Stepwise surface encoding for high-throughput assembly of nanoclusters

Facile and efficient preparation of anisotropic DNA-functionalized gold nanoparticles and their regioselective assembly

Anisotropic nanoparticles can provide considerable opportunities for assembly of nanomaterials with unique structures and properties. However, most reported anisotropic nanoparticles are either difficult to prepare or to functionalize. Here we report a facile one-step solution-based method to prepare anisotropic DNA-functionalized gold nanoparticles (a-DNA-AuNP) with 96% yield and with high DNA density (120 ± 20 strands on the gold hemisphere). The method is based on the competition between a thiolated hydrophilic DNA and a thiolated hydrophobic phospholipid and has been applied to prepare a-DNA-AuNPs of different sizes and with a variety of DNA sequences. In addition, DNA strands on the a-DNA-AuNPs can be exchanged with other DNA strands with a different sequence. The anisotropic nature of the a-DNA-AuNPs allows regioselective hetero- and homonuclear assembly with high monodispersity, as well as regioselective functionalization of two different DNA strands for more diverse applications.

Epitaxial growth of DNA-assembled nanoparticle superlattices on patterned substrates

DNA-functionalized nanoparticles, including plasmonic nanoparticles, can be assembled into a wide range of crystalline arrays via synthetically programmable DNA hybridization interactions. Here we demonstrate that such assemblies can be grown epitaxially on lithographically patterned templates, eliminating grain boundaries and enabling fine control over orientation and size of assemblies up to thousands of square micrometers. We also demonstrate that this epitaxial growth allows for orientational control, systematic introduction of strain, and designed defects, which extend the range of structures that can be made using superlattice assembly. Ultimately, this will open the door to integrating self-assembled plasmonic nanoparticle materials into on-chip optical or optoelectronic platforms.

A general strategy for the DNA-mediated self-assembly of functional nanoparticles into heterogeneous systems

Nanoparticles coated with DNA molecules can be programmed to self-assemble into three-dimensional superlattices. Such superlattices can be made from nanoparticles with different functionalities and could potentially exploit the synergetic properties of the nanoscale components. However, the approach has so far been used primarily with single-component systems. Here, we report a general strategy for the creation of heterogeneous nanoparticle superlattices using DNA and carboxylic-based conjugation. We show that nanoparticles with all major types of functionality--plasmonic (gold), magnetic (Fe2O3), catalytic (palladium) and luminescent (CdSe/Te@ZnS and CdSe@ZnS)--can be incorporated into binary systems in a rational manner. We also examine the effect of nanoparticle characteristics (including size, shape, number of DNA per particle and DNA flexibility) on the phase behaviour of the heterosystems, and demonstrate that the assembled materials can have novel optical and field-responsive properties.

A molecular logical switching beacon controlled by thiolated DNA signals

A logical switching MB is established, with an "ON/OFF" switching function. In this study, thiolated DNA can participate as a switching controller to regulate the fluorescent increments of other DNA input signals. Assisted by gold nanoparticles and DNA branch migration, one and two-switch systems have been achieved.

Phase behavior and critical activated dynamics of limited-valence DNA nanostars

Colloidal particles with directional interactions are key in the realization of new colloidal materials with possibly unconventional phase behaviors. Here we exploit DNA self-assembly to produce bulk quantities of "DNA stars" with three or four sticky terminals, mimicking molecules with controlled limited valence. Solutions of such molecules exhibit a consolution curve with an upper critical point, whose temperature and concentration decrease with the valence. Upon approaching the critical point from high temperature, the intensity of the scattered light diverges with a power law, whereas the intensity time autocorrelation functions show a surprising two-step relaxation, somehow reminiscent of glassy materials. The slow relaxation time exhibits an Arrhenius behavior with no signs of criticality, demonstrating a unique scenario where the critical slowing down of the concentration fluctuations is subordinate to the large lifetime of the DNA bonds, with relevant analogies to critical dynamics in polymer solutions. The combination of equilibrium and dynamic behavior of DNA nanostars demonstrates the potential of DNA molecules in diversifying the pathways toward collective properties and self-assembled materials, beyond the range of phenomena accessible with ordinary molecular fluids.

