Liquid Interfaces Generated in a Microfluidic Device

Article pubs.acs.org/JPCC

Droplet Liquid/Liquid Interfaces Generated in a Microfluidic Device for Assembling Janus Inorganic Nanohybrids Natalia Hassan,†,‡,# Antonio Stocco,§,∥,⊥ and Ali Abou-Hassan*,†,‡ †

Sorbonne Universités, UPMC Université Paris 06, Laboratoire de PHysico-chimie des Electrolytes et Nanosystèmes InterfaciauX (PHENIX), UMR 8234, Équipe Colloïdes Inorganiques, Bat F(74), case 51, 4 place Jussieu, F-75252 Paris Cedex 05, France ‡ CNRS, Laboratoire de Physico-chimie des Electrolytes et Nanosystèmes InterfaciauX (PHENIX), UMR 8234, F-75252, Paris Cedex 05, France § Université Montpellier, Laboratoire Charles Coulomb UMR 5221, F-34095, Montpellier, France ∥ CNRS, Laboratoire Charles Coulomb UMR 5221, F-34095, Montpellier, France ⊥ DWI − Leibniz-Institut für Interaktive Materialien, Aachen 52056, Germany S Supporting Information *

ABSTRACT: One among other remarkable methods to produce multifunctional assemblies with different spatial organizations is the use of liquid−liquid (L−L) interfaces. Herein, a droplet microfluidic-based method is reported as a strategy for the assembly of asymmetrical inorganic nanohybrid structures. As a proof of concept and motivated by their wide applications in different fields, we studied the assembly of two building nanoblocks, which are fluorescent silica (160 nm diameter) and gold nanoparticles (15 nm diameter). In this strategy, droplets of an aqueous solution of citrated gold nanoparticles are generated in a continuous flow of amine functionalized fluorescent silica nanoparticles dispersed in cyclohexane using the microdevice. The electrostatic attraction between the two nanoparticles confined at the water/cyclohexane interface to form a Pickering emulsion allowed their assembly. We show that Janus nanohybrids can only be observed when the residence time in the microdevice was less than 30 min, thus avoiding the formation of solid shells for longer residence times. Transmission and scanning electron microscopies, optical microscopies, and UV−vis spectroscopy were used to characterize the resulting assemblies. The results were compared to experiments in bulk which showed that microfluidics offers a higher control over the assembly and reduces the time for their elaboration. Moreover, an analytical model based on transport of nanoparticles and their adsorption onto interfaces is used to rationalize our observations. Both flow recirculation inside and outside the droplets in the microchannel and the confinement effect seem to be relevant for the enhanced nanoparticle transport to the interfaces.

1. INTRODUCTION

ideal templates for the assembly of different structures including dissymmetrical micro and nanoassemblies.4−11 However, the transport of nanoparticles toward the interface is a very slow process and the adsorption to obtain the final interfacial assemblies can last very long.12−14 Additionally, for planar or macroscopic droplet interfaces, due to limited areas, low amount of assemblies can be produced using these strategies, which represents the main constraint for further applications.15,16 Digital or droplet microfluidic devices may help to overcome these limitations. Indeed droplets with high reproducibility, monodispersity and controlled interfaces resulting from shear forces and interfacial tension at L−L interfaces can be generated at rates of up to several kHz, while maintaining exquisite control over both droplet size and droplet compositions.17,18 Microfluidic strategies have been successfully

Hybrid nanoparticles (NPs) composed of multiple components with several functionalities have attracted remarkable interest due to their unique properties and applications that are difficult to find from single-component nanoparticles. Hybrid nanostructures with discrete domains of different materials arranged in a controlled fashion can be obtained using self-assembly of individual nanoblocks. Thus, different functionalities can be integrated into a single nanoparticle to form multifunctional materials.1 Gold NPs (AuNPs) and their assemblies have attracted a lot of attention from fundamental and applied perspectives. They are used in several applications ranging from rapid assessment of water quality to biomedical imaging and photothermal therapy.2,3 One among other remarkable methods to produce multifunctional assemblies with different spatial organizations is to use liquid−liquid (L−L) interfaces. In this field, planar and curved macroscopic immiscible L−L interfaces such as the ones forming the droplets of Pickering emulsions have been used as © XXXX American Chemical Society

