The assemble, grow and lift-off (AGLO) strategy to construct complex gold nanostructures with pre-designed morphologies

The construction of metallic nanostructures with customizable morphologies and complex shapes has been an essential pursuit in nanoscience. DNA nanotechnology has enabled the fabrication of increasingly complex DNA nanostructures with unprecedented specificity, programmability and sub-nanometer precision, which makes it an ideal approach to rationally organize metallic nanostructures. Here we report an Assemble, Grow and Lift-Off (AGLO) strategy to construct robust standalone gold nanostructures with pre-designed customizable shapes in solution, using only a simple 2D DNA origami sheet as a versatile transient template. Gold nanoparticle (AuNP) seeds were firstly assembled onto the pre-designed binding sites of the DNA origami template and then additional gold was slowly deposited onto the AuNP seeds. The growing seed surfaces eventually merge with adjacent seeds to generate one continuous gold nanostructure in a pre-designed shape, which can then be lifted off the origami template. Diverse customized patterns of templated AuNP seeds were successfully transformed into corresponding gold nanostructures with the target structure transformation percentage over 80%. Moreover, the AGLO strategy can be incorporated with a magnetic bead separation platform to enable the easy recycling of the excess AuNP seeds and DNA components.

Detailed cellular assessment of albumin-bound oligonucleotides: Increased stability and lower non-specific cell uptake

Conjugation of lipid moieties to nucleic-acid therapeutics increases their interaction with cellular membranes, enhances their uptake and influences in vivo distribution. Once injected in biological fluids, such modifications trigger the binding of various serum proteins, which in turn play a major role in determining the fate of oligonucleotides. Yet, the role played by each of these proteins, more than 300 in serum, remains to be elucidated. Albumin, the most abundant circulating protein is an attractive candidate to study, as it was previously used to enhance the therapeutic effect of various drugs. Herein, we present a thorough fluorescent-based methodology to study the effect of strong and specific albumin-binding on the fate and cellular uptake of DNA oligonucleotides. We synthesized a library of molecules that exhibit non-covalent binding to albumin, with affinities ranging from high (nanomolar) to none. Our results revealed that strong albumin binding can be used as a strategy to reduce degradation of oligonucleotides in physiological conditions caused by enzymes (nucleases), to reduce uptake and degradation by immune cells (macrophages) and to prevent non-specific uptake by cells. We believe that introducing protein-binding domains in oligonucleotides can be used as a strategy to control the fate of oligonucleotides in physiological environments. While our study focuses on albumin, we believe that such systematic studies, which elucidate the role of serum proteins systematically, will ultimately provide a toolbox to engineer the next-generation of therapeutic oligonucleotides, overcoming many of the barriers encountered by these therapeutics, such as stability, immunogenicity and off-target effects.

Design Strategy to Access siRNA-Encapsulating DNA "Nanosuitcases" That Can Conditionally Release Their Cargo

DNA nanotechnology enables the design and assembly of DNA nanostructures with unprecedented control over their size and shape. Additionally, the programmable base-pairing alphabet of DNA allows the incorporation of responsive units within these DNA nanostructures. Here, we describe a general design strategy to construct responsive DNA prisms that can encapsulate and selectively release an encapsulated siRNA upon recognition of an oligonucleotide trigger. This prismatic DNA scaffold design is adaptable and can encapsulate oligonucleotides of any length and type. Moreover, the prism can be made to respond to an oligonucleotide trigger of interest like a messenger RNA (mRNA) or a microRNA (miRNA), thus enabling dual or synergistic therapeutic strategies. We present an overview of the design strategy used to access these DNA nanostructures, followed by the steps involved in DNA sequence generation, assembly, and validation of this construct.

