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Carbon quantum dot

Carbon quantum dots (CQDs), also known as carbon dots (CDs), are zero-dimensional, quasi-spherical carbon-based nanomaterials with diameters typically less than 10 nm, characterized by their strong photoluminescence, high water solubility, low toxicity, and excellent biocompatibility.^[1] These nanoparticles, first discovered accidentally in 2004 during the purification of single-walled carbon nanotubes via electrophoresis, represent a promising class of fluorescent carbon materials that bridge the gap between traditional semiconductor quantum dots and organic dyes.^[2] CQDs are broadly classified into subtypes such as graphene quantum dots (GQDs), carbon nanodots (CNDs), and carbonized polymer dots (CPDs), each distinguished by their internal structure and surface functionalization.^[1] The unique properties of CQDs stem from their quantum confinement effects and surface states, enabling tunable optical characteristics including excitation-dependent emission across the visible spectrum and quantum yields ranging from 10% to over 78% depending on synthesis and doping.^[1] Chemically, they exhibit robust stability in acidic or basic environments, high electrical conductivity, and strong adsorption capabilities for pollutants, while biologically, their low cytotoxicity makes them ideal for in vivo applications.^[1] Additional features like up-conversion photoluminescence—where shorter wavelengths are emitted upon near-infrared excitation—further enhance their utility in deep-tissue imaging.^[1] Synthesis of CQDs can be achieved through top-down approaches, such as arc discharge (yielding particles with 16% quantum yield in 40–60 minutes) or laser ablation (producing 3–5 nm dots with up to 12.2% quantum yield), which break down larger carbon structures, or bottom-up methods like hydrothermal treatment (achieving 46% quantum yield at 150°C for 8 hours) and microwave-assisted processes (24% quantum yield in 35 minutes at 180°C), which build nanostructures from molecular precursors including citric acid, biomass, or natural sources like ginger juice.^[1] These versatile preparation routes allow for precise control over size, composition (e.g., nitrogen or phosphorus doping), and functionality. Due to their multifaceted properties, CQDs find applications across diverse fields: in optoelectronics, they serve as electron acceptors in solar cells with power conversion efficiencies up to 9.15% and as emitters in LEDs with brightness exceeding 250 cd·cm⁻²; in biomedicine, they enable high-resolution bioimaging with photostability superior to traditional dyes, targeted drug delivery (e.g., 69.2% loading capacity for anticancer agents), and therapies like photothermal (efficiencies of 38.7%–87.9%) and photodynamic cancer treatments via reactive oxygen species generation; additionally, they excel in environmental sensing and remediation, detecting ions like Hg²⁺ at limits as low as 4.2 nM and degrading pollutants such as methyl orange by over 91%.^[1] Ongoing research as of 2025 continues to explore their potential in chiral variants for stereoselective sensing and advanced photocatalysis.^[3]^[4]^[5]

Fundamentals

Definition and Structure

Carbon quantum dots (CQDs) are zero-dimensional fluorescent carbon nanoparticles typically smaller than 10 nm in size, characterized by their photoluminescent properties and biocompatibility.^[6] They consist primarily of sp²-hybridized carbon domains forming the core, combined with surface functional groups that enhance solubility and enable further modification.^[7] These nanomaterials are distinguished from broader carbon dot categories by their defined nanoscale dimensions and pronounced quantum effects, making them suitable for optical applications.^[8] The structure of CQDs generally features a graphitic crystalline core composed of sp²-hybridized carbon atoms, often with an amorphous shell incorporating sp³-hybridized carbons and heteroatoms.^[7] This core-shell architecture includes surface passivation by oxygen- and nitrogen-containing functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and amine (-NH₂), which stabilize the particles and influence their electronic properties.^[6] The core typically exhibits lattice spacing around 0.2 nm, indicative of graphitic domains, while the shell provides emissive trap states.^[7] Quantum confinement in CQDs arises from their nanoscale size, leading to a size-induced increase in the bandgap energy that enables tunable photoluminescence across the visible spectrum.^[6] Smaller CQDs (e.g., 1.5–3.0 nm) exhibit blue-shifted emission due to a larger bandgap, while larger ones (up to ~10 nm) show red-shifted emission.^[8] This effect can be approximated by the particle-in-a-box model:

