Structural DNA nanotechnology: state of the art and future perspective.

Over the past three decades DNA has emerged as an exceptional molecular building block for nanoconstruction due to its predictable conformation and programmable intra- and intermolecular Watson-Crick base-pairing interactions. A variety of convenient design rules and reliable assembly methods have been developed to engineer DNA nanostructures of increasing complexity. The ability to create designer DNA architectures with accurate spatial control has allowed researchers to explore novel applications in many directions, such as directed material assembly, structural biology, biocatalysis, DNA computing, nanorobotics, disease diagnosis, and drug delivery. This Perspective discusses the state of the art in the field of structural DNA nanotechnology and presents some of the challenges and opportunities that exist in DNA-based molecular design and programming.

Uncovering the self-assembly of DNA nanostructures by thermodynamics and kinetics.

CONSPECTUS: DNA nanotechnology is one of the most flourishing interdisciplinary research fields. DNA nanostructures can be designed to self-assemble into a variety of periodic or aperiodic patterns of different shapes and length scales. They can be used as scaffolds for organizing other nanoparticles, proteins, and chemical groups, leveraging their functions for creating complex bioinspired materials that may serve as smart drug delivery systems, in vitro or in vivo biomolecular computing platforms, and diagnostic devices. Achieving optimal structural features, efficient assembly protocols, and precise functional group positioning and modification requires a thorough understanding of the thermodynamics and kinetics of the DNA nanostructure self-assembly process. The most common real-time measurement strategies include monitoring changes in UV absorbance based on the hyperchromic effect of DNA, and the emission signal changes of DNA intercalating dyes or covalently conjugated fluorescent dyes/pairs that accompany temperature dependent structural changes. Thermodynamic studies of a variety of DNA nanostructures have been performed, from simple double stranded DNA formation to more complex origami assembly. The key parameters that have been evaluated in terms of stability and cooperativity include the overall dimensions, the folding path of the scaffold, crossover and nick point arrangement, length and sequence of single strands, and salt and ion concentrations. DNA tile-tile interactions through sticky end hybridization have also been analyzed, and the steric inhibition and rigidity of tiles turn out to be important factors. Many kinetic studies have also been reported, and most are based on double stranded DNA formation. A two-state assumption and the hypothesis of several intermediate states have been applied to determine the rate constant and activation energy of the DNA hybridization process. A few simulated models were proposed to represent the structural, mechanical, and kinetic properties of DNA hybridization. The kinetics of strand displacement reactions has also been studied as a special case of DNA hybridization. The thermodynamic and kinetic characteristics of DNA nanostructures have been exploited to develop rapid and isothermal annealing protocols. It is conceivable that a more thorough understanding of the DNA assembly process could be used to guide the structural design process and optimize the conditions for assembly, manipulation, and functionalization, thus benefiting both upstream design and downstream applications.

Multifactorial modulation of binding and dissociation kinetics on two-dimensional DNA nanostructures.

We use single-particle fluorescence resonance energy transfer (FRET) to show that organizing oligonucleotide probes into patterned two-dimensional arrays on DNA origami nanopegboards significantly alters the kinetics and thermodynamics of their hybridization with complementary targets in solution. By systematically varying the spacing of probes, we demonstrate that the rate of dissociation of a target is reduced by an order of magnitude in the densest probe arrays. The rate of target binding is reduced less dramatically, but to a greater extent than reported previously for one-dimensional probe arrays. By additionally varying target sequence and buffer composition, we provide evidence for two distinct mechanisms for the markedly slowed dissociation: direct hopping of targets between adjacent sequence-matched probes and nonsequence-specific, salt-bridged, and thus attractive electrostatic interactions with the DNA origami pegboard. This kinetic behavior varies little between individual copies of a given array design and will have significant impact on hybridization measurements and overall performance of DNA nanodevices as well as microarrays.

DNA gridiron nanostructures based on four-arm junctions.

