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Combining Magnetic Field Induced Locomotion and Supramolecular Interaction to Micromanipulate Glass Fibers: Toward Assembly of Complex Structures at Mesoscale Mengjiao Cheng,† Haitao Gao, ‡ Yajun Zhang, § Wolfgang Tremel, ‡ Jian-Feng Chen, § Feng Shi,*,† and Wolfgang Knoll|| State Key Laboratory of Chemical Resource Engineering and §State Key Laboratory of OrganicInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China ‡ Johannes Gutenberg - University of Mainz, Institute of Inorganic and Analytical Chemistry, Duesbergweg 10-14, Mainz, 55099 Germany Austrian Research Centers, Donau-City-Strasse 1, Vienna 1220, Austria
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ABSTRACT: The formation of ordered complex structures is one of the most challenging fields in the research of biomimic materials because those structures are promising with respect to improving the physical and mechanical properties of man-made materials. In this letter, we have developed a novel approach to fabricating complex structures on the mesoscale by combining magnetic-fieldinduced locomotion and supramolecular-interaction-assisted immobilization. We have employed a magnetic field to locomote the glass fiber, which was modified by the layer-by-layer self-assembly of magnetic nanoparticles, to desired positions and have exploited the supramolecular interaction to immobilize glass fiber onto the appointed position. By magnetically induced micromanipulation, we can drive another fiber across the former one and finally obtain a crossing structure, which can lead to more complex structures on the mesocale. Moreover, we have constructed a mesoscale structure, termed “CHEM”, to demonstrate further the application of this method.
1. INTRODUCTION Self-assembly, together with supramolecular chemistry, has bridged modern chemistry and biology via exploring noncovalent intermolecular forces.1,2 From molecules to living organisms, selfassembly as a concept not only plays a crucial role in the understanding of various natural structures at all scales but also provides an effective solution to the problem of synthesizing structures beyond molecular levels.3,4 Taking the fabrication of thin films as an example, the layer-by-layer (LbL)5 self-assembled technique employs many building blocks, such as nanoparticles,6 carbon nanotubes,7 enzymes,8 and polyelectrolytes9,10 by using supramolecular interactions (noncovalent or weak covalent interactions such as electrostatic interactions,11,12 hydrogen bonds,13 and coordination bonds14) as driving forces. In the self-assembly of larger components on meso- or macroscopic scales, interactions such as gravitational attraction, external electromagnetic fields, and magnetic, capillary, and entropic interactions, which are not important on the molecular level, have been involved.15 Among the driving forces stated above, the magnetic interaction enjoys certain advantages over others.16 Magnetic fields act over a much longer range than van der Waals, surface tension, and electrostatic fields in polar solvents, leading to a noncontact interaction between the magnet and the magnetic responsive targets. Applying magnetic fields is a clean, effective, environmentally benigh approach and has little influence on the chemical r 2011 American Chemical Society
systems involved. Besides, the highly nonlinear behavior of magnetic materials in magnetic fields can be well applied in terms of controlled movement and the precise location and manipulation of targeted magnetic objectives, which have not been widely employed in assembled techniques.17 Recently, Kriha and co-workers presented a controlled movement of short electrospun polymer microfibers containing superparamagnetic nanoparticles by the interconnection of hippocampal neurons.18 We also developed a facile method to drive a glass fiber on the water surface through combining LbL self-assembly of Fe3O4 magnetic nanoparticles (MNPs) with the inducement of an external magnetic field and determined the force load of one magnetic nanoparticle.19 Whitesides’ group reported a process for the fabrication and positioning of magnetically responsive nanowires embedded in thin epoxy slabs through magnetic mooring;20 they also revealed a strategy for assembly using magnetic forces between ferromagnetic rods to organize and stabilize the microstructures.21 Recently, they used surface forces and magnetic forces for the assembly and registration of optical components with small footprints.22 Lieber et al. showed an approach to the hierarchical assembly of 1D nanostructures into well-defined functional networks by combining fluidic alignment Received: April 17, 2011 Revised: April 30, 2011 Published: May 04, 2011 6559
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Scheme 1. Flow Chart of Surface Modification and LbL Self-Assembly of PAA and MNPsa
a
Building blocks for LbL self-assembly are displayed above: PAA, PDDA, and oleic acid-capped Fe3O4 magnetic nanoparticles.
