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We demonstrate a purely solvent-based approach to assembling CdSe nanorods into vertically aligned, hexagonally packed monolayers in solution. Nanorods were dispersed in a mixture of good solvent with high vapor pressure and bad solvent with low vapor pressure, and preferential evaporation of the good solvent led to ordered assembly under conditions of continuously decreasing solvent quality. No applied external bias, extensive control of drying conditions, exceptionally monodisperse nanoparticles, or high concentrations of additives were required. This clean and facile method yielded ordered nanorod sheets of up to 7.5 μm wide with potential use as active materials in unique applications., (© 2011 American Chemical Society)

With the high demand for nanoelectronic devices, extensive research has focused on the use of single walled carbon nanotubes (CNTs) due to their high electron carrier mobility, large tensile strength, and single nanometer dimensions. Despite their promise, however, their applicability has been greatly hindered by the inherent difficulties of both separating nanotubes of different chiralities and diameters and positioning them from metallic tubes and positioning them in a precise location on a surface. In recent years, single stranded DNA (ssDNA) has been identified as a potential solution for both of these problems since DNA can be used to both separate the different types of CNTs as well as direct their organization. We demonstrate here the first principles on how to guide CNT assembly directly on surfaces from solution by specific DNA hybridization. It was found that the specific DNA sequence used to disperse the carbon nanotubes greatly influences the adsorption and specificity of nanotube binding to the surface. Furthermore, we demonstrate here that thermal annealing can correct misaligned tubes or incorrect binding. These studies provide an excellent foundation for employing two-dimensional DNA templates for CNT organization for nanoelectronic logic and memory based applications. Furthermore, using a single biomaterial to both sort and place CNTs in minimal steps would greatly help the throughput, manufacturability, and cost of such devices.

A new method to engineer unique, solution-based protein diagnostics with femotomole sensitivies from modified bacteriophage is reported. These sensors are highly facile to use, rapid to run, possible to read without any spectroscopic or microscopic analysis, and do not require thermally unstable enzymes. These sensing platforms should be functional in locations with limited access to equipment and facilities.

Artificial DNA nanostructures such as DNA origami have garnered significant interest as templates for sub-20 nm lithography because their rational design allows for the incorporation of binding sites to assemble nanocomponents with 6 nm resolution. In addition, their overall size of 100 nm is easily accessible by top-down lithographic methods. Combining the strengths of top-down lithography and bottom-up self-assembly using DNA nanostructures may provide a commercially viable route to fabricating electronic and photonic devices with nanometer-scale features. We have demonstrated just such a comprehensive process in which 5 nm gold nanoparticles are first assembled in high yield on DNA origami. The constructs are then organized, rinsed, and dried on patterned silicon substrates, yielding large area arrays of both origami and nanoparticles.

This paper reports the one-pot synthesis of perfluorocarbon microbubbles with crosslinked shells of poly(acrylic acid) and phospholipid that boast excellent ultrasound contrast enhancement, enhanced loading capacity, and the ability to retain or release their contents through variation in the level of ultrasound exposure.

In the last decade, "small" and "integrated" have been keywords in the field of device fabrication as the desire to exploit nanoscale phenomena and make electronic, photonic and magnetic arrays has grown. In an effort to improve resolution and control costs, much work has been dedicated to developing alternatives to conventional microfabrication technology. For this purpose, biomolecular assembly and DNA nanotechnology in particular are appealing owing to their inherent size and capacity for molecular recognition. Herein, we review recent achievements in DNA-based directed assembly on substrates. These include novel methods for patterning and depositing nanomaterials on DNA-modified surfaces as well as using synthetic DNA nanostructures such as DNA tiles and origami as templates to direct the assembly of nanoscale components. Particular attention is paid to integrating self-assembly with top-down lithography, and some possible directions for future work are discussed.

