Continuous flow synthesis, based on the manipulation of continuous liquid flows through micro- fabricated channels is by far the most used strategy in microfluidics because its high level of controllability. Actuation of the liquid flow is realized, either by external pressure sources, external mechanical pumps, integrated mechanical micro-pumps, or by combinations of capillary forces and electro-kinetic mechanisms. Syringe pumps provide a quick and simple method for pumping fluids through micro-channels. Continuous-flow microfluidic operation is easy to implement and it is adequate for many applications, and for certain additional post-synthesis operations. However, it is less suitable when a high degree of flexibility in the fluid manipulations is required. The closed-channel systems are inherently difficult to integrate and scale because the parameters that govern the flow field vary along the flow path, making the fluid flow at one location dependent on the properties of the entire system. Permanently etched micro-structures also lead to limited possibilities to reconfigure the system. In addition, substantial volumes of reagents and relatively long channels have to be used [12]. In continuous-flow systems, highly sensitive microfluidic flow sensors, based on MEMS technology, are used for monitoring the flow.

The simplest micro-reactors used by Mulvaney et al. and Maeda et al. for the synthesis of semiconductor nanoparticles were capillary-tube reactors [13,14].

A water or oil bath is used to control the temperature, and, because the small dimensions of the channel, very short times are necessary to heat a liquid from room temperature to 300 °C [13]. Different materials such as glass, polymers and stainless steel can be used to fabricate the capillary tubes. For the fabrication of composite materials such as CdSe/ZnS, two-step reactors are used. It was observed that sometimes, because of the poor mixing of reagents in the short linear channels, unstable nanoparticles are produced. In this case, a serpentine micromixer is used to increase the length of the channel. This setup has been used by Gomez de Pedro et al. for the synthesis of gold nanoparticles in a ceramic microfluidic device by using a low-temperature co-fired ceramics technology (LTCC) (Figure 3).

This microreactor takes advantage of the substrate material (ceramic tapes) and its associated microfabrication technology. By using ceramic tapes, multilayers can be fabricated easily and, hence, fluidic components can be integrated in a single device. In this work, gold nanoparticles are obtained by a simple one-phase reaction in which the gold ions are reduced by sodium borohydride.

Droplet-based microfluidic systems called also segmented flow or multiphase microfluidic flow, is an emerging area of microfluidic research which has been widely used in nanoparticle synthesis. In order to induce the flow instability necessary for forming the droplets, multiple laminar streams of aqueous reagents are injected into an immiscible carrier fluid. There are several distinctive advantages in droplet-based microfluidic systems. First, the systems promise a new high-throughput technology that enables the generation of thousands of discrete micro-droplets per second. The reagents are confined inside the mono-dispersed droplets or plugs in water-in-oil (w/o) emulsions and the transport occurs with no dispersion. Furthermore, due to the short diffusion distance and chaotic mixing within droplets, fast mixing can occur within minute volumes of micro-droplets of nano-liter to femto-liter range by stretching, folding, and reorienting the fluid. The variation of the channel dimensions can regulate the droplet volumes and decrease volumes considerably, compared to the assays in conventional micro-titer plates. It is the convergence of these features and the ability to manipulate the droplet motions that provides a good approach to synthesis [16]. A flow-focusing microcapillary device showing the formation of droplets can be seen in Figure 4.

Figure 4a illustrates the co-flow geometry—a three dimensional co-axial flow, in which one fluid is flowing inside the circular capillary and the second one flows through the square capillary in the same direction.

In devices using the flow-focusing geometry (Figure 4b), the two fluids are introduced from the two ends of the same square capillary, from opposite directions. The inner fluid is hydrodynamically focused by the outer fluid through the narrow orifice of the tapered round capillary [17].

Figure 5 shows the formation of hexadecane droplets with a diameter of approximately 40 μm and a volume of 25 pL. The size of the droplet can be increased or reduced by changing the flow rate of the two phases.

When the channels are coated with a hydrophobic material, water droplets can be kept inside an organic phase. Figure 6 shows a T-shaped droplet generator chip fabricated by Micronit Microfluidics.

