In this study, the investigation of attachment of DNA molecular wires and ropes to high aspect-ratio three-dimensional (3D) metal microelectrodes and their subsequent electrical characterization as part of a bionanoelectronics platform is reported. The 3-D microelectrode architecture consists of mainly high aspect-ratio microelectrode structures (75 μm height and above) patterned from relatively thick layers of negative tone photoresist and covered by sputtered gold on their top surface. DNA attachments on 3-D microelectrode structures was demonstrated using oligonucleotide-DNA self-assembly and thiol-gold covalent bonding. Further, DC and AC electrical characterization of double-stranded λ-DNA molecular wires in a dry environment and suspended between high aspect-ratio 3D microelectrodes 75 μm away from the substrate (to heights unprecedented so far in the literature which thereby eliminate interference of substrate) is presented. Electrical characterizations based on I-V and AC impedance analysis of several repeatable data points of attachment with varying λ-DNA concentration (500 ng/μL to 1.5 ng/μL) showed measurable and significant conductivity of λ-DNA molecular wires with some band-gap; thereby establishing it as semi-conductor at low-frequencies (<100 Hz) and a very good conductor at high-frequencies (∼1 MHz). We believe that the research presented here represents a significant departure from previous studies and makes unique contributions through (i) more accurate direct conductivity measurement of DNA molecular wires facilitated by suspension of the DNA away from the substrate, and (ii) AC impedance measurement of DNA molecular wires in dry-state attachment (relevant for long-term viability studies) that suggest metal-type low impedance at high-frequencies. The significant conductivity of λ-DNA molecular wires (similar to metals) observed at high-frequencies (|Z| < 5 KΩ) opens up substantial opportunities.

In this study, AC characterisation of DNA molecular wires, effects of frequency, temperature and UV irradiation on their conductivity is presented. λ-DNA molecular wires suspended between high aspect-ratio electrodes exhibit highly frequency-dependent conductivity that approaches metal-like behaviour at high frequencies (∼MHz). Detailed temperature dependence experiments were performed that traced the impedance response of λ-DNA until its denaturation. UV irradiation experiments where conductivity was lost at higher and longer UV exposures helped to establish that it is indeed λ-DNA molecular wires that generate conductivity. The subsequent renaturation of λ-DNA resulted in the recovery of current conduction, providing yet another proof of the conducting DNA molecular wire bridge. The temperature results also revealed hysteretic and bi-modal impedance responses that could make DNA a candidate for nanoelectronics components like thermal transistors and switches. Further, these experiments shed light on the charge transfer mechanism in DNA. At higher temperatures, the expected increase in thermal-induced charge hopping may account for the decrease in impedance supporting the 'charge hopping mechanism' theory. UV light, on the other hand, causes damage to GC base-pairs and phosphate groups reducing the path available both for hopping and short-range tunneling mechanisms, and hence increasing impedance--this again supporting both the 'charge hopping' and 'tunneling' mechanism theories.

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