Liquid-based gating mechanism with tunable multiphase selectivity and antifouling behaviour.

Living organisms make extensive use of micro- and nanometre-sized pores as gatekeepers for controlling the movement of fluids, vapours and solids between complex environments. The ability of such pores to coordinate multiphase transport, in a highly selective and subtly triggered fashion and without clogging, has inspired interest in synthetic gated pores for applications ranging from fluid processing to 3D printing and lab-on-chip systems. But although specific gating and transport behaviours have been realized by precisely tailoring pore surface chemistries and pore geometries, a single system capable of controlling complex, selective multiphase transport has remained a distant prospect, and fouling is nearly inevitable. Here we introduce a gating mechanism that uses a capillary-stabilized liquid as a reversible, reconfigurable gate that fills and seals pores in the closed state, and creates a non-fouling, liquid-lined pore in the open state. Theoretical modelling and experiments demonstrate that for each transport substance, the gating threshold-the pressure needed to open the pores-can be rationally tuned over a wide pressure range. This enables us to realize in one system differential response profiles for a variety of liquids and gases, even letting liquids flow through the pore while preventing gas from escaping. These capabilities allow us to dynamically modulate gas-liquid sorting in a microfluidic flow and to separate a three-phase air-water-oil mixture, with the liquid lining ensuring sustained antifouling behaviour. Because the liquid gating strategy enables efficient long-term operation and can be applied to a variety of pore structures and membrane materials, and to micro- as well as macroscale fluid systems, we expect it to prove useful in a wide range of applications.

Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna.

Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.

Bacterial flagella explore microscale hummocks and hollows to increase adhesion.

Biofilms, surface-bound communities of microbes, are economically and medically important due to their pathogenic and obstructive properties. Among the numerous strategies to prevent bacterial adhesion and subsequent biofilm formation, surface topography was recently proposed as a highly nonspecific method that does not rely on small-molecule antibacterial compounds, which promote resistance. Here, we provide a detailed investigation of how the introduction of submicrometer crevices to a surface affects attachment of Escherichia coli. These crevices reduce substrate surface area available to the cell body but increase overall surface area. We have found that, during the first 2 h, adhesion to topographic surfaces is significantly reduced compared with flat controls, but this behavior abruptly reverses to significantly increased adhesion at longer exposures. We show that this reversal coincides with bacterially induced wetting transitions and that flagellar filaments aid in adhesion to these wetted topographic surfaces. We demonstrate that flagella are able to reach into crevices, access additional surface area, and produce a dense, fibrous network. Mutants lacking flagella show comparatively reduced adhesion. By varying substrate crevice sizes, we determine the conditions under which having flagella is most advantageous for adhesion. These findings strongly indicate that, in addition to their role in swimming motility, flagella are involved in attachment and can furthermore act as structural elements, enabling bacteria to overcome unfavorable surface topographies. This work contributes insights for the future design of antifouling surfaces and for improved understanding of bacterial behavior in native, structured environments.

Optomechanical and photothermal interactions in suspended photonic crystal membranes.

We present here an optomechanical system fabricated with novel stress management techniques that allow us to suspend an ultrathin defect-free silicon photonic-crystal membrane above a Silicon-on-Insulator (SOI) substrate with a gap that is tunable to below 200 nm. Our devices are able to generate strong attractive and repulsive optical forces over a large surface area with simple in- and out- coupling and feature the strongest repulsive optomechanical coupling in any geometry to date (gOM/2π ≈65 GHz/nm). The interplay between the optomechanical and photo-thermal-mechanical dynamics is explored, and the latter is used to achieve cooling and amplification of the mechanical mode, demonstrating that our platform is well-suited for potential applications in low-power mass, force, and refractive-index sensing as well as optomechanical accelerometry.

Structural transformation by electrodeposition on patterned substrates (STEPS): a new versatile nanofabrication method.

Arrays of high-aspect-ratio (HAR) nano- and microstructures are of great interest for designing surfaces for applications in optics, bio-nano interfaces, microelectromechanical systems, and microfluidics, but the difficulty of systematically and conveniently varying the geometries of these structures significantly limits their design and optimization for a specific function. This paper demonstrates a low-cost, high-throughput benchtop method that enables a HAR array to be reshaped with nanoscale precision by electrodeposition of conductive polymers. The method-named STEPS (structural transformation by electrodeposition on patterned substrates)-makes it possible to create patterns with proportionally increasing size of original features, to convert isolated HAR features into a closed-cell substrate with a continuous HAR wall, and to transform a simple parent two-dimensional HAR array into new three-dimensional patterned structures with tapered, tilted, anisotropic, or overhanging geometries by controlling the deposition conditions. We demonstrate the fabrication of substrates with continuous or discrete gradients of nanostructure features, as well as libraries of various patterns, starting from a single master structure. By providing exemplary applications in plasmonics, bacterial patterning, and formation of mechanically reinforced structures, we show that STEPS enables a wide range of studies of the effect of substrate topography on surface properties leading to optimization of the structures for a specific application. This research identifies solution-based deposition of conductive polymers as a new tool in nanofabrication and allows access to 3D architectures that were previously difficult to fabricate.

