Droplet Microreactors Research Articles

Droplet microreactor continuous synthesis of hierarchical Rh on CdZnS snowflakes for enhanced photocatalytic hydrogen evolution

Promising droplet microreactor-cation exchange strategy is utilized to construct the unique Rh hierarchical structure (RhHS) and Rh nanoclusters (RhNCs) on CdZnS (CZS) snowflakes. The fabricated RhHS/RhNCs/CZS achieves an optimal hydrogen evolution activity of 926.0μmol h−1 g−1, which is 5.9 and 10.5 folds enhancement that of CZS and CdS, respectively. The enhanced activity for H2 generation can be attributed to the RhHS/RhNCs (accelerating electron transfer), as well as the hierarchical structure of RhHS and CZS, which enhances reflection–absorption of visible light. Theoretical calculations reveal that Rh species can serve as new H* generation-release sites, reducing the energy barrier for hydrogen evolution. Notably, in situ Raman confirmed the Rh (Rh-H) and S (S-H) two main active sites for proton reduction during hydrogen generation. This paper provides a novel approach to constructing hierarchical structure photocatalyst (RhHS/RhNCs/CZS) for enhanced photocatalytic H2 production.

Rapid Concentration of Ga-68 and Proof-of-Concept Microscale Labeling of [68Ga]Ga-PSMA-11 in a Droplet Reactor.

The radiometal gallium-68 (Ga-68) has garnered significant interest due to its convenient production via compact and widely available generators and the high performance of 68Ga-labeled compounds for positron-emission tomography (PET) imaging for cancer diagnosis and management of patients undergoing targeted radionuclide therapy. Given the short half life of Ga-68 (68 min), microfluidic-based radiosynthesis is a promising avenue to establish very rapid, efficient, and routine radiolabeling with Ga-68; however, the typical elution volume of Ga-68 from a generator (4-10 mL) is incompatible with the microliter reaction volumes of microfluidic devices. To bridge this gap, we developed a microscale cartridge-based approach to concentrate Ga-68. By optimizing cartridge design, resin type, resin mass, and eluent composition, Ga-68 was reliably concentrated from ~6 mL to ~80 µL with high recovery efficiency (>97%, n = 14). Furthermore, this method is suitable for both single- and dual-generator setups. To demonstrate suitability of the concentrated radiometal for radiolabeling, we performed microdroplet synthesis of [68Ga]Ga-PSMA-11, achieving high radiochemical yield (83 ± 11%, n = 3), excellent radiochemical purity (>99%), and high apparent specific activity (255-320 MBq/μg). The entire process, including Ga-68 concentration, radiosynthesis, purification, and formulation, was completed in 12 min. Starting with activity of 0.81-0.84 GBq, 0.51-0.64 GBq of product was produced, sufficient for multiple patient doses. This work paves the way to clinical-scale production of other 68Ga-labeled compounds using droplet microreactor methods, or high-throughput labeling optimization or compound screening of 68Ga-labeled probes using droplet reaction arrays.

Photoresponsive microfluidic three-phase emulsions for tandem reactions

The long-term objective of tandem reaction is to investigate ‘intelligent’ tandem reaction systems akin to living cells. Here we’ve prepared an ‘intelligent’ droplet microreactor for the tandem reaction using droplet microfluidic technology constructing a photoresponsive microfluidic tandem reaction system. The method leverages a three-phase emulsion’s ‘two membranes and three phases’ structure to segregate incompatible reagents, and introduces a stimulating surfactant to control the structure of this emulsion structure for intelligent reaction sequence control, enablingefficient,orderlytandem reactionwithin the same system. Validating this concept, we applied the droplet microreactor to the condensation-reduction cascade reaction, as envisioned, incompatible condensation and reduction reactions, were coordinated by cycling the UV light on /off for a smooth cascade reaction. Compared to batch reaction, the droplet microreactor showed sixfold catalytic efficiency enhancement and > 99 % reaction selectivity. Our work offers novel insights into realizing intelligent cascade reactions.

Rational Design of Liquid-Liquid Microdispersion Droplet Microreactors for the Controllable Synthesis of Highly Uniform and Monodispersed Dextran Microspheres.

