Development of an affinity sensor for ochratoxin A

Ochratoxin A is a contaminant in wine and known to be immunosuppressive and possibly carcinogenic. Therefore, the development of a rapid and sensitive method for field analysis is required for risk assessment and management. The work presented in this thesis reports the construction of a sensor platform capable of fulfilling these requirements. As a sensor platform, screen-printed thick film electrodes and microelectrodes on a silicone support were investigated for sensor development. As biological recognition elements, an antibody specifically binding ochratoxin A and a peptide receptor that was designed using computational modelling were examined. A disposable immunosensor for ochratoxin A was developed based on screen-printing technology. An indirect competitive immunoassay format was used on bare screen printed gold electrode (SPGE). The performance of this sensor was compared to carboxmethylated dextran (CMD) modified SPGE. Detection was performed by chronoamperometry monitoring the reaction of tetramethylbenzidine and hydrogen peroxide catalysed by horseradish peroxidase. The SPGE-based immunosensor achieved a detection limit of 100 ng L-1 and the CMD-modified SPGE immunosensor 10 ng L-1. The latter has been used for ochratoxin A determination in wine samples and was validated against standard HPLC and a commercial immunoassay test kit. Wine sample analysis involved the sample pre-treatment using immunoaffinity chromatography, electrochemical wine component characterisation and interference control. The immunosensor format was transferred to a gold microelectrode array based on a silicone support for the purpose of signal sensitivity enhancement and miniaturisation in the prospect of field analysis. Preliminary data showed the characterisation of the microelectrode array immunosensor construction and characterisation. Further optimisation is needed to establish a calibration curve with the required sensitivity. The second part of the work comprised the design of a peptide receptor for ochratoxin A using computational methods by screening de novo designed peptide libraries. An octapeptide (CSIVEDGL) and a 13-peptide (GPAGIDGPAGIRC) were selected for synthesis and affinity characterised for ochratoxin A recognition using a surface plasmon resonance biosensor (BiacoreTM). The peptide receptors showed good sensitivity for ochratoxin A of 10 μg L-1. Preliminary affinity characterisation resulted in KA = 63 mM-1 for the 13-mer peptide and KA = 84 mM-1 for the octapeptide, which appears to be binding with higher strength to ochratoxin A. The affinity values correspond to the binding score (binding energy) calculated by computational modelling. This work shows the potential of designing peptide receptors for small molecules (e.g. ochratoxin A) and suggests their application in affinity sensors for detecting ochratoxin A contamination.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Electronic transport and vibrational modes in the smallest molecular bridge: H2 in Pt nanocontacts

We present a state-of-the-art first-principles analysis of electronic transport in a Pt nanocontact in the presence of H2 which has been recently reported by Smit et al. in Nature 419, 906 (2002). Our results indicate that at the last stages of the breaking of the Pt nanocontact two basic forms of bridge involving H can appear. Our claim is, in contrast to Smit et al.'s, that the main conductance histogram peak at G approx 2e^2/h is not due to molecular H2, but to a complex Pt2H2 where the H2 molecule dissociates. A first-principles vibrational analysis that compares favorably with the experimental one also supports our claim .Comment: 5 pages, 3 figure

Structure and conductance histogram of atomic-sized Au contacts

Many experiments have shown that the conductance histograms of metallic atomic-sized contacts exhibit a peak structure, which is characteristic of the corresponding material. The origin of these peaks still remains as an open problem. In order to shed some light on this issue, we present a theoretical analysis of the conductance histograms of Au atomic contacts. We have combined classical molecular dynamics simulations of the breaking of nanocontacts with conductance calculations based on a tight-binding model. This combination gives us access to crucial information such as contact geometries, forces, minimum cross-section, total conductance and transmission coefficients of the individual conduction channels. The ensemble of our results suggests that the low temperature Au conductance histograms are a consequence of a subtle interplay between mechanical and electrical properties of these nanocontacts. At variance with other suggestions in the literature, our results indicate that the peaks in the Au conductance histograms are not a simple consequence of conductance quantization or the existence of exceptionally stable radii. We show that the main peak in the histogram close to one quantum of conductance is due to the formation of single-atom contacts and chains of gold atoms. Moreover, we present a detailed comparison with experimental results on Au atomic contacts where the individual channel transmissions have been determined.Comment: 11 pages, 10 figures, version to be published in Phys. Rev. B. The paper has been thoroughly revised and several figures have been replaced by new one