with surface-patterning techniques.23 Christianen’s group demonstrated the controlled magnetic deformation of spherical nanocapsules, assembled from bolaamphiphilic sexithiophenes into oblate spheroids, and the deformed capsules could be fixed in a compatible organogel, preserving their shape outside a magnetic field;24 they reported that high magnetic fields are effective at orienting self-assembled 2,3-bis-n-decyloxyanthracene (DDOA) fibers during organogel preparation.25 These above methods provide novel avenues to construct useful complex geometry and functional structures and present a huge potential for micromanipulation. However, only the locomotion of the building blocks is not sufficient to obtain stable functional structures; it is the immobilization of building blocks and freezing the formed structures that are essential to those structures. In this letter, we have developed a novel approach to fabricating complex structures on the mesoscale by combining magnetic-field-induced locomotion and supramolecular interaction. We have employed a magnetic field to locomote the glass fiber modified by LbL self-assembly technique of magnetic nanoparticles to desired positions and have exploited supramolecular interaction to immobilize the glass fiber onto an appointed position. By benefitting from magnetically induced micromanipulation and the immobilization of supramolecular interaction, we can move and freeze the modified glass fibers into more complex structures on the mesoscale, termed CHEM.
2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as supplied: poly(diallyldimethylammonium chloride) (PDDA, Mw = 400 000 g/mol) and poly(acrylic acid) (PAA, Mw = 2000 g/mol) from Acros Organics.
The oleic acid-capped Fe3O4 magnetic nanoparticles (MNPs) were provided by Dr. Junfeng Liu and Prof. Tierui Zhang, which were synthesized according to a literature method.26 The substrates, glass fibers (17 μm in diameter), were pretreated with piranha solution (the volume ratio of sulfuric acid to 30% hydrogen peroxide is 7:3). 2.2. Magnetic Modification. The magnetic modification process of Fe3O4 MNPs and polyelectrolyte on glass fibers was carried out as follows (shown in Scheme 1): (i) immersion of the pretreated glass fibers covered with hydroxyl groups into an aqueous solution of PDDA (1 mg/ mL) for 30 min, followed by rinsing with deionized water and drying in a flow of nitrogen, resulting in a layer of positive charges as a preassembled step; (ii) deposition of the preassembled glass fibers into an aqueous solution of PAA (1 mg/mL) for 30 min, identically followed by washing and drying, to obtain a layer of carboxyl groups via electrostatic interaction; (iii) modification of the substrates with carboxyl groups immersed in a hexane solution of Fe3O4 MNPs (0.2 mg/mL) for 20 min, washing with hexane, and drying with nitrogen, giving a bilayer of PAA and MNPs based on cooperative coordination interactions between the carboxyl groups and Fe3O4 MNPs; (iv) deposition of carboxyl groups in an ethanol solution of PAA (1 mg/mL) for 20 min, rinsing with ethanol, and drying in a flow of nitrogen. Repeat steps iii and iv to achieve the expected magnetic multilayers. The same magnetic modification was also carried out with the substrates of quartz glass to characterize the process with a UVvisible spectrometer (Hitachi model U-3900H). 2.3. Micromanipulation. The micromanipulation of glass fibers within water was compared on two kinds of glass slides as operating platforms, and the procedures were observed with a microscope: (i) water was dropped onto a modified glass slide as a platform and a number of glass fibers (at a diameter of 17 μm over a length range of 0.11 mm after cutting) modified with 20 bilayers of PAA/MNPs were placed on the water surface; (ii) fiber was driven into the microscopic 6560
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Figure 1. (a) UVvis spectra of PAA/MNP multilayer films measured after each (PAA/MNP) bilayer was deposited. The inset shows the dependence of the absorbance of Fe3O4 MNPs at 320 nm on the number of bilayers. (b) Untreated glass fibers. (c) Glass fibers of b after modification with 20 bilayers of PAA/MNPs.
Figure 2. (a) One magnetic fiber was induced into the field by a magnet, and another fiber was driven in any direction. (b) Two fibers finally were oriented parallel to each other. The above magnet formed a local magnetic field in the direction parallel to the glass fibers. view by a magnet at an average interactive distance of 1 cm with a field strength of 1.4 T; (iii) the glass fibers were immobilized by pressing with a needle; (iv) another fiber was drive to cross the former one by the magnet. A controlled experiment proceeded following the above steps on glass slides without any modification. On the basis of this, through step-by-step locomotion and freezing, single fibers modified one by one on a glass slide, and a more complex structure, CHEM was constructed. 2.4. Modification of Glass Slides. The above glass slides used as operating platforms for micromanipulation were modified through a layer-by-layer self-assembly technique by alternately depositing PDDA and PAA for five cycles on the surface. PDDA and PAA were both aqueous solutions at a concentration of 1 mg/mL.
