Continuous and scalable production of well-controlled noble-metal nanocrystals in milliliter-sized droplet reactors

Continuous-Flow and Scalable Synthesis of Pd@PtnL Core-Shell Nanocrystals with Enhanced Activity toward Oxygen Reduction

We report a scalable method based on continuous-flow reactors for conformally coating the surfaces of facet-controlled Pd nanocrystals with uniform, ultrathin shells made of Pt. The key to the success of such an approach is the identification of a proper polyol to generate the Pt atoms at a relatively slow rate to ensure adequate surface diffusion and thus the formation of uniform shells in a layer-by-layer fashion. We first demonstrate the concept using the production of Pd@PtnL (n = 2-5) core-shell icosahedral nanocrystals and then have the strategy successfully extended to the syntheses of Pd@PtnL cubic and octahedral nanocrystals. All these core-shell nanocrystals showed great enhancement in catalytic activity toward the oxygen reduction reaction. Our results suggest that seed-mediated growth can be combined with a continuous-flow reactor to achieve scalable production of bimetallic and even trimetallic nanocrystals with controlled sizes, shapes, compositions, and properties.

Continuous production of bimetallic nanoparticles on carbon nanotubes based on 3D-printed microfluidics

Nanoparticle-functionalized carbon nanotubes are promising in many research fields, especially in sensing, due to their intriguing performance in catalysis. However, these nanomaterials are mainly produced through batch processes under harsh conditions, thus encountering inherent limitations of low throughput and uncontrollable morphology of functional nanoparticles (NPs). In this work, we propose a method for high-yield and continuous production of bimetallic (Pt-Pd) NPs on multi-walled carbon nanotubes (MWCNTs) at room temperature through a custom 3D-printed microfluidic platform. A homogenous particle nucleation and growth environment could be created on the microfluidic platform that was equipped with two 3D-printed micromixers. Pt-Pd NPs loaded on MWCNTs were prepared in the microfluidic platform with high throughput and controlled size, dispersity and composition. The synthetic parameters for these nanocomposites were investigated to optimize their electrocatalytic performance. The optimized nanocomposites exhibited excellent electrocatalytic activity with exceptional sensitivity and wide detection range, superior to their counterparts prepared via conventional approaches. This method proposed here could be further adapted for manufacturing other catalyst support materials, opening more avenues for future large-scale production and catalytic investigation of functional nanomaterials.

High-Throughput Microfluidic Production of Bimetallic Nanoparticles on MXene Nanosheets and Application in Hydrogen Peroxide Detection

Nanoparticle-functionalized transition-metal carbides and nitrides (MXenes) have attracted extensive attention in electrochemical detection owing to their excellent catalytic performance. However, the mainstream synthetic routes rely on the batch method requiring strict experimental conditions, generally leading to low yield and poor size tunability of particles. Herein, we report a high-throughput and continuous microfluidic platform for preparing a functional MXene (Ti₃C₂T_x) with bimetallic nanoparticles (Pt-Pd NPs) at room temperature. Two 3D micromixers with helical elements were integrated into the microfluidic platform to enhance the secondary flow for promoting transport and reaction in the synthesis process. The rapid mixing and strong vortices in these 3D micromixers prevent aggregation of NPs in the synthesis process, enabling a homogeneous distribution of Pt-Pd NPs. In this study, Pt-Pd NPs loaded on the MXene nanosheets were synthesized under various hydrodynamic conditions of 1-15 mL min^-1 with controlled sizes, densities, and compositions. The mean size of Pt-Pd NPs could be readily controlled within the range 2.4-9.3 nm with high production rates up to 13 mg min^-1. In addition, synthetic and electrochemical parameters were separately optimized to improve the electrochemical performance of Ti₃C₂T_x/Pt-Pd. Finally, the optimized Ti₃C₂T_x/Pt-Pd was used for hydrogen peroxide (H₂O₂) detection and shows excellent electrocatalytic activity. The electrode modified with Ti₃C₂T_x/Pt-Pd here presents a wide detection range for H₂O₂ from 1 to 12 000 μM with a limit of detection down to 0.3 μM and a sensitivity up to 300 μA mM^-1 cm^-2, superior to those prepared in the traditional batch method. The proposed microfluidic approach could greatly enhance the electrochemical performance of Ti₃C₂T_x/Pt-Pd, and sheds new light on the large-scale production and catalytic application of the functional nanocomposites.

