DNA-Based Self-Assembly

Historically, nanoparticles have been synthesised for their intriguing colours, with one of the most famous examples, the Lycurgus Cup, exhibiting different colours depending upon whether it is illuminated from the inside or the outside. The origin of this phenomenon was unknown at the time and explained only much later on, notably thanks to the development of electron microscopes. The optical properties of the Lycurgus Cup are imparted by the presence of noble metal nanoparticles (composed of a gold–silver alloy) within the glass. With particle sizes typically ranging from 1–100 nanometres, nanoparticles possess electronic properties which are intermediate between the bulk material and their constituent atoms. For metallic nanoparticles, their optical properties are a consequence of their ability to support a localised surface plasmon resonance (LSPR). For example, 10 nm gold spheres are red, while 10 nm silver spheres are yellow. Moreover, the colour depends strongly on the shape, size, material and environment of the particles. Therefore, nanoparticles can exhibit a wide range of colours. Yet, a significantly wider range of optical properties can be accessed via the coupling of the plasmon resonances of particles in close proximity

Plasmonic isomers via DNA-based self-assembly of gold nanoparticles

Developments in DNA nanotechnology offer control of the self-assembly of materials into discrete nanostructures. Within this paradigm, pre-assembled DNA origami with hundreds of DNA strands allows for precise and programmable spatial positioning of functionalised nanoparticles. We propose an alternative approach to construct multiple, structurally different, nanoparticle assemblies from just a few complementary nanoparticle-functionalised DNA strands. The approach exploits local minima in the potential energy landscape of hybridised nanoparticle-DNA structures by employing kinetic control of the assembly. Using a four-strand DNA template, we synthesise five different 3D gold nanoparticle (plasmonic) tetrameric isomers, akin to molecular structural isomers. The number of different structures formed using this approach for a set of DNA strands represents a combinatorial library, which we summarise in a hybridisation pathway tree and use to achieve deposition of tetrahedral assemblies onto substrates in high yield. The ability to program nanoparticle self-assembly pathways gives unprecedented access to unique plasmonic nanostructures.</p

Control of electric field localization by three-dimensional bowtie nanoantennae

The three-dimensional morphological control of the individual metallic nanocrystals in a coupled structure imposes an electric field localization and enhancement in all three dimensions. We exploit the unique morphology of chemically synthesized nanotriangle monomers to form assembled dimers with a bowtie-like morphology in two orthogonal planes, effectively minimizing the volume of the interaction space to a point. The antenna has a longitudinal mode at 893 nm (1.39 eV), a 294 nm (0.68 eV) red shift compared to a monomer of equivalent size. This is indicative of extremely strong coupling because of the three-dimensional confinement of the electric field within the nanogap. By changing the geometry of the nanotriangle dimer, the longitudinal mode is tunable within a 220 nm (0.45 eV) range. The distribution of the electric field in the interparticle space transitions from a localized point for a bowtie to a more distributed line for an inverted bowtie.</p

Flexible synthesis of high-purity plasmonic assemblies

The self-assembly of nanoparticles has attracted a vast amount of attention due to the ability of the nanostructure to control light at the sub-wavelength scale, along with consequent strong electromagnetic field enhancement. However, most approaches developed for the formation of discrete assemblies are limited to a single and homogeneous system, and incorporation of larger or asymmetrical nanoparticles into assemblies with high purity remains a key challenge. Here, a simple and versatile approach to assemble nanoparticles of different sizes, shapes, and materials into various discrete homo- or hetero-structures using only two complementary deoxyribonucleic acid (DNA) strands is presented. First, surface functionalisation using DNA and alkyl-polyethylene glycol (PEG) enables transformation of as-synthesised nanoparticles into readily usable plasmonic building blocks for self-assembly. Optimisation of the DNA coverage enables the production of different assembly types, such as homo- and hetero-dimers, trimers and tetramers and core-satellite structures, which are produced in high purity using electrophoresis purification. The approach is extended from purely plasmonic structures to incorporate (luminescent) semiconductor nanoparticles for formation of hybrid assemblies. The deposited assemblies form a high yield of specific geometrical arrangements, attributed to the van der Waals attraction between particles. This method will enable the development of new complex colloidal nanoassemblies for biological and optical applications.[Figure not available: see fulltext.]</p

Copper assisted symmetry and size control of gold nanobars

Shape and size control of metal nanocrystals has become a powerful tool in tuning their physicochemical properties for applications, however, the growth mechanisms controlling shape and size are not fully elucidated. Gold nanocuboids provide a simple nanoparticle system for investigating nanocrystal growth mechanisms. Here, we study the control over size and shape anisotropy of gold nanocuboids by copper additives. We first optimize the synthesis and yield. We find that, in the presence of copper additives, symmetry is broken and anisotropic growth can occur, leading to nanobars, rather than nanocubes, accompanied by a significant reduction in particle size. We show that symmetry breaking is caused by a combination of rapid deposition on {111} facets coupled with the slow surface diffusion rate introduced by surface copper. This reveals a mechanism by which metal additives can cause symmetry breaking and control shape in nanoparticle growth.</p

