Carbon dots-based fluorescent turn off/on sensor for highly selective and sensitive detection of Hg2+ and biothiols.

In this work, sodium salicylate and ethylenediamine (EDA) are used as the precursors to synthesize green fluorescent carbon dots (CDs). The CDs have some attractive properties, including better oxidation resistance, good water solubility, and excellent stability in high ionic strength solutions in a pH range of 6.0-10.0. Compared to other metal ions, only Hg2+ can quench the fluorescence of CDs, and with the introduction of biothiols, the fluorescence of the CDs/Hg2+ system can be recovered. Therefore, a turn off/on fluorescent sensor is constructed using CDs as a fluorescent probe, and the sensor is applied to the detection of Hg2+ and biothiols (glutathione, homocysteine and cysteine). In addition, the fluorescent sensor exhibits excellent selectivity and sensitivity. The linear range of Hg2+ is 0.05-10 µM with the detection limit of 44 nM. Glutathione, homocysteine, and cysteine have a linear response in the range of 0.5-10 µM with the limit of detection of 80, 76, and 69 nM, respectively. Furthermore, the fluorescence method is successfully used to detect Hg2+ in actual water samples and biothiols in human plasma.

Proton-controlled synthesis of red-emitting carbon dots and application for hematin detection in human erythrocytes.

The Red-emitting nitrogen-doped carbon dots (N-CDs) are synthesized using o-phenylenediamine by a one-step method, and can serve as a fluorescent probe for "turn off" detection of hematin in human red cells. The red-emitting N-CDs can be obtained only in acidic conditions and the emission of the red-emitting N-CDs is pH-dependent, indicating proton-controlled synthesis and emission. The red-emitting N-CDs are 2.7 nm in mean size and have a uniform dispersion and exhibit a high quantum yield (12.8%) and great optical properties. The developed sensing system for hematin displays a linear response from 0.4 to 32 µM with a detection limit of 0.18 µM. Importantly, this fluorescent probe demonstrates a good potential practicability for the quantitative detection of hematin in complex matrixes. Graphical abstract ᅟ.

Green fluorescent carbon quantum dots as a label-free probe for rapid and sensitive detection of hematin.

Hematin is an oxidized form of heme, and the abnormal levels of hematin in the human body can lead to various inflammatory lesions. Hence, there is still a need to establish a rapid, sensitive and efficient method for hematin detection. Herein, the green fluorescent carbon quantum dots (CQDs) are synthesized by using L-cysteine and hydrogen peroxide as precursors. The synthesized CQDs exhibit some fascinating characters including excellent water solubility, high fluorescence quantum yield, and good stability in a broad pH range of 7.0-11.0 and high ionic strength solution. Excitingly, the fluorescence of CQDs can be rapidly and selectively quenched by hematin via the inner filter effect. Moreover, the detection of hematin by the CQDs fluorescent probe shows a good linearity in the concentration range of 0.5-30 µM with a minimum detection limit of 0.1 µM. Finally, the proposed approach is successfully applied to detect hematin in human blood samples.

Green Synthesis of Blue Fluorescent P-doped Carbon Dots for the Selective Determination of Picric Acid in an Aqueous Medium.

A fluorescence method for the determination of picric acid (PA) using phosphorus-doped carbon dots (P-CDs), synthesized from ß-cyclodextrin and sodium pyrophosphate, is described. The P-CDs are very uniform and monodisperse with a diameter of about 2.8 nm. Under an excitation of 350 nm, the P-CDs emit bright blue fluorescence with an emission peak at 440 nm. The as-synthesized P-CDs serve as a sensitive, selective, and label-free fluorescent probe for the detection of PA. Based on an inner filter effect between PA and P-CDs, a linear response is obtained for PA from 0.1 to 10 µM with a detection limit of 82 nM. Finally, this sensing system has been demonstrated to have practicability for PA detection in the environmental water samples.

pH-mediated reversible fluorescence nanoswitch based on inner filter effect induced fluorescence quenching for selective and visual detection of 4-nitrophenol.