Hydrogel microspheres are biocompatible materials widely used in biological and medical fields. Emulsification and stirring are the commonly used methods to prepare hydrogels. However, the size distribution is considerably wide, the monodispersity and the mechanical intensity are poor, and the stable operation conditions are comparatively narrow to meet some sophisticated applications. In this paper, a T-shaped stepwise microchannel combined with a simple side microchannel structure is developed to explore the liquid-liquid dispersion mechanism, interfacial evolution behavior, satellite droplet formation mechanism and separation, and the eventual successful synthesis of dextran hydrogel microspheres. The effect of the operation parameters on droplet and microsphere size is comprehensively studied. The flow pattern and the stable operation condition range are given, and mathematical prediction models are developed under three different flow regimes for droplet size prediction. Based on the stable operating conditions, a microdroplet-based method combined with UV light curing is developed to synthesize the dextran hydrogel microsphere. The highly uniform and monodispersed dextran microspheres with good mechanical intensity are synthesized in the developed microfluidic platform. The size of the microsphere could be tuned from 50 to 300 μm with a capillary number in the range of 0.006-0.742. This work not only provides a facile method for functional polymeric microsphere preparation but also offers important design guidelines for the development of a robust microreactor.

Portable Triboelectric Electrostatic Tweezer for External Manipulation of Droplets within a Closed Femtosecond Laser-Treated Superhydrophobic System.

Controllable droplet manipulation has diverse applications; however, limited methods exist for externally manipulating droplets in confined spaces. Herein, we propose a portable triboelectric electrostatic tweezer (TET) by integrating electrostatic forces with a superhydrophobic surface that can even manipulate droplets in an enclosed space. Electrostatic induction causes the droplet to be subjected to an electrostatic force in an electrostatic field so that the droplet can be moved freely with the TET on a superhydrophobic platform. Characterized by its high precision, flexibility, and robust binding strength, TET can manipulate droplets under various conditions and achieve a wide range of representative fluid applications such as droplet microreactors, precise self-cleaning, cargo transportation, the targeted delivery of chemicals, liquid sorting, soft droplet robotics, and cell labeling. Specifically, TET demonstrated the ability to manipulate internal droplets from the outside of a closed system, such as performing cell labeling experiments within a sealed Petri dish without opening the culture system.

Complex Droplet Microreactor for Highly Efficient and Controllable Esterification and Cascade Reactions.

A highly efficient complex emulsion microreactor has been successfully developed for multiphasic water-labile reactions, providing a powerful platform for atom economy and spatiotemporal control of reaction kinetics. Complex emulsions, composing a hydrocarbon phase (H) and a fluorocarbon phase (F) dispersed in an aqueous phase (W), are fabricated in batch scale with precisely controlled droplet morphologies. A biphasic esterification reaction between 2-bromo-1,2-diphenylethane-1-ol (BPO) and perfluoro-heptanoic acid (PFHA) is chosen as a reversible and water-labile reaction model. The conversion reaches up to 100 % under mild temperature without agitation, even with nearly equivalent amounts of reactants. This efficiency surpasses all reported single emulsion microreactors, i. e., 84~95 %, stabilized by various emulsifiers with different catalysts, which typically necessitate continuous stirring, a high excess of one reactant, and/or extended reaction time. Furthermore, over 3 times regulation threshold in conversion rate is attained by manipulating the droplet morphologies, including size and topology, e. g., transition from completely engulfed F/H/W double to partially engulfed (F+H)/W Janus. Addition-esterification, serving as a model for triple phasic cascade reaction, is also successfully implemented under agitating-free and mild temperature with controlled reaction kinetics, demonstrating the versatility and effectiveness of the complex emulsion microreactor.

Selective Sparse Sampling of Water Droplets in Oil with Acoustic Tweezers.

In microfluidics, water droplets are often used as independent biochemical microreactor units, enabling the implementation of massively parallel screening assays where only a few of the reacting water droplets yield a positive result. However, sampling the product of these few successful reactions is an unsolved challenge. One possible solution is to use acoustic tweezers, which are lab-free, easily miniaturized, and biocompatible manipulation tools, and existing acoustic tweezers manipulating particles or cells, and water droplet manipulation in oil with an acoustic tweezer is absent. The first challenge in attempting to recover a few water droplets from a large batch is the selective manipulation of water droplets in an oil system. In this paper, we trap and manipulate single water droplets in oil using integrated single-beam (focused beam/vortex beam) acoustic tweezers for the first time. We find that water droplets with a diameter smaller than half a wavelength are trapped by acoustic vortices, while larger ones are better captured by focused acoustic beams. It is the first step to extract the target water droplet microreactors (positive ones) in an oil system and analyze their content. Compared to previous techniques, such as fluorescence-activated cell sorting (FACS), our technique is sparse, meaning that the sampling time is proportional to the number of droplets required and very insensitive to the total number of microreactors, making it well suited for large-scale screening assays.

