Emulsification-based dispersive liquid microextraction and HPLC determination of carbazole-based explosives

We have developed a simple method for the highly selective colorimetric detection of dissolved mercury(II) ions via direct formation of gold nanoparticles (AuNPs). The dithia-diaza ligand 2-[3-(2-amino-ethylsulfanyl)-propylsulfanyl]-ethylamine (AEPE) was used as a stabilizer to protect AuNPs from aggregation and to impart highly selective recognition of Hg(II) ion over other metal ions. A solution of Au(III) ion is directly reduced by sodium borohydride in the presence of AEPE and the detergent Triton X-100. This results in the formation of AEPE-AuNPs and a red coloration of the solution. On the other hand, in the presence of Hg(II), the solution turns blue within a few seconds after the addition of borohydride. This can be detected spectrophotometrically or even visually. The method was successfully applied to quantify Hg(II) levels in water sample, with a minimum detectable concentration as low as 35?nM.

Ultrasensitive detection of mercury(II) ion using CdTe quantum dots in sol-gel-derived silica spheres coated with calix[6]arene as fluorescent probes

We have developed a simple method for the preparation of highly fluorescent and stable, water-soluble CdTe quantum dots in sol-gel-derived composite silica spheres that were coated with calix[6]arene. The resulting nanoparticles (NP) were characterized in terms of UV, fluorescence and FT-IR spectroscopy and by TEM. The results show that the new NPs display more intense fluorescence intensity and are more stable than its precursors of the type SiO₂/CdTe. In addition, the new NPs exhibit a higher selectivity for the determination of Hg²⁺ than for other metal ions. Under the optimum conditions, the relative fluorescence intensity decreases with the concentration of Hg²⁺ in the range from 2.0 to 14.0?nmol?L^?1 and the detection limit is 1.55?nmol?L^?1. The method is based on the quenching of fluorescence by Hg²⁺ and expected to serve as a practical fluorescence test for rapid detection of Hg²⁺. A mechanism is suggested to explain the inclusion process by a Langmuir binding isotherm, and fluorescence quenching is best described by the Stern-Volmer equation.

Interaction of mercury(II) ions with immobilized apo-metallothioneins studied by scanning electrochemical microscopy combined with surface plasmon resonance

Scanning electrochemical microscopy (SECM) was combined with surface plasmon resonance (SPR) and applied for in-situ monitoring of the incorporation of Hg²⁺ by apo-metallothionein (apo-MT) immobilized on the SPR substrate. Hg²⁺ was anodically stripped from the Hg-coated SECM Pt tip and sequestered by apo-MT upon its diffusion to the SPR substrate. The high sensitivity of the SPR instrument enabled the detection of the change in the composition and structure of apo-MT molecules that was induced by the metal sequestration of Hg²⁺. The SPR response revealed that the saturation co-ordination number of Hg²⁺ binding to apo-MT was 18. Moreover, an unexpected collapse of the structure of MT was observed when the stoichiometric ratio of Hg²⁺/MT was ~70, and the structure cannot be further altered even by adding a large excess of Hg²⁺. This collapse was also confirmed by Raman spectroscopy. The results are potentially useful for a deeper understanding of the detoxification mechanism of MT to mercury ion.

Water-soluble homo-oligonucleotide stabilized fluorescent silver nanoclusters as fluorescent probes for mercury ion

Water–soluble fluorescent silver nanoclusters (Ag NCs) were prepared with the assistance of commercially available polyinosinic acid (PI) or polycytidylic acid (PC). The fluorescence of the Ag NCs is effectively quenched by trace mercury(II) ions, which can be applied for their detection. The response of the Ag NCs prepared with PI to Hg(II) ion is linear in the Hg(II) concentration range from 0.05 to 1.0 μM (R²?=?0.9873), and from 0.5 to 10 μM of Hg(II) (R²?=?0.9971) for Ag NCs prepared with PC. The detection limits are 3.0 nM and 9.0 nM (at an S/N of 3), respectively. The method is simple, sensitive and fairly selective.

