Electromechanical Properties of DNA-CNT Complexes—Toward Functional Bio-Electronic Switches

DNA’s versatility in molecular engineering stems from its scalability and programmability. However, building DNA-based electronic devices is still a distant prospect, primarily due to two contributing factors, low conductivity and challenges with contact engineering. Although progress has been demonstrated towards engineering the intrinsic conductance of DNA, designing optimum molecular scale contacts remains challenging. In this study, our primary focus lies in tackling the challenge of contacts for DNA electronics. Through detailed computational studies, we identify carbon nanotubes (CNT) as a promising alternative to the commonly used metal contacts. A fundamental understanding of the interaction between DNA and CNT is developed. We present a comprehensive analysis of the DNA-CNT complex connected via a linker (amino and ester) with molecular dynamics simulations and charge transport calculations.
Our large-scale computational experiments over a time window of 100 ns highlights the impact of linkers on the mechanical stability. Based on the results, we predict that these systems prefer a stable configuration where DNA orients perpendicular to the CNT axis. This configuration is achieved after around 35 ns in the case of the amino linker and 30 ns for the ester linker. The stability of these configurations is confirmed by root mean square deviation (RMSD) values that drop below 6 and 4 Angstroms for the amino and ester linkers, respectively. The shorter ester linker also reduces the fluctuations, suggesting it provides greater mechanical stability compared to the amino linker.
Further investigation into the electronic properties revealed that the transmission probability is highly sensitive to the relative orientation and geometry of the DNA and CNT. Notably, in the system with the amino linker, a 500-fold increase in transmission was observed at the HOMO level as the DNA moved from a parallel to a perpendicular orientation relative to the CNT, with the transmission increasing from 8.18 × 10^(-7) to 3.75 × 10^(-4). In comparison, the system with the ester linker showed a more modest increase in transmission from 3.32 × 10^(-5) to 2.11 × 10^(-4). The results highlight that the dominant conduction pathway in such systems is direct hopping between the DNA and CNT, with the linker playing a less significant role in electron transport but contributing to the structural stability of the system.
Orbital analysis reveals enhanced delocalization in the final configuration for the amino linker, leading to improved coupling between DNA and CNT. Furthermore, the density of states (DOS) for the initial configuration indicates limited available states across the system, correlating with weaker orbital overlap. In contrast, the final configuration shows a notable increase in the DOS, especially at the DNA-CNT interface. This increased DOS suggests stronger π-π interactions between the DNA bases and the CNT sidewall, enhancing the system's electronic properties and making it more conducive to electron transport.
Our results indicate that application of strain breaks the strong direct electronic coupling between the nanotube and the DNA bases, leading to electromechanical switching with a conductance contrast of over one hundred. This forms the basis for electromechanical switching that should be experimentally observable. We envision that stable DNA-CNT configurations can serve as a foundational building block for larger bio-electronic circuits. Recent experimental demonstrations of CNT contact-based conductance measurements of DNA further reinforce the potential of CNT contacts in the bio-electronic domain. The outcomes of this large-scale computational study not only validate their candidacy but also pave the way for future advancements.