Yueqi Li

Scanning tunneling microscopy and atomic force microscopy studies of biomaterials at a liquid–solid interface

We have compared the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) using a clean gold surface under electrochemical potential control, finding that the STM usually yields higher resolution, although this may be a consequence of better production methods for STM tips. New methods for tethering DNA molecules to substrates have yielded many excellent AFM images of DNA, even under water. The highest resolution has been obtained with specially sharpened tips, but recent work suggests that this may not be necessary. Control of contamination, and tip–sample interactions (i.e., adhesion) are also important factors. We describe a scheme of magnetic control of AFM cantilevers which should overcome the mechanical instability that places a lower limit on contact force. This should permit active control of interaction forces with the instrument operated in water. We illustrate local force measurements made under water with examples of the measurements of the microelastic properties of bone and ...

Initial stage of underpotential deposition of Pb on reconstructed and unreconstructed Au(111)

Abstract We have studied the initial stage of Pb nucleation and growth on both reconstructed and unreconstructed Au(111) from 5.7mM Pb(NO 3 ) 2 + 50mM HClO 4 using scanning tunneling microscopy. We have found that Pb clusters grow preferentially along the 23 × √3 reconstruction of Au(111), a result that is similar to studies of metal deposition on reconstructed Au(111) in ultrahigh vacuum. A superperiodic structure of monolayer Pb was also observed and can be explained as a result of the rotation of the hexagonal-close packed Pb monolayer relative to the Au lattice. The coverage and average diameter of the growing Pb clusters were found to increase in two steps which correlate with the double peak in the voltammogram from underpotential deposition.

Engineering nanometre-scale coherence in soft matter

Electronic delocalization in redox-active polymers may be disrupted by the heterogeneity of the environment that surrounds each monomer. When the differences in monomer redox-potential induced by the environment are small (as compared with the monomer-monomer electronic interactions), delocalization persists. Here we show that guanine (G) runs in double-stranded DNA support delocalization over 4-5 guanine bases. The weak interaction between delocalized G blocks on opposite DNA strands is known to support partially coherent long-range charge transport. The molecular-resolution model developed here finds that the coherence among these G blocks follows an even-odd orbital-symmetry rule and predicts that weakening the interaction between G blocks exaggerates the resistance oscillations. These findings indicate how sequence can be exploited to change the balance between coherent and incoherent transport. The predictions are tested and confirmed using break-junction experiments. Thus, tailored orbital symmetry and structural fluctuations may be used to produce coherent transport with a length scale of multiple nanometres in soft-matter assemblies, a length scale comparable to that of small proteins.

Thermoelectric effect and its dependence on molecular length and sequence in single DNA molecules.

Studying the thermoelectric effect in DNA is important for unravelling charge transport mechanisms and for developing relevant applications of DNA molecules. Here we report a study of the thermoelectric effect in single DNA molecules. By varying the molecular length and sequence, we tune the charge transport in DNA to either a hopping- or tunnelling-dominated regimes. The thermoelectric effect is small and insensitive to the molecular length in the hopping regime. In contrast, the thermoelectric effect is large and sensitive to the length in the tunnelling regime. These findings indicate that one may control the thermoelectric effect in DNA by varying its sequence and length. We describe the experimental results in terms of hopping and tunnelling charge transport models.

Studies of the electrical properties of large molecular adsorbates

The scanning tunneling microscope (STM) was used to investigate the conductivity of organic aggregates on gold electrodes. DNA fragments can form very stable aggregates in the presence of tris(hydroxymethyl)aminomethane buffer salt. With a submonolayer coverage, the tip can be moved back and forth between ‘‘clean’’ gold (in contact with the electrolyte) and an organic adsorbate patch. In these conditions, a systematic variation of the current voltage (I–V) characteristics indicates that the electrical characteristics are not dominated by a contaminant particle on the STM tip. The dependence of image contrast on tip bias was also studied over the range +0.3 to −0.3 V. It is found that: (a) the image contrast does not depend strongly on tip bias; (b) the I–V curves over what appears to be ‘‘clean’’ gold under the electrolyte are similar to those observed over clean gold maintained in an inert atmosphere; (c) the I–V curves over an adsorbate patch are diode‐like; (d) the I–V curves show no sign of a large vo...

Gate-controlled conductance switching in DNA

Extensive evidence has shown that long-range charge transport can occur along double helical DNA, but active control (switching) of single-DNA conductance with an external field has not yet been demonstrated. Here we demonstrate conductance switching in DNA by replacing a DNA base with a redox group. By applying an electrochemical (EC) gate voltage to the molecule, we switch the redox group between the oxidized and reduced states, leading to reversible switching of the DNA conductance between two discrete levels. We further show that monitoring the individual conductance switching allows the study of redox reaction kinetics and thermodynamics at single molecular level using DNA as a probe. Our theoretical calculations suggest that the switch is due to the change in the energy level alignment of the redox states relative to the Fermi level of the electrodes.

Tuning the Electromechanical Properties of Single DNA Molecular Junctions

Understanding the interplay between the electrical and mechanical properties of DNA molecules is important for the design and characterization of molecular electronic devices, as well as understanding the role of charge transport in biological functions. However, to date, force-induced melting has limited our ability to investigate the response of DNA molecular conductance to stretching. Here we present a new molecule-electrode linker based on a hairpin-like design, which prevents force-induced melting at the end of single DNA molecules during stretching by stretching both strands of the duplex evenly. We find that the new linker group gives larger conductance than previously measured DNA-electrode linkers, which attach to the end of one strand of the duplex. In addition to changing the conductance the new linker also stabilizes the molecule during stretching, increasing the length a single DNA molecule can be stretched before an abrupt decrease in conductance. Fitting these electromechanical properties to a spring model, we show that distortion is more evenly distributed across the single DNA molecule during stretching, and thus the electromechanical effects of the π-π coupling between neighboring bases is measured.

Charge splitters and charge transport junctions based on guanine quadruplexes

Self-assembling circuit elements, such as current splitters or combiners at the molecular scale, require the design of building blocks with three or more terminals. A promising material for such building blocks is DNA, wherein multiple strands can self-assemble into multi-ended junctions, and nucleobase stacks can transport charge over long distances. However, nucleobase stacking is often disrupted at junction points, hindering electric charge transport between the two terminals of the junction. Here, we show that a guanine-quadruplex (G4) motif can be used as a connector element for a multi-ended DNA junction. By attaching specific terminal groups to the motif, we demonstrate that charges can enter the structure from one terminal at one end of a three-way G4 motif, and can exit from one of two terminals at the other end with minimal carrier transport attenuation. Moreover, we study four-way G4 junction structures by performing theoretical calculations to assist in the design and optimization of these connectors.Guanine-quadruplex motifs form multi-ended DNA junctions that transport electrical charges with minimal losses.

Mechanical Stretching-Induced Electron-Transfer Reactions and Conductance Switching in Single Molecules

A central idea in electron-transfer theories is the coupling of the electronic state of a molecule to its structure. Here we show experimentally that fine changes to molecular structures by mechanically stretching a single metal complex molecule via changing the metal-ligand bond length can shift its electronic energy levels and predictably guide electron-transfer reactions, leading to the changes in redox state. We monitor the redox state of the molecule by tracking its characteristic conductance, determine the shift in the redox potential due to mechanical stretching of the metal-ligand bond, and perform model calculations to provide insights into the observations. The work reveals that a mechanical force can shift the redox potential of a molecule, change its redox state, and thus allow the manipulation of single molecule conductance.