High performance flow-focusing droplet microreactor. Extractive separation of rare earths as case of study

This work advances the knowledge of the design and manufacture of microdroplet reactors for reactive liquid–liquid systems assisted by advanced simulation techniques (CFD). The mathematical model is based on the integrated analysis of the fluid dynamics for multiphase systems, passive mixing of reactants inside and outside the microdroplet and interfacial reaction rate. To validate the results obtained with the predictive model a spiral microdevice with droplet generation using flow-focusing geometry has been designed and fabricated by additive manufacturing. First, the influence of fluid flowrate, hold-up and viscosity on the droplets frequency and size has been evaluated with the model and assessed experimentally. Next, the performance in the separation of a binary Dysprosium-Lanthanum system has been tested, working with a dispersed aqueous phase containing the rare earth elements (REEs) solution and a continuous organic phase constituted of a solution of the extractant Cyanex® 572 in Shellsol® D70. The extraction experiments have been conducted at residence times between 3 and 60 s to generate aqueous phase monodispersed droplets with high interfacial area that varies between 61.4 and 49.2 cm2·cm−3 depending on the operating conditions. At pH 1, 90 % of dysprosium has been extracted, and almost complete separation of both REEs has been achieved. Very good agreement between simulated and experimental results has been reached with an error lower than 12 %. Therefore, here we provide the tools to design and predict the microdroplet enhanced performance of extractive liquid–liquid microreactors.

Comparison of three droplet microreactors for the continuous production of nano and micro particles

In this study we compare three droplet microreactors for the synthesis of cerium oxide nanoparticles (CONs) and calcium phosphate crystals (CaPs) and discuss their suitability for long-term, continuous operation. The synthesis conditions were controlled by manipulating the flow rate ratio of the continuous phase to the dispersed (Qc/Qd), which is a crucial factor influencing droplet formation rate (DF) and droplet size. After identifying appropriate operating conditions (droplet size between 400 and 500 μm), synthesis experiments were conducted to test the continuous operation of the microreactors. Among the three designs, one flow focusing device with a buffer stream, demonstrated superior resilience to particle fouling, and seemed to be appropriate for long-term, continuous particle synthesis. Particles synthesized using microfluidics were consistently smaller and exhibited less crystallinity than those produced during conventional batch synthesis due to the short residence time in the microreactor.

Population dynamics of cross-protection against β-lactam antibiotics in droplet microreactors.

Bacterial strains that are resistant to antibiotics may protect not only themselves, but also sensitive bacteria nearby if resistance involves antibiotic degradation. Such cross-protection poses a challenge to effective antibiotic therapy by enhancing the long-term survival of bacterial infections, however, the current understanding is limited. In this study, we utilize an automated nanoliter droplet analyzer to study the interactions between Escherichia coli strains expressing a β-lactamase (resistant) and those not expressing it (sensitive) when exposed to the β-lactam antibiotic cefotaxime (CTX), with the aim to define criteria contributing to cross-protection. We observed a cross-protection window of CTX concentrations for the sensitive strain, extending up to approximately 100 times its minimal inhibitory concentration (MIC). Through both microscopy and enzyme activity analyses, we demonstrate that bacterial filaments, triggered by antibiotic stress, contribute to cross-protection. The antibiotic concentration window for cross-protection depends on the difference in β-lactamase activity between co-cultured strains: larger differences shift the 'cross-protection window' toward higher CTX concentrations. Our findings highlight the dependence of opportunities for cross-protection on the relative resistance levels of the strains involved and suggest a possible specific role for filamentation.

