Tunable Plasmon Coupling Research Articles

DNA-Condensed Plasmonic Supraballs Transparent to Molecules.

DNA nanotechnology offers an unrivaled programmability of plasmonic nanoassemblies based on encodable Watson-Crick basepairing. However, it is very challenging to build rigidified three-dimensional supracolloidal assemblies with strong electromagnetic coupling and a self-confined exterior shape. We herein report an alternative strategy based on a DNA condensation reaction to make such structures. Using DNA-grafted gold nanoparticles as building blocks and metal ions with suitable phosphate affinities as abiological DNA-bonding agents, a seedless growth of spheroidal supraparticles is realized via metal-ion-induced DNA condensation. Some governing rules are disclosed in this process, including kinetic and thermodynamic effects stemming from electrostatic and coordinative forces with different interaction ranges. The supraballs are tailorable by adjusting the volumetric ratio between DNA grafts and gold cores and by overgrowing extra gold layers toward tunable plasmon coupling. Various appealing and highly desirable properties are achieved for the resulting metaballs, including (i) chemical reversibility and fixation ability, (ii) stability against denaturant, salt, and molecular adsorbates, (iii) enriched and continuously tunable plasmonic hotspots, (iv) permeability to small guest molecules and antifoulingness against protein contaminates, and (v) Raman-enhancing and photocatalytic activities. Innovative applications are thus foreseeable for this emerging class of meta-assemblies in contrast to what is achieved by DNA-basepaired ones.

Periodic Arrays of 3D AuNP‐Capped VO2 Shells and Their Temperature‐Tunable SERS Performance

AbstractIn this work, periodic arrays of 3D composite structures consisting of gold nanoparticle (AuNP) capped VO2 shells on self‐assembled highly‐ordered SiO2 microspheres have been designed and fabricated, and their temperature‐tunable surface‐enhanced Raman scattering (SERS) performance is investigated. The results show that the SERS intensity enhancement factor (EF) for R6G molecules adsorbed on the AuNP‐capped VO2 shells increases from 8.3 × 105 to 3.9 × 106 when triggering the semiconductor‐to‐metal phase transition of VO2 by temperature rise, which strengthens the plasmonic coupling between the AuNPs and VO2 shells and consequently leads to a larger electric near‐field enhancement at the gaps between neighboring AuNPs. Such periodic arrays of AuNP‐capped VO2 shells with tunable plasmonic coupling can provide great opportunities for various optical applications, such as quantitative SERS detection of analytes, photothermal nano‐regulators, and biosensing in general.

Ultrasensitive critical-state of upconversion nanoparticles assembled on plasmonic antenna arrays for rapid and accurate determination of NH3

Plasmon enhanced upconversion luminescence has drawn extensive attention due to its advances in the fields of optical sensors, bio-detection and medical diagnosis. However, the coupling state between plasmon and upconversion emission is never clearly revealed. Here, we report a novel critical state in plasmon coupled upconversion nanoparticles (UCNPs) that has a long lifetime and is highly stimuli-responsive. The critical state is generated through the precise control over the coupling between plasmonic local field and optical near field. Platinum (Pt) film with thickness of ~7 nm is deposited on photonic crystals to form a periodic plasmonic antenna arrays. UCNPs with inert shells are embedded in plasmonic antenna arrays with a tunable plasmon coupling distance via the thickness control of inert shells. An ultrasensitive critical state is observed for the coupling distance of ~10 nm between Pt film and NaGdF4:Yb/Er luminescent core, accompanied with light confinement by NaYF4/NaGdF4 heterojunction shells. This critical-state can be employed as an ultrasensitive route for fluorescent detection of NH3 and the lowest detection of 0.47 ppm can be achieved that is about 34 times lower than ever reported.

Tunable 2D Group‐III Metal Alloys

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-Inx Ga1- x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Nanoparticles exploring the macroscopic world

Nanoparticles exploring the macroscopic world

Magnetically Tunable Plasmon Coupling of Au Nanoshells Enabled by Space-Free Confined Growth.

We report the unconventional space-free confined growth of Au nanoshells with well-defined plasmonic properties and active tuning of their plasmon coupling by the nanoscale magnetic assembly. The seeded growth of Au exclusively occurred at the hard-soft interfaces between the Fe3O4 core and phenolic resin without the need of creating a limiting space, which represents a general and elegant approach to various core-shell nanostructures. The deformability of permeable phenolic layers plays an essential role in regulating the interfacial growth of Au nanoshells. While the polymer elasticity suppresses the radial deposition of Au atoms, their high deformability can afford enough spaces for the formation of conformal metallic shells. The coupled magnetic-plasmonic properties allow active tuning of the plasmon coupling and the resonant scattering of Au nanoshells by the magnetic assembly of the hybrid nanoparticles into plasmonic chains, whose potentials in applications have been demonstrated in designing transparent displays and anticounterfeiting devices.

Preparation of polymer gold nanoparticle composites with tunable plasmon coupling and their application as SERS substrates.

