Turning colloidal gold into clathrates

Clathrates contain extended pore structures that can trap other molecules. Lin et al. created colloidal analogs of clathrates in which bipyramidal gold nanoparticles functionalized with DNA molecules assembled into polyhedral clusters to create open-pore structures (see the Perspective by Samanta and Klajn). These clathrate colloidal crystals exhibit extraordinary structural complexity and substantially broaden both the scope and the possibilities provided by DNA-inspired methodologies.

Science, this issue p. 931; see also p. 912

Controlled colloid bonding using DNA

Colloidal particles can act as analogs of atoms for studying crystallization and packing behavior, but they don't naturally bond together the way atoms do. Short strands of DNA are one versatile way to link together colloidal particles (see the Perspective by Tao). Kim et al. designed a series of gold colloids with DNA ligands that reversibly bound to or released neighboring particles via DNA strands that opened or closed hairpin loops. Liu et al. devised a set of DNA strands that pack into origami structures. Inside each structure were strands that cage a gold nanoparticle. These were further linked to other uncaged nanoparticles to assemble a diamond-like structure. Changing the strand design yielded a wide range of sparsely packed colloidal crystals.

Science, this issue p. 561, p. 579; see also p. 582

Multimetal nanoparticle synthesis

Multicomponent nanoparticles can be difficult to synthesize. Rather than mixing in one type of particle, the compounds often separate and form distinct particles. Using dip-pen lithography, Chen et al. show how adding reactants to very small volumes forces the reactants to form single particles containing various combinations of five different transition metal ions. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy revealed the shapes of the nanoparticles and how metallic composition varied within them. For example, the quinary particle containing gold, silver, cobalt, copper, and nickel consisted of three domains of binary alloys.

Science, this issue p. 1565

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Valency and bonding on a larger scale

In molecular systems, valency describes the number of bonds an atom can make with its neighbors. Larger objects such as colloids can be linked together to make connected structures in which the number of connections, or valency, is controlled by the central object. Jones et al. review the two main approaches to creating stiff bonds, based on DNA-based materials synthesis. These approaches allow the construction of molecular-like objects from building blocks much larger than single atoms.

Science, this issue 10.1126/science.1260901

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Sticking with DNA

One strategy for creating superlattices from nanoparticles is to coat the particles with DNA strands that have sticky ends that can be exploited to control the assembly of the lattice. This method can create binary lattices, but now Macfarlane et al. (p. 1222, published online 22 August) have succeeded in inserting a third type of nanoparticle into a predetermined site by tuning the strength of the relative DNA binding interactions.

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siRNA-based spherical nucleic acid (SNA) nanoparticle conjugates silence antiapoptosis signaling and impair GBM progression.

Summary

In 2010, we and our colleagues on the President's Council of Advisors on Science and Technology (PCAST) released a report entitled Prepare and Inspire: K-12 Education in Science, Technology, Engineering and Math (STEM) Education for America's Future. This important report advocates preparing all students to use STEM in their personal and professional lives and inspiring them to learn STEM subjects and pursue STEM careers. But in the United States, over 60% of students who enter college intending to major in a STEM field fail to graduate with a STEM degree. Because economic analyses forecast that the United States will need 1 million more STEM graduates over the next decade than will be produced by our current modes of education,* reducing this dropout rate is the focus of a second report we released last month, called Engage to Excel: Producing One Million Additional College Graduates with Degrees in Science, Technology, Engineering, and Mathematics.†

Design rules allow the synthesis of nanoparticle-DNA superlattices in nine different lattices.

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Nanostructures fabricated by a novel technique, termed On-Wire Lithography (OWL), can be combined with organic and biological molecules to create systems with emergent and highly functional properties. OWL is a template-based, electrochemical process for forming gapped cylindrical structures on a solid support, with feature sizes (both gap and segment length) that can be controlled on the sub-100 nm length scale. Structures prepared by this method have provided valuable insight into the plasmonic properties of noble metal nanomaterials and have formed the basis for novel molecular electronic, encoding, and biological detection devices.

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We report a low-cost, high-throughput scanning probe lithography method that uses a soft elastomeric tip array, rather than tips mounted on individual cantilevers, to deliver inks to a surface in a “direct write” manner. Polymer pen lithography merges the feature size control of dip-pen nanolithography with the large-area capability of contact printing. Because ink delivery is time and force dependent, features on the nanometer, micrometer, and macroscopic length scales can be formed with the same tip array. Arrays with as many as about 11 million pyramid-shaped pens can be brought into contact with substrates and readily leveled optically to ensure uniform pattern development.