A crescent-shaped imprinted microgel adsorbent with near-infrared light-responsive performance for selective adsorption of Lead(II)

Smart hydrogel adsorbent with high selectivity and“on–off”switching ability is vital for the effective elimination of heavy metal pollution, but its simple fabrication remains a serious task. To address this, a facile strategy via an oil-in-water-in-oil (O1/W/O2) droplet microreactor was proposed to prepare polydopamine-coated imprinted microgel adsorbents (DMHIIPs) with crescent shape, and then as-prepared smart adsorbent with near-infrared (NIR) light-responsive performance was used to selectively remove lead ions (Pb(II)) from aqueous solution. The critical solution temperature (LCST) of DMHIIPs is 44.2 °C, resulting from the increase of thermal change and decrease of entropy change during swelling. DMHIIPs have high photothermal conversion efficiency and stability, and the temperature of DMHIIPs dispersion (0.5 mg/L) can be rapidly heated to above 50 °C under NIR (808 nm, 1.0 W cm−2) irradiation for 10 min. The adsorption capacity of Pb(II) on DMHIIPs elevates from 128.0 mg g−1 to 135.2 mg g−1, when the solution temperature increases from 20 °C to 40 °C, but this value significantly reduces to 59.2 mg g−1 at 60 °C. Pseudo-second-order can be well described the kinetic results of DMHIIPs, confirming chemisorption is the rate-limiting step. Desorption ratio of DMHIIPs is 36.5% in 30 min under NIR irradiation, and it can be increased to 63.2% When combining NIR irradiation with the using of elution agent. In addition, the obtained DMHIIPs has excellent selectivity coefficient (k) for competitive Cu(II) (7.20), Cd(II) (23.31), and Ni(II) (39.33), respectively. This work demonstrates a new strategy to NIR-responsive imprinted microgel adsorbent provides a new perspective for controlling the adsorption process of heavy metal pollution.

Droplet microreactor for high-throughput fluorescence-based measurements of single catalyst particle acidity

The particles of heterogeneous catalysts differ greatly in size, morphology, and most importantly, in activity. Studying these catalyst particles in batch typically results in ensemble averages, without any information at the level of individual catalyst particles. To date, the study of individual catalyst particles has been rewarding but is still rather slow and often cumbersome1. Furthermore, these valuable in-depth studies at the single particle level lack statistical relevance. Here, we report the development of a droplet microreactor for high-throughput fluorescence-based measurements of the acidities of individual particles in fluid catalytic cracking (FCC) equilibrium catalysts (ECAT). This method combines systematic screening of single catalyst particles with statistical relevance. An oligomerization reaction of 4-methoxystyrene, catalyzed by the Brønsted acid sites inside the zeolite domains of the ECAT particles, was performed on-chip at 95 °C. The fluorescence signal generated by the reaction products inside the ECAT particles was detected near the outlet of the microreactor. The high-throughput acidity screening platform was capable of detecting ~1000 catalyst particles at a rate of 1 catalyst particle every 2.4 s. The number of detected catalyst particles was representative of the overall catalyst particle population with a confidence level of 95%. The measured fluorescence intensities showed a clear acidity distribution among the catalyst particles, with the majority (96.1%) showing acidity levels belonging to old, deactivated catalyst particles and a minority (3.9%) exhibiting high acidity levels. The latter are potentially of high interest, as they reveal interesting new physicochemical properties indicating why the particles were still highly acidic and reactive.

Plasma-Activated Mist: Continuous-Flow, Scalable Nitrogen Fixation, and Aeroponics

Nitrogen fixation underpins diverse biological and industrial processes and is currently one of the leading producers of carbon emissions at a global scale. Radical solutions to decarbonize nitrogen fixation include plasma-electrified ammonia (NH3) synthesis using water and atmospheric nitrogen. Here, we resolve the intrinsic limitations of the state-of-the-art solutions in (i) energy efficiency; (ii) scalability; (iii) direct use of clean renewable energy; and (iv) direct sustainable application of water-based nitrogen fixation by the novel, scalable plasma-enabled process and system for the continuous-flow, scalable nitrogen fixation, and aeroponics using plasma-activated mist (PAM). The new approach is based on the continuous, scalable, in-flow generation of PAM containing NH4+, NO3–, NO2–, and other nitrogen-fixation species through the reaction of large-area uniform air plasma and high-flux water mist containing large area-to-volume ratio micron-sized droplets. Through the solar-energy-driven plasma-assisted oxidation confined within the droplet micro-reactors, a liquid-phase nitrogen fixation product dominated by NO3– is produced. This PAM nitrogen fixation system is applied for the continuous delivery of nitrogen-based nutrients directly to the roots of living plants using a custom-designed aeroponic system which increases the leaf emergence rate and leaf area of bean sprouts by more than 30%. The unique energy concentration in droplet microreactors leads to the much increased (from 2 to 39 times) energy efficiency at a larger scale compared to the key types of common lab-scale plasma reactors. Compatibility with direct solar power, flow-reactor chemistry, and electrified processes at industry-relevant scales make this low-cost, low-carbon-footprint, renewable-energy-driven process and system promising for the widespread sustainable and distributed production and applications of plasma-enabled nitrogen fixation.

