Molecular transport in a crowded volume created from vertically aligned carbon nanofibres: a fluorescence recovery after photobleaching study

Fabrication of nanoporous membranes for tuning microbial interactions and biochemical reactions

New strategies for combining conventional photo- and soft-lithographic techniques with high-resolution patterning and etching strategies are needed in order to produce multiscale fluidic platforms that address the full range of functional scales seen in complex biological and chemical systems. The smallest resolution required for an application often dictates the fabrication method used. Micromachining and micropowder blasting yield higher throughput, but lack the resolution needed to fully address biological and chemical systems at the cellular and molecular scales. In contrast, techniques such as electron beam lithography or nanoimprinting allow nanoscale resolution, but are traditionally considered costly and slow. Other techniques such as photolithography or soft lithography have characteristics between these extremes. Combining these techniques to fabricate multiscale or hybrid fluidics allows fundamental biological and chemical questions to be answered. In this study, a combination of photolithography and electron beam lithography are used to produce two multiscale fluidic devices that incorporate porous membranes into complex fluidic networks in order to control the flow of energy, information, and materials in chemical form. In the first device, materials and energy were used to support chemical reactions. A nanoporous membrane fabricated with e-beam lithography separates two parallel, serpentine channels. Photolithography was used to pattern microfluidic channels around the membrane. The pores were written at 150 nm and reduced in size with silicon dioxide deposition from plasma enhanced chemical vapor deposition and atomic layer deposition. Using this method, the molecular weight cutoff of the membrane can be adapted to the system of interest. In the second approach, photolithography was used to fabricate 200 nm thin pores. The pores confined microbes and allowed energy replenishment from a media perfusion channel. The same device can be used for study of intercellular communication via the secretion and uptake of signal molecules. Pore size was tested with 750 nm fluorescent polystyrene beads and fluorescein dye. The 200 nm polydimethylsiloxane pores were shown to be robust enough to hold 750 nm beads while under pressure, but allow fluorescein to diffuse across the barrier. Further testing showed that extended culture of bacteria within the chambers was possible. These two examples show how lithographically defined porous membranes can be adapted to two unique situations and used to tune the flow of chemical energy, materials, and information within a microfluidic network.

Fabrication of nanoporous membranes for tuning microbial interactions and biochemical reactions

New strategies for combining conventional photo- and soft-lithographic techniques with high-resolution patterning and etching strategies are needed in order to produce multiscale fluidic platforms that address the full range of functional scales seen in complex biological and chemical systems. The smallest resolution required for an application often dictates the fabrication method used. Micromachining and micropowder blasting yield higher throughput, but lack the resolution needed to fully address biological and chemical systems at the cellular and molecular scales. In contrast, techniques such as electron beam lithography or nanoimprinting allow nanoscale resolution, but are traditionally considered costly and slow. Other techniques such as photolithography or soft lithography have characteristics between these extremes. Combining these techniques to fabricate multiscale or hybrid fluidics allows fundamental biological and chemical questions to be answered. In this study, a combination of photolithography and electron beam lithography are used to produce two multiscale fluidic devices that incorporate porous membranes into complex fluidic networks in order to control the flow of energy, information, and materials in chemical form. In the first device, materials and energy were used to support chemical reactions. A nanoporous membrane fabricated with e-beam lithography separates two parallel, serpentine channels. Photolithography was used to pattern microfluidic channels around the membrane. The pores were written at 150 nm and reduced in size with silicon dioxide deposition from plasma enhanced chemical vapor deposition and atomic layer deposition. Using this method, the molecular weight cutoff of the membrane can be adapted to the system of interest. In the second approach, photolithography was used to fabricate 200 nm thin pores. The pores confined microbes and allowed energy replenishment from a media perfusion channel. The same device can be used for study of intercellular communication via the secretion and uptake of signal molecules. Pore size was tested with 750 nm fluorescent polystyrene beads and fluorescein dye. The 200 nm polydimethylsiloxane pores were shown to be robust enough to hold 750 nm beads while under pressure, but allow fluorescein to diffuse across the barrier. Further testing showed that extended culture of bacteria within the chambers was possible. These two examples show how lithographically defined porous membranes can be adapted to two unique situations and used to tune the flow of chemical energy, materials, and information within a microfluidic network.

