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In the wild, microbial rhodopsin proteins convert solar energy into a transmembrane voltage, which provides energy for their host. We engineered microbial rhodopsins to run backward: to convert membrane potential into a readily detectable optical signal. When expressed in a neuron or a cardiac myocyte, these voltage-indicating proteins convert electrical action potentials into visible flashes of fluorescence, allowing us to make movies of electrical activity in cells. Upon expression of the voltage indicator in E. coli, we discovered that bacteria generate electrical spikes too. These voltage-indicating proteins are a new class of environmentally sensitive fluorescent proteins that emit in the near infrared, are highly photostable, and have no homology to GFP or to any other fluorescent indicator.

J. Kralj, D. R. Hochbaum, A. D. Douglass, A. E. Cohen, “Electrical spiking in Escherichia coli probed with a fluorescent voltage-indicating protein,” Science, 333, 345-348 (2011)
J. Kralj*, A. D. Douglass*, D. R. Hochbaum*, D. Maclaurin, A. E.
Cohen, “Optical recording of action potentials in mammalian neurons using a microbial rhodopsin," Nature Methods, 9, 90-95 (2012)

Understanding the Electronic Structure of Metal-Organic Interfaces through Quantum-Mechanical Modeling

Egbert Zojer
Institute of Solid State Physics, Graz University of Technology, Graz, Austria

The absolutely crucial role that interfaces play for applications like organic (opto)electronic devices is increasingly acknowledged. In the present contribution, quantum-mechanical simulations are used to gain an in-depth understanding of the electronic properties of such interfaces, in particular those formed between metal electrodes and molecular monolayers. The focus is on understanding the fundamental differences between covalently (typically thiolate-)bonded self-assembled monolayers and layers consisting of strong donors or acceptors that undergo a charge-transfer reaction with the substrate. The electronic properties of the former are often dominated by collective/cooperative effects that electronically decouple the various parts of the SAM and result in SAM-properties qualitative differing from those of the individual molecules. Such effects can also be exploited to realize unexpected transport characteristics of suitably designed layers. The properties of charge-transfer monolayers, on the other hand, are typically determined by Fermi-level pinning. The first part of the talk will focus on reviewing these fundamental aspects for a number of examples; subsequently, deviations from the “conventional” behavior will be discussed. These include Fermi-level pinning in SAMs, the underlying mechanism, workarounds, and how it can lead to an anti-correlation between molecular dipole moments and SAM-induced work-function changes. Additionally, a coverage induced transition from a charge-transfer monolayer type situation to an upright-standing SAM with markedly different electronic properties will be mentioned and the talk will be concluded by discussing, how the internal electric fields in a “distributed-dipole” SAM can impact its electronic structure in a way reminiscent of the quantum-confined Stark effect commonly observed in semiconductor heterostructures.

We study the displacement of immiscible fluids in deformable, non-cohesive granular media by experiments and simulations. Experimentally, we inject air into a thin bed of water-saturated glass beads and observe the invasion morphology. Numerically, we develop a pore-scale model that captures the dynamic pressure redistribution at the invasion front and the feedback between fluid invasion and microstructure rearrangement. We identify three invasion regimes: capillary fingering, viscous fingering, and ``capillary fracturing'', where capillary forces induce the opening of conduits. We derive two dimensionless numbers that govern the transition among the different regimes: a modified capillary number and a fracturing number. We predict the emergence of fracturing in fine-grained media under low confining stress, a phenomenon that likely plays a fundamental role in many natural processes such as primary oil migration, methane venting from lake sediments, and the formation of desiccation cracks.