Investigations On The Simultaneously Fabrication And Applications Of Bi-morphological Nitrogen,Sulfur-doped Graphene Quantum Dots

Graphene quantum dots(GQDs)is a kind of graphene material with size less than 100 nm.Due to the strong quantum confinement and edge effects,GQDs are merited with photoluminescence properties.The excellent biocompatibility and low cytotoxicity gain GQDs extensive popularity in the fields of fluorescence sensors and bioimaging.In present study,the simultaneously fabrication of free and solidified N,S-doped graphene quantum dots are achieved and their applications in sensing of Fe3+ and detecting of fingermarks are investigated.Firstly,a facile one-step solvent-free synthesis approach is proposed for the simultaneous fabrication of free and solidified N,S-doped graphene quantum dots(N,S-GQDs)by using citric acid as precursor and L-cysteine as dopant.Graphene nucleus is firstly formed via the intermolecular dehydration of citric acid,N and S are then incorporated into the graphene structure by attacking the margin of graphene nucleus.The cross-linking among the graphene nucleus via the intermolecular condensation leads to the generation of free N,S-GQDs,while the intermolecular amidation among L-cysteine molecules and grapheme nucleuses contribute to the solid-state fluorescence graphene quantum dots(SSF-GQDs).The results of high resolution transmission electron microscopy(HRTEM)indicate that the obatined free N,S-GQDs are of uniform distribution with sizes of ca.3?11 nm,and the obtained SSF-GQDs show a net structure with clear nodes.Atomic force microscopy(AFM)analyse illustrates that the obtained N,S-GQDs are of multi-layers with a topographic height near to 3.8 nm.X-ray photoelectron spectroscopy(XPS)reveals that N and S atoms have been successfully doped into graphene structure.N atom is bound to the C atom in the structure of GQDs with the form of five-member ring structure and the S atom is mainly doped at defects of graphene in the form of thiophene-like structure or thiazolidline ring.The fourier transform infrared spectra(FT-IR)well suggest the existence of acylamino bonds in SSF-GQDs,which further confirm the successfully doping of N and S into the strcuture of GQDs.Raman spectrum shows that the ID/IG ratio of N,S-GQDs is 0.60,suggesting that N,S-GQDs have highly crystalline nature and integrated graphite structure.The doping with N and S changes the surface state and tune electronic density of free N,S-GQDs,offerring it bright blue fluorescence under UV light at 365 nm.The free N,S-GQDs exhibit a excitation-independent PL behaviors with an excitation/emission maximum at 340/417 nm,and a high fluorescence quantum yield of 74.5%.The fluorescence intensity of free N,S-GQDs keeps virtuall stable in the pH range of 5.0?13.0 and an ionic strength high to 1.0 mol L-1 NaCl.Based on the quenching effect of Fe3+ on the fluorescence of the free N,S-GQDs,a sensitive and selective approach is developed for the detection of Fe3+.Fe3+ could be accurately determined in a linear range of 0.01-3 ?mol L-1,with a detection limit of 3.3 nmol L-1.This developed method has been successfully applied to the determination of Fe3+ content in human serum samples.The special mesh intercrossing structure of SSF-GQDs effectively reduce the aggregation-induced quenching effect of GQDs,offering the obtained SSF-GQDs strongyellow-green fluorescencegood thermal stability.The fluorescence of SSf-GQDs is deduced to be 10.6%,which makes the SSF-GQDs as excellent fluorescent powder for the detection of fingermarks.