Surfactant-Driven Assembly of Poly(ethylenimine)-Coated

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Surfactant-Driven Assembly of Poly(ethyleneimine)Coated Microparticles at the Liquid Crystal/Water Interface Lian Hao Ong, and Kun-Lin Yang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b10265 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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The Journal of Physical Chemistry

Surfactant-Driven Assembly of Poly(ethyleneimine)-Coated Microparticles at the Liquid Crystal/Water Interface Lian Hao Ong, Kun-Lin Yang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 Email addresses: [email protected] (corresponding author: K.L. Yang*), [email protected] (L.H. Ong) Additional contact information of corresponding author: Phone: +65 6516-6614. Fax: +65 6779-1936. Postal address: Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Block E5, 03-07, Singapore 117585

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Abstract Microparticles sitting at interfaces formed by liquid crystal (LC) and water are known to selfassemble into distinct patterns. In this study, we observe that poly(ethyleneimine) (PEI)coated microparticles are able to self-assemble at LC/water interfaces decorated with surfactants such as Tween 20 or sodium dodecylsulfate (SDS). Interestingly, assemblies of microparticles strongly depend on types of surfactants used, and how surfactants adsorb on the PEI-coated microparticles. For example, adsorption of Tween 20 on the PEI-coated microparticles causes the microparticles to form short chains that follow the director field of LC. In contrast, adsorption of SDS causes microparticles to assemble into circular rings which encompass domains saturated with SDS. Such surfactant-driven assembly of microparticles offers a possible method to direct assembly of microparticles. It can also be applied for visual detection of lipases which hydrolyze Tween 20 in water. Keywords: microparticles assembly; liquid crystal; liquid crystal/water interface


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Introduction Self-assembly of microparticles into uniform 2-dimensional or 3-dimensional structures has been exploited intensively in recent years because it has many applications in microphotonics,1 microelectromechanical systems,2 and tissue engineering.3 For these applications, there are strong motivations to influence and control the final assemblies. Surface morphologies, shapes and chemical compositions of microparticles are all important factors that determine the final form of assemblies. For example, Dendukuri et al. assembled amphiphilic microparticles at water or oil interfaces.4 They demonstrated self-assembly of amphiphilic microparticles by minimizing microparticles’ surface energy. This resulted in preferential location of hydrophobic and hydrophilic portions of microparticles in oil and water phase, respectively. To form more distinct microstructures, Shields et al. synthesized anisotropic microparticles and formed linear or zigzagged chains under external electric and magnetic fields.5 The metal-coatings resulted in highly polarizable microparticles, allowing external magnetic and electric fields to influence the final assembly. These examples illustrated how synthesis of microparticles with specially designed shapes and chemical composition can influence assembly at liquid interfaces. Another crucial consideration for microparticles assembly is supporting interfaces of these assemblies. Precise engineering of these supporting interfaces can provide templates to direct microparticles assemblies, especially in fluid interfaces such as liquids or liquid crystal (LC). Due to the size of microparticles, dominance of surface tension causes microparticles to be trapped at fluid interfaces.6 Consequently, deformation of fluid interfaces by microparticles and curvatures of fluid interfaces can direct microparticles assembly.7-8 For example, Cavallaro et al. assembled rod-like microparticles at regions of high interfacial curvatures created at water interfaces by microposts.9 They demonstrated how these microstructures and anisotropic microparticles drove microparticles assembly by long-range capillary forces. To control microparticles 3 ACS Paragon Plus Environment


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assemblies more actively, Chen et al. created unique microstructures of large surface area (>mm2) at liquid interfaces using standing waves as templates.10 Interestingly, the global microstructure patterns formed were dependent on drift energy gradients caused by the standing waves rather than capillary forces. These water interfaces provide soft templates with great flexibility for creation of distinct microstructures. LC has also been utilized as soft templates for microparticles assemblies. Anisotropic nature of LC allows creation of unique microparticles assembly without intricate control of interfacial curvatures. Instead, microparticles self-assemble depending on orientation of LC molecules (or LC director field) in the LC phase. A variety of microparticles assemblies controlled by LC director field had been reported by various research groups.11-15

