1 Introduction
In recent years, due to the unique chemical and physical properties of light, heat, electricity and magnetism, nano-materials have shown great application prospects in the field of photons, medical treatment, detection and analysis
[1⇓⇓⇓⇓⇓~7]. The physical and chemical properties of nanoparticle materials, including chemical composition, particle surface properties, particle morphology, particle size and dispersion, are important determinants of nanoparticle applications. Precise control and clear definition of each step in the synthesis process is therefore essential.
However, the traditional preparation methods of nano-materials have some problems, such as low yield, poor repeatability, easy to agglomerate and so on, which are not conducive to the improvement of the performance of nano-materials and hinder the subsequent application of nano-particle materials
[8]. Compare with that traditional reactor, the microfluidic system can realize the precise regulation and control of the whole reaction process due to the miniaturization and integration of the device; on the one hand, the microfluidic synthesis system can realize the precise regulation and control of the whole reaction proces on a micro scale,The operation and control of that reaction solution has the characteristic of small reagent consumption, fast mixing speed, independent control and the like, thereby bee widely applied to the preparation of nano materials and composite materials
[9⇓⇓~12][13]. On the other hand, through the design and adjustment of the device, the microfluidic system can also complete multi-step chemical reactions, that is, multi-stage reactions. These advantages of microfluidic systems provide a broad space and platform for the precise synthesis of nanomaterials.
Based on the importance of the microfluidic synthesis system for the preparation of nanomaterials. In this paper, the following four aspects are discussed. Firstly, the devices are classified and discussed according to the factors such as the materials used and the manufacturing methods. Secondly, the characteristics of the laminar flow state and droplet morphology reaction in the microfluidic synthesis system were discussed, and the formation mechanism was discussed. Thirdly, the reported nanomaterials prepared by microfluidic synthesis system were reviewed. Finally, the research and prospect of microfluidic synthesis system were prospected.
2 Microfluidic synthesis system
Microfluidic synthesis system is a miniaturized reaction system that reduces the traditional reaction system to a micron-scale channel to operate nanoliter to microliter fluid. Compared with the conventional reaction vessel (flask), the chemical reaction in the microfluidic synthesis system has the characteristics of fast, simple and easy to operate accurately. The whole system includes a liquid delivery device (such as a syringe pump) for quantitative delivery of the reaction solution, a microreaction device (a reaction site, generally a specially designed microchannel or tube), and a collection device (for collection of the solution and products after the reaction) (fig. 1). Because the whole reaction takes place in the micro-reaction device, the scientific and reasonable design of the micro-channel device is conducive to accurately controlling the reaction, accelerating the reaction of reagents, and independently controlling different experimental stages, so the design of the micro-reaction device is a key part.
With the continuous development of microfluidic technology, and in order to meet the higher experimental requirements, the design of microfluidic devices shows diversity, which is mostly reflected in the design and adjustment of microreaction devices. Considering the design idea, reaction strategy and materials of the microreaction device, the device is divided into microchannel flow synthesis device and tubular microfluidic synthesis device.
2.1 Microchannel flow synthesis device
The microchannel flow synthesis device is a kind of microreaction device which provides a micrometer-scale reaction channel for the flow of solution by processing the matrix material through chemical, laser and other etching technologies or precision processing means
[14⇓~16][17⇓~19]. Matrix materials include conventional materials (silicon, glass, ceramics, etc.) And polymeric materials (polydimethylsiloxane (PDMS), polystyrene (PS), polymethylmethacrylate (PMMA), etc.)
[17][20,21]. According to the experimental requirements, the microchannel flow synthesis device can adjust and customize the channel area, channel diameter and channel length, or adjust the particle size, morphology and yield of the material by adjusting the injection rate, mixing process, reaction route, time and temperature of the reaction solution.