Engineering wireframe architectures and scaffolds of increasing complexity is one of the important challenges in nanotechnology. We present a design strategy to create gridiron-like DNA structures. A series of four-arm junctions are used as vertices within a network of double-helical DNA fragments. Deliberate distortion of the junctions from their most relaxed conformations ensures that a scaffold strand can traverse through individual vertices in multiple directions. DNA gridirons were assembled, ranging from two-dimensional arrays with reconfigurability to multilayer and three-dimensional structures and curved objects.

Self-assembly of DNA rings from scaffold-free DNA tiles.

We report a scaffold-free approach in which four- and six-helix DNA bundle units, assembled from a small number of single stranded DNA oligonucleotides precisely arranged in networks of contiguous and semicrossover strands, are connected into DNA nano rings. Nearly uniform structures with well-defined diameters of 53 ± 7, 81 ± 9, 85 ± 8, and 166 ± 13 nm were achieved by introducing uniform, in-plane curvature to the repeating units. We demonstrate that precise higher order assemblies can be achieved by fine tuning the particular features of the individual building blocks.

Mapping the thermal behavior of DNA origami nanostructures.

Understanding the thermodynamic properties of complex DNA nanostructures, including rationally designed two- and three-dimensional (2D and 3D, respectively) DNA origami, facilitates more accurate spatiotemporal control and effective functionalization of the structures by other elements. In this work fluorescein and tetramethylrhodamine (TAMRA), a Förster resonance energy transfer (FRET) dye pair, were incorporated into selected staples within various 2D and 3D DNA origami structures. We monitored the temperature-dependent changes in FRET efficiency that occurred as the dye-labeled structures were annealed and melted and subsequently extracted information about the associative and dissociative behavior of the origami. In particular, we examined the effects of local and long-range structural defects (omitted staple strands) on the thermal stability of common DNA origami structures. The results revealed a significant decrease in thermal stability of the structures in the vicinity of the defects, in contrast to the negligible long-range effects that were observed. Furthermore, we probed the global assembly and disassembly processes by comparing the thermal behavior of the FRET pair at several different positions. We demonstrated that the staple strands located in different areas of the structure all exhibit highly cooperative hybridization but have distinguishable melting temperatures depending on their positions. This work underscores the importance of understanding fundamental aspects of the self-assembly of DNA nanostructures and can be used to guide the design of more complicated DNA nanostructures, to optimize annealing protocol and manipulate functionalized DNA nanostructures.

Super-resolution fingerprinting detects chemical reactions and idiosyncrasies of single DNA pegboards.

We employ the single-particle fluorescence nanoscopy technique points accumulation for imaging in nanoscale topography (PAINT) using site-specific DNA probes to acquire two-dimensional density maps of specific features patterned on nanoscale DNA origami pegboards. We show that PAINT has a localization accuracy of ~10 nm that is sufficient to reliably distinguish dense (>10(4) features µm(-2)) sub-100 nm patterns of oligonucleotide features. We employ two-color PAINT to follow enzyme-catalyzed modification of features on individual origami and to show that single nanopegboards exhibit stable, spatially heterogeneous probe-binding patterns, or "fingerprints." Finally, we present experimental and modeling evidence suggesting that these fingerprints may arise from feature spacing variations that locally modulate the probe binding kinetics. Our study highlights the power of fluorescence nanoscopy to perform quality control on individual soft nanodevices that interact with and position reagents in solution.

Robust DNA-functionalized core/shell quantum dots with fluorescent emission spanning from UV-vis to near-IR and compatible with DNA-directed self-assembly.

The assembly and isolation of DNA oligonucleotide-functionalized gold nanoparticles (AuNPs) has become a well-developed technology that is based on the strong bonding interactions between gold and thiolated DNA. However, achieving DNA-functionalized semiconductor quantum dots (QDs) that are robust enough to withstand precipitation at high temperature and ionic strength through simple attachment of modified DNA to the QD surface remains a challenge. We report the synthesis of stable core/shell (1-20 monolayers) QD-DNA conjugates in which the end of the phosphorothiolated oligonucleotide (5-10 nucleotides) is "embedded" within the shell of the QD. These reliable QD-DNA conjugates exhibit excellent chemical and photonic stability, colloidal stability over a wide pH range (4-12) and at high salt concentrations (>100 mM Na(+) or Mg(2+)), bright fluorescence emission with quantum yields of up to 70%, and broad spectral tunability with emission ranging from the UV to the NIR (360-800 nm).