Being a common hazardous waste, 4-nitrophenol (4-NP) has caused a serious threat to humans and environment. Therefore, rapid and selective detection of 4-NP, especially using a simple and portable instrument, is highly desired for human health and environmental monitoring. Herein, we develop a novel pH-mediated reversible fluorescence nanoswitch for selectively detecting 4-NP by using water-soluble fluorescent polymer carbon dots (PCDs) as a probe. The fluorescence of PCDs can be quenched by 4-NP via inner filter effect (IFE) because its excitation spectrum well overlaps with the absorption spectrum of 4-NP under alkaline condition. However, an obvious blue shift of the absorption peak of 4-NP occurs under acidic condition, causing the fluorescence recovery of PCDs due to the disappearance of IFE. On the basis of this principle, a pH-mediated reversible fluorescence nanoswitch was constructed and a broad linear range was obtained from 0.5 to 60 µM with a detection limit of 0.26 µM for 4-NP. Furthermore, this approach was successfully applied to detect 4-NP in real water samples and a portable polyamide film-based sensor was developed for visual detection of 4-NP, which offers a promising platform for the detection of 4-NP in on-site and resource-poor settings.

Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots.

p-Nitrophenol (p-NP) contaminants seriously endanger environmental and living beings health, hence to establish a sensitive and selective method is of great importance for the determination of p-NP. In this work, boron and nitrogen co-doped carbon dots (B,N-CDs) were synthesized by one-step hydrothermal method using 3-aminophenylboronic acid as the sole precursor. The product was characterized through high-resolution transmission electron microscopy, fluorescence spectroscopy, UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Without any functionalized modification, B,N-CDs can be directly applied as a 'turn-off' fluorescent probe for rapid, highly selective, and sensitive detection of p-NP. The fluorescent sensor based on the B,N-CDs exhibited a broad linear response to the concentration of p-NP in the range of 0.5 - 60 µM and 60 - 200 µM, respectively, and provided a detection limit of 0.2 µM. It was found that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues. The response mechanism was due to the inner filter effect and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP, which endowed the sensing platform with the rapid response and high selectivity to p-NP. Finally, the sensor showed the practicability of p-NP determination in environmental water samples.

Adenosine-derived doped carbon dots: From an insight into effect of N/P co-doping on emission to highly sensitive picric acid sensing.

The various synthetic routes of carbon dots (C-dots) feature a considerable step toward their potential use in chemical sensors and biotechnology. Herein, by coupling phosphorus and nitrogen element introduction, the adenosine-derived N/P co-doped C-dots with fluorescence enhancement were achieved. By separately employing adenosine, adenosine monophosphate, adenosine diphosphate, and adenosine-5'-triphosphate as precursors, the effect of N/P co-doping on the fluorescence emission is discussed in detail. The formed C-dots with adenosine monophosphate exhibited strong blue fluorescence with a high quantum yield of 33.81%. Then the C-dots were employed as a fluorescent probe and utilized to develop a fast, sensitive, and selective picric acid sensor. The fluorescence of C-dots can be quenched by picric acid immediately, giving rise to a picric acid determination down to 30 nM. The possible mechanism of fluorescence quenching was discussed, which was proved to be inner filter effect and static quenching. Moreover, this method has the potential to detect picric acid in environmental water samples.

A fluorescence and colorimetric dual-mode assay of alkaline phosphatase activity via destroying oxidase-like CoOOH nanoflakes.

Nanozymes are increasingly exploited as components in bioanalysis and diagnostics. Here, we report a fluorescence and colorimetric dual-mode assay for alkaline phosphatase (ALP) activity employing oxidase-like cobalt oxyhydroxide (CoOOH) nanoflakes. Colorless o-phenylenediamine (OPD), a substrate for oxidase, can be oxidized to a product (OxOPD) with yellow color and orange fluorescence in a CoOOH nanoflake solution. But, ascorbic acid (AA) is able to reduce CoOOH to cobalt ion (Co2+), which causes the decomposition and collapse of the CoOOH nanoflakes, and thereby the CoOOH nanoflakes are deprived of the oxidase-like property. Based on this principle, the fluorescence and colorimetric dual-mode detection of AA was achieved. Alkaline phosphatase (ALP) can make l-ascorbic acid-2-phosphate (AAP) hydrolyze to yield AA. As a result, with the help of AAP, the selective and sensitive dual-mode assay of ALP activity has been realized successfully by using fluorescence and UV-vis absorption spectroscopies. Quantitative analysis of ALP in human serum samples and an ALP inhibitor investigation were performed using this sensing system. Given the economical and sensitive properties, the proposed method based on CoOOH nanoflakes has great potential for not only probing ALP activity in biological systems but also screening potential ALP inhibitors. Meanwhile, in this study, a new insight has been provided into the application of CoOOH nanoflakes in the development of sensitive and convenient sensors.