# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Preparation of Microcapsules  
Multifunctionalized polymer microcapsules, which had a monitoring property of intracellular environments, were presented by [PERSON] et al. [30]. Moreover, it was demonstrated that microcapsules can encapsulate a large variety of molecules such as enzymes, proteins and drugs and release encapsulated compounds in a controlled manner upon an external trigger. Functionalization of nano and microcapsules with luminescent cadmium telluride (CdTe) QDs quantum dots gave the ability to facilitate imaging and labelling of various capsules. For the formation of capsule-based pH sensor, pH indicator dye (SNARF-1) was encapsulated via covalent bonding. The knowledge of local pH values gave important information of cell activity for the reason that changes in the pH were associated with numerous enzymatic reactions and many metabolic processes. Additionally, capsules were targeted to desired cells by applying different surface coatings. Incorporation of magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles into capsule shells allowed the control of the microcapsules by applying external magnetic field. It was shown that microcapsules containing embedded magnetic nanoparticles can be transported with a magnet and be carried to a particular place inside a living cell. Then, the microcapsules were activated or opened to release encapsulated molecules by the laser-induced remote, which gave the possibility to deliver compounds into the cell with precise control. In order to detect proteins and nucleic acids, [PERSON] et al. [31] developed a microcapsule-based assay that featured simplicity, selectivity and sensitivity. Four layers of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) were sequential deposited by electrostatic forces on the 6 \(\upmu\)m size CaCO\({}_{3}\) particle to form the (PAH/PAA)\({}_{2}\) microcapsules. 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide (EDC) was used to covalently cross-link carboxylic groups of PAA to amine groups of PAHs. By coating the surface of microcapsules with protein A or streptavidin, it was possible to further attach a target biomolecule--for example antibodies, biotinylated oligonucleotides or MHC class I proteins. Prepared microcapsules were stable and did not aggregate under storage or experimental conditions. The detection was carried out on the surface of the microcapsule, when the analyte was squeezed in between a binder and a detector molecule. For evaluating the assay's sensitivity and selectivity, biomarker protein (h\(\beta_{2}\)M) and oligonucleotide molecules were used as analytes. As a result, the assay was highly sensitive to h\(\beta_{2}\)M in the concentration range from picogram/L to nanogram/L, despite the presence of irrelevant proteins (streptavidin and conalbumin) in the sample. Figure 6: (**A**) Schematic illustration of composition of sensor microparticles. (**B**) Scheme showing the light cascade inside a sensor particle. Adapted from [28]. Copyright © 2015, Springer Nature. Moreover, it was shown that biotinylated ligands were strongly bound to the streptavidin-coated (PAH/PAA)\({}_{2}\) capsules. [PERSON] et al. [32] reported a method for enhancing the targeting of cell surface receptors by applying biofunctionalization of polyelectrolyte (PAH/PAA)\({}_{2}\) microcapsules. In this study, two antibody immobilization strategies on the (PAH/PAA)\({}_{2}\) microcapsule surface were compared (Figure 7). In the first case, the specific cell-targeting antibodies were immobilized randomly due to direct covalent coupling of the amino groups of an antibody to the carboxyl groups of a microcapsule surface via EDC/sulfo-NHS reaction. In the second case, before antibody immobilization, microcapsules were coated with protein A, which formed a supporting layer for more effective and optimized coupling of antibodies. The cell surface MHC class I receptors were chosen for a selective binding to antibody-functionalized (PAH/PAA)\({}_{2}\) microcapsules.