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Electricity-free picoinjection assisted droplet microfluidics
Sensors & Actuators: B. Chemical 298 (2019) 126766
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Electricity-free picoinjection assisted droplet microfluidics Hao Yuan a b
a,b
, Yi Pan
a,b
a,b
, Jingxuan Tian
, Youchuang Chao
a,b
a,b
, Jingmei Li
, Ho Cheung Shum
a,b,⁎
T
HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, 518057, China Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Droplet microfluidics Picoinjection Droplet microfluidic hydrodynamics Droplet microfluidic material syntheses
Using droplet microfluidic picoinjection as a reactant dosing technique is of great importance in assembling artificial cells and performing multistep reactions. However, the utilization of electricity in the existing picoinjection complicates the device fabrication and operation, and compromises the bioactivity of the encapsulated bio-ingredients. In this work, we propose an electricity-free picoinjection technique as an alternative to address these issues. Specifically, by precisely controlling the pressures inside the microfluidic channel, we can inject one reactant into the flowing droplets that contain another reactant without applying the electric field. Furthermore, the dosed volumes can be tuned by controlling the value of external pressure or the ratio of flow rates between the continuous and droplet phases. To demonstrate the robustness of the proposed picoinjection, we apply it to synthesize crystals and nanoparticles. In the synthesis of crystals, the proposed picoinjection eliminates the problem of device fouling that occurs in the current reactant dosing devices. In the synthesis of nanoparticles, the proposed picoinjection generates nanoparticles that are highly monodispersed. As a result, this simplified picoinjection potentially extends the application of droplet microfluidics to investigate reaction dynamics or biochemical processes in cells. Besides, by eliminating the electricity, the proposed picoinjection avoids the usages of large equipment such as large power supplies or complicate devices, enhancing the accessibility of the proposed picoinjection.
1. Introduction In droplet microfluidic techniques, droplets can be controllably generated and manipulated through immiscible multiphase flows in a custom-designed microchannel, and each of these droplets can be regarded as a tiny “test tube” to perform (bio)chemical reactions [1–7]. Due to the small volumes of these “test tubes”, the reaction time is significantly shortened and the reaction throughput is greatly improved, which enable a wide application of the droplet microfluidics in material fabrication and biochemical analysis [3,4,8–13]. Before performing (bio)chemical reaction, reactants need to be dosed into these “test tubes”, requiring a precise control of the reactant volume and dosing time. An ideal dosing technique must be able to generate reaction products with uniform and tunable chemical and physical properties, as well as with robust operation [14,15]. Typically, two different methods are applied to dose reactants in droplet microfluidics, including (1) droplet fusion via merging paired droplets that contain different reactants, and (2) direct injection via injecting reactants into the flowing droplets [12,16–19]. However, for applying droplet fusion, reductant oils need to be drained to achieve the coalescence of the
paired droplets, which requires delicate control of droplet synchronization and thus complicates the fabrication and operation of devices [14,17,20]. For using direct injection, the capillary number (Ca = μu/γ, where μ is the viscosity, u is the linear velocity of the continuous phase fluid, and γ is the interfacial tension between the continuous and droplet phases) is required to be very small to favor the reactant injection, and also the amount of reactants that could be injected is very small when compared with that of flowing droplets, thus hampering the injection efficiency [14,15,19]. Therefore, an effective reactant dosing method, which possess a simplified device fabrication process and a high injection rate, is critically important. Recently, picoinjection is introduced to inject solutions into flowing droplets directly, which largely simplifies the device fabrication process and improve the injection rate [14,15,20,21]. Completion of picoinjection relies on a “picoinjector”, which generally consists of electrodes and a pressurized channel containing an injection liquid [15,21]. When a droplet flows by the picoinjector, the electrodes are activated and destabilize the water/oil interface, triggering the coalescence between the flowing droplet and injection liquid, and thus allow the injection liquid to enter the droplet. Despite the use of an applied electric
⁎ Corresponding author at: Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, China. E-mail address: [email protected] (H. Cheung Shum).
https://doi.org/10.1016/j.snb.2019.126766 Received 19 March 2019; Received in revised form 1 July 2019; Accepted 1 July 2019 Available online 29 July 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 298 (2019) 126766
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field to destabilize the water/oil interfaces works effectively, however, the use of an external power supply makes the systems more complicated and, in addition, the introduction of electrical filed compromises the bioactivity of delicate encapsulated bio-ingredients [15,21]. More recently, picoinjection in the absence of electricity has been reported, where a Venturi junction is developed to avoid electrical charging during picoinjection [14]. However, this advance still requires a glass capillary and constructed Venturi junction that need specific design, and manual assembly of different device components requires delicate handling. In this study, we introduce a way to achieve picoinjection by precisely tuning pressures inside a microfluidic device. With our approach, reactants can be dosed into the droplets with constant volume and time without applying the electric fields or secondary manual device components such as Venturi junction. In addition, by increasing the value of external pressure or the ratio of flow rates between the continuous and droplet phases, we can adjust the dosed volume from a few to several hundred picoliters. Furthermore, by integrating with a serpentine channel, the resultant device is capable of performing synthetic reactions that require a fast mixing, for instance, the fabrication of nanoparticles, and that require a long incubation time, for instance, the synthesis of crystals. The proposed picoinjection method and the insight gained into the on-chip reaction will enable investigation of the synthetic mechanisms and control of the relevant chemical syntheses. In particular, the avoidance of additional electricity inputs also significantly enhance the simplicity and controllability. Furthermore, by eliminating the use of electric field, our strategy will allow the investigation of cellular behaviors that are sensitive to changes in the electrical environments.
the instructions of Microchem (Microchem, USA). After UV exposure and development, the photoresist-coated wafer was heated to consolidate the photoresist pattern. Afterwards, PDMS base and curing agent (Dow Corning Sygard 184 silicone elastomer, Dow Corning, USA) at the weight ratio of 10:1, were poured onto the mold. After degassing and curing at 65 ℃ for 2 h, the PDMS slab was peeled off, with the target section cut out. Holes (diameter of 1 mm) were punched for connecting the channels to the syringes through plastic tubing. A glass slide spin-coated with a thin layer of PDMS was used as the bottom layer of the device. Oxygen plasma (PDC-002, Harrick Plasma, USA) was applied to facilitate subsequent bonding of the bottom layer and the patterned PDMS device. 2.3. Picoinjection setup The picoinjection microfluidic device comprised of a PDMS layer and a PDMS-coated glass layer, which provided a hydrophobic surface for stably generating w/o emulsions [23]. The picoinjection device consisted of a droplet generator and a picoinjector. For the picoinjector, the injection channel was connected with a tubing (Tubing (a) marked with yellow color in Fig. 1) immersed in a sealed bottle containing the injection liquid, as shown in Fig. 1. Driven by an air flow controlled by a regulator, the injection liquid was introduced from the sealed bottle to the injection channel for picoinjection. 2.4. Characterization of injected volumes The injected volume was calculated by measuring the changes in droplet size. Specifically, we recorded images of the droplets before and after picoinjection, and then measured the distance between the leading and trailing interface using Image J (NIH, USA) by assuming the droplets as cylinders with rounded ends [15].