Received: March 16, 2015 Revised: April 24, 2015

A

DOI: 10.1021/acs.jpcc.5b02527 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Schematic Representation of the Microfluidic Strategy Used to Elaborate Assemblies at the Interface of Microdroplets Produced by Emulsification of an Aqueous Suspension of Gold Nanoparticles (AuNPs) in an Organic Phase Composed of a Suspension of Fluorescent Silica Nanoparticles (SiO2 NPs) in Cyclohexanea

a

The nanoparticles meet at the interface and assemble by electrostatic interaction to form Janus nanoparticles.

Milli-Q water. The mixture was added to 47.5 mL of boiling Milli-Q. Refluxing was continuing until a change to a reddish color was observed. This procedure resulted in a citratestabilized colloid of AuNPs well-dispersed in water with a final concentration of 2 × 1018 Nps/L; the particles had a narrow size distribution, as confirmed by transmission electron micrscopy (TEM) (Supporting Information (SI), Figure S-1). 2.3. Synthesis and Functionalization of RITC-Doped Silica Nanoparticles (165 ± 8 nm). Silica nanoparticles were prepared by a variation of the typical Stöber-based synthesis method changing fluorescein isothiocyanate (FITC) for rodhamine B isothyocyanate (RITC, λabsorption, max= 555 nm).31,32 To prepare the silica cores, RITC was first covalently linked to the silane coupling agent (3-aminopropyl)triethoxysilane (APTS). APTS (0.20 mL) was added to RITC (0.0404 g) in anhydrous ethanol (5 mL) medium. The reaction proceeded for 20 h in the dark with magnetic stirring. Then, a solution of the prepared APTS-RITC conjugates (0.5 mL) was added to a clean glass reaction vessel containing anhydrous ethanol (0.20 mL) and ammonium hydroxide (28%, 2.0 mL). TEOS (1 mL) was added and the mixture was stirred for 24 h in the dark (SI, Figure S-2). 2.4. Functionalization of the Silica Surface. To modify the surface with propyl groups and amine groups, we followed the work of Westcott et al.33 with some modifications. 0.04 mL (0.287 mmol) of propyltrimetoxysilane (PTMS) followed by 0.01 mL (0.045 mmol) of (3-aminopropyl)triethoxysilane (APTS) were added to the prepared silica suspensions, thus giving a molar ratio of PTMS/APTS = 6.4. The suspensions were stirred for another 24 h. After the reaction, the prepared samples were centrifuged at 10 000 rpm for 15 min to collect the silica cores. The cores were further washed with ethanol and deionized water after centrifugation and decantation several times to remove the unreacted chemicals. The obtained fluorescent silica particles were dried in a desiccator before

implemented for the generation of particle-stabilized emulsions, foams, colloidosomes, and capsules from micro- and nanoparticles, and the adsorption dynamics have been well established and quantified.8,19−23 A superior level of control was demonstrated due to shear interfacial forces and advection that enhance the mass transfer of nanoparticles and increase their ability to stabilize fluid interfaces.8,19−23 Motivated by different works on the preparation of Janus nanoparticles from Pickering emulsions,10,11,24,25 herein, we investigated the possibility to generate such Janus nanoassemblies using microfluidic L−L droplet interfaces by confining at the interface two types of nano building blocks of different chemical and reactive surface functionalities. As a concept of proof, fluorescent silica NPs having surface amine group and citrated AuNPs are used in this study in order to study their assembly at the interfaces. The motivation behind these combinations is the wide applications resulting from the arrangement of the two kinds of NPs in different applications including the fields of medicine and biology.26−29

