Synthesis of folic acid, histidine, and serine-functionalized and boron and phosphorus-doped graphene quantum dots with excellent yellow luminescence behavior in aqueous/solid states and their use for the fluorescence detection of Fe2+ in urine

Low luminous efficiency at visible light excitation restricts many applications of graphene quantum dots in bioanalysis, bioimaging, and light-emitting diodes. Herein, we report the synthesis of folic acid, histidine, and serine-functionalized and boron and phosphorus-doped graphene quantum dots (FHS-GQD-B/P) using a one-step thermal treatment. The resulting FHS-GQD-B/P consists of tiny graphene sheets with an average size of 4.6 +/- 0.28 nm. The synergy of multiple functional groups and multiple heteroatoms achieved excellent yellow luminescence behavior in the aqueous state and in the solid state. FHS-GQD-B/P can produce the strongest yellow fluorescence upon excitation with visible light at 490 nm. The fluorescence quantum yield reached 60.2%, which is higher than that of the other graphene quantum dots emitting yellow light. The fluorescence emission wavelength is independent of excitation wavelength, FHS-GQD-B/P concentration, and medium acidity. Interestingly, the yellow fluorescence can be quenched by Fe2+ in the presence of o-phenanthroline. Based on this, a method was developed for the fluorescence detection of Fe2+. The fluorescence intensity linearly decreases with the increase of Fe2+ concentration in the range of 0.01-50 mu M with a detection limit of 0.0042 mu M and a signal-to-noise ratio of 3. The as-proposed analytical method was satisfactorily applied in the fluorescence detection of Fe2+ in urine.