Determination of thiourea based on the reversion of fluorescence quenching of nitrogen doped carbon dots by Hg2

Highly dispersive PEI-modified CDs@ZIF-L dual-emitting fluorescent sensor for detecting metal ions

The leaf-like zeolitic imidazolate framework (ZIF-L) is a promising porous nanomaterial that has attracted increasing attention as an ideal host material to encapsulate functional fluorescent nanoparticles for designing fluorescent sensors. However, owing to the large particle size, gravity readily facilitates the precipitation of the ZIF-L from the aqueous solution, and thus lead to imperfect experimental results. Herein, we report a simple and rapid synthetic method which uses the polyethyleneimine (PEI)-modified ZIF-L as a host to solve the precipitation problem and construct a dual-emitting system that combines its fluorescence with carbon dots (CDs). Furthermore, CDs@ZIF-L/PEI with dual-emitting centres could be utilised as a ratio fluorescence sensor to detect Hg2+ ions. The sensor exhibited excellent dispersibility and good selectivity for sensing Hg2+ ions, with a limit of detection (LOD) of 14.5 nM. Furthermore, experimental results reveal that the CDs@ZIF-L/PEI fluorescent sensor could be effectively dispersed into agarose and less polar organic solvents such as DMF, MeOH, EtOH and CH3CN, expanding the application scope of the fluorescent sensor.

A novel and simple naphthol azo dye chemosensor as a naked eye detection tool for highly selective, sensitive and accurate determination of thiourea in tap water, juices and fruit skins

In the present study, a highly accurate and sensitive azo-dye-based colorimetric sensor based on Eriochrome Black T (EBT) was proposed to detect and determine thiourea (TU). TU is truly an important toxic and carcinogenic hazardous pollutant as approved by EPA and IARC. This chemosensor shows a distinct color change from blue to pink during interaction with TU in aqueous medium. So EBT is capable as an applied tool for naked eye detection of TU as its color change is easily observed without any means. The sensing mechanism was also investigated using UV-vis absorption and FT-IR spectra. The linear range and the detection limit of TU sensing were respectively 0.15-18.5 μmol/L and 0.02 μmol/L. In addition, the relative standard deviation (RSD) based on ten repetitions calculated for two different TU concentrations 4.4 and 9.0 μmol/L were 2.3 % and 1.8 %, respectively. Besides its useful application as a naked eye detection tool, the advantages of the developed method include simplicity, elimination of tedious separation and pre-concentration steps, executable in neutral aqueous media, low costs, high accuracy, linear response for wide range of concentrations, low detection limit, high sensitivity, compatibility, and excellent selectivity. The concentration of TU in tap water, fruit juices or fruit skin samples can be visually detected and determined easily using this method. The results showed that EBT is an ideal colorimetric chemosensor for TU, which has been reported for the first time.

A fluorescent probe based on FRET effect between carbon nanodots and gold nanoparticles for sensitive detection of thiourea

Illegal abuse results in the presence of thiourea (TU) in soil, wastewater, and even fruits, which is harmful for the environment and human health. It has urgent practical significance to design an efficient and reliable probe for TU detection. Herein, a sensitive fluorescent probe with off-on response for harmful TU was reported. The probe was designed with fluorescent carbon nanodots (CNDs) and gold nanoparticles (AuNPs) based on fluorescence resonance energy transfer (FRET) effect. Firstly, the CNDs were pre-combined with AuNPs and the fluorescence of CNDs was quenched due to the FRET effect. Upon addition of TU, the fluorescence of CNDs recovered due to the unbinding of CNDs and AuNPs, since the coordination interaction between TU and AuNPs is stronger than the electrostatic interaction among CNDs and AuNPs. Under the optimum parameters, a linear relationship was found between the relative fluorescence intensity of the probe and the concentration of TU in the range of 5.00 × 10^-8-1.00 × 10^-6 M (R² = 0.9958), with the limit of detection (LOD) calculated to be 3.62 × 10^-8 M. This proposed method is easy to operate and has excellent selectivity and sensitivity for TU, which can be effectively applied in environmental water and compound fruit-vegetable juice.

