Carbon quantum dots as a turn-on fluorescent probe for the sensitive detection of Cd2.

In this study, fluorescent carbon dots were synthesized for the first time using ammonium citrate and glutamic acid as precursors via a one-pot hydrothermal method. The synthesized carbon dots emit blue fluorescence at 436 nm (excited at 320 nm) and demonstrate excellent photobleaching resistance and fluorescence stability in high salt environments. Within the range of 1-25 µM, the fluorescence of CDs gradually increases with the increasing concentration of Cd2+, reaching a limit of detection as low as 13 nM. This phenomenon could be ascribed to the chelation-enhanced fluorescence, a result of Cd2+ forming complexes with the abundant surface functional groups such as CN-, -COOH, -OH, -NH2 in CDs. Furthermore, this turn-on fluorescent probe has been successfully used for the detection of Cd2+ in tap water and lake water, providing an efficient and sensitive method for the analysis of environmental metals.

Pyrene-acylhydrazone-based Turn-on Fluorescent Probe for Highly Sensitive Detection Cu2+ and Application in Bioimaging.

The development of highly selective and sensitive, low detection limits, and biocompatible turn-on copper ion fluorescent probes is of great significance for the environment and life sciences. In this study, a novel turn-on fluorescent probe T based on pyrene-acylhydrazone was synthesized via an efficient one-step condensation reaction and characterized by 1H NMR, 13C NMR and HRMS. The probe T exhibited high selectivity with a low detection limit of 0.304 nM towards Cu2+ in DMSO/H2O (v/v = 1 : 1) medium by a PET-TICT dual interplaying sensing mechanisms. Job's plot analysis and HRMS data confirmed the 1 : 1 binding stoichiometry between T and Cu2+ with an association constant of 5.7×103 M-1. Additionally, the binding model was investigated by 1H NMR titration and FT-IR spectra. Furthermore, probe T exhibits low cellular toxicity and excellent membrane permeability, and has been successfully applied for fluorescent imaging of copper ions in live HT-22 cells.

Design and synthesis of tetrazine bioorthogonal fluorogenic probes / 药学学报

Bioorthogonal fluorogenic probes are becoming an ideal tool for live-cell fluorescence imaging. With the tetrazine bioorthogonal fluorogenic probe that displays fluorescence enhancement, the tetrazine plays the dual-role of a bioorthogonal reaction unit and the fluorescence quenching unit. The "off" and "on" states of the fluorescence probe are mainly controlled through inverse electron demand Diels-Alder (IEDDA) bioorthogonal reaction. We designed a series of turn-on tetrazine fluorescent probes with Donor-π-Acceptor (D-π-A) structure to achieve a high signal-to-noise ratio and specificity of fluorescence imaging. This series of probes reacted with the dienophile bicyclononyne, and then generated pyridazine structure in-situ that acted as an electron acceptor, resulting in a new D-π-A effect of fluorescent dyes, turning on the intramolecular charge transfer (ICT) effect. By adjusting the electron-donating groups and the degree of conjugation, tunable fluorescence spectra between 400-647 nm with fluorescence turn-on enhanced up to 500-fold have been achieved. This research lays the foundation for the further optimization of tetrazine bioorthogonal fluorescent probes and their applications in molecular imaging and biomedical fields.

Cyclization Reaction-Based Turn-on Probe for Covalent Labeling of Target Proteins.

Fluorescent molecules have contributed to basic biological research but there are currently only a limited number of probes available for the detection of non-enzymatic proteins. Here, we report turn-on fluorescent probes mediated by conjugate addition and cyclization (TCC probes). These probes react with multiple amino acids and exhibit a 36-fold greater emission intensity after reaction. We analyzed the reactions between TCC probes and nuclear receptors by electrospray ionization mass spectrometry, X-ray crystallography, spectrofluorometry, and fluorescence microscopy. In vitro analysis showed that probes consisting of a protein ligand and TCC could label vitamin D receptor and peroxisome proliferator-activated receptor γ. Moreover, we demonstrated that not only a ligand unit but also a peptide unit can label the target protein in a complex mixture.

A Fluorescent Probe Based on Pyrene Ring for Detecting Cys and its Application in Biology.

The identification of thiols has become a research hotspot due to its role in biological systems. In this work, we simply constructed a turn-on fluorescent probe named 3-(5-bromopyridin-3-yl)-1-(pyren-1-yl) prop-2-en-1-one, that a combination of pyrene ring and substituted pyridine via the connection of α, ß-unsaturated ketone. Cys can destroy the space effect by Michael addition reaction, which makes the fluorescence intensity changes. Furthermore, the probe featured excellent selectivity and high sensitivity (the detection limit was 0.52 µM) by addition of Cys. Moreover, this probe suggested a potential for imaging in vivo owing to the successful imaging of the probe in HepG2 cells, zebrafish, and Arabidopsis thaliana.

A highly selective and ultrafast near-infrared fluorescent turn-on and colorimetric probe for hypochlorite in living cells.