Wang et al carved a continuous U-shaped microchannel on a ceramic substrate and covered it with glass to form a completely closed channel in the device as a reaction site
[22]. By changing the solvent and adjusting the liquid flow rate, a stable liquid-liquid interface was formed in the microchannel, the solution was in a laminar state, and the synthesis reaction took place at the interface.TiO
2 nanoparticles with a particle size of less than 10 nm were successfully synthesized.
Compared with the traditional matrix materials, the polymer matrix is easier to process, and can realize the diversity of channel design to meet various experimental requirements and different flow States of fluids in the microchannel. Zhao et al. And Yang et al. Use photolithography to etch PDMS or PMMA substrates to form T-shaped channels and cross-shaped channels (Fig. 2). When the liquid passes through the channels, the continuous phase is cut by the carrier phase.The reaction solution exists in the form of droplets in the microchannel, and each droplet can be regarded as an independent microreactor to synthesize raspberry-shaped SiO
2 and Ag nanoparticles, which provides a new idea for the controllable synthesis of nanoparticles
[23][24].
In order to achieve precise operation and meet the requirements of complex experimental conditions, the design of microchannel flow synthesis devices has been diversified. Stroock et al first proposed that the herringbone microfluidic chip could mix liquids in a very short time (about 3 ms), and the special chute designed at the bottom of the channel could produce chaotic flow, which could fully mix the reaction solution and enhance the diffusion of substances in the system. Belliveau et al synthesized lipid cubic nanoparticles based on a similar microchannel device
[25][26]. Ka Kašpar et al. Designed Z-row channels to achieve the mixing of reagents. These devices highlight the full mixing of reagents in the microchannel device, which increases the repeatability of the experiment and is conducive to obtaining products with uniform particle size
[27].
In addition, the microchannel reaction device can also realize the integration of a series of chemical reaction steps, including sample injection, mixing, reaction, separation and the like. When Uson et al. manufactured the microchannel flow synthesis device, the device was divided into a mixing area and a reaction area. The mixing area used a serpentine channel to accelerate the mixing of reagents, and the reaction area used a spiral channel to reduce the deposition of reagents in the microchannel. The two units were independent of each other
[28]. Two separate microchannel devices are connected in series and heated by an external aluminum block, so that the temperature can be controlled in sections, the reagents can react at different temperatures, and the nucleation and growth processes of the particles are separated and independently controlled, thereby realizing the regulation and control of the preparation of the nanoparticles. Jensen et al. Also designed the microchannel device as a cooling mixing zone and a heating reaction zone, which can quickly and easily synthesize various metal nanostructures: monometallic, bimetallic, anisotropic growth and heterostructure by adjusting the liquid flow pattern and synthesis parameters, showing technical advantages over conventional synthesis
[29].
Because of the small size of the channel, when the liquid is in a laminar flow state in the channel, the reactant on the inner wall of the channel will directly contact the inner wall of the channel and flow, which is easy to form deposition and cause channel blockage. In order to avoid similar problems, the inside of the microchannel is usually pretreated according to the requirements of the experiment before using the microchannel reaction device
[30⇓~32].
Abou Hassan et al. cured polydimethylsiloxane (PDMS) using a capillary with a diameter of 1.7 mm as a template, and then removed the capillary to obtain a microchannel formed by PDMS as an outflow channel.Then a thinner capillary is inserted as an inner flow channel to obtain a coaxial microchannel reaction device, and the flow rates of the two reagents flowing in the inner channel and the outer channel are adjustable, so that the reaction occurs at the junction of the two fluids
[33]. Quan et al. Used a similar three-dimensional coaxial microfluidic device, in which the coaxial flow mode and high Reynolds number of the liquid in the channel ensured a very short mixing time, thus producing spherical drug nanoparticles with uniform particle size at high throughput
[34]. After that, Seibt et al designed PDMS by photolithography and prepared a three-dimensional double-focusing microchannel device, which avoided the contact and premixing between the reaction flow and the inner wall of the channel compared with the previous device.The advantage of this kind of microchannel flow synthesis device is that the reaction takes place at the interface of two streams of fluid, and the synthesized nanoparticles are compressed in the middle of the channel in the centrally focused water flow and do not contact the channel wall, thus preventing precipitation that may lead to blockage on the inner wall of the channel
[35,36]. In addition, a single coaxial microchannel reaction device is connected in series to realize the one-step preparation of the composite nano material
[37,38]2.2 Tubular microfluidic synthesis device
The method for processing and manufacturing the microchannel reaction device on the solid phase substrate by using etching or precision instruments has higher requirements on equipment and processing means, the manufactured microchannel is not easy to adjust after being formed, and the manufacturing cost is high.Therefore, stainless steel tubes, alloy tubes, quartz capillary glass tubes and polytetrafluoroethylene tubes (with an inner diameter of ~ 500 μm to ~ 3 mm) were used as liquid flow and reaction sites
[39⇓~41][42][43,44][45,46]. A suitable liquid driving device (pump) and a heating and collecting device are selected to form a tubular microfluidic synthesis device. The device is characterized by simple construction, easily available materials and high flexibility.
