The interplay between ionic and electronic charge carriers in mixed conductors offers rich physics and unique device potential. In light-emitting electrochemical cells (LEECs), for example, the redistribution of ions assists the injection of electronic carriers and leads to efficient light emission. The mechanism of operation of LEECs has been controversial, as there is no consensus regarding the distribution of electric field in these devices. Here, we probe the operation of LEECs using electric force microscopy on planar devices. We show that obtaining the appropriate boundary conditions is essential for capturing the underlying device physics. A patterning scheme that avoids overlap between the mixed-conductor layer and the metal electrodes enabled the accurate in situ measurement of the electric-field distribution. The results show that accumulation and depletion of mobile ions near the electrodes create high interfacial electric fields that enhance the injection of electronic carriers.

Fabrication of hierarchical materials, with highly optimized features from the millimeter to the nanometer scale, is crucial for applications in diverse areas including biosensing, energy storage, photovoltaics, and tissue engineering. In the past, complex material architectures have been achieved using a combination of top‐down and bottom‐up fabrication approaches. A remaining challenge, however, is the rapid, inexpensive, and simple fabrication of such materials systems using bench‐top prototyping methods. To address this challenge, the properties of hierarchically structured electrodes are developed and investigated by combining three bench‐top techniques: top‐down electrode patterning using vinyl masks created by a computer‐aided design (CAD)‐driven cutter, thin film micro/nanostructuring using a shrinkable polymer substrate, and tunable electrodeposition of conductive materials. By combining these methods, controllable electrode arrays are created with features in three distinct length scales: 40 μm to 1 mm, 50 nm to 10 μm, and 20 nm to 2 μm. The electrical and electrochemical properties of these electrodes are analyzed and it is demonstrated that they are excellent candidates for next generation low‐cost electrochemical and electronic devices.

Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube--with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.

We have electrospun light-emitting nanofibers from ruthenium(II) tris(bipyridine)/polyethylene oxide mixtures. The electroluminescent fibers were deposited on gold interdigitated electrodes and lit in a nitrogen atmosphere. The fibers showed light emission at low operating voltages (3-4 V), with turn-on voltages approaching the band gap limit of the organic semiconductor. Because of the fiber size, emission from electrospun light-emitting nanofibers is confined to nanoscale dimensions, an attractive feature for sensing applications and lab-on-a-chip integration where highly localized excitation of molecules is required.

Zero mode waveguides (ZMWs), subwavelength optical nanostructures with dimensions ranging from 50 to 200 nm, have been used to study systems involving ligand-receptor interactions. We show that under proper conditions, lipid membranes will invaginate into the nanostructures, which confine optical excitation to subattoliter volumes. Fluorescence correlation spectroscopy (FCS) was used to characterize the diffusion of fluorescently tagged lipids in liquid-disordered phase 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and gel phase 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) membranes incubated on the nanostructured surface. In contrast to the POPC, DSPC membranes did not appear to enter the structures, suggesting that invagination is dependent on membrane rigidity. Although correlation curves obtained from POPC membranes conformed to previously derived models for diffusion in the evanescent field within the nanostructure, the diffusion constants obtained were systematically lower than expected. The validity of the one-dimensional diffusion model for membrane diffusion is discussed and it is concluded that the erroneous diffusion constants are a result of nontrivial membrane conformation within the ZMWs. Additionally, FCS was used to characterize the fraction of fluorescently labeled tetanus toxin C fragment bound to a ganglioside-populated POPC membrane within the ZMWs. This allowed the determination of the toxin's equilibrium binding constant at a concentration of 500 nM; higher than possible with diffraction-limited FCS. To our knowledge, the results presented here are the first reported for supported lipid bilayers in nanostructured devices. Furthermore, they open the possibility of studying membrane imbedded receptors and proteins at physiological concentrations with single-molecule resolution.

Cotton-source cellulose nanocrystals (CNCs) with a range of surface charge densities were fluorescently labeled with 5-(4, 6-dichlorotriazinyl) aminofluorescein (DTAF) in a facile, one-pot reaction under alkaline conditions. Three CNC samples were labeled: (I) anionic CNCs prepared by sulfuric acid hydrolysis with a sulfur content of 0.47 wt %, (II) a partially desulfated, sulfuric acid-hydrolyzed CNC sample, which was less anionic with an intermediate sulfur content of 0.21 wt %, and (III) uncharged CNCs prepared by HCl hydrolysis. The DTAF-labeled CNCs were characterized by dynamic light scattering, atomic force microscopy, fluorescence spectroscopy and microscopy, and polarized light microscopy. Fluorescent CNCs exhibited similar colloidal stability to the starting CNCs, with the exception of the HCl-hydrolyzed sample, which became less agglomerated after the labeling reaction. The degree of labeling depended on the sulfur content of the CNCs, indicating that the presence of sulfate half-ester groups on the CNC surfaces hindered labeling. The labeling reaction produced CNCs that had detectable fluorescence, without compromising the overall surface chemistry or behavior of the materials, an aspect relevant to studies that require a fluorescent cellulose substrate with intact native properties. The DTAF-labeled CNCs were proposed as optical markers for the dispersion quality of CNC-loaded polymer composites. Electrospun polyvinyl alcohol fibers loaded with DTAF-labeled CNCs appeared uniformly fluorescent by fluorescence microscopy, suggesting that the nanoparticles were well dispersed within the polymer matrix.

Plasma membranes are highly dynamic structures, with key molecular interactions underlying their functionality occurring at nanometre scales. A fundamental challenge in biology is to observe these interactions in living cells. Although fluorescence microscopy has enabled advances in characterizing molecular distributions in cells, optical techniques are restricted by the diffraction limit. We address this limitation with an approach based on zero-mode waveguides (ZMWs), which are optical nanostructures that confine fluorescence excitation to sub-diffraction volumes. Successful use of ZMWs with cell membranes is reported in this paper. We demonstrate that plasma membranes from live cells penetrate these nanostructures. Cellular exploration of the nanoapertures depends heavily on actin filaments but not on microtubules. Thus, membranes enter the confined excitation volume, and diffusion of individual fluorescent lipids can be monitored. Through fluorescence correlation spectroscopy, we compared DiIC12 and DiIC16 fluorescent labels incorporated into plasma membranes and found distinctive diffusion behaviours. These results show that the use of optical nanostructures enables the measurement of membrane events with single molecule resolution in sub-diffraction volumes.

We prepared highly oriented, multi-lamellar stacks of human red blood cell (RBC) membranes applied on silicon wafers. RBC ghosts were prepared by hemolysis and applied onto functionalized silicon chips and annealed into multi-lamellar RBC membranes. High resolution X-ray diffraction was used to determine the molecular structure of the stacked membranes. We present direct experimental evidence that these RBC membranes consist of nanometer sized domains of integral coiled-coil peptides, as well as liquid ordered (lo) and liquid disordered (ld) lipids. Lamellar spacings, membrane and hydration water layer thicknesses, areas per lipid tail and domain sizes were determined. The common drug aspirin was added to the RBC membranes and found to interact with RBC membranes and preferably partition in the head group region of the lo domain leading to a fluidification of the membranes, i.e., a thinning of the bilayers and an increase in lipid tail spacing. Our results further support current models of RBC membranes as patchy structures and provide unprecedented structural details of the molecular organization in the different domains.