Tunable Plasmon Resonance Wavelength Research Articles

Spectrum and size controllable synthesis of high-quality gold nanorods using 1,7-dihydroxynaphthalene as a reducing agent.

The spectrum and size controllable synthesis of gold nanorods is of great value for their widely applicable aspect ratio dependence of anisotropic surface plasmon resonance. Herein, 1,7-dihydroxynaphthalene with a relatively strong reducibility is proposed as a reducing agent for the controllable synthesis of gold nanorods. The result indicated that gold nanorods with high monodispersity, high shape yield, relatively small diameters, and maximum plasmon resonance wavelength of above 1000 nm can be acquired. More importantly, by virtue of the reducing agent used, fine and precise controls over the plasmon wavelength and diameter of the rod can be achieved via changes in experimental conditions. In particular, increases in the concentration of both silver ions and cetyltrimethylammonium bromide (CTAB) can increase the plasmon wavelength from around 600 nm to 1000 nm but respectively show a decreased diameter with the smallest value of around 14.3 nm and a mildly increased diameter from around 9.0 nm to 14.3 nm; moreover, increasing the concentration of reducing agents and gold seeds can simultaneously cause decreases in the plasmon wavelength from around 1000 nm to 800 nm and the diameters from around 14.3 nm to 9.0 and 7.3 nm, respectively. This powerful and efficient method of controllable synthesis of AuNRs could be valuable and attractive for the application of the as-obtained particles.

Au Nanobottles with Synthetically Tunable Overall and Opening Sizes for Chemo-Photothermal Combined Therapy.

Highly asymmetric Au nanostructures, such as split Au nanorings and Au nanocups, exhibit attractive plasmonic properties because of their asymmetric geometries. To facilitate their plasmonic applications, effective and facile synthetic methods for producing asymmetric Au nanostructures with controllable sizes and uniform shapes are highly desirable. Herein, we report on an approach for the synthesis of largely asymmetric colloidal Au nanobottles with synthetically tunable overall and opening sizes. Au nanobottles with overall sizes in the range of ∼100-230 nm are obtained through sacrificial templating with differently sized PbS nano-octahedra. The opening sizes of the produced Au nanobottles can be tailored from ∼10 to ∼120 nm by either adjusting the Au/PbS molar ratio in the growth process or controlling the oxidation degree. The achieved size tunability allows the plasmon resonance wavelength of Au nanobottles to be varied in the range of ∼600-900 nm. Our uniform Au nanobottles, which possess controllable sizes, large cavity volumes, and tunable plasmon resonance wavelengths in the visible to near-infrared range, have been further applied for anticancer drug delivery and photothermal therapy. The effects of surface coating and the opening size of Au nanobottles on the drug encapsulation efficiency (EE) and initial burst drug release are systemically evaluated. A high doxorubicin EE and low initial burst drug release are realized with the dense silica-coated Au nanobottles having an opening size of 44 nm. In addition, chemo-photothermal combined therapy has been demonstrated with the doxorubicin-loaded Au nanobottles. Our results will be helpful for the design of Au nanobottles with different sizes and plasmonic properties as well as provide ample opportunities for exploring various plasmon-enabled applications of Au nanobottles.

3D Plasmon Coupling Assisted Sers on Nanoparticle-Nanocup Array Hybrids

Unique colorimetric optical properties of nanomaterials can effectively influence the light absorption or emission of molecules. Here, we design plasmonic substrate for surface-enhanced Raman scattering (SERS) by inducing three-dimensional (3D) hot spots on the sensing surface. The 3D hot spots are formed by the self-assembly of plasmonic nanoparticles (NPs) on a 3D plasmonic nanocup array structure. This 3D hot spot formation on the periodic nanocup arrays achieves much higher SERS enhancement factor than the 2D NP arrays, which have been conventionally sought SERS substrates. We also utilize the colorimetric properties of the nanocup arrays for an additional degree of SERS enhancement. Colorimetry, achieved by tunable plasmon resonance wavelength by controlling dielectric property on the nanocup array surface, eases the modulation of the plasmonic resonance condition without modifying the nanostructure design. By continuously monitoring the shifts of the plasmon resonance condition and its effect on the light absorption and emission of the nearby molecules, we verify that larger SERS enhancement is achieved when the plasmon resonance wavelength is matched with the Raman excitation wavelength. The ease of plasmon resonance tuning of this nanocup array-nanoparticle hybrid structure allows versatile SERS enhancement for a variety of different Raman measurement conditions.

Preparation of Au–Ag bimetallic nanoparticles for enhanced solar photothermal conversion

In this study, the solar photothermal conversion performance of bimetallic Au–Ag nanoparticles (NPs) was investigated experimentally. NPs were prepared using a sodium citrate thermal reduction method, using the same procedure and materials to maintain the same particle surface properties. NPs with a tunable plasmon resonance wavelength and demonstrating a high intensity in the visible region were obtained by adjusting the molar ratio of Ag to Au precursors. Experimental results show that blended Au–Ag NPs exhibit a solar photothermal conversion efficiency of 31.41%, which lies between the values of pure Au and Ag NPs under the same NP volume fraction due to the independently scatter light of the Au and Ag NPs in this composite system. Bimetallic Au–Ag NPs demonstrated the highest solar photothermal conversion efficiency of 41.37%, a 31.7% increase over that of blended Au–Ag NPs. The efficiency of the bimetallic Au–Ag NP system was also higher than that of the pure Au NPs (37.81%) with a 40% decrease in Au consumption.

Bromide (Br−) ion-mediated synthesis of anisotropic palladium nanocrystals by laser ablation

Anisotropic growth of Pd nanoparticles in bromine (Br) containing solution has been studied by pulsed laser ablation. For size and shape control different solutions like water, sodium dodecyl sulphate (SDS) (anionic surfactant), and (Br−) ion containing cetyltrimethylammonium bromide (CTAB) (cationic surfactant) and electrolyte (KBr) were used. In laser ablation surrounding liquid plays a dominant role in controlling size and directional growth. Absorption spectra of as-generated Pd nanoparticles undergo modification with time in different solutions due to Br− ion-mediated directional growth. In water and SDS quasi-spherical and spherical Pd nanoparticles with mean size of 14 and 8nm, respectively, and in CTAB decahedron and icosahedron shape Pd nanocrystals with mean size 65nm were observed. When strong Br− ion source KBr was used sharp edged cuboid shaped large Pd nanoparticles were observed. Surface energy modification due to preferential chemisorption of Br− ions onto {100} planes of Pd resulted in formation anisotropic Pd nanostructures enclosed with {100} planes. The nanocubes exhibit broad plasmon resonance around 250–280nm. Further, size of nanocuboids were controlled by using mixed solutions of KBr with SDS and CTAB for tunable plasmon resonance wavelength from 230 to 550nm.

Localized Surface Plasmon Resonance and Refractive Index Sensitivity of Metal–Dielectric–Metal Multilayered Nanostructures

The localized surface plasmon resonances of multilayered nanostructures are studied using finite difference time domain simulations and plasmon hybridization method. Concentric metal–dielectric–metal (MDM) structure with metal core and nanoshell separated by a thin dielectric layer exhibits a strong coupling between the core and nanoshell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the dielectric layer thickness and composition of core and outer layer metal. The aluminum-based MDM structures show lower plasmon wavelength compared with Ag- and Au-based MDM nanostructures. The calculated refractive index sensitivity (RIS) factor is in the order Ag–Air–Ag>Au–Air–Au>Al–Air–Al for monometallic multilayered nanostructures. Bimetallic multilayered nanostructures support strong and tunable plasmon resonance wavelengths as well as high RIS factor of 510 nm/refractive index unit (RIU) and 470 nm/RIU for Al–Air–Au and Ag-Air-Au, respectively. The MDM structures not only exhibit higher index sensitivity but also cover a wide ultraviolet–near-infrared wavelengths, making these structures very promising for index sensing, biomolecule sensing, and surface-enhanced Raman spectroscopy.

Investigation of shell aggregate gold nanostructures

The unique optical properties of noble metal nanostructures have led to an increased interest into their potential uses for various biological applications. For many medical investigations, it would be beneficial to use near infrared (NIR) excitation as well as small gold nanospheres, which can easily reach the cytoplasm and cell nucleus. To fulfil both requirements simultaneously, this paper proposes a novel nanostructure, the aggregate, which consists of small nanospheres aggregate around a core such as an intracellular organelle. The extinction efficiency of such monolayer and bilayer shell aggregates is thoroughly investigated with appropriate simulations. The extinction spectra appear to depend heavily on the distance between the small nanospheres. The monolayer shell aggregate could be a good candidate for use in various biological, intracellular, applications since it provides a reasonably tunable plasmon resonance wavelength while the small size of its components can be exploited for intracellular distribution.

Tunable plasmon resonance wavelength of the periodic gold film with converging and diverging shaped slits and grooves

We investigate a periodic slit–groove metal array with converging and diverging shaped channels by using the finite-difference time-domain (FDTD) method. We show that the peaks and dips of the transmission spectra can be tuned by the geometry parameters of the groove. And the resonance modes are strongly correlated with the groove depth and width of both ends of the groove, and the transmission dip between the resonance modes can be modulated by these parameters. However, the distance between the slit and the groove has little influence on the transmission. We show that the periodic slit–groove with diverging and converging shaped channels is useful for the design of plasmonic devices.