Low Concentration Limit Research Articles

Poly(luminol) based sensor array for determination of dissolved chlorine in water

An optical sensor for determination of free chlorine content of drinking water was prepared and tested. The function of the sensor is based on detecting chemiluminescence signal provided by thin immobilized poly(luminol) reagent layer. The poly(luminol) reagent film was prepared by electropolymerization of luminol onto planar indium-tin-oxide (ITO) electrode. Different methods, like electrode potential cyclization (cyclic voltammetry, CV), pulsed potential electrolysis (pulsed amperometry, PA) and potentiostatic electrolysis (constant potential electrolysis, PSE) were employed for preparation of the poly(luminol) layer. The chemoluminescence (chemiluminescence) of the differently prepared films was investigated in the poly(luminol) – hypochlorite – hydrogen peroxide reaction. Highest luminescence signal was obtained by the films prepared with CV. Poly(luminol) layers deposited with pulsed potential showed 80% less luminescence while almost no signal was obtained in case of films made with constant potential technique. The effects of the buffer composition and pH on the analytical properties of the electro polymerized sensing layer were investigated. The lower concentration limit of free chlorine detection was 5×10−7M in phosphate buffer at pH=8.0. It was found that the chemiluminescence signal decreased significantly when hypochlorite concentrations over 1mM were applied. An array of 24 micro wells was fabricated on ITO glass slab of about microscope slide size. The individual micro wells had identical volume and the poly(luminol) layer immobilized on their bottom had identical activity. The wells could be used for “single shot” determination of free chlorine content of drinking water. The long storage stability, the simple measurement procedure and low feasible concentration range makes the array an attractive analytical tool. Its applicability was proved measuring dissolved chlorine concentration of tap water samples.

The Reduced Hartree–Fock Model for Short-Range Quantum Crystals with Nonlocal Defects

In this article, we consider quantum crystals with defects in the reduced Hartree–Fock framework. The nuclei are supposed to be classical particles arranged around a reference periodic configuration. The perturbation is assumed to be small in amplitude, but need not be localized in a specific region of space or have any spatial invariance. Assuming Yukawa interactions, we prove the existence of an electronic ground state, solution of the self-consistent field equation. Next, by studying precisely the decay properties of this solution for local defects, we are able to expand the density of states of the nonlinear Hamiltonian of a system with a random perturbation of Anderson–Bernoulli type, in the limit of low concentration of defects. One important step in the proof of our results is the analysis of the dielectric response of the crystal to an effective charge perturbation.

Interplay between surface anisotropy and dipolar interactions in an assembly of nanomagnets

We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization, taking into account temperature, applied field, core and surface anisotropy, and dipolar interparticle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$ nanoparticle assemblies in the low concentration limit.

Profiling of urinary formestane and confirmation by isotope ratio mass spectrometry

Formestane (F, androst-4-en-4-ol-3,17-dione) is an irreversible aromatase inhibitor with the ability to suppress the estrogen production from anabolic steroids. Consequently, F is mentioned on the World Anti-Doping Agency (WADA) prohibited list and because studies have shown that F is produced endogenously in small amounts, a threshold for urinary excreted F of 150ng/mL was introduced. Lower concentrations could be due to endogenous production and need further investigation to prove the exact origin through determination of the carbon isotope ratio.However, because the current screening methods are a lot more sensitive, F is detected in practically every urine sample. A strict implementation of this WADA rule would imply that almost every urine sample needs additional investigation to verify an exogenous or endogenous origin. The main aim of this study was to propose and introduce a lower concentration limit of 25ng/mL beneath which the detected F is considered as being endogenous and no further investigation is needed. The data presented in this paper suggests that this threshold provides a good balance between a sufficiently large detection window and not having to perform isotope ratio mass spectrometry (IRMS) analyses on negative urine samples.

Low temperature operating In2−xNixO3 sensors with high response and good selectivity for NO2 gas

Pure In2O3 and solid state solution In2−xNixO3 nanofibers with different Ni-doping concentration were prepared via electrospinning method. Sensor based on In2−xNixO3 nanofibers with 6(mol)% Ni-doping concentration exhibited the highest response, good selectivity and low detectable concentration limit down to ppb levels of NO2 at a relatively low temperature of 90°C. These properties make the In2−xNixO3 nanofibers good candidates for NO2 detection.

