DNA-based applications in molecular electronics

This thesis is mainly focused on DNA molecules and especially on self-assembled DNA constructs and their potential applications in nanotechnology and molecular electronics. In the field of molecular electronics the conductivity of DNA is a crucial - yet open - question, and it is of great concern, since DNA is a very promising molecule in a context of bottom-up based nanodevices due to its superior selfassembly characteristics. A key tool in all the experiments presented in this thesis is a dielectrophoretic trapping technique, which was exploited in spatial manipulation of individualDNA molecules, DNA constructs and also semiconducting quantum dots. In the case of DNA, the technique provides immobilization and connection of DNA molecules and constructs to nanoscale electrodes enabling characterization of the electrical properties at the single construct level, and on the other hand, bridging of the electrodes by growing DNA molecules locally from trapped oligonucleotides. By exploiting dielectrophoresis phenomenon, two structurally distinct DNA constructs, DNA origamis and defined-sized TX tile complexes, were directed between lithographically fabricated nanoelectrodes. These structures could potentially serve as molecular scale circuit boards in nanoelectronics since they make controlled organization of wide range of materials possible. Thus, the electrical properties of a single rectangular origami and a TX tile construct were investigated by utilizing alternating current impedance spectroscopy with a full equivalent circuit modeling. The electrical measurements were carried out in high humidity conditions revealing that various environmental factors and in particular adsorbed water molecules surrounding the DNA had a significant influence to the observed conductance. In addition, a high throughput field-induced lithography method for nanoobjects was introduced, and its feasibility was demonstrated using quantum dots in a proof-of-principle experiment. The method allows dynamic control over the manipulation of any kinds of polarizable objects by dielectrophoresis and thus also over the formed pattern: one universal and reusable master stamp can be exploited in order to produce a desired multicomponent pattern on the specific target plate by recurrently trapping of certain components on the master followed by efficient mechanical transfer of the created pattern(s) to the target plate. The robust technique could be utilized in mass production and it is readily extendable to other nanoscale objects as well. Moreover, a method to grow single double-stranded DNA molecules at certain locations on a chip is proposed and demonstrated. The technique is based on immobilization of single-stranded DNA molecules to the ends of a desired set of nanoelectrodes by dielectrophoresis, and further elongation of them by polymerase chain reaction. Finally, the extended single-strand molecules can form a complete doublestranded DNA by binding together via hybridization. By means of this method one can grow single DNA molecules at specific spatial positions on a substrate or bridge the neighboring electrodes by DNA, and therefore, it may be utilized in detecting and sensing applications as well as in molecular electronics.