E_g \approx E_{bulk} + \frac{\hbar^2 \pi^2}{2 m^* r^2}

where E_g is the bandgap energy, E_{bulk} is the bulk bandgap, \hbar is the reduced Planck's constant, m^* is the effective mass of the exciton, and r is the radius of the CQD.^[9] The confinement enhances electronic transitions within the sp² domains, contributing to excitation-dependent emission.^[7] In terminology, CQDs are differentiated from graphene quantum dots (GQDs), which are planar, graphene-derived fragments with lateral confinement and edge effects, and from carbon nanodots (CNDs), which are more amorphous and exhibit weaker quantum confinement without the distinct core-shell structure.^[6] CQDs emphasize spherical morphology and stronger size-tunable fluorescence compared to these relatives.^[7]

Physical and Optical Properties

Carbon quantum dots (CQDs) demonstrate exceptional physical stability, including high photostability under continuous irradiation and chemical stability in various solvents, which stems from their robust carbon core structure.^[10] Their water solubility is primarily due to abundant surface functional groups such as hydroxyl (-OH) and carboxyl (-COOH), enabling facile dispersion in aqueous media without aggregation.^[11] Furthermore, the small size of CQDs (typically less than 10 nm) and their all-carbon composition contribute to inherently low toxicity, as evidenced by minimal cytotoxicity in cellular assays compared to heavy metal-containing nanomaterials. The optical properties of CQDs are dominated by excitation-dependent photoluminescence (PL), where the emission spectrum shifts to longer wavelengths as the excitation wavelength increases, allowing color-tunable fluorescence. Absorption bands appear in the ultraviolet-visible (UV-Vis) region, typically between 200 and 400 nm, arising from π-π* transitions in the sp² carbon domains.^[11] Emission can be tuned from blue (~450 nm) to near-infrared (~700 nm) by adjusting surface states or doping, with Stokes shifts generally ranging from 100 to 200 nm. Quantum yields for PL reach up to 80% in nitrogen-doped CQDs, approaching those of traditional organic dyes. CQDs also exhibit upconversion fluorescence, converting lower-energy photons to higher-energy emission via multi-photon processes, and support electrochemiluminescence through coreactant annihilation or co-reactant pathways.^[12] In terms of chemical properties, CQDs display high biocompatibility, facilitated by oxygen- and nitrogen-containing surface groups like -OH and -COOH, which promote interactions with biological systems without eliciting immune responses. Their conjugated π-system enables efficient electron transfer, supporting roles in redox processes. Additionally, CQDs show pH-responsive behavior, with fluorescence intensity or wavelength altering in response to environmental pH changes due to protonation/deprotonation of surface groups.^[13] Compared to semiconductor quantum dots, CQDs are metal-free and eco-friendly, avoiding heavy metal toxicity while exhibiting non-blinking PL for stable emission over time.^[11]

History

Discovery

The discovery of carbon quantum dots (CQDs) occurred serendipitously in 2004 during efforts to purify single-walled carbon nanotubes (SWCNTs) synthesized via arc-discharge. Researchers led by X. Xu employed preparative gel electrophoresis on oxidized SWCNTs, isolating fluorescent fractions that appeared as brightly glowing bands under UV light.^[14] These emissions were initially puzzling, leading to speculation that the fluorescence might stem from silica nanoparticle contaminants commonly present in nanotube preparations.^[14] Subsequent characterization confirmed the fluorescent entities as carbon-based nanoparticles rather than silica. Transmission electron microscopy (TEM) revealed discrete particles with diameters below 10 nm, exhibiting a graphitic lattice structure, while selected-area electron diffraction supported their crystalline carbon nature.^[14] Spectroscopic analyses, including UV-visible absorption and fluorescence emission, further verified the carbon composition and size-dependent optical properties, ruling out inorganic impurities.^[14] Initially termed "fluorescent carbon nanoparticles" or "nanotube fragments," these materials marked the inadvertent identification of a new class of nanoscale carbon fluorophores.^[14] By 2006, the nomenclature evolved to emphasize their quantum confinement effects and photoluminescent potential. In a seminal study, Y.-P. Sun and colleagues synthesized similar nanoparticles via laser ablation of graphite in the presence of water vapor and argon, followed by surface passivation with organic solvents to enhance emission efficiency.^[11] This work formalized the designation "quantum-sized carbon dots" (or CQDs), highlighting their tunable, colorful photoluminescence across the visible spectrum.^[11] This publication solidified CQDs as a distinct nanomaterial, distinct from traditional semiconductor quantum dots.^[11]

