the driving force34 of fluids for droplet emulsification, and (iii) online droplet solidification. On the basis of this method, monodisperse droplet templates and PCBs can be produced constantly. The achieved PCBs have good morphology, low polydispersity, brilliant structural colors, and narrow stop bands. It was also demonstrated that a series of PCBs with distinctively encoded stop bands could be achieved by using various sizes of colloidal nanoparticles. Thus, the PCBs are very promising as microcarriers in biomedical applications where multiplexing is needed. In addition, materials with PCB components show angle-independent structural colors that could be useful for many potential optical applications, such as wide-viewing-angle displays.
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RESULTS AND DISCUSSION The gravity-driven microfluidic device used for the generation of the droplets was composed of a fluidic module and an electromechanical controller, as shown in Figure 1. The fluidic Received: March 3, 2013 Revised: May 16, 2013
A
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Figure 1. Gravity-driven microfluidics and online drying system: (a) schematic illustration of the workflows of the system; (b) pictures of the overall system; (c, d) magnified pictures of the reservoir (c) and the funnel (d); and (e) images of droplet formation in the microfluidics.
where K1 = K(uo/uw)M also represents a constant. For a fixed microfluidic device, the vw (or vo) value in the orifice section of the droplet formation tends to have a linear relationship with the height of the corresponding water column hw (or the height of the oil column ho); that is, vo/vw = K2*ho/hw (K2 represents a constant). By calculating this (assume K′= K1K2M), eq 2 could be shown as
module is a coflow microfluidic chip connected to two sets of funnels (Figure 1d) through transparent soft tubes. Each funnel was kept in a filled state by continuously drawing liquid from the reservoirs (Figure 1c). In this process, the flow rate from the reservoir should be slightly faster than the flow rate of the liquid out of the funnel. The electromechanical controller has two motors that can raise or drop the funnels from a height of 5 to 100 cm with a minimal step of 0.5 cm. In the fixed height of the funnels of both water and oil phases, the columns of liquids in the device were also fixed, and the gravitational driving of the fluids in the microfluidics was kept constant, providing ideal conditions for the generation of the droplets. The droplets in our research were formed in a glass microfluidic chip, which was assembled by aligning two cylindrical capillaries coaxially inside a square capillary. The dispersed water phase was driven through one tapered round capillary, and the continuous oil flowed through the region between the inner round capillary and the outer square capillary in the same direction. By employing the coflow geometry, the chip could generate monodisperse water-in-oil (W/O) emulsions in the collection capillary, as shown in Figure 1e. The diameters of the droplets (D0) were determined by the orifice sizes of the microfluidic inner channel (d), the velocities of the water and oil phases (vw and vo), and the viscosities of the two phases (uw and uo). Their relationship could be described by the empirical law35 ⎛ Ca ⎞ M D0 = K⎜ o ⎟ d ⎝ Ca w ⎠
⎛ h ⎞M D0 = K ′d⎜ o ⎟ ⎝ hw ⎠
The method of the control variable was employed to verify the relationship between D0 and hw, ho, and d, as described in eq 3. Three sets of experiments were performed in the same microfluidic device by using a 15% w/v silica nanoparticle solution as the water phase and n-hexadecane as the oil phase. Figure 2a shows the results when d = 112 μm, ho = 10 cm, and hw increased from 5 to 65 cm. It was found that D0 exhibited exponential growth from 260 to 480 μm. In comparison, when ho increased from 5 to 100 cm under the condition of hw = 10 cm, D0 exhibited an exponential decay from 420 to 210 μm, as shown in Figure 2b. It was also found that at the fixed heights of hw and ho, the D0 and d values were linearly related, as recorded in Figure 2c. By using the above three groups of relationships in eq 3, the values of K′ and M were 3.021 and −0.1865. Thus, a simplified empirical law describing our microfluidic system is given below: ⎛ h ⎞−0.1865 D0 = 3.021d⎜ o ⎟ ⎝ hw ⎠
(1)
(4)
On the basis of the microfluidic device and empirical law, the diameters of the droplet templates could be precisely controlled from 30 μm to several millimeters by choosing the microfluidic chip with an appropriate orifice and adjusting the height of the water and oil funnels. For example, a microfluidic chip with an orifice diameter of 112 μm was employed, and the water and oil funnels were set to 40 and 25 cm, respectively. This resulted in monodisperse droplet templates hexagonally arranged in a thin layer of oil, as shown in Figure 3a. The size probability