2. MATERIALS AND METHODS 2.1. Materials. Chloroauric acid (HAuCl4·nH2O), silver nitrate (AgNO3) and trisodium citrate (Na3C16H5O7), Rodhamine B isothyocyanate, (3-Aminopropyl)triethoxysilane (APTES), ammonium hydroxide (28%), tetraethyl orthosilicate (TEOS), propyltrimetoxysilane (PTMS), absolute ethanol and cyclohexane were purchased from Sigma-Aldrich and used as received. 2.2. Synthesis of AuNPs. AuNPs with a diameter of 16.5 ± 3.3 nm were prepared according to the work of Xia et al.30 Briefly, 0.5 mL of 1 wt % of HAuCl4 and 42.5 μL of 0.1 wt % of AgNO3 and 1.5 mL 1% of trisodium citrate (Na3C16H5O7) were mixed, and the volume was completed to 2.5 mL with B


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fluorescent, silica NPs adsorption onto the L−L interface can be monitored by fluorescence microscopy. As depicted in Scheme 1, the aqueous suspension of AuNPs was injected in the center of the microdevice as the dispersed phase, while the suspension of silica NPs in cyclohexane (organic phase) was the continuous phase. At the point of confluence aqueous droplets of AuNPs dispersed in the suspension of silica NPs in cyclohexane are continuously generated. Little difference was observed in the deformation of drops freshly formed in cylclohexane or particle dispersions. Such results have been observed by Priest et al.22 and suggest that the silica and AuNPs do not play a central role in the early stages of droplet formation. Once the droplets have been created, it takes a predictable amount of time for the interface to become fully populated with particles. The surface coverage of droplet interfaces with silica NPs can be tuned independently by changing the concentration c of silica NPs or the size of the droplets (see section 3.3). Herein we operated in slug flow regime to maximize the adsorption of silica NPs to the interface. The size of slugs and the concentration of silica NPs were maintained constant; however, we varied the residence time (tR) by modifying the length of the Teflon tube. Slugs with approximately 800 μm length were generated by setting the volumetric flow rate ratio of silica NPs to AuNPs to one. The total volumetric flow rate was fixed to Q = 20 μL/min, which is the minimum flow rate that allowed experimentally for observing stable flow and reproducible slug generation. Herein the length of the Teflon tube was varied between 16 cm and 6 m, which gives tR ranging between 3 and 115 min, respectively. Once silica Nps reach the interface, their protonated amine groups will attract negatively charged AuNps. Figure 1a shows an epifluorescence optical microscopy image of the slugs (length ≈ 800 μm) during their continuous generation in the microfluidic device. The interior of the droplets appears darker as it contains AuNPs while the exterior of the droplets is fluorescent due to the presence of silica NPs. A small intensity increase in the shell of the slugs can be detected, which is due to NP adsorption at the interface; however, the size of slugs is unchanged compared to freshly generated ones. In contrast, after 70 min, the fluorescence was more pronounced in the shell of the slugs reflecting a high adsorption of silica NPs at interfaces. Moreover, the accumulation of NPs at the interface did not result in any change in the size or form of slugs. No breakup of slugs in this regime was observed as reported at high flow rates for air bubbles by other groups.19,20 At the exit of Teflon tubes of different lengths (for different tR) the flow was collected and the different phases were characterized by fluorescence and confocal fluorescence microscopies, UV−vis spectroscopy, TEM, and SEM. As tR decreases, the number of free silica and AuNPs in the cyclohexane and aqueous phases (deduced from UV−vis spectroscopy) increases. For tR* < 30 min and at the exit of the channel, droplets formed in the microchannel are not stable. Demulsification and coalescence can be observed after ≈1 h that resulted in the formation of a thin particle layer at the cyclohexane−aqueous interface. This was confirmed by the presence of emulsions undertaken coalescence when the interface separating the two phases was transferred on a glass slide and checked by epifluorescence optical microscopy (Figure 1b). When the residence time increases, the number of free nanoparticles decreases, and for tR ≈ 75 min the number

