Biomedical subjects

Douglas S English

Publications and source records attributed to Douglas S English.

8 recordsLinked to original sources

PubMed2007

Reversible vesicle restraint in response to spatiotemporally controlled electrical signals: a bridge between electrical and chemical signaling modes.

Microelectronic devices employ electrons for signaling whereas the nervous system signals using ions and chemicals. Bridging these signaling differences would benefit applications that range from biosensing to neuroprosthetics. Here, we report the use of localized electrical signals to perform an operation common to chemical signaling in the nervous system. Specifically, we employ electrical signals to restrain vesicles reversibly. We perform this operation using the stimuli-responsive aminopolysaccharide chitosan that is able to electrodeposit onto cathode surfaces in response to localized electrical stimuli. We show that surfactant-vesicles and liposomes can be co-deposited with chitosan and are entrapped (i.e., restrained) within the deposited film's matrix. Vesicle co-deposition could be controlled spatially and temporally using microfabricated wafers with independent electrode addresses. Finally, we show that vesicles restrained within the deposited chitosan matrix can be mobilized under mildly acidic conditions (pH <6.5) that resolubilize chitosan. Potentially, the ability to restrain and mobilize chemical signals that are segregated within vesicles may allow microfluidic systems to access the rich diversity offered by chemical signaling.

Animals↗

PubMed2006

Solution-phase single quantum dot fluorescence resonance energy transfer.

We present a single particle fluorescence resonance energy transfer (spFRET) study of freely diffusing self-assembled quantum dot (QD) bioconjugate sensors, composed of CdSe-ZnS core-shell QD donors surrounded by dye-labeled protein acceptors. We first show that there is direct correlation between single particle and ensemble FRET measurements in terms of derived FRET efficiencies and donor-acceptor separation distances. We also find that, in addition to increased sensitivity, spFRET provides information about FRET efficiency distributions which can be used to resolve distinct sensor subpopulations. We use this capacity to gain information about the distribution in the valence of self-assembled QD-protein conjugates and show that this distribution follows Poisson statistics. We then apply spFRET to characterize heterogeneity in single sensor interactions with the substrate/target and show that such heterogeneity varies with the target concentration. The binding constant derived from spFRET is consistent with ensemble measurements.

Cadmium Compounds↗

PubMed2006

Toward efficient photomodulation of conjugated polymer emission: optimizing differential energy transfer in azobenzene-substituted PPV derivatives.

We present fluorescence studies of quenching behavior in photoaddressable azobenzene-substituted derivatives of the fluorescent conjugated polymer poly(p-phenylenevinylene) (PPV). The azobenzene side chains partially quench the PPV fluorescence, and we have shown previously that the quenching efficiency is greater when the azobenzene side chains are cis than when they are trans. This effect provides a photoaddressable means of modulating the fluorescence intensity of PPV derivatives. To optimize the efficiency of photoinduced intensity modulation, it is important to understand the molecular nature of quenching by both trans- and cis-azobenzene side chains. Here we investigate the photophysical origins of quenching by the two isomers using steady-state and time-resolved fluorescence spectroscopy. We present results from the azobenzene-modified PPV derivative poly(2-methoxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (MPA-10-PPV) and two new related polymers, a copolymer lacking half of the azobenzene side chains and an analogue of MPA-10-PPV with a tert-butyl-substituted azobenzene. These studies reveal that steric interactions influence the extent of PPV emission quenching by trans-azobenzene but do not affect the efficient quenching by cis-azobenzene. The difference in dynamic quenching efficiencies between trans- and cis-azobenzene isomers is consistent with fluorescence resonance energy transfer. These results show that it is possible to control the efficiency of photoswitchable fluorescence modulation through specific structural variations designed to encourage or block quenching by trans-azobenzene. This is a promising approach to providing useful general guidelines for designing photomodulated PPV derivatives.

Journal Article↗

PubMed2006

Highly efficient capture and long-term encapsulation of dye by catanionic surfactant vesicles.

Vesicles formed from the cationic surfactant, cetyltrimethylammonium tosylate (CTAT) and the anionic surfactant, sodium dodecylbenzenesulfonate (SDBS), were used to sequester the anionic dye carboxyfluorescein. Carboxyfluorescein was efficiently sequestered in CTAT-rich vesicles via two mechanisms: encapsulation in the inner water pool and electrostatic adsorption to the charged bilayer. The apparent encapsulation efficiency (22%) includes both encapsulated and adsorbed fractions. Entrapment of carboxyfluorescein by SDBS-rich vesicles was not observed. Results show the permeability of the catanionic membrane is an order of magnitude lower than that of phosphatidylcholine vesicles and the loading capacity is more than 10 times greater.

Benzenesulfonates↗

PubMed2006

Magnetic iron oxide nanoparticles for biorecognition: evaluation of surface coverage and activity.

