Ellipsoidal Colloids with a Controlled Surface Roughness via

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Ellipsoidal colloids with a controlled surface roughness via bio-inspired surface engineering: building blocks for liquid marbles and superhydrophobic surfaces Pengjiao Zhang, Lu Yang, Qiang Li, Songhai Wu, Shaoyi Jia, Zhanyong Li, Zhenkun Zhang, and Linqi Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16733 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Applied Materials & Interfaces

Ellipsoidal colloids with a controlled surface roughness via bio-inspired surface engineering: building blocks for liquid marbles and superhydrophobic surfaces Pengjiao Zhanga,+, Lu Yangb,+, Qiang Lia, Songhai Wub ,Shaoyi Jiab, Zhanyong Lia,*, Zhenkun Zhanga, *, Linqi Shia a) Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China. b) School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail: [email protected]; [email protected] + These authors contributed equally to this work. KEYWORDS: surface roughness • ellipsoid • polydopamine • superhydrophobicity • liquid marble ABSTRACT: Understanding the important role of the surface roughness of nano-/colloidal particles and harnessing them for practical applications need novel strategies to control the particles’ surface topology. Although there are many examples of spherical particles with a specific surface roughness, non-spherical ones with similar surface features are rare. The current work report a one-step, straightforward and bio-inspired surface engineering strategy to prepare ellipsoidal particles with a controlled surface roughness. By manipulating the unique chemistry inherent to the

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oxidation induced self-polymerization of dopamine into polydopamine (PDA), PDA coating of polymeric ellipsoids leads to a library of hybrid ellipsoidal particles (PS@PDA) with a surface that decorates with nanoscale PDA protrusions of various densities and sizes. Together with the advantages originated from the anisotropy of ellipsoids and rich chemistry of PDA, such surface feature endows these particles with some unique properties. Evaporative drying of fluorinated PS@PDA particles produces a homogeneous coating with superhydrophobicity that arises from the two-scale hierarchal structure of microscale inter-particle packing and nanoscale roughness of the constituent ellipsoids. Instead of water repelling that occurs to most of the lotus leaf-like superhydrophobic surfaces, such coating exhibits strong water adhesion that is observed with certain species of rose pedals. In addition, the as-prepared hybrid ellipsoids are very efficient in preparing liquid marbles-isolated droplets covered with solid particles. Such liquid marbles can be placed onto many surfaces and might be useful for the controllable transport and manipulation of small volumes of liquids.

1. INTRODUCTION Surface properties of particles with a size ranging from nano- to micrometers play paramount roles in performing their functions in catalysis, water treatments, energy, nanomedicines, etc.1-3 Traditionally, much attention has been paid to surface functional groups, stabilizing ligands and accompanied chemical modifications while the particle surface is normally assumed to be perfectly smooth. However, particles with such smooth surfaces do not exist in reality. Notices to the potential impacts of


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the particle surface roughness on many fundamental colloidal phenomena can date back to decades ago and then have been largely ignored.4 Several recent works with the help of advanced characterization techniques have revealed that the surface roughness, even on the scale of few nanometers, play critical roles in the discrepancy between experimental results and theories.5-9 Especially, a recent work found that colloidal particles with extreme surface ruggedness-the ‘hedgehog’ particles can be dispersed in both polar and nonpolar solvents.10 This observation challenges the conventional DLVO theory that rationalizes colloidal stability in suspension.10 In practical applications, evidences have also been accumulating about the important roles of the surface roughness of particles in diverse fields. For instance, the surface roughness can dramatically influence the behavior of colloidal particles at liquid-liquid interfaces which have important implications for particle-stabilized emulsions and foams.9,11 Crystallization of DNA-grafted colloids into photonic materials can also be slowed down or even inhibited by the surface roughness.12 Some abnormal behaviors in the cellular uptake,13 plasmonic optical properties,14 and interactions with proteins of particles have also been tentatively attributed to the surface roughness.2 Together with these passive observations, intentionally manipulating and harnessing the surface roughness of a particle have also started in recent years. For instance, drying particles with a controlled surface roughness into particulate films is a convenient way to construct superhydrophobic surfaces.15 Imparting the surface roughness onto specific locations of particles or mesoscopic objects has been used to create directional interactions that can guide their preferential



