Lipid-modulated assembly of magnetized iron-filled carbon nanotubes in millimeter-scale structures

Biomolecule-functionalized carbon nanotubes (CNTs) combine the molecular recognition properties of biomaterials with the electrical properties of nanoscale solid state transducers. Application of this hybrid material in bioelectronic devices requires the development of methods for the reproducible self-assembly of CNTs into higher-order structures in an aqueous environment. To this end, we have studied pattern formation of lipid-coated Fe-filled CNTs, with lengths in the 1–5 µm range, by controlled evaporation of aqueous CNT-lipid suspensions. Novel diffusion limited aggregation structures composed of end-to-end oriented nanotubes were observed by optical and atomic force microscopy. Significantly, the lateral dimension of assemblies of magnetized Fe-filled CNTs was in the millimeter range. Control experiments in the absence of lipids and without magnetization indicated that the formation of these long linear nanotube patterns is driven by a subtle interplay between radial flow forces in the evaporating droplet, lipid-modulated van der Waals forces, and magnetic dipole–dipole interactions. Keywords

Mapping nanomechanical properties of live cells using multi-harmonic atomic force microscopy

The nanomechanical properties of living cells, such as their surface elastic response and adhesion, have important roles in cellular processes such as morphogenesis(1), mechano-transduction(2), focal adhesion(3), motility(4,5), metastasis(6) and drug delivery(7-10). Techniques based on quasi-static atomic force microscopy techniques(11-17) can map these properties, but they lack the spatial and temporal resolution that is needed to observe many of the relevant details. Here, we present a dynamic atomic force microscopy(18-28) method to map quantitatively the nanomechanical properties of live cells with a throughput (measured in pixels/minute) that is similar to 10-1,000 times higher than that achieved with quasi- static atomic force microscopy techniques. The local properties of a cell are derived from the 0th, 1st and 2nd harmonic components of the Fourier spectrum of the AFM cantilevers interacting with the cell surface. Local stiffness, stiffness gradient and the viscoelastic dissipation of live Escherichia coli bacteria, rat fibroblasts and human red blood cells were all mapped in buffer solutions. Our method is compatible with commercial atomic force microscopes and could be used to analyse mechanical changes in tumours, cells and biofilm formation with sub-10 nm detail

Designer cantilevers for even more accurate quantitative measurements of biological systems with multifrequency AFM

Multifrequency excitation/monitoring of cantilevers has made it possible both to achieve fast, relatively simple, nanometre-resolution quantitative mapping of mechanical of biological systems in solution using atomic force microscopy (AFM), and single molecule resolution detection by nanomechanical biosensors. A recent paper by Penedo et al [2015 Nanotechnology 26 485706] has made a significant contribution by developing simple methods to improve the signal to noise ratio in liquid environments, by selectively enhancing cantilever modes, which will lead to even more accurate quantitative measurements

Controlled ionic condensation at the surface of a native extremophile membrane

At the nanoscale level biological membranes present a complex interface with the solvent. The functional dynamics and relative flexibility of membrane components together with the presence of specific ionic effects can combine to create exciting new phenomena that challenge traditional theories such as the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory or models interpreting the role of ions in terms of their ability to structure water (structure making/breaking). Here we investigate ionic effects at the surface of a highly charged extremophilemembrane composed of a proton pump (bacteriorhodopsin) and archaeal lipids naturally assembled into a 2D crystal. Using amplitude-modulation atomic force microscopy (AM-AFM) in solution, we obtained sub-molecular resolution images of ion-induced surface restructuring of the membrane. We demonstrate the presence of a stiff cationic layer condensed at its extracellular surface. This layer cannot be explained by traditional continuum theories. Dynamic force spectroscopy experiments suggest that it is produced by electrostatic correlation mediated by a Manning-type condensation of ions. In contrast, the cytoplasmic surface is dominated by short-range repulsive hydration forces. These findings are relevant to archaeal bioenergetics and halophilic adaptation. Importantly, they present experimental evidence of a natural system that locally controls its interactions with the surrounding medium and challenges our current understanding of biological interfaces

Lateral coupling and cooperative dynamics in the function of the native membrane protein bacteriorhodopsin

