Spark-Discharge-Activated 3D-Printed Electrochemical Sensors.

3D printing technology is a tremendously powerful technology to fabricate electrochemical sensing devices. However, current conductive filaments are not aimed at electrochemical applications and therefore require intense activation protocols to unleash a suitable electrochemical performance. Current activation methods based on (electro)chemical activation (using strong alkaline solutions and organic solvents and/or electrochemical treatments) or combined approaches are time-consuming and require hazardous chemicals and dedicated operator intervention. Here, pioneering spark-discharge-activated 3D-printed electrodes were developed and characterized, and it was demonstrated that their electrochemical performance was greatly improved by the effective removal of the thermoplastic support polylactic acid (PLA) as well as the formation of sponge-like and low-dimensional carbon nanostructures. This reagent-free approach consists of a direct, fast, and automatized spark discharge between the 3D-electrode and the respective graphite pencil electrode tip using a high-voltage power supply. Activated electrodes were challenged toward the simultaneous voltammetric determination of dopamine (DP) and serotonin (5-HT) in cell culture media. Spark discharge has been demonstrated as a promising approach for conductive filament activation as it is a fast, green (0.94 GREEnness Metric Approach), and automatized procedure that can be integrated into the 3D printing pipeline.

Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings.

The first racemization-stable helicene derivatives fluorinated at terminal rings, 1,2,3,4-tetrafluoro[6]helicene (6) and 1,2,3,4,13,14,15,16-octafluoro[6]helicene (15), were synthesized via the Wittig reaction followed by oxidative photocyclization in an overall yield of 41% of 6 and 76% of 15. The changed electronic structure in fluorinated helicenes was reflected in a slight shift of UV-vis absorption, fluorescence excitation, and emission spectra maxima when compared to unsubstituted [6]helicene. Cyclic voltammetry revealed a moderate decrease in the HOMO-LUMO gap with increasing fluorination. The specific rotation of tetrafluoro[6]helicene 6 enantiomers was found to be approximately 25% lower than that of unsubstituted [6]helicene. The theoretical study of the racemization barrier suggested a reasonable shift toward higher energy with increasing fluorination. The increasing fluorination also significantly affected the intermolecular interactions in the crystal lattice. The observed CH···F interactions led to the formation of 1D-molecular chains in the crystal structures of both fluorinated helicenes.

eL-Chem Viewer: a freeware package for the analysis of electroanalytical data and their post-acquisition processing.

In electrochemical sensing, a number of voltammetric or amperometric curves are obtained which are subsequently processed, typically by evaluating peak currents and peak potentials or wave heights and half-wave potentials, frequently after background correction. Transformations of voltammetric data can help to extract specific information, e.g., the number of transferred electrons, and can reveal aspects of the studied electrochemical system, e.g., the contribution of adsorption phenomena. In this communication, we introduce a LabView-based software package, 'eL-Chem Viewer', which is for the analysis of voltammetric and amperometric data, and enables their post-acquisition processing using semiderivative, semiintegral, derivative, integral and elimination procedures. The software supports the single-click transfer of peak/wave current and potential data to spreadsheet software, a feature that greatly improves productivity when constructing calibration curves, trumpet plots and performing similar tasks. eL-Chem Viewer is freeware and can be downloaded from www.lchem.cz/elchemviewer.htm.

Electrochemical HPLC Determination of Piperazine Antihistamine Drugs Employing a Spark-Generated Nickel Oxide Nanoparticle-Modified Carbon Fiber Microelectrode.

In this work, we demonstrate a sensitive high-performance liquid chromatography (HPLC) method for the determination of piperazine antihistamine drugs employing innovative electrochemical detection based on a spark-generated nickel oxide nanoparticle-modified carbon fiber microelectrode built into a miniaturized electrochemical detector. The direct carbon fiber-to-nickel plate electrode spark discharge, carried at 0.8 kV DC, with the nickel electrode connected to the negative pole of the high-voltage power supply, provides extremely fast (1 s) in situ tailoring of the carbon fiber microelectrode surface by nickel oxide nanoparticles. It has been found that nickel oxide nanoparticles exhibit an electrocatalytic effect toward the piperazine moiety electrooxidation process, as confirmed by voltammetric experiments, revealing the shift in the peak potential from 1.25 to 1.09 V versus Ag/AgCl. Cetirizine, cyclizine, chlorcyclizine, flunarizine, meclizine, and buclizine were selected as sample piperazine antihistamine drugs, while diclofenac served as an internal standard. The isocratic reversed-phase separation of the above set was achieved within 15 min using an ARION-CN 3 µm column with a binary mobile phase consisting of 50 mM phosphate buffer (pH 3) and methanol (45/55, v/v). The limits of detection (LOD) were within the range of 3.8-120 nM (for cyclizine and buclizine) at E = +1500 mV (vs Ag/AgCl), while the response was linear within the concentration range measured up to 5 µmol L-1. The method was successfully applied to the determination of piperazine antihistamine drugs in spiked plasma samples.

