72
K. Illath et al.
easy to add reagents downstream and easy to quench the reaction once the product
is formed [132, 133]. NPs in microfluidics are synthesized using a single solvent
(single-phase) or multiple miscible solvents (various phases). For the synthesis of
MNPs, two flow types with different mixing geometries are used. Diffusion mixing
of reagents happens in continuous flow microfluidic, whereas segmented flow uses
either gas-liquid or liquid-liquid flow. Gas-phase is due to inlet air, or air evolved
through reaction, whereas the liquid phase is due to the injection of immiscible liquid
[134–136]. An important segmented flow device is based on droplet formation. Shear
force and interfacial tension within two immiscible liquid generate droplets. It can be
achieved with the geometries such as flow focusing, T-junction, and co-flow. Winding
and distributed mixing modules are used in laminar flow mixing. T and Y-junction
geometries are simple and easy to handle, where particle size can be controlled with
channel width as well as flow rate. Geometry and flow condition determine particle
size in cross channel geometry microfluidics [137].
Compared to other methods described above, particles prepared with microfluidics
technology revealed superior performance in targeted cancer imaging, enhanced therapeutic efficiency, low sensing detection limit and higher catalytic activity [132, 138,
139]. For this, smart construction and design of the microfluidic device are required
so that MNPs with desired shape, size, and crystallinity can be achieved. Parameters
such as concentration ratio of reactants, flow rate, seed quantity, seed ageing time,
residence time, temperature, heating time, and microchannel wall thickness influence
the controlled synthesis of MNPs.
3.6 Other Methods of Synthesis
Other methods of synthesis include electrochemical deposition, microwave-assisted
synthesis, supercritical fluid technology, and ultrasound technique. In an electrochemical deposition, baths containing metal salts are used. Baths can be either acidic
or basic, and an electrode, namely cathode, is used, where metals are deposited. A
three-terminal potentiostat and Ag/AgCl electrode (anode) is used, which acts as a
reference electrode. A voltage is applied for a specific time for the deposition of metal.
The advantages of this technique are simple, inexpensive, and particle synthesis with
uniform morphology and size directly on the substrate. This method can quickly
synthesize nanowires of Au, Co, Ni, and Pt [34, 140].
Recently, Ag NPs were successfully synthesized with the application of
microwave of 300 MHz to 300 GHz. With this, polar molecules such as water are
oriented with the electric field, creates friction and loss of energy in the form of
heat. Usually, a metallic salt solution is irradiated with a reducing agent, and with or
without a stabilizer. Sometimes this technique is employed along with plant-mediated
synthesis. Compared to other methods, homogeneous heating in this method can
speed up the reaction, leads to shorter crystallization time and homogeneous nucleation [34, 141]. Another technique is supercritical fluid technology, in which reaction
K. Illath et al.
easy to add reagents downstream and easy to quench the reaction once the product
is formed [132, 133]. NPs in microfluidics are synthesized using a single solvent
(single-phase) or multiple miscible solvents (various phases). For the synthesis of
MNPs, two flow types with different mixing geometries are used. Diffusion mixing
of reagents happens in continuous flow microfluidic, whereas segmented flow uses
either gas-liquid or liquid-liquid flow. Gas-phase is due to inlet air, or air evolved
through reaction, whereas the liquid phase is due to the injection of immiscible liquid
[134–136]. An important segmented flow device is based on droplet formation. Shear
force and interfacial tension within two immiscible liquid generate droplets. It can be
achieved with the geometries such as flow focusing, T-junction, and co-flow. Winding
and distributed mixing modules are used in laminar flow mixing. T and Y-junction
geometries are simple and easy to handle, where particle size can be controlled with
channel width as well as flow rate. Geometry and flow condition determine particle
size in cross channel geometry microfluidics [137].
Compared to other methods described above, particles prepared with microfluidics
technology revealed superior performance in targeted cancer imaging, enhanced therapeutic efficiency, low sensing detection limit and higher catalytic activity [132, 138,
139]. For this, smart construction and design of the microfluidic device are required
so that MNPs with desired shape, size, and crystallinity can be achieved. Parameters
such as concentration ratio of reactants, flow rate, seed quantity, seed ageing time,
residence time, temperature, heating time, and microchannel wall thickness influence
the controlled synthesis of MNPs.
3.6 Other Methods of Synthesis
Other methods of synthesis include electrochemical deposition, microwave-assisted
synthesis, supercritical fluid technology, and ultrasound technique. In an electrochemical deposition, baths containing metal salts are used. Baths can be either acidic
or basic, and an electrode, namely cathode, is used, where metals are deposited. A
three-terminal potentiostat and Ag/AgCl electrode (anode) is used, which acts as a
reference electrode. A voltage is applied for a specific time for the deposition of metal.
The advantages of this technique are simple, inexpensive, and particle synthesis with
uniform morphology and size directly on the substrate. This method can quickly
synthesize nanowires of Au, Co, Ni, and Pt [34, 140].
Recently, Ag NPs were successfully synthesized with the application of
microwave of 300 MHz to 300 GHz. With this, polar molecules such as water are
oriented with the electric field, creates friction and loss of energy in the form of
heat. Usually, a metallic salt solution is irradiated with a reducing agent, and with or
without a stabilizer. Sometimes this technique is employed along with plant-mediated
synthesis. Compared to other methods, homogeneous heating in this method can
speed up the reaction, leads to shorter crystallization time and homogeneous nucleation [34, 141]. Another technique is supercritical fluid technology, in which reaction