Advances in Structural Modifications and Properties of Graphene Quantum Dots for Biomedical Applications

Abstract

Graphene quantum dots (GQDs) are carbon-based, zero-dimensional nanomaterials and unique due to their astonishing optical, electronic, chemical, and biological properties. Chemical, photochemical, and biochemical properties of GQDs are intensely being explored for bioimaging, biosensing, and drug delivery. The synthesis of GQDs by top-down and bottom-up approaches, their chemical functionalization, bandgap engineering, and biomedical applications are reviewed here. Current challenges and future perspectives of GQDs are also presented.

A Review of Advances in Graphene Quantum Dots: From Preparation and Modification Methods to Application

“…For example, in the preparation of GQDs from carbon-containing wastes, Wang et al [62] used coffee grounds as raw material, Ding et al [63] used renewable lignin as raw material, and Tran et al [64] used gelatin as raw material, realizing the effective utilization of solid waste. A technique applicable to the preparation of large-scale nonfunctional GQDs has also been proposed, based on the principle that graphite suspensions are passed through micrometer-sized z-channels at high speeds and pressures to generate high shear rates, whereas millimetersized suspensions of graphite flakes are first exfoliated into micrometer-sized graphene, which is then further shrunken and fragmented into nanometer-sized GQDs [65]. Methods of preparing graphene quantum dots are still being researched, and we may see more efficient and environmentally friendly methods in the future.…”

“…The doped nitrogen will resonate with the effective orbitals of GQDs and improve the electronic activity, which makes it easier to be doped into GQDs. It has been found that doping N atoms into the original nanomaterial enhances its PL behavior, leading to enhanced applications in biosensing, fluorescence imaging, and several other areas [65]. Common nitrogen sources generally include ammonia [74], dimethylformamide (DMF), nitric acid [74], melamine, and quadrol [75].…”

“…Sulfur can change the band structure of GQDs to increase the path of electron transition, which has great potential in optics. It has been demonstrated in the literature that doping S atoms can provide more energy for photoexcited electrons, which can better regulate the electronic structure and optical properties of nanoparticles [65]. The most common sulfur sources are: 3-cymbal propyate (MPA), Na 2 S [84], cysteine, sulfuricate, sulfate [85] and ammonium sulfate.…”

“…The possibility of utilizing GQDs as fluorophores has been demonstrated by labeling multiple cell types. [65] Zhu et al [23] reported a method of large-scale synthesis of GQDs with strong green fluorescent GQDs. The generated GQDs exhibited significant fluorescence with a fluorescence quantum yield of 11.4%.…”

“…GQDs have very good photoluminescence properties due to their nanosize, which, coupled with their low cytotoxicity, high biocompatibility, and photostability, has led to their attention as new fluorophores for bioimaging. The possibility of utilizing GQDs as fluorophores has been demonstrated by labeling multiple cell types [65]. Zhu et al [23] reported a method of large-scale synthesis of GQDs with strong green fluorescent GQDs.…”