Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy.

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

An Indole-Based Fluorescent Chemosensor for Detecting Zn2+ in Aqueous Media and Zebrafish.

An indole-based fluorescent chemosensor IH-Sal was synthesized to detect Zn2+. IH-Sal displayed a marked fluorescence increment with Zn2+. The detection limit (0.41 µM) of IH-Sal for Zn2+ was greatly below that suggested by the World Health Organization. IH-Sal can quantify Zn2+ in real water samples. More significantly, IH-Sal could determine and depict the presence of Zn2+ in zebrafish. The detecting mechanism of IH-Sal toward Zn2+ was illustrated by fluorescence and UV-visible spectroscopy, DFT calculations, 1H NMR titration and ESI mass.

A Thiourea-Containing Fluorescent Chemosensor for Detecting Ga3.

A thiourea-based fluorescent chemosensor NADA, (E)-2-(4-(diethylamino)-2-hydroxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, has been designed and synthesized. NADA could detect Ga3+ through a fluorescent turn-on with a low detection limit (0.29 µM). Importantly, NADA could effectively discriminate Ga3+ from Al3+ and In3+. The binding mechanism of NADA with Ga3+ was identified by ESI-mass, NMR titration, and DFT calculations.

Determination of Zinc Ion by a Quinoline-Based Fluorescence Chemosensor.

A novel fluorescence chemosensor XYQ for detecting Zn(II) was synthesized. XYQ showed fluorescence turn-on to Zn(II) with high sensitivity and selectivity in aqueous media among 19 metal ions. Its binding structure was demonstrated by ESI-MS, Job plot, and 1H NMR titration. The detection limit of XYQ to Zn(II) was 0.53 µM. It is much below WHO drinking water standard (76.0 µM). XYQ could be applied successfully to the test kit and real samples. The fluorescence turn-on process was possibly explained as a chelation-enhanced fluorescence (CHEF) effect with theoretical calculations.

A Novel Thiophene-Based Fluorescent Chemosensor for the Detection of Zn2+ and CN-: Imaging Applications in Live Cells and Zebrafish.

A novel fluorescent turn-on chemosensor DHADC ((E)-3-((4-(diethylamino)-2-hydroxybenzylidene)amino)-2,3-dihydrothiophene-2-carboxamide) has been developed and used to detect Zn2+ and CN-. Compound DHADC displayed a notable fluorescence increase with Zn2+. The limit of detection (2.55 ± 0.05 µM) for zinc ion was far below the standard (76 µM) of the WHO (World Health Organization). In particular, compound DHADC could be applied to determine Zn2+ in real samples, and to image Zn2+ in both HeLa cells and zebrafish. Additionally, DHADC could detect CN- through a fluorescence enhancement with little inhibition with the existence of other types of anions. The detection processes of compound DHADC for Zn2+ and CN- were demonstrated with various analytical methods like Job plots, 1H NMR titrations, and ESI-Mass analyses.

Highly Sensitive Dansyl-Based Chemosensor for Detection of Cu2+ in Aqueous Solution and Zebrafish.

A new dansyl-based chemosensor (2-(4-((5-(dimethylamino)naphthalen-1-yl)sulfonyl)piperazin-1-yl)-N-(quinolin-8-yl)acetamide) (DC) for detecting Cu2+ was synthesized and characterized. DC showed great selectivity to Cu2+ by a fluorescent "on-off" detection method. Job plot, ESI-mass spectroscopy, and 1H NMR titration suggested a 1 to 1 binding mode between DC and Cu2+. The detection limit was determined to be 43 nM, which is greatly below the WHO guidelines. In addition, DC can be applied to real samples and zebrafish imaging. The fluorescence quenching mechanism was proposed as the enhancement of intramolecular charge transfer with calculations.

Fluorescent determination of zinc by a quinoline-based chemosensor in aqueous media and zebrafish.

A quinoline-based fluorescence sensor QDTD was developed for Zn2+. QDTD can detect Zn2+ by fluorescence turn-on. Detecting limit (0.27 µM) of QDTD for Zn2+ was far below WHO standard (76.0 µM). For the practical application, compound QDTD could be used to determine Zn2+ in real samples and applied to the test kit. More importantly, QDTD was expertly applied for Zn2+ imaging in HeLa cells and zebrafish with good membrane-permeability. Detection mechanism of Zn2+ ion by compound QDTD was suggested through the analytical tools like 1H NMR titration, ESI-MS, Job plot, fluorescent and UV-vis titration, and theoretical calculations, and through the synthesis and applications of a model compound AAQA.

A visible chemosensor based on carbohydrazide for Fe(ii), Co(ii) and Cu(ii) in aqueous solution.