3. RESULT AND DISCUSSION The stepwise LbL assembly of the multilayer film was characterized by UVvis absorption spectroscopy. In Figure 1a, the absorption signals of the multilayers increased depending on the number of bilayers, which showed a broad absorption spanning the UVvis wavelength range, centered on 320 nm. This broad absorption was due to the mixture of the ligand-field transition and charge-transfer transition of the Fe3O4 MNPs.27 The inset of Figure 1a showed a correlation analyzed with the absorbance intensities of Fe3O4 MNPs at 320 nm and the number of bilayers, which gave a good linear increase. This linear fitting indicated an identical Fe3O4 MNPs content within each bilayer. Because the hexane solution of Fe3O4 MNPs was brown, the appearance of the glass fibers grew darker during the LbL self-assembly process and the originally colorless glass fiber in Figure 1b finally turned golden, as shown in Figure 1c after modification with 20 bilayers of PAA/MNPs. The movement of the as-prepared glass fibers on the water surface induced by a magnetic field was demonstrated in our
previous work.19 Because the value of fluid drag within water would be much higher than that on the water surface, we wondered whether the magnetically driven movement of the identically treated glass fibers was possible. To investigate the micromanipulation of glass fibers within water, we carried out the following experiment in Figure 2. First, we dropped some water onto a clean glass slide and placed a number of glass fibers (with a length range of 0.11 mm after cutting) modified with 20 bilayers of PAA/MNPs onto the water surface. These glass fibers could penetrate water with the aid of an external pressing force by a needle or by using a liquid mixture of water and ethanol. Second, one fiber was driven into microscopic view by a magnet with field strength of 1.4 T. This fiber was free to reach any corner within water guided by the magnet at an average interactive distance of 1 cm. In the optical image of Figure 2a, one magnetic fiber was induced into the microscopic field by a magnet and another fiber was driven in any direction. As a result, two fibers finally oriented parallel to each other in the presence of the magnetic field, as Figure 2b shows. This was reasonable because several identical paramagnetic rodlike objects with small magnetic torques tended to orient in the direction of magnetic lines and would be parallel to each other in a small range where magnetic lines distorted little. In this sense, we confronted challenges in an attempt to get useful complex geometry by manipulating magnetically modified glass fibers. Wherever we induced a second fiber, its orientation would be consistent with the pre-existing one; namely, what we obtained depended totally on the distribution of magnetic lines. Because only the locomotion of the building blocks was not enough to obtain designed structures such as a crossing pattern, the immobilization of glass fibers and the fixation of the designed 6561
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Figure 3. (a) Without immobilization, the glass fiber was free to reach anywhere within water micromanipulated by the magnet, which was defined as the ON state. (b) The immobilized state of the glass fiber was named the OFF state. (c) The glass fiber was moved and left no sign on the glass slide without modification. (d) A little displacement of the fiber left a golden sign on the modified glass slide during pressing.
Figure 4. (a) The glass fiber in the left corner was immobilized, and another fiber was induced in any direction. (b) Another identical fiber was placed across the assembled fiber, resulting in a crossing pattern.
Figure 5. “CHEM” was constructed with magnetically modified glass fibers as building strokes combining supramolecular interaction and magnetically induced immobilization. Note that each letter was fabricated independently on a separate glass slide.
pattern were essential to the desired structures. One possible strategy was using a supramolecular interaction to freeze the glass fiber onto the substrate as illustrated in Figure 3. Without modification, the glass slide covered with hydroxyl groups had very weak interactions with the glass fiber, which was not strong enough to immobilize the glass fiber. Thus, the glass fiber was free to reach anywhere within the water manipulated by the magnet. We defined this situation as the ON state (Figure 3a). In contrast, when the glass slide was modified with carboxyl groups, it was possible to attach the glass fiber to the substrate based on cooperative coordination between carboxylic groups and Fe3O4 MNPs. This situation was named the OFF state correspondingly (Figure 3b), in which the glass fiber did not respond to the movement of the magnet at an average interactive distance of 1 cm. To confirm that cooperative coordination existed between the
glass fiber and the slide, controlled experiments were carried out. When the glass slide was modified with carboxyl groups, the glass fiber was vertically pressed by a needle to provide a close-enough distance for the effective interaction of the coordination binding. Sometimes when the pressing force was not appropriately applied, a slight displacement of the fiber occurred, leaving golden residues at the original position as shown in Figure 3d. These golden residues could be attributed to a fragment of PAA/MNPs multilayers that were peeled off of the modified fibers because only MNPs had such color. Furthermore, the golden residues stayed immobile when we moved around the magnet above them, which implied that the fragment of PAA/MNPs had been attached to the substrate by cooperative coordination. However, when we performed identical micromanipulations on the glass slide without any treatment, the glass fiber was under its ON state and no sign was left on the glass slide as Figure 3c showed. These phenomena demonstrated that cooperative coordination did exist between the glass fiber and the slide and could be used to immobilize the glass fiber at the desired positions. However, it was still uncertain as to whether the noncovalent bonding was strong enough to stick a glass fiber at micro and millimeter scales onto glass substrates. Therefore, we proceeded with experiments of the immobilization between glass fibers and the slides. First, through LbL assembly, the glass slide was adsorbed with five bilayers of PDDA/PAA to produce a surface of carboxyl groups, which could interact with Fe3O4 MNPs by 6562
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Figure 6. Magnetic hysteresis loop of (a) the as-prepared glass fibers with 20 bilayers of PAA/MNPs and (b) pure magnetic nanoparticles. The two insets are abstracts from the corresponding figures with shortened coordinates.