Using temperature-sensitive smart polymers to regulate DNA-mediated nanoassembly and encoded nanocarrier drug release

In this paper we describe the use of a temperature-responsive polymer to regulate DNA interactions in both a DNA-mediated assembly system and a DNA-encoded drug delivery system. A thermoresponsive pNIPAAm-co-pAAm polymer, with a transition temperature (TC) of 51 °C, was synthesized with thiol modification and grafted onto gold nanoparticles (Au NPs) also containing single-stranded oligonucleotides (ssDNA). The thermoresponsive behavior of the polymer regulated the accessibility of the sequence-specific hybridization between complementary DNA-functionalized Au NPs. At T < TC, the polymer was hydrophilic and extended, blocking interaction between the complementary sequences at the periphery of the hydrodynamic diameter. In contrast, at T > TC, the polymer shell undergoes a hydrophilic to -phobic phase transition and collapses, shrinking below the outer ssDNA, allowing for the sequence-specific hybridization to occur. The potential application of this dynamic interface for drug delivery is shown, in which the chemotherapy drug doxorubicin (DOX) is bound to double-stranded DNA (dsDNA)-functionalized Au NPs whose sequences are known to be high-affinity intercalation points for it. The presence of the polymer capping is shown to decrease drug release kinetics and equilibrium at T < TC, but increase release at T > TC, thus improving the cytotoxicity of the encoded nanocarrier design.

Kinetics of DNA-coated sticky particles

DNA-functionalized particles are promising for complex self-assembly due to their specific controllable thermoreversible interactions. However, there has been little work on the kinetics and the aggregation rate, which depend on the rate of particle encounters and the probability that an encounter results in particles sticking. In this study, we investigate theoretically and experimentally the aggregation times of micron-scale particles as a function of DNA coverage and salt concentration. Our 2-μm colloids accommodate up to 70,000 DNA strands. For full coverage and high salt concentration, the aggregation time is 5 min while for 0.1 coverage and low salt it is 4 days. A simple model using reaction-limited kinetics and experimental oligomer hybridization rates describes the data well. A controlling factor is the Coulomb barrier at the nanometer scale retarding DNA hybridization. Our model allows easy measurements of microscopic hybridization rates from macroscopic aggregation and enables the design of complex self-assembly schemes with controlled kinetics.

In situ plasmonic counter for polymerization of chains of gold nanorods in solution

Self-assembly of gold nanorods (NRs) in linear, polymer-like chains offers the ability to test and validate theoretical models of molecular polymerization. Practically, NR chains show multiple promising applications in sensing of chemical and biological species. Both areas of research can strongly benefit from the development of a quantitative tool for characterization of the structure of NR chains in the course of self-assembly, based on the change in ensemble-averaged optical properties of plasmonic polymers; however, quantitative correlation between the extinction spectra and the structural characteristics of NR chains has not been reported. Here, we report such a tool by a quantitatively correlating the red shift of the longitudinal surface plasmon band of gold NRs and the average aggregation number of NR chains. The generality of the method is demonstrated for NRs with different aspect ratios, for varying inter-rod distances in the chains, and for varying initial concentrations of NRs in solution. We modeled the extinction spectra of the NR chains by combining the theory of step-growth polymerization with finite-difference time-domain simulations and a resistor-inductor-capacitor model, and obtained agreement between the theoretical and experimental results. In addition to capturing quantitatively the ensemble physics of the polymerization, the proposed 'plasmonic counter' approach provides a real-time cost- and labor-efficient method for the characterization of self-assembly of plasmonic polymers.

Smart nanomachines based on DNA self-assembly

DNA-based nanomachines are self-assembled DNA superstructures that harness chemical free energy to perform mechanical work. The development of DNA machines has benefited greatly from the achievements in both structural and dynamic DNA nanotechnology. In this review, the configurations of DNA machines, fuel systems, and operations are discussed to outline the evolving paths of DNA machines. The focus is on the smart mechanical behavior of DNA machines, from the standpoint of upgrading the complexity of DNA nanostructures, cooperative activation of multimachinary systems, and the establishment of a network of the mechanical states. In the end, the challenges are highlighted and possible solutions are proposed to push forward smart DNA nanomachines, with the goal of creating biomimicking systems. Insights are also provided into the potential applications of the DNA machines with designable intelligence.