Advancing Wireframe DNA Nanostructures Using Single-Molecule Fluorescence Microscopy Techniques

DNA nanotechnology relies on the molecular recognition properties of DNA to produce complex architectures through self-assembly. The resulting DNA nanostructures allow scientists to organize functional materials at the nanoscale and have therefore found applications in many domains of materials science over the past several years. These scaffolds have been used to position proteins, nanoparticles, carbon nanotubes, and other nanomaterials with high spatial resolution. In addition to their remarkable performance as frameworks for other species, DNA constructs also possess interesting dynamic properties, which have led to their use in logic circuits, drug delivery vehicles, and molecular walkers. Although DNA nanostructures have become increasingly complex, the development of tools to study them has lagged. Currently, gel electrophoresis, dynamic light scattering, and ensemble fluorescence measurements are widely used to characterize DNA-based assemblies. Unfortunately, ensemble averaging in these methods obscures malformed structures and may mask properties associated with structure, length, and shape in polydisperse samples. While atomic force microscopy allows for the determination of morphology at the single-molecule level, this technique cannot typically be used to assess the dynamic properties of these constructs. To analyze the function of DNA-based devices such as molecular motors and reconfigurable nanostructures in real time, new single-molecule techniques are required. This Account details the work from our laboratories toward developing single-molecule fluorescence (SMF) methodologies for the structural and dynamic characterization of wireframe DNA nanostructures, one at a time. The methods described herein provide us with two separate yet related sets of information: First, we can statically examine the nanostructures one by one to assess their robustness, structural fidelity, and morphology. This is primarily done using two-color stepwise photobleaching, wherein we can examine the subunit stoichiometry of our assemblies before and after various perturbations to the structures. For example, we can introduce length mismatches to cause the nanotube to bend or perform strand displacement reactions to generate single-stranded, flexible analogues of our materials. Second, due to the unmatched spatiotemporal resolution of SMF techniques, we can study the dynamic character of these assemblies by implementing structural changes to the nanotube and monitoring them in real time. With this structural and dynamic information in hand, our groups have additionally developed new tools for the improved construction of DNA nanotubes, inspired by solid-phase DNA synthesis. By assembling the nanotubes in a stepwise manner, highly monodisperse nanostructures of any desired length can be made without a template strand. In this way, unique building blocks can also be added sequence-specifically, allowing for the production of user-defined scaffolds to organize nanoscale materials in three dimensions. This method, in combination with our imaging and analysis protocols, may be extended to assemble and inspect other supramolecular constructs in a controlled manner. Overall, by combining synthesis, characterization, and analysis, these single-molecule techniques hold the potential to advance the study of DNA nanostructures and dynamic DNA-based devices.

Uptake and Fate of Fluorescently Labeled DNA Nanostructures in Cellular Environments: A Cautionary Tale

Fluorescent dye labeling of DNA oligonucleotides and nanostructures is one of the most used techniques to track their fate and cellular localization inside cells. Here, we report that intracellular fluorescence, and even FRET signals, cannot be correlated with the cellular uptake of intact DNA structures. Live cell imaging revealed high colocalization of cyanine-labeled DNA oligos and nanostructures with phosphorylated small-molecule cyanine dyes, one of the degradation products from these DNA compounds. Nuclease degradation of the strands outside and inside the cell results in a misleading intracellular fluorescent signal. The signal is saturated by the fluorescence of the degradation product (phosphorylated dye). To test our hypothesis, we synthesized a range of DNA structures, including Cy3- and Cy5-labeled DNA cubes and DNA tetrahedra, and oligonucleotides with different stabilities toward nucleases. All give fluorescence signals within the mitochondria after cellular uptake and strongly colocalize with a free phosphorylated dye control. Kinetics experiments revealed that uptake of stable DNA structures is delayed. We also studied several parameters influencing fluorescent data: stability of the DNA strand, fixation methods that can wash away the signal, position of the dye on the DNA strand, and design of FRET experiments. DNA nanostructures hold tremendous potential for biomedical applications and biotechnology because of their biocompatibility, programmability, and easy synthesis. However, few examples of successful DNA machines in vivo have been reported. We believe this contribution can be used as a guide to design better cellular uptake experiments when using fluorescent dyes, in order to further propel the biological development, and application of DNA nanostructures.

