Tiago Almeida Silva

The application of graphene for in vitro and in vivo electrochemical biosensing.

Advances in analysis are required for rapid and reliable clinical diagnosis. Graphene is a 2D material that has been extensively used in the development of devices for the medical proposes due to properties such as an elevated surface area and excellent electrical conductivity. On the other hand, architectures have been designed with the incorporation of different biological recognition elements such as antibodies/antigens and DNA probes for the proposition of immunosensors and genosensors. This field presents a great progress in the last few years, which have opened up a wide range of applications. Here, we highlight a rather comprehensive overview of the interesting properties of graphene for in vitro, in vivo, and point-of-care electrochemical biosensing. In the course of the paper, we first introduce graphene, electroanalytical methods (potentiometry, voltammetry, amperometry and electrochemical impedance spectroscopy) followed by an overview of the prospects and possible applications of this material in electrochemical biosensors. In this context, we discuss some relevant trends including the monitoring of multiple biomarkers for cancer diagnostic, implantable devices for in vivo sensing and, development of point-of-care devices to real-time diagnostics.

Electrochemical Performance of Porous Diamond-like Carbon Electrodes for Sensing Hormones, Neurotransmitters, and Endocrine Disruptors

Porous diamond-like carbon (DLC) electrodes have been prepared, and their electrochemical performance was explored. For electrode preparation, a thin DLC film was deposited onto a densely packed forest of highly porous, vertically aligned multiwalled carbon nanotubes (VACNT). DLC deposition caused the tips of the carbon nanotubes to clump together to form a microstructured surface with an enlarged surface area. DLC:VACNT electrodes show fast charge transfer, which is promising for several electrochemical applications, including electroanalysis. DLC:VACNT electrodes were applied to the determination of targeted molecules such as dopamine (DA) and epinephrine (EP), which are neurotransmitters/hormones, and acetaminophen (AC), an endocrine disruptor. Using simple and low-cost techniques, such as cyclic voltammetry, analytical curves in the concentration range from 10 to 100 μmol L(-1) were obtained and excellent analytical parameters achieved, including high analytical sensitivity, good response stability, and low limits of detection of 2.9, 4.5, and 2.3 μmol L(-1) for DA, EP, and AC, respectively.

A digital image-based method employing a spot-test for quantification of ethanol in drinks

A low-cost analytical method for quantification of ethanol in drinks based on the combination of a colorimetric spot-test and a digital image-based (DIB) method is proposed. The digital images from spot-test reactions were captured using a digital camera in a portable plastic chamber designed with internal lighting control. The images were decomposed by a RGB approach using freely available software. The R channel showed the best linearity, with two linear ethanol concentration ranges: from 1.0% to 20.0% v/v (r = 0.999) and from 25.0% to 50.0% v/v (r = 0.980), with limits of detection and quantification of 0.25% and 0.85% v/v, respectively, for the first analytical curve. The developed method was applied to quantification of ethanol in alcoholic drink samples with results in close agreement with those obtained using a spectrophotometric method at a confidence level of 95%, and with low waste generation (835 μL/spot-test). Thus, we believe that the DIB method can be useful with regard to environmental and social impacts, once the method has a low waste generation and uses an easily available instrumentation with potential for in situ determination during the alcoholic beverage production and its quality control.

A digital image analysis method for quantification of sulfite in beverages

A novel, simple and low-cost analytical procedure for sulfite determination in beverage samples is presented. The approach proposed consists of image capture from a sulfite colorimetric reaction, using a system built with low-cost materials for luminosity control and digital image decomposition into the primary colors red (R), green (G) and blue (B). The colorimetric reaction is based on the reduction of Fe(III) to Fe(II) in the presence of sulfite and further reaction with o-phenanthroline to form the red complex [Fe(C12H8N2)3]2+. Under optimized reaction and system conditions, the analytical curve was linear in a sulfite concentration range of 8.0 to 140 mg L−1, with limits of detection and quantification of 2.6 mg L−1 and 8.0 mg L−1, respectively. The analytical method was applied to sulfite quantification in different beverage samples such as white wine, vinegar, rose wine, cashew juice and coconut water. The results acquired were in close agreement with those obtained using iodometric titration as a comparative method, with a confidence level of 95%. Moreover, the method can be useful with regard to social and environmental impacts, due to the low generated residues (800 μL per spot-test) and employs easily available instrumentation with the potential for in situ determination during the application of sulfites as additives during beverage production and quality control.

Electrochemical sensor based on graphene oxide and ionic liquid for ofloxacin determination at nanomolar levels.

New insights into the design of highly sensitive, carbon-based electrochemical sensors are presented in this work by exploring the interesting properties of graphene oxide (GO) and ionic liquids (ILs). An electrochemical sensor based on the carbon paste electrode (CPE) modified with GO and IL was developed for the sensitive detection of ofloxacin using square-wave adsorptive anodic stripping voltammetry (SWAdASV). GO sheets were obtained from the acid treatment of graphene and characterized by scanning and transmission electronic microscopy (SEM and TEM) and selected area electron diffraction (SAED), and the electrochemical behavior of the modified GO-IL/CPE was explored by electrochemical impedance spectroscopy studies. The CPE modification with GO and IL allowed an 8.2 fold increase in the analytical sensitivity for ofloxacin sensing compared to the unmodified CPE. Under the optimized experimental conditions using the SWAdASV technique, the GO-IL/CPE sensor provided an analytical curve for ofloxacin in the concentration range of 7.0×10-9 to 7.0×10-7molL-1, with a sensitivity of 7.7×106μALmol-1 and limit of detection of 2.8×10-10molL-1 (0.28nmolL-1). The proposed sensor was successfully applied for the ofloxacin determination in human urine and ophthalmic samples, with recoveries near 100%. The results were similar those obtained by a spectrophotometric comparative method.