Discrete nanocubes as plasmonic reporters of molecular chirality

One of the most intriguing structural properties, chirality, is often exhibited by organic and bio-organic molecular constructs. Chiral spectral signatures, typically appearing in the UV range for organic materials and known as circular dichroism (CD), are widely used to probe a molecular stereometry. Such probing has an increasingly broad importance for biomedical and pharmacological fields due to synthesis/separation/detection of homochiral species, biological role of chiral organization, and structural response to environmental conditions and enantiomeric drugs. Recent theoretical and experimental works demonstrated that the CD signal from chiral organic molecules could appear in the plasmonic (typically, visible) band when they coupled with plasmonic particles. However, the magnitude of this CD signal, induced by discrete nonchiral plasmonic particles, and its native molecular analog were found to be comparable. Here we show that shaped nonchiral nanoparticles, namely, gold/silver core/shell nanocubes, can act as plasmonic reporters of chirality for attached molecules by providing a giant, 2 orders of magnitude CD enhancement in a near-visible region. Through the experimental and theoretical comparison with nanoparticles of other shapes and materials, we demonstrate a uniqueness of silver nanocube geometry for the CD enhancement. The discovered phenomenon opens novel opportunities in ultrasensitive probing of chiral molecules and for novel optical nanomaterials based on the chiral elements.

DNA-functionalized quantum dots: fabrication, structural, and physicochemical properties

We have systematically investigated the effect of physicochemical conditions, such as pH, salt concentration, and DNA/nanoparticle ratio, on the chemical conjugation process and structural and optical stability of carboxyl-functionalized quantum dots (QDs) functionalized with amino-modified DNA. We reveal the relationship between aqueous conditions and the amount of DNA conjugated on QDs, colloidal stability, and yield of the final QD-DNA conjugates. By carefully adjusting the environmental variables we have successfully achieved up to 20 DNA strands conjugated per QD, and demonstrated how this number can be tuned. The fabricated QD-DNA conjugates are dispersed and optically stable in salted solutions for over a month. We have also evaluated the involved interparticle interactions to explain the solution behavior of QD-DNA conjugates. Our results provide a basic understanding of the physiochemical processes governing a nanoparticle-biomolecule conjugation and the structural stability of the formed conjugates. Such fabricated QD-DNA conjugates might be of great benefit for programmable assemblies of optically active nanomaterials and for emerging biosensing methods based on nanomaterials.

Correct spectral conversion between surface-enhanced raman and plasmon resonance scattering from nanoparticle dimers for single-molecule detection

Simultaneous measurement of surface-enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR) in nanoparticle dimers presents outstanding opportunities in molecular identification and in the elucidation of physical properties, such as the size, distance, and deformation of target species. SERS-LSPR instrumentation exists and has been used under limited conditions, but the extraction of SERS and LSPR readouts from a single measurement is still a challenge. Herein, the extraction of LSPR spectra from SERS signals is reported and a tool for measuring the interparticle distance from Raman enhancement data by the standardization of the SERS signal is proposed. The SERS nanoruler mechanism incorporates two important aspects (the LSPR scattering peak shift and the Raman shift for measuring interparticle distance), and signifies their exact one-to-one correspondence after spectral correction. The developed methodology is applied to calculate the interparticle distance between nanoparticle dimers from SERS signals, to detect and quantify DNA at the single-molecule level in a base-pair-specific manner. It is also shown that the SERS nanoruler concept can be used in structural analysis for the specific detection of the interaction of immunoglobulin G (IgG) with its target from bianalyte Raman signals with identical shaping at single-molecule resolution. The SERS profile shaping approach not only offers a new detection mechanism for single molecules, but also has excellent potential for studying protein interactions and the intracellular detection of mRNA.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.

Three-dimensional gold nanoparticle clusters with tunable cores templated by a viral protein scaffold

Assembling nanoparticles (NPs) into ordered architectures remains a challenge in the field of nanotechnology. Templated strategies have been widely utilized for NP assembly. As typical biological nanostructures, virus-based NPs (VNPs) have shown great promise in templating NP assembly. Here it is illustrated that the VNP of simian virus 40 (SV40) is a powerful scaffold in directing the assembly of 3D hybrid nanoarchitectures with one NP encapsulated inside as a core and a cluster of gold NPs (AuNPs) on the outer surface of the SV40 VNP as a shell, in which the core NPs can be CdSe/ZnS quantum dots (QDs), Ag(2)S QDs, or AuNPs. The assembling of AuNPs onto the SV40 VNP surface is determined by the interactions between the AuNPs and the amine groups on the outer surface of SV40 VNPs. It is expected that the VNP guided 3D hybrid nanoarchitectures provide ideal models for NP interaction studies and open new opportunities for integrating various functionalities in NP assemblies.