used. The silica nanoparticles were very stable in cyclohexane, which proves the presence of PTMS groups on the surface. A stock solution was prepared with a final concentration of 1.06 × 1014 Nps/L or 0.5 g/L (considering a density of silica of 2 g/ mL). The presence of amine groups on the surface of the silica was confirmed by IR spectroscopy: two broad N−H stretching bands above 3000 cm−1, and a very broad and intense feature between 3000 and 2000 cm−1, which is caused by stretching vibrations of H-bonded NH2 and NH3+ groups, and an N−H deformation vibration at 1595 cm−1 were observed. 2.5. Characterizations. Optical spectra were obtained on an Avaspec-USB2 UV−vis−NIR Spectrometer. Fluorescence microscopy images were observed by using a Zeiss Axiovert 200 microscope (X 40, NA 0.65, HBO 100) and rhodamine filter set with pictures taken by a high speed CCD camera (300 kHz) and digitalized on a computer. Confocal fluorescent images were acquired on Zeiss LSM 510 Meta microscope equipped with an Axiovert 200 M microscope. All TEM images were obtained by using a JEOL 10 CX instrument (100 kV). Scanning electron microscopy (SEM) images were observed on a SEM-FEG ZEISS ULTRA 55 apparatus. 2.6. Microreactor (μR). For the assembly of fluorescent and plasmonic NPs (silica and Au) a T-junction (UpChurch Scientific, ref: P-728) with a 0.50 mm thru-hole and including ferrules for 1/16″ = 1.59 mm outer diameter capillaries (ref: F151) was purchased from UpChurch Scientific. The internal capillary was a fused silica tube (inside diameter ID = 150 μm; outer diameter OD = 360 μm; Polymicro technologies). The external capillary was a Teflon tube (UpChurch Scientific, ID = 700 μm; OD = 1/16″ = 1.59 mm, and length L = 40 cm). The alignment of the internal capillary in the outer capillary was possible by inserting the internal tube in a tubing sleeve (UpChurch Scientific, ref: 185X). The cyclohexane phase containing the silica nanoparticles (continuous phase) was injected perpendicular to the main axis of the T-junction while the citrated aqueous suspension of Au nanoparticles (dispersed phase) was injected via the fused silica capillary using syringe pumps (Harvard Apparatus, USA, PHD 2000 series). The flow rates of the aqueous phase continuous phase were varied from 10 to 100 μL/min to find the best parameters for slug generation.

3. RESULTS AND DISCUSSION 3.1. Assembly of Bifunctional Pickering Emulsions. In a first step we studied the formation of Pickering emulsions in the microfluidic channel. Scheme 1 presents the process of assembly of fluorescent silica NPs and plasmonic AuNPs at L− L interfaces. Teflon microreactor was used in this study since high adsorption of silica NPs on glass microreactors can be observed even after hydrophibization of the channel. Moreover working with Teflon microreactors and compared to glass ones allow us easily and when needed to increase residence times basically by injecting lower flow rates or simply by increasing the length of Teflon tube. Silica NPs were functionalized with propyltrimetoxysilane (PTMS) and (3-aminopropyl)triethoxysilane (APTES)33 to allow their dispersion in cyclohexane, while maintaining a possible efficient interaction through electrostatic attraction at the cyclohexane/water interface between the positively charged silica NPs and the negatively charged AuNPs. Also, as documented, the presence of hydrophobic silane groups can increase the interfacial activity of silica NPs and their adsorption at the oil/water interface.34−38 Moreover being C


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droplets that create a thin particle layer at the interface for short residence times (tR* < 30 min). In this way we avoided the formation of highly stable solid shells. The interface obtained after 1 h (time necessary for complete demulsification) was isolated. After several cycles of ultrasonication, centrifugation, and washing in water, the resulting NPs were transferred on a copper grid and analyzed by TEM. TEM images (Figure 2a−f) confirmed the attachment of

