## 3 Results and Discussion
Figure 3: Simulated morphology of HNTs:Ag-NPs for randomly distributed Ag-NPs (d = 10 nm) (left) and fragments of the cross-sectional maps of the electric field distribution (right) under linearly polarized illumination with a wavelength of 632.8 nm; (**a**) the electric field is along the HNT’ axis, and (**b**) the electric field is in the perpendicular the HNT’ axis. The electric field and wavevector directions are indicated by blue and red arrows in the left-side images. The dashed lines in the right-side images indicate borders of the external and internal surfaces of the nanotube, while its center corresponds to X = 0. The wide and long-wavelength-shifted extinction spectra of HNTs with densely attached plasmonic NPs are obviously related to the NP grouping ensembles and aggregates, which can create numerous \"hot spots\" near the HNT's surface where the electric field is enhanced by an order of magnitude at least as shown in Figure 2. The broadening and long-wavelength shift of the extinction spectra of the samples with smaller NPs can be explained by their closer location on the HNT's surface in qualitative agreement with the simulation results (see Figure 2). Indeed, for the same volume concentration, the number of NP groping ensembles increase with decreasing the NP's size, which should lead to a change in the integral shape of LSPR, as shown in Figure 4.  
According to the optical transmittance measurements, HNTs with deposited Ag- and Au-NPs exhibit pronounced LSPR in the corresponding spectral regions. Typical extinction spectra of aqueous suspensions of HNT:Ag-NPs and HNT:Au-NPs are shown in Figure 5a,b, respectively. For instance, the LSPR peak in HNTs: Au-NPs is located between 500 and 600 nm and it is more pronounced in the case of bigger Au-NPs (Figure 5b), which is consistent with the results of [29] and our simulations shown in Figure 4. Additionally, when the Au-NPs' diameter decreases from 10 to 5 nm, the LSPR spectrum becomes weaker and broader, which indicates an effect of the interaction between neighboring NPs on the HNT's surface. Figure 4: (**a**) Simulated extinction spectra of HNTs with monodisperse (black curve) and size-distributed (red curve) Au-NPs (d = d\({}_{0}\) = 10 nm), which are immobilized on the outer surface of HNTs with surface density of 300 \(\mu\)m\({}^{-1}\); (**b**) the same spectra for size-distributed Au-NPs (d\({}_{0}\) = 5 nm) for different surface densities; (**c**) simulated extinction spectra of HNTs:Ag-NPs for randomly distributed monodisperse NPs (d = 5 nm) with different surface density and (**d**) for Ag-NPs with the same mean diameter and size distribution from Figure 1d. Plasmonic fields from NPs deposited on HNTs are well-observed with different biophotonic methods. For example, HNTs:Ag-NPs with added small amounts of probe dye molecules showed a remarkable SERS activity of Ag-NPs even after one week of storage in water, which indicates the improved stability of AgNPs on the HNTs' surfaces [11]. The spatial distribution of Ag-NPs on the surface of HNTs forms numerous 'hot spots\" between NPs that leads to an increase in the SERS efficiency [11; 12; 13; 14; 15; 16; 17; 18; 19]. In order to demonstrate the applicability of HNTs:Ag-NPs for biophotonics in vitro, four samples were prepared (see Methods). Figure 6a shows a schematic illustration of the prepared samples and fibroblasts cells cultivated with HNTs:Ag-NPs and Figure 6b,c show optical microscopy images of fibroblast cells and those with added HNTs, respectively. Typical Raman spectra of fibroblasts obtained for the above-mentioned samples of HNTs:Ag-NPs are shown in Figure 6d. In the case of HNTs:Ag-NPs (set C), the Raman lines, which correspond to the well-known vibrations of proteins in fibroblasts [30; 31], are five to six times stronger in comparison to the control sample. This effect was well-reproducible for all obtained spectra. However, even in the presence of bare HNTs (set A), the protein peak intensity was two to three times higher than that in the control sample.