Raman study on defective graphene: Effect of the excitation energy, type, and amount of defects

We present a detailed Raman study of defective graphene samples containing specific types of defects. In particular, we compared ${\mathit{sp}}^{3}$ sites, vacancies, and substitutional Boron atoms. We find that the ratio between the $D$ and $G$ peak intensities, $I$($D$)/$I$($G$), does not depend on the geometry of the defect (within the Raman spectrometer resolution). In contrast, in the limit of low defect concentration, the ratio between the ${D}^{\ensuremath{'}}$ and $G$ peak intensities is higher for vacancies than ${\mathit{sp}}^{3}$ sites. By using the local activation model, we attribute this difference to the term $C$${}_{S,x}$, representing the Raman cross section of $I$($x$)/$I$($G$) associated with the distortion of the crystal lattice after defect introduction per unit of damaged area, where $x$ $=$ $D$ or ${D}^{\ensuremath{'}}$. We observed that ${C}_{S,D}=0$ for all the defects analyzed, while ${C}_{S,{D}^{\ensuremath{'}}}$ of vacancies is 2.5 times larger than ${C}_{S,{D}^{\ensuremath{'}}}$ of ${\mathit{sp}}^{3}$ sites. This makes $I$($D$)/$I$(${D}^{\ensuremath{'}}$) strongly sensitive to the nature of the defect. We also show that the exact dependence of $I$($D$)/$I$(${D}^{\ensuremath{'}}$) on the excitation energy may be affected by the nature of the defect. These results can be used to obtain further insights into the Raman scattering process (in particular for the ${D}^{\ensuremath{'}}$ peak) in order to improve our understanding and modeling of defects in graphene.

Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm(-1) on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm(-1) below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti(4+) sites. At a higher density of defects, adsorption on Ti(3+) sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti(4+) sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm(-1). These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.

The important role of reactions between atoms and radicals in flame propagation in cylinder reactors

Results from studying features of the combustion of hydrogen in narrow cylinder reactors with diameters of <5.0 cm are presented. It is determined that the lower concentration limit of flame propagation in a reactor with a diameter of 1.1 cm is higher than 9% H2. The dependence of the principles of combustion on the method of initiation is studied. Mathematical simulations are performed that prove the impossibility of describing the combustion of hydrogen-deficient mixtures without assuming the cell structure of a flame. It is experimentally determined that increasing the termination of chains during the propagation of a flame results in its attenuation. It is noted that in addition to conductive heat abstraction, the recombination of atoms and radicals on the surface of the reactor plays an important role. It is concluded that these heterogeneous reactions representing the termination of reaction chains also result in the removal of recombination energy by the reactor’s walls.

Why are the Actions of Anesthetics on GABA-A Universal?

There is a long-standing debate on whether general anesthetics act through a non-specific perturbation of bilayer physical properties or through binding to specific sites within ion channels, particularly the GABA-A receptor. In this study, we demonstrate that a series of liquid n-alcohol general anesthetics lower phase transition temperatures in giant plasma membrane vesicles, which have previously been shown to sit close to a miscibility critical point. All n-alcohols depress critical temperatures (Tc) by 4±1°C when added to vesicles at their anesthetic dose. Current work is investigating if transition temperatures are also depressed when n-alcohols are added to synthetic vesicles with critical lipid compositions. We also performed simulations of simplified receptors embedded in a nearly-critical membrane. In this model, receptor channels can be in two distinct internal states (conducting or non-conducting), and the occupancy of these states is allosterically regulated by their local lipid environment as well as the availability of ligand. We show that model channels with dimensions comparable to that of GABA-A could have their conductance increased by 50% when Tc is lowered by 4°C in the limit of low ligand concentration. This is in good agreement with experimental observations of GABA-A channels in the presence of general anesthetics (compare to Figure 2b in [1]). Taken together, these findings suggest that general anesthetics can have dramatic effects on the internal states of membrane bound proteins without requiring that they directly bind to specific sites. Instead, we propose that anesthetics may act by lowering the critical temperature of the membrane which in turn allosterically regulates ion channel function.1. N. Franks and W. Lieb, Molecular and cellular mechanisms of general anesthesia. Nature, 367, 607 (1994).