Figure 1. (a) Epifluorescence optical microscopy image showing the first steps (after the point of confluence) during generation of microdroplets inside the microdevice with AuNPs and fluorescent silica NPs inside and outside the droplets, respectively. (b) Epifluorescence optical microscopy image of the interface during spontaneous breaking of emulsion (demulsification) and after transfer on a glass slide for tR* < 30 min. (c) Epifluorescence microscopy image of a bifunctionnal Pickering emulsion. (d) Confocal microscopy image taken at the middle of the Pickering emulsion during evaporation. (e) Low magnification SEM image of the Pickering emulsion after drying under vacuum showing a collapsed and hollow colloidosome. (f) SEM zoom on image e; the same image and more SEM images are provided at higher magnification in the SI (Figure S3). (g) High magnification SEM image showing some dissymmetrical assemblies.

of silica NPs and AuNPs was ≈5% and 56% (as deduced from UV−vis) in the organic and aqueous phases, respectively. Moreover, a third turbid middle fluorescent layer separating the cyclohexane and water phases was observed, which was formed by Pickering droplets armored by fluorescent silica as it can be seen by epifluorescence and confocal microscopies (Figure 1c,d). SEM was used to image the resulting Pickering emulsions after vacuum drying (Figure 1e−g). Low magnification images confirmed the presence of AuNPs and silica NPs after drying of the Pickering emulsion and formation of tightly packed and bridged silica in the shell of droplets. Moreover, high magnification images clearly show the presence of silica NPs decorated with AuNPs (Figure 1f,g, and Figure S-3 in the SI). Bridges between silica NPs may have been created by condensation of amine groups or due to different interactions of nanoparticles at fluid interfaces.39,40 Also some artifacts may be brought by the evaporation of solvents during the preparation of samples. 3.2. From Pickering Emulsions to Janus Nanoparticles. It was demonstrated in a very recent work that Janus nanoparticles can be obtained from solid shells.41 Thus, motivated by this work, the bifunctional Pickering emulsions were ultrasonicated for several hours, centrifuged, and washed several times with water. However, because of the strong interaction between particles at the fluid interface (and in agreement with SEM images in Figure 1), we could not obtain any individual and separated Janus silica−Au nanostructures. Thus, in the next step we focused our attention on nonstable

Figure 2. (a−f) Representative TEM images of SiO2−Au Janus NPs formed at water−cyclohexane interface of microdroplets generated in our microfluidic device for tR* < 30 min. More representatitive TEM images are provided in the SI.

AuNPs on the silica and the formation of Janus nanostructures in the microfluidic system (SI, Figure S-4) as only one side of the surface of silica NPs (≈ 20−30% estimated from TEM) is covered with AuNPs.42−44 Since the assembly is driven essentially by electrostatic interaction between AuNPs and silica NPs at the L−L interface, the control of the surface coverage is due to the interplay between electrostatic attraction and the contact angle of silica NPs at the oil−water interface, θOW. Actually, we can distinguish two different areas of the silica NPs where AuNPs are present or not. Hence one could estimate the area of the silica NPs wetted by water, where AuNPs were dispersed, that gives an estimate of the contact angle θOW. From Figure 2, θOW agrees with the value of ca. 110° (measured from the water side) reported by Binks and Clint for similar silica particles at the cyclohexane−water interface.45 Note also that AuNPs might not be fully packed at the (silica NPs and also the fluid) interface given their small sizes, R ≈ 8 nm. As a matter of fact, the adsorption energy ΔE scales with R2 and a value of ΔE ≤ −20 kT46 might lead to a incomplete coverage and a reversible adsorption.13 In contrast for silica NPs, R ≈ 82 nm corresponds to a highly irreversible adsorption D