To understand the effects of LC on microparticles, early works were carried out by Poulin et al. and Muševič et al. using water droplets or solid microparticles dispersed in LC.16-17 Presence of microparticles in bulk LC caused formation of topological defects in the LC phase near surfaces of microparticles. Attraction among topological defects and interactions with LC director field eventually led to different types of microparticles assemblies as shown in the literature.16-22 Based on these understandings, studies involving microparticles assembly at LC/water or LC/air interfaces were carried out to overcome confinement of microparticles in the bulk LC phase. Initial work by Koenig et al. demonstrated assembly of N, N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP, Figure 1a) coated silica microparticles at the LC/water interface.13 They reported self-assembly of microparticles as elongated chains or hexagonal lattices, depending on the presence of surfactant in the water phase. Later on, Ghabi et al. studied the assembly of DMOAP -coated silica microparticles that imposed strong homeotropic (perpendicular) boundary conditions at the LC/air interface.12 They concluded that relative positions of defects formed near the


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microparticles led to distinct patterns such as elongated chains or hexagonal lattices. More recently, uncoated polystyrene microparticles that imposed planar (tangential) boundary conditions were employed by Mondiot et al. and Wang et al.,23-25 where they studied the ordering of microparticles at defect points of LC-in-water emulsion droplets. From these studies, there are strong interests to control microparticles assembly through interactions between microparticles and LC at LC/water interfaces. In previous works, however, researchers only focused on uncoated surfaces or surfaces coated with a single layer of DMOAP. To further extend the possibility of forming assembly of microparticles at the LC/water interfaces, two coating layers on the surfaces of microparticles were employed. The first layer was PEI polymer (Figure 1b) adsorbed on the surfaces of microparticles, whereas the second layer was surfactants which reversibly bound to PEI through different interactions (electrostatic attraction or hydrogen bonding). Because of this unique two-layer structure, it is possible to obtain different assembly patterns of microparticles. This approach also allowed us to understand how binding strength of surfactants affect the assembly patterns.

Figure 1. Chemical structures of (a) N, N-dimethyl-n-octadecyl-3aminopropyltrimethoxysilylchloride (DMOAP), (b) poly(ethyleneimine) (PEI), (c) sodium dodecylsulfate (SDS), (d) Tween 20 and (e) cetyltrimethylammonium bromide (CTAB). 5 ACS Paragon Plus Environment


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Experimental Section Materials. Glass slides were purchased from Fisher Scientific (U.S.A.). 5% (w/v) Silica microparticles (4.40±0.24 µm) were purchased from Microparticles GmbH (Germany). N, Ndimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP), poly(ethyleneimine) (PEI) (Mw ~ 750,000, 50% w/v in water), Tween 20, cetyltrimethylammonium bromide (CTAB), lipase from candida rugose (type VII, activity >700 U/mg protein), and fluorescein 5-isothiocyante (FITC) were purchased from Sigma-Aldrich (Singapore). Phosphate buffered saline (PBS) and sodium dodecylsulfate (SDS) were purchased from 1st Base (Singapore). LC 4-cyano-4’-n-pentyl-biphenyl (5CB) was purchased from Merck (Singapore). 100 mesh transmission electron microscope (TEM) copper grids were purchased from Ted Pella (U.S.A.).

Surface Modifications. Glass slides were soaked in a 5% (v/v) Decon 90 solution overnight to clean the surfaces. After this, the glass slides were sonicated thrice for 15 min in deionized (DI) water. Subsequently, the glass slides were immersed in water solution containing 0.1% (v/v) of DMOAP for 5 min, and then rinsed with copious amount of DI water. Finally, DMOAP-coated glass slides were dried under a stream of nitrogen, followed by heating in a vacuum oven at 100°C for 15 min.

Coating of Microparticles with PEI. Microparticles (0.5% w/v) were immersed into an ethanol solution containing 1% (w/v) of PEI for 3 h. After this, the solution was centrifuged briefly to collect the PEI-coated microparticles. Subsequently, DI water was added to wash the PEI-coated microparticles 3 times to remove remaining PEI solution. The PEI-coated microparticles were dispersed in DI water before use.