Ismagilov et al. have categorized the segmented flows according to the phase where the reaction takes place [19].

In the first type, the carrier fluid encapsulates the droplets in which the reactions occur and the reagents are separated from the walls (Figure 7a). In the second type of segmented flow, droplets are separated by discrete gas bubbles and the reaction occurs in the continuous phase (Figure 7b).

In this section, the microfluidic synthesis of metal and semiconductor nanoparticles as well as other colloidal materials will be briefly reviewed. When possible, the micro-scale synthesis will be compared to the conventional methods used for the same material. This section is not extensive. Several representative syntheses will be discussed for each material.

The most important advantage of the microfluidic devices for the synthesis of metal nanoparticles is the capability of controlling the temperature along the flow. This fact, together with an efficient mixing of reagents and the possibility to add reagents in any part of the channel, contribute to the generation of homogeneous nanoparticles in a well-controlled fashion. Three examples regarding the synthesis of gold nanoparticles in continuous flow systems are presented below in a chronological order to show the progress realized for a duration of less than a decade. Wagner and Köhler have used a simple microfluidic device (as shown in Figure 8) to synthesize gold nanoparticles by using a mild reducing agent and poly(vinylpyrrolidone) as a capping agent [20].

A mild reducing agent, ascorbic acid is used in this synthesis, in the presence of poly(vinylpyrrolidone) (PVP) as capping agent, and the size distribution of the gold nanoparticles is found considerably narrower than that obtained by conventional synthesis methods. The mixing is based on split and recombination of the laminar streams. Smaller gold nanoparticles (4–7 nm) were obtained when, instead the ascorbic acid, a much stronger reducing agent such as sodium borohydride was used [21]. It seems that in this work, the size of nanoparticles was not influenced by the flow rate, or by the reagents ratios.

Thiol-passivated gold nanoparticles, generally called monolayer protected clusters, have been prepared for the first time, using a continuous flow microfluidic device. The authors (de Mello et al.) have shown that nanoparticles generated by the device are smaller than those prepared by the batch method and, that their size distribution is significantly narrower [22]. The schematic of the mixer is shown in Figure 9.

The size histogram (Figure 10) shows the narrow distribution of the thiol functionalized gold nanoparticles synthesized in the device using the mixer. The size distribution is compared to that of gold nanoparticles prepared through a bulk synthesis shown in Figure 10c.

It can be seen that, while the bulk synthesis produced gold nanoparticles with non-uniform sizes, those produced in the microfluidic device provided with a mixer, show a narrow size distribution for both flow rates.

More recently, the nucleation and growth of gold nanoparticles prepared by reduction of gold ions by sodium borohydride was studied with small-angle X-ray scattering online, at a very high time resolution (100 ms) [23]. The experimental setup used for the synthesis is shown in Figure 11.

The ability to analyze online the nanoparticles is a considerable advantage. Coupling the microstructured mixer with small-angle X-ray scattering enabled online analysis of the formed nanoparticles, without requiring a synchrotron radiation facility. In general, by using this setup, the particle size is identical to sizes obtained in batch experiments. The main advantage of this setup is that kinetic information can be provided for rapid particle formation processes. In combination with XANES, the data show that initially, the gold precursor is converted into gold nuclei within less than 100 ms, followed by particle growth by coalescence as shown in Figure 12.

As in all the processes based on gold salts reduction, the final size of the particles is reached only after the precursor species are completely consumed.

Noble metal nanorods can be prepared from spherical metal seed crystals in a ‘growth solution’ containing the metal salt (HAuCl₄) in millimolar concentrations, a mild reducing agent (ascorbic acid) and a high concentration of cetyl-trimethyl-ammonium-bromide (CTAB), which forms rod-shaped micelles as a template for the anisotropic particle growth [24]. The seed and growth solutions are injected with independent flow rates. For the preparation of silver rods, NaOH is added shortly after the mixing of seed and growth solutions. The flow is then directed through temperature-controlled tubing, allowing the crystals to grow at a fixed temperature. The extinction of the final product is measured by a flow-through spectrometer. The experimental results for gold and silver nanorod synthesis demonstrated the potential of the continuous flow setup for the synthesis of anisotropic particles as well. The continuous flow synthesis with online optical monitoring allows the reproducible and controlled synthesis of particles with specific shapes.

