Liquid Marbles Stabilized by Fluorine-Bearing Cyclomatrix

Jan 30, 2016 - Thomas C. Draper , Claire Fullarton , Neil Phillips , Ben P.J. de Lacy Costello , Andrew Adamatzky. Materials Today 2017 20 (10), 561-5...
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Liquid Marbles Stabilized by Fluorine-Bearing Cyclomatrix Polyphosphazene Particles and Their Application as High-Efficiency Miniature Reactors Wei Wei, Rongjie Lu, Weitao Ye, Jianhua Sun, Ye Zhu, Jing Luo, and Xiaoya Liu* The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China S Supporting Information *

ABSTRACT: Increasing attention has been paid to fabricate multifunctional stabilizers of liquid marbles for expanding their application. Here, a kind of hydrophobic cyclomatrix polyphosphazene particles (PZAF) were facilely prepared using a one-step precipitation polycondensation of hexachlorocyclotriphosphazene and 4,4′-(hexafluoroisopropylidene)diphenol, and their ability to stabilize liquid marbles was first investigated. The Ag nanoparticledecorated PZAF particles (Ag/PZAF) were then fabricated by an in situ reduction of silver nitrate onto PZAF particles and used to construct catalytic liquid marbles. The results revealed that the reduction of methylene blue (MB) in aqueous solution by sodium borohydride could be highly efficiently catalyzed in the catalytic liquid marbles, even with a large volume. An excellent cycle use performance of the catalytic liquid marbles without losing catalytic efficiency was also present. The high catalytic activity is mainly attributed to the uniform immobilization of Ag nanoparticles onto PZAF particles and the adsorption behavior of PZAF particles toward MB, which may play an effect on allowing high catalytic surface area and effective accelerating the mass transfer of MB to the Ag catalytic active sites, respectively. Therefore, the combination of Ag nanoparticles with PZAF particles has been demonstrated clearly to be a facile and effective strategy to obtain the functional stabilizer for preparing the catalytic liquid marbles as promising miniature reactors used in heterogeneous catalytic reactions.

1. INTRODUCTION Liquid marbles are commonly known as the spherical structures formed by adsorption of hydrophobic micro/nanometer-sized particles to the liquid/gas interface. Since the pioneering work of Aussillous and Quéré in 2001 preparing the liquid marbles by simply rolling sessile water droplets over a bed of hydrophobic fluorinated silane-covered lycopodium grains,1 there has been a flourish of studies on the formation, properties, and potential application of liquid marbles during the past decade.2−6 The utilization of liquid marbles as miniature reactors has aroused extensive interest owing to their fascinating advantages, compared to the traditional miniature reactors, in minimizing chemical reagent usage, providing a confined microenvironment, allowing the mass transfer of reactants and products, and accurately controlling reaction capacity, facile fabrication, and versatility for various reaction liquids, especially for the reactions involving highly toxic and/or costly reagents as well as the biological processes.7−9 The liquid marble-based miniature reactors have been successfully exploited for rapid blood typing,10 nanocomposite synthesis,11 chemiluminescence reactions, photochemical polymerization, and acid−base reactions.12 However, most of the previous studies on the application of liquid marbles as miniature reactors only focus © XXXX American Chemical Society

on the role of the particle-formed shells as inert isolating layers to produce a confined reaction compartment, while the participation of the encapsulating layers in the reactions has seldom been reported. Recently, Miao et al. prepared a catalytic liquid marbles by employing the perfluorodecanethiol-grafted Ag nanowires as stabilizer, which, could simultaneously play a role in catalyzing the chemical reactions in the liquid marbles.13 Gao et al. fabricated a graphene-based liquid marbles and utilized the photothermal properties of graphene to achieve precise temperature and reaction kinetic control in the miniature reactors.14 Sheng et al. demonstrated a participation of the shell of silica-particle-stabilized liquid marbles in regulating the classical silver mirror reaction. 15 These contributions reveal the attractive potential of the encapsulating shells in regulating reactions and enhancing the suitability of liquid marbles as miniature reactors. Synthetic polymer particles draw increasing attention for preparing liquid marbles due to their designable functions and surface chemistries,16−20 although most of the current literature is concerned with natural and inorganic powders including Received: December 23, 2015 Revised: January 29, 2016