2. Material and methods 2.1. Materials and equipment
2.5. Application of picoinjection in synthesizing calcium carbonate crystals Mineral oil (Sigma-Aldrich, USA) with 2.5 wt% Abil EM 90 (Evonik, China) was used as a continuous phase in all the experiments. For the synthesis of calcium carbonate crystals, 0.1 M sodium carbonate (Aladdin, China) aqueous solution was used as injection liquid, and 0.1 M calcium chloride (Aladdin, China) aqueous solution was used as the droplet phase as shown in Table 1. For the synthesis of silver nanoparticles, 0.01 M sodium borohydride (Shanghai Aladdin Bio-Chem Technology, China) aqueous solution was used as the inner phase (prepared according to the description by Shalom et al. [22]), while 0.7 mM silver nitrate (Uni-chem, USA) aqueous solution was used as injection liquid in the picoinjection channel (Table 1). For other experiments, water was used as inner phase while 1%wt methylene blue (Sigma-Aldrich, USA) was used as injection liquid to visualize the picoinjection process. Syringe pumps (Cetoni, Deutschland) were used to inject the solutions into the microfluidic devices at controlled flow rates.
In the synthesis of calcium carbonate crystals, we picoinjected aqueous solution containing 0.1 M sodium carbonate into the droplets containing 0.1 M calcium chloride, which were emulsified by mineral oils with 2.5 wt% EM 90. The resultant droplets were collected into a petri dish, and demulsified by adding surfactant-free mineral oil. 2.6. Application of picoinjection in silver nanoparticle synthesis In the synthesis of silver nanoparticles, the continuous phase, dispersed phase and injection liquid were mineral oils with 2.5 wt% EM90, aqueous solution with 0.01 M sodium borohydride, and aqueous solution with 0.7 mM silver nitrate, respectively. After droplet generation and picoinjection, the resultant droplets were collected using a 2 ml centrifuge tube, and demulsified by centrifuging at the speed of 10,000rmp. The aqueous solution formed by coalesced droplets was extracted for subsequent characterization.
2.2. Fabrication of microfluidic chips 2.7. Characterization and imaging of droplets, crystals and silver nanoparticles
Patterns for fabricating picoinjection device were designed using AutoCAD (Auto Desk, Inc., USA). Standard soft lithography was applied to fabricate the PDMS devices. Specifically, a clean silicon wafer was spin-coated with 3025 SU-8 photoresist (Microchem, USA) according to
The picoinjection processes were recorded using a high speed camera (Phantom v9.1, USA) connected to a customized microscope
Table 1 Aqueous solutions applied for synthesizing crystals and nanoparticles. Product
Injection liquid
Droplet phase
Calcium carbonate crystals Silver nanoparticles
0.1 M Sodium carbonate 0.7 mM Silver nitrate (containing 0.03M sodium hydroxide and 0.1M isopropanol)
0.1 M Calcium chloride 0.1 M Sodium borohydride (containing 6.5 mM trisodium citrate dehydrate and 7.6 μm hydrogen peroxide)
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Fig. 1. Schematic of the proposed picoinjection system. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
Fig. 2. (a-b) Schematic showing the distributions of various pressures exerted on the interfaces during picoinjection. The direction of disjoining pressure changes as a function of the distance between interfaces. (For detailed information, please see Part 1 Pressure Analysis in Supporting Information). (b1-4) Optical microscope images of picoinjection processes. The scale bar is 50 μm. (c) The variations of volume of picoinjected liquids for 1068 different injection trials. (d) The variations of picoinjected time for 91 different injection trials.
3. Results and discussions
(Thorlabs, USA). Droplets and crystals were imaged under an optical microscope (Leica Microsystems Inc.). The size of nanoparticles was analyzed using a particle size analyzer (Nanotrac Wave, Microtrac. USA), and characterized using a TEM (FEI Tecnai G2 20 S-TWIN, Spain).
3.1. Hydrodynamics of the proposed picoinjection An injector, which comprises of a pressurized channel containing injection liquid (Fig. 2(a)), is crucial to achieve a successful 3
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Fig. 3. (a-c) Schematic of expected distance (a) and actual distance (b, c). In the undesired case (b), driven by a relatively small external pressure, the injection liquid cannot contact with the flowing droplets, which leads to a failed picoinjection (e1). In the case shown in (c), driven by a relatively large external pressure, the injection liquid moves beyond the orifice which leads to either a successful trial (e2) or a failed trial (e3) depending on the relationship between the actual distance and expected distance. (d) A plot showing the successful rate of picoinjection as a function of the expected and actual distances between the interface of injection fluids and the orifice. Each point represents one experimental trial via recording at least 20 consecutive droplets. (e1-3) Microscope images of pico-injection processes when Lexpected > Lactual, Lexpected < Lactual < 5.2Lexpected and Lactual＞5.2Lexpected. Scale bars are 50 μm.