With the increasing demand in recent years for high-performance devices for both energy and health applications, there has been extensive research to direct the assembly of nanoparticles into meso- or macroscale single two- and three-dimensional crystals of arbitrary configuration or orientation. Inorganic nanoparticle arrays can have intriguing physical properties that differ from either individual nanoparticles or bulk materials. For most device applications, it is necessary to fabricate two-dimensional nanoparticle superlattices at programmed sites on a surface. However, it has remained a significant challenge to generate patterned arrays with long-range positional order because most highly ordered close-packed nanocrystal arrays are typically obtained by kinetically driven evaporation processes. In this report, we demonstrate a method to generate patterned nanocrystal superlattices by confining nanoparticles to geometrically defined 2-D DNA sites on a surface and using associative biomolecular interparticle interactions to produce thermodynamically stable arrays of hexagonally packed nanocrystals with significant long-range order observed over 1-2 μm. We also demonstrate the role of chemical and geometrical confinement on particle packing and obtaining long-range order. Finally, we also demonstrate that the formation of DNA-mediated nanocrystal superlattices requires both interparticle DNA hybridization and solvent-less thermal annealing.

The development of nanoscale electronic and photonic devices will require a combination of the high throughput of lithographic patterning and the high resolution and chemical precision afforded by self-assembly. However, the incorporation of nanomaterials with dimensions of less than 10 nm into functional devices has been hindered by the disparity between their size and the 100 nm feature sizes that can be routinely generated by lithography. Biomolecules offer a bridge between the two size regimes, with sub-10 nm dimensions, synthetic flexibility and a capability for self-recognition. Here, we report the directed assembly of 5-nm gold particles into large-area, spatially ordered, two-dimensional arrays through the site-selective deposition of mesoscopic DNA origami onto lithographically patterned substrates and the precise binding of gold nanocrystals to each DNA structure. We show organization with registry both within an individual DNA template and between components on neighbouring DNA origami, expanding the generality of this method towards many types of patterns and sizes.

Artificial DNA nanostructures show promise for the organization of functional materials to create nanoelectronic or nano-optical devices. DNA origami, in which a long single strand of DNA is folded into a shape using shorter 'staple strands', can display 6-nm-resolution patterns of binding sites, in principle allowing complex arrangements of carbon nanotubes, silicon nanowires, or quantum dots. However, DNA origami are synthesized in solution and uncontrolled deposition results in random arrangements; this makes it difficult to measure the properties of attached nanodevices or to integrate them with conventionally fabricated microcircuitry. Here we describe the use of electron-beam lithography and dry oxidative etching to create DNA origami-shaped binding sites on technologically useful materials, such as SiO(2) and diamond-like carbon. In buffer with approximately 100 mM MgCl(2), DNA origami bind with high selectivity and good orientation: 70-95% of sites have individual origami aligned with an angular dispersion (+/-1 s.d.) as low as +/-10 degrees (on diamond-like carbon) or +/-20 degrees (on SiO(2)).

One of the most challenging but potentially rewarding goals in nanoscience is the ability to direct the assembly of nanoscale materials into functional architectures with high yields, minimal steps, and inexpensive procedures. Despite their unique physical properties, the inherent difficulties of engineering wafer-level arrays of useful devices from nanoscale materials in a cost-effective manner have provided serious roadblocks toward technological impact. To address nanoscale features while still maintaining low fabrication costs, we demonstrate here an inexpensive printing method that enables repeated patterning of large-area arrays of nanoscale materials. DNA strands were patterned over 4 mm areas with 50 nm resolution by a soft-lithographic subtraction printing process, and DNA hybridization was used to direct the assembly of sub-20 nm materials to create highly ordered two-dimensional nanoparticle arrays. The entire printing and assembly process was accomplished in as few as three fabrication steps and required only a single lithographically templated silicon master that could be used repeatedly. The low-cost procedures developed to generate nanoscale DNA patterns can be easily extended toward roll-to-roll assembly of nanoscale materials with sub-50 nm resolution and fidelity.