Continuous and Scalable Production of Platinum Nanocubes with Uniform and Controllable Sizes in Air-Free Droplet Reactors

Platinum (Pt) nanocrystals hold the key to a variety of catalytic applications, and those with a cubic shape are attractive as a reference catalyst due to their well-defined {100} facets on the surface. Here we demonstrate the use of droplet reactors as a viable platform for the continuous and scalable production of Pt nanocubes with uniform and controllable sizes. The synthesis was found to be sensitive to the O₂ from air because of the oxidative etching associated with the O₂/Br^- pair. As such, either silicone oil or an inert gas had to be employed as the carrier phase to keep the droplets isolated from air. By controlling the amounts of the precursor and halide ions, the edge length of the Pt nanocubes could be tuned from 5-7 nm. In the setting of a millifluidic device, the droplet reactors could be used to achieve a production rate as high as 31.8 mg min^-1, about 10-100 times greater than what has been reported in the literature. We also evaluated the electrocatalytic properties of the as-obtained Pt nanocubes toward the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). For the Pt nanocubes of 6 nm in edge length, they showed a specific activity of 0.27 mA cm^-2 toward ORR at 0.9 V and a specific activity of 0.96 mA cm^-2 toward MOR at the anodic potential.

Sustainable, Green, and Continuous Synthesis of Fivefold Palladium Nanorods Using l-Ascorbic Acid in a Segmented Millifluidic Flow Reactor

Pd nanorods (PdNRs) have recently come to attention due to their wide array of applications. The green synthesis of PdNR with a relatively high yield and high aspect ratio is challenging. A continuous millifluidic flow reactor (CMFR) has been explored to precisely control mass and heat transfer as well as mixing in the PdNR synthesis processes. CMFRs demonstrate a few drawbacks, such as the presence of parabolic velocity profile in the laminar flow of the reaction solution, causing uneven axial residence time distribution. The CMFRs are likely to show irreversible fouling, which may cause the product quality to deteriorate or result in the channel being clogged. These shortcomings can be avoided or minimized using a segmented millifluidic flow reactor (SMFR) that consists of the solution forming a train of individual segments in another inert medium. This study explores the use of a sustainable reducing agent (l-ascorbic acid) in the presence of potassium bromide (KBr) as the capping agent and poly(vinyl pyrrolidone) (PVP) as the stabilizing agent for PdNR synthesis in an SMFR employing compartmentalized flow of a reaction solution, in which liquid segments consisting of a reaction solution will be immersed in the steam generated by boiling of the solvent water. The effect of reaction parameters such as reagent concentration has been studied on the size and morphology of synthesized Pd nanostructures. A kinetic study has been conducted to calculate the rate of reduction that can be used as a quantitative measure for manipulation of the type and relative concentration of initially formed seeds. It has been shown that the initial reduction rate during the first 45 min of residence time of the millifluidic reactor is about 66% faster compared to the rest of the reaction. A filtration procedure has been utilized to separate Pd nanostructures other than nanorods synthesized in the SMFR.

Scalable Production of High-Quality Silver Nanowires via Continuous-Flow Droplet Synthesis

Silver nanowires (Ag NWs) have shown great potential in next-generation flexible displays, due to their superior electronic, optical, and mechanical properties. However, as with most nanomaterials, a limited production capacity and poor reproduction quality, based on the batch reaction, largely hinder their application. Here, we applied continuous-flow synthesis for the scalable and high-quality production of Ag NWs, and built a pilot-scale line for kilogram-level per day production. In addition, we found that trace quantities of water could generate sufficient vapor as a spacer under high temperature to efficiently prevent the back-flow or mixed-flow of the reaction solution. With an optimized synthetic formula, a mass production of pure Ag NWs of 36.5 g/h was achieved by a multiple-channel, continuous-flow reactor.

Microfluidics-enabled functional 3D printing

Microfluidic technology has established itself as a powerful tool to enable highly precise spatiotemporal control over fluid streams for mixing, separations, biochemical reactions, and material synthesis. 3D printing technologies such as extrusion-based printing, inkjet, and stereolithography share similar length scales and fundamentals of fluid handling with microfluidics. The advanced fluidic manipulation capabilities afforded by microfluidics can thus be potentially leveraged to enhance the performance of existing 3D printing technologies or even develop new approaches to additive manufacturing. This review discusses recent developments in integrating microfluidic elements with several well-established 3D printing technologies, highlighting the trend of using microfluidic approaches to achieve functional and multimaterial 3D printing as well as to identify potential future research directions in this emergent area.

Continuous Flow Synthesis of Persistent Luminescent Chromium-Doped Zinc Gallate Nanoparticles

Near-infrared persistent luminescent (or afterglow) nanoparticles with the biologically appropriate size are promising materials for background-free imaging applications, while the conventional batch synthesis hardly allows for reproducibility in controlling particle size because of the random variations of reaction parameters. Here, highly efficient chemistry was matched with an automated continuous flow approach for directly synthesizing differently sized ZnGa₂O₄:Cr³⁺ (ZGC) nanoparticles exhibiting long persistent luminescence. The key flow factors responsible for regulating the particle formation process, especially the high pressure-temperature and varied residence time, were investigated to be able to tune the particle size from 2 to 6 nm and to improve the persistent luminescence. Upon silica shell encapsulation of the nanoparticles accompanied by an annealing process, the persistent luminescence of the resulting particles was remarkably enhanced. High-fidelity automated flow chemistry demonstrated here offers an alternative for producing ZGC nanoparticles and will be helpful for other compositionally complex metal oxide nanoparticles.

