A novel method for generation of amphiphilic PDMS particles by ...

Microfluid Nanofluid (2011) 10:453–458 DOI 10.1007/s10404-010-0673-5

SHORT COMMUNICATION

A novel method for generation of amphiphilic PDMS particles by selective modification Li-Bo Zhao • Si-Zhe Li • Hao Hu • Zhi-Xiao Guo • Feng Guo • Nan-Gang Zhang • Xing-Hu Ji • Wei Liu Kan Liu • Shi-Shang Guo • Xing-Zhong Zhao

•

Received: 14 May 2010 / Accepted: 4 July 2010 / Published online: 20 July 2010 Ó Springer-Verlag 2010

Abstract We reported a novel method to fabricate both spherical and nonspherical PDMS (Polydimethylsiloxane) particles of amphiphilic property. PDMS particles were first obtained using microfluidic devices. A layer of PAA (poly (acrylic acid)) polymer was then selectively grafted on a section of particle surface. After modification, PDMS particles exhibited an amphiphilic property. We demonstrated the assembly of spherical particles at the water/oil interface as well as the deformation of cuboidal particles which possessed a bowl-shaped structure after deformation. Keywords PDMS

Janus particle Amphiphilic Microfluidics

1 Introduction In the past few years, a number of new approaches have been reported for the syntheses of anisotropic particles (Glotzer and Solomon 2007). These particles are useful because they usually possess peculiar natures, such as unusual shape (Dendukuri et al. 2006), multilayer structure (Chu et al. 2007), anisotropic compositions or surface property (Nie et al. 2006; Nisisako et al. 2006; Shah et al. 2009; Dendukuri et al. 2007; Kim et al. 2010). One of the potential applications that L.-B. Zhao S.-Z. Li H. Hu Z.-X. Guo F. Guo N.-G. Zhang X.-H. Ji W. Liu K. Liu S.-S. Guo (&) X.-Z. Zhao (&) Department of Physics, Key Laboratory of Artificial Microand Nano-Structures of the Ministry of Education and School of Physics Science and Technology, Wuhan University, Wuhan 430072, People’s Republic of China e-mail: [email protected] X.-Z. Zhao e-mail: [email protected]

receive much concern is the assembly of these particles into complex structure. Under the inspiration of cell membrane structure, a kind of micro-particle possessing amphiphilic property has been prepared in microfluidic device (Nie et al. 2006; Dendukuri et al. 2007; Zhao et al. 2009). These particles with one part hydrophobic and the other part hydrophilic behave differently from isotropic particles at the water/oil interface (Dendukuri et al. 2007). So far, these studies usually focus on integrating the process of preparing anisotropic droplets and the process of solidification within one microfluidic device, which requires to fabricate microchip of extremely high accuracy, construct complex experiment setups and precisely control each parameter. Recently, Shin-Hyun Kim and his group introduced a two-step approach to prepare amphiphilic microspheres. In this study, porous microspheres were first generated using co-flowing method, and then reactive ion etching (RIE) was applied to achieve surface modification. Microspheres obtained in this way had one part superhydrophobic (contact angle 120°) and the other part hydrophilic (contact angle 72°) (Kim et al. 2010). It is possible to simplify the experiment setups by conducting modification process outside microchip and maintain the monodispersion at the same time. However, this method is not available for the preparation of nonspherical particles. Herein, we reported a flexible approach to fabricate amphiphilic particles using PDMS (Polydimethylsiloxane) as major composition. PDMS is a useful polymer in many industrial and medical applications due to its attractive properties, such as optical transparency, non-toxicity, chemical inertness and thermal stability. Microfluidic devices were employed to generate PDMS particles of different shapes. A layer of polymer (poly (acrylic acid), PAA) was then selectively grafted on a part of particle surface. As a result, this part became hydrophilic, while the other part without PAA polymer maintained the hydrophobic property.

