Bio-functional 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. We show that the lipid bilayers in our devices are continuous, have very high mobile fraction, and are resistant to fouling. We also demonstrate that our platform allows rapid membrane exchange. Finally we use this device for detection of specific DNA sequences in solution by anchoring complementary DNA target strands in the lipid bilayer. This evanescent wave sensing architecture holds great potential for portable, all-optical detection systems

Persistence Length Control of the Polyelectrolyte Layer-by-Layer Self-Assembly on Carbon Nanotubes

One-dimensional inorganic materials such as carbon nanotubes1 and semiconductor nanowires have been central to important advances in materials science in the last decade. Unique mechanical and electronic properties of these molecular-scale wires enabled a variety of applications ranging from novel composite materials, to electronic circuits, to new sensors. Often, these applications require non-covalent modification of carbon nanotubes with organic compounds, DNA and biomolecules, and polymers to change nanotube properties or to add new functionality. We recently demonstrated a versatile and flexible strategy for non-covalent modification of carbon nanotubes using layer-by-layer self-assembly of polyelectrolytes. Researchers used this technique extensively for modification of flat surfaces, micro-, and nano-particles; however, little is known about the mechanism and the factors influencing layer-by-layer self-assembly in one-dimensional nanostructures. The exact conformation of polyelectrolyte chains deposited on single-walled carbon nanotubes (SWNT) is still unknown. There are two possible configurations: flexible polymers wrapping around the nanotube and stretched, rigid chains stacked parallel to the nanotube axis. Several factors, such as polymer rigidity, surface curvature, and strength of polymer-surface interactions, can determine the nature of assembly. Persistence length of the polymer chain should be one of the critical parameters, since it determines the chain's ability to wrap around the nanotube. Indeed, computer simulations for spherical substrates show that polymer rigidity and substrate surface curvature can influence the deposition process. Computational models also show that the persistence length of the polymer must fall below the threshold values determined by target surface curvature in order to initiate polyelectrolyte deposition process. Although these models described the effects of salt concentration and target surface curvature, they considered only nano-particles with radius 5 nanometer and larger. One-dimensional materials, such as carbon nanotubes, provide an even more interesting template for studying self-assembly mechanisms, since they give us access to even smaller surface curvatures down to 1 nm. We have examined the role of the polymer persistence length in layer-by-layer self-assembly process on carbon nanotubes by observing formation of multilayer polyelectrolyte shells around carbon nanotubes at different ionic strength. Persistence length of polyelectrolytes varies with solution ionic strength, due to screening of the electrostatic repulsion between the polymer Figure 1. TEM images of single-walled carbon nanotubes after polymer deposition for ionic strengths of (A) 0.05M, (B) 0.1M, (C) 0.2M, (D) 0.4M, (E) 0.65M, and (F) 1.05M. Scale bar corresponds to 10 nm. backbone charges; therefore changing ionic strength is a convenient way to alter the configuration of the polymer molecule systematically. We have used the layer-by-layer self-assembly technique to form 5-layer thick coating of the alternating polyallylamine hydrochloride (PAH) and sodium poly(styrenesulfonate) (PSS) layers on the surfaces of the pristine single-wall carbon nanotubes. For our experiments, we grew the nanotubes across copper TEM grid openings using catalytic chemical vapor deposition. The deposition solutions contained different amounts of NaCl to vary the ionic strength. After polymer multilayer formation we examined the resulting coating in high-resolution TEM

Electric Field Induced Biomimetic Transmembrane Electron Transport Using Carbon Nanotube Porins

Cells modulate their homeostasis through the control of redox reactions via transmembrane electron transport systems. These are largely mediated via oxidoreductase enzymes. Their use in biology has been linked to a host of systems including reprogramming for energy requirements in cancer. Consequently, our ability to modulate membrane redox systems may give rise to opportunities to modulate underlying biology. The current work aimed to develop a wireless bipolar electrochemical approach to form on-demand electron transfer across biological membranes. To achieve this goal, we show that using membrane inserted carbon nanotube porins that can act as bipolar nanoelectrodes, we could control electron flow with externally applied electric fields across membranes. Before this work, bipolar electrochemistry has been thought to require high applied voltages not compatible with biological systems. We show that bipolar electrochemical reaction via gold reduction at the nanotubes could be modulated at low cell-friendly voltages, providing an opportunity to use bipolar electrodes to control electron flux across membranes. We provide new mechanistic insight into this newly describe phenomena at the nanoscale. The results presented a give rise to a new method using CNTPs to modulate cell behavior via wireless control of membrane electron transfer

High-resolution ab initio three-dimensional X-ray diffraction microscopy

Coherent X-ray diffraction microscopy is a method of imaging non-periodic isolated objects at resolutions only limited, in principle, by the largest scattering angles recorded. We demonstrate X-ray diffraction imaging with high resolution in all three dimensions, as determined by a quantitative analysis of the reconstructed volume images. These images are retrieved from the 3D diffraction data using no a priori knowledge about the shape or composition of the object, which has never before been demonstrated on a non-periodic object. We also construct 2D images of thick objects with infinite depth of focus (without loss of transverse spatial resolution). These methods can be used to image biological and materials science samples at high resolution using X-ray undulator radiation, and establishes the techniques to be used in atomic-resolution ultrafast imaging at X-ray free-electron laser sources.Comment: 22 pages, 11 figures, submitte

Real-time dynamics of carbon nanotube porins in supported lipid membranes visualized by high-speed atomic force microscopy.

In-plane mobility of proteins in lipid membranes is one of the fundamental mechanisms supporting biological functionality. Here we use high-speed atomic force microscopy (HS-AFM) to show that a novel type of biomimetic channel-carbon nanotube porins (CNTPs)-is also laterally mobile in supported lipid membranes, mimicking biological protein behaviour. HS-AFM can capture real-time dynamics of CNTP motion in the supported lipid bilayer membrane, build long-term trajectories of the CNTP motion and determine the diffusion coefficients associated with this motion. Our analysis shows that diffusion coefficients of CNTPs fall into the same range as those of proteins in supported lipid membranes. CNTPs in HS-AFM experiments often exhibit directed diffusion behaviour, which is common for proteins in live cell membranes.This article is part of the themed issue Membrane pores: from structure and assembly, to medicine and technology