Luiz C. S. Figueiredo-Filho

Freestanding three-dimensional graphene foam gives rise to beneficial electrochemical signatures within non-aqueous media

Freestanding three-dimensional (3D) graphene foam has been fabricated via a Chemical Vapour Deposition (CVD) methodology which has a macroscopic structure with microscopic (graphene) features. The 3D graphene macrostructure is characterised with SEM, EDX, XPS and Raman spectroscopy and is found to comprise pristine graphene (O/C of 0.05) which is in the range of mono- to few-layered graphene sheets and is thus termed quasi-graphene. This unique 3D graphene foam is electrochemically explored in both aqueous and non-aqueous solutions and compared to a freestanding 3D reticulated vitreous carbon (RVC) foam alternative. In aqueous solutions, the 3D graphene foam exhibits poor voltammetric responses. Contact angle measurements reveal the 3D graphene to exhibit a value of 120° representing quasi-super-hydrophobicity. Consequently the freestanding 3D graphene foam is found to give rise to significantly improved voltammetric signatures in non-aqueous media (ionic liquids) over that of a freestanding 3D RVC alternative. The 3D graphene foam provides a promising and beneficial architecture over 2D pristine graphene due to its ease of use and macroscopic/microscopic structure, which will have wide implementation in the field of electrochemistry.

A biosensor based on gold nanoparticles, dihexadecylphosphate, and tyrosinase for the determination of catechol in natural water.

In this work, a biosensor using a glassy carbon electrode modified with gold nanoparticles (AuNPs) and tyrosinase (Tyr) within a dihexadecylphosphate film is proposed. Cystamine and glutaraldehyde crosslinking agents were used as a support for Tyr immobilization. The proposed biosensor was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cyclic voltammetry in the presence of catechol. The determination of catechol was carried out by amperometry and presented a linear concentration range from 2.5×10(-6) to 9.5×10(-5)molL(-1) with a detection limit of 1.7×10(-7)molL(-1). The developed biosensor showed good repeatability and stability. Moreover, this novel amperometric method was successfully applied in the determination of catechol in natural water samples. The results were in agreement with a 95% confidence level for those obtained using the official spectrophotometric method.

Inexpensive and disposable copper mini-sensor modified with bismuth for lead and cadmium determination using square-wave anodic stripping voltammetry

The fabrication and evaluation of a disposable copper mini-sensor ex situ modified bismuth film for the sensing of lead(II) and cadmium(II) via square-wave anodic stripping voltammetry (SWASV) technique is presented. The sensor was ex situ modified with a bismuth film via electro-deposition through applying a potential of −0.18 V vs. Ag/AgCl (3.0 mol L−1 KCl) for 200 seconds in a 0.02 mol L−1 Bi(NO3)3, 0.15 mol L−1 sodium citrate in 1.5 mol L−1 HCl solution. Under the optimal experimental conditions, the voltammetric response was linearly dependent on the analyte concentrations over the range from 1.3 × 10−6 to 1.3 × 10−5 mol L−1 and from 9.9 × 10−7 to 1.2 × 10−5 mol L−1 with a limit of detection of 8.3 × 10−7 mol L−1 and 5.3 × 10−7 mol L−1 for Pb(II) and Cd(II), respectively. The determination of both toxic metallic species was carried out in natural waters using the sensor obtaining results which are in close agreement with those obtained using Flame Atomic Absorption Spectrometry at a 95% confidence level.

Forensic electrochemistry: sensing the molecule of murder atropine

We present the electroanalytical sensing of atropine using disposable and economic screen printed graphite sensors. The electroanalytical determination of atropine is found to be possible over the concentration range of 5 μM to 50 μM with a detection limit of 3.9 μM (based on 3-sigma) found to be possible. We demonstrate proof-of-concept that this approach provides a rapid and inexpensive sensing strategy for determining the molecule of murder atropine in diet Coca-Cola samples.

An improved flow system for chloride determination in natural waters exploiting solid-phase reactor and long pathlength spectrophotometry

A single and sensitive spectrophotometric method for chloride ions determination based on a commuted flow system with a 100cm optical path flow cell and a solid-phase reactor containing immobilized silver chloranilate was proposed. This procedure exploited the AgCl formation in the solid-phase reactor leading the chloranilate ions, monitored spectrophotometrically at 530nm. The analytical signals were 75-fold higher and the sensitivity was 12-fold than that achieved with a 1cm flow cell, allowed a chloride determination in the 0.5-100mgl(-1) range. The R.S.D. was 1.1% (n=20) with a sample throughput of 80h(-1) and a waste generated of ca. 100ng of chloranilate ions per determination. Four samples of natural waters from São Carlos and Araraquara cities were evaluated using the proposed method. Results agreed with the obtained by a reference method at the 95% confidence level.

