Graphene Quantum Dots (Free Article)

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Graphene quantum dots (GQDs) are a class of zero-dimensional nanomaterials that have attracted significant research interest in recent years. GQDs are graphene sheets with lateral dimensions smaller than 100 nm and possess unique size-dependent properties. GQDs exhibit exceptional optical and electronic properties due to quantum confinement and edge effects. They demonstrate pronounced photoluminescence across the visible and near-infrared regions. Their fluorescence emission can be tuned by controlling the size and surface chemistry of the GQDs. These unique optical properties make GQDs promising materials for various optoelectronics applications.

There are several key advantages of GQDs over other fluorescent nanomaterials:

  • High photoluminescence quantum yield
  • Resistance to photobleaching
  • Excellent biocompatibility
  • Low toxicity
  • Ability to be functionalized
  • Water solubility
  • Environmental friendliness

GQDs can be produced from different carbon sources using various synthesis methods:

Method

Carbon Source

Hydrothermal Graphene oxide, citric acid
Electrochemical Graphite rods
Microwave Carbon nanotubes, candle soot
Ultrasonic Carbon fibers

The most common route is the hydrothermal method using graphene oxide as the precursor. The optical, electronic, and chemical properties of GQDs can be tuned by controlling the synthesis conditions.

The unique attributes of GQDs make them promising materials for a broad range of potential applications:

  • Bioimaging and sensing
  • Photocatalysis
  • Light emitting diodes
  • Solar cells
  • Energy storage devices
  • Electrocatalysis
  • Drug delivery

Optical Properties of Graphene Quantum Dots

The versatile optical properties of graphene quantum dots arise from quantum confinement and edge effects. The electronic structure and bandgap can be tuned by controlling the size and shape of GQDs during synthesis. This enables their photoluminescence properties to be tailored for different applications.

Photoluminescence is the predominant optical characteristic of GQDs. They demonstrate strong fluorescence under UV excitation across the entire visible spectrum and into the near-infrared. Some key optical properties include:

  • Broad absorption spectra
  • Excitation-dependent emission
  • Large fluorescence quantum yields up to 90%
  • Resistance to photobleaching
  • Tunable fluorescence based on size and surface chemistry

The main factors influencing the PL of GQDs are:

  • Size of the GQDs
  • Surface defects and functional groups
  • Purity
  • Dispersion level
  • Excitation wavelength

Smaller GQDs tend to show blue-shifted PL peaks due to wider bandgaps caused by quantum confinement. The PL can span the visible spectrum by controlling the GQD size from 2-10 nm during synthesis. Surface passivation with agents like PEG can enhance PL intensity. Oxygen-containing groups on GQD surfaces enable excitation-dependent emissions. The highest quantum yields are achieved with very pure and uniformly dispersed samples. GQDs also demonstrate electroluminescence for lighting applications. Electroluminescent GQDs can be integrated into LEDs, displays, and other optoelectronic devices.

The strong PL makes GQDs excellent candidate materials for:

  • Bioimaging
  • Fluorescent sensing
  • Photocatalysis
  • Security inks
  • Optical coding
  • LEDs
  • Lasers
  • Photovoltaics

The versatile photoluminescence of GQDs arising from quantum confinement enables their properties to be tailored for diverse optoelectronics, sensing, and imaging applications. Further research is focused on achieving uniform quantum yields close to 100% across the entire visible spectrum.

Synthesis Methods for Graphene Quantum Dots

Various synthesis methods have been developed to produce graphene quantum dots (GQDs) with tunable optical, electronic, and surface properties. The most common synthesis routes include:

Hydrothermal Method

This is the most widely used technique to produce GQDs. It involves heating a carbon source like graphene oxide in an aqueous solution using an autoclave. Reaction parameters like temperature, pressure, and time can be varied to control GQD size and surface chemistry.

Advantages:

  • Simple process
  • Facile control over optical properties
  • Environmentally friendly
  • Scalable

Disadvantages:

  • Long reaction times
  • Difficult morphology control

Electrochemical Synthesis

GQDs are produced by electrochemical oxidation of graphite rods or other carbon sources. The size can be tuned by controlling voltage, current density and electrolyte pH.

