Ultrasensitive,Rapid, and Selective Detection of Mercury Using Graphene Assisted Laser Desorption/Ionization Mass Spectrometry

We report an extremely sensitive and specific detection of mercuric ions (Hg²⁺) based on graphene assisted laser desorption/ionization mass spectrometry (GALDI-MS). Combining the highly selective coordination interactions between thymine (T) and Hg²⁺, we present a simple, effective, and novel approach, based on π–π interactions of the T-Hg²⁺-T complex and G that can serve as a platform and matrix for GALDI-MS. The present sensor not only exhibits high selectivity and sensitivity (picomolar) to Hg²⁺ in aqueous solution, but also can elucidate the chemical structures of the metal complexes. The significant advantage in the current approach is that there is no need for a sophisticated instrument, and no sample pretreatment is required to detect the Hg²⁺ ions.

Binding kinetics of human cellular prion detection by DNA aptamers immobilized on a conducting polypyrrole

We developed a biosensor based on the surface plasmon resonance (SPR) method for the study of the binding kinetics and detection of human cellular prions (PrP^C) using DNA aptamers as bioreceptors. The biosensor was formed by immobilization of various biotinylated DNA aptamers on a surface of conducting polypyrrole modified by streptavidin. We demonstrated that PrP^C interaction with DNA aptamers could be followed by measuring the variation of the resonance angle. This was studied using DNA aptamers of various configurations, including conventional single-stranded aptamers that contained a rigid double-stranded supporting part and aptamer dimers containing two binding sites. The kinetic constants determined by the SPR method suggest strong interaction of PrP^C with various DNA aptamers depending on their configuration. SPR aptasensors have a high selectivity to PrP^C and were regenerable by a brief wash in 0.1 M NaOH. The best limit of detection (4 nM) has been achieved with this biosensor based on DNA aptamers with one binding site but containing a double-stranded supporting part.

Direct surface plasmon resonance immunosensing of pyraclostrobin residues in untreated fruit juices

A surface plasmon resonance (SPR) immunoassay for on-line detection of the strobilurin fungicide pyraclostrobin in untreated fruit juices is presented. The analysis of pyraclostrobin residues is accomplished in apple, grape, and cranberry samples by monitoring the recognition events occurring separately in a two-channel home-made SPR biosensor. Covalent coupling of the analyte derivative results in a reversible method, enabling more than 80 measurements on the same sensor surface. Optimization of the immunoassay conditions provides limits of detection as low as 0.16?μg?L^?1. The selectivity and reproducibility of the analysis is ensured by studying both non-specific interactions with unrelated compounds and inter-assay coefficients of variation. Excellent recovery ranging from 98 to 103?% was achieved by a simple 1:5 dilution of fruit juice with assay buffer before the analysis. The lack of previous cleaning and homogenization procedures reduces the analysis time of a single food sample to only 25?min, including the regeneration cycle.

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.

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.

A surface plasmon resonance study on the optical properties of gold nanoparticles on thin gold films

We report on an investigation of the optical properties of gold nanoparticles assembled as thin films of different thickness. The nanoparticles were linked to the surface of a gold chip by dithiol reagents and studied by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy. There is good correlation between the experimental findings and theoretical simulation, and the respective data reveal the presence of ordered nanostructures in the assemblies. The shift in the SPR angle is linearly dependent on the particle size and the ratio of the different particles. SPR spectroscopy also reveals important information in terms of the optical constants of such films. This shall be further applied to in-situ quality control in the fabrication of optoelectronic, solar cell and semiconductor devices.

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.

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.

Detection of mismatched caspase-3 DNA oligonucleotides with an SPR biosensor following amplification by Taq polymerase

We demonstrate that base mismatches of caspase-3 DNA sequences can be detected by surface plasmon resonance (SPR) following signal amplification by polymerase from Thermus aquaticus (Taq). The concentration of magnesium ions and the respective dNTPs for polymerase binding to the oligonucleotides on the sensing surface were optimized. Taq polymerase binds to double-stranded DNA that is self-assembled on the gold surface of the biosensor to induce an SPR signal. Experiments are presented on the effect of Mg(II) and dNTP concentrations on the activity of the polymerase on the sensing surface. The detection limits are 50 pM, 0.1 nM, 0.7 nM, 7 nM, and 20 nM for correctly matched, single-base mismatched, two-base mismatched, three-base mismatched and four-base mismatched DNA of caspase-3, respectively. This is attributed to the optimized experimental conditions, with samples containing 2 μM of Mg(II) and 0.3 mM of dNTP.

Surface plasmon resonance, fluorescence, and circular dichroism studies for the characterization of the binding of BACE-1 inhibitors

The mechanism of action underlying β-secretase 1 (BACE-1) inhibition was characterized by a surface plasmon resonance (SPR) method using primary amino groups to immobilize OM99-2, a well-known highly potent peptidic BACE-1 inhibitor, on the carboxyl groups of the dextran layer of a sensor chip. The diluted BACE-1 was mixed with buffer or the test compound and the mixture was flushed through the chip. BACE-1 binding to the immobilized peptide inhibitor was quantified. This SPR method was used to identify BACE-1 inhibitor binding sites and the mechanism of action (competitive/noncompetitive) and to validate findings of fluorescence resonance energy transfer (FRET) inhibition studies. To support this, a multimethodological approach (circular dichroism and fluorescence spectroscopy) was applied in parallel to FRET inhibition studies to characterize the binding modes of peptidic and nonpeptidic BACE-1 inhibitors. Circular dichroism spectroscopy served to correlate the conformation of BACE-1 with enzymatic activity and to monitor secondary structure changes upon ligand binding. In a complementary approach, direct fluorescence spectroscopy was used to characterize different BACE-1 inhibitor binding sites. The influence of pH and inhibitors on BACE-1 secondary structure was also elucidated. This multimethodological approach was applied to identify binding modes of bis(7)-tacrine and myricetin in comparison with well-known peptidic inhibitors.

