Toward high throughput optical metamaterial assemblies.

Optical metamaterials have unique engineered optical properties. These properties arise from the careful organization of plasmonic elements. Transitioning these properties from laboratory experiments to functional materials may lead to disruptive technologies for controlling light. A significant issue impeding the realization of optical metamaterial devices is the need for robust and efficient assembly strategies to govern the order of the nanometer-sized elements while enabling macroscopic throughput. This mini-review critically highlights recent approaches and challenges in creating these artificial materials. As the ability to assemble optical metamaterials improves, new unforeseen opportunities may arise for revolutionary optical devices.

Virus-templated plasmonic nanoclusters with icosahedral symmetry via directed self-assembly.

The assembly of plasmonic nanoparticles with precise spatial and orientational order may lead to structures with new electromagnetic properties at optical frequencies. The directed self-assembly method presented controls the interparticle-spacing and symmetry of the resulting nanometer-sized elements in solution. The self-assembly of three-dimensional (3D), icosahedral plasmonic nanosclusters (NCs) with resonances at visible wavelengths is demonstrated experimentally. The ideal NCs consist of twelve gold (Au) nanospheres (NSs) attached to thiol groups at predefined locations on the surface of a genetically engineered cowpea mosaic virus with icosahedral symmetry. In situ dynamic light scattering (DLS) measurements confirm the NSs assembly on the virus. Transmission electron micrographs (TEM) demonstrate the ability of the self-assembly method to control the nanoscopic symmetry of the bound NSs, which reflects the icosahedral symmetry of the virus. Both, TEM and DLS show that the NCs comprise of a distribution of capsids mostly covered (i.e., 6-12 NS/capsid) with NSs. 3D finite-element simulations of aqueous suspensions of NCs reproduce the experimental bulk absorbance measurements and major features of the spectra. Simulations results show that the fully assembled NCs give rise to a 10-fold surface-averaged enhancement of the local electromagnetic field.

Molecular sensing: modulating molecular conduction through intermolecular interactions.

We observe changes in the molecular conductivity of individual oligophenylene-vinylene (OPV) molecules due to interactions with small aromatic molecules. Fluorescence experiments were correlated with scanning tunneling microscopy measurements in order to determine the origin of the observed effect. Both nitrobenzene and 1,4-dinitrobenzene decreased fluorescence intensity and molecular conductivity, while toluene had no effect. The observed changes in the fluorescence and conduction of OPV correlate well with the electron withdrawing ability of the interacting aromatic molecules. These results demonstrate the potential usefulness of OPV as a sensor for aromatic compounds containing electron withdrawing groups.

Biotemplating rod-like viruses for the synthesis of copper nanorods and nanowires.

BACKGROUND: In the past decade spherical and rod-like viruses have been used for the design and synthesis of new kind of nanomaterials with unique chemical positioning, shape, and dimensions in the nanosize regime. Wild type and genetic engineered viruses have served as excellent templates and scaffolds for the synthesis of hybrid materials with unique properties imparted by the incorporation of biological and organic moieties and inorganic nanoparticles. Although great advances have been accomplished, still there is a broad interest in developing reaction conditions suitable for biological templates while not limiting the material property of the product. RESULTS: We demonstrate the controlled synthesis of copper nanorods and nanowires by electroless deposition of Cu on three types of Pd-activated rod-like viruses. Our aqueous solution-based method is scalable and versatile for biotemplating, resulting in Cu-nanorods 24-46 nm in diameter as measured by transmission electron microscopy. Cu2+ was chemically reduced onto Pd activated tobacco mosaic virus, fd and M13 bacteriophages to produce a complete and uniform Cu coverage. The Cu coating was a combination of Cu0 and Cu2O as determined by X- ray photoelectron spectroscopy analysis. A capping agent, synthesized in house, was used to disperse Cu-nanorods in aqueous and organic solvents. Likewise, reactions were developed to produce Cu-nanowires by metallization of polyaniline-coated tobacco mosaic virus. CONCLUSIONS: Synthesis conditions described in the current work are scalable and amenable for biological templates. The synthesized structures preserve the dimensions and shape of the rod-like viruses utilized during the study. The current work opens the possibility of generating a variety of nanorods and nanowires of different lengths ranging from 300 nm to micron sizes. Such biological-based materials may find ample use in nanoelectronics, sensing, and cancer therapy.

Critical field strength in an electroclinic liquid crystal elastomer.

