Article pubs.acs.org/Langmuir
Cascaded Assembly of Complex Multiparticle Patterns Songbo Ni,†,‡ Mona J. K. Klein,†,§ Nicholas D. Spencer,‡ and Heiko Wolf*,† †
IBM Research − Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland
‡
S Supporting Information *
ABSTRACT: A method for the cascaded capillary assembly of different particle populations in a single assembly cycle is presented. The method addresses the increasing need for fast and simple fabrication of multicomponent arrays from colloidal micro- and nanoscale building blocks for constructing nanoelectronic, optical, and sensing devices. It is based on the use of a microfluidic device from which two independent capillary bridges extend. The menisci of the capillary bridges are pulled over a template with trapping sites that receive the colloidal particles. We describe the parameters for simultaneous, highyield assembly from both menisci and demonstrate the applicability of the process by means of the size-selective assembly of particles of different diameters and also by the fabrication of two-component particle clusters with defined shape and composition. This approach allows the fabrication of multifunctional particle clusters having different functionalities at predetermined positions.
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INTRODUCTION A multitude of micro- and nanoscale building blocks, such as functionalized polymer beads,1 metallic2 or semiconducting3 nanoparticles, or carbon nanotubes,4 is becoming available for the fabrication of nanoelectronic, optical, and sensing devices. The application of such building blocks in a device requires effective ways for multicomponent, heterogeneous integration into ordered arrays on substrates. Yet there is only a very limited number of examples for the directed assembly of more than one micro- or nanoscale component in a single assembly step and in a controlled manner.5 Even in the most advanced demonstrations of directed assembly of different building blocks, a series or a “cascade” of assembly cycles is used in a multistep sequence. Different groups of assembly sites are activated in a sequence of several steps, for example, by changing their wetting properties,6,7 by switching pairs of electrodes on and off,8 or by initiating specific DNA hybridization reactions.9 A directed assembly method for the selective integration of different components in a single assembly cycle would be of great advantage. The assembly of colloidal objects from a receding meniscus onto a structured template has proved its applicability and advantages in many fields of research, such as plasmonics,10 nanoelectronics,11 as well as biochemistry and biophysics.12,13 Capillary assembly exploits capillary forces acting during meniscus break-off to position and align micro- or nanoscale objects at the desired positions in a template. The possibility to form particle clusters 14,15 or to perform size-selective assemblies16 has been demonstrated. However, it is difficult to assemble two different populations of particles in a single © 2013 American Chemical Society
assembly cycle from the same meniscus by capillary assembly. Clusters of higher complexity or trap-selective assemblies have so far been prepared in time-consuming sequential assembly cycles,16 sometimes after an additional fixation step of the first particle population.14 In most common setups for capillary assembly, variations of a drying drop,17 dip-coating,18,19 or sandwiching a drop of colloid between the template and a cover slide are applied.14,15,20 Some feature a passive14 or actively controlled device11,15,21 to facilitate high-yield trapping of particles by capillary forces. However, such setups typically work with a limited volume of the colloidal suspension and do not offer the possibility to change the colloid composition during the assembly process. We recently described a device based on a microfluidic chip with a continuous supply of colloidal suspension and particle assembly from a geometrically well-defined capillary bridge that overcomes some of the shortcomings of conventional capillary assembly22 but still works with a single meniscus. Here, we present a new and efficient method to assemble two particle populations in a single assembly cycle, while keeping the colloidal suspensions separated in two different capillary bridges formed from a single microfluidic chip. We demonstrate the feasibility of this “cascaded assembly” method by performing a size-selective assembly of two components in a single cycle and by fabricating two-component particle clusters in individual traps. Received: October 29, 2013 Revised: December 12, 2013 Published: December 19, 2013 90
dx.doi.org/10.1021/la403956e | Langmuir 2014, 30, 90−95
Langmuir
Article
Figure 1. Schematics of (a) the assembly head and (b) the cascade chip (top view). Inset in (a): Side view of a cascade chip with two capillary bridges.
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served to remove any excess of surfactant that might have been codeposited with the beads in the assembly step. The temperature then was reduced below the dew point, which initiated condensation of water and formation of droplets on the traps. The sample was brought back to room temperature to evaporate the droplets.
