Recent Advances in Reactive Microdroplets for Clean Water and Energy

Abstract

Microdroplets have emerged as powerful and sustainable platforms for the design and synthesis of functional materials under mild and environmentally friendly conditions. Their unique physicochemical environments - characterized by high surface-to-volume ratios and confined internal space - enable precise control over mass and heat transfer, interfacial energy conversion, and chemical reactions. These features have been harnessed in two main ways: first, by employing microdroplets as microreactors for the fabrication of advanced materials such as polymeric microlenses, artificial compound eyes, metal oxide nanoparticles, and metal-organic framework microstructures; and second, by using microdroplets as reactive entities to accelerate interfacial reactions relevant to hydrogen and biodiesel production, as well as nitrogen and carbon dioxide fixation. Together, these strategies have driven significant advances in clean energy generation, environmental monitoring, and water treatment. This review provides a critical overview of recent progress in microdroplet-assisted synthesis of functional materials and their integration in energy and environmental technologies. An emerging direction in the integration of microdroplet-based systems into adaptive sensing and human-machine interfaces driven by artificial intelligence is also highlighted.

Keywords: clean energy; functional materials; materials synthesis; microdroplets; microreactors; water remediation.

Figure 1

Overview of microdroplet‐based strategies for functional material synthesis, energy carrier production, self‐propelled microswimmers, and miniaturized analytical devices.

Figure 2

A) Schematic of the classical dispersive liquid–liquid microextraction (DLLME) process.^{[ 42 ]} Copyright 2022, American Institute of Physics (AIP). B) Schematic illustration showing the extraction of analytes from the sample flow into the surface nanodroplets. Reproduced with permission.^{[ 104 ]} Copyright 2021, Royal Society of Chemistry. C) A fluorescent image showing the octanol microdroplets pinned on the inner wall of the capillary tube.^{[ 105 ]} Copyright 2023, Elsevier, and the collected octanol in the capillary tube.^{[ 101 ]} Copyright 2022, American Chemical Society. D) A scheme and corresponding photos for the description of the extraction procedure of Rhodamine B from water (left) to 1‐octanol (right). Along with the extraction of varied analytes between neighboring droplets. (scale bar: 1 mm).^{[ 106 ]} Copyright 2023, Wiley. E) Schematic showcasing the capture of microplastics by surface nanodroplets in the (i) aqueous and (ii) air environment; (iii) A fluorescence microscopic image displaying microplastics on the nanodroplet interface. Reproduced with permission.^{[ 35 ]} Copyright 2024, American Chemical Society. F) Schematic demonstration of enhanced fluorescence intensity of molecules extracted into encapsulated nanodroplets from external solution, with bright‐field and fluorescence optical image of compartmentalized droplets in contact with a mixture of hydrophobic (red) and hydrophilic (green). Reproduced with permission.^{[ 107 ]} Copyright 2020, Wiley. G) (i) Optical microscopy images of acoustically patterned PDDA/ATP coacervate microdroplets; (ii) Fluorescence microscopy images of coacervate microdroplets containing varied analytes, including glucose oxidase (GOx), amyloglucosidase (AGx), or horseradish peroxidase (HRP).^{[ 108 ]} Copyright 2016, Nature. H) Schematic of the fluorescence generation by the accumulation of probe plus miRNA in the glucan phase and undergoing hybridization.^{[ 109 ]} Copyright 2024, Royal Society of Chemistry. I) Optical images of polydimethylsiloxane (PDMS) microwells (top) and transferred LiCl (3 m) aqueous solution microlenses (bottom), scale bar: 10 µm, and a schematic of the microlens‐substrate set‐up for detection.^{[ 90 ]} Copyright 2021, Wiley.