metalorganic coordination. Afterwards, a magnetically modified fiber was manipulated by the magnet and fixed through supramolecular interactions assisted with pressing with a needle. In Figure 4a, the glass fiber in the left corner was in its OFF state and another fiber was induced in any direction. Because the first fiber was immobilized and did not respond to the magnetic field, it was accessible to move another identical one across the frozen fiber, resulting in a crossing pattern in Figure 4b. However, when the fibers were both in their ON states, they tended to orient parallel to each other and such a crossing pattern was impossible. Moreover, benefitting from magnetically induced micromanipulation and the immobilization of supramolecular interaction, we could move and freeze the modified glass fibers into complex structures on the mesoscale. We introduced the abbreviation CHEM in Figure 5 by using the magnetically modified glass fibers as building strokes combining supramolecular interaction and magnetically induced immobilization. To enable a better understanding of the magnetically induced behaviors of glass fibers, we carried out a calculation concerned with the driving force in the magnetic field. In that case, the equation F = Vμ0M 3 rH (1) was applied in the calculation,28 where the symbol F(N) represents the magnetic driving force that induces the locomotion of the glass fiber under a magnetic gradient; V(m3) is the volume of the glass fiber; μ0 (= 4π 107 N/A2) is the magnetic permeability of vacuum; M (A/m) is the intensity of magnetization; H (A/m, m/(4πμ0μsr2)) is the applied external magnetic field; and rH (A/m2) gives a gradient of the magnetic field. From Figure 6a measured with superconducting quantum interference devices (a Quantum Design MPMS-XL SQUID magnometer), we could obtain that the saturated value of the magnetic moment of the as-prepared fibers at 300 K was 0.43 emu/g. Thus, the magnetic force F was finally determined at a magnitude of 105 N. Moreover, with the value of the magnetic force, we could evaluate how much weight magnetic micromanipulators can deliver. In most drug-delivery systems involving MNPs, the weight of the MNPs was much larger than that of the delivered cargo. Our previous results indicated that through suitable surface modification the Fe3O4 MNPs could be used to deliver cargo up to 10 000 times the weight of themselves on the water surface.19 Herein, from the magnetic hysteresis loops we could observe that the saturated values of the magnetic moment of the magnetically modified glass fibers and the pure MNPs at 300 K were 0.43 emu/g in Figure 6a and 30.7 emu/g in Figure 6b, respectively. The ratio of the two values was about 1.4%, which indicated that
the percentage of the magnetic nanoparticles assembled on the glass fiber was 1.4%. That meant only a 1.4% content of magnetic nanoparticles could drag the fiber in water. To prove that our magnetic fiber was viable for potential drug-delivery applications, we carried out the same experiment in solutions with different ionic strengths (e.g., PBS buffer), and there was almost no influence on our results as observed in pure water. In this sense, our PAA/Fe3O4 magnetic multilayer system was very promising because magnetic micromanipulators can deliver cargo of up to 100 times the weight of the MNPs.
4. CONCLUSIONS We have put forward a new method to fabricate complex structures on the mesoscale by combining magnetic-field-induced locomotion and supramolecular interaction. We have employed a magnetic field to locomote the glass fiber modified by the LbL selfassembly technique of magnetic nanoparticles to desired positions and have exploited supramolecular interactions to immobilize glass fibers and freeze the designed complex structures on the mesoscale. It is anticipated that the obtained geometry may open a huge potential for the design of novel devices or new material combinations, such as the mimic of the rat incisor, which has two sets of prisms or rods set at an angle of some 6470° from each other.29 Moreover, this approach may provide novel solutions to fine micromanipulation with magnetic assistance. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant no. 50903005), the Beijing Nova Program of China (grant no. 2009B011), and the Key Project of the Chinese Ministry of Education (grant no. 109012). We thank Dr. Junfeng Liu from Beijing University of Chemical Technology and Prof. Tierui Zhang from Technical Institute of Physics and Chemistry, Chinese Academy of Science, for providing Fe3O4 magnetic nanoparticles. ’ REFERENCES (1) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2009. 6563
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