Janus Photonic Crystal Microspheres: Centrifugation-Assisted

Article pubs.acs.org/Langmuir

Janus Photonic Crystal Microspheres: Centrifugation-Assisted Generation and Reversible Optical Property Yuandu Hu, Jianying Wang, Chengnian Li, Qin Wang, Hong Wang, Jintao Zhu,* and Yajiang Yang* Key Laboratory of Large-Format Battery Materials and Systems of the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: A new strategy to prepare core/shell Janus photonic crystal (PC) microspheres with reversible optical spectrum property is demonstrated. The microfluidic technique was employed to generate the uniform core/shell PC microspheres containing nanogels aqueous suspension. Under centrifugal force, the nanogel particles homogeneously dispersed in the core of microspheres would aggregate in the half of the microspheres, leading to Janus PC microspheres with varied reflection spectra at the different side of the spheres. More interestingly, such Janus structure of PC microspheres and their reflection spectrum were significantly reversible when the centrifugation was employed and removed alternatively. In addition, due to the soft and thermal-responsive nature of the building blocks (e.g., nanogels), Janus structures and optical properties of the PC microspheres are highly influenced by the temperature, centrifugal speed, and time, providing the other parameters on the manipulation of properties of the PC microspheres. This strategy provides a new concept for the preparation of Janus PC microspheres with tunable structures and optical properties, which will find potential applications in the field of sensors, optical devices, barcodes, etc.

1. INTRODUCTION Janus particles possess a unique feature; namely, both sides of the particle exhibit different chemical or physical properties.1 Janus particles have been extensively studied due to their potential applications in the fields of emulsions,2,3 displays,4,5 anisotropic functional materials,6 and others. Meanwhile, colloidal photonic crystals (PC), which are periodic dielectric arrangements of colloidal nanoparticles on the optical wavelength scale, demonstrate unique optical properties, including photonic badgap and structural colors. Generally, stop band position and structural color of the PCs can be tailored through controlling the refractive index or lattice spacing. In recent years, Janus PC microspheres have attracted increasing attention because of their potential application in optical devices,7,8 barcodes,9 sensors,10 etc. Among the various fabricating methods of Janus microspheres, the microfludic technique has proved to be a powerful and effective approach.7,9,11−14 For instance, a microcapillary device was employed to prepare resin-based photonic balls with Janus microspheres by dispersion of silica particles in a photopolymerizable monomer (ethoxylated trimethylolpropane triacrylate (denoted as ETPTA)).15 Chen et al. demonstrated the micromanipulation of the magnetic responsive Janus PC microspheres in a magnetic field, forming rewritable full-color photonic patterns in a triphase microfluidic device.16 In addition, Gu et al. developed a new kind of Janus PC microsphere with multiplexed features, such as different boss arrays and wettability compartmentalized on the same surface, and an anisotropic color and magnetic properties.17 The © 2013 American Chemical Society

prepared Janus microspheres can be anchored at the air−water interface and act as a highly flexible barrier for preventing coalescence of water droplets. Although these Janus PC microspheres show good optical properties, the fixed or irreversible optical properties are hard to be adjustable once formed. Recently, centrifugation assisted sedimentation has been used to measure size and separate of colloidal particles with different shape and size by manipulation of the centrifugation parameters.18−20 In this work, we demonstrated a simple yet robust method to generate Janus PC microspheres by a centrifugation method assisted by the microfluidic technique. Interestingly, the resultant Janus microspheres exhibit a tunable and reversible optical property. Herein, the core/shell microspheres were first prepared by a double emulsion microfluidic technique,21,22 in which, a photopolymerizable resin was used to form the shell and an aqueous dispersion of deformable nanogels formed by the copolymerization of N-isopropylacrylamide and acrylic acid (denoted as PNIPAM-co-AAc) was used to form the core (see Scheme 1). The aqueous dispersion of nanogels with well-ordered structures endows the core/shell microspheres properties of PC. Based on the temperature sensitivity of PNIPAM-co-AAc nanogels, tunable and reversible optical properties of the microspheres could be realized by the variation of temperature.21 In comparison with PC consisting of Received: October 23, 2013 Revised: November 19, 2013 Published: November 23, 2013 15529

