Adsorptive Stripping Voltammetry and/or Chronopotentiometry. Comparison and Assessment

How to overcome the difficulty in assaying Ni in biodiesel samples? Extraction induced by microemulsion breaking and square wave adsorptive stripping voltammetry could be the answer

Herein, a simple, green, and relatively inexpensive approach to determine nickel (Ni) in biodiesel samples by square wave adsorptive cathodic stripping voltammetry (SWAdCSV) is presented. A method based on the accumulation of Ni as Ni(II)-dimethylglyoxime (Ni(II)(HDMG)2) on the glassy carbon electrode was carried out in a solution containing the aqueous phase extract (APhEx) obtained from an extraction induced by microemulsion breaking (EIMB), which was achieved by adding a few microliters of ultrapure water to a microemulsion composed of biodiesel, n-propanol and a diluted HNO3 solution. The LOD and LOQ were 0.2 μg L-1 and 0.8 μg L-1, respectively, and the accuracy was evaluated by recovery assays of spiked samples and by analyzing a standard reference material. Results obtained from a comparative method (HR-CS GF AAS) were also used for this evaluation. The method was applied to biodiesel samples produced from different feedstocks. To the best of the authors knowledge, it is the first time that: 1) Ni in biodiesel is determined by a voltammetric method; 2) EIMB is applied to extract Ni from this matrix and 3) this type of sample preparation method is used with adsorptive stripping voltammetry.

A simple, fast and inexpensive approach to quantify low concentrations of iron in biodiesel by voltammetry after extraction induced by microemulsion breaking

An alternative approach to assay iron (Fe) in biodiesel by differential pulse adsorptive cathodic stripping voltammetry (DPAdCSV) is presented herein. The sample treatment involved a simple, rapid, but effective extraction of Fe from biodiesel into an aqueous phase after microemulsion (ME) breaking. Then, Fe was determined as the complex Fe(III)-PAN (1-(2-pyridylazo)-2-naphthol) on a glassy carbon electrode (GCE) in the presence of bismuth (Bi(III)). The extraction induced by microemulsion breaking (EIMB) was achieved by adding 0.80 mL of ultrapure water into a water-in-oil ME containing 7.00 mL biodiesel, 2.70 mL n-propanol and 0.30 mL of 0.25 mol L-1 HNO3 solution. No deliberate addition of surfactant was necessary to form and maintain the ME. The EIMB resulted in a 1.30 mL lower aqueous phase extract (APhEx) and an upper oily phase. DP voltammograms were recorded with a portable potentiostat, showing the potentiality of carrying out the determination out of a central laboratory. Another feature was the non-necessity of deaerating the solution to eliminate the dissolved O2. The limits of detection (LOD) and quantification (LOQ) were 1.7 μg L-1 (140 mg kg-1) and 5.5 μg L-1 (455 mg kg-1), respectively. The accuracy of the method was evaluated by recovery assays of spiked samples, by analyzing a standard reference material and by comparisons with high-resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GF AAS).

Chronopotentiometric sensing of specific interactions between lysozyme and the DNA aptamer

Specific DNA-protein interactions are vital for cellular life maintenance processes, such as transcriptional regulation, chromosome maintenance, replication and DNA repair, and their monitoring gives valuable information on molecular-level organization of those processes. Here, we propose a new method of label-free electrochemical sensing of sequence specific binding between the lysozyme protein and a single stranded DNA aptamer specific for lysozyme (DNA_apta) that exploits the constant current chronopotentiometric stripping (CPS) analysis at modified mercury electrodes. Specific lysozyme-DNA_apta binding was distinguished from nonspecific lysozyme-DNA interactions at thioglycolic acid-modified mercury electrodes, but not at the dithiothreitol-modified or bare mercury electrodes. Stability of the surface-attached lysozyme-DNA_apta layer depended on the stripping current (I_str) intensity, suggesting that the integrity of the layer critically depends on the time of its exposure to negative potentials. Stabilities of different lysozyme-DNA complexes at the negatively polarized electrode surface were tested, and it was shown that structural transitions of the specific lysozyme-DNA_apta complexes occur in the I_str ranges different from those observed for assemblies of lysozyme with DNA sequences capable of only nonspecific lysozyme-DNA interactions. Thus, the CPS allows distinct discrimination between specific and non-specific protein-DNA binding and provides valuable information on stability of the nucleic acid-protein interactions at the polarized interfaces.

Early stage of nucleic acid electrochemistry. Detection of DNA damage in X-ray-irradiated rats

First papers on electroactivity of DNA and RNA were published more then 50 years ago. For about 8 years oscillographic polarography at controlled a.c. (OP, proposed by J. Heyrovský already in 1941) was the method of choice for DNA analysis. Since approximately 1954 Robert Kalvoda developed OP for wide application in various fields. It is shown that already before 1960 it was possible to detect damage to DNA in X-ray-irradiated rats by means of OP. DNA samples from irradiated animals produced significantly larger OP anodic guanine signal indicating changes in the DNA structure. At present, radiation-induced strand breaks and damage to bases in DNA can be electrochemically detected at high sensitivity.

Effect of Polycyclic Aromatic Hydrocarbons on Detection Sensitivity of Ultratrace Nitroaromatic Compounds

Polycyclic aromatic hydrocarbons (PAHs) with different numbers of pi-electrons and geometric symmetry of pi-systems, including anthracene, phenanthrene, pyrene, triphenylene, perylene, benzo[ghi]perylene, and coronene, were chosen to modify glassy carbon electrodes (GCEs) by self-assembling. The self-assembled monolayer of PAHs was investigated by STM and was used in the electrochemical detection of nitroaromatic compounds (NACs). The results indicate that PAH-modified GCE shows higher sensitivity to NACs than an unmodified one. Among the seven different PAHs, coronene-modified GCE exhibits the highest sensitivity to 2,4,6-trinitrotoluene and 1,3,5-trinitrobenzene.

Palladium and the electrochemical quartz crystal microbalance: a new method for the in situ analysis of the precious metal in aqueous solutions

A new method for the quantitative determination of palladium(II) by the electrochemical quartz crystal microbalance (EQCM) technique has been developed. Using a bare carbon-coated quartz crystal, Pd(II) ions are directly deposited from aqueous solution as palladium metal onto the crystal surface, and the Pd(II) concentration is determined with a detection limit of 0.0156 mM, or 1.66 ppm. No complexing agent or preconcentration of palladium is required for the analysis. The palladium is stripped from the crystal through its electrochemical oxidation, regenerating the crystal for subsequent multi-cycle palladium analyses. A conventional gold-coated quartz crystal was incapable of carrying out the same measurements. The EQCM technique presented is simple, sensitive, and reproducible for the detection of this widely used precious metal.