In a simple tubular microfluidic synthesis device, a mixed precursor solution is injected into a tubular channel through an injection pump, heated by an oil bath or a salt bath, and then flows out and is collected through a pipeline
[39,40,49]. For the reaction pipe, the reaction time can be controlled by controlling the pipe length and the flow rate, or the pipe can be heated in stages. Y-type channels (Fig. 3A) can also be added to the tubular microfluidic synthesis device to achieve instant mixing of the solution, or the polymer tube can be designed to prepare a tubular micromixer with a surface screw pattern (Fig. 3B) to enhance the lateral mixing in the tube. The improved performance of the helical tube mixer is attributed to the forced rotational flow around the central axis
[47,50][48]. The tubular microfluidic device can also realize the automation of reactions and the integration of multi-step experimental reactions, and the T-junction is designed on the pipeline, so that reagents can be added at different stages to realize sectional control, thereby being more suitable for the continuous synthesis of composite materials
[51]. Based on this, Xia et al. designed a set of tubular microfluidic synthesis device based on droplet reaction with automation potential, which includes four units, namely, special functions for reaction, cooling, water and oil separation and purification, and can realize continuous, automatic and convenient production of Pd nanoparticles
[52].
In the tubular microfluidic synthesis reaction device, Utada et al. designed a capillary microfluidic device, which can be used to form small droplets in larger droplets by stretching the front end of a glass capillary to form a smaller inner diameter and assembling it with a square capillary, and can control the number of encapsulated small droplets and the size and shape of the droplets
[53][54]. This device is mostly used to produce emulsion structures, but in recent years, researchers have also used this device to produce nanoparticles, or to combine with other materials to produce composite microspheres with special properties of nanoparticles, such as Bandulasena.The large velocity difference between the gold precursor stream injected through the inner capillary and the ascorbic acid stream provided through the outer capillary creates a strong internal flow within the droplet, enhancing reagent mixing and reducing the size of the synthesized Au nanoparticles
[55]. Zhao et al. combined magnetic Fe
3O
4 and Ag nanoparticles in porous hydrogel microspheres as a multifunctional micromotor, which has many possibilities in the field of targeting and delivery
[56]. The appearance of the device can combine the properties of various nanoparticles and provide a new synthesis method for the preparation of multifunctional composite nanomaterials.
3 Fluid state of fluid in microfluidic synthesis system
In the microfluidic synthesis system, the fluid is used as the medium to realize the transportation of various substances, and the reaction occurs in the fluid flow. The flow state of the fluid in the microfluidic synthesis system is of great significance to the synthesis of nanomaterials. According to the different flow States of the liquid in the channel of the microfluidic synthesis system, it is divided into single-phase laminar flow and multiphase droplet flow for discussion (Figure 4).
3.1 Monophasic laminar flow
The laminar flow of single-phase liquid is usually in direct contact with the inner wall of the channel, which requires a stable state for reaction. Through the calculation of a series of parameters, the researchers predict the flow state of the fluid in the microchannel to guide the synthesis of materials
[57⇓~59].