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affect the reflection spectra of microspheres, such as positions and intensities of reflection peaks. As we reported previously, the structural colors of PC microspheres mainly depend upon the concentration of nanogels.21 Taking the PC microspheres with red color as an example (shown in Figure 1), the mass fraction of the nanogels within the microspheres was ∼2.5 wt %. Before the centrifugation, the wavelength of maximum reflection peak was found to be ∼600 nm at 20 °C (Figure 1a), while after the centrifugation, the wavelengths show the blue-shift from 600 to 562 nm, and the corresponding color also changed from red to light green (one pole of the Janus PCs microsphere) (Figure 1b). The reason for the structural color change can be ascribed to the change of distance between nanogel particles. Based on the Bragg law

Scheme 1. (a) Illustration Showing the Synthesis Strategy of the Photonic Crystal (PC) Microspheres through Microfluidics Technique;a (b) Illustration Showing the Formation of Janus PC Microspheres through Sedimentation of the Microgels by Centrifugationb

λ = 2nd sin θ

(1)

where n is the effective refractive index, d is the center-to-center space of the crystalline planes next to each other, and θ is the viewing angle. Here, d is a key parameter to determine the position of the reflection peaks and structural colors. Apparently, d decreased with a decrease of distance between the nanogels, leading to a blue-shift of the reflection peaks, and vice versa. This result is in accord with the case where hard nanoparticles were applied.15 Under centrifugation processing, there exists a force balance on a particle, including centrifugation force, buoyant force, Brownian fluctuating force, and viscous drag force.18 The hydrodynamic behavior of particles depends upon their density and size. In our case, uniformity of the nanogels allows the similar sedimentation velocity. Because of the action of centrifugal force, dispersion state of nanogels (e.g., lattice spacing of the nanogels) was changed, leading to a clear boundary observed within the microspheres (Figure 1b). Both sides of the boundary exhibit structural colors as shown in the inset of Figure 1b, which can be attributed to the sedimentation of the nanogels to one side of the microspheres under centrifugation. In this case, the density of nanogel dispersion within the microspheres may vary under the centrifugation as shown in an inset optical microscopy image in reflection mode, which directly affects the structural color and reflection spectrum of the microspheres. Even in the half sphere, varied structural color still existed, indicating that slight difference of the density/size among the nanogels can affect their sedimentation velocity and the final structural color. As shown in Figure 1c, the single sharp reflection peaks based on the Bragg diffraction of well-ordered nanogel arrays were observed at different sites of the microspheres indicated as the

a Photocurable ETPTA resin was solidified by UV irradiation to obtain the core−shell microspheres with encapsulated microgel crystalline aqueous suspension. b

Various Janus PC microspheres can be generated by tuning the centrifugation speed.

hard silica (or polystyrene) nanoparticles,5,17,23 PC from deformable and soft nanogels is beneficial to sense the imposed stress,24 such as centrifugal force. Thus, the aggregation state of nanogels within the core of microspheres could be finely tuned by adjusting the centrifugal speed, time, and temperature.

2. RESULTS AND DISCUSSION To fabricate the core/shell Janus microspheres with PC properties, monodispersed W/O/W double emulsions were prepared by injecting the mixed solution of poly(vinyl alcohol)/glycerol/water as outer phase, ETPTA with initiator (2-hydroxy-2-methylpropiophenone) as middle phase, and concentrated aqueous dispersions of PNIPAM-co-AAc nanogels as inner phase in a specially designed microfluidic device (Scheme 1). The shell parts of the obtained double emulsion droplets were subsequently polymerized under the UV irradiation. The typical structural color of nanogels located in the core of microspheres can be observed because the shell is consisted of transparent ETPTA resin.25,26 The nanogel dispersions inside the microspheres were highly stable mainly due to the electrostatic interactions between nanogel particles.27,28 The aggregation state of nanogels would directly

