# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Preparation of Sensor Particles in Microfluidic Systems  
By using the microfluidic technique, various nanoparticles such as organic, metal, oxide nanoparticles and composites of organic-inorganic nanoparticles can be produced in reproducible and controllable manner. Additionally, during microfluidic synthesis, particles surface can be functionalized with different functional groups; hence, particles have unique physical and chemical properties [2]. With application of continuous single flow systems for multistep reactions, the agents for reaction can be easily added and precisely controlled. [PERSON]'s group [11] presented a multistep continuous single-phase microfluidic system for synthesis and functionalization of gold nanorods. By using this platform, formation of seeds and different stages of nanorod growth were separately controlled. Due to this, the reproducibility and quality of the particles were improved. Additionally, the amount of poly-(ethylene glycol)-methyl ether thiol (SH-PEG) used for surface functionalization was reduced 100 times in comparison to batch process. In addition, the microfluidic system was used for tuning the concentration of halide (chloride and bromide) ions at various stages of synthesis. Variable ion \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fabrication Method** & **Particle Type** & **Main Advantage** & **Ref.** \\ \hline Continuous single-phase microfluidic synthesis & Gold nanorods & Improved reproducibility and quality, 100 times reduced consumption of SH-PEG, ability to tune halide concentration at various stages of synthesis. & [11] \\ \hline Continuous micro segmented flow synthesis & Au/Ag core/shell & Efficient reactants mixing in droplets, high homogeneity, narrow size distribution, smaller particle diameter. & [12] \\ \hline Microdroplet-based synthesis & Silica nanoparticles & Improvement of nanoparticle size uniformity (down to 3\% relative standard deviation), ability to tune particle size by adjusting reaction time or reagent concentrations & [13] \\ \hline Gas–liquid droplet–based synthesis & Silver nanocrystals & tunable edge lengths of Ag nanocubes from 30 to 100 nm & [9] \\ \hline Batch synthesis & Silver core–silica shell nanoparticles & Used as the immobilization matrix for pH-sensitive dyes & [14] \\ \hline Surfactant-free emulsion polymerization & Polystyrene particles & Good size distribution, used as the immobilization matrix for oxygen-sensitive dyes & [6,15] \\ \hline Continuous flow synthesis & Polyacrylamide hydrogel particles & Narrow size distribution (standard deviation 2–3\%), used as the immobilization matrix for lucigenin & [16] \\ \hline Frens–Turkevich method & Gold nanoparticles & 17 nm size particles were functionalized with Rhodamine & [17] \\ \hline Hole mask colloidal lithography & nanoparticle arrays & Scalable and inexpensive fabrication of uniform nanoparticle arrays, ability to tune size of nanoparticles & [18,19] \\ \hline Physical vapor deposition followed by a rapid thermal annealing (RTA) & Gold nanostructures & Simple and rapid fabrication & [20] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Various methods for sensor particle fabrication with examples of special particle types. concentrations influenced the final nanorods' aspect ratio and accordingly the optical properties of nanorods. By utilizing continuous-flow systems, nanoparticles can be synthesized with better reaction conditions control, fast heat and mass transfer as well as high reproducibility [9]. [PERSON] et al. [12] presented a segmented flow method for fabrication of noble metal core/shell and multishell nanoparticles in colloidal solutions. The particles were synthesized by reduction of HAuCl4 and AgNO3 salts at the surface of seed particles by ascorbic acid. An improvement of the particle quality was achieved due to the efficient reactants mixing in droplets because of internal convection. It led to a narrower size distribution and lower particle diameter in comparison to the particles produced by conventional batch synthesis.