Membrane proteins are laterally coupled to the surrounding cell membrane through complex interactions that can modulate their function. Here, we directly observe and quantify the dynamics of functioning bacteriorhodopsin (bR) in its native membrane, a crystalline aggregate of bR trimers. We show that much of a monomer's isomerization energy is mechanically redistributed into the membrane, producing cooperative activity within the trimer while simultaneously generating functionally relevant long-range lateral pressure waves. Our results provide evidence of coordinated short and long-range effects in the cell membrane. © 2009 The Royal Society of Chemistry

Amphiphilic DNA tiles for controlled insertion and 2D assembly on fluid lipid membranes: Effect on mechanical properties

Future lipid membrane-associated DNA nanostructures are expected to find applications ranging from synthetic biology to nanomedicine. Here we have designed and synthesized DNA tiles and modified them by amphiphilic covalent moieties. dod-DEG groups, which consist of a hydrophilic diethylene glycol (DEG) and a hydrophobic dodecyl group, are introduced at the phosphate backbone to create amphiphilic DNA strands which are subsequently introduced into one face of DNA tiles. In this way the tile becomes able to stably bind to lipid membranes by insertion of the hydrophobic groups inside the bilayer core. The functionalized tiles do not aggregate in solution. Our results show that these amphiphilic DNA tiles can bind and assemble into 2D lattices on both gel and fluid lipid bilayers. The binding of the DNA structures to membranes is dependent on the lipid phase of the membrane, the concentration of Mg2+ cation, the length of the amphiphilic modifications to the DNA as well as on the density of the modifications within the tile. Atomic force microscopy–based force spectroscopy is used to investigate the effect of the inserted DNA tiles on the mechanical properties of the lipid membranes. The results indicate that the insertion of DNA tiles produces an approx. 20% increase of the bilayer breakthrough force

Amphiphilic DNA tiles for controlled insertion and 2D assembly on fluid lipid membranes: Effect on mechanical properties

Future lipid membrane-associated DNA nanostructures are expected to find applications ranging from synthetic biology to nanomedicine. Here we have designed and synthesized DNA tiles and modified them by amphiphilic covalent moieties. dod-DEG groups, which consist of a hydrophilic diethylene glycol (DEG) and a hydrophobic dodecyl group, are introduced at the phosphate backbone to create amphiphilic DNA strands which are subsequently introduced into one face of DNA tiles. In this way the tile becomes able to stably bind to lipid membranes by insertion of the hydrophobic groups inside the bilayer core. The functionalized tiles do not aggregate in solution. Our results show that these amphiphilic DNA tiles can bind and assemble into 2D lattices on both gel and fluid lipid bilayers. The binding of the DNA structures to membranes is dependent on the lipid phase of the membrane, the concentration of Mg2+ cation, the length of the amphiphilic modifications to the DNA as well as on the density of the modifications within the tile. Atomic force microscopy–based force spectroscopy is used to investigate the effect of the inserted DNA tiles on the mechanical properties of the lipid membranes. The results indicate that the insertion of DNA tiles produces an approx. 20% increase of the bilayer breakthrough force

Atomic surface characterisation and modification of the layered compounds Bi2Se3, Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3.

In this work, we show atomic STM images of the layered compound Bi2Se3. We study the effect in the surface of the substitution of 5% and 20% of the Bi atoms for Sb in Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3. The images of the three samples show similar trigonal structures corresponding probably to the van der Waals Se atoms. The distance measured between surface atoms in Bi2Se3 is 4.04 A, in Bi1.9Sb0.1Se3 is 4.16 A and in Bi1.6Sb0.4Se3 is 4.26 A. In Bi1.6Sb0.4Se3 some atomic sites appear brighter than others. The effect is accentuated at higher tunnelling currents and is not observed in the other compounds. Nanoscopic range depressions on the sample might be related to the skeletal crystal structure since the images show atomic corrugations that align slightly in one direction. We explain the results as the effects of the interactions between tip and sample, and discuss two interpretations: on the one hand, localised depression of the individual atomic sites, and on the other the possible elevation of the atoms of the surface due to a phase transition of the compounds induced by STM