Solid‐phase extraction with metal–organic frameworks for the analysis of chiral compounds

Metal–organic frameworks (MOFs) are excellent porous materials with nanoscale cavities and high surface areas, which make them promising as novel adsorbents in solid‐phase extraction (SPE). In this article we report a new application of the chiral MOF [Zn₂(D‐Cam)₂(4,4′‐bpy)]_n in SPE used for the measurement of the enantiomeric excess (ee) of (±)‐1,1′‐bi‐2‐naphthol. Several important experimental parameters that may influence the extraction efficiency were investigated and optimized. Under the optimum conditions, a good linearity (R² > 0.999) was found between the ee value and the reciprocal of the peak areas. When compared with the actual ee measured using chiral HPLC, the SPE‐based assay also showed good accuracy and precision. The results showed that SPE based on chiral MOFs as adsorbents is a simple and effective method for the determination of the ee values of chiral compounds.     

Hg2+‐selective ratiometric and colorimetric probe based on dansyl–rhodamine and its staining function in cell imaging

A new ratiometric probe composed of a dansyl–rhodamine dyad for the detection of Hg²⁺ via fluorescence resonance energy transfer was designed and synthesized. Rhodamine, dansyl chloride, and hydrazide were selected as the acceptor, donor, and reaction site, respectively. It displayed high selectivity and sensitivity to Hg²⁺ with obvious colour change and fluorescence change due to Hg²⁺‐assisted hydrolysis of rhodamine hydrazide. A good linear relationship ranging from 0 to 16 μM and 0–28 μM for the Hg²⁺ concentration was found based on absorbance and fluorescence assay, respectively. Detection limits of absorbance and fluorescence for Hg²⁺ were calculated to be 1.22 μM and 9.10 μM, respectively.     

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2+

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺. Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇/F₅₉₀ and the concentration of Hg²⁺. The detection range was 10–70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.     

An aqueous fluorescent probe for Hg2+ detection with high selectivity and sensitivity

An aqueous fluorescent probe, 1, was developed for the rapid detection of Hg²⁺ with high sensitivity and excellent selectivity. Upon the addition of Hg²⁺ in pure aqueous media, the Hg²⁺‐mediated hydrolysis of vinyl ether and subsequent cyclization reactions converted probe 1 into the corresponding iminocoumarin dye, which is strongly fluorescent when excited. The application of this probe for the detection of intracellular Hg²⁺ was successfully demonstrated in living cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Detection of mercury(II) by DNA templated gold nanoclusters based on forming thymidine–Hg2+–thymidine duplexes

We have successfully synthesized gold nanoclusters (AuNCs) templated with DNA (5′‐CCCCCCCCCCCCTTTTTT‐3′), and subsequently employed the fluorescent DNA‐AuNCs as a novel probe for sensitive detections of mercury ions (Hg²⁺). Basically, the procedure is due to the formation of thymidine–Hg²⁺–thymidine duplexes between DNA‐AuNCs and Hg²⁺, thus leading to aggregations of DNA‐AuNCs described here occurring, and facilitating their fluorescence decrease. Significantly, this decrease of fluorescent signals permitted sensitive detection of Hg²⁺ in a linear range of 0.1–100 µmol L^?1, with a detection limit of 0.083 µmol L^?1 at a signal‐to‐noise ratio of 3. Additionally, the practicality of this probe for assaying Hg²⁺ in human urine and lake water samples was further validated, and showed various advantages including simplicity, selectivity, sensitivity and low cost, demonstrating its potential to broaden ways for assaying Hg²⁺ in real samples. Copyright © 2014 John Wiley & Sons, Ltd.     

A highly selective colorimetric and long‐wavelength fluorescent probe for the detection of Hg2+

Currently, the fluorescent probe is an important method for detecting heavy metal ions, especially mercury ion (Hg²⁺), which is harmful to the health of humans and the environment due to its toxicity and extensive use. In this paper, we designed and synthesized a colorimetric and long‐wavelength fluorescent probe Hg‐P with high sensitivity and excellent selectivity, which could detect Hg²⁺ by the changes of visual color, fluorescence and absorption spectroscopy. With the addition of Hg²⁺ to probe Hg‐P solution, its color changed from yellow to pink, and showed a 171 nm red‐shifted absorption spectrum. Probe Hg‐P was used in real water and soil solution samples to detect Hg²⁺, and the result is satisfactory. Therefore, this new probe shows great value and application in detecting Hg²⁺ in the environment.     