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The Journal of Physical Chemistry C being ΔE ≈ 105 kT. UV−vis spectroscopy of the Janus suspension in water demonstrated the presence of a peak at 525 nm (see SI, Figure S-5). This peak was originally absent in the spectrum of silica NPs, and, compared to free AuNPs, it shifted, confirming thus the presence of closely associated AuNPs on the surface of silica NPs.33 UV−vis spectra and TEM images of the same Janus nanostructures, remained unchanged after several cycles of ultrasonication, centrifugation, and washing, which prove that the nanostructures were formed by irreversible assembly and not by a random adsorption of AuNps during centrifugation or evaporation on the TEM grids. We compared our microfluidic method for elaborating Janus nanohybrids to the bulk method by using L−L interfaces after emulsification.24,47 Silica nanoparticles (165 nm average diameter) were functionalized with PTMS:APTES (80:20 v/ v, used in this investigation) and 100% PTMS were used in each study. Also, silica nanoparticles (165 nm average diameter) functionalized with 100% APTES served as a reference for symmetrical assembly. The colloidal suspensions of silica in cyclohexane and citrated AuNPs in functionalized used in the microfluidic study were introduced in a volumetric ratio (vSiO2/vAu ≈ 1) and emulsified by mechanical mixing for 30 min. They were kept in contact for 2 h. The interface between the two immiscible liquids (or the mixture in the case of 100%APTES) was transferred on a carbon grid and analyzed by TEM after different contact times. As can be seen from TEM (Figure 3a), for silica functionalized with 100% APTES, the surface is symmetrically

surface coverage of AuNPs to the surface of silica NPs was detected, only after 2 h (Figure 3b). The difference between the two elaborations indicates that in the absence of microfluidic flow recirculation (see Scheme 1), silica NPs and AuNPs cannot adsorb efficiently onto the L−L interface to form Janus hybrids. For macroscopic curved interfaces generated by bulk emulsification, NPs tranfer to the interface is not a finely controlled process due to the broad distribution of flows and forces and the polydispersity of generated drops.48,49 Our results show clearly that only Janus structures are dominant when using our microfluidic approach for short residence times. 3.3. Nanoparticle Adsorption onto the L−L Interface. In order to rationalize our findings, in this section we estimate the NP adsorption onto the L−L interface and the contribution due to flows present in the microfluidic channels. In a first approximation, this process can be modeled in two steps.50 The first one consists in the particle transport along the streamlines due to advection, which enables the nanoparticles to come close to the interface. The second step is the adsorption onto the interface, which occurs normal to the streamlines and it is due to diffusion. Hence the contribution of hydrodynamic flows in microchannels is to accelerate the first step of the transport but it does not influence the final adsorption step.50 In absence of any driven flow, in the early stages of adsorption by diffusion, the surface coverage Θ of nanoparticles at the fluid interface changes as43

dΘ = −πR2j dt

(1)

where R is the nanoparticle radius and j is the flux of NPs (in NPs/(m2·s) units), j depends only on the concentration c of the interfacial subphase, which in absence of any driven flow is the bulk concentration cBULK, and the diffusion coefficient D, j = −c(D/πt)1/2; see Figure 4. Thus, the surface coverage becomes43 Θ = −πR2

∫0

t

j dt = 2πR2c

Dt π

(2)

The diffusion coefficient D = kT/(6πηR) (where k is the Boltzmann constant, T is the temperature, η is the viscosity of the fluid in which the particles are suspended) of AuNPs and silica NPs are respectively 2.7 × 10−11 m2/s and 2.8 × 10−12 m2/s in water and cyclohexane at the temperature of the experiment (20 °C). From eq 2 we can estimate the diffusion time needed to obtain a significant coverage of the interface. Given the relatively high concentration (2 × 1018 Nps/L) and diffusion, AuNPs will require less than a second to significantly cover the interface (see Figure 5). While for silica NPs (c = 1 × 1014 NPs/L) to achieve a surface coverage Θ = 0.2, about 40 min will be required, as it is shown in Figure 5. The latter time clearly represents a limitation for an efficient adsorption and assembly of NPs at the interface. Note that these times are estimated by accounting that the concentration of nanoparticles in the subphase remains equal to the bulk concentration. This is clearly not true in our microfluidic device because advection tends to increase the concentration close to the surface. Accounting for the contribution of hydrodynamic flows to the whole adsorption process, we calculate the Peclet number, which describes the competition between advection and

