Gregory G. Wildgoose

Sensitive electrochemical detection of arsenic (III) using gold nanoparticle modified carbon nanotubes via anodic stripping voltammetry

Gold nanoparticles were deposited electrolessly on multiwalled carbon nanotubes (CNTs) via in situ reduction of HAuCl(4) by NaBH(4). The resulting gold covered nanotubes were immobilised onto the surface of a glassy carbon electrode via evaporation of a suspension in chloroform. Anodic stripping voltammetry was performed with the modified electrode in As(III) solutions. A limit of detection (LOD based on 3 sigma) of 0.1 microgL(-1) was obtained but more importantly a sensitivity of 1985 microA microM(-1) was obtained with square wave voltammetry (SWV) in an optimised system with a deposition time of 120s. These values, particularly the high sensitivity compare favourably with previously reported methods in the area of electrochemical arsenic detection.

Apparent ‘electrocatalytic’ activity of multiwalled carbon nanotubes in the detection of the anaesthetic halothane: occluded copper nanoparticles

The electrocatalytic detection of the anaesthetic halothane on a multiwalled carbon nanotube modified glassy carbon electrode is reported with a low limit of detection of 4.6 microM. A thorough investigation of the underlying cause of this apparent catalytic effect is undertaken by comparing the response of various carbon electrodes including glassy carbon, basal- and edge-plane pyrolytic graphite electrodes (bppg and eppg respectively) to increasing additions of halothane. The reduction of halothane is shifted by 250-300 mV to more negative potentials at an eppg electrode than that observed at the GC-CNT electrode. Therefore the results of this investigation show that, surprisingly, the electrocatalysis is not solely due to the introduction of edge-plane-like defect sites on the carbon nanotubes as is commonly found for many other substrates showing favourable voltammetry at nanotube modified electrodes. Instead, we reveal that in this unusual case the electroactive sites for the reduction of halothane are due to the presence of copper nanoparticles occluded within the carbon nanotubes during their production, which are never completely removed by standard purification techniques such as acid washing. This is only the third known case where apparent electrocatalysis by carbon nanotube modified electrodes is due to occluded metal-related nanoparticles within the nanotube structure, rather than the active sites being the edge-plane-like defect sites on the nanotubes. Furthermore this is the first case where the active sites are nanoparticles of copper metal, rather than metal oxide nanoparticles (namely oxides of iron(II)/(III)) as was found to be the case in the previous examples.

Carbon nanotube-based electrochemical sensors for quantifying the ‘heat’ of chilli peppers: the adsorptive stripping voltammetric determination of capsaicin

A sensitive electroanalytical methodology for the determination of capsaicin using adsorptive stripping voltammetry (AdsSV) at a multiwalled carbon nanotube modified basal plane pyrolytic graphite electrode (MWCNT-BPPGE) is presented. This analytical method is then further developed using a multiwalled carbon nanotube screen-printed electrode (MWCNT-SPE) demonstrating the proof-of-concept that this approach can easily be incorporated into a sensing device which is both facile to use and inexpensive to produce. Capsaicin is the chemical responsible for the hot taste of chilli peppers, and measuring the concentration of capsaicin is an indicator of how hot any given chilli pepper, hot sauce and other related foodstuffs are. Standard additions plots for AdsSV of capsaicin at open circuit potential at a MWCNT-BPPGE exhibits two linear ranges, from 0.5 to 15 microM and from 15 to 60 microM. Using the first range of calibration curve, a detection limit of 0.31 microM (based on 3sigma) is obtained. The plot of standard additions of capsaicin determined using the disposable MWCNT-SPE shows a linear range between 0.5 and 35 microM and a detection limit of 0.45 microM. MWCNT-BPPGE and MWCNT-SPE are successfully utilized for the determination of capsaicin in real samples, such as a few commercially available hot pepper sauces, and the determined values are in excellent agreement and correlation with the average Scoville unit values reported in the literature for these sauces. To the best of our knowledge, this is the first electroanalytical method using MWCNT-BPPGE or MWCNT-SPE reported for the determination of capsaicin. This method offers advantages such as precision and objectivity over the well-known but potentially subjective Scoville method (based on organoleptic testing by human tasting panels) and is facile and inexpensive compared to existing HPLC methods.

Anthraquinone-derivatised carbon powder: reagentless voltammetric pH electrodes

A simple pH probe is developed based upon the covalent chemical derivatisation of carbon particles with anthraquinone. The amperometric response of electrodes constructed from this material is examined and shown to produce a Nernstian linear response to pH from 1 to 9, over a range of temperatures from 20 to 70 degrees C, consistent with a two-electron, two-proton electrochemical process.

Derivatised carbon powder electrodes: reagentless pH sensors

In this report, we derivatise carbon powder with anthracene, azobenzene, diphenylamine, 9,10-diphenylanthracene, methylene blue, 3-nitrofluoranthene, 6-nitrochrysene, 9-nitroanthracene, 9,10-phenanthraquinone (PAQ), thionin, and fast black K (2,5-dimethoxy-4-[(4-nitrophenyl)azo]benzenediazonium chloride) and separately immobilise the resulting material onto a bppg electrode. We use cyclic voltammetry (CV) to demonstrate that the observed voltammetric response for each derivatised carbon is consistent with that of an immobilised species. Further, we use CV and square wave voltammetry (SWV) to investigate the effect of pH on the peak potentials of each compound studied over the range pH 1-12 and at elevated temperatures up to 70 degrees C in order to demonstrate the versatility of derivatised carbon electrodes as reagentless pH sensors.

