Polymer Composite and Remotely

ARTICLE pubs.acs.org/JPCC

Magnetic Nickel
Phosphorus/Polymer Composite and Remotely Driven Three-Dimensional Micromachine Fabricated by Nanoplating and Two-Photon Polymerization Wei-Kang Wang,||,†,‡ Zheng-Bin Sun,||,§ Mei-Ling Zheng,*,† Xian-Zi Dong,† Zhen-Sheng Zhao,† and Xuan-Ming Duan*,† †

Laboratory of Organic NanoPhotonics and Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancunbeiyitiao No. 2, Beijing 100190, People's Republic of China ‡ Graduate University of Chinese Academy of Sciences, Zhongguancunbeiyitiao No. 2, Beijing 100190, People's Republic of China § College of Materials Science and Opto-Electronic Technology, Graduate University of Chinese Academy of Sciences, Yuquanlu No. 19, Beijing 100049, People's Republic of China

bS Supporting Information ABSTRACT: We have prepared and characterized nickel
phosphorus (Ni
P)/polymer composite and demonstrated a method for fabricating three-dimensional (3D) micromachines by combining two-photon polymerization (TPP) of polymer and electroless plating of Ni
P alloy to achieve high mechanical performance and remotely controllable capability. Ni
P electroless plating process has been optimized. The mechanical performance of the Ni
P/polymer composite film, such as the hardness and modulus, was improved to 1.74 and 34.93 GPa with a Ni
P alloy layer, respectively. The Ni
P alloy layer deposited on the surface of the 3D micromachine provides good response capability in the magnetic field. Furthermore, we successfully demonstrate that the as-prepared magnetic 3D micromachine was able to be remotely manipulated expeditiously by the external magnetic field. This study would open up a broad prospect for developing remote manipulated 3D micro/nanomachine with excellent mechanical performance.

1. INTRODUCTION Micromachines have been attracting much interest owing to their extensive applications in integrated circuits,1 wireless communication,2,3 microfluidic pumps,4,5 chemical or biological sensors,6
8 and disease diagnosis.9
11 Much effort has been made to develop remotely manipulated micromachines to meet the requirement for precisely targeted treatment in narrow enclosures, harsh environments, and even inside the human body.12
14 Well-designed true three-dimensional (3D) microstructures are preferred to satisfy these requirements.15,16 As an emerging micro/nanofabrication technique, two-photon polymerization (TPP) of polymers is promising for fabricating 3D micro/nanomachines due to its capability of 3D micro/ nanofabrication17
21 and high spatial resolution at nanometric scale.22
25 Thus, TPP technique has been applied in the fields of micro/nanodevices and microelectromechanical systems (MEMS).14,24,26,27 Magnetically driven micromachines have been proved promising in manipulation by remote and noncontact control.28
30 Sun and and co-workers31 presented an available magnetic force driven micromachine prepared by TPP of ferropolymer, indicating the unique advantages for potential applications in health care. However, the mechanical performance of the ferropolymer material at micronanoscale, such as hardness and strength, which has been considered important for evaluating micromachines, is still unidentified.32 Micromachines r 2011 American Chemical Society

made of polymers exhibited intrinsically weak mechanical performance originating from the polymer materials, which would limit potential applications in fields that require mechanical strength of a micromachine, for example, the elimination of thrombus in blood vessels. Moreover, the introduction of the driven force into the polymer material was essential for advanced application of smart micromachines, but it was hard to achieve. Therefore, it is of great importance to exploit a strategy for fabricating precise 3D micromachines with both excellent mechanical strength and manipulated features. Constructing composite materials by plating is a facile method for improving the performance of materials. Electroless plating has been utilized in a wide range of fields since there is no requirement for the conductivity of the substrate.33,34 Nickel
phosphorus (Ni
P) alloy deposit has been extensively studied owing to its extremely high hardness, modulus, wear resistance, and corrosion resistance.35
37 Furthermore, Ni
P alloy exhibits the characteristics of soft magnetic material, which is sensitive to external magnetic field without any hysteresis and shows no remanence after removal of the external magnetic field.38 Therefore, coating a layer of Ni
P alloy on polymer microstructures Received: March 21, 2011 Revised: April 22, 2011 Published: May 18, 2011 11275