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assembly.16,17 In the field of active colloids, the propulsion speed of the micromotors can be enhanced by the surface roughness, resulting in four-fold increase in their propulsion speed compared to conventional particles with a smooth surface.18 In nanomedicine, the particle roughness has also been recruited to enhance the cellular delivery performance of nanoscale drug delivery particles.19,20 In the above and many other works, particles with a defined surface roughness have been prepared via some specific and material-dependent physiochemical mechanisms such as phase separation, heterogeneous aggregations, etc.21-23 Among the reported strategies, the most popular way is to construct raspberry-like particles that have a structure of a large particle decorated with many small nodules. Such kinds of particles can be prepared by putting small particles onto a pre-formed large particle via either physical interactions or in situ chemical procedures.24-26 This normally involves multiple steps and stringent chemistry, leading to low yield that limits their practical applications. Furthermore, the surface roughness is often ill-defined and cannot be varied systematically, due to difficulties in controlling the number of the small nodules and their distribution around the curved surface of the large one.24,25 In most cases, the surface chemistry of the small nodules is different from that of the large particles and such chemical heterogeneity might stand behind some contradictory observations, for instance, in the case of cellular uptake behavior of rough particles.13,19,27 In addition, most of such kinds of raspberry-like particles have a spherical shape. Anisotropic particles, like ellipsoidal particles with a raspberry-like surface feature, have barely been reported, although particles with a


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non-spherical shape are expected to have some advantages in terms of packing,28 self-assembly,29,30 coating formation and fluid interfacial behaviors,31 etc.32 Fabricating methods realized with spherical raspberry-like particles cannot be directly translated to non-spherical particles due to the various surface curvatures of the latter that leads to inhomogeneous physio-chemical interactions.33,34 Therefore, a method that can form conformal coating layers on any particle surface regardless of the shape and simultaneously introduce surface roughness is needed. In the current contribution, we report a one-pot and straightforward method to fine-tune the surface roughness of ellipsoidal particles via the mussel inspired surface engineering (Scheme 1).35 During coating of polystyrene ellipsoidal particles (PSs) with a polydopamine (PDA) layer through the oxidation-induced self-polymerization of dopamine,36 it is unexpectedly found that the surface of the resulted PS@PDA hybrid particles is decorated with nanoscale protrusions (nanoprotrusions). Coating conditions have therefore been thoroughly investigated to control the size and the density of these nanoprotrusions, leading to a library of ellipsoidal particles with a well-controlled surface roughness. Such surface feature, together with the advantages inherent to the anisotropy of ellipsoids and the rich chemistry of polydopamine, is expected to endow these particle with some peculiar properties that promise many applications, as will be demonstrated by using them as building blocks for superhydrophobic surfaces and liquid marbles. The former application mainly exploits the specific evaporative drying behavior of anisotropic particles, such as anti-“coffee ring” effects.37 Microscale inter-particle packing of ellipsoids in the dried film and the



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nanoscale surface roughness of the constituent particle together are expected to create a two-scale hierarchical structure that fulfils the requirements of superhydrophobic surfaces. Liquid marbles refer to isolated liquid droplets surrounded by solid particles, which can be used for the controllable transport and manipulation of small volumes of liquids for miniaturized systems in many applications.38 Compared to spherical counterparts, anisotropic particles have rich behavior at fluid-fluid interfaces and are very efficient in stabilizing emulsions or foams.31,32 We shall show that the as-prepared hybrid ellipsoids can be used to create liquid marbles that exhibit reasonable mechanic strength and can be placed on many surfaces.

Scheme 1. Schematic illustration of polydopamine coating of polymeric ellipsoids and their applications. The drawing is not to scale for clarity. (I) Polydopamine coating; (II) Chemical modification (e.g. Fluorination); (III) Evaporative drying of ellipsoids in ethanol; (IV) Water droplets on the ellipsoidal particulate films. 2. EXPERIMENTAL SECTION 2.1. Material.

Dopamine hydrochloride (DA), fluoresceinamine isomer I,

1H,1H,2H,2H-perfluorodecanethiol (PFDT) and tris(hydroxymethyl)aminomethane


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(Tris) were purchased from Sigma-Aldrich. All other materials were obtained from J&K (Beijing, China). The preparation of the spherical polystyrene particles with a dimeter of 1.70 µm and thermo-stretching of these particles into ellipsoids are detailed in the supporting information. 2.2 Polydopamine coating of polystyrene (PS) ellipsoids. The polystyrene ellipsoids used herein have a long axis of 6.0±0.3 µm and a short axis of 0.85±0.08

µm (See SI for the preparation method). PS ellipsoids were suspended in Tris-HCl buffer (pH = 8.5, 20 mM) to a concentration of 10 mg mL-1. To 10 mL such suspensions, dopamine was directly added under strong stirring. After dopamine dissolved completely, the reaction was carried out for certain times under stirring that is enough to prevent the sedimentation of the ellipsoids. After coating, the mixture was centrifuged at 2000g to spin down the coated ellipsoids, which were then redispersed in ethanol. This procedure was repeated three times. Hereafter, the PDA coated particles will be referred as PS@PDA ellipsoids. To investigate the influence of the dopamine concentration (CDA), various amount of dopamine was added to the PS ellipsoidal suspension in Tris-HCl buffer to obtain CDA of 0.04, 0.1, 0.5, 1 and 2 mg mL-1 while the coating time was kept at 10 hours. Similarly, while CDA was fixed at 0.1 mg mL-1, aliquots of the reaction mixture were taken and purified at preset intervals to investigate the influence of the coating time. To prepare ellipsoidal PDA capsules, 500 µL of the PDA coated ellipsoids were centrifuged to remove solvents. To the sediments, 2 mL THF was added. The resulted suspension was stirred for 5 hours. The PDA capsules were collected by low-speed