Key Developments

The late 2000s saw the introduction of hydrothermal synthesis as a versatile, eco-friendly route for producing carbon quantum dots (CQDs), enabling the use of green precursors such as citric acid and facilitating scalable, bottom-up fabrication. In 2010, Pan et al. demonstrated this method by treating graphene oxide under hydrothermal conditions to yield blue-fluorescent CQDs with a quantum yield of approximately 10%, highlighting its potential for controlled size and morphology without harsh chemicals.^[15] This approach rapidly gained traction, shifting focus from top-down methods like laser ablation to sustainable alternatives that minimized environmental impact.^[16] By 2012, heteroatom doping strategies, particularly with nitrogen (N), sulfur (S), and phosphorus (P), emerged to address limitations in intrinsic quantum yields, often below 10%. Zhu et al. reported graphene quantum dots (a subset of CQDs) with tunable surface oxidation prepared via solvothermal treatment of graphene oxide in DMF, achieving quantum yields up to 12.2% through controlled surface states.^[17] These advancements enabled tunable emission and improved charge transfer, though brief references to property enhancements like excitation-dependent emission are detailed elsewhere.^[18] From 2015 to 2020, research emphasized biocompatibility testing, revealing CQDs' low cytotoxicity compared to heavy-metal quantum dots, which sparked regulatory interest from bodies like the FDA for potential biomedical approvals. Pires et al. synthesized CQDs from cashew gum via a green route, demonstrating high cell viability (>90%) in live-cell imaging assays and minimal oxidative stress in mammalian cells.^[19] Concurrently, microwave-assisted synthesis rose for its rapidity and scalability, reducing reaction times from hours to minutes while yielding uniform CQDs from biomass precursors; for instance, gram-scale production with quantum yields exceeding 15% has been achieved, facilitating industrial prototyping.^[20] In the 2021-2025 period, carbohydrate-derived CQDs advanced as biocompatible alternatives, leveraging abundant sugars like glucose for sustainable synthesis, as reviewed in a 2025 analysis showing quantum yields up to 40% and applications in sensing.^[21] Integration with perovskites for optoelectronics also progressed, with Alqahtani et al. (2023) demonstrating CQD-perovskite composites that improved charge extraction and stability in solar cells, achieving power conversion efficiencies over 18%.^[22] Commercialization efforts intensified for CQDs in displays and sensors through scalable nanotechnology processes. Ongoing research as of 2025 explores kilogram-scale production from sustainable sources. Ongoing challenges include achieving reproducible photoluminescence (PL) due to variations in precursor purity and reaction conditions, with studies showing batch-to-batch quantum yield fluctuations of 10-20%.^[23] Toxicity standardization remains critical, as surface functional groups influence biocompatibility; a 2023 protocol established rapid assays confirming low-dose safety (<100 mg/L) but highlighting needs for long-term in vivo metrics to ensure regulatory compliance.^[24]^[25]

Synthesis and Fabrication

Synthetic Methods

Carbon quantum dots (CQDs) are primarily synthesized through top-down and bottom-up approaches, with the former breaking down bulk carbon materials and the latter assembling from small molecular precursors via carbonization, nucleation, and growth processes.^[7] Top-down methods often yield crystalline CQDs but require harsh conditions, while bottom-up routes are more versatile and eco-friendly, enabling control over composition through precursor selection.^[16] Among top-down techniques, laser ablation of graphite targets was pioneered in 2006 by Sun et al., involving irradiation with a 532 nm Nd:YAG laser in the presence of water vapor and argon carrier gas at room temperature, which fragments the carbon source into nanoscale particles approximately 5 nm in diameter with photoluminescence quantum yields of 4–10%.^[11] Arc discharge, first yielding fluorescent CQDs serendipitously in 2004 by Xu et al. during electrophoretic purification of arc-synthesized single-walled carbon nanotubes, employs a high-voltage electric arc (around 20–30 V) between graphite electrodes in an inert helium atmosphere at 500–700°C, producing CQDs alongside other carbon nanostructures.^[14] Electrochemical oxidation represents another top-down strategy, where carbon precursors such as graphite rods or carbon fibers are anodically oxidized in an aqueous electrolyte (e.g., NaOH or phosphate buffer) at potentials of 10–30 V for 1–10 hours, resulting in exfoliation and formation of oxygen-functionalized CQDs typically 2–5 nm in size.^[26] Bottom-up synthesis commonly employs hydrothermal or solvothermal carbonization of organic precursors like glucose or citric acid in sealed autoclaves under aqueous or solvent conditions at 180–250°C for 4–12 hours, promoting dehydration, polymerization, aromatization, and subsequent nucleation-growth to form CQDs with sizes around 2–10 nm.^[7] For instance, hydrothermal treatment of glucose at 200°C yields blue-emissive CQDs with a quantum yield of approximately 7%, derived from the progressive carbonization of the sugar molecules.^[27] Microwave-assisted pyrolysis accelerates this process, heating mixtures of citric acid and ethylenediamine at 500–800 W for 5–30 minutes to rapidly achieve carbonization and passivation, as demonstrated by Zhai et al. in 2012, who obtained nitrogen-doped CQDs with a quantum yield up to 30%. Conventional pyrolysis, often at 220–400°C for 1–4 hours under inert atmosphere, similarly uses citric acid and ethylenediamine precursors to form highly luminescent CQDs through thermal decomposition and condensation.^[7] Green synthesis variants leverage sustainable biomass feedstocks such as fruit peels, agricultural waste, or algae via hydrothermal or pyrolysis routes, minimizing chemical inputs and enhancing environmental viability, as emphasized in 2024 reviews on biomass-derived CQDs.^[28] These methods typically produce CQDs with quantum yields of 10–50%, depending on the precursor. Hydrothermal approaches generally offer mass yields of 20–50% and superior scalability compared to semiconductor quantum dot synthesis, as they avoid toxic heavy metals and organometallic reagents.^[29] Across these methods, the core mechanism entails initial carbonization to form sp²-hybridized cores, followed by nucleation of aromatic clusters and surface growth, with reaction conditions influencing size variability.^[16]