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Labeling of PEI-coated Microparticles with FTIC. PEI-coated microparticles were immersed in 1x PBS buffer containing 1 µg/ml of FITC. After 1 h, the FITC-labeled microparticles were briefly centrifuged, and the FITC solution was removed. DI water was then added to wash the microparticles twice. After this, the microparticles were observed under a fluorescence microscope. Quantification of fluorescence signal was done by using software (Image J), and readings from 20 microparticles were used to obtain an average value for each sample. A calibration curve of the fluorescence intensity was obtained by measuring known concentrations of FITC solutions in a capillary tube (height 0.2 × width 2.0 mm).

Zeta Potential Measurements of Microparticles. Firstly, 0.5% (w/v) PEI-coated microparticles were diluted in 10mM sodium chloride solutions (pH 7.0) to a final concentration of 0.05% (w/v). Sodium chloride was used to maintain ionic strength of the solution.26 Surfactants were added to the aqueous phase, and zeta potential of the microparticles were measured by using Malvern Zetasizer Nano ZS. For each data point, the average and standard deviation of 60 readings were used.

Microparticles Assembly at LC/water Interfaces. Firstly, copper grids (100 mesh) were supported on DMOAP-coated glass slides. The grids were filled with LC (5CB,) and excess LC was removed using capillary tubes. The LC-filled copper grids were immersed into water containing 90 µM of Tween 20 for 30 min to allow adsorption of Tween 20 at the LC/water interface. Next, PEI-coated microparticles were added to the water phase to a final concentration of 0.1% (w/v), allowing them to sediment to the LC/water interface (Scheme 1a). Optical appearance of microparticles assembly were observed using an optical microscope (Nikon ECLIPSE LV100POL, Japan), and images were taken 10 min after the



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introduction of microparticles. Inter-microparticle distances were measured based on centerto-center distance of 2 microparticles, and the average and standard deviation of 40 readings were obtained using Image J software.

Visual Detection of Lipase Activity by Using Microparticles Assembly. To detect lipase activity through microparticles assembly, LC-filled copper grids were immersed into water containing 90 µM of Tween 20 solutions and lipases for 35 min. Next, PEI-coated microparticles were added to the water phase to a final concentration of 0.1% (w/v), allowing them to settle on the LC/water interface. Optical appearance of microparticles assembly was observed using an optical microscope (Nikon ECLIPSE LV100POL, Japan), and images were taken 10 min after the microparticles were added.

Scheme 1. (a) Experimental setup for microparticles assembly at LC/water interface. (b) Selfassembly of PEI-coated microparticles in the presence of Tween 20, which imposed a weak homeotropic boundary condition on the LC. (c) Self-assembly of PEI-coated microparticles in the presence of SDS, which imposed a strong homeotropic boundary condition on the LC. A hedgehog defect (represented by a black dot) was formed near each microparticle.


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Results and Discussion Surface Coating of Microparticles. To confirm that microparticles were coated with PEI by using fluorescence, microparticles coated with different concentrations of PEI were immersed into 1 µg/ml of FITC solution. After rinsing to remove unreacted FITC, fluorescence images of the microparticles were taken, and fluorescence intensities were analyzed by using Image J. Figure 2 shows that the fluorescence intensity increases with increasing PEI coating concentration until 1% (w/v). This result suggests that surfaces of the microparticles were coated with PEI successfully, and density of surface amine groups increases with increasing PEI coating concentration. When the PEI coating concentration was higher than 1% (w/v), the fluorescence intensity reached a plateau, suggesting that surfaces of microparticles were saturated with PEI already. Therefore, 1% (w/v) of PEI solution was used for coating microparticles in all subsequent experiments. Under this coating condition, we estimated that surface density of reactive amine group is approximately ~0.03/nm2 (see Supporting Information). Stability of the coated PEI layer was tested with repetitive rinsing. After 2 more washing cycles, only ~10% decrease in fluorescence intensity was observed. This result implies that the PEI layer is stable.



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Figure 2. Effect of PEI coating concentration on the surface density of reactive amine group. Insert: Calibration plot of fluorescence intensity obtained by using standard FITC solutions.

