Carbon nanotube transistors have outstanding potential for electronic detection of biomolecules in solution. The physical mechanism underlying sensing however remains controversial, which hampers full exploitation of these promising nanosensors. Previously suggested mechanisms are electrostatic gating, changes in gate coupling, carrier mobility changes, and Schottky barrier effects. We argue that each mechanism has its characteristic effect on the liquid gate potential dependence of the device conductance. By studying both the electron and hole conduction, the sensing mechanisms can be unambiguously identified. From extensive protein-adsorption experiments on such devices, we find that electrostatic gating and Schottky barrier effects are the two relevant mechanisms, with electrostatic gating being most reproducible. If the contact region is passivated, sensing is shown to be dominated by electrostatic gating, which demonstrates that the sensitive part of a nanotube transistor is not limited to the contact region, as previously suggested. Such a layout provides a reliable platform for biosensing with nanotubes.

In many naturally occurring habitats, bacteria live in micrometersize confined spaces. Although bacterial growth and motility in such constrictions is of great interest to fields as varied as soil microbiology, water purification, and biomedical research, quantitative studies of the effects of confinement on bacteria have been limited. Here, we establish how Gram-negative Escherichia coli and Gram-positive Bacillus subtilis bacteria can grow, move, and penetrate very narrow constrictions with a size comparable to or even smaller than their diameter. We show that peritrichously flagellated E. coli and B. subtilis are still motile in microfabricated channels where the width of the channel exceeds their diameters only marginally (ϳ30%). For smaller widths, the motility vanishes but bacteria can still pass through these channels by growth and division. We observe E. coli, but not B. subtilis, to penetrate channels with a width that is smaller than their diameter by a factor of approximately 2. Within these channels, bacteria are considerably squeezed but they still grow and divide. After exiting the channels, E. coli bacteria obtain a variety of anomalous cell shapes. Our results reveal that sub-micron size pores and cavities are unexpectedly prolific bacterial habitats where bacteria exhibit morphological adaptations.biophysics ͉ confinement ͉ microbiology ͉ microfluidics

We report an experimental study of 1/f noise in liquid-gated graphene transistors. We show that the gate dependence of the noise is well described by a charge-noise model, whereas Hooge's empirical relation fails to describe the data. At low carrier density, the noise can be attributed to fluctuating charges in close proximity to the graphene, while at high carrier density it is consistent with noise due to scattering in the channel. The charge noise power scales inversely with the device area, and bilayer devices exhibit lower noise than single-layer devices. In air, the observed noise is also consistent with the charge-noise model.

Field-effect transistors based on single-walled carbon nanotubes (SWNTs) and graphene can function as highly sensitive nanoscale (bio)sensors in solution. Here, we compare experimentally how SWNT and graphene transistors respond to changes in the composition of the aqueous electrolyte in which they are immersed. We show that the conductance of SWNTs and graphene is strongly affected by changes in the ionic strength, the pH, and the type of ions present, in a manner that can be qualitatively different for graphene and SWNT devices. We show that this sensitivity to electrolyte composition results from a combination of different mechanisms including electrostatic gating, Schottky-barrier modifications, and changes in gate capacitance. Interestingly, we find strong evidence that the sensor response to changes in electrolyte composition is affected by a high density of ionizable groups on both the underlying substrate and the carbon surfaces. We present a model based on the (regulated) surface charge associated with these ionizable groups that explains the majority of our data. Our findings have significant implications for interpreting and optimizing sensing experiments with nanocarbon transistors. This is particularly true for complex biological samples such as cell extracts, growth media, or bodily fluids, for which the complete composition of the solution needs to be considered.

Cell division in Escherichia coli starts with assembly of FtsZ protofilaments into a ring-like structure, the Z-ring. Positioning of the Z-ring at midcell is thought to be coordinated by two regulatory systems, nucleoid occlusion and the Min system. In E. coli, nucleoid occlusion is mediated by the SlmA proteins. Here, we address the question of whether there are additional positioning systems that are capable of localizing the E. coli divisome with respect to the cell center. Using quantitative fluorescence imaging we show that slow growing cells lacking functional Min and SlmA nucleoid occlusion systems continue to divide preferentially at midcell. We find that the initial Z-ring assembly occurs over the center of the nucleoid instead of nucleoid-free regions under these conditions. We determine that Z-ring formation begins shortly after the arrival of the Ter macrodomain at the nucleoid center. Removal of either the MatP, ZapB, or ZapA proteins significantly affects the accuracy and precision of Z-ring positioning relative to the nucleoid center in these cells in accordance with the idea that these proteins link the Ter macrodomain and the Z-ring. Interestingly, even in the absence of Min, SlmA, and the putative Ter macrodomain – Z-ring link, there remains a weak midcell positioning bias for the Z-ring. Our work demonstrates that additional Z-ring localization systems are present in E. coli than are known currently. In particular, we identify that the Ter macrodomain acts as a landmark for the Z-ring in the presence of MatP, ZapB and ZapA proteins.