Außen ist Silber, innen ist Gold: Mit Goldnanopartikeln als Keimen wurde der Einfluss der Plasmonenanregung beim photochemischen Wachstum von Silbernanoprismen untersucht. Dieser Ansatz ergibt neuartige Kern-Schale-Nanostrukturen, in denen ein sphärischer (siehe TEM-Bild; Einschub: Seitenansicht) oder dreieckig-prismatischer Goldkern von einer dreieckig-prismatischen Silberschale umgeben ist. Die Partikelstruktur lässt sich über die Anregungswellenlänge und den Durchmesser des Goldkerns steuern.

Color is everything: Hg²⁺ in aqueous media is detected by the formation of thymidine–Hg²⁺–thymidine coordination complexes, which raises the melting temperature of the DNA-hybridized gold nanoparticle probes and thus the temperature at which the probes disperse and effect a purple-to-red color change. The method has very high sensitivity and selectivity, and it provides a simple and fast colorimetric readout (see picture).

A pHunction of pH: The silver nanoprism fusion process can be turned on and off as a function of the pH value during photochemical synthesis. With appropriate pH regulation (see TEM images, middle and right), one can achieve excellent control over the nanoprism edge length with a fixed 10-nm thickness and the corresponding plasmon bands, which span the visible and NIR range (see extinction spectra, left).

On the small screen: Aggregates of gold nanoparticles interconnected by DNA duplexes (see scheme, left) are bluish-purple. Cleavage of the duplexes by deoxyribonuclease I (DNase I) releases the nanoparticles (right), producing a bluish-purple-to-red color change. This color change can be used to monitor the activity (and inhibition) of the enzyme.

Outside silver, inside gold: Gold nanoparticles are used as seeds to probe the role of plasmon excitation in the photomediated growth of silver nanoprisms. This approach generates novel core–shell nanostructures that contain a spherical (see TEM image, inset shows side view) or triangular-prism gold core with a triangular silver prism shell. The architecture of these particles can be tuned by controlling the excitation wavelength and gold-core diameter.

The spontaneous formation of the heteroligated complex [PtCl(κ²-Ph₂PCH₂CH₂SMe)(Ph₂PCH₂CH₂SPh)]Cl (8 a) by a novel ligand rearrangement process has been observed. By using the weak-link approach, the relative arrangement of the alkyl and aryl groups can be controlled by abstraction of chloride from 8 a to form the closed complex [Pt(κ²-Ph₂PCH₂CH₂SMe)(κ²-Ph₂PCH₂CH₂SPh)][BF₄]₂ (5) and reopening using halide ions to form semi-open complexes [PtX(κ²-Ph₂PCH₂CH₂SMe)(Ph₂PCH₂CH₂SPh)]BF₄ (8 b; X=Cl⁻) and (8 c; X=I⁻). Analogous procedures using Ph₂PCH₂CH₂SMe and 1,4-(Ph₂PCH₂CH₂S)₂C₆H₄ lead to heteroligated bimetallic complexes 7 and 9, illustrating that this ligand rearrangement process can be used as a tool for the assembly of complementary metallosupramolecular structures.

Designer nanoparticle conjugates. Locked nucleic acid (LNA) oligonucleotides can be used to functionalize nanoparticles to yield a novel conjugate that is highly stable and has a remarkably high affinity for complementary nucleic acids. When tested in a cell line model, these conjugates were found to be effective at controlling survivin gene expression.

Ganz nach Wahl: Das Verschmelzen von Silber-Nanoprismen kann als Funktion des pH-Werts bei der photochemischen Synthese an- und ausgeschaltet werden. Bei geeigneter pH-Regulierung (siehe TEM-Bilder, Mitte und rechts) lassen sich die Kantenlänge der Nanoprismen bei konstanter Dicke von 10 nm und die entsprechenden Plasmonenbanden, die den sichtbaren und NIR-Bereich überstreichen (siehe Extinktionspektren, links), genau einstellen.

Farbnachweis: Aggregate von Goldnanopartikeln, die durch DNA-Duplexe verknüpft sind (siehe Schema, links), sind violett. Beim Spalten der Duplexe durch die Desoxyribonuclease I (DNase I) werden die Nanopartikel freigesetzt (rechts), was eine Farbänderung von Violett nach Rot zur Folge hat. Diese Farbänderung kann zum Verfolgen der Aktivität (und Inhibierung) des Enzyms genutzt werden.