Adsorption of Surfactants on PEI-Coated Microparticles. To study adsorption of surfactants on PEI-coated microparticles, we measured zeta potentials of PEI-coated microparticles in the presence of different surfactants at different concentrations. In the absence of surfactant, the zeta potential of the microparticles was 62.0±1.2 mV (Figure 3). Since PEI is positively charged at neutral pH, the positive zeta potential agrees well with the model. In the presence of SDS, the zeta potential decreased quickly with increasing SDS concentration. This result suggests that negatively charged adsorbed onto the PEI-coated microparticles because of electrostatic attraction. For Tween 20, the zeta potential also decreased with increasing Tween 20 concentration. Since PEI contains nitrogen atoms and Tween 20 contains oxygen atoms, the decrease in zeta potential was probably caused by adsorption of Tween 20 onto the PEI-coated microparticles through hydrogen bonding. This result is consistent with earlier work showing that non-ionic surfactants adsorbed on hydrophilic surfaces.27-29 For instance, Zhang et al. proposed adsorption of non-ionic surfactants on hydrophilic surfaces through hydrogen bonding. They also observed slight decrease in zeta potential after the addition of non-ionic surfactants.30 In contrast, the zeta potential did not decrease in the presence of CTAB. This is reasonable because both PEI and 10 ACS Paragon Plus Environment

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CTAB are positively charged and repelled each other. These results suggest that only SDS and Tween 20 can adsorb on the surfaces of PEI-coated microparticles.

Figure 3. Influence on zeta potential due to the adsorption of surfactants on PEI-coated microparticles. Surfactants studied were (a) cationic CTAB, (b) non-ionic Tween 20 and (c) anionic SDS.

Self-Assembly of PEI-Coated Microparticles at LC/water Interfaces. To initiate selfassembly of PEI-coated microparticles, we immersed LC-filled grid into water. The LC/water interface was decorated with 90 µM of Tween 20. Next, we added PEI-coated particles to the solution to a final concentration of 0.1 % (w/v). Within 5 min, microparticles settled down to the LC/water interface and started to assemble into short chains (each chain contained up to 20 microparticles) as shown in Figure 4a. Inter-microparticles distance was 4.61±0.21µm, which is very close to the diameter of microparticles used (4.40±0.24µm). This showed that there is no spacing between any two microparticles in the short chains. In contrast, when the LC/water interface was not decorated with Tween 20 (Figure 4b), or when the microparticles were not coated with PEI (Figure 4c), the microparticles did not assemble into short chains. From Figure 4b and 4c, the microparticles appeared to organize into 2-dimensional hexagonal structures. The inter-microparticle distances were 4.74±0.14µm and 4.71±0.28µm in Figure



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4b and 4c, respectively (there was little or no space between any two microparticles). This resulted in close packing of microparticles at center of the grid hole (due to a concave meniscus). Moreover, if the LC was heated above its clearing temperature (35oC for 5CB), the microparticles also did not assemble into short chains (Figure 4d). Instead, they either moved to the center of the grid (due to a concave meniscus) or to the side (Figure 4e). These controlled experiments led us to conclude that all three elements (PEI coating, Tween 20 and an LC phase) are essential to the assembly of microparticles into short chains.

Figure 4. Bright field images for assembly of PEI-coated microparticles at the LC/water interfaces decorated (a) with 90 µM of Tween 20 or (b) without Tween 20. PEI-coated microparticles assembled into short chains only in the presence of Tween 20. Control experiments in the presence of 90 µM Tween 20 were also conducted by using (c) uncoated microparticles or (d) PEI-coated microparticles at 40°C. 5CB became isotropic liquid at this temperature. (e) The grid holes were completely with LC to remove the concave meniscus and PEI-coated microparticles were introduced without Tween 20. The scale bar represents 100µm.


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Figure 4a also revealed that the directions of short chains all pointed to a single point in the LC. This behavior is similar to LC director field when a point defect is present in the LC. To obtain the LC field, the LC sample was rotated under cross polars (Figure 5a) to determine the direction of LC director field. Figure 5b shows that the LC director field also converges to a single point defect in the center of the square. By comparing Figure 5a and 5b, we can see that the directions of the short chains followed the LC director field. This result is consistent with past studies showing that microparticles with homeotropic boundary conditions assembled into short chains that followed LC director field.12-13, 22

Figure 5. Assembly of PEI-coated microparticles that followed the LC director at the LC/water interface. (a) Cross-polarized image of the PEI-coated microparticles at the LC/water interface decorated with 90 µM of Tween 20. (b) Observed LC director field. Scale bar represents 100µm. The arrows indicate the orientation of polarizer and analyzer.