Stabilizing Pt-Based Electrocatalysts for Oxygen Reduction Reaction: Fundamental Understanding and Design Strategies

Proton exchange membrane fuel cells (PEMFCs) with high efficiency and nonpollution characteristics have attracted massive attention from both academic and industrial communities due to their irreplaceable roles in building the future sustainable energy system. However, the stability issue of Pt-based catalysts for oxygen reduction reaction (ORR) has become a central constraint to the widespread deployment of the devices relative to the catalytic activity. This review aims to provide comprehensive insights into how to improve the stability of Pt-based catalysts for ORR. First, the basic physical chemistry behind the catalyst degradation, including the fundamental understandings of carbon corrosion, catalyst dissolution, and particle sintering, is highlighted. After a discussion of advanced characterization techniques for the catalyst degradation, the design strategies for improving the stability of Pt-based catalysts are summarized. Finally, further insights into the remaining challenges and future research directions are also provided.

Nanocatalysts in electrosynthesis

Electrosynthesis is to use electricity to drive chemical reactions for chemical synthesis and is potentially a green approach to fuel and energy sustainability. Nanostructured catalysts play an important role in promoting electrochemical reactions under green chemistry conditions. This perspective first provides a brief tutorial on electrosynthesis and the roles the nanocatalysts play in the synthesis. It then outlines the common strategies used to develop nanocatalysts for hydrogen evolution reaction, CO2 reduction reaction, and biomass upgrading. The perspective further summarizes the current methodologies that have been developed for scaling-up synthesis of nanocatalysts, which will be essential for the electrosynthesis to become a viable industry approach.

Generation and Dynamics of Janus Droplets in Shear-Thinning Fluid Flow in a Double Y-Type Microchannel

Droplets composed of two different materials, or Janus droplets, have diverse applications, including microfluidic digital laboratory systems, DNA chips, and self-assembly systems. A three-dimensional computational study of Janus droplet formation in a double Y-type microfluidic device filled with a shear-thinning fluid is performed by using the multiphaseInterDyMFoam solver of the OpenFOAM, based on a finite-volume method. The bi-phase volume-of-fluid method is adopted to track the interface with an adaptive dynamic mesh refinement for moving interfaces. The formation of Janus droplets in the shear-thinning fluid is characterized in five different states of tubbing, jetting, intermediate, dripping and unstable dripping in a multiphase microsystem under various flow conditions. The formation mechanism of Janus droplets is understood by analyzing the influencing factors, including the flow rates of the continuous phase and of the dispersed phase, surface tension, and non-Newtonian rheological parameters. Studies have found that the formation of the Janus droplets and their sizes are related to the flow rate at the inlet under low capillary numbers. The rheological parameters of shear-thinning fluid have a significant impact on the size of Janus droplets and their formation mechanism. As the apparent viscosity increases, the frequency of Janus droplet formation increases, while the droplet volume decreases. Compared with Newtonian fluid, the Janus droplet is more readily generated in shear-thinning fluid due to the interlay of diminishing viscous force, surface tension, and pressure drop.

Microfluidic High-Throughput Platforms for Discovery of Novel Materials

High-throughput screening is a potent technique to accelerate the discovery and development of new materials. By performing massive synthesis and characterization processes in parallel, it can rapidly discover materials with desired components, structures and functions. Among the various approaches for high-throughput screening, microfluidic platforms have attracted increasing attention. Compared with many current strategies that are generally based on robotic dispensers and automatic microplates, microfluidic platforms can significantly increase the throughput and reduce the consumption of reagents by several orders of magnitude. In this review, we first introduce current advances of the two types of microfluidic high-throughput platforms based on microarrays and microdroplets, respectively. Then the utilization of these platforms for screening different types of materials, including inorganic metals, metal alloys and organic polymers are described in detail. Finally, the challenges and opportunities in this promising field are critically discussed.

Microfluidic synthesis of high-valence programmable atom-like nanoparticles for reliable sensing

Synthesis of programmable atom-like nanoparticles (PANs) with high valences and high yields remains a grand challenge. Here, a novel synthetic strategy of microfluidic galvanic displacement (μ-GD) coupled with microfluidic DNA nanoassembly is advanced for synthesis of single-stranded DNA encoder (SSE)-encoded PANs for reliable surface-enhanced Raman scattering (SERS) sensing. Notably, PANs with high valences (e.g., n-valence, n = 12) are synthesized with high yields (e.g., >80%) owing to the effective control of interfacial reactions sequentially occurring in the microfluidic system. On the basis of this, we present the first demonstration of a PAN-based automatic analytical platform, in which sensor construction, sample loading and on-line monitoring are carried out in the microfluidic system, thus guaranteeing reliable quantitative measurement. In the proof-of-concept demonstration, accurate determination of tetracycline (TET) in serum and milk samples with a high recovery close to 100% and a low relative standard deviation (RSD) less than 5.0% is achieved by using this integrated analytical platform.

