Absorbance spectra were acquired after diluting the GO samples prepared for fluorescence measurements by a factor of 10 in water (2.5 -20 μg/mL), using a Cary 50 Bio UV/Vis spectrophotometer (Varian Inc., Agilent Technologies, UK). These samples were transferred to a 1 mL quartz cuvette with 1 cm path length. Measurements were performed at room temperature, using a dual beam mode with baseline correction based on water as a blank control.