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centrifuge (2000g) and washed with ethanol for thee times. 2.3 Surface chemical modifications of the PDA coated ellipsoids. The ellipsoids coated with PDA obtained under the conditions of 0.1 mg mL-1 dopamine and coating for 10 hours were used for the surface modification. For the dye labeling, PS@PDA ellipsoids were dispersed in 2 mL phosphate buffer (10 mM, pH = 8.2) to a concertation of 10 mg mL-1, to which, fluoresceinamine was added to achieve a dye concentration of 10 µg mL-1. The reaction was carried out for 24 hour in dark. After this step, the reaction mixture was dialyzed against water to remove unreacted dyes. The particles were collected by centrifugation and further washed in distilled water by centrifuge and redispersion. For fluorination, the PS@PDA ellipsoids were first suspended in ethanol and centrifuged down for several times. To the sediment, 2 mL 1H,1H,2H,2H-perfluorodecanethiol in ethanol (50 mg mL-1) was added. The resulted suspension was standby for 12h. Centrifugation at 2000g and re-suspension in ethanol was used to purify the products. As the control, the original PS ellipsoids were subjected to the same procedures of dye labeling and fluorination. 2.4 Preparation of superhydrophobic surfaces and liquid marbles. Fluorinated PS@PDA ellipsoids (20 µL, 1 mg mL-1) in ethanol were casted onto a cleaned glass slide and the solvent was evaporated at room temperature. Droplets of ellipsoidal suspensions were applied to the same stain repeatedly until a dense film was formed. To test the rolling off behavior of water, a large water droplet (15 µL) was placed on the stain and the slide was turned around in different positions. To prepare liquid marbles, powders of fluorinated PS@PDA ellipsoids were first


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prepared by drying the ellipsoids in ethanol in a plastic vial. The particles were then collected in the form of powders by mechanically vibrating off the ellipsoids from the vial wall. The powder was placed onto a glass plate to form a bed of particles, onto which a water droplet (20 µL) was placed and rolled back and forward by tilting the glass slide, until the droplet was covered with a layer of the powder. The marble was then placed onto several surfaces by a tweezer. 2.5 Characterization. A JSM-7500F scanning electron microscope (SEM) (JEOL, Japan) was used to characterize the surface roughness of the particles. Samples for SEM were prepared by spin coating a diluted suspension of particles in ethanol to clean silicon wafers to make sure that the ellipsoids lie on the surface in a side-on fashion. The ellipsoidal particles were also checked with a JEM-100CXII transmission electron microscope (TEM) (JEOL, Japan) operated at an accelerating voltage of 100 kV. Samples were prepared by placing a drop of the particle suspension in ethanol on a carbon-coated copper grid, and then evaporating at ambient temperature. Fourier transform infrared (FT-IR) spectra were collected on a Bio-rad FTS6000 spectrometer (Bio-rad, USA) by embedding the ellipsoids into KBr pellets. X-ray photoelectron spectroscopy (XPS) analysis of the particles was conducted on a GENESIS 60S X-ray photoelectron spectroscope (EDAX, USA). Thermogravimetric analysis (TGA) was performed on a TG 209 F3 (Netzsch, Germany) under N2 atmosphere and in the temperature range from 50 to 800 °C. The contact angles of the dried film of the ellipsoidal particles were measured on an OCAH200 equipment (Data Physics instruments, Germany) and analyzed by the



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snake-based algorithm.39 The fluorescent images of the fluorescently labeled ellipsoids were recorded on a Zeiss Z2 fluorescent microscopy (Zeiss, Germany). The zeta potentials of the particles were measured by the Malvern Zetasizer Nano ZS90 (Worcestershire, UK) and the samples was suspended in 1 mM phosphate buffer (pH 7.4). 3. RESULTS AND DISCUSSION 3.1 Coating of polystyrene ellipsoidal particles with polydopamine (PDA) in a one-pot procedure under mild conditions. Polystyrene (PS) ellipsoidal particles with an aspect ratio of ca. 7 and amino surface groups were prepared following a well-established thermo-stretching of PS spheres embedded in PVA films (See SI).40 Thanks to the simplicity of the surface engineering via self-polymerization of dopamine, PDA coating of the as-prepared ellipsoids is very straightforward: by simply mixing the PS ellipsoidal template with the dopamine solution in Tris buffer (20 mM, pH = 8.5) under gentle stirring for several hours, PDA coated ellipsoidal particles (PS@PDA) with specific surface characteristics can be obtained if the concentration of dopamine and the coating time are controlled properly, as discussed later (Figure 1). During coating, the reaction aqueous mixture gradually turned from colorless, to pink and into black, which is a convenient indication of the progressing coating reaction. The coating procedure was also performed at the scale of 500 mL, resulting in grams of PDA coated PS ellipsoids, which is beneficial for latter applications.