Advantages:

  • Fast and facile synthesis
  • Tight control over size and shape
  • Scalable

Disadvantages:

  • Requires complex instrumentation
  • Limited yield1

Microwave-Assisted Method

Microwave irradiation of graphitic carbon sources leads to rapid exfoliation and cutting to form GQDs. Microwave power and time can control the GQD size.

Advantages:

  • Rapid and easy
  • Tunable GQD size
  • High yields

Disadvantages:

  • Poor size uniformity
  • Requires specialized equipment

Other Methods

  • Sonication/Ultrasonication
  • Solvothermal
  • Chemical Oxidation
  • Photo-Fenton Reaction

Fine-tuning the synthesis conditions enables control over GQD properties for targeted applications. More work is focused on developing sustainable, scalable methods with precise morphology control.

Applications in Optoelectronics

The excellent optical properties of graphene quantum dots (GQDs) make them promising materials for various optoelectronics applications including light emitting diodes (LEDs), displays, lasers, and solar cells.

Light Emitting Diodes

GQDs can be used as the luminescent material in LEDs. Their tunable photoluminescence spanning the visible spectrum allows emission colors to be adjusted by controlling the GQD size and surface chemistry. GQDs have been incorporated into quantum dot-LEDs, achieving pure and stable color with high brightness and efficiency. The solution processability of GQDs enables low-cost, large-area LED fabrication.

Photovoltaics

When combined with electron acceptors like TiO2, the excited electrons in GQDs can be transferred to generate current. GQDs have been used in quantum dot solar cells with efficiencies over 10% .

Advantages over dyes:

  • Broad absorption spectrum
  • High stability against photobleaching
  • Large extinction coefficient
  • Easy to fabricate and low cost

Further research is focused on improving charge transfer efficiency in GQD-based solar cells.

Displays

GQDs can serve as downconverters for LCD backlights. They absorb UV light and emit white light that enhances brightness and efficiency compared to traditional phosphors. Their high photostability is beneficial for display applications. GQDs are promising fluorophores for next-generation displays, solid-state lighting, and photovoltaics. Further advances in synthesis and device integration will help realize their full potential in optoelectronics.

Applications in Bioimaging and Sensing

The excellent fluorescence properties and biocompatibility of graphene quantum dots (GQDs) make them ideal probes for bioimaging and fluorescent sensing.

Bioimaging

GQDs have emerged as next-generation fluorescent labels for cellular and in vivo bioimaging. Their high photostability enables long-term tracking of cells. GQDs perform well in challenging in vitro and in vivo environments.

The mechanisms of cellular uptake of GQDs. Confocal fluorescent images... | Download Scientific Diagram

Fluorescence image of GQDs in HeLa cells. 

Key advantages over organic dyes and quantum dots:

  • Resistance to photobleaching
  • Low toxicity
  • Stable fluorescence across broad pH range

Targeted GQD probes have been developed by attaching biomolecules like antibodies, peptides, or aptamers. This enables specific labeling and bioimaging of cancer cells, bacteria, nucleic acids, enzymes, and biomarkers.

Fluorescent Sensing

The fluorescence of GQDs can be quenched by electron transfer or energy transfer processes. This provides the basis for fluorescent sensors that can detect metal ions, biomolecules, and environmental pollutants.

GQDs have been integrated into test strips, microfluidic devices, and wearables for rapid on-site detection of compounds at low concentrations. Their broad sensitivity and high quenching efficiency make them versatile sensing platforms.

The excellent optical properties and biocompatibility of GQDs give them significant advantages for fluorescence-based bioimaging and chemical sensing. Further research aims to improve their quantum yield, targeting specificity, and integration into point-of-use devices.

Energy Storage Applications

Graphene quantum dots (GQDs) have shown promising potential for energy storage applications including supercapacitors, lithium-ion batteries, and fuel cells due to their unique structure and properties.

Supercapacitors

GQDs with their large specific surface area, high electrical conductivity and tunable properties can enhance the performance of supercapacitor electrodes.

GQDs incorporated into carbon-based electrodes have shown improved specific capacitance and cycling stability. GQD/conducting polymer composites as electrode materials also display excellent capacitive performance and charge-discharge rates.

Lithium-Ion Batteries

GQDs have been widely researched as anode materials for Li-ion batteries. They can enhance charge transfer kinetics and withstand volume changes during cycling.

