## 2 Metallic Nanoparticles in Conjugation with Chlorin Derivatives
### Upconverted Nanoparticles  
Upconverted nanoparticles (UNs), consisting of host lattices of ceramic materials embedded with transition metals, actinide, or lanthanide ions, such as Yb\({}^{3+}\), Er\({}^{3+}\), and Tm\({}^{3+}\), display a unique feature that is based on a process of the emission of light at a shorter wavelength in comparison to the excitation wavelength after the sequential absorption of two or more photons [46]. The use of upconversion nanoparticles brings one significant advantage--created material can be irradiated with near-infrared (NIR) wavelengths. The energy absorbed is converted and emitted as fluorescence in the visible or UV region upon NIR irradiation. The desired emission wavelength of the photosensitizer can be triggered by appropriately selected doped upconversion nanoparticles. Matching the emission and excitation wavelengths of the two components so that they overlap is critical to unleashing the cascade of energy transfers (Figure 6). Such an approach has received much attention recently. [PERSON] et al., prepared \(\beta\)-NaYF\({}_{4}\):Yb, Er(Tm) nanoparticles coated with a PEGylated polymer, functionalized with folic acid for targeting [47]. The material was mainly studied as a vehicle for the enhanced delivery of doxorubicin, but it was also shown to be a suitable carrier for Ce6 and _meso_-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP). Hexagonal NaYF\({}_{4}\):Yb,Er/NaGdF\({}_{4}\) core-shell nanoparticles bearing Ce6 were proposed by [PERSON] et al. for cancer diagnosis and treatment (Figure 7) [48]. Ce6 molecules were incorporated into the hydrophobic layer surrounding core-shell nanoparticles, which allowed to enhance their water solubility. Moreover, the carboxy groups present in the Ce6 molecules were applied in their conjugation with amino-modified particles with amide bond formation. The mechanism of action of the discussed nanomaterial was based on an upconverting phenomenon. Ytterbium was responsible for light absorption at the wavelength of 980 nm and energy transfer to the Erbium, which produced fluorescence. In addition, the PS Ce6 molecules were excited with the red light emitted by Erbium, which  
Figure 6: Schematic presentation of energy transfer in UNs–Ce6 conjugates. produced singlet oxygen responsible for cancer cell death. A nanoparticle shell bearing gadolinium ions was responsible for the magnetic resonance signal. The advantage of optical imaging was its sensitivity and the MRI spatial resolution. The authors performed in vitro and in vivo experiments, and for both biological models, noted the high imaging potentialwith significant green emission within the tumor up to 6 h after injection. The authors also noted high biocompatibility with no negative response up to 0.25 mg/mL of rare earth metals. The assessed half-time of designed nanoparticles equaled 21.6 min, and they accumulated in the liver and spleen at a low level. The nano particles applied in vitro against cancer U87 MG cells and in vivo, irradiated with light at 980 nm,revealed significant cell death and tumor growth inhibition [48]. An interesting photocatalytic material was invented by [PERSON] et al., who combined the Fe\({}_{3}\)O\({}_{4}\)/MnO\({}_{2}\)-doped NaYF\({}_{4}\):Yb/Er/Nd upconversion nanoparticles with black phosphorus nanosheets (BPNs) and linked Ce6 via polyacrylic acid [49]. The material revealed several advantages in in vivo studies with HeLa-bearing mice, of which the most significant arethe following: (_i_) it was activated only with single light at 808 nm leading to the activation of both PTT and PDT mechanisms (catalyzed both by BPNs and Ce6); (_ii_) MnO\({}_{2}\) decomposition overexpressed hydrogen peroxide in the tumor microenvironment leading to the generation of oxygen species enhancing PDT effects; and finally (_iii_) the doped Fe\({}_{3}\)O\({}_{4}\) and MnO\({}_{2}\) nanoparticles were able to facilitate T1 and T2 MRI imaging, leading to a multimodal imaging system accompanied by the Ce6 fluorescence imaging [49].