We elucidate the polymer dynamics of a liquid crystal elastomer based on the time-dependent response of the pendent liquid crystal mesogens. The molecular tilt and switching time of mesogens are analyzed as a function of temperature and cross-linking density upon application of an electric field. We observe an unexpected maximum in the switching time of the liquid crystal mesogens at intermediate field strength. Analysis of the molecular tilt over multiple time regimes correlates the maximum response time with a transition to entangled polymer dynamics at a critical field strength.

Nanoparticle networks as chemoselective sensing devices.

We theoretically analyzed transport properties of a molecular network constructed of gold nanoparticles linked with oligophenylenevinulene (OPV) molecules. We showed that the conductance of such system was strongly reduced when trinitrotoluene (TNT) became attached to the OPV linkers in the network. The reported results are based on the ab initio electronic structure calculations. These results corroborate and elucidate experiments which revealed significant drops in the conductance the network while the latter was exposed to TNT vapors. The results suggest that the detected sensitivity of transport characteristics of the considered nanoparticle network to TNT may be used to design a sensing nanodevice.

Toward single molecule detection of staphylococcal enterotoxin B: mobile sandwich immunoassay on gliding microtubules.

An immunoassay based on gliding microtubules (MTs) is described for the detection of staphylococcal enterotoxin B. Detection is performed in a sandwich immunoassay format. Gliding microtubules carry the antigen-specific "capture" antibody, and bound analyte is detected using a fluorescent viral scaffold as the tracer. A detailed modification scheme for the MTs postpolymerization is described along with corresponding quantification by fluorescence spectroscopy. The resultant antibody-MTs maintain their morphology and gliding capabilities. We report a limit of detection down to 0.5 ng/mL during active transport in a 30 min assay time and down to 1 ng/mL on static surfaces. This study demonstrates the kinesin/MT-mediated capture, transport, and detection of the biowarfare agent SEB in a microfluidic format.

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.

A cowpea mosaic virus nanoscaffold for multiplexed antibody conjugation: application as an immunoassay tracer.

Cowpea mosaic virus (CPMV), an icosahedral 30 nm virus, offers a uniquely programmable biological nanoscaffold. This study reports initial optimization of the simultaneous modification of two CPMV mutants with AlexaFluor 647 fluorescent dyes and either IgG proteins or antibodies at specific sites on the virus scaffold. The capacity of CPMV as a simultaneous carrier for different types of molecules was demonstrated, specifically, when applied as a tracer in direct and sandwich immunoassays. The ability to label the virus capsid with antibody and up to 60 fluorescent dyes resulted in an improved limit of detection in SEB sandwich immunoassays, when used as a tracer, relative to a mole equivalent of dye-labeled antibody.

An engineered virus as a bright fluorescent tag and scaffold for cargo proteins--capture and transport by gliding microtubules.

We have demonstrated substantial capture and transport of fluorescently-labeled engineered cowpea mosaic virus (CPMV) using Drosophila kinesin-driven microtubules (MTs). The capture occurred through both NeutrAvidin (NA)-biotin and antibody (IgG)-antigen interactions. The MTs were derivatized with rabbit anti-chicken IgG or biotin, and the virus was conjugated with chicken IgG or NA. The CPMV conjugate was introduced into standard MT motility assays via convective flow at concentrations as high as 1.36 nM, and became bound to the MTs in densities as high as one virus per microm of MT length. When the CPMV conjugate was present at 17 pM, the average speed of the MTs bearing the NA-virus was 0.59 +/- 0.08 microm/sec, and that of those bearing IgG-virus was 0.52 +/- 0.15 microm/sec. These speeds are comparable to those of the unladen MTs (0.61 +/- 0.09 microm/sec), the presence of the virus on the MT causing only a small decrease in MT gliding speeds. The fluorescent CPMV appears to be superior to fluorescent polystyrene spheres of the same size, as both a reporter tag and a scaffold for MT-transported cargo proteins, because of its negligible non-specific adsorption and superior brightness. This work is important for the development of sensors based on nanolocomotion and biological recognition, or new strategies for the nanoassembly of biological structures.

A technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates.

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface. The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates. The flow is mediated by a surface tension gradient and creates macroscopic, high-density, monolayer nanoparticle-ligand films. This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

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.

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.

Virus hybrids as nanomaterials for biotechnology.

