Electrochemically reduced graphene oxide films from Zn-C battery waste for the electrochemical determination of paracetamol and hydroquinone.

Contributing to the development of sustainable electroanalytical chemistry, electrochemically reduced graphene oxide (ERGO) films obtained from residual graphite of discharged Zn-C batteries are proposed in this work. Graphite from the cathode of discarded Zn-C batteries was recovered and used in the synthesis of graphene oxide (GO) by the modified Hummer's method. The quality of the synthesized GO was verified using different characterization methods (FT-IR, XRD, SEM, and TEM). GO films were deposited on a glassy carbon electrode (GCE) by the drop coating method and then electrochemically reduced by cathodic potential scanning using cyclic voltammetry. The electrochemical features of the ERGO films were investigated using the ferricyanide redox probe, as well as paracetamol (PAR) and hydroquinone (HQ) molecules as model analytes. From the cyclic voltammetry assays, enhanced heterogeneous electron transfer rate constants (k0) were observed for all redox systems studied. In analytical terms, the ERGO-based electrode showed higher analytical sensitivity than the bare and GO-modified GCE. Using differential pulse voltammetry, wide linear response ranges and limits of detection of 0.14 µmol L-1 and 0.65 µmol L-1 were achieved for PAR and HQ, respectively. Furthermore, the proposed sensor was successfully applied to the determination of PAR and HQ in synthetic urine and tap water samples (recoveries close to 100%). The outstanding electrochemical and analytical properties of the proposed ERGO films are added to the very low cost of the raw material, being presented as a green-based alternative for the development of electrochemical (bio)sensors with unsophisticated resources.

Study and voltammetric determination of fipronil in bovine lactose-free milk by differential pulse voltammetry using a carbon paste electrode.

A novel voltammetric screening method has been developed for the rapid determination of fipronil (FPN) residues in lactose-free milk samples with the use of a carbon-paste electrode (CPE) by differential-pulse voltammetry (DPV). Cyclic voltammetry indicated the occurrence of an irreversible anodic process at approximately +0.700 V (vs. Ag|AgCl, 3.0 mol L-1 KCl) in a 0.100 mol L-1 NaOH supporting electrolyte prepared as a 30% (v/v) ethanol-water solution. Quantification of FPN was carried out by DPV and analytical curves were constructed. In the absence of a matrix, the limits of detection (LOD) and quantification (LOQ) were 0.568 mg L-1 and 1.89 mg L-1, respectively. In the presence of a lactose-free skim milk matrix, the values of LOD and LOQ were 0.331 mg L-1 and 1.10 mg L-1. The recovery percentages for three different concentrations of FPN in lactose-free skim milk samples ranged between 95.3% and 109%. All assays could be conducted with milk samples without any prior extraction steps or pre-concentration of FPN, making this novel method rapid, simple, and relatively cheap.

Direct determination of boscalid in grape samples by differential pulse voltammetry using a carbon paste electrode.

A new methodology to determine directly the fungicide boscalid (BSC) was developed and successfully applied in red grape 100% juice, peel extracts, pulp and purple grape seeds (Vitis labrusca L.) with a working carbon paste electrode (CPE) without sample preparation. Cyclic voltammetry (CV) indicated the presence of an irreversible cathodic process of BSC at -1.21 V vs. Ag|AgCl (KCl 3.0 mol L-1) in a solution of 0.100 mol L-1 HCl/acetone 70 : 30 (v/v). This behavior was also observed using Square Wave Voltammetry (SWV). The Differential Pulse Voltammetry (DPV) technique proved to be more sensitive and with higher selectivity for BSC quantification. The influence of pH on the reduction of BSC was investigated in Britton-Robinson Buffer (BRB), 0.01 mol L-1 (pH 2.00-12.00). The limit of detection (LOD) values obtained from calibration curves for different samples were as follows: 0.107 mg L-1 for deionized water; 0.146 mg L-1 for red grape 100% juice; 0.922 mg kg-1 for peel extracts; 0.818 mg kg-1 for grape pulp and 0.691 mg kg-1 for grape seeds. The corresponding Limit of Quantification (LOQ) values for the same samples were as follows: 0.358 mg L-1; 0.486 mg L-1; 2.87 mg kg-1; 2.73 mg kg-1 and 2.51 mg kg-1, respectively. In addition, the recovery rates for the different concentration levels in the investigated range varied between 97.13 and 103.4%. All tests performed with the samples did not require extraction or pre-concentration steps of BSC, resulting in a fast, simple and cheap methodology.

A voltammetric screening method to determine ronidazole in bovine meat.