Thermal reduction of a few precursors of noble metals is possible by using the continuous flow approach shown in Figure 13.

A continuous flow tubular micro-reactor is suitable for the synthesis of silver nanoparticles when a single-phase precursor such as silver pentafluoropropionate in isoamyl ether is thermally reduced at moderate temperatures. The produced silver nanoparticles have a narrow size distribution and, because of the nature of the procedure, there is no need for a mixer. Flow rates between 0.08 and 0.7 mL min⁻¹ were tested and the TEM study showed that average diameters of silver nanoparticles prepared under these conditions were 8.6 +/− 0.9 and 8.6 +/− 1.0 nm for the molar ratios of TOA/silver pentafluoropropionate of 6 and 12, respectively.The experiments showed that low flow rates have to be used in order to obtain a narrow size distribution of silver nanoparticles. It is demonstrated that single-source precursors favour the formation of mono-disperse metal nanoparticles

Small copper nanoparticles with a narrow size distribution were produced in a microfluidic device by Kumar et al. [26], and Packirisamy et al. [27]. The most important property of microdevice-prepared copper nanoparticles is an improved stability to oxidation, very important for any application.

Cu nanoparticles with narrower size-distribution and a smaller size as shown in Figure 14a could be synthesized by using the sulfobetaine-Cu(I) complex as the starting material, in comparison with Cu obtained by the conventional batch process by using CuCl₂. (Figure 14b) UV-Vis spectroscopy, reaction calorimetry and XANES (X-ray absorption near edge structure) investigations showed that stable and water-soluble sulfobetaine-Cu(I) complex can be formed in a THF solution by using 3-(N,N-dimethyldodecylammonia) propanesulfonate as a stabilizer during the Cu nanocolloid formation from CuCl₂/THF solution. In this work, the important role of the starting material and stabilizer is emphasized.

In a recent work, copper nanoparticles have been synthesized in a microfluidic reactor and the spectra were compared to those corresponding to Cu nanoparticules prepared through the traditional batch method.

            Cu²⁺ + BH₄⁻ + H₂O → Cu⁰ + B(OH)₃ + H₂ → formation of Cu nanoparticles           (1)

and their oxidation:

                                              2Cu + ½O₂ + H₂O → Cu₂O                                               (2)

and

                                                  Cu₂O + O₂→ 2CuO                                                       (3)

The schematic of the microfluidic reactor is shown in Figure 15. The total volume of the microreactor is approximately 20 µL³.

Copper nanoparticles were synthesized in the authors’ laboratory by using a poly(dimethylsiloxane) (PDMS) device.

The copper sulfate solution (2.7 × 10⁻³ M) is mixed with 4 mL of a sodium citrate solution (1%) outside the micro-reactor and then introduced into the micro-reactor, simultaneously with a sodium borohydride solution (0.02 M) through inlet 1 and 2, respectively. The spectra show the immediate formation of Cu nanoparticles. In the microfluidic environment, the Cu LSPR band is observed around 520 nm and corresponds to small (less than 5 nm) nanoparticles. In Figure 16 the spectra of Cu synthesized inside and outside the micro-reactor are compared.

Copper nanoparticles were synthesized through a “flask” process as well and their morphology is shown in Figure 16.

The AFM image shows the presence of large particles with a broad size distribution.

Because of the anaerobic conditions, Cu nanoparticles are stable in the microfluidic environment.

Another important application of microfluidic synthesis, the synthesis of nanoparticles with two or more crystalline phases, realized by Kumar et al. is shown in the example below (Figure 17) for cobalt nanoparticles [29].

The different crystal structures are synthesized through manipulation of parameters such as flow rates, reaction times and quenching procedures. The formation route suggested by the authors is shown below (Figure 18). They show that, by using high flow rates (high kinetic energy), the Co nanoparticles that are formed, have mainly a face-centered cubic structure, while at low flow rates, particles with hcp structures are favored. It is found that the flow rate has a marked effect only on the nucleation process as the growth of the particles, in this work, happens outside of the micro-reactor.