Fabrication and characterization of high-quality-factor silicon nitride nanobeam cavities.

We present the fabrication and characterization of high-quality-factor (Q) Si3N4 photonic crystal nanobeam cavities at visible wavelengths for coupling to nitrogen-vacancy centers in a cavity QED system. Confocal microphotoluminescence analysis of the nanobeam cavities demonstrates quality factors up to Q ~ 55,000, which are limited by the resolution of our grating spectrometer. This is a 1-order-of-magnitude improvement over previous SiNx cavities at this important wavelength range. We also demonstrate coarse tuning of cavity resonances across 600-700 nm by lithographically scaling the size of fabricated devices.

Effects of Growth Surface Topography on Bacterial Signaling in Coculture Biofilms.

Bacteria form interface-associated communities called biofilms, often comprising multiple species. Biofilms can be detrimental or beneficial in medical, industrial, and technological settings, and their stability and function are determined by interspecies communication via specific chemical signaling or metabolite exchange. The deterministic control of biofilm development, behavior, and properties remains an unmet challenge, limiting our ability to inhibit the formation of detrimental biofilms in biomedical settings and promote the growth of beneficial biofilms in biotechnology applications. Here, we describe the development of growth surfaces that promote the growth of commensal Escherichia coli instead of the opportunistic pathogen Pseudomonas aeruginosa. Periodically patterned growth surfaces induced robust morphological changes in surface-associated E. coli biofilms and influenced the antibiotic susceptibilities of E. coli and P. aeruginosa biofilms. Changes in the biofilm architecture resulted in the accumulation of a metabolite, indole, which controls the competition dynamics between the two species. Our results show that the surface on which a biofilm grows has important implications for species colonization, growth, and persistence when exposed to antibiotics.

Buckling-induced reversible symmetry breaking and amplification of chirality using supported cellular structures.

Buckling-induced reversible symmetry breaking and amplification of chirality using macro- and microscale supported cellular structures is described. Guided by extensive theoretical analysis, cellular structures are rationally designed, in which buckling induces a reversible switching between achiral and chiral configurations. Additionally, it is demonstrated that the proposed mechanism can be generalized over a wide range of length scales, geometries, materials, and stimuli.

Enriching libraries of high-aspect-ratio micro- or nanostructures by rapid, low-cost, benchtop nanofabrication.

We provide a protocol for transforming the structure of an array of high-aspect-ratio (HAR) micro/nanostructures into various new geometries. Polymeric HAR arrays are replicated from a Bosch-etched silicon master pattern by soft lithography. By using various conditions, the original pattern is coated with metal, which acts as an electrode for the electrodeposition of conductive polymers, transforming the original structure into a wide range of user-defined new designs. These include scaled replicas with sub-100-nm-level control of feature sizes and complex 3D shapes such as tapered or bent columnar structures bearing hierarchical features. Gradients of patterns and shapes on a single substrate can also be produced. This benchtop fabrication protocol allows the production of customized libraries of arrays of closed-cell or isolated HAR micro/nanostructures at a very low cost within 1 week, when starting from a silicon master that otherwise would be very expensive and slow to produce using conventional fabrication techniques.

A diamond nanowire single-photon source.

The development of a robust light source that emits one photon at a time will allow new technologies such as secure communication through quantum cryptography. Devices based on fluorescent dye molecules, quantum dots and carbon nanotubes have been demonstrated, but none has combined a high single-photon flux with stable, room-temperature operation. Luminescent centres in diamond have recently emerged as a stable alternative, and, in the case of nitrogen-vacancy centres, offer spin quantum bits with optical readout. However, these luminescent centres in bulk diamond crystals have the disadvantage of low photon out-coupling. Here, we demonstrate a single-photon source composed of a nitrogen-vacancy centre in a diamond nanowire, which produces ten times greater flux than bulk diamond devices, while using ten times less power. This result enables a new class of devices for photonic and quantum information processing based on nanostructured diamond, and could have a broader impact in nanoelectromechanical systems, sensing and scanning probe microscopy.