Figure 3. TEM images of SiO2−Au assemblies synthesized in bulk after 2h with a volumetric ratio of silica on AuNps vSiO2/vAu ≈ 1 with (a) silica nanoparticles functionalized 100% APTES, (b) silica functionalized with 80% PTMS and 20% APTES (used in this study) at the cyclohexane/water interface, and (c) silica functionalized with 100% PTMS at the cyclohexane/water interface.

covered with AuNPs. While in the case of silica functionalized with 100% PTMS (Figure 3c), TEM images show no interaction with AuNPs. In the case of silica functionalized with a mixture of PTMS:APTES, after a contact time ≈30 min, similar to the residence time in the microdevice, only free silica and AuNPs could be observed. However, random attachment and a low E


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m/s ≈ Vdrop is the velocity (Q = 20 μL/min is the volumetric flow; see section 3.1). Vdrop − Vext ≈ Ca2/3 Vdrop, corresponds to a small (the velocity difference is 0.07% of the drop velocity, (Vdrop − Vext) ≈ 5.8 × 10−7 m/s) but significant flux QE = S (Vdrop − Vext) between the droplets and the external fluid. Now we can estimate the Peclet numbers in the lubrication film and end-cap regions: Pe = (Vdrop − Vext)l /D

(3)

where l is a characteristic length. In the lubrication film region, the characteristic length is the film thickness h, Pelf = 0.05, whereas in the end-cap region, the characteristic length is the microchannel radius ID/2, Peec = 71.20 In the lubrication film region, Pelf < 1, and diffusion rules the transport and nanoparticle attachment onto the interface. Note the film thickness ≈234 nm is of the same order of magnitude of the silica nanoparticle (165 nm). Hence this adsorption process must be very efficient, and once a silica nanoparticle enters in the lubrication film region, only a very small amount of time t ≈ (h2/2D)1/2 = 0.1 s is needed to attach onto the interface. Going back to the end-cap region, one could estimate a transfer coefficient within the framework of a phenomenological boundary layer model, which describes the coupling between advection and diffusion at Pe > 1. Following the same approach of Kotula and Anna,20 the particle flux in the boundary layer is jBL = DcBULK/d, where d is the thickness of the boundary layer d = (4/3Pe)1/3 ID/2 = 93 μm. The corresponding surface coverage is

Figure 4. Sketch of the lubrication film and end-cap regions and the control parameters of adsorption. In adsorption by diffusion, the surface concentration (i.e., particles per unit area) is proportional to the subphase concentration and on a characteristic length of diffusion L ∼ (Dt)1/2. The subphase concentration is equal to the bulk concentration when advection is absent. In the presence of advection, particles can accumulate in the subphase, increasing the concentration c.

Θ = πR2

Dc BULK t d

(4)

The surface coverage calculated using eq 4 is plotted in Figure 5. One can see that the estimation made by the boundary layer model can not explain the enhanced attachment of nanoparticle onto the interface we observed in our microfluidic device, being the time needed for adsorption of the same order of magnitude the time calculated from a purely diffusion model (i.e., 40 min, eq 2). This might be due to an overestimation of the boundary layer thickness. Hence, we reconsider eq 2, and we estimate the concentration c of the subphase in the presence of advection (see Figure 4). To calculate this concentration, we consider a volume of section S and thickness equal to the silica nanoparticle diameter (2R). Now we make the crude assumption that due to the net flux QE = S(Vdrop − Vext), all NPs accumulate on the subphase in the end-cap region. Within this approximation, one finds that

Figure 5. Nanoparticle surface coverage at the L−L interface for AuNPs (small red circles) and silica NPs (big gray circles) calculated by a diffusion model (see eq 2). The solid line is the surface coverage calculated from a boundary layer model in the end-cap region. The dash line considers a diffusion model accounting for accumulation of nanoparticle in the subphase due to advection.

