## 2 Metallic Nanoparticles in Conjugation with Chlorin Derivatives
### Gold Nanoparticles
Firstly, two gold nanorods were linked by two complementary oligonucleotides and formed nanorod dimers. Further Yb, Er-doped NaGdF\({}_{4}\) upconversion nanoparticles were connected with Ce6 by an oligonucleotide. Next, the nucleotide linker was introduced to the nanorod dimers to bind the previously obtained upconversion nanoparticles with Ce6. The activity of the developed nanosystem was studied in vitro using HeLa cells. In the TEM analysis, the attachment of HeLa cells around the dimer nanosystem was observed. Irradiation of the system with light at 980 nm provided the photodynamic response, which, when used immediately after 808 nm irradiation, also gave a photothermal effect. The combined PDT/PTT effect caused 0% HeLa cell viability in the in vitro studies, whereas only a partial response was achieved when PDT and PTT were utilized separately. For the in vivo experiments, a nude mouse model bearing a HeLa tumor was applied. Nanoparticles were administered via injection into the tail vein, and first, the designed system was studied in terms of imaging. Bright images of the tumor were recorded after 24 h of accumulation. The multimodal diagnostic features, as well as the treatment efficacy, were confirmed. Irradiation with 980 nm was performed for PDT. Next, the PTT effect was studied when the irradiation source was switched from 980 nm to 808 nm. This time, a significant decrease in tumor volume was also noted. Simultaneously, no body weight loss was observed, and nanoparticles within 15 days were wholly eliminated [33]. Another example of a Hernasotic approach with gold nanoparticle/chlorin conjugates was presented by [PERSON] et al. [34], who prepared 21 nm gold nanoparticles functionalized with Ce6 and epidermal growth factor (EGF). The in vitro studies performed with the MDS-MB-468 and MCF-10 cell lines revealed that the obtained bifunctional theranostic nanoprobes were highly cytotoxic to cancer cells (86%) with no toxic effect on normal cells. It was also found that with the most effective concentration of 0.2 ug/mL, nanoprobes caused both necrosis and apoptosis to tumor cells with a ratio of 3:2. Some benefits of combining gold nanoparticles with Ce6 were underlined by [PERSON] et al. [35]. These authors applied human serum corona protein to cover gold nanorods previously covalently bound with Ce6 via the sulfur bridge. In the in vitro experiments with CAL 27 oral squamous cell carcinoma, 95.2% of dead cells were observed in the treated culture. It is worth mentioning that the authors used nanoparticles at an extremely low concentration of 50 pM corresponding to 7.67 ug/mL of Au and 4.83 nM of Ce6. Moreover, a 57-fold increase in the cellular uptake of Ce6-loaded nanoparticles compared to that with bare Ce6 was observed [35]. The studies of the theranostic systems involving gold nanoparticles and chlorins were continued by [PERSON] et al. [36], who developed a photo-theranostic agent for PDT and PDD. The gold nanoparticles linked with an aliphatic linker with Ce6 generated singlet oxygen under illumination, significantly reducing living melanoma MDA-MB-435 cells in the in vitro experiment. The authors found that the gold-based nanoclusters entered the cells based on the endocytosis pathway and spread within the cytoplasm, whereas some particles entered the lysosomes. The preferential accumulation of the system in cancerous tissue and high light emission allowed them to consider the nanoparticles for visualization of the tumor [36]. [PERSON] et al. utilized pH Low Insertion Peptide (pHLIP) to insert attached gold nanoparticles bearing Ce6 into the cells upon decreased pH [37; 38]. Firstly, PEG was introduced to the gold nanoparticles, and then, particles were modified with amine groups. Finally, Ce6 and pHLIP were adsorbed onto hollow gold nanospheres via electrostatic forces. The physicochemical, in vitro, and in vivo experiments allowed them to conclude that the nanoagent under excitation with light efficiently forms ROS and generates a photothermal effect.