Potentiometric characterization of weak acids by multiple sample addition-I. Linear equations and intrinsic performance of the method

The suitability of linearized multiple sample-addition for the potentiometric characterization of monoprotic weak acids is examined. Linear equations for the separate or simultaneous determination of the acidity constant and of the sample concentration by treatment of experimental data are introduced. The intrinsic performance of the method and the application range of the different equations are discussed with reference to the theoretical effect of measurement errors on the values of the quantities to be determined.     

Comparison of chromium speciation by CZE and ion exchange followed by AAS

The hydrogen chromate anion (HCrO4-), which is the predominant species in acidic solutions and solutions with low chromium concentration, was determined by capillary zone electrophoresis (CZE) using UV detection on-column at 200 nm. A fused-silica capillary (55 cm x 50 microm i.d.) was employed with a high negative voltage of 20 kV. Total chromium was determined after reduction by H2O2 and its complexation by EDTA. The use of H2O2 as reducing agent is advantageous, as it does not increase the conductivity of the solution. Detection limits achieved (for 200 s injection time) were 30 and 8 microg/L for Cr(VI) and Cr(III), respectively. The CZE results obtained for Cr(III) and Cr(VI) were compared with those obtained by ion exchange with subsequent AAS.     

Nanomaterials and lab-on-a-chip technologies

Lab-on-a-chip (LOC) platforms have become important tools for sample analysis and treatment with interest for DNA, protein and cells studies or diagnostics due to benefits such as the reduced sample volume, low cost, portability and the possibility to build new analytical devices or be integrated into conventional ones. These platforms have advantages of a wide set of nanomaterials (NM) (i.e. nanoparticles, quantum dots, nanowires, graphene etc.) and offer excellent improvement in properties for many applications (i.e. detectors sensitivity enhancement, biolabelling capability along with other in-chip applications related to the specificities of the variety of nanomaterials with optical, electrical and/or mechanical properties). This review covers the last trends in the use of nanomaterials in microfluidic systems and the related advantages in analytical and bioanalytical applications. In addition to the applications of nanomaterials in LOCs, we also discuss the employment of such devices for the production and characterization of nanomaterials. Both framed platforms, NMs based LOCs and LOCs for NMs production and characterization, represent promising alternatives to generate new nanotechnology tools for point-of-care diagnostics, drug delivery and nanotoxicology applications.     

Correlations and Fluctuations of Charged Colloids as Determined by Anomalous Small‐Angle X‐Ray Scattering

Summary: We performed molecular dynamics simulation of a charged colloidal particle with explicit counterions. Our work provides a direct comparison between simulations and ASAXS‐experiments, offering insight into the counterion distribution of charged colloidal suspensions. We give a detailed constitution of the appearing scattering terms with their physical meaning. It is shown that the cross‐correlation between a macroion and its counterions gives the meanfield approximation of the counterion density even if the counterion system is highly fluctuating. Furthermore, it is shown that cross‐correlations can be negative due to oscillations of the density amplitudes of the macroion and counterions and, therefore, must be distinguished from other scattering contributions. These oscillations become more pronounced if the counterions exhibit a fixed shape and if the size of the macroion and that of the counterion system are different.

Simulation sanpshot of a charged colloid (big central sphere) with counterions (small spheres).     

A Practical Approach to Potentiometric Biosensors Based on Consolidated Composites: Construction and Evaluation of a D-Amygdalin Biosensor

A simple procedure to be used in an analytical chemistry laboratory by undergraduate students to prepare a potentiometric biosensor for D-amygdalin is given. The membrane material is prepared by simple compression of a solid sensing mixture (-glucosidase mixed with Ag₂S and AgI). This new technology has some advantages. It presents a simple way to prepare a biosensor membrane and this methodology adapts well to mass production technology. Simple polishing before each new measurement can renew the membrane. This type of biosensor produced by consolidated biocomposites can serve as a base material for different biosensing schemes.Using this technique, students can easily envision the functioning of a potentiometric biosensor where the classic detection mechanism as of an I-ISE is combined with the biological recognition of an enzyme.     