Size and Morphology Control

The size and morphology of carbon quantum dots (CQDs) are critical parameters that influence their electronic and optical properties, primarily through quantum confinement effects, where smaller sizes lead to larger bandgaps and blue-shifted emissions. Precise control over these attributes is achieved during synthesis by modulating reaction conditions and employing specialized techniques, enabling tailored applications in optoelectronics and sensing. Typical CQD sizes range from 1 to 10 nm, with monodispersity essential for consistent performance; non-uniform distributions can broaden emission spectra and reduce quantum yields.^[30]^[7] Key factors influencing CQD size include reaction temperature, duration, and precursor concentration. Higher temperatures promote particle growth and graphitization, often resulting in larger CQDs (e.g., ~6 nm at 180°C versus smaller sizes at lower temperatures), while extended reaction times allow for progressive nucleation and Ostwald ripening, increasing average diameter from 2 nm to 4-5 nm over hours. Similarly, elevated precursor concentrations favor larger particles by accelerating aggregation, though excessive amounts can lead to polydispersity. These parameters are particularly effective in bottom-up methods like hydrothermal and solvothermal synthesis, where temperatures of 120-200°C and times of 4-24 hours yield sizes tunable from 2-6 nm.^[7]^[30] Advanced methods further refine size and morphology. Template-assisted synthesis, such as using silica nanoparticles or columns, confines growth to produce highly uniform CQDs below 5 nm, with template removal via etching yielding monodisperse spherical dots ideal for precise bandgap engineering. pH control exploits surface charge effects; acidic conditions (pH < 4) stabilize smaller nuclei through protonation, favoring CQDs of 2-3 nm, while neutral or basic media promote growth to 4-6 nm. Solvent choice also plays a role, with polar solvents like water yielding compact spherical morphologies (~3 nm), whereas less polar ones like DMF enable slight elongation. For morphology variations, spherical shapes dominate due to isotropic growth in most syntheses, but rod-like structures can be induced via anisotropic conditions, such as directed assembly in solvothermal processes with linear precursors, achieving aspect ratios up to 3:1 and lengths of 10-20 nm.^[31]^[7]^[30] Achieving monodispersity is vital for applications requiring narrow emission lines, often accomplished post-synthesis via ultrafiltration with membranes (e.g., 10 kDa cutoff) to separate fractions by hydrodynamic radius, reducing polydispersity index from >0.3 to <0.1. The resulting size-dependent emission follows quantum confinement, where the emission wavelength \lambda_{em} generally increases with particle diameter d (e.g., blue emission at ~450 nm for 2 nm CQDs shifting to ~550 nm for 5 nm). This tunability underscores the importance of control strategies in optimizing CQD functionality.^[7]^[30]