In the microfluidic synthesis system, dimensionless numbers are used to describe the interaction and influence of various forces in the microchannel, such as viscous force, inertial force, surface tension and so on. Due to the reduction of the characteristic scale, the size effect of the fluid in microfluidic synthesis makes the above force more important than that of the macroscopic fluid. Due to the larger specific surface area of microchannels, the forces closely related to the surface, such as heat transfer and mass transfer, are significantly enhanced. In the microchannel flow synthesis system, because the fluid often flows at the micron scale, buoyancy, gravity and inertia are no longer the decisive factors, but viscosity and surface tension are more important.
The commonly used Reynolds number is a very important parameter in fluid mechanics, which is defined as:
It reflects the ratio of inertial force to viscous force, U is the average velocity of fluid, DH is the diameter of fluid in the channel, μ is the viscosity of liquid, ρ is the density of liquid. In addition, Table 1 lists other relevant dimensionless numbers in the microfluidic synthesis system, as well as their physical meanings and calculation formulas.
It is worth noting that when the liquid flows and reacts in a laminar state in the tubular fluidic synthesis device, the fluid will show a parabolic velocity curve (Figure 5), that is, the velocity of the reaction solution flowing along the center is greater than that of the reaction solution flowing near the wall.This will lead to different Residence time of particles in the reactor, resulting in uneven particle size distribution, which is the Residence time distribution (RTD) theory
[60]. Although researchers have reduced RTD by modifying the surface pattern inside the microreactor, the most effective of these methods is to reduce the linear velocity of the fluid and to segment the flow of the liquid
[61,62][63].
3.2 Multiphase droplet
Droplet-based microfluidic synthesis reaction is based on immiscible two-phase flow (Figure 6), in which the continuous phase is the reaction solution, the carrier phase carries the droplets to flow in the tube, the monodisperse droplets are used as the reaction site, and the droplets are wrapped by the oil phase or the carrier phase which has a phase interface with the droplets of the reaction system.The invention realizes the separation of liquid drops and the inner wall of the reactor, effectively avoids the deposition of materials on the inner wall, realizes the rapid mixing of liquid, and better overcomes the problem that a reagent is deposited on the inner wall of a channel in a microfluidic synthesis system based on single-phase liquid laminar flow reaction.
The key of the microfluidic synthesis system based on droplets is the formation of droplets in the microreaction site, which is the place where the reaction takes place. According to the formation mode of reaction droplets, they can be divided into two categories:
The first is the direct generation of droplets in the carrier phase. The continuous phase first exists in the microchannel as a continuous fluid, and then the carrier phase shears the continuous phase to form reaction droplets, helping to accelerate reagent mixing within the droplets (fig. 7)
[64⇓~66]. Kumar et al pierced the glass capillary tube from both sides of the polytetrafluoroethylene tube, the capillaries intersected at 90 degrees, the liquid converged at the front section of the glass capillary tube, and formed droplets in the carrier phase due to the combined effect of radial force and axial force on the continuous phase.Extrusion sections with different inner diameters can also be formed by extruding the polytetrafluoroethylene tube, and the droplets are extruded and deformed when passing through, which can help the substances in the droplets to be quickly and effectively mixed
[67][68].
The second type is to generate continuous phase droplet pairs with different compositions in the carrier phase, and fuse the droplet pairs into one droplet by electropolymerization or collision as the reaction droplet
[69⇓~71][72,73]. Frenz et al. Used the hydrodynamic coupling method of two spatially separated nozzles to generate droplet pairs, and then applied voltage to regulate the surface tension of the droplets, so that the droplets merged at a very fast speed when they passed through the electrodes
[74]. Droplets are formed by colliding droplet pairs, and the process is roughly divided into four stages, including droplet dispersion formation, droplet collision, droplet pairing, and droplet fusion and reaction. By controlling the flow rate of two monodisperse droplets, the precise pairing and fusion of droplets in the microchannel are realized
[73]. Another form of droplet formation by collision is injection, where a new reagent is inject into that channel and a droplet (which does not disperse in the carry phase) is formed at the injection point, and when the droplet in the carrier phase passes through the injection point, the two droplet contact and mix to form a new droplet (fig. 8)
[75⇓~77]. The characteristic of this method is that the injection method does not form two monodisperse droplets in the carrier phase, but forms a small droplet at the injection point waiting for another droplet, collides and fuses to form a new droplet, and continues to move forward in the carrier phase to react.
