Fig. 6: FTIR spectrum of NK20a@Au nanoparticles and validation of gold particles as drug delivery materials through in vitro confocal microscopy.

a Representative absorption FTIR spectra of streptavidin@Au binding to biotinylated NK20a (red line) compared with the spectra of streptavidin@Au surface (blue line). Typical vibration of biotin-block streptavidin was observed at peaks located at 1637 and 1547 cm⁻¹. The peak at 1652 cm⁻¹ represents the NK20a amide I band. The peaks at 2849 and 2925 cm⁻¹ are related to the stretching vibration of C–H and N–H bonds, respectively. b B-lymphoma cells were incubated with NK20a@Au and NK20a at room temperature for 20 min. Images were taken after the cells were fixed in glutaraldehyde for 20 min and then washed with 1× PBS. The cell nuclei were counterstained with DAPI staining (blue). The cell membrane was stained with Cytopainter (red). NK20a was labeled with FITC (green). Merged images represent the nuclei, cell membrane, and NK20a locations. NK20a single staining and merged images show that the peptide could bind to the IL-10 receptor on the plasma membrane as indicated by Cytopainter colocalization with FITC fluorescence. NK20a@Au fluorescence was distributed in the cytoplasm but not in the nuclei and cell membrane; FITC fluorescence was not observed in the nucleus area. These results indicate that NK20a@Au successfully penetrated the membrane into the cytoplasm through cell uptake. Scale bar: 10 µm.