Key advantages over carbon materials:

  • Higher theoretical capacity
  • Better electrochemical utilization
  • Faster Li-ion transport

Coating Si or metal oxide nanoparticles with GQDs improves stability and cycling performance. Further research aims to build advanced GQD-composite anodes.

Fuel Cells

GQDs are promising metal-free catalysts for the oxygen reduction reaction in fuel cells due to their high surface area and tunable catalytic properties [5]. They also show potential as hydrogen storage materials.

GQDs present new opportunities to enhance the performance and durability of supercapacitors, Li-ion batteries, and fuel cells through the unique optoelectronic properties derived from quantum confinement.

Electrocatalytic Applications

Graphene quantum dots (GQDs) have emerged as promising metal-free electrocatalysts for reactions including the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).

Hydrogen Evolution Reaction

The HER is a key reaction for clean hydrogen fuel production from water splitting. GQDs can catalyze this reaction with performances rivalling platinum-based catalysts. Factors influencing the HER activity include:

  • Edge site density
  • Surface defects
  • Oxygen content
  • Heteroatom doping

Nitrogen-doped GQDs show excellent HER catalytic activity in acidic media, with tunable properties based on the N-doping levels.

Oxygen Reduction Reaction

The ORR is vital for fuel cells and metal-air batteries. GQDs have shown remarkable ORR catalytic performances exceeding commercial Pt/C catalysts.

Their metal-free nature makes GQDs promising sustainable ORR catalysts. The ORR activity can be tuned via morphology control and heteroatom doping with N, S, P etc. during synthesis.

GQDs are emerging as efficient metal-free electrocatalysts for clean energy reactions like HER and ORR. Further research on controlled synthesis and advanced composite catalysts aims to fully exploit their potential for energy applications.

Photocatalytic Applications

Graphene quantum dots (GQDs) have emerged as a new class of photocatalysts for energy conversion and environmental remediation driven by their unique properties.

Fundamentals of Photocatalysis

When GQDs absorb light, electron-hole pairs are generated which drive reduction and oxidation reactions. The photocatalytic activity is influenced by [1]:

  • Light absorption range
  • Charge separation efficiency
  • Surface reactive sites
  • Stability

GQDs can address limitations of traditional photocatalysts like TiO2 and CdS through:

  • Broad spectral absorption extending into visible/NIR region
  • Facile charge transport due to high conductivity
  • Tunable bandgap and surface properties
  • High stability against photocorrosion

Solar Energy Conversion

GQDs are promising co-catalysts for dye-sensitized, quantum-dot and perovskite solar cells where they can enhance light absorption, charge transfer and stability [2].

Environmental Remediation

GQDs can enable degradation of organic pollutants via reactions with photogenerated reactive oxygen species [3]. They also catalyze CO2 reduction for solar fuels.

Disinfection

Photoexcited GQDs can produce bactericidal reactive oxygen species for water disinfection [4]. Their high photostability enables durable disinfection under solar irradiation.

GQDs are promising next-generation photocatalysts for renewable energy and environmental applications based on their broad light absorption, efficient charge transfer, high stability and tunable properties.

Toxicity and Biocompatibility

The toxicity and biocompatibility of graphene quantum dots (GQDs) is a critical consideration for their use in bioimaging, drug delivery, medical diagnostics, and other in vivo applications.

Several studies have investigated the toxicity of GQDs in cells and animal models. Key findings show:

  • Size-dependent toxicity with smaller GQDs showing lower toxicity [1,2].
  • Surface chemistry affects toxicity – more oxidation increases biocompatibility [3].
  • Most GQDs display low toxicity at functional dosages.
  • Toxicity arises mainly from oxidative stress and lipid peroxidation [4].
  • GQDs show much lower toxicity than graphene oxide and carbon nanotubes.

Improving Biocompatibility

Strategies to further improve GQD biocompatibility include [5]:

  • Size control to keep GQDs under 5 nm diameter.
  • Surface passivation with biocompatible polymers.
  • Functionalization with target molecules for specific delivery.
  • Careful purification to remove contaminants.

More in vivo toxicity studies are vital to establish the long-term safety profile of GQDs for clinical use. Overall, most well-designed GQDs show good biocompatibility at functional dosages for biomedical applications.

For further information on markets and companies see The Global Market for Graphene Quantum Dots.

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