Determination of Dynamic Interfacial Tension during the Generation of Tiny Droplets in the Liquid-Liquid Jetting Flow Regime

Liquid-liquid dispersion coupled with droplet formation and mass transfer of surfactants is one of the most typical phenomena in many chemical processes. As in every aspect of this process, the interfacial tension variation caused by the unsaturated adsorption of surfactants on the droplet surface plays an important role. This article focuses on microdroplet formation and the dynamic interfacial behavior of surfactants in the jetting regime. In a capillary embedded step T-junction device, controllable preparation of monodisperse droplets is achieved, and a correlation for predicting droplet sizes is established. A method for measuring the dynamic interfacial tension is provided. Mass transfer coefficients are then calculated for Tween 20 during the droplet formation process by a semiempirical correlation. The results indicate that dynamic interfacial tensions are lower than those obtained when the surfactant is adsorbed to equilibrium. Based on the tip-streaming phenomenon, mass transfer coefficients for Tween 20 can reach up to ∼10^-3 m/s, higher than those obtained in coaxial microfluidic devices. All the preliminary results shed light on the nature of droplet formation and will be of significance for application in industrial apparatuses.

Modular and Integrated Systems for Nanoparticle and Microparticle Synthesis-A Review

Nanoparticles (NPs) and microparticles (MPs) have been widely used in different areas of research such as materials science, energy, and biotechnology. On-demand synthesis of NPs and MPs with desired chemical and physical properties is essential for different applications. However, most of the conventional methods for producing NPs/MPs require bulky and expensive equipment, which occupies large space and generally need complex operation with dedicated expertise and labour. These limitations hinder inexperienced researchers to harness the advantages of NPs and MPs in their fields of research. When problems individual researchers accumulate, the overall interdisciplinary innovations for unleashing a wider range of directions are undermined. In recent years, modular and integrated systems are developed for resolving the ongoing dilemma. In this review, we focus on the development of modular and integrated systems that assist the production of NPs and MPs. We categorise these systems into two major groups: systems for the synthesis of (1) NPs and (2) MPs; systems for producing NPs are further divided into two sections based on top-down and bottom-up approaches. The mechanisms of each synthesis method are explained, and the properties of produced NPs/MPs are compared. Finally, we discuss existing challenges and outline the potentials for the development of modular and integrated systems.

Continuous Synthesis of Nanocrystals via Flow Chemistry Technology

Modern nanotechnologies bring humanity to a new age, and advanced methods for preparing functional nanocrystals are cornerstones. A considerable variety of nanomaterials has been created over the past decades, but few were prepared on the macro scale, even fewer making it to the stage of industrial production. The gap between academic research and engineering production is expected to be filled by flow chemistry technology, which relies on microreactors. Microreaction devices and technologies for synthesizing different kinds of nanocrystals are discussed from an engineering point of view. The advantages of microreactors, the important features of flow chemistry systems, and methods to apply them in the syntheses of salt, oxide, metal, alloy, and quantum dot nanomaterials are summarized. To further exhibit the scaling-up of nanocrystal synthesis, recent reports on using microreactors with gram per hour and larger production rates are highlighted. Finally, an industrial example for preparing 10 tons of CaCO₃ nanoparticles per day is introduced, which shows the great potential for flow chemistry processes to transfer lab research to industry.

Microfluidics and catalyst particles

In this review article, we discuss the latest advances and future perspectives of microfluidics for micro/nanoscale catalyst particle synthesis and analysis. In the first section, we present an overview of the different methods to synthesize catalysts making use of microfluidics and in the second section, we critically review catalyst particle characterization using microfluidics. The strengths and challenges of these approaches are highlighted with various showcases selected from the recent literature. In the third section, we give our opinion on the future perspectives of the combination of catalytic nanostructures and microfluidics. We anticipate that in the synthesis and analysis of individual catalyst particles, generation of higher throughput and better understanding of transport inside individual porous catalyst particles are some of the most important benefits of microfluidics for catalyst research.

Facile synthesis and functionalization of color-tunable Ln3+-doped KGdF4 nanoparticles on a microfluidic platform

Hyaluronic acid (HA)-functionalized lanthanide-doped KGdF₄ nanoparticles were synthesized through two steps on a microfluidic platform. This microfluidic synthesis method allows better control of experimental conditions with lower labor and energy input than traditional beaker synthesis methods for large-scale production of nanoparticles with higher uniformity. First, Ln³⁺-doped KGdF₄ nanoparticles were ultrafast (in minutes) and continuously synthesized using a four-inlets microfluidic chip at room temperature. Then, HA is continuously functionalized on the surface of Ln³⁺-doped KGdF₄ nanoparticles using a T-shape chip through electrostatic adsorption. The synthesized nanoparticles show good uniformity, high biocompatibility, targeted cellular uptake, photoluminescence (PL) and magnetic resonance (MR) properties. This work highlights the potential of microfluidic platform for the development of multifunctional nanoparticles in biomedicine.

