Isotopically induced variation in the stability of FMN-wrapped carbon nanotubes.

Isotopic, hydrogen-to-deuterium substitution has been an invaluable tool in the characterization of small molecules and biological nanostructures. The natural variability of most inorganic nanomaterials has hindered the use of isotopic substitution in gaining meaningful insights into their structure. The ideal helical wrapping of a flavin mononucleotide (FMN) around (8,6)-SWNTs (single-walled carbon nanotubes) is presently utilized to probe isotopically dependent intermolecular interactions. The facile proton-to-deuterium exchange of the imide group of FMN enabled us to alter the intermolecular stability of the helix depending on the surrounding solvent (i.e., H2O vs D2O). Our studies show that FMN-dispersed (8,6)-SWNTs exhibit greater stability in D2O than in H2O. The higher complex stability in D2O was verified on the basis of (i) FMN helix replacement with SDBS (sodium dodecylbenzenesulfate) and (ii) thermal- and (iii) pH-induced helix dissociation. This is in agreement with the previously observed stronger amide H-bonding of proteins in D2O, and to the best of our knowledge, it demonstrates the architectural fidelity of FMN-wrapped SWNTs, which is expected to enhance the assembly repertoire of carbon nanotubes further.

Microsphere erosion in outer hydrogel membranes creating macroscopic porosity to counter biofouling-induced sensor degradation.

Biofouling and tissue inflammation present major challenges toward the realization of long-term implantable glucose sensors. Following sensor implantation, proteins and cells adsorb on sensor surfaces to not only inhibit glucose flux but also signal a cascade of inflammatory events that eventually lead to permeability-reducing fibrotic encapsulation. The use of drug-eluting hydrogels as outer sensor coatings has shown considerable promise to mitigate these problems via the localized delivery of tissue response modifiers to suppress inflammation and fibrosis, along with reducing protein and cell absorption. Biodegradable poly (lactic-co-glycolic) acid (PLGA) microspheres, encapsulated within a poly (vinyl alcohol) (PVA) hydrogel matrix, present a model coating where the localized delivery of the potent anti-inflammatory drug dexamethasone has been shown to suppress inflammation over a period of 1-3 months. Here, it is shown that the degradation of the PLGA microspheres provides an auxiliary venue to offset the negative effects of protein adsorption. This was realized by: (1) the creation of fresh porosity within the PVA hydrogel following microsphere degradation (which is sustained until the complete microsphere degradation) and (2) rigidification of the PVA hydrogel to prevent its complete collapse onto the newly created void space. Incubation of the coated sensors in phosphate buffered saline (PBS) led to a monotonic increase in glucose permeability (50%), with a corresponding enhancement in sensor sensitivity over a 1 month period. Incubation in serum resulted in biofouling and consequent clogging of the hydrogel microporosity. This, however, was partially offset by the generated macroscopic porosity following microsphere degradation. As a result of this, a 2-fold recovery in sensor sensitivity for devices with microsphere/hydrogel composite coatings was observed as opposed to similar devices with blank hydrogel coatings. These findings suggest that the use of macroscopic porosity can reduce sensitivity drifts resulting from biofouling, and this can be achieved synergistically with current efforts to mitigate negative tissue responses through localized and sustained drug delivery.

The role of H2O2 outer diffusion on the performance of implantable glucose sensors.

The performance of an implantable glucose sensor is strongly dependent on the ability of their outer membrane to govern the diffusion of the various participating species. In this contribution, using a series of layer-by-layer (LBL) assembled outer membranes, the role of outwards of H(2)O(2) diffusion through the outer membrane of glucose sensors has been correlated to sensor sensitivity. Glucose sensors with highly permeable humic acids/ferric cations (HAs/Fe(3+)) outer membranes displayed a combination of lower sensitivities and better linearities when compared with sensors coated with lesser permeable outer membranes (namely HAs/poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS)/PDDA). On the basis of a comprehensive evaluation of the oxygen dependence of these sensors in conjunction with the permeability of H(2)O(2) through these membranes, it was concluded that the outer diffusion of H(2)O(2) is crucial to attain optimized sensor performance. This finding has important implications to the design of various bio-sensing elements employing perm-selective membranes.

Resonance Raman spectroscopy characterization of single-wall carbon nanotube separation by their metallicity and diameter.

