Electrochemical Immunosensors and peptide self-assembled monolayers for cancer biomarker protein detection

This Thesis concerns the development of a very efficient immunosensor electrochemical device for detecting cancer biomarker proteins, and possible ways to improve its efficiency by modifying parts of the system that are relevant to improve stability and ET through the modified electrode surface. For the part concerning the study of the ultrasensitive electrochemical immunosensor (Figure 13) for early cancer biomarkers detection, I worked with the research group of Prof. James F. Rusling (Department of Chemistry, University of Connecticut, Storrs, CT, USA). The research associated with the study of the stability and structure of Aib-peptide self-assembled monolayers to be used for improving the sensor performance was carried out in the group of Prof. Flavio Maran. The two topics are related because these peptides will be used to anchor the antibody onto AuNPs. This, however, is an aspect that for time limitations could not be covered during my Thesis work. Details on the Thesis scope and structure are as follows. We focused our attention on Nanog detection. Nanog is a protein that may be involved in carcinogenesis of cervix and progression of cervical carcinoma. Nowadays, the researchers still do not know the detection limit of this biomarker and the difference of concentration between healthy individuals and patients with cancer. Therefore, we aimed at making an electrochemical sensor capable of displaying very high-sensitivity immunoarrays and low detection limit. Sensors were prepared and, particularly, several conditions to make Nanog-based electrodes were essayed. Eventually, we could optimize the conditions and obtain a nice calibration plot (Amperometric current versus Nanog concentration). Most of the initial work was carried out using the sample handling technology, but then we integrated the system into a microfluidic device, the goal being to automate the method as much as possible. To improve the efficiency, we are about to further optimize the immunosensor by changing some elements of the transducer, particularly by using a SAM formed by peptides allowing very fast ET and by increasing the active superficial area thanks to nanostructured gold electrodes as an alternative to a bed of AuNPs. We carried out an investigation of related issues by using SAMs formed with thiolated Aib peptides of different lengths. The effect of the orientation of the peptide dipole moment was studied by attaching the thiolated moiety to either the nitrogen or carbon terminus. The stability and conformational properties of such SAMs were assessed by an extensive IRRAS investigation, in comparison with the IR absorption spectroscopy of the free peptides. This study showed that in these SAMs Aib peptides form 310-helices, form interchain C=O···H-N hydrogen bonds, and pack tightly, the surface coverage depending on both the peptide length and orientation. We also found that short peptides may undergo helix disruption, with formation of structures where the n umber of inter-chain interactions increases. The results nicely supported what recently found concerning the chemical and electrochemical stability of these SAMs as well as the efficiency of ET through them. Main outcome of this study is that we now know which peptides should provide the best transducer substrate supporting the actual Nanog-sensor architecture