Continuous Microfluidic Synthesis of Pd Nanocubes and PdPt Core-Shell Nanoparticles and Their Catalysis of NO2 Reduction

Faceted colloidal nanoparticles are currently of immense interest due to their unique electronic, optical, and catalytic properties. However, continuous flow synthesis that enables rapid formation of faceted nanoparticles of single or multi-elemental composition is not trivial. We present a continuous flow synthesis route for the synthesis of uniformly sized Pd nanocubes and PdPt core-shell nanoparticles in a single-phase microfluidic reactor, which enables rapid formation of shaped nanoparticles with a reaction time of 3 min. The PdPt core-shell nanoparticles feature a dendritic, high surface area with the Pt shell covering the Pd core, as verified using high-resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The Pd nanocubes and PdPt core-shell particles are catalytically tested during NO₂ reduction in the presence of H₂ in a flow pocket reactor. The Pd nanocubes exhibited low-temperature activity (i.e., <136 °C) and poor selectivity performance toward production of N₂O or N₂, whereas PdPt core-shell nanoparticles showed higher activity and were found to achieve better selectivity during NO₂ reduction retaining its basic structure at relatively elevated temperatures, making the PdPt core-shell particles a unique, desirable synergic catalyst material for potential use in NO_x abatement processes.

Producing protein-nanoparticle co-assembly supraparticles by the interfacial instability process

Originally discovered in fundamental research of nanomaterial-biomolecule interactions, protein-nanoparticle co-assembly supraparticles (PNCAS) have become an emerging class of nanomaterials with various biological applications. We apply the interfacial instability process, which was originally reported for forming nanoparticles-encapsulated polymeric micelles, to produce PNCAS. By doing so hydrophobic nanoparticles, which are often the product formed from the upstream nanoparticle synthesis step, can be directly used as the raw materials of the production process of PNCAS. On the other hand, we take advantage of the structural features of protein molecules, in comparison with amphiphilic block copolymers, to mitigate two common problems encountered in the original interfacial instability-mediated nanoparticle encapsulation process, namely (1) poor encapsulation number control and (2) inconvenience and high cost to vary the assembly size. Additionally, we achieve semi-continuous and scalable production of PNCAS by combining the electrospray process and the interfacial instability process. We also conduct proof-of-concept studies of biological applications of the PNCAS products.

Microdroplet Synthesis of Silver Nanoparticles with Controlled Sizes

A method was developed to synthesize silver nanoparticles with controlled size and Localized Surface Plasmon Resonance (LSPR) wavelength. In a microchip, droplets with high monodispersity and stability were produced. Using droplets as microreactors, seed-mediated growth approach was successfully applied for silver nanoparticles preparation. It was observed that nanoparticles size and LSPR wavelength could be optimized via adjusting synthesis conditions, such as droplets heating temperature, reaction time, and concentration of silver seeds and silver nitrate in aqueous phase. These results indicated that the proposed microdevices could provide a convenient and inexpensive approach for preparing nanoparticles with optimum properties.

Superstructure of a Metal-Organic Framework Derived from Microdroplet Flow Reaction: An Intermediate State of Crystallization by Particle Attachment

Understanding the crystallization pathway is of fundamental importance in controlling structures and functionalities for metal-organic frameworks (MOFs), but only few studies have been reported on the mechanism of crystallization for MOFs to date. Here, by using a microdroplet flow (MF) reaction technique, we successfully revealed the different status of HKUST-1 during its crystal growth process. The morphologies and structures of crystals at different stages were recorded and characterized by scanning electron microscopy, transmission electron microscopy, and small-angle X-ray diffraction. Experimental observations clearly demonstrate a process of crystallization by particle attachment (CPA) for crystal growth of HKUST-1 under MF conditions. The superstructure of HKUST-1, which is assembled from oriented attachment of nanosized particles of HKUST-1, is observed at early stage of crystal growth. This type of superstructure gradually transforms to true single crystals through a ripening effect upon increasing residence time, accompanied by increase in dimensions of crystals. Thus, the superstructure is the intermediate state during crystallization and acts as the bridge between disordered reactants and highly ordered single crystals. Based on these findings, the crystal growth of HKUST-1 in MF reaction can be elucidated as a process involving three steps: the generation of nanosized primary particles, the following assembly of the primary particles into a superstructure, and the ripening of superstructure into a crystal. Furthermore, the superstructure of HKUST-1 shows superior performance for CO₂ and CH₄ adsorptions. The CPA mechanism in the crystallization of HKUST-1 demonstrated in this work is in clear contrast to the monomer-by-monomer addition mechanism in classic models of crystal growth. This mechanism could have important reference meaning for understanding the crystal growth mechanism of other type of MOFs or other special morphologies.