Farbe ist alles: Hg²⁺ lässt sich in wässrigen Medien durch die Bildung von Thymidin-Hg²⁺-Thymidin-Koordinationskomplexen nachweisen, die die Schmelztemperatur der DNA-hybridisierten Goldnanopartikelsonden erhöht und damit die Temperatur, bei der die Sonden unter Farbänderung von Violett zu Rot dispergieren. Das Verfahren ist sehr empfindlich und selektiv und ermöglicht ein einfaches und schnelles kolorimetrisches Auslesen (siehe Bild).

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We describe the use of gold nanoparticle-oligonucleotide complexes as intracellular gene regulation agents for the control of protein expression in cells. These oligonucleotide-modified nanoparticles have affinity constants for complementary nucleic acids that are higher than their unmodified oligonucleotide counterparts, are less susceptible to degradation by nuclease activity, exhibit greater than 99% cellular uptake, can introduce oligonucleotides at a higher effective concentration than conventional transfection agents, and are nontoxic to the cells under the conditions studied. By chemically tailoring the density of DNA bound to the surface of gold nanoparticles, we demonstrated a tunable gene knockdown.

Amine-active N-hydroxysuccinimide-terminated alkyl thiol templates are generated using parallel dip-pen nanolithography (DPN) and are used to covalently couple protein A/G. The protein arrays generated (see figure) are used to capture antibodies through affinity binding, while preserving their biological recognition properties. The versatility of the parallel DPN method for making many similar structures in a relatively high-throughput manner (14 000 dots in 10 min) is described.

The synthesis of nanoparticles asymmetrically functionalized with oligonucleotides is reported. The method provides excellent control over the placement of oligonucleotides on the surface of only one hemisphere of each particle (see figure). This new synthetic capability allows one to introduce valency into such structures and then use that valency to direct particle-assembly events.

A straightforward method is investigated for controlling and reinitiating the growth of single-crystalline Au nanoprisms. This work is based on seeding methodology, and depends on the slow reduction of metal ions onto the surface of a growing nanoprism. In this manner, we can tailor the edge length of Au nanoprisms between 100 and 300 nm without changing their thickness or crystallinity. Each nanoprism size has been characterized by UV-vis-NIR (NIR: near-IR) spectroscopy, transmission electron microscopy (TEM) techniques, and statistical analysis. Based on this work and existing silver halide crystal-growth theories, a preliminary mechanism is proposed which comments on the interplay between crystal growth and surface chemistry that ultimately dictates the morphology of the resulting nanostructure.

Sorting the mail: A highly selective assay for the detection of multiple DNA targets in solution was developed based on gold nanoparticles with oligonucleotide reporters. Four DNA targets associated with infectious viruses were detected simultaneously without enzymatic amplification at mid-femtomolar concentrations.

Immobile but active. Unmodified antibodies can be immobilized in an active state on arrays of patterned features coupled with metal-ions by using dip-pen nanolithography and microcontact printing. Their immunoreactivity was determined by atomic force and fluorescence microscopy after the arrays had been exposed to their corresponding protein or virus antigens. An example of a functional antibody array used to capture the influenza virus is shown in the AFM image.

Schreibstube plus: Einfach und flexibel präsentiert sich eine zweidimensionale Anordnung von 55 000 Rasterkraftmikroskop(AFM)-Federarmen als „Stifte“ bei der Hochdurchsatzbemusterung großer Flächen durch direktes Aufbringen von Molekülen. Dank pyramidaler Spitzen mit niedrigem Seitenverhältnis, gebogener Federarme und einer neuartigen Ausrichtungsmethode unter Nutzung der Schwerkraft können 1 cm² große Molekülmuster mit einer Auflösung unter 100 nm erzeugt werden.

DNA-verknüpfte Nanopartikelaggregate eignen sich zum colorimetrischen Screening DNA bindender Moleküle. Die Schmelztemperatur der Nanopartikelaggregate steigt in Gegenwart von Molekülen, die DNA stark binden. Die Stärke der Bindung an DNA ergibt sich daher aus der Verschiebung dieses scharfen Schmelzübergangs, der sich in einem Farbwechsel von Blau nach Rot ausdrückt (siehe Bild).

