# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Oxygen Sensing
Dissolved oxygen concentration, cell density and endogenous autofluorescence intensity were simultaneously sensed in nl-sized droplets by utilizing a microflow-through fluorimeter and photometer. For that purpose, polystyrene beads (PS) doped with oxygen-sensitive dye PtTPBP were mixed with culture liquid in microfluid segments. Two bacterial strains _Streptomyces acidisachies_ E13 and _Psychrochacillus psychrochurans_ UrPLO1, which were resistant to heavy metal ions, were investigated. A model of the microfluidic experimental setup is shown schematically in Figure 10A. Microfluid droplets were generated through PEEK 7-port manifold. A syringe pump system controlled 5 syringes filled with the effector CuCl\({}_{2}\), suspension of sensor beads, cultivation medium, cell suspension and carrier liquid PP9. By varying flow rates of the effector solution and the cultivation medium, reactants were dosed and, therefore, it was possible to tune the composition of the microfluid segments inside a large sequence. After generation of micro fluid segments, they were transported with a fixed 55 uL/min flow rate to the detection unit (Figure 10B). For determining a distance between two segments, droplet size and number, light scattering measurements were performed by using two diodes with peak wavelengths of 470 and 505 nm. For evaluation of the bacterial growth and the oxygen consumption, the sample was excited under visible light with wavelengths of 470 and 405 nm. The results showed that the phosphorescence intensity measurements were sufficient for monitoring oxygen concentration inside the droplets. During microtoxicological investigation on the effect of copper (II) chloride, it was proved that the investigated strain of _Psychrobasillus psychrodurants_ UrPLO1 had a strong tolerance to Cu\({}^{2+}\). However, uneven distribution of the dye-labelled sensor particles inside the droplet, photobleaching and also droplet geometry could have a relatively high impact on the accuracy of an intensity-based luminescence measurements. [PERSON] et al. [56] proposed a non-toxic and contactless technique for estimating the absolute values of oxygen concentration in a cell culture microfluidic device. Pd-octaethylporphine (Pd-OEP) dye loaded polystyrene microparticles were embedded in dimethylpolysiloxane (PDMS) by creating a thin oxygen sensing film in order to avoid direct contact of the dyes with the living cells. After the oxygen-sensing film was installed into the microfluidic device, the 4T1 cells were cultured on it. Due to this, monitoring of oxygen consumption was performed stably and over long periods of time. It was determined that the response time of the oxygen concentration changes depended on microparticle size and oxygen-sensing film thickness. The smaller microparticles and thicker film led to the shorter response time. Due to the fact that phosphorescence lifetime depends on the oxygen concentration,  
Figure 10: (**A**) Model of the microfluidic experimental set-up. (**B**) Schematic illustration of the optical detector units that consisted of channel microflow-through photometry and fluorimetry. Reprinted from [6] with permission from Springer-Verlag Wien 2014.  
pO\({}_{2}\) changes were quantified from the lifetime/phosphorescence quantum yield, respectively, and the oxygen consumption quantity was estimated from pO\({}_{2}\) shifts. It was shown that the oxygen gradient downshifted due to cellular respiration process. In addition, by transplanting tumor fragments, which were excised from the mouse, on the sensor film, oxygen consumption was screened at various oxygen concentrations. As a result, oxygen concentration decreased in the parts in which the tumor tissue fragments were placed. [PERSON] et al. [57] presented a simple and low-priced method for integrating an oxygen-sensitive sensor particle into a glass microfluidic device. A sensing strategy relied on the formation of a sensing layer by physical adsorption of nanoparticles.