Amy Szuchmacher Blum

Critical phenomena of water bridges in nanoasperity contacts

This article discusses capillary forces measured by scanning force microscopy (SFM), which, as recently reported, show a discontinuous behavior at a low relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible phase change or restructuring transition of bulk water in the interfacial solid–liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusses the origin of the adhesive interactions as a function of relative humidity with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in relative humidity. In the course of this article, we will discuss roughness effects on capillary forces and provide a modified capillary force equation for asperity nanocontacts.

Electrochemically controlled conductance switching in a single molecule: quinone-modified oligo(phenylene vinylene).

Reversible conductance switching in single quinone-oligo(phenylene vinylene) (Q-OPV) molecules was demonstrated using electrochemical STM. The switching was achieved by application of electrochemical potential to the substrate supporting the molecule. The ratio of conductances between the high- and low-conductivity states is over 40. The high-conductivity state is ascribed to strong electron delocalization of the fully conjugated hydroquinone-OPV structure, whereas the low-conductivity state is characterized by disruption of electron delocalization in the quinone-OPV structure.

Comparing the conductivity of molecular wires with the scanning tunneling microscope

Current-voltage characteristics as measured by scanning tunneling microscopy for several different molecular backbones are presented. It is demonstrated that isolated oligo(phenylene ethynylene) molecules have the same measured conductance as oligo(phenylene ethynylene) molecules in a crystalline self-assembled monolayer. This result suggests that previous studies involving relatively large surface areas of self-assembled monolayers can be applied to molecular electronics devices employing small numbers of molecules. In addition, gap resistance measurements are used to rank the molecular conductance of oligo(phenylene ethynylene), oligo(phenylene vinylene), and dodecanedithiol. The observed trend for isolated molecules agrees with earlier large-scale measurements.

Role of Hexahistidine in Directed Nanoassemblies of Tobacco Mosaic Virus Coat Protein

A common challenge in nanotechnology is the fabrication of materials with well-defined nanoscale structure and properties. Here we report that a genetically engineered tobacco mosaic virus (TMV) coat protein (CP), to which a hexahistidine (His) tag was incorporated, can self-assemble into disks, hexagonally packed arrays of disks, stacked disks, helical rods, fibers, and elongated rafts. The insertion of a His tag to the C-terminus of TMV-CP was shown to significantly affect the self-assembly in comparison to the wild type, WT-TMV-CP. Furthermore, the His tag interactions attributed to the alternative self-assembly of His-TMV-CP can be controlled through ethanol and nickel-nitrilotriacetic acid (Ni-NTA) additions as monitored with atomic force microscopy.

Solution Phase Gold Nanorings on a Viral Protein Template

Current studies on materials that exhibit metamaterial properties are mainly focused on lithography-generated 2D substrates. Here we report the successful fabrication of 22 nm gold nanoparticle rings with and without a central nanoparticle assembled on Tobacco Mosaic Virus coat protein disks. These structures are one of the first examples of nanorings produced independently of a substrate and represent the first steps toward the realization of a solution-phase or coatings-based metamaterial.

Templated self-assembly of quantum dots from aqueous solution using protein scaffolds

Short, histidine-containing peptides can be conjugated to lysine-containing protein scaffolds to controllably attach quantum dots (QDs) to the scaffold, allowing for generic attachment of quantum dots to any protein without the use of specially engineered domains. This technique was used to bind quantum dots from aqueous solution to both chicken IgG and cowpea mosaic virus (CPMV), a 30?nm viral particle. These quantum dot?protein assemblies were studied in detail. The IgG?QD complexes were shown to retain binding specificity to their antigen after modification. The CPMV?QD complexes have a local concentration of quantum dots greater than 3000?nmol?ml?1, and show a 15% increase in fluorescence quantum yield over free quantum dots in solution.

Molecular electronics based nanosensors on a viral scaffold

Assembling and interconnecting the building blocks of nanoscale devices and being able to electronically address or measure responses at the molecular level remains an important challenge for nanotechnology. Here we show the usefulness of bottom-up self-assembly for building electronic nanosensors from multiple components that have been designed to interact in a controlled manner. Cowpea mosaic virus was used as a scaffold to control the positions of gold nanoparticles. The nanoparticles were then interconnected using thiol-terminated conjugated organic molecules, resulting in a three-dimensional conductive network. Biotin molecules were attached to the virus scaffold using linkers to act as molecular receptors. We demonstrated that binding avidin to the biotin receptors on the self-assembled nanosensors causes a significant change in the network conductance that is dependent on the charge of the avidin protein.

Quantum Dot Fluorescence as a Function of Alkyl Chain Length in Aqueous Environments

In this work, we examine the dependence of the fluorescence quantum yield of water-soluble CdSe/ZnS quantum dots on the local environment. The hydrophobicity of the local environment was modified by using different alkyl chain lengths in a set of oligo-ethylene glycols. Our results show that the quantum yield of CdSe/ZnS quantum dots is highest for the longest alkyl chain length, suggesting that a more hydrophobic environment is beneficial for generating bright, water-soluble quantum dots.

Electronic Properties of Molecular Memory Circuits on a Nanoscale Scaffold

Significant challenges exist in assembling and interconnecting the building blocks of a nanoscale device and being able to electronically address or measure responses at the molecular level. Here we demonstrate the usefulness of engineered proteins as scaffolds for bottom-up self-assembly for building nanoscale devices out of multiple components. Using genetically engineered cowpea mosaic virus, modified to express cysteine residues on the capsid exterior, gold nanoparticles were attached to the viral scaffold in a specific predetermined pattern to produce specific interparticle distances. The nanoparticles were then interconnected using thiol-terminated conjugated organic molecules, resulting in a three-dimensional network. Network properties were engineered by using molecular components with different I-V characteristics. Networks consisting of molecular wires alone were compared with networks containing voltage controlled molecular switches with two stable conductance states. Using such bistable molecules enabled the formation of switchable molecular networks that could be used in nanoscale memory circuits.

Short ligands offer long-term water stability and plasmon tunability for silver nanoparticles

Four short ligands; cysteine, cysteamine, dithiothrietol and glycine are examined and compared in their ability to stabilize and assemble silver nanoparticles (AgNPs). Transmission Electron Microscopy (TEM) and UV-visible spectroscopy are used to characterize these nanoparticles in terms of their size (7–16 nm), stability, and capacity for inter-particle assembly and plasmon coupling enforced by hydrogen-bonding. The results show that both sulfhydryl and amine groups can interact with the silver nanoparticle surface. The polydispersity of glycine-stabilized AgNPs can be significantly reduced by centrifugal filtration using an appropriate membrane pore size.