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The Journal of Physical Chemistry C should provide us a great opportunity to develop magnetically driven micromachines. In this study, we have prepared and characterized Ni
P/ polymer composite and proposed a strategy for fabricating 3D smart micromachines by the combination of polymer TPP and electroless plating of Ni
P alloy to achieve high mechanical performance and remotely controllable capability. The experimental condition of electroless plating was studied to satisfy nanoplating for tiny sections at nanoscale. Ni
P alloy layer contributed excellent mechanical performance and magnetism to the polymer material, which could be fabricated to 3D micromachine by TPP. We demonstrated the fabrication of a 3D micromachine combined by TPP and Ni
P alloy nanoplating and the remotely driven motion process of the as-prepared 3D micromachine by external magnetic field. The proposed strategy for the development of magnetically driven micromachines with excellent mechanical performance would open up good prospects for the broad applications in MEMS.

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Scheme 1. Procedure for Fabricating 3D Magnetically Driven Micromachines

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation of Composite Films. All chemicals including dipentaerythritol hexacrylate (DPE-6A), poly(methacrylic acid) (MA), SnCl2, PdCl2, NiSO4 3 6H2O, NH4Ac, NaH2PO2 3 H2O, NaOH, photosensitizer benzil, and photoinitiator 2-benzyl-2-(dimethylamino)-40 -morpholinobutyrophenone were purchased and used without any purification. The photoresist was prepared by mixing MA (0.422 g, 4.9 mmol), DPE-6A (1.060 g, 1.8 mmol), 16.7 mg of photoinitiator, and 18.1 mg of photosensitizer, respectively. After being stirred for 2 h, the liquid resin was sandwiched between a pair of glass slides and polymerized under the irradiation of a high-voltage mercury lamp (power 32.5 mW 3 cm
2, wavelength 365 nm) for 3 min to prepare the polymer film. Then the polymer film was soaked in 1.0 g/L SnCl2 aqueous solution for 30 min and subsequently in 0.9 g/L PdCl2 aqueous solution for another 30 min at room temperature to provide a sensitized surface by synthesizing Pd nanoparticles on the polymer film. The aqueous solubility of PdCl2 was improved by adding an amount of 1 M HCl and the final pH of PdCl2/HCl mixed aqueous solution was about 1.5. The electroless plating solution was prepared by involving 100 μL of 10 wt % NaOH aqueous solution/mL of plain plating solution, which was composed of 0.1 M NiSO4 3 6 H2O, 0.4 M NH4Ac, and 0.2 M NaH2PO2 3 H2O.39 The pH of the plating solution was 7.98 at 22.3 °C and the plating process was implemented at 60 °C. 2.2. Laser Microfabrication. For the fabrication of 3D microstructures, a mode-locked Ti-sapphire laser was used. The center wavelength, pulse width, and repetition rate were 780 nm, 80 fs, and 82 MHz, respectively. The lasing source was tightly focused by a 60, oil-immersion objective lens with a high numerical aperture (NA = 1.42, Olympus) on the liquid photopolymerizable sample. The laser beam was scanned in the horizontal directions by a pair of galvano-mirrors (Scanlab, HurrySCAN 14). A computer was used to control the scanning operation of the galvano-mirrors according to the preprogrammed pattern. After laser fabrication, the unpolymerized resins were washed away with ethanol. The obtained microstructures were used for electroless plating process. 2.3. Characterization. pH measurement of the plating solution was assessed by a portable pH meter (FG2, Mettler-Toledo). X-ray diffraction (XRD) was measured by a polycrystalline X-ray

diffractometer (Bruker D8 Focus) using a monochromatic Cu KR X-ray beam. The surface morphology of the composite film and the micromachine were characterized by scanning electron microscopy (SEM) (Hitachi S-4300 FEG) with an energydispersive spectrometer (EDS) accessory. The thickness of electroless layer was identified by high-resolution transmission electron microscopy (HR-TEM) (JEOL JEM-2100). The hardness and modulus were investigated by a nanoindentor (Nano Indenter XP, Nano Instruments). Magnetic characterization was demonstrated by a superconducting quantum interference device magnetometer (Quantum Design MPMS). The remote manipulation of the magnetic micromachine was recorded by an optical microscope (BX-51, Olympus).

