Randolph S. Duran

Superparamagnetic Fe3O4/SiO2 Nanocomposites: Enabling the Tuning of Both the Iron Oxide Load and the Size of the Nanoparticles

Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.

A torsional dilatometer for volume change measurements on deformed glasses: Instrument description and measurements on equilibrated glasses

An automated mercury dilatometer has been designed and built for the purpose of making volume change measurements on cylindrical samples subjected to torsional deformations. In its current configuration the instrument takes readings of torque, normal force, and volume change upon application of a twist at one end of the sample. Volume change sensitivity of approximately 2× 10−6cm3 makes possible relative volume change measurements of the order of ΔV/V0≂10−7 given the sample geometry. Over long times temperature stability limits this to approximately 2.5×10−5. The instrument is described and preliminary measurements are presented for epoxy glasses which have been equilibrated by annealing near to the glass transition. The results show that the volume of the sample tested at temperatures ranging from 10 K below the conventionally measured glass transition temperature Tg to Tg increases upon application of a torsional deformation. This result is contrary to results reported in the literature for samples test...

Functional ion channels in tethered bilayer membranes - Implications for biosensors

The demand for rapid in situ detection of chemical and biological analytes has increased the interest in the development of biosensors, which combine biological sensing elements with physicochemical transducers. Engineered membrane-bound ion channels are one promising class of biological receptors because they allow for highly sensitive stochastic detection of analytes, and produce a well-defined read-out that is inherently suitable for digitization. However, in order to perform stochastic sensing, it is necessary to measure the ion currents associated with single ion channel opening and closing events. Although, sensors based on supported tethered bilayers that contain various pore forming proteins have been described, there is still great limitations in creating a signal-to-noise ratio that is high enough to allow for single-channel activity detection. An alternative way to design bilayers on a chip, in which the lipid membrane covers an aperture, has been proposed. This technique has proven sensitive enough for detection of single ionchannel activity. However, this approach is fundamentally different to the tethering of bilayers onto a stable solid surface, and is likely to cause problems due to low mechanical stability. Here, we present a biosensor based on modulation of single ion-channel activity, with the ability to detect analytes in the micromolar range. The ion channels were interfaced to a gold surface, where they were reconstituted into tethered bilayer lipid membranes (tBLMs), which were in turn formed at multiple individual pixels of a microelectrode array device. The limited size of the gold sense pad surface (100;100 mm) and the electrical stability of the overlying lipid bilayer membrane made each pixel sensitive enough to measure single ion-channel currents in the picoampere range, and yet the device is convenient for monolithically integrated fabrication schemes. The biosensor is illustrated in Figure 1. Recently we were able to measure, for the first time, single ion-channel activity by using gramicidin A (gA), which was ACHTUNGTRENNUNGdirectly interfaced to a gold device surface. Even though gA can be modified to be used as a sensor there are still limitations in its use due to its relatively simple chemical struc-

Three routes to modulate the pore size of the Mscl channel/nanovalve

MscL is a bacterial mechanosensitive channel that protects cells from lysis upon acute decrease in external osmotic environment. It is one of the best characterized mechanosensors known, thus serving as a paradigm of how such molecules sense and respond to stimuli. In addition, the fact that it can be genetically modified, expressed, isolated, and manipulated has led to its proposed use as a triggered nanovalve for various functions including sensors within microelectronic array chips, as well as vesicular-based targeted drug release. X-ray crystallography reveals a homopentameric complex with each subunit containing two transmembrane α-helices (TM1 and TM2) and a single carboxyl terminal α-helix arranging within the complex to form a 5-fold cytoplasmic bundle (CB), whose function and stability remain unclear. In this study, we show three routes that throttle the open channel conductance. When the linker between the TM2 and CB domain is shortened by deletions or constrained by either cross-linking or heavy metal coordination, the conductance of the channel is reduced; in the later two cases, even reversibly. While they have implications for the stability of the CB, these data also provide routes for engineering MscL sensors that are more versatile for potential nanotech devices.

Interfacial binding dynamics of bee venom phospholipase A2 investigated by dynamic light scattering and quartz crystal microbalance.

Bee venom phospholipase A(2) (bvPLA(2)) is part of the secretory phospholipase A(2) (sPLA(2)) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA(2) activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA(2) to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA(2) does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA(2) to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA(2) adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA(2) is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.