Figure 3

A) Schematic of the experimental setup of sprayed droplets coupled mass spectrum detection.^{[ 126 ]} Copyright 2024, American Chemical Society. B) (i) Schematic showing the collisions of emulsion droplets on Pt microelectrode and subsequent oxidation of H₂O₂; (ii) The characteristic transient of currents resulted from the collision of a single droplet with the microelectrode (rapid rise followed by an exponential decay).^{[ 80 ]} Copyright 2024, National Academy of Sciences. C) (i) Schematics of the color changes during enzymatic cascades in aqueous microdroplet; (ii) Time‐dependent evolution of the cascade reaction progress tracked via optical images of aqueous microdroplets. Reproduced with permission.^{[ 128 ]} Copyright 2021, Royal Society of Chemistry. D) (i) Decolorizatoin process in single surface nanodroplets for two Chinese spirits with varied hoard years, which can be monitored by the portable device; (ii) Sketch of nanoextraction and colorimetric reaction in surface microdroplets; (iii) The decoloration time (t) as a function of the nanodroplet lateral radius (R). Reproduced with permission.^{[ 124 ]} Copyright 2020, American Chemical Society, and^{[ 129 ]}. Copyright 2022, American Chemical Society. E) Workflow sketch of Surface‐enhanced Raman Spectroscopy detection of single‐cell‐secreted growth factors in microdroplets based on the cell surface bioconjugation. Reproduced with permission.^{[ 91 ]} Copyright 2022, American Chemical Society.

Figure 4

A) (i) Schematic illustrating the formation of a VE film and surface nanodroplets on PDMS via a solvent exchange process in a 3D‐printed microchamber; (ii) FESEM image of Ag nanoparticles formed on the PDMS substrate; (iii) 2D Raman mapping of Rhodamine 6G (R6G) at 612 cm⁻¹ on the Ag nanoparticles–PDMS substrate. Reproduced with permission.^{[ 145 ]} Copyright 2024, American Chemical Society. B) (i) Schematic overview of the solvent exchange process; (ii) Biphasic reaction mechanism between VE nanodroplets and Ag⁺ ions for Ag nanostructure synthesis; (iii) VE surface nanodroplets on a patterned substrate and corresponding scanning electron microscopy (SEM) image of Ag nanoring arrays; (iv) 2D Raman intensity mapping of the Ag nanorings. Reproduced with permission.^{[ 146 ]} Copyright 2023, Elsevier. C) (i) SEM image of a single MIL‐100/Ag dome; (ii) Elemental mapping of the MIL‐100/Ag structure; (iii) Linear correlation between methylene blue concentration and Raman intensity at 1625 cm⁻¹. Reproduced with permission.^{[ 31 ]} Copyright 2025, Elsevier.

Figure 5

A) (i) Microwell array on the surface of the PDMS film; (ii) SEM image of Ag nanostructures formed within the microwells; (iii) Confocal image of VE droplets confined in microwells (scale bar: 200 µm); (iv) Schematic illustrating the nucleation and growth of Ag nanoparticles at the VE droplet interface in the presence of AgNO₃ precursor. B) Illustration of droplet evaporation on the microwell‐patterned PDMS film, with arrows marking the receding contact line that leads to stronger SERS signals at the rim. C) SERS spectra acquired from 100 different locations across the plasmonic PDMS film. D) Linear relationship between SERS intensity at 612 cm⁻¹ and Rhodamine 6G concentration (error bars represent >10 spectra from varied positions). E) Demonstration of the flexibility and mechanical robustness of the plasmonic PDMS film (scale bar: 2 cm), applied on an apple surface for in situ pesticide detection. Reproduced with permission.^{[ 34 ]} Copyright 2024, Wiley.

Figure 6

A) (i) Optical image of an insect eye; (ii) Schematic of compound eye (CE) fabrication using a microlens array (MLA) template derived from surface microdroplets; (iii) Photograph of the resulting CE featuring microwells on a curved surface; (iv) SEM image of the CE top surface. B) Optical projection of a glass bead with and without CE. C) Focal points observed at 0° (top), +59° (middle), and −59° (bottom) using a 5‐mm CE. D) Fluorescence signals of Rhodamine 6G (10⁻⁸ and 10⁻⁹ m) imaged through glass, single lens (no ommatidia), and CE. E) Application of CE for Surface‐Enhanced Raman Spectroscopy (SERS) detection and representative Raman spectrum of adenine. F) Comparison of SERS signal intensity for different adenine concentrations, with and without CE. Reproduced with permission.^{[ 36 ]} Copyright 2024, Wiley.

