# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Preparation of Composite Particles  
There are several motives for assembling particles for sensing. Sensor NPs can be densely packed into a swellable or nanoporous matrix by forming submicron- or micrometer-sized sensor particles. The preparation of metal/polymer micro and nanoparticles for SERS sensing gives a higher sensor signal because a large amount of small metal particles is incorporated in the composite [1]. [PERSON] et al. [25] applied a microcontinuous-flow process in order to prepare hydrogel and silver nanoparticle composites. First of all, silver seed particles were formed in microsegmented flow by reducing silver nitrate via sodium borohydride. Then, silver seeds were used to fabricate silver nanoprisms by using ascorbic acid and silver nitrate. Prepared silver nanoprisms were mixed with an acrylamide monomer, cross-linker and photoinitiator, and droplets were generated in micro co-flow arrangement when polydimethyl-siloxane was used as a viscous carrier phase. The composite particles were produced by a photopolymerization step under UV irradiation. By applying microfluidic synthesis, high reproducibility and narrow size distribution of particles were achieved. Furthermore, washed composite particles were mixed with silver nitrate and ascorbic acid solution for nanoparticle-catalyzed silver deposition on the particle surface in order to improve sensitivity for SERS sensing. [PERSON] et al. [26] reported the microfluidic assisted formation of nanoassembly particles. In this robust approach, PMMA-polyDADMAC/gold (Au) nanoassemblies were formed under a flow condition. For formation of copolymer nanoparticles, the microflow-through platform was applied (Figure 4). Therefore, the particle size was tunable in the range of 200 nm-1 \(\upmu\)m by varying the flow rate ratio and concentrations of monomer and carrier phases. The formation of nanoassemblies was based on electrostatic adsorption of negatively charged 30 nm size Au nanoparticles were attracted on the surface of the positively charged polymer particle. In order to improve SERS sensing properties, Ag deposition in the presence of silver precursor and reducing was initiated by the adsorbed AuNPs on the composite particle. Formed PMMA-polyDADMAC/Au/Ag composite was used for adenine sensing. Due to the small size, sensor particles could be applied for SERS sensing not only in the batch condition but also in the flow arrangement. [PERSON]'s group [27] reported a two-step microfluidic assisted self-assembly formation of plasmonic, fluorescent and magnetic nanostructures. Rhodamine isothiocyanate (RITC) doped silica particles, superparamagnetic \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) nanocrystals and gold NPs were self-assembled under continuous  
Figure 4: Schematic representation of emulsion polymerization of the copolymer nanoparticles [26]. Copyright Wiley-VCH GmbH. Reproduced with permission. flow in the microfluidic system that consisted of two Y-shaped microreactors made from glass (Figure 5). Due to the fact that the assembly processes are dynamic, the desired structures can be obtained by varying flow rates of fluids and residence time. By comparing the assemblies prepared in microfluidic process and in bulk conditions, it was demonstrated that the lower density of Au NPs on silica nanoparticles surface was found in bulk. As a result, the high potential of microfluidics in nanoparticle synthesis for imaging purposes was achieved. A second motivation for assembling can be given by combination of sensor signals. [PERSON] et al. [28] prepared multifunctional composite particles that combined oxygen sensing, magnetic and luminescence properties. Luminescence-based composites for the oxygen sensing consisted of the matrix material (PPSQ), upconverting nanoparticles (UCLNPs), magnetic nanoparticles (EMG1300), mediator dye (MFR) and a dye with far-red emission (PtTPBP). All components were swollen and dissolved in chloroform (Figure 6). After solvent evaporation, the mixture was heated for 4 h at 230 \({}^{\circ}\)C temperature.