A

DOI: 10.1021/acs.langmuir.5b04697 Langmuir XXXX, XXX, XXX−XXX

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Langmuir lycopodium,1 silica,2 metal oxide,21 and carbon black.22 Cyclomatrix polyphosphazene (CPPz) particles are the versatile organic−inorganic hybrid polymer particles prepared by a onestep precipitation polycondensation of hexachlorocyclotriphosphazene (HCCP) with the aromatic organic monomers bearing dual-nucleophilic groups.23−34 The chemical structure, functions, and surface properties of the particles can be readily tailored by altering functional monomers for intended applications such as fluorescent probe,25−27 superhydrophobic surface,28 drug delivery,29−31 and cell imaging.32 It is desirable to note that the coordination ability of the nitrogen atoms in HCCP moieties usually allows the particles to be good supports for stabilizing noble metal nanoparticles.35,36 Hence, in this study we demonstrate the fabrication of liquid marbles with the particles of a fluorine-bearing CPPz, poly[cyclotriphosphazene-co-4,4′-(hexafluoroisopropylidene)diphenol] (PZAF), which were facilely prepared using a onestep precipitation polymerization method.28 Further, Ag nanoparticle-decorated PZAF particles (Ag/PZAF) composite particles, obtained by an in situ reduction of silver nitrate with sodium borohydride on the surface of PZAF particles, were served as the encapsulating shells to construct liquid marblebased catalytic miniature reactors. The Ag-catalyzed reduction of methylene blue by sodium borohydride was employed as a model reaction to test the heterogeneous catalytic capacity of the miniature reactors. Our study contributes a simple and effective strategy for preparing the polymer-based stabilizer of liquid marbles. The combination of Ag nanoparticles with PZAF particles endows the encapsulating layers with highefficiency catalytic ability besides the regular function for stabilizing liquid marbles, making the liquid marbles be a desired miniature reactor for heterogeneous catalytic reactions. In addition, compared with the direct use of metal catalyst as the building blocks for the catalytic liquid marbles, our strategy involving the immobilization of Ag nanoparticles onto PZAF particles has the advantage in effectively reducing the catalyst aggregation, as well as lowing the catalyst dosage.

Scheme 1. Illustration for the Preparation Route of PZAF Particles (a) and Ag/PZAF Particles (b)

kHz) for 60 min. After that, 0.4 mL of NaBH4 ethanol solution (3 mg mL−1) was added to the above mixture dropwise and the reaction system was magnetically stirred for 30 min at room temperature. Then the resulting product was separated by centrifugation, washed several times using deionized water, and finally dried in vacuum at 65 °C for 24 h to yield Ag/PZAF composite particles as a yellow powder. 2.4. Preparation of Liquid Marbles. The liquid marbles were formed by rolling sessile water droplets onto the powder bed of PZAF particles until complete encapsulation. The volume of the dispensed water droplets was controlled from 5 μL to 2 mL using a micropipette. The preparation process of liquid marbles is schematic illustrated in Scheme 2a.

2. EXPERIMENTAL SECTION Scheme 2. Illustration for the Formation of Liquid Marbles Using PZAF Particles as Stabilizer (a) and the Catalytic Reduction of MB in the Liquid Marble-Based Miniature Reactors Stabilized by Ag/PZAF Particles (b)