picoinjection. When the flowing droplet approaches to the injection liquid, the opposing interfaces become flat due to the existence of a surfactant layer, as shown in Fig. 2(a) [24]. To induce the coalescence of the injection liquids with the flowing droplets, the injector needs to impose a significantly high pressure: i) to form a convex meniscus, equivalent to Laplace pressure (PLaplace) inside the meniscus, and ii) to overcome the disjoining pressure (PDisjoinning) from the stabilizing surfactants. PLaplace is given by Young-Laplace equation: PLaplace≈2γ/R, where γ is the interfacial tension and R is the size of orifice, respectively. PDisjoinning refers to a sum of repulsive stresses and attractive stresses between two interfaces, changing with distance and commonly reaching its threshold (maximal) value (Pthreshold) at ˜100 nm (see Fig. S1(b) in Supporting information) [15,24–27]. Therefore, to successfully inject reagents into the flowing droplets, the pressure from the injector needs to be larger than the sum of PLaplace and Pthreshold. In the originally proposed picoinjection, PLaplace and Pthreshold are overcome by external pressure and electrical stresses, whereas an external pressure is imposed on the injection channel to balance PLaplace and stabilize the injection liquid at the orifice [15]. When the flowing droplet approaches the injection liquid, the electric stresses overcome Pthreshold, inducing the coalescence between the injection liquid and the flowing droplet [15]. However, the additional electric field requires embedded electrodes inside the device and external electric sources. Here, we controllably apply a sufficiently large external pressure to overcome both PLaplace and Pthreshold without resorting to any electric stresses (Fig. 2(a)). Since PLaplace inside the injected liquid meniscus near the orifice is too small to balance the large external pressure, the
resultant pressure drives the injection liquid to approach the orifice, as shown in Fig. 2(b1). If the pressure difference is large enough to overcome Pthreshold, the injection liquid will merge with the flowing droplet, and form a liquid bridge shown in Fig. 2(b2-b3). After the flowing droplet leaves the orifice, the liquid bridge will snap, as shown in Fig. 2(b4). Comparing with the existing picoinjection devices, this novel strategy enables liquids to be picoinjected into surfactant-stabilized droplets independent of an applied electric field or Venturi junctions (see Video 1 in Supporting Information). Besides, the injection volume can be maintained at 0.021 ± 0.003 μl and the injection time can be maintained at 0.018 s, as shown in Fig. 2(c, d). The timing of picoinjection of each droplet can be potentially controlled by adding a computer-aided control system into the existing setup. 3.2. Factors affect the successful rate of picoinjection and picoinjected volumes To ensure a stable droplet injection, the injection liquid needs to be in contact with the droplet at the orifice. Specifically, when the preceding picoinjection is completed, the injection liquid in the injection channel needs to be refilled before the arrival of next flowing droplet at the orifice. We refer to this distance between the injection liquid and the orifice as “expected distance (Lexpected)”, depending on the value of external pressure and the inner/outer phase flow rates in each experiment as shown in Fig. 3(a). In practice, depending on the value of the applied pressure, the injection liquid may move towards the flowing droplets but do not come into contact with the droplet (Fig. 3(b)); the 4
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outer phase flow rate is below 20μL/h, picoinjection fails due to its high sensitivity to the fluctuations of the system including those from pneumatic source and flow rates. When the flow rate of outer phase is larger than 70 μL/h, the injected volumes remain relatively unchanged, as shown in Fig. S3 in Supporting information. In addition to the dependence on the external pressure value, the injected volume also changes with the ratio of flow rates between the continuous and droplet phases. We confirm this by adjusting the ratio from 1 to 3 (See Table S1 in Supporting information), which leads to an increase in the picoinjected volume, causing by a change in the starting point of picoinjection. Specifically, when the ratio equals to 1, picoinjection starts at the middle of the droplet, resulting in shorter picoinjection time and ultimately smaller picoinjection volume (Fig. 4(b1)). After increasing the flow rate ratio to 1.5 or 2, the starting point of picoinjection shifts to the front of the droplet, leading to an increase of the picoinjection volume (Fig. 4(b2, 3)). When the flow rate ratio reaches 3, the injection liquid arrives and accumulates at the orifice before the flowing droplet arriving at the orifice, hence significantly increasing the picoinjected volume, as shown in Fig. 4(b4). Consequently, using our picoinjection, the external pressure and the flow rate ratio between the outer and inner phases, determine the picoinjected volume, while in a previously reported picoinjection, the picoinjected volume only depends on flow rate ratio between the inner and outer phase [14]. 3.3. Applications in synthesis of crystals and nanoparticles
Fig. 4. (a) Relationship between the external pressure and picoinjection volume. (a1-4) Microscope images showing the picoinjection processes at different external pressure values. (b) The relationship between the ratio of flow rates and picoinjection volume. (b1-4) Microscope images capturing picoinjection process at different ratios of flow rates between the droplet and continuous phases. All the scale bars are 50 μm.
After characterizing the picoinjection mechanism and process, we demonstrate the advantages of the proposed picoinjection by applying it to perform synthetic reactions, such as crystal syntheses, that require a certain time of incubation, and nanoparticle syntheses that require rapid mixing. In these processes, the proposed picoinjector is used to dose one reactants into the droplet containing another reactant. A serpentine channel located downstream from the injector is used to thoroughly mix the reactants inside the resultant droplets with a constant mixing time of 0.18 s (see Fig. S4(a), (b) and Video 3 in Supporting information). The combination of a picoinjection junction and a serpentine channel enables reactants to be controllably dosed and mixed for performing crystal and nanoparticle syntheses with various advantages:
injection liquid may also move beyond the orifice (Fig. 3(c)). We define this distance that the injection liquid practically moving through as “actual distance (Lactual)”, as shown in Fig. 3(b) and (c). When the Lexpected exceeds the Lactual (Lexpected > Lactual) in Fig. 3(e1), the injection liquid do not come into contact with the flowing droplet, leading to the successful rate of picoinjection of only 20%–40%, as shown in Fig. 3(d) and Video 2 in Supporting Information. For Lactual > Lexpected, the injection liquid arrives the orifice before the flowing droplet does, and the successful rate increases into 80% to 100%, as shown in Fig. 3(d) and (e2). Furthermore, a highly stable picoinjection or a picoinjection with 90%–100% success rate can be potentially achieved by adding a precise pressure control system into the existing setup. However, when Lactual > 5.2Lexpected, as empirically deduced from experimental data, the success rate drops to 0–20%, as shown in Fig. 3(d). In this regime, injection liquid is hardly introduced into the flowing droplet even when they come into contact with the droplet, as shown in Fig. 3(e3). In the unlikely event that picoinjection occurs, the injected volume varies greatly, as shown in Fig. S2 in Supporting information. These different results suggest that the value of external pressure should be optimized to achieve the effective picoinjection. In the regimes where Lexpected < Lactual < 5.2Lexpected, we could control the injected volume by varying the value of external pressure, and ratio of flow rates between the continuous and droplet phases. In particular, when we keep the flow rate at 40 μL/h, we could observe that the injected volume increases approximately with increasing value of external pressure, as shown in Fig. 4(a). The picoinjected volumes vary from 9.8 pL to 125 pL when we increase the external pressure from 0.005 MPa to 0.01 MPa. A large error bar is observed when the external pressure is 0.01 MPa, as shown in Fig. 4(a). We attribute this large error bar to the low picoinjection successful rate since the injection liquid fails to coalesce with the flowing droplet in most cases. Besides, this linear relationship between the injected volume and external pressure is valid only when the flow rates range from 20 μL/h to 70 μL/h. When the
(a) Crystal synthesis. Droplet microfluidics provides an ideal environment for growing crystals due to the ability to control the physiochemical parameters of crystal growth [14,28–30]. Most of the existing droplet microfluidic crystal syntheses rely on “Y shaped” channel to dose the required reactants into droplets for crystallization [30–33]. However, only a limited number of crystallizations can be completed using the “Y shaped” channels [14]. In particular, when a fast reaction rate and an insoluble product such as calcium carbonate are involved, device fouling and clogging will lead to reaction failure [see Fig. S5 in Supporting information]. Therefore, a glass capillary-assisted picoinjector has been introduced to alleviate the fouling and clogging by injecting reactants into the droplet containing another reactant [14]. However, a long time operation still remains challenging. To demonstrate the robustness of our approach, we combine our picoinjector and serpentine channel to perform calcium carbonate crystallization, which is achieved by injecting the calcium chloride solutions into the droplets that contain sodium carbonate solutions. Since the reaction of two reagents are confined within droplets, no precipitate contaminates are observed in the device after a continuous operation for over 4 h (Fig. 5(a)). This surprisingly long operation time highlights the superior advantage of our picoinjection. Besides, calcium carbonate crystals can be successfully fabricated after incubating the resultant droplets for 15 min [see Fig. 5(b) and (c)]. These results demonstrate the great potential of our picoinjection 5
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Fig. 5. (a) Schematic showing the capability of the proposed method in synthesizing crystals and nanoparticles. Scale bar in the CaCO3 crystal figure and in the Ag nanoparticle figure are 10 μm and 5 nm respectively. (b) Microscope image of calcium carbonate crystals synthesis using the proposed picoinjection method after running continuously for 4 h. Scale bars are 50 μm. (c) Microscope image of the droplets with synthesized crystals and droplets undergoing crystal synthesis. Scale bar is 100 μm. (d) Microscope image showing the synthesized calcium carbonate crystals. Scale bar is 50 μm (e-f) TEM image and size distribution of synthesized silver nanoparticles using the picoinjection method (e) and the bulk phase method (f). Scale bars are 100 nm.