Pyromellitic diimide and naphthalene diimide moieties were incorporated into hemilabile phosphanylalkyl thioether ligands. These ligands reacted with [Cu(CH₃CN)₄]PF₆ and [Rh(NBD)Cl]₂ (NBD=norbornadiene) by the weak-link approach to form condensed intermediates. Upon reaction of each diimide ligand with these transition-metal precursors, the two diimide units became cofacially aligned within a supramolecular macrocyclic architecture. The introduction of ancillary ligands to each of these condensed intermediates caused the weak thioether–metal bonds to break, thus generating a large macrocycle in which the distance between diimide units is significantly larger than for the condensed intermediates. The two Rh^I cationic condensed intermediates were characterized by single-crystal X-ray diffraction studies, and the electrochemical activity of these macrocycles was demonstrated with the naphthalene diimide–Cu^I macrocycles.

In the science and engineering communities, the nanoscience revolution is intensifying. As many types of nanomaterials are becoming more reliably synthesized, they are being used for novel applications in all branches of nanoscience and nanotechnology. Since it is sometimes desirable for single nanomaterials to perform multiple functions simultaneously, multicomponent nanomaterials, such as core–shell, alloyed, and striped nanoparticles, are being more extensively researched. Nanoscientists hope to design multicomponent nanostructures and exploit their inherent multiple functionalities for use in many novel applications. This review highlights recent advances in the synthesis of multisegmented one-dimensional nanorods and nanowires with metal, semiconductor, polymer, molecular, and even gapped components. It also discusses the applications of these multicomponent nanomaterials in magnetism, self-assembly, electronics, biology, catalysis, and optics. Particular emphasis is placed on the new materials and devices achievable using these multicomponent, rather than single-component, nanowire structures.

Schwache Verknüpfungen wurden bei der Synthese eines vierfach koordinierten Cu^I-Komplexes zur chiralen Erkennung von Mandelsäure genutzt. Die Chelatbildung mit 2,2′-Bipyridin in einer Reaktion, bei der zugleich die schwachen Cu-S-Verknüpfungen gebrochen und die kondensierte Struktur zu einem 27-gliedrigen Makrocyclus geöffnet werden, wandelt den Komplex in einen enantioselektiven fluoreszierenden Rezeptor um.

Postsortierung: Ein hochselektiver Assay für den Nachweis mehrerer DNA-Zielmoleküle in Lösung wurde auf der Grundlage von Goldnanopartikeln mit Oligonucleotidreportern entwickelt. Vier solche Zielmoleküle, die zu infektiösen Viren gehören, wurden simultan und ohne enzymatische Amplifizierung in mittleren femtomolaren Konzentrationen nachgewiesen.

Nanomaterialien gewinnen in den Natur- und Ingenieurwissenschaften rasant an Stellenwert. Für viele derartige Materialien sind inzwischen zuverlässige Synthesen verfügbar, die vielfältige Anwendungen in allen Bereichen von Nanowissenschaften und Nanotechnologie ermöglicht haben. Bisweilen ist es erwünscht, dass ein einziges Nanomaterial mehrere Funktionen zugleich ausübt, sodass sich die Forschung zu Mehrkomponenten-Nanomaterialien wie Kern-Schale- oder Legierungsnanopartikeln sowie gestreiften Nanodrähten intensiviert. Solche multifunktionalen Strukturen bieten vielversprechende Perspektiven für die Entwicklung von neuartigen Anwendungen. Dieser Aufsatz präsentiert aktuelle Fortschritte bei der Synthese vielsegmentiger eindimensionaler Nanostäbe und -drähte mit Metallen, Halbleitern, Polymeren, Molekülen und sogar Lücken als Komponenten. Zudem werden Anwendungen dieser Mehrkomponenten-Nanomaterialien in den Bereichen Magnetismus, Selbstorganisation, Elektronik, Biologie, Katalyse und Optik vorgestellt. Ein besonderer Schwerpunkt liegt auf neuen Materialien und Anwendungen, bei denen die Vorteile von Mehrkomponenten-Nanodrähten gegenüber Einkomponentenstrukturen zutage treten.

Ionenbälle: Die Zusammensetzung von Nano- und Mikropartikeln aus kontinuierlichen Koordinationspolymeren kann chemisch durch Ionenaustauschreaktionen verändert werden, ohne dass sich Partikelgröße und -morphologie wesentlich ändern. Auf diese Art lässt sich ein Satz an Partikelvorstufen einfach erhalten, dessen Zusammensetzung gezielt in die eines neuen Satzes an Verbindungen mit anderen Eigenschaften überführt werden kann.