Effect of Tween 20 Concentration on Microparticles Assembly. To understand the effect of Tween 20 concentration on self-assembly of PEI-coated microparticles, we varied the concentration of Tween 20. Figure 6 shows that 90 µM of Tween 20 was the minimal concentration required to trigger self-assembly of PEI-coated microparticles into short chains. Interestingly, when the Tween 20 concentration was 45 µM, the texture of LC was not disrupted by the microparticles (Figure 6a). The LC texture was very continuous even in the region with microparticles. Additionally, adjustment of focal planes showed that microparticles and the LC/water interface were not at the same focal plane. This result implies that microparticles did not penetrate the LC interface when the Tween 20 13 ACS Paragon Plus Environment


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concentration was too low. In contrast, Figure 6b (and the magnified view in Figure 6g) shows that when the Tween 20 concentration was increased to 90 µM, microparticles started to form short chains and they were surrounded by white halo in the LC phase. The white halo is a strong evidence showing that these microparticles were able to submerge into the LC and distort its orientation. When Tween 20 concentration was between 900 µM and 4.5 mM, short chains of microparticles aggregated together and formed bands of 3 to 5 chains (Figure 6c and 6d). This can be explained by the increasing surface density of Tween 20 adsorbed on the PEI-coated microparticles, leading to aggregation of PEI-coated microparticles to form thicker bands. Again, white halo surrounding the microparticles are clearly visible in these cases.

However, when the Tween 20 concentration was 18 mM, the LC/water interface was fully covered by Tween 20. This was evident from the uniformly dark image of LC under cross polars (Figure 6e). 31-32 Under this condition, all PEI-coated microparticles assembled at the center of the grid hole (due to a concave meniscus) as shown in Figure 6f.33-34 This implies that when the LC/water interface was fully covered with Tween 20, PEI-coated microparticles did not submerge into LC. This is supported by the uniform texture of LC under cross-polars (Figure 6e) and bright field (Figure 6f). Both figures strongly suggest that when the LC interface was saturated with Tween 20, it prevented the microparticles breaking the LC interface.


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Figure 6. Effect of Tween 20 concentration on the assembly of PEI-coated microparticles at the LC/water interfaces decorated with Tween 20. Concentrations of Tween 20 are (a) 45 µM, (b) 90 µM, (c) 900 µM, (d) 4.5 mM and (e) 18 mM, respectively. (a) to (e) are crosspolarized images and (f) is a bright-field image of (e). (g) is a magnified area of (b) to illustrate white halo surrounding microparticles in a short chain. Scale bar in (a) and (g) represents 100 µm and 10 µm, respectively.

Previous reports had suggested that non-ionic surfactants (like Tween 20) are expected to have a weak anchoring energy (~10-6 J/m2) compared to ionic surfactants (~10-2 J/m2).18, 35-36 Therefore, we propose that the short-chain assembly was due to a weak homeotropic boundary condition imposed by microparticles covered with Tween 20. When such a microparticle was placed at the LC/water interface, it was able to penetrate into the LC phase and caused distortion of the director as shown in Scheme 1b. However, the anchoring energy 15 ACS Paragon Plus Environment


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was not strong enough to cause a hedgehog defect as in the case of strong homeotropic anchoring. Therefore, we hypothesize that LC director around the microparticles resemble surface rings suggested by Mondian-Monval et al. and Stark et al., previously.35, 37 For surface rings, LC director field is minimally distorted by the microparticles, as compared to the case of hedgehog defects. Although Saturn ring defects are also probable, we did not observe these defects in our experiments. When several microparticles were present with surface rings, they aggregated together to form short chains as shown in Scheme 1b.

Visual Detection of Lipase Activity in Water. Since assembly of microparticles is dictated by surfactants adsorbed on PEI-coated microparticles, this phenomenon can be applied to detect any lipases which hydrolyze Tween 20.38-40 We anticipated that after the hydrolysis of Tween 20, microparticles can no longer assemble into short chains as shown in Figure 6b. To test this hypothesis, we added lipase to water with a concentration between 500ng/ml and 10ng/ml. From Figure 7a to 7d, when the lipase concentration was 100ng/ml or higher, microparticles did not form short chains. Instead, they assembled at the center of the grid hole, similar to the case of Figure 6a (when Tween 20 was lower than the minimal concentration). However, when the lipase concentration was 10ng/ml, microparticles still assembled into short chains. This result shows that 100ng/ml of lipase is sufficient to hydrolyze Tween 20 and make its concentration fall below its minimal concentration for microparticles to form short chains.