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Figure 1. Polystyrene ellipsoids (A) and typical examples of hybrid ellipsoids with specific surface features after PDA coating (B ~ E). The dopamine coating conditions in (B) ~ (E) are as following: 0.04 mg mL-1 and 10 hours (B), 0.1 mg mL-1 and 10 hours (C), 0.5 mg mL-1 and 10 hours (D), 1 mg mL-1 and 10 hours (E). Scale bar: 200 nm. Several techniques were utilized to confirm the successful PDA coating of the ellipsoids (Figure 2). The PS template with surface-anchored amino groups is slightly positively charged with a zeta potential of +5 mV. After the PDA coating and regardless of the coating conditions, the zeta potential changes to ca. -20 mV, a typical value for PDA coated surfaces (Table S1 in SI). In addition, compared to the XPS spectra of the PS ellipsoids, the intensities of the O 1s and N 1s peaks in that of the PS@PDA ellipsoid increase (Figure 2A). Furthermore, the C 1s peak of the PS@PDA ellipsoid can be deconvoluted into four subpeaks at 284.8eV (C-C/C-H), 285.8eV (C-N), 286.4eV (C-O) and 288.0 eV (C=N), respectively (Figure 2B). The peak at



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288.0 eV (C=N) should be due to the oxidized intermediate compounds, such as 5,6-indolequinone that exists in the PDA layer.41 The subtraction of the FTIR spectra of the PS template and the PS@PDA ellipsoids features a peak at 1510 cm -1 that is also assigned to the C=N vibration mode of the oxidized intermediate compounds of dopamine (Figure 2C).42 Thermogravimetric analysis (TGA) of PS@PDA ellipsoids reveals a two-stage decomposition behavior that is the combination of pure PS ellipsoids and polydopamine (Figure 2D): an initial fast decomposing process occurred in the temperature range of 386 ~ 437 ℃, during which the PS templates decompose completely, together with parts of PDA; Following this step is a slow procedure up to 600 ℃ that mainly stems from decompensation of the PDA components. From TGA, the PS@PDA ellipsoid typically consists of 19% PDA and 81% PS. These analysis results are comparable to the polydopamine coated PS spheres from previous works.43,44 In addition, the PS ellipsoid template has an average aspect ratio of 7, which decreases to ca. 6.4 after PDA coating. The conical ends of the PS@PDA ellipsoids become blunted while that of the PS template is sharp (Figure 1). Finally, the PS ellipsoidal template can be dissolved away by THF, producing PDA capsules with a shell thickness of ca. 20 nm (Figure S1 in SI). These results together confirm that a conformal PDA layer successfully forms around the PS ellipsoidal template, regardless of the surface curvature difference.


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Figure 2. Physiochemical characterization of polydopamine coated polystyrene (PS) ellipsoids. (A) and (B) are XRD analysis of the PS template (A) and PS@PDA ellipsoids (B), respectively. (C) FT-IR spectra of the PS and PS@PDA ellipsoids. (D) TGA curves of the PS template (b), PS@PDA ellipsoids (c) and pure polydopamine (a). The PS@PDA ellipsoids were prepared under the conditions of CDP = 0.1 mg mL-1 and coating time = 10 h. 3.2 Typical surface topologies and roughness analysis of the PDA coated ellipsoids. During PDA coating of the PS ellipsoid, the most intriguing observation is that different dopamine concentrations and coating times can lead to several kinds of particles with a specific surface topology (Figure 1, 4 and 5, more are presented in SI). In contrast to the PS ellipsoid which has a relatively smooth surface, PDA coated particles have a rough surface decorated with many nanoscale protrusions (nanoprotrusions). The nanoprotrusions are homogeneously distributed around the



