Mathew M. Maye

DNA-guided crystallization of colloidal nanoparticles

Many nanometre-sized building blocks will readily assemble into macroscopic structures. If the process is accompanied by effective control over the interactions between the blocks and all entropic effects, then the resultant structures will be ordered with a precision hard to achieve with other fabrication methods. But it remains challenging to use self-assembly to design systems comprised of different types of building blocks—to realize novel magnetic, plasmonic and photonic metamaterials, for example. A conceptually simple idea for overcoming this problem is the use of ‘encodable’ interactions between building blocks; this can in principle be straightforwardly implemented using biomolecules. Strategies that use DNA programmability to control the placement of nanoparticles in one and two dimensions have indeed been demonstrated. However, our theoretical understanding of how to extend this approach to three dimensions is limited, and most experiments have yielded amorphous aggregates and only occasionally crystallites of close-packed micrometre-sized particles. Here, we report the formation of three-dimensional crystalline assemblies of gold nanoparticles mediated by interactions between complementary DNA molecules attached to the nanoparticles’ surface. We find that the nanoparticle crystals form reversibly during heating and cooling cycles. Moreover, the body-centred-cubic lattice structure is temperature-tuneable and structurally open, with particles occupying only ∼4% of the unit cell volume. We expect that our DNA-mediated crystallization approach, and the insight into DNA design requirements it has provided, will facilitate both the creation of new classes of ordered multicomponent metamaterials and the exploration of the phase behaviour of hybrid systems with addressable interactions.

Switching binary states of nanoparticle superlattices and dimer clusters by DNA strands.

Nanoscale components can be self-assembled into static three-dimensional structures, arrays and clusters using biomolecular motifs. The structural plasticity of biomolecules and the reversibility of their interactions can also be used to make nanostructures that are dynamic, reconfigurable and responsive. DNA has emerged as an ideal biomolecular motif for making such nanostructures, partly because its versatile morphology permits in situ conformational changes using molecular stimuli. This has allowed DNA nanostructures to exhibit reconfigurable topologies and mechanical movement. Recently, researchers have begun to translate this approach to nanoparticle interfaces, where, for example, the distances between nanoparticles can be modulated, resulting in a distance-dependent plasmonic response. Here, we report the assembly of nanoparticles into three-dimensional superlattices and dimer clusters, using a reconfigurable DNA device that acts as an interparticle linkage. The interparticle distances in the superlattices and clusters can be modified, while preserving structural integrity, by adding molecular stimuli (simple DNA strands) after the self-assembly processes has been completed. Both systems were found to switch between two distinct rigid states, but a transition to a flexible device configuration within a superlattice showed a significant hysteresis.

Stepwise surface encoding for high-throughput assembly of nanoclusters

Self-assembly offers a promising method to organize functional nanoscale objects into two-dimensional (2D) and 3D superstructures for exploiting their collective effects. On the other hand, many unique phenomena emerge after arranging a few nanoscale objects into clusters, the so-called artificial molecules. The strategy of using biomolecular linkers between nanoparticles has proven especially useful for construction of such nanoclusters. However, conventional solution-based reactions typically yield a broad population of multimers or isomers of clusters; furthermore, the efficiency of fabrication is often limited. Here, we describe a novel high-throughput method for designing and fabricating clusters using DNA-encoded nanoparticles assembled on a solid support in a stepwise manner. This method efficiently imparts particles with anisotropy during their assembly and disassembly at a surface, generating remarkably high yields of well-defined dimer clusters and Janus (two-faced) nanoparticles. The method is scalable and modular, assuring large quantities of clusters of designated sizes and compositions.

Photoluminescence enhancement in CdSe/ZnS–DNA linked–Au nanoparticle heterodimers probed by single molecule spectroscopy

Photoluminescence enhancement of up to 20 fold is demonstrated at the single molecule level for heterodimers composed of a core/shell CdSe/ZnS semiconductive quantum dot and a gold nanoparticle of 60 nm size separated by a 32 nm-long dsDNA linker when employing optical excitation at wavelengths near the surface plasmon resonance of the gold nanoparticle.

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.

Designing quantum rods for optimized energy transfer with firefly luciferase enzymes

The bioluminescence resonance energy transfer (BRET) between firefly luciferase from Photinus pyralis (Ppy) with core/shell semiconductive quantum rods (QRs) has been studied as a function of QR aspect ratio and internal microstructure. The QRs were found to be ideal energy acceptors, and Ppy-to-core distances were optimized using rod-in-rod microstructures that were achieved by the synthetic control of rod morphology, surface chemistry, and Ppy:QR loading. The BRET ratios of >44 measured are the highest efficiencies to date.

