Understanding the Physicoelectrochemical Properties of Carbon Nanotubes: Current State of the Art

Electroanalytical overview: the sensing of carbendazim

Carbendazim is a broad-spectrum systemic fungicide that is used to control various fungal diseases in agriculture, horticulture, and forestry. Carbendazim is also used in post-harvest applications to prevent fungal growth on fruits and vegetables during storage and transportation. Carbendazim is regulated in many countries and banned in others, thus, there is a need for the sensing of carbendazim to ensure that high levels are avoided which can result in potential health risks. One approach is the use of electroanalytical sensors which present a rapid, but highly selective and sensitive output, whilst being economical and providing portable sensing platforms to support on-site analysis. In this minireview, we report on the electroanalytical sensing of carbendazim overviewing recent advances, helping to elucidate the electrochemical mechanism and provide conclusions and future perspectives of this field.

Electroanalytical Overview: The Determination of Levodopa (L-DOPA)

L-DOPA (levodopa) is a therapeutic agent which is the most effective medication for treating Parkinson's disease, but it needs dose optimization, and therefore its analytical determination is required. Laboratory analytical instruments can be routinely used to measure L-DOPA but are not always available in clinical settings and traditional research laboratories, and they also have slow result delivery times and high costs. The use of electroanalytical sensing overcomes these problems providing a highly sensitivity, low-cost, and readily portable solution. Consequently, we overview the electroanalytical determination of L-DOPA reported throughout the literature summarizing the endeavors toward sensing L-DOPA, and we offer insights into future research opportunities.

Electroanalytical overview: utilising micro- and nano-dimensional sized materials in electrochemical-based biosensing platforms

Research into electrochemical biosensors represents a significant portion of the large interdisciplinary field of biosensing. The drive to develop reliable, sensitive, and selective biosensing platforms for key environmental and medical biomarkers is ever expanding due to the current climate. This push for the detection of vital biomarkers at lower concentrations, with increased reliability, has necessitated the utilisation of micro- and nano-dimensional materials. There is a wide variety of nanomaterials available for exploration, all having unique sets of properties that help to enhance the performance of biosensors. In recent years, a large portion of research has focussed on combining these different materials to utilise the different properties in one sensor platform. This research has allowed biosensors to reach new levels of sensitivity, but we note that there is room for improvement in the reporting of this field. Numerous examples are published that report improvements in the biosensor performance through the mixing of multiple materials, but there is little discussion presented on why each nanomaterial is chosen and whether they synergise well together to warrant the inherent increase in production time and cost. Research into micro-nano materials is vital for the continued development of improved biosensing platforms, and further exploration into understanding their individual and synergistic properties will continue to push the area forward. It will continue to provide solutions for the global sensing requirements through the development of novel materials with beneficial properties, improved incorporation strategies for the materials, the combination of synergetic materials, and the reduction in cost of production of these nanomaterials.

A label-free electrochemical system for comprehensive monitoring of o-chlorophenol

o-Chlorophenol (OCP) is a priority pollutant that poses serious health threats to the public. The following study designs a simple electrochemical system to monitor the concentration and toxicity of OCP. This system was primarily characterized by the integration of both physicochemical and biological monitoring procedures that had a synergistic effect between the functionalized carbon nanotubes and rhodamine B. This resulted in excellent electrocatalytic activities toward OCP and cellular purine bases. The peak current of OCP was linear with concentrations ranging from 0.05-125.0 μM and the detection limit was 0.028 μM under optimal testing conditions. There was an enhanced voltammetric signal detected that was caused by the guanine/xanthine of human hepatoma (HepG2) cells. The cytotoxicity of OCP to HepG2 cells was assessed using the proposed system. The obtained IC₅₀ value was 512.86 μM. This study provided a fast, label-free, and low-cost platform for the comprehensive assessment of OCP. This is highly beneficial for simplifying the environmental monitoring process.