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Figure 7. Visual detection of lipase activity using microparticles at the LC/water interface decorated with 90 µM of Tween 20. Lipase concentrations are (a) 500 ng/ml, (b) 250ng/ml, (c) 100ng/ml and (d) 10ng/ml, respectively. (a) to (d) are bright-field images, and scale bar in (d) represents 50 µm.

Effects of Cationic and Anionic Surfactants. To further study whether PEI-coated microparticles can self-assemble at LC/water interfaces decorated with cationic or anionic surfactants, we deposited PEI-coated microparticles at LC/water interfaces decorated with either SDS (anionic surfactant) or CTAB (cationic surfactant, Figure 1e). The concentrations of SDS and CTAB were 90 µM and 9 µM, respectively, which correspond to approximately 1% of their critical micelle concentrations. From Figure 8a and 8b, PEI-coated microparticles assembled into circular rings at the LC/water interface decorated with SDS. Interestingly, this microparticles assembly pattern is distinctively different from those observed in the presence of Tween 20 (Figure 6b). On the other hand, when CTAB was used, microparticles assembled at the center of the grid hole, independent of LC director field (Figure 8c and 8d). This was probably because positively charged CTAB cannot adsorb onto surfaces of PEIcoated microparticles as shown in zeta potential measurements.

To understand how microparticles assemble in the presence of SDS, we observed the microparticles under a higher magnification (Figure 8e) and found that LC surrounding the microparticles appeared dark under crossed polars. This phenomenon implies that



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microparticles imposed a strong homeotropic boundary condition on the LC. Since the PEImicroparticles are positively charged, they were able to attract negatively charged SDS to their surfaces as demonstrated in the zeta potential measurement (In the presence of 100 µM of SDS, zeta potential decreased from 62.0 ± 1.2 mV to -54.4 ± 1.8mV). Moreover, hedgehog defects near the microparticles can be seen in Figure 8e. In the literature, it has been reported that microparticles with a strong homeotropic boundary condition can form hedgehog defects near the surface of microparticles.17, 22 Additionally, the inter-microparticle distance was 5.9 ±0.7µm, which agrees well with previous studies showing an inter-microparticle distance of 1.3d (where d is diameter of microparticle) for microparticles that impose strong homeotropic boundary condition.13, 16 Another notable difference is the significantly larger intermicroparticle distance compared to the case of short chains formed in the presence of Tween 20. Based on these observations, we propose a model showing how PEI-coated microparticles assembled in the presence of SDS. As shown in Scheme 1c, after the formation of a hedgehog defect near a microparticle, the microparticle behaves like dipoles in the LC field and attracts other microparticles. Eventually, these microparticles chain up together and form a linear pattern as shown in Scheme 1c.


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Figure 8. Assembly of PEI-coated microparticles at the LC/water interfaces decorated with (a-b) 90 µM of SDS or (c-d) 9 µM of CTAB. Each pair consists of a bright-field and a crosspolarized image for comparison. (e) Magnified cross-polarized image of PEI-coated microparticles in (b). Hedgehog defects were indicated by arrows. Scale bar in (a) and (e) represents 100 µm and 5 µm, respectively.

Another interesting phenomenon shown in Figure 8b was the dark domains surrounded by microparticles. We attributed the dark domains to a saturated monolayer of SDS adsorbed at the LC/water interface.31-32 This phenomenon strongly implies that PEI-coated microparticles at the LC/water interface were able to “squeeze” SDS into a saturated domain. Since SDS is a surfactant, it can lower surface tension and compact SDS into a saturated domain through surface tension gradient. Formation of surfactant-rich domain at the LC/water interface was also observed by other research groups. For example, Brake et al. and Gupta et al. discovered phase separation of phospholipids and ionic surfactants at LC/water interfaces.41-43 In particular, Gupta and Abbott demonstrated phase separation of ionic surfactants in a high salt concentration (1M NaBr).43 They proposed that the high salt concentration is needed to screen electrostatic repulsion of charged surfactants, allowing them to aggregate and form