Synthesis of biocatalyst in microfluidic reactor for β-sitosterol esterification

The goal of the current study is to propose and study an innovative method in synthesizing biocatalyst within a microfluidic reactor and later use it in the esterification of β-sitosterol. A droplet microreactor was specially designed and fabricated using soft-lithography technique and was used for formulation of silica nanoparticles. Sol-gel reaction method was adapted in the microreactor to produce nanoparticles which were used as the support for lipase from Rhizomucor miehei. Another microfluidics reactor was also designed and fabricated for the immobilization of the Lipase on the silica nanoparticles through a physical adsorption technique producing a biocatalyst. The catalytic efficacy of the biocatalyst was next evaluated by esterifying β-sitosterol (the main component of phytosterol) at two different catalyst loadings and comparing it to free lipase under the same experimental parameters. Samples were taken periodically and diluted with n-hexane before injecting them into gas chromatography for analysis. Spherical silica nanoparticles synthesized from the microreactor have good monodispersity with an average size of 480 nm. The silica nanoparticles produced in the microflow system showed large BET surface area, pore volume, and pore size of 125.08 m2/g, 0.42 cm3/g, and 135.70 Å, respectively. Rhizomucor miehei lipase was successfully adsorbed on the support with lipase activity around 16160 U/g. Increasing the catalyst loading increased the esterification rate from 79% to 97.3% when utilized 2.0 and 2.5 mg/ml of immobilized lipase, respectively. The findings also revealed that, under comparable reaction parameters, immobilized lipase had a higher esterification rate than free lipase.

A novel isothermal digital amplification system and its application for absolute quantification of respiratory infectious virus

Herein, we establish a novel isothermal digital amplification system termed digital nicking and extension chain reaction system-based amplification (dNESBA) by utilizing the isothermal NESBA technique and the newly developed miniaturized fluorescence monitoring system (mFMS). dNESBA enables parallel isothermal NESBA reactions in more than 10,000 localized droplet microreactors and read the fluorescence signals rapidly in 150 s by mFMS. This system could identify the genomic RNA (gRNA) extracted from target respiratory syncytial virus A (RSV A) as low as 10 copies with remarkable specificity. The practical applicability of dNESBA was also successfully verified by reliably detecting the gRNA in the artificial sputum samples with excellent reproducibility and accuracy. Due to the intrinsic advantages of isothermal amplifying technique including the elimination of the requirement of thermocycling device and the enhanced portability of the miniaturized read-out equipment, the dNESBA technique equipped with mFMS could serve as a promising platform system to achieve point-of-care (POC) digital molecular diagnostics, enabling absolute and ultra-sensitive quantification of various infectious pathogens even in an early stage.

Induced flow inside a droplet by static electrical charge

Introducing controlled fluid motion in a droplet turns out to be of outstanding scientific importance, hallmarked by a plethora of applications ranging from engineering to biology. While internal mechanisms such as interfacial tension or buoyancy-driven dynamics may trigger fascinating flow structures inside a droplet, controllability of the same without external forcing remains questionable. On the other hand, in an electrically forced environment, complex fabrication steps and special choices of the ionic liquid are often demanded. Circumventing these limits, here we bring out a new method of flow manipulation inside a sessile droplet by simply deploying a static charge produced by the triboelectric effect. This is physically actuated by charge transfer between the two lateral electrodes within which the droplet is entrained, triggering a strong ionized air current. The flow inside the droplet is generated due to the shear exerted at the interface by the charge-induced ionized airflow around the droplet, a paradigm that has hitherto remained unexplored. The strength of the fluid flow can be controlled by adjusting the supplied static charge. Such unique controllability without sacrificing the physical simplicity opens up new possibilities for flow manipulation in a multitude of applications ranging from droplet microreactors to digital microfluidics.

Continuous synthesis of BaFe2O4 and BaFe12O19 nanoparticles in a droplet microreactor for efficient detection of antihistamine drugs in oral fluid using surface-assisted laser desorption/ionization mass spectrometry.