Determination of thiourea by terbium (III)/ prulifloxacin sensitized potassium permanganate-sulfite chemiluminescence with quenching method

Based on the thiourea quenching of the chemiluminescence of Tb³⁺/ prulifloxacin (PUFX) sensitized KMnO₄-Na₂SO₃ system, a convenient and rapid chemiluminescence method for the determination of thiourea was proposed. The reaction between KMnO₄ and Na₂SO₃ brought only weak chemiluminescence, but the chemiluminescence increased sharply in the presence of sensitizer Tb³⁺/ PUFX. Addition of thiourea can prevent the reaction between KMnO₄ and Na₂SO₃, thus the chemiluminescence intensity was significantly decreased. Under the optimum conditions, the calibration graphs for thiourea were linear in the range of 1.0 × 10^-7 to 4.0 × 10^-5 mol•L^-1. The limit of detection was 6.4 × 10^-8 mol•L^-1. The method was applied satisfactorily to the determination of thiourea in tap water, lake water and rice noodles and the spiked recoveries were between 104.7 ~ 113.4%. The possible mechanism of sensitization and quenching was also proposed.

A Lanthanide Complex Fluorescent Probe for the Detection of Melamine

Melamine has been illegally adulterated in dairy food because of the rich nitrogen content and stable chemical properties in recent years. Therefore, the detection of melamine is of great significance for food safety supervision and human health protection. As melamine is a weak fluorescent substance, it is difficult to detect melamine directly by fluorescence spectroscopy. In this work, we found that melamine can significantly enhance the emission of the tetracycline-europium (EuTC) complex at 616 nm. Therefore, we took EuTC complex as a fluorescent probe to detect melamine. According to the characterizations of absorption spectra, molecular electrostatic potential distribution, and the time-resolved spectra, we speculated that tetracycline and melamine may form a complex through hydrogen bonding interaction in the melamine-EuTC reaction system, causing the melamine closer approach to Eu³⁺ and reducing the non-radiative energy loss of water molecules to Eu³⁺, which significantly enhanced the fluorescence intensity of EuTC. The fluorescence intensity of EuTC complex with melamine concentration in the range of 0.5-40.0 μM shows a good linear relationship, and the correlation coefficient is 0.9951 with the detection limit of 7.85 × 10^-8M. It shows a high sensitivity for the EuTC complex as a fluorescent probe to detect melamine, which provides a supplement and extension for the detection of melamine by fluorescence spectroscopy.

Fluorescent reversible regulation of cysteamine-capped ZnSe quantum dots successively induced by photoinduced electron transfer of herring sperm DNA and intercalation binding of ethidium bromide

A fluorescent reversible regulation was studied by fluorescence spectra, ultraviolet-visible spectra in the combination of molecular docking, which based on the photoinduced electron transfer(PET) from hsDNA (herring sperm DNA) to CA (cysteamine)-capped ZnSe QDs (quantum dots) and intercalation of ethidium bromide (EB) into hsDNA. It was proven that the QDs bound with the adding hsDNA by electrostatic force and formed 1:1 hsDNA-QDs complexes, leading to the PET from hsDNA to QDs, and consequently the fluorescence quenching of the QDs; with EB being added in the complex solution, it bound with hsDNA by intercalation interaction and caused hsDNA releasing from hsDNA-QDs complex with forming 2.5:1 EB-hsDNA complex, leading to the recovery of fluorescence, based on the greater binding constant (1.74 × 106 L·mol-1) of hsDNA with the embedded EB comparing to that of QDs with the captured hsDNA (4.25 × 104 L·mol-1). A good linear relationship existed between the fluorescence recovery yield and the EB concentrations under the range of 1.0-12.0 × 10-6 mol·L-1 with bare interference of related substances. This work provided some useful insights into the study of binding mechanism between DNAs with their intercalators and fluorescence bi-direction regulation, and showed great potential for the determination of trace EB.