Heterogeneous nanoclusters assembled by PNA-templated double-stranded DNA

Heterogeneous nanoclusters with trimeric and core-shell architectures containing nanoparticles of different size and composition have been fabricated via site-specific PNA-"invasion" of DNA double helix. This novel strategy facilitates the incorporation of double-stranded DNA into the nanoparticle assembly design.

Analysis of DNA-guided self-assembly of microspheres using imaging flow cytometry

Imaging flow cytometry was used to analyze the self-assembly of DNA-conjugated polystyrene microspheres. This technique enables quantitative analysis of the assembly process and thereby enables detailed analysis of the effect of structural and process variables on the assembly yield. In a demonstration of the potential of this technique, the influence of DNA strand base pair (bp) length was examined, and it was found that 50 bp was sufficient to drive the assembly of microspheres efficiently, forming not only dimers but also chainlike structures. The effect of stoichiometry on the yield was also examined. The analysis demonstrated that self-assembly of 50 bp microspheres can be driven nearly to completion by stoichiometric excess in a manner similar to Le Chatelier's principle in common chemical equilibrium.

Internal structure of nanoparticle dimers linked by DNA

We construct nanoparticle dimers linked by DNA. These dimers are basic units in a possible multiscale, hierarchical assembly and serve as a model system to understand DNA-mediated interactions, especially in the nontrivial regime when the nanoparticle and DNA are comparable in their sizes. We examine the structure of nanoparticle dimers in detail by a combination of scattering experiments and molecular simulations. We find that, for a given DNA length, the interparticle separation within the dimer is controlled primarily by the number of linking DNA. We summarize our findings in a simple model that captures the interplay of the number of DNA bridges, their length, the particle's curvature, and the excluded volume effects. We demonstrate the applicability of the model to our results, without any free parameters. As a consequence, the increase of dimer separation with increasing temperature can be understood as a result of changing the number of connecting DNA.

Controlled assembly and plasmonic properties of asymmetric core-satellite nanoassemblies

The assembly of noble metal nanoparticles offers an appealing means to control and enhance the plasmonic properties of nanostructures. However, making nanoassemblies with easily modifiable gap distances with high efficiency has been challenging. Here, we report a novel strategy to assemble gold nanoparticles (AuNPs) into Janus-type asymmetric core-satellite nanostructures. Markedly different desorption efficiency between large and small AuNPs in ethanol allows us to prepare the asymmetric core-satellite nanoassemblies in a dispersed colloidal state with near 100% purity. The resulting nanoassemblies have well-defined structures in which a core AuNP (51 nm) is covered by an average of 13 ± 3 satellite AuNPs (13 nm) with part of the core surfaces left unoccupied. Strong surface plasmon coupling is observed from these nanoassemblies as a result of the close proximity between the core and the satellites, which appears significantly red-shifted from the surface plasmon resonance frequencies of the constituting nanoparticles. The dependence of the surface plasmon coupling on a gap distance of less than 3 nm is systematically investigated by varying the length of the alkanedithiol linkers. The asymmetric core-satellite nanoassemblies also serve as an excellent surface-enhanced Raman scattering substrate with an enhancement factor of ~10(6). Finally, we demonstrate that the presented assembly method is extendible to the preparation of compositionally heterogeneous core-satellite nanoassemblies.

Dispersible gold nanorod dimers with sub-5 nm gaps as local amplifiers for surface-enhanced Raman scattering

We report the synthesis of solution-dispersible, 35 nm diameter gold nanorod dimers with gaps as small as ∼2 nm for surface-enhanced Raman scattering (SERS). Using on-wire lithography (OWL), we prepared tailorable dimers in high yield and high monodispersity (∼96% dimers) that produce both large and reproducible SERS signals with enhancement factors of (6.8 ± 0.7) × 10(8) for single dimers in air and 1.2 × 10(6) for ensemble-averaged solution measurements. Furthermore, we show that these structures, which are the smallest ever made by OWL, can be used to detect molecules on flat surfaces and in aqueous solutions. When combined, these attributes with respect to sensitivity, reproducibility, and tailorability lead to a novel and powerful local amplification system for SERS applications.