DNA origami as a carrier for circumvention of drug resistance.

Although a multitude of promising anti-cancer drugs have been developed over the past 50 years, effective delivery of the drugs to diseased cells remains a challenge. Recently, nanoparticles have been used as drug delivery vehicles due to their high delivery efficiencies and the possibility to circumvent cellular drug resistance. However, the lack of biocompatibility and inability to engineer spatially addressable surfaces for multi-functional activity remains an obstacle to their widespread use. Here we present a novel drug carrier system based on self-assembled, spatially addressable DNA origami nanostructures that confronts these limitations. Doxorubicin, a well-known anti-cancer drug, was non-covalently attached to DNA origami nanostructures through intercalation. A high level of drug loading efficiency was achieved, and the complex exhibited prominent cytotoxicity not only to regular human breast adenocarcinoma cancer cells (MCF 7), but more importantly to doxorubicin-resistant cancer cells, inducing a remarkable reversal of phenotype resistance. With the DNA origami drug delivery vehicles, the cellular internalization of doxorubicin was increased, which contributed to the significant enhancement of cell-killing activity to doxorubicin-resistant MCF 7 cells. Presumably, the activity of doxorubicin-loaded DNA origami inhibits lysosomal acidification, resulting in cellular redistribution of the drug to action sites. Our results suggest that DNA origami has immense potential as an efficient, biocompatible drug carrier and delivery vehicle in the treatment of cancer.

DNA origami with double-stranded DNA as a unified scaffold.

Scaffolded DNA origami is a widely used technology for self-assembling precisely structured nanoscale objects that contain a large number of addressable features. Typical scaffolds are long, single strands of DNA (ssDNA) that are folded into distinct shapes through the action of many, short ssDNA staples that are complementary to several different domains of the scaffold. However, sources of long single-stranded DNA are scarce, limiting the size and complexity of structures that can be assembled. Here we demonstrated that dsDNA (double-stranded DNA) scaffolds can be directly used to fabricate integrated DNA origami structures that incorporate both of the constituent ssDNA molecules. Two basic principles were employed in the design of scaffold folding paths: folding path asymmetry and periodic convergence of the two ssDNA scaffold strands. Asymmetry in the folding path minimizes unwanted complementarity between staples, and incorporating an offset between the folding paths of each ssDNA scaffold strand reduces the number of times that complementary portions of the strands are brought into close proximity with one another, both of which decrease the likelihood of dsDNA scaffold recovery. Meanwhile, the folding paths of the two ssDNA scaffold strands were designed to periodically converge to promote the assembly of a single, unified structure rather than two individual ones. Our results reveal that this basic strategy can be used to reliably assemble integrated DNA nanostructures from dsDNA scaffolds.

Reconfigurable DNA origami to generate quasifractal patterns.

The specificity of Watson-Crick base pairing, unique mechanical properties of DNA, and intrinsic stability of DNA double helices makes DNA an ideal material for the construction of dynamic nanodevices. Rationally designed strand displacement reactions can be used to produce dynamic reconfiguration of DNA nanostructures postassembly. Here we describe a 'fold-release-fold' strategy of multiple strand displacement and hybridization reactions to reconfigure a simple DNA origami structure into a complex, quasifractal pattern, demonstrating a complex transformation of DNA nanoarchitectures.

Steric crowding and the kinetics of DNA hybridization within a DNA nanostructure system.