EXPERIMENTAL SECTION
Chip and Template Preparation. The microfluidic chips were fabricated by standard optical lithography and prepared as described in ref 22. The chips are 16 × 20 mm2 in size, and the channels are 50 μm deep and 150 μm wide. After fabrication, the surface of the chips was rendered hydrophobic by silanization with 1H,1H,2H,2H-perfluorododecyltrichlorosilane. Before use, the top side was made hydrophilic by oxygen plasma (Tepla 200 plasma system, 1 mbar, 200 W, 40 s) while protecting the bottom side by covering it with a blue tape. The fluidic channel was sealed with a piece of PDMS (2.4 mm, Sylgard 184 silicone elastomer, Dow Corning). Holes aligned to the tubing port were punched through the PDMS piece by a 500-μm (diameter) punch tool. To further confine the geometry of the capillary bridge, two stripes of tape were affixed at two sides of each aperture. Further details of chip design are given in the Supporting Information. Structured stamps that serve as templates resulted from the replication of silicon masters with PDMS (polydimethylsiloxane). The masters were fabricated by a DWL (direct-write laser) (DWL 2000, Heidelberg Instruments, Heidelberg, Germany). PDMS templates were prepared as described earlier.23 Particle Assembly. Fluorescent spherical polystyrene (PS) beads (1.0% solids in water; blue, red, and green; diameters, 1.0 and 0.49 μm) were purchased from Thermo Scientific. For some of the experiments, particle suspensions were diluted (0.1−1.0 wt %). An aqueous mixture of Triton X-45 (Fluka Chemie AG, 0.1 wt %) and sodium dodecyl sulfate (SDS, Fluka Chemie AG, 1 mM) was used to replace the original solvent, resulting in a receding contact angle of the particle suspension on the template of about 50°. Two different colloidal suspensions were dispensed into the specific microfluidic channels through tubes connected to the inlet ports on the chip and to syringes (#1750 Hamilton, Bonaduz, Switzerland) at the other end. The assembly setup has been described earlier.22 A dosing rate of 0.005 μL/s was chosen initially and adjusted in the course of the experiments to keep the volume of the capillary bridge constant. The stage temperature (i.e., assembly temperature) was set to 25 K above the dew point unless stated otherwise. Cascaded assembly experiments were carried out at 2 μm/s. ImageJ (Rasband, W.S., ImageJ, National Institutes of Health, Bethesda, MD) was used to prepare merged images from different fluorescence channels. Particle Printing. The assembled particles on the PDMS template were transferred to a silicon substrate at room temperature. A 35-nm adhesion layer of polyisobutylene (0.75 wt % in chlorobenzene) was formed by spin coating at 3000 rpm for 100 s. Particles were transferred by placing the PDMS template on top of the substrate and pressing it down manually. Particle Rearrangement. Pairs of particles of 1 μm diameter were assembled in a 2.2 × 2.2 μm2 square trap, with the meniscus sweeping over the squares at an angle of 45°. Before condensation of water was performed, a capillary bridge of pure water was pulled over the assembled array in the same direction. The contact with pure water
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RESULTS AND DISCUSSION A dedicated microfluidic chip was developed to perform cascaded capillary assembly from two independent menisci. The chip is schematically depicted in Figure 1. It has two inlet ports and two channels guiding liquid to two separate apertures that form the capillary bridges. The apertures have a width of 0.8 mm and a length of 5 mm. For reasons of clarity, we will refer to the capillary bridge formed close to the front edge of the chip as the “outer capillary bridge” or “outer meniscus”, whereas the capillary bridge formed further inside the chip area is referred to as the “inner capillary bridge” or “inner meniscus”. Note that in an assembly experiment, the template is moved under the chip in such a way that the traps of the template are first swept over by the inner capillary bridge and then by the outer one. Thus, the particles from the inner meniscus will always be assembled before the particles from the outer one. The apertures for the capillary bridges are separated by 2.3 mm. Between them is a larger opening, which serves as a window for observation of the inner meniscus and is also intended to facilitate the exchange of air and humidity between the two capillary bridges with the environment (venting). The mechanism of capillary assembly requires a volume of highly concentrated particles to be formed at the meniscus to achieve high-yield assembly. This accumulation zone (AZ) is established from a constant flow of colloid suspension toward the meniscus, replenishing the water that is evaporating and transporting additional particles toward the meniscus.15,24 As a consequence, particles are densely packed in the AZ (see Supporting Information Figure S1). The AZ only forms above a critical temperature Tc, which depends on the concentration of the particle suspension, the particle-depletion rate (i.e., the density of trapping sites on the template), the particle size, the assembly speed, and the ambient humidity (i.e., the dew point).15 In the case of assembly of 1-μm PS beads (1 wt %) in a hexagonal array of holes with a pitch of 10 μm (see Supporting Information Figure S1), the critical temperature was found to be 10 K above the dew point at an assembly speed of 2 μm/s. For temperatures lower than the critical temperature, the assembly yield quickly drops to values
