Preferential end-to-end assembly of gold nanorods by biotin-streptavidin connectors

Synthesis and bioanalytical applications of specific-shaped metallic nanostructures: a review

Many successful synthesis routes for producing different shapes of metallic nanostructures, including sphere, rod, cube, and hollow shapes, have been developed in the past few decades. Many applications using these nanostructures have been studied because the outstanding properties of the nanostructures are not exhibited by their bulk-state counterparts. This review paper reports some recent developments in clinical and biosensor applications. The first part focused on the synthesis methods of metallic nanostructures having various shapes along with their optical properties. The second and third part is an introduction of the gold nanoparticle assemblies and arrays, explaining the conjugation methods of metallic nanostructures with biological entities. The final part reviews on the recent bioanalytical applications using various shapes of metallic nanostructures.

Synthetically programmable nanoparticle superlattices using a hollow three-dimensional spacer approach

Crystalline nanoparticle arrays and superlattices with well-defined geometries can be synthesized by using appropriate electrostatic, hydrogen-bonding or biological recognition interactions. Although superlattices with many distinct geometries can be produced using these approaches, the library of achievable lattices could be increased by developing a strategy that allows some of the nanoparticles within a binary lattice to be replaced with 'spacer' entities that are constructed to mimic the behaviour of the nanoparticles they replace, even though they do not contain an inorganic core. The inclusion of these spacer entities within a known binary superlattice would effectively delete one set of nanoparticles without affecting the positions of the other set. Here, we show how hollow DNA nanostructures can be used as 'three-dimensional spacers' within nanoparticle superlattices assembled through programmable DNA interactions. We show that this strategy can be used to form superlattices with five distinct symmetries, including one that has never before been observed in any crystalline material.

Using silver nanowire antennas to enhance the conversion efficiency of photoresponsive DNA nanomotors

Plasmonic near-field coupling can induce the enhancement of photoresponsive processes by metal nanoparticles. Advances in nanostructured metal synthesis and theoretical modeling have kept surface plasmons in the spotlight. Previous efforts have resulted in significant intensity enhancement of organic dyes and quantum dots and increased absorption efficiency of optical materials used in solar cells. Here, we report that silver nanostructures can enhance the conversion efficiency of an interesting type of photosensitive DNA nanomotor through coupling with incorporated azobenzene moieties. Spectral overlap between the azobenzene absorption band and plasmonic resonances of silver nanowires increases light absorption of photon-sensitive DNA motor molecules, leading to 85% close-open conversion efficiency. The experimental results are consistent with our theoretical calculations of the electric field distribution. This enhanced conversion of DNA nanomotors holds promise for the development of new types of molecular nanodevices for light manipulative processes and solar energy harvesting.

Reversible assembly and disassembly of gold nanorods induced by EDTA and its application in SERS tuning

A facile and reversible method for assembling and disassembling gold nanorods (GNRs) using a common chelating agent, ethylenediaminetetraacetic acid (EDTA), is reported. Assembly was induced by the electrostatic interaction between the cetyltrimethylammonium bromide (CTAB) bilayer present on GNRs and EDTA. At lower concentrations of EDTA, end-to-end assembled chains were formed. At higher concentrations of EDTA, these chains come together to form sheet-like structures. The complex of CTAB and EDTA, being labile, disassembles in the presence of stronger chelating agents. Upon addition of metal ions having higher formation constants, EDTA detaches from the GNRs and forms stronger complexes with metal ions, resulting in disassembly. Characteristic changes were observed in the UV/vis spectra. Addition of EDTA resulted in a red shift of longitudinal surface plasmon (LSP) resonance at lower concentrations, indicating an end-to-end assembly. At higher concentrations, the characteristic of side-by-side assembly was seen in the UV/vis spectra. TEM analysis proved the existence of end-to-end chains at lower concentrations of EDTA and side-by-side assembled sheet-like structures at higher concentrations. The addition of metal ions induced disassembly. Even 2 ppb of metal ion was detected using the spectral changes. Disassembly was studied in detail, taking Pb(II) as the model system. Upon addition of Pb(II), TSP showed a blue shift and decreased in intensity while the LSP showed a red shift and increased in intensity. A new peak at a higher wavelength region emerged, pointing to the existence of both side-by-side and end-to-end assembly in the system. TEM analysis showed that the disassembly involves the formation of bundled chains which may be the reason for the observed spectral changes. Surface-enhanced Raman scattering (SERS) activity of the system could be tuned by controlling the concentration of EDTA and the metal ion, Pb(II).

