# BODIPY-Based Nanomaterials--Sensing and Biomedical Applications
## 1 Introduction
### Brief Characterization of Selected Nanoparticles
In addition to the antimicrobial effect, silver nanoparticles also exhibit radiosensitizing properties. In this case, the mechanism of action relies on the induction of apoptosis, activation of oxidative stress, and increase in the permeability of the cell membrane [17]. Magnetic nanoparticles constitute another type of interesting nanomaterial. Their core, formed of Fe\({}_{3}\)O\({}_{4}\) or \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), can be coated with polyvinyl alcohol (PVA), dextran, PEG, PVP, or chitosan to protect them from rapid clearance within the blood system. The synthesis of magnetic nanoparticles is based on alkaline coprecipitation of iron(II) or iron(III) ions [30]. Thanks to their supermagnetic properties, these nanoparticles can be transported to thedesired location when an external magnetic field is applied. In addition, they can also release heat at their location, causing cell death. Similar to other metallic nanoparticles, the toxicity of the magnetic nanoparticles is also under investigation. The reaction of iron ions with hydrogen peroxide or oxygen generates ROS, leading to DNA damage. Despite this, magnetic nanoparticles are considered as attractive drug carriers due to their high loading capacity and the possibility of surface modification [31]. Nowadays, they are also widely used in nanomedicine and nanobiotechnology as biological sensors and contrast agents in magnetic resonance imaging. Quantum dots (QDs) belong to interesting metallic and semiconducting nanostructures with a size range of 2-10 nm and are made of inorganic nanocrystals usually composed of a CdSe core or ZnS shell and coated with PEG [32]. QDs are photostable and possess broader excitation spectra in comparison to narrow excitation-emission spectra used in imaging techniques. Moreover, their excited state after light irradiation is prolonged [33]. They are often encapsulated in micelles or attached to amphiphilic molecules and polysaccharides to provide good water solubility. The quantum dots can penetrate tissues well due to their small size and the EPR effect. QDs conjugated with tumor-specific antibodies can be applied in imaging techniques in oncology. Their fluorescence properties allow only one light source to be used, which results in lower costs and easier data analysis. Moreover, in imaging with QDs, no signal amplification is needed [34]. QDs can also be applied in theranostics as both drug carriers and imaging agents. Despite this, there are some controversies about the use of QDs in medicine due to their potential toxicity related to the release of toxic cations. QDs can also act as energy donors and acceptors in fluorescence resonance energy transfer (FRET) when conjugated with photosensizers [35]. Titanium dioxide (TiO\({}_{2}\)) also belongs to the group of semiconducting metallic nanoparticles. This naturally occurring mineral is commonly used as a white pigment. Its utilization in medicine or environmental protection is based on its photocatalytic properties [36]. After excitation of TiO\({}_{2}\) with UVA light, electron emission is observed, accompanied by a positively charged hole left behind in the molecule. The free electron can react with oxygen to generate superoxide, whereas the hole can produce hydroxyl radicals from water [27]. The production of ROS via irradiation of TiO\({}_{2}\) can be used for photocatalytic an antimicrobial effect or degradation of organic pollutants. Nowadays, TiO\({}_{2}\) nanoparticles are also used as photosensitizing agents in hybrid materials [37]. Upconverting nanoparticles (UCNs) belong to modern metallic nanoparticles, which after sequential absorption of two or more photons can emit light at shorter wavelengths in comparison to the excitation wavelength [27]. UCNs can convert low energy light (near-infrared wavelengths) to high energy light through the anti-Stokes emission process.