Carbon Nanomaterials in Electrochemical Detection

This chapter overviews the use of carbon nanomaterials in the field of electroanalysis and considers why carbon-based nanomaterials are widely utilized and explores the current diverse range that is available to the practising electrochemist, which spans from carbon nanotubes to carbon nanohorns through to the recent significant attention given to graphene.

Batch-injection analysis with amperometric detection of the DPPH radical for evaluation of antioxidant capacity

This work proposes the application of batch-injection analysis with amperometric detection to determine the antioxidant capacity of real samples based on the measurement of DPPH radical consumption. The efficient concentration or EC50 value corresponds to the concentration of sample or standard required to scavenge 50% DPPH radicals. For the accurate determination of EC50, samples were incubated with DPPH radical for 1h because many polyphenolic compounds typically found in plants and responsible for the antioxidant activity exhibit slow kinetics. The BIA system with amperometric detection using a glassy-carbon electrode presented high precision (RSD = 0.7%, n = 12), low detection limit (1 μmol L(-1)) and selective detection of DPPH (free of interferences from antioxidants). These contributed to low detection limits for the antioxidant (0.015 and 0.19 μmol L(-1) for gallic acid and butylated hydroxytoluene, respectively). Moreover, BIA methods show great promise for portable analysis because battery-powered instrumentation (electronic micropipette and potentiostats) is commercially available.

MWCNT–CTAB modified glassy carbon electrode as a sensor for the determination of paracetamol

An electrochemical sensor for the sensitive detection of paracetamol (PCM) was developed by constructing a glassy carbon electrode (GCE) modified with multiwalled carbon nanotube–cetyltrimethyl ammonium bromide (MWCNT–CTAB).

Electrochemical behavior of the 1,10-phenanthroline ligand on a multiwalled carbon nanotube surface and its relevant electrochemistry for selective recognition of copper ion and hydrogen peroxide sensing

1,10-Phenanthroline (Phen) is a well-known benchmark ligand and has often been used in the coordination chemistry for the complexation of transition metal ions, such as Fe(2+), Ni(2+), and Co(2+). Because the electro-oxidation potential of Phen is much higher (>2 V versus Ag/AgCl) than the water decomposition potential, i.e., ∼1.5 V versus Ag/AgCl, in pH 7, it is practically difficult to electro-oxidize Phen in aqueous medium using any conventional electrodes, such as glassy carbon electrode (GCE), gold, and platinum. Interestingly, herein, we report an unexpected oxidation of Phen to a highly redox active 1,10-phenanthroline-5,6-dione (Phen-dione) and its confinement on a multiwalled carbon nanotube (MWCNT)-modified glassy carbon electrode (GCE/MWCNT@Phen-dione) surface by potential cycling of Phen-adsorbed GCE/MWCNT (GCE/MWCNT@Phen) from -1 to 1 V versus Ag/AgCl in pH 7 phosphate buffer solution. GCE/MWCNT@Phen-dione showed selective recognition of copper ion (GCE/MWCNT@Phen-dione-Cu(2+)) by catalyzing the hydrogen peroxide reduction reaction in a neutral pH solution. The precise structure of the Phen electro-oxidized product has been identified after characterizing the electrode and/or ethanolic extract of the product by various techniques, such as Raman, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) (for copper complex), liquid chromatography-mass spectrometry (LC-MS), electrospray ionization-mass spectrometry (ESI-MS) (for copper complex), cyclic voltammetry (CV), and in situ electrochemical quartz crystal microbalance (EQCM) and comparing electrochemical behavior of several control compounds, such as phenanthrene and 9,10-phenanthrenequinone. It is concluded that the product formed is 1,10-phenanthroline-5,6-dione, wherein the dione position is ortho to each other and the copper ion is complexed with nitrogen of the phenanthroline ring. With extended electrochemical oxidation of a structurally similar ligand, 2,2'-bipyridine failed to show any such electrochemical dynamics. Finally, applicability of GCE/MWCNT@Phen-dione-Cu(2+) for electrochemical sensing of hydrogen peroxide in a couple of real samples is successfully demonstrated.