The current review describes advances in the field of bionanotechnology in which viruses are used to fabricate nanomaterials. Viruses are introduced as protein cages, scaffolds, and templates for the production of biohybrid nanostructured materials where organic and inorganic molecules are incorporated in a precise and a controlled fashion. Genetic engineering enables the insertion or replacement of selected amino acids on virus capsids for uses from bioconjugation to crystal growth. The variety of nanomaterials generated in rod-like and spherical viruses is highlighted for tobacco mosaic virus (TMV), M13 bacteriophage, cowpea chlorotic mottle virus (CCMV), and cowpea mosaic virus (CPMV). Functional biohybrid nanomaterials find applications in biosensing, memory devices, nanocircuits, light-harvesting systems, and nanobatteries.

Strain analysis of a chiral smectic-A elastomer.

We present a detailed analysis of the molecular packing of a strained liquid crystal elastomer composed of chiral mesogens in the smectic-A phase. X-ray diffraction patterns of the elastomer collected over a range of orientations with respect to the x-ray beam were used to reconstruct the three-dimensional scattering intensity as a function of tensile strain. We show that the smectic domain order is preserved in these strained elastomers. Changes in the intensity within a given scattering plane are due to reorientation, and not loss, of the molecular order in directions orthogonal to the applied strain. Incorporating the physical parameters of the elastomer, a nonlinear elastic model is presented to describe the rotation of the smectic-layered domains under strain, thus providing a fundamental analysis to the mechanical response of these unique materials.

Role of surfactant in the stability of liquid crystal-based nanocolloids.

We examine the dependence of liquid crystalline nanocolloid formation and stability on surfactant. Nanocolloids composed of polymerizable liquid crystal mesogens and cross-linking agents and capped with either ionic or nonionic surfactants are prepared via the miniemulsion technique. Colloids synthesized with anionic surfactant were stable and displayed 2D hexagonal packing when deposited via slow vertical pulling of the silicon substrate from an aqueous suspension. Liquid crystal nanocolloids stabilized with the nonionic, polar polymer polyvinyl alcohol (PVA) were stable in aqueous environments but coalesced upon drying to form relatively large, well-defined crystal-like structures with uniform birefringence. SEM images reveal that the coalesced structures have mesalike features. Polarized light, atomic force, and polarized Raman microscopy of these structures indicate that the liquid crystal molecules are arranged with their long molecular axis slightly tilted with respect to the surface normal. A mechanism is proposed to explain the formation of the mesalike structures from the nanocolloids. These studies provide fundamental insight into the incorporation and stabilization of polymerizable liquid crystal molecules into nanovolumes and open up opportunities for the incorporation of functionality and anisotropy into isotropically shaped nanocolloids.

Spectral tuning of organic nanocolloids by controlled molecular interactions.

The controlled self-assembly of molecules and interactions between them remain a challenge in creating tunable and functional organic nanostructures. One class of molecular systems that has proven useful for incorporating tunable functionality at different length scales is liquid crystals (LCs) due to its ability to inherently self-organize. Here we present a novel approach to utilize the self-assembly of polymerizable liquid crystals to control the molecular aggregation of stable fluorescent chromophores and create a unique class of organic fluorescent nanocolloids. By adjusting the ratio between the dye and LC molecules inside the nanocolloids, we demonstrate the ability to control the molecular interactions and tune the fluorescent emission spectra of nanocolloid populations under single wavelength excitation. The single absorption spectrum and multiple emission spectra are highly desirable and reminiscent of the spectroscopic signature of quantum dots. These novel fluorescent nanocolloids have broad potential applications in fluorescent imaging and biological labeling.

Cowpea mosaic virus nanoscaffold as signal enhancement for DNA microarrays.

Previous studies have shown that a functionalized viral nanoparticle can be used as a fluorescent signal-generating element and enhance detection sensitivity for immunoassays and low density microarrays. In this study, we further tested this ability in commercial DNA microarrays, including Affymetrix high density resequencing microarray. Optimum conditions for NeutrAvidin and dye coupling to a double-cysteine mutant of cowpea mosaic virus (CPMV) were found to be comparable to the commonly used streptavidin-phycoerythrin (SAPE) for high density resequencing microarray. A 3-fold signal enhancement in comparison to Cy5-dCTP controls was obtained when using nanoparticles on control scorecard expression microarrays. Hybridization results from commercially available 8000 rat expression arrays indicate an increment of 14% on the detected features when the virus complex was used as the staining reagent in comparison to Cy5-dCTP controls. The current work shows the utility of the CPMV-dye nanoparticles as a detection reagent in well-established detection platforms.

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.