Purple membranes from Halobacterium salinarum as building blocks for nanobiotechnology: The importance of mechanical and thermal properties for matrix and surface applications

Bacteriorhodopsin (BR) is a light-driven proton pump and the key protein in halobacterial photosynthesis. In its native host, the archaeon Halobacterium salinarum, BR trimers arrange into a 2-D crystalline lattice, the so-called purple membranes (PMs) which comprise BR and lipids only. Along with the PM assembly BR is astonishingly stable against thermal and chemical stress which makes it an excellent candidate for a variety of technical applications. Many technical applications involve immobilization of PM in a matrix or at a surface. Sugar glasses are frequently used matrixes for the stabilization of biomolecules against thermal stress as well as dehydration. In the following work temperature-dependent interactions between PM and sugar glasses were analyzed. Above T > 60°C a blue membrane is formed the so-called sugar-induced membrane (SIBM). This thermochromism was explained by a model which is based on the release of divalent cations from PM and the chelating properties of carbohydrates. Due to their high mass adsorption is an appropriate method for the immobilization of PMs at surfaces. In this work self-assembled monolayers (SAMs) of alkanethiols on gold were used and analyzed for this purpose. To fabricate selective adsorption sites the alkanethiol-SAMs have to be structured. An appropriate procedure for the patterning of alkanethiol-SAMs has been developed which has been named “submerged laser ablation” (SLAB). Knowledge about the mechanical properties of freely suspended membranes is essential for nanobiotechnological applications of PM. In each PM thousands of BR molecules are arranged in a unidirectional manner and are strongly coupled due to the crystalline assembly. Therefore, collective conformational changes of single BRs should influence the topology of freely suspended PMs. In this work it was demonstrated by direct imaging of freely suspended native PMs via cryogenic high-resolution scanning electron microscopy (cryo-SEM) that the flat disk-like shape of PM changes dramatically as soon as most of the BRs are in the M2 state which is characterized by wedge-shaped BRs due to collective opening of the cytoplasmatic half-channels. Light- as well as pH-induced shape changes are easily observed with mutated BRs which accumulate wedge-shaped BR molecules. These results open the way for further nanobiotechnological applications of PM, i. e. as supramolecular actuator

Directed deposition of silicon nanowires using neopentasilane as precursor and gold as catalyst

In this work the applicability of neopentasilane (Si(SiH3)4) as a precursor for the formation of silicon nanowires by using gold nanoparticles as a catalyst has been explored. The growth proceeds via the formation of liquid gold/silicon alloy droplets, which excrete the silicon nanowires upon continued decomposition of the precursor. This mechanism determines the diameter of the Si nanowires. Different sources for the gold nanoparticles have been tested: the spontaneous dewetting of gold films, thermally annealed gold films, deposition of preformed gold nanoparticles, and the use of “liquid bright gold”, a material historically used for the gilding of porcelain and glass. The latter does not only form gold nanoparticles when deposited as a thin film and thermally annealed, but can also be patterned by using UV irradiation, providing access to laterally structured layers of silicon nanowires

Energy-filtered transmission electron microscopy of biological samples on highly transparent carbon nanomembranes

Ultrathin carbon nanomembranes (CNM) comprising crosslinked biphenyl precursors have been tested as support films for energy-filtered transmission electron microscopy (EFTEM) of biological specimens. Due to their high transparency CNM are ideal substrates for electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) of stained and unstained biological samples. Virtually background-free elemental maps of tobacco mosaic virus (TMV) and ferritin have been obtained from samples supported by ~ 1 nm thin CNM. Furthermore, we have tested conductive carbon nanomembranes (cCNM) comprising nanocrystalline graphene, obtained by thermal treatment of CNM, as supports for cryoEM of ice-embedded biological samples. We imaged ice-embedded TMV on cCNM and compared the results with images of ice-embedded TMV on conventional carbon film (CC), thus analyzing the gain in contrast for TMV on cCNM in a quantitative manner. In addition we have developed a method for the preparation of vitrified specimens, suspended over the holes of a conventional holey carbon film, while backed by ultrathin cCNM

Single-walled carbon nanotubes and nanocrystalline graphene reduce beam-induced movements in high-resolution electron cryo-microscopy of ice-embedded biological samples

