DNA-linked metal nanosphere materials: Fourier-transform solutions for the optical response

Localized surface plasmon resonance based biosensing

INTRODUCTION

Bioanalytical sensing based on the principle of localized surface plasmon resonance experiences is currently an extremely rapid development. Novel sensors with new kinds of plasmonic transducers and innovative concepts for the signal development as well as read-out principles were identified. This review will give an overview of the development of this field. Areas covered: The focus is primarily on types of transducers by preparation or dimension, factors for optimal sensing concepts and the critical view of the usability of these devices as innovative sensors for bioanalytical applications. Expert commentary: Plasmonic sensor devices offer a high potential for future biosensing given that limiting factors such as long-time stability of the transducers, the required high sensitivity and the cost-efficient production are addressed. For higher sensitivity, the design of the sensor in shape and material has to be combined with optimal enhancement strategies. Plasmonic nanoparticles from bottom-up synthesis with a post-synthetic processing show a high potential for cost-efficient sensor production. Regarding the measurement principle, LSPRi offers a large potential for multiplex sensors and can provide a high-throughput as well as highly paralleled sensing. The main trends are expected towards optimal LSPR concepts which represent cost-efficient and robust point-of-care solutions, and the use of multiplexed devices for clinical applications.

Plasmonic behavior of ionic liquid stabilized gold nanoparticles in molecular solvents

In this paper, we have demonstrated the facile synthesis of stable gold nanoparticles (AuNPs) using imidazolium ionic liquids (ImILs) as a stabilizer as well as a surfactant and their surface plasmon resonance (SPR) in different molecular solvents with varying dielectric properties.

Magneto-Optical Response of Cobalt Interacting with Plasmonic Nanoparticle Superlattices

The magneto-optical Kerr effect is a striking phenomenon whereby the optical properties of a material change under an applied magnetic field. Though promising for sensing and data storage technology, these properties are typically weak in magnitude and are inherently limited by the bulk properties of the active magnetic material. In this work, we theoretically demonstrate that plasmonic thin-film assemblies on a cobalt substrate can achieve tunable transverse magneto-optical (TMOKE) responses throughout the visible and near-infrared (300-900 nm). In addition to exhibiting wide spectral tunability, this response can be varied in sign and magnitude by changing the plasmonic volume fraction (1-20%), the composition and arrangement of the assembly, and the shape of the nanoparticle inclusions. Of particular interest is the newly discovered sensitivity of the sign and intensity of the TMOKE spectrum to collective metallic plasmonic behavior in silver, mixed silver-gold, and anisotropic superlattices.

Defect tolerance and the effect of structural inhomogeneity in plasmonic DNA-nanoparticle superlattices

Bottom-up assemblies of plasmonic nanoparticles exhibit unique optical effects such as tunable reflection, optical cavity modes, and tunable photonic resonances. Here, we compare detailed simulations with experiment to explore the effect of structural inhomogeneity on the optical response in DNA-gold nanoparticle superlattices. In particular, we explore the effect of background environment, nanoparticle polydispersity (>10%), and variation in nanoparticle placement (∼5%). At volume fractions less than 20% Au, the optical response is insensitive to particle size, defects, and inhomogeneity in the superlattice. At elevated volume fractions (20% and 25%), structures incorporating different sized nanoparticles (10-, 20-, and 40-nm diameter) each exhibit distinct far-field extinction and near-field properties. These optical properties are most pronounced in lattices with larger particles, which at fixed volume fraction have greater plasmonic coupling than those with smaller particles. Moreover, the incorporation of experimentally informed inhomogeneity leads to variation in far-field extinction and inconsistent electric-field intensities throughout the lattice, demonstrating that volume fraction is not sufficient to describe the optical properties of such structures. These data have important implications for understanding the role of particle and lattice inhomogeneity in determining the properties of plasmonic nanoparticle lattices with deliberately designed optical properties.

Conformal, macroscopic crystalline nanoparticle sheets assembled with DNA

A novel method for preparing conformal silica-embedded crystalline nanoparticle sheets via DNA programmable assembly provides independent control over nanoparticle size, nanoparticle spacing, and film thickness. The conformal materials retain the nanoparticle crystallinity and spacing after being transferred to flat or highly curved substrates even after being subjected to various mechanical, physical, and chemical stimuli.

Nanoscale form dictates mesoscale function in plasmonic DNA-nanoparticle superlattices

The nanoscale manipulation of matter allows properties to be created in a material that would be difficult or even impossible to achieve in the bulk state. Progress towards such functional nanoscale architectures requires the development of methods to precisely locate nanoscale objects in three dimensions and for the formation of rigorous structure-function relationships across multiple size regimes (beginning from the nanoscale). Here, we use DNA as a programmable ligand to show that two- and three-dimensional mesoscale superlattice crystals with precisely engineered optical properties can be assembled from the bottom up. The superlattices can transition from exhibiting the properties of the constituent plasmonic nanoparticles to adopting the photonic properties defined by the mesoscale crystal (here a rhombic dodecahedron) by controlling the spacing between the gold nanoparticle building blocks. Furthermore, we develop a generally applicable theoretical framework that illustrates how crystal habit can be a design consideration for controlling far-field extinction and light confinement in plasmonic metamaterial superlattices.