3. RESULTS AND DISCUSSION 3.1. Strategy for Fabricating Magnetically Driven 3D Micromachines. The procedure of fabricating 3D polymer

microstructures by TPP and the subsequent Ni
P electroless plating is illustrated in Scheme 1. In the components of photoresist, 69.8 wt % cross-linker dipentaerythritol hexacrylate (DPE6A) was employed to form a dense polymer network to improve polymer mechanical strength for creating microstructures. Methacrylic acid (MA) provided a hydrophilic group to assist the deposition of Ni
P alloy on the surface of the fabricated polymer microstructures. Consequently, the photoresist was used to fabricate the 3D polymer micromachine by the TPP using a fem tosecond laser. The 3D polymer micromachine was sensitized by use of SnCl2 and PdCl2 aqueous solution to form a homogeneous layer, providing activation centers for the deposition of Ni. Then the deposition procedure of the successive electroless plating of Ni
P alloy was carried out, which led to the formation of Ni
P alloy layer on the surface of 3D polymer micromachine. The 3D micromachine achieved by using the nanoplating methodology would exhibit strong mechanical strength, such as hardness and modulus. Moreover, the Ni
P alloy on the surface of the 3D micromachine make it attractive for the remote manipulation by the external magnetic field. 11276

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Table 1. Composition of the Deposit Layer for Different Plating Times plating time (min) phosphorus content (wt %) nickel grain size (nm)

10 3.3

15 3.5

20 4.3

30 4.8

45 5.4

60 5.7

6.1

6.1

6.1

6.5

7.8

3.2. Preparation and Characterization of Ni
P/Polymer Composite. It was realized that remote magnetic manipulation

of 3D micromachines was significant for applications in micro/ nano science. The procedures for developing this kind of 3D micromachine had been studied, which involved magnetic optimization of the plating solution, characterization of the phosphorus content and the grain size of nickel, influence of the surface morphology, enhancement of the mechanical performance of the Ni
P deposit layer, and demonstration of the magnetic behavior. Based on the understanding of these experimental conditions, it was promising to achieve remotely driven micromachines. 3.2.1. Optimization for Preparing Ni
P/Polymer Composite. Polymer film was used to optimize electroless plating conditions for preparation of Ni
P/polymer composite. The polymer film was prepared by sandwiching the liquid resin between a pair of glass slides and then polymerized under the irradiation of the high-voltage mercury lamp for 3 min. Consequently, the nanoplating process of Ni
P composite was carried out by using the plating solution with a pH of 7.98 at 22.3 °C. Energy-dispersive spectrometer (EDS) analysis showed that the deposit layer prepared by this plating solution was composed of Ni
P alloy with 8.8 wt % phosphorus (Table S1, Supporting Information). It was considered that the magnetic capabilities decreased with increasing phosphorus content and became nonmagnetic when the plating layer contained more than 8 wt % phosphorus.40 Therefore, this plating solution had to be modified in order to achieve a magnetic deposit layer. According to the electrochemical mechanism of Ni
P alloy electroless plating, as shown in eq 1,41,42 a plating solution with higher pH would accelerate the synthesis of nickel and reduce that of phosphorus. We attempted to improve the reduction rate of nickel by adding NaOH aqueous solution into the plain plating solution. However, an excessively high reaction rate would result in the production of a large amount of H2 bubbles, which would decrease the adhesion between Ni
P electroless layer and the polymer substrate. Our experiments suggested that the Ni
P/ polymer composite exhibited apparently strong magnetism when the pH of the plating solution was optimized to 7.98 at 22.3 °C. H2 PO2
þ H2 O f H2 PO3
þ 2Hþ þ 2e


