Sulfur Doped Carbon Quantum Dots: Synthesis, Characterization, And Advanced Applications In Sensing And Bioimaging

Molecular Structure And Doping Mechanisms Of Sulfur Doped Carbon Quantum Dots

The structural characteristics of sulfur doped carbon quantum dots are fundamentally defined by the integration of sulfur heteroatoms into the sp²-hybridized carbon lattice. Transmission electron microscopy (TEM) analysis reveals that S-CQDs typically exhibit spherical morphology with diameters ranging from 4.5 to 8.5 nm, displaying high crystallinity with lattice parameters of approximately 0.21 nm1. X-ray photoelectron spectroscopy (XPS) characterization demonstrates that sulfur incorporation occurs primarily through thiophenic sulfur bonding (characteristic peaks between 162-166 eV in S2p spectra), which provides enhanced electron delocalization and improved photoluminescent properties19.

The doping mechanism involves several key structural modifications. First, sulfur atoms substitute carbon atoms within the graphitic domains, creating electron-rich sites that modify the electronic band structure2. Second, sulfur-containing functional groups (such as sulfonic acid, thiol, and sulfoxide moieties) decorate the quantum dot surface, enhancing water solubility and providing reactive sites for further functionalization3. Third, the introduction of sulfur increases the density of surface states and quantum confinement effects, leading to enhanced fluorescence quantum yields that can reach 32.96% in optimized synthesis conditions4.

X-ray diffraction (XRD) patterns of S-CQDs typically show broad peaks corresponding to the (002) plane of graphitic carbon, with slight shifts attributable to lattice distortion induced by sulfur incorporation3. Fourier-transform infrared spectroscopy (FTIR) reveals characteristic absorption bands at 1050-1150 cm⁻¹ (C-S stretching), 1620-1680 cm⁻¹ (C=O stretching from carboxyl groups), and 3200-3500 cm⁻¹ (O-H and N-H stretching from surface functional groups)3. These structural features collectively contribute to the unique optical and chemical properties that distinguish S-CQDs from pristine carbon quantum dots.

Synthesis Methodologies For Sulfur Doped Carbon Quantum Dots

Hydrothermal And Solvothermal Synthesis Routes

Hydrothermal synthesis represents the most widely adopted method for producing sulfur doped carbon quantum dots due to its simplicity, cost-effectiveness, and ability to control particle size and doping level3. The typical procedure involves dissolving carbon precursors (such as citric acid, glucose, or jaggery) and sulfur sources (commonly sulfuric acid, thiourea, or cysteine) in deionized water, followed by heating in an autoclave at temperatures ranging from 160°C to 240°C for 4 to 12 hours318. The mass ratio of carbon source to sulfur source critically influences the final sulfur content, with optimal ratios typically between 1:0.1 and 1:0.5 to achieve sulfur incorporation of 0.5-6 wt%19.

A representative synthesis protocol reported in patent literature describes mixing 5 grams of jaggery with 5-10 ml of sulfuric acid, followed by hydrothermal treatment at 180°C for 6 hours3. The resulting solution is diluted, centrifuged at 8000 rpm for 20 minutes, and filtered through 0.22 μm syringe filters to obtain pale yellow to light greenish S-CQDs with fluorescence quantum yields exceeding 25%3. The hydrothermal conditions promote carbonization of organic precursors while simultaneously facilitating sulfur incorporation through nucleophilic substitution and condensation reactions.

Solvothermal methods employ organic solvents instead of water, enabling synthesis at higher temperatures and providing better control over surface chemistry2. For instance, using N,N-dimethylformamide (DMF) or ethanol as solvents with sulfur-containing precursors at 200-250°C yields S-CQDs with enhanced solvatochromic properties—exhibiting different fluorescence colors depending on the solvent polarity2. This characteristic makes solvothermally synthesized S-CQDs particularly valuable for organic solvent detection and environmental monitoring applications.

Microwave-Assisted And Pyrolytic Synthesis Approaches

Microwave-assisted synthesis offers significant advantages in terms of reaction time and energy efficiency compared to conventional heating methods34. The process typically involves mixing carbon and sulfur precursors in aqueous solution, then subjecting the mixture to microwave irradiation at 600-1200 W for 30 seconds to 5 minutes314. The rapid and uniform heating provided by microwave energy accelerates carbonization and doping processes, yielding S-CQDs with narrow size distributions and high fluorescence intensities.

Patent US20140116867A1 describes a microwave-assisted method where jaggery and sulfuric acid are heated at 800-1200 W for 30 seconds, producing highly fluorescent S-CQDs with diameters of 5-9 nm and quantum yields approaching 30%3. The short reaction time minimizes thermal degradation of fluorescent centers while maximizing sulfur incorporation efficiency. Post-synthesis purification involves dialysis or column chromatography to remove unreacted precursors and low-molecular-weight byproducts.

