# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Temperature Sensing  
Temperature control is among the important functionalities, which could be integrated in a microfluidic device. By applying non-invasive spectroscopic techniques, temperature can be monitored at multiple locations inside microfluidic devices [47]. In order to efficiently perform various chemical as well as biochemical reactions and assays, it is very important to control temperature properly and accurately. Polymerase chain reaction (PCR) is among the examples of a temperature-controlled process, which requires an accurate temperature measurement, and is widely used for DNA sequencing, pathogen detection and genetic analysis. [PERSON] et al. [48] presented an interferometric temperature control system with closed-loop temperature heating and cooling for direct temperature measurement in order to monitor PCR process. Non-contact temperature sensing was performed using Extrinsic Fabry-Perot interferometry (EFPI). A schematic representation of an interferometric temperature control system is shown in Figure 9. During the experiment, optical fiber emitted the NIR radiation (850 nm) into the to the microfluidic chamber, whose surfaces were modified with a platinum nanoparticle monolayer. Interference pattern was formed by reflected lights, which were collected from the upper and lower chamber surfaces, and was used for determining the optical path length. By monitoring the change in the optical path length, the temperature inside the chamber was determined because the refractive index of the medium decreased with increasing temperature. The presented interferometric temperature control system was applied in the PCR process for amplification of a portion \(\lambda\)-phage DNA and human genomic DNA. As a result, the temperature was monitored with detectable temperature change of 0.3 \({}^{\circ}\)C. This universal thermocyling system can be applied for a variety of disposable miniaturized platforms such as cell-based devices or sequencing tools. Separate fluorescent dyes in solution can also be used as temperature indicators. Generally, not only quenching agents but also changes in pH can strongly affect spectroscopic properties of the dyes; therefore, it is challenging to measure temperature precisely. Another disadvantage of fluorescent dye application is that their fluorescence properties depend on the temperature linearly only in a narrow temperature range [47]. Moreover, the concentration of fluorescent dyes influences the final fluorescence intensity. Therefore, the ratiometric technique, which is based on the measurement of intensities at two or more peaks of an excitation or emission spectrum and does not depend of temperature probe concentration, can be applied for temperature monitoring. [PERSON] et al. [49] applied a ratiometric band shape luminescence thermometry in microfluidic systems by using NaYF\({}_{4}\):Yb,Er nanoparticles, which convert near infrared radiation of emission into visible radiation, as an accurate temperature probe. In order to perform various temperature sensing experiments, temperature-dependent luminescence of NaYF4:Yb,Er was measured. Sensor NaYF\({}_{4}\):Yb,Er particles were formed by doping NaYF\({}_{4}\) NPs with 18% Yb\({}^{3+}\) for efficient upconversion along with 2% of emitting ion Er\({}^{3+}\). Efficient upconversion in Er\({}^{3+}\)-Yb\({}^{3+}\) couple caused a green fluorescence emission from Er\({}^{3+}\) ion under infrared (980 nm) excitation. To increase particle dispersibility in polar solvents and chemical stability, a protective SiO\({}_{2}\) layer of approximately 10 nm thickness was grown and core/shell structure was formed. By comparing luminescence spectra of particles with and without a protective layer, it was shown that the SiO\({}_{2}\) layer did not affect the temperature dependence of luminescence. It was demonstrated that by dispersing NaYF4:Yb,Er particles in the fluids, temperature could be monitored inside different microfluidic devices made of silicon/glass, glass/glass or PDMS/glass.