123

454

Microfluid Nanofluid (2011) 10:453–458

2 Experimental 2.1 Materials PDMS gel RTV-615 was obtained from GE. Benzophenone (99%), acetone (99.5%) and NaIO4 (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Benzyl alcohol (99.7%) and acrylic acid (99.5%) were obtained from Tianjin Chemical Reagent Institute, Tianjin, China. DI water was produced by a Millipore Direct-Q3 water purification system (Millipore, Worcester, MA). All reagents were used with no additional purification. The glass slab with Cr/Photoresist layers was obtained from Changsha Shaoguang Chrome Blank Co., Ltd. 2.2 Fabrication of spherical PDMS particles A co-flowing method was used to generate spherical PDMS particles (Utada et al. 2005). Figure 1 shows the setup of our chip, which consists of coaxially aligned metal/glass tubes. The fabrication processes are schematically shown in Fig. 1a. (I) A glass tube (inner diameter 0.9 mm, outer diameter 1.1 mm) and a small piece of metal bulk were placed on a culture dish to form a mold. (II) PDMS gel was poured on this mold, evacuated in a vacuum and placed in an oven heated at 80°C. (III) After curing for 2 h, the PDMS slab was peeled off and two holes as fluid inlets were punched at the chamber where used to be held by the metal bulk. The passage of the glass tube might be blocked by PDMS, therefore the tube was replaced with a new one. (IV) Two metal tubes (stainless steel, inner diameter 0.4 mm, outer diameter 0.7 mm) were inserted to the inlet holes as connectors to syringe pump. One end of the L-shape metal tube was inserted into the glass tube, as shown in Fig. 1a, to form a coaxial channel. Finally this PDMS slab was bonded to another piece of PDMS after oxygen PLASMA treatment and then annealed in 80°C for 30 min. Newly prepared PDMS with a ratio of 10:1 was used as dispersed phase and injected into the metal tube from inlet B with a syringe pump. Soybean oil was injected from the inlet A as continuous phase. Under the shear force stressed by continuous phase, PDMS fluid ruptured and formed droplets in the glass tube. The formation of PDMS droplets can approximately be divided into two stages as shown in Fig. 1b, which was in accord with a common model (Zhang 1999). In stage I, net force on PDMS droplet attached to the metal tube was nearly zero, and a growth of drop volume can be observed. When this initial equilibrium no more existed, it came to stage II which corresponded to the necking and breaking of PDMS fluid. PDMS droplets suspended in soybean oil were delivered to a container, and placed in an oven heated at 80°C for 3 h to conduct crosslinking. Then PDMS particles were

123

Fig. 1 a shows the processes of chip fabrication. b is a schematic diagram of the droplet generation. A and B in this figure are, respectively, inlets for soybean oil and PDMS phase. Inserted pictures at bottom left corresponds to two stages of droplet formation

separated from oil using centrifugation. After rinsing them three times with hexane, these particles were dried in a fume hood and then collected. 2.3 Fabrication of cuboidal PDMS particles A soft-lithography technique was employed to fabricate cuboidal PDMS particles (Qiu et al. 2007). A glass slab with Cr/Photoresist layers was used to form a template. First, a photoresist protective layer was transferred from an optical template by photolithography. After removing the uncovered Cr layer by wet etching, a metal mask with the same structure of optical template was made on the glass slab. Then HF etching was used to construct a micro-well array on the glass. Finally, Cr/Photoresist left on the glass slab was entirely removed. PDMS (ratio 10:1) was poured on the glass mold and evacuated in vacuum to ensure micro-wells were completely filled. Extra PDMS gel was swept away and then cured in an oven. To release PDMS particles, the glass mold was immersed in hexane and treated with ultrasonics. 2.4 Particle modification Since PDMS had a hydrophobic surface, in order to obtain an amphiphilic particles we employed a method to graft PAA polymer on PDMS particles (Hu et al. 2002). The operation process can be summarized as follows. First, PDMS particles were immersed in benzophenone (10 wt% in acetone) for 15 min to adsorb photoinitiator on the surface (benzophenone). After rinsing three times with DI water, PDMS particles were dispersed in a monomer solution containing 10 wt% acrylic acid, 0.05 mM NaIO4 and 0.5 wt% benzyl alcohol. After exposing to UV irradiation, the particles were collected. However, the modification of spherical and cuboidal particles is slightly different in the operations.


For the modification of spherical PDMS, a process assisted with buoyancy and capillary force was applied. After absorbing photoinitiator and being rinsed with DI water, the spherical particles were dispersed in monomer solution. Since the particles produced by our microchip had a small diameter (\1 mm), capillary force was no longer insignificant (Pitois and Chateau 2002). In a static condition, the forces acting on the particle are: Weight force, * * * G ¼ 43 pR3 q1 z (q1 is the density of PDMS, z is the unit vector pointing vertically upward). Buoyancy force, * * Fb ¼ ph2 R h3 q0 z (q0 is the density of monomer solution). Capillary force, * * * Fc ¼ Fs cos½p ðh þ /Þ ¼ 2pRr sin / sinðh /Þz ; (where r is the liquid/gas surface tension). As shown in Fig. 2, gravity force was balanced by buoyancy and capillary forces, as expressed by equation * * * G ¼ Fb þ Fc (Pitois and Chateau 2002). Under the coactions of buoyancy and capillary forces, it is possible to form a partial contact between PDMS particle and monomer solution. Finally, UV exposal was conducted to induce photopolymerization at the PDMS/liquid interface. For the modification of cuboidal PDMS particles, these particles were first released in a glass culture dish containing hexane. After the evaporation of hexane, PDMS particles became attached at the dish bottom. Benzophenone (10 wt% in acetone) was filled in and then drained out after 15 min. However, absorption of photoinitiator could not occurred at the bottom side of PDMS particle, since this part was firmly attached to the bottom of culture dish. After rinsing with DI water to remove extra benzophenone, monomer solution was added. Finally, PDMS particles were exposed to UV irradiation to induce modification.