Conductive Metal–Organic Frameworks Selectively Grown on Laser‐Scribed Graphene for Electrochemical Microsupercapacitors

Conductive 2D metal–organic frameworks (MOFs) have merits beyond traditional MOFs for electrochemical applications, but reports on using MOFs as electrodes for electrochemical microsupercapacitors (MSCs) are practically non‐existent. In this work, a Ni‐catecholate‐based MOF (Ni‐CAT MOF) having good conductivity and exhibiting redox chemistry in the positive and negative voltage windows is developed. A novel process is developed to selectively grow the conductive Ni‐CAT MOF on 3D laser scribed graphene (LSG). The LSG with its superior wettability serves as a functional matrix‐current collector for the hybridization of conductive Ni‐CAT MOF nanocrystals. Impressively, MSCs fabricated using the hybrid LSG/Ni‐CAT MOF show significant improvement compared with MOF‐free LSG electrodes. Specifically, the LSG/Ni‐CAT MOF electrodes can deliver MSCs with a wide operating voltage (1.4 V), high areal capacitance (15.2 mF cm^?2), energy density (4.1 µWh cm^?2), power density (7 mW cm^?2), good rate performance, and decent cycling stability. This work opens up an avenue for developing electrochemical microsupercapacitors using conductive MOF electrodes.     

Ratiometric fluorescent detection of Cu2+ based on dual‐emission ZIF‐8@rhodamine‐B nanocomposites

Zeolitic imidazolate framework‐8 (ZIF‐8) loading rhodamine‐B (ZIF‐8@rhodamine‐B) nanocomposites was proposed and used as ratiometric fluorescent sensor to detect copper(II) ion (Cu²⁺). Scanning electron microscopy, Fourier transform infrared spectroscopy, X‐ray powder diffraction, nitrogen adsorption/desorption isotherms and fluorescence emission spectroscopy were employed to characterize the ZIF‐8@rhodamine‐B nanocomposites. The results showed the rhodamine‐B was successfully assembled on ZIF‐8 based on the π‐π interaction and the hydrogen bond between the nitrogen atom of ZIF‐8 and –COOH of rhodamine‐B. The as‐obtained ZIF‐8@rhodamine‐B nanocomposites were octahedron with size about 150–200 nm, had good water dispersion, and exhibited the characteristic fluorescence emission of ZIF‐8 at 335 nm and rhodamine‐B at 575 nm. The Cu²⁺ could quench fluorescence of ZIF‐8 rather than rhodamine‐B. The ZIF‐8 not only acted as the template to assemble rhodamine‐B, but also was employed as the signal fluorescence together with the fluorescence of rhodamine‐B as the reference to construct a novel ratiometric fluorescent sensor to detect Cu²⁺. The resulted ZIF‐8@rhodamine‐B nanocomposite fluorescence probe showed good linear range (68.4 nM to 125 μM) with a low detection limit (22.8 nM) for Cu²⁺ combined with good sensitivity and selectivity. The work also provides a better way to design ratiometric fluorescent sensors from ZIF‐8 and other fluorescent molecules.     

Core–shell structured CdTe/CdS@SiO2@CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection

Core–shell structured quantum dot (QD)–silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core–shell structured CdTe/CdS@SiO₂@CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red‐emissive CdTe/CdS QDs (630 nm) were used as a built‐in reference signal and green‐emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S^2?, owing to a S^2?‐induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S^2?‐quenched CdTe/CdS@SiO₂@CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric ‘off–on’ fluorescent probe for the detection of Cd²⁺. The sensing mechanism was suggested to be: the newly formed CdS‐like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.     

