UC Davis

The potential role of DNA in nanoelectronics and nano sensors arises from its distinct self-assembly properties, charge transport characteristics, and significance in essential chemical and biological processes. This necessitates a comprehensive understanding of charge transport and transfer mechanisms, essential for both nano electronic applications like DNA-based memories and the utilization of DNA-based platforms as bioelectronic sensors. In our work, we initially explored the application of DNA molecules as biosensors. Using h RNA-based single-molecule conductance experiments, we successfully detected distinct variants of the SARS-CoV-2 virus. Our approach, adaptable for emerging variants, relied on a methodology to select target DNA sequences from specific variant's RNA segments. Employing single-molecule break junction (SMBJ) measurements, we achieved sensitivity and specificity in detecting RNA:DNA hybrids via their conductance. Our approach demonstrated the potential to detect SARS-CoV-2 variants, including B.1.1.7 (Alpha), B.1.351 (Beta), B.1.617.2 (Delta), and B.1.1.529 (Omicron). Understanding charge transport within DNA is crucial in designing sensitive and efficient DNA sensors. Previous studies have revealed that charge flow in DNA is through pi-stacking between adjacent base pairs. Yet, factors influencing this transport within DNA require further exploration. Thus, the latter part of this thesis investigates what could enhance or disrupt a charge moving across a single DNA molecule junction. We first investigated the influence of nearest neighbors within DNA sequences. Our findings reveal that the arrangement of bases in a sequence substantially impacts coupling in hopping transport, leading to significant changes in conductance values in short DNA molecules. Building on these insights, we proceed to design longer conductive DNA sequences, spanning 7 nanometers. This device length enables interactions with other biological systems such as proteins, paving the way for novel sensing applications. Next, we explored the integration of non-canonical bases to modulate energy and density of state distribution within dsDNA molecules, broadening the genetic alphabet beyond the classic Watson-Crick (GCAT) base pairs. Although these modifications alter the distribution of DOS and HOMO-LUMO energies, the molecule's conductance remains relatively unaffected. This shows the potential for expanding DNA’s electronic design while preserving its structural and electronic integrity. In the final segment of our work, we conducted temperature-dependent charge transport studies under vacuum and cryogenic condition, shedding light on the intricacies of DNA's charge transport phenomena. We measured DNA conductance using the SMBJ technique at varying temperatures from 77 K to 290 K, alongside IV characteristics. Among many models proposed for explaining DNA conductance temperature dependence our findings align with the variable range hopping model, indicating that DNA charge transport emerges as a multifaceted interplay of tunneling and hopping, resulting in intricate quantum phenomena within a single molecule. In summary, our study provides a systematic exploration of charge transport in DNA molecules and its application in biosensors. Through carefully designed sequences and SMBJ measurement techniques, we unravel conductance and current-voltage behaviors. Leveraging these insights, we not only developed DNA for COVID variant detection but also engineered highly conductive DNA sequences, thus facilitating efficient charge transport. This endeavor substantially contributes to unraveling the intricacies of DNA's charge transport and its implications for ultra-high-sensitive sensors and extended DNA wires.