2.1. Materials. HCCP (99% purity), supplied by Wanduoxin Chemical Co., Ltd. (Jinan, China), was recrystallized from dry hexane followed by sublimation (60 °C, 0.05 mmHg) thrice prior to use. 4,4′(hexafluoroisopropylidene)diphenol (BPAF, AnalaR grade) was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China) and used directly. Triethylamine (TEA), acetonitrile, acetone, ethanol, sodium borohydride (NaBH4), methylene blue (MB), and silver nitrate (AgNO3) were of AnalaR grade and used as received from Sinopharm Chemical Regent Co. Ltd. (Shanghai, China). 2.2. Preparation of PZAF Particles. The PZAF particles were prepared by a one-pot precipitation polymerization method.28 Typically, 0.20 g (0.575 mmol) of HCCP and 0.58 g (1.725 mmol) of BPAF were placed in a 250 mL round-bottomed flask and dissolved in 100 mL of acetonitrile first. Then the reaction began with adding 4 mL of TEA to the flask and was agitated in an ultrasonic water bath (200 W, 40 kHz) at about 50 °C for 5 h. When the reaction finished, the solid product was isolated by centrifugation, followed by washing thrice with acetone and deionized water, respectively. Finally, the PZAF particles were obtained as a white powder after drying in vacuum at 35 °C for 24 h. The preparation route of PZAF particles is depicted in Scheme 1a. 2.3. Preparation of Ag/PZAF Particles. The Ag/PZAF composite particles were prepared by an in situ reduction as follows (Scheme 1b): In a 100 mL round-bottomed flask, 20 mg of PZAF particles, 2.72 mL of AgNO3 aqueous solution (2 mg mL−1), and 40 mL of ethanol were mixed under ultrasonic irradiation (200 W, 40 B

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cross-linking mechanism.24,28,37 Briefly, the oligomers generated at the initial stage of polymerization aggregated spontaneously due to their poor solubility in acetonitrile to form primary stable cores in the solution system. With the collision and incorporation between oligomers and primary stable cores, the size of the primary particles continued to grow. Inside the particles, cross-linking occurred and the polymeric particles with a highly cross-linked structure were finally produced. The chemical structure of PZAF particles was characterized by FT-IR (Figure S1a) and XPS measurements (Figure S1b and Table S1). The results demonstrate that PZAF particles are the polycondensation product of HCCP and BPAF and own a possible cyclomatrix structure as suggested in Scheme 1a. Notably there are remnant chlorine atoms in the particle structure due to the steric hindrance effects resulting in an incomplete substitution reaction.24,26,27 The remarkable thermal stability of PZAF particles revealed by TGA (Figure S2) also indicates their highly cross-linked organic−inorganic hybrid structure. Figure 1a shows the morphology of PZAF particles investigated by SEM. The particles possess an almost

2.5. Preparation of Liquid Marble-Based Miniature Reactors for Catalytic Reduction of MB. In a typical experiment, the MB aqueous solution (12.5 mg L−1) was first mixed with the NaBH4 aqueous solution (2.5 mg mL−1) by the volume ratio of 4:1 to obtain a mixture solution. Then, 25 100 μL droplets of the as-obtained solution were successively dropped on the powder bed of Ag/PZAF particles and subsequently rolled to form the catalytic liquid marbles. The reduction of MB was conducted in the catalytic liquid marbles. At given time intervals, the internal solutions of the 25 liquid marbles were extracted out and collected together to measure the optical absorbance at a wavelength of 664 nm (the characteristic absorption wavelength of MB in aqueous solution) by a UV−visible spectrophotometer. Scheme 2b gives the illustration of the catalytic reduction of MB using liquid marbles as miniature reactors. To evaluate the catalytic efficiency of the liquid marble-based miniature reactors, the traditional catalytic mode was served as a contrast, in which the hydrophobic Ag/PZAF particles were directly spread over the liquid surface of the MB/NaBH4 aqueous solution. The dosages of Ag/PZAF particles and the solution used in the two catalytic mode were consistent. The reduction of MB by NaBH4 in solution droplets without Ag/PZAF particles was also carried out as a reference. The effect of the size of the liquid marble-based miniature reactors on catalytic reaction rate was investigated by varying the volume of solution droplets (25, 50, 100, and 200 μL). The cycle use performance of the catalytic liquid marbles was tested by withdrawing the solution inside liquid marbles after the reduction, drying the recycled Ag/PZAF particles in vacuum at 60 °C for 20 min, and then rerolling the new droplets of the MB/NaBH4 aqueous solution on the powder bed for ten cycles. 2.6. Characterization. The Fourier-transform infrared (FT-IR) analyses were carried out by a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, U.S.A.) at room temperature (25 °C). The X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250 instrument (Thermo Fisher Scientific, U.S.A.) with a monochromatic Al Kα X-ray source (1486.60 eV). The scanning electron microscopy (SEM) images were obtained from an S-4800 field-emission scanning electron microscope (Hitachi, Japan) at an accelerating voltage of 2 kV. The transmission electron microscopy (TEM) images were taken by a JEM-2100 (HR) LaB6 transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. The static water contact angles were measured on an OCA15EC video contact angle goniometer (Data Physics Instruments, Germany). The thermogravimetric analyses (TGA) were conducted on a TGA-1 thermogravimetric analyzer (Mettler Toledo, Switzerland) in the temperature range from ambient temperature to 800 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere. The X-ray diffraction (XRD) patterns were recorded on a D8 X-ray powder diffractometer (Bruker AXS, Germany) with a Cu Kα radiation source (λ = 1.5418 Å). The digital photographs and optical microscopy images were collected by a P500 digital camera (Nikon, Japan) and a DM-BA450 optical microscope (Motic, China) fitted with a digital camera, respectively. The UV− visible absorbance spectra were measured using a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China).

