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Crafted from sheets of graphene, graphene quantum dots have extraordinarily unique qualities and have been revolutionizing the field of fluorescence microscopy.
Graphene quantum dots are superior to standard quantum dots in terms of their photostability, biocompatibility, and low toxicity. Furthermore, the distinct graphene structure of these dots gives them the characteristics of standard graphene, such as high strength and conductivity.
Making Graphene Quantum Dots
The preparation techniques of graphene quantum dots can be categorized into two classes: top-down and bottom-up. The intention of top-down processes is to trim down graphene sheets, carbon nanotubes, carbon fibers or graphite into quantum dots. Common top-down methods involve nanolithography, acidic oxidation, electrochemical techniques, and plasma oxidation.
In the bottom-up techniques, small molecules are used as source materials to create graphene quantum dots. Common bottom-up tactics include using benzene derivatives and unsubstituted hexaperihexabenzocoronene in multi-step chemical processes.
Typically, the composite elements of carbon, oxygen, and hydrogen, along with the functional surface groups of graphene are also found in graphene quantum dots. The forms of most graphene quantum dots are round and elliptical. However, graphene quantum dots that are triangular, quadrate or hexagonal have also been created.
Using Graphene Quantum Dots to Facilitate Fluorescence Microscopy
While high-quality chemical and biological labels are effectively used for fluorescence microscopy, their use is primarily restricted to fixed and permeabilized cells, since they cannot easily pass through the cell membrane of living cells.
Genetically fabricating fluorescent proteins has become the main labeling technique for the fluorescent microscopy of live cells. However, the issues of over-expression of artifacts, insufficient spectral range, lack of brightness and photo-instability are significant restrictions on the technology.
Having low toxicity, high photostability and the capability to pass through a cell membrane makes graphene quantum dots appealing candidates for biological and medical imaging. In fact, graphene quantum dots have been used in many applications to expedite intercellular fluorescence microscopy. One study group produced nanocomposites with graphene quantum dots and gold nano-cubes to effectively produce fluorescence probes for the precise imaging of membrane-bound proteins in a living cell. In another recent study, graphene quantum dots were used to both deliver anti-tumor drugs and act as fluorescent markers for real-time imaging purposes.
Graphene quantum dots have also been used to add fluorescence to a cell via a photothermal technique for intracellular delivery of materials: the formation of vapor nanobubbles on a cell membrane. Upon irradiation with short laser bursts, plasmonic graphene quantum dots become very hot, to the point that the nearby water in the tissue evaporates, creating vapor nanobubbles. The mechanical pressure of the nanobubbles around each quantum dot locally ruptures the cell membrane, enabling external compounds to enter the cytoplasm. Notably, being a laser-based technology, this technique can facilitate light microscopy and can be easily used on cells commonly used in fluorescence microscopy.
Graphene quantum dots are quite tolerant of pulsed laser irradiation and can create a number of vapor nanobubbles, thus enabling multiple perforations of the cells and precise governing of the quantity of fluorescent material that could be put into cells. Using graphene quantum dots in this manner, scientists have been able to demonstrate that fluorescent compounds can be precisely delivered into cells to achieve the desired results.
Furthermore, short-wavelength ultraviolet and visible light does not penetrate deep into tissue, restricting their application in deep tissue microscopy. To tackle the issue of deep tissue imaging, a team of scientists developed nitrogen-doped graphene quantum dots that were an effective two-photon fluorescent probe.
This development and others are prompting the expanded usage of graphene quantum dots in many types of biomedical imaging applications.
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