Figure 7

A) Schematic of the formation of triethylamine‐functionalized polystyrene/methyl methacrylate (PMMA) microspheres from microdroplets, with optical images showing the transition from microdroplets (top) to microspheres (bottom). Reproduced with permission.^{[ 24 ]} Copyright 2025, Elsevier. B) Photopolymerization of a poly(anhydride) network and subsequent photodegradation in a dye aqueous solution using catalyst‐loaded microcapsules under UV light exposure (40 min). Reproduced with permission.^{[ 160 ]} Copyright 2020, Royal Society of Chemistry. C) Sketch of the solvent exchange method to form surface microdroplets followed by local photopolymerization to fabricate microlenses (ML). Reproduced with permission.^{[ 39 ]} Copyright 2024, Wiley. D) Optical images of random PMMA ML (MLR) and microlens arrays (MLA) on homogeneous and prepatterned substrates.^{[ 38 ]} Copyright 2023, Elsevier. E) Simulated cross‐sectional light intensity profiles of single ML from PMMA MLR (left) and MLA (right).^{[ 23 ]} Copyright 2022, American Chemical Society. F) Correlation between photodegradation efficiency (η) and ML focusing effect strength (I_Total).^{[ 173 ]} Copyright 2024, Wiley. G) Schematic of a surface ML‐functionalized curved substrate using a flower‐shaped reactor as an example.^{[ 173 ]} Copyright 2024, Wiley. H) MLA‐functionalized reactor for photocatalytic degradation tested in real river water under varying solar intensities.^{[ 38 ]} Copyright 2023, Elsevier. I) Sketch of a non‐contact‐mode reactor integrated with a concave MLA PDMS film, including a photo of the film in natural light and SEM image of the surface. J) Simulated cross‐sectional (top) and top‐view (bottom) intensity profiles of concave MLA.^{[ 39 ]} Copyright 2024, Wiley.

Figure 8

A) Fabrication procedure and photograph of ZnO nanocaps‐functionalized vials using solvent exchange and hydrothermal treatment. Reproduced with permission.^{[ 40 ]} Copyright 2025, Elsevier. B) Color change of methyl orange solution in a ZnO nanocaps‐functionalized vial under simulated indoor sunlight. Reproduced with permission.^{[ 26 ]} Copyright 2023, American Chemical Society. C) SEM (top view) and D) PFIB‐SEM (cross‐sectional view) images of ZnO nanorods. Reproduced with permission.^{[ 40 ]} Copyright 2025, Elsevier. E) Schematic illustration of ZnO‐coated hollow micro glass bubbles, with FESEM images of F) uncoated MGB and G) ZnO‐coated MGB. Reproduced with permission.^{[ 203 ]} Copyright 2024, Elsevier.

Figure 9

A) A sketch of the microfluidic device for the synthesis of Pt nanoparticles, with silicone oil as the carrier phase. TEM and HRTEM images of Pt cubes were displayed. Reproduced with permission.^{[ 209 ]} Copyright 2022, American Chemical Society. B) Schematic of the microdroplet generated with a microfluidic device for metallacage preparation.^{[ 210 ]} Copyright 2023, Wiley. C) (i) Schematic of the biphasic reaction at surface nanodroplets interface to form flower‐shaped surface gold nanostructures; (ii) Confocal microscopy images of the binary surface nanodroplets at 500 s and 1000 s; (iii) Rate of catalytic reaction of the gold nanostructures as a function of the droplet radius. Reproduced with permission.^{[ 211 ]} Copyright 2024, Wiley. D) (i) Reaction schematic of HAuCl₄ precursor interaction with the 1‐dodecanthiol surface nanodroplets; (ii) Optical images of the array of gold nanocraters (GNCs). Reproduced with permission.^{[ 212 ]} Copyright 2022, Elsevier. E) (i) Formation of MOF at the liquid–liquid interface of surface microdroplets; (ii) 3D‐stacked confocal fluorescence microscopy image of surface microdroplets dyed with FITC‐Dextran (up) and AFM image of a single MIL‐100 dome (down); (iii) SEM‐EDX images of a single MIL‐100 dome; (iv) Reusability tests of the MIL‐100 substrate for model compounds removal in water. Reproduced with permission.^{[ 31 ]} Copyright 2025, Elsevier.

