Weihai Ni

Shape- and Size-Dependent Refractive Index Sensitivity of Gold Nanoparticles

Gold nanoparticles of different shapes and sizes, including nanospheres, nanocubes, nanobranches, nanorods, and nanobipyramids, were dispersed into water-glycerol mixtures of varying volume ratios to investigate the response of their surface plasmon peaks to the refractive index of the surrounding medium. The refractive index sensitivities and figures of merit were found to be dependent on both the shape and the size of the Au nanoparticles. The index sensitivities generally increase as Au nanoparticles become elongated and their apexes become sharper. Au nanospheres exhibit the smallest refractive index sensitivity of 44 nm/RIU and Au nanobranches exhibit the largest index sensitivity of 703 nm/RIU. Au nanobipyramids possess the largest figures of merit, which increase from 1.7 to 4.5 as the aspect ratio is increased from 1.5 to 4.7.

Tailoring Longitudinal Surface Plasmon Wavelengths, Scattering and Absorption Cross Sections of Gold Nanorods

Tailoring the longitudinal surface plasmon wavelengths (LSPWs), scattering, and absorption cross sections of gold nanorods has been demonstrated by combining anisotropic shortening and transverse overgrowth and judiciously choosing starting Au nanorods. Shortening yields Au nanorods with decreasing lengths but a fixed diameter, while overgrowth produces nanorods with increasing diameters but a nearly unchanged length. Two series of Au nanorods with LSPWs varying in the same spectral range but distinct extinction coefficients are thus obtained. The systematic changes in the LSPW and extinction for the two series of Au nanorods are found to be in good agreement with those obtained from Gans theory. Dark-field imaging performed on two representative nanorod samples with similar LSPWs shows that the scattering intensities of the overgrown nanorods are much larger than those of the shortened nanorods. The experimental results are found to be in very good agreement with those obtained from finite-difference time-domain (FDTD) calculations. FDTD calculations further reveal that the scattering-to-extinction ratio increases linearly as a function of the diameter for Au nanorods with a fixed aspect ratio.

pH‐Controlled Reversible Assembly and Disassembly of Gold Nanorods

Gold nanorods exhibit rich surface-plasmon-resonance (SPR)derived properties, which have made discrete nanorods useful for many interesting applications such as optical data storage, submicrometer metallic barcodes, sensing, biological imaging, and controlled gene delivery. Future scientific and technological applications of Au nanorods require the capability to assemble into complex one-, two-, or even three-dimensional (3D) functional architectures. The assembly of Au nanorods also allows for the utilization of their collective properties that result from the coupling of the optical and electronic properties between neighboring individual nanorods. Several approaches have been developed for the assembly of Au nanorods in either end-to-end (EE) or side-by-side (SS) orientations. They include i) assembly through electrostatic interactions, hydrogen bonding, or covalent bonding, ii) antibody/antigen and streptavidin/biotin biorecognitions, iii) use of carbon nanotubes and silica nanofibers as templates, and iv) interactions between functionalized polymers in selective solvents. Au nanorods assembled by these approaches are generally difficult to disassemble. Even though significant progress has beenmade in the organizationof nanomaterials, reversible assembly and disassembly of Au nanorods in either EE or SS orientations has remained a big challenge. So far, reversible aggregation of spherical Au nanoparticles has been demonstrated by functionalizing them with thiol-modified DNA oligomers. Here, we report on a robust strategy for the reversible assembly and disassembly of Au nanorods in both EE and SS fashion. Thiol-containing bifunctional molecules are selectively bound to the end or side surface of individual Au nanorods. The bound molecules induce the assembly of Au nanorods if the pH of the nanorod solution is adjusted within an optimal range. Outside the optimal pH range, Au nanorods are disassembled. This pH-controlled assembly and disassembly is reversible and can be repeated many times. Moreover, the distances between assembled nanorods are estimated to vary from 0.080 to 1.8 nm for different assemblingmolecules and assembly orientations.

A General Approach to the Synthesis of Gold–Metal Sulfide Core–Shell and Heterostructures†

Cores and effect: Water-dispersible core-shell structures and heterostructures incorporating gold nanocrystals of different shapes (polyhedra, cubes, and rods) and a variety of transition metal sulfide semiconductors (ZnS, CdS, NiS, Ag(2)S, and CuS) are synthesized using cetyltrimethylammonium bromide-encapsulated gold nanocrystals and metal thiobenzoates as starting materials.

Coupling between molecular and plasmonic resonances in freestanding dye-gold nanorod hybrid nanostructures.

Hybrid nanostructures of organic dyes and inorganic gold nanorods are constructed using the layer-by-layer assembly method via electrostatic interactions. Strong coupling is observed between the molecular resonance of dyes and the plasmonic resonance of gold nanorods when their spectra overlap strongly. The coupling strength is tuned by choosing gold nanorods with longitudinal plasmon wavelengths varying from 570 to 870 nm. The resonance coupling-induced plasmon shift is found to be strongly dependent on the dye concentration and the spacing between the dye and nanorod. Moreover, the resonance coupling can be switched off by laser illumination to decompose adsorbed dyes. We believe this is the first time that the coupling between molecular and plasmonic resonances is observed for freestanding hybrid nanostructures constructed out of dyes and colloidal gold nanorods. These results will be helpful in understanding the fundamental interactions between molecular and plasmonic resonances and useful for the design of resonance coupling-based chemical and biological sensors.