Screening interaction between ochratoxin A and aptamers by fluorescence anisotropy approach

By taking advantage of the intrinsic fluorescence of ochratoxin A (OTA), we present a fluorescence anisotropy approach for rapid analysis of the interactions between OTA and aptamers. The specific binding of OTA with a 36-mer aptamer can induce increased fluorescence anisotropy (FA) of OTA as the result of the freedom restriction of OTA and the increase of molecular volume, and the maximum FA change is about 0.160. This FA approach enables an easy way to investigate the effects of buffer compositions like metal ions on the affinity binding. FA analysis shows the interaction between OTA and aptamer is greatly enhanced by the simultaneous presence of Ca²⁺ and Na⁺, while the binding affinity of aptamer decreases more than 18-fold when only Ca²⁺ exists, and the binding is completely lost when Ca²⁺ is absent. Crucial region of the aptamer for binding can be mapped through FA analysis and aptamer mutation. The demonstrated FA approach maintains the advantages of FA in simplicity, rapidity, and robustness. This investigation will help the development of aptamer-based assays for OTA detection in optimizing the binding conditions, modification of aptamers, and rational design.

On-line species-unspecific isotope dilution analysis in the picomolar range reveals the time- and species-depending mercury uptake in human astrocytes

In order to reveal the time-depending mercury species uptake by human astrocytes, a novel approach for total mercury analysis is presented, which uses an accelerated sample introduction system combined on-line with an inductively coupled plasma mass spectrometer equipped with a collision/reaction cell. Human astrocyte samples were incubated with inorganic mercury (HgCl₂), methylmercury chloride (MeHgCl), and thimerosal. After 1-h incubation with Hg²⁺, cellular concentrations of 3 μM were obtained, whereas for organic species, concentrations of 14–18 μM could be found. After 24 h, a cellular accumulation factor of 0.3 was observed for the cells incubated with Hg²⁺, whereas the organic species both showed values of about 5. Due to the obtained steady-state signals, reliable results with relative standard deviations of well below 5 % and limits of detection in the concentration range of 1 ng L^?1 were obtained using external calibration and species-unspecific isotope dilution analysis approaches. The results were further validated using atomic fluorescence spectrometry.

Study on the interaction between dinaphthosulfide-substituted macrocyclic diamides and some metal ions: experimental measurements vs. quantum mechanical calculations

Complexation properties of dinaphthosulfide-substituted macrocyclic diamides 1 and 2 with some metal cations that have been obtained by conductometric method are described with quantum mechanics calculations. To do this, the most stable structures of ligands, Hg²⁺-ligand complexes, ligand-MeOH and ClO₄ ^?–MeOH are optimized at HF/Lanl2dz level of theory and the most important interactions are analyzed by atoms in molecules (AIM) theory. These calculations predict the existence of strong interaction between Hg²⁺ cation and ligands 1 and 2, particularly, S–Hg²⁺ interaction. The different conductometric behaviors of complexation of ligands 1 and 2 with metal ions are interpreted on the basis of the calculated intramolecular hydrogen bonds in ligands and intermolecular hydrogen bonds between ligands and methanol as a solvent and perchlorate as a counter ion. In addition, binding energies between Hg²⁺ and ligands are also calculated by HF/Lanl2dz level of theory. Results show that all theoretical predictions are in line agreement with the experimental data.     

Untersuchungen über das Gleichgewicht des Hg2+-Hg 2 2+ -Hg-Systems in konzentrierten Perchloratlösungen

The formal potentials of the Hg²⁺/Hg ₂ ²⁺ , Hg ₂ ²⁺ /Hg and Hg²⁺/Hg redox couples and the apparent equilibrium constants of the reaction Hg²⁺ + Hg ∝ Hg ₂ ²⁺ in conc. aqueous solutions of Mg(ClO₄)₂ and Ca(ClO₄)₂ have been determined from emf measurements performed using cells with liquid junction. Based on these data, the hydration numbers of the Hg²⁺ and Hg ₂ ²⁺ ions were estimated.     

