Square Wave Voltammetry Experiments Research Articles

A Square Wave Voltammetry Study on the Antioxidant Interaction and Effect of Extraction Method for Binary Fruit Mixture Extracts

Square wave voltammetry (SWV) analysis was used to assess the antioxidant interactions (synergism, addition, and antagonism) of fruit mixture extracts from grape (G), lemon (L), and blueberry (B) obtained by conventional extraction, ultrasound-assisted extraction, and high hydrostatic pressure extraction. The experimental results showed antagonistic antioxidant effects in all binary mixture extracts (L-G, L-B, and G-B). In DPPH and FRAP assays, the greatest antioxidant capacity was found in the G-B mixture (108.7 and 108.8 μmol TE g−1dry extract, respectively) obtained by high hydrostatic pressure extraction; however, there were no significant differences when measured by ultrasound-assisted extraction. For TPC and TFC assays, the greatest values were for G-B (6.67 mg GA g−1dry extract) and L-G (1.63 mg QE g−1dry extract), respectively. SWV experiments showed antagonistic behavior in the mixtures. Among the different ratios of the fruit mixture extracts evaluated by SWV, 1 : 1 (w/w) combination showed the greatest antagonistic antioxidant effects. SWV suggests the components of the mixture with the highest antioxidant capacity oxidize after mixing. The results indicate that the presence of natural bioactive antioxidants in fruit mixtures does not guarantee that the interactions are synergistic.

Development of peptide biosensor for the detection of dengue fever biomarker, nonstructural 1.

Dengue virus (DENV) nonstructural 1 (NS1) protein is a specific and sensitive biomarker for the diagnosis of dengue. In this study, an efficient electrochemical biosensor that uses chemically modified affinity peptides was developed for the detection of dengue virus NS1. A series of amino acid-substituted synthetic peptides was rationally designed, chemically synthesized and covalently immobilized to a gold sensor surface. The sensor performance was monitored via square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS). Potential affinity peptides specific for NS1 were chosen according to the dynamic current decrease in SWV experiments. Using circular dichroism, the molar ellipticity of peptides (DGV BP1–BP5) was determined, indicating that they had a mostly similar in random coil structure, not totally identical. Using SWV, DGV BP1 was selected as a promising recognition peptide and limit of detection for NS1 was found to be 1.49 μg/mL by the 3-sigma rule. DGV BP1 showed good specificity and stability for NS1, with low signal interference. The validation of the sensor to detect NS1 proteins was confirmed with four dengue virus culture broth (from serotype 1 to 4) as proof-of-concept. The detection performance of our sensor incorporating DGV BP1 peptides showed a statistically significant difference. These results indicate that this strategy can potentially be used to detect the dengue virus antigen, NS1, and to diagnosis dengue fever within a miniaturized portable device in point-of-care testing.

Development of DNA aptamer-based sensor for electrochemical detection of C-reactive protein

C-reactive protein (CRP) is a crucial biomarker of cardiovascular diseases and for its detection both optical and electrochemical techniques were applied. This study concerns the application of DNA aptamer as recognition layer for CRP detection. For that purpose aptamer immobilization method on gold surface was selected and the content of receptor layer was optimized to ensure an efficient binding to target protein. The quality of the monolayer was verified by the application of chronocoulometry and atomic force microscopy. Using thiolated aptamers provided the formation of layers of highest density and stability. The square-wave voltammetry experiments performed in the presence of methylene blue redox indicator revealed a linear response of aptasensor towards CRP in the range from 1 to 100 pM. Moreover, a DNA aptamer – based sensor showed good selectivity towards C-reactive protein in comparison to interfering proteins including BSA and IgE. Finally, the analysis of CRP in serum sample was conducted using the developed aptasensor.