Ion balls: The composition of nano- and microparticles made from infinite coordination polymers can be chemically transformed through ion-exchange reactions without substantially changing the particle size and morphology. This approach is a straightforward and useful way of creating one precursor set of particles whose composition can be controllably transformed into that of a new set of compounds with different properties.

Lots of writing on the wall: A simple and flexible approach for performing high-throughput, large-area, direct-write molecular patterning, without tip feedback, is demonstrated by using a 55 000-pen two-dimensional array of atomic force microscope (AFM) cantilevers. The use of low-aspect-ratio pyramidal tips, curved cantilevers, and a novel gravity-driven alignment method allows parallel patterning of molecules across 1-cm² substrate areas at sub-100-nm resolution.

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We report a high-throughput procedure for lithographically processing one-dimensional nanowires. This procedure, termed on-wire lithography, combines advances in template-directed synthesis of nanowires with electrochemical deposition and wet-chemical etching and allows routine fabrication of face-to-face disk arrays and gap structures in the range of five to several hundred nanometers. We studied the transport properties of 13-nanometer gaps with and without nanoscopic amounts of conducting polymers deposited within by dip-pen nanolithography.

Silver nanoprisms with a unimodal size distribution are synthesized through a novel thermal route that permits some control over nanoprism-edge length (see Figure). The influences of citrate, poly(vinylpyrrolidone), H₂O₂, and NaBH₄ on the nanostructures are described. This protocol allows control over nanoprism thickness, an architectural parameter not accessible via known preparative methods for such structures.

Polyelectrolyte multilayer (PEM) organic thin films with diameters ranging from 80 to 200 nm can be generated from dip-pen nanolithography (DPN) fabricated templates. Through the use of multi-pen AFM cantilever probes, parallel fabrication of PEM features with nanoscale resolution can be achieved. This demonstrates the versatility of the parallel DPN approach and its applicability to building nano-/microscale structures in conjunction with a layer-by-layer method.

Ein Mosaikmuster: Nanoskalige Anordnungen aus einzelnen Viruspartikeln lassen sich durch das Positionieren und Orientieren von Viruspartikeln auf Templatoberflächen erzeugen, die durch Dip-Pen-Nanolithographie hergestellt wurden. Die Immobilisierung wurde durch Antikörper-Virus-Erkennung und IR-Spektroskopie nachgewiesen (ein Beispiel ist im AFM-Bild zu sehen).

Biologische Template: Diatomeenzellwände wurden mit Metallfilmen überzogen und anschließend aufgelöst, was nanostrukturierte Metallmikroschalen mit den Strukturmerkmalen der Diatomeentemplate lieferte. Wegen ihrer einzigartigen Nanostrukturdetails (siehe Bild) sind diese Mikroschalen nützliche Substrate für die oberflächenverstärkte Raman-Streuung.

A halide-induced ligand rearrangement around a Rh^I center can be used for the preparation of heteroligated tweezer-type complexes. By using a variety of P,S- and P,O-hemilabile ligands, the complexes can be interconverted between four different Rh^I-coordination environments (see scheme).

Biological templates: Diatom cell walls were coated with metallic films and then subsequently dissolved to yield nanostructured metallic microshells that exhibit the structural characteristics of the diatom templates. The unique nanostructural details of the microshells (see image) make them useful as substrates for surface-enhanced Raman scattering.

Eine Halogenid-induzierte Ligandenumlagerung an einem Rh^I-Zentrum eignet sich zur Synthese heterokoordinierter Komplexe vom Pinzettentyp. Mithilfe einer Vielzahl an P,S- und P,O-hemilabilen Liganden lassen sich Umwandlungen zwischen vier verschiedenen Rh^I-Koordinationsumgebungen erreichen (siehe Schema).

Mikroorganismen weisen den Weg: Mit einfachen chemischen Verfahren wurden die Silicatzellwände mehrerer Diatomeen kovalent mit DNA funktionalisiert, um als Template für die sequenzspezifische Zusammenlagerung und 3D-hierarchische Anordnung vorab gebildeter DNA-modifizierter Gold-Nanopartikel zu fungieren. Die DNA diente außerdem dazu, mehrere Nanopartikelschichten auf den Diatomeentemplaten aufzubauen (siehe Bild).

Microorganisms point the way: The silica cell walls of various diatoms were covalently functionalized with DNA by using straightforward chemical procedures and then used as templates for the sequence-specific assembly and 3D hierarchical arrangement of prefabricated DNA-modified gold nanoparticles. The DNA was used to further direct the assembly of multiple nanoparticle layers onto the diatom templates (see picture).