Flow-Injection spectrophotometric system for captopril determination in pharmaceuticals

A simple, accurate and precise flow-injection spectrophotometric procedure is reported for the determination of captopril in pharmaceutical formulations. In this procedure, captopril was oxidized by iron(III) and the iron(II) produced was spectrophotometrically monitored as iron(II)-1,10-phenantroline complex at 540 nm. The analytical curve for captopril was linear in the concentration range from 1.0 × 10-5 to 8.0 × 10-4 mol L-1 with a detection limit of 5.0 × 10-6 mol L-1. The recovery of this analyte in five samples ranged from 98.5 to 102.4%. The analytical frequency was sixty determinations per hour and the RSD was less than 0.2% for a captopril concentration of 4.0 × 10-4 mol L-1 (n = 10). A paired t-test showed that all results obtained for captopril in commercial formulations using the proposed flow injection procedure and a potentiometric procedure agreed at the 95% confidence level.

High temperature low vacuum synthesis of a freestanding three-dimensional graphene nano-ribbon foam electrode

The fabrication of a freestanding three-dimensional (3D) graphene nano-ribbon open cell foam electrode is reported based upon a facile high temperature (1700 °C) low vacuum (50 Torr) process. The graphene nano-ribbon (GNR) foam comprises on average 4 graphene layers and has an O/C ratio of 0.14; a quasi-graphene structure. This unique material is demonstrated to be electrochemically useful, with the electrochemical properties and resultant electroanalytical performance of the novel freestanding 3D GNR foam electrode reported for the first time. Electrochemical characterisation is performed via cyclic voltammetry in aqueous solutions using a range of electro-active redox probes and biologically relevant analytes, namely potassium ferrocyanide(II), hexaammineruthenium(III) chloride, uric acid (UA), acetaminophen (AP) and dopamine hydrochloride (DA). Analytical performance is evaluated and benchmarked through comparisons of the 3D GNR foam to other carbon based 3D foam electrodes, namely pristine graphene and reticulated vitreous carbon (RVC) alternatives. We show that the 3D GNR foam electrode possesses favourable heterogeneous electron transfer (HET) properties when compared to the alternative carbon based 3D foams, likely due to improved coverage of reactive edge plane like-sites/defects on its structure. In terms of the electroanalytical response of the 3D GNR foam electrode, it is found to give rise to an improved linear range and limit of detection towards some analytes; however, in certain cases the alternative carbon based 3D foams out-performed the GNR foam. These findings question the need of ‘only’ fast HET properties and suggest a compromise is required (for improved sensing capabilities to be realised) between HET speeds, the presence/absence of oxygenated species and the accessibility of the electrodes active surface area. This work offers insight to those working in the field of electrochemistry, particularly electroanalysis and those searching for new carbon based 3D foam electrode materials.

Flow Injection Spectrophotometric System for Ranitidine Determination in Pharmaceuticals Using Cerium(IV) and Ferroin

An indirect flow injection spectrophotometric procedure for the determination of ranitidine in pharmaceutical formulations is proposed. The method was based on the oxidation of the ranitidine with cerium(IV) in acidic medium and subsequent determination of cerium(IV) excess by reaction with ferroin which was monitored spectrophotometrically at 500 nm. The linearity of the analytical curve for ranitidine ranged from 2.5 x 10 -4 to 1.25 x 10 -3 mol L -1 . The detection limit of 1.6 x 10 -6 mol L -1 and recoveries between 96.6 and 103 % were obtained. The sampling rate of 60 determinations per hour and the RSD smaller than 0.3% for 7.1 x 10 -4 mol L -1 ranitidine were also obtained. The method was applied with success in the determination of ranitidine in several commercial formulations.

Electroanalytical determination of the linuron herbicide using a cathodically pretreated boron-doped diamond electrode: comparison with a boron-doped diamond electrode modified with platinum nanoparticles

The determination of linuron using differential-pulse voltammetry (DPV) and a cathodically pretreated boron-doped diamond electrode is proposed. Cyclic voltammetry results showed one irreversible oxidation peak for linuron at 1.29 V (vs. Ag/AgCl (3.0 mol L−1 KCl)) in 0.04 mol L−1 Britton–Robinson (BR) buffer solution (pH 2.0). Under optimized DPV conditions, a linear analytical curve was obtained for the linuron concentration range of 0.61–26.0 μmol L−1 with a detection limit of 0.18 μmol L−1. Similar responses (linearity and sensitivity) were obtained employing a boron-doped diamond electrode modified with platinum nanoparticles. The proposed method was successfully applied in the determination of linuron in natural water samples with recoveries ranging from 90.9% to 104%.

Development of a carbon nanotube paste electrode modified with zinc phosphate for captopril determination in pharmaceutical and biological samples

We report a novel electrode composite comprising zinc phosphate within a multiwalled carbon nanotube paste electrode matrix which is applied for the detection of trace amounts of captopril (CAP) via square-wave adsorptive anodic stripping voltammetry (SWAdASV); to the best of our knowledge this is the first report on such a composite electrode. Under optimum experimental conditions, the peak current of CAP was found to be linear over the concentration range from 3.7 to 67 μmol L−1 with a limit of detection of 0.1 μmol L−1. The proposed method was applied for the successful determination of CAP in commercial tablet excipients and biological samples using the electroanalytical protocol evaluated against the laboratory standard iodometric procedure. Excellent comparability between the two is found suggesting that the former can replace the latter within laboratory settings or can be applied to in-the-field applications.