Figure 10

A) Schematic illustration showing the film zone where the transesterification reaction occurs and the subsequent formation of a glycerol layer at the interface. Reproduced with permission.^{[ 232 ]} Copyright 2019, Elsevier Ltd. B) Mechanism of the enzymatic transesterification of soybean oil: (i) the closed (inactive) and open (active) conformations of the enzyme structure, and (ii) schematic of enzymatic interfacial activation. Reproduced with permission.^{[ 237 ]} Copyright 2024, Elsevier B.V. C) Schematic highlighting the interfacial localization of microcapsule systems with varied wettability. Reproduced with permission.^{[ 238 ]} Copyright 2022, American Chemical Society. D) Catalytic conversion of substrates in different biphasic systems. E) Schematic representation of interfacial biocatalysis and lipase recycling using O/W/O Pickering double emulsions co‐stabilized by PNIPAM‐co‐4VP microgels and lipase. Reproduced with permission.^{[ 239 ]} Copyright 2022, John Wiley and Sons. F) Magneto‐optical responsive behavior of the Pickering emulsion. Reproduced with permission.^{[ 240 ]} Copyright 2024, Elsevier.

Figure 11

A) Schematic representation of the programmable reaction process via CO₂/vacuum treatment.Reproduced with permission.^{[ 249 ]} Copyright 2019, American Chemical Society. B) Histogram of organosilane conversion efficiency in different systems: Pickering emulsion, the presence of organic solvent, and two‐phase system. Reproduced with permission.^{[ 249 ]} Copyright 2019, American Chemical Society. C) Schematic of dispersed organosilane microdroplets in an aqueous environment. Reproduced with permission.^{[ 53 ]} Copyright 2025, American Chemical Society. D) Ternary phase diagram of the PMH‐acetone‐water system used to prepare surfactant‐free Ouzo emulsions. Reproduced with permission.^{[ 54 ]} Copyright 2025, Elsevier Inc. E) Effect of surface‐to‐volume ratio on H₂ conversion yield ($\eta$). Reproduced with permission.^{[ 54 ]} Copyright 2025, Elsevier Inc. F) Schematic of OH⁻ ion transfer from the aqueous to the organic phase mediated by CTAB.Reproduced with permission.^{[ 53 ]} Copyright 2025, American Chemical Society. G) Influence of surfactant type on the interfacial hydrogen evolution reaction: (left) cationic surfactants enable fast hydroxide ion diffusion; (right) anionic and non‐ionic surfactants hinder diffusion. Reproduced with permission.^{[ 53 ]} Copyright 2025, American Chemical Society.

Figure 12

A) Time evolution of H₂ volume at varying mixing factors (n) for binary (i) PMH‐octanol and (ii) PMH‐decanol microdroplets. Reproduced with permission.^{[ 254 ]} Copyright 2025, Elsevier B.V. B) Schematic of a single organosilane droplet reacting in a surrounding sodium hydroxide solution, where dehydrogenation proceeds within the droplet via alcoholysis and at the droplet‐water interface via hydrolysis. Reproduced with permission.^{[ 254 ]} Copyright 2025, Elsevier B.V. C) Schematic of an energy conversion system utilizing a single‐unit fuel cell powered by H₂ generated from a bulk reaction. Reproduced with permission.^{[ 254 ]} Copyright 2025, Elsevier B.V.