Ni2þ þ 2e
f Ni 2Hþ þ 2e
f H2 H2 PO2
þ 2Hþ þ 2e
f P þ 2H2 O

ð1Þ

A series of plating times, from 10 to 60 min, with the pHmodified plating solution were investigated to seek proper deposit condition for microstructures. EDS of the obtained Ni
P electroless plating layers indicated 3.3
5.7 wt % phosphorus, as shown in Table 1, implying a magnetic enhancement corresponding to the much lower phosphorus content (P wt %). P wt % increased obviously when the plating time was extended, which could be explained by the continual increase of pH of the plating solution resulting from the consumption of OH
in the electroless plating process. As lower P wt % induced stronger

Figure 1. XRD of composite films at different plating times.

magnetism, the EDS analysis implied that the plating time should be optimized to achieve strong magnetism. 3.2.2. Characterization of Ni
P/Polymer Composite. The corresponding XRD exhibited a broadened diffractive peak at 44.5°, which could be assigned to the reflecting plane (111) of nickel (Figure 1). The grain size of Ni was calculated according to the Debye
Scherrer method43 by using the following Debye
Scherrer equation: D¼

kλ β cos θ

Where D was the size of NPs, k was the shape factor, in which 0.93 was for cubic crystalline structure; λ was the X-ray wavelength, typically 1.54 Å; and θ was the Bragg angle, for which we chose 2θ = 44.5° because of the strongest signals here. β corresponded to the broadened full width at half-maximum (fwhm) intensity in radians. By use of β values obtained from Figure 1, the average sizes of Ni grain with different reduction times were calculated (Table 1). There was no calculating result for the sample with plating time 10 min due to the extremely low signal. The grain size of the deposited Ni was only several nanometers, which was much smaller than that of pure nickel because of the incorporation of phosphorus.37 Moreover, the grain size grew very slowly, which was maintained smaller than 10 nm even when the plating time was extended to 60 min. It was acceptable that the size of the particle should be optimized to form the deposit layer with a certain thickness. Otherwise, it would result in an unsatisfactory deposit. Therefore, the grain obtained at 10-nm scale in our experiment was promising for further applications of nanoscale plating. The electroless Ni
P alloy layers were visually characterized by SEM and TEM images. A typical surface morphology of the deposit layer with plating time of 20 min showed a layer of densely packed nanoparticles with diameter less than 100 nm (Figure 2a). A few larger crystallites were synthesized during the electroless plating, suggesting a well-controlled process and a surface-smooth deposit layer at nanoscale. The surface-smooth deposit layer was directly demonstrated by the corresponding TEM image of the ultrathin section sliced from the composite film with plating time 20 min (Figure 2b). The average thickness was 336 ( 17 nm. The selected-area electron diffraction (SAED) pattern indicated a face-centered cubic (fcc) monocrystalline nickel with the allowed reflecting plane (111) (inset of Figure 2b). The corresponding high-resolution transmission 11277

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Figure 2. (a) SEM image of surface morphology of deposit layer with plating time 20 min. (b) TEM image of deposit layer with plating time 20 min (inset: corresponding SAED). (c) HR-TEM of deposit layer surface. (d) Dependence of deposit layer thickness on plating time.

