Polymerizable surfactant design for transmission electron microscopy

Polymerized liposomes and vesicles are under close scrutiny as long-lived, stable substitutes for their natural and synthetic unpolymerized counterparts. The monomer surfactant, which contains one or more polymerizable groups, is dispersed in water at the proper temperature and concentration to form the lyotropic liquid crystalline phase of interest and polymerized while in the liquid crystalline state. In addition to their applications to slow-release and site-specific drug delivery, membrane-mediated chemistry, artificial photosynthesis, etc., polymerized surfactant liposomes and vesicles hold great promise as model systems for TEM investigations of lamellar liquid crystal structure. One such model polymerizable surfactant is DBPAl, or N,N-dimethyl-N,N-bis(1,3-pentadecadienyl-carbonyloxyethyl) ammonium iodide. Polarized light microscopy and differential scanning calorimetry (DSC) confirm that DBPAI forms lamellar liquid crystalline liposomes in water. The DBPAI liposomes were polymerized while in the liquid crystalline state by ultraviolet (UV) irradiation. The DBPAI liposomes were shown to be identical in structure before and after polymerization by a combination of X-ray diffraction and freeze-fracture TEM. However, turbidity measurements showed that the polymerized DBPAI liposomes were much more stable in acetone and ethanol than the monomer DBPAI liposomes, demonstrating that the chemical nature of the surfactant in the liposome had changed. The combination of structural preservation and enhanced chemical stability makes DBPAI a natural choice for TEM thin-sections. A method of preparing DBPAI liposomes for thin-section TEM is outlined and bilayer resolution images of the DBPAI liposomes are presented. Polymerized bilayers in thin-section TEM promise the enhanced resolution required to answer many important structural questions left unresolved by freeze-fracture TEM.     

Tunneling estimates of paramagnon suppression of superconductivity in transition metals

New experimental estimates are given of the separate electron-phonon (λ_ph) and paramagnon (λ_S) contributions to mass renormalization for the transition metals Nb and V, from recent tunneling results. Our analysis, which is the first of its kind, is based on the recent demonstration of Daams, Mitrovic, and Carbotte of a simple rescaling of the Eliashberg equations using an effective coupling parameter $\text{λ}{}_{\mathrm{ph}}\text{(1+λ}{}_{\mathrm{S}}\text{)}$. The results provide a consistent picture of superconductivity in these transition elements from tunneling, in substantially improved agreement with other measurements. We conclude that spin fluctuations reduce the electron-phonon T_c values in Nb and V by 25% and 45%, respectively.     

Quantitative proximity tunneling spectroscopy

Using the new Green's function calculation of Arnold and an appropriate modification of the McMillan-Rowell procedure, it is demonstrated that tunneling characteristics of suitable proximity junctions may be inverted to determine the properties of superconductors which do not oxidize satisfactorily. The junctions employed are of the form M-Al₂O₃-Al/S (where S is the superconductor of interest). The Al thickness is less than 100 Å and the Al/S interface is specularly transmitting. Quantitative values for Δ_S(E) and Δ_N(E) are obtained for S=Nb, N=Al.     

Remotely triggered liposome release by near-infrared light absorption via hollow gold nanoshells

An elusive goal for systemic drug delivery is to provide both spatial and temporal control of drug release. Liposomes have been evaluated as drug delivery vehicles for decades, but their clinical significance has been limited by slow release or poor availability of the encapsulated drug. Here we show that near-complete liposome release can be initiated within seconds by irradiating hollow gold nanoshells (HGNs) with a near-infrared (NIR) pulsed laser. Our findings reveal that different coupling methods such as having the HGNs tethered to, encapsulated within, or suspended freely outside the liposomes, all triggered liposome release but with different levels of efficiency. For the underlying content release mechanism, our experiments suggest that the microbubble formation and collapse due to the rapid temperature increase of the HGN is responsible for liposome disruption, as evidenced by the formation of solid gold particles after the NIR irradiation and the coincidence of a laser power threshold for both triggered release and pressure fluctuations in the solution associated with cavitation. These effects are similar to those induced by ultrasound and our approach is conceptually analogous to the use of optically triggered nano-"sonicators" deep inside the body for drug delivery. We expect HGNs can be coupled with any nanocarriers to promote spatially and temporally controlled drug release. In addition, the capability of external HGNs to permeabilize lipid membranes can facilitate the cellular uptake of macromolecules including proteins and DNA and allow for promising applications in gene therapy.