The ability to generate precisely designed molecular networks and modulate the surrounding environment is vital for fundamental studies of chemical reactions. DNA nanotechnology simultaneously affords versatility and modularity for the construction of tailored molecular environments. We systematically studied the effects of steric crowding on the hybridization of a 20 nucleotide (nt) single-stranded DNA (ssDNA) target to a complementary probe strand extended from a rectangular six-helix tile, where the number and character of the surrounding strands influence the molecular environment of the hybridization site. The hybridization events were monitored through an increase in the quantum yield of a single reporter fluorophore (5-carboxyfluorescein) upon hybridization of the 20-nt ssDNA, an effect previously undocumented in similar systems. We observed that as the hybridization site moved from outer to inner positions along the DNA tile, the hybridization rate constant decreased. A similar rate decrease was observed when noncomplementary single- and double-stranded DNA flanked the hybridization site. However, base-pairing interactions between the hybridization site of the probe and the surrounding DNA resulted in a reduction in the reaction kinetics. The decreases in the hybridization rate constants can be explained by the reduced probability of successful nucleation of the invading ssDNA target to the complementary probe.

DNA-directed artificial light-harvesting antenna.

Designing and constructing multichromophoric, artificial light-harvesting antennas with controlled interchromophore distances, orientations, and defined donor-acceptor ratios to facilitate efficient unidirectional energy transfer is extremely challenging. Here, we demonstrate the assembly of a series of structurally well-defined artificial light-harvesting triads based on the principles of structural DNA nanotechnology. DNA nanotechnology offers addressable scaffolds for the organization of various functional molecules with nanometer scale spatial resolution. The triads are organized by a self-assembled seven-helix DNA bundle (7HB) into cyclic arrays of three distinct chromophores, reminiscent of natural photosynthetic systems. The scaffold accommodates a primary donor array (Py), secondary donor array (Cy3) and an acceptor (AF) with defined interchromophore distances. Steady-state fluorescence analyses of the triads revealed an efficient, stepwise funneling of the excitation energy from the primary donor array to the acceptor core through the intermediate donor. The efficiency of excitation energy transfer and the light-harvesting ability (antenna effect) of the triads was greatly affected by the relative ratio of the primary to the intermediate donors, as well as on the interchromophore distance. Time-resolved fluorescence analyses by time-correlated single-photon counting (TCSPC) and streak camera techniques further confirmed the cascading energy transfer processes on the picosecond time scale. Our results clearly show that DNA nanoscaffolds are promising templates for the design of artificial photonic antennas with structural characteristics that are ideal for the efficient harvesting and transport of energy.

DNA origami: a quantum leap for self-assembly of complex structures.

The spatially controlled positioning of functional materials by self-assembly is one of the fundamental visions of nanotechnology. Major steps towards this goal have been achieved using DNA as a programmable building block. This tutorial review will focus on one of the most promising methods: DNA origami. The basic design principles, organization of a variety of functional materials and recent implementation of DNA robotics are discussed together with future challenges and opportunities.

DNA origami with complex curvatures in three-dimensional space.

We present a strategy to design and construct self-assembling DNA nanostructures that define intricate curved surfaces in three-dimensional (3D) space using the DNA origami folding technique. Double-helical DNA is bent to follow the rounded contours of the target object, and potential strand crossovers are subsequently identified. Concentric rings of DNA are used to generate in-plane curvature, constrained to 2D by rationally designed geometries and crossover networks. Out-of-plane curvature is introduced by adjusting the particular position and pattern of crossovers between adjacent DNA double helices, whose conformation often deviates from the natural, B-form twist density. A series of DNA nanostructures with high curvature--such as 2D arrangements of concentric rings and 3D spherical shells, ellipsoidal shells, and a nanoflask--were assembled.

DNA nanostructures as models for evaluating the role of enthalpy and entropy in polyvalent binding.

DNA nanotechnology allows the design and construction of nanoscale objects that have finely tuned dimensions, orientation, and structure with remarkable ease and convenience. Synthetic DNA nanostructures can be precisely engineered to model a variety of molecules and systems, providing the opportunity to probe very subtle biophysical phenomena. In this study, several such synthetic DNA nanostructures were designed to serve as models to study the binding behavior of polyvalent molecules and gain insight into how small changes to the ligand/receptor scaffolds, intended to vary their conformational flexibility, will affect their association equilibrium. This approach has yielded a quantitative identification of the roles of enthalpy and entropy in the affinity of polyvalent DNA nanostructure interactions, which exhibit an intriguing compensating effect.