Determination of Ascorbic Acid in Commercial Tablets Using Pencil Drawn Electrochemical Paper-based Analytical Devices

This study describes the use of electrochemical paper-based analytical devices (ePADs) drawn with graphite pencil for the determination of ascorbic acid (AA) in commercial tablets. ePADs were fabricated using vegetal paper and graphite pencil. First, the three-electrode electrochemical cell drawn using a graphical software and toner lines were laser printed on the vegetal paper surface to delimit the electrode areas. Then, the electrode regions were manually painted with graphite pencil. Afterwards, the pseudo-reference electrode was defined with the deposition of silver ink over the graphite surface. Cyclic voltammetry and square wave voltammetry (SWV) experiments were performed to optimize the electroanalytical parameters as well as to quantitatively determine the AA concentration in two commercial tables. ePADs exhibited linear behavior for a concentration range between 0.5 and 3.0 mmol L-1. The achieved limit of detection and sensitivity were 70 μmol L-1 and 0.47 μA/mmol L-1, respectively. The AA concentration levels found by SWV experiments in both CenevitTM and Energil CTM were 2.80 ± 0.02 and 3.10 ± 0.01 mmol L-1, respectively. The accuracy of the proposed devices was investigated through recovery experiments in three concentration levels and it presented values between 95 and 115%.

Study of Electrochemical Oxidation and Quantification of the Pesticide Pirimicarb Using a Boron-Doped Diamond Electrode

An electrochemical study of the carbamate pesticide pirimicarb (PMC), which acts on the central nervous system, was performed using a boron-doped diamond working electrode. Cyclic, differential pulse, and square-wave voltammetry experiments across a wide pH range (2.0 to 8.0) showed three irreversible oxidation processes in the voltammetric behavior of PMC. The two first processes were pH-dependent, while the third was not. The three oxidation process were independent of each other, and each involved the transfer of one electron. A reaction proposal for the electrochemical oxidation of PMC is shown, and it is supported by mass spectrometry experiments. After this, an analytical method for PMC quantification in water samples by differential pulse (DP) voltammetry is proposed. The optimal DP voltammetric parameters (step potential, amplitude potential, and scan rate) were optimized using experimental design, and an analytical curve was obtained from 2.0 to 219μmolL−1 with an estimated detection limit of 1.24μmolL−1. The accuracy of the proposed method was evaluated by the addition and recovery method, with recoveries ranging from 88.6 to 96.3%. Some highlights of the proposed analytical method are its simplicity, reliability, and portability.

Substituent Inductive Effects on the Electrochemical Oxidation of Flavonoids Studied by Square Wave Voltammetry and Ab Initio Calculations.

Flavonoids are natural products commonly found in the human diet that show antioxidant, anti-inflammatory and anti-hepatotoxic activities. These nutraceutical properties may relate to the electrochemical activity of flavonoids. To increase the understanding of structure–electrochemical activity relations and the inductive effects that OH substituents have on the redox potential of flavonoids, we carried out square-wave voltammetry experiments and ab initio calculations of eight flavonoids selected following a systematic variation in the number of hydroxyl substituents and their location on the flavan backbone: three flavonols, three anthocyanidins, one anthocyanin and the flavonoid backbone flavone. We compared the effect that the number of –OH groups in the ring B of flavan has on the oxidation potential of the flavonoids considered, finding linear correlations for both flavonols and anthocyanidins (). We analyzed the effects that position and number of –OH substituents have on electron density distributions via ab initio quantum chemical calculations. We present direct correlations between structural features and oxidation potentials that provide a deeper insight into the redox chemistry of these molecules.

