# BODIPY-Based Nanomaterials--Sensing and Biomedical Applications
## 1 Introduction
### Nanoparticles Involved in the Photodynamic Therapy  
The conjugation between metallic nanoparticles and porphyrinoid-type photosensizers is an interesting issue for medical, photocatalytic, and sensing purposes. Metallic nanoparticles conjugated with porphyrinoids are of great interest for medical applications since they could overcome the limitations of classic PS used in PDT. Nanoparticles can be categorized into passive and active nanoparticles depending on their involvement in photosensitizer excitation. Nanoparticles as PS carriers offer some benefits, such as the hydrophilicity of hybrid material and passive targeting of tumor tissues via the EPR effect. Direct targeting of nanoparticles to specific tissues can be reached via nanoparticles' surface functionalization with monoclonal antibodies or specific tumor-targeting molecules (proteins, peptides, and aptamers) [39; 46]. Unlike typical drug delivery systems, the release of PS by metallic nanoparticles is not required [47]. In PDT, gold nanoparticles, due to their localized surface plasmon resonance, could be used to increase the excitation efficiency of the accompanying PS [48]. Another type of nanoparticle which is useful in PDT is the group known as QDs, the surface of which can be modified and functionalized to make it water-soluble and more biocompatible. Due to the large transition of the dipole moment, they are strong light absorbers and can act as energy donors and transfer energy, finally leading to the generation of ROS. The transfer of energy from QD to PS via FRET can also be potentially applied in PDT due to deeper tissue penetration of near-infrared (NIR) light irradiation [35]. Apart from QDs, UCNs can also be used in photodynamic reactions to convert NIR excitation to lower energies, which are capable of activating the accompanied PS. What is more, the surface of TiO\({}_{2}\) nanoparticles can be modified with metal porphyrins using impregnation based on simple incubation of the synthesized TiO\({}_{2}\) nanoparticle powder in an organic solution of metal porphyrins. The adsorption of metal porphyrins on the TiO\({}_{2}\) surface can be realized in different ways, including (i) chemical functionalization, (ii) axial coordination of bridging ligands with metal atoms, (iii) intermolecular \(\pi\)-\(\pi\) interactions between planar porphyrin surfaces, and (iv) intermolecular hydrogen bonds due to the introduction of functional groups in porphyrin molecules [49]. Via the conjugation of metal porphyrinoids with TiO\({}_{2}\), the absorption of visible light also becomes possible [50]. Metallic nanoparticles in conjunction with PSs reveal many advantages for their prospective use in PDT: (i) the amount of PS around tumor tissue can be increased due to a large surface-to-volume ratio of nanoparticles, (ii) nanoparticles may prevent tissue from fast elimination and inactivation of PSs via plasma components, (iii) nanoparticles may prevent normal tissues from nonspecific accumulation of PSs and reduce their overall photosensitivity, (iv) the amphiphilicity of PS can be obtained via conjugation with nanoparticles, allowing them to be distributed in the cardiovascular system and be further localized in the tumor tissue, (v) nanocomposites delivery and retention in tumor tissue is combined with the EPR effect, (vi) the surface modification of nanoparticles with functional groups or targeting agents can improve their biodistribution, pharmacokinetics, cellular uptake, and targeting abilities, and (vii) nanoparticles allow the carrying of multiple components to the target tissue; specifically imaging agents, drugs, targeting ligands, and agents preventing interference with the immune system (Figure 4) [39]. To sum up:  
* In PDT, the nanoparticles can be used as passive drug carriers or active co-photosensitizers. * To the most frequently studied nanoparticles for PDT belong gold nanoparticles, quantum dots, upconverted nanoparticles, and titanium dioxide nanoparticles.