Ms. Pushpavalli, S.1, Dr. Suresh Kumar. P2*, Birundha.M1, Research Scholar1
Introduction
Nanotechnology is the science, engineering, and application of materials at the nanoscale, typically between 1 and 100 nanometers. At this scale, materials often exhibit unique physical, chemical, and biological properties that differ significantly from their bulk counterparts. One of the most promising developments in nanotechnology is the carbon-based nanomaterials, particularly Carbon Dots (CDs). These zero-dimensional nanomaterials have garnered significant attention due to their unique properties and wide range of applications. Carbon Dots are small carbon nanoparticles, generally less than 10 nanometers in size, that exhibit strong photoluminescence. They are composed primarily of carbon, oxygen, and hydrogen atoms, and sometimes include other elements like nitrogen or sulfur.
Unlike traditional semiconductor quantum dots, CDs are free from toxic heavy metals, making them more environmentally friendly and biocompatible. The global impact of carbon dots is profound, especially when integrated into nanocomposites. Nanocomposites are materials that combine a matrix (such as polymers, metals, or ceramics) with nanoparticles to enhance their properties. The inclusion of carbon dots into these matrices can significantly improve optical, mechanical, and electrical properties. Normally nanocomposite are biomedical Device, Sensors, Energy Storage device, Electronics devices etc.
Methods of Carbon Dot Synthesis
- Top-Down Methods
The method involve the breakdown of larger carbon materials into nanoscale particles.
Laser Ablation: Laser ablation is a method of separating layers of material by treating its surface with high-energy quanta of laser radiation. It has attracted much interest in the synthesis of carbon quantum dots because of its simplicity and speed. The dots produced by this method have no contaminants and adopt very small sizes (from 2 to 5 nm)
Arc Discharge: A high-voltage arc is struck between carbon electrodes in a gaseous environment. Electrochemical Oxidation: Electrochemical synthesis widely used in the synthesis of CDs, where carbon sources such as graphite, are used as electrodes together with a counter electrode.
- Bottom-Up Methods
The methods develop carbon dots from smaller molecular precursors. Different types bottom up method available but most common method.
Hydrothermal Method: Material are heated in a sealed autoclave at high temperature creating high pressure in the presence or absence of water as solvent. The formation mechanism includes hydrolysis of starch to glucose, and further condensation of glucose by ring closure. The produced CQDs were hydrophilic in nature, about of 2.25–3.50 nm in diameter, with potential for biological applications.
Solvothermal Synthesis: Solvothermal method involves carbonization of organic molecules in the presence of organic solvents such as dimethylformamide (DMF) and octadecene . The reaction mechanism involves formation of CDs with chemical reaction of the reactants and crystallization of excess trisodium citrate around the formed CDs. Then, crystal growth results in the embedment of CDs into the trisodium citrate matrix. In another report, CDs with multicolour emission were prepared using citric acid and urea in DMF using solvothermal reaction. CDs have been synthesized from synthetic materials though using natural sources as precursors can aid to decrease the pollution and cost.
Microwave assisted synthesis: Method involves homogenous, rapid, and uniform heating to give high reaction yield in a short time. The microwave assisted CD synthesis mechanism includes acidic oxidation of epoxy groups leading to the formation of mixed epoxy and carbonyl groups on the lattice resulting in fragile graphitic domains that can be easily attacked and cleaved. GQDs obtained by acidic oxidation give greenish yellow emission with 11.7% QY and exhibited blue emission with 22.9% QY after the reduction of GQDs by NaBH 4.
Thermal Decomposition is Organic materials are heated to decompose and form carbon dots. The method have multiple advantages like cost, scalability, and control over the properties of the resulting carbon dots.
Types of carbon dot
The carbon dot are classified into based on the structure and composition.
- Carbon Quantum Dots (CQDs)
- Graphene Quantum Dots (GQDs
- Nitrogen-Doped Carbon Dots (N-CDs)
- Polymer-Derived Carbon Dots (P-CDs)
Carbon Quantum Dots (CQDs)
CQDs are spherical, crystalline nanoparticles. It produced using both method. The interlayer spacing in CQDs is around 0.34 nm, corresponding to that of graphite. Their atomic structure consists mostly of sp²-hybridized carbon, though some sp³-hybridized atoms may be present. Quantum confinement and edge effects contribute to their photoluminescence, which is also influenced by surface states and dopants.
Graphene Quantum Dots (GQDs
GQDs are crystalline structures. GQD composed of one or more graphene, with dimensions of the carbon dot less than 20 nm. The interlayer spacing is approximately 0.24 nm. GQDs are typically synthesized via top-down methods, involving the cutting of larger carbon structures. The optical characteristics of GQDs are governed not only by quantum confinement and edge effects but also by the number and size of π-conjugated domains and the presence of surface functional groups.
Nitrogen-Doped Carbon Dots (N-CDs)
Nitrogen-doped carbon dots (N-CDs) are a class of carbon-based nanomaterials where nitrogen atoms are incorporated into the carbon matrix, significantly enhancing their electronic, optical, and chemical properties. The presence of nitrogen introduces surface defects and active sites, improving conductivity, photoluminescence, and chemical reactivity. Due to these unique characteristics, N-CDs find wide applications in bioimaging, drug delivery, photocatalysis, sensors, and energy storage devices such as supercapacitors and batteries. Their excellent biocompatibility and tunable fluorescence also make them promising candidates for biomedical diagnostics and environmental monitoring.
Polymer-Derived Carbon Dots (P-CDs)
Polymer-derived carbon dots (P-CDs) are nanomaterials formed from polymer precursors, featuring a hybrid structure with a partially carbonized core surrounded by polymer chains and functional groups. This unique configuration enhances their stability, solubility, and tunability, allowing precise control over their size, surface chemistry, and optical properties. As a result, P-CDs are widely used in drug delivery, biosensing, imaging, and light-emitting devices. Their ease of surface functionalization and biocompatibility make them especially suitable for targeted biomedical applications and flexible optoelectronic systems.
Properties of Carbon Dots
Carbon dots (CDs) possess remarkable properties such as high quantum yield, excellent photostability, low toxicity, and strong resistance to photobleaching and degradation—features that set them apart from traditional organic dyes. Their strong and tunable photoluminescence makes them highly effective for imaging purposes, while their robust chemical stability ensures durability under various conditions. CDs are also biocompatible, making them safe for biomedical use, and their surfaces can be easily functionalized for targeted applications. Additionally, their high solubility in both water and organic solvents broadens their usability. These combined attributes make CDs highly versatile for diverse fields including environmental sensing, biomedical imaging, food packaging, and electronic devices.
Sustainable Synthesis of Carbon Dots from Agricultural Waste
Agricultural wastes such as fruit peels, leaves, husks, and other biomass residues are rich in carbon, making them excellent precursors for carbon dot synthesis. Through green synthesis methods like hydrothermal or pyrolysis processes, these agro-wastes are thermally treated under controlled conditions to break down their organic structure. This results in the formation of nanoscale carbon dots with desirable properties such as fluorescence, biocompatibility, and chemical stability. The process not only adds value to low-cost waste materials but also supports sustainable nanomaterial production with minimal environmental impact. Key highlights include the use of renewable, carbon-rich agro-waste as a raw material, adoption of eco-friendly synthesis techniques, generation of functional carbon dots with high stability and fluorescence, economic valorization of agricultural by products, and alignment with green and sustainable nanotechnology practices.
Applications of Carbon Dots
Carbon dots (CDs) have emerged as versatile nanomaterials with a wide range of applications across multiple fields. In biomedical applications, they are widely used for bioimaging due to their excellent fluorescence properties, both in vivo and in vitro. They also function as drug delivery systems, enabling controlled release of therapeutic agents. CDs serve as sensitive biosensors for detecting biomolecules and pathogens, and they show promise in cancer therapy, particularly in photothermal and photodynamic approaches because of their strong light-absorbing capacity.
In the environmental domain, CDs are employed in water purification, where they adsorb heavy metals and organic pollutants. They are also used to detect environmental toxins and serve as catalysts in photocatalytic degradation of various contaminants.
Within the energy and electronics sector, carbon dots contribute to improving the efficiency of solar cells and are used in the fabrication of LEDs for efficient lighting. They also enhance the performance of supercapacitors, contributing to better energy storage and delivery in electronic devices.
In the food industry, especially in food packaging, carbon dots are crucial for developing biodegradable, UV-resistant, antibacterial, antioxidant, and biocompatible films that extend the shelf life and preserve the quality of food. They also help in preventing microbial contamination through ROS generation, and can function as sensors for real-time monitoring of freshness and temperature.
For food safety and quality monitoring, CDs are effective in detecting food additives, leveraging their strong fluorescence and the linearity between fluorescence intensity and analyte concentration. For example, sulfur-doped CDs synthesized from rosemary and thiourea can detect lemon yellow through a fluorescence-quenching effect, with a detection limit of 0.45 µmol/L. CDs are also used for detection of foodborne pathogens such as E. coli, Salmonella, and H. pylori, with specific binding mechanisms enabling rapid and sensitive quantification, even in real samples like water and juices.
In metal ion detection, CDs have shown remarkable sensitivity. For instance, CDs synthesized using sodium citrate and formamide demonstrated a quantum yield of 35.3% and could detect Cu²⁺ ions with a limit as low as 5 nmol/L. Similarly, Ag⁺ ions were effectively detected using sulfur and nitrogen co-doped CDs prepared via a hydrothermal method using citric acid and guanidine thiocyanate.
Conclusion
Carbon dots represent a promising class of nanomaterials with wide-ranging applications thanks to their unique optical, chemical, and biological characteristics. Their ease of synthesis, biocompatibility, and functional versatility make them ideal candidates for future advancements in healthcare, environmental protection, food safety, and electronic devices.
Author’s Bio
Principal Scientist2, ICAR-National Research Centre for Banana, Tiruchirappalli.
*e-mail: [email protected]