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separate domains. In our system, however, the phase separation was initiated by the introduction of PEI-coated microparticles. Furthermore, we note that a long chains of microparticles circled the perimeters of the saturated SDS domains (Figure 8b). A possible explanation is that LC oriented homeotropically in the saturated SDS domain whereas LC oriented planarly outside these domains. Therefore, orientation of LC experiences homeotropic-to-planar deformation (Figure 8b) which comes with high energy penalty. Previous works had suggested that microparticles can be attracted to regions with greatest LC deformation to minimize the energy.37, 44 Therefore, this is why all microparticles moved to the perimeters of saturated SDS domains.

Effect of SDS Concentration on Microparticles Assembly. To understand the effect of SDS concentration on self-assembly of PEI-coated microparticles, we varied the concentration of SDS. At a lower SDS concentration (0.9 µM), no distinct microparticles assembly was observed (Figure 9a). This was likely due to limited coverage of SDS on the PEI-coated microparticles. When the SDS concentration was 9 µM, PEI-coated microparticles assembled into short chains. However, no dark domains were formed (Figure 9b). As shown in our model (Scheme 1c), the short chains were caused by hedgehog defects attracting each other (Figure 9b). At 90 µM - 270µM SDS, PEI-coated microparticles elongated and formed dark domains as seen in Figure 9c and 9d. These dark domains appeared to increase as SDS concentration was increased. The increase in SDS concentration can cause more SDS to adsorb on PEI-coated microparticles and higher SDS coverage at the LC/water interface. Both factors are responsible for the expanding dark domains. At 450 µM of SDS, PEI-coated microparticles assembled at the center of the grid hole and independent of LC director field (Figure 9f). From Figure 9e, dark cross-polarized image at 450 µM SDS indicated that the LC/water interface was saturated with SDS. The saturated coverage of SDS 20 ACS Paragon Plus Environment

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at LC/water interface shielded the LC phase and prevented interactions between the microparticles and the LC phase. Such observation is similar to the case of microparticles assembly at LC/water interface decorated with 18 mM of Tween 20 (Figure 6f).

Figure 9. Effect of SDS concentration on assembly of PEI-coated microparticles at the LC/water interfaces decorated with SDS. Concentrations of SDS are (a) 0.9 µM, (b) 9 µM, (c) 90 µM, (d) 270 µM and (e) 450 µM. (a) to (e) are cross-polarized images and (f) is a bright-field image of (e). Scale bar represents 100 µm.

Conclusion In this study, we show that poly(ethyleneimine) (PEI)-coated microparticles were able to assemble into different patterns at the liquid crystal (LC)/water interface. However, this phenomenon was only observed when the interface was decorated with surfactants Tween 20 or sodium dodecylsulfate (SDS), and the assembly pattern was very sensitive to the types of surfactants used. Both surfactants are able to adsorb on the surfaces of PEI-coated 21 ACS Paragon Plus Environment


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microparticles and interact with the director field of LC to give different assembly patterns. Previous studies on microparticles assembly at the LC/water interface had adopted the approach of controlling the LC director field to achieve different assembly patterns.12-13 However, such approach relies on N, N-dimethyl-n-octadecyl-3aminopropyltrimethoxysilylchloride (DMOAP)-coated microparticles that impose strong homeotropic boundary condition on LC. This limitation has been addressed by this study, where we make use of PEI-coated microparticles that interact with surfactants present in the water phase to achieve different assembly patterns. Significantly, this study also suggests the potential of utilizing microparticles assembly at the LC/water interface for sensing applications. This is exemplified through the detection of lipase activity by examining the assembly patterns, with a detection limit of 100ng/ml of lipase.


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AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. Phone: +65 6516-6614. Fax: +65 67791936. E-mail: [email protected] Funding Sources This work was supported by the National Research Foundation (NRF) in Singapore under grant (NRF 2009 NRF-CRP 001-039). ACKNOWLEDGMENT We would like to thank A/P Saif Khan, Dr. Zhu Qingdi and our group members for the fruitful discussions.

Supporting Information Available Real-time videos recorded using cross-polarized light source for microparticles assembly at the LC/water interfaces decorated with Tween 20 and SDS, respectively. Details and video captions are found in SI file.



