Improving organic electrochemical sensing by aptamer functionalization

Abstract

One of the most critical point for the best prognosis is an early accurate diagnosis. The ability to detect the onset of a disease, in fact, is crucial for the patients’ survival. To this end, it is essential to find reliable tools that help in illness prediction and diagnosis. Since the human body responds to the development of diseases by producing or changing concentration levels of specific biomolecules, the monitoring of this biological markers (biomarkers) can be used to predict a disease or its evolution. Biomarkers can be traced in fluids like urine, blood or saliva by using various detection methods. Unfortunately, most of the approaches used still suffer from lack of accuracy, sensitivity and specificity. The research activity of this PhD thesis is positioned in the need to develop more efficient and reliable sensing technologies in the framework of the field of biosensors that are promising solutions in terms of sensitivity and fast and cost-effective measurements. In particular, this PhD project aims at developing class of biosensors based on Organic Electrochemical Transistors (OECTs) as the transduction element and aptamers as bioreceptors. In our OECTs, we used as key material poly(3,4-ethylendioxythiophene):polystyrene sulfonate (PEDOT:PSS), a flexible and biocompatible conductive organic polymers. This polymer efficiently transduce an ionic signal into an electronic one with high amplification levels allowing the detection of very low concentrations of biomarkers. In the present work, the major innovation consists in the combination of OECTs structure with Aptamers, as the selective element, which are short RNA or single-stranded DNA able to recognize with unprecedented selectivity a great variety of targets even compared to antibodies, which represent the gold standards in biosensors. Integrating different competences of IMEM in Parma and Turin, a full-plastic and flexible OECT device has been developed exploiting the high flexibility of PEDOT:PSS and using as flexible gate electrode a multi-layers graphene deposited on low density polyethylene (PMLG) film supplied by the collaboration with the Dr. Gianfranco Carotenuto from IPCB-CNR of Naples. The aptamers functionalization of 2 OECTs has been performed in collaboration with the Dr. Laura Pasquardini from the Fondazione Bruno Kessler (FBK) of Trento. Generally, for biological applications, the selectivity of an OECT can be engineered through the functionalization of the PEDOT:PSS or of the gate electrode. In this project, we decided to functionalize the PMLG gate electrode with thiolated-aptamers, through the non-covalent modification of the graphene surface with gold nanoparticles (AuNPs), which in turn have been used as binding site exploiting the high affinity between gold and sulfur. This OECT has been engineered for the selective detection of thrombin, used as a test case because it is a protein of great interest since is involved in blood coagulation and it plays an essential role for homeostasis in many physiological systems. The sensing performance of the as designed OECT has been largely investigated by carrying out several accurate electrical characterizations from which it is possible to extract some key parameters. We have demonstrated that the AuNPs-PMLG composite electrode leads to several advantages compared to the metallic electrodes generally used for this type of applications and that we could achieve a limit of detection high enough for monitoring thrombin at its physiological concentrations.