Plasma IgM levels differentiate between survivors and non-survivors of culture-positive and culture-negative sepsis and SIRS: a pilot study

Immunoglobulin IgM is important for controlling viral and bacterial infections, and low immunoglobulin levels have been found in sepsis. There is a clear need to stratify sepsis patients according to the presence of an invading organism, compared to no organism identified, and SIRS patients, where organ dysfunction is a result of a non-infective process. The aim of this pilot study in a small cohort of patients with sepsis was to evaluate the association between IgM plasma levels and survival in 47 patients with sepsis and 11 patients diagnosed with organ failure without the identification of a pathogen (SIRS). Patients were admitted to the intensive care unit (ICU) at The Royal Glamorgan Hospital, Llantrisant, UK between 2010 and 2014. We found that low IgM levels were associated with sepsis, but not SIRS. IgM levels did not differ significantly for culture-positive (CP) compared with culture-negative (CN, no organism found) sepsis samples. Kaplan–Meier analysis was used to compare survival curves according to IgM levels, with no significant difference. We observed significantly higher survival in the CP samples when comparing with CN. Cut-off value for IgM (266 μg/mL) for diagnosis of sepsis patients was determined using receiver operator characteristic (ROC) curves with 70% sensitivity, 69% specificity and 92% negative predictive values (NPV), respectively. The corresponding area under the curve (AUC) for the discrimination of sepsis patients was AUC = 0.73, and in a subgroup analysis of CP was AUC = 0.77 and for CN was AUC = 0.79. We confirm IgM as a good diagnostic marker of sepsis. These findings indicate a difference in the pathology between culture-positive versus negative sepsis, SIRS and survival. This indicates that IgM is likely relevant to pathology, because of its role in the early immune response against pathogens, the potentially protective role of natural IgM antibodies, and supports its application in immunoglobulin therapy

Dark states of dressed Bose-Einstein condensates

We combine the ideas of dressed Bose-Einstein condensates, where an intracavity optical field allows one to design coupled, multicomponent condensates, and of dark states of quantum systems, to generate a full quantum entanglement between two matter waves and two optical waves. While the matter waves are macroscopically populated, the two optical modes share a single photon. As such, this system offers a way to influence the behaviour of a macroscopic quantum system via a microscopic ``knob''.Comment: 6 pages, no figur

Subharmonic Shapiro steps and assisted tunneling in superconducting point contacts

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n) \hbar \omega_r/2e, where n,m are integer numbers and \omega_r is the radiation frequency. The subharmonic steps (n different from 1) are a consequence of the ocurrence of multiple Andreev reflections (MAR) and provide an unambiguous signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.Comment: Revtex, 4 pages, 4 figure

Electrical transport through single-molecule junctions: from molecular orbitals to conduction channels

We present an atomistic theory of electronic transport through single organic molecules that reproduces the important features of the current-voltage characteristics observed in recent experiments. We trace these features to their origin in the electronic structure of the molecules and their local atomic environment. We demonstrate how conduction channels arise from the molecular orbitals and elucidate which specific properties of the individual orbitals determine their contribution to the current.Comment: Revtex4, 4 pages, 4 figures. Version with color figures in http://www-tfp.physik.uni-karlsruhe.de/~cuevas/Publications.htm

The number of transmission channels through a single-molecule junction

We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting $\pi$-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level.Comment: 9 pages, 8 figure

Geometric Metasurfaces for Ultrathin Optical Devices

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