Figure 1. (a) SEM image of PZAF particles. Inset: magnified image of a single particle. (b) Static water contact angle image of a glass slide surface coated with PZAF particles. (c) Digital photo of a liquid marble prepared from PZAF particles. The droplet volume is 25 μL. Inset: cartoon illustration of the liquid marble structure. (d) Optical microscopy image of the liquid marble surface.

monodispersed spherical shape. The particle size and size distribution were obtained from the SEM images by counting more than 300 particles. The mean diameter of PZAF particles is 1.47 μm, and the size distribution result is shown in Figure S3. It can be seen from the inset of Figure 1a, a magnified SEM image of one PZAF particle, that, the particle surface is rough with numerous nanosized protuberances rather than smooth. Therefore, the particles are highly hydrophobic and exhibit a water contact angle of about 148° (Figure 1b), due to the combined contribution of such surface roughness and the abundant low surface energy fluorine groups on the particle surface as confirmed by XPS (Table S1). The superior antiwetting property of PZAF particles is essential for the subsequent preparation of liquid marbles. The liquid marbles were formed by rolling water droplets over a bed of PZAF particles, as illustrated in Scheme 2a.

3. RESULTS AND DISCUSSION 3.1. Liquid Marbles Stabilized by PZAF Particles. The PZAF particles were facilely prepared by a one-pot precipitation polycondensation of HCCP and BPAF in acetonitrile (Scheme 1a). At initial stage, the phosphorus nuclei of HCCP were attacked by the phenolic hydroxyl groups of BPAF, which were activated by TEA first, to proceed nucleophilic replacement reaction to generate oligomers and hydrogen chloride (HCl). Then the excess TEA acted as acid acceptor to absorb the resultant HCl, accelerating the polymerization.24 About the formation process and colloidal stabilizing mechanism of PZAF particles, it obeyed an oligomeric species self-assembling and C

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Langmuir During the process, the PZAF particles spontaneously assembled at the surface of water droplets as a result of the minimization of the surface free energy.1,20 The formed liquid marbles remained integrity after being transferred onto a glass slide. A digital photo of a typical PZAF particle-stabilized liquid marble on the glass slide is shown in Figure 1c. Optical microscopy was adopted to investigate the adsorption of PZAF particles at the liquid/air interface. As can be seen in Figure 1d, the dark regions are the aggregates of PZAF particles adsorbed on the surface of water droplet, and the bright regions belong to the exposed droplet surface without being covered by the particles. It is obvious that PZAF particles form a loosely packed and porous shell encapsulating the water droplet, which is a unique structural feature of liquid marbles allowing the volatilization of internal water (Figure S4) and the transport of external gas (Figure S5). Because of the prevention of direct contact between the liquid core and the surface of glass slide by the hydrophobic particle-formed shell, the liquid marbles are endowed with a nonwetting nature.6 Thus, they exhibit low friction and low adhesion on the glass slide, and can roll freely without the leakage of water. Figure 2 shows the relationship between the shape of the PZAF particle-based liquid marbles and the internal water

