Article pubs.acs.org/Langmuir
Ternary Asymmetric Particles with Controllable Patchiness Zhiyuan Zhao, Zengmin Shi, Ye Yu, and Gang Zhang* State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P.R. China S Supporting Information *
ABSTRACT: This work demonstrated a facile approach to the fabrication of the ternary asymmetric silica particles that consisted of different components on the opposite poles, such as metal or fluorescent quantum dots, with the aid of the mask−unmask method and double-sided etching and modifying. By controlling the thickness of the polymer mask, the surface of the particle was precisely further controlled for functional modification. At the same time, as-prepared ternary particles could self-assemble into dimers and trimers. Asymmetric patch could be clearly distinguished by SEM, EDS mapping, TEM, and fluorescence microscopy. The asymmetric particles are stable and show the potential applications in supraparticle assembly and catalysis, etc.
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INTRODUCTION As a special class of colloidal particles, asymmetric particles that consist of at least two different regions are various building blocks for complicated structures. During the last ten years, asymmetric particles have attracted considerable attention because of their unique performance that cannot be achieved by symmetric particles. For this reason, asymmetric particles have novel applications in a number of fields, such as building blocks for complex structures,1 surfactants,2 drug delivery,3 imaging probes,4 electronic devices,5 catalysis,6 and sensors.7 Typical examples of asymmetric particles are Janus particles that have two hemispheres with dissimilar properties,8 such as electrical, chemical, or physical properties. The traditional methods have been utilized for preparing Janus particles, including hydrodynamic techniques,9 biphasic interface techniques,10 controlled phase separation and surface nucleation,11 and toposelective surface modification.12 Several new methods have been reported, including interfacial engineering technique,13 surface-initiated polymerization,14 flame synthesis approach,15 and glancing angle deposition.16 To meet the needs of various functional applications, asymmetric particles with more feature areas are of interest. Multiple asymmetric particles, which are more complex than Janus geometry, can make up for deficiency of Janus particles, and therein, ternary asymmetric particles are the basic examples. Currently, there are several methods to prepare ternary asymmetric particles. For example, microcontact printing (μCP) involves the preparation of fluorescent asymmetric particles in which the two sides of particles could be modified in a single step.17 In polymer microfibers as a mask, the temperature can be used to control silica particle submergence and production of the ternary asymmetric particles.18 Otherwise, microfluidic and metal deposition © 2012 American Chemical Society
methods also have been used to prepare these asymmetric particles.19 Herein, we combined the mask−unmask method and double-sided etching and modifying for the formation of ternary asymmetric particles with controllable patchiness (a polystryene (PS) film acts as a mask for partially protecting particles from being modified, and then, the PS film is removed to expose the part of the particles that is masked, so it is called mask−unmask method. In the process of fabricating ternary asymmetric particles, we used twice etching and modifying at two hemispheres of particles, so we called double-sided etching and modifying). By plasma etching to control the size of the patch, particles could be precisely controlled for further functional modification. We fabricated ternary asymmetric particles with different components, such as metal and fluorescent quantum dots (QD), using controlled deposition and assembly, respectively. We could thoroughly distinguish different components on particles by energy dispersive X-ray spectrometer (EDS) mapping, and the particles with nickel patches provided magnetic response since these patches could be oriented in a magnetic field. All asymmetric areas of particles could be clearly distinguished in fluorescence microscopy, and self-assembly of particles arose in a certain degree. Furthermore, the ternary asymmetric particles could provide a basis for building a hierarchy structure and catalytic application, etc.
