Carbon Dots (CDs)

Updated 25 August 2025

Carbon Dots (CDs) are quasi-spherical, nanoscale carbon particles with tunable photoluminescence and diverse surface functionalities.
They are synthesized using methods like pyrolysis, electrochemical etching, sol–gel entrapment, and hydrothermal processes to control size and surface chemistry.
Their unique optical properties and robust processability enable applications in photonics, sensing, bioimaging, catalysis, and electronic devices.

Carbon dots (CDs) are quasi-spherical, nanoscale carbon-based particles exhibiting strong and tunable photoluminescence. With typical diameters ranging from approximately 2 nm to 8 nm—dependent on synthesis route and precursor—they are composed of amorphous or crystalline cores based on sp² and sp³ hybridized carbon, frequently functionalized with a variety of oxygen, nitrogen, or other groups at the surface. Their unique combination of chemical versatility, photostability, quantum confinement, and robust processability has positioned them as subjects of intense investigation for applications spanning optoelectronics, sensing, photocatalysis, bioimaging, and next-generation electronics.

1. Synthetic Methodologies and Structural Features

CDs are fabricated via several top-down and bottom-up approaches, each imparting distinct structural characteristics and surface chemistry. Widely adopted protocols include:

Solvent-assisted pyrolysis (e.g., thermal decomposition of citric acid with urea (Tian et al., 2019), or citric acid alone for carboxyl-rich CCQDs (Hao et al., 2018)), yielding CDs with mixed sp²/sp³ character and high density of surface carboxylate and hydroxyl groups.
Electrochemical etching of graphite rods in an alkaline alcoholic medium (50 mL ethanol/NaOH) under controlled current (30–45 mA), producing CQDs with size-dependent optical properties; addition of surfactant such as sodium dodecyl sulfate (SDS) caps growth and allows precise control over dot diameter (Gaurav et al., 2020).
Direct electrooxidation of ethanol on Ni foam (1 M NaOH, 30 V, 60 min) producing highly crystalline CQDs (~2.8 nm) with controlled domain sizes; subsequent post-synthesis aging modifies both absorption and emission characteristics (Barrionuevo et al., 2023).
Sol–gel entrapment into silica matrices, where CDs are directly introduced into tetraethyl orthosilicate (TEOS)/F127 copolymer hybrid films under acidic conditions and subsequently heat-treated to yield free-standing, mesoporous structures (mean CD size ~3 nm) (Vassilakopoulou et al., 2016).
Hydrothermal methods using waste-derived carbon sources (e.g., cork industry wastewater), yielding C-dot/titanate nanotube hybrids (Alves et al., 2022).
Pyrolysis or combustion of natural materials (e.g., coal burning dust (Zhao et al., 3 Nov 2024) or cigarette smoke (Li et al., 2022)), which generates CDs through self-assembly of PAHs or bond cleavage under high temperatures.

Table 1: Common Synthesis Routes and CD Structural Features

Method	Typical Core Structure	Common Surface Groups
Thermal Decomposition	sp²/sp³	–COOH, –OH, –NH₂, –CONH₂
Electrochemical Etching	sp²-dominated	–COOH, –OH, –CO, –OSO₃⁻
Direct Electrooxidation (EtOH)	sp² segments, O-functional	epoxide, carbonyl, hydroxyl
Sol–gel Entrapment	sp² core in silica cage	–CONH₂, carboxyl, polymeric
Hydrothermal (Wastewater)	sp²/sp³, TNT hybrid	Ti–O–C linkage, amines

Surface chemical states are routinely characterized by XPS (e.g., C 1s peaks at 284.6, 286.4, 288.8 eV), FTIR (CO, OH stretches), TEM (core lattice spacing, typically 0.21 nm for graphene-like structures), and UV–vis/PL spectroscopy.