Figure 1. (a) Optical microscopy (OM) image of the microspheres without centrifugation. Inset is the representative OM image of the microspheres in reflection mode. (b) OM image of the microspheres treated by centrifugation under transmission mode. Inset is the representative OM image of the microspheres in reflection mode. (c) Reflection spectra measured at different sites indicated in the inset image of the microsphere in (b). 15530

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Figure 2. (a, b) Gradual change of structural colors of PC microspheres versus centrifugal speed at 20 °C (a) and 40 °C (b), respectively. The scale bars are 20 μm. (c, d) Variation of corresponding wavelengths of reflection peak (c) and lattice spacing (d) with the increase of centrifugal speed.

Figure 3. (a, b) Gradual change of structural colors of PC microspheres with an increase of centrifugal time at 20 °C (a) and 40 °C (b), respectively. The centrifugal speed is fixed at 12 000 rpm for all the samples. The scale bars are 20 μm. (c, d) Variation of corresponding wavelengths of reflection peak versus centrifugal time. The original reflection spectra of the PC microspheres can be found in the Supporting Information (Figures S3 and S4).

sites of 1−5. We thus recorded the spectrum of pole on the Janus PCs in the following part in order to compare the spectrum variation. On the basis of the spectrum at different sites of the PC spheres, we calculated the lattice spacing of the nanogels in the Janus PC microspheres according to the simplified Bragg equation (see the Experimental Section). From the results, we can see clearly that the lattice spacing of the nanogels increases from site 1 to 5, which agrees well with the corresponding spectrum. We note that when the centrifugal force was removed, the aggregation state of nanogels can gradually return to its original state because of the Brownian motion and Coulomb repulsion force of the nanogels.28

PNIPAM-co-AAc is a typical temperature-sensitive copolymer.29 Its low critical solution temperature (LCST) is ∼30−35 °C depending upon the composition.30 Hence, we investigated the effect of temperature and centrifugal speed (which directly corresponds to the centrifugal force) on the formation of Janus PC microspheres at 20 and 40 °C. The hydrodynamic diameters of PNIPAM-co-AAc nanogels measured by dynamic light scattering were found to be ∼320 nm at 20 °C and ∼130 nm at 40 °C (see the Supporting Information, Figure S1). Apparently, nanogels with small size aggregated more easily in the half of microsphere under the centrifugation at high temperature due to the higher density and the volume effect of the nanogels. As shown in Figures 2a and 2b, the boundary 15531