γ=

ρgH2 4

(1) −1

where ρ is the density of the internal water (1.00 g cm ), g is gravitational acceleration (9.81 m s−2), and H is the height of liquid marbles. For our study, the stable liquid marbles could be obtained by encapsulating up to 1.0 mL of water, and the maximum H is 5.3 mm (Figure 2). Therefore, the corresponding γ was calculated to be 68.89 mN m−1 by eq 1. This value is lower than the surface tension of pure water (71.97 mN m−1 at 25 °C),39 as a result of the adsorption of hydrophobic PZAF particles on the surface of water droplet. The capillary length (κ−1), a characteristic length scale for fluid interface subject to a body force from gravity and a surface force due to surface tension, was further estimated to be 2.65 mm based on the γ value of 68.89 mN m−1 by use of the following equation40 κ −1 =

γ ρg

(2) 38

According to the contribution by Aussillous and Quéré, the morphology of water droplet is related to the relative sizes between the quasi-spherical radius (R0) of the droplet and κ−1. For R0 ≪ κ−1, the gravity is negligible and the droplet takes on a quasi-spherical shape. If the R0 is much larger than κ−1, the gravity predominates and the droplet becomes a “puddle”. Here, R0 = (3V/4π)1/3, where V is the volume of the water droplet. Hence in our case, the liquid marbles with the volume less than 25 μL (R0 = 1.81 mm) are suggested to exhibit a nearspherical shape, while the larger liquid marbles (>100 μL) are considered to adopt a puddle-like shape, which is consistent with the result revealed in Figure 2. 3.2. Ag/PZAF Particle-Based Liquid Marbles as Catalytic Miniature Reactors. The PZAF particles were loaded with Ag nanoparticles to obtain Ag/PZAF composite particles by an in situ reduction of AgNO3 using NaBH4 as reductant (Scheme 1b). Figure 3a,b shows the TEM images of the as-prepared Ag/PZAF particles. It is clear that the PZAF particles were decorated with Ag nanoparticles successfully. The Ag nanoparticles with the mean size of 20 nm are uniformly dispersed on the surface of PZAF particles. The composite particles were further characterized by XRD, as shown in Figure 3c. Compared with the XRD pattern of pristine PZAF particles, there appears five distinct diffractions at 38.1°, 44.2°, 64.5°, 77.3°, and 81.5° in the XRD pattern of Ag/PZAF particles besides the broad characteristic band (10− 20°) of PZAF particles. The five diffractions are assigned to the (111), (200), (220), (311), and (222) planes of Ag, respectively,41 indicating the presence of Ag nanoparticles. The content of Ag nanoparticles in the composite particles is 6.72 wt %, which was determined by TGA (Figure 3d). It can also be confirmed by TEM observation that few Ag nanoparticles are free and detached from the PZAF particles after a vigorous stirring of Ag/PZAF particles in ethanol for 30 min (the specific images are not shown here), suggesting a strong adhesion of the Ag nanoparticles on the surface of PZAF particles. A distinct shift of the UV−vis absorption band of AgNO3 (302 nm) to long wavelength (312 nm) was observed by mixing the AgNO3 with PZAF particles in a mixed solvent of ethanol and water (Vethanol/Vwater = 4:1), as shown in Figure S6, implying the possible coordination of the N and/or O atoms of PZAF particles to Ag+ ions.35 Such interaction, together with the rough surface of PZAF particles (Figure 1a), makes the

Figure 2. Shape evolution of the PZAF particle-stabilized liquid marbles with increasing the volume of internal water: height (a), width (b), and diameter of the contact area of liquid marbles and glass substrate (c). The solid curve represents a theoretical relationship of marble diameter (a′) with internal water volume if it is assumed that the liquid marbles maintain a perfectly spherical shape independent of water volume. The inset above shows the digital photographs of the liquid marbles containing different volumes of water.

volume. The height (a) and width (b) of liquid marbles, as well as the diameter of the contact area between liquid marbles and glass substrate (c), were plotted with the volume of internal water. The solid curve indicates the diameter (a′) of the liquid marbles with a perfectly spherical morphology against water volume, which is based on an assumption that the shape of the liquid marbles is independent of the water volume. As can be seen, the morphology of the liquid marbles gradually evolves from a quasi-spherical shape to a puddle shape as the water volume increases. The significant deformation (or dimension deviation) of the liquid marbles from a standard sphere is caused by the gravitational force. The effective surface tension (γ) of liquid marbles can be calculated using the following equation38 D

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Figure 3. (a,b) TEM images of Ag/PZAF particles with different magnification. (c) XRD patterns of PZAF particles and Ag/PZAF particles. (d) TGA curves of PZAF particles and Ag/PZAF particles measured under nitrogen atmosphere. (e) Static water contact angle image of a glass slide surface coated with Ag/PZAF particles.