Ag@SiO2 Core-Shell Nanostructures: Distance-Dependent Plasmon Coupling and SERS Investigation

Enhancement of Raman signals of pyrene due to the enhanced electric fields on the surface of silver nanoparticles has been investigated by controlling the thickness of the silica shell. Dimeric nanostructures having well-defined gaps between two silver nanoparticles were prepared, and the gap size (d) was varied from 1.5 to 40 nm. The molecules trapped at the dimeric junctions showed higher Raman signal enhancements when the gap was less than 15 nm due to the presence of amplified electric field, in agreement with our theoretical studies. The experimental Raman enhancement factors at the hot spots follow a 1/d(n) dependence, with n = 1.5, in agreement with the recent theoretical studies by Schatz and co-workers. Experimental results presented here on the distance dependence of surface enhanced Raman spectroscopy (SERS) enhancement at the hot spots can provide insight on the design of newer plasmonic nanostructures with optimal nanogaps.

Photoswitchable oligonucleotide-modified gold nanoparticles: controlling hybridization stringency with photon dose

We describe a new class of stimulus-responsive DNA-functionalized gold nanoparticles that incorporate azobenzene-modified oligonucleotides. Beyond the classic directed assembly and sensing behaviors associated with oligonucleotide-modified nanoparticles, these particles also exhibit reversible photoswitching of their assembly behavior. Exposure to UV light induces a trans-cis isomerization of the azobenzene which destabilizes the DNA duplex, resulting in dissociation of the nanoparticle assemblies. The isomerization is reversible upon exposure to blue light, resulting in rehybridization and reassembly of the DNA-linked nanoparticle clusters. We show that perfectly complementary and partially mismatched strands exhibit clearly distinguishable photoinduced melting properties, and we demonstrate that photon dose can thus be used in place of temperature or ionic strength to control hybridization stringency with the ability to discriminate single-base mismatches.

Biochemical investigation of the formation of three-dimensional networks from DNA-grafted large silica particles

DNA is used to rationally build up networks of silica nanoparticles (SiNPs) based on the molecular recognition properties of complementary sequences. Network self-assembly is controlled from DNA covalently grafted at the surface of chemically modified SiNPs. Two strategies are compared, where grafted DNA sequences are designed in a three-strand system using noncomplementary sequences and an extra DNA linker, or in a two-strand approach for direct hybridization. In this paper, both systems are compared in terms of DNA hybridization stability, network size, and three-dimensional organization using a combination of dynamic light scattering and electron microscopy. The observed differences are discussed in terms of hybridization interactions between DNA sequences in particle-free systems through fluorescence, circular dichroism, and UV spectroscopy techniques.

Regiospecific plasmonic assemblies for in situ Raman spectroscopy in live cells

Multiple properties of plasmonic assemblies are determined by their geometrical organization. While high degree of complexity was achieved for plasmonic superstructures based on nanoparticles (NPs), little is known about the stable and structurally reproducible plasmonic assemblies made up from geometrically diverse plasmonic building blocks. Among other possibilities, they open the door for the preparation of regiospecific isomers of nanoscale assemblies significant both from a fundamental point of view and optical applications. Here, we present a synthetic method for complex assemblies from NPs and nanorods (NRs) based on selective modification of NRs with DNA oligomers. Three types of assemblies denoted as End, Side, and Satellite isomers that display distinct elements of regiospecificity were prepared with the yield exceeding 85%. Multiple experimental methods independently verify various structural features, uniformity, and stability of the prepared assemblies. The presence of interparticle gaps with finely controlled geometrical parameters and inherently small size comparable with those of cellular organelles fomented their study as intracellular probes. Against initial expectations, SERS intensity for End, Side, and Satellite isomers was found to be dependent primarily on the number of the NPs in the superstructures rationalized with the help of electrical field simulations. Incubation of the label-free NP-NR assemblies with HeLa cells indicated sufficient field enhancement to detect structural lipids of mitochondria and potentially small metabolites. This provided the first proof-of-concept data for the possibility of real-time probing of the local organelle environment in live cells. Further studies should include structural optimization of the assemblies for multitarget monitoring of metabolic activity and further increase in complexity for applications in transformative optics.

