Fig. 3. Typical confocal (A) and STED (B) microscopy images of Atto647N–Tf NPs immobilized on glass surfaces. Scale bars: 1 μm. (C) Intensity profiles of the NP images marked as white squares in A and B, taken in confocal (blue) and STED (red) modes. Full width at half maximum (FWHM) values were determined by Gaussian fits (solid lines). (D) Statistical analysis of FWHM values obtained for >100 individual emitting particles in both confocal (blue) and STED (red) microscopy images; the black curves depict the corresponding Gaussian fits.