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has received considerable attention as a complementary approach to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), offering substantial potential for analyzing molecules in the low-mass region. Herein, we propose a facile method, a microreactor for the synthesis of two types of barium ferrite (BaFe2O4 and BaFe12O19) nanoparticles (NPs) within moving droplets for detecting antihistamine (AH) drugs in oral fluid (OF). The synthesized BaFe2O4 and BaFe12O19 NPs exhibited small particle size, good ultraviolet absorption, and excellent performance in SALDI-MS, as determined by survival yield measurements. The limits-of-detection for AH drugs were in the range of 1 pg mL-1 to 100 ng mL-1, and spot-spot reproducibility of the SALDI substrates was satisfactory. Moreover, when analyzing cetirizine in OF, the obtained recoveries of cetirizine were 101% and 99% using BaFe2O4 and BaFe12O19 NP, respectively. Furthermore, the proposed method was validated by analyzing OF samples from a healthy volunteer who consumed a 5 mg levocetirizine tablet for seven days. SALDI-MS analysis confirmed the successful detection of endogenous components, the parent ion of cetirizine, and other exogenous substances. This study reports an advanced application of droplet microreactor technology for designing and synthesizing a wide range of novel and efficient SALDI-MS substrates for various applications.

Stretchable superhydrophobic elastomers with on-demand tunable wettability for droplet manipulation and multi-stage reaction

We report a facile approach to fabricating stretchable superhydrophobic surfaces with different microstructures (arc-shaped or V-shaped air pockets) for multi-stage liquid droplet micro-reactors.

Imprinted polymer beads featuring both predefined multiple-point interaction and accessible binding sites for precise recognition of 2′-deoxyadenosine

“Tailor-made” formation of complementary binding sites via lock-and-key molecular binding mechanism is critical for selective adsorption of target molecules in environmental and chemical engineering fields. However, the innovative design of molecularly imprinted polymers (MIPs) remains a challenging task. Here, photo-crosslinked functional polymer building blocks consisting of monomeric methacrylic acid (MAA), 1-(vinylbenzyl)thymidine (VBT) and post-crosslinked monomeric cinnamoyloxyethyl methacrylate (CEMA) were firstly synthesized. MIPs beads (PC-MIPs) were then prepared by emulsion droplet microreactor, and finally their precise recognition of 2'-deoxyadenosine (dA) was investigated by batch mode adsorption. The prepared PC-MIPs microspheres have a large number of voids on the surface with interconnected cavities inside, and the bubble-like framework inside may be caused by the full encapsulation of bubbles from the low boiling point isolated phase. Due to the high density of accessible imprinted sites and strong interaction with dA, the maximum monolayer adsorption capacity of PC-MIPs was calculated to be 134.5 μmol g−1 for dA, surpassing most of the reported dA-imprinted sorbents. In addition, unique advantages of the post-crosslinking strategy combined with the emulsion template method exhibited good selectivity (imprinting factor higher than 2.71) and high stability, as well as excellent recyclability toward dA. Moreover, 82.4 % of dA was extracted by PC-MIPs from spiked human urine samples. Therefore, our work demonstrated an alternate strategy to design polymers bearing precise molecular recognition capability, and provided a new perspective for dA enrichment from biological samples.

Analysis of double-emulsion droplets with ESI mass spectrometry for monitoring lipase-catalyzed ester hydrolysis at nanoliter scale

Microfluidic double-emulsion droplets allow the realization and study of biphasic chemical processes such as chemical reactions or extractions on the nanoliter scale. Double emulsions of the rare type (o1/w/o2) are used here to realize a lipase-catalyzed reaction in the non-polar phase. The surrounding aqueous phase induces the transfer of the hydrophilic product from the core oil phase, allowing on-the-fly MS analysis in single double droplets. A microfluidic two-step emulsification process is developed to generate the (o1/w/o2) double-emulsion droplets. In this first example of microfluidic double-emulsion MS coupling, we show in proof-of-concept experiments that the chemical composition of the water layer can be read online using ESI–MS. Double-emulsion droplets were further employed as two-phase micro-reactors for the hydrolysis of the lipophilic ester p-nitrophenyl palmitate catalyzed by the Candida antarctica lipase B (CalB). Finally, the formation of the hydrophilic reaction product p-nitrophenol within the double-emulsion droplet micro-reactors is verified by subjecting the double-emulsion droplets to online ESI–MS analysis.Graphical abstract