DNA and DNA-based polymers are of interest in molecular electronics because of their versatile and programmable structures. However, transport measurements have produced a range of seemingly contradictory results due to differences in the measured molecules and experimental set-ups, and transporting significant current through individual DNA-based molecules remains a considerable challenge. Here, we report reproducible charge transport in guanine-quadruplex (G4) DNA molecules adsorbed on a mica substrate. Currents ranging from tens of picoamperes to more than 100 pA were measured in the G4-DNA over distances ranging from tens of nanometres to more than 100 nm. Our experimental results, combined with theoretical modelling, suggest that transport occurs via a thermally activated long-range hopping between multi-tetrad segments of DNA. These results could re-ignite interest in DNA-based wires and devices, and in the use of such systems in the development of programmable circuits.

We describe a method for the preparation of novel long (hundreds of nanometers), uniform, inter-molecular G4-DNA molecules composed of four parallel G-strands. The only long continuous G4-DNA reported so far are intra-molecular structures made of a single G-strand. To enable a tetra-molecular assembly of the G-strands we developed a novel approach based on avidin–biotin biological recognition. The steps of the G4-DNA production include: (i) Enzymatic synthesis of long poly(dG)-poly(dC) molecules with biotinylated poly(dG)-strand; (ii) Formation of a complex between avidin-tetramer and four biotinylated poly(dG)-poly(dC) molecules; (iii) Separation of the poly(dC) strands from the poly(dG)-strands, which are connected to the avidin; (iv) Assembly of the four G-strands attached to the avidin into tetra-molecular G4-DNA. The average contour length of the formed structures, as measured by AFM, is equal to that of the initial poly(dG)-poly(dC) molecules, suggesting a tetra-molecular mechanism of the G-strands assembly. The height of tetra-molecular G4-nanostructures is larger than that of mono-molecular G4-DNA molecules having similar contour length. The CD spectra of the tetra- and mono-molecular G4-DNA are markedly different, suggesting different structural organization of these two types of molecules. The tetra-molecular G4-DNA nanostructures showed clear electrical polarizability. This suggests that they may be useful for molecular electronics.

Over the past decades, DNA, the carrier of genetic information, has been used by researchers as a structural template material. Watson-Crick base pairing enables the formation of complex 2D and 3D structures from DNA through self-assembly. Various methods have been developed to functionalize these structures for numerous utilities. Metallization of DNA has attracted much attention as a means of forming conductive nanostructures. Nevertheless, most of the metallized DNA wires reported so far suffer from irregularity and lack of end-to-end electrical connectivity. An effective technique for formation of thin gold-coated DNA wires that overcomes these drawbacks is developed and presented here. A conductive atomic force microscopy setup, which is suitable for measuring tens to thousands of nanometer long molecules and wires, is used to characterize these DNA-based nanowires. The wires reported here are the narrowest gold-coated DNA wires that display long-range conductivity. The measurements presented show that the conductivity is limited by defects, and that thicker gold coating reduces the number of defects and increases the conductive length. This preparation method enables the formation of molecular wires with dimensions and uniformity that are much more suitable for DNA-based molecular electronics.

Transparent conductive networks are important for flexible electronics and solar cells. Often interwire (junction) conductivity is the limiting factor for network conductivity and can be improved by various treatments. The conductivity of individual junctions in a single walled carbon nanotube network was measured by conductive atomic force microscopy before and after exposure to nitric acid. The measurements show that this exposure improves the conductivity of each one of the junctions within the network. Our results suggest that the acid improves the conductivity by p-type charge transfer doping and by surfactant degradation.