Semiconductor nanocrystals can also be synthesized through the continuous flow approach as shown in Figure 19.

Alivisatos et al. showed that the size of CdSe nanocrystals can be tuned by changing the parameters [30].

The tuning of the size of CdSe nanocrystals is done in this work by independently changing the temperature, the flow rate through heated microchannels, and the concentration of the precursor. The microfluidic reactor employed by the author works in a continuous flow regime as shown in Figure 20. They make use of the major advantage of microfluidic reaction systems, that is, the ability to change rapidly the concentration and temperature on the scale of micrometers. The authors show that, while, similar data could have been obtained through flask synthesis, the experimental work would be very tedious.

The examples shown in this section have demonstrated the advantages of the devices based on continuous flow for the synthesis of high-quality Au, Ag, Cu, Co, nanoparticles. However, for obtaining nanoparticles with a narrow size distribution, the reactors should be integrated with mixers, pumps, and sometimes optical devices for characterizing the nanoparticles online.

The principal drawback of the continuous flow-based methods is the deposition of nanoparticles on the walls. As it has been shown in Section 2.2, an alternative approach is the use of droplet microfluidics, where the confinement within small droplets prevents the contact of the nanoparticles with the walls, and hence, their deposition.

Gold nanorods of varying aspect ratios have been successfully prepared by implementing the T-junction droplet-based synthesis as shown in Figure 21 [31]. The authors adapted small-scale batch method [32] to a flow version in a microfluidic environment (Figure 19). The formation of droplets at the junction is determined by the microchannel inlet geometry, fluid viscosities, and interfacial tension, and the relative flow rates of the two immiscible fluids. Pico-liter droplets are formed containing both seeds and growth reagents and silicon oil is used as the continuous fluid. The most interesting results achieved in this study is the tunability of the shapes and thus of the UV-Visible absorption by changing parameters such as the concentration of reagents and their flow rates (Figure 22). The authors (Duraiswamy and Khan) further intend to develop an integrated synthesis method that incorporates online synthesis of gold nanoparticle seeds, and extend the residence time of growing nanocrystals on-chip, so as to enable a fully continuous process. The microfluidic synthesis proves to be quite difficult in the case of using both a seed and a growth solution.

The size (aspect ratio) of the nanorods prepared through the droplet method appears to be uniform as shown in Figure 23. It can be seen that, in addition to nanorods, a number of nanospheres are formed as well. However, this is a general occurrence, hardly avoidable in both batch methods and continuous synthesis as well.

The spectra exhibit two resonance maxima, at 520 nm that corresponds to transverse plasmon resonance (TPR) of anisotropic particles and the second peak, usually obtained between 550 nm to near-IR wavelengths (1200 nm), represents the longitudinal plasmon resonance (LPR) of anisotropic particles. The width of the band determines the polydispersity in the particles.

Because of the rapid mixing in the discrete slugs, flow segmentation has proved to generate more uniformly sized nanoparticles than the continuous flow approach [33]. By using a silicon/Pyrex microreactor (Figure 23), Jensen et al. demonstrated that the narrow size distribution of gold nanoparticles is due to the homogeneous mixing in two-phase flow systems [33].

The system studied in this work is the rapid formation of gold nuclei with sodium borohydride, a very strong reducing agent. The particle size distribution diagram for three different reaction times shows that by using short times (10 s), a very narrow size distribution can be obtained when the two phases are carefully chosen.

It can be seen (Figure 24d) that for a short reaction time (10 s) the particles are very small (less than 5 nm) and their size distribution is very narrow.

The examples shown in this section show some of the advantages of droplet-based methods. In spite of the complexity of the design and procedure, once the methodology well-established, the results in terms of size distribution are very gratifying.