Pyrolytic synthesis methods involve thermal decomposition of sulfur-containing organic precursors at elevated temperatures (250-900°C) in inert atmospheres14. For example, heating poultry feather waste (naturally rich in sulfur-containing amino acids like cysteine and methionine) at 350-550°C under nitrogen flow, followed by microwave treatment, produces S-CQDs with excellent fluorescence stability and metal ion sensing capabilities14. The high-temperature treatment ensures complete carbonization and formation of stable C-S bonds, while the subsequent microwave step enhances surface passivation and quantum confinement effects.

Co-Doping Strategies With Nitrogen And Other Heteroatoms

Co-doping sulfur with nitrogen represents a powerful strategy to synergistically enhance the optical and electronic properties of carbon quantum dots24. Nitrogen-sulfur co-doped CQDs (N,S-CQDs) exhibit superior fluorescence quantum yields (often exceeding 40%) compared to single-doped variants due to the complementary effects of electron-donating nitrogen and electron-withdrawing sulfur atoms24. The synthesis typically employs dual precursors: nitrogen sources such as urea, ethylenediamine, or amino acids combined with sulfur sources like thiourea or sulfuric acid24.

Patent KR20230138479A describes a method for synthesizing N,S-CQDs by mixing citric acid (carbon source), cysteine (providing both nitrogen and sulfur), and additional thiourea (supplementary sulfur source) in a 1:1:0.5 mass ratio, followed by hydrothermal treatment at 180°C for 8 hours2. The resulting N,S-CQDs display bright blue-green fluorescence with quantum yields of 38-45% and exhibit excellent photostability under continuous UV irradiation for over 24 hours2. XPS analysis confirms nitrogen content of 8-12 wt% (primarily pyridinic and pyrrolic nitrogen) and sulfur content of 3-6 wt% (predominantly thiophenic sulfur)2.

Triple-doping with boron, nitrogen, and sulfur has been explored to further optimize electronic structure and catalytic activity419. Patent IDA202102050A describes synthesizing B,N,S-tri-doped CQDs from citric acid, boric acid, urea, and thiourea via microwave-assisted pyrolysis, achieving quantum yields up to 32.96% with enhanced reactive oxygen species generation for photocatalytic applications4. The boron incorporation (0.5-2 wt%) introduces additional electron-deficient sites that complement the electron-rich nitrogen centers, creating efficient charge separation pathways beneficial for sensing and catalysis419.

Optical Properties And Photoluminescence Mechanisms Of Sulfur Doped Carbon Quantum Dots

Excitation-Dependent And Independent Emission Behaviors

Sulfur doped carbon quantum dots exhibit complex photoluminescence behaviors that can be categorized into excitation-dependent and excitation-independent emission patterns12. Excitation-dependent emission, where the emission wavelength shifts with changing excitation wavelength, arises from the heterogeneous distribution of particle sizes and surface states within the S-CQD population3. This property enables multicolor emission from a single S-CQD sample: excitation at 340 nm typically produces blue emission (450-480 nm), 380 nm excitation yields cyan emission (490-510 nm), and 420 nm excitation generates green emission (520-550 nm)23.

In contrast, excitation-independent emission—where the emission peak position remains constant regardless of excitation wavelength—indicates a more uniform electronic structure and is generally associated with higher crystallinity and narrower size distributions14. Silicon-doped carbon quantum dots (Si-CQDs) prepared via photochemical synthesis exhibit excitation-independent bright blue-green fluorescence centered at 510 nm with full-width-half-maximum (FWHM) of only 45 nm, attributed to their highly regular spherical structure and uniform lattice parameters of 0.21 nm1. Similar excitation-independent behavior has been observed in optimally synthesized S-CQDs with narrow size distributions (standard deviation <0.8 nm)3.

The photoluminescence quantum yield (PLQY) of S-CQDs varies significantly depending on synthesis conditions, precursor selection, and doping concentration. Reported PLQY values range from 15% to 45%, with the highest values achieved through careful optimization of sulfur content (typically 3-5 wt%), surface passivation, and co-doping strategies234. For comparison, undoped CQDs typically exhibit PLQY of 5-20%, demonstrating the beneficial effect of sulfur incorporation3. The enhanced quantum yield results from sulfur-induced modifications to the electronic band structure, increased radiative recombination rates, and reduced non-radiative decay pathways2.

Mechanisms Of Fluorescence Enhancement Through Sulfur Doping

The fluorescence enhancement mechanism in sulfur doped carbon quantum dots involves multiple synergistic effects at the molecular and electronic levels. First, sulfur atoms introduce mid-gap states within the bandgap of the carbon framework, creating additional radiative recombination pathways that increase fluorescence intensity23. Density functional theory (DFT) calculations reveal that thiophenic sulfur incorporation reduces the HOMO-LUMO gap by 0.3-0.5 eV compared to pristine CQDs, facilitating visible-light absorption and emission2.