The controlled surface functionalisation of polystyrene beads (200 nm) with a lipoic acid derivative is used to assemble composites with between 4 to 20% loadings of citrate stabilised gold nanoparticles (13 nm-30 nm), which exhibit variable optical properties arising from interactions of the nanoparticle surface plasmon resonance (SPR). The decrease in average interparticle distance at higher loadings results in a red-shift in the SPR wavelength, which is well described by a universal ruler equation. The composite particles are shown to act as good SERS substrates for the standard analyte 4-mercaptophenol. The direct assessment of the SERS activity for individual composite particles solution is achieved by Raman optical tweezer measurements on 5.3 μm composite particles. These measurements show an increase in performance with increasing AuNP size. Importantly, the SERS activity of the individual particles compares well with the bulk measurements of samples deposited on a surface, indicating that the SERS activity arises primarily from the composite and not due to composite-composite interactions. In both studies the optimum SERS response is obtained with 30 nm AuNPs.

Tunable Optical Properties of Ag–TiO2 Nanorod Composites Based on Interparticle Plasmon Coupling

The metal–semiconductor composites with unique optical properties are highly desirable. Here we report that Ag–TiO2 nanorod composites with two distinctive absorption peaks exhibit tunable plasmon coupling behaviors. The intensity of the long-wave absorption could be enhanced by modifying the geometry of TiO2 particles, while such absorption is due to the plasmon coupling between the Ag particles. The long-wave absorption bands can be tuned by varying the reaction time and changing the amount of silver acetate. Furthermore, the Ag–TiO2 composites were also prepared by photochemical reduction method which confirmed the previous properties. These results may help to design metal–semiconductor systems with desirable optical properties.

Designing and fabricating composites of PNIPAM@Au nanorods with tunable plasmon coupling for highly sensitive SERS detection

In this study, hybrid substrates composed of poly(N-isopropylacrylamide) hydrogels and Au nanorods (PNIPAM@AuNRs) that exhibit tunable plasmon coupling are designed and fabricated. In this process, Au NRs are deposited on the surface of a PNIPAM hydrogel, and the temperature is increased, causing the hydrogel template to shrink, thereby driving the Au NRs closer to each other. The resulting surface plasmon resonances (SPR) of the composite are observed as a large red-shift. The formation of more hot spots increases the already high sensitivity of surface-enhanced Raman scattering (SERS) detection, and the tunable optical properties enable closer matching of the laser to the near-infrared excitation required. Moreover, these hybrid substrates can be conveniently identified by optical microscopy, enabling in situ SERS detection with high sensitivity and reproducibility.

Tunable Plasmonic Coupling in Self-Assembled Binary Nanocrystal Superlattices Studied by Correlated Optical Microspectrophotometry and Electron Microscopy

We study the plasmonic properties of self-assembled binary nanocrystal superlattices (BNSLs) using correlated optical microspectrophotometry and electron microscopy performed on individual BNSL domains. The strength of near-field couplings between adjacent plasmonic nanocrystals (NCs) can be systematically engineered by varying the NC size, composition, and the lattice symmetry of BNSLs, leading to broadband spectral tunability of the collective plasmonic response of BNSLs across the entire visible spectrum. Self-assembled multicomponent NC superlattices represent a versatile platform for the rational design of macroscopic three-dimensional plasmonic metamaterials with emergent optical characteristics.

Silver nanoparticles loading pH responsive hybrid microgels: pH tunable plasmonic coupling demonstrated by surface enhanced Raman scattering

The pH responsive hybrid microgels containing homogeneously distributed silver nanoparticles (Ag NPs) were created by in situ reduction of Ag+ ions coordinated into the microgels with semi-interpenetrating polymer network structure composed of linear poly(acrylic acid) and crosslinked poly(N-isopropylacrylamide). When the pH value of the aqueous medium lowers from 6.0 to 4.0, the localized surface plasmon resonance (LSPR) band of the entrapped Ag NPs is pronouncedly red-shifted because of the decreased spatial distances between them as a result of shrinkage of the microgels, leading to their plasmonic coupling. The pH tunable plasmonic coupling is demonstrated by pH dependence of the surface enhanced Raman scattering (SERS) signal of 4-mercapto benzoic acid anchored on the Ag NPs surfaces. Differing from static plasmonic coupling modes from nanostructured assembly or array system of noble metals, the here reported plasmonic coupling can be dynamically controlled by environmental pH value. Therefore, the pH responsive hybrid microgels have potential applications in mobile LSPR or SERS microsensors for probing pH values of living tissues or cells.

Specific glucose-to-SPR signal transduction at physiological pH by molecularly imprinted responsive hybrid microgels

We design a class of imprinted hybrid microgels that can optically monitor glucose levels with high sensitivity and selectivity in complex media at physiological pH, acting like a “glucose-indicator”. Such imprinted hybrid microgels were made of Ag nanoparticles (NPs) in situ immobilized in molecularly imprinted glucose-responsive polymeric microgel templates containing phenylboronic acids in such a way that the Ag NPs were confined in the immediate vicinity to each other, thus enabling their efficient plasmon coupling. The glucose-responsive gel-actuated tunable plasmon coupling effects among the Ag NPs immobilized inside the microgels were investigated in both phosphate buffer solution and artificial tear fluid. The visually evident color shift from yellow to red of the hybrid microgel dispersion in response to a glucose concentration change from 0 to 20.0 mm allows one to see the glucose levels without instrumental aid. The surface plasmon resonance (SPR) response of the imprinted hybrid microgels at appropriate loading amount of Ag NPs is free of the significant interferences from the major non-glucose constituents, enabling the optical glucose sensing in artificial tear fluids with the achieved root-mean-squared error of predication (RMSