Water-assisted growth of uniform 100 mm diameter SWCNT arrays.

We report a simple method for growing high-quality single-walled carbon nanotube (SWCNT) arrays on 100 mm wafers via the addition of water vapor to highly purified gases during the CNT growth step. We show that adding a small amount of water during growth helps to create a uniform catalyst distribution and yields high-quality (Raman G/D of 26 ± 3), high-density (up to 6 × 10(11) cm(-2)) and uniform SWCNT arrays on 100 mm large wafers. We rationalize our finding by suggesting that the addition of water decreases catalyst mobility, preventing its coarsening at higher temperatures.

Thermally switchable aligned nanopores by magnetic-field directed self-assembly of block copolymers.

A scalable approach for developing large area polymer films, with stimuli responsive vertically aligned nanopores is reported. Magnetic fields are used to create highly aligned hexagonally packed block copolymer cylindrical microdomains with order parameters exceeding 0.95.

Molecular Design of Liquid Crystalline Brush-Like Block Copolymers for Magnetic Field Directed Self-Assembly: A Platform for Functional Materials.

We report on the development of a liquid crystalline block copolymer with brush-type architecture as a platform for creating functional materials by magnetic-field-directed self-assembly. Ring-opening metathesis of -alkyloxy cyanobiphenyl and polylactide (PLA) functionalized norbornene monomers provides efficient polymerization yielding low polydispersity block copolymers. The mesogenic species, spacer length, monomer functionality, brush-chain length, and overall molecular weight were chosen and optimized to produce hexagonally packed cylindrical PLA domains which self-assemble and align parallel to an applied magnetic field.

Slow unfolded-state structuring in Acyl-CoA binding protein folding revealed by simulation and experiment.

Protein folding is a fundamental process in biology, key to understanding many human diseases. Experimentally, proteins often appear to fold via simple two- or three-state mechanisms involving mainly native-state interactions, yet recent network models built from atomistic simulations of small proteins suggest the existence of many possible metastable states and folding pathways. We reconcile these two pictures in a combined experimental and simulation study of acyl-coenzyme A binding protein (ACBP), a two-state folder (folding time ~10 ms) exhibiting residual unfolded-state structure, and a putative early folding intermediate.

Microfluidic mixers for studying protein folding.

The process by which a protein folds into its native conformation is highly relevant to biology and human health yet still poorly understood. One reason for this is that folding takes place over a wide range of timescales, from nanoseconds to seconds or longer, depending on the protein. Conventional stopped-flow mixers have allowed measurement of folding kinetics starting at about 1 ms.

Evidence of multiple folding pathways for the villin headpiece subdomain.

The defining property of two-state models of protein folding is that the measured relaxation rates are independent of the starting conditions and only depend on the final conditions. In this work we compare the kinetics of the very fast folding villin subdomain measured after a large change in denaturant concentration using an ultrarapid microfluidic mixer with the kinetics measured after a small temperature change in a laser T-jump experiment and find a significant difference in the observed folding kinetics. The final conditions of temperature and denaturant concentration and the use of tryptophan fluorescence as a probe are the same in both experiments, while the initial conditions are very different.

pH-tunable ion selectivity in carbon nanotube pores.

The selectivity of ion transport in nanochannels is of primary importance for a number of physical, chemical, and biological processes ranging from fluid separation to ion-channel-regulated cellular processes. Fundamental understanding of these phenomena requires model nanochannels with well-defined and controllable structural properties. Carbon nanotubes provide an ideal choice for nanofluidic studies because of their simple chemistry and structure, the atomic scale smoothness and chemical inertness of the graphitic walls, and the tunability of their diameter and length.

Extremely slow intramolecular diffusion in unfolded protein L.

A crucial parameter in many theories of protein folding is the rate of diffusion over the energy landscape. Using a microfluidic mixer we have observed the rate of intramolecular diffusion within the unfolded B1 domain of protein L before it folds. The diffusion-limited rate of intramolecular contact is about 20 times slower than the rate in 6 M GdnHCl, and because in these conditions the protein is also more compact, the intramolecular diffusion coefficient decreases 100-500 times.

Direct observation of downhill folding of lambda-repressor in a microfluidic mixer.

The protein lambda(6-85) has been implicated in barrierless folding by observations of kinetic relaxation after nanosecond T-jump. In this work we observed folding of this protein after dilution of a high denaturant in an ultrarapid microfluidic mixer at temperatures far below the thermal midpoint. The observations of total intensity and spectral shift of tryptophan fluorescence yielded distinctly different kinetics and activation energies.

Ruggedness in the folding landscape of protein L.

By exploring the folding pathways of the B1 domain of protein L with a series of equilibrium and rapid kinetic experiments, we have found its unfolded state to be more complex than suggested by two-state folding models. Using an ultrarapid mixer to initiate protein folding within approximately 2-4 microseconds, we observe folding kinetics by intrinsic tryptophan fluorescence and fluorescence resonance energy transfer. We detect at least two processes faster than 100 mus that would be hidden within the burst phase of a stopped-flow instrument measuring tryptophan fluorescence.

Biofunctional subwavelength optical waveguides for biodetection.

We report a versatile biofunctional subwavelength photonic device platform for real-time detection of biological molecules. Our devices contain lipid bilayer membranes fused onto metal oxide nanowire waveguides stretched across polymeric flow channels. The lipid bilayers incorporating target receptors are submersed in the propagating evanescent field of the optical cavity.

Mechanism and kinetics of growth termination in controlled chemical vapor deposition growth of multiwall carbon nanotube arrays.