Figure 4. Digital photos depicting the color change of MB/NaBH4 mixed aqueous solutions in the form of (a) bare droplet without Ag catalysis, (b) traditional catalytic mode in which the hydrophobic Ag/PZAF particles were directly spread over the liquid surface of the MB/NaBH4 aqueous solution, and (c) Ag/PZAF particle-based catalytic liquid marbles at different time intervals. For all the three cases, the concentrations of MB and NaBH4 in the mixed aqueous solution are 10 and 0.5 mg L−1, respectively. The dosages of Ag/PZAF particles and the solution used in (b,c) are consistent. (d−f) UV−vis absorption spectra of the MB/NaBH4 aqueous solutions measured at the given time intervals of (a−c), respectively.

good dispersion and stable attachment of Ag nanoparticles on the surface of PZAF particles. The result of water contact angle tests shows that Ag/PZAF particles are still highly hydrophobic and exhibit a water contact angle of about 143° (Figure 3e), though the value declines a little from that of pristine PZAF particles (Figure 1b). It is suggested the potential of Ag/PZAF particles for stabilizing liquid marbles subsequently. The liquid marble-based catalytic miniature reactors were fabricated by rolling the droplets of reactant solutions over the powder bed of Ag/PZAF particles (Figure S7). The Ag/PZAF particles assembled at the liquid/air interface spontaneously and served as the encapsulating shell to not only isolate a smallvolume compartment but also heterogeneously catalyze the chemical reaction in the interior of the marble. To investigate

the performance of the Ag/PZAF particle-based liquid marbles as catalytic miniature reactors, the catalytic reduction of encapsulated MB aqueous solution by NaBH4 was carried out (Scheme 2b). Typically, 25 100 μL catalytic liquid marbles containing 10 mg L−1 MB aqueous solution and 0.5 mg L−1 NaBH4 aqueous solution were prepared. The catalytic liquid marbles were then artificially ruptured at predefined timings and the encapsulated solutions were collected to measure the UV−vis absorption spectra. It is visible from the digital photo in Figure 4c that, the blue color of the solutions encapsulated in the catalytic liquid marbles fades rapidly within 5 min and eventually changes into colorless by 25 min. The corresponding UV−vis absorbance of the solutions at a wavelength of 664 nm, the characteristic absorption band of MB, decreases dramatiE

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equation, as shown in Figure 5b. The good linear relation between ln(A/A0) and time suggests that the reduction of MB follows pseudo-first order kinetics. The apparent rate constants (k) can be obtained from the slopes of the linear relations. It is obvious that the liquid marble-based miniature reactors have a much larger k value of 0.179 min−1 than the other two cases, indicating their high-efficiency catalytic capacity for rapid reduction of MB. This value is also superior to that reported for Ag nanowire-based liquid marbles with equal volume as catalytic miniature reactors.13 The remarkable catalytic performance of Ag/PZAF particlebased liquid marbles on the reduction of MB may be attributed to three reasons. First, using liquid marbles as miniature reactors provides a confined microenvironment and high surface area/volume ratio and the loosely packed and porous shell facilitates the exposure of catalytic sites for efficient reduction reaction.13 Second, the good dispersion and stable immobilization of Ag nanoparticles onto PZAF particles effectively reduce the catalyst aggregation and allow the high catalytic surface area, compared with the direct use of Ag nanoparticles as the building blocks for the catalytic liquid marbles,13 which is in favor of the role of Ag as an efficient electron relay between electrophilic MB and nucleophilic NaBH4 for the catalytic reduction process.43 Third, it has been confirmed in our previous study that there usually presents a unique adsorption behavior of the nitrogen-enriched CPPz particles toward acidic dyes such as MB by an acid−base interaction.44 Thus, in this case the observable effective adsorption of MB by PZAF particles may play an effect in accelerating the diffusion and enrichment of MB molecules around the Ag catalysts, as described by the cartoon shown in Scheme 3, which is an important factor for achieving the rapid