Streptavidin-enhanced surface plasmon resonance biosensor for highly sensitive and specific detection of microRNA

We are presenting a method for sensitive and specific detection of microRNA (miRNA) using surface plasmon resonance. A thiolated capture DNA probe with a short complete complementary sequence was immobilized on the gold surface of the sensor to recognize the part sequence of target miRNA, and then an oligonucleotide probe linked to streptavidin was employed to bind the another section of the target. The use of the streptavidin-oligonucleotide complex caused a ~5-fold increase in signal, improved the detection sensitivity by a factor of ~24, and lowered the detection limit to 1.7 fmol of miR-122. This specificity allowed a single mismatch in the target miRNA to be discriminated. The whole assay takes 30 min, and the surface of the sensor can be regenerated at least 30 times without loss in performance. The method was successfully applied to the determination of miRNA spiked into human total RNA samples.

Angle-dependent resonance of localized and propagating surface plasmons in microhole arrays for enhanced biosensing

The presence of microhole arrays in thin Au films is suited for the excitation of localized and propagating surface plasmon (SP) modes. Conditions can be established to excite a resonance between the localized and propagating SP modes, which further enhanced the local electromagnetic (EM) field. The co-excitation of localized and propagating SP modes depends on the angle of incidence (θ _exc) and refractive index of the solution interrogated. As a consequence of the enhanced EM field, enhanced sensitivity and an improved response for binding events by about a factor of 3 to 5 was observed with SPR sensors in the Kretschmann configuration for a set of experimental conditions (λ _SPR, θ _exc, and η). Thus, microhole arrays can improve sensing applications of SPR based on classical prism-based instrumentation and are suited for SP-coupled spectroscopic techniques.

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.     

Comparative SPR study on the effect of nanomaterials on the biological activity of adsorbed proteins

Bioactivity of proteins is evaluated to test the adverse effects of nanoparticles interjected into biological systems. Surface plasmon resonance (SPR) spectroscopy detects binding affinity that is normally related to biological activity. Utilizing SPR spectroscopy, a concise testing matrix is established by investigating the adsorption level of bovine serum albumin (BSA) and anti-BSA on the surface covered with 11-mercaptoundecanoic acid (MUA); magnetic nanoparticles (MNPs) and single-walled carbon nanotubes (SWCNTs), respectively. The immunoactivity of BSA on MNPs and SWCNT decreased by 18?% and 5?%, respectively, compared to that on the gold film modified with MUA. This indicates that MNPs cause a considerable loss of biological activity of adsorbed protein. This effect can be utilized for practical applications on detailed biophysical research and nanotoxicity studies.

Cytosine triphosphate-capped silver nanoparticles as a platform for visual and colorimetric determination of mercury(II) and chromium(III)

This work reports on a colorimetric platform for determination of chromium ions (Cr³⁺) and mercury ions (Hg²⁺) using silver nanoparticles (AgNPs) capped with cytosine triphosphate (CTP). The capped AgNPs were synthesized one-step by reduction of AgNO₃ in the presence of CTP. It was found that such AgNPs aggregate in the presence of Cr³⁺. This results in a decrease in the intensity of the surface plasmon resonance (SPR) band at 390 nm and the formation of a new red-shifted band at 510 nm, and consequently a color change from yellow to red. Different from the Cr³⁺-induced aggregation of AgNPs, exposure to Hg²⁺ causes the formation of a mercury layer around the surface of the AgNPs. This, in turn, causes the SPR absorption of the AgNPs to decrease and to undergo a slight blue shift, and this results in a fading of the yellow color. The findings are the basis of developing a new method for quantification of either Cr³⁺ or Hg²⁺, with detection limits of 6.25 μM for Cr³⁺ and of 0.125 μM for Hg²⁺, respectively. The method was applied to the determination of the two ions in spiked drinking water and lake water samples, and recoveries ranged from 94.5% to 101.3% for Cr³⁺, and from 96% to 108% for Hg²⁺, which is satisfactory for quantitative assays performed in water samples.

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Graphical abstract Cytosine triphosphate-capped silver nanoparticles (cAgNPs) are shown to represent a viable probe for visual and colorimetric detection of Hg²⁺ and Cr³⁺ via two different mechanisms: aggregation of cAgNPs in case of Cr³⁺; and amalgamation of cAgNPs in case of Hg²⁺.

     

A bifunctional,colorimetric and fluorescent probe for recognition of Cu2+ and Hg2+ and its application in molecular logic gate

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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.     

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.

Highly sensitive SERS probe for mercury(II) using cyclodextrin-protected silver nanoparticles functionalized with methimazole

We have developed a surface-enhanced Raman scattering (SERS) probe for the determination of mercury(II) using methimazole-functionalized and cyclodextrin-coated silver nanoparticles (AgNPs). These AgNPs in pH 10 solution containing sodium chloride exhibit strong SERS at 502 cm^?1. Its intensity strongly decreases in the presence of Hg(II). This effect serves as the basis for a new method for the rapid, fast and selective determination of trace Hg(II). The analytical range is from 0.50 μg L^?1 to 150 μg L^?1, and the limit of detection is 0.10 μg L^?1. The influence of 11 metal ions commonly encountered in environmental water samples was found to be quite small. The method was applied to the determination of Hg(II) in spiked water samples and gave recoveries ranging from 98.5 to 105.2 % and with relative standard deviations of <3.5 % (n?=?5). The total analysis time is <10 min for a single sample.