DNA-directed assembly of asymmetric nanoclusters using Janus nanoparticles

Asymmetric assembly of nanomaterials has attracted broad interests because of their unique anisotropic properties that are different from those based on the more widely reported symmetric assemblies. Despite the potential advantages, programmable fabrication of asymmetric structure in nanoscale remains a challenge. We report here a DNA-directed approach for the assembly of asymmetric nanoclusters using Janus nanoparticles as building blocks. DNA-functionalized spherical gold nanoparticles (AuNSs) can be selectively attached onto two different hemispheres of DNA-functionalized Janus nanoparticle (JNP) through DNA hybridization. Complementary and invasive DNA strands have been used to control the degree and reversibility of the assembly process through programmable base-pairing interactions, resulting in a series of modular and asymmetric nanostructures that allow systematic study of the size-dependent assembly process. We have also shown that the attachment of the AuNSs onto the gold surface of the Janus nanoparticle results in red shifting of the UV-vis and plasmon resonance spectra.

High-throughput analysis of concentration-dependent antibody self-association

Monoclonal antibodies are typically monomeric and nonviscous at low concentrations, yet they display highly variable associative and viscous behavior at elevated concentrations. Although measurements of antibody self-association are critical for understanding this complex behavior, traditional biophysical methods are not capable of characterizing such concentration-dependent self-association in a high-throughput manner. Here we describe a nanoparticle-based method, termed self-interaction nanoparticle spectroscopy, that is capable of rapidly measuring concentration-dependent self-interactions for three human monoclonal antibodies with unique solution behaviors. We demonstrate that gold nanoparticles conjugated with antibodies at low protein concentrations (<40 μg/mL) display self-association behavior (as measured by the interparticle distance-dependent plasmon wavelength) that is well correlated with static light-scattering measurements obtained at three orders of magnitude higher antibody concentrations. Using this methodology, we find that the antibodies display a complex pH-dependent self-association behavior that is strongly influenced by the solution ionic strength. Importantly, we find that a polyclonal human antibody is nonassociative for all solution conditions evaluated in this work, suggesting that antibody self-association is more specific than previously realized. We expect that our findings will guide rational manipulation of antibody phase behavior, and enable studies that elucidate sequence and structural determinants of antibody self-association.

Heterogeneous ligand-nanoparticle distributions: a major obstacle to scientific understanding and commercial translation

Nanoparticles conjugated with functional ligands are expected to have a major impact in medicine, photonics, sensing, and nanoarchitecture design. One major obstacle to realizing the promise of these materials, however, is the difficulty in controlling the ligand/nanoparticle ratio. This obstacle can be segmented into three key areas: First, many designs of these systems have failed to account for the true heterogeneity of ligand/nanoparticle ratios that compose each material. Second, studies in the field often use the mean ligand/nanoparticle ratio as the accepted level of characterization of these materials. This measure is insufficient because it does not provide information about the distribution of ligand/nanoparticle species within a sample or the number and relative amount of the different species that compose a material. Without these data, researchers do not have an accurate definition of material composition necessary both to understand the material-property relationships and to monitor the consistency of the material. Third, some synthetic approaches now in use may not produce consistent materials because of their sensitivity to reaction kinetics and to the synthetic history of the nanoparticle. In this Account, we describe recent advances that we have made in under standing the material composition of ligand-nanoparticle systems. Our work has been enabled by a model system using poly(amidoamine) dendrimers and two small molecule ligands. Using reverse phase high-pressure liquid chromatography (HPLC), we have successfully resolved and quantified the relative amounts and ratios of each ligand/dendrimer combination. This type of information is rare within the field of ligand-nanoparticle materials because most analytical techniques have been unable to identify the components in the distribution. Our experimental data indicate that the actual distribution of ligand-nanoparticle components is much more heterogeneous than is commonly assumed. The mean ligand/nanoparticle ratio that is typically the only information known about a material is insufficient because the mean does not provide information on the diversity of components in the material and often does not describe the most common component (the mode). Additionally, our experimental data has provided examples of material batches with the same mean ligand/nanoparticle ratio and very different distributions. This discrepancy indicates that the mean cannot be used as the sole metric to assess the reproducibility of a system. We further found that distribution profiles can be highly sensitive to the synthetic history of the starting material as well as slight changes in reaction conditions. We have incorporated the lessons from our experimental data into the design of new ligand-nanoparticle systems to provide improved control over these ratios.