Figure 13

A) Ultrasonic spray setup coupled with mass spectrometry for real‐time monitoring of ammonia formation. Reproduced with permission.^{[ 50 ]} Copyright 2023, Proceedings of the National Academy of Sciences. B) Schematic of the microdroplet‐based system for urea synthesis from CO₂ and N₂.^{[ 261 ]} C) Schematic of the ultrasonic atomization method for nitrogen fixation. Time‐dependent evolution of nitrogen fixation products under D) air and E) N₂ atmospheres. Reproduced with permission.^{[ 51 ]} Copyright 2025, American Chemical Society. F) Experimental setup for HNO₃ production. G) ${\mathrm{NO}}_3^{-}$ yield as a function of N₂ bubbling time. Reproduced with permission.^{[ 262 ]} Copyright 2024, American Chemical Society.

Figure 14

A) Schematic representation of polymer‐based self‐swimming microdroplet. Reproduced with permission.^{[ 302 ]} Copyright 2025, American Chemical Society. B) Mechanistic interpretation of vacuole growth rates and popping‐based motility. Reproduced with permission.^{[ 303 ]} C) (i) Schematic representation of a chemically active protein condensate. Polymers act as depletants, triggering condensation, while enzyme‐rich droplets function as micro‐chemical reactors. (ii) Image sequence of chemically active droplets on a non‐wetting surface, showing the primary droplet (p), satellite droplets (s), and the direction of motion indicated in the first panel. Reproduced with permission.^{[ 304 ]} D) Schematic illustration of different motile behaviors involving biomolecules. Reproduced with permission.^{[ 305 ]} Copyright 2023, Elsevier Inc.

Figure 15

A) Schematic representations of various mechanisms underlying light‐induced droplet motion: (i) A wettability gradient induces a difference in contact angles between the rear and front of the droplet, generating a Laplace pressure gradient and internal flow. Reproduced with permission.^{[ 306 ]} Copyright 2012, Royal Society of Chemistry. (ii) Photodeformation‐driven motion of a fully wetting liquid slug confined in a tubular microactuator. Reproduced with permission.^{[ 307 ]} Copyright 2016, Macmillan Publishers Limited. (iii) Trajectories of tracer beads within a water droplet subjected to near‐infrared (NIR) irradiation at one edge. Reproduced with permission.^{[ 308 ]} Copyright 2018, Wiley–VCH. (iv) Light‐guided droplet manipulation on a photo‐induced charged surface enabling real‐time, in situ generation of free surface charges. Reproduced with permission.^{[ 309 ]} B,C) Schematic and time‐lapse snapshots of the experimental setup and motion mechanism of a light‐driven water droplet in an oil phase. Scale bar: 5 mm. Reproduced with permission.^{[ 310 ]} Copyright 2020, Wiley–VCH. D) Schematic illustration of interconnected droplet–filament networks with photocontrolled drain droplets. Reproduced with permission.^{[ 311 ]} E) Schematic illustration of a light‐induced charged slippery surface (LICS) for droplet manipulation. Reproduced with permission.^{[ 312 ]}

Figure 16

A) (i) Schematic representation of the detachment and rise of a PS‐DMCHA microdroplet from a solid substrate. (ii) Archimedes (Ar) numbers for the non‐rising and rising droplet. (iii) A sketch of setting time for launching the drop with varied initial conditions. Reproduced with permission.^{[ 316 ]} Copyright 2023, Wiley‐VCH. B) NaOH‐catalyzed dehydrogenation of polymethylhydrosiloxane (PMH) to produce hydrogen gas (H₂). C) Side view photo and D) (i) Schematic illustration and snapshots of (i) H₂ bubble nucleation, growth, and in‐drop bubble formation. (ii) Bond numbers for numerous droplets and various NaOH concentrations. Reproduced with permission.^{[ 317 ]} Copyright 2024, Wiley‐VCH. E) Snapshots showing the growth of H₂ bubbles from binary PMH–alcohol microdroplets of varying composition. Molecular simulations illustrating nanobubble nucleation and growth at three different interfacial tensions ($\gamma$), left: ${\gamma }_{12}$ = 40 mN m⁻¹, middle: ${\gamma }_{12}$ = 23 mN m⁻¹, and right: ${\gamma }_{12}$ = 10 mN m⁻¹. Reproduced with permission.^{[ 254 ]} Copyright 2025, Elsevier B.V.