A rhodamine-based Hg2+-selective fluorescent probe in aqueous solution

A new rhodamine-based Hg²⁺-selective fluorescent probe (I) was designed and synthesized. Compound I displays excellent selective and sensitive response to Hg²⁺ over other transition metal ions in neutral aqueous solutions. I itself is a colorless, nonfluorescent compound. Upon addition of Hg²⁺ to its solution, the thiosemicarbazide moiety of I undergoes an irreversible desulfurization reaction to form the corresponding 1,3,4-oxadiazole (II), a colorful and fluorescent product, causing instantaneous development of visible color and strong fluorescence emission. Based on this mechanism, a fluorogenic probe for Hg²⁺ was developed. The fluorescence increases linearly with the Hg²⁺ concentration up to 0.8 μmol L⁻¹ with the detection limit of 9.4 nmol L⁻ (3σ).     

Hydration of Hg2+ in aqueous solution studied by neutron diffraction with isotopic substitution

The structural parameters of Hg2+ hydration were studied in 0.225 mol/L solutions of Hg2+ in DNO3/D2O by means of neutron diffraction with isotopic substitution of 199Hg for natHg. It was found that Hg2+ is hydrated by a first solvation shell of six water molecules. The observed Hg-O and Hg-H distances are equal to 2.48+/-0.05 and 3.08+/-0.05 A, respectively. The angle phi between the plane of the water molecule and the cation-water oxygen axis is approximately 35 degrees . The solvation of Hg2+ therefore mimics very closely that of Ca2+ (the Ca-O and Ca-H distances are 2.40 and 3.03 A, respectively) and helps to account for the extreme toxicity of mercury(II). We note also that the Hg-O distance obtained in the neutron diffraction experiment is larger by approximately 0.1 A than that obtained by X-ray diffraction. This difference is consistent with a shift of the oxygen electron density toward the mercury cation due to the covalency of the Hg-O interaction.     

Anthroneamine based chromofluorogenic probes for Hg2+ detection in aqueous solution

Anthroneamine derivatives 1–3 (H₂O:DMSO; 9:1, HEPES buffer, pH 7.0 ± 0.1) undergo highly selective fluorescence quenching with Hg²⁺. The observed linear fluorescence intensity change allows the quantitative detection of Hg²⁺ between 200 nM/40 ppb—12 μM/2.4 ppm even in the presence of interfering metal ions viz. Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Cr³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Pb²⁺. Probes 1–3 and their Hg²⁺ complexes also show the broad pH resistance for their practical applicability.     

Rhodamine-based Hg2+-selective chemodosimeter in aqueous solution: fluorescent OFF-ON

[reaction: see text] N-(Rhodamine-6G)lactam-N'-phenylthiourea-ethylenediamine (1) was developed as a fluorescent and colorimetric chemodosimeter in aqueous solution with a broad pH span (5 approximately 10) and high selectivity toward Hg2+ but no significant response toward other competitive cations, such as Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Ca2+, Mg2+, K+, Na+, etc. The Hg2+-promoted ring opening of spirolactam of the rhodamine moiety induced cyclic guanylation of the thiourea moiety, which resulted in the dual chromo- and fluorogenic observation (OFF-ON).     

Amin-functionalized magnetic-silica core-shell nanoparticles for removal of Hg2+ from aqueous solution

Magnetic nanoparticles with monodisperse shape and size were prepared by a simple method and covered by silica. The prepared core-shell Fe₃O₄@silica nanoparticles were functionalized by amino groups and characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer-Emmett-Teller (BET) and Fourier transform infrared spectroscopy (FT-IR) techniques. The synthesized nanoparticles were employed as an adsorbent for removal of Hg²⁺ from aqueous solutions, and the adsorption phenomena were studied from both equilibrium and kinetic point of views. The adsorption equilibriums were analyzed using different isotherm models and correlation coefficients were determined for each isotherm. The experimental data were fitted to the Langmuir–Freundlich isotherm better than other isotherms. The adsorption kinetics was tested for the pseudo-first-order, pseudo-second-order and Elovich kinetic models at different initial concentrations of the adsorbate. The pseudo-second-order kinetic model describes the kinetics of the adsorption process for amino functionalized adsorbents. The maximum adsorption occurred at pH 5.7 and the adsorption capacity for Fe₃O₄@silica-NH₂ toward Hg²⁺ was as high as 126.7 mg/g which was near four times more than unmodified silica adsorbent.     

A highly selective and sensitive fluorescent chemosensor for Hg2+ in neutral buffer aqueous solution

A selective and sensitive fluorescent chemosensor for Hg2+, which was composed of two aminonaphthalimide fluorophores and a receptor of 2,6-bis(aminomethyl)pyridine, was synthesized through the reaction of 2,6-bis(chloromethyl)pyridine and N-[2-(2-hydroxyethoxy)ethyl]-4-piperazino-1,8-naphthalimide. The chemosensor showed an about 17-fold increase in fluorescence quantum yield upon addition of 1 equiv of Hg2+ in neutral buffer aqueous solution, and the other common metal ions did not notably disturb the detection of Hg2+.     