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of Hong Kong.
for dosing reactants for synthesizing crystals. (b) Nanoparticle synthesis: Nanoparticles with high monodispersity exhibit uniform properties and are demanded in various applications, such as drug delivery and biomedical sensing [16,34–37]. To synthesize monodisperse nanoparticles, the reaction parameters need to be rigorously controlled, however, this is hardly achieved using the traditional bulk phase synthesis method [31,38–40]. In comparison, the proposed strategy, that combines picoinjector and serpentine channel, is capable of controlling reaction parameters by keeping the injected volume and mixing time constant, and thus provides an alternative to achieve monodispersed nanoparticles. As a demonstration, we synthesize silver nanoparticles by picoinjecting silver nitrate solutions into the flowing droplets containing sodium borohydride solutions. Afterwards, these reactants are mixed inside the droplets via the flow in the serpentine channel. During the picoinjection and mixing, the external pressure and the inner/outer flow rate are kept constant, rendering constant reaction volume and reaction time. Consequently, the synthesized silver nanoparticles have a narrow size distribution, with a polydispersity of 0.005, as shown in Fig. 5(d). We use TEM to confirm the monodispersity of silver nanoparticles (Figs. 5(d) and S6). In contrast, using bulk phase method under the same condition, we find that the polydiserpsity is as high as 0.437, as shown in Fig. 5(e). The successful synthesis of crystals that without device clogging and nanoparticles that with high monodispersity demonstrates the great promise of our electricity-free picoinjection in performing reactions that require long incubation time or/and rapid mixing.
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4. Conclusions In conclusion, we develop a novel picoinjection-based droplet microfluidic method. By precisely controlling the pressures inside the device, we inject controlled volume of liquid into surfactant-stabilized droplets in the absence of electricity or Venturi effect. The picoinjected volume can be tuned from a few to hundreds of picoliters by adjusting the value of external pressure or adjusting the ratio of flow rates between droplet and continuous phase. By demonstrating the syntheses of silver nanoparticles and calcium carbonate crystals, we address the problems of device fouling and clogging, which are commonly observed during microfluidic synthesis of crystals and insoluble compounds, and enhance the product monodispersity of the synthesized nanoparticles. In addition, this advanced electricity-free picoinjection has great potential to replace the existing picoinjection approaches. Specifically, compared with the traditional electric picoinjection, the proposed picoinjection complete reactions in the absence of large instruments such as power supply or complicated device structures that involved electrodes. Besides, the absence of electricity potentially enables the proposed picoinjection to perform cell-related experiment as a noninvasive technique. Declaration of Competing Interest There are no conflicts to declare. Acknowledgement We thank Prof. Shien-Ping Feng from The University of Hong Kong, for kindly providing us the particle size analyzer (Nanotrac Wave, Microtrac. USA). We thank Dr. Tiantian Kong and Xiaoxue Yao from Shenzhen University, for their kind help on the nanoparticle analysis. This work was supported by the General Research Fund (Nos. 17329516, 17304017, 17304514, 17305518 and 17306315) from the Research Grants Council of Hong Kong, as well as the Seed Fund for Basic Research (Nos. 201711159249, 201611159205 and 201511159280), Seed Fund for Translational and Applied Research (201711160016) and Platform Technology Funding from the University 7
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(2011) 1221–1227. [30] B. Zheng, J.D. Tice, L.S. Roach, R.F. Ismagilov, A droplet‐based, composite PDMS/ glass capillary microfluidic system for evaluating protein crystallization conditions by microbatch and vapor‐diffusion methods with on‐chip X‐ray diffraction, Angew. Chem. Int. Ed. Engl. 43 (2004) 2508–2511. [31] Y.-F. Su, H. Kim, S. Kovenklioglu, W. Lee, Continuous nanoparticle production by microfluidic-based emulsion, mixing and crystallization, J. Solid. State Chem. 180 (2007) 2625–2629. [32] B. Zheng, C.J. Gerdts, R.F. Ismagilov, Using nanoliter plugs in microfluidics to facilitate and understand protein crystallization, Curr. Opin. Struct. Biol. 15 (2005) 548–555. [33] B. Zheng, L.S. Roach, R.F. Ismagilov, Screening of protein crystallization conditions on a microfluidic chip using nanoliter-size droplets, J. Am. Chem. Soc. 125 (2003) 11170–11171. [34] M. Grzelczak, J. Pérez-Juste, P. Mulvaney, L.M. Liz-Marzán, Shape control in gold nanoparticle synthesis, Chem. Soc. Rev. 37 (2008) 1783–1791. [35] J. Nebu, J.A. Devi, R. Aparna, B. Aswathy, G. Lekha, G. Sony, Fluorescence turn-on detection of fenitrothion using gold nanoparticle quenched fluorescein and its separation using superparamagnetic iron oxide nanoparticle, Sensors Actuators B: Chem. 277 (2018) 271–280. [36] X. Wang, F. Chen, M. Yang, L. Guo, N. Xie, X. Kou, et al., Dispersed WO3 nanoparticles with porous nanostructure for ultrafast toluene sensing, Sensors Actuators B: Chem. 289 (2019) 195–206. [37] E.-S.M. Duraia, S. Das, G.W. Beall, Humic acid nanosheets decorated by tin oxide nanoparticles and there humidity sensing behavior, Sensors Actuators B: Chem. 280 (2019) 210–218. [38] X. Hou, Y.S. Zhang, G. Trujillo-de Santiago, M.M. Alvarez, J. Ribas, S.J. Jonas, et al., Interplay between materials and microfluidics, Nat. Rev. Mater. 2 (2017) 17016. [39] I. Shestopalov, J.D. Tice, R.F. Ismagilov, Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system, Lab Chip 4 (2004) 316–321. [40] C.-X. Zhao, L. He, S.Z. Qiao, A.P. Middelberg, Nanoparticle synthesis in
Hao Yuan received her M. S. degree in 2014 from Fudan University. Now she is a Ph.D candidate in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on droplet microfluidics, DNA amplification and soft matter. Yi Pan received his M.Eng. degree in 2016 from Sun Yat-sen University. Now he is a Ph.D. candidate in Mechanical Engineering, The University of Hong Kong. His current research mainly focuses on designing multifunctional microreactor for synthesis application. Jingxuan Tian received her M. S. degree in 2016 from Hong Kong University of Science and Technology. Now she is a Ph.D candidate in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on instability-driven liquid jets and thir potential applications. Youchuang Chao received his M. S. degree in Electrical Engineering from Xi'an Jiaotong University in 2015. Now he is a PhD candidate in Mechanical Engineering, The University of Hong Kong. His current research mainly focuses on interfacial flows and microfluidics. Jingmei Li received her Ph.D degree in Mechanical Engineering from The University of Hong Kong in 2017. Now she is a postdoctor in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on 3D printing, droplet microfluidics and fluid dynamics. Dr. Ho Cheung Shum received his B.S.E. degree in Chemical Engineering from Princeton University, S.M. and Ph.D. in Applied Physics from Harvard University. He is currently an associate professor in the Department of Mechanical Engineering and the Biomedical Engineering Programme at the University of Hong Kong. His research interests include emulsions, biomicrofluidics, biomimetic materials and soft matter.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Electricity-free picoinjection assisted droplet microfluidics Hao Yuan a b
