Boric acid modified S and N co-doped graphene quantum dots as simple and inexpensive turn-on fluorescent nanosensor...

A new fluorescent nanosensor based on S and N co-doped graphene quantum dots (S,N-GQDs) modified by boric acid was designed for glucose detection. First, the S,N-GQDs was prepared via one pot hydrothermal process utilizing citric acid and thiourea as precursors. Then, S,N-GQDs was modified by boric acid to fabricate (B)/S,N-GQDs. The excitation dependent photoluminescence spectra of (B)/S,N-GQDs confirmed the heteroatom (S,N) dopant effect on GQDs emission. FT-IR and energy dispersive X-ray (EDX) spectroscopies confirmed the modification of S,N-GQDs with boric acid. The optical and electrochemical band gaps of the obtained (B)/S,N-GQDs were found to be 2.7 and 2.5 eV, respectively. The boric acid functionalized S,N-GQDs exhibited fluorescent enhancement at 455 nm upon addition of glucose. Such fluorescence response was used for glucose quantification with a detection limit of 5.5 μM which is comparable with previous boronic acid based fluorescent sensing systems. However, compared with earlier reported expensive boronic acid based glucose sensors, this modified system is simpler, more economical, and efficient. A mechanism was proposed for fluorescence enhancement based on the reaction of cis-diol units of glucose with the boric acid groups of (B)/S,N-GQDs which creates rigid (B)/S,N-GQDs-glucose structures, restricting the non-radiative intramolecular motions and results in the fluorescent enhancement.