Surface Modification and Doping

Surface passivation of carbon quantum dots (CQDs) involves coating the particles with polymers such as polyethylene glycol (PEG) or silica to mitigate surface defects, reduce fluorescence blinking, and enhance aqueous solubility. For instance, PEG passivation stabilizes surface energy traps, leading to improved photostability and quantum yields (QY) up to 40%. Silica coatings similarly passivate the surface, preventing aggregation and enabling better dispersion in biological media. These strategies are typically applied post-synthesis through methods like sol-gel processes or ligand exchange.^[32] Heteroatom doping introduces nitrogen (N), sulfur (S), or phosphorus (P) into the CQD structure during or after synthesis, altering the electronic configuration and optical properties. Common methods include co-pyrolysis with urea for N-doping or hydrothermal treatment with phosphoric acid for P-doping, which can boost QY by up to 50% for N-doped variants compared to undoped CQDs. N-doping creates mid-gap states that facilitate multi-color emission, while S- and P-doping induce red-shifts in the emission spectrum, for example, shifting from blue to green light. S/N co-doping has been shown to achieve QY as high as 65.1%, enhancing charge separation and photocatalytic efficiency. These modifications influence the HOMO-LUMO gap, with N-doping reducing it from 2.01 eV to 0.64 eV in some cases.^[33]^[33]^[32] Functionalization of CQDs often involves covalent attachment of biomolecules, such as antibodies, to surface carboxylic acid (-COOH) groups activated via EDC/NHS chemistry, enabling targeted applications like biosensing. This approach leverages the inherent -COOH groups from citric acid precursors, allowing conjugation without compromising core integrity. Doping and passivation collectively improve biocompatibility, though detailed evaluations are covered in biomedical contexts.^[34]^[35] Recent advances in 2025 have focused on eco-friendly heteroatom doping using carbohydrate-derived precursors via hydrothermal or microwave-assisted synthesis to produce sustainable N- and S-doped CQDs with QY up to 83%. For example, ionic liquid-functionalized CQDs from grape skin achieved detection limits of 0.001 nM for Fe³⁺, while CQDs from Jengkol peels reached 30 nM for Hg²⁺, highlighting their environmental benignity and scalability.^[36] As of November 2025, carbohydrate-derived CQDs via green methods show promise in analytical sensing, with enhanced scalability.^[21] These variants maintain high photostability while minimizing synthetic toxicity.^[36]

Characterization Techniques

Structural Characterization

Structural characterization of carbon quantum dots (CQDs) primarily relies on electron microscopy, diffraction, and surface analysis techniques to elucidate their nanoscale morphology, crystallinity, and composition. These methods confirm the quasi-spherical shape, graphitic core, and surface functionalities typical of CQDs, often revealing a core-shell structure with sp²-hybridized carbon domains. Sample preparation is crucial, involving dispersion of CQDs in solvents like water or ethanol and deposition onto supports such as copper grids or mica substrates to minimize aggregation artifacts during imaging.^[37] Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) are essential for visualizing CQD morphology and internal structure. TEM images typically show quasi-spherical particles with diameters of 2-10 nm, while HR-TEM reveals lattice fringes with interplanar spacings of approximately 0.21 nm, corresponding to the (100) plane of graphitic carbon, and sometimes 0.34 nm for the (002) plane, indicating a crystalline core amid amorphous regions. Scanning electron microscopy (SEM) complements these by assessing larger-scale aggregation and surface topography, often showing clustered particles due to van der Waals interactions.^[38]^[30] X-ray diffraction (XRD) provides insights into the bulk crystallinity of CQDs, typically exhibiting a broad diffraction peak at 2θ angles of 20-30°, reflective of a mix of amorphous and crystalline phases with interlayer spacing around 0.34-0.45 nm. Selected area electron diffraction (SAED) patterns obtained during TEM analysis further confirm local crystallinity, displaying diffuse rings or spots consistent with graphitic ordering. These diffraction techniques highlight the disordered yet partially ordered nature of CQD structures.^[39]^[30] Atomic force microscopy (AFM) measures the topographic height and surface roughness of individual CQDs, with height profiles confirming thicknesses below 10 nm, often 1-5 nm, supporting their zero-dimensional character and distinguishing them from thicker graphene sheets. In tapping mode, AFM avoids deformation and provides precise dimensional data.^[40] X-ray photoelectron spectroscopy (XPS) elucidates elemental composition and surface chemistry, with the C 1s spectrum deconvoluting into peaks at 284-285 eV (C-C/C=C), 286 eV (C-O/C-N), and 288 eV (C=O/O-C=O), indicating oxygenated and potentially nitrogen-doped functionalities. O 1s peaks appear around 532 eV for C-O and adsorbed water, while N 1s at ~400 eV confirms doping in modified CQDs, quantifying surface passivation layers.^[39]^[37]