where K and M are constants, Cao and Caw are the capillary numbers for the oil and water phases, respectively, and Cao = uovo/γ and Caw = uwvw/γ. (γ is the interfacial tension of the water−oil interface, and uo and uw were fixed in our system.) Equation 1 could be simplified to ⎛ vo ⎞ M D0 = K1⎜ ⎟ d ⎝ vw ⎠
(3)
(2) B
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high-quality droplets could be bulk generated for further applications. The droplets were used as templates for the generation of PCBs. Generally, this was carried out by evaporating the droplet solvent in an oven. This process was usually very slow and took dozens of hours. Thus, the PCB production was inefficient at limiting the practical applications of the PCBs where bulk beads were needed. To solve this problem, we developed an online drying module in the gravity-driven microfluidic device, as shown in Figure 4a. It was constructed by putting a hydrophobic container on a tilted hot plate. The container was covered with a thin layer of oil in which the collection tube was just immersed. The outflow together with the formed droplets runs out of the tube and converges continuously into the oil of the container. As the oil in the container was heated constantly by the hot plate, the water in the droplets started to evaporate as soon as the droplets entered the container, as shown in Figure 4b,c. It was found that the newly generated droplets are white and opaque because of the Tyndall scattering of the silica nanoparticles in the droplets. When the water evaporates, the silica nanoparticles gradually self-assemble into ordered lattices and the structural colors appear. Eventually, brilliant, transparent PCBs were obtained when the water was evaporated completely. The generation of PCBs by integrating the online drying module into the device has many advantages. It simplified the process, reduced the time cost, and improved the yield of bead fabrication. Moreover, because the transportation of the container to the oven and the washing step of the solidified beads from the oil were avoided, all vibrations during these processes were eliminated. Thus, the self-assembled silica nanoparticle clusters could form PCBs with lower polydispersity. The microstructures of the PCBs were characterized by a scanning electron microscope (SEM), as shown in Figure 5. It can be observed that the PCBs have excellent spherical structures and smooth surfaces (Figure 5a). The enlarged SEM image in Figure 5b shows that the nanoparticles on the PCB surface form predominantly hexagonal symmetry, similar to that observed in colloidal crystal films. This structure was also extended to the inside of the whole beads (Figure 5c,d). Because the PCBs are derived from the silica droplet templates, the size of the PCBs can also be detemined before fabrication by controlling the droplet diameter and the concentration of the silica nanoparticles in the droplet templates. Characteristic reflection peaks are a remarkable property of the PCBs. Under normal incidence, the peak value can be estimated from the law
Figure 2. Relationships between the droplet sizes D0 and (a) the height of the water column hw (with d = 112 μm, ho = 10 cm), (b) the height of the oil column ho (with d = 112 μm, hw = 10 cm), and (c) the water orifice diameter d (with hw = 10 cm, ho = 10 cm). The solid lines are the fitting curves. The variation in the droplet diameter corresponds to the standard deviation (σ) of the droplet diameter; each point was an average diameter of 100 droplets.
distribution of these droplets gave a polydispersity (σ/d) of less than 0.3%, as calculated in Figure 3b. The low polydispersity of the droplets should be ascribed to their stable generation process, where the surfactant stabilized the W/O interface of the droplets and the ultrastable gravity was used as the driving force for droplet emulsification. More importantly, these monodisperse droplets could be produced constantly in our device because the fluids of the two phases could be continuously supported without suspending the flows. Thus,
Figure 3. (a) Picture of the hexagonally arranged droplet templates. (b) Size distribution of 200 droplets derived from gravity-driven microfluidics. C
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Figure 4. Online evaporation of water from droplet templates: pictures of (a) the online drying module and (b) the drying process. (c) Magnified image of the selected region in b.
