Near-infrared emitting graphene quantum dots synthesized from reduced graphene oxide for in vitro/in vivo/ex vivo bioimaging applications

Abstract

Near-infrared (NIR) emissive nanomaterials are desired for bioimaging and drug delivery applications due to the high tissue penetration depth of NIR light, enabling in vitro/ex vivo/in vivo fluorescence tracking. Considering the scarcity of NIR-fluorescing biocompatible nanostructures, we have for the first-time synthesized nanometer-sized reduced graphene oxide-derived graphene quantum dots (RGQDs) with NIR (950 nm) emission highly biocompatible in vitro with no preliminary toxic response in vivo. RGQDs are obtained in a high-yield (∼90%) top-down sodium hypochlorite/ultraviolet-driven synthetic process from non-emissive micron-sized reduced graphene oxide (RGO) flakes. This oxidation of RGO yields quantum dots with an average size of 3.54 ± 0.05 nm and a highly crystalline graphitic lattice structure with distinguishable lattice fringes. RGQDs exhibit excitation-independent emission in the visible and NIR-I region with a maximum NIR quantum yield of ∼7%. Unlike their parent material, RGQDs show substantial biocompatibility with ∼75%–80% cell viability up to high (1 mg ml−1) concentrations verified via both MTT and luminescence-based cytotoxicity assays. Tracked in vitro via their NIR fluorescence, RGQDs exhibit efficient internalization in HeLa cells maximized at 12 h with further anticipated excretion. In vivo, RGQDs introduced intravenously to NCr nude mice allow for fluorescence imaging in live sedated animals without the need in sacrificing those at imaging time points. Their distribution in spleen, kidneys, liver, and intestine assessed from NIR fluorescence in live mice, is further confirmed by excised organ analysis and microscopy of organ tissue slices. This outlines the potential of novel RGQDs as NIR imaging probes suitable for tracking therapeutic delivery in live animal models. A combination of smaller size, water-solubility, bright NIR emission, simple/scalable synthesis, and high biocompatibility gives RGQDs a critical advantage over a number of existing nanomaterials-based imaging platforms.

Graphene Quantum Dots as Intracellular Imaging-Based Temperature Sensors

“…Transmission electron microscopy (TEM) performed to characterize these nanomaterials in this (Figure S2) and previous works indicates the average size of N-GQDs and RGQDs of 5.5 nm and 3.5 nm, respectively, and confirms their graphitic lattice structure and spacing [33,41,42]. The structures of N-GQDs and RGQDs are also confirmed via Raman spectroscopy with observable D and G bands (Figure S3a,b), representing the sp 2 hybridized carbon and disordered structure, respectively [33,41]. Both RGQDs and N-GQDs are well-characterized and possess distinct optical signatures in absorption (Figure S4a) and fluorescence (Figure S4b,c) that can be utilized effectively for intensity luminescence nanothermometry.…”

“…N-GQDs are synthesized from an aqueous solution of glucosamine-HCl undergoing one-step hydrothermal reaction using a commercially available microwave [33]. For the production of RGQDs via the top down approach, an aqueous solution of micrometer-sized rGO flakes is treated with NaOCl, and exposed to UV light to accelerate the reaction rate allowing for oxidation and scission of rGO [41]. Transmission electron microscopy (TEM) performed to characterize these nanomaterials in this (Figure S2) and previous works indicates the average size of N-GQDs and RGQDs of 5.5 nm and 3.5 nm, respectively, and confirms their graphitic lattice structure and spacing [33,41,42].…”

“…For the production of RGQDs via the top down approach, an aqueous solution of micrometer-sized rGO flakes is treated with NaOCl, and exposed to UV light to accelerate the reaction rate allowing for oxidation and scission of rGO [41]. Transmission electron microscopy (TEM) performed to characterize these nanomaterials in this (Figure S2) and previous works indicates the average size of N-GQDs and RGQDs of 5.5 nm and 3.5 nm, respectively, and confirms their graphitic lattice structure and spacing [33,41,42]. The structures of N-GQDs and RGQDs are also confirmed via Raman spectroscopy with observable D and G bands (Figure S3a,b), representing the sp 2 hybridized carbon and disordered structure, respectively [33,41].…”

“…Both N-GQDs and RGQDs exhibit broad excitation-dependent visible emission attributed to the size-dependent quantum confinement effects [33,36]. Additionally, N-GQDs possess a weak NIR fluorescence originating from their surface defect states [33,36], while RGQDs exhibit substantial NIR fluorescence at~950 nm with up to 8% quantum yields obtained from an absolute quantum yield measurement in an aqueous solution using an integrating sphere [41].…”

Dual-Emission Fluorescence Probe Based on CdTe Quantum Dots and Rhodamine B for Visual Detection of Mercury and Its Logic Gate Behavior

“…Among them, QDs fascinate researchers and reveal promising prospects in the fluorescence detection field due to their distinctive photophysical properties, such as high quantum efficiency, size-dependent photoluminescence, broad excitation window, high extinction coefficients and tunable emission spectra [ 13 ]. The utilization of QDs as fluorescence labels in the analytical industry has been reported widely [ 15 , 16 , 17 , 18 ]. Most QD-based labels employ a single optical output for the detection.…”