DNA-capped nanoparticles designed for doxorubicin drug delivery

The anticancer drug, doxorubicin (DOX), was loaded onto DNA-capped gold nanoparticles (AuNP) designed for specific DOX intercalation. Drug binding was confirmed by monitoring DNA melting temperature, AuNP plasmon resonance maximum, and hydrodynamic radius increase, as a function of [DOX]/[DNA] ratio. The capacity for drug release to target DNA was confirmed.

Novel multistep BRET-FRET energy transfer using nanoconjugates of firefly proteins, quantum dots, and red fluorescent proteins

Sequential bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) from firefly luciferase to red fluorescent proteins using quantum dot or rod acceptor/donor linkers is described. The effect of morphology and tuned optical properties on the efficiency of this unique BRET-FRET system was evaluated.

A Modular Phase Transfer and Ligand Exchange Protocol for Quantum Dots

In this paper, we describe a quantum dot (qdot) phase transfer protocol using ligand exchange and the amino acid histidine. The phase transfer from nonpolar solvents to aqueous buffers is homogeneous, and no appreciable precipitation occurs. The molecule histidine was chosen in order to first displace the organic encapsulation and second to provide a weakly chemisorbing intermediate at the qdot ionic interface. This allows the histidine to act as an intermediate shell upon which further direct ligand exchange can occur. Since this intermediate encapsulation is easily displaced by an assortment of different molecules while in aqueous buffers, we refer to this approach as modular. Characterization via FTIR and NMR revealed the extent of ligand exchange, and provides insights into the interfacial binding mechanism. The colloidal stability and photostability of the qdots was probed via UV-vis and steady state fluorescence, which revealed promising quantum yield stability of greater than 1 year. The qdots have hydrodynamic diameters of <12 nm and surface charges dependent upon ligand type and coverage. The modularity of this approach is shown by tailoring the qdot surface charge via sequential ligand exchange using mixed monolayers of carboxylic acid and poly(ethylene glycol)-terminated thiols.

Multifunctional DNA-Gold Nanoparticles for Targeted Doxorubicin Delivery

In this report we describe the synthesis, characterization, and cytotoxic properties of DNA-capped gold nanoparticles having attached folic acid (FA), a thermoresponsive polymer (p), and/or poly(ethylene glycol) (PEG) oligomers that could be used to deliver the anticancer drug doxorubicin (DOX) in chemotherapy. The FA-DNA oligomer used in the construction of the delivery vehicle was synthesized through the reaction of the isolated folic acid N-hydroxysuccinimide ester with the amino-DNA and the conjugated DNA product was purified using high performance liquid chromatography (HPLC). This approach ultimately allowed control of the amount of FA attached to the surface of the delivery vehicle. Cytotoxicity studies using SK-N-SH neuroblastoma cells with drug loaded delivery vehicles were carried out using a variety of exposure times (1-48 h) and recovery times (1-72 h), and in order to access the effects of varying amounts of attached FA, in culture media deficient in FA. DOX loaded delivery vehicles having 50% of the DNA strands with attached FA were more cytotoxic than when all of the strands contained FA. Since FA stimulates cell growth, the reduced cytotoxicity of vehicles fully covered with FA suggests that the stimulatory effects of FA can more than compensate for the cytotoxic effects of the drug on the cell population. While attachment of hexa-ethylene glycol PEG(18) to the surface of the delivery vehicle had no effect on cytotoxicity, 100% FA plus the thermoresponsive polymer resulted in IC50 = 0.48 ± 0.01 for an exposure time of 24 h and a recovery time of 1 h, which is an order of magnitude more cytotoxic than free DOX. Confocal microscopic studies using fluorescence detection showed that SK-N-SH neuroblastoma cells exposed to DOX-loaded vehicles have drug accumulation inside the cell and, in the case of vehicles with attached FA and thermoresponsive polymer, the drug appears more concentrated. Since the biological target of DOX is DNA, the latter observation is consistent with the high cytotoxicity of vehicles having both FA and the thermoresponsive polymer. The study highlights the potential of DNA-capped gold nanoparticles as delivery vehicles for doxorubicin in cancer chemotherapy.