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EXPERIMENTAL SECTION
Materials. Silica particles with a uniform of diameter of 2.0 μm were purchased from Fluka. Polystyrene (PS; Mw = 280 000), N-(3dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), NReceived: September 19, 2011 Revised: January 1, 2012 Published: January 5, 2012 2382
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hydroxysuccinimide (NHS), sodium dodecylsulfate (SDS), 3-aminopropyltriethoxysilane (APS), and octadecyltrichlorosilane (OTS) were purchased from Sigma-Aldrich. Powders of gold, chromium, and nickel were employed for the deposition on silica particles. Carboxylatefunctionalized quantum dots were synthesized according to literature procedures.20 Deionized water was used in all experiments. Preparation of Single-Layer Silica Particles. Hydrophobic silica particles were obtained by modifying commercial silica particles with OTS. A toluene solution containing a small amount of OTS was added to an ethanol solution containing silica particles. The reactant mixture was stirred for 8 h at room temperature and then washed with ethanol. Subsequently, the hydrophobic silica particles were dispersed in ethanol. The suspension was spread on the water surface by injector. A small amount of SDS aqueous solution (10 wt %) was then dropped into water along the brim of a Petri dish. Silica particles self-assembled into 2D arrays, which were transferred onto a silicon substrate (silicon substrates were cleaned by immersion in piranha solution, H2SO4 and H2O2, in a 7:3 volume ratio). Fabrication of Ternary Asymmetric Particles. A PS film (30 mg/mL in toluene) was spin-coated onto the silica particles layer only to fully embed the particles at a speed of 3000 rpm. Oxygen plasma etching with different duration was preformed on the sample, which controlled the thickness of the PS mask film (total gas pressure was 5 mTorr; the etching temperature was set at 20 °C; the oxygen flow rate was 20 sccm; both the RF power and the ICP power were 30 W). Four nanometers of Cr and 30 nm of Au were sequently deposited on silica particles that were partially embedded in the PS film. Except for metal, other materials were also used for modification to fabricate the ternary asymmetric particles. Here, we took CdTe quantum dots; for example, the bare parts of a silica particle were functionalized with APS by gas phase chemisorption in a container (60 °C, 3 h), forming amionfunctionalized silica particles. Then, the substrate was immersed in a solution of carboxylate-functionalized CdTe quantum dots (pH = 7.4), which contained EDC and NHS. The reactant mixture was stirred for 5 h at room temperature. PS solution (100 mg/mL in toluene) was drop-coated on the asprepared sample that was modified with metal or quantum dots; after drying, the PS film embedding silica particles layer was peeled and turned over, so the modified pole of particles was turned upside-down to contact the substrate. Oxygen plasma etching was taken to the PS mask film again for 60 s, 120 s, 180 s, and 240 s (total gas pressure was 5 mTorr; the etching temperature was set at 20 °C; the oxygen flow rate was 50 sccm; the RF power and the ICP power were 30 and 100 W, respectively). Nickel or CdTe quantum dots were modified on bare silica particles again in the same ways that were mentioned above. The PS film was removed by ultrasonication, centrifugation, and annealing to get dispersed particles. Characterization. Scanning electron microscopy (SEM) micrographs were performed on a JEOL FESEM 6700F electron microscope with a primary electron enery of 3 kV. An EDS (energy-dispersive Xray spectroscopy) detector coupled with SEM (XL 30 ESEM FEG Scanning Electron Microscope, FEI Company) was used for elemental analysis. Transmission electron microscopy (TEM) was conducted with a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV. Fluorescence microscopy images were recorded on an Olympus BX51 microscope. Photographs were taken with a Canon A590 camera.
Figure 1. Schematic illustration of the procedure for the fabrication of ternary asymmetric particles via mask−unmask method and doublesided etching and modifying. (a) Etching and modifying, (b) dropcoating PS mask film, then peeling and turning over the array of particles with the film, (c) etching and modifying again, and (d) dispersing the ternary particles.
Plasma Etching for the Control of Patch. Prior to oxygen plasma etching, the particle layer was fully embedded within the polymer matrix. The PS-embedded silica particles almost retained the morphology of bare silica particles (Figure
Figure 2. SEM images of the PS-embedded SiO2 particles upon oxygen plasma etching for 0 s (a), 50 s (b), 80 s (c), and 120 s (d). Scale bars: 1.0 μm.