Luminescent sensor for Cd2+, Hg2+ and Ag+ in water based on a sulphur‐containing receptor: quantitative binding–softness relationship

A water‐soluble, high‐output fluorescent sensor, based on a lumazine ligand with a thiophene substituent for Cd²⁺, Hg²⁺ and Ag⁺ metal ions, is reported. The sensor displays fluorescence enhancement upon Cd²⁺ binding (log  β = 2.79 ± 0.08) and fluorescence quenching by chelating with Ag⁺ and Hg²⁺ (log β = 4.31 ± 0.15 and 5.42 ± 0.1, respectively). The mechanism of quenching is static and occurs by formation of a ground‐state non‐fluorescent complex followed by rapid intersystem crossing. The value of the Stern–Volmer quenching rate constant (k_q) by Ag⁺ ions is close to 6.71 × 10¹² mol/L/s at 298 K. The thermodynamic parameters (ΔG, ΔH and ΔS) were also evaluated and indicated that the complexation process is spontaneous, exothermic and entropically favourable. The quantitative linear relationship between the softness values of Klopman (σ_K) or Ahrland (σ_A) and the experimental binding constants (β) being in the order of Hg²⁺ > Ag⁺ > Cd²⁺ suggests that soft–soft interactions are the key for the observed sensitivity and selectivity in the presence of other metal ions, such as: Pb²⁺, Ni²⁺, Mn²⁺, Cu²⁺, Co²⁺, Zn²⁺ and Mg²⁺ ions. Copyright © 2008 John Wiley & Sons, Ltd.     

Multifunctional peptide‐based fluorescent chemosensor for detection of Hg2+, Cu2+ and S2− ions

A novel multifunctional fluorescent peptide sensor based on pentapeptide dansyl‐Gly‐His‐Gly‐Gly‐Trp‐COOH (D‐P5) was designed and synthesized efficiently using Fmoc solid‐phase peptide synthesis (SPPS). This fluorescent peptide sensor shows selective and sensitive responses to Hg²⁺ and Cu²⁺ among 17 metal ions and six anions studied in N‐2‐hydroxyethylpiperazine‐N‐2‐ethane sulfonic acid (HEPES) buffer solution. The peptide probe differentiates Hg²⁺ and Cu²⁺ ions by a ‘turn‐on’ response to Hg²⁺ and a ‘turn‐off’ response to Cu²⁺. Upon addition of Hg²⁺ or Cu²⁺ ions, the sensor displayed an apparent color change that was visible under an ultraviolet lamp to the naked eye. The limits of detection (LOD) of DP‐5 were 25.0 nM for Hg²⁺ and 85.0 nM for Cu²⁺; the detection limits for Cu²⁺ were much lower than the drinking water maximum contaminant levels set out by the United States Environmental Protection Agency (USEPA). It is noteworthy that both D‐P5‐Hg and D‐P5‐Cu systems were also used to detect S^2? successfully based on the formation of ternary complexes. The LODs of D‐P5‐Hg and D‐P5‐Cu systems for S^2? were 217.0 nM and 380.0 nM, respectively. Furthermore, the binding stoichiometry, binding affinity and pH sensitivity of the probe for Hg²⁺ and Cu²⁺ were investigated. This study gives new possibilities for using a short fluorescent peptide sensor for multifunctional detection, especially for anions.     

A carbonothioate‐based highly selective fluorescent probe with a large Stokes shift for detection of Hg2+

Mercury (Hg) is one of the heavy metal pollutants in the environment. Even a very small amount of mercury can cause serious harm to human beings. Herein, we reported a new carbonothioate‐based fluorescent probe for the detection of Hg²⁺ without interference from other metal ions. This probe possessed a very large Stokes shift (192 nm), which could improve the detection sensitivity by minimizing the interferences resulted from self‐absorption or auto‐fluorescence. With the addition of Hg²⁺ to the probe solution, considerable fluorescence enhancement was observed. Additionally, the Hg²⁺ concentration of 0–16 μM and fluorescence intensity showed a good linear relationship (y = 22106× + 53108, R² = 0.9955). Finally, the proposed probe was used to detect Hg²⁺ in real water samples, and its result was satisfactory. Therefore, our proposed probe would provide a promising method for the determination of Hg²⁺ in the environment.     

A new Schiff base as a turn‐off fluorescent sensor for Cu2+ and its photophysical properties

A new Schiff base receptor 1 was synthesized and its photophysical properties were investigated by absorption, emission and excitation techniques. Furthermore, its chromogenic and fluorogenic sensing abilities towards various metal ions were examined. Receptor 1 selectively detects Cu²⁺ ion through fluorescence quenching and detection was not inhibited in the presence of other metal ions. From fluorescence titration, the limit of detection of receptor 1 as a fluorescent ‘turn‐off’ sensor for the analysis of Cu²⁺ was estimated to be 0.35 μM.     

