Sulfur Co-doped Carbon Quantum Dots Research Articles

Developing a fluorometric aptasensor based on carbon quantum dots and silver nanoparticles for the detection of adenosine

A new simple fluorometric aptasensor was developed for the determination of adenosine (AD) in human urine samples. A mixture of new and green nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) as fluorescents and silver nanoparticles (AgNPs) as quenchers were utilized for aptasensing. The binding of AD to anti-AD aptamer (AP) provided greater protection for unmodified AgNPs against salt-induced aggregation than AP alone. In this case, the fluorescence signal of N,S-CQDs was quenched by the dispersed AgNPs based on the inner filter effect. In the absence of AD, AgNPs tended to aggregate in a high salt solution. Hence, the fluorescence spectrum of N,S-CQDs no longer overlapped with the absorption spectrum of the aggregated AgNPs, and consequently N,S-CQDs did not quench. The linear range of the method was in the range of 50–200 nM, and the limit of detection was 13 nM. The relative standard deviations ranged from 5.2 to 7.3% (n = 6). In addition, the method had high selectivity toward AD in the presence of other interferences. The applicability of the method was examined by the quantitation of AD in urine samples of two patients through the standard addition method with recoveries in the range of 89–93%.

N, S Co-Doped Carbon Quantum Dots for the Selective and Sensitive Fluorescent Determination of N-Acetyl-l-Cysteine in Pharmaceutical Products and Urine

To design a probe with “turn-on” sensing, nitrogen and sulfur co-doped carbon quantum dots (N, S-CQDs) were prepared and screened against some metallic cations to first induce “turn-off” fluorescence. The ferric iron (Fe3+) was shown to be the most responsive and effective in the fluorescence quenching of the N, S-CQDs based on a proposed photo-induced electron transfer mechanism. In addition, the fluorescence of N, S-CQDs-Fe3+ system was well recovered using N-acetyl-l-cysteine (NAC) (turn-on) due to a redox reaction, suggesting that the N, S-CQDs-Fe3+ system acts as a highly sensitive and selective sensor for the determination of N-acetyl-l-cysteine with a low limit of detection equal to 65.0 nmol/L and wide linear ranges from 0.67 to 25.56 and 25.56 to 193.55 μmol/L. The “turn-off/on” fluorescence method was successfully employed to monitor N-acetyl-l-cysteine in pharmaceutical products and human urine samples with a recovery range from 99.2 to 101.3%. In addition, the fluorescence switch properties of the nitrogen and sulfur co-doped carbon quantum dots were also investigated by alternate additions of Fe3+ and N-acetyl-l-cysteine.

A multifunctional sensor for selective and sensitive detection of vitamin B12 and tartrazine by Förster resonance energy transfer.

We used thiamine nitrate (TN) as single material to fabricate nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) with a quantum yield of 10.4% through one-pot hydrothermal method, and its properties were characterized by TEM, XPS, FTIR, fluorescence (FL) and UV-vis spectrophotometer, respectively. The fluorescence of N,S-CQDs was effectively quenched in the presence of vitamin B12 (VB12)/tartrazine due to Förster resonance energy transfer (FRET). Moreover, the rate (KT) and efficiency (E%) of energy transfer from N,S-CQDs (as a donor) to VB12/tartrazine (as an acceptor) enhanced with increasing the concentrations of acceptor, and the KT and E% were also varied with the change of excitation wavelengths (from 338 to 408 nm). Based on this principle, a multifunctional fluorescence probe was designed for selective and sensitive detection of VB12/tartrazine with a detection limit (3σ/slope) of 15.6/18.0 nmol/L. Meanwhile, the proposed method was successfully employed to detect VB12/tartrazine in milk and several beverages with a recovery range of 97.5-104.2%/91.0-110.6%.

A green one-pot synthesis of nitrogen and sulfur co-doped carbon quantum dots for sensitive and selective detection of cephalexin

The article reports a simple, economic, and green method for preparing water-soluble, nitrogen and sulfur co-doped carbon quantum dots via a one-step hydrothermal method. Pomegranate juice served as the carbon source, and the L-cysteine provided nitrogen and sulfur. Co-doped carbon dots were characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy techniques. The co-doped carbon dots served as fluorescent probes for sensitive and selective detection of cephalexin. Briefly, the co-doped carbon dot systems showed quenching of photoluminescence intensity in the presence of cephalexin. The decrease of fluorescence intensity made it possible to analyze cephalexin with satisfactory detection limits and linear ranges. The Sterne–Volmer plot showed a linear relationship (R2 = 0.998) between F0/F and the concentration of cephalexin over the range from 0.3 to 10 μmol L−1. The limit of detection (LOD) was estimated to be 1 × 10−7 mol L−1 (at a signal to noise ratio of 3). To validate the applicability, the described method was successfully applied for the detection of cephalexin in human urine and raw milk samples.

Nitrogen and sulfur co-doped carbon quantum dots for highly selective and sensitive fluorescent detection of Fe(III) ions and L-cysteine

Nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) with a high quantum yield (69%) and excellent photoluminescent properties were synthesized via a facile hydrothermal method using citric acid and cysteine as precursors. The cytotoxicity of N,S-CQDs was evaluated through the MTT assays using HepG2 cells as the target. The cell viability is still >95% after a 24 h incubation with particles in a concentration of up to 300 μg·mL−1, thereby indicating low cytotoxicity and excellent biocompatibility of N,S-CQDs. The emission spectra of the N,S-CQDs are nearly independent of the excitation wavelengths in the range from 280 to 380 nm and have the strongest fluorescence emission centered at 415 nm. Co-doping with N and S promotes the electron-transfer rate and coordination interaction between N,S-CQDs and Fe(III) ions which acts as a quencher of fluorescence. Fluorescence can be recovered, however, by addition of L-Cys. Hence, the N,S-CQDs act as a highly sensitive and selective “turn-off-on” probe for the determination of Fe(III) or Cys. The limits of detection are 14 nM for Fe(III) and 0.54 μM for Cys. The interferences by 17 common metal ions and heavy metal ions, 12 other quenchers and 16 amino acids were investigated and found to be tolerable. The probe was successfully applied for cellular imaging of Fe(III) in HepG2 cells by using fluorescence microscopy.

Nitrogen and sulfur co-doped chiral carbon quantum dots with independent photoluminescence and chirality

Chiral CQDs exhibit strong PL and outstanding enantioselective electrochemical recognition ability. Moreover, chiral CQDs possess independent PL and chiral properties.