G-quadruplex DNA and ligand interaction in living cells using NMR spectroscopy

Gathering structural information from biologically relevant molecules inside living cells has always been a challenging task. In this work, we have used multidimensional NMR spectroscopy to probe DNA G-quadruplexes inside living Xenopus laevis oocytes. Some of these structures can be found in key regions of chromosomes. G-quadruplexes are considered potential anticancer therapeutic targets and several lines of evidence indirectly point out roles in key biological processes, such as cell proliferation, genomic instability or replication initiation. However, direct demonstrations of the existence of G-quadruplexes in vivo are scarce. Using SOFAST-HMQC type spectra, we probed a tetramolecular G-quadruplex model made of d(TG₄T)₄ inside living Xenopus laevis oocytes. Our observations lead us to conclude that the quadruplex structure is formed within the cell and that the intracellular environment preferentially selects a conformation that most resembles the one found in vitro under KCl conditions. We also show for the first time that specific ligands targeting G-quadruplexes can be studied using high resolution NMR directly inside living cells, opening new avenues to study ligand binding discrimination under physiologically relevant conditions with atomic detail.     

The one electron reduction of Hg2+ by 1-hydroxyalkyl radicals in aqueous solution: a pulse radiolysis study

The rate constants for the reduction of the divalent mercury ion Hg²⁺ by alcohol radicals, which are produced in acidic aqueous solutions containing methanol, ethanol or 2-propanol, are determined by optical absorption measurements to be 7.0×10⁸, 3.8×10⁸ or 3.1×10⁸ dm³ mol⁻¹ s⁻¹, respectively.     

A methionine-based turn-on chemical sensor for selectively monitoring Hg2+ ions in 100% aqueous solution

Dansyl-labeled methionine is synthesized by solid-phase synthesis, and found to be a highly sensitive and selective sensor for Hg(2+). The sensor sensitively detects Hg(2+) ions in aqueous solution by a turn-on response; however, the sensor detects Hg(2+) ions by a turn-off response in organic and mixed aqueous-organic solutions. We investigated the binding stoichiometry, binding constant, and binding mode of the sensor under various solvent conditions. In 100% aqueous solution, 2 : 1 complexation of the sensor with Hg(2+) ions is more favorable than 1 : 1 complexation, whereas the sensor preferentially forms a 1 : 1 complex in 100% CH(3)CN or in 50% CH(3)CN-aqueous solutions. Results reveal that the stoichiometry of the sensor-Hg(2+) complex plays an important role in the type of response to Hg(2+) ions, and that 2 : 1 complexation is required for a turn-on response to Hg(2+) ions in aqueous solution.     

Salacca zalacca skin and its modified form as biosorbents for Hg2+ removal from aqueous solution

ABSTRACT

A chemically modified-biosorbent was prepared by attaching dithizone onto Salacca zalacca skin waste for Hg²⁺ bioremoval. The material was synthesized by refluxing dithizone 5% with the Salacca zalacca skin powder followed by drying. The material was characterized through scanning electron microscopy, gas sorption analysis, and Boehm titration. The applicability of the material as biosorbent was tested for Hg²⁺ adsorption at room temperature. Findings suggested that the modification altered the surface properties of the biosorbent as indicated by the increased values for such surface parameters as specific surface area, pore volume, and quantitative functional groups. Particularly, the material demonstrated a high removal efficiency during Hg²⁺ adsorption, which fit the pseudo-second-order kinetics. The removal efficiency of Hg²⁺ was not influenced by the adsorbent dosage of 1–4?g/L.     

A medium-controlled fluorescence dual-responsive probe for Cu2+ and Hg2+ in aqueous solutions

In this study, we report a histidine-based fluorescence probe for Cu(2+) and Hg(2+), in which the amino group and imino group were modified by two common protective groups, 9-fluorenylmethoxycarbonyl and trityl group, respectively. In a water/methanol mixed solution, the probe displayed a selective fluorescence "turn-off" response to Cu(2+) when the ratio of CH(3)OH/H(2)O was higher than 1:1. Specifically, when the solvent is changed to 1:1 methanol/water, the 304 nm fluorescence peak is enhanced, while the 317 nm peak is weakened, upon addition of either Cu(2+) or Hg(2+) ions. The mechanism for such distinct responses of the probe to Cu(2+) and Hg(2+) was further clarified by using NMR and molecular simulation. The experiment results indicated that the polarity of solvent could influence the coordination mode of 1 with Cu(2+) and Hg(2+), and control the fluorescence response as a "turn-off" or ratiometric probe.     