Optical and Spectroscopic Analysis

Ultraviolet-visible (UV-Vis) absorption spectroscopy is a fundamental technique for characterizing the electronic transitions in carbon quantum dots (CQDs). Typically, CQDs exhibit a strong absorption peak around 230 nm attributed to π-π* transitions within the sp²-hybridized carbon core, reflecting the aromatic π-electron system.^[41] A weaker shoulder or peak near 340 nm corresponds to n-π* transitions involving non-bonding electrons from surface heteroatoms such as oxygen or nitrogen in carbonyl or amine groups.^[42] These spectral features provide insights into the core structure and surface functionalization, with absorption extending into the visible range for larger or doped CQDs. Photoluminescence (PL) spectroscopy reveals the emission properties of CQDs, often displaying excitation-dependent behavior due to multiple emissive sites. Excitation-emission matrices (EEMs) generated from PL scans show a broad range of emission wavelengths, typically from blue (around 450 nm) to red (up to 650 nm), shifting with excitation wavelength from 300 to 500 nm; this wavelength-dependent emission arises from size variations, surface states, and defect-related recombination pathways.^[43] Such matrices are essential for mapping the luminescent landscape and optimizing excitation conditions for applications requiring specific emission colors.^[44] Fourier-transform infrared (FTIR) and Raman spectroscopy probe the vibrational modes and carbon lattice defects in CQDs. FTIR spectra commonly feature a sharp peak at approximately 1700 cm⁻¹ assigned to the C=O stretching vibration from surface carbonyl groups, alongside broader bands at 1000-1400 cm⁻¹ for C-O and C-N stretches indicating oxygen- and nitrogen-containing functionalities.^[45] Raman spectra display characteristic D and G bands at around 1350 cm⁻¹ and 1580 cm⁻¹, respectively, where the D band signifies disordered sp³ carbon or defects, and the G band represents graphitic sp² domains; an intensity ratio (I_D/I_G) near 1 indicates moderately defect-rich structures typical of many synthesized CQDs.^[46] Nuclear magnetic resonance (NMR) spectroscopy elucidates the chemical environment of carbon atoms in CQDs. In ¹³C NMR spectra, signals between 120 and 150 ppm are indicative of sp²-hybridized aromatic carbons in the graphitic core, while peaks below 60 ppm correspond to sp³-hybridized aliphatic carbons from surface chains or defects; the relative intensities help quantify the sp²/sp³ ratio, which influences optical properties.^[47] Solid-state ¹³C NMR is particularly useful for insoluble CQDs, revealing dynamic structural heterogeneity.^[48] Advanced time-resolved photoluminescence (TRPL) measures the excited-state dynamics of CQDs, with decay lifetimes typically on the nanosecond scale (1-10 ns), reflecting radiative and non-radiative recombination processes from core states or surface traps.^[49] Multi-exponential fits to TRPL curves often reveal components from 1-5 ns for prompt fluorescence and longer tails up to 20 ns for delayed emission due to triplet states or energy transfer. Electrogenerated chemiluminescence (ECL) studies further probe mechanistic pathways, where CQDs act as luminophores in coreactant systems like peroxydisulfate, involving electron transfer and radical annihilation to produce light; ECL spectra mirror PL but with enhanced intensity from electrochemical excitation.^[50] Recent investigations into fluorescence quenching mechanisms in CQDs distinguish between inner filter effects (IFE), where analyte absorption overlaps with excitation/emission wavelengths, and dynamic quenching via collisional electron transfer or energy transfer.^[51] In 2023-2024 studies, IFE predominates for high-concentration quenchers like heavy metal ions, causing apparent non-linear quenching without altering lifetimes, whereas dynamic quenching shortens decay times through direct interaction with surface states.^[52] Doping can modulate these mechanisms by shifting absorption bands, enhancing selectivity in quenching-based assays.^[53]

