Fábio de Lima

A remarkably simple characterization of glassy carbon-supported films of graphite, graphene oxide, and chemically converted graphene using Fe(CN)3−6/Fe(CN)4−6 and O2 as redox probes

This study investigated the behavior of thin films of graphene oxide (GO) and chemically converted graphene (CCG) on a glassy carbon surface in the presence of two redox probes (Fe(CN)3−6/Fe(CN)4−6 and O2), employing cyclic voltammetry, electrochemical impedance spectroscopy, and hydrodynamic voltammetry as a simple procedure for characterizing these films. The feasibility of using these electrochemical techniques for this purpose opens up the possibility of applying them to biosensors and electrocatalysts using surface-supported GO and CCG materials. Raman spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy were employed to characterize GO and CCG.

Determination of thiodicarb using a biosensor based on alfalfa sprout peroxidase immobilized in self-assembled monolayers

A biosensor based on alfalfa sprout (Medicago sativa) homogenate as a source of peroxidase is proposed for the determination of thiodicarb by square-wave voltammetry. This enzyme was immobilized in self-assembled monolayers of l-cysteine on a gold electrode. Several parameters were investigated to evaluate the optimum conditions for operation of the biosensor. The analytical curve was linear for thiodicarb concentrations of 2.27 x 10(-6) to 4.40 x 10(-5) mol L(-1) with a detection limit of 5.75 x 10(-7) mol L(-1). The lifetime of the Au-alfalfa sprout-SAMs was 20 days (at least 220 determinations). The average recovery of thiodicarb from samples of vegetable extracts ranged from 99.02 to 101.04%. The results obtained for thiodicarb in vegetable extracts using the proposed method are in close agreement with those using a high performance liquid chromatography procedure at the 95% confidence level.

Electrodeposition of reduced graphene oxide on a Pt electrode and its use as amperometric sensor in microchip electrophoresis

This report describes the development and application of a novel graphene‐modified electrode to be used as amperometric sensor in microchip electrophoresis (ME) devices. The modified electrode was achieved based on electroreduction of graphene oxide on an integrated Pt working electrode of a commercial ME device. The surface modification was characterized by SEM and cyclic voltammetry techniques. The results indicated that graphene sheets were successfully deposited exhibiting higher surface conductivity and greater electrode sensitivity. The performance of the modified electrode for the amperometric detection on ME devices has been demonstrated by the separation and detection of an anionic mixture containing iodide and ascorbate. The graphene‐modified electrode provided significantly higher sensitivity (896.7 vs. 210.9 pA/μM for iodide and 217.8 vs. 127.8 pA/μM for ascorbate), better separation efficiencies (3400 vs. 700 plates/m for iodide and 10 000 vs. 2400 plates/m for ascorbate), enhanced peak resolutions (1.6 vs. 1.0), and LODs (1.5 vs. 5.3 μM for iodide and 3.1 vs. 7.3 μM for ascorbate) in comparison with the unmodified Pt electrode. The proposed amperometric sensor was successfully applied for the analysis of ascorbic acid (through its anionic form) in a commercial medicine sample, and the results achieved were in agreement with the value provided by the supplier. Based on the data here presented, the modified graphene electrode shows great promise for ME applications.

Ultrasensitive determination of carbendazim in water and orange juice using a carbon paste electrode

ABSTRACT A carbon paste electrode was used for the electrochemical quantification of carbendazim in water and orange juice samples. Carbendazim oxidation on the electrode surface was found to be controlled by adsorption. The novel electrochemical procedure for carbendazim quantification employed differential pulse voltammetry using a carbon paste electrode under optimal conditions. Carbendazim oxidation currents were linear at concentrations of 2.84 to 45.44 µg L−1, with a limit of detection of 0.96 µg L−1. The proposed method was applied to carbendazim quantification in ultrapurified water, river water, and orange juice. Recovery rates in water and orange juice samples were in the 97%–101% range, indicating that the method can be employed to determine carbendazim in these matrices, with advantages including shorter analysis time and lower cost than routine methods such as chromatography or spectroscopy. The electrode showed good reproducibility, remarkable stability, and especially good surface renewability by simple mechanical polishing. The recovery rates observed were highly concordant with those obtained for high-performance liquid chromatography, having a relative standard deviation of less than 1.3%.

Direct electron transfer from alcohol dehydrogenase

This study employed hydrodynamic cyclic voltammetry (HCV) with a glassy carbon (GC) rotating disk electrode (RDE), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) to investigate the direct electron transfer (DET) behavior of alcohol dehydrogenase (ADH, EC 1.1.1.1) embedded or otherwise in polymyxin (PM) and adsorbed concomitantly with cofactor NAD+ or NADH, or none on GC and covered with Nafion®. The hybrid GC/PM-ADH-NAD+/Nafion electrode thus constructed (and visualized by scanning electron microscopy (SEM)) persistently bioelectrocatalyzed ethanol oxidation and performed DET involving bakers yeast ADH, GC, and ethanol—a key finding in the present study—making this system a promising anode for use in biofuel cells. A rate constant (ks) of 0.82 s−1 was obtained for this electrode at a potential scan rate (ν) of 120 mV s−1. EIS experiments, particularly those conducted after higher potential HCV scans, allowed resistance to electron hopping (Reh) between redox centers in ADH and between these centers and ethanol molecules to be estimated as 84 kΩ lower for the GC/PM-ADH-NAD+/Nafion electrode during ethanol oxidation than for bare GC in the presence of ethanol. Nuclear magnetic resonance (NMR) unequivocally confirmed acetaldehyde production from ethanol oxidation by ADH DET.