In this work, we have developed 11-mercaptoundecanoic acid (MUA)-polypeptide "bilayer" systems by adsorbing poly(diethylene glycol-l-lysine)-poly(l-lysine) (PEGLL-PLL) diblock copolypeptide molecules of various architectures onto MUA-functionalized gold substrates. An objective of our present work is to use the PEGLL-PLL/MUA bilayer as a model system for studying the interfacial phenomena that occur when PEGLL-PLL molecules interact with carboxylic acid (COOH) moieties of nanoparticle ligands. Specifically, we have elucidated the nature of the interactions between the PEGLL-PLL and COOH moieties as well as the resulting polypeptide conformation and organization, using a combination of surface techniques-grazing-incidence IR spectroscopy, ellipsometry, and contact angle. We have also thoroughly characterized other film properties such as the packing and graft density of the polypeptide molecules as a function of the PEGLL-PLL architecture. From the IR data, the adsorption process occurs primarily by means of electrostatic interaction between the protonated PLL residues (pKa approximately 10.6) and carboxylate moieties of the MUA self-assembled monolayer (SAM) (pKa approximately 6) that is enhanced by H-bonding. The PLL block is thought to adopt a random-coil (extended) conformation, while the PEGLL block that is not interacting with the MUA molecules is found to adopt an alpha-helical conformation with an average tilt angle of -60 degrees. The PEGLL-PLL molecules have also been deduced to form a heterogeneous film and adopt liquidlike/disordered packing on the surface. The average contact angle of the MUA-polypeptide bilayer systems is -40 degrees, which implies that the diethylene glycol (EG2) side chains of the PEGLL residues may be oriented somewhat toward the surface normal. From ellipsometry measurements, it is found that PEGLLx-PLLy molecules with a longer alpha-helical block are associated with a lower graft density on the MUA surface compared to those with a shorter alpha-helical block. This observation may be attributed to the greater repulsion-steric and H-bonding effects-that is imposed by the EG2 side chains found on and projected area occupied by the longer PEGLL block. The bilayer systems have been found to be extremely stable over a 2-week period with no changes in the contact angle, thickness, polypeptide tilt angle, or conformation. Beyond that, there is a gradual decrease in the thickness and increase in the contact angle of the bilayer that could be attributed to the oxidation of the MUA SAM molecules.

We report the analogy between the self-assembly properties of amphiphilic phospholipids and the similar behavior observed for quantum dot (CdSe/CdS)-diblock copolypeptide hybrid systems, and the effect of the self-assembly on secondary structures of the polypeptides. At neutral pH, the diblock copolypeptide, poly(diethyleneglycol-l-lysine)-poly(l-lysine), comprises a positively charged poly-l-lysine (PLL) block and a hydrophilic and uncharged poly(diethyleneglycol-l-lysine) (PEGLL) block. By itself, the copolypeptide is not amphiphilic. However, when the polymers are mixed with water-soluble, negatively charged, citrate-functionalized quantum dots (QDs) in water, shell-like structures or dense aggregates are spontaneously formed. Electrostatic and hydrogen-bonding interactions between the positively charged PLL residues and the negatively charged ligands on the QDs lead to charge neutralization of the PLL block, while the PEGLL block remains hydrophilic. As a result, a pseudo "amphiphilic" molecular unit is formed in which the "hydrophobic" and hydrophilic sections constitute the charge-neutralized PLL residues together with the associating QD and the remaining polypeptide residues that are not neutralized, respectively. The generation of these "amphiphilic" molecular units in turn drives the formation of the QD-polypeptide assemblies. Support for this analogy comes from the observed transition in the shape of the assembly from a shell-like structure to a dense aggregate that is very much analogous to the vesicle-to-micelle transition observed in lipid systems. Furthermore, this shape transition can be explained qualitatively using a concept that is analogous to the surfactant number (N = a(hc)/a(hg)), which has been applied extensively in amphiphilic lipid systems. Specifically, as the ratio of the "hydrophobic" area (a(hc)) to the hydrophilic area (a(hg)) decreases, a shape transition from the shell-like structure to the dense aggregate occurs. In addition, the size of the shell-like structure changes as a function of the dimensions of the "amphiphilic" molecular unit in a manner that is similar to how the size of the lipid vesicle changes with the dimensions of the lipid molecule. Circular dichroism (CD) measurements have shown that the PEGLL-PLL molecule has a well-defined secondary structure (alpha-helical PEGLL block and random coil PLL block) that remains virtually unchanged after reacting with the QDs. This finding is consistent with the hypothesis that it is the electrostatic interaction between the amines on the PLL block and the citrate ligands on the QDs that drives the self-assembly.