Sensitive riboflavin sensing using silver nanoparticles deposited onto screen-printed electrodes via controlled-energy spark discharges.

In this work, we elaborated the graphite screen-printed electrodes (SPEs) modification with metal nanoparticles formed as a result of spark discharges produced between a metal wire electrode and SPE that are connected to an Arduino board-based DC high voltage power supply. This sparking device allows, on the one hand, the toposelective formation of NPs of controlled dimensions through a direct and liquid-free approach, and on the other hand, controls the number and energy of the discharges delivered to the electrode surface during a single spark event. This way, the potential damage to the SPE surface by the action of heat evolved during the sparking process is considerably minimized compared with the standard setup in which each spark event consists of multiple electrical discharges. Data demonstrated that the sensing properties of the resulting electrodes are significantly improved compared with those achieved when conventional spark generators are employed, as demonstrated for silver-sparked SPEs that exhibit enhanced sensitivity to riboflavin. Sparked AgNp-SPEs were characterized using scanning electron microscopy and voltammetric measurements in alkaline conditions. The analytical performance of sparked AgNP-SPEs was evaluated by various electrochemical techniques. Under optimum conditions, the detection range for DPV was from 1.9 (LOQ) to 100 nM riboflavin (R2 = 0.997), while a limit of detection (LOD, S/N 3) of 0.56 nM was achieved. The analytical utility is demonstrated for the determination of riboflavin in the real matrices of B-complex pharmaceutical preparation and an energy drink.

Electrochemical behavior and determination of rutin on modified carbon paste electrodes.

The performances of ionic liquid (1-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide, IL/CPE) and iron phthalocyanine (IP/CPE) modified carbon paste electrodes in electroanalytical determinations of rutin were evaluated and compared to the performance of unmodified carbon paste electrode (CPE). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), differential pulse adsorptive stripping voltammetry (DPAdSV), and amperometry were used for rutin analysis. The best current responses of rutin were obtained at pH 4.0 for all tested techniques. IL/CPE electrode was found to perform best with DPAdSV technique, where a detection limit (LOD) as low as 5 nmol L(-1) of rutin was found. On the other hand, IP/CPE showed itself to be an optimum choice for DPV technique, where LOD of 80 nmol L(-1) was obtained. Analytical applicability of newly prepared electrodes was demonstrated on determination of rutin in the model samples and the extracts of buckwheat seeds. To find an optimum method for buckwheat seeds extraction, a boiling water extraction (BWE), Soxhlet extraction (SE), pressurized solvent extraction (PSE), and supercritical fluid extraction (SFE) were tested.

Electrochemically pretreated carbon microfiber electrodes as sensitive HPLC-EC detectors.

The paper focuses on the analysis and detection of electroactive compounds using high-performance liquid chromatography (HPLC) combined with electrochemical detection (EC). The fabrication and utilization of electrochemically treated carbon fiber microelectrodes (CFMs) as highly sensitive amperometric detectors in HPLC are described. The applied pretreatment procedure is beneficial for analytical characteristics of the sensor as demonstrated by analysis of the model set of phenolic acids. The combination of CFM with separation power of HPLC technique allows for improved detection limits due to unique electrochemical properties of carbon fibers. The CFM proved to be a promising tool for amperometric detection in liquid chromatography.

Simultaneous determination of guanine and adenine in human saliva with graphite sparked screen-printed electrodes.