2a). Etching could control the thickness of the PS mask film precisely. As shown in Figure 2b−d, after exposure to oxygen plasma etching in different duration, openings with various sizes were created on the particle surfaces due to the change of the relative position of the PS mask film with a particle layer. This result indicates that the exposed particle surface area can be controlled by varying etching duration. For 50 s plasma etching, the size of opening (d) was 0.7 ± 0.1 μm (Figure 2b). With the increase of etching time, the opening size increased correspondingly. For 80 s etching duration, d was 1.1 ± 0.1 μm (Figure 2c). When the etching duration reached 120 s, d was 1.6 ± 0.1 μm; almost half of the particles surface area has
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RESULTS AND DISCUSSION The process to fabricate ternary asymmetric particles is shown in Figure 1. First, an array of single-layer silica particles was prepared via the gas/liquid interface method. Then, a PS film was spin-coated onto the silica particle layer. Oxygen plasma etching was preformed on the sample to control the size of the patch. By controlled deposition and assembly, the ternary asymmetric particles with component a/silica/component b were fabricated. 2383
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been exposed (Figure 2d). Hence, this procedure allows precise control of the patch and maintains the homogeneity of the exposed surface area. Furthermore, these particles could be precisely controlled for further functional modification, such as deposition and assembly. Controllable Deposition for Ternary Asymmetric Particles. In the entire process of fabricating ternary asymmetric particles, the etching that can control the thickness of a PS mask film was used twice. So, it can be achieved to control the size of deposited material on both sides of asymmetric particles. As shown in Figure 3, after twice oxygen
time. (During the etching, a lot of energy is generated, so the temperature of the sample surface increases. Moreover, the longer the etching duration is, the higher the temperature, so it is called thermal effect.) Upon the increase of the twice etching duration, the sizes of the naked areas on particles were found to increase correspondingly. Hence, the sizes of two patches on each particle could be controlled. The ternary asymmetric particles modified with gold and nickel on both sides could be prepared via this method. The patch edges of asymmetric particles could be seen clearly in SEM and TEM images. In Figure S1a of the Supporting Information, we can see that the patch edge on the lower left of the particle was wavelike, and the upper right one was smooth. In our experiment, the first PS mask film that was spin-coated onto the monolayer of particles only to cover the particles for controlling the advanced modification was about 400 nm in thickness. In the etching process, the edge of the first deposited patch was wavelike because of the lithography effect of contact area between particles (When two particles make contact with each other, the contact area between particles is masked off. The area cannot be modified. The lithography effect is wellknown as contact area lithography.)12e The second PS mask film was prepared through drop-coating, used not only to embed particles but also to peel and turn over the particle layer, which need a thicker mask film. PS solution could infiltrate into the voids within particle array during the drop-coating. So, the edge of the second deposited patch was smooth. The thickness of a flat PS mask film fully embedding particle array was more than 1.0 μm. After peeling off the film, the silica particle layer was put on a substrate again for the second etching that was for controlling the size of the second patch. The fabricated PS mask film by drop-coating could keep the flatness and completeness of the particle layer, which was indispensable to achieve the homogeneity of each asymmetric particle. So the second PS mask film was prepared by drop-coating. On the basis of our experiment process, we may conclude that the lower left side of the particle was covered by gold, and the upper right side was nickel (Figure S1a, Supporting Information). In addition, the TEM image more clearly revealed the asymmetric morphology of as-prepared particles (Figure S1b, Supporting Information). In this TEM image, the upper left side of the particle was covered by gold, and the lower right side was nickel. Finally, the PS mask film with asprepared ternary asymmetric particles was dispersed in THF with several ultrasonications and centrifugations, and then, the ternary asymmetric particles were dispersed on another substrate. When the particles were detached in the early stage
Figure 3. Cross-section SEM images of the ternary asymmetric particles upon different oxygen plasma etching duration. The lower patch is produced in first etching and depositing, and the upper one is prepared in the second etching and depositing. The duration of the first and second etchings are (a) 60 and 60 s, (b) 80 and 120 s, (c) 100 and 180 s, and (d) 120 and 240 s. The samples were treated with annealing at 400 °C for removing PS film. Scale bars: 2.0 μm.
