Patterned forests of vertically-aligned multiwalled carbon nanotubes using metal salt catalyst solutions.

A simple method for producing patterned forests of multiwalled carbon nanotubes (MWCNTs) is described. An aqueous metal salt solution is spin-coated onto a substrate patterned with photoresist by standard methods. The photoresist is removed by acetone washing leaving the acetone-insoluble catalyst pattern on the substrate. Dense forests of vertically aligned (VA) MWCNTs are grown on the patterned catalyst layers by chemical vapour deposition. The procedures have been demonstrated by growing MWCNT forests on two substrates: silicon and conducting graphitic carbon films. The forests adhere strongly to the substrates and when grown directly on carbon film, offer a simple method of preparing MWCNT electrodes.

Reproducible fabrication of robust, renewable vertically aligned multiwalled carbon nanotube/epoxy composite electrodes.

We describe the reproducible fabrication of robust, vertically aligned multiwalled carbon nanotube (VACNT)/epoxy composite electrodes. The electrodes are characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopies. Low background currents are obtained at the electrodes, and common redox probe molecules and NADH show excellent voltammetric behavior. When electrode performance deteriorates due to fouling, the electrode surfaces can be reproducibly renewed by mechanical polishing followed by O(2) plasma treatment. The electrochemical performance of the electrodes is maintained after more than 100 cycles of use and renewal.

Electrochemical detection of oestrogen binding protein interaction with oestrogen in Candida albicans cell lysate.

Previously we reported an electrochemical method to quantitatively detect vertebrate oestrogens using wild type Saccharomyces cerevisiae cells. That assay required the use of a double mediator system, a five-hour incubation period and had a maximum detection limit of around 11 nM 17ß-oestradiol. In the work reported here we have sought to systematically increase the utility and decrease the complexity of the whole cell assay. The steps we took to achieve this goal were in order; lysing the cells to remove transport constraints, removing the lipophilic mediator and conducting the assay with the hydrophilic mediator only and finally performing the assay in a complex medium to demonstrate its specificity. Linear sweep voltammetry was used to investigate the interaction of mediators with NADH. The assay is now cell free and functions in a complex substrate. The linear response range upper limit has been raised to 100 nM with a calculated limit of detection of 0.005 nM with a limit of determination of 0.014 nM and the assay period has been reduced to 20 min.

Characterisation of yeast microbial fuel cell with the yeast Arxula adeninivorans as the biocatalyst.

Yeast microbial fuel cells have received little attention to date. Yeast should be ideal MFC catalyst because they are robust, easily handled, mostly non-pathogenic organisms with high catabolic rates and in some cases a broad substrate spectrum. Here we show that the non-conventional yeast Arxula adeninvorans transfers electrons to an electrode through the secretion of a reduced molecule that is not detectable when washed cells are first resuspended but which accumulates rapidly in the extracellular environment. It is a single molecule that accumulates to a significant concentration. The occurrence of mediatorless electron transfer was first established in a conventional microbial fuel cell and that phenomenon was further investigated by a number of techniques. Cyclic voltammetry (CV) on a yeast pellet shows a single peak at 450 mV, a scan rate study showed that the peak was due to a solution species. CVs of the supernatant confirmed a solution species. It appears that, given its other attributes, A. adeninivorans is a good candidate for further investigation as a MFC catalyst.

Electron transfer from Proteus vulgaris to a covalently assembled, single walled carbon nanotube electrode functionalised with osmium bipyridine complex: application to a whole cell biosensor.

We report the fabrication and use of electrodes constructed from single walled carbon nanotubes (SWCNTs) chemically assembled on a carbon surface and functionalised with an osmium(II) bipyridine complex (Osbpy). The ability of the electrodes to transduce biologically generated currents from Proteus vulgaris has been established. Our investigations show that there are two contributions to the current: one from electroactive species secreted into solution and another from cell redox sites. The modified electrode can be used to monitor cell metabolism, thereby acting as a whole cell biosensor. The biosensor was used in a 1-h assay to investigate the toxicity of ethanol, sodium azide and the antibiotic ampicillin and gave quantitative data that were closely correlated with standard cell plate viability assays. The results provide proof of principle that the whole cell biosensor could be used for high throughput screening of antimicrobial activity. One of the modified electrodes was used for approximately 1000 measurements over four months demonstrating the robustness of the system.

Robust forests of vertically aligned carbon nanotubes chemically assembled on carbon substrates.

Forests of vertically aligned carbon nanotubes (VACNTs) have been chemically assembled on carbon surfaces. The structures show excellent stability over a wide potential range and are resistant to degradation from sonication in acid, base, and organic solvent. Acid-treated single-walled carbon nanotubes (SWCNTs) were assembled on amine-terminated tether layers covalently attached to pyrolyzed photoresist films. Tether layers were electrografted to the carbon substrate by reduction of the p-aminobenzenediazonium cation and oxidation of ethylenediamine. The amine-modified surfaces were incubated with cut SWCNTs in the presence of N,N'-dicyclohexylcarbodiimide (DCC), giving forests of vertically aligned carbon nanotubes (VACNTs). The SWCNT assemblies were characterized by scanning electron microscopy, atomic force microscopy, and electrochemistry. Under conditions where the tether layers slow electron transfer between solution-based redox probes and the underlying electrode, the assembly of VACNTs on the tether layer dramatically increases the electron-transfer rate at the surface. The grafting procedure, and hence the preparation of VACNTs, is applicable to a wide range of materials including metals and semiconductors.

Patterned arrays of vertically aligned carbon nanotube microelectrodes on carbon films prepared by thermal chemical vapor deposition.

A straightforward procedure is described for preparation of arrays of microdisk electrodes comprising bundles of vertically aligned carbon nanotubes (VACNTs). The arrays are fabricated by thermal chemical vapor deposition synthesis directly on a planar carbon film support. Use of standard micro- and nanolithography procedures for patterning the bilayer catalyst spots enables arrays to be grown with controlled electrode diameters and spacings. The minimum accessible VACNT bundle diameter, and hence microelectrode diameter, is 2 microm. After insulating the arrays with SU-8 epoxy and exposing the VACNT ends by polishing or treating with O2 plasma, the microdisk electrodes exhibit attractive electrochemical properties.

Electrochemical detection of wild type Saccharomyces cerevisiae responses to estrogens.

The presence of an estrogen binding protein (EBP) and an endogenous ligand in three yeast species was first reported in 1982/1983. The ligand was shown to be 17beta-estradiol and the binding affinities of EBP were demonstrated to be similar to those of rat estrogen receptors. This report describes detection of the behaviour of a putative estrogen binding protein in Saccharomyces cerevisiae using a double mediator electrochemical detection system. The response to estrogen is shown to be quantitative with signals detectable from 10(-8) to 10(-14)M. An incubation period of 5h is established and a method to block electrochemical signals produced by the catabolism of exogenous substrates is demonstrated to be effective. The system provides a method that permits the use of wild type S. cerevisiae to quantify estrogens.