# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Polymer and Hydrogel Particles with Embedded Sensor Dyes  
Fluorescent nanosized sensors are the most attractive for sensing purposes. Some materials (quantum dots and fluorescent proteins) show intrinsic fluorescence, but more often, NP fluorescence is created as a result of immobilization of fluorescent or phosphorescent dyes. In order to obtain extrinsic fluorescence, the particles need to incorporate one or more fluorophore dyes on its surface or inside the matrix by entrapping physically or via covalent attachment. Fluorescence sensing is based on changes in fluorescence signals due to fluorescence component interaction with an analyte molecule [1]. Generally, pH-sensor particles consist of two essential parts: a polymer matrix which has appropriate adhesive properties and a pH-indicator, which can be immobilized inside the particle matrix or on the particle surface. [PERSON] et al. [21] presented pH-sensors that consisted of 1,4-diketopyrrolo-[3,4-]pyrroles (DPPs) as a pH-sensitive dye. As the immobilization matrices, different polymer hydrogels were used in order to achieve high sensitivity, brightness and wide operational range from pH 5 to 12. For the preparation of sensor nanoparticles, 100 mg of positively charged acrylate polymer Eudragit RL100 was dissolved in 50 mL of acetone. Later, indicator dye (DPP) and Macrolex Yellow (1.25 mg) were added with 300 mL of water. By rotary evaporator, acetone was removed from the solution and the particle suspension was prepared. Two fluorescence quenching mechanisms were associated with the ability of DPP indicators to change fluorescence properties in different pH media. Due to the deprotonation of phenolic group, the photoinduced electron transfer (PET) dominated from pH 5.9 to 9.3. The deprotonation of DPP core (lactam nitrogen) occurred at higher pH media (9.7-11.6). Unlike the static planar sensors, which were limited to short-time applications, it was possible to use sensor beads dispersed in the sample for applications in microfluidic systems, fluorescence imaging or microscopy. Another example of fluorescent pH nanosensors was presented by [PERSON] et al. [22]. Sensors were prepared by staining the poly(styrene-block-vinylpyrrolidone) (PS-PVP) beads with pH dyes consisted of fluorescein and 1-hydroxypre-3,6,8-trisulfonate derivatives. By changing substituents on the pH indicator, it was possible to modify the pH-sensing properties. Lipophilic fluorescein derivatives were easily immobilized into the PS-PVP particles without any dyes leaching. This made various beads promising for application in biotechnology and many other fields. For ratiometric pH imaging, [PERSON] et al. [14] reported silver core-silica shell (Ag@SiO2) nanoparticles covalently incorporated with fluorescein isothiamate (FiTC) fluorophore. The pH-sensitive sensor nanoparticles were made in two steps. First of all, the seed-growth method of reduction for silver nitrate by sodium citrate was applied for formation of metallic cores with an average size of 80 nm. Secondly, tetraethylorthosilicate (TEOS) was co-condensed with a silanized molecule of fluorescein isothianate (FiTC-APS). In this study, fluorescein was used as a pH indicator because its spectroscopic properties (absorption spectra and fluorescence quantum yield) depend on the pH. By varying the pH value, fluorescein can be in protonated or negatively charged forms and, therefore, applied for quantitative pH measurements. Oxygen sensitive dyes are usually embedded in the polymer matrix in order to form sensor particles or oxygen sensitive layers. Monodispersed fluorescent platinum octaethylporphyrin-polystyrene (PtOEP/PS) spheres for oxygen monitoring were presented by [PERSON] et al. [15]. Fluorescent PtOEP/PS particles, whose size varied about 251-384 nm, were prepared by a soap-free emulsion polymerization.