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whole particle surface and ellipsoidal particles with either sparse or densely packed nanoprotrusions on the surface can be obtained by varying the coating conditions. The diameter of the nanoprotrusions is in the range of 30 ~ 70 nm in the cases of the ellipsoids with a homogenous surface feature. Under certain dopamine concentrations as will be demonstrated later, the particle surface is decorated with both small and large protrusions that have a size of more than 200 nm, leading to irregular surface topologies. The PS@PDA ellipsoids with such an hierarchical surface topology can be classified into the raspberry-like particles that normally consist of a big particle decorated with many small nodules.21 The surface feature of each kind of particles was represented by the 3D surface plot of the central area of the ellipsoids with a length of 3000 nm and a width of 500 nm (Figure 3). The resulted surface feature was then analyzed by the SurfCharJ plugin of ImageJ developed by G. Chinga to extract the root-mean-square roughness, .45,46 Due to complexity of the surface roughness analysis in the cases of nano- or colloidal particles, these data are not the absolute quantification of the surface roughness but serve to indicate the amplitude of the roughness. As shown in Fig.3, the surface plot representation of each kind of ellipsoids clearly reveals the different surface roughness that consists of hills and valleys with various degrees of density and homogeneity. Although the PS ellipsoid template has a relatively smooth surface roughness with < 10 nm, of the PDA coated particles is more than 50 nm and the exact values depends on the coating conditions, as will be further investigated in the following section.


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Figure 3. Surface roughness analysis of polydopamine coated ellipsoids. (A) An ellipsoid highlighted with a yellow area which is used for the analysis of the surface roughness. (B) ~ (F) Representation of the surface topology of the ellipsoids listed in Figure 1. (G) Root-mean-square roughness, at different coating time when the dopamine concentration (CDP) is fixed at 0.1 mg mL-1 (square) or different CDP when the coating time is fixed (circle). 3.3 Influence of the coating conditions on the surface roughness of PS@PDA ellipsoids. Many previous works have concluded that the initial dopamine concentration and reaction time are two important parameters that govern the self-polymerization of dopamine and the resulted morphology of the PDA coating.41,47-51 These two parameters were investigated herein to obtain insights into the mechanism of the above results and to optimize the coating conditions toward a specific surface roughness. Influence of initial dopamine concentrations. The dopamine concentration, CDA, was varied from 0.04 to 2 mg mL-1 while the coating time was fixed (Figure 4 and S3-S6 in SI). When CDA is less than 0.1 mg mL-1, the surface of the PS@PDA



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ellipsoids is covered with many nanoprotrusions that have an average diameter in the range of 30 ~ 70nm (Figure 4A and B). The nanoprotrusions homogenously distribute around the whole surface and their density increases when CDA increase from 0.04 to 0.1 mg mL-1. In addition, the whole system is also free from any pure PDA particles. However, when CDA > 0.5 mg mL-1, the surface feature becomes irregular and is decorated with polydisperse protrusions that have a broad size in the range of several hundred nanometers (Figure 4C and D). The locations of the large protrusions on the particle surface are random. By-products of pure PDA particles start appearing and their amount increases with increasing dopamine concentrations. At CDA = 2 mg mL-1, there are a large amount of by-products of pure PDA aggregates, together with some ellipsoids that have a relatively smooth surface (as indicated by the arrow in Figure S10 in SI). Analysis of the surface roughness indicates that initially increases with increasing CDA and then become broad when further increasing concentration (Figure 3G). Therefore, the proper initial concentration of dopamine is paramount in order to achieve a good control of the surface properties and to avoid contamination of side-products of pure PDA.41 Among the dopamine concentration, 0.1 mg mL-1 is an optimal concentration in terms of the homogeneity of the surface topology and reaction time.


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Figure 4. Influence of the initial dopamine concentrations (CDP) on the surface topology of polydopamine coated ellipsoidal particles. The coating time was fixed at 10 h. CDP in (A) ~ (E) is 0.04, 0.1, 0.5, 1 and 2 mg mL-1, respectively. Images in the left, middle and right column are TEM, SEM images and the magnified SEM view of the area highlighted in the middle column, respectively. Influence of the coating time. Based on the above results, PDA coating of PS ellipsoids was performed by varying the coating time up to 25 h while fixing CDA at 0.1 mg mL-1 (Figure 5 and S7-S10 in SI). During the first five hours, hybrid ellipsoids



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with a well-controlled surface topology can be obtained, which are decorated with monodisperse nanoprotritions (Figure 5A). With increasing coating time, the areal density of the nanoprotrusions slightly increases while theirs size and distribution barely change (Figure 5B-D). At longer time, the density of nanoprotusions increases further and the particle surface indeed becomes relatively smooth (Figure 5E). Analysis of the surface roughness also indicates that first slightly increases and then decreases with elongating coating time. In general, PDA coating for 10 to 15 h gives rise to a clearly defined surface roughness.


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Figure 5. Influence of the coating time on the surface topology of polydopamine coated ellipsoidal particles. The coating time in (A) ~ (E) was 5, 10, 14, 20 and 25 h, respectively. The dopamine concentration was fixed at 0.1 mg mL-1. Images in the left, middle and right column are TEM, SEM images and the magnified SEM view of the area highlighted in the middle column, respectively.