Challenges and opportunities for structural DNA nanotechnology

DNA molecules have been used to build a variety of nanoscale structures and devices over the past 30 years, and potential applications have begun to emerge. But the development of more advanced structures and applications will require a number of issues to be addressed, the most significant of which are the high cost of DNA and the high error rate of self-assembly. Here we examine the technical challenges in the field of structural DNA nanotechnology and outline some of the promising applications that could be developed if these hurdles can be overcome. In particular, we highlight the potential use of DNA nanostructures in molecular and cellular biophysics, as biomimetic systems, in energy transfer and photonics, and in diagnostics and therapeutics for human health.

Design and application of inorganic nanoparticle superstructures: current status and future challenges

Self-assembly of inorganic nanoparticles (NPs) into superstructures, which is used as a general way to integrate functional inorganic NPs into macroscale devices, has attracted much research interest. This review will summarize the recent progress and discuss future challenges of the inorganic NP superstructures. Examples include both DNA-based and polymer-based NP assemblies with controlled positioning and geometries, and quasicrystalline ordered structures from the self-assembly of binary or ternary NPs. Different from their individual NP counterparts, these self-assembled superstructures possess unique properties, such as optical chirality and dynamic structural change under an external stimulus. Due to their diversified structures and functionalities, inorganic NP superstructures have shown a wide range of promise for applications in electronic and photonic devices, such as field-effect transistors, magnetoresistive components, optical information recording, and solar cells.

Orthogonally dual-clickable Janus nanoparticles via a cyclic templating strategy

Synthetic asymmetrical systems, Janus particles and patchy particles, are capable of undergoing hierarchical assembly processes that mimic those of Nature, to serve as switchable devices, optical probes, phase-transfer catalysts, and multifunctional drug carriers, each of which benefits from opposing surface patterns that behave differently. Production of nanometer-sized Janus particles that are equipped with efficient chemistries remains a challenge. A robust Janus-faced polymer nanoparticle framework that presents two orthogonally click-reactive surface chemistries has been generated by a recyclable strategy that involves reactive functional group transfer by templating against gold nanoparticle substrates. This anisotropic functionalization approach is compatible with a wide range of soft materials, providing Janus nanoparticles for the construction of dual-functionalized devices by accurately controlling chemical functionality at the nanoscopic level.

Effect of gold nanoparticle aggregation on cell uptake and toxicity

Aggregation appears to be a ubiquitous phenomenon among all nanoparticles and its influence in mediating cellular uptake and interactions remain unclear. Here we developed a simple technique to produce transferrin-coated gold nanoparticle aggregates of different sizes and characterized their uptake and toxicity in three different cell lines. While the aggregation did not elicit a unique toxic response, the uptake patterns were different between single and aggregated nanoparticles. There was a 25% decrease in uptake of aggregated nanoparticles with HeLa and A549 cells in comparison to single and monodisperse nanoparticles. However, there was a 2-fold increase in MDA-MB 435 cell uptake for the largest synthesized aggregates. These contrasting results suggest that cell type and the mechanism of interactions may play a significant role. This study highlights the need to investigate the behavior of aggregates with cells on a case-by-case basis and the importance of aggregation in mediating targeting and intracellular trafficking.

Disassembly of a core-satellite nanoassembled substrate for colorimetric biomolecular detection

The disassembly of a core-satellite nanostructured substrate is presented as a colorimetric biosensor observable under dark-field illumination. The fabrication method described herein utilizes thiol-mediated adsorption and streptavidin-biotin binding to self-assemble core-satellite nanostructures with a sacrificial linking peptide. Biosensing functionality is demonstrated with the protease trypsin, and the optical properties of the nanoassemblies are characterized. A figure of merit is presented to determine the optimal core and satellite size for visual detection. Nanoassemblies with 50 nm cores and 30 or 50 nm satellites are superior as these structures achieve an orange to green color shift greater than 70 nm that is easily discernible by the naked eye. This colorimetric substrate may prove to be a favorable alternative to liquid-based colloidal sensors and a useful visual readout mechanism for point-of-care microfluidic diagnostic assays.