Rhodamine-based highly sensitive colorimetric off-on fluorescent chemosensor for Hg2+ in aqueous solution and for live cell imaging

A novel rhodamine-based highly sensitive and selective colorimetric off-on fluorescent chemosensor for Hg(2+) ions is designed and prepared by using the well-known thiospirolactam rhodamine chromophore and furfural hydrazone as signal-reporting groups. The photophysical characterization and Hg(2+)-binding properties of sensor RS1 in neutral N, N-dimethylformamide (DMF) aqueous solution are also investigated. The signal change of the chemosensor is based on a specific metal ion induced reversible ring-opening mechanism of the rhodamine spirolactam. The response of the chemosensor for Hg(2+) ions is instantaneous and reversible. And it successfully exhibits a remarkably "turn on" response toward Hg(2+) over other metal ions (even those that exist in high concentration). Moreover, this sensor is applied for in vivo imaging in Rat Schwann cells to confirm that RS1 can be used as a fluorescent probe for monitoring Hg(2+) in living cells with satisfying results, which further demonstrates its value of practical applications in environmental and biological systems.     

Highly sensitive and selective detection of Hg(2+) in aqueous solution with mercury-specific DNA and Sybr Green I

Sybr Green I efficiently discriminates mercury-specific DNA and mercury-specific DNA/Hg(2+) complex, which provides a label-free, fast, fluorescence turn on assay for Hg(2+) detection with high sensitivity and selectivity.     

Tertiary amine mediated aerobic oxidation of sulfides into sulfoxides by visible-light photoredox catalysis on TiO2

The selective oxidation of sulfides into sulfoxides receives much attention due to industrial and biological applications. However, the realization of this reaction with molecular oxygen at room temperature, which is of importance towards green and sustainable chemistry, remains challenging. Herein, we develop a strategy to achieve the aerobic oxidation of sulfides into sulfoxides by exploring the synergy between a tertiary amine and titanium dioxide via visible-light photoredox catalysis. Specifically, titanium dioxide can interact with triethylamine (TEA) to form a visible-light harvesting surface complex, preluding the ensuing selective redox reaction. Moreover, TEA, whose stability was demonstrated by a turnover number of 32, plays a critical role as a redox mediator by shuttling electrons during the oxidation of sulfide. This work suggests that the addition of a redox mediator is highly functional in establishing visible-light-induced reactions via heterogeneous photoredox catalysis.     

Rhodamine-based chemodosimeter for fluorescent determination of Hg in 100% aqueous solution and in living cells

A rhodamine spirolactam derivative (1) bearing a hydrophilic carboxylic acid group is developed as a fluorescent chemodosimeter for bivalent mercury ions (Hg²⁺) in 100% aqueous solution. It exhibits a highly sensitive “turn-on” fluorescent response toward Hg²⁺ with a 42-fold fluorescence intensity enhancement under 1 equiv. of Hg²⁺ added. The chemodosimeter can be applied to the quantification of Hg²⁺ with a linear range covering from 3.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M and a detection limit of 9.7 × 10⁻⁸ M. Most importantly, the fluorescence changes of the chemodosimeter are remarkably specific for Hg²⁺ in the presence of other metal ions, which meet the selective requirements for practical application. Moreover, the experiment results show that the response behavior of 1 towards Hg²⁺ is pH independent in neutral condition (pH 5.0–8.0) and the response is fast (response time less than 3 min). Furthermore, the ring-opening mechanism of the rhodamine spirolactam induced by Hg²⁺ was supported by NMR, MS, and DFT theoretical calculations. In addition, the proposed chemodosimeter has been used to detect Hg²⁺ in water samples and image Hg²⁺ in living cells with satisfying results.     

Two dansyl fluorophores bearing amino acid for monitoring Hg in aqueous solution and live cells

A series of compounds (1-4) bearing one or two dansyl fluorophore(s) based on a Lys amino acid were synthesized in solid phase synthesis. Among them, two dansyl labeled Lys amino acid 3 detected Hg²⁺ in a 100% aqueous solution with high sensitivity (K_d=4.3 nM) via a turn-on response. Compound 3 was applied for monitoring Hg²⁺ in environmental and biological fields. 3 showed a hypersensitive response to Hg²⁺ without interferences from other metal ions and satisfied the requirements for monitoring the maximum allowable level (2 ppb) of mercury ions in drinking water demanded by EPA. In addition, 3 penetrated living HeLa cells and detected intracellular Hg²⁺. The organic spectroscopic data revealed the two sulfonamide and amide groups of 3 played a key role in stabilizing the 3-Hg²⁺ complex.     