Hypochlorous acid (HOCl)/hypochlorite (OCl-), important reactive oxygen species, play essential roles in many physiological and pathological progresses. Accordingly, we developed a novel dicyanomethylene-4H-pyran (DCM)-based probe DCM-OCl for colorimetric and near-infrared fluorescent turn-on detection of OCl-. The probe exhibited excellent selectivity and sensitivity for OCl- over other bio-related analytes with a detection limit of 80 nM. The excellent selectivity of DCM-OCl for OCl- was ascribed to specific oxidative cleavage of the dimethylthiocarbamate (DMTC) recognition unit by OCl-. Moreover, DCM-OCl exhibited an ultrafast turn-on response (<3 s) to OCl-, potentially allowing real-time detection of OCl-. Furthermore, DCM-OCl was successfully used to image endogenous/exogenous OCl- in living cells.

Selective and sensitive detection of cyanate using 3-amino-2-naphthoic acid-based turn-on fluorescence probe.

The cyanate anion (CNO-), formed spontaneously within cells from urea and carbamoyl phosphate, usually functions as a biomarker of some diseases such as chronic kidney disease. Therefore, accurate determination of CNO- is highly demanded. Herein, a 3-amino-2-naphthoic acid-based "turn-on" fluorescence probe was developed for specific detection of CNO-. Upon the addition of sodium cyanate, the weak-fluorescent 3-amino-2-naphthoic acid could react with CNO-, which triggered intense emission of green fluorescence. And up to 9-fold fluorescence enhancement was observed. The fluorescence enhancement ratios displayed a good linear relationship with the concentrations of CNO- in the range of 0.5-200 µM. The high selectivity and sensitivity for CNO- detection were investigated with the detection limit as low as 260 nM. The probe was further successfully applied to determine CNO- in real samples such as tap water, human urine and serum samples, which offered a promising approach in practical applications. Graphical abstract.

Cationic cyanine chromophore-assembled upconversion nanoparticles for sensing and imaging H2S in living cells and zebrafish.

Elevated hydrogen sulfide (H2S) level is closely associated with various diseases. So the sensing of H2S is noteworthy for divulging its role in diagnosing these diseases. Herein, we proposed poly(acrylic acid)-modified upconversion nanoparticles assembled with cationic near-infrared cyanine chromophores (Cy7-Cl) as the nanoprobe (Cy7-UCNPs) for monitoring H2S based on thiolation reactions. The presence of H2S resulted into about five-fold enhancement in the luminescence intensity of Cy7-UCNPs and the nanoprobe showed a good linearity (R2 =0.9952) over the range of 1.0 - 90 µM. Furthermore, Cy7-UCNPs were successfully employed in sensing and imaging of exogenous and endogenous H2S in live cells and zebrafish. The system shows great potential in the field of nanobiomedicine because of the many excellent properties including high sensitivity, good selectivity, and low cytotoxicity.

A Hybrid Coumarin-Semifluorescein-Based Fluorescent Probe for the Detection of Cysteine.

A new type of turn-on fluorescent probe CF-AC for the detection of Cys was firstly reported. The probe exhibited an excellent response to Cys with high selectively and sensitivity. In the presence of Cys, two fluorescence emission peaks at 525 nm and 650 nm appeared accompanied by the fluorescence color change from blue to red. Morever, the probe had good biocompatibility and could be successfully used for fluorescence imaging of Cys in MCF-7 cells.

"Turn-On" Fluorescent Probe for Mercury(II): High Selectivity and Sensitivity and New Design Approach by the Adjustment of the π-Bridge.

By intelligent design, a new "turn-on" fluorescent probe (1-CN) was obtained based on the deprotection reaction of the dithioacetal promoted by Hg2+ ions, which could sense mercury ions sensitively and selectively, with the detection limit of 8×10(-7) M. Thanks to the apparent turn-on signal, 1-CN has been successfully applied to rapidly detect trace amounts of mercury ions as test strips and cell image.

A novel "Turn-On" fluorescent probe for F(-) detection in aqueous solution and its application in live-cell imaging.

A novel probe incorporating quaternized 4-pyridinium group into a BODIPY molecule was synthesized and studied for the selective detection of fluoride ions (F(-)) in aqueous solution. The design was based on a fluoride-specific desilylation reaction and the "Turn-On" fluorescent response of probe 1 to F(-) was ascribed to the inhibition of photoinduced electron transfer (PET) process. The probe displayed many desired properties such as high specificity, appreciable solubility, desirable response time and low toxicity to mammalian cells. There was a good linearity between the fluorescence intensity and the concentrations of F(-) in the range of 0.1-1mM with a detection limit of 0.02 mM. The sensing mechanism was confirmed by the NMR, electrospray ionization mass spectrum, optical spectroscopy and the mechanism of "Turn-On" fluorescent response was also determinated by a density functional theory (DFT) calculation using Gaussian 03 program. Moreover, the probe was successfully applied for the fluorescence imaging of F(-) in human epithelial lung cancer (A549) cells and alveolar type II (ATII) cells under physiological conditions.