You Don't Learn That in School: An Updated Practical Guide to Carbon Quantum Dots

Carbon quantum dots (CQDs) have started to emerge as candidates for application in cell imaging, biosensing, and targeted drug delivery, amongst other research fields, due to their unique properties. Those applications are possible as the CQDs exhibit tunable fluorescence, biocompatibility, and a versatile surface. This review aims to summarize the recent development in the field of CQDs research, namely the latest synthesis progress concerning materials/methods, surface modifications, characterization methods, and purification techniques. Furthermore, this work will systematically explore the several applications CQDs have been subjected to, such as bioimaging, fluorescence sensing, and cancer/gene therapy. Finally, we will briefly discuss in the concluding section the present and future challenges, as well as future perspectives and views regarding the emerging paradigm that is the CQDs research field.

A colorimetric sensor for detecting thiourea based on inhibiting peroxidase-like activity of gold-platinum nanoparticles

In this study, gold-platinum nanoparticles (Au@PtNPs) with peroxidase-like activity were synthesized. In the absence of thiourea (TU), the Au@PtNPs can catalyze the decomposition of hydrogen peroxide, and oxidize 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB, colorless) into oxidized 3,3',5,5'-tetramethylbenzidine dihydrochloride (oxTMB, blue). The peroxidase-like activity of the Au@PtNPs is inhibited in the presence of TU, and TMB cannot be oxidized to oxTMB effectively, and no blue color could be observed. Based on this finding, a novel colorimetric sensor for detecting TU is proposed. The absorbance response curve showed a good linearity for the concentration of TU in the range of 10 nmol L^-1 to 10 μmol L^-1 with a correlation coefficient of R² = 0.999, and the limit of detection is 9.57 nmol L^-1. The colorimetric sensor possesses excellent selectivity, anti-interference ability, and application value in actual samples.

Controllable Synthesis of Biocompatible Fluorescent Carbon Dots From Cellulose Hydrogel for the Specific Detection of Hg2

Heavy metal ions overload can seriously harm human health. Simple and effective strategies for the specific detection of heavy metal ions are of great important. In this work, using different pretreatment methods, a series of carbon dots (CDs) with different particle sizes and doped with varying amounts of elements (O, N, S) were prepared based on the natural polymer, cellulose hydrogel. The CDs exhibit excellent fluorescence and biocompatibility. When the particle size decreased from 8.72 to 2.11 nm, the fluorescence quantum yield increased from 0.029 to 0.183. In addition, doping with elements (N) also effectively enhanced the fluorescent performance of the CDs. The fluorescence of the CDs, especially for the smallest, CD-4a, was significantly quenched in the presence of the heavy metal ion, Hg2+. Thus, CD-4a may be used as a fluorescence sensor for the detection of Hg2+. The fluorescence intensity of CD-4a exhibited a two-stage, concentration-dependent fluorescence response in the range 0.2-10 and 10-100 μmol/L Hg2+, with each stage having different slopes; the detection limit was 0.2 μM. More importantly, even in the presence of interfering metal ions, the detection of Hg2+ using the CDs-4a remained stable. Therefore, these biocompatible CDs may serve as a promising candidate for the specific detection of Hg2+.

Hydrothermal synthesis of highly fluorescent nitrogen-doped carbon quantum dots with good biocompatibility and the application for sensing ellagic acid

Blue emissive nitrogen-doped carbon quantum dots (N-CQDs) with a high quantum yield as high as 84.79% were successfully synthesized via the hydrothermal treatment of citric acid and diethylenetriamine in one pot. The as-prepared N-CQDs displayed excellent stability in high-salt conditions, good photostability, promising the N-CQDs as potential probes for selectively detecting ellagic acid with a linear range of 0.01-50 μM on the basis of inner filter effect. And the hydroponics experiment of gardenia with N-CQDs suggested the good biocompatibility of the N-CQDs, indicating the potential applications in biomedical fields.