A colorimetric sensor with pyridyl and carbohydrazide components has been synthesized and visibly turns blue in the presence of Fe(ii). The colorless sensor also changes color when exposed to Co(ii) and Cu(ii), but its color becomes yellow. The sensor shows no visible response to other metal ions such Ca2+, Cr3+, Mn2+, Fe3+, Ni2+, Zn2+, Cd2+, Ag+, Hg2+, and Pb2+. The binding ratio of the sensor to Fe(ii), Co(ii), and Cu(ii) is 2 sensors to 1 metal ion. The binding constants of the sensor are: Fe(ii): 1.0 × 109 M-2, Co(ii): 2 × 109 M-2, and Cu(ii): 3.0 × 109 M-2. The sensor works well at neutral pH and micromolar concentrations of Fe(ii), Co(ii), and Cu(ii) can be detected in water samples. The sensor's color response to Cu(ii) is uniquely attenuated by glutathione.

A multiple target chemosensor for the sequential fluorescence detection of Zn2+ and S2- and the colorimetric detection of Fe3+/2+ in aqueous media and living cells.

A novel multiple target sensor, (E)-5-((4-(diethylamino)-2-hydroxybenzyldene)amino)-1H-imidazole-4-carboxamide (DHIC), was synthesized for fluorescence detection of Zn2+ and S2- and colorimetric detection of Fe3+/2+ in aqueous media. DHIC can operate as a turn "on-off" sequential fluorescent sensor for Zn2+ and S2-. Detection limits (1.59 µM and 8.03 µM) for Zn2+ and S2- are below the WHO standards (76.0 µM and 14.7 µM). The DHIC-Zn2+ complex could be reversibly reused with ethylenediaminetetraacetic acid. Importantly, DHIC could image sequentially Zn2+ and S2- in living cells. Moreover, DHIC displayed a discriminatory color change from pale yellow to orange yellow to Fe3+/2+. The detection limit of DHIC for Fe3+/2+ (0.73 µM and 1.11 µM) is far below the EPA drinking water standard (5.37 µM). The sensor DHIC could be applied to analyze Fe3+ in real samples.

A Highly Selective Fluorescent Chemosensor for Detecting Indium(III) with a Low Detection Limit and its Application.

A highly selective chemosensor BHC ((E)-N-benzhydryl-2-((2-hydroxynaphthalen-1-yl)methylene)hydrazine-1-carbothioamide) for detecting indium(III) was synthesized. Sensor BHC can detect In(III) by a fluorescence turn-on method. The detection limit was analyzed to be 0.89 µM. Importantly, this value is the lowest among those previously known for fluorescent turn-on In(III) chemosensors. Based on the analytical methods like ESI-mass, Job plot, and theoretical calculations, the detection mechanism for In(III) was illustrated to be chelation-enhanced fluorescence (CHEF) effect. Additionally, sensor BHC was successfully applied to test strips.

A multi-functional chemosensor for highly selective ratiometric fluorescent detection of silver(I) ion and dual turn-on fluorescent and colorimetric detection of sulfide.

A multi-functional chemosensor 1 as silver and sulfide detector was synthesized by the combination of octopamine and 4-dimethylaminocinnamaldehyde. Sensor 1 exhibited a ratiometric fluorescence emission for Ag+ from blue to sky. The binding mode of 1 and Ag+ turned out to be a 1 : 1 ratio as determined using Job plot and electrospray ionization (ESI) mass spectral analyses. The sensing mechanism of 1 with silver ion was unravelled by 1H NMR titrations and theoretical calculations. Sensor 1 also discerned sulfide by enhancing fluorescence intensity and changing colour from yellow to colourless in aqueous solution. The sensing properties of 1 toward S2- were investigated by using ESI-mass analysis, Job plot and 1H NMR titrations. Moreover, 1 could be used as a detector for sulfide in a wide pH range.

Sequential detection of Fe3+/2+ and pyrophosphate by a colorimetric chemosensor in a near-perfect aqueous solution.

A novel colorimetric chemosensor 1 was designed and synthesized for Fe3+/2+ and pyrophosphate. Sensor 1 showed a selective color change toward both Fe3+ and Fe2+ from yellow to brown in a near-perfect aqueous solution. The detection limits (0.36 µM and 0.37 µM) for Fe3+ and Fe2+ were much lower than the guideline (5.37 µM) set by the Environmental Protection Agency (EPA) for iron in drinking water. Sensor 1 could be used to quantify Fe3+ in real water samples. Moreover, the resulting Fe3+-2·1 complex can detect pyrophosphate selectively over various anions especially including phosphate-based anions through a metal-complex displacement method. Based on UV-vis titrations, Job plot and ESI-mass spectrometry analyses, the sensing mechanisms of Fe3+, Fe2+ and PPi were proposed.