Applications

Biomedical Applications

Carbon quantum dots (CQDs) have emerged as promising nanomaterials in biomedical applications due to their excellent biocompatibility, low toxicity, and tunable optical properties, enabling their use in imaging, drug and gene delivery, and theranostics. Their small size (<10 nm) facilitates cellular uptake, while surface modifications enhance targeting specificity. Studies have demonstrated that CQDs exhibit high biocompatibility, with median lethal doses (LD50) exceeding 100 mg/kg in mice, indicating minimal acute toxicity at therapeutic concentrations. However, recent investigations in zebrafish models have revealed potential neurological effects at higher doses, including dopamine level alterations and behavioral changes, underscoring the need for dose-dependent safety assessments.^[1] In bioimaging, CQDs serve as effective fluorescent probes for in vitro and in vivo cell tracking owing to their bright, stable photoluminescence and minimal photobleaching. Folate-conjugated CQDs enable targeted tumor imaging by binding to folate receptors overexpressed on cancer cells, facilitating selective accumulation and enhanced contrast in fluorescence microscopy. A 2024 review highlights advances in multi-modal hybrids combining CQD fluorescence with magnetic resonance imaging (MRI), such as gadolinium-doped CQDs, which provide complementary anatomical and functional information for precise tumor detection.^[1] For drug delivery, CQDs act as pH-responsive carriers, particularly for chemotherapeutic agents like doxorubicin (DOX), exploiting the acidic tumor microenvironment (pH ~5.5-6.5) for controlled release. High encapsulation efficiencies have been achieved in CQD-based nanocarriers, ensuring effective payload delivery while reducing off-target effects. Surface functionalization, such as with polyethyleneimine or lipids, further improves stability and tumor-specific uptake.^[1] Recent advances in gene delivery utilize CQD vectors for siRNA and plasmid transfection, offering low cytotoxicity compared to traditional polyethylenimine (PEI) carriers. PEI-coated CQDs demonstrate efficient nucleic acid complexation and endosomal escape, achieving improved transfection efficiencies in cancer cell lines with high cell viability, outperforming unmodified PEI.^[1] In theranostics, CQDs integrate imaging and therapy, notably in photodynamic therapy (PDT) where they generate reactive oxygen species (ROS) under near-infrared light irradiation to induce cancer cell apoptosis. Doped CQDs, such as nitrogen or phosphorus variants, enhance ROS production while enabling real-time fluorescence monitoring, improving treatment efficacy and reducing systemic toxicity.^[1]

Optoelectronic and Sensing Applications

Carbon quantum dots (CQDs) have emerged as promising materials in optoelectronic devices due to their tunable photoluminescence, high quantum yields, and solution-processability. In light-emitting diodes (LEDs), CQDs serve as efficient emitters, enabling high-performance electroluminescent devices. For instance, doped CQD frameworks have achieved external quantum efficiencies (EQEs) up to 20%, with low efficiency roll-off, attributed to enhanced charge balance and radiative recombination. These advancements position CQDs as alternatives to traditional quantum dots in solid-state lighting, offering cost-effective and environmentally friendly options.^[1] In solar cells, CQDs function as down-converters to broaden light absorption spectra and reduce energy loss from UV photons. When integrated into perovskite solar cells, CQDs convert high-energy UV light to visible wavelengths, improving power conversion efficiency (PCE) by up to 10% in some configurations, as demonstrated in studies using CQD-embedded PMMA films for interfacial light management. Additionally, CQD-based flexible displays benefit from their mechanical robustness and multicolor emission tunability; phenylenediamine-derived CQDs in polyvinyl alcohol matrices enable bendable emissive films with stable photoluminescence under deformation, suitable for wearable electronics.^[1] Turning to sensing applications, CQDs exploit fluorescence quenching mechanisms for detecting environmental analytes with high sensitivity. For heavy metal ions like Hg²⁺, CQDs act as turn-off probes through photoinduced electron transfer or inner filter effects, achieving detection limits as low as 4.2 nM in aqueous media, with linear responses over relevant ranges. This selectivity arises from strong coordination between Hg²⁺ and surface carboxyl groups on CQDs, enabling rapid, on-site monitoring in water samples.^[1] pH sensing with CQDs relies on emission wavelength shifts due to protonation of surface functional groups, providing ratiometric detection. Green-emitting CQDs exhibit a 30 nm bathochromic shift below pH 5.6, allowing qualitative identification of acidic conditions such as acid rain, with robustness against interferences like temperature variations. Electrochemiluminescence (ECL) sensors incorporating CQDs have been developed for analytes like glucose and DNA, leveraging coreactant-enhanced emission for portable diagnostics. Recent reviews highlight CQD-based ECL platforms achieving sub-micromolar detection limits for glucose in point-of-care devices, with 2024 advancements focusing on miniaturized, battery-operated systems for real-time analysis.^[1] In forensic applications, CQDs facilitate latent fingerprint recovery by adhering to ridge residues and emitting under UV excitation. Post-cyanoacrylate fuming, ~2 nm CQDs bind via hydrogen bonding to ester and nitrile groups, producing bright blue fluorescence at 410 nm that reveals fine details like pores, remaining stable even after washing. CQD-polymer composites further enhance these capabilities; for example, CQD-polyvinyl alcohol hybrids improve sensing sensitivity for metal ions like Cu²⁺ to 10 nM limits through amplified quenching and mechanical stability, while in optoelectronics, they boost quantum yields up to 47% for flexible LED prototypes.