Core−Shell Nanoparticles: Characterization and Study of Their Use for the Encapsulation of Hydrophobic Fluorescent Dyes

Core-shell nanocapsules intended to be used as drug scavengers were prepared using a surfactant mixture containing octadecyltrimethoxysilane (OTMS) as a reactive amphiphile, to form spherical templates. A siloxane shell was grown on the surface of the templates by reacting tetramethoxysilane (TMOS) with the silanol groups obtained after the hydrolysis and condensation of OTMS. Dynamic light scattering (DLS) showed that particles with diameters in the range of 100-200 nm were obtained, with core and shell sizes controlled by varying component compositions. Atomic force microscopy (AFM) was used to study the effect of the silica coating of the templates on their robustness after deposition on a substrate. Subsequently, we present studies on the encapsulation of two hydrophobic fluorescent dyes, which are sensors of polarity and rigidity. Steady-state fluorescence spectroscopy was used to examine the fluorescence response of the dyes before and after shell growth. Changes in the emission of the encapsulated dyes were related to changes in the polarity and rigidity of the microenvironment where the dyes were located and correlated to the AFM results. Finally, dye-free core-shell particles were used to sequester the dyes from aqueous suspensions. Fluorescence of the sequestered species was compared to the dye-loaded particles to determine the final fate of the fluorophores in the nanoparticles.

Mesoporous TMOS–MTMS copolymer silica gels catalyzed by fluoride

Abstract Mesoporous HF-catalyzed tetramethoxysilane-methyltrimethoxysilane (TMOS–MTMS) copolymer silica gels with average pore diameter from 80 to 100 A and narrow pore size distribution have been obtained. Mesoporous TMOS–MTMS copolymer gels with tetramethoxysilane ranging from 10% to 100% were also produced with tetrabutylammonium fluoride (TBAF) catalyst. The pore sizes of TBAF-catalyzed copolymer gels were controlled over a range from 25 to 194 A. Their pore sizes increased with increasing fluoride content to R 2 =0.112 ( R 2 , molar ratio of F/Si) and then level off. The catalytic effect of fluoride on the sol–gel process was investigated. The results suggest that multi-fluorinated silicon species catalyze the silica polymerization.

Ferroelectricity in a Langmuir–Blodgett multilayer film of a liquid crystalline side‐chain polymer

The observation of ferroelectric behavior in a Langmuir–Blodgett (LB) film of a liquid crystalline polymer is reported. It is established that a 30‐layer film exhibits spontaneous polarization and electro‐optic switching. The magnitude of the polarization of the LB film is found to be similar to that of the bulk material. The polarization current in the LB film follows the applied triangular wave field to higher frequencies as compared to the same material in a surface stabilized (sandwich) cell.

Aging in glasses subjected to large stresses and deformations

Abstract Physical aging studies near the conventional glass transition temperature Tg were made using a model epoxy glass. Non-linear viscoelastic responses were measured after quenching the samples from above Tg to below it. The physical aging response in creep in simple extension was studied as a function of stress magnitude and in stress relaxation in torsion as a function of deformation magnitude. In each test the time, t ∗ , for the aging time shift factor to approach a constant value was determined and found to be independent of the stress or deformation magnitude. The torsional experiments were performed in a torsional dilatometer and the volume recovery response during the physical aging experiment was measured in addition to the torque response. It was found that the underlying volume recovery kinetics were not changed by the mechanical stimuli, i.e. were independent of the applied deformation. These results are interpreted to mean that large mechanical stimuli do not alter the underlying thermodynamic state of the glass and aging is not ‘erased’ by the large stresses or deformations.

Prolonged stochastic single ion channel recordings in S-layer protein stabilized lipid bilayer membranes.

S-layer proteins are commonly found in bacteria and archaea as two-dimensional monomolecular crystalline arrays as the outermost cell membrane component. These proteins have the unique property that following disruption by chemical agents, monomers of the protein can re-assemble to their original lattice structure. This unique property makes S-layers interesting for utilization in bio-nanotechnological applications. Here, we show that the addition of S-layer proteins to bilayer lipid membranes increases the lifetime and the stability of the bilayer. M2delta ion channels were functionally incorporated into these S-layer stabilized membranes and we were able to record their activity for up to 20 h. Transmission electron microscopy (TEM) was used to visualize the 2D crystalline pattern of the S-layer and the M2delta ion channel characteristics in bilayer lipid membranes were compared in the presence and absence of S-layers.