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Mechanism that leads to the controlled surface roughness. The mechanism of self-polymerization of dopamine into polydopamine and coating onto surfaces is highly elusive.41,42,47-50 Generally, dopamine is oxidized into some polymerizable precursors, such as 5,6-dihydroxyindole (DHI) and indole-5,6-quinone (IQ) that can then

covalently

polymerize

into

insoluble

oligomers.50

Eventually,

self-polymerization or addition of the monomer or small oligomers to the growing species leads to nanoscale polydopamine particles that are very monodisperse (Figure S17 in SI). In the presence of other particulate or macroscopic templates, the early polymerizable species can adsorb onto the surface of the templates, mainly via the excellent adhesive properties of the catechol groups of the intermediate species.47 Further polymerization or aggregation of these intermediates at the surface results in polydopamine coating. Concomitantly, the intermediate species in the bulk solvent continue polymerizing into nanoscale PDA particles, which can further deposit onto the surface of the PDA layer.41,52 These procedures are supported by recent works on the PDA coating of macroscopic surfaces and can explain the fact that PDA coatings normally have a rough surface.9,47,51,53 We speculate that the formation of PS@PDA ellipsoids with a controlled surface roughness follows similar scenario: adsorption and polymerization of early polymerizible species onto the PS ellipsoids leads to a relatively smooth and conformal coating layer while the nanoprotrusions are introduced via deposition of nanoscale PDA particles formed in bulk. Indeed, removing the PS templates of PDA coated ellipsoids that were separated from the


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early stage of the coating procedure produces PDA capsules with a relatively smooth surface (Figure S1A in SI). Particles separated from the later stage of the coating procedure, however, produces flattened PDA capsules that consist of a smooth layer decorated with many nanoparticles (Figure S1B in SI). Self-polymerization and condensation of dopamine are influenced by many factors, among which the initial concentration of dopamine, CDA, is the most critical one.41 When CDA is high, the concentration of the polymerizable species is very high, leading to fast self-polymerization and condensation of the species into large PDA particles with a broad size distribution (Figure S17 in SI). Adsorption of polymerizable species such as monomers and oligomers onto the PS ellipsoids occurs but might be inextricably coupled with deposition of polydisperse PDA particles, giving rise to an ill-defined surface feature. This is the case when CDA is above 0.5 mg mL-1: the surface of the PS@PDA ellipsoids obtained at this CDA is randomly attached with PDA particles of various sizes and large amount of pure PDA particles exist at the end of the PDA coating (Figure 4D and E). The size and distribution of both surface attached and free PDA particles increase with the DA concentration (Figure S3-S6 in SI). At 2 mg mL-1, besides particles with ill-defined roughness and large amount of irregular aggregates, there exit some ellipsoids with a smooth surface that is different from the original PS ellipsoids (as indicated by the arrow in Figure S6 in SI). EDS analysis on such particles reveals that they are indeed coated with a PDA layer (Figure S14 in SI). This result further verifies the coating scenario as suggested above. However, the self-polymerization and condensation in bulk at 2 mg mL-1 is so



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dominant that most of dopamine ends up as irregular aggregates and the deposition of PDA nanoparticles onto the smooth ellipsoids is interrupted. Therefore, to achieve a well-controlled surface feature, it is needed to find a subtle balance between the surface deposition of the early polymerizable species and formation of the PDA nanoparticles in bulk by choosing proper dopamine concentrations. The feasibility of such method was also tested with spherical polystyrene particles. Direct PDA coating on polystyrene spheres can also produce spherical hybrid particles with controlled surface roughness at the proper dopamine concentrations (Figure S11 in SI). It is noted here that PDA coating have been extensively applied to various kinds of particles and macroscopic surfaces in recent years.54,55 Such coating has often been used as the auxiliary intermediate layer for in situ formation of metal nanoparticles or deposition of other particles for enhanced catalysis and construction of superhydrophobic surfaces.44,56-61 Only in a few cases of PDA coating of macroscopic materials, the ill-defined roughness of the PDA surface has been noted and directly exploited for functional applications.41,62,63 Our results demonstrate that the self-polymerization of dopamine not only just works as versatile surface coatings but also can be harnessed to impart well-controlled surface topologies onto anisotropic particles through manipulating the unique chemistry inherent to the polymerization of dopamine. 3.4 Chemical Modifications of the PDA coated PS. Compared to other surface engendering techniques, vast possibilities for chemical modifications are the attractive aspects of the PDA coating.64 Such post-chemical functionalization was demonstrated


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herein by preparing fluorescent or fluorinated PS@PDA ellipsoids (Figure 6). Amino functionalized fluorescein was simply mixed with PDA coated ellipsoids under stirring

and

free

dyes

were

washing

away

by

several

arounds

of

centrifugation/redipsersion. The resulted particles have a fluorescent halo, suggesting successful

labeling

of

the

fluorescent

dyes.