For single particle electron cryo-microscopy (cryoEM), contrast loss due to beam-induced charging and specimen movement is a serious problem, as the thin films of vitreous ice spanning the holes of a holey carbon film are particularly susceptible to beam-induced movement. We demonstrate that the problem is at least partially solved by carbon nanotechnology. Doping ice-embedded samples with single-walled carbon nanotubes (SWNT) in aqueous suspension or adding nanocrystalline graphene supports, obtained by thermal conversion of cross-linked self-assembled biphenyl precursors, significantly reduces contrast loss in high-resolution cryoEM due to the excellent electrical and mechanical properties of SWNTs and graphene

Purple membranes from Halobacterium salinarum as building blocks for nanobiotechnology: The importance of mechanical and thermal properties for matrix and surface applications

Bacteriorhodopsin (BR) is a light-driven proton pump and the key protein in halobacterial photosynthesis. In its native host, the archaeon Halobacterium salinarum, BR trimers arrange into a 2-D crystalline lattice, the so-called purple membranes (PMs) which comprise BR and lipids only. Along with the PM assembly BR is astonishingly stable against thermal and chemical stress which makes it an excellent candidate for a variety of technical applications. Many technical applications involve immobilization of PM in a matrix or at a surface. Sugar glasses are frequently used matrixes for the stabilization of biomolecules against thermal stress as well as dehydration. In the following work temperature-dependent interactions between PM and sugar glasses were analyzed. Above T > 60°C a blue membrane is formed the so-called sugar-induced membrane (SIBM). This thermochromism was explained by a model which is based on the release of divalent cations from PM and the chelating properties of carbohydrates. Due to their high mass adsorption is an appropriate method for the immobilization of PMs at surfaces. In this work self-assembled monolayers (SAMs) of alkanethiols on gold were used and analyzed for this purpose. To fabricate selective adsorption sites the alkanethiol-SAMs have to be structured. An appropriate procedure for the patterning of alkanethiol-SAMs has been developed which has been named “submerged laser ablation” (SLAB). Knowledge about the mechanical properties of freely suspended membranes is essential for nanobiotechnological applications of PM. In each PM thousands of BR molecules are arranged in a unidirectional manner and are strongly coupled due to the crystalline assembly. Therefore, collective conformational changes of single BRs should influence the topology of freely suspended PMs. In this work it was demonstrated by direct imaging of freely suspended native PMs via cryogenic high-resolution scanning electron microscopy (cryo-SEM) that the flat disk-like shape of PM changes dramatically as soon as most of the BRs are in the M2 state which is characterized by wedge-shaped BRs due to collective opening of the cytoplasmatic half-channels. Light- as well as pH-induced shape changes are easily observed with mutated BRs which accumulate wedge-shaped BR molecules. These results open the way for further nanobiotechnological applications of PM, i. e. as supramolecular actuator

Atmospheric Pressure, Temperature-Induced Conversion of Organic Monolayers into Nanocrystalline Graphene

Rhinow D, Weber N-E, Turchanin A. Atmospheric Pressure, Temperature-Induced Conversion of Organic Monolayers into Nanocrystalline Graphene. The Journal of Physical Chemistry C. 2012;116(22):12295-12303.Atomically thin free-standing nanomembranes belong to the emerging class of materials with a great promise for basic research of two-dimensional (2D) systems and applications in nanotechnology. However, their synthesis and characterization remain a frontier challenge in chemical and physical research. Here, we demonstrate that atmospheric pressure (Ar/H-2) annealing in the range from 500 to 1000 degrees C of similar to 1 nm thick carbon nanomembranes (CNMs) made of cross-linked aromatic self-assembled monolayers results in their conversion into free-standing sheets of covalently bonded, in-plane oriented graphene nanocrystals. Upon this transformation the electrical characteristics of CNMs evolve from insulating to conducting, which is accompanied by a change of the room temperature sheet resistivity by more than 5 orders of magnitude. We analyze this atmospheric pressure, temperature-induced transformation of CNMs employing various complementary spectroscopic and microscopic techniques, such as X-ray photoelectron spectroscopy, Raman spectroscopy, electron energy loss spectroscopy, optical microscopy, helium ion microscopy, atomic force microscopy, and transmission electron microscopy. In particular we studied the chemical, structural, and electronic properties of CNMs. We provide a comparative analysis of these data with the properties of pristine graphene, graphene oxide, and reduced graphene oxide sheets, which reveals both similarities and differences between these 2D materials