The Application of Nafion Metal Catalyst Free Carbon Nanotube Modified Gold Electrode: Voltammetric Zinc Detection in Serum

Metal catalyst free carbon nanotube (MCFCNT) whiskers were first used as an electrode modification material on a gold electrode surface for zinc voltammetric measurements. A composite film of Nafion and MCFCNT whiskers was applied to a gold electrode surface to form a mechanically stable sensor. The sensor was then used for zinc detection in both acetate buffer solution and extracted bovine serum solution. A limit of detection of 53 nM was achieved for a 120 s deposition time. The zinc in bovine serum was extracted via a double extraction procedure using dithizone in chloroform as a zinc chelating ligand. The modified electrode was found to be both reliable and sensitive for zinc measurements in both matrices.

Carbon nanotubes and metalloporphyrins and metallophthalocyanines-based materials for electroanalysis

We discuss here the state of the art on hybrid materials made from single (SWCNT) or multi (MWCNT) walled carbon nanotubes and MN4complexes such as metalloporphyrins and metallophthalocyanines. The hybrid materials have been characterized by several methods such as cyclic voltammetry (CV), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electrochemical microscropy (SECM). The materials are employed for electrocatalysis of reactions such as oxygen and hydrogen peroxide reduction, nitric oxide oxidation, oxidation of thiols and other pollutants.

Prussian Blue Modified Solid Carbon Nanorod Whisker Paste Composite Electrodes: Evaluation towards the Electroanalytical Sensing ofH2O2

Metallic impurity free solid carbon nanorod “Whiskers” (SCNR Whiskers), a derivative of carbon nanotubes, are explored in the fabrication of a Prussian Blue composite electrode and critically evaluated towards the mediated electroanalytical sensing of H2O2. The sensitivity and detection limits for H2O2on the paste electrodes containing 20% (w/w) Prussian Blue, mineral oil, and carbon nanorod whiskers were explored and found to be 120 mA/(M cm2) and 4.1 μM, respectively, over the concentration range 0.01 to 0.10 mM. Charge transfer constant for the 20% Prussian Blue containing SCNR Whiskers paste electrode was calculated, for the reduction of Prussian Blue to Prussian White, to reveal a value of1.8±0.2 1/s (α=0.43,N=3). Surprisingly, our studies indicate that these metallic impurity-free SCNR Whiskers, in this configuration, behave electrochemically similar to that of an electrode constructed from graphite.

Carbon composite micro- and nano-tubes-based electrodes for detection of nucleic acids

The first aim of this study was to fabricate vertically aligned multiwalled carbon nanotubes (MWCNTs). MWCNTs were successfully prepared by using plasma enhanced chemical vapour deposition. Further, three carbon composite electrodes with different content of carbon particles with various shapes and sizes were prepared and tested on measuring of nucleic acids. The dependences of adenine peak height on the concentration of nucleic acid sample were measured. Carbon composite electrode prepared from a mixture of glassy and spherical carbon powder and MWCNTs had the highest sensitivity to nucleic acids. Other interesting result is the fact that we were able to distinguish signals for all bases using this electrode.

Sequential layer analysis of protein immunosensors based on single wall carbon nanotube forests

Electrochemical immunosensors using vertically aligned single wall carbon nanotube (SWNT) forests can provide ultrasensitive, accurate cancer biomarker protein assays. Herein we report a systematic investigation of the structure, thickness, and functionality of each layer of these immunosensors using atomic force microscopy (AFM), quartz crystal microbalance (QCM), and scanning white light interferometry (SWLI). This provides a detailed picture of the surface morphology of each layer along with surface concentration and thickness of each protein layer. Results reveal that the major reasons for sensitivity gain can be assigned to the dense packing of carboxylated SWNT forest tips, which translate to a large surface concentration of capture antibodies, together with the high quality of conductive SWNT forests.