Boron-doped diamond modified with gold nanoparticles for the characterization of bovine serum albumin protein

Boron-doped diamond (BDD) has been modified with bare gold nanoparticles (AuNPs) for the investigation of bovine serum albumin (BSA) protein films. First, the BDD electrode was amino-terminated via silanization with (3-aminopropyl)triethoxysilane (APTES). Then, the AuNPs were synthesized using a stainless steel-assisted method, and immobilized at the APTES-modified BDD surface (BDD/APTES/AuNPs). The resulting morphology of both modification steps was characterized using atomic force microscopy (AFM). Besides, the synthesis of AuNPs was in-situ monitored via infrared attenuated total reflection (IR-ATR) spectroscopy using a combined IR-ATR-EC-AFM system taking advantage of surface enhanced IR absorption (SEIRA) effects induced by the AuNPs. The modified conductive IR-ATR BDD waveguide was employed to investigate spectroelectrochemically films of globular and fibrilized BSA, which were deposited onto the AuNPs-modified surface. SEIRA spectroscopy enabled the discrimination between globular and fibril aggregates of BSA via curve fitting of amide I band revealing a major contributions of crossed β-sheet structures within the fibril aggregates. Finally, preliminary square-wave voltammetry experiments showed that the spectroelectrochemical setup has potential for in-situ electrochemical characterization of such protein films.

The effect of composition of solid silver amalgam electrodes on their electrochemical response

The main purpose of this work was to evaluate the effect of the silver to mercury ratio on the voltammetric responses of silver solid amalgam electrodes (AgSAE’s). For this, the AgSAE were prepared by mechanical mixing the metals in the following mass ratios of silver to mercury: 30/70, 40/60, 50/50, 60/40, and 70/30. The resulting AgSAE’s were physically characterized by energy dispersive X-ray analysis, X-ray diffraction and scanning electron microscopy, confirming the mass percentages of the silver and mercury, the total absence of liquid mercury and a globular structure of all AgSAE’s. Furthermore, it was observed that the AgSAE 30/70 contained only one single phase (Ag2Hg3), and no metallic silver or mercury oxides. Additionally, the resulting AgSAE’s were chemically characterized with respect to the influence of the electrode composition on the reproducibility and electrochemical signals of a hexamine-ruthenium (III) chloride solution by use of electrochemical impedance spectroscopy and cyclic voltammetry. The separation between anodic and cathodic peaks, and consequently, the charge transfer resistance across the electrode/solution interface, and the electroactive area were calculated demonstrating that the 30/70 composition is the best surface for practical applications. Finally, square-wave voltammetry experiments were performed in 4-nitrophenol solution, with a previous optimization of the experimental and voltammetric parameters. The calculated detection limit shows that the AgSAE 30/70 is suitable for determining any contamination by p-nitrophenol, minimizing the toxic residues in case of using liquid mercury electrodes.

Development of biosensors containing laccase and imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid for the determination of rutin

Biosensors based on hydrophobic ionic liquids (ILs) derived from the bis(trifluoromethylsulfonyl)imide [(CF 3SO 2) 2N − = Tf 2N −] anion associated with three different imidazolium cations: 1-butyl-3-methylimidazolium (BMI·Tf 2N), 1-decyl-3-methylimidazolium (DMI·Tf 2N) and 1-tetradecyl-3-methylimidazolium (TDMI·Tf 2N), along with laccase from Aspergillus oryzae, were constructed and optimized for determination of rutin. The laccase catalyzes the oxidation of rutin to the corresponding o-quinone, which is electrochemically reduced back to rutin. The best performance was obtained with 50:20:15:15% (w/w/w/w) as the graphite powder:laccase:Nujol:ILs composition in 0.1 mol L −1 acetate buffer solution (pH 5.0). The parameters for the square-wave voltammetry experiments and scanning electron microscopy images of the biosensors were studied. Under the selected conditions, the cathodic peak current increased linearly in the rutin concentration ranges of 4.77 × 10 −6 to 4.62 × 10 −5 mol L −1, 5.84 × 10 −6 to 5.36 × 10 −5 mol L −1 and 5.84 × 10 −6 to 5.36 × 10 −5 mol L −1 using the (I) BMI·Tf 2N-laccase, (II) DMI·Tf 2N-laccase and (III) TDMI·Tf 2N-laccase, respectively. The rutin contents of commercial samples of pharmaceuticals were successfully determined by the biosensors and the results compared well with those obtained using the official method. The studies on rutin recovery from these samples gave values of 96.9–104.6%.

