Microfluidic Nanoparticle Synthesis: A short review

1. Introduction into nanoparticle synthesis by microfluidics

3. Nanoparticle production with microfluidics

Microfluidic devices are designed to manipulate fluids in channels on the microscale, and have widely been used in the field of nanotechnology. Nanoparticle synthesis in microfluidic systems is performed in microreactors that have inner dimensions usually smaller than 1 mm. Microreactors can be of tubular designs and perhaps more commonly of lab-on-chip designs. Chips are often fabricated in polymers such as PDMS or glass. Xu et al. [5] demonstrated the advantages of microfluidic devices over bulk synthesis in terms of reaction yield and improved size and shape distribution. Also various shapes of non-spherical particles can be synthesized with microfluidic systems [6].

Microfluidic systems for nanoparticle synthesis can be divided into two general categories:
1. Single-phase continuous flow systems
2. Multiphase flow systems

Figure 2. Schematic illustration of different nanoparticle synthesis approaches. (a) batch approach; (b) single-phase flow; (c) gas-liquid multiphase flow; (d) liquid-liquid multiphase flow. Image from Ma et al. “Controllable synthesis of functional nanoparticles by microfluidic platforms for biomedical applications” [7].

Single-phase continuous flow systems

The single-phase continuous flow system was the first type of microfluidic device utilized for nanoparticle fabrication. Various types of nanoparticles, such as metal colloids [8], quantum dots [9] and liposomes [10], have been effectively fabricated through self-assembly and nanoprecipitation in single-phase systems.

Figure 3. Picture of a single-phase continuous flow system for nanosynthesis. Image from Wang et al. ”A rapid pathway toward a superb gene delivery system: programming structural and functional diversity into a supramolecular nanoparticle library” [11]

In a single-phase system, continuous laminar flow streams of single or multiple fluids flow through microchannels where nucleation and growth take place. The system ensures a homogenous environment throughout the reaction, as well short distances for diffusion and fast mixing and stirring. Mixing occurs primarily through diffusion across laminar flow streams. The geometries of the microfluidic channel can be utilized to precisely control mixing and reaction times, resulting in reproducible formation of nanoparticles of desired size and shape. Single-phase systems offer continuous modification in terms of allowing subsequent addition of reagents throughout the reaction and multi-step synthesis. They can also be scaled up as multiple reactions can be carried out in parallel, even on the same chip.

Multiphase systems

Microfluidic devices with multiphase flows, also referred to as segmented flows or droplet microfluidics, comprise two or several immiscible fluids in segmented phases. The flow phases can be liquid-liquid flows (often water-in-oil or oil-in-water) or gas-liquid flows. The discrete segments act as small individual reaction chambers, where mixing is generated within each segment as the segments move inside the channels [12]. Like in single-phase systems, the geometries of the microchannels are designed to optimize reaction time and mixing. The very small volume of the droplet, typically on the picoliter scale, favours efficient thermal exchange and high reaction speed.

Compartmentalisation of reactions reduces the risk of contamination and channel clogging, resulting in a highly reproducible nanoparticle production with a small size distribution [13]. However, the multiphase systems are less flexible than the single-phase systems in terms of continuous modification, as it is difficult to add reagents to the encapsulated droplets. Like single-phase systems, multiphase systems can be parallelized to increase throughput.

Figure 4. Optical images of a device for gold nanoparticle synthesis; (1) Droplet formation, (2) Picoinjection, (3) Mixing and (4) Droplet collection. Image from Abalde-Cela et al. “Droplet microfluidics for the highly controlled synthesis of branched gold nanoparticles” [14].

4. Advantages of microfluidic nanoparticle production

Microfluidic systems have presented several advantages for nanoparticle production over conventional batch synthesis. A list of advantages and disadvantages of microfluidic devices for nanoparticle synthesis compared to conventional synthesis is summarized below [7].

Spatial separation

Importantly, in flow systems the nucleation, growth and formation stages can be spatially separated to occur at different places, whereas in batch synthesis they occur simultaneously in the reactor. In batch reactors, agglomeration inevitably starts to occur at a certain point of growth due to lack of mixing and separation of particles, resulting in a wider size and shape distribution [15]. Another advantage of separating the phases in microfluidic devices is that each part of the device can be adapted for the specific conditions of the synthesis phase that occurs there. This facilitates high-throughput and process scale-up.

Fine-tuning of parameters

In microfluidic systems various experimental parameters including temperature, pH, concentration of reagents and flow velocity can be conveniently controlled and fine-tuned to fabricate nanoparticles of desired characteristics [16]. Due to the small dimensions of microfluidic devices, fast and efficient temperature changes or can be induced. An easy way of varying parameters enables better control of the reactions and the possibility for fast screening to find optimal reaction conditions. Microfluidic devices have also opened up for new possibilities of sensor integration and online characterization, for process optimization and real-time feedback control.

Microfluidic mixing

Passive and active mixing are key concepts in microfluidic nanoparticle formation. As most microfluidic devices work at laminar flow, the primary mixing is diffusion between flow streams. The mixing time in a continuous microfluidic channel can be estimated from the equation:

where w is the channel width, D is the diffusivity of the solvent in the core stream, R is the ratio of the core stream rate to the total flow rate of surrounding streams [17]. Hence mixing time can be controlled by appropriate chip design and manipulating flow rates. To improve mixing performance, single-phase systems often comprise structures such as t-junctions, y-junctions, capillary and coaxial tubes, hydrodynamic flow focusing, herringbone mixer and micro mixing-based reactors. Some of these mixing techniques are illustrated in figure 4.