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within the microspheres gradually became clear with an increase of centrifugal speed from 9000 to 14 000 rpm, implying that the feature of Janus particles is more remarkable, particularly in the case of 40 °C. Figure 2 also illustrates the relationship between reflection spectra of microspheres (and lattice spacing of the nanogels) and centrifugal speed at 20 and 40 °C (the original reflection spectra can be found in Figure S2). From Figure 2, the gradient change of structural colors also demonstrated the formation of the Janus structure. Interestingly, the initial homogeneous red color before centrifugation gradually evolved into yellow, green or even blue with the increase of centrifugal speed. In general, the reflection spectrum of the PC nanogels is relative to the lattice spacing of the ordered nanogels. With an increase of centrifugal speed, the lattice spacing decreased due to the dense aggregation in the half of microsphere (Figure 2c,d). Meanwhile, the wavelength of reflection peak blue-shifted from 586 to 563 nm at 20 °C (Figure 2c) and from 557 to 491 nm at 40 °C (Figure 2d). Significantly, the wavelengths of reflection peak versus centrifugal speed exhibit a nearly linear relationship whatever at 20 or 40 °C. Centrifugal time is also an important parameter for the formation of Janus structure. Figures 3a and 3b illustrate the effect of centrifugal time on the formation of Janus PC microspheres at 20 and 40 °C under the constant centrifugal speed (12 000 rpm), respectively. Typical Janus PC microspheres can be observed as shown in the photographs of Figure 3. In comparison with the effect of centrifugal speed, the centrifugal time seems to be more beneficial to form Janus structure because the boundary between two distinct structural colors within the microspheres is more remarkable. With an increase of centrifugal time from 15 to 60 min, the microspheres show an obvious color gradient from the initial homogeneous red to green and even blue. The wavelength of reflection peaks blue-shifted from 600 to 551 nm at 20 °C (Figure 3c) and from 600 to 555 nm at 40 °C (Figure 3d). Similar to the results of Figure 2, the wavelengths of reflection peaks versus centrifugal time also exhibit an excellent linear relationship at 20 °C. In contrast, such a blue-shift shows a smooth curve in the case of 40 °C, indicating a slow blue-shift. This may be attributed to a fact that the size of nanogels is small at 40 °C as discussed above. Hence, a relatively dense aggregation of the nanogels could be realized within ∼25 min, and equilibrium state was reached under the centrifugation condition. The further aggregation could be more difficult with an increase of centrifugal time, resulting in a slow blue-shift. Nevertheless, the results from Figures 2 and 3 indicate that the resultant Janus PC microspheres could potentially be used as a sensor to measure centrifugal force (speed). In the case of removal of the centrifugation, the dense aggregation state of nanogels within the microsphere could not be maintained for a long time due to the effect of Brownian motion and Coulomb repulsion.28 As shown in Figure 4a, the clearly visible boundary within the microspheres treated by centrifugation gradually became unclear with an increase of time when the centrifugation was removed. It was completely undistinguishable after ∼2 h, implying disappearance of the Janus structure of PC microspheres. The corresponding wavelength of reflection peaks shifted back from ∼491 to ∼600 nm (Figure 4b). Significantly, such Janus structure of PC microspheres is reversible in the case of centrifugation employed and removed alternatively. It was found that the

Figure 4. (a) Variation of Janus structure of the formed PC microspheres as a function of time after removal of centrifugation. The scale bars are 20 μm. (b) Red-shift of corresponding wavelength of reflection peaks after removal of centrifugation. The original reflection spectra of the PC microspheres during this process can be found in the Supporting Information (Figure S4). (c) Reversibility of Janus structure of PC microspheres in the case of the centrifugation employed and removed alternatively.

good reversibility of Janus structure can be obtained at least five cycles, as displayed in Figure 4c.

3. CONCLUSIONS In summary, a novel centrifugation approach to prepare core/ shell PC microspheres with reversible Janus structure is proposed in this work. The core/shell microspheres with PC property were fabricated by a special designed microfluidic device. Under the action of centrifugal force, the nanogel particles homogeneously dispersed in the core of microspheres would aggregate in the half of the microspheres, leading to the two distinct structural colors within a microsphere. The effects of temperature, centrifugal speed, and time on the formation of such Janus structure were investigated. It was found that such Janus structure of PC microspheres and their reflection spectrum is significantly reversible in the case of centrifugation employed and removed alternatively. 4. EXPERIMENTAL SECTION Synthesis of PNIPAM-co-AAc Nanogels. Briefly, 1 g of NIPAM, 0.07 g of MBA, 0.08 g of AAc, and 0.03 g of sodium dodecylsulfate 15532