Fig. 2 The force analysis schematic diagram of PDMS particle in monomer solution. h is the height of sphere crown coated with PAA, R is radius of spherical particle, / defines the angular*position of * * contact line on the particle, and h is the contact angle, Fb ; Fs and G are, respectively, the buoyancy force, surface tension and gravity

455

3 Results and discussion 3.1 Formation of spherical PDMS particles In the process of droplet preparation, suppose the oil flux c was 10 ml/h, Reynolds number Re ¼ Dvq gc (where D is glass tube inner diameter, v, qc, and gc are velocity, density and dynamic viscosity of continuous flow, respectively) could be around 0.05, apparently, the shear force dominated. So, adjusting the flow rate of soybean oil while other parameters remained unchanged would alter the hydrodynamic force and influence droplet volume (Zhang 1999). As shown in Fig. 3, with the increase of oil flux from 1 to 10 ml/h, shear force grew accordingly and resulted in a reduction of droplet diameter from 870 to 400 l. According to the results, a weakening of size tunability of continuous phase can be observed at both ends of the curve. At the high flux end, though the shear force grew, the diameter of PDMS droplets would hardly increase because the PDMS droplet attached to metal tube would stay in stage I until it exceeded a threshold value which was greater than or at least equal to the inner diameter of metal tube (0.4 mm). At the low flux end, the weakening can be attributed to the wall effect which became remarkable as droplets diameter increased and finally inhibited the growth of PDMS droplets. Therefore, using this microchip, generation of PDMS particles with diameter between 400 and 870 lm was available. 3.2 Formation of cuboidal PDMS particles Because the size and shape of cuboidal particle were simply determined by the glass template, it was possible to obtain particle with different size by changing the design of optical mask (Fig. 4). However, considering the isotropy of glass etching, the length of micro wells in glass template

Fig. 3 The relation between diameter of PDMS droplet (before curing) and the flux of soybean oil. The inserted picture is the microscopic image of PDMS droplets

123

456


3.3 Modification of PDMS particles

Fig. 4 The microscopic image of glass template. The inserted figure is the microscopic image of PDMS particles of different sizes. The scale bars in this figure are 500 lm

would be greater than the original mask. According to our experiment, the length of optical mask was 200 lm, while the obtained micro wells had a length of 244.7 lm and a depth of 29.4 lm, which were related approximately by the equation L = L0 ? 1.5D (where L represents the final length of micro well, L0 is the length of optical mask and D is the depth of micro well). Fig. 5 a and c are SEM images of an amphiphilic PDMS particle. b is the SEM image of an ordinary PDMS particle. d is the distribution of PDMS particles at the interface of a w/o (water in oil) droplet, scale bars are all 200 lm

123

In the modification process, benzophenone adsorbed on the PDMS particles worked as photoinitiator to induce free radical polymerization. NaIO4 in monomer solution was used to scavenge oxygen which could compete with the monomer for free radicals, and benzyl alcohol was included in because it acted as a chain-transfer agent during the reaction and enhanced the grafting efficiency (Qiu et al. 2007). To illustrate the surface topography of spherical particles, SEM images are given in Fig. 5. As can be seen in Fig. 5b, ordinary spherical particle possessed a smooth surface. Figure 5a, c are SEM images of an amphiphilic spherical particle. As shown in Fig. 5a, the top end of this particle had a distorted and rough surface. This part was hydrophilic because a layer of PAA was coated on it. While on the bottom end, the particle presented a smooth surface indicating there was no PAA grafted, therefore this part maintained the hydrophobic property. Figure 5c is a magnified view of the boundary between the hydrophobic and hydrophilic areas. It is clear that only the top part had a layer of irregular PAA polymer cladding on the surface. To further demonstrate the amphiphilic property of these particles and illustrate their application potential, one more experiment was carried out to study their behaviour in two