Wanted: a positive control for anomalous subdiffusion

Anomalous subdiffusion in cells and model systems is an active area of research. The main questions are whether diffusion is anomalous or normal, and if it is anomalous, its mechanism. The subject is controversial, especially the hypothesis that crowding causes anomalous subdiffusion. Anomalous subdiffusion measurements would be strengthened by an experimental standard, particularly one able to cross-calibrate the different types of measurements. Criteria for a calibration standard are proposed. First, diffusion must be anomalous over the length and timescales of the different measurements. The length-scale is fundamental; the time scale can be adjusted through the viscosity of the medium. Second, the standard must be theoretically well understood, with a known anomalous subdiffusion exponent, ideally readily tunable. Third, the standard must be simple, reproducible, and independently characterizable (by, for example, electron microscopy for nanostructures). Candidate experimental standards are evaluated, including obstructed lipid bilayers; aqueous systems obstructed by nanopillars; a continuum percolation system in which a prescribed fraction of randomly chosen obstacles in a regular array is ablated; single-file diffusion in pores; transient anomalous subdiffusion due to binding of particles in arrays such as transcription factors in randomized DNA arrays; and computer-generated physical trajectories.

Surface charge- and space-dependent transport of proteins in crowded environments of nanotailored posts

The reaction and diffusion of molecules across barriers and through crowded environments is integral to biological system function and to separation technologies. Ordered, microfabricated post arrays are a promising route to creating synthetic barriers with controlled chemical and physical characteristics. They can be used to create crowded environments, to mimic aspects of cellular membranes, and to serve as engineered replacements of polymer-based separation media. Here, the translational diffusion of fluorescein isothiocyante and various forms of green fluorescent protein (GFP), including "supercharged" variants, are examined in a silicon-based post array environment. The technique of fluorescence recovery after photobleaching (FRAP) is combined with analytical approximations and numerical simulations to assess the relative effects of reaction and diffusion on molecular transport, respectively. FRAP experiments were conducted for 64 different cases where the molecular species, the density of the posts, and the chemical surface charge of the posts were varied. In all cases, the dense packing of the posts hindered the diffusive transport of the fluorescent species. The supercharged GFPs strongly interacted with oppositely charged surfaces. With similar molecular and surface charges, transport is primarily limited by hindered diffusion. For conventional, enhanced GFP in a positively charged surface environment, transport was limited by the coupled action of hindered diffusion and surface interaction with the posts. Quantification of the size-, space-, time-, and charge-dependent translational diffusion in the post array environments can provide insight into natural processes and guide the design and development of selective membrane systems.

[Microchip electrochromatography: the latest developments and applications]

This review summarizes recent developments and applications of microchip electrochromatography (microCEC) mainly in the past five years between 2005 and 2009 with a focus on column technologies. In addition, some new improvements in the chip design and fabrication, sample preconcentration, electroosmotic flow control as well as mechanisms that govern electrochromatographic separation are described and reviewed. The features and limitations of several practical aspects of their applications are highlighted.

An optimized nanoparticle separator enabled by electron beam induced deposition

Size-based separations technologies will inevitably benefit from advances in nanotechnology. Direct-write nanofabrication provides a useful mechanism for depositing/etching nanoscale elements in environments otherwise inaccessible to conventional nanofabrication techniques. Here, electron beam induced deposition was used to deposit an array of nanoscale features in a 3D environment with minimal material proximity effects outside the beam-interaction region. Specifically, the membrane component of a nanoparticle separator was fabricated by depositing a linear array of sharply tipped nanopillars, with a singular pitch, designed for sub-50 nm nanoparticle permeability. The nanopillar membrane was used in a dual capacity to control the flow of nanoparticles in the transaxial direction of the array while facilitating the sealing of the cellular-sized compartment in the paraxial direction. An optimized growth recipe resulted which (1) maximized the growth efficiency of the membrane (which minimizes proximity effects) and (2) preserved the fidelity of the spacing between nanopillars (which maximizes the size-based gating quality of the membrane) while (3) maintaining sharp nanopillar apexes for impaling an optically transparent polymeric lid critical for device sealing.

Carbon nanotubes integrated in electrically insulated channels for lab-on-a-chip applications

A fabrication process for monolithic integration of vertically aligned carbon nanotubes in electrically insulated microfluidic channels is presented. A 150 nm thick amorphous silicon layer could be used both for anodic bonding of a glass lid to hermetically seal the microfluidic glass channels and for de-charging of the wafer during plasma enhanced chemical vapor deposition of the carbon nanotubes. The possibility of operating the device with electroosmotic flow was shown by performing standard electrophoretic separations of 50 microM fluorescein and 50 microM 5-carboxyfluorescein in a 25 mm long column containing vertical aligned carbon nanotubes. This is the first demonstration of electroosmotic pumping and electrokinetic separations in microfluidic channels with a monolithically integrated carbon nanotube forest.