Nightingale et al. Added inert gas phase on this basis, and built a three-phase flow droplet reaction device (as shown in Fig. 9a), so that the monodisperse droplets wrapped by each carrier phase were separated by inert gas, in which the inert gas had a supporting effect on the injected reagent, and then combined with the droplets after passing through the inert gas section
[78]. Compared with the two-phase droplet reaction device, the three-phase droplet reaction device has the advantages that the combination of the droplets is more precise due to the existence of the inert gas phase,Multi-step chemical reactions can also be achieved by controlling the flow rate of the inert gas to form gas segments of different lengths, thereby controlling the amount of reagent added each time, as well as the fractional injection of reagents in the channel (Fig. 9B).
4 Preparation of Nanoparticles by Microfluidic Synthesis System
Due to their unique optical, electrical, thermal, chemical and mechanical properties, nanomaterials have been widely used in many fields, such as biology, environment, medicine and manufacturing.However, in the booming field of nanoscience and technology, it is still very difficult to obtain high-performance, monodisperse and high-stability nanomaterials. Microfluidic synthesis system has many excellent characteristics, such as low reagent consumption, high yield and strong operability, and researchers have done a lot of work on the preparation of nanomaterials in microfluidic synthesis system.
4.1 Noble metal nanoparticles
Noble metal nanoparticles, including gold (Au), silver (Ag), palladium (Pd) and platinum (Pt), have been widely used in the fields of biology, sensing, catalysis and detection due to their unique physicochemical properties of surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS)
[79,80][81][82][83]. How to synthesize noble metal nanoparticles with good dispersion stability and morphology is the key to achieve subsequent applications. At present, the microfluidic synthesis system has been widely used in the synthesis of noble metal nanoparticles because of its simple, controllable process and stable performance of the synthesized particles.
For the tubular microfluidic device, Huh et al. Based on the droplet flow state reaction, the size of Ag nanoparticles can be accurately controlled by changing the flow rate of the precursor and the reducing agent, and the particle size can be adjusted between 4.37 nm and 11.45 nm. Huang et al. Synthesized 3 nm Au nanoparticles in the tube using a continuous flow state, showing stronger SERS performance than the traditional method
[84][85]. For the microchannel reaction device, Zhang et al used this device to continuously synthesize gold nanoparticles with precisely controlled size (4.5 ~ 7.0 nm) in high throughput. Because this system has the advantage of large-scale, continuous and stable preparation of gold nanoparticles, it also provides a new scheme for large-scale industrial production
[86]. Compared with the traditional batch reaction, the particle size and size distribution of the nanoparticles synthesized by the microfluidic system become smaller. In order to control the dispersion degree of the nanoparticles, besides the traditional surface modifiers such as PVP, thiol, PEG and citrate,Lazarus et al. Also used ionic liquids as modifiers to obtain spherical nanoparticles with smaller particle size and better dispersibility, which could maintain long-term dispersion stability without agglomeration
[87][88][89][90][91,92].