Tuning Photoinduced Energy‐ and Electron‐Transfer Events in Subphthalocyanine–Phthalocyanine Dyads

A series of subphthalocyanine-phthalocyanine dyads has been prepared by means of palladium-catalyzed cross-coupling reactions between a monoalkynylphthalocyanine and different monoiodosubphthalocyanines. Electronic coupling between the two photoactive units is ensured by a rigid and pi-conjugated alkynyl spacer. In addition, the electronic characteristics of the subphthalocyanine moiety were modulated by the introduction of different peripheral substituents. Cyclic and Osteryoung square-wave voltammetry experiments revealed that the reduction potential of this subunit can be decreased by about 400 mV on going from thioether or no substituents to nitro groups. As a consequence, the energy level of the charge-transfer state could be fine-tuned so as to gain control over the fate of the photoexcitation energy in each subunit. The diverse steady-state and time-resolved photophysical techniques employed demonstrated that, when the charge-transfer state lies high in energy, a quantitative singlet-singlet energy-transfer mechanism from the excited subphthalocyanine to the phthalocyanine takes place. On the contrary, stabilization of the radical pair by lowering the redox gap between electron donor and acceptor results in a highly efficient photoinduced electron-transfer process, even in solvents of low polarity such as toluene (Phi(ET) approximately 0.9). These features, together with the extraordinary absorptive cross section that these molecular ensembles display across the whole UV/Vis spectrum, make them model candidates for application in situations where broadband light sources are needed.

Ohmic drop effects in square-wave voltammetry response for an ion transfer process at a liquid–liquid interface

The effect of ohmic drop on square-wave voltammetry experiments (SWV) at a liquid–liquid interface is analyzed. The study is based on the distortion of SW voltammetric profiles of the tetraethyl ammonium transfer process across the water/1,2-dichloroethane interface when an uncompensated resistance, R Ω , is present. Depending on the experimental conditions significant R Ω values arise as a consequence of the low electrical conductivity of the organic solvent characteristic of these systems, and efficient IR compensation is needed. When R Ω is not totally removed, a non-linear dependence of peak current (Δ I p) on the square root of the frequency, an increase of half-peak width (Δ E p/2) and a shift of peak potential ( E p) towards more positive values are observed in the SW profiles. The influence of signal frequency and R Ω values on SWV parameters (Δ I p, E p and Δ E p/2) is evaluated.

Square wave voltammetric determination of paraquat and diquat in aqueous solution

A rapid, sensitive method for the determination of paraquat and diquat in aqueous media using square wave voltammetry (SWV) is discussed on the basis of the analyte behaviour in cyclic voltammetry. Cation-radicals formed upon the one-electron reduction of the analyte(s) adsorb on the mercury electrode surface with both flat and vertical orientations. With low concentrations only the flat oriented deposit occurs and the adsorption becomes progressively diffusion-controlled, especially at high scan rates. SWV experiments were based on this diffusion-controlled adsorption process. Effects on square wave frequency f, staircase step height ΔE s, scanning direction, presence of surfactants and supporting electrolyte concentration were examined for the optimization of experimental conditions. The optimal parameters found were f = 100 Hz and ΔE s = 2 mV for scanning potentials from −0.85 to −0.25 V in the presence of 0.003% gelatin and with supporting electrolyte concentrations ranging from 0.02 to 0.05 M. Calibration curves were linear in the range 5 × 10 −8 to 1 × 10 −5 M with a detection limit of 1.5 × 10 −8 M. SWV is a sensitive technique, more rapid than all other described procedures for determining these pesticides, and allows analysis to be performed in the presence of an ionic matrix. The method was extended to the analysis of paraquat and diquat in potatoes after digestion in hot sulphuric acid, neutralization and solid phase extraction. The detection limit has been found to be 1 μg g −1 for both pesticides.