Effect of sulfur doping on fluorescence and quantum yield of graphene quantum dots: an experimental and theoretical investigation

Abstract

Graphene quantum dots (GQDs) are one of the most promising luminescent carbon derived nanomaterials decorated with multiple useful functional groups and remarkable optoelectronic properties. Heteroatom doping of hexagonal carbon sheet of GQDs is an effective strategy to tailor their properties to meet desired application. In this work, sulfur doped GQDs (S-GQDs) were synthesized by simply pyrolyzing citric acid (CA) as a source of carbon and 3-Mercaptopropionic acid as a source of sulfur dopant. The optimal reaction conditions (ratio of the carbon to dopant source, temperature and time of reaction) were obtained while investigating their effect on the quantum yield and fluorescence properties of GQDs and, are hereby, reported for the first time. The as-synthesized S-GQDs were extensively characterized by different analytical techniques such as transmission electron microscopy (TEM), UV-vis Spectroscopy (UV), Fourier transform infrared spectroscopy, photoluminescence (PL) and x-ray Photoelectron Spectroscopy. S-GQDs were found uniform in size (∼4 nm) and spherical in shape with strong blue fluorescence. Further, for in-depth analysis of experimental results and underlying phenomena, theoretical studies based on density functional theory were performed for chemical structure optimization, possible sites of doping and density of states calculation. The synthesized S-GQDs exhibited excellent solubility in water, a stronger fluorescence and desirably higher quantum yield (57.44%) as compared to that of previously reported undoped GQDs. These successfully demonstrated unique and improved properties of S-GQDs present them as a potential candidate for biomedical, optical, electrical and chemical applications.

Targeted bioimaging and sensing of folate receptor-positive cancer cells using folic acid-conjugated sulfur-doped graphene quantum dots

“…Therefore, the development of appropriate non-toxic, water-soluble, and cost-effective fluorescent nanomaterials for targeted bioimaging of cancerous cells is highly desirable. Recently, graphene quantum dots (GQDs) and carbon dots (CDs) have emerged as a promising zero-dimensional fluorescent nanostructure for bioimaging applications by virtue of their fascinating properties such as high aqueous solubility, low cytotoxicity, high quantum yield, and effortless surface modification [21,22]. To expand their use in bioimaging applications for differentiating cancerous and non-cancerous cells, several reports have been consigned to covalent conjugation of FA with GQDs and CDs.…”

Recent advances in graphene quantum dot-based optical and electrochemical (bio)analytical sensors

“…Similarly, another group described the N, S co-doped GQDs synthesis via carbonization of CA and L-cystine 96 . Likewise, the synthesis of sulphur doped GQDs has been reported by Kadian et al using CA and 3-mercaptopropionic acid (MPA) as raw precursors 20 . Liu described the pyrolysis of hexa-peri-hexabenzocoronene (Figure 2C) into multicolor photoluminescent GQDs 97 .…”

“…Chua and coworkers demonstrated the alteration of C60 into photoluminescent GQDs via cage-opening procedure using concentrated H2SO4, sodium nitrate, and potassium permanganate 83 . Over the past decade, several studies reported the synthesis of GQDs via carbonization or pyrolysis method 20,27,[84][85][86][87][88][89] . In line with this, firstly, Dong et al tuned the carbonization degree of carbon precursor (i.e., CA) for the preparation of blue fluorescent A D C B GQDs (Figure 2B) .…”

“…Graphene is known for its exceptional set of tunable properties: high mechanical strength, high elasticity, thermal stability, larger surface area, and good electrical conductivity 17,18 . However, compared to graphene, GQDs have an added advantage of higher aqueous solubility, tunable bandgap, and ease of surface functionalization due to edge and quantum confinement effects 13,19,20 . Additionally, GQDs have excellent biocompatibility and the existence of multiple functional groups (hydroxyl, carboxyl, carbonyl, or epoxide) on their surface, which act as active sites on the edges or basal plane and thus offers unparalleled benefits in the fabrication of biosensing platforms [21][22][23] .…”

Allium sativum derived carbon dots as a potential theranostic agent to combat the COVID-19 crisis

“…Interestingly, CDs are helpful for cell labeling, tracking cellular events, targeting specific cells due to their intrinsic fluorescent properties, and a vehicle for intracellular transport of drugs/genes [ 29 , 30 ]. Different nanomaterials such as graphene quantum dots (GQDs), carbon quantum dots (CQDs), fluorescence polymeric nanoparticles, partially graphitized core-shell carbon nanoparticles, and amorphous carbon nanoparticles are included in the family of CDs [ 29 , 31 , 32 , 33 , 34 ]. The surface functional groups of CDs provide easy conjugation with numerous therapeutic agents, prevailing in anti-viral applications [ 35 , 36 , 37 ].…”