In addition to spherical nanoparticles, microfluidic synthesis systems also show unique advantages in the synthesis of anisotropic nanoparticles. Knauer et al. Synthesized Ag nanoprisms in a two-step method using a tubular microfluidic synthesis device
[93]. The method comprises the following steps of: firstly, synthesizing an Ag nano seed; and then, introducing a solution of the nano Ag seed, ascorbic acid and silver nitrate into a microfluidic device to grow into a silver nano prism in a channel. The average particle size of the silver nanoparticles synthesized in the first step is (3.8 ± 2.3) nm, which is smaller than the average particle size of the silver nanoparticles synthesized in the traditional liquid phase (4.2 ± 7.5) nm. The experimental results show that the initial seed concentration and the concentration of silver nitrate have a direct effect on the thickness and diameter of the prismatic particles. Under the same conditions, the silver nanoparticles synthesized by the conventional synthesis method and the microfluidic system were compared, and it was found that the size uniformity and particle dispersion of the silver nanoparticles synthesized by the microfluidic system were significantly improved. In addition, hollow nanoparticles, gold nanocubes, gold nanorods with different aspect ratios, gold nanotrees and gold nanobicones can also be obtained by using the microfluidic synthesis system
[49][94][89,95][96][97].
As an important metal catalyst, Pd has shown excellent performance in the fields of fuel cells and carbon dioxide reduction, and the morphology and stability of Pd nanoparticles are very important. Khan et al. Successfully synthesized spherical Pd nanoparticles less than 5 nm by using a three-phase flow tube flow microfluidic device and adding T junction to introduce reducing agent
[51]. Besides spherical Pd nanoparticles, cubic Pd nanoparticles with good dispersion and Pd nanowires with adjustable aspect ratio can also be synthesized by using this system
[68][98]. Xia et al. Changed the amount of reducing agent and capping agent to control the morphology of Pd nanoparticles, such as 16 nm Pd cubic seeds, 35 nm Pd octahedron and Pt nanowires, icosahedral Pd nanoparticles, threaded/chain-like Pd nanoparticles, Pt nanoparticles and Ru nanoparticles
[52][99][100,101]4.2 Quantum dot
Quantum dots (QDs) are a new type of fluorescent nanomaterials. Due to their unique optical properties, such as long and strong fluorescence and good photostability, they are widely used in electronic devices, sensor
[103~105], biological imaging, immunoassay and so on
[102][106][107]. The structure determines the properties, so the preparation of the morphology and size of quantum dot materials has attracted much attention.
Quantum dots have been used in microfluidic synthesis systems for a long time. Table 2 summarizes the quantum dots prepared by microfluidic reaction devices built with different materials. Nakamura et al. First used a tubular microfluidic device to synthesize CdSe nanocrystals with Cd(CH
3COO)
2 as the Cd precursor. Shestopalov et al. Used a microchannel device to control the synthesis of CdS quantum dots in milliseconds. Through the improvement of the microfluidic device, the nucleation and growth stages of quantum dots were separated. The quantum dots synthesized by this method have higher photoluminescence quantum yield than those synthesized by traditional methods
[108][109][110]. At present, CdSe quantum dots with controllable particle size and good dispersion can be synthesized by microfluidic synthesis system under normal pressure, but the particle size distribution is easy to be wide due to the limited amount of high boiling point solvent, slow mixing speed and long residence time. For this reason, Chakrabart et al. Established a high-pressure microfluidic synthesis reactor and used supercritical fluid to prepare high-quality CdSe quantum dots with narrow particle size distribution, high quantum yield and good crystallinity.It overcomes the problems of expensive solvent, high viscosity and low diffusion coefficient encountered in atmospheric pressure synthesis, which also shows that the use of microfluidic synthesis devices to prepare quantum dots has developed towards the direction of scale
[111].
In order to clarify the growth mechanism of quantum dots in the microfluidic synthesis system and explore the influencing factors of nucleation and growth, Seibt et al. Used PDMS as the substrate, made a three-dimensional microfluidic synthesis device, combined with in-situ microscopy and spectroscopy technology, observed the formation of quantum dots in millisecond time scale for the first time, and successfully synthesized CdS nanoparticles with particle sizes of 2.6 nm and 3.2 nm
[35].
