## 3 Conclusions  
In the presented review, numerous types of conjugates consisting of diverse metallic nanoparticles with Ce6 and its derivatives were described, and the studies clearly indicate a significant advantage of iron oxide and gold nanoparticles. In addition, many variants of binding between porphynioids and nanoparticles were presented, including electrostatic interactions, covalent linking, and encapsulation. Considering the above-described data, one can conclude that there are several advantages of conjugating metallic nanoparticles with chlorin-type photosensizers. Some of the advantages are as follows: (_i_) the conjugates usually facilitate the performance of in vitro or in vivo multimodal therapy, where PDT can be combined with the photo-/magnetothermal therapy or with the classic chemotherapy driven by doxorubicin or cisplatin, (_ii_) the conjugation of the photosensitizer with metallic nanoparticles can enhance reactive oxygen species formation by catalyzing the decomposition of H\({}_{2}\)O\({}_{2}\) and an increase of the oxygen concentration, and (_iii_) apart from therapeutic applications, the obtained conjugates can be utilized in cancer diagnostics based on the intense fluorescence of chlorin macrocycle (PDD) and/or amplification of the MRI intensity.  
In general, the use of nanoparticles as carriers enables the attachment of additional targeting molecules while simplifying the synthetic procedures, most notably the purification step. Additionally, nanoparticles functionalized with targeting moieties increase the selectivity and therapeutic efficacy of the systems in in vivo studies, allowing for the effectivecombination of different therapies. It is also worth noting that in combined therapy, more than one trigger for ROS or cytotoxicity is used, which leads to higher activation yields and better therapeutic results. A combination of metallic nanoparticles and porphyrinoids usually increases singlet oxygen generation. Moreover, metallic nanoparticles open access to imaging alongside increased therapeutic outcomes. However, despite the abovementioned advantages of these conjugates, some risks can also accompany the usage of functionalized nanoparticles in cancer treatment. There are some biosafety concerns regarding NP metabolism, degradation, and accumulation. For example, the passive absorption of nanoparticles can cause acute hypersensitivity in healthy tissue since they cannot be distinguished from neoplastic cells [72]. To avoid the negative impact of metallic nanoparticles on healthy tissue in PDT, carrier-free nanomedicines are constructed via the nano-assembly of photosensitizing compounds in simple procedures, allowing for the delivery of dual or multi-components for combination therapy PDT/PTT/chemotherapy [73].  
Conceptualization, T.K., T.G. and L.S.; investigation, T.K., A.G.-S., D.T.M. and M.M.; resources, M.M. and L.S.; writing--original draft preparation, T.K., M.M. and L.S.; writing--review and editing, E.G., T.G. and L.S.; visualization, L.S.; supervision, T.G. and L.S.; project administration, T.G.; funding acquisition, T.G. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the funding provided by the National Science Centre, Poland, under Grant No. 2020/39/O/NZ7/00351.  
Not applicable.  
The authors would like to thank [PERSON] and [PERSON] for their support.  
The authors declare no conflict of interest.