plasma etchings, gold was deposited onto the exposed area of the particle surface, and then, the PS mask film was removed at 400 °C in air. The asymmetric particles modified with gold on their opposite poles were fabricated. Since the second PS mask film was thicker than the first one, more etching was necessary. So, the first etching was performed under low power conditions and the second etching should be carried out under high power conditions to avoid the thermal effect during etching for a long
Figure 4. SEM image and EDS mapping results for ternary asymmetric particles on silicon substrate. The red dots in EDS mapping represented gold, and the green ones were nickel. Scale bar: 1.0 μm. 2384
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through ultrasonication and centrifugation, some metal overhang could stay on the particle surface. So, the samples were treated with an annealing method for 2 h at a temperature of 500 °C in air for dewetting of the metal overhangs and removing residual polymer that was precipitated on particles and substrate surface (Figure S2, Supporting Information). It is obvious that almost all overhangs have been removed after annealing. Heterogeneous Regions Mapping and Assembly of Asymmetric Particles. To further confirm the chemical composition of heterogeneous regions, particles were characterized by EDS mapping. Figure 4 shows a typical SEM image and EDS mapping of the as-prepared gold/silica/nickel ternary asymmetric particles. In the EDS mapping, red parts represented gold, and green ones indicated nickel. The three particles in the center of EDS mapping showed red and green simultaneously; it proved these particles were the ternary asymmetric particles modified with gold and nickel. After the comparison of the particle positions between the SEM image and EDS mapping, the morphology and component of each particle were confirmed. As we know, nickel with strong magnetism is widely used in some magnetic materials. The ternary asymmetric silica particles that were fabricated using our approach contained nickel element so that these particles had a response to magnetic field. Figure S3a of the Supporting Information shows the asymmetric silica particles dispersed in ethanol that is slightly turbid dispersion. The dispersion was subjected to a strong magnetic field, and then the particles were completely separated from the dispersion within seconds, gathered near the container wall, as shown in Figure S3b, Supporting Information. Slight agitation brought the dispersion back to the original state after the magnetic field was removed. This reveals that magnetic response of these ternary asymmetric particles provides conditions for related applications such as magnetic imaging, drug delivery, etc. Similarly, the ternary asymmetric particles with fluorescent QDs and silica were fabricated since quantum dots could be assembled on the bare particle surface through covalent bonding. First, the exposed silica particle surface area was functionalized with APS to form amino-functionalized surface by gas phase chemisorption. Then, in the presence of EDC and NHS, the amino-functionalized surface reacted with carboxylate-functionalized CdTe quantum dots with the aim of forming an amide bond by carbodiimide coupling; so, the QDs were modified onto the particle surface. Fluorescence microscopy images of the resulting ternary asymmetric particles with two kinds of QDs are shown in Figure 5a. The QDs with red and green fluorescence were modified onto the opposite poles of a silica particle. Fluorescence from the two poles was easily distinguished, and the middle part of the unlabeled central silica portion was dark (Figure 5b). Owing to controllable etching, the size of the ternary area could be controlled accurately. Several particles aggregated together, the red and dark fluorescence areas of the particles contacted with each other in Figure 5a and 5c; in addition, the green fluorescence areas were adjacent to each other in Figure 5d. During the process of drying, capillary force may lead to aggregation of the particles, but it is not directional but random. To a certain extent, the self-assembly of the ternary asymmetric particles happened in solution and were held on the substrate during the process of drying. We may use the hydrophobic interaction and the electrostatic attraction to achieve targeted
Figure 5. (a) Fluorescence microscopy images of the ternary asymmetric particles modified with two different CdTe quantum dots via the method of double-sided etching and modifying. (b) Single asymmetric particles, (c) dimer asymmetric particles, and (d) trimer asymmetric particles. Scale bars: 2.0 μm.
ordered self-assembled structures through the patch particles. The asymmetric region could be clearly distinguished. According to the experiences of fabricating the asymmetric particles, the preparation of asymmetric particles with the diametet above 100 nm was similar. The fluorescence and nonfluorescence area could be more clearly distinguished by single particle fluorescence microscopy images. With the help of hydrophobic interaction and electrostatic force, it is hopeful to achieve more complex self-assembly of the asymmetric particles. Asymmetric particles could be induced to selfassemble into a complex predetermined colloidal crystal or clusters through decoration of their surfaces with a simple pattern of hydrophobic domains.21
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CONCLUSIONS In summary, a facile method combining mask−unmask and double-sided etching and modifying has been demonstrated to prepare the ternary asymmetric silica particles with different components such as metal and fluorescent quantum dots. This facile method for fabricating ternary asymmetric particles is suitable for the particles of a wide size range and different functional materials that can be modified on two poles, such as metal, fluorescent dye, polymer, and so on. The low-cost approach is achieved at room temperature, and the mask film can be removed by organic solvent or annealing. These 2385
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heterogeneous asymmetric particles make it possible to multifunctionalize and assemble into unique structures. We believe that the highly multifunctionalized asymmetric particles will play an important role in wide applications of various fields, such as photochemistry, drug delivery, catalysis, and magnetic devices.
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ASSOCIATED CONTENT
S Supporting Information *
Distinction of edge between two patches modified with Au and Ni after annealing (Figure S1), ternary asymmetric particles shown before annealing with overhangs and after annealing at temperature of 500 °C, and magnetic response of these ternary asymmetric particles containing nickel element (Figure S3). 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:
[email protected].
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51173068, 51073070, and 50703015) and the National Basic Research Program of China (2007CB936402).
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