4.3 Silicon dioxide
Nanometer SiO2 has been widely used in the fields of electronics, metallurgy, aerospace, chemical industry, medicine and health because of its strong adsorption, good plasticity, high magnetoresistance and low thermal conductivity. Nanometer SiO2 particles with small particle size, high chemical purity and good dispersion are of practical significance for their application.
Knossalla et al. Synthesized a variety of spherical silica structures, such as 300-400 nm mesoporous core-shell spheres (SiO
2@mSiO
2), mesoporous spheres (mSiO
2) and gold-encapsulated spheres (Au@SiO
2), using a tubular microfluidic device with gas and liquid flow
[122]. Chung et al. prepared a microchannel reaction device by laser etching the PDMS substrate to mix the reaction fluids in a laminar flow state. By regulating the flow rate of the fluids, nano-SiO
2 particles ranging from (88 ± 11) nm to (250 ± 50) nm can be synthesized
[123]. The synthesis of SiO
2 nanoparticles by various microfluidic devices has higher yield and narrower particle size distribution compared with traditional reactions. Because of the absence of axial dispersion at high linear velocity, the segmented liquid flow reaction can produce nanoparticles with more uniform particle size than the laminar liquid reaction device.
In addition to the use of microfluidic synthesis system to synthesize spherical SiO
2 nanoparticles, Zhang et al.Ynthesizing mesoporous silicon dioxide fiber, hollow spherical silicon dioxide with a layered spongy macroporous shell, two-dimensional hollow sandwich-shaped silicon dioxide nanosheets and silicon dioxide microflowers, and the length-diameter ratio and the appearance structure of the nano material can be easily regulated and controlled by controlling the flow rate and the concentration of reactants through the device
[124][125][21][126]. In addition, based on this system, researchers have also synthesized solid, hollow, hollow shell with holes on the surface and asymmetric SiO
2 microspheres with good dispersion, micron size and neat arrangement, honeycomb silica, hollow mesoporous silica, elliptical hollow mesoporous silica and plate silica
[127][128][129][130][131].
4.4 Magnetic nanoparticles
Fe
3O
4 nanoparticles exhibit superparamagnetism and unique physical and chemical properties under the action of external magnetic field, which has attracted wide attention in the fields of medical imaging, drug delivery, biosensing, catalysis and so on
[132][133]. However, the dispersion of particles, the controllability of morphology and particle size, and the response to external magnetic field play a decisive role in the subsequent application.Microfluidic synthesis system can minimize local concentration differences and achieve rapid mixing, which provides a good condition for the synthesis of Fe
3O
4 nanoparticles with smaller and more uniform particle size distribution.
Based on the excellent characteristics of the microfluidic synthesis system, the researchers used the co-precipitation method to synthesize Fe
3O
4 nanoparticles by mixing the precursor and the reducing agent quickly and uniformly. Frenz et al. Regulated the surface tension of droplets by voltage, so that the precursor phase and the reducing phase droplets were mixed and co-precipitated to form superparamagnetic Fe
3O
4 nanoparticles
[74]. In addition to coprecipitation method, Fe
3O
4 nanoparticles can also be synthesized by pyrolysis method in microfluidic synthesis system, in which the molar ratio of ferric oleate to oleic acid is the decisive factor affecting the particle size of Fe
3O
4 nanoparticles
[39]. Uson et al. Used a microchannel device to separate the nucleation stage from the growth stage, and obtained Fe
3O
4 nanoparticles with high crystallinity and purity at high temperature. The total reaction time was about 76 s, and the particle size was less than 4 nm
[28].
Other magnetic nanoparticles, such as magnetic nickel nanoparticles, have been successfully synthesized by hydrothermal reduction method using hydrazine hydrate as reducing agent in a microfluidic synthesis device. The nickel nanoparticles are spherical, with a good normal distribution of particle size and an average particle size of 8.76 nm
[134,135]. Cobalt nanoparticles were also successfully synthesized in the microfluidic synthesis system, and the products were three different crystal structures of face-centered cubic (β) phase, HCP and ε-Co
[136].