experiment, the reflectivity ranges from 28 to 35%, and the reflectivity was not influenced by the forming sphere size and bead diameter. Thus, the diffraction peak positions can be derived for encoding. Because the coded reflection of the PCBs originates from their structural periodicity, they are very stable and do not suffer from the defects of traditional fluorescence coding, such as fading, bleaching, quenching, and chemical instability. These, together with the absence of background signal such as fluorescence, suggest that the PCBs are ideal microcarriers for multiplex-encoded analysis. The construction of novel 3D PhC materials is another important application of the PCBs. Because the PCBs have spherical symmetry, they can show identical photonic responses that are independent of the rotation of the axes. Thus, the PCBs were angle-independent and exhibited identical structural colors from all observing perspectives. Here they were used as elements to construct 3D PhC film materials. In contrast to the traditional colloidal PhC film, which was directly self-assembled from silica nanoparticles and showed an obvious structural color variation at different viewing angle positions, the PCBcomposed 3D PhC film material had structural colors similar to those of their PCB elements and showed excellent angle independence and constant structural colors at different viewing angles (Figure 7). This angle-independent property could be achieved in different colors and 3D morphologies of PhC materials by using the PCBs as the structural elements. With these characteristics, PCB-composed 3D PhC materials should find important applications in the development of novel optical devices, such as angle-independent reflective displays.
Figure 5. SEM images of the PCBs: (a) low-magnification image of a PCB and (b) high-magnification images of the PCB surface and (c, d) the inner structures.
λ = 1.633d particlenaverage
(5)
where λ is the peak wavelength, dparticle is the center-to-center distance between two neighboring silica nanoparticles, and naverage is the average refractive index of the PCBs. Because the naverage of our PCBs is a constant, the peak value λ mainly depends on the size of the silica nanoparticles. Therefore, by introducing differently sized silica nanoparticle solutions into the gravity-driven microfluidics, a series of PCBs with different diffraction peak positions could be obtained (Figure 6). In the
Figure 6. Microscope images and reflection spectra of 10 kinds of PCBs, where measurements were performed with the beads in pure water. From left to right, the sizes of the silica nanoparticles were 213, 227, 238, 243, 250, 267, 280, 287, 302, and 309 nm. Each image corresponds to a spectrum. D
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Figure 7. Images of (a) purple, (b) green, (c) orange, and (d) red PCB-composed films in a cuvette, where the PCBs consisted of silica nanoparticles that were (a) 200, (b) 243, (c) 267, and (d) 280 nm in diameter. The pictures were taken from various angles.
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CONCLUSIONS We present a gravity-driven microfluidic device for the bulk and continuous generation of droplets. Because the ultrastable gravity was employed to drive the fluids in the microfluidics, the resultant droplets exhibited low polydispersity. These droplets were used as templates for the self-assembly of silica nanoparticles into PCBs. An online drying module was integrated into the gravity driven microfluidic device to accelerate the process of nanoparticle assembly. This integration could also improve the yield of the PCBs. Moreover, because the vibrations during the bead generation were all but eliminated, self-assembled PCBs can be fabricated with better morphology, higher monodispersity, and more vivid structural colors. A series of distinctive PCBs with stably encoded reflections could be achieved by using different sizes of nanoparticles to assemble, which makes them highly promising as microcarriers for multiplexing assays. Moreover, the spherical symmetry imparted the PCBs with angle-independence,
suggesting their use in constructing novel optical devices that can be seen over a wide range of viewing angles.
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EXPERIMENTAL SECTION
Materials. The monodisperse silica nanoparticles (polydispersity
