Recent research in the field of nanometer-scale electronics has focused on the operating principles of small-scale devices and schemes to realize useful circuits. In contrast to established ''topdown'' fabrication techniques, molecular self-assembly is emerging as a ''bottom-up'' approach for fabricating nanostructured materials. Biological macromolecules, especially proteins, provide many valuable properties, but poor physical stability and poor electrical characteristics have prevented their direct use in electrical circuits. Here we describe the use of self-assembling amyloid protein fibers to construct nanowire elements. Self-assembly of a prion determinant from Saccharomyces cerevisiae, the N-terminal and middle region (NM) of Sup35p, produced 10-nm-wide protein fibers that were stable under a wide variety of harsh physical conditions. Their lengths could be roughly controlled by assembly conditions in the range of 60 nm to several hundred micrometers. A genetically modified NM variant that presents reactive, surfaceaccessible cysteine residues was used to covalently link NM fibers to colloidal gold particles. These fibers were placed across gold electrodes, and additional metal was deposited by highly specific chemical enhancement of the colloidal gold by reductive deposition of metallic silver and gold from salts. The resulting silver and gold wires were Ϸ100 nm wide. These biotemplated metal wires demonstrated the conductive properties of a solid metal wire, such as low resistance and ohmic behavior. With such materials it should be possible to harness the extraordinary diversity and specificity of protein functions to nanoscale electrical circuitry.

I introduce an algorithm for subpixel localization of imaged objects based on an analytic, non-iterative calculation of the best-fit radial symmetry center. This approach yields tracking accuracies that are near theoretical limits, similarly to Gaussian fitting, but with orders-of-magnitude faster execution time, lower sensitivity to nearby particles and applicability to any radially symmetric intensity distribution. I demonstrate the method with several types of data, including super-resolution microscopy images.

We investigate the impact of structural disorder on electronic transport in gold nanocrystal monolayers. Arrays ranging from void-filled networks to well-ordered superlattices show clear voltage thresholds (VT) due to Coulomb blockade, and temperature-independent conduction indicative of quantum tunneling. Current-voltage characteristics of arrays with and without long-range structural order were found to collapse onto distinct scaling curves. The former follow a single power-law: I ∼ (V − VT) ζ , ζ = 2.25 ± 0.1. The latter show additional structure, reflecting the underlying disordered topology.

Cellular membranes exhibit a variety of controlled curvatures, with filopodia, microvilli, and mitotic cleavage furrows being only a few of many examples. Coupling between local curvature and chemical composition in membranes could provide a means of mechanically controlling the spatial organization of membrane components. Although this concept has surfaced repeatedly over the years, controlled experimental investigations have proven elusive. Here, we introduce an experimental platform, in which microfabricated surfaces impose specific curvature patterns onto lipid bilayers, that allows quantification of mechanochemical couplings in membranes. We find that, beyond a critical curvature value, membrane geometry governs the spatial ordering of phase-separated domain structures in membranes composed of cholesterol and phospholipids. The curvature-controlled ordering, a consequence of the distinct mechanical properties of the lipid phases, makes possible a determination of the bending rigidity difference between cholesterol-rich and cholesterol-poor lipid domains. These observations point to a strong coupling between mechanical bending and chemical organization that should have wide-reaching consequences for biological membranes. Curvature-mediated patterning may also be useful in controlling complex fluids other than biomembranes.

The gut microbiota is a complex consortium of microorganisms with the ability to influence important aspects of host health and development. Harnessing this “microbial organ” for biomedical applications requires clarifying the degree to which host and bacterial factors act alone or in combination to govern the stability of specific lineages. To address this issue, we combined bacteriological manipulation and light sheet fluorescence microscopy to monitor the dynamics of a defined two-species microbiota within a vertebrate gut. We observed that the interplay between each population and the gut environment produces distinct spatiotemporal patterns. As a consequence, one species dominates while the other experiences sudden drops in abundance that are well fit by a stochastic mathematical model. Modeling revealed that direct bacterial competition could only partially explain the observed phenomena, suggesting that a host factor is also important in shaping the community. We hypothesized the host determinant to be gut motility, and tested this mechanism by measuring colonization in hosts with enteric nervous system dysfunction due to a mutation in the ret locus, which in humans is associated with the intestinal motility disorder known as Hirschsprung disease. In mutant hosts we found reduced gut motility and, confirming our hypothesis, robust coexistence of both bacterial species. This study provides evidence that host-mediated spatial structuring and stochastic perturbation of communities can drive bacterial population dynamics within the gut, and it reveals a new facet of the intestinal host–microbe interface by demonstrating the capacity of the enteric nervous system to influence the microbiota. Ultimately, these findings suggest that therapeutic strategies targeting the intestinal ecosystem should consider the dynamic physical nature of the gut environment.

We investigate how temperature affects transport through large networks of nonlinear conductances with distributed thresholds. In monolayers of weakly coupled gold nanocrystals, quenched charge disorder produces a range of local thresholds for the onset of electron tunneling. Our measurements delineate two regimes separated by a crossover temperature T*. Up to T* the nonlinear zero-temperature shape of the current-voltage curves survives, but with a threshold voltage for conduction that decreases linearly with temperature. Above T* the threshold vanishes and the low-bias conductance increases rapidly with temperature. We develop a model that accounts for these findings and predicts T*.

The excitation spectrum of a model magnetic system, LiHoF4, was studied with the use of neutron spectroscopy as the system was tuned to its quantum critical point by an applied magnetic field. The electronic mode softening expected for a quantum phase transition was forestalled by hyperfine coupling to the nuclear spins. We found that interactions with the nuclear spin bath controlled the length scale over which the excitations could be entangled. This generic result places a limit on our ability to observe intrinsic electronic quantum criticality.

Most materials freeze when cooled to sufficiently low temperature. We find that magnetic dipoles randomly distributed in a solid matrix condense into a spin liquid with spectral properties on cooling that are the diametric opposite of those for conventional glasses. Measurements of the non-linear magnetic dynamics in the low temperature liquid reveal the presence of coherent spin oscillations composed of hundreds of spins with lifetimes up to ten seconds. These excitations can be labeled by frequency, manipulated by the magnetic fields from a loop of wire, and permit the encoding of information at multiple frequencies simultaneously.Magnetic solids offer arrays of quantum degrees of freedom, or spins, interacting with each other in a manner and strength ranging from the long-range ferromagnetism of iron and nickel to the nano-antiferromagnetism of vortices in high temperature superconductors.Unfortunately, there is a large barrier to exploiting quantum effects in magnetic solids: namely, the rarity of coherence effects that can be simply manipulated and observed (1). In particular, it is difficult to create the magnetization oscillations corresponding to prepared superpositions of states, which are so straightforwardly created in liquid phase nuclear magnetic resonance (NMR) experiments. The "decoherence" for the solid magnets is generally attributed to disorder and to the coupling of the electronic spins to other degrees of freedom, such as nuclear spins, atomic motion, and conduction electrons. The present paper describes coherence effects in a magnet,