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A nanocomposite formed from graphene nanoplatelets (GNP) and manganese oxide (MnO2) nanoparticles (GNP/MnO2) was proposed as a novel and suitable support for enzyme immobilisation. The performances of screen-printed carbon electrodes (SPCEs) was highly improved after modification with GNP/MnO2 (SPCE/GNP/MnO2). The polyphenol index biosensor was prepared by surface modification of SPCE/GNP/MnO2 with drop coating of the laccase (from Trametes Versicolor) and Nafion®. All electrochemical measurements were carried out in acetate buffer, pH=4.60. The developed laccase biosensor shows fast and reliable amperometric response toward caffeic acid, as model compound, at operating potential of +0.40 V (vs. Ag/AgCl), with a linear range from 5 μmol L-1 to 2.75 mmol L-1 (r2 = 0.9997), with detection limit of 2.38 μmol L-1. Moreover, effects of possible interfering compounds were investigated. The developed procedure was successfully applied for the determination of total polyphenol content in red and white wine samples. In order to validate the proposed method, the polyphenol content in wine samples, under optimized parameters, was determined using a glassy carbon electrode. Recovery tests (95.7-97.5%) shows satisfactory accuracy and precision of the developed method, concluding that proposed construction of biosensor can offer fast, stable and reproducible determination of the polyphenol index. 26th Young Investigators’ Seminar on Analytical Chemistry, June 24 – 27, 2019, Pardubice, Czech Republic

A simple and highly sensitive electrochemical biosensor based on laccase immobilized onto a gold nanoparticle/graphene nanoplatelet-modified screen-printed carbon electrode (AuNP/GNPl/SPCE) was developed for the determination of hydroquinone (HQ) and other polyphenolic compounds. The biosensor shows excellent electro- catalytic activity towards oxidation of hydroquinone at a potential of -0.05 V (vs Ag/AgCl, 3M KCl) in phosphate buffer as supporting electrolyte (0.1 M, pH 7.0) using hydrodynamic amperometry. Analytical characteristics uncover that the AuNP/GNPl/SPCE comprises a wide linear range for the dependence of the signal on hydroquinone concentrations from 4 to 120 µM with a detection limit (3σ) of 1.5 µM. The repeatability (5 measurements, 100 µM hydroquinone) is ±2 % and the reproducibility (5 biosensors, 100 µM hydroquinone) is ±3 %. Interference studies of most common compounds with the determination of hydroquinone demonstrated negligible effects. Finally, the biosensor and the proposed analytical method were applied to the determination of total antioxidant capacity (TAOC) in wine, sirup and pharmaceutical drugs based on Trolox (6-hydroxy-2, 5, 7, 8- tetramethylchromane-2-carboxylic acid) and hydroquinone equivalents. The results were evaluated by using their calibration curves which were satisfactory and agreed well with the results obtained by the reference method Trolox Equivalent Antioxidant Capycity assay (TEAC- Assay).

The performance of screen-printed electrodes modified in situ with tellurium film for the anodic stripping voltammetric (ASV) determination of Cu(II) is reported. It was found that two types of screen-printed substrates, namely carbon and mesoporous carbon, were optimal for this application. The selected in situ tellurium film modified electrodes were applied for the square wave ASV determination of copper at μg L−1 concentration levels. Well-defined and reproducible Cu oxidation stripping peaks were produced at a potential more negative than the anodic dissolution of tellurium. The highest sensitivity of Cu determination was achieved in 0.05 M HCl containing 50 μg L−1 Te(IV) after 300 s of accumulation at −0.5 V. Using the optimized procedure, a linear range from 2 to 35 μg L−1 of Cu(II) was obtained with a detection limit of 0.5 μg L−1 Cu(II) (S/N=3) for 300 s of deposition time. Both sensors, carbon TeF-SPE and mesoporous carbon TeF-SPE, were successfully applied for the quantification of Cu in a certified reference surface water sample. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work a new electrochemical method will be presented for the determination of enalapril in pharmaceutical tablets using unmodified screen printed electrode (SPE). The studies were done using amperommetric detection. Enalapril provides well defined, ovalshape oxidation peak at +1.05 V (vs. Ag/AgCl, 3.0 M KCl) in Britton-Robinson buffer solution (BRBS) at pH 5.0. After optimization of the experimental conditions, the influence of most common interferent compounds was tested. Under optimized experimental conditions, the signals were linearly proportional to the concentration of enalapril in the range from 2.5 to 90 mu M with a detection limit of 0.9 mu M. Repeatability of ten replicate measurements of 5 mu M enalapril was estimated to be 1.5%. Proposed method was successfully applied for the determination of the total amount of enalapril content in pharmaceutical preparations. Nevertheless, proposed method could be beneficial for the quick quantifications of enalapril in drugs using unmodified SPE electrode without any further treatment.

In the present paper a glassy carbon electrode was successfully applied for the sensitive and selective determination of the indole alkaloid reserpine by differential pulse voltammetry (DPV). The electrochemical behavior of reserpine was investigated using cyclic voltammetry and best conditions for its quantification were examined by differential pulse voltammetry. Cyclic voltammetry has shown that the oxidation process is irreversible over the pH range 1 - 12 and that the electrode process is mainly diffusion controlled. The proposed procedure has wide linear concentration range for determination of reserpine by DPV from 2 mu M to 75 mu M with a detection limit 0.6 mu M. The sensor in combination with optimized electrochemical parameters was successfully applied for the determination of reserpine in model human urine with good recovery.

Hippocampal volume loss has been related to chronic stress as well as genetic factors. Although genetic and environmental variables affecting hippocampal volume have extensively been studied and related to mental illness, limited evidence is available with respect to G × E interactions on hippocampal volume. The present MRI study investigated interaction effects on hippocampal volume between three well-studied functional genetic variants (COMT Val158Met, BDNF Val66Met, 5-HTTLPR) associated with hippocampal volume and a measure of environmental adversity (life events questionnaire) in a large sample of healthy humans (n = 153). All three variants showed significant interactions with environmental adversity with respect to hippocampal volume. Observed effects were additive by nature and driven by both recent as well as early life events. A consecutive analysis of hippocampal subfields revealed a spatially distinct profile for each genetic variant suggesting a specific role of 5-HTTLPR for the subiculum, BDNF Val66Met for CA4/dentate gyrus, and COMT Val158Met for CA2/3 volume changes. The present study underscores the importance of G × E interactions as determinants of hippocampal volume, which is crucial for the neurobiological understanding of stress-related conditions, such as mood disorders or post-traumatic stress disorder (PTSD).