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Hierarchical assembly of gold nanorod stripe patterns for sensing and cells alignment

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Abstract

Hierarchical assemblies of nanomaterial superstructures with controlled orientation affords a multitude of novel properties of plasmonics and broad applications. Yet constructing multi-functional superstructures with nanoparticles positioned in desired locations remains challenging. Herein, gold nanorods (GNRs) assembled in stripe patterns with controlled orientation and structures in millimeter scale for versatile application have been achieved. Applications of patterned GNRs in sensing enhancement and engineering mammalian cells alignment are investigated experimentally. The performance of patterned GNRs in surface enhanced Raman scattering (SERS) and electrical sensing are found in orientational dependence. The SERS signals of vertically arranged GNR arrays exhibit double the folder intensity than those horizontally arranged. In contrast, the horizontally arranged GNRs exhibit twice as much electrical conductivity. The system is further explored to pattern mammalian cells. For the first time, we reveal the nanostructured topography of GNR confined cells to a specific region, and direct the adhesion and extension of living cells, which opens up broad applications in tissue engineering and biosensing.

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... Au NRs in solutions can spontaneously fill in almost every channel and finally assemble into a stripe pattern. 744 One can realize the assembly of Au NR arrays with various characteristic sizes and periodicities using PDMS stamps. Au NRs dispersed in ethanol−water mixtures can be arranged into SS-packed lamellae (Figure 13a). ...
... SERS can be used for chemical analysis, detection of pollutants or bacteria, photochemical studies or vibrational spectroscopy [20] down to the single-emitter level [21,22]. The search for efficient and inexpensive SERS metallic substrates is a very active field and many different substrates and fabrication methods have been proposed, as exemplified most recently by Au nanoparticles obtained by femtosecond exposure of a Au film [23], by bipyramid Au nanoparticles [24], by Agnanoparticles growth on Si by replacement method [25] or by stripes of vertical or horizontal Au nanorods [26]. The key element is the presence of sharp tips or dips on the surface of the metallic substrate, where plasmonic modes can be strongly confined and create "hot spots". ...
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The use of near-infrared plasmon resonance of gold nanorods (NR) to offer an integrated platform for multiplexed Raman detection and remote-controlled photothermal heating is analyzed. Surface enhanced Raman scattering (SERS)-coded NRs were efficiently detected following subcutaneous or intratumoral injection and enabled remote photothermal tumor heating to ablative temperatures. Cetyltrimethylammonium bromide (CTAB)-coated gold NRs were bought from Nanopartz with a peak plasmon resonance at 790 nm. NRs were purified further with multiple rounds of centrifugation using molecular weight cutoff centrifugal filters and stored at 200 pm at 4°C. The SERS intensities of NRs coated with different dyes were compared using ethanol as an internal standard. The best NRs were selected by comparing the peak height ratio of the dye's most intense SERS peak to the peak height of the ethanol vibration at 879 cm-1 used as an internal standard.
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Radiative coupling of induced plasmonic fields in metal nanoparticles has drawn increasing attention in the recent literature due to a combination of improved experimental methods to study such phenomena and numerous potential applications, such as plasmonic nanoparticle rulers and plasmonic circuitry. Many groups, including ours, have used a near-exponential fit to express the size scaling of plasmonic coupling. First, we show experimental agreement between previously simulated nanorod coupling and plasmonic coupling in electron beam lithography (EBL) fabricated nanorods using the near-exponential expression. Next, we study the effect of nanoparticle orientation on plasmonic coupling using EBL and DDA simulations. We develop a mathematical relationship that is consistent with our findings and quantitatively describes plasmonic coupling between nanorods as a function of orientation, separation, induced dipole strength, and the dielectric constant of the medium. For applications utilizing plasmonic coupling to become viable with particle shapes that do not have spherical symmetry, such as nanoprisms and nanorods, comparison of the experimental and theoretical results of how particle orientation affects plasmonic coupling is essential.
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The experimentally determined scattering spectra of discrete, crystalline, gold nanorod dimers arranged side-to-side, end-to-end, at right angles in different orientations and also with longitudinal offsets are reported along with the electron micrographs of the individual dimers. The spectra exhibit both red- and blue-shifted surface plasmon resonances, consistent with the plasmon hybridization model. However, the plasmon coupling constant for gold dimers with less than a few nanometers separation between the particles does not obey the exponential dependence predicted by the Universal Plasmon Ruler equation. The experimentally determined spectra are compared with electrodynamic calculations and the interactions between the individual rod plasmons in different dimer orientations are elucidated.
Article
(Figure Presented) The order of gold: Ordered assemblies of gold nanostructures are produced by droplet evaporation from single- and two-component systems. The ordering of the assemblies is highly dependent on the shape and size of gold nanostructures and includes nanorods, nanocubes, polyhedra, and bipyramids. The two-photon-excited luminescence of ordered gold nanorod assemblies is larger than that of disordered nanorod assemblies.
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The shape anisotropy of nanorods gives rise to two distinct orientational modes by which nanorods can be assembled, i.e., end-to-end and side-by-side, analogous to the well-known H and J aggregation in organic chromophores. Optical absorption spectra of gold nanorods have earlier been observed to show a red-shift of the longitudinal plasmon band for the end-to-end linkage of nanorods, resulting from the plasmon coupling between neighboring nanoparticles, similar to the assembly of gold nanospheres. We observe, however, that side-by-side linkage of nanorods in solution shows a blue-shift of the longitudinal plasmon band and a red-shift of the transverse plasmon band. Optical spectra calculated using the discrete dipole approximation method were used to simulate plasmon coupling in assembled nanorod dimers. The longitudinal plasmon band is found to shift to lower energies for end-to-end assembly, but a shift to higher energies is found for the side-by-side orientation, in agreement with the optical absorption experiments. The strength of plasmon coupling was seen to increase with decreasing internanorod distance and an increase in the number of interacting nanorods. For both side-by-side and end-to-end assemblies, the strength of the longitudinal plasmon coupling increases with increasing nanorod aspect ratio as a result of the increasing dipole moment of the longitudinal plasmon. For both the side-by-side and end-to-end orientation, the simulation of a dimer of nanorods having dissimilar aspect ratios showed a longitudinal plasmon resonance with both a blue-shifted and a red-shifted component, as a result of symmetry breaking. A similar result is observed for a pair of similar aspect ratio nanorods assembled in a nonparallel orientation. The internanorod plasmon coupling scheme concluded from the experimental results and simulations is found to be qualitatively consistent with the molecular exciton coupling theory, which has been used to describe the optical spectra of H and J aggregates of organic molecules. The coupled nanorod plasmons are also suggested to be electromagnetic analogues of molecular orbitals. Investigation of the plasmon coupling in assembled nanorods is important for the characterization of optical excitations and plasmon propagation in these nanostructures. The surface plasmon resonance shift resulting from nanorod assembly also offers a promising alternative for analyte-sensing assays.