Minimalist Design of a Stimuli-Responsive Spherical Nucleic Acid for Conditional Delivery of Oligonucleotide Therapeutics

In this work, we report a component-minimal spherical nucleic acid (SNA) from monodisperse DNA-polymer conjugates that can load and release nucleic acid therapeutics in a stimuli-responsive manner. We show that this vehicle assembles from only four strands, and conditional release of its antisense therapeutic cargo can be induced upon recognition of specific oligonucleotide triggers via strand displacement. The latter (triggers) may be a microRNA that offers additional synergistic therapy, in addition to the previously shown ability of the SNA to load hydrophobic drugs. The SNA is easy to prepare, has dynamic character, releases its cargo only upon the presence of both triggers, and can survive biological conditions while protecting its cargo. The gene silencing potency of the cargo was tested in live cells and shown to be suppressed when loaded in the SNA, and its activity was restored only upon release with the two triggers. This vehicle has the essential characteristics of versatility, ease of synthesis, low cost, highly responsive behavior, and ability to support combination therapies, making it a promising candidate for cell-selective drug delivery and clinical transition.

Kinetics of Strand Displacement and Hybridization on Wireframe DNA Nanostructures: Dissecting the Roles of Size, Morphology, and Rigidity

Dynamic wireframe DNA structures have gained significant attention in recent years, with research aimed toward using these architectures for sensing and encapsulation applications. For these assemblies to reach their full potential, however, knowledge of the rates of strand displacement and hybridization on these constructs is required. Herein, we report the use of single-molecule fluorescence methodologies to observe the reversible switching between double- and single-stranded forms of triangular wireframe DNA nanotubes. Specifically, by using fluorescently labeled DNA strands, we were able to monitor changes in intensity over time as we introduced different sequences. This allowed us to extract detailed kinetic information on the strand displacement and hybridization processes. Due to the polymeric nanotube structure, the ability to individually address each of the three sides, and the inherent polydispersity of our samples as a result of the step polymerization by which they are formed, a library of compounds could be studied independently yet simultaneously. Kinetic models relying on mono-exponential decays, multi-exponential decays, or sigmoidal behavior were adjusted to the different constructs to retrieve erasing and refilling kinetics. Correlations were made between the kinetic behavior observed, the site accessibility, the nanotube length, and the structural robustness of wireframe DNA nanostructures, including fully single-stranded analogs. Overall, our results reveal how the length, morphology, and rigidity of the DNA framework modulate the kinetics of strand displacement and hybridization as well as the overall addressability and structural stability of the structures under study.

Mapping the energy landscapes of supramolecular assembly by thermal hysteresis

Understanding how biological macromolecules assemble into higher-order structures is critical to explaining their function in living organisms and engineered biomaterials. Transient, partly-structured intermediates are essential in many assembly processes and pathway selection, but are challenging to characterize. Here we present a simple thermal hysteresis method based on rapid, non-equilibrium melting and annealing measurements that maps the rate of supramolecular assembly as a function of temperature and concentration. A straightforward analysis of these surfaces provides detailed information on the natures of assembly pathways, offering temperature resolution beyond that accessible with conventional techniques. Validating the approach using a tetrameric guanine quadruplex, we obtain strikingly good agreement with previous kinetics measurements and reveal temperature-dependent changes to the assembly pathway. In an application to the recently discovered co-assembly of polydeoxyadenosine (poly(A)) and cyanuric acid, we show that fiber elongation is initiated when an unstable complex containing three poly(A) monomers acquires a fourth strand.

Complex assembly pathways often involve transient, partly-formed intermediates that are challenging to characterize. Here, the authors present a simple and rapid spectroscopic thermal hysteresis method for mapping the energy landscapes of supramolecular assembly.