Particle-based detection of DNA hybridization using electrochemical stripping measurements of an iron tracer

Two particle-based procedures for monitoring DNA hybridization based on electrochemical stripping detection of an iron tracer are described. The first protocol involves probes labeled with gold-coated iron core-shell nanoparticles, while the second route relies on detecting the iron content of magnetic-sphere tags. In both cases, the captured iron-containing particles are dissolved following the hybridization, and the released iron is quantified by cathodic-stripping voltammetry in the presence of the 1-nitroso-2-naphthol ligand and a bromate catalyst. Both protocols offer high sensitivity, a well-defined concentration dependence, and minimal contributions from non-complementary nucleic acids. The iron-containing particle signal amplifiers thus represent a very useful addition to the arsenal of metal tracers employed in electrical bioassays.     

Amino Acid Determination Using Screen-Printed Electrochemical Sensors

The use of a screen-printed electrochemical sensor (SPES) strip, consisting of a two-electrode system (a carbon working electrode and Ag/AgCl reference electrode), for amino acid determination is reported.The behaviour of the SPES toward cysteine and tyrosine is investigated using linear sweep and hydrodynamic voltammetries. The SPES operate at a lower oxidation potential (vs. Ag/AgCl) compared with traditional carbon and platinum electrodes, in a buffer solution (pH = 7) prepared using 0.1 M phosphates and 0.1 M KCl. The linear response lies between 5 * 10⁻⁵ M and 5 * 10⁻⁴ M for both amino acids while the sensitivity is 2.95±0.03 µA mM⁻¹ for cysteine and 8.00±0.01 µA mM ⁻¹for tyrosine; the correlation coefficient is higher than 0.9980. The sensors were applied to the analysis of some commercial pharmaceutical samples, and the results suggested that the devices hold promise in this application area.     

Potentiometric characterization of weak acids by multiple sample addition II. The effect of chemical interferences and the practical performance of linearization methods

Potentiometric multiple addition of a sample containing a pure weak acid to a solution of supporting electrolyte has been previously shown [C. Maccà and A. Merkoci, Talanta, 41 (1994) 2033] to be formally suitable for the determination of the dissociation constant and concentration of the sample acid. Linear equations have been developed for the treatment of experimental data to yield, simultaneously or separately, the chemical parameters of the acid solution. These equations are now tested on real samples together with analogous equations for titrations, and the results are compared with those obtained with rigorous statistical methods. For the determination of the acidity constant with samples of known concentration, multiple samples addition is comparable with titration. When the sample concentration is unknown and must be determined simultaneously to the acidity constant, the results obtained by linearized multiple sample addition can be seriously affected by impurities present, even at low level, either in the sample or in the supporting solution. Linear equations accounting for the effects of basic or acidic impurities in the sample or in the supporting solution are developed. Sample addition is confirmed to be a useful complement to pH-metric titration for the determination of acidity constants of moderately weak acids by non-linear regression; linearization of data is a convenient technique for screening purposes and a powerful means of detecting and correcting some common pitfalls, interferences and contaminations, whose effects are enhanced in linearized sample addition.     

Using genetic algorithm for dynamic and multiple criteria web-site optimizations

In today’s competitive electronic marketplace, companies try to create long-lasting relations with their online customers. Log files and registration forms generate millions of online transactions. Companies use new techniques to “mine” these data and establish optimal online storefronts to maximize their web presence. Several criteria, such as minimization of download time, maximization of web-site visualization and product association level, can be used for the optimization of virtual storefronts. This paper introduces a genetic algorithm, to be used in a model-driven decision-support system for web-site optimizations. The algorithm ensures multiple criteria web-site optimizations, and the genetic search provides dynamic and timely solutions independent of the number of objects to be arranged.