Blinking of CdSe/Cd_.33Zn_.67S semiconductor nanoplatelets

Unstable photoluminescence quantum yield is important because it indicates changes in the transition rates between excited states. We synthesized 4.5 monolayer CdSe core, Cd.33Zn.67S gradient shell semiconductor nanoplatelets. The platelets exhibit a variety of blinking behaviors. Change points in the brightness of the platelets were investigated with frequentist and Bayesian techniques. We measured blinking power law constants ranging from 1.4 to 2.3. The brightness levels of blinking quantum particles are important because they are an accessible, if ambiguous, way to study surface photochemistry. Using histograms and a clustering algorithm, we determined that the number of brightness levels in the nanoplatelets is in the range of two to nine, with the lower end of that range appearing most likely and common. We conclude that the thickness and ensemble spectra are insufficient information to understand the evolving coupling between the excited states of platelets. Models of the interplay of excited state localization and reaction kinetics that span 10−10 m to 10−8 m and 10−10 s to 102 s are needed.</p

Controlling Photoluminescence for Optoelectronic Applications via Precision Fabrication of Quantum Dot/Au Nanoparticle Hybrid Assemblies

The distance-dependent interaction of an emitter with a plasmonic nanoparticle or surface forms the basis for the application of such systems within optoelectronics. Semiconductor quantum dots (QDs) are robust emitters due to their photostability. A key challenge is the formation of well-defined assemblies containing QDs and plasmonic nanoparticles in high purity. Here, we present the translation of DNA-based self-assembly to assemble metal and semiconductor nanocrystals into hybrid structures. The high purity of the assemblies, including dimers and higher-order core-satellite structures, allows fundamental investigation of the plasmon-exciton interaction. In contrast to the increase in the QD emission rate and enhancement in steady-state photoluminescence observed for overlap between the QD emission and localized surface plasmon resonance, significant detuning of these energies leads to lengthening of the QD emission lifetime (reduction of the emission rate) up to 1.7-fold and enhancement in steady-state photoluminescence of 15-75%. These results are understood in terms of the Purcell effect, where the gold nanoparticle acts as a damped, nanoscale cavity. We show that the response is driven by the interference experienced by the emitter for parallel and perpendicular field orientations. This provides a mechanism for control of the emission rate of a QD by a metal nanoparticle and a much wider range of lifetimes than previously understood.</p

Control of Symmetry Breaking Size and Aspect Ratio in Gold Nanorods:Underlying Role of Silver Nitrate

Single crystal gold nanorods remain one of the most important and intensively studied anisotropic nanocrystals. The aspect ratio of the nanorods is controlled during the colloidal synthesis using silver nitrate; however, the mechanisms for the underlying control are not well understood. Here, we investigate the growth of gold nanocrystals at the stage where they break symmetry and begin anisotropic growth into nanorods. Using high resolution electron microscopy, we determine directly the size and atomic structure of the nanocrystals at the symmetry breaking point. We find that silver nitrate controls the size of the crystal at which symmetry breaking occurs. The seed crystal undergoes a symmetry breaking event at a critical diameter between 4 and 6 nm that depends upon the [HAuCl4]:[AgNO3] ratio. The smallest diameter for symmetry breaking, ∼4 nm, is observed at the lowest [HAuCl4]:[AgNO3] ratio (i.e., the highest AgNO3 concentration) corresponding to the minimum size at which a "truncation" can form, a precursor to a {110} facet. The diameter of the nanocrystal at the symmetry breaking point becomes the width of the nascent nanorod, and this in turn determines the final nanorod width. Surprisingly, the [HAuCl4]:[AgNO3] ratio has little effect on the final nanorod length. Our observations explain why the nanorod aspect ratio is constrained within a limited range. This provides a rational framework for controlling width and aspect ratio in the growth of single crystal gold nanorods.</p

Study of polycarbonate–polystyrene interfaces using scanning transmission electron microscopy spectrum imaging (STEM-SI)

Polymer blends are important for both commercial utility and scientific understanding. The degree of interfacial mixing in polymer blends is important since it influences the blends' mechanical properties. Understanding bulk properties in multiphase polymeric materials requires knowledge of the interfacial properties of the materials. The characterization of the interface, in terms of its width and composition profile, provides insight about the bulk behaviour of the material. Chemical microscopy through electron energy-loss spectroscopy (EELS) in a transmission electron microscope is gaining popularity to characterize narrow polymer–polymer interfaces. In this work, we show how scanning transmission electron microscopy spectrum imaging, a spatially resolved energy-loss spectroscopy, can be employed to calculate the interfacial width in a pair of immiscible polymers, taking a polycarbonate–polystyrene (PC-PS) bilayer as an example. By mapping peaks unique to each of the blend constituents at several points across the interface, we show how the interfacial profile concentrations can be determined. With this method we calculated the interfacial width in the PC-PS bilayer sample to be approximately 32 nm, even utilizing low resolution spectrometers, which are more widely available. Using the technique described with higher resolution EELS instruments having a better signal-to-noise ratio, a higher spatial resolution can be achieved. Using EELS chemical fingerprints of polymers that have been developed earlier, the technique presented here has the potential for effective visualization and morphological measurements of phase-differentiated polymer blends. This paper is an attempt to enable a new user to characterize polymer–polymer interfaces using chemical microscopy.</p