Similarly,

1H,

1H,

2H,

2H-perfluorodecanethiol (PFDT) was mixed with PS@PDA ellipsoids in ethanol for fluorination and PFDT can be coupled to the surface of the particles via the reaction of the thiols with the PDA layer. This latter modification can transfer the surface from hydrophilic into hydrophobic. After purification, the particles can only be dispersed into organic solvents such as ethanol. XPS analysis of the fluorinated hybrid particles revealed a sharp peak at 690 ev that can be ascribed to the F atom (Figure 6B). The two weak peaks around 160 ev are originated from the S atom. These results confirm the coupling of PFDT onto the surface of the particles. Such modifications barely change the surface topology of the particles (Figure 6C).



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Figure 6. Surface modification of polydopamine coated ellipsoids. (A) Fluorescent optical microscopy image of fluoresceinamine labeled ellipsoids. (B) and (C) XPS analysis and SEM image of 1H, 1H, 2H, 2H-perfluorodecanethiol (PFDT) modified ellipsoids. 3.5 Superhydrophobic surfaces and liquid marbles constructed by fluorinated PS@PDA ellipsoids with a controlled surface roughness. As stated in Introduction, compared to their spherical counterparts, anisotropic particles, like ellipsoids, have peculiar behaviors in self-assembly, packing and at fluid interfaces, etc. The controlled surface roughness achieved herein is expected to endow the current PS@PDA ellipsoids with other unique properties. By taking advantage of these properties, we shall present some applications of the current PS@PDA ellipsoids as building blocks for superhydrophobic surfaces and liquid marbles. Evaporation-driven drying of colloidal particles is a cost-effect and convenient way


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to construct functional coatings, for instance, for superhydrophobic surfaces.15,21,26 Drying of sensible droplets containing spherical particles often leads to an uneven coating due to the coffee ring effect while ellipsoidal particles can suppress such effect under certain conditions.37,65-67 In the current work, homogeneous particulate films on glass slides can be obtained by drying a droplet of the fluorinated PS@PDA ellipsoids in ethanol (Figure 7A). Inside the dense film, the packing of the ellipsoids are random but locally several ellipsoids pack with a preferential side-by-side configuration. Some ellipsoids point out of the film surface (Figure 7B and see Figure S13 in SI for a large overview). The microscale structural feature due to such inter-particle packing, together with the nanoscale surface roughness of the constituent particles, endows the films of the hybrid ellipsoids with a two-scale hierarchal structure. Furthermore, the particle surface becomes low surface energy due to the convenient fluorination of PDA. With these properties together, the dried fluorinated PS@PDA ellipsoidal film should fulfill the requirements of a superhydrophobic surface.68 As expected, water has an apparent static contact angle (CA) of more than 155 ± 3° on the films prepared from ellipsoids presented in Figure 1C prepared under the conditions of CDP = 0.1 mg mL-1 and coating time = 10 h (Figure S15 in SI). The advantage of the shape effect was confirmed by comparing with the behavior of fluorinated spherical counterparts. Drying of the spherical particles with similar surface roughness produces a coffee ring like structure under the same drying conditions as the ellipsoids, in which most of the particles locate at the outskirts of the stain (Figure S12A in SI). Measurements of the water CA on the area



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of densely packed fluorinated sphere particles gives apparent static CA values of around 110°. Compared to the random and open structure of the film formed by ellipsoids, the spheres closely arrange into dense structures in the multi-layered film (Figure S12B in SI). Such mesoscale structure difference might contribute to the different hydrophobicity performance. The influence of the surface feature of the ellipsoidal particles on the wettability was also compared. The films formed by the particles in Figure 1B and 1E have an apparent static water CA that is less than 150o (Figure S16 in SI). The former has a larger water CA (130o) than that of the latter (120o). The packing of the ellipsoids, i.e. the mesoscale structure of the films are similar. The particles in Figure 1B have a surface that is only decorated with sparse nanoprotrusions and the surface roughness feature is not pronounced enough to fulfil two-scale hierarchical structure requirements of superhydrophobic surfaces. In the case of particles in Figure 1E, only parts of the surface of ellipsoidal particles are unevenly covered with polydisperse PDA particles with sizes of hundreds of nm while other parts are relative smooth. Such nanoscale surface irregularity may stand behind the poor superhydrophobic performance. Therefore, the nanoscale surface feature of the ellipsoids particles play a critical role in determining the water wettability of the particulate films. Interestingly, although most of superhydrophobic coatings derived from spherical particles with rough surfaces have a very low rolling-off angle of water that is similar to lotus leaves,15,21,56 the ellipsoidal film exhibits strong adhesion to water. A 15 µL water droplet can firmly stick to the surface when the glass slide was put vertically