Saliva represents one of the most useful biological samples for non-invasive testing of health status and diseases prognosis and therefore, the development of advanced sensors enabling the determination of biomarkers in unspiked human whole saliva is of immense importance. Herein, we report on the development of a screen-printed graphite sensor modified with carbon nanomaterials generated by spark discharge for the determination of guanine and adenine in unspiked human whole saliva. The designed sensor was developed with a "green", extremely simple, fast (16 s), fully automated "linear mode" sparking process implemented with a 2D positioning device. Carbon nanomaterial-modified surfaces exhibit outstanding electrocatalytic properties enabling the determination of guanine and adenine over the concentration range 5 - 1000 nM and 25 - 1000 nM, while achieving limits of detection (S/N 3) as low as 2 nM and 8 nM, respectively. The sensor was successfully applied to the determination of purine bases in unspiked human whole saliva following a simple assay protocol based on ultrafiltration that effectively alleviates biofouling issues. Recovery was 96-108%.

Flavin-Helicene Amphiphilic Hybrids: Synthesis, Characterization, and Preparation of Surface-Supported Films.

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin-[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin-2,4-dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2-nitroso[6]helicene and 6-methylamino-3-methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1-monoolein) matrix was developed.

Vanadocene arsenate complexes: evidence of super-hyperfine coupling with (75)As.

The reaction of vanadocene dichloride with sodium arsenate gives Cp(2)V(O(2)AsO(2)H). This compound was identified on the basis of the super-hyperfine coupling observed in the solution EPR spectrum. The effect of substitution was studied on the ring-substituted and ansa-bridged compounds.

Sensors and microarrays in protein biomarker monitoring: an electrochemical perspective spots.

The development of clinically applicable portable sensors and multiplex protein biomarker assays is one of the most important goals of laboratory medicine today. Sensing strategies based on electrochemical devices are discussed in this overview, with special emphasis on detection principles derived from voltammetry, electrogenerated chemiluminescence, bipolar electrochemistry and impedance-based measurements. Up-to-date examples of electrochemical methods in biomedical research and development are highlighted here, including critical evaluation and future directions of the analysis, development and validation of new protein biomarkers.

Chiral Electrochemistry: Anodic Deposition of Enantiopure Helical Molecules.

Chirality is a fascinating phenomenon for recent electrochemistry and materials research, allowing for the preparation of detection platforms based on analyte enantiodiscrimination and the development of advanced chiroptical devices and chiral electrodes. In this Viewpoint, we highlight new directions in the field of chiral helical polyaromatic molecules (mainly helicenes) that are useful for the preparation of optically and redox-active polymers and/or self-assembled thin layers, nanostructures and functional electrode surfaces. Instead of the previously reported chiral materials prepared by molecular imprinting, a concept based on the preparation of inherently chiral helicene-based materials with (opto)electrochemical applicability is presented. A short overview of well-established electrochemical methods for the research of chiral molecules is also outlined.

A compact bipolar electrochemistry device utilizing a liquid free catholyte and eye visual indication of the reporting event for the determination of antioxidant capacity in real-world samples.

Here we report for the first time the development of a compact, closed bipolar electrochemistry (BPE) device for the determination of antioxidant capacity in real-world samples by recording the onset driving voltage at which an eye-visible reporting event at the cathode pole of bipolar electrode (BE) occurs. The BPE cell consists of two distinct anode and cathode compartments that are electrically connected through a platinum wire BE. The anode pole of the BE is covered by a cellulose acetate thin film to prevent the fouling of the surface, while the cathode pole is embedded into an agarose hydrogel containing phenolphthalein, i.e., an acid-base indicator. The method relies on the oxidation of the reducing compounds in the sample by a fixed amount of H2O2 and the electro oxidation of the remaining H2O2 in the anode pole of the BE. This reaction triggers the reduction of water at the cathodic pole accompanied by the production of hydroxyl ions at different, H2O2 concentration-dependent onset driving voltages. The resulting increase of pH at the cathode pole results in the formation of a pink spot (reporting event), which is visible by the unaided eye. The applicability of the BPE device for the determination of antioxidant capacity in fruit juices and sodas is demonstrated. The analytical results, expressed as Trolox (TEAC) or Vitamin C (CVEAC) equivalent antioxidant capacity, correlate well with those obtained by the CUPRAC method.

Lipids modulate H(2)S/HS(-) induced NO release from S-nitrosoglutathione.