Diffusion of Lithium in Bulk Amorphous Silicon: A Theoretical Study

The rate performance of lithium-ion secondary batteries depends critically on the kinetic transport of Li within the anode material. Here we use first-principles theoretical calculations to study the diffusion of Li in the low-concentration limit, using model electrodes of crystalline and four-fold coordinated bulk amorphous silicon. We identify Li diffusion pathways that have relatively low energy barriers ( 1 nm) distances depends on the atomic-scale features of the silicon host. We find that both the energy barriers for diffusion and the topology of the atomic structure control the diffusion. We estimate the diffusion rate in amorphous Si anode to be comparable to the rate in crystalline Si anodes. These findings shed light on the wide range of reported experimental results for Li diffusion in Si anodes.

Determination of the lower concentration limits of the spread of a flame through aerosols of nano-disperse powders

Information is presented for the relationship between fire and explosion risk of dusts and the equivalent particle diameter. Theoretical premises as to the variation of the lower concentration limit of the spread (LCLS) of the flame as the size of the disperse phase of the aerosol diminishes to “nano” dimensions, and also experimental data permitting comparison of LCLS values for aerosols of micro- and nano-particle organic substances are examined. A brief analysis and conclusion concerning the data obtained is presented. One of the problems encountered in lowering the fire- and explosion risk of production processes is strict control of the concentration of aerosols of solid combustible substances. Having information on the lower concentration limit of the spread (LCLS) of a flame will make it possible to adopt timely measures for prevention of fires and explosions at production facilities. A sufficient amount of informative data on indicators of the fires and explosion risk of various substances currently exists [1, 2 (Table 7.3.4) 3, 4]. Refinement of protective measures is required, however, in connection with the development of procedures. The particle size of the disperse phase is one of the most significant parameters affecting the combustion of aerosols [5]. For a majority of substances with an equivalent particle diameter of more than 500 μm, the ignition of an airborne suspension is impossible. And if the energy of the source can be increased for ignition of the mixture, acceleration of the initial heating, an increase in the completeness of combustion of the particles, and a lengthening of their dwell time in the suspended state are required for spread of the flame throughout the entire volume of a coarse-fraction aerosol.

Investigation of energy transfer in terbium doped Y 2SiO5 phosphor particles

The kinetics of luminescence of sol–gel synthesized terbium doped Y 2SiO5 (YSO) phosphor particles is investigated in detail with reference to Tb concentration in the 0.001%–10% range. By increasing the dopant concentration, the luminescence profile changes from a blue to a green peaked emission spectrum because of the energy transfer among centers. The inter-center energy transfer mechanism is well accounted for by the Inokuti–Hirayama (IH) kinetic model which is based on a statistical average of inter-center distance dependent decay modes of the donor luminescence. The distribution of the decay modes is implemented from the Förster–Dexter resonance theory of energy transfer by assuming a rate constant for the energy transfer by multipolar interactions between donors and acceptors. However, the experimental results recorded in the low concentration limit show the presence of green emission contributions in the luminescence spectrum which cannot be related to the Tb concentration; for this reason an additional internal energy transfer mechanism, occurring among levels of the same center, is proposed to account for the recorded emission properties. Thus, a new and more exhaustive model which includes both the internal and external energy transfer processes is considered; the proposed model allows a better explanation of the spectroscopic features of Tb related centers in YSO crystals and discloses the critical concentration and the quantum yields of the different energy transfer mechanisms.

2D Capillary Electrophoresis Hyphenated with Spectral Detection for the Determination of Quinine in Human Urine