Removal of UO2 2+ from aqueous solution by plasma functionalized MWCNTs

The study was undertaken to evaluate the feasibility of functionalized multi-walled carbon nanotubes (MWCNTs) for the removal of UO₂ ²⁺ from aqueous solutions. The MWCNTs was treated by oxygen plasma and characterized by FTIR and XPS. The characterization indicates that MWCNTs is successfully functionalized of oxygen groups such as –COOH on its surface (denote as P-MWCNTs). The sorption of UO₂ ²⁺ from aqueous solution on P-MWCNTs was studied as a function of contact time, solid contents, pH, ionic strength and temperature under ambient conditions using batch experiment. Two simplified kinetic models of pseudo-first-order and pseudo-second-order were tested to determine kinetic parameters such as rate constants, equilibrium sorption capacities and related correlation coefficients for kinetic models of the sorption process. It can be seen that the UO₂ ²⁺ sorption on P-MWCNTs could be described more favorably by the pseudo-second-order model. The thermodynamic parameters (?G°, ?S°, ?H°) calculated from the temperature-dependent sorption isotherms indicated that the sorption of UO₂ ²⁺ on P-MWCNTs were an endothermic and spontaneous processes. The results of the present study suggest that P-MWCNTs can be used beneficially in treating industrial effluents containing radioactive and heavy metal ions.     

Thiourea catalysis of MeHg ligand exchange between natural dissolved organic matter and a thiol-functionalized resin: a novel method of matrix removal and MeHg preconcentration for ultratrace Hg speciation analysis in freshwaters

Ultratrace analysis of dissolved MeHg in freshwaters requires both dissociation of MeHg from strong ligands in the sample matrix and preconcentration for detection. Existing solid phase extraction methods generally do not efficiently adsorb MeHg from samples containing high concentrations of natural dissolved organic matter. We demonstrate here that the addition of 10–60 mM thiourea (TU) quantitatively releases MeHg from the dissolved matrix of freshwater samples by forming a more labile complex (MeHgTU⁺) that quantitatively exchanges MeHg with thiol-functionalized resins at pH∼3.5 during column loading. The contents of these columns were efficiently eluted with acidified TU and MeHg was analyzed by Hg–TU complex ion chromatography with cold-vapor atomic fluorescence spectrometry detection. Routinely more than 90% of MeHg was recovered with good precision (average relative standard deviation of 6%) from natural waters—obtained from pools and saturated sediments of wetlands and from rivers—containing up to 68.7 mg C L⁻¹ dissolved organic matter. With the preconcentration step, the method detection limit of 0.29 pg absolute or 0.007 ng L⁻¹ in 40-mL samples is equivalent to that of the current state-of-the- art as practiced by skilled analysts. MeHg in 20–50-mL samples was completely trapped. On the basis of our knowledge of the chemistry of the process, breakthrough volume should depend on the concentrations of TU and H⁺. At a TU concentration of 12 mM breakthrough occurred between 50 and 100 mL, but overall adsorption efficiency was still 85% at 100 mL. Formation of artifactual MeHg is minimal; only about 0.7% of ambient MeHg is artifactual as estimated from samples spiked with 4 μg L⁻¹ Hg^II.     

A highly selective redox, chromogenic, and fluorescent chemosensor for Hg2+ in aqueous solution based on ferrocene-glycine bioconjugates

The synthesis, electrochemical, optical, and metal-cation-sensing properties of ferrocene-glycine conjugates C(30)H(38)O(8)N(8)Fe (2) and C(20)H(24)O(4)N(4)Fe (3) have been documented. Both compounds 2 and 3 behave as very selective redox (ΔE(1/2) = 217 mV for 2 and ΔE(1/2) = 160 mV for 3), chromogenic, and fluorescent chemosensors for Hg(2+) cations in an aqueous environment. The considerable changes in their absorption spectra are accompanied by the appearance of a new low-energy peak at 630 nm (2, ε = 1600 M(-1) cm(-1); 3, ε = 822 M(-1) cm(-1)). This is also accompanied by a strong color change from yellow to purple, which allows a prospective for the "naked eye" detection of Hg(2+) cations. These chemosensors present immense brightness and fluorescence enhancement (chelation-enhanced fluorescence = 91 for 2 and 42 for 3) following Hg(2+) coordination within the limit of detection for Hg(2+) at 7.5 parts per billion.     

Detection of Hg2+ in aqueous solutions with a foldamer-based fluorescent sensor modulated by surfactant micelles

[structure: see text] A hybrid foldamer constructed from six cholate units and two methionines was labeled with a DANSYL (DNS) group. The foldamer was solubilized by surfactant micelles to allow its usage as a fluorescent sensor for mercury ions present in the micromolar range in aqueous solutions. Its sensitivity was largely independent of the concentration of nonionic surfactants but was strongly influenced by both the nature and the concentration of ionic surfactants.