Microfluidic synthesis of polymer particles provides the same advantages over the conventional methods as in the case of nanoparticles, that is, an improved control over their sizes, size distributions, morphologies as well as molecular weight distribution or average molecular weight of linear polymers [34]. The batch methods, especially the emulsion and suspension polymerizations, totally lack of control of size and shape of particles and, composite materials cannot be prepared directly by using these methods. The convergence of particle technologies and microfluidics has enabled a considerable progress in the field of controlled synthesis of polymeric particles, especially important for applications such as painting formulation and drug delivery. However, before describing the use of microfluidic devices for polymer particle synthesis, the linear and branched polymers synthesis is briefly reviewed in this section.

Most of the work on the microfluidic synthesis of linear polymers was done by radical polymerization or copolymerization, initiated by thermal or photoinitiators [35]. The authors (Wu et al.) demonstrated the effect of the flow rate—that is the polymerization time—on the molecular mass of the polymer.

Cationic and anionic polymerization were also performed in a microfluidic format [36]. There are two different methods for producing polymer particles in microfluidic devices: single-phase (continuous flow) and multiphase (droplet) flow [4,37]. The method of continuous flow (projection photolithography technique) developed by Dendukuri et al. utilizes the objective of an optical microscope to UV irradiate through a mask, the solution flowing through the microchannel as shown in Figure 25 [38].

Through the mask, the desired particle shape can be “printed” into the monomer solution that will polymerize under the UV light, in the presence of a photo-initiator.

Monodispersed particles with diameters ranging from 20 to 1000 nm were generated by using a flow-focusing geometry by Whitesides et al. The schematic of the flow-focusing geometry used for droplet formation is described by Kumacheva et al. [39].

In a recent work, size-tunable polymeric nanoparticles were synthesized by Langer et al. in a novel 3D flow focusing in both the horizontal and the vertical dimensions [40,41].

PLGA-PEG nanoparticles were synthesized by hydrodynamic flow focusing in a controlled nanoprecipitation process.

The figure below (Figure 26) shows the microfluidic device as well as the TEM image of nanoparticles.

The authors showed that microfluidics may be used to tune the nanoparticle size and drug loading and release.

The polymer particles synthesized within the microfluidic device shown in the figure, are composed of poly(lactide-co-glycolide)-b-polyethyleneglycol (PLGA-PEG) block copolymers, a model biomaterial for drug delivery and, because they are biodegradable and biocompatible, are suitable for controlled drug release for therapeutic applications. The authors showed that the polydispersity of the small nanoparticles (30–230 nm), made with different concentrations of polymer precursor and molecular weights, has been found very low when 3D flow focusing was used. Flow focusing enables the rapid mixing of the polymer solutions with water. Drug loading and release of the resulting nanoparticles can be precisely controlled.

Non-spherical microparticles (plugs and disks) as shown in Figure 27, were synthesized in a microfluidic device by Dendukuri et al. [42].

By using appropriate channel geometries, curable polymer droplets formed at a T-junction, are constrained to adopt nonspherical shapes and are subsequently, irradiated with UV light to photopolymerize them and preserve the shapes. Two microchannels with different heights are used to form the droplets having the two shapes, 38 µm for the plugs and 16 µm for the disks.

As seen in Figure 28, the non-spherical polymer particles (larger than 10 µm), fabricated through this method, are monodisperse plugs and disks. By varying the flow rates of the two phases, the dimensions of the two nonspherical microparticles can be tuned.

Recently, a new microfluidic route allowing the incorporation of inorganic nanoparticles in polymer microparticles was reported by Köhler et al. [43].

Simple composite microparticles have been fabricated in the capillary-based system shown in Figure 29.

In this work, Au and ZnO nanoparticles are synthesized previously in a microfluidic device and, in a second step, integrated into the polymer. The monomer phase was a mixture of an acrylate-based monomer, a photoinitiator and the gold nanoparticle (or ZnO) solution in toluene.

The droplets are hardened through thermal- or UV-induced polymerization. The polymer particles are in the size range of a few to hundreds of micrometers but the size distribution was found very narrow as shown in Figure 30.

Au nanoparticles with a size around 13 nm are found to be dispersed randomly in the polymer matrix. The micrograph shows that ZnO nanoparticles are concentrated in the central part of the polymer bead.