Nickel(II) tetra-aminophthalocyanine modified MWCNTs as potential nanocomposite materials for the development of supercapacitors†

The supercapacitive properties of nickel(II) tetraaminophthalocyanine (NiTAPc)/multi-walled carbon nanotube (MWCNT) nanocomposite films have been interrogated for the first time and found to possess a maximum specific capacitance of 981 ± 57 F g−1 (200 ± 12 mF cm−2), a maximum power density of 700 ± 1 Wkg−1, a maximum specific energy of 134 ± 8 Wh kg−1 and excellent stability of over 1500 charge-discharge continuous cycling. Impedimetric study proves that most of the stored energy of the MWCNT-NiTAPc nanocomposite can be accessible at high frequency (720 Hz). When compared to MWCNTs modified with unsubstituted nickel(II) phthalocyanine (MWCNT-NiPc) or nickel(II) tetra-tert-butylphthalocyanine (MWCNT-tBuNiPc), MWCNT-NiTAPc exhibited superior supercapacitive behaviour, possibly due to the influence of nitrogen-containing groups on the phthalocyanine rings.

Voltammetric and X-ray photoelectron spectroscopic fingerprinting of carboxylic acid groups on the surface of carbon nanotubes via derivatisation with arylnitro labels

4-Nitrophenol (4-NP)or 4-nitrobenzyl alcohol (4-NBA) are used as voltammetric and X-ray photoelectron spectroscopic (XPS) labels for carboxylic acid groups on the surface of single-walled carbon nanotubes, “bamboo-like” and “hollow-tube” multi-walled carbon nanotubes. The surface carboxyl groups are first converted to the corresponding acyl chlorides, and coupled to the labels via the formation of an ester linkage. The voltammetric reduction of the arylnitro label allows the number of carboxyl groups on the surface to be estimated. Alternatively the percentage of the elemental surface composition corresponding to the carboxyl groups can be measured using XPS via the emission from the N1s level of the label. The number of electroactive quinonyl groups present on the surface relative to the number of carboxyl groups can also be determined voltammetrically by measuring the area under the voltammetric wave corresponding to the surface quinone groups. By combining these voltammetric and XPS results the relative percentages of the O1s spectral peak, which correspond to carboxyl and quinonyl groups, can then be determined, which also allows, by difference, an estimate of the percentage elemental surface composition of total other oxygen-containing functionalities present such as lactones, ethers, aliphatic ketones etc. The effect of standard acid pre-treatments used to introduce carboxyl groups onto the CNT surface (such as stirring in relatively concentrated mixtures of nitric and sulfuric acids) on the number of each type of oxygen functionality is compared. In all cases the number of carboxyl groups is found to increase, but the number of quinonyl and other oxygen-containing functionalities is also found to increase, often to a greater extent than the carboxyl groups.

Characterising chemical functionality on carbon surfaces

This feature article introduces the reader to the surface chemistry and structure of graphitic carbon materials, including carbon nanotubes. Recent work involving the development of dual labels that allow us to selectively and quantitatively label carboxyl and general carbonyl groups (such as quinones, ketones and aldehydes) and to distinguish between ortho- and para-quinone groups is reviewed. In addition, the mechanisms of covalent, chemical derivatisation of these surfaces and the reactive sites towards attack by radical and cationic intermediates are discussed, as well as the interesting effects on the pKa values of organic molecules that attachment to a carbon surface can induce. When combined, the methods described herein allow one to differentiate and explore the chemical functionality and reactive sites on graphitic carbon surfaces.

Graphite powder derivatised with poly-L-cysteine using “building-block” chemistry—a novel material for the extraction of heavy metal ions

Graphite powder derivatised with 4-nitrophenyl moieties (NPcarbon) can be used to successively attach other chemical species to the graphite surface in a controlled and selective manner. Characterisation of these novel materials was undertaken using electrochemical and X-ray photoelectron spectroscopic (XPS) techniques. We demonstrate the proof of concept of this “building-block” chemistry by first reducing the nitro groups on the NPcarbon using a Sn–HCl reduction to the corresponding aniline-like moieties, and then coupling p-nitrobenzoic acid to these via an amide-linkage. Next we demonstrate the utility of this novel derivatisation method by coupling poly-S-benzyl-L-cysteine to the reduced NPcarbon in an analogous manner. After deprotection of the thiol groups using Na in liquid ammonia, the poly-L-cysteine derivatised carbon (PCcarbon) was found to complex cadmium(II) ions. Quantitative analysis of the uptake of Cd2+ from aqueous solutions at pH 5.0 by PCcarbon was carried out using stripping-voltammetry at a boron doped diamond (BDD) electrode and revealed that PCcarbon complexed 137 ± 20 mg of Cd2+ per gram of modified PCcarbon.

Oxygen reduction catalysis at anthraquinone centres molecularly wired via carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) have been chemically derivatised via the reduction of anthraquinone-1-diazonium chloride with hypophosphorous acid to attach 1-anthraquinonyl groups to the MWCNTs, most likely at edge plane like defects. The covalently attached quinone moiety attached to the nanotubes (‘molecular wire’) acts as an effective mediator for the electrocatalytic reduction of oxygen.