The possibilities of a column coupling two-dimensional capillary electrophoresis (2D CE) combined with fiber-based diode array detection (DAD) for the direct, highly reliable and ultrasensitive quantitative determination of quinine in real multicomponent ionic matrices (human urine) are demonstrated in this work. The capillary isotachophoresis (CITP) stage provided an on-line sample pretreatment (elimination of interfering matrix constituents, preseparation and preconcentration of the analyte) before the capillary zone electrophoresis (CZE) separation. Due to the large volume (30 µL) sample injection and CITP sample preconcentration, a simple absorbance photometric detection was sufficient for obtaining very low concentration limits of detection (∼8.6 ng/mL). The combination of the different separation mechanisms (CITP and CZE) resulted in enhanced separation selectivity. This enabled us to obtain a pure analyte zone in the directly injected real samples suitable for qualitative and quantitative evaluation. The spectral DAD allowed (i) characterization of the purity (i.e., spectral homogeneity) of the analyte zone; and (ii) preliminary indication of structurally related compounds (i.e., potential biodegradation products of quinine), via characteristic spectra recorded in intervals of 200-800 nm. The CITP-CZE-DAD method was characterized by favorable performance parameters that are suitable for its routine biomedical use. One of the primary benefits of the CITP-CZE-DAD method is the possibility of performing direct injections of real biological samples while avoiding external sample preparation procedures and, therefore, enhancing the reliability and applicability of analyses and the potential for method automatization and miniaturization.

Secondary Electrospray Ionization of Complex Vapor Mixtures. Theoretical and Experimental Approach

In secondary electrospray ionization (SESI) systems, gaseous analytes exposed to an electrospray plume become ionized after charge is transferred from the charging electrosprayed particles (the charging agent) to the vapor species. Currently available SESI models are valid for simplified systems having only one type of electrosprayed species, which ionizes only one single vapor species, and for the limit of low vapor concentration. More realistic models require considering other effects. Here we develop a theoretical model that accounts for the effects of high vapor concentration, saturation effects, interferences between different vapor species, and electrosprays producing different types of species from the liquid phase. In spite of the relatively high complexity of the problem, we find simple relations between the different ionic species concentrations that hold independently of the particular ion source configuration. Our model suggests that an ideal SESI system should use highly concentrated charging agents composed preferably of only one dominating species with low mobility. Experimental measurements with a MeOH-H(2)O-NH(3) electrospray and a mixture of fatty acids and lactic acid served to test the theory, which gives good qualitative results. These results also suggest that the SESI ionization mechanism is primarily based on ions rather than on charged droplets.

Rules governing flame propagation in fuel-deficient hydrogen-air mixtures in cylindrical reactors

The special features of flame propagation in hydrogen-air mixtures with low hydrogen contents in horizontal cylindrical reactors with different diameters were studied. The dependences of the lower concentration limit of flame propagation and flame propagation velocity on the diameter of reactors and the chemical properties of the surface were determined. The character of the dependence of the flame cellular structure on mixture composition and reactor diameter was studied.

Internal Diffusion-Controlled Enzyme Reaction: The Acetylcholinesterase Kinetics.

Acetylcholinesterase is an enzyme with a very high turnover rate; it quenches the neurotransmitter, acetylcholine, at the synapse. We have investigated the kinetics of the enzyme reaction by calculating the diffusion rate of the substrate molecule along an active site channel inside the enzyme from atomic-level molecular dynamics simulations. In contrast to the previous works, we have found that the internal substrate diffusion is the determinant of the acetylcholinesterase kinetics in the low substrate concentration limit. Our estimate of the overall bimolecular reaction rate constant for the enzyme is in good agreement with the experimental data. In addition, the present calculation provides a reasonable explanation for the effects of the ionic strength of solution and the mutation of surface residues of the enzyme. The study suggests that internal diffusion of the substrate could be a key factor in understanding the kinetics of enzymes of similar characteristics.

Mathematical modelling and computer simulation of Brownian motion and hybridisation of nanoparticle–bioprobe–polymer complexes in the low concentration limit

A wide variety of nano-biotechnological applications are being developed for nanoparticles based on in vitro diagnostic and imaging systems. Some of these systems allow highly sensitive detection of molecular biomarkers. Frequently, the very low concentration of the biomarkers makes very difficult the mathematical simulation of the motion of nanoparticles based on classical, partial differential equations. We address the issue of incubation times for low concentration systems using Monte Carlo simulations. We describe a mathematical model and computer simulation of Brownian motion of nanoparticle–bioprobe–polymer contrast agent complexes and their hybridisation to immobilised targets. We present results for the dependence of incubation times on the number of particles available for detection, and the geometric layout of the DNA-detection assay on the chip.