NiFe‐Based Metal–Organic Framework Nanosheets Directly Supported on Nickel Foam Acting as Robust Electrodes for Electrochemical Oxygen Evolution Reaction

It is of great significance to develop highly efficient and superior stable oxygen evolution reaction (OER) electrocatalysts for upcoming electrochemical conversion technologies and clean energy systems. Here, an assembled 3D electrode is synthesized by a one‐step solvothermal process using such an original OER electrocatalyst. During the solvothermal process, Ni ions released from Ni foam in acidic solution and Fe ions added exogenously act as metal centers and coordinate with terephthalic acid (TPA) organic molecules by robust coordinate bonds, and finally, NiFe‐based metal–organic framework (MOF) nanosheets in situ grown on Ni foam, i.e., MIL‐53(FeNi)/NF, are prepared. This binder‐free 3D electrode shows superior OER activity with high current density (50 mA cm^?2) at an overpotential of 233 mV, a Tafel slope of 31.3 mV dec^?1, and excellent stability in alkaline aqueous solution (1 m KOH). It is discovered that introduction of Fe into MIL‐53 structure increases electrochemically‐active areas as well as reaction sites, accelerated electron transport capability, and modulated electronic structure to enhance catalytic performance. Besides, first principles calculations show that MIL‐53(FeNi) is more favorable for foreign atoms' adsorption and has increased 3d orbital electron density boosting intrinsic activity. This work elucidates a promising electrode for electrocatalysts and enriches direct application of MOF materials.     

Metal–Organic Frameworks‐Derived Tunnel Structured Co3(PO4)2@C as Cathode for New Generation High‐Performance Al‐Ion Batteries

Despite great progress in aluminum ion batteries (AIBs), the commercialization and performance improvement of AIBs‐based carbon cathodes is greatly impeded by sluggish intercalation/extraction and redox kinetics due to large‐sized AlCl₄^? anions. Phosphates with tunnel channels and much larger d‐spacing than the radius of Al³⁺ could be an alternative candidate as a cathode for potential high‐performance AIBs. Herein, elaborately designed porous tunnel structured Co₃(PO₄)₂@C composites derived from ZIF‐67 as AIBs cathodes are demonstrated, showing increased active sites, high ionic mobility, and high Al³⁺ ion diffusion coefficient, leading to remarkably enhanced discharge–charge redox reaction kinetics. Furthermore, the carbon shell and porous structure performs as armor to alleviate volume change and maintain the structure integrity of the cathodes. As expected, the rationally constructed Co₃(PO₄)₂@C composite exhibits a superior capacity of 111 mA h g^?1 at a high current density of 6 A g^?1 and 151 mA h g^?1 at 2 A g^?1 after 500 cycles with capacity decay of 0.02% per cycle. This innovative strategy could be a big step forward for long‐term cycle stable AIBs and reveals significant insights into the redox reaction mechanism for high‐performance AIBs based on Al³⁺ rather than large‐sized AlCl₄^?.     

Construction of a novel turn‐on–off fluorescence sensor used for highly selective detection of thiamine via its quenching effect on o‐phen‐Zn2+ complex

In this work, a simple and selective fluorescence sensor approach called ‘turn‐on–off’ for the determination of thiamine (TM) has been developed. As known, the o‐phenanthroline (o‐phen) has inner fluorescence, though when reacted with zinc ions to form the o‐phen–Zn²⁺ complex the fluorescence intensity was enhanced effectively, while upon addition of TM into the o‐phen–Zn²⁺ complex solution, the intensity of the system was gently quenched, which was termed the ‘turn‐on–off’ probe. Notably, the method possessed highly selective, sensitive determination for TM with a detection limit of 0.25 μmol L^?1 and the reduced fluorescence intensity was proportional to the concentration of TM in the range 0.84–80.0 μmol L^?1. Besides, the proposed mechanism was also investigated through exploring the Fourier transform infrared (FT‐IR), nuclear magnetic resonance (NMR) spectroscopy. Furthermore, this manner was successfully applied into practical samples for TM detection with satisfactory results.     