Determination of mercury(II) by surface-enhanced Raman scattering spectroscopy based on thiol-functionalized silver nanoparticles

Silver nanoparticles (Ag NPs) modified with sodium 2-mercaptoethanesulfonate (mesna) exhibit strong surface-enhanced Raman scattering (SERS). Their specific and strong interaction with heavy metal ions led to a label-free assay for Hg(II). The covalent bond formed between mercury and sulfur is stronger than the one between silver and sulfur and thus prevents the adsorption of mesna on the surface of Ag NPs. This results in a decrease of the intensity of SERS in the presence of Hg(II) ions. The Raman peak at 795?cm^?1 can be used for quantification. The effect of the concentration of mesna, the concentration of sodium chloride, incubation time and pH value on SERS were optimized. Under the optimal conditions, the intensity of SERS decreases with increasing concentration of Hg(II). The decrease is linear in the 0.01 and 2?μmol L^?1 concentration range, with a correlation coefficient (R²) of 0.996 and detection limit (S/N?=?3) is 0.0024?μmol L^?1. The method was successfully applied to the determination of the Hg(II) in spiked water samples.

Role of Cationization and Multimers Formation for Diastereomers Differentiation by Ion Mobility-Mass Spectrometry

Stereochemistry plays an important role in biochemistry, particularly in therapeutic applications. Indeed, enantiomers have different biological activities, which can have important consequences. Many analytical techniques have been developed in order to allow the identification and the separation of stereoisomers. Here, we focused our work on the study of small diastereomers using the coupling of traveling wave ion mobility and mass spectrometry (TWIMS-MS) as a new alternative for stereochemistry study. In order to optimize the separation, the formation of adducts between diastereomers (M) and different alkali cations (X) was carried out. Thus, monomers [M + X]⁺ and multimers [2M + X]⁺ and [3M + X]⁺ ions have been studied from both experimental and theoretical viewpoints. Moreover, it has been shown that the study of the multimer [2Y + M + Li]⁺ ion, in which Y is an auxiliary diastereomeric ligand, allows the diastereomers separation. The combination of cationization, multimers ions formation, and IM-MS is a novel and powerful approach for the diastereomers identification. Thus, by this technique, diastereomers can be identified although they present very close conformations in gaseous phase. This work presents the first TWIMS-MS separation of diastereomers, which present very close collision cross section thanks to the formation of multimers and the use of an auxiliary diastereomeric ligand.

A colorimetric method for the determination of lead(II) ions using gold nanoparticles and a guanine-rich oligonucleotide

We have developed a colorimetric method for the determination of Pb(II) ions. It is based on the use of gold nanoparticles and a guanine-rich synthetic oligonucleotide. On addition of Pb(II), the color of the solution turns from red to blue. The ratio of the UV-vis absorption at 630?nm and 525?nm is proportional to the concentration of Pb(II) ions in the range from 10 to 100?nM, and the detection limit is 20?nM. Other metal ions do not interfere if present in up to a 10-fold molar excess. The method was successfully applied to the detection of Pb(II) in lake water and urine. The recovery in case of spiked samples is 92%. The results show that this method is sensitive, simple and fast.

Fluorine Bonding Enhances the Energetics of Protein-Lipid Binding in the Gas Phase

This paper reports on the first experimental study of the energies of noncovalent fluorine bonding in a protein-ligand complex in the absence of solvent. Arrhenius parameters were measured for the dissociation of gaseous deprotonated ions of complexes of bovine β-lactoglobulin (Lg), a model lipid-binding protein, and four fluorinated analogs of stearic acid (SA), which contained (X =) 13, 15, 17, or 21 fluorine atoms. In all cases, the activation energies (E_a) measured for the loss of neutral XF-SA from the (Lg + XF-SA)^7– ions are larger than for SA. From the kinetic data, the average contribution of each?>?CF₂ group to E_a was found to be ~1.1 kcal mol^–1, which is larger than the ~0.8 kcal mol^–1 value reported for?>?CH₂ groups. Based on these results, it is proposed that fluorocarbon–protein interactions are inherently stronger (enthalpically) than the corresponding hydrocarbon interactions.