a,b
, Yi Pan
a,b
a,b
, Jingxuan Tian
, Youchuang Chao
a,b
a,b
, Jingmei Li
, Ho Cheung Shum
a,b,⁎
T
HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), Shenzhen, 518057, China Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Droplet microfluidics Picoinjection Droplet microfluidic hydrodynamics Droplet microfluidic material syntheses
Using droplet microfluidic picoinjection as a reactant dosing technique is of great importance in assembling artificial cells and performing multistep reactions. However, the utilization of electricity in the existing picoinjection complicates the device fabrication and operation, and compromises the bioactivity of the encapsulated bio-ingredients. In this work, we propose an electricity-free picoinjection technique as an alternative to address these issues. Specifically, by precisely controlling the pressures inside the microfluidic channel, we can inject one reactant into the flowing droplets that contain another reactant without applying the electric field. Furthermore, the dosed volumes can be tuned by controlling the value of external pressure or the ratio of flow rates between the continuous and droplet phases. To demonstrate the robustness of the proposed picoinjection, we apply it to synthesize crystals and nanoparticles. In the synthesis of crystals, the proposed picoinjection eliminates the problem of device fouling that occurs in the current reactant dosing devices. In the synthesis of nanoparticles, the proposed picoinjection generates nanoparticles that are highly monodispersed. As a result, this simplified picoinjection potentially extends the application of droplet microfluidics to investigate reaction dynamics or biochemical processes in cells. Besides, by eliminating the electricity, the proposed picoinjection avoids the usages of large equipment such as large power supplies or complicate devices, enhancing the accessibility of the proposed picoinjection.
1. Introduction In droplet microfluidic techniques, droplets can be controllably generated and manipulated through immiscible multiphase flows in a custom-designed microchannel, and each of these droplets can be regarded as a tiny “test tube” to perform (bio)chemical reactions [1–7]. Due to the small volumes of these “test tubes”, the reaction time is significantly shortened and the reaction throughput is greatly improved, which enable a wide application of the droplet microfluidics in material fabrication and biochemical analysis [3,4,8–13]. Before performing (bio)chemical reaction, reactants need to be dosed into these “test tubes”, requiring a precise control of the reactant volume and dosing time. An ideal dosing technique must be able to generate reaction products with uniform and tunable chemical and physical properties, as well as with robust operation [14,15]. Typically, two different methods are applied to dose reactants in droplet microfluidics, including (1) droplet fusion via merging paired droplets that contain different reactants, and (2) direct injection via injecting reactants into the flowing droplets [12,16–19]. However, for applying droplet fusion, reductant oils need to be drained to achieve the coalescence of the
paired droplets, which requires delicate control of droplet synchronization and thus complicates the fabrication and operation of devices [14,17,20]. For using direct injection, the capillary number (Ca = μu/γ, where μ is the viscosity, u is the linear velocity of the continuous phase fluid, and γ is the interfacial tension between the continuous and droplet phases) is required to be very small to favor the reactant injection, and also the amount of reactants that could be injected is very small when compared with that of flowing droplets, thus hampering the injection efficiency [14,15,19]. Therefore, an effective reactant dosing method, which possess a simplified device fabrication process and a high injection rate, is critically important. Recently, picoinjection is introduced to inject solutions into flowing droplets directly, which largely simplifies the device fabrication process and improve the injection rate [14,15,20,21]. Completion of picoinjection relies on a “picoinjector”, which generally consists of electrodes and a pressurized channel containing an injection liquid [15,21]. When a droplet flows by the picoinjector, the electrodes are activated and destabilize the water/oil interface, triggering the coalescence between the flowing droplet and injection liquid, and thus allow the injection liquid to enter the droplet. Despite the use of an applied electric
⁎ Corresponding author at: Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, China. E-mail address: [email protected] (H. Cheung Shum).
https://doi.org/10.1016/j.snb.2019.126766 Received 19 March 2019; Received in revised form 1 July 2019; Accepted 1 July 2019 Available online 29 July 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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field to destabilize the water/oil interfaces works effectively, however, the use of an external power supply makes the systems more complicated and, in addition, the introduction of electrical filed compromises the bioactivity of delicate encapsulated bio-ingredients [15,21]. More recently, picoinjection in the absence of electricity has been reported, where a Venturi junction is developed to avoid electrical charging during picoinjection [14]. However, this advance still requires a glass capillary and constructed Venturi junction that need specific design, and manual assembly of different device components requires delicate handling. In this study, we introduce a way to achieve picoinjection by precisely tuning pressures inside a microfluidic device. With our approach, reactants can be dosed into the droplets with constant volume and time without applying the electric fields or secondary manual device components such as Venturi junction. In addition, by increasing the value of external pressure or the ratio of flow rates between the continuous and droplet phases, we can adjust the dosed volume from a few to several hundred picoliters. Furthermore, by integrating with a serpentine channel, the resultant device is capable of performing synthetic reactions that require a fast mixing, for instance, the fabrication of nanoparticles, and that require a long incubation time, for instance, the synthesis of crystals. The proposed picoinjection method and the insight gained into the on-chip reaction will enable investigation of the synthetic mechanisms and control of the relevant chemical syntheses. In particular, the avoidance of additional electricity inputs also significantly enhance the simplicity and controllability. Furthermore, by eliminating the use of electric field, our strategy will allow the investigation of cellular behaviors that are sensitive to changes in the electrical environments.
