## 2 Metallic Nanoparticles in Conjugation with Chlorin Derivatives
### Gold Nanoparticles
The translocation of the nanoparticles into a liposomal sphere through the interaction of pHLP with membrane lipids was confirmed using the liposomal model. Translocation into cells occurred only when the pH value decreased from neutral 7.4 to the typical acidic pH for most cancer tissues, around 6.2. Moreover, it was found that upon irradiation, the generated heat caused the disintegration of gold nanospheres and Ce6. In the in vitro experiments on HeLa cells, the authors observed a significant decrease in cell viability upon irradiation. In vivo tests indicated significant inhibition of tumor growth measured after three weeks. The possibility of cancer imaging based on photothermal and fluorescence phenomena was confirmed [37; 38]. Aside from Ce6, some other chlorin derivatives were utilized in conjugations with diverse gold nanoforms. [PERSON] et al. prepared gold nanoparticles with a diameter 12 nm linked to _mese_-tetrakis(3-hydroxyphenyl)chlorin (mTHPC) using 3-mercaptoethanol appropionic acid [39]. It was found that after conjugation to gold, the singlet oxygen generation quantum yield was slightly lower than that noted for the chlorin only. Conversely, the in vitro experiments using SH-SY5Y human neuroblastoma cells indicated that conjugation of the chlorin to gold nanoparticles increased their photocytotoxicity. Additionally, the combination with gold nanoparticles significantly decreased the dark toxicity of mTHPC at higher concentrations [39]. A combined photothermal and photodynamic effect was obtained with good results for CeO\({}_{2}\)-coated gold nanorods encapsulated within a temperature-sensitive polymer structure and conjugated with Ce6 [40]. The near-infrared irradiation at 808 nm caused an increase in the temperature up to 60 \({}^{\circ}\)C resulting in significant tumor tissue ablation. What is more, at 50 \({}^{\circ}\)C, the cumulative release rate of Ce6 was 95.31%. The CeO\({}_{2}\)-coating effectively alleviates the anaerobic tumor microenvironment, which was proven in in vitro and in vivo tests on HepG2 cells and tumor-bearing mice [40]. Thiolated chlorin derivatives were also adsorbed on the surface of the bimetallic gold-platinum nanoparticles by [PERSON] et al. [41]. The obtained conjugates displayed lower dark cytotoxicity in in vitro studies based on the murine melanoma cell line B16F10-Nex2. In addition, higher generation of singlet oxygen and cell internalization compared to those with unmodified chlorin conjugated to bare AuNPs was observed. An interesting nanomaterial based on gold/Ce6 nanoclusters targeted with folic acid was prepared [PERSON] et al. [42]. In the first step, the authors fabricated gold nanoclusters with a diameter of ca. 2 nm. In the next step, the polyethylene glycol (PEG) chains linked to the folic acid and bearing anchor NHS groups were conjugated with gold nanoparticles. Finally, the photosensitizer Ce6 was introduced into hydrophobic pockets formed by PEG. The authors assessed cellular uptake into gastric cancer cells with an overexpressed folic receptor MGC-803 and human gastric epithelial folic receptor-negative cells--GES 1 cells. Synthesized gold nanoparticles accumulated 4.8-fold more efficiently in the MGC-803 cells compared to that with Ce6 only, whereas nanoparticles in the normal FR-free GES 1 cells reached a negligible level in contrast to that with free Ce6. Therefore, the authors concluded that the developed therapeutics are highly selective. It was confirmed that the introduction of Ce6 into the PEG-pockets within the nanoagent did not decrease the ability of singlet oxygen formation in comparison to that with the free Ce6. In dark conditions, only low cytotoxicities of Ce6@GNC-PEG-FA, similar to those of the bare nanogold and Ce6 only, were reported. It is worth noting that upon irradiation with 633 nm, nanoparticles produced 2-3 times more dead cancer cells than the free Ce6. Pharmacokine parameters were also evaluated using an in vivo BALB/c mouse model. Synthesized nanoparticles followed two-compartment pharmacokinetics.