D-penicillamine-functionalized graphene quantum dots for fluorescent detection of Fe3+ in iron supplement oral liquids

The introduction of functional groups may drastically alter the electronic characteristics of graphene quantum dots (GQD), thus leading to unusual properties and related applications. The paper reported synthesis of functional GQD via the thermal pyrolysis of citric acid and d-penicillamine (DPA) or l-penicillamine (LPA). The result shows that DPA-GQD provides a stronger fluorescence and more sensitive and selective fluorescent response to Fe3+ compared to LPA-GQD, verifying that the configuration of functional groups is an important factor for influencing optical behavior of GQD. The fluorescent quantum yield of DPA-GQD reaches 99.8%, which is much better than that of the reported GQDs. Its oxygen and nitrogen-rich groups can selectively coordinate with Fe3+ and lead to an obvious fluorescence quenching via the nonradiative electron-transfer. The peak fluorescence intensity linearly decreases with increasing concentration of Fe3+ in the range of 0.004–1.8 mM with the detection limit of 1.2 × 10−6 M (S/N = 3). The DPA-GQD as a novel fluorescent probe also possesses excellent selectivity for the optical detection of Fe3+. All metal and nonmetallic ions and amino acids tested don’t interfere with the fluorescent detection of Fe3+. It has been successfully applied in the detection of Fe3+ in iron supplement oral liquids.