the instructions of Microchem (Microchem, USA). After UV exposure and development, the photoresist-coated wafer was heated to consolidate the photoresist pattern. Afterwards, PDMS base and curing agent (Dow Corning Sygard 184 silicone elastomer, Dow Corning, USA) at the weight ratio of 10:1, were poured onto the mold. After degassing and curing at 65 ℃ for 2 h, the PDMS slab was peeled off, with the target section cut out. Holes (diameter of 1 mm) were punched for connecting the channels to the syringes through plastic tubing. A glass slide spin-coated with a thin layer of PDMS was used as the bottom layer of the device. Oxygen plasma (PDC-002, Harrick Plasma, USA) was applied to facilitate subsequent bonding of the bottom layer and the patterned PDMS device. 2.3. Picoinjection setup The picoinjection microfluidic device comprised of a PDMS layer and a PDMS-coated glass layer, which provided a hydrophobic surface for stably generating w/o emulsions [23]. The picoinjection device consisted of a droplet generator and a picoinjector. For the picoinjector, the injection channel was connected with a tubing (Tubing (a) marked with yellow color in Fig. 1) immersed in a sealed bottle containing the injection liquid, as shown in Fig. 1. Driven by an air flow controlled by a regulator, the injection liquid was introduced from the sealed bottle to the injection channel for picoinjection. 2.4. Characterization of injected volumes The injected volume was calculated by measuring the changes in droplet size. Specifically, we recorded images of the droplets before and after picoinjection, and then measured the distance between the leading and trailing interface using Image J (NIH, USA) by assuming the droplets as cylinders with rounded ends [15].
2. Material and methods 2.1. Materials and equipment
2.5. Application of picoinjection in synthesizing calcium carbonate crystals Mineral oil (Sigma-Aldrich, USA) with 2.5 wt% Abil EM 90 (Evonik, China) was used as a continuous phase in all the experiments. For the synthesis of calcium carbonate crystals, 0.1 M sodium carbonate (Aladdin, China) aqueous solution was used as injection liquid, and 0.1 M calcium chloride (Aladdin, China) aqueous solution was used as the droplet phase as shown in Table 1. For the synthesis of silver nanoparticles, 0.01 M sodium borohydride (Shanghai Aladdin Bio-Chem Technology, China) aqueous solution was used as the inner phase (prepared according to the description by Shalom et al. [22]), while 0.7 mM silver nitrate (Uni-chem, USA) aqueous solution was used as injection liquid in the picoinjection channel (Table 1). For other experiments, water was used as inner phase while 1%wt methylene blue (Sigma-Aldrich, USA) was used as injection liquid to visualize the picoinjection process. Syringe pumps (Cetoni, Deutschland) were used to inject the solutions into the microfluidic devices at controlled flow rates.
In the synthesis of calcium carbonate crystals, we picoinjected aqueous solution containing 0.1 M sodium carbonate into the droplets containing 0.1 M calcium chloride, which were emulsified by mineral oils with 2.5 wt% EM 90. The resultant droplets were collected into a petri dish, and demulsified by adding surfactant-free mineral oil. 2.6. Application of picoinjection in silver nanoparticle synthesis In the synthesis of silver nanoparticles, the continuous phase, dispersed phase and injection liquid were mineral oils with 2.5 wt% EM90, aqueous solution with 0.01 M sodium borohydride, and aqueous solution with 0.7 mM silver nitrate, respectively. After droplet generation and picoinjection, the resultant droplets were collected using a 2 ml centrifuge tube, and demulsified by centrifuging at the speed of 10,000rmp. The aqueous solution formed by coalesced droplets was extracted for subsequent characterization.
2.2. Fabrication of microfluidic chips 2.7. Characterization and imaging of droplets, crystals and silver nanoparticles
Patterns for fabricating picoinjection device were designed using AutoCAD (Auto Desk, Inc., USA). Standard soft lithography was applied to fabricate the PDMS devices. Specifically, a clean silicon wafer was spin-coated with 3025 SU-8 photoresist (Microchem, USA) according to
The picoinjection processes were recorded using a high speed camera (Phantom v9.1, USA) connected to a customized microscope
Table 1 Aqueous solutions applied for synthesizing crystals and nanoparticles. Product
Injection liquid
Droplet phase
Calcium carbonate crystals Silver nanoparticles
0.1 M Sodium carbonate 0.7 mM Silver nitrate (containing 0.03M sodium hydroxide and 0.1M isopropanol)
0.1 M Calcium chloride 0.1 M Sodium borohydride (containing 6.5 mM trisodium citrate dehydrate and 7.6 μm hydrogen peroxide)
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Fig. 1. Schematic of the proposed picoinjection system. (For interpretation of the references to colour in this figure text, the reader is referred to the web version of this article.)
Fig. 2. (a-b) Schematic showing the distributions of various pressures exerted on the interfaces during picoinjection. The direction of disjoining pressure changes as a function of the distance between interfaces. (For detailed information, please see Part 1 Pressure Analysis in Supporting Information). (b1-4) Optical microscope images of picoinjection processes. The scale bar is 50 μm. (c) The variations of volume of picoinjected liquids for 1068 different injection trials. (d) The variations of picoinjected time for 91 different injection trials.
3. Results and discussions
(Thorlabs, USA). Droplets and crystals were imaged under an optical microscope (Leica Microsystems Inc.). The size of nanoparticles was analyzed using a particle size analyzer (Nanotrac Wave, Microtrac. USA), and characterized using a TEM (FEI Tecnai G2 20 S-TWIN, Spain).
3.1. Hydrodynamics of the proposed picoinjection An injector, which comprises of a pressurized channel containing injection liquid (Fig. 2(a)), is crucial to achieve a successful 3
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Fig. 3. (a-c) Schematic of expected distance (a) and actual distance (b, c). In the undesired case (b), driven by a relatively small external pressure, the injection liquid cannot contact with the flowing droplets, which leads to a failed picoinjection (e1). In the case shown in (c), driven by a relatively large external pressure, the injection liquid moves beyond the orifice which leads to either a successful trial (e2) or a failed trial (e3) depending on the relationship between the actual distance and expected distance. (d) A plot showing the successful rate of picoinjection as a function of the expected and actual distances between the interface of injection fluids and the orifice. Each point represents one experimental trial via recording at least 20 consecutive droplets. (e1-3) Microscope images of pico-injection processes when Lexpected > Lactual, Lexpected < Lactual < 5.2Lexpected and Lactual＞5.2Lexpected. Scale bars are 50 μm.