diffusion. In our geometry, we can distinguish two different regions where nanoparticle adsorption occurs. One is the lubrication film region, and the other is the end-cap region (see Figure 4). Due to the presence of the lubrication film, a net flux is created between the external fluid and the droplet, which move at two different mean velocities Vext and Vdrop, respectively. The velocity difference is proportional to the thickness of the lubrication film, which for low capillary numbers is51 h ≈ Ca2/3 (ID/2) = 234 nm, where Ca is the capillary number (Figure 4). Ca = ηU/γ ≈ η(Q/S)/γ = 1.73 × 10−5 (η ≈ 10−3 Pa.s, and the surface tension γ ≈ 50 × 10−3 N/m), S = π (ID/2)2 = 3.85 × 10−7 m2 is the channel’s section, and U = Q/S = 8.65 × 10−4

c = c BULK + c BULK

S(Vdrop − Vext) S(2R )

⎛ (Vdrop − Vext) ⎞ t ⎟⎟ = c BULK ⎜⎜1 + 2R ⎝ ⎠

t

(5)

Clearly, the linear increase of the subphase concentration will be not infinite, and it will depend on the reservoir of silica nanoparticles present between two droplets. In our case, the distance between two droplets is Ld = 100−200 μm (see Figure 1a), which correspond to a saturation time Ld/(Vdrop − Vext) = 170−340 s. F


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Sandra Casal for TEM. A.S. gratefully acknowledges the Alexander von Humboldt foundation for a research fellowship.

Plugging the subphase concentration estimated in eq 5 in eq 2, one finds a strongly increase of the surface coverage when compared to the one predicted by assuming that the subphase concentration remains equal to the bulk (see Figure 5). Now accounting for advection, the time needed to obtain a significant coverage of silica NPs at the L−L interface reduces from 40 min to a few seconds. The latter time should be considered as the fastest possible time obtainable when all NPs accumulate on the subphase and the particle attachment onto the surface follows a purely diffusive process. It is clear that this will be not the usual case since nanoparticles could diffuse in the lubrication film region or they could be moved away from the interface due to recirculation flows. Moreover, the attachment of NPs onto the fluid interface may be slowed down by adsorption energy barrier and friction due to wetting dynamics.12,36

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4. CONCLUSION In conclusion, we have demonstrated for the first time the elaboration of Janus fluorescent and plasmonic nanohybrids by using L−L interfaces of microdroplets generated in a microfluidic device. Bifunctional Pickering emulsions were obtained by confining the silica and gold nanoparticles at the interface. By manipulating the residence time in the microdevice, and for low residence times Pickering emulsions demulsified and individual Janus nanohybrids can be isolated. Compared to the assembly in bulk, the time necessary to observe Janus nanostructures in the microdevice was reduced, which can be explained by the effect of both flow recirculations inside and outside the droplets in the microchannel and the confinement effect given by the lubrication film that enhanced nanoparticle transport to the interfaces. These results confirm again the high potential of microfluidic reactors for the fast elaboration of dissymmetrical nanomaterials. Moreover, our approach can be generalized and applied to the engineering of particles with specific surface modifications, for example, using chemical reactions or by grafting of functional molecules or polymers.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of silica and gold nanoparticles, SEM images of dried Pickering emulsions, TEM images of Janus nanohybrids, UV−vis spectra of Janus nanoparticles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02527.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+33) 1-44-27-3174. Fax: (+33) 1-44-27-32- 28. Present Address

Laboratorio de Nanobiotecnologia,́ Facultad de Ciencias ́ Quimicas y Farmacéuticas, Universidad de Chile, Santiago, Chile. #

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from Émergence-UPMC-2011. We thank Dr. Sophie Neveu, Aude Michel from PHENIX lab, and G


Article

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Liquid Interfaces Generated in a Microfluidic Device

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