Recently we observed that a gas messenger H(2)S/HS(-) released NO from S-nitrosoglutathione (Ondrias et al., Pflugers Arch. 457 (2008) 271-279). However, the effect of biological compounds on the release is not known. Measuring the NO oxidation product, which is nitrite, by the Griess reaction, we report that unsaturated fatty acid-linoleic acid and lipids having unsaturated fatty acids: asolectin, dioleoylphosphocholine and dioleoylphosphoserine depressed the H(2)S/HS(-) induced NO release from S-nitrosoglutathione. On the other hand, a depression effect of the saturated fatty acid-myristic acid and lipids having saturated fatty acids, dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine was less pronounced. The inhibition effect increased with the decreasing gel-to-liquid phase transitions temperature of the fatty acids and lipids. We suggest that lipid composition of biological membranes modulates NO release from nitrosoglutathione induced by H(2)S/HS(-), assuming that a reaction of H(2)S/HS(-) with unsaturated bonds of fatty acids may be partially responsible for the effect.

Continuous electrochemical monitoring of nitric oxide production in murine macrophage cell line RAW 264.7.

In this study, we realized the continual and long-term electrochemical detection of NO production by stimulated macrophages using modified porphyrinic microsensor. The NO release from RAW 264.7 cells stimulated by lipopolysaccharide started 5 h after the lipopolysaccharide administration. After reaching its maximum at the sixth hour, the stable level of NO production was observed between the seventh and 12th hour of the experiment. This phase was followed by a gradual decline in NO production. A close correlation between the NO signal detected with microelectrode and nitrite accumulation, which had been determined in supernatants removed from stimulated cells, was observed. This finding was utilized for the calibration of the electrochemical experiment. The presence of iNOS enzyme, which constitutes a main requirement for NO production by stimulated macrophages, was confirmed by Western blot analysis of iNOS protein expression at key time points of the corresponding electrochemical experiment. The capability of our microsensor to instantaneously monitor the changes in the NO production by stimulated RAW 264.7 cells was demonstrated by the immediate decrease in the signal due to NO as a response to the addition of iNOS inhibitor into the cell culture medium.

Lab-on-a-screen-printed electrochemical cell for drop-volume voltammetric screening of flunitrazepam in untreated, undiluted alcoholic and soft drinks.

Flunitrazepam, also known as "Rohypnol" or "Rophy" among other trade and street names, is an extremely potent benzodiazepine that is prescribed to treat severe insomnia. Due to these attributes, flunitrazepam, when is surreptitiously administered to an alcoholic or soft drink, is associated with "drug-facilitated sexual assault". We report here for the first time, a low cost lab-on-a-screen-printed electrochemical cell (SPC) based on iron-sparked graphite working electrode modified with glucose oxidase (GOx) and glucose hydrogel droplets (GluHD) for the detection of flunitrazepam. Iron-spark modification increases the response of the sensor by ca. 3-fold compared with that of the plain electrode, while an in situ deoxygenation process, based on GOx-glucose enzyme reaction, depletes dissolved oxygen. As a result, the method enables interference free voltammetric measurements of the electro reduction of the nitro group of flunitrazepam at ca. -0.71 to -0.78 V vs. Ag printed pseudo reference electrode depending on the sample's matrix, and the detection of the drug at the sub-millimolar level. GOx/GluHD-FeSPC was directly applied to the drop-volume (∼60 µL) detection of flunitrazepam to a wide range of untreated and undiluted spiked samples (Pepsi cola®, Vodka, Whisky, Tequila, Gin, and Rum) of different acidity (pH 2.3-8.4), and alcohol content up to 40% v/v. Data demonstrate the excellent performance of the sensor for point-of-need screening of flunitrazepam and suggest that GOx/GluHD-FeSPC holds promise as an effective analytical tool to prevent phenomena of covert drug administration.

Mechanism of the antioxidant action of silybin and 2,3-dehydrosilybin flavonolignans: a joint experimental and theoretical study.