picoinjection. When the flowing droplet approaches to the injection liquid, the opposing interfaces become flat due to the existence of a surfactant layer, as shown in Fig. 2(a) [24]. To induce the coalescence of the injection liquids with the flowing droplets, the injector needs to impose a significantly high pressure: i) to form a convex meniscus, equivalent to Laplace pressure (PLaplace) inside the meniscus, and ii) to overcome the disjoining pressure (PDisjoinning) from the stabilizing surfactants. PLaplace is given by Young-Laplace equation: PLaplace≈2γ/R, where γ is the interfacial tension and R is the size of orifice, respectively. PDisjoinning refers to a sum of repulsive stresses and attractive stresses between two interfaces, changing with distance and commonly reaching its threshold (maximal) value (Pthreshold) at ˜100 nm (see Fig. S1(b) in Supporting information) [15,24–27]. Therefore, to successfully inject reagents into the flowing droplets, the pressure from the injector needs to be larger than the sum of PLaplace and Pthreshold. In the originally proposed picoinjection, PLaplace and Pthreshold are overcome by external pressure and electrical stresses, whereas an external pressure is imposed on the injection channel to balance PLaplace and stabilize the injection liquid at the orifice [15]. When the flowing droplet approaches the injection liquid, the electric stresses overcome Pthreshold, inducing the coalescence between the injection liquid and the flowing droplet [15]. However, the additional electric field requires embedded electrodes inside the device and external electric sources. Here, we controllably apply a sufficiently large external pressure to overcome both PLaplace and Pthreshold without resorting to any electric stresses (Fig. 2(a)). Since PLaplace inside the injected liquid meniscus near the orifice is too small to balance the large external pressure, the
resultant pressure drives the injection liquid to approach the orifice, as shown in Fig. 2(b1). If the pressure difference is large enough to overcome Pthreshold, the injection liquid will merge with the flowing droplet, and form a liquid bridge shown in Fig. 2(b2-b3). After the flowing droplet leaves the orifice, the liquid bridge will snap, as shown in Fig. 2(b4). Comparing with the existing picoinjection devices, this novel strategy enables liquids to be picoinjected into surfactant-stabilized droplets independent of an applied electric field or Venturi junctions (see Video 1 in Supporting Information). Besides, the injection volume can be maintained at 0.021 ± 0.003 μl and the injection time can be maintained at 0.018 s, as shown in Fig. 2(c, d). The timing of picoinjection of each droplet can be potentially controlled by adding a computer-aided control system into the existing setup. 3.2. Factors affect the successful rate of picoinjection and picoinjected volumes To ensure a stable droplet injection, the injection liquid needs to be in contact with the droplet at the orifice. Specifically, when the preceding picoinjection is completed, the injection liquid in the injection channel needs to be refilled before the arrival of next flowing droplet at the orifice. We refer to this distance between the injection liquid and the orifice as “expected distance (Lexpected)”, depending on the value of external pressure and the inner/outer phase flow rates in each experiment as shown in Fig. 3(a). In practice, depending on the value of the applied pressure, the injection liquid may move towards the flowing droplets but do not come into contact with the droplet (Fig. 3(b)); the 4
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outer phase flow rate is below 20μL/h, picoinjection fails due to its high sensitivity to the fluctuations of the system including those from pneumatic source and flow rates. When the flow rate of outer phase is larger than 70 μL/h, the injected volumes remain relatively unchanged, as shown in Fig. S3 in Supporting information. In addition to the dependence on the external pressure value, the injected volume also changes with the ratio of flow rates between the continuous and droplet phases. We confirm this by adjusting the ratio from 1 to 3 (See Table S1 in Supporting information), which leads to an increase in the picoinjected volume, causing by a change in the starting point of picoinjection. Specifically, when the ratio equals to 1, picoinjection starts at the middle of the droplet, resulting in shorter picoinjection time and ultimately smaller picoinjection volume (Fig. 4(b1)). After increasing the flow rate ratio to 1.5 or 2, the starting point of picoinjection shifts to the front of the droplet, leading to an increase of the picoinjection volume (Fig. 4(b2, 3)). When the flow rate ratio reaches 3, the injection liquid arrives and accumulates at the orifice before the flowing droplet arriving at the orifice, hence significantly increasing the picoinjected volume, as shown in Fig. 4(b4). Consequently, using our picoinjection, the external pressure and the flow rate ratio between the outer and inner phases, determine the picoinjected volume, while in a previously reported picoinjection, the picoinjected volume only depends on flow rate ratio between the inner and outer phase [14]. 3.3. Applications in synthesis of crystals and nanoparticles
Fig. 4. (a) Relationship between the external pressure and picoinjection volume. (a1-4) Microscope images showing the picoinjection processes at different external pressure values. (b) The relationship between the ratio of flow rates and picoinjection volume. (b1-4) Microscope images capturing picoinjection process at different ratios of flow rates between the droplet and continuous phases. All the scale bars are 50 μm.
After characterizing the picoinjection mechanism and process, we demonstrate the advantages of the proposed picoinjection by applying it to perform synthetic reactions, such as crystal syntheses, that require a certain time of incubation, and nanoparticle syntheses that require rapid mixing. In these processes, the proposed picoinjector is used to dose one reactants into the droplet containing another reactant. A serpentine channel located downstream from the injector is used to thoroughly mix the reactants inside the resultant droplets with a constant mixing time of 0.18 s (see Fig. S4(a), (b) and Video 3 in Supporting information). The combination of a picoinjection junction and a serpentine channel enables reactants to be controllably dosed and mixed for performing crystal and nanoparticle syntheses with various advantages:
injection liquid may also move beyond the orifice (Fig. 3(c)). We define this distance that the injection liquid practically moving through as “actual distance (Lactual)”, as shown in Fig. 3(b) and (c). When the Lexpected exceeds the Lactual (Lexpected > Lactual) in Fig. 3(e1), the injection liquid do not come into contact with the flowing droplet, leading to the successful rate of picoinjection of only 20%–40%, as shown in Fig. 3(d) and Video 2 in Supporting Information. For Lactual > Lexpected, the injection liquid arrives the orifice before the flowing droplet does, and the successful rate increases into 80% to 100%, as shown in Fig. 3(d) and (e2). Furthermore, a highly stable picoinjection or a picoinjection with 90%–100% success rate can be potentially achieved by adding a precise pressure control system into the existing setup. However, when Lactual > 5.2Lexpected, as empirically deduced from experimental data, the success rate drops to 0–20%, as shown in Fig. 3(d). In this regime, injection liquid is hardly introduced into the flowing droplet even when they come into contact with the droplet, as shown in Fig. 3(e3). In the unlikely event that picoinjection occurs, the injected volume varies greatly, as shown in Fig. S2 in Supporting information. These different results suggest that the value of external pressure should be optimized to achieve the effective picoinjection. In the regimes where Lexpected < Lactual < 5.2Lexpected, we could control the injected volume by varying the value of external pressure, and ratio of flow rates between the continuous and droplet phases. In particular, when we keep the flow rate at 40 μL/h, we could observe that the injected volume increases approximately with increasing value of external pressure, as shown in Fig. 4(a). The picoinjected volumes vary from 9.8 pL to 125 pL when we increase the external pressure from 0.005 MPa to 0.01 MPa. A large error bar is observed when the external pressure is 0.01 MPa, as shown in Fig. 4(a). We attribute this large error bar to the low picoinjection successful rate since the injection liquid fails to coalesce with the flowing droplet in most cases. Besides, this linear relationship between the injected volume and external pressure is valid only when the flow rates range from 20 μL/h to 70 μL/h. When the