Size-selectivity and anomalous subdiffusion of nanoparticles through carbon nanofiber-based membranes

A simulation is presented here that serves the dual functions of generating a nanoporous membrane replica and executing the Brownian motion of nanoparticles through the virtual membrane. Specifically, the concentration profile of a dilute solution of fluorescent particles in a stochastic and SiO(2)-coated carbon nanofiber (oxCNF), nanoporous membrane was simulated. The quality of the simulated profile was determined by comparing the results with experimental concentration profiles. The experimental concentration profiles were collected adjacent to the oxCNF membrane surface from time-lapse fluorescence microscopy images. The simulation proved ideal as an accurate predictor of particle diffusion-the simulated concentration profile merged with the experimental profiles at the inlet/exit surfaces of the oxCNF membrane. In particular, the oxCNF barrier was found to hinder the transport of 50 and 100 nm particles and transmembrane trajectories were indicative of anomalous subdiffusion; the diffusion coefficient was found to be a function of time and space.

Actuatable membranes based on polypyrrole-coated vertically aligned carbon nanofibers

Nanoporous membranes are applicable to a variety of research fields due to their ability to selectively separate molecules with high efficiency. Of particular interest are methods for controlling membrane selectivity through externally applied stimuli and integrating such membrane structures within multiscale systems. Membranes comprised of deterministically grown, vertically aligned carbon nanofibers (VACNFs) are compatible with these needs. VACNF membranes can regulate molecular transport by physically selecting species as they pass between the fibers. Defined interfiber spacing allows for nanoscale control of membrane pore structure and resultant size selectivity. Subsequent physical or chemical modification of VACNF structures enables the tuning of physical pore size and chemical specificity allowing further control of membrane permeability. In this work, the dynamic physical modulation of membrane permeability that results when VACNFs are coated with an electrically actuatable polymer, polypyrrole, is demonstrated. Electrochemical reduction of polypyrrole on the VACNFs results in controlled swelling of the diameter of the nanofibers that in turn decreases the pore size. Dynamic control of membrane pore size enables selective transport and gating of nanoscale pores.

Positional control of catalyst nanoparticles for the synthesis of high density carbon nanofiber arrays

Precise arrangement of nanoscale elements within larger systems, is essential to controlling higher order functionality and tailoring nanophase material properties. Here, we present findings on growth conditions for vertically aligned carbon nanofibers that enable synthesis of high density arrays and individual rows of nanofibers, which could be used to form barriers for restricting molecular transport, that have regular spacings and few defects. Growth through plasma-enhanced chemical vapor deposition was initiated from precisely formed nickel catalyst dots of varying diameter and spacing that were patterned through electron beam lithography. Nanofiber growth conditions, including power, precursor gas ratio, growth temperature and pressure were varied to optimize fiber uniformity and minimize defects that result from formation and migration of catalyst particles prior to growth. It was determined that both catalyst dot diameter and initial plasma power have a considerable influence on the number and severity of defects, while growth temperature, gas ratio (C(2)H(2):NH(3)) and pressure can be varied within a considerable range to fine-tune nanofiber morphology.

Nano-enabled synthetic biology

Biological systems display a functional diversity, density and efficiency that make them a paradigm for synthetic systems. In natural systems, the cell is the elemental unit and efforts to emulate cells, their components, and organization have relied primarily on the use of bioorganic materials. Impressive advances have been made towards assembling simple genetic systems within cellular scale containers. These biological system assembly efforts are particularly instructive, as we gain command over the directed synthesis and assembly of synthetic nanoscale structures. Advances in nanoscale fabrication, assembly, and characterization are providing the tools and materials for characterizing and emulating the smallest scale features of biology. Further, they are revealing unique physical properties that emerge at the nanoscale. Realizing these properties in useful ways will require attention to the assembly of these nanoscale components. Attention to systems biology principles can lead to the practical development of nanoscale technologies with possible realization of synthetic systems with cell-like complexity. In turn, useful tools for interpreting biological complexity and for interfacing to biological processes will result.