Tubulin-Blocked State of VDAC Probed by Polymer Partitioning and Bilayer Surface Charge

Recently reported functional interaction between voltage-dependent anion channel of the outer mitochondrial membrane, VDAC, and dimeric tubulin is observed as reversible channel blockage. The tubulin-blocked state is only partially closed for small ions and, in 1 M KCl solutions, its conductance stays at about 40 % of that for the open channel. Reconstituting VDAC into planar lipid membranes we have studied the nature of the tubulin-blocked state of the channel using partitioning of poly-(ethylene glycol)s, reversal potential measurements, and manipulation of the membrane surface charge. We found that judged by polymer partitioning an estimated reduction of the open-state channel radius upon tubulin blockage is about 26 %, but the channel selectivity reverses from anionic to cationic. In the limit of low salt concentrations (down to 0.05 M KCl) the current of the blocked state is decreased by positive and increased by negative charge of membrane lipids; open state of the VDAC is virtually insensitive to the surface charge. This behavior can be understood taking into account the cationic selectivity of the blocked state and the increased proximity of ion pathway(s) to the nearest lipid headgroups as compared with the open state of the channel. Interestingly, the sensitivity of the tubulin-VDAC blockage to the applied transmembrane voltage characterized by the effective gating charge practically does not depend on salt concentration in the bulk or on lipid charge, staying within the range of about 10–14 elementary charges.

Measurement of absolute concentrations of individual compounds in metabolite mixtures by gradient-selective time-zero 1H-13C HSQC with two concentration references and fast maximum likelihood reconstruction analysis.

Time-zero 2D (13)C HSQC (HSQC(0)) spectroscopy offers advantages over traditional 2D NMR for quantitative analysis of solutions containing a mixture of compounds because the signal intensities are directly proportional to the concentrations of the constituents. The HSQC(0) spectrum is derived from a series of spectra collected with increasing repetition times within the basic HSQC block by extrapolating the repetition time to zero. Here we present an alternative approach to data collection, gradient-selective time-zero (1)H-(13)C HSQC(0) in combination with fast maximum likelihood reconstruction (FMLR) data analysis and the use of two concentration references for absolute concentration determination. Gradient-selective data acquisition results in cleaner spectra, and NMR data can be acquired in both constant-time and non-constant-time mode. Semiautomatic data analysis is supported by the FMLR approach, which is used to deconvolute the spectra and extract peak volumes. The peak volumes obtained from this analysis are converted to absolute concentrations by reference to the peak volumes of two internal reference compounds of known concentration: DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid) at the low concentration limit (which also serves as chemical shift reference) and MES (2-(N-morpholino)ethanesulfonic acid) at the high concentration limit. The linear relationship between peak volumes and concentration is better defined with two references than with one, and the measured absolute concentrations of individual compounds in the mixture are more accurate. We compare results from semiautomated gsHSQC(0) with those obtained by the original manual phase-cycled HSQC(0) approach. The new approach is suitable for automatic metabolite profiling by simultaneous quantification of multiple metabolites in a complex mixture.

Determination of quinine in beverages by online coupling capillary isotachophoresis to capillary zone electrophoresis with UV spectrophotometric detection

The present study illustrates the possibilities of capillary isotachophoresis (CITP) online coupled with capillary zone electrophoresis (CZE) and hyphenated with fiber-based spectrophotometric diode array detection (DAD) for the direct, highly reliable, and ultrasensitive determination of quinine (QUI) in real multicomponent ionic matrices (beverages). Here, the CITP provided an effective online sample pretreatment (preseparation and preconcentration) prior to the CZE separation. Due to the CITP sample preconcentration, a simple UV-visible absorbance spectrophotometric detection was sufficient for obtaining very low concentration limits of detection (~2.3 ng/mL). Enhanced separation selectivity due to the combination of different separation mechanisms (CITP vs. CZE) enabled to obtain a pure analyte zone, suitable for its detection and quantitation in the directly injected real samples. The spectrophotometric DAD, unlike single wavelength UV detection, enabled to characterize the purity (i.e. spectral homogeneity) of the analyte zone and preliminary data indicate structurally related compounds via characteristic spectra recorded in the interval of 200-600 nm. The proposed CITP-CZE-DAD method was characterized by favorable performance parameters (sensitivity, linearity, precision, recovery, accuracy, robustness, and selectivity) and successfully applied to the control of QUI and potential QUI impurities in commercial beverages. This method is proposed as a routine automatized method for the highly reliable quality food control.