# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles  
Many conventional large-scale synthesis methods suffer from low efficiency of reactant loading and deficiency of flexibility for tuning parameters of the fabrication. Moreover, due to poor control over mixing and nucleation conditions, it is difficult to form reproducible nanoparticles by conventional bulk synthesis. In recent decades, usage of microfluidic systems has created the ability to fabricate nanoparticles with better control of size distribution, improved reproducibility, high encapsulation efficiency, low sample/solution consumption, high surface to volume ratio, efficient mixing and better control of mass and heat transfer [7]. Generally, the microfluidic technique handles and controls liquid at the microscale; therefore, nanoliter or even smaller volumes [2] of fluids can be precisely mixed, and the chemical reactions take place in short time and space scales. A comparison of microfluidic and conventional bulk synthesis is presented in Table 1.  
Microfluidic devices offer several advantages for the production of polymer, polymer composite and metal particles. By using them, the improvement of the particle size, its shape, morphology, composition and size distribution can be achieved. In order to prepare emulsion of a liquid monomer, two different strategies can be used (Figure 1). In the first strategy, fluids of continuous and dispersed phases flow inside microchannels and both are in direct contact with the device's wall. Another strategy describes capillary-based devices, where continuous phase flows inside a tube around a dispersed phase, which is applied by a smaller capillary in the way that the diameter of the formed droplets is much smaller than the channel diameter [8]. In both cases, the device material (hydrophobic for hydrophilic droplets or vice versa) must be carefully selected or modified to reduce a desired wall contact or a phase inversion. The most popular microfluidic geometries used for emulsification in microchannel-based devices include a T-junction and a flow-focusing unit. In the T-junction configuration, the dispersed phase reaches the continuous phase perpendicularity in the way that shear forces elongate the dispersed phase. Formed segment transforms into a sphere because of interfacial tension. The mechanism of breakup (dripping, jetting) and the droplet size depend on the aspect ratios of the widths and height of the channels or the nozzle, respectively; the flow rates of the two fluids; their viscosities; and the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Synthesis Type** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Challenges**} \\ \hline \multirow{6}{*}{Microfluidic synthesis} & \(\bullet\) & high reproducibility & \\  & \(\bullet\) & high encapsulation efficiency & \(\bullet\) & channel clogging \\  & \(\bullet\) & large surface to volume ratio & \(\bullet\) & preparation costs \\ \cline{1-1}  & \(\bullet\) & low sample consumption & \(\bullet\) & compatibility of fluid (solvent) and \\ \cline{1-1}  & \(\bullet\) & efficient mixing & \(\bullet\) & devise surface properties \\ \cline{1-1}  & \(\bullet\) & narrow particle size distribution & \(\bullet\) & limitations for heating \\ \cline{1-1}  & \(\bullet\) & control of mass and heat transfer & & \\ \hline \multirow{6}{*}{Conventional bulk synthesis} & \(\bullet\) & relatively basic setup & \(\bullet\) & low efficiency \\ \cline{1-1}  & \(\bullet\) & wide usage in applications & \(\bullet\) & low reproducibility \\ \cline{1-1}  & \(\bullet\) & easier preparation & \(\bullet\) & poor control over mixing and \\ \cline{1-1}  & & & nucleation conditions \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and challenges of microfluidic synthesis and conventional bulk synthesis of nanoparticles. interfacial tension between them and the nozzle materials. Devices for flow-focusing are based on the hydrodynamic focusing technique, in which a stream of one fluid is focused using the flow of another, immiscible fluid by a high flow rate ratio.