cally with time, as shown in Figure 4f. In contrast, there is no distinct color change for the bare droplet of MB/NaBH4 aqueous solutions in the absence of the encapsulating Ag/ PZAF particle shell (Figure 4a,d) and only partial color fading for the traditional catalytic mode in which the hydrophobic Ag/ PZAF particles were directly spread over the liquid surface of the MB/NaBH4 aqueous solution (Figure 4b,e), in 30 min. It is suggested qualitatively that the reduction of MB to leuco methylene blue by NaBH442 can be highly efficiently catalyzed in the Ag/PZAF particle-based liquid marbles. In order to evaluate the catalytic capacity of the liquid marble-based miniature reactors quantitatively, the kinetic curves of MB reduction were plotted, as shown in Figure 5a.

Scheme 3. Illustration for the Efficient Mass Transfer of MB from Bulk Solution to the Ag Catalytic Active Sites Facilitated by the Adsorption of MB onto PZAF Particles via Acid−Base Interaction

Figure 5. Reduction kinetic curves (a) and first-order linear plots of ln(A/A0) versus time (b) of MB solutions in the form of bare droplet without Ag catalysis, traditional catalytic mode, and Ag/PZAF particlebased catalytic liquid marbles.

The reduction efficiency (%) was determined using the following equation ⎛ A⎞ Reduction efficiency (%) = ⎜1 − ⎟ × 100% A0 ⎠ ⎝

(3)

where A is the absorbance of the solutions at 664 nm at time t, and A0 is the initial absorbance. It can be observed that the nonencapsulated droplet exhibits unconspicuous reduction of MB by NaBH4 due to the lack of heterogeneous Ag-based catalysis. The reduction efficiency is as low as 25.6% even after 25 min. The MB reduction rate of the traditional catalytic mode is also slow, and only 57.7% of MB is catalytically reduced to nonabsorbing products within 25 min. For the Ag/PZAF particle-based catalytic liquid marbles, by contrast a drastic reduction of MB is depicted. At t ≥ 25 min, the reduction efficiency reaches nearly 100%. The kinetic data of the MB reduction were further fitted to first-order reaction rate

kinetics of a heterogeneous catalytic reaction. Although the catalytic efficiency of the Ag/PZAF particle-based liquid marbles is lower than that of the silver colloids dispersed directly in the reaction solution,42 the utilization of the catalytic liquid marbles in heterocatalysis has a significant strength in the easy recovery of catalysts. The catalytic performance of the Ag/PZAF particle-based liquid marbles with different volumes (25, 50, 100, and 200 μL) F

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Langmuir toward the MB reduction was investigated. The UV−vis absorption spectra of the MB solutions extracted from the four catalytic liquid marbles at 0 and 14 min are recorded in Figure S8, and the obtained catalytic efficiency of the four different liquid marbles are compared as shown in Figure 6. After 14

Figure 7. Cycle use performance of the Ag/PZAF particle-based catalytic liquid marbles on MB reduction for 10 cycles. The volume of the liquid marbles is 100 μL and the reduction time is 25 min for each cycle. Figure 6. Correlation of the MB reduction efficiency with the size of the Ag/PZAF particle-based catalytic liquid marbles. The reduction was conducted for 14 min.

By further characterizing the recycled Ag/PZAF particles after 10 cycles with FT-IR and UV−vis spectroscopy, it is found that there are no visible characteristic absorption peaks belonging to MB in both the FT-IR spectrum (Figure S9a) and UV−vis absorption spectrum (Figure S9b) of the recycled Ag/PZAF particles. It is strongly demonstrated that the blue color fading of the MB solution in the liquid marbles is a result of catalytic MB reduction instead of pure MB adsorption by PZAF particles. The adsorption behavior of PZAF particles toward MB just produces an effect on accelerating the reduction kinetics. Once the adsorbed MB is reduced to the leuco form, it can no longer be adsorbed effectively on the PZAF particles by acid−base interaction, due to the lack of cation structure.44 It makes the PZAF particles maintain their adsorption capacity toward MB in the following reaction cycles. Thus, the Ag/PZAF particle-based liquid marbles can highly efficiently catalyze the reduction of MB with good cycling stability.

