Given the improved linear relationship between fluorescence intensity and concentration of GO at 525 nm, we used this wavelength, alongside the absorbance peak at 230 nm, to characterise the amount of electronic transitions taking place (Figure , insets). The similar calibration curves obtained for the three different types at λexc = 525 nm indicate that the production steps (including sonication) did not affect significantly the optical properties of GO. Understanding the photochemical properties of GO not only allows the quantification of material in colloidal dispersion but also enables their exploitation in biomedicine. One recently reported example is the use of time-lapse confocal fluorescence microscopy to visualise the behaviour of cells in real time after exposure to the material .