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Scheme 1. (a) Experimental setup for microparticles assembly at LC/water interface. (b) Self-assembly of PEI-coated microparticles in the presence of Tween 20, which imposed a weak homeotropic boundary condition on the LC. (c) Self-assembly of PEI-coated microparticles in the presence of SDS, which imposed a strong homeotropic boundary condition on the LC. A hedgehog defect (represented by a black dot) was formed near each microparticle. 80x78mm (300 x 300 DPI)

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Figure 1. Chemical structures of (a) N, N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP), (b) poly(ethyleneimine) (PEI), (c) sodium dodecylsulfate (SDS), (d) Tween 20 and (e) cetyltrimethylammonium bromide (CTAB). 82x85mm (300 x 300 DPI)



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Figure 2. Effect of PEI coating concentration on the surface density of reactive amine group. Insert: Calibration plot of fluorescence intensity obtained by using standard FITC solutions. 66x54mm (600 x 600 DPI)


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Figure 3. Influence on zeta potential due to the adsorption of surfactants on PEI-coated microparticles. Surfactants studied were (a) cationic CTAB, (b) non-ionic Tween 20 and (c) anionic SDS. 82x62mm (300 x 300 DPI)



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Figure 4. Bright field images for assembly of PEI-coated microparticles at the LC/water interfaces decorated (a) with 90 µM of Tween 20 or (b) without Tween 20. PEI-coated microparticles assembled into short chains only in the presence of Tween 20. Control experiments in the presence of 90 µM Tween 20 were also conducted by using (c) uncoated microparticles or (d) PEI-coated microparticles at 40°C. 5CB became isotropic liquid at this temperature. (e) The grid holes were completely with LC to remove the concave meniscus and PEI-coated microparticles were introduced without Tween 20. The scale bar represents 100µm. 82x126mm (300 x 300 DPI)


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Figure 5. Assembly of PEI-coated microparticles that followed the LC director at the LC/water interface. (a) Cross-polarized image of the PEI-coated microparticles at the LC/water interface decorated with 90 µM of Tween 20. (b) Observed LC director field. Scale bar represents 100µm. The arrows indicate the orientation of polarizer and analyzer. 82x36mm (300 x 300 DPI)



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Figure 6. Effect of Tween 20 concentration on the assembly of PEI-coated microparticles at the LC/water interfaces decorated with Tween 20. Concentrations of Tween 20 are (a) 45 µM, (b) 90 µM, (c) 900 µM, (d) 4.5 mM and (e) 18 mM, respectively. (a) to (e) are cross-polarized images and (f) is a bright-field image of (e). (g) is a magnified area of (b) to illustrate white halo surrounding microparticles in a short chain. Scale bar in (a) and (g) represents 100 µm and 10 µm, respectively. 82x164mm (300 x 300 DPI)


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Figure 7. Visual detection of lipase activity using microparticles at the LC/water interface decorated with 90 µM of Tween 20. Lipase concentrations are (a) 500 ng/ml, (b) 250ng/ml, (c) 100ng/ml and (d) 10ng/ml, respectively. (a) to (d) are bright-field images, and scale bar in (d) represents 50 µm. 43x44mm (300 x 300 DPI)



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Figure 8. Assembly of PEI-coated microparticles at the LC/water interfaces decorated with (a-b) 90 µM of SDS or (c-d) 9 µM of CTAB. Each pair consists of a bright-field and a cross-polarized image for comparison. (e) Magnified cross-polarized image of PEI-coated microparticles in (b). Hedgehog defects were indicated by arrows. Scale bar in (a) and (e) represents 100 µm and 5 µm, respectively. 80x110mm (300 x 300 DPI)


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Figure 9. Effect of SDS concentration on assembly of PEI-coated microparticles at the LC/water interfaces decorated with SDS. Concentrations of SDS are (a) 0.9 µM, (b) 9 µM, (c) 90 M, (d) 270 µM and (e) 450 µM. (a) to (e) are cross-polarized images and (f) is a bright-field image of (e). Scale bar represents 100 µm. 82x124mm (300 x 300 DPI)



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Table of Contents Image 80x33mm (300 x 300 DPI)


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Surfactant-Driven Assembly of Poly(ethylenimine)-Coated

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