Molecular behavior of DNA origami in higher-order self-assembly.

DNA-based self-assembly is a unique method for achieving higher-order molecular architectures made possible by the fact that DNA is a programmable information-coding polymer. In the past decade, two main types of DNA nanostructures have been developed: branch-shaped DNA tiles with small dimensions (commonly up to ∼20 nm) and DNA origami tiles with larger dimensions (up to ∼100 nm). Here we aimed to determine the important factors involved in the assembly of DNA origami superstructures. We constructed a new series of rectangular-shaped DNA origami tiles in which parallel DNA helices are arranged in a zigzag pattern when viewed along the DNA helical axis, a design conceived in order to relax an intrinsic global twist found in the original planar, rectangular origami tiles. Self-associating zigzag tiles were found to form linear arrays in both diagonal directions, while planar tiles showed significant growth in only one direction. Although the series of zigzag tiles were designed to promote two-dimensional array formation, one-dimensional linear arrays and tubular structures were observed instead. We discovered that the dimensional aspect ratio of the origami unit tiles and intertile connection design play important roles in determining the final products, as revealed by atomic force microscopy imaging. This study provides insight into the formation of higher-order structures from self-assembling DNA origami tiles, revealing their unique behavior in comparison with conventional DNA tiles having smaller dimensions.

DNA origami: a history and current perspective.

Researchers have been using DNA for the rational design and construction of nanoscale objects for nearly 30 years. Recently, 'scaffolded DNA origami' has emerged as one of the most promising assembly techniques in DNA nanotechnology with a broad range of applications. In the past two years alone, DNA origami has been used to assemble water-soluble probe tiles for label-free RNA hybridization, to study single-molecule chemical reactions, to probe distance-dependent multivalent ligand-protein binding effects, and to organize a variety of relevant molecules including proteins, carbon nanotubes, and metal nanoparticles. This review will recount the origin, evolution, and current status of this extremely versatile assembly technique.

Molecular robots guided by prescriptive landscapes.

Traditional robots rely for their function on computing, to store internal representations of their goals and environment and to coordinate sensing and any actuation of components required in response. Moving robotics to the single-molecule level is possible in principle, but requires facing the limited ability of individual molecules to store complex information and programs. One strategy to overcome this problem is to use systems that can obtain complex behaviour from the interaction of simple robots with their environment. A first step in this direction was the development of DNA walkers, which have developed from being non-autonomous to being capable of directed but brief motion on one-dimensional tracks. Here we demonstrate that previously developed random walkers-so-called molecular spiders that comprise a streptavidin molecule as an inert 'body' and three deoxyribozymes as catalytic 'legs'-show elementary robotic behaviour when interacting with a precisely defined environment. Single-molecule microscopy observations confirm that such walkers achieve directional movement by sensing and modifying tracks of substrate molecules laid out on a two-dimensional DNA origami landscape. When using appropriately designed DNA origami, the molecular spiders autonomously carry out sequences of actions such as 'start', 'follow', 'turn' and 'stop'. We anticipate that this strategy will result in more complex robotic behaviour at the molecular level if additional control mechanisms are incorporated. One example might be interactions between multiple molecular robots leading to collective behaviour; another might be the ability to read and transform secondary cues on the DNA origami landscape as a means of implementing Turing-universal algorithmic behaviour.

A replicable tetrahedral nanostructure self-assembled from a single DNA strand.

We report the design and construction of a nanometer-sized tetrahedron from a single strand of DNA that is 286 nucleotides long. The formation of the tetrahedron was verified by restriction enzyme digestion, Ferguson analysis, and atomic force microscopy (AFM) imaging. We further demonstrate that synthesis of the tetrahedron can be easily scaled up through in vivo replication using standard molecular cloning techniques. We found that the in vivo replication efficiency of the tetrahedron is significantly higher in comparison to in vitro replication using rolling-circle amplification (RCA). Our results suggest that it is now possible to design and replicate increasingly complex, single-stranded DNA nanostructures in vivo.