Interfacial electrical properties of DNA-modified diamond thin films: intrinsic response and hybridization-induced field effects

We have investigated the frequency-dependent interfacial electrical properties of nanocrystalline diamond films that were covalently linked to DNA oligonucleotides and how these properties are changed upon exposure to complementary and noncomplementary DNA oligonucleotides. Frequency-dependent electrical measurements at the open-circuit potential show significant changes in impedance at frequencies of >10(4) Hz when DNA-modified diamond films are exposed to complementary DNA, with only minimal changes when exposed to noncomplementary DNA molecules. Measurements as a function of potential show that at 10(5) Hz, the impedance is dominated by the space-charge region of the diamond film. DNA molecules hybridizing at the interface induce a field effect in the diamond space-charge layer, altering the impedance of the diamond film. By identifying a range of impedances where the impedance is dominated by the diamond space-charge layer, we show that it possible to directly observe DNA hybridization, in real time and without additional labels, via simple measurement of the interfacial impedance.     

Test strip enzyme immunoassays and the fast screening of nortestosterone and clenbuterol residues in urine samples at the parts per billion level

The preliminary results of an investigation into the development of "on-site" test strip enzyme immunoassays for the screening of urine samples for the presence of growth promoters, such as 17 beta, 19-nortestosterone and clenbuterol at the parts per billion level are described. Urine samples, enzyme-labelled analyte and a nitrocellulose test strip, containing immobilized antibodies, are incubated together, after which the strip is placed in a chromogen-containing substrate solution for colour reaction. Using prefabricated strips, the tests can be performed in 45-60 min. A similar assay was worked out using a dot-blotting device, allowing the test to be performed in 20-50 min. The tests are simple and easy to perform outside the laboratory. Urine samples identified positive by gas chromatography mass spectrometry were also found to be positive with these test strips and, so far, no false-positive results have been encountered. With standard additions to blank urine samples, positive samples could be distinguished above the 5 ng ml level. However, samples from treated calves contain one or more metabolites of the parent compound, which increase the sensitivity of the assays. Although the tests described can be improved and still have to be evaluated further by analysing more urine samples, the preliminary results are very promising and give a lead to further research into the applicability of such "on-site" tests in residue analysis.