Using nanoscale and mesoscale anisotropy to engineer the optical response of three-dimensional plasmonic metamaterials

The a priori ability to design electromagnetic wave propagation is crucial for the development of novel metamaterials. Incorporating plasmonic building blocks is of particular interest due to their ability to confine visible light. Here we explore the use of anisotropy in nanoscale and mesoscale plasmonic array architectures to produce noble metal-based metamaterials with unusual optical properties. We find that the combination of nanoscale and mesoscale anisotropy leads to rich opportunities for metamaterials throughout the visible and near-infrared. The low volume fraction (<5%) plasmonic metamaterials explored herein exhibit birefringence, a skin depth approaching that of pure metals for selected wavelengths, and directionally confined waves similar to those found in optical fibres. These data provide design principles with which the electromagnetic behaviour of plasmonic metamaterials can be tailored using high aspect ratio nanostructures that are accessible via a variety of synthesis and assembly methods.

Engineering the metathesis and oxidation-reduction reaction in solid state at room temperature for nanosynthesis

It is a long-standing goal to explore convenient synthesis methodology for functional materials. Recently, several multiple-step approaches have been designed for photocatalysts Ag(n)X@Ag (X = Cl(-), PO4(3-), etc.), mainly containing the ion-exchange (metathesis) reaction followed by photoreduction in solution. But they were obsessed by complicated process, the uncontrollability of composition and larger sizes of Ag particles. Here we show a general solid-state route for the synthesis of Ag(n)X@Ag catalysts with hierarchical structures. Due to strong surface plasmon resonance of silver nanoparticles with broad shape and size, the Ag(n)X@Ag showed high photocatalytic activity in visible region. Especially, the composition of Ag(n)X@Ag composites could be accurately controlled by regulating the feed ratio of (NH2OH)2 ·H2SO4 to anions, by which the performance were easily optimized. Results demonstrate that the metathesis and oxidation-reduction reactions can be performed in solid state at room temperature for nanosynthesis, greatly reducing the time/energy consumption and pollution.

Structures of DNA-linked nanoparticle aggregates

The room-temperature structure of DNA-linked gold nanoparticle aggregates is investigated using a combination of experiment and theory. The experiments involve extinction spectroscopy measurements and dynamic light scattering measurements of aggregates made using 60 and 80 nm gold particles and 30 base-pair DNA. The theoretical studies use calculated spectra for models of the aggregate structures to determine which structure matches the observations. These models include diffusion-limited cluster-cluster aggregation (DLCA), reaction-limited cluster-cluster aggregation (RLCA), and compact (nonfractal) cluster aggregation. The diameter of the nanoparticles used in the experiments is larger than has been considered previously, and this provides greater sensitivity of spectra to aggregate structure. We show that the best match between experiment and theory occurs for the RLCA fractal structures. This indicates that DNA hybridization takes place under irreversible conditions in the room-temperature aggregation. Some possible structural variations which might influence the result are considered, including the edge-to-edge distance between nanoparticles, variation in the diameter of the nanoparticles, underlying lattice structures of on-lattice compact clusters, and positional disorders in the lattice structures. We find that these variations do not change the conclusion that the room-temperature structure of the aggregates is fractal. We also examine the variation in extinction at 260 nm as temperature is increased, showing that the decrease in extinction at temperatures below the melting temperature is related to a morphological change from fractal toward compact structures.

Complexes of DNA bases and Watson-Crick base pairs with small neutral gold clusters

The nature of the DNA-gold interaction determines and differentiates the affinity of the nucleobases (adenine, thymine, guanine, and cytosine) to gold. Our preliminary computational study [Kryachko, E. S.; Remacle, F. Nano Lett. 2005, 5, 735] demonstrates that two major bonding factors govern this interaction: the anchoring, either of the Au-N or Au-O type, and the nonconventional N-H...Au hydrogen bonding. In this paper, we offer insight into the nature of nucleobase-gold interactions and provide a detailed characterization of their different facets, i.e., geometrical, energetic, and spectroscopic aspects; the gold cluster size and gold coordination effects; proton affinity; and deprotonation energy. We then investigate how the Watson-Crick DNA pairing patterns are modulated by the nucleobase-gold interaction. We do so in terms of the proton affinities and deprotonation energies of those proton acceptors and proton donors which are involved in the interbase hydrogen bondings. A variety of properties of the most stable Watson-Crick [A x T]-Au3 and [G x C]-Au3 hybridized complexes are described and compared with the isolated Watson-Crick A x T and G x C ones. It is shown that enlarging the gold cluster size to Au6 results in a rather short gold-gold bond in the Watson-Crick interbase region of the [G x C]-Au6 complex that bridges the G x C pair and thus leads to a significant strengthening of G x C pairing.