Single-stranded templates as railroad tracks for hierarchical assembly of DNA origami

DNA origami is one of the most effective tools for bottom-up construction of novel objects and devices at the nanometer-scale. However, many applications require larger structures than can be obtained with the conventional single-stranded scaffold, typically 7249 nucleotides. Here, we address this limitation by developing custom-made single-stranded scaffolds that bind pre-assembled origami tiles and induce their one-dimensional organization in high yields. Our synthetic method allows the conversion of multiple repetitive and unique sequences into correctly assembled, large backbones, and to finely tune the position and frequency of each building block. Granted with these regions, three and five origami tiles were successfully arranged in 1-D with the aid of one or two scaffolds, forming a nano-"railroad track". This new method increases length scale in DNA origami without increasing cost and complexity, and is anticipated to increase the yield of other approaches aiming to assemble large origami structures.

Recent advances in DNA nanotechnology

DNA is a powerful guiding molecule to achieve the precise construction of arbitrary structures and high-resolution organization of functional materials. The combination of sequence programmability, rigidity and highly specific molecular recognition in this molecule has resulted in a wide range of exquisitely designed DNA frameworks. To date, the impressive potential of DNA nanomaterials has been demonstrated from fundamental research to technological advancements in materials science and biomedicine. This review presents a summary of some of the most recent developments in structural DNA nanotechnology regarding new assembly approaches and efforts in translating DNA nanomaterials into practical use. Recent work on incorporating blunt-end stacking and hydrophobic interactions as orthogonal instruction rules in DNA assembly, and several emerging applications of DNA nanomaterials will also be highlighted.

DNA Nanotubes with Hydrophobic Environments: Toward New Platforms for Guest Encapsulation and Cellular Delivery

Natural systems combine different supramolecular interactions in a hierarchical manner to build structures. In contrast, DNA nanotechnology relies almost exclusively on DNA base pairing for structure generation. Introducing other supramolecular interactions can expand the structural and functional range of DNA assemblies, but this requires an understanding of the interplay between these interactions. Here, an economic strategy to build DNA nanotubes functionalized with lipid-like polymers is reported. When these polymers are linked to the nanotube using a spacer, they fold inside to create a hydrophobic environment within the nanotube; the nanotube can encapsulate small molecules and conditionally release them when specific DNA strands are added, as monitored by single-molecule fluorescence microscopy. When the polymers are directly linked to the nanostructure without spacers, they interact intermolecularly to form a network of DNA bundles. This morphological switch can be directly observed using a strand displacement strategy. The two association modes result in different cellular uptake behavior. Nanotubes with internal hydrophobic association show dye-mediated mitochondrial colocalization inside cells; while the bundles disassemble into smaller polymer-coated structures that reduce the extent of nonspecific cellular uptake. This approach uncovers parameters to direct the hierarchical assembly of DNA nanostructures, and produces promising materials for targeted drug delivery.

Encapsulation of Gold Nanoparticles into DNA Minimal Cages for 3D-Anisotropic Functionalization and Assembly

Gold nanoparticles (AuNPs) endowed with anisotropic DNA valency are an important class of materials, as they can assemble into complex structures with a minimal number of DNA strands. However, methods to encode 3D DNA strand patterns on AuNPs with a controlled number of unique DNA strands in a predesigned spatial arrangement remain elusive. In this work, a simple one-step method to yield such DNA-decorated AuNPs is demonstrated, through encapsulating AuNPs into DNA minimal nanocages. The AuNP@DNA cage encapsulation complex inherits the 3D anisotropic molecular information from the DNA nanocage with enhanced structural stability. The DNA nanocage can be further functionalized and used as a building block for the self-assembly of complex architectures, such as dimers and trimers, programmed assemblies with sequential growth DNA backbones and DNA origami.

Templated synthesis of spherical RNA nanoparticles with gene silencing activity

Bioactive RNA nanoparticles with gene silencing activity have been synthesized in high yield using a complementary spherical DNA nanoparticle.