Highly organized supercrystals of Au nanorods with plasmonic antennae enhancement of electrical field have made possible fast direct detection of prions in complex biological media such as serum and blood. The nearly perfect three-dimensional organization of nanorods render these systems excellent surface enhanced Raman scattering spectroscopy substrates with uniform electric field enhancement, leading to reproducibly high enhancement factor in the desirable spectral range.S urface enhanced Raman scattering (SERS) spectroscopy is not only one of the most sensitive analytical techniques but also can be used under biological conditions. Additionally, SERS signals are strongly dependent on conformational changes in macromolecules such as proteins (1). Unfortunately, although SERS of proteins has been consistently investigated during the last decade (2-6), enhancement factors (EFs) obtained for most conventional (nonfluorescent) proteins are still insufficient for their direct detection in complex biological media (7). There are two additional very serious challenges as well. Both quantitative detection by SERS and reproducible geometry of the "hot spots" necessary for SERS are difficult to achieve. The way to solve these challenges is to design and fabricate a highly organized photonic structure (8) that provides a high electromagnetic field enhancement in a reproducible geometry (9, 10). Recent demonstration of near-field focalization by nanoantennas (11,12) has paved the way for development of ultrasensitive SERS substrates that can concentrate the near field within certain confined regions, allowing one to obtain extremely high EFs (13-15). Such a nanoantenna effect was predicted and found for nanorod (NR) dimers, where the maximum focalization is present at the NR tips (16,17). One can hypothesize, therefore, that a highly organized system of NRs (18-21) acting as an extended nanoantenna may provide resolution for the SERS challenges of proteins or their segments. In turn, this hypothesis can lead to significant technological development for relevant biomedical problems. One example of those problems is the presymptomatic detection of scrambled prions directly in biological fluids.Prions are hard-to-detect infectious agents that cause a number of fatal neurodegenerative diseases in mammalians such as bovine spongiform encephalopathy (BSE), scrapie of sheep, and Creutzfeldt-Jakob disease (CJD) of humans (22), and recently traced as well to other neurodegenerative syndromes as Alzheimer's (23) and Parkinson (24). Invariably, all of these diseases involve the modification of the endogenous and functional prion protein (PrP C ) into a nonfunctional but much more stable form (PrP SC ) giving rise to the so-called amyloid plaques in the brain and other nervous tissues (25). Detection of its presence for contention in cattle or diagnosis in humans or blood transfusion banks (26) is very difficult even by state-of-the-art immunological methods such as fluorescence immunoassay, RIA, or ELISA (27) or protein misfolding c...

It is widely accepted that the physical properties of nanostructures depend on the type of surface facets. For Au nanorods, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there has been controversy regarding facet indexing. Aberration-corrected electron microscopy is the ideal technique to study the atomic structure of nanomaterials. However, these images correspond to two-dimensional (2D) projections of 3D nano-objects, leading to an incomplete characterization. Recently, much progress was achieved in the field of atomic-resolution electron tomography, but it is still far from being a routinely used technique. Here we propose a methodology to measure the 3D atomic structure of free-standing nanoparticles, which we apply to characterize the surface facets of Au nanorods. This methodology is applicable to a broad range of nanocrystals, leading to unique insights concerning the connection between the structure and properties of nanostructures.

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Seed-mediated growth is the most efficient methodology to control the size and shape of colloidal metal nanoparticles. In this process, the final nanocrystal shape is defined by the crystalline structure of the initial seed as well as by the presence of ligands and other additives that help to stabilize certain crystallographic facets. We analyze here the growth mechanism in aqueous solution of silver shells on presynthesized gold nanoparticles displaying various well-defined crystalline structures and morphologies. A thorough three-dimensional electron microscopy characterization of the morphology and internal structure of the resulting core–shell nanocrystals indicates that {100} facets are preferred for the outer silver shell, regardless of the morphology and crystallinity of the gold cores. These results are in agreement with theoretical analysis based on the relative surface energies of the exposed facets in the presence of halide ions.

Exceptional magnetic properties of magnetite, Fe3O4, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe3O4 nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe3O4 composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe3O4+δ composition, which could be attributed to the formation of Fe3O4–γ-Fe2O3 solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia.

Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research.[1] The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, [2] nanoscale organization of readily accessible spherical gold nanoparticles [3,4] has been manipulated to produce a diverse range of topologies [5] with interesting optical and electrical properties. [6,7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report [8] demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases [9][10][11][12] with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant.[13] Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties.The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, [14] which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale.[15] Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, [16] based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes.[17] Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion.[18] This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent-air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies.To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents [19,20] and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-func...

This work describes the control and manipulation of the optical properties of multiresponsive organic/inorganic hybrid colloids, which consist of thermo-responsive poly-(NIPAM-co-allylacetic acid) microgel cores and gold nanorods assembled on their surface. These composites are multifunctional, in the sense that they combine the interesting optical properties of the rod-shaped gold particles--exhibiting two well-differentiated plasmon modes--with the sensitivity of the copolymer microgel toward external stimuli, such as temperature or solution pH. It is shown that the collapse of the microgel core, induced by changes in either temperature or pH, enhances the electronic interactions between the gold nanorods on the gel surface, as a result of the subsequent increase of the packing density arising from the surface decrease of the collapsed microgel. Above a certain nanorod density, such interactions lead to remarkable red-shifts of the longitudinal plasmon resonance.

Changing faces: The shape of gold nanorods can be finely tuned by controlled growth under sonication in DMF in the presence of poly(vinylpyrrolidone). Reshaping involves the formation of rods with sharp tips and strongly faceted lateral faces, and ultimately leads to perfect, single‐crystal octahedrons (see images). Mechanistic considerations indicate a shape‐inducing effect of the polymer through different binding interactions for the different faces.