Modifying the surfaces of magnetic nanoparticles (MNPs) by the covalent attachment of biomolecules will enable their implementation as contrast agents for magnetic resonance imaging or as media for magnetically assisted bioseparations. In this paper we report both the surface coverage and the activity of IgG antibodies on MNPs. The antibodies were immobilized on gamma-Fe2O3 nanoparticles by conventional methods using aminopropyltriethoxy silane and subsequent activation by glutaraldehyde. Novel fluorescence methods were used to provide a quantitative evaluation of this well-known approach. Our results show that surface coverage can be stoichiometrically adjusted with saturated surface coverage occurring at approximately 36% of the theoretical limit. The saturated surface coverage corresponds to 34 antibody molecules bound to an average-sized MNP (32 nm diameter). We also show that the immobilized antibodies retain approximately 50% of their binding capacity at surface-saturated levels.

Antibodies↗

PubMed2005

Observing capillarity in hydrophobic silica nanotubes.

The development of template-synthesized silica nanotubes has created a unique opportunity for studying confined fluids by providing nanometer-scale containers in which the inner diameter (i.d.) and surface chemistry can be systematically and independently varied. An interesting question to be answered is the following: do solvents wet nanometer-scale tubes in the same way they wet ordinary capillaries? To answer this question, we have conducted studies to explore the wettability of the hydrophobic interiors of individual nanotubes. In these studies, single nanotubes with i.d.'s of either 30 or 170 nm were investigated over a range of water/methanol mixtures. These studies provide a direct route for comparing wetting phenomena in nanotubes with conventional macroscopic theories of capillarity. Our observations reveal four important aspects of capillary wetting in the 30-170 nm regime, a size range where the application of the Young-Laplace theory has not been experimentally investigated for hydrophobic pores. They are (i) a sharp transition between wetting and nonwetting conditions induced by addition of a cosolvent, (ii) invariance of this transition between nanotubes of 30 and 170 nm pore diameter, (iii) failure of the Young-Laplace equation to accurately predict the cosolvent's (methanol) mol fraction where the transition occurs, and (iv) reversibility of the observed wetting. The first two aspects conform to conventional capillarity (Young-Laplace), but the latter two do not. These measurements were complemented with ensemble experiments. The difference between theory and experiment is likely due to reliance on macroscopic values of contact angles or to liquid-phase instability within the hydrophobic pore.

Journal Article↗

PubMed2005

Single-molecule spectroscopic determination of lac repressor-DNA loop conformation.

The Escherichia coli lactose repressor protein (LacI) provides a classic model for understanding protein-induced DNA looping. LacI has a C-terminal four-helix bundle tetramerization domain that may act as a flexible hinge. In previous work, several DNA constructs, each containing two lac operators bracketing a sequence-induced bend, were designed to stabilize different possible looping geometries. The resulting hyperstable LacI-DNA loops exist as both a compact "closed" form with a V-shaped repressor and also a more "open" form with an extended hinge. The "9C14" construct was of particular interest because footprinting, electrophoretic mobility shift, and ring closure experiments suggested that it forms both geometries. Previous fluorescence resonance energy transfer (FRET) measurements gave an efficiency of energy transfer (ET) of 70%, confirming the existence of a closed form. These measurements could not determine whether open form or intermediate geometries are populated or the timescale of interconversion. We have now applied single-molecule FRET to Cy3, Cy5 double-labeled LacI-DNA loops diffusing freely in solution. By using multiple excitation wavelengths and by carefully examining the behavior of the zero-ET peak during titration with LacI, we show that the LacI-9C14 loop exists exclusively in a single closed form exhibiting essentially 100% ET.

Bacterial Proteins↗

PubMed2005

Chitosan-mediated and spatially selective electrodeposition of nanoscale particles.

Nanoscale particles offer a variety of interesting properties, and there is growing interest in their assembly into higher ordered structures. We report that the pH-responsive aminopolysaccharide chitosan can mediate the electrodeposition of model nanoparticles. Chitosan is known to electrodeposit at the cathode surface in response to a high localized pH. To demonstrate that chitosan can mediate nanoparticle deposition, we suspended fluorescently labeled latex nanoparticles (100 nm diameter spheres) in a chitosan solution (1%) and performed electrodeposition (0.05 mA/cm2 for several minutes). Results demonstrate that chitosan is required for nanoparticle electrodeposition; chitosan confers spatial selectivity to electrodeposition; and nanoparticles distribute throughout the electrodeposited chitosan film. Additionally, we observed that the deposited films reversibly swell upon rehydration. This work indicates that chitosan provides a simple means to assemble nanoparticles at addressable locations and provides further evidence that stimuli-responsive biological materials may facilitate fabrication at the microscale.

Biocompatible Materials↗