Several techniques were recently reported for the bulk separation of metallic (M) and semiconducting (S) single wall carbon nanotubes (SWNTs), using optical absorption and resonance Raman spectroscopy (RRS) as a proof of the separation. In the present work, we develop a method for the quantitative evaluation of the M to S separation ratio, and also for the SWNT diameter selectivity of the separation process, based on RRS. The relative changes in the integrated intensities of the radial-breathing mode (RBM) features, with respect to the starting material, yield the diameter probability distribution functions for M and S SWNTs in the separated fractions, accounting for the different resonance conditions of individual SWNTs, while the diameter distribution of the starting material is obtained following the fitting procedure developed by Kuzmany and coworkers. Features other than the RBM are generally less effective for characterization of the separation process for SWNTs.

Layer-by-layer growth of CdSe-based nanocrystal light-emitting diodes.

The partial exchange of surface-passivating trioctylphosphine oxide (TOPO) on CdSe and ZnS-clad CdSe (CdSe/ZnS) nanocrystals with primary amines was utilized to grow ultra-thin films of these nanocrystals under nonaqueous conditions. This growth was achieved using 1,12-diaminododecane in a layer-by-layer assembly format, where one of the amino groups binds with the nanocrystal surface and the other regenerates the interface for further binding of nanocrystals. The nature of the growth is dependent on the relative surface affinity between the TOPO and the primary amine toward the zinc or cadmium sites on the nanocrystals. Using this technique, high-quality luminescent films of these nanocrystals can be built with well-defined thicknesses. Electroluminescent devices have been fabricated using this methodology.

Characterization and biocompatibility studies of novel humic acids based films as membrane material for an implantable glucose sensor.

Multilayered films of humic acids (HAs) (naturally occurring biopolymers) were investigated as a potential semipermeable membrane for implantable glucose sensors. These films were grown using a layer-by-layer self-assembly process of HAs and oppositely charged ferric ions. The growth of these assemblies exhibited strong dependence on the pH and ionic strength of HAs solutions, which correlated with the degree of ionization of the carboxyl groups and neutralization-induced surface spreading. Quartz crystal microbalance (QCM) and ellipsometric studies have shown repeatable, stepwise increase in mass (as high as 5.63 microg/cm(2)) and in film thickness (ca. 24.3 nm per layer) for these assemblies. The permeability of glucose through these membranes can be regulated by varying the number of self-assembled HAs/Fe(3+) layers. Moreover, a 200 nm thick HAs/Fe(3+) film (in its hydrated state) had a shear modulus of about 80 MPa, implying stability upon implantation. These films were determined to be biocompatible since in vivo studies indicated only mild tissue reaction along with some neovascularization.

Calcification-resistant Nafion/Fe3+ assemblies for implantable biosensors.

An electrostatic layer-by-layer deposition technique was employed for the formation of thin films consisting of alternating layers of perfluorinated ionomer (Nafion) and ferric ions. UV-vis spectroscopic and ellipsometric data indicate a stepwise growth that in certain cases is as high as 47 nm per dip cycle. The growth characteristics of these assemblies can be correlated with Nafion's hydrodynamic radius, iron content, as well as the ionic strength and pH of Nafion and the wash solution. When these assemblies were compared to cast Nafion films, they exhibit the following advantages: (i) increased hydrolytic stability, attained without thermal treatment required for pristine Nafion films, and (ii) resistance to calcification, by more than an order of magnitude. These results, along with the ability to control glucose permeability by varying the number of Nafion/Fe3+ layers, could prove vital in prolonging the lifetime of implantable biosensors.

Design and application of electron-transporting organic materials.

Operating lifetime is the main problem that complicates the use of polymeric light-emitting diodes (LEDs). A class of electron transport (ET) polymers [poly(aryl acrylate) and poly(aryl ether)s] is reported in which moieties with high electron affinities are covalently attached to stable polymer backbones. Devices based on poly(p-phenylenevinylene) (PPV) prepared with these materials exhibited a 30-fold improvement in stability and, in one case, dramatically lower (10 volts versus about 30 volts) operating voltage relative to those having conventional ET layers. The current-carrying capacity of indium tin oxide-PPV-polymeric ET layer-aluminum LEDs was also increased by a factor of 30. These improvements lead to an enhancement in power efficiency of nearly an order of magnitude. Choosing polymers with high glass transition temperatures increases device lifetime.