Synthetically directed self-assembly and enhanced surface-enhanced Raman scattering property of twinned crystalline Ag/Ag homojunction nanoparticles

A synthetically directed self-assembly strategy to the aqueous-phase synthesis of twinned crystalline silver/silver homojunction nanoparticles (Ag/Ag HJNPs) is demonstrated. In the self-assembly, ethylenediamine tetraacetic acid disodium (EDTA) and solution pH values play a crucial role in the formation of Ag/Ag HJNPs while the sizes of Ag nanoparticles (NPs) in the Ag/Ag HJNPs depend on the reductant concentrations of ascorbic acid. Surface-enhanced Raman scattering (SERS) measurements indicate that the SERS intensity acquired from the Ag/Ag HJNP colloidal solution is about 200 times stronger than that obtained from isolated Ag NP colloid solution. The plasmonic and SERS behaviors of Ag/Ag HJNPs were simulated by discrete-dipole approximation (DDA) and three-dimensional finite-difference time domain (3D-FDTD) methods, respectively. Theoretical calculation results disclose that surface plasmon resonance (SPR) properties of the Ag/Ag HJNPs are different from those of isolated Ag nanospheres, and their maximal SERS enhancement is about 2 orders of magnitude higher than that of isolated Ag nanospheres, which is in good agreement with the experimental results. The extra SERS enhancement can be explained by the hot spots at homojunction structures between Ag particles because of near-field coupling effect.

Anisotropic nanomaterials: structure, growth, assembly, and functions

Comprehensive knowledge over the shape of nanomaterials is a critical factor in designing devices with desired functions. Due to this reason, systematic efforts have been made to synthesize materials of diverse shape in the nanoscale regime. Anisotropic nanomaterials are a class of materials in which their properties are direction-dependent and more than one structural parameter is needed to describe them. Their unique and fine-tuned physical and chemical properties make them ideal candidates for devising new applications. In addition, the assembly of ordered one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) arrays of anisotropic nanoparticles brings novel properties into the resulting system, which would be entirely different from the properties of individual nanoparticles. This review presents an overview of current research in the area of anisotropic nanomaterials in general and noble metal nanoparticles in particular. We begin with an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. Then we describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies, and conclude with major applications.

Peptide-tailored assembling of Aunanorods

A simple and fast way to study the interaction between the Au NRs and peptides was developed, which makes potential peptide recognition and detection possible.

A kinetic Monte Carlo analysis for the production of singularly tethered carbon nanotubes

Nanoparticles that possess a single covalent tether to either another particle or a surface play an increasingly important role in nanotechnology, serving as a foundation for aggregation-based plasmonic sensors, chemically assembled framework structures, and scanning probe tips. Using a theoretical approach, we explore the reaction conditions necessary to maximize singular tethering for several cases of homogeneously dispersed nanoparticles, with a particular focus on single-walled carbon nanotubes. In the limit of particles of monodisperse size and equal site reactivity, the number of tethers versus the reaction conversion is statistically described by the well-known binomial distribution, with a variance that is minimal for the single tether case. However, solutions of nanoparticles often deviate from this ideal, and reaction events can introduce steric hindrance to neighboring sites or alter particle electronic properties, both of which can influence local reactivity. In order to study these cases we use the electron transfer reactions of single-walled carbon nanotubes. We find that the distribution in the number of monofunctional tubes, as a function of conversion, is largely dependent on the distribution of nanotube rate constants, and therefore tube chiralities, in the initial solution. As a contemporary example, we examine the implications of this result on the metallic-semiconductor separation of carbon nanotubes using electron transfer chemistry.

High-sensitivity biosensors fabricated by tailoring the localized surface plasmon resonance property of core-shell gold nanorods

An enhanced sensitive biosensor has been developed to detect biological targets by tailoring the localized surface plasmon resonance property of core-shell gold nanorods. In this new concept, a shell layer is produced on gold nanorods by generating a layer of chalcogenide on the gold nanorod surface after attachment of the recognition reagent, namely, goat IgG and antigen of schistosomiasis japonica. The bioactivity of these attached biomolecules is retained and the sensitivity of this biosensor is thus enhanced significantly. The plasmonic properties of the gold nanorods attached with the biomolecules can be adjusted and the plasmon resonance wavelength can be red-shifted up to several hundred nanometers in the visible or near infrared (NIR) region, which is extremely important to biosensing applications. This leads to a lager red-shift in the localized surface plasmon resonance absorption compared to the original gold nanorod-based sensor and hence offers greatly enhanced sensitivity in the detection of schistosomiasis japonica. The human serum infected with schistosomiasis japonica diluted to 1:50,000 (volume ratio, serum/buffer solution) can be detected readily. The technique offers enhanced sensitivity and can be easily extended to other sensing applications based on not only immuno-recognition but also other types of specific reactions.

