Nanoscale electrical conductivity imaging using a nitrogen-vacancy center in diamond.

The electrical conductivity of a material can feature subtle, non-trivial, and spatially varying signatures with critical insight into the material's underlying physics. Here we demonstrate a conductivity imaging technique based on the atom-sized nitrogen-vacancy (NV) defect in diamond that offers local, quantitative, and non-invasive conductivity imaging with nanoscale spatial resolution. We monitor the spin relaxation rate of a single NV center in a scanning probe geometry to quantitatively image the magnetic fluctuations produced by thermal electron motion in nanopatterned metallic conductors. We achieve 40-nm scale spatial resolution of the conductivity and realize a 25-fold increase in imaging speed by implementing spin-to-charge conversion readout of a shallow NV center. NV-based conductivity imaging can probe condensed-matter systems in a new regime not accessible to existing technologies, and as a model example, we project readily achievable imaging of nanoscale phase separation in complex oxides.

Toward a Minimal Artificial Axon.

The electrophysiology of action potentials is usually studied in neurons, through relatively demanding experiments which are difficult to scale up to a defined network. Here we pursue instead the minimal artificial system based on the essential biological components-ion channels and lipid bilayers-where action potentials can be generated, propagated, and eventually networked. The fundamental unit is the classic supported bilayer: a planar bilayer patch with embedded ion channels in a fluidic environment where an ionic gradient is imposed across the bilayer. Two such units electrically connected form the basic building block for a network. The system is minimal in that we demonstrate that one kind of ion channel and correspondingly a gradient of only one ionic species is sufficient to generate an excitable system which shows amplification and threshold behavior.

Artificial phosphorylation sites modulate the activity of a voltage-gated potassium channel.

The KvAP potassium channel is representative of a family of voltage-gated ion channels where the membrane potential is sensed by a transmembrane helix containing several positively charged arginines. Previous work by Wang and Zocchi [A. Wang and G. Zocchi, PLoS ONE 6, e18598 (2011)] showed how a negatively charged polyelectrolyte attached in proximity to the voltage sensing element can bias the opening probability of the channel. Here we introduce three phosphorylation sites at the same location and show that the response curve of the channel shifts by about 20 mV upon phosphorylation, while other characteristics such as the single-channel conductance are unaffected. In summary, we construct an artificial phosphorylation site which confers allosteric regulation to the channel.

Dissipative dynamics of enzymes.

We explore enzyme conformational dynamics at sub-Å resolution, specifically, temperature effects. The ensemble-averaged mechanical response of the folded enzyme is viscoelastic in the whole temperature range between the warm and cold denaturation transitions. The dissipation parameter γ of the viscoelastic description decreases by a factor of 2 as the temperature is raised from 10 to 45 °C; the elastic parameter K shows a similar decrease. Thus, when probed dynamically, the enzyme softens for increasing temperature. Equilibrium mechanical experiments with the DNA spring (and a different enzyme) also show, qualitatively, a small softening for increasing temperature.