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were dissolved in 70 mL of deionized water to form a homogeneous aqueous solution. The mixed solution was bubbled with nitrogen for 1 h to remove oxygen. Subsequently, the solution was heated in an oil bath to ∼70 °C. The aqueous solution of potassium persulfate (0.03 g in 10 mL of water) was dropwise added to initiate the polymerization. The reaction was carried out for another 6 h under stirring. After cooling to room temperature, the resultant nanogel suspension was further stirred overnight and subsequently evaporated in an oven at 70 °C to obtain a concentrated suspension.21 Microfluidic Fabrication of Core/Shell PC Microspheres. A specially designed microfluidic device, as described in our previous report,21 was used in this work. The colorful concentrated suspensions containing crystalline nanogel arrays were used as the inner fluid. The photocurable monomer ETPTA with an UV-sensitive initiator was used as the middle fluid (oil phase). The outer phase was an aqueous solution containing poly(vinyl alcohol) (PVA) which acts as surfactant to stabilize the interface between the middle oil and the outer aqueous phase. To increase the viscosity of the aqueous solution, a desirable amount of glycerol was added to the PVA aqueous solution. Three types of fluids were pumped into the microfluidic device, resulting in the formation of the water (suspension of crystalline nanogel arrays)/ oil/water (aqueous solution of PVA) double emulsion droplets. The flow rates of the three phases were adjusted to an appropriate value to form stable W/O/W double emulsions under a dripping mode. The W/O/W double emulsion droplets were solidified by photopolymerization under the irradiation of a 400 W UV lamp (PortaRay 400, Uvitron International Inc.) for ∼40 s and collected in a beaker. The distance between the samples and the UV lamp should be accurately controlled to avoid the formation of micropores on the shell surfaces, as discussed in our previous report.31 Centrifugation of the Core/Shell PC Microspheres. The PC microspheres suspensions were centrifuged in a centrifugal machine (KDC-140HR, Anhui Zhongke Zhongjia Science Apparatus Company Ltd.) with varied speed and time at 20 and 40 °C. After the centrifugation, the samples were then transferred onto a glass slide to observe the morphology of microspheres by using an inverted optical microscope (IX71, Olympus) in bright field or phase-contrast modes. The images were captured by a high-speed CCD connected to the microscope. The optical properties were measured by using a fiberoptic spectrometer (USB4000, Ocean Optics Inc.) equipped with an optical microscope (MA2001, Chongqing Optical Instrument Inc.). Estimation of Interparticle Spacing of the Nanogels in the Janus PC Microspheres. Photonic crystals, fabricated by drying a colloidal suspension, usually have a (111)-plane-orientated facecentered cubic (fcc) structure. Position of the reflection peak is expressed by a modified Bragg’s law, combined with Snell’s law:32,33

mλmax = 2d111 neff 2 − sin 2 θ

d=

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* Supporting Information

Additional figures, including TEM/optical microscopy images and reflection spectra of the PC microspheres. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail [email protected] (Y.Y.). *E-mail [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Basic Research Program of China (2012CB932500 and 2012CB821500) and the National Natural Science Foundation of China (51073062, 51103050, and 91127046). We also thank HUST analytical and Testing Center for the TEM measurements.

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REFERENCES

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(3)

Generally, the value of m is 1, and the value of θ is 90°; thus, sin θ equals 1. The reflective index (RI) is 1.34 for PNIPAM when the temperature (T) of the experiment is lower than the LCST while RI is 1.37 when T is greater than LCST.34 In our case, all of the spectrum measurement experiments were conducted under LCST (∼20 °C); thus, the RI of PNIPAAm is 1.34. Moreover, the RI value of water is 1.333 at room temperature.35 On the other hand, neff can be obtained from the eq 4: 2

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mλmax

neff = nNIPAM × φNIPAM + nH2O × φH O

ASSOCIATED CONTENT

S

(2)

2 neff 2 − sin 2 θ

(5)

Thus, the corresponded interparticle spacing (d) of the nanogels in the specific position in the Janus PC microsphere can thus be obtained from eq 5 by introducing the λmax recorded from the spectrum of specific site on the PC microspheres.

where λ is the reflection peak wavelength, θ is the incident angle of the light, d111 is the interplanar spacing of the (111) plane, and neff is the effective reflective index of the photonic crystals. From eq 2, we can obtain interplanar (interparticle) spacing from d=

λmax 1.761

(4)

In our study, the concentration of PNIPAM nanogels is 2.5 wt % and the volume fraction of the nanogels was ∼52%; thus, neff can be calculated to be 1.337. Therefore, eq 2 can be simplified into 15533

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