Polymer composite fluorescent hydrogel film based on nitrogen‐doped carbon dots and their application in the detection of Hg2+ ions

A simple microwave‐assisted solvothermal method was used to prepare fluorescent nitrogen‐doped carbon dots (N‐CDs) with high fluorescence quantum yield (79.63%) using citric acid and N‐(2‐hydroxyethyl)ethylenediamine as starting materials. The PVAm‐g‐N‐CDs grafted products were synthesized by amide bond formation between the carboxylic groups of N‐CDs and amine groups of polyvinylamine (PVAm). Fluorescent hydrogel films (PVAm‐g‐N‐CDs/PAM) were synthesized by interpenetration polymer network polymerization of PVAm‐g‐N‐CDs and acrylamide (AM). When used for ion detection, we found that the fluorescence of the hydrogel films was clearly quenched by addition of Hg²⁺. Repeatability tests on using the hydrogel films for Hg²⁺ detection showed that they could be applied at least three times. The PVAm‐g‐N‐CDs/PAM could serve as an effective fluorescent sensing platform for sensitive detection of Hg²⁺ ions with a detection limit of 0.089 μmol/L. This work may offer a new approach for developing recoverable and sensitive N‐CDs‐based sensors for biological and environmental applications.     

Flexible Quasi‐Solid‐State Sodium‐Ion Capacitors Developed Using 2D Metal–Organic‐Framework Array as Reactor

Achieving high‐performance Na‐ion capacitors (NICs) has the particular challenge of matching both capacity and kinetics between the anode and cathode. Here a high‐power NIC full device constructed from 2D metal–organic framework (MOFs) array is reported as the reactive template. The MOF array is converted to N‐doped mesoporous carbon nanosheets (mp‐CNSs), which are then uniformly encapsulated with VO₂ and Na₃V₂(PO₄)₃ (NVP) nanoparticles as the electroactive materials. By this method, the high‐power performance of the battery materials is enabled to be enhanced significantly. It is discovered that such hybrid NVP@mp‐CNSs array can render ultrahigh rate capability (up to 200 C, equivalent to discharge within 18 s) and superior cycle performance, which outperforms all NVP‐based Na‐ion battery cathodes reported so far. A quasi‐solid‐state flexible NIC based on the NVP@mp‐CNSs cathode and the VO₂@mp‐CNSs anode is further assembled. This hybrid NIC device delivers both high energy density and power density as well as a good cycle stability (78% retention after 2000 cycles at 1 A g^?1). The results demonstrate the powerfulness of MOF arrays as the reactor for fabricating electrode materials.     

Covalent Encapsulation of Sulfur in a MOF‐Derived S,N‐Doped Porous Carbon Host Realized via the Vapor‐Infiltration Method Results in Enhanced Sodium–Sulfur Battery Performance

Practical applications of room temperature sodium–sulfur batteries are still inhibited by the poor conductivity and slow reaction kinetics of sulfur, and dissolution of intermediate polysulfides in the commonly used electrolytes. To address these issues, starting from a novel 3D Zn‐based metal–organic framework with 2,5‐thiophenedicarboxylic acid and 1,4‐bis(pyrid‐4‐yl) benzene as ligands, a S, N‐doped porous carbon host with 3D tubular holes for sulfur storage is fabricated. In contrast to the commonly used melt‐diffusion method to confine sulfur physically, a vapor‐infiltration method is utilized to achieve sulfur/carbon composite with covalent bonds, which can join electrochemical reaction without low voltage activation. A polydopamine derived N‐doped carbon layer is further coated on the composite to confine the high‐temperature‐induced gas‐phase sulfur inside the host. S and N dopants increase the polarity of the carbon host to restrict diffusion of sulfur, and its 3D porous structure provides a large storage area for sulfur. As a result, the obtained composite shows outstanding electrochemical performance with 467 mAh g^?1 (1262 mAh g^?1_(sulfur)) at 0.1 A g^?1, 270 mAh g^?1 (730 mAh g^?1_(sulfur)) after 1000 cycles at 1 A g^?1 and 201 mAh g^?1 (543 mAh g^?1_(sulfur)) at 5.0 A g^?1.