Figure 4. Common microfluidic mixing techniques (a) Y-shaped microfluidic device, image from [18] (b) T-shaped microfluidic device, image from [19] (c) Hydrodynamic flow focusing device, image from [3] (d) Herringbone micromixer, image from [20].

5. Application of nanoparticles synthesized in microfluidic systems

As previously mentioned, several types of organic and inorganic nanoparticles have successfully been fabricated in microfluidic devices. To illustrate the versatility and improved outcome of nanoparticles produced in microfluidic systems, a short selection of examples of their biomedical applications is presented here. Several comprehensive review articles have already been published on the topic, and the examples below relies on some of the sources presented therein [7, 15, 21].

Microfluidic synthesis of nanoparticles for imaging

Metallic nanoparticles are of great interest in biomedical imaging due to their optical and plasmonic properties. Their potential has been realized for applications in fluorescence imaging or as contrast agents in magnetic resonance imaging (MRI). The common approach to microfluidic metal nanoparticle synthesis has been mixing streams of a metal salt with a reducing agent. Wagner et al. [22] synthesized gold nanoparticles in a microfluidic device using a single-phase flow, from the reagents gold salt (HAuCl₄) and a reducing agent (ascorbic acid). The device allowed convenient optimisation of parameters to obtain a narrower size distribution compared to conventional batch synthesis. Another group, Hassan et al. [23], first reported a proof-of-concept for continuous multistep microfluidic self-assembly of fluorescent, plasmonic, and magnetic nanoparticles. Their device was made up of two coupled Y-shaped glass microreactors (figure 5.). They observed a reduced synthesis time from hours in batch synthesis to a few minutes with the microfluidic device.

Figure 5. Illustration of a multistep device for continuous-flow synthesis of magnetic and fluorescent nanoparticles. Image from Hassan et al. “Multistep continuous-flow microsynthesis of magnetic and fluorescent gamma-Fe2O3@SiO2 core/shell nanoparticles [23].

Microfluidic synthesis of nanoparticles for drug delivery

Biodegradable polymers that release a drug after their degradation have been widely used for drug delivery [24]. Microfluidic-synthesized nanoparticles have shown better drug loading capabilities compared to batch-synthesized nanoparticles [25]. The first polymeric drug loaded nanoparticles fabricated in microfluidic devices, were synthesized by precipitation in a hydrodynamic flow focusing chip in PDMS by the group Farokhadz et al. [3]. Their method has been used to fabricate various types of nanoparticles. Also, lipid-polymer nanoparticles, which are problematic to synthesize with batch approaches, have been fabricated with the desired characteristics in microfluidic devices [26].  The recently emerging field of “organ-on-a-chip” has introduced a new type of platforms for evaluation and screening of drug delivery systems. Organ-on-a-chip platforms are a group of microfluidic systems that aim to mimic specific functions of one or multiple organs. They can be used for fast screening to study the performance of drug delivery systems, aiming to accelerating the clinical translation of drug delivery nanoparticles [27].

PLGA nanoparticles have also been used for drug delivery thanks to good biocompatibility and biodegradability. Find out how to make plga nanoparticle preparation using Elveflow instruments in our dedicated application note.

Microfluidic synthesis of nanoparticles for biosensing

Introducing nanomaterials to biosensing applications has led to enhanced performance in terms of increased sensitivity, lower detection limits and broad detection ranges. Plasmonic nanoparticles (often gold or silver) has lately been a subject of interest, due to their localized surface plasmon resonance (SPR) phenomena which can be utilized for biosensing. As their SPR frequency is dependent on size, shape and local atmosphere of the particles (among other factors), particle size and shape must be precisely controlled during synthesis.

Microfluidic synthesis of these metal nanoparticles have shown advantages over the conventional synthesis methods, including better size and shape control, increase in reaction yield, and rapid mixing [28]. Polydiacetylene is another type of nanomaterial that has been used for biosensing as it experiences a change in color in response to various factors such as variations in pH and temperature. Baek et al. [29] successfully synthesized polydiacetylene nanoparticles in a microfluidic chip with high reproducibility and adjustable size by varying the flow rate (figure 6).

Figure 6. Illustration of the experimental setup for single-phase continous flow synthesis of polydiacetylene nanoparticles for sensing applications in a herringbone micromixer microfluidic chip. Image from Baek et al. ”Nanoscale diameter control of sensory polydiacetylene nanoparticles on microfluidic chip for enhanced fluorescence signal” [29].

6. Conclusions on microfluidics nanoparticle synthesis

Microfluidic systems have demonstrated great potential as platforms for synthesis of both organic and inorganic nanoparticles with well-controlled size and shape. Improved techniques for synthesis resulting in higher quality of produced nanoparticles is expected to accelerate the research of nanoparticles for biomedical applications. The obvious challenge of microfluidic nanoparticle production lies in the translation of proof-of-concept stage to clinical and industrial practice. Other practical challenges in fabrication, automation and channel clogging must also be addressed. Nevertheless, microfluidic systems are very promising for solving some of the current challenges and speed up the development of up-scale, cost-effective nanoparticle synthesis.

References