The quest for conductive molecular nanowires for nanoelectronic devices has prompted the study of the electrical properties of DNA as a possible electrical conduit or template, primarily due to its molecular recognition and self-assembly properties. [1] Non-contact techniques, such as electrostatic force microscopy (EFM), can provide valuable information on the charge distribution, thus indicating on charge mobility, polarization and migration, within a single molecule by measuring its response to an external applied field with an oscillating probe above the molecule. [2][3][4] Here we report on comparative atomic force microscopy (AFM) and EFM measurements of two forms of guanine-based quadruplex DNA molecules, tetra-and intramolecular G4-DNA. The tetra-molecular G4-DNA used in this work is made of four singlestrands of guanine nucleotides that run parallel to each other [5] . Each strand is attached to a biotin molecule and four such strands are linked to an avidin tetramer. We label this type of tetra-molecular G4-DNA as BA-G4-DNA. Intra-molecular G4-DNA is obtained by self-folding of a single strand of guanines. Such folding leads to an anti-parallel configuration, in which two strands run in one direction and the other two strands run in the opposite direction. [6] When using the same number of tetrads for the construction of the tetra-molecular G4-DNA and the 2 intra-molecular G4-DNA, the former are thicker and shorter than the latter molecules. [5] This suggests that the folding orientation of the strands, which form the backbone, affects the molecular structure, i.e. the tetrad unit and the tetrad-tetrad stacking. By comparing adjacent molecules of both types, co-adsorbed on the same mica surface, we circumvent the problem of phase calibration, showing that the EFM signal is twice as strong in the parallel configuration as compared with the anti-parallel G4-DNA, possibly because of greater charge mobility in tetra-molecular G4-DNA, thus making tetra-molecular G4-DNA a better candidate for conductivity measurements.Theoretical [7,8] and experimental [4,9] studies showed that out of the four natural bases, guanine may form a π-stacking with the greatest chance of providing a conducting bridge between bases, due to its lowest oxidation potential. [9] Moreover, the robust quadruple helix, in which each tetrad (Figure 1a) is formed by eight hydrogen bonds rather than by two or three as in dsDNA, is more rigid than the duplex dsDNA helix and may withstand surface deformations in solid-state molecular devices. Such rigidity is particularly appealing for the realization of conducting molecular bridges. Previously, we reported the synthesis [6,10] and EFM measurements [4] of intra-molecular G4-DNA (Figure 1b, left) which was stabilized by K⁺ cations. This type of intra-molecular G4-DNA possessed distinct polarizability, in contrast to native dsDNA, which gave no discernible signal. [4] That study was followed by the synthesis and EFM measurements of tetra-molecular G4-DNA [5] (Figure 1b, right), which is stable ...

Liquid metals have attracted attention as functional components for moldable electronics, such as soft flexible connectors, wires or conductive ink. The relatively high surface tension (> 400 mN m−1) and the fact that liquid metals do not readily wet ceramic or oxide surfaces have led to devising unique techniques to spread the liquid and mold its shape. These techniques include surface modification, electrowetting and vacuum filling of channels. This work presents an injection technique based on pressurized fountain pen lithography with glass nanopipettes developed to directly pattern liquid metal on flat hard substrates. The liquid metals were eutectic alloys of Gallium, including Gallium-Indium (EGaIn), Gallium-Indium-Zinc and Gallium-Indium-Tin. The nanopipettes were coated internally with gold, acting as a sacrificial layer and facilitating the wetting of the pipette down to its pore, with an inner diameter of ~ 100–300 nm. By applying hydrodynamic pressure to the connected end of the pipette, the metal was extruded through the pore, forming long continuous (> 3 mm) and narrow (~ 1–15 µm) metal lines on silicon oxide and gold surfaces at room temperature and ambient conditions. With this robust platform, it is possible to pattern liquid metals on a variety of substrates and geometries down to the micron range.

We report that long double-stranded DNA confined to quasi-1D nanochannels undergoes superdiffusive motion under the action of the enzyme T4 DNA ligase in the presence of necessary co-factors. Inside the confined environment of the nanochannel, double-stranded DNA molecules stretch out due to self-avoiding interactions. In absence of a catalytically active enzyme, we see classical diffusion of the center of mass. However, cooperative interactions of proteins with the DNA can lead to directed motion of DNA molecules inside the nanochannel. Here we show directed motion in this configuration for three different proteins (T4 DNA ligase, MutS, E. coli DNA ligase) in the presence of their energetic co-factors (ATP, NAD+).

The morphological structure of G4-DNA wires having four parallel strands, made by a novel synthesis method, was revealed by high-resolution scanning tunneling microscopy (STM). This imaging technique is a unique alternative to obtain information on the molecular structure of these wires in the absence of X-ray and NMR data. Imaging reveals a periodic structure seen as repeating "bulbs" along the molecule. The bulbs correspond to the helical structure of the molecules. The STM imaging shows a molecular chain structure. This structure exhibits an average periodicity of 3.5 nm and an average molecular apparent height of 1.4 nm.