Second, sulfur-containing surface functional groups (such as sulfonic acid and thiol groups) enhance surface passivation, reducing non-radiative recombination centers caused by dangling bonds and defects3. Time-resolved photoluminescence spectroscopy shows that S-CQDs exhibit longer fluorescence lifetimes (average 8-12 ns) compared to undoped CQDs (4-7 ns), indicating suppressed non-radiative decay processes23. The increased lifetime also suggests that sulfur doping promotes the formation of triplet excited states, which can participate in delayed fluorescence mechanisms.

Third, the electron-withdrawing nature of sulfur atoms creates localized charge redistribution within the carbon lattice, enhancing π-electron delocalization and oscillator strength for electronic transitions2. This effect is particularly pronounced in N,S-co-doped systems, where the push-pull electronic interaction between electron-donating nitrogen and electron-withdrawing sulfur creates intramolecular charge-transfer states that exhibit strong fluorescence with large Stokes shifts (80-120 nm)24. The large Stokes shift minimizes self-absorption and improves signal-to-noise ratios in sensing and imaging applications.

Photostability And Environmental Stability Characteristics

Sulfur doped carbon quantum dots demonstrate exceptional photostability compared to organic dyes and conventional semiconductor quantum dots12. Continuous UV irradiation (365 nm, 100 mW/cm²) for 24 hours results in less than 10% fluorescence intensity decrease for optimally synthesized S-CQDs, whereas organic fluorophores like fluorescein and rhodamine B lose 60-80% of their initial intensity under identical conditions13. This superior photostability stems from the robust carbon framework and the absence of photobleaching-prone chromophores found in organic dyes.

Chemical stability tests reveal that S-CQDs maintain stable fluorescence across a wide pH range (pH 4-12), with optimal emission intensity observed at neutral to slightly alkaline conditions (pH 7-9)34. At extreme acidic conditions (pH <3), protonation of surface carboxyl and amino groups can cause fluorescence quenching through aggregation-induced non-radiative decay3. Conversely, at highly alkaline conditions (pH >12), deprotonation and increased electrostatic repulsion enhance colloidal stability but may slightly reduce quantum yield due to changes in surface state energetics3.

Ionic strength effects have been systematically investigated, showing that S-CQDs remain stable in salt concentrations up to 0.5 M NaCl, with fluorescence intensity variations less than 15%4. However, the presence of heavy metal ions (such as Fe³⁺, Cu²⁺, and Hg²⁺) can cause significant fluorescence quenching through electron or energy transfer mechanisms, forming the basis for metal ion sensing applications314. Temperature stability studies demonstrate that S-CQDs retain over 85% of their initial fluorescence intensity after heating at 80°C for 2 hours, and show no significant degradation after storage at room temperature for 6 months13.

Applications Of Sulfur Doped Carbon Quantum Dots In Fluorescent Sensing And Detection

Metal Ion Detection And Environmental Monitoring

Sulfur doped carbon quantum dots have emerged as highly sensitive and selective fluorescent probes for detecting heavy metal ions in aqueous environments314. The detection mechanism primarily involves fluorescence quenching through static or dynamic processes when target metal ions interact with sulfur-containing functional groups on the CQD surface3. For mercury (Hg²⁺) detection, S-CQDs exhibit exceptional selectivity due to the strong affinity between mercury and sulfur atoms, forming stable Hg-S coordination complexes that facilitate non-radiative electron transfer and fluorescence quenching14.

Patent INA220005695A describes S-CQDs synthesized from poultry feather waste that demonstrate a linear detection range for Hg²⁺ from 0.1 to 100 μM with a detection limit of 0.05 μM (10 ppb)14. The Stern-Volmer quenching constant (Ksv) for Hg²⁺ reaches 1.2 × 10⁵ M⁻¹, significantly higher than for other metal ions (Fe³⁺: 3.5 × 10³ M⁻¹; Cu²⁺: 5.8 × 10³ M⁻¹), confirming excellent selectivity14. The detection process is rapid (response time <5 minutes) and can be performed in real water samples with minimal matrix interference after simple filtration14.

For multi-ion detection capabilities, S-CQDs synthesized from jaggery and sulfuric acid show simultaneous sensing of Fe³⁺, Cu²⁺, and Cr⁶⁺ with detection limits of 0.8 μM, 1.2 μM, and 2.5 μM respectively3. The differential quenching responses enable semi-quantitative discrimination of mixed metal ion solutions through fluorescence intensity ratio analysis3. XPS characterization confirms that metal ion binding occurs primarily through coordination with surface sulfonic acid groups (-SO₃H) and thiol groups (-SH), with binding constants ranging from 10⁴ to 10⁶ M⁻¹ depending on the metal ion3.

Organic Pollutant And Pharmaceutical Detection

Beyond metal ions, sulfur doped carbon quantum dots serve as effective sensors for organic pollutants and pharmaceutical compounds14. The detection of 4-nitrophenol (4-N