Nanomechanical Microfluidic Mixing and Rapid Labeling of Silica Nanoparticles using Allenamide-Thiol Covalent Linkage for Bioimaging

Rapid surface functionalization of nanomaterials using covalent linkage following "green chemistry" remains challenging, and the quest for developing simple protocols is persisting. We report a nanomechanical microfluidic approach for the coupling of allenamide functionalized organic derivatives on the surface of thiol-modified silica nanoparticles using allenamide-thiol chemistry. The coupling principle involves the use of a microfluidic surface acoustic wave device that generates acoustic streaming-based chaotic fluid micromixing that enables mixing of laterally flowing fluids containing active components. This approach was used to demonstrate the direct surface labeling of thiol-modified silica nanoparticles using a selected group of modified fluorescent tags containing allenamide handles and achieved a total labeling efficiency of 83-90%. This green approach enabled a highly efficient surface functionalization under aqueous conditions, with tunable control over the conjugation process via the applied field. The dye-labeled silica particles were characterized using various analytical techniques and found to be biocompatible with potential in live cell bioimaging. It is envisaged that this bioconjugation strategy will find numerous applications in the field of bioimaging and drug delivery.

Gas-phase synthesis of morphology-controlled Pt nanoparticles and their impact on cinnamaldehyde hydrogenation

Pt nanoparticles of which morphology is controlled by gas-phase synthesis using carbon monoxide as a protective agent show high catalytic activity and selectivity for cinnamaldehyde hydrogenation.

Size and shape control of metal nanoparticles in millifluidic reactors

Engineered metal nanoparticles (metal NPs) possess unique size -dependent optical and electronic properties that could enable new applications in biomedicine, energy generation, microelectronics, micro-optics, and catalysis. For metal NPs to make a mark in these fields, however, new synthetic strategies must be developed that permit NP synthesis on the kilogram scale, while maintaining precise control over NP physiochemical properties (size, shape, composition, and surface chemistry). Currently, NP batch syntheses produce product on the milligram scale and rely on synthetic strategies that are not readily amenable to scale-up. Flow reactor systems (including lab-on-a-chip devices) provide a synthesis platform that can circumvent many of the traditional limitations of batch-scale NP syntheses. These reactors provide more uniform reagent mixing, more uniform heat transfer, opportunities to interface in situ monitoring technology, and allow product yield to be scaled up simply by running multiple reactors in parallel. While many NP syntheses have been successfully transferred to microfluidic reactor systems, microfluidic reactor fabrication is time intensive and typically requires sophisticated lithography facilities. Consequently, millifluidic flow reactors (reactors with channel dimensions of 0.5–10.0 mm) are gaining popularity in NP synthesis. These millifluidic reactors provide many of the same synthetic advantages as microfluidic devices, but are simpler to construct, easier to reconfigure, and more straightforward to interface with in situ monitoring techniques. In this chapter, we will discuss the progress that has been made in developing millifluidic reactors for functionalized metal NP synthesis. First, we will review the basic wet-chemical strategies used to control metal NP size and shape in batch reactors. We will then survey some of the basic principles of millifluidic device design, construction, and operation. We will also discuss the potential for incorporating in situ monitoring for quality control during synthesis. We will conclude by highlighting some particularly relevant examples of millifluidic metal NP synthesis that have set new standards for metal NP size, shape, and surface chemistry control.

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Hollow Metal Nanocrystals with Ultrathin, Porous Walls and Well-Controlled Surface Structures

Recent developments of a novel class of catalytic materials built on hollow nanocrystals having ultrathin, porous walls, and well-controlled surface structures are discussed, with a focus on platinum and the oxygen reduction reaction (ORR). An introduction is given to the critical role of platinum in the proton exchange membrane fuel cells, and the pressing need to develop a strategy for achieving cost-effective and sustainable use of this precious metal. How to maximize the mass activity of ORR catalysts based on platinum by rationally engineering the surface structure while increasing the utilization efficiency of atoms is then discussed. After reporting on the synthetic methods involving galvanic replacement and seed-mediated growth followed by etching, respectively, a number of examples to demonstrate the enhancement in activity and durability for this new class of catalytic materials are showcased. The feasibility to have the methodology extended from platinum to other precious metals such as gold and ruthenium is highlighted. In conclusion, some of the remaining issues and emerging solutions are examined.

