Unravelling the Potential of Graphene Quantum Dots in Biomedicine and Neuroscience

Abstract

Quantum dots (QDs) are semiconducting nanoparticles that have been gaining ground in various applications, including the biomedical field, thanks to their unique optical properties. Recently, graphene quantum dots (GQDs) have earned attention in biomedicine and nanomedicine, thanks to their higher biocompatibility and low cytotoxicity compared to other QDs. GQDs share the optical properties of QD and have proven ability to cross the blood-brain barrier (BBB). For this reason, GQDs are now being employed to deepen our knowledge in neuroscience diagnostics and therapeutics. Their size and surface chemistry that ease the loading of chemotherapeutic drugs, makes them ideal drug delivery systems through the bloodstream, across the BBB, up to the brain. GQDs-based neuroimaging techniques and theranostic applications, such as photothermal and photodynamic therapy alone or in combination with chemotherapy, have been designed. In this review, optical properties and biocompatibility of GQDs will be described. Then, the ability of GQDs to overtake the BBB and reach the brain will be discussed. At last, applications of GQDs in bioimaging, photophysical therapies and drug delivery to the central nervous system will be considered, unraveling their potential in the neuroscientific field.

Keywords: blood brain barrier; graphene; quantum dots; theranostics.

Figure 1

Properties of quantum dots. (A) Correlation between optical properties and the size of quantum dots. Reproduced with permission [5]; (B) Typical structure of quantum dots with a central core in which electrons occupy only discrete energy levels, and an insulator shell; (C) 3T3 mouse fibroblasts labeled with cadmium selenide/cadmium sulfide quantum dots (CdSe-CdS QDs) [12]. Reproduced with permission from American Association for the Advancement of Science.

Figure 2

Graphene quantum dots’ optical properties. (A) Hexagonal and triangular-shaped graphene quantum dots (GQDs) in two distinct conformations: armchair (left) and zigzag (right), and their relative charge distribution (negative in red and positive in blue). Adapted with permission [6]; (B) Typical absorption peaks of GQDs at 230 and 300 nm. Adapted with permission from The Royal Society of Chemistry [31]; (C) Photoluminescence (PL) intensity at different excitation wavelengths. Adapted with permission from The Royal Society of Chemistry [31]; (D) Reduction of energy gap (red shift) as a function of sp² domains. Reproduced with permission [34]; (E) Influence of functional groups on PL properties. Amination increases quantum yield, while carboxylation causes blue shifts due to the clusterization of sp² domains. Adapted with permission [38]; (F) Upconversion of GQDs, showing a reduction in emitted wavelength at higher excitations [41]; (G) HOMO-LUMO transitions for big GQDs (b,c) and smaller GQDs (a,d). F and G were reproduced with permission from The Royal Society of Chemistry [41].

Scheme 1

Representation of the main strategies for the synthesis of GQDs.

Figure 3

Toxicity of GQDs. (A) Cell viability tests and reactive oxygen species (ROS) production on HeLa cells. Reprinted from [64]. Copyright 2014, permission from Elsevier; (B) Cell viability on A549 (top) and C6 (bottom) after the treatment with aminated GQDs (aGQDs), carboxylated GQDs (cGQDs) and DMF GQDs (dGQDs). Reproduced with permission [38]; (C) Oxidative stress on zebrafish after the treatment with MMP and GQDs with relative fluorescence intensity on head and heart. Reprinted from [65]; (D) Fluorescence intensity of caspase-3 after the treatment with GQDs and MMP [65]; (E) In vivo and ex vivo imaging of mouse after injection with carboxylated GQDs [67]; (F) Accumulation of different concentrations of carboxylated GQDs in mouse organs, 160× magnification. E and F were reprinted with permission [67]. Copyright 2013, American Chemical Society.

Figure 4

GQDs across the blood-brain barrier (BBB). (A) Structure of the BBB [75]; (B) In vitro permeability via transwell in a co-culture system. A and B were reprinted with permission [75]. Copyright 2008, American Chemical Society; (C) Transcellular transport mechanism across a biological barrier made of canine kidney MDCK cells with different sizes of GQDs. Scalebar 50 µm. * was for P < 0.05, ** was for P < 0.01, and *** was for p < 0.001 over control. Reproduced with permission from The Royal Society of Chemistry [73]; (D) Uptake of PEG-GQDs functionalized with a ligand of CD13, capable of recognizing glioma and tumor vessels. *** was for p < 0.001 over control. Adapted from [84]. Copyright 2017, permission from Elsevier.

Figure 5

Neuroimaging with GQDs. (A) Confocal imaging of neurospheres with GQDs. Adapted with permission from The Royal Society of Chemistry [93]; (B) Confocal imaging of human neural stem cells with GQDs (green) and nuclear staining (blue). Adapted with permission from The Royal Society of Chemistry [94]; (C) Confocal imaging of U87 cells excited at 407 nm (blue) and 488 nm with N-doped carbon dots. Reprinted with permission [37]. Copyright 2014, American Chemical Society; (D) Deep tissue confocal imaging of GO nanoparticles inside mouse brain. Reproduced with permission [95]; (E) PH-responsive GQDs injected in mice bearing different tumors and adjacent muscles. Scale bar 40 μm. Adapted with permission from The Royal Society of Chemistry [39].

Figure 6

Phototherapy with GQDs. (A) Photodynamic therapy by irradiating U251 cells with blue light (470 nm), with relative ROS production (top) and autophagic vacuole formation (bottom, TEM). Adapted from [102]. Copyright 2012, permission from Elsevier; (B) NIR-II imaging (top) on xenograft C6 glioma mouse model and photothermal therapy (bottom), showing tumor growth reduction (in vivo) and loss in cell viability (in vitro). Adapted from [100]. Copyright 2019, permission from Elsevier; (C) Combination of PTT and chemotherapy with doxorubicin. GSPI = functionalized graphene sheets; GSPID = graphene sheets loaded with doxorubicin. Reprinted with permission [109]. Copyright 2013, American Chemical Society. Scale bar 75 μm.

Figure 7

Drug delivery with GQDs. (A) Carbon dots functionalized with transferrin and conjugated with doxorubicin. The nanocomplex was administered to SJGBM2 pediatric glioblastoma, resulting in a higher uptake of the chemotherapeutic agent. Adapted with permission from The Royal Society of Chemistry [116]; (B) U251 cell treated with GQDs functionalized with a targeting peptide and loaded with doxorubicin. Adapted from [101]. Copyright 2017, permission from Elsevier; (C) Neuroprotective activity of GQDs on Alzheimer’s disease, showing a reduction in the fibrillation (top), and the increase in neurogenesis in vivo (bottom, scalebar 60 µm). Reproduced with permission from The Royal Society of Chemistry [117]. Adapted from [118]. Copyright 2016, permission from Elsevier.