Plasmonic signatures in the composite crystals of gold nanoparticles and p-hydroxyacetanilide (paracetamol)

A new type of inorganic nanoparticle-organic hybrid crystalline material consisting of gold nanoparticles (Au NPs) and p-hydroxyacetanilide (pHA) is reported. The composite crystals were on the order of several millimeters in dimensions. They could be grown from a solution of Au NPs and pHA at 35 °C. The optical properties of the crystals not only reflected the presence of Au NPs but also their degree of association inside the crystals. Single crystal X-ray diffraction data indicated that the crystal motifs were those of pHA. Transmission electron microscopic images indicated Au NPs being dispersed randomly in the crystal with increase in their density when crystallized in the presence of low concentration of pHA. FTIR measurements indicated attachment of -NH group to the NPs. Optical microscopic investigation revealed the presence of Au NP crystals, the color of which represented their density, being red at low concentration of NPs and purple at their high concentration.

Biological surface effects of metallic nanomaterials for applications in assembly and catalysis

Recent experimental evidence has suggested that bioinspired techniques represent promising avenues toward the production of functional nanomaterials that possess a high degree of activity. These materials are prepared under synthetically simple and efficient conditions, thus making them attractive alternatives to many traditional methods that employ hazardous and harsh conditions. Many biomimetic methods employ peptide and amino acid binding events on the surfaces of nanostructures to generate materials that are stable in solution. The basis of both the stability and activity of these materials is likely to be controlled by the biotic/abiotic interface, which is mediated by the bioligand binding process. Unfortunately, most readily available techniques are unable to be used to study this intrinsic process; however, very recent studies have begun to shed light on this important event. In this feature article, an overview of the understanding of peptide and amino acid binding events to nanomaterials and how these motifs can be exploited for activities in nanoparticle assembly and catalytic reactivity is discussed. From both 2D surface studies and computational modeling analyses, different biomolecule binding characteristics have been elucidated. These results indicate that the amino acid sequence and peptide secondary structure play important roles in the binding capability. Furthermore, these studies suggest that the peptides are able to form specific patterns and motifs once bound to the nanoparticle surface. This attribute could affect the nanoparticle electronics and can play a significant role in their activities to generate functional materials. From these binding motifs, the ability of reagents to interact with the metallic surface is possible, thus affecting many of the properties of these materials.

Phototransport in networks of tetrapod-shaped colloidal semiconductor nanocrystals

Tetrapod-shaped CdSe(core)/CdTe(arms) colloidal nanocrystals, capped with alkylphosphonic acids or pyridine, were reacted with various small molecules (acetic acid, hydrazine and chlorosilane) which induced their tip-to-tip assembly into soluble networks. These networks were subsequently processed into films by drop casting and their photoconductive properties were studied. We observed that films prepared from tetrapods coated with phosphonic acids were not photoconductive, but tip-to-tip networks of the same tetrapods exhibited appreciable photocurrents. On the other hand, films prepared from tetrapods coated with pyridine instead of phosphonic acids were already highly photoconductive even if the nanocrystals were not joined tip-to-tip. Based on the current-voltage behavior under light we infer that the tunneling between tetrapods is the dominant charge transport mechanism. In all the samples, chemically-induced assembly into networks tended to reduce the average tunneling barrier. Additionally, pyridine-coated tetrapods and the tip-to-tip networks made out of them were tested as active materials in hybrid photovoltaic devices. Overall, we introduce an approach to chemically-induced tip-to-tip assembly of tetrapods into solution processable networks and demonstrate the enhancement of electronic coupling of tetrapods by various ligand exchange procedures.

Uniform and controllable preparation of Au-Ag core-shell nanorods using anisotropic silver shell formation on gold nanorods

Anisotropic and controllable silver shell formation on gold nanorods was realized in a micellar solution of hexadecytrimethylammonium chloride. Uniformity of the anisotropic Au-Ag core-shell particles contributes separation of four extinction bands. The ability to manipulate the shapes and sizes of these nanoparticles offers a wide-range control of the surface extinction from the visible to the near infrared regions (450-800 nm).

Step-growth polymerization of inorganic nanoparticles

Self-organization of nanoparticles is an efficient strategy for producing nanostructures with complex, hierarchical architectures. The past decade has witnessed great progress in nanoparticle self-assembly, yet the quantitative prediction of the architecture of nanoparticle ensembles and of the kinetics of their formation remains a challenge. We report on the marked similarity between the self-assembly of metal nanoparticles and reaction-controlled step-growth polymerization. The nanoparticles act as multifunctional monomer units, which form reversible, noncovalent bonds at specific bond angles and organize themselves into a colloidal polymer. We show that the kinetics and statistics of step-growth polymerization enable a quantitative prediction of the architecture of linear, branched, and cyclic self-assembled nanostructures; their aggregation numbers and size distribution; and the formation of structural isomers.