Continuous synthesis of gold nanoparticles in micro- and millifluidic systems

Gold nanomaterials have diverse applications ranging from healthcare and nanomedicine to analytical sciences and catalysis. Microfluidic and millifluidic reactors offer multiple advantages for their synthesis and manufacturing, including controlled or fast mixing, accurate reaction time control and excellent heat transfer. These advantages are demonstrated by reviewing gold nanoparticle synthesis strategies in flow devices. However, there are still challenges to be resolved, such as reactor fouling, particularly if robust manufacturing processes are to be developed to achieve the desired targets in terms of nanoparticle size, size distribution, surface properties, process throughput and robustness. Solutions to these challenges are more effective through a coordinated approach from chemists, engineers and physicists, which has at its core a qualitative and quantitative understanding of the synthesis processes and reactor operation. This is important as nanoparticle synthesis is complex, encompassing multiple phenomena interacting with each other, often taking place at short timescales. The proposed methodology for the development of reactors and processes is generic and contains various interconnected considerations. It aims to be a starting point towards rigorous design procedures for the robust and reproducible continuous flow synthesis of gold nanoparticles.

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A Droplet-Reactor System Capable of Automation for the Continuous and Scalable Production of Noble-Metal Nanocrystals

Noble-metal nanocrystals with well-controlled shapes or morphologies are of great interest for a variety of applications. To utilize these nanomaterials in consumer products, one has to produce the colloidal nanocrystals in large quantities while maintaining good control over their physical parameters and properties. Droplet reactors have shown great potential for the continuous and scalable production of colloidal nanocrystals with controlled shapes. However, the efficiencies of most previously reported systems are still limited because of the complex post-treatment procedures. For example, the mixture of silicone oil and an aqueous suspension of solid products has to be separated by leveraging their miscibility and difference in density, while the solid products often need to be purified and concentrated by centrifugation. Herein, we report the design and construction of a droplet-reactor system that include new features such as a homemade unit for the automatic separation of silicone oil from the aqueous phase as well as a cross-flow filtration unit for the effective purification and concentration of the nanocrystals. Using various types of Pd nanocrystals as examples, we have demonstrated the feasibility of using this system to automatically produce and collect samples with uniform sizes and well-controlled shapes.

Oscillating dispersed-phase co-flow microfluidic droplet generation: Multi-droplet size effect

Controllable generation of microdroplets at desired sizes and throughputs is important in many applications. Many biological assays require size-optimized droplets for effective encapsulation of analytes and reagents. To perform size optimization, different-size droplets must be generated from identical sources of samples to prevent potential cross-sample variations or other sources of error. In this paper, we introduce a novel alteration of the co-flow droplet generation technique to achieve multi-size generation of monodispersed droplets. Using a custom-made mechanism, we oscillate the disperse-phase (d-phase) flow nozzle perpendicular to the continuous phase (c-phase) flow in a co-flow channel. Oscillation of the d-phase nozzle introduces an additional lateral drag force to the growing droplets while exposing them to various levels of axial drag owing to the parabolic velocity distribution of the c-phase flow. Superimposing both effects results in simultaneous and repeatable generation of monodispersed droplets with different sizes. The effect of nozzle oscillation frequency (f = 0-15 Hz) on droplet generation at different d-phase (Qd = 0.05, 0.10, and 0.50 ml/min) and c-phase (Qc = 2, 5, and 10 ml/min) flow rates was studied. A wide range of monodispersed droplets (4nl-4 μl) were generated using this method. Droplet sizes were directly proportional to the We number and inversely proportional to the Ca number and oscillation frequency. Our technique is promising for applications such as aqueous two-phase systems, where due to inherently low interfacial tension, the d-phase flow forms a long stable jet which can be broken into droplets using the additional oscillatory drag in our device.

Trapping a moving droplet train by bubble guidance in microfluidic networks

Trapping a train of moving droplets into preset positions within a microfluidic device facilitates the long-term observation of biochemical reactions inside the droplets. In this paper, a new bubble-guided trapping method, which can remarkably improve the limited narrow two-phase flow rate range of uniform trapping, was proposed by taking advantage of the unique physical property that bubbles do not coalescence with two-phase fluids and the hydrodynamic characteristic of large flow resistance of bubbles. The flow behaviors of bubble-free and bubble-guided droplet trains were compared and analyzed under the same two-phase flow rates. The experimental results show that the droplets trapped by bubble-free guided trapping exhibit the four trapping modes of sequentially uniform trapping, non-uniform trapping induced by break-up and collision, and failed trapping due to squeezing through, and the droplets exhibit the desired uniform trapping in a relatively small two-phase flow rate range. Compared with bubble-free guided droplets, bubble-guided droplets also show four trapping modes. However, the two-phase flow rate range in which uniform trapping occurs is increased significantly and the uniformity of the trapped droplet array is improved. This investigation is beneficial to enhance the applicability of microfluidic chips for storing droplets in a passive way.

Kinetically controlled synthesis of bimetallic nanostructures by flowrate manipulation in a continuous flow droplet reactor

We show that different Au–Pd nanoparticles, ranging from sharp-branched octopods to core@shell octahedra, can be achieved by inline manipulation of reagent flowrates in a microreactor for seeded growth.