The synthesis of composite materials demonstrated a good control over the size and size distribution.

In the authors’ laboratory, gold-PDMS nanocomposite was fabricated both at the macro scale (Figure 31) and in the channel, by introducing a gold chloride solution directly in the channel of a PDMS microfluidic device as shown in Figure 32.

By immersing a piece of PDMS in a gold chloride solution, the gold ions are reduced by the curing agent in the polymer and the gold nanoparticles formed by reduction are embedded in the surface layers of the polymer. The figure shows that the absorbance of the Au Localized Surface Plasmon Resonance band (LSPR) increases when the solution is left in the channel for a longer time (Figure 33).

Sensing experiments carried out with a Au-PDMS platform, demonstrated the sensitivity of the gold-PDMS nanocomposite to antigen-antibody interactions carried out in an immunoassay format.

The device shown in Figure 34 was used for biosensing of a hormone polypeptide molecule by using an immunosensing format. Gold nanoparticles were functionalized by pumping a linker molecule solution through the channel and, subsequently, adsorbing the antibody and the antigen onto the immobilized gold nanoparticles. By using the shift of the AuLSPR band, the antigen can be quantified.

However, the outlook is promising; the direct use of microfluidics for biosensing is still in an incipient stage. For the time being, only the continuous flow microfluidics is used for biosensing, droplet-based methods are not yet developed for this application. For the development of analytical microfluidic chip, new functionalization methods have to be developed.

Multi-functional polycaprolactone size-controlled microcapsules entrapping an anticancer drug, together with fluorescent and supermagnetic nanoparticles, were produced by microfluidic emulsification by Chang et al. [46]. This work demonstrates a proof-of-concept approach for fabrication of multifunctional materials. The drug delivery system enabled magnetic targeting through the super paramagnetic nanoparticles, fluorescence imaging through QDs and drug controlled release properties.

Microfluidics offers new strategies for fabricating polymer particles. With traditional methods it is difficult to prepare monodisperse spherical or non-spherical polymer microparticles. To be used in commercial applications, the microfluidic production of polymer particles will have to be scaled up. In addition, efforts will have to be made to synthesize nanometer-sized polymer particles as well.

Microgels are promising materials in biomedicine and drug delivery applications as the release rate and profile of the drug molecules can be optimized for specific applications [47]. De Geest et al. fabricated 10 µm sized mono-disperse microgels in a PDMS device by emulsifying an aqueous dextran-hydroxyethyl methacrylate solution (dex-HEMA) within an oil phase (Figure 35).

The particularity of this work is that the curing by UV irradiation is done in a vial, outside of the device.

Microfluidic technologies, mostly laminar flow-based multiple phase coaxial flowing systems have been used for various applications in tissue engineering for mimicking the tissue architecture [48].

Doyle et al. [49] have synthesized magnetic hydrogel particles with different shapes (spheres, disks, and plugs) in a T-junction microfluidic device (Figure 36 and Figure 37).

In order to improve the polymerization conditions, the device is used in combination with a UV light reflector (aluminium foil). The hydrogel particles, (both spherical and non-spherical), are monodisperse and have magnetic nanoparticles uniformly distributed inside the hydrogel. The hydrogelparticles synthesized in this way show super paramagnetic behavior.

Porous microfibers were also fabricated using a simple PDMS-based microfluidic device in which an amphiphilic ABA triblock copolymer, poly(p-dioxanone-co-caprolactone)-block-poly(ethylene oxide)-block-poly(pdioxanone-co-caprolactone) (PPDO-co-PCL-b-PEG-b- PPDO-co-PCL) in dichloromethane and de-ionized water were introduced into the device and porous fibers were extruded through the outlet utilizing immersion precipitation and solvent evaporation.

Amphiphilic triblock copolymeric microfibers with a wide range of diameters (from 2 to 200 mm) were successfully extruded using the microfluidic device shown in Figure 38. The microfiber scaffolds were incorporated with fibronectin and the release behavior has been studied. The microfluidic technology is promising for use in biomedical applications such as tissue engineering scaffolds and drug delivery vehicles.