electron microscopy (HR-TEM) image provided more detailed information on the components of the deposit layer (Figure 2c). Numerous microcrystals at several-nanometer scale formed the deposit layer. The crystalline parameter of a typical microcrystal was around 0.205 nm, approaching 0.204 nm, which was the standard parameter for the reflecting plane (200) of a fcc structure. The lattice expansion was caused by the decreased intracrystalline pressure in small particles, which originated from the electrostatic interaction between the elements in the crystal charge lattice.44 The thickness of electroless Ni
P deposit with different plating times, from 10 to 60 min, is summarized in Figure 2d. The thickness was increasing with the continual deposition of Ni
P alloy during the evolution time of plating. The largest thickness was controlled within the nanoscale even when the plating time reached to 60 min, declaring a good nanoplating process. 3.2.3. Mechanical and Magnetic Properties. Furthermore, the mechanical performance of Ni
P/polymer composite film was determined by a nanoindenter. Several Ni
P/polymer composite films with typical plating times of 10, 15, 20, and 60 min were investigated. The polymer film was also studied as a control. The dynamic variation of hardness and modulus versus the displacement into surface is shown in Figure 3. The polymer film exhibited very low hardness and modulus. In contrast, all the composite films exhibited much better mechanical performance. The composite films presented obvious mechanical behavior, namely, much higher hardness and modulus at the first dozens of nanometers of the displacement into surface on the surface of the

Ni
P deposited layer, and then rapidly decreased and was close to that of polymer film at deep depth. The plating time, related to the thickness of the deposited layers, effectively affected the hardness and modulus of the composite materials. As shown in Figure 3, longer plating times resulted in better mechanical behavior. The largest enhancement of hardness and modulus of the composite film with plating time 60 min was demonstrated. The hardness for composite film and polymer film at 50 nm was 1.74 and 0.17 GPa, respectively, providing an enhancement factor of more than 10-fold. The modulus of the composite film at 60 min showed almost 9-fold enhancement compared to the polymer film at 50 nm. However, it was interesting that the composite film at 20 min presented a little weaker hardness and modulus than that at 60 min, although the thickness at 60 min was nearly twice as large as that at 20 min (Figure 2d). This phenomenon was reasonable because the mechanical behavior of the composite film at superficial displacement was considered to be contributed entirely by the surface materials when the deposit layer reached a certain thickness.45 After plating for 20 min, the Ni
P deposit layer achieved an optimum thickness possessing excellent mechanical performance that resulted from Ni
P alloy but not Ni
P/polymer composite. The magnetic characteristic of Ni
P/polymer composite materials was demonstrated by a superconducting quantum interference device (SQUID) magnetometer. Since only the surface-coated electroless Ni
P alloy contributed to magnetism in our study, we chose magnetic moment per square centimeters (emu/cm2) instead of magnetic moment per gram (emu/g) as 11278

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Figure 3. (a) Hardness and (b) modulus of the composite film with plating times of 10, 15, 20, and 60 min, respectively. Polymer film was studied as a control.

Figure 5. (a) Model of the micromachine. (b) Polymer micromachine with electroless layer. (c, d) Detailed sections of the micromachine. (e
h) Continual position change of the micromachine, which rolled to right side. Figure 4. Magnetic hysteresis loops of composite films with plating times of 10, 15, 20, and 60 min at 300 K.

the unit to evaluate the magnetic behavior. The magnetic hysteresis loops at 300 K of the composite films with typical plating times of 10, 15, 20, and 60 min are presented in Figure 4. Coercivity was hardly observed in these curves, suggesting the soft magnetic nature of the Ni
P/polymer composites. The saturation magnetization increased obviously with the amount of deposited Ni
P alloys at the first 20 min. Note that the saturation magnetic density of the composite film with the longest plating time of 60 min decreased unexpectedly and became smaller than 9.5  10
4 emu/cm2 at 20 min. This abnormal phenomenon could be explained by the higher weight content of phosphorus in Ni
P alloys analyzed by EDS. Although the prolonged plating time provided a large amount of magnetic Ni and P nanoparticles, on the other hand, the higher P wt % resulted from the longer plating time would decrease the magnetism. Thus, these were two contrary effects on the apparent magnetism. Under our experimental conditions, a plating time of 20 min was preferred to achieve a strong apparent magnetism. 3.3. Fabrication of 3D Magnetic Micromachine and Remote Manipulation. The fabrication and remote control of magnetic micromachines were carried out based on the understanding of the Ni
P/polymer composite materials. The polymer micromachine was created by two-photon polymerization according to the preprogrammed model as shown in Figure 5a. The model was designed to contain densely packed pieces with six claws at the edge and a hole in the center, as well as a long thin

