Thesis presented November 13, 2017

Abstract:
The detection of small molecules contributes to the development of many fields such as food safety, homeland security, diagnose, environment control, etc. However, their small size and low concentration are the usual cause of limitations in their detection. In order to improve the detection, biosensors with appropriate probes and signal amplification strategies are required. Amongst the commonly used recognition elements, aptamer has the advantage of easier mass production and modification, reversible denaturation at high temperature and high tolerance of salt concentration and pH in the working environment. More importantly its small size made it an ideal choice for creating delicate structures for the detection of small targets. The possibility of splitting the aptamer sequence has provided more approaches for amplification purpose. There are two categories of detecting methods based on aptamers: heterogeneous analyzation where the aptamer is immobilized on a surface or homogeneous analyzation where the assay is performed in solution. In this thesis, we proposed an amplification method useful for both heterogeneous and homogeneous assays. Adenosine was used as a proof of concept target. The detection of Adenosine was achieved by combining the self-assembly of oligonucleotide dimers with split-aptamer dangling ends. We constructed self-assembled DNA structures (from 1D to 3D) with Adenosine as the trigger for a structural change. The heterogeneous assay is based on in Surface Plasmon Resonance imaging (SPRi). SPRi is a method sensitive to the change of refraction index created by the interaction between the probes immobilized on the gold surface and the targets in the flowing solution. With the presence of Adenosine in the solution, the DNA structure is self-assembled on the gold surface and the signal was created. The detection limit achieved by this method was 10 µM. The second homogeneous assay is based on the melting profile of the solution determined from the absorbance of UV light (260 nm wavelength). The UV absorbance of single strand DNA and hybridized DNA duplex is different. Due to this effect, the melting temperature could be obtained from the UV absorbance measured. The DNA structures combining self-complementary oligonucleotides and split-aptamer dangling ends have two melting temperatures, one correspond to the oligonucleotides and the other to the split-aptamer. In presence of Adenosine in the solution the strength in the binding is increased. As a result, the melting peak of the split-aptamer shifted to higher temperature while the second melting peak correspond the oligonucleotide remains the same as an internal reference. The detection limit achieved for this method was 1 µM. The DNA structures we proposed varied from 1D to 3D: the 1D structure was a DNA chain formed by a series of dimers connected through split-aptamer dangling ends; the 2D structure was a Y shape structure formed by three single-strand DNA with a split-aptamer dangling end on each branch of the “Y”; the 3D structure was a tetrahedron formed by four single-strand DNA with split-aptamer dangling ends on the four vertexes. With presence of Adenosine, 2D and 3D structures can further form a network with the dangling ends. The 1D structure has been maturely developed for the two detection methods, the 2D and 3D structures have been proven effective for detection but still require more efforts to reach perfection.

Keywords:
DNA, Aptamers, Self-Assembly, Biosensor

On-line thesis.