## 4 Conclusions  
The obtained results show that gold and silver NPs with sizes from 5 nm to 10 nm randomly distributed on the external surface of HNTs exhibit the localized plasmon resonances in the corresponding spectral regions. The simulated extinction spectra of HNTs with gold and silver NPs are in qualitative agreement with the experimental ones. The simulations reveal an appearance of numerous \"hot spots\" of the local electric fields nearby NPs. In vitro studies have demonstrated the possibility of enhancing the Raman signal from fibroblast cells by using HNTs:Ag-NPs. Optimization of parameters such as the size of the nanoparticles, their surface density on HNTs, and the concentration of HNTs:Ag-NPs composites added to cells seems to be prospective for an improvement of their properties for the SERS diagnostics of biosystems. Furthermore, the SERS diagnostics can be combined with NP-promoted phototherapy because of the enhanced local electric fields, which stimulate both the direct field-matter interaction in biosystems and the photoinduced heating under irradiation with a wavelength close to the localized surface plasmon resonance. It seems that the biophotonic modalities of HNTs:(Ag, Au)-NPs should be combined with necessary biofunctionalization of their surfaces to ensure selective binding of those nanocomposites with investigated biosystems. The pronounced plasmonic responses of those nanocomposites allow us to consider them also as potential labels in optical reflection microscopy. Besides their biophotonic applications, HNT-based nanocomposites are of continuing interest in various applications [32; 33; 34; 35]. The discussed diagnostics and therapeutic abilities of HNTs with attached plasmonic nanoparticles make them attractive for the development of new biophonic and biomedical methods. Figure 6: (**a**) Schematic illustration of the sample preparation and fibroblast cells cultivated with HNTs:Ag-NPs; (**b**) optical microscopy images of fibroblasts; (**c**) optical microscopy images of fibroblasts cultivated with HNTs:Ag-NPs; (**c**) Raman spectra of fibroblasts with and without HNTs:Ag-NPs. Arrows in panel (**d**) indicate the Raman peaks characteristic of fibroblasts. **Author Contributions:** Conceptualization, I.V.L., A.I.S., A.A.N. and P.S.T. ; Data curation, S.B.I. ; Formal analysis, I.V.L., A.V.S., A.A.N., S.B.I., P.S.T., A.V.A. and V.S.V. ; Investigation, A.V.K., S.M.N., G.A.K., A.N.V. and V.Y.T. ; Methodology, A.V.K., S.M.N., G.A.K., S.J., A.I.S., A.V.S. and M.V.G. ; Software, A.V.A. ; Supervision, V.Y.T. ; Validation, S.J., A.N.V. and V.Y.T. ; Visualization, S.M.N., S.J. and V.S.V. ; Writing--original draft, G.A.K. and V.Y.T. ; Writing--review & editing, A.V.K., S.M.N. and V.Y.T. All authors have read and agreed to the published version of the manuscript. **Funding:** [PERSON] and [PERSON] acknowledge the financial support of their part of the research (in vitro experiments on the cell culturing and fixation) by the Government of the Russian Federation Megagrant, 2020-220-08-5262. [PERSON] acknowledges support from the Russian Foundation for Basic Research (RFBR, grant No. 20-07-00840a). [PERSON], [PERSON], and [PERSON] performed their part of the research (synthesis of nanoparticles and halloysite-nanoparticles composites) in the Functional Aluminosilicate Nanomaterials Lab supported by the Government of the Russian Federation (grant 14.Z50.31.0035). [PERSON] acknowledges financial support from the Russian Science Foundation (RNF, grant number 18-79-10208), [PERSON] acknowledges the financial support of his part of the research (the SERS analysis) from the Ministry of Science and Higher Education of the Russian Federation (grant No. 0714-2020-0002) and [PERSON] thanks the support from the same Ministry (grant FSWU-2020-0035) for the optical characterization of the samples. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable.