Carbon Quantum Dots: Recent Advances and Biological Insights

Carbon quantum dots (CQDs) are attracting attention for their unique optical properties, biocompatibility, and applications in bioimaging, drug delivery, and environmental sensing. Their nanoscale size and tunable fluorescence make them promising alternatives to traditional semiconductor quantum dots, which often contain toxic heavy metals. Advancements in synthesis methods and structural modifications continue to expand their functionality.

Research is refining CQD performance through improved synthesis techniques, heteroatom doping, and surface functionalization. These developments are essential for enhancing their use in biological and medical sciences.

Structural Composition And Optical Behavior

CQDs consist of sp² and sp³ hybridized carbon atoms forming a core-shell structure that influences their electronic and optical properties. The core, either graphitic or amorphous carbon, is surrounded by oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy moieties. These groups enhance solubility in water and modulate electronic transitions, affecting fluorescence characteristics. A higher degree of graphitization generally results in stronger fluorescence due to more efficient electron delocalization.

Fluorescence emission in CQDs stems from quantum confinement effects, surface state emissions, and molecular fluorophore contributions. Unlike traditional semiconductor quantum dots, which exhibit size-dependent bandgap tuning, CQDs display excitation-dependent fluorescence, meaning emission shifts based on excitation energy. This is due to diverse emissive sites, including surface defects, edge states, and embedded molecular fluorophores. Controlling oxidation levels can fine-tune emission spectra, with higher oxygen content often leading to red-shifted fluorescence.

Photoluminescence quantum yield (PLQY) varies depending on synthesis conditions and post-processing modifications. While pristine CQDs typically exhibit moderate PLQY (5–20%), surface engineering has enabled enhancements exceeding 80%. Passivating non-radiative recombination centers and introducing electron-donating or withdrawing groups stabilize excitonic transitions. Heteroatom doping, such as nitrogen or sulfur incorporation, introduces new energy levels, further modifying fluorescence intensity and stability.

Green Synthesis Techniques

Environmentally friendly synthesis methods for CQDs aim to minimize hazardous reagents and energy-intensive processes. Green synthesis leverages naturally derived precursors and sustainable reaction conditions to produce CQDs with desirable properties. Biomass, plant extracts, and other renewable sources reduce environmental impact while introducing functional groups that enhance biocompatibility and fluorescence efficiency.

Hydrothermal and solvothermal treatments enable controlled decomposition of organic materials into CQDs under mild conditions. These methods involve heating biomass-derived precursors, such as fruit peels, algae, or polysaccharides, in water or ethanol at 150–250°C. This process facilitates carbonization while preserving oxygen-containing functional groups that contribute to solubility and fluorescence modulation. Plant-based CQDs exhibit tunable emission properties due to naturally occurring heteroatoms influencing electronic transitions.

Microwave-assisted synthesis significantly reduces reaction times and energy consumption by rapidly heating carbon precursors, promoting uniform nucleation and particle formation within minutes. Compared to conventional thermal treatments, this approach enhances quantum yield by preventing excessive carbonization and preserving surface functional groups. CQDs derived from honey, coffee grounds, and citrus extracts using microwave irradiation demonstrate high luminescence efficiency, highlighting the scalability of this technique.

Ultrasonic-assisted synthesis employs high-energy acoustic waves to break down organic precursors into nanoscale carbon structures. The cavitation effect generates localized high temperatures and pressures, forming CQDs without harsh chemical reagents. This method has been successfully applied to biowaste materials like coconut husks and soybean residues, yielding particles with stable fluorescence and minimal aggregation. The rapid nature of ultrasonic synthesis makes it attractive for large-scale applications.

Characterization Methods

Accurate characterization of CQDs is essential for understanding their structural, optical, and chemical properties. A combination of spectroscopic, microscopic, and crystallographic techniques assesses particle size distribution, surface chemistry, and fluorescence behavior.

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) determine size, morphology, and structural uniformity. TEM imaging reveals core structure and graphitization, often showing lattice fringes indicative of sp²-hybridized domains. High-resolution TEM (HRTEM) distinguishes amorphous and crystalline regions, correlating structural order with quantum yield efficiency. AFM provides topographical data, offering precise height measurements that complement lateral size distributions obtained from TEM.

Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) identify surface functional groups and elemental composition. FTIR detects vibrational modes associated with hydroxyl, carboxyl, and amine groups, revealing surface oxidation and hydrophilicity. XPS provides quantitative elemental state information, verifying heteroatom doping and surface passivation success.

Optical characterization relies on ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectroscopy to evaluate electronic transitions and emission behavior. UV-Vis spectra show absorption peaks corresponding to π–π and n–π transitions, influenced by core size and functionalization. PL spectroscopy reveals excitation-dependent fluorescence, allowing emission wavelengths to be tuned by adjusting synthesis conditions. Time-resolved fluorescence spectroscopy provides decay lifetime data, informing energy transfer mechanisms and photostability.

Common Sources Of Precursor Materials

Precursor selection significantly influences CQD properties. Naturally derived sources, such as biomass and organic waste, offer sustainability and inherent surface functionalities that enhance fluorescence. Agricultural byproducts, including fruit peels, sugarcane bagasse, and coffee grounds, contain high carbon content and diverse heteroatoms, making them ideal for CQD synthesis. These materials reduce environmental waste while introducing oxygen- and nitrogen-containing groups that improve solubility and emission efficiency.

Amino acid-rich precursors, such as gelatin, chitosan, and casein, contribute nitrogen to the carbon matrix, altering electronic transitions and enhancing quantum yield. Polysaccharide-based sources like starch and cellulose facilitate CQD synthesis by undergoing controlled carbonization, where hydroxyl-rich structures influence particle dispersity and stability. Starch-derived CQDs exhibit strong blue fluorescence, which can be fine-tuned by modifying reaction parameters like temperature and pH.

Role Of Heteroatom Doping

Heteroatom doping modifies CQD electronic and optical properties by incorporating elements such as nitrogen, sulfur, phosphorus, or boron. These elements influence charge distribution, bandgap energies, and photoluminescence efficiency. Nitrogen doping improves quantum yield by introducing additional π-electrons that enhance electron delocalization.

Sulfur and phosphorus doping introduce localized energy levels affecting fluorescence behavior. Sulfur-doped CQDs often exhibit red-shifted emissions due to sulfur-carbon bonds altering charge carrier dynamics. Phosphorus doping enhances electron mobility and stability, making CQDs more suitable for bioimaging. Heteroatom doping can also improve reactive oxygen species (ROS) generation, relevant for photodynamic therapy applications. Optimizing dopant elements and synthesis conditions allows researchers to tailor CQD performance for biomedical and environmental applications.

Surface Passivation Approaches

Surface passivation enhances CQD optical performance and stability by modifying the surface with organic molecules, polymers, or inorganic coatings. This process reduces non-radiative recombination centers and enhances fluorescence intensity. Passivation is particularly useful for CQDs with surface defects that act as charge traps quenching photoluminescence.

Polyethylene glycol (PEG), amines, and thiol-containing compounds are commonly used for passivation. PEGylation enhances biocompatibility and prolongs circulation time in biological systems, making CQDs suitable for in vivo imaging and drug delivery. Amino-functionalized passivation introduces electron-donating groups that stabilize excitonic transitions, increasing fluorescence intensity. Thiol-based coatings improve resistance to oxidative degradation, ensuring stability in physiological environments. The choice of passivating agent depends on the intended application, enabling CQDs to function optimally across scientific domains.

Material Functionalization Strategies

Functionalization extends CQD versatility by enabling targeted interactions with biological molecules, enhancing selectivity in diagnostics and therapeutics. This involves grafting functional groups or biomolecules onto the CQD surface to facilitate binding with cellular targets, improve solubility, or introduce new properties. Covalent and non-covalent functionalization methods offer distinct advantages depending on the application.

Covalent approaches, such as amide or ester bond formation, provide strong and stable linkages, ideal for attaching drug molecules or antibodies. Non-covalent functionalization, including electrostatic interactions and π–π stacking, allows reversible modifications that maintain CQD integrity while enabling dynamic biomolecular interactions. This strategy is useful for biosensing applications, where CQDs detect specific analytes through fluorescence quenching or enhancement mechanisms. Functionalized CQDs have been engineered with DNA probes, peptides, and small-molecule ligands to develop highly sensitive detection platforms for disease biomarkers. Tailoring functionalization strategies allows CQDs to be optimized for nanomedicine, environmental monitoring, and advanced imaging technologies.