We have investigated growth kinetics of multiwall carbon nanotube (MWCNT) arrays produced by catalytic thermal decomposition of ethylene gas in hydrogen, water, and argon mixture. The MWCNT growth rate exhibits a nonmonotonic dependence on total pressure and reaches a maximum at approximately 750 Torr of total pressure. Water concentrations in excess of 3000 ppm lead to the decrease in the observed growth rate.

High-speed, temperature programmable gas chromatography utilizing a microfabricated chip with an improved carbon nanotube stationary phase.

A new growth recipe for producing carbon nanotubes (CNTs) combined with a new bonding technique was implemented in a microfabricated gas chromatography (micro-GC) chip. Specifically, the micro-GC chip contained a 30-cm (length) microfabricated channel with a 50 microm x 50 microm square cross-section. A CNT stationary phase "mat" was grown on the bottom of the separation channel prior to the chip bonding.

Microfluidic mixers for the investigation of rapid protein folding kinetics using synchrotron radiation circular dichroism spectroscopy.

We have developed a microfluidic mixer optimized for rapid measurements of protein folding kinetics using synchrotron radiation circular dichroism (SRCD) spectroscopy. The combination of fabrication in fused silica and synchrotron radiation allows measurements at wavelengths below 220 nm, the typical limit of commercial instrumentation. At these wavelengths, the discrimination between the different types of protein secondary structure increases sharply.

Ion exclusion by sub-2-nm carbon nanotube pores.

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform.

Improvements in mixing time and mixing uniformity in devices designed for studies of protein folding kinetics.

Using a microfluidic laminar flow mixer designed for studies of protein folding kinetics, we demonstrate a mixing time of 1 +/- 1 micros with sample consumption on the order of femtomoles. We recognize two limitations of previously proposed designs: (1) size and shape of the mixing region, which limits mixing uniformity and (2) the formation of Dean vortices at high flow rates, which limits the mixing time. We address these limitations by using a narrow shape-optimized nozzle and by reducing the bend of the side channel streamlines.

Protein hydrophobic collapse and early folding steps observed in a microfluidic mixer.

We demonstrate that the sub-millisecond protein folding process referred to as "collapse" actually consists of at least two separate processes. We observe the UV fluorescence spectrum from naturally occurring tryptophans in three well-studied proteins, cytochrome c, apomyoglobin, and lysozyme, as a function of time in a microfluidic mixer with a dead time of approximately 20 mus. Single value decomposition of the time-dependent spectra reveal two separate processes: 1), a spectral shift which occurs within the mixing time; and 2), a fluorescence decay occurring between approximately 100 and 300 micros.

Mapping protein collapse with single-molecule fluorescence and kinetic synchrotron radiation circular dichroism spectroscopy.

We have used the combination of single-molecule Förster resonance energy transfer and kinetic synchrotron radiation circular dichroism experiments to probe the conformational ensemble of the collapsed unfolded state of the small cold shock protein CspTm under near-native conditions. This regime is physiologically most relevant but difficult to access experimentally, because the equilibrium signal in ensemble experiments is dominated by folded molecules. Here, we avoid this problem in two ways.

Controlled electrostatic gating of carbon nanotube FET devices.

Carbon nanotube transistors are a promising platform for the next generation of nonoptical biosensors. However, the exact nature of the biomolecule interactions with nanotubes in these devices remains unknown, creating one of the major obstacles to their practical use. We assembled alternating layers of oppositely charged polyelectrolytes on the carbon nanotube transistors to mimic gating of these devices by charged molecules.

Ultrafast gas chromatography on single-wall carbon nanotube stationary phases in microfabricated channels.

The key to rapid temperature programmed separations with gas chromatography are a fast, low-volume injection and a short microbore separation column with fast resistive heating. One of the major problems with the reduction of column dimensions for micro gas chromatography is the availability of a stationary phase that provides good separation performance. In this report, we present the first integration of single-wall carbon nanotubes (SWNTs) as a stationary phase into 100 mum x 100 mum square and 50-cm-long microfabricated channels.

Optimization of a microfluidic mixer for studying protein folding kinetics.

We have applied an optimization method in conjunction with numerical simulations to minimize the mixing time of a microfluidic mixer developed for protein folding studies. The optimization method uses a semideterministic algorithm to find the global minimum of the mixing time by varying the mixer geometry and flow conditions. We describe the minimization problem and constraints and give a brief overview of the optimization algorithm.

Bio-assay based on single molecule fluorescence detection in microfluidic channels.

A rapid bioassay is described based on the detection of colocalized fluorescent DNA probes bound to DNA targets in a pressure-driven solution flowing through a planar microfluidic channel. By employing total internal reflection excitation of the fluorescent probes and illumination of almost the entire flow channel, single fluorescent molecules can be efficiently detected leading to the rapid analysis of nearly the entire solution flowed through the device. Cross-correlation between images obtained from two spectrally distinct probes is used to determine the target concentration and efficiently reduces the number of false positives.

Fast mass transport through sub-2-nanometer carbon nanotubes.

We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations.

Functional one-dimensional lipid bilayers on carbon nanotube templates.

Use of biological machines and environments in novel bioinorganic nanostructures is critical for development of new types of biosensors, bio-NEMS devices, and functional materials. Lipid bilayers that mimic a cell membrane have already played an important role in such applications. We present supported lipid bilayers that spontaneously assemble in a continuous nanoshell around a template of a carbon nanotube wrapped with hydrophilic polymer cushion layers.