2. Optical Properties and Photoluminescence Mechanisms

CDs display prominent excitation-dependent photoluminescence (PL), with emission maxima and quantum yield modulated by core size, domain structure, and surface functionalization. Representative behaviors include:

Excitation-dependent PL: Wide emission bands arising from interplay between sp² core states and sp³ or heteroatom-enriched surface states (Maiti et al., 2018).
PL peak shifts: Thermal or chemical treatments induce blue- or red-shifts in emission (e.g., silica encapsulation/thermolysis causes a blue shift from 485 to 426 nm, indicating amide-to-ketone/carboxylate conversion (Vassilakopoulou et al., 2016); sodium borohydride reduction induces a 7 nm red-shift and >7-fold intensity enhancement via carbonyl-to-hydroxyl transformation and surface Na incorporation (Li et al., 2022)).
Quantum yield: Ranges from ~3.6% (coal dust-derived CDs (Zhao et al., 3 Nov 2024)) to 50% (CDs embedded in silica/F127 (Vassilakopoulou et al., 2016)).
Up- and down-conversion PL: CDs exhibit both down-conversion (UV to visible) and up-conversion (two-photon absorption under red excitation) fluorescence; the latter is confirmed by quadratic scaling of emission intensity with excitation power, as in coal burning dust-derived CDs (Zhao et al., 3 Nov 2024), and is leveraged in two-photon bioimaging (Barhum et al., 2023).

Table 2: PL Characteristics in Representative Systems

Sample Context	PL Max (nm)	Quantum Yield (%)	Emission Shift (nm)
CDs aqueous solution	~485	40	–
Silica/F127–CD hybrid	466	50	Blue shift (–19 nm)
Reduced cigarette smoke CDs	463	8.86	Red shift (+7 nm)
Coal dust-derived	410	3.6	Strong up- and down-conversion
Titanate nanotube hybrid	varies	–	Redshift with C-dot content

3. Structural, Chemical, and Environmental Control of Spectroscopic Properties

DFT/TD-DFT studies reveal the spectroscopic signature is highly sensitive to chemical composition and conformational changes in multi-layered, multi-defect CDs (Mehmood et al., 22 Aug 2025):

Oxidizing groups (carbonyl, acetate, p-dipyran): Impose pronounced redshifts by stabilizing excited states and introducing charge-transfer character, e.g. carbonyl acetate yields ΔE ≈ –0.69 eV. Such groups act “dominantly” versus spectators like hydroxyl, which exert minimal direct effect unless strongly coupled.
Conformational dynamics: Motions such as twisting (70° dipole reorientation, 0.27 eV shift), sliding (≈0.07 eV shift, ≈50% oscillator strength modulation), and linker-mediated folding (0.12 eV shift, ≈27° dipole angle change) alter excitation energies and PL emission.
pH and protonation: Surface group protonation states (e.g., carboxylate versus carboxylic acid) result in marked emission energy changes (e.g., up to 1.62 eV drop for carboxylate vs. protonated acid), and n→π* character emerges upon deprotonation, yielding strong environmental responsiveness.

Such fine-tuning mechanisms are responsible for blinking and polarization fluctuations in single-particle emission, offering design levers for application-driven spectral tailoring.

4. Functional Applications: Photonics, Sensing, and Catalysis

CDs provide advanced materials solutions for:

Photonics: Silica/F127–CD hybrid films demonstrate high transparency (91.8% transmittance, 250 μm thickness), robust PL (maintained post-550℃ anneal), and ultra-low shrinkage (2%; 1 μm features replicated as 0.98 μm) suitable for soft lithography, photonic device patterning, and optical sensor platforms (Vassilakopoulou et al., 2016).
UV photodetectors: CND/n-Si heterojunctions yield broadband UV response (peak responsivity 1.25 A/W at 300 nm, rectification ∼5×10³, dark current ~500 pA), offering CMOS-compatible, room-temperature deep UV detection with low cost and high scalability (Maiti et al., 2018).
Scale inhibition: CCQDs synthesized from citric acid exhibit >95% inhibition of CaSO₄ scaling at dosages as low as 25 mg/L (40°C) and >99% at higher doses (200 mg/L, 80°C); inhibition is realized via carboxylate chelation and lattice distortion, with high industrial applicability and environmental safety (Hao et al., 2018).
Strain sensing: CD/polyurethane composites exhibit strain-responsive PL (intensity increases 1.4–1.8× for 0–250% strain) via an 'anisotropic anti-quenching' mechanism, enabling large-range, visually detectable strain mapping for health monitoring and wearables (Tian et al., 2019).
Photocatalysis: C-dot/TNT hybrid materials (via hydrothermal embedding) shift absorption edges redward (from 3.50 ± 0.07 eV to 2.96 ± 0.03 eV for increasing C-dot content), minimize electron–hole recombination, and accelerate pollutant degradation (e.g., caffeine: k_ap = 23.0×10⁻³ min⁻¹; t₁/₂ = 30.2 min, complete removal in 120 min) by hydroxyl radical and hole-mediated pathways (Alves et al., 2022).
Bio-imaging and theragnostics: CDs from cigarette smoke (after NaBH₄ reduction) offer strong, biocompatible blue emission (QY = 8.86%), with cellular/nuclear targeting in onion and BEAS-2B cells (Li et al., 2022). Phenylenediamine-based CDs display nonlinear multiphoton absorption (peak cross-section ≈50 GM, NIR excitation 775–895 nm), facilitating two-photon in vivo brain imaging and drug tracking via vaterite nanoparticle cargoes (Barhum et al., 2023).

5. Quantum Mechanical Modeling, Transport, and Environmental Interactions

Recent theoretical frameworks model fluorescence lifetimes in pH sensing as weighted averages over protonated/deprotonated species (Dilshener et al., 8 May 2024):

Lifetime pH dependence: $\Gamma(p) = [\Gamma_{HD} + \Gamma_D \cdot 10^{p-pK_a} ] / [1 + 10^{p-pK_a}]$ for transition rates; maximal sensitivity when $| \Gamma_{HD} - \Gamma_D |$ is large and $pK_a \approx pH$ .
Environmental (Purcell) effect: Transition rates modulated by the Green function $G(r,r',\omega)$ and local dielectric environment, incorporating reflection coefficients and local-field corrections.
Transport effects: CQD-based electronic devices show nonlinear current–voltage characteristics (Schottky diode, Coulomb blockade, negative differential conductance) due to quantum confinement and strong charging energy, captured by master equation modeling including dot/trap state interactions and tunneling rates (Copeland et al., 26 May 2025).

Such quantum mechanical modeling provides rigorous guidelines for device optimization and fundamental understanding of nanoscale optoelectronic and sensing phenomena.

6. Environmental and Sustainability Implications

CDs produced from industrial, natural, or waste sources (e.g., coal combustion dust, cigarette smoke, cork wastewater) present unique opportunities for:

Pollution mitigation: Extraction from coal combustion dust not only prevents environmental dispersal of potentially harmful nanoscale particulates, but also enables mass production of functional nanomaterials (Zhao et al., 3 Nov 2024).
Green chemistry: Low-cost, phosphorus-free syntheses (e.g., from citric acid (Hao et al., 2018)) reduce ecological impact while maintaining high biocompatibility.

These strategies position CDs as scalable, low-toxicity materials for green technologies in optical sensing, catalysis, and environmental remediation.

7. Future Directions and Research Opportunities

Open research frontiers and technological possibilities include:

Molecular and mesoscale tailoring: Controlling functional group type, protonation state, and interlayer dynamics (twisting, sliding, folding) for tunable PL, emission intermittency, and polarization (blinking/polarization fluctuations) (Mehmood et al., 22 Aug 2025).
Integration in bio- and optoelectronic devices: Expanding multiphoton imaging modalities, optimizing electronic device architectures for room-temperature quantum phenomena, and engineering hybrid systems (metal oxides, polymers) to exploit charge-transfer and spectral tunability.
Scalable environmental deployment: Valorization of waste (coal dust, industrial effluents) as CD sources for catalyst and sensor manufacturing.
Advanced quantum models: Development and validation of theoretical frameworks linking environmental modification, quantum confinement, and device-level function, including complex pH sensing and electronic transport.

As theoretical, computational, and experimental insights converge, CDs will continue to offer a platform for foundational advances and cross-disciplinary innovations in materials science, chemistry, engineering, photonics, and environmental technology.