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(Figure 7C, Video S1 in SI). A smaller volume (2 µL) of water on the film can be put upside down. Such behavior is similar to some red rose pedals or peanut leaves that have superhydrophobicity but also strong water adhesion behaviors.69 The superhydrophobicity of both lotus leaves and certain species of rose pedals are due to their two-scale hierarchal structure that consist of microscale wax pillars decorated with nanoscale features. Jang and collaborators have attributed the distinct water adhesion behavior between them to the density and size difference of their two-scale structures.69 While water penetration into the microscale structure is prevented by air pockets existing in the surface structure of lotus leaves, certain species of rose pedals have a microscale surface structure with a size and density that allow water to penetrate into the inter-space among wax pillars, giving rise to the pronounced water adhesion behavior. In the case of our ellipsoids with a long axis of 7 µm and a diameter of 1 µm, random packing of the ellipsoids after drying into films creates microscale voids, into which water may penetrate (Figure S13 in the SI). Such high adhesive superhydrophobic surfaces might be used for no-loss microdroplet transportation and collection of water.70



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Figure 7. Superhydrophobic coatings and liquid marbles prepared from fluorinated polydopamine coated ellipsoids. (A) Dense particulate films by evaporative drying of concentrated PS@PDA ellipsoids in ethanol. (B) SEM image of the ellipsoids inside the film of (A). (C) Water droplets on the dense film of (A). In the inset is a rose pedal with water droplets attached. (D) and (E) are examples of liquid marbles. One of the marbles was squeezed into flat shape (E). Similar to superhydrophobic coatings, liquid marbles, i.e, isolated liquid droplets encapsulated in a layer of hydrophobic particles, have also attracted broad interest.38 Spherical particles are often of the choice for constructing liquid marbles while recent works have demonstrated that ellipsoidal or rod-like particles are very efficient in stabilizing emulsion droplets or foam bubbles, by attaching to the fluid interface in a side-on fashion.31,32 Therefore, the current particles were also tested for preparing


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liquid marbles. It is found that water droplets can stand on the loose particle bed of fluorinated PS@PDA ellipsoidal powders. Rolling of such droplets leads to the encapsulation of the intact water droplet inside a layer of such particles. The as-prepared ellipsoid-stabilized marbles can be picked up by a tweezer and squeezed into a non-spherical shape without any leakage, indicating reasonable mechanical strength. In addition, the marble can be transferred onto various kinds of substrates and roll off the slightly titling solid surfaces (Figure S2 and Video S2 in SI). Therefore, the as-prepared ellipsoids, with their controlled surface roughness and peculiar film formation behavior, are good building blocks for bottom-up fabrication of superhydrophibic coatings and liquid marbles. 4. CONCLUSIONS We demonstrate that self-polymerization of dopamine into polydopamine offers a mild and versatile means for the surface engineering of anisotropic particles, such as colloidal ellipsoids, through forming a surface curvature-independent conformal coating. With insights into the self-polymerization of dopamine under basic conditions, this work also presents the possibility to further control the surface roughness of the polydopmaine coating. Under proper dopamine concentrations and coating times, deposition of PDA nanoparticles formed in bulk onto the PDA layer of the ellipsoids leads to hybrid particles decorated with many nanoscale protrusions. After surface fluoridation via rich chemical modifications inherent to the PDA coating, the ellipsoids with controlled surface topologies can be further used as building blocks to construct superhydrophobic surfaces with strong water adhesion and liquid marbles



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with reasonable mechanics. Compared to previous fabrication methods of raspberry-like particles that normally involve multiple steps and stringent chemistry, the unique chemical nature inherent to self-polymerization of dopamine can not only produce uniform coating but also offer a means to simultaneously control the surface roughness under proper conditions. ASSOCIATED CONTENT Supporting Information

Video_S1: A 15 µL droplet stands on a film formed by drying fluorinated polydopamine coated polystyrene ellipsoids. Video_S2: A liquid marble stabilized by fluorinated polydopamine coated polystyrene ellipsoids rolls off a tilted glass slide. Preparation of the polystyrene particles and thermo-stretching of these particles into ellipsoids, more SEM images of PDA coated ellipsoids and polydopamine coating of spherical polystyrene particles. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Authors E-mail: [email protected]；Fax: +86 22 23503510. Tel: +86 22 23501945. [email protected] Author Contributions + These authors contributed equally to this work.


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Ellipsoidal Colloids with a Controlled Surface Roughness via

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