(a) Crystal synthesis. Droplet microfluidics provides an ideal environment for growing crystals due to the ability to control the physiochemical parameters of crystal growth [14,28–30]. Most of the existing droplet microfluidic crystal syntheses rely on “Y shaped” channel to dose the required reactants into droplets for crystallization [30–33]. However, only a limited number of crystallizations can be completed using the “Y shaped” channels [14]. In particular, when a fast reaction rate and an insoluble product such as calcium carbonate are involved, device fouling and clogging will lead to reaction failure [see Fig. S5 in Supporting information]. Therefore, a glass capillary-assisted picoinjector has been introduced to alleviate the fouling and clogging by injecting reactants into the droplet containing another reactant [14]. However, a long time operation still remains challenging. To demonstrate the robustness of our approach, we combine our picoinjector and serpentine channel to perform calcium carbonate crystallization, which is achieved by injecting the calcium chloride solutions into the droplets that contain sodium carbonate solutions. Since the reaction of two reagents are confined within droplets, no precipitate contaminates are observed in the device after a continuous operation for over 4 h (Fig. 5(a)). This surprisingly long operation time highlights the superior advantage of our picoinjection. Besides, calcium carbonate crystals can be successfully fabricated after incubating the resultant droplets for 15 min [see Fig. 5(b) and (c)]. These results demonstrate the great potential of our picoinjection 5
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Fig. 5. (a) Schematic showing the capability of the proposed method in synthesizing crystals and nanoparticles. Scale bar in the CaCO3 crystal figure and in the Ag nanoparticle figure are 10 μm and 5 nm respectively. (b) Microscope image of calcium carbonate crystals synthesis using the proposed picoinjection method after running continuously for 4 h. Scale bars are 50 μm. (c) Microscope image of the droplets with synthesized crystals and droplets undergoing crystal synthesis. Scale bar is 100 μm. (d) Microscope image showing the synthesized calcium carbonate crystals. Scale bar is 50 μm (e-f) TEM image and size distribution of synthesized silver nanoparticles using the picoinjection method (e) and the bulk phase method (f). Scale bars are 100 nm.
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of Hong Kong.
for dosing reactants for synthesizing crystals. (b) Nanoparticle synthesis: Nanoparticles with high monodispersity exhibit uniform properties and are demanded in various applications, such as drug delivery and biomedical sensing [16,34–37]. To synthesize monodisperse nanoparticles, the reaction parameters need to be rigorously controlled, however, this is hardly achieved using the traditional bulk phase synthesis method [31,38–40]. In comparison, the proposed strategy, that combines picoinjector and serpentine channel, is capable of controlling reaction parameters by keeping the injected volume and mixing time constant, and thus provides an alternative to achieve monodispersed nanoparticles. As a demonstration, we synthesize silver nanoparticles by picoinjecting silver nitrate solutions into the flowing droplets containing sodium borohydride solutions. Afterwards, these reactants are mixed inside the droplets via the flow in the serpentine channel. During the picoinjection and mixing, the external pressure and the inner/outer flow rate are kept constant, rendering constant reaction volume and reaction time. Consequently, the synthesized silver nanoparticles have a narrow size distribution, with a polydispersity of 0.005, as shown in Fig. 5(d). We use TEM to confirm the monodispersity of silver nanoparticles (Figs. 5(d) and S6). In contrast, using bulk phase method under the same condition, we find that the polydiserpsity is as high as 0.437, as shown in Fig. 5(e). The successful synthesis of crystals that without device clogging and nanoparticles that with high monodispersity demonstrates the great promise of our electricity-free picoinjection in performing reactions that require long incubation time or/and rapid mixing.
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4. Conclusions In conclusion, we develop a novel picoinjection-based droplet microfluidic method. By precisely controlling the pressures inside the device, we inject controlled volume of liquid into surfactant-stabilized droplets in the absence of electricity or Venturi effect. The picoinjected volume can be tuned from a few to hundreds of picoliters by adjusting the value of external pressure or adjusting the ratio of flow rates between droplet and continuous phase. By demonstrating the syntheses of silver nanoparticles and calcium carbonate crystals, we address the problems of device fouling and clogging, which are commonly observed during microfluidic synthesis of crystals and insoluble compounds, and enhance the product monodispersity of the synthesized nanoparticles. In addition, this advanced electricity-free picoinjection has great potential to replace the existing picoinjection approaches. Specifically, compared with the traditional electric picoinjection, the proposed picoinjection complete reactions in the absence of large instruments such as power supply or complicated device structures that involved electrodes. Besides, the absence of electricity potentially enables the proposed picoinjection to perform cell-related experiment as a noninvasive technique. Declaration of Competing Interest There are no conflicts to declare. Acknowledgement We thank Prof. Shien-Ping Feng from The University of Hong Kong, for kindly providing us the particle size analyzer (Nanotrac Wave, Microtrac. USA). We thank Dr. Tiantian Kong and Xiaoxue Yao from Shenzhen University, for their kind help on the nanoparticle analysis. This work was supported by the General Research Fund (Nos. 17329516, 17304017, 17304514, 17305518 and 17306315) from the Research Grants Council of Hong Kong, as well as the Seed Fund for Basic Research (Nos. 201711159249, 201611159205 and 201511159280), Seed Fund for Translational and Applied Research (201711160016) and Platform Technology Funding from the University 7
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Hao Yuan received her M. S. degree in 2014 from Fudan University. Now she is a Ph.D candidate in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on droplet microfluidics, DNA amplification and soft matter. Yi Pan received his M.Eng. degree in 2016 from Sun Yat-sen University. Now he is a Ph.D. candidate in Mechanical Engineering, The University of Hong Kong. His current research mainly focuses on designing multifunctional microreactor for synthesis application. Jingxuan Tian received her M. S. degree in 2016 from Hong Kong University of Science and Technology. Now she is a Ph.D candidate in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on instability-driven liquid jets and thir potential applications. Youchuang Chao received his M. S. degree in Electrical Engineering from Xi'an Jiaotong University in 2015. Now he is a PhD candidate in Mechanical Engineering, The University of Hong Kong. His current research mainly focuses on interfacial flows and microfluidics. Jingmei Li received her Ph.D degree in Mechanical Engineering from The University of Hong Kong in 2017. Now she is a postdoctor in Mechanical Engineering, The University of Hong Kong. Her current research mainly focuses on 3D printing, droplet microfluidics and fluid dynamics. Dr. Ho Cheung Shum received his B.S.E. degree in Chemical Engineering from Princeton University, S.M. and Ph.D. in Applied Physics from Harvard University. He is currently an associate professor in the Department of Mechanical Engineering and the Biomedical Engineering Programme at the University of Hong Kong. His research interests include emulsions, biomicrofluidics, biomimetic materials and soft matter.
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