min, the reduction efficiency are 97.8, 92.1, 88.6, and 75.5% for 25, 50, 100, and 200 μL liquid marbles, respectively. That is, the catalytic efficiency of the liquid marble-based miniature reactors decreases with increasing the liquid marble size. It can be attributed to a decrease of surface area-to-volume ratio with increasing the volume of reaction solutions. Nevertheless, it is found that the decline of catalytic efficiency with respect to the increase of liquid marble volume in our cases is not as steep as the previously reported Ag nanowire-based catalytic liquid marbles.13 For the literature, the MB reduction efficiency with 14 min showed a near 49% decline when the volume of the catalytic liquid marbles was increased from 20 to 80 μL. In contrast, there is only a 23% decrease in reduction efficiency from 25 μL to even 200 μL, a much larger size, in our study. Such distinction may be due to the unique adsorption of MB by the particle shell for our catalytic liquid marbles, which can allow a high-efficiency mass transfer of MB to the catalytic active sites even at large volumes of marbles. Although the smaller liquid marbles are expected to exhibit higher catalytic efficiency, the larger ones, such as 100 μL liquid marbles, are more suitable as miniature reactors for practical application, by considering their more accurate control in volume as well as easier manipulation. It is also suggested that the catalytic capacity of the Ag/PZAF particle-based liquid marbles can be modulated facilely by changing the marble size. The cycle use performance of the Ag/PZAF particle-based catalytic liquid marbles with 100 μL volume was examined. After the reduction of MB for 25 min, the Ag/PZAF particle powder adsorbed on the liquid marbles was recycled easily by withdrawing the interior solution, followed by a rapid drying. Then the new catalytic liquid marbles were formed by rerolling the new droplets of the MB/NaBH4 aqueous solution on the powder bed of the recycled particles. The cyclic experiment was carried out for 10 cycles. As shown in Figure 7, the Ag/PZAF particles exhibits remarkable recyclability and reusability. Using the recycled Ag/PZAF particles for the subsequent formation of catalytic liquid marbles, the reduction efficiency toward MB can maintain a rather high value about 99% after 10 reaction cycles, indicating a long cycle life without losing the catalytic efficiency.

4. CONCLUSIONS In summary, hydrophobic cyclomatrix polyphosphazene particles, PZAF, demonstrated their ability to serve as the stabilizers for forming liquid marbles. Catalytic liquid marbles were further prepared successfully using Ag/PZAF as the encapsulating shells. The catalytic liquid marbles exhibited high-efficiency catalytic performance on the reduction of encapsulated MB aqueous solution by NaBH4. The remarkable catalytic capacity is mainly contributed by the good dispersion and stable immobilization of Ag nanoparticles onto PZAF particles allowing the high catalytic surface area, as well as the adsorption of MB by PZAF particles effectively accelerating the mass transfer of MB to the Ag catalytic active sites. The ensemble of benefits also enables the catalytic liquid marbles to show a relatively high catalytic efficiency at large volumes of marbles and excellent reusability over 10 reaction cycles. Therefore, the Ag/PZAF particle-based catalytic liquid marbles have been demonstrated to be a kind of promising miniature reactors for heterogeneous catalytic reactions.

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FT-IR, XPS, TGA, and size distribution of PZAF particles; properties of internal water volatilization and external gas transport of the PZAF particle-based liquid marbles; UV−vis characterization of the coordination of PZAF particles to Ag+ ions; fabrication process of the Ag/PZAF particle-based catalytic liquid marble traced by digital photographs; UV−vis comparison of the catalytic efficiency of the Ag/PZAF particle-based liquid marbles with different volumes; FT-IR and UV−vis characterization of the recycled Ag/PZAF particles after 10 reduction cycles. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-510-85917763. Tel: 86510-85917763. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Nature Science Foundation of China (21404049), the Fundamental Research Funds for the Central Universities (JUSRP51305A), and MOE and SAFEA for the 111 Project (B13025).

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DOI: 10.1021/acs.langmuir.5b04697 Langmuir XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.langmuir.5b04697 Langmuir XXXX, XXX, XXX−XXX