Controllable synthesis of nanocrystals in droplet reactors

In recent years, a broad range of nanocrystals have been synthesized in droplet-based microfluidic reactors which provide obvious advantages, such as accurate manipulation, better reproducibility and reliable automation. In this review, we initially introduce general concepts of droplet reactors followed by discussions of their main functional regions including droplet generation, mixing of reactants, reaction controlling, in situ monitoring, and reaction quenching. Subsequently, the enhanced mass and heat transport properties are discussed. Next, we focus on research frontiers including sequential multistep synthesis, intelligent synthesis, reliable scale-up synthesis, and interfacial synthesis. Finally, we end with an outlook on droplet reactors, especially highlighting some aspects such as large-scale production, the integrated process of synthesis and post-synthetic treatments, automated droplet reactors with in situ monitoring and optimizing algorithms, and rapidly developing strategies for interfacial synthesis.

Synthesis of Core@Shell Nanostructures in a Continuous Flow Droplet Reactor: Controlling Structure through Relative Flow Rates

Bimetallic nanostructures are primarily synthesized in small volume batches. However, droplet-based reactors are receiving attention due to their ability to maintain thermal and compositional equilibrium within and between droplets, enabling flow operations for inline analyses and the scale-up of nanomaterial syntheses. Here, the syntheses of shape-controlled core@shell Au@Pd nanostructures with variable shell thicknesses are reported through control of the relative flow rates of reagents within the microreactor. Specifically, Pd shells were grown on cubic or octahedral Au seeds, selected as a model system. In batch reactions, shell thickness is determined by precursor concentration; however, as shown here, precursor feedstock concentration can be held constant, with the precursor concentration within the droplets being controlled through relative flow rates. This approach allows process conditions to be modified inline rather than from batch to batch to achieve particles with different shell thicknesses, and this procedure should be applicable to other multicomponent systems.

Emerging Droplet Microfluidics

Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation. The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development. We believe this review will promote communications among biology, chemistry, physics, and materials science.

Continuous In-Flight Synthesis for On-Demand Delivery of Ligand-Free Colloidal Gold Nanoparticles

We demonstrate an entirely new method of nanoparticle chemical synthesis based on liquid droplet irradiation with ultralow (<0.1 eV) energy electrons. While nanoparticle formation via high energy radiolysis or transmission electron microscopy-based electron bombardment is well-understood, we have developed a source of electrons with energies close to thermal which leads to a number of important and unique benefits. The charged species, including the growing nanoparticles, are held in an ultrathin surface reaction zone which enables extremely rapid precursor reduction. In a proof-of-principle demonstration, we obtain small-diameter Au nanoparticles (∼4 nm) with tight control of polydispersity, in under 150 μs. The precursor was almost completely reduced in this period, and the resultant nanoparticles were water-soluble and free of surfactant or additional ligand chemistry. Nanoparticle synthesis rates within the droplets were many orders of magnitude greater than equivalent rates reported for radiolysis, electron beam irradiation, or colloidal chemical synthesis where reaction times vary from seconds to hours. In our device, a stream of precursor loaded microdroplets, ∼15 μm in diameter, were transported rapidly through a cold atmospheric pressure plasma with a high charge concentration. A high electron flux, electron and nanoparticle confinement at the surface of the droplet, and the picoliter reactor volume are thought to be responsible for the remarkable enhancement in nanoparticle synthesis rates. While this approach exhibits considerable potential for scale-up of synthesis rates, it also offers the more immediate prospect of continuous on-demand delivery of high-quality nanomaterials directly to their point of use by avoiding the necessity of collection, recovery, and purification. A range of new applications can be envisaged, from theranostics and biomedical imaging in tissue to inline catalyst production for pollution remediation in automobiles.

Controllable synthesis of functional nanoparticles by microfluidic platforms for biomedical applications - a review

Nanoparticles have drawn significant attention in biomedicine due to their unique optical, thermal, magnetic and electrical properties which are highly related to their size and morphologies. Recently, microfluidic systems have shown promising potential to modulate critical stages in nanosynthesis, such as nucleation, growth and reaction conditions so that the size, size distribution, morphology, and reproducibility of nanoparticles are optimized in a high throughput manner. In this review, we put an emphasis on a decade of developments of microfluidic systems for engineering nanoparticles in various applications including imaging, biosensing, drug delivery, and theranostic applications.

Robust, non-fouling liters-per-day flow synthesis of ultra-small catalytically active metal nanoparticles in a single-channel reactor

Robust, non-fouling, litres-per-day continuous synthesis of catalytically active palladium nanoparticles using triphasic segmented flow in a hybrid milli-meso flow reactor.

Liquid–liquid microflow reaction engineering

Engineering characteristics of liquid–liquid microflow and its advantages in chemical reactions.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.