We investigated the photonic crystal structure inside iridescent scales of the weevil Lamprocyphus augustus. By combining a high-resolution structure analysis technique based on sequential focused ion beam milling and scanning electron microscopy imaging with theoretical modeling and photonic band-structure calculations, we discovered a natural three-dimensional photonic structure with a diamond-based crystal lattice operating at visible wavelengths. Moreover, we found that within individual scales, the diamond-based structure is assembled in the form of differently oriented single-crystalline micrometer-sized pixels with only selected lattice planes facing the scales' top surface. A comparison of results obtained from optical microreflectance measurements with photonic band-structure calculations reveals that it is this sophisticated microassembly of the diamond-based crystal lattice that lends Lamprocyphus augustus its macroscopically near angle-independent green coloration.

This article highlights areas of research at the interface of nanotechnology, the physical sciences, and biology that are related to energy conversion: specifically, those related to photovoltaic applications. Although much ongoing work is seeking to understand basic processes of photosynthesis and chemical conversion, such as light harvesting, electron transfer, and ion transport, application of this knowledge to the development of fully synthetic and/or hybrid devices is still in its infancy. To develop systems that produce energy in an efficient manner, it is important both to understand the biological mechanisms of energy flow for optimization of primary structure and to appreciate the roles of architecture and assembly. Whether devices are completely synthetic and mimic biological processes or devices use natural biomolecules, much of the research for future power systems will happen at the intersection of disciplines.

Because of their nanometer sizes and molecular recognition capabilities, biological systems have garnered much attention as vehicles for the directed assembly of nanoscale materials.(1-6) One of the greatest challenges of this research has been to successfully interface biological systems with electronic materials, such as semiconductors and metals. As a means to address some of these issues, Sarikaya, Belcher, and others have used a combinatorial technique called phage display(7-9) to discover new families of peptides that showed binding affinities to various substrates. More recently, Zheng and co-workers used combinatorial DNA libraries to isolate short DNA oligomers (30-90 bases) that could disperse single-walled carbon nanotubes (SWCNT) in water.(10) Through a systematic analysis, they found that short oligonucleotides having repeating sequences of gunanines and thymines (dGdT)(n) could wrap in a helical manner around a CNT with periodic pitch.(11) Although helix formation around SWCNTs having regular pitches is an effective method for dispersing and separating CNTs, the need for specific repeating sequences limits use to non-natural DNA that must be synthesized with optimal lengths of less than 150 bases. In contrast, we demonstrate here that long genomic single-stranded DNA (>>100 bases) of a completely random sequence of bases can be used to disperse CNTs efficiently through the single-stranded DNA's (ssDNA) ability to form tight helices around the CNTs with distinct periodic pitches. Although this process occurs irrespective of the DNA sequence, we show that this process is highly dependent on the removal of complementary strands. We also demonstrate that although the helix pitch-to-pitch distances remain constant down the length of a single CNT, the distances are variable from one DNA-CNT to another. Finally, we report initial work that shows that methods developed to align long dsDNA can be applied in a similar fashion to produce highly dense arrays of aligned ssDNA-CNT hybrids.

Coassemblies of block copolymers and inorganic precursors offer a path to ordered inorganic nanostructures. In thin films, these materials combined with domain alignment provide highly robust nanoscopic templates. We report a simple path to control the morphology, scaling, and orientation of ordered mesopores in organosilicate thin films through the coassembly of a diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an oligomeric organosilicate precursor that is selectively miscible with PEO. Continuous films containing cylindrical or spherical pores are generated by varying the mixing composition of symmetric PS-b-PEO and an organosilicate precursor. Tuning interfacial energy at both air/film and film/substrate interfaces allows the control of cylindrical pore orientation normal to the supported film surfaces. Our method provides well-ordered mesoporous structures within organosilicate thin films that find broad applications as highly stable nanotemplates.