Sharp melting in DNA-linked nanostructure systems: thermodynamic models of DNA-linked polymers

Sharp melting that has been found for DNA-linked nanostructure systems such as DNA-linked gold nanoparticles enhances the resolution of DNA sequence detection enough to distinguish between a perfect match and single base pair mismatches. One intriguing explanation of the sharp melting involves the cooperative dehybridization of DNA strands between the nanostructures. However, in the DNA-linked gold nanoparticle system, strong optical absorption by the gold nanoparticles hinders the direct observation of cooperativity. Here, with a combination of theory and experiment, we investigate a DNA-linked polymer system in which we can show that the optical profile of the system at 260 nm is directly related to the individual DNA dehybridization profile, providing a clear distinction from other possible mechanisms. We find that cooperativity plays a crucial role in determining both the value of the melting temperature and the shape of the melting profile well away from the melting temperature. Our analysis suggests that the dehybridization properties of DNA strands in confined or dense structures differ from DNA in solution.

The Supramolecular Chemistry of Organic–Inorganic Hybrid Materials

The combination of nanomaterials as solid supports and supramolecular concepts has led to the development of hybrid materials with improved functionalities. These "hetero-supramolecular" ideas provide a means of bridging the gap between molecular chemistry, materials sciences, and nanotechnology. In recent years, relevant examples have been reported on functional aspects, such as enhanced recognition and sensing by using molecules on preorganized surfaces, the reversible building of nanometer-sized networks and 3D architectures, as well as biomimetic and gated chemistry in hybrid nanomaterials for the development of advanced functional protocols in three-dimensional frameworks. This approach allows the fine-tuning of the properties of nanomaterials and offers new perspectives for the application of supramolecular concepts.

Plasmon interactions between gold nanoparticles in aqueous solution with controlled spatial separation

The effects of interparticle distance on the UV-visible absorption spectrum of gold nanocrystals aggregates in aqueous solution have been investigated. The aggregates were produced by ion-templated chelation of omega-mercaptocarboxylic acid ligands covalently attached to the nanoparticles surface. Variation of the ligand chain length provides control over the interparticle separation in the aggregates. The UV-visible spectra consist typically of a single particle band and a secondary band at higher wavelengths associated with the formation of aggregates in solution. The position of the latter depends on interparticle separation up to distances of approximately 8 nm, in accordance with existing models. Potential applications therefore include distance sensitive labels or proximity probes. Conversely, variation of the ligand length allows the preparation of nanostuctured materials with tuned optical properties.

One-step synthesis of amino-dextran-protected gold and silver nanoparticles and its application in biosensors

A sensitive method for the detection of the lectin protein concanavalin A (Con A) was developed using amino-dextran (AD)-protected gold (AD-Au) and silver nanoparticles (AD-Ag) as sensitive optical probes. The AD-Au and AD-Ag nanoparticles were synthesized by directly applying amino-dextran as a reductive and protective reagent. The size of the nanoparticles could be altered by changing the molar ratio of AD to the metal salt. The amino-dextran bound to Con A by forming a 4:1 Au-Con A complex at neutral pH, and the nanoparticles were induced to aggregate by Con A. The absorption intensity of the nanoparticles decreased linearly with as the Con A concentration was increased from 3.85 x 10(-8) to 6.15 x 10(-7) M. The Au-Con A complex was dissociated by the disaccharide isomaltose, which has a higher affinities for Con A than Au; this competitive strategy could also be used to detect similar types of saccharides.

Ultrafast electronic relaxation and coherent vibrational oscillation of strongly coupled gold nanoparticle aggregates

We report the first direct observation of the ultrafast electronic relaxation and coherent vibrational oscillation of strongly interacting gold nanoparticle aggregates measured by femtosecond laser spectroscopy. The electronic relaxation, reflected as a fast decay component with a time constant of 1.5-2.5 ps, becomes faster with decreasing pump power, similar to earlier observations of isolated gold nanoparticles. Surprisingly, periodic oscillations have been observed in the transient absorption/bleach signal and have been attributed to the coherent vibrational excitation of the gold nanoparticle aggregates. The oscillation period has been found to depend on the probe wavelength. As the probe wavelength is varied from 720 to 850 nm, the period changes from 37 to 55 ps. This suggests that the broad extended plasmon band (EPB) contains contributions from gold nanoparticle aggregates with different sizes and/or different fractal structures. Each of the different probe wavelengths therefore interrogates one subset of the aggregates with similar size or structure. Interestingly, the observed oscillation period for a given aggregate size determined by dynamic light scattering is longer than that predicted based on a elastic sphere model. One possible explanation is that the actual size of the aggregates is larger than what was observed from dynamic light scattering. An alternative, perhaps more likely, explanation is that the vibration of the aggregates is "softer" than that of hard spherical gold nanoparticles possibly because the longitudinal speed of sound is lower in the aggregates than in bulk gold. Persistent spectral hole burning was performed and yielded a hole in the nanoparticle aggregate's extended plasmon band, further supporting that the near-IR band is composed of absorption subbands from differently sized/structured aggregates.