# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Polymer and Hydrogel Particles with Embedded Sensor Dyes
It was shown that the fluorescence intensity significantly decreased with the increased dissolved oxygen concentration due to fact that PtOEP molecules fluorescence was quenched by dissolved oxygen, which could easily interact with dye because of porous PS particles structure. Moreover, PtOEP/PS sensors were stable, highly sensitive and showed a fast response time and linear dependence on dissolved oxygen concentration. After 3 months of the preparation of PtOEP/PS  
Figure 2: Schematic illustration of the generation of droplets of ethylene glycol (EG) solutions, which were separated by air. Adapted from [9]. Copyright © 2013 American Chemical Society. spheres, their photostability was investigated. PtOEP/PS particles were illuminated for 1 h under irradiation at 380 nm, and fluorescence intensity of PtOEP decreased only 2.2%. Photo-stability of PtOEP embedded in the PS spheres was much better in comparison to PtOEP in THF solution. Oxygen nanosensor particles PS-PtTPBP were used by [PERSON] et al. [6] for non-invasive monitoring of oxygen during growth of bacteria. For the preparation of polystyrene (PS) nanoparticles doped with oxygen-sensitive dye PtTPBP, first PS particles were synthesized from styrene/water mixture and K\({}_{2}\)S\({}_{2}\)O\({}_{8}\) under temperature control. After the dialysis process, a suspension of nanoparticles was dispersed in methanol/water (2:1) solution and then PtTPTPBP, dissolved in chloroform, was added. The mixture was shaken for 4 h and afterwards filtrated and dialyzed. Produced particles were \(357\pm 53\) nm in size. Core\(-\)shell poly(styrene-block-vinylpyrrolidone) (PSPVP) particles were synthesized by [PERSON] et al. [23]. For simultaneous detection of O\({}_{2}\) and pH, the oxygen indicator (PtTPTPBPF) was embedded in the core and a pH sensitive BF\({}_{2}\)-chelated tetraarylazadi-pyrromethene dye (aza-BODIPY) and antenna dye (Zn-Schiff base), which was used for enhancement of brightness, were incorporated into the shell of the particle. Ideally, all dyes should be embedded into a single particle but due to interactions between dyes and their migration in the core and shell, it was not achieved. Therefore, nanoparticles embedded with oxygen and pH sensitive dyes were prepared separately and mixed together for m-DLR measurements. In addition to pH, oxygen monitoring, the detection of organic molecules and inorganic ions are also very important for chemical analytics. Therefore, fluorescent micro and nanoparticles are widely used for local chemical sensing. For detection of chloride ions, [PERSON] et al. [16] synthesized polyacrylamide hydrogel particles loaded with lucigenin dye due to the fact that its fluorescence is quenched by chloride ions. The continuous flow synthesis of hydrogel sensor particles is shown in Figure 3. For droplets generation, a monomer and photoinitiator mixture was injected into the carrier phase (silicone oil). After the formation, droplets were irradiated under UV light, which initiated a free radical polymerization of hydrogel particles. Particles were produced with narrow size distribution and an average size of 800 \(\upmu\)m. After washing and drying, the size of particles decreased by up to 480 \(\upmu\)m. Dry particles were added to the aqueous solution of lucigenin for swelling. After the lucigenin was immobilized into the polyacrylamide matrix, particles were dried again and used for chloride ion detection. It was demonstrated that swelling of sensor particles inside a chloride-containing aqueous solution dramatically influenced the fluorescence quantum yield. Due to the chloride ions, fluorescence intensity decreased in the swollen part of the particle. Figure 3: Experimental set-up for microfluidic generation of hydrogel sensor particles. Adapted from [16]. Copyright © 2020 Springer Nature. [PERSON] et al. [17] reported a microfluidic sensor for the fast detection of environmental contaminants. The heavy metal Hg and the pesticides ziram were detected by using fluorescent gold nanoclusters and particles.