A highly selective colorimetric sensor for Hg2+ based on a copper (II) complex of thiosemicarbazone in aqueous solutions

By applying an indirect strategy, a new copper (Ⅱ) complex of a thiosemicarbazone L has been successfully developed as a colorimetric chemosensor for the sensitive detection of mercury (Ⅱ) ions. In the presence of copper (Ⅱ) ions, the colorless solution of L became yellow; however, upon the addition of traces of mercury (Ⅱ) ions, the yellow color faded to colorless immediately. Other ions, including Fe3+ , Ag+ , Ca2+ , Zn2+ , Pb2+ , Cd2+ , Ni2+ , Co2+ , Cr3+ and Mg2+ had a negligible influence on the probe behavior. The detection limits were 5.0×10 -6 M and 3.0×10 -7 M of Hg2+ using the visual color changes and UV-vis changes respectively. Test strips based on Cu-L were fabricated, which could act as a convenient and efficient Hg2+ test kits.     

Soft Ionization of Saturated Hydrocarbons, Alcohols and Nonpolar Compounds by Negative-Ion Direct Analysis in Real-Time Mass Spectrometry

Large polarizable n-alkanes (approximately C18 and larger), alcohols, and other nonpolar compounds can be detected as negative ions when sample solutions are injected directly into the sampling orifice of the atmospheric pressure interface of the time-of-flight mass spectrometer with the direct analysis in real time (DART) ion source operating in negative-ion mode. The mass spectra are dominated by peaks corresponding to [M + O₂]￣^?. No fragmentation is observed, making this a very soft ionization technique for samples that are otherwise difficult to analyze by DART. Detection limits for cholesterol were determined to be in the low nanogram range.

Structural Analysis of Guanylyl Cyclase-Activating Protein-2 (GCAP-2) Homodimer by Stable Isotope-Labeling, Chemical Cross-Linking, and Mass Spectrometry

The topology of the GCAP-2 homodimer was investigated by chemical cross-linking and high resolution mass spectrometry. Complementary conducted size-exclusion chromatography and analytical ultracentrifugation studies indicated that GCAP-2 forms a homodimer both in the absence and in the presence of Ca²⁺. In-depth MS and MS/MS analysis of the cross-linked products was aided by ¹⁵ ? N-labeled GCAP-2. The use of isotope-labeled protein delivered reliable structural information on the GCAP-2 homodimer, enabling an unambiguous discrimination between cross-links within one monomer (intramolecular) or between two subunits (intermolecular). The limited number of cross-links obtained in the Ca²⁺-bound state allowed us to deduce a defined homodimeric GCAP-2 structure by a docking and molecular dynamics approach. In the Ca²⁺-free state, GCAP-2 is more flexible as indicated by the higher number of cross-links. We consider stable isotope-labeling to be indispensable for deriving reliable structural information from chemical cross-linking data of multi-subunit protein assemblies.

Colorimetric determination of copper ions based on the catalytic leaching of silver from the shell of silver-coated gold nanorods

We have developed a method for the colorimetric determination of copper ions (Cu²⁺) that is based on the use of silver-coated gold nanorods (Au@Ag NRs). Its outstanding selectivity and sensitivity result from the catalytic leaching process that occurs between Cu²⁺, thiosulfate (S₂O₃ ^2?), and the surface of the Au@Ag NRs. The intrinsic color of the Au@Ag NRs changes from bright red to bluish green with decreasing thickness of the silver coating. The addition of Cu²⁺ accelerates the leaching of silver from the shell caused in the presence of S₂O₃ ^2?. This result in a decrease in the thickness of the silver shell which is accompanied a change in color and absorption spectra of the colloidal solution. The shifts in the absorption maxima are linearly related to the concentrations of Cu²⁺ over the 3–1,000 nM concentration range (R?=?0.996). The method is cost effective and was applied to the determination of Cu²⁺ in real water samples.