457

Fig. 6 a The microscopic image of an ordinary particle, b and c are SEM images of an ordinary particle (scale bars are, respectively, 100 lm in b and 10 lm in c), d is the microscopic image of an

amphiphilic particle, e and f are SEM images of an amphiphilic particle (scale bars are, respectively, 100 lm in e and 10 lm in f)

phase condition. Figure 5d shows the situation of spherical particles assembly at the interface between oil and water. The hydrophilic parts of particle (dark side) tended to attach on the water droplet while the hydrophobic part (transparent side) was exposed in the oil phase. Therefore, it was possible to utilize these spherical amphiphilic particles as particle stabilizer. Unlike those spherical particles which were floating in monomer solution, cuboidal particles were stuck at the bottom of culture dish before modification. When exposing to UV irradiation, the polymerization of PAA occurred at the particle surface except the bottom side. A deformation of particle was observed during the modification. As shown in Fig. 6a, an ordinary cuboidal PDMS particle had a flat and smooth surface. This result was also supported by SEM images as shown in Fig. 6b, c. After modification, the top side of particle coated with PAA polymer became rough and bulged out, as shown in Fig. 6d. SEM images of a particle’s bottom side are shown in Fig. 6e, f. According to the SEM results, the bottom side had a smooth and recessed surface. However, at the edge we can see PAA polymer cladding on the other side of the particle. This deformation can be attributed to the swelling of PAA hydrogel. Therefore, cuboidal particles after modification can not only

obtain an amphiphilic property, but also a bowl-shaped 3D structure. Considering PAA hydrogel had the ability to response to changing environmental conditions, this kind of particles might possibly be utilized as particle sensors.

4 Conclusion We have presented a method for fabricating both spherical and nonspherical PDMS particles of amphiphilic property with simple experiment setups. By grafting a layer of PAA polymer partially on their surfaces, ordinary PDMS particles become partially hydrophilic and the rest of the particles’ surface maintain hydrophobic property. Spherical anisotropic PDMS particles display an ability to assemble at the water/oil interface. Cuboidal anisotropic PDMS particles possess a bowl-shaped 3D structure besides the amphiphilic property. We believe these particles have potential applications as particle stabilizer or particle sensor. Acknowledgments This study is supported by the Microfluidic Lab in Physics Department of Wuhan University. Financial support from the National Natural Science Foundation of the National Young Scientists Fund (funding No. 50125309) and National Natural Science Foundation of China (under Grant No. 10804087) is acknowledged.

123

458

References Chu LY, Utada AS, Shah RK, Kim JW, Weitz DA (2007) Controllable monodisperse multiple emulsions. Angew Chem Int Ed 46(47):8970–8974 Dendukuri D, Pregibon DC, Collins J, Hatton TA, Doyle PS (2006) Continuous-flow lithography for high-throughput microparticle synthesis. Nat Mater 5(5):365–369 Dendukuri D, Hatton TA, Doyle PS (2007) Synthesis and selfassembly of amphiphilic polymeric microparticles. Langmuir 23(8):4669–4674 Glotzer SC, Solomon MJ (2007) Anisotropy of building blocks and their assembly into complex structures. Nat Mater 6(8):557–562 Hu SW, Ren XQ, Bachman M, Sims CE, Li GP, Allbritton N (2002) Surface modification of poly(dimethylsiloxane) microfluidic devices by ultraviolet polymer grafting. Anal Chem 74(16): 4117–4123 Kim SH, Lee SY, Yang SM (2010) Janus microspheres for a highly flexible and impregnable water-repelling interface. Angew Chem Int Ed 49(14):2535–2538 Nie ZH, Li W, Seo M, Xu SQ, Kumacheva E (2006) Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly. J Am Chem Soc 128(29):9408–9412

123

Microfluid Nanofluid (2011) 10:453–458 Nisisako T, Torii T, Takahashi T, Takizawa Y (2006) Synthesis of monodisperse bicolored janus particles with electrical anisotropy using a microfluidic co-flow system. Adv Mater 18(9): 1152–1156 Pitois O, Chateau X (2002) Small particle at a fluid interface: effect of contact angle hysteresis on force and work of detachment. Langmuir 18(25):9751–9756 Qiu C, Chen M, Yan H, Wu HK (2007) Generation of uniformly sized alginate microparticles for cell encapsulation by using a softlithography approach. Adv Mater 19(12):1603–1607 Shah RK, Kim JW, Weitz DA (2009) Janus supraparticles by induced phase separation of nanoparticles in droplets. Adv Mater 21(19):1949–1953 Utada AS, Lorenceau E, Link DR, Kaplan PD, Stone HA, Weitz DA (2005) Monodisperse double emulsions generated from a microcapillary device. Science 308(5721):537–541 Zhang XG (1999) Dynamics of drop formation in viscous flows. Chem Eng Sci 54(12):1759–1774 Zhao LB, Pan L, Zhang K, Guo SS, Liu W, Wang Y, Chen Y, Zhao XZ, Chan HLW (2009) Generation of janus alginate hydrogel particles with magnetic anisotropy for cell encapsulation. Lab Chip 9(20):2981–2986