# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### pH Sensing  
In addition to the temperature control, the information about ion concentrations--for example, pH--is very important due to the fact that medium pH can strongly affect many metabolic processes and enzymatic reactions. Additionally, the intracellular pH is involved in cellular events such as cell growth, endocytosis, transport of ions, activity of enzymes and evolving of some diseases--for example, cancer [50]. [PERSON] et al. [51] presented a pH sensor system based on polymer capsules for monitoring pH changes in human breast cancer cells and fibroblast environments. Multifunctional capsules were formed by incorporating magnetite nanoparticles and pH-indicator dye into a polymer microcapsule shell. SNARF-1-dextran fluorophore was incorporated in spherical CaCO\({}_{3}\) particles by a coprecipitation method [50]. Later, SNARF-1-loaded particles were embedded in the shells of polyelectrolytes. A multi-layered (onion-like) shell was formed by consecutively depositing 10 layers of polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH). Therefore, SNARF-1-dextran fluorophore was retained in the microcapsule shell and was not dispersed throughout the sample. A pH-sensitive fluorophore exhibited a significant emission shift under different pH conditions: in acidic medium, dye showed a green emission and in basic medium, the fluorescence peak was at the red spectral region. Due to the incorporation of magnetite nanoparticles, it was possible to transport microcapsules to a specific location or easily separate them from the sample by applying magnetic field. In order to observe local pH changes under cell culture conditions, cancer cells and fibroblasts were incubated with sensor capsules. During endocytic uptake, capsules got inside the cells, and fluorescence measurements proved that inside the endosomal/lysosomal compartments  
Figure 9: Schematic representation of interferometric temperature control system. Adapted from [48]. Copyright 2012 © The Royal Society of Chemistry. medium was acidic and did not depend on the varied pH value outside the cells. Microcapsules, which were left outside the cells, corresponded well to the pH changes. However, the distribution of SNARF-dextran in microcapsules was inhomogeneous and the amount of dye varied in different capsules. Moreover, due to photobleaching, the absolute fluorescence intensity decreased. Another microfluidic platform for pH monitoring was presented by [PERSON] et al. [14]. By applying a fluorescent nanoparticle-based sensor, ratiometric pH imaging of oral bacteria _Streptococcus salivarius_ biofilms was performed in real-time. For this purpose, Ag@SiO\({}_{2}\) + FiTC nanoparticles were covalently patterned on a planar glass slide and formed a thin sensing surface that featured good photostability and ability to preserve Ag@SiO\({}_{2}\) + FiTC nanoparticles under flow conditions. _Streptococcus salivarius_ bacteria were used for co-aggregation, surface attachment and formation of biofilm directly at the sensing surface inside a Y-channel microfluidic device due to hair-like protein-based appendages. In this study, fluorescein was used as a pH-sensitive fluorescent dye and was covalently attached inside the 15 nm silica layer for quantitative pH measurements. It was demonstrated that the glucose concentration changes in the nutrient solution stimulated pH changes at the attachment surface of a biofilm, which were monitored by ratiometric pH images. Due to good biocompatibility, it was possible to perform experiments, which lasted longer than one week. Additionally, density and porosity of biofilms were investigated by varying flow rates of the nutrient solution and monitoring the pH changes. In order to monitor pH changes or determine the pH value more precisely, luminescence lifetime-based measurements offer this opportunity since the lifetime of a fluorophore does not depend on the fluorescence intensity or wave-length interferences.