Flavonolignans from silymarin, the standardized plant extract obtained from thistle, exhibit various antioxidant activities, which correlate with the other biological and therapeutic properties of that extract. To highlight the mode of action of flavonolignans as free radical scavengers and antioxidants, 10 flavonolignans, selectively methylated at different positions, were tested in vitro for their capacity to scavenge radicals (DPPH and superoxide) and to inhibit the lipid peroxidation induced on microsome membranes. The results are rationalized on the basis of (i) the oxidation potentials experimentally obtained by cyclic voltammetry and (ii) the theoretical redox properties obtained by quantum-chemical calculations (using a polarizable continuum model (PCM)-density functional theory (DFT) approach) of the ionization potentials and the O-H bond dissociation enthalpies (BDEs) of each OH group of the 10 compounds. We clearly establish the importance of the 3-OH and 20-OH groups as H donors, in the presence of the 2,3 double bond and the catechol moiety in the E-ring, respectively. For silybin derivatives (i.e., in the absence of the 2,3 double bond), secondary mechanisms (i.e., electron transfer (ET) mechanism and adduct formation with radicals) could become more important (or predominant) as the active sites for H atom transfer (HAT) mechanism are much less effective (high BDEs).

Natural feed additive of Macleaya cordata: safety assessment in rats a 90-day feeding experiment.

Macleaya cordata (Willd.) (Papaveraceae) is used as an active component in the natural feed additive Sangrovit. Sangrovit contains mixture of the intact aerial parts and the fraction of quaternary benzo[c]phenanthridine alkaloids from M. cordata (FQBA). In a 90-day pilot toxicity trial, Sangrovit and the FQBA were tested for safety. Male Wistar rats were fed for 90 days with 100, 7000 or 14000mg of Sangrovit or 600mg of FQBA in 1kg of feed. Body and organ weights, clinical chemistry and hematology markers, oxidative stress parameters, morphological structure of tongue, liver, ileum, kidney and heart samples, and total cytochrome P450 in liver were monitored. The results showed no statistically significant alterations in any parameter between control and treated animals, except for the group treated with 14000ppm Sangrovit that resulted in elevation of reduced glutathione level and superoxide dismutase activity in liver.

Nitric oxide sensor based on carbon fiber covered with nickel porphyrin layer deposited using optimized electropolymerization procedure.

Electropolymerization regime of meso-tetrakis(3-methoxy-4-hydroxyphenyl) porphyrin is optimized to yield films possessing both electrocatalytical and permselective properties towards nitric oxide oxidation. The sensor composed of electrochemically oxidized carbon fiber, covered solely with nickel porphyrin derivative layer electropolymerized using our method, is characterized by high selectivity towards nitrite (1:600), ascorbate (1:8000) and dopamine (>1:80), determined by constant potential amperometry at 830 mV (vs. Ag/AgCl). Selectivity for ascorbate and dopamine as well as detection limit for NO (1.5 nM at S/N=3) is 5-10 times better than parameters usually reported for Nafion coated porphyrinic sensors. Nafion coating can further enhance selectivity properties as well as aids to the stability of the sensors' responses.

Glucose sensing on graphite screen-printed electrode modified by sparking of copper nickel alloys.

Electric spark discharge was employed as a green, fast and extremely facile method to modify disposable graphite screen-printed electrodes (SPEs) with copper, nickel and mixed copper/nickel nanoparticles (NPs) in order to be used as nonenzymatic glucose sensors. Direct SPEs-to-metal (copper, nickel or copper/nickel alloys with 25/75, 50/50 and 75/25wt% compositions) sparking at 1.2kV was conducted in the absence of any solutions under ambient conditions. Morphological characterization of the sparked surfaces was performed by scanning electron microscopy, while the chemical composition of the sparked NPs was evaluated with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The performance of the various sparked SPEs towards the electro oxidation of glucose in alkaline media and the critical role of hydroxyl ions were evaluated with cyclic voltammetry and kinetic studies. Results indicated a mixed charge transfer- and hyroxyl ion transport-limited process. Best performing sensors fabricated by Cu/Ni 50/50wt% alloy showed linear response over the concentration range 2-400µM glucose and they were successfully applied to the amperometric determination of glucose in blood. The detection limit (S/N 3) and the relative standard deviation of the method were 0.6µM and <6% (n=5, 2µM glucose), respectively. Newly devised sparked Cu/Ni graphite SPEs enable glucose sensing with distinct advantages over existing glucose chemical sensors in terms of cost, fabrication simplicity, disposability, and adaptation of green methods in sensor's development.