Discrimination of fluorescein isothiocyanate (FITC) labeled SiO2-NP (55 ± 6 nm in diameter) from the autofluorescent background of skin and subcutaneous tissue using fluorescence lifetime imaging microscopy (FLIM). False color coded sections of mouse skin including subcutis following subcutaneous particle injection from the site of injection (+SiO2-NP, (a)) and from the contralateral without NP (b) are presented. Cyan colored areas represent the autofluorescent background, while the yellow colored areas indicate a different lifetime species, which was identified as FITC-SiO2-NP. Scale bar = 50 µm. (c) Fluorescence lifetime curves of the tissue autofluorescence (mean lifetime curve) and of the FITC-SiO2-NP.