2D nanomaterials as the basis for DNA biosensors with SNP detection capabilities

Abstract

Developments in biosensing techniques have provided a wide range of diagnostic solutions in recent years for a variety of sectors such as medicine, agriculture and biotechnology. The subject is so broad in fact, that only a small fragment of one of the main categories, namely label-free electrochemical sensors, will be discussed in the present thesis. The development of a highly selective and sensitive biosensor platform for SNPs would be a perfect addition to present clinical diagnostics in view of the increase in old-age population worldwide and especially in first-world countries. SNPs have been correlated to most autoimmune diseases, as well as neurodegenerative disease, including Alzheimer disease as well as Parkinson’s disease. Recently, nanomaterial-based biosensors have been used for the electro-chemical detection of biomolecules which possess advantageous characteristics because of their non-invasive and label-free nature. Unfortunately, the need for reproducibility as well as their high costs, have been major hurdles for the commercialization of the new biosensors. The present dissertation focuses on the use of 2-dimensional nanomaterials such as graphene based materials (GBMs) and molybdenum disulfide (MoS2) as the basis for cost-effective, sensitive and selective biosensors with electronic readout possibilities. A special focus has been placed on scalable fabrication techniques with near-identical sensing characteristics. The candidate materials are produced in easy and inexpensive chemical synthesis processes from their bulk counterparts. In the case of the graphite oxide (GrO) flakes several reduction techniques were explored in order to achieve partial reduction of the material and restore an optimal conductivity for sensing purposes. Gold micro-electrode pairs and interdigitated micro-electrodes were fabricated on glass, Si/SiO2 and polymer wafers. Looking to avoid toxicity in subsequent cell experiments, titanium was used as the adhesion material between the electrodes and the substrate. One of the most challenging aspects of working with nanomaterials is their correct manipulation and placement over the electrodes. Using dielectrophoresis (DEP), the aforementioned flakes were selectively deposited on top of the different arrays before being annealed for device homogenization. The deposited nanomaterials were physically characterized via atomic force microscopy (AFM) as well as scanning electron microscopy (SEM) to ensure their structural homogeneity. Subsequently, the nanomaterials were electrochemically characterized through impedance spectroscopy (IS) and field-effect measurements. In the specific case of the rGrO samples these electrochemical techniques allowed us to determine the level of reduction of the flakes after the different reduction protocols. Because the sensors were meant to work in biological buffers, their corresponding contact lines had to be passivated in order to insulate them from the liquid. Several approaches were studied including PDMS micro-channels as well as nickel electro and electroless plating. In the end however, the contact lines were passivated using a silicon oxide and silicon nitrite layering process. Finally, specific biomolecules or cells were functionalized on the sensors in order to perform cell adhesion, glucose detection and DNA hybridization experiments down to single nucleotide polymorphism detection. The changes in the impedance and/or the field-effect measurements provided a label-free and sensitive quantification method for different analytes, with first proof-of-concept results that compare to previously published sensors, with the added advantage of being cost-effective and extremely robust. Moreover, the use of DEP and refined annealing methods have resulted in near-identical biosensor performance.