A colorimetric probe for the rapid and selective determination of mercury(II) based on the disassembly of gold nanorods

We report on a novel mercury(II)-controlled approach for the disassembly of gold nanorods (AuNRs) that has led to a detection system for Hg(II). The modified AuNRs were fabricated by functionalizing AuNRs with L-cysteine via a thiol group chemisorption-type of interaction. L-cysteine induces the assembly of AuNRs through cooperative electrostatic interaction upon which the color of the solution of the AuNRs changes from blue-green to gray dark. The addition of Hg(II), in turn, causes the disassembly of the modified AuNRs and the color of the solution returns to blue-green. This effect enables the optical determination of Hg(II) in aqueous solution, with a linear response in the 0.5 to 250 μM Hg(II) concentration range, an excellent selectivity for Hg(II), and with recoveries ranging from 99 % to 106 % in spiked environmental water samples.

Sensitive and selective colorimetric detection of cadmium(II) using gold nanoparticles modified with 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole

We have developed a simple, sensitive and selective colorimetric method for the detection of cadmium(II) (Cd²⁺) using gold nanoparticles modified with 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole. Organic solvents or additives are not required. It is found that Cd²⁺ induces the aggregation of the modified Au-NPs via chelation, leading to a color change from red to blue. This change can be seen with bare eyes, and monitored by UV–vis spectroscopy, transmission electron microscopy and dynamic light scattering. The detection limit is 30 nM (at a signal-to-noise ratio of 3). The new approach was successfully applied to the detection of Cd²⁺ in spiked samples of tap water and lake water, and the results agree well with those obtained by flame atomic absorption spectroscopy.

Highly Selective Fluorescence Detection for Mercury (II) Ions in Aqueous Solution Using Water Soluble Conjugated Polyelectrolytes

A highly selective assay method has been developed to detect mercury (II) (Hg²⁺) ions using cationic conjugated polymer (CCP). The transduction mechanism is based on a Hg²⁺ promoted reaction. In the absence of Hg²⁺ ions, the CCP can form the complex with an anionic 1,3‐dithiole‐2‐thione derivative through electrostatic interactions. The fluorescence of CCP is efficiently quenched by 1,3‐dithiole‐2‐thione derivative via an electron transfer process. Upon adding Hg²⁺ ions, the transformation of 1,3‐dithiole‐2‐thione into 1,3‐dithiole‐2‐one inhibits the quenching, and the fluorescence of CCP is recovered. Distinguishing aspects of this assay include the signal amplification of CCPs and a specific Hg²⁺ promoted reaction. By triggering the change in the emission intensity of CCP, it is possible to detect Hg²⁺ ions in aqueous solution.

     

Evidence for Sequence Scrambling in Collision-Induced Dissociation of y-Type Fragment Ions

Sequence scrambling from y-type fragment ions has not been previously reported. In a study designed to probe structural variations among b-type fragment ions, it was noted that y fragment ions might also yield sequence-scrambled ions. In this study, we examined the possibility and extent of sequence-scrambled fragment ion generation from collision-induced dissociation (CID) of y-type ions from four peptides (all containing basic residues near the C-terminus) including: AAAAHAA-NH₂ (where “A” denotes carbon thirteen (¹³C₁) isotope on the alanine carbonyl group), des-acetylated-α-melanocyte (SYSMEHFRWGKPV-NH₂), angiotensin II antipeptide (EGVYVHPV), and glu-fibrinopeptide b (EGVNDNEEGFFSAR). We investigated fragmentation patterns of 32 y-type fragment ions, including y fragment ions with different charge states (+1 to +3) and sizes (3 to 12 amino acids). Sequence-scrambled fragment ions were observed from ~50 % (16 out of 32) of the studied y-type ions. However, observed sequence-scrambled ions had low relative intensities from ~0.1 % to a maximum of ~12 %. We present and discuss potential mechanisms for generation of sequence-scrambled fragment ions. To the best of our knowledge, results on y fragment dissociation presented here provide the first experimental evidence for generation of sequence-scrambled fragments from CID of y ions through intermediate cyclic “b-type” ions.