An unusual aggregation phenomenon that involves positively charged poly(L-lysine) (PLL) and negatively charged gold nanoparticles (Au NPs) is reported. Discrete, submicrometer-sized spherical aggregates are found to form immediately upon combining a PLL solution with gold sol (diameter approximately 14 nm). These PLL-Au NP assemblies grow in size with time, according to light scattering experiments, which indicates a dynamic flocculation process. Water-filled, silica hollow microspheres (outer diameter approximately microns) are obtained upon the addition of negatively charged SiO2 NPs (diameter approximately 13 nm) to a suspension of the PLL-Au NP assemblies, around which the SiO2 NPs form a shell. Structural analysis through confocal microscopy indicates the PLL (tagged with a fluorescent dye) is located in the interior of the hollow sphere, and mostly within the silica shell wall. The hollow spheres are theorized to form through flocculation, in which the charge-driven aggregation of Au NPs by PLL provides the critical first step in the two-step synthesis process ("flocculation assembly"). The SiO2 shell can be removed and re-formed by decreasing and increasing the suspension pH about the point-of-zero charge of SiO2, respectively.

Nanoparticle vesicles were spontaneously assembled from homopolymer polyamine polyelectrolytes and water-soluble, citrate-stabilized quantum dots. The further addition of silica nanoparticles to a solution of quantum dot vesicles generated stable micrometer-sized hollow spheres whose walls were formed of a thick, inner layer of close-packed quantum dots followed by an outer layer of silica. The method employed here to assemble both the nanoparticle vesicles and the hollow spheres is in direct contrast to previous syntheses that use either tailored block copolymers or oil-in-water emulsion templating. We propose that the formation of charge-stabilized hydrogen bonds between the positively charged amines of the homopolymer polyelectrolytes and the negatively charged citrate molecules stabilizing the quantum dots is responsible for the macroscopic phase separation in this completely aqueous system. The ease and processibility of the present approach gives promise for the production of a diverse array of materials ranging in applications from drug delivery to catalysis to micrometer-scale optical devices.

In biological systems such as diatoms and sponges, the formation of solid silica structures with precisely controlled morphologies is directed by proteins and polysaccharides and occurs in water at neutral pH and ambient temperature. Laboratory methods, in contrast, have to rely on extreme pH conditions and/or surfactants to induce the condensation of silica precursors into specific morphologies or patterned structures. This contrast in processing conditions and the growing demand for benign synthesis methods that minimize adverse environmental effects have spurred much interest in biomimetic approaches in materials science. The recent demonstration that silicatein-a protein found in the silica spicules of the sponge Tethya aurantia--can hydrolyse and condense the precursor molecule tetraethoxysilane to form silica structures with controlled shapes at ambient conditions seems particularly promising in this context. Here we describe synthetic cysteine-lysine block copolypeptides that mimic the properties of silicatein: the copolypeptides self-assemble into structured aggregates that hydrolyse tetraethoxysilane while simultaneously directing the formation of ordered silica morphologies. We find that oxidation of the cysteine sulphydryl groups, which is known to affect the assembly of the block copolypeptide, allows us to produce different structures: hard silica spheres and well-defined columns of amorphous silica are produced using the fully reduced and the oxidized forms of the copolymer, respectively.

A protein isolated from a biosilica (shown schematically) catalyzes alkoxysilane polycondensation at neutral pH values and low temperatures. Replacement of either of two specific side chain functionalities (Ser-26 and His-165) significantly diminishes catalysis, supporting a reaction mechanism analogous to that of a well-known enzyme that is highly homologous to the silica protein. These results may be useful in the development of synthetic catalysts for environmentally benign synthesis of polysiloxanes., (© 1999 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.)

Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the "silicatein" (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.