Observing plasmonic-molecular resonance coupling on single gold nanorods.

Strong plasmonic-molecular resonance coupling occurs between noble metal nanocrystals and organic adsorbates when the plasmonic resonance is degenerate with the molecular one. This interaction forms the basis for many fundamental studies and practical applications. We describe here the first direct measurement of the resonance coupling on single gold nanorods. The dark-field scattering technique is employed. The nanorods are embedded in hydrogel to facilitate uniform dye adsorption. The adsorbed dye molecules exhibit both monomer and H-aggregate absorption bands. The same gold nanorods are measured before and after the dye adsorption. Both strong and weak coupling are investigated by selecting nanorods with different longitudinal plasmon bands. Excellent agreement between the experiments and an analytic theory is obtained. The resonance coupling reveals a unique three-band structure. The tunability of the coupling on individual nanorods is further demonstrated by photodecomposing the adsorbed dye molecules.

Bifacial DNA Origami-Directed Discrete, Three-Dimensional, Anisotropic Plasmonic Nanoarchitectures with Tailored Optical Chirality

Discrete three-dimensional (3D) plasmonic nanoarchitectures with well-defined spatial configuration and geometry have aroused increasing interest, as new optical properties may originate from plasmon resonance coupling within the nanoarchitectures. Although spherical building blocks have been successfully employed in constructing 3D plasmonic nanoarchitectures because their isotropic nature facilitates unoriented localization, it still remains challenging to assemble anisotropic building blocks into discrete and rationally tailored 3D plasmonic nanoarchitectures. Here we report the first example of discrete 3D anisotropic gold nanorod (AuNR) dimer nanoarchitectures formed using bifacial DNA origami as a template, in which the 3D spatial configuration is precisely tuned by rationally shifting the location of AuNRs on the origami template. A distinct plasmonic chiral response was experimentally observed from the discrete 3D AuNR dimer nanoarchitectures and appeared in a spatial-configuration-dependent manner. This study represents great progress in the fabrication of 3D plasmonic nanoarchitectures with tailored optical chirality.

Identification of the Optimal Spectral Region for Plasmonic and Nanoplasmonic Sensing

We present a theoretical and experimental study involving the sensing characteristics of wavelength-interrogated plasmonic sensors based on surface plasmon polaritons (SPP) in planar gold films and on localized surface plasmon resonances (LSPR) of single gold nanorods. The tunability of both sensing platforms allowed us to analyze their bulk and surface sensing characteristics as a function of the plasmon resonance position. We demonstrate that a general figure of merit (FOM), which is equivalent in wavelength and energy scales, can be employed to mutually compare both sensing schemes. Most interestingly, this FOM has revealed a spectral region for which the surface sensitivity performance of both sensor types is optimized, which we attribute to the intrinsic dielectric properties of plasmonic materials. Additionally, in good agreement with theoretical predictions, we experimentally demonstrate that, although the SPP sensor offers a much better bulk sensitivity, the LSPR sensor shows an approximately 15% better performance for surface sensitivity measurements when its FOM is optimized. However, optimization of the substrate refractive index and the accessibility of the relevant molecules to the nanoparticles can lead to a total 3-fold improvement of the FOM in LSPR sensors.

Plasmon coupling in clusters composed of two-dimensionally ordered gold nanocubes.

Gold nanocubes are assembled into clusters of varying numbers and ordering on indium tin oxide substrates. The plasmon coupling in the clusters is studied with both dark-field imaging and finite-difference time-domain calculations. Generally, as a cluster becomes larger and more asymmetric, it exhibits more scattering peaks towards longer wavelengths. The coupling of the vertically oriented dipole in the nanocube with its image dipole in the substrate generates two scattering peaks. One is fixed in energy and the other red-shifts with increasing cluster size. The coupling of horizontally oriented dipoles among different nanocubes produces multiple scattering peaks at lower energies. Their positions and intensities are highly dependent on the number and ordering of nanocubes in the cluster. Au nanocubes in the clusters are further welded together by thermal treatment. The scattering peaks of the thermally treated clusters generally become sharper. The lower-energy scattering peaks arising from dipolar oscillations are red-shifted.

Emission enhancement from metallodielectric-capped ZnO films

Emission enhancement from ZnO thin films coated with Al∕AlOx has been studied by photoluminescence spectroscopy. While Al exhibits a moderate enhancement on ZnO, the introduction of an ultrathin AlOx spacer can increase the luminescence significantly. By examining the dependence of light emission of Al∕AlOx∕ZnO on AlOx thickness, we found, other than the surface plasmon mediation, a short-ranged nonradiative channel that plays a crucial role in determining that the overall emission enhancement is also present. Based on a simple analytical model, it is found that the nonradiative process is strongly dependent on (AlOxthickness)−6 and therefore its origin is suggested to be of Forster type. Finally, an AlOx spacer with thickness of 5nm is found to be adequate to eliminate the unwanted quenching effect.