shape, indicating the potential application in narrow environments to cut or remove objects. It was about 70 μm in length, 40 μm at the widest section, and 25 μm at the narrowest section. In our study, we have fabricated this 3D polymer micromachine from the photoresist by using the TPP technique. The fabricated 3D polymer micromachine was electrolessplated for 20 min in order to achieve strong mechanical and magnetic performance as per our previous investigations of composite materials on hardness, modulus, and magnetism. The corresponding Ni
P deposited 3D polymer micromachine was visualized by SEM (Figure 5b). Detailed sections of the micromachine with higher magnification are shown in Figure 5c, d, which suggested a well-controlled electroless plating process with smooth deposited surface and acute edges. EDS in the selected area proved that the whole 3D microstructure was coated with Ni
P alloys (Figure S1, Supporting Information), which indicated possible remote manipulation by the external magnetic field. As shown in Figure 5d, the 3D micromachine held sharp-angled claws and exhibited an angle of 56.8°, which was in agreement with the predesigned 3D microstructure model. The sharp-angled claws could be utilized for potential applications in the elimination of jams in microenvironments, in which a micromachine with enough hardness could be manipulated optionally. We have demonstrated the manipulation of the as-prepared 3D micromachine by an external magnetic field. First, the 3D micromachine was put in a glass case with a droplet of water. With the forces of the water surface tension, gravity, and water buoyancy, the 3D micromachine kept a vertical state at the edge 11279

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The Journal of Physical Chemistry C of the water droplet (Figure 5e). Then we added an external field under the micromachine. When the external ferromagnetism rotated and moved from left to right, the magnetic micromachine rolled and moved from left to right simultaneously. The whole process was recorded, as shown in Figure 5e
h. The rolling and moving processes had been vividly observed by video (Supporting Information). This 3D micromachine, designed with six claws at the edge and controllable remote manipulation capability by the external magnetic field, could provide a practical tool for removing or cutting objects in narrow environments.

4. CONCLUSIONS In conclusion, we have prepared and characterized Ni
P/ polymer composite and demonstrated a facile strategy for fabricating 3D magnetically driven micromachines by combining the nanoplating and two-photon polymerization techniques. Ni
P electroless plating process was well-optimized and applied for fabricating 3D micromachines of Ni
P/polymer composite with excellent magnetic properties. The mechanical performance of the composite materials, such as the hardness and the modulus, were 10- and 9-fold improved by the Ni
P alloy deposited layer compared to polymer film. Moreover, the Ni
P deposited layer provided polymer microstructure good response capability in the magnetic field, which indicated the micromanipulated property. The 3D micromachine is able to be remotely manipulated expeditiously by the external magnetic field. The combination of TPP and nanoplating would provide a promising protocol for developing smart-micro/nanomachines with excellent mechanical performance and remotely manipulated capability. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional text, one figure, and one table with details of chemicals, EDS of composite films, and selected-area EDS of magnetically driven micromachine, and a video of rolling and moving processes. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected] (M.-L.Z.) or xmduan@ mail.ipc.ac.cn (X.-M.D.); tel/fax þ86-10-82543597. )

Author Contributions

These authors contributed equally to this work.

’ ACKNOWLEDGMENT We are grateful to the NSFC (Grants 50773091, 50973126, 60907019, and 61077028), 973 (2010CB934103), ICP programs of MOST (2008DFA02050 and 2010DFA01180), and China Postdoctoral Science Foundation project (20100480510) for financial support. ’ REFERENCES (1) Dickey, M. D.; Russell, K. J.; Lipomi, D. J.; Narayanamurti, V.; Whitesides, G. M. Small 2010, 6, 2050. (2) Nishino, T.; Hangai, M.; Yoshida, Y.; Lee, S. S. IEICE Trans. Electron. 2010, E93C, 1111.

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