4.5 Composite nanoparticles
In addition to the above noble metal nanoparticles, quantum dots, and oxide nanoparticles, other composite nanoparticles with core-shell structure have also been prepared in the microfluidic synthesis device. In the experiment, nanoparticles with small particle size were synthesized as seeds, and then the reaction phase containing the core seeds was mixed with the material to be coated to obtain the core-shell structure. As mentioned earlier, SiO
2 nanoparticles are easily surface-coated, coated with metal shells or formed into metal oxide shells to form SiO
2@Au, Fe
2O
3@SiO
2, Ag@SiO
2 or SiO
2@TiO
2 structures
[137][37][138][139].
These different kinds of core-shell composites, whose cores and shells have different properties, have better application prospects through structural reorganization. In addition, metal @ metal core-shell nanoparticles, such as Au @ Ag nanoparticles with star or round shape, metal @ metal oxide Cu@Cu
2O, Fe
3O
4@Au, Ag@Fe
3O
4, Pd@Fe
3O
4, Ag@Cu
2O, organic @ metal core-shell structure, such as chitosan @ Au, were also prepared by microfluidic synthesis system.The synthesis of composite quantum dot CuInS
2/ZnS, InP/ZnS, InP/CdS, and multilayer composite ZnS @ CdSe @ ZnS nanoparticles and Au @ Ag @ Au nanoparticles all proved that the microfluidic synthesis system has great application potential in the synthesis of core-shell or composite materials, and completed the synthesis reaction which is difficult to achieve by traditional batch reaction
[140,141][142][77][143][144][145][146][147][148][149].
5 Conclusion and prospect
From the point of view of material synthesis, because the microfluidic synthesis system can realize the precise control of synthesis conditions and the adjustment of physical and chemical properties of reactants in small batches, the morphology of the obtained materials can be controlled.Compared with the material prepared by the traditional batch synthesis method, the particle size distribution is uniform, the particle dispersion degree is obviously improved, and the application value of the nano material is greatly improved. Therefore, the microfluidic synthesis system has been widely used in the preparation of a variety of nanomaterials and composite nanomaterials.
From the point of view of the device of microfluidic synthesis system, (1) microchannel reaction device has the advantages of miniaturization, designability of channel shape, accurate internal size and easy operation after design and processing. However, under different reaction conditions (such as temperature, acidity and alkalinity), in addition to the need to select the type of substrate material, it is also necessary to treat the surface of the channel with hydrophilicity and hydrophobicity, which also involves the problem that the channel is not easy to adjust after processing. And (2) the materials for building the tubular microfluidic reaction device are simple and easy to obtain, the reaction device can be adjusted according to requirements in the experiment process, and the integration of multi-step chemical reactions and operation steps is realized. However, in addition to the same need to adjust the hydrophilicity and hydrophobicity of the inner wall of the pipeline, it also lacks the flexibility to meet the needs of more complex chemical reactions.
From the flow state of liquid in the microfluidic reaction system, the liquid in the laminar flow state has the inherent problems of parabolic velocity distribution and residence time distribution, and the direct contact of reagents with the inner wall of the reactor will inevitably lead to reagent contamination and material deposition in the channel.Droplet-based microfluidic reaction devices can solve this problem well, but their devices should take into account that the stable and continuous formation of droplets may hinder the amplification of the synthesis process.
The choice of the type of microfluidic reaction device should be based on the type of target material, reaction type and conditions, and the experimental process and feasibility should be considered comprehensively. At the same time, the combination of microchannel reaction device and tubular microfluidic reaction device can also be used to meet the experimental operation under specific conditions.
In the future, the development of microfluidic synthesis system will inevitably integrate various online analysis methods into the system to realize the integration of material synthesis, real-time monitoring and analysis. In addition, how to realize the industrialization and large-scale production of nanomaterials is still facing enormous challenges.
the National Natural Science Foundation of China(21505089)
the 61th Batch of National Postdoctoral Funds Second-Class, China(202101710)
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