Haiying Xu

End-to-end and side-by-side assemblies of gold nanorods induced by dithiol poly(ethylene glycol)

The assemblies of gold nanorods (Au NRs) exhibit unique properties distinct from the isolated Au NR. We report an effective and simple method for the end-to-end (E-E) and side-by-side (S-S) assemblies of Au NRs with a molecularly defined nanogap (1–2 nm) only in the presence of dithiol poly(ethylene glycol) (HS-PEG-SH). The assembled methods need neither the pH value adjustments nor the addition of other organic solvent. With increasing amount of dithiol molecules, assembled modes of Au NRs experience an interesting procedure, changing from E-E to S-S orientation. The experimental results indicate that when the concentration of HS-PEG-SH is less than 0.25 μM, electrostatic repulsion of positive-charged CTA+ is stronger than the affinity of the Au-S binding, resulting in the E-E oriented assembly. Otherwise, the S-S oriented mode is predominated. The current assembled method will be potentially useful for the optoelectronics and biomedical engineering.

Plasmonic Properties of the End-to-End and Side-by-Side Assembled Au Nanorods

Well-defined assemblies of Au nanorods (NRs) exhibit outstanding optical and electric properties, including surface plasmon resonance (SPR) of an individual nanorod and the coupling SPR among the assemblies. We present a direct approach for end-to-end (E-E) and side-by-side (S-S) assembly of Au NRs using poly (ethylene glycol) dithiol (HS-PEG-SH) and cysteine (Cys), respectively. The coupling SPR properties between the neighboring Au NRs are studied through experimental measurements and finite-different time-domain (FDTD) simulations. The simulated SPR tunability over the assembly agree with that of the experimental results, and both of the longitudinal SPR shifts for E-E and S-S assemblies are well fitted with the exponential function. The present assembled method provides a way for directing anisotropic nanostructures into well-defined orientations, and will be potentially useful in optoelectronics and biomedical engineering.

Synthesis of high-purity silver nanorods with tunable plasmonic properties and sensor behavior

Through anisotropic Ag overgrowth on the surface of Au nanobipyramids (AuNBPs), high-purity and size-controlled Ag nanorods (Au/AgNRs) are obtained by a simplified purification process. The diameters of the Au/AgNRs are determined by the size of the as-prepared AuNBPs, and the lengths of the Au/AgNRs are tunable using different amounts of Ag precursor in the growth solution. Surface-enhanced Raman scattering (SERS) studies using Rhodamine-6G (R6G) as a test molecule indicate that the Au/AgNRs have excellent sensing potential. The tunable optical properties and strong electromagnetic effect of the Au/AgNRs, along with their superior SERS signal enhancement, show that Au/AgNRs are promising for further applications in plasmon sensing and biomolecular detection.

Heat generation and stability of a plasmonic nanogold system

The surface plasmon resonance (SPR) of Au nanostructures can be precisely tuned in the visible to near-infrared (vis–NIR) region with the size and morphology. The photothermal effect induced by the SPR can raise the temperature of Au nanostructures and the surrounding matrix under external illumination. In this work, hollow Au nanostructures such as nanoboxes and nanorings with a tunable SPR in the region of 650–1100 nm were obtained by a replacement reaction between HAuCl4 and the as-prepared Ag nanostructures as the sacrificed templates. Compared with the solid Au nanorods, studies on the photothermal conversion and stability of hollow Au nanostructures were systematically carried out with the assistance of the near-infrared (NIR) lasers available. Under NIR laser irradiation, the temperatures of the colloidal Au nanostructures increased rapidly from ~30 °C to ~65 °C. Combining the experimental results with a finite-different time-domain (FDTD) numerical simulation, the heat generation of different Au nanostructures was investigated. With the consideration of the concentration of the Au nanostructures, it is indicated that hollow Au nanostructures are superior to solid Au nanorods in photothermal conversion. On increasing the NIR laser power (3 W), Au nanorods undergo a shape deformation from nanorods to spherical nanoparticles, while the SPR and morphology of hollow Au nanoboxes and nanorings maintain high stability, promising to be candidates for nanoheaters. This work provides a standard to design optimized plasmonic nanoheaters.

Synthesis and Plasmonic Property of Ag Nanorods

In this paper, Ag nanorods (AgNRs) with different aspect ratios (ARs) were prepared by a seed-mediated fast growth approach. The possible growth mechanism of Ag nanostructures was proposed. With a strong interaction between CTAB and Ag seeds, the reduced Ag atoms agglomerated and attached to the performed Ag nanoparticles, where CTAB molecule layer plays a role of rod-like micelles template, leading to one dimensional growth of Ag atoms into AgNRs. The influences of the reaction conditions were discussed on the yield of AgNRs. The surface plasmon resonance (SPR) of AgNRs was studied by finite-difference time-domain (FDTD) simulations. The simulated results indicate that the transverse surface plasmon resonance (SPRT) has no obvious shifting, whereas the longitudinal surface plasmon resonance (SPRL) shows a significant redshift with the increase of AR of AgNRs, which agrees well with the experimental variation trend. It is also found that the absorption and scattering of AgNRs are stronger than that of Au nanorods (AuNRs), which is in accordance with the Raman signal enhancement by AgNRs compared with that of AuNRs, indicating that AgNR is a promising candidate in bio-molecular detection.

Facile synthesis and heteroepitaxial growth mechanism of Au@Cu core–shell bimetallic nanocubes probed by first-principles studies

Bimetallic Au@Cu core–shell nanocrystals have been synthesized by a two-step seed-mediated growth method through controlled heteroepitaxial growth of Cu shells on Au nanorods serving as seeds. The final crystal structures have cubic morphology with edge lengths from 50 to 100 nm due to a strong affinity of hexadecylamine, which is a selective capping agent for the {100} facets of Cu. Various characterizations reveal that the epitaxial growth mechanism is based on block-type growth mode, rather than layer-by-layer growth mode, owing to the larger mismatch in the different lattice constants between Au core and Cu shell. Moreover, contrastive experiments using Au nanobipyramids as seeds were carried out to gain further insight into the heteroepitaxial growth mechanism. Combining first-principles density-functional theory with experimental results, the binding energies of Cu atoms at different sites on Au surfaces are explored to clarify the aspects of nanostructure formation. This study provides useful insights into the effective synthesis method and the further investigation of the proposed growth mechanism.

Dependence of plasmon coupling on curved interfaces

The optical properties of coupled plasmon systems can be tuned by individual material and geometry, gap distance, and surrounding dielectric. This paper reports a dramatic effect of a curved interface in the nanoparticles dimer on the optical responses. Compared with gold nanorod (AuNR) monomer, AuNR dimers with different assembly types (such as end-to-end and side-by-side) can manipulate the longitudinal surface plasmon resonance (SPRL) to red/blueshift. The electromagnetic field of the dimer is further enhanced in the interactive region. Under the incident polarization along the gap, a new resonance mode will be excited when AuNR dimers touch each other, and the SPR mode turns to blueshift from redshift due to the formation of the conductive coupling. It can be assumed that when one of the interactive surfaces is curved, an additional plasmon resonance can be stimulated under the polarization of incident light along the gap. The particular phenomenon can be explained by the plasmon hybridization theory. Silver nanocubes dimers (with sharp or smooth corners and edges) also possess the same property. Supported by finite-difference time-domain solutions, the coupled plasmon resonance mode represents high sensitivity to structural geometry.