Catalytic and Environmental Applications

Carbon quantum dots (CQDs) have emerged as promising metal-free catalysts in photocatalysis, particularly for hydrogen evolution reactions (HER), due to their ability to enhance charge separation and extend light absorption in hybrid systems. In composites with TiO₂, CQDs act as sensitizers, facilitating electron transfer from the conduction band of CQDs to TiO₂, which suppresses recombination and boosts photocatalytic efficiency. For instance, CQD/TiO₂ nanohybrids have achieved enhanced hydrogen evolution rates under visible light, significantly outperforming bare TiO₂ by promoting band alignment and upconversion luminescence. Similarly, in graphitic carbon nitride (g-C₃N₄) hybrids, CQDs introduce built-in electric fields that drive spatial charge separation, enabling selective CO₂ reduction to CO with high selectivity (>90%) under simulated solar irradiation. These mechanisms rely on CQDs' π-conjugated structure mediating electron shuttling in band-aligned heterojunctions, generating reactive oxygen species (ROS) like hydroxyl radicals for enhanced redox processes.^[1] In electrocatalysis, CQDs serve as efficient oxygen reduction reaction (ORR) catalysts in fuel cells, offering a sustainable alternative to platinum-based materials through their high surface area and tunable heteroatom doping. N- and S-doped CQDs exhibit four-electron ORR pathways with onset potentials around 0.9 V vs. RHE and limiting current densities comparable to commercial Pt/C, attributed to active edge sites that lower the energy barrier for O₂ adsorption and reduction. Their metal-free nature ensures biocompatibility and cost-effectiveness, with recyclability demonstrated over 10,000 cycles in alkaline media without significant activity loss. For environmental remediation, CQDs excel in water purification by adsorbing and degrading organic pollutants through π-π interactions and ROS-mediated photocatalysis. CQD-modified membranes or composites remove dyes like methylene blue with efficiencies >95% via electrostatic attraction and hydroxyl radical generation, enabling degradation rates of 0.05 min⁻¹ under UV-visible light. In soil enhancement, CQDs facilitate nutrient delivery by modulating microbial activity and pH, with applications showing up to 30% improvement in plant growth metrics such as biomass yield in nutrient-deficient soils. These benefits stem from CQDs' biocompatibility and ability to chelate essential ions like nitrogen and phosphorus, promoting rhizospheric enzyme activity without toxicity at doses below 100 mg kg⁻¹ soil. Overall, the recyclability and low environmental footprint of CQDs position them as versatile agents for sustainable catalysis and eco-remediation.^[1]

Emerging Applications

Carbon quantum dots (CQDs) have shown promise in energy storage devices, particularly as electrode materials in supercapacitors. Recent advancements demonstrate that CQD-modified electrodes can achieve specific capacitances exceeding 300 F/g, attributed to their high surface area and enhanced conductivity, enabling superior charge storage and cycling stability.^[54] For instance, CQD-based electrodes have demonstrated over 90% retention after thousands of cycles.^[55] In battery applications, CQDs serve as anodes or additives in sodium-ion and lithium-ion systems, improving ion transport and structural integrity to deliver high reversible capacities and rate performance.^[56]^[57] In agriculture, CQDs are emerging as nano-fertilizers that enable controlled nutrient release, enhancing soil health and plant stress tolerance. A 2025 review highlights their role in mitigating abiotic stressors like drought and salinity by improving water retention and nutrient uptake in soil, leading to increased crop yields without excessive chemical inputs.^[58] Biobased CQDs, derived from sustainable sources, further support this by promoting root growth and photosynthesis efficiency in various plants.^[59] For food safety, CQDs facilitate sensitive detection of pesticides and mycotoxins through fluorescent biosensors, offering low detection limits and rapid response times compared to traditional methods.^[60]^[61] CQD composites have been developed for on-site monitoring of contaminants like organophosphates and aflatoxins in agricultural products.^[31] Additionally, CQD-infused coatings exhibit antibacterial properties, inhibiting pathogens such as Escherichia coli and Staphylococcus aureus on food packaging surfaces, thereby extending shelf life.^[62] Beyond these, CQDs are utilized in anti-counterfeiting inks due to their tunable fluorescence under UV light, enabling secure patterns invisible to the naked eye for document protection and product authentication.^[63]^[64] As dopants in 3D printing, CQDs enhance material properties like mechanical strength and bioactivity in printed structures, such as bactericidal soft robotics.^[65] In 2025, carbohydrate-derived CQDs have advanced sustainable electronics by serving as eco-friendly emitters in flexible devices, reducing reliance on rare-earth materials.^[36] Looking ahead, the integration of CQDs with artificial intelligence holds potential for designing smart materials that adapt to environmental stimuli, such as self-healing composites optimized via machine learning algorithms.^[66] However, scalability remains a key challenge, including uniform synthesis at industrial volumes and maintaining performance in hybrid systems for widespread adoption.^[67]

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