Promising electrochemical performance of high-surface-area boron-doped diamond/carbon nanotube electroanalytical sensors

Porous boron-doped diamond (p-BDD) electrodes of high-surface-area have been prepared on vertically aligned carbon nanotube substrates, and their electrochemical performance has demonstrated promising results for application in electroanalysis. The electrochemical features of the p-BDD electrodes were investigated and compared with those of a conventional flat BDD electrode (f-BDD). From cyclic voltammetry studies performed for the electrochemical probes [Fe(CN)6]3− and N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD), a fast charge transfer was observed at the p-BDD/electrolyte interface. For the [Fe(CN)6]3− redox probe, the heterogeneous electron-transfer rate constant (k0) value obtained for p-BDD was 10.9 times higher than that obtained using a f-BDD electrode. Moreover, the p-BDD electrodes also gave a smaller peak potential separation, ΔEp, and larger analytical signal magnitude for different biomolecules, such as dopamine (DA), acetaminophen (AC), and epinephrine (EP). These set of results demonstrated that the p-BDD electrode is a suitable candidate for applications in electroanalytical chemistry.

Electrochemical sensing of levodopa or carbidopa using a glassy carbon electrode modified with carbon nanotubes within a poly(allylamine hydrochloride) film

In this study, the performance of a glassy carbon electrode (GCE) modified with functionalized multi-walled carbon nanotubes (MWCNT) immobilized within a poly(allylamine hydrochloride) (PAH) film for the electrochemical determination of the catecholamines, levodopa (LD) and carbidopa (CD) is evaluated. The electrochemical behaviour of the MWCNT-PAH/GCE sensor was investigated via cyclic voltammetry. Under the optimum experimental conditions, the LD and CD analytes were determined by differential pulse voltammetry (DPV) using their oxidation redox processes at −24 mV and +490 mV (versus Ag/AgCl (3.0 mol L−1 KCl)), respectively. The obtained analytical curves were linear from 2.0 to 27 μmol L−1 for LD and from 2.0 to 23 μmol L−1 for CD, with limits of detection of 0.84 μmol L−1 for LD and 0.65 μmol L−1 for CD. The proposed DPV method was successfully employed for LD and CD determination in commercial pharmaceutical formulation samples, and the collected results were consistent with those obtained by the respective standard methods at a confidence level of 95%. In addition, the potentiality of the DPV method was tested for LD and CD quantification in human serum samples.

Square-wave adsorptive anodic stripping voltammetric determination of ramipril using an electrochemical sensor based on nanostructured carbon black

A novel and alternative electroanalytical method for the determination of ramipril (RAM) using a glassy carbon electrode (GCE) modified with nanostructured carbon black (CB) is reported. The electrochemical sensor was obtained by a simple drop coating strategy, and the designed architecture consisted of a GCE modified with CB within a dihexadecylphosphate film. The remarkable electrocatalytic activity of CB towards RAM electrooxidation was verified, and from different electrochemical assays, the number of electrons involved in the RAM oxidation was determined and, therefore, an electrooxidation reaction for RAM molecules was proposed. Under optimised experimental conditions, RAM was determined by square-wave adsorptive anodic stripping voltammetry. The analytical curve of RAM was linear from 1.98 × 10−6 to 2.42 × 10−5 mol L−1, with a limit of detection of 4.27 × 10−7 mol L−1. The proposed voltammetric procedure was successfully applied for the determination of RAM in pharmaceutical products and biological samples.

Preparation and electroanalytical applications of vertically aligned carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in the development of electrochemical devices in recent years. The rapid growth of scientific interest in the application of CNTs in different electrochemistry research fields has resulted from a set of chemical and physical features of these nanostructures, including high electrical conductivity, chemical stability and versatility for the immobilisation of chemical and biological species. The electrochemical performance is dependent upon various structural factors of the CNTs, such as chemical functionalisation, number of concentric tubes, exposed CNT area, and orientation. In particular, a number of recent publications have shown some beneficial effects when using vertically aligned CNTs (VACNT). Thus, this book chapter presents an overview of the processes of synthesis, functionalisation and characterisation of VACNT for applications as electrode materials for the electroanalytical determination of different target analytes.

Simultaneous determination of isoproterenol, acetaminophen, folic acid, propranolol and caffeine using a sensor platform based on carbon black, graphene oxide, copper nanoparticles and PEDOT:PSS

We explored the use of carbon black (CB), graphene oxide (GO), copper nanoparticles (CuNPs) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as electrode materials for the simultaneous determination of isoproterenol, acetaminophen, folic acid, propranolol and caffeine. The designed nanostructured surface was widely characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), contact angle measurements and electrochemistry. From electrochemical characterization assays carried out towards the potassium ferricyanide redox probe, fast electron transfer kinetics and a considerably higher electroactive surface area were observed for the modified electrodic surface based on CB, GO, CuNPs and PEDOT:PSS film. Using square-wave voltammetry (SWV), well defined and resolved anodic peaks were detected for isoproterenol, acetaminophen, folic acid, propranolol and caffeine, with peak-to-peak potential separation not less than 170 mV. Then, the SWV technique was explored for the simultaneous determination of quinary mixtures of these analytes, resulting in analytical curves with linear ranges and limits of detection at micromolar concentration levels. The practical viability of the proposed voltammetric sensor was illustrated in the analysis of human body fluid samples. The proposed sensor showed good repeatability and a successful application using urine and serum matrices, with recoveries close to 100%.