An original voltammetric screening method, employing glassy carbon electrode (GCE) with the differential-pulse voltammetry technique (DPV), has been developed to determine residues of the anti-parasitic agent Ronidazole (RNZ) in bovine meat. By using cyclic voltammetry (CV), it has been demonstrated that an irreversible cathodic process occurs at approximately -0.740 V (vs. Ag|AgCl, KCl 3 mol L-1) in a 0.100 mol L-1 phosphate buffer at pH 6.5 as supporting electrolyte. Furthermore, the behavior of RNZ in CV indicates the occurrence of a diffusion mass transfer process to the working electrode surface. The RNZ reduction mechanism was proposed as a 6-electron transfer, similar to Metronidazole under the same pH range. Quantification of RNZ and method validation were then carried out by DPV. The relative standard deviation (RSD) were 3.21% for intraday precision of 10 consecutive repetitions and 6.78% for interday precision after five analysis. Limits of detection and quantification were also obtained, and the values were 0.107 and 0.358 mg kg-1, respectively. The recovery percentage for three different concentrations of RNZ in the bovine meat matrix ranged between 98.1% and 100.3%. The method proved to be efficient for screening RNZ in bovine meat.

Fast screening for antioxidant properties of flavonoids from Pterogyne nitens using electrochemical methods.

A fast, low-cost, convenient, and especially sensitive voltammetric screening approach for the study of the antioxidant properties of isoquercitrin and pedalitin from Pterogyne nitens is suggested in this work. These flavonoids were investigated for their redox properties using cyclic voltammetry in nonaqueous media using N,N-dimethylformamide and tetrabutylammonium tetrafluorborate as the supporting electrolyte, a glassy carbon working electrode, A6(see symbol in text)AgCI reference electrode, and Pt bare wire counter electrode. The comparative analysis of the activity of rutin has also been carried out. Moreover, combining HPLC with an electrochemical detector allowed qualitative and quantitative detection of micromolecules (e.g., isoquercitrin and pedalitin) that showed antioxidant activities. These results were then correlated to the inhibition of beta-carotene bleaching determined by TLC autographic assay and to structural features of the flavonoids.

Voltammetric determination of Δ9-THC in glassy carbon electrode: An important contribution to forensic electroanalysis.

A new voltammetric method for the determination of Δ(9)-tetrahydrocannabinol (Δ(9)-THC) is described. The voltammetric experiments were accomplished in N-N dimethylformamide/water (9:1, v/v), using tetrabutylammonium tetrafluoroborate (TBATFB) 0.1mol/L as supporting electrolyte and a glassy carbon disk electrode as the working electrode. The anodic peak current was observed at 0.0V (vs. Ag/AgCl) after a 30s pre-concentration step under an applied potential of -1.2V (vs. Ag/AgCl). A linear dependence of Δ(9)-THC detection was obtained in the concentration range 2.4-11.3ng/mL, with a linear correlation coefficient of 0.999 and a detection limit of 0.34ng/mL. The voltammetric method was used to measure the content of Δ(9)-THC in samples (hemp and hashish) confiscated by the police. The elimination of chemical interferences from the samples was promptly achieved through prior purification using the TLC technique, by employing methanol/water (4:1, v/v) as the mobile phase. The results showed excellent correlation with results attained by HPLC.

Rapid and sensitive method for the determination of acetaldehyde in fuel ethanol by high-performance liquid chromatography with UV-Vis detection.

A high-performance liquid chromatography (HPLC) method for the determination of acetaldehyde in fuel ethanol was developed. Acetaldehyde was derivatized with 0.900 mL 2,4-dinitrophenylhydrazine (DNPHi) reagent and 50 microL phosphoric acid 1 mol L(-1) at a controlled room temperature of 15 degrees C for 20 min. The separation of acetaldehyde-DNPH (ADNPH) was carried out on a Shimadzu Shim-pack C18 column, using methanol/LiCl((aq)) 1.0 mM (80/20, v/v) as a mobile phase under isocratic elution and UV-Vis detection at 365 nm. The standard curve of ADNPH was linear in the range 3-300 mg L(-1) per injection (20 microL) and the limit of detection (LOD) for acetaldehyde was 2.03 microg L(-1), with a correlation coefficient greater than 0.999 and a precision (relative standard deviation, RSD) of 5.6% (n = 5). Recovery studies were performed by fortifying fuel samples with acetaldehyde at various concentrations and the results were in the range 98.7-102%, with a coefficient of variation (CV) from 0.2% to 7.2%. Several fuel samples collected from various gas stations were analyzed and the method was successfully applied to the analysis of acetaldehyde in fuel ethanol samples.

Simultaneous determination of zinc, copper, lead, and cadmium in fuel ethanol by anodic stripping voltammetry using a glassy carbon-mercury-film electrode.

A new, versatile, and simple method for quantitative analysis of zinc, copper, lead, and cadmium in fuel ethanol by anodic stripping voltammetry is described. These metals can be quantified by direct dissolution of fuel ethanol in water and subsequent voltammetric measurement after the accumulation step. A maximum limit of 20% ( v/ v) ethanol in water solution was obtained for voltammetric measurements without loss of sensitivity for metal species. Chemical and operational optimum conditions were analyzed in this study; the values obtained were pH 2.9, a 4.7-microm thickness mercury film, a 1,000-rpm rotation frequency of the working electrode, and a 600-s pre-concentration time. Voltammetric measurements were obtained using linear scan (LSV), differential pulse (DPV), and square wave (SWV) modes and detection limits were in the range 10(-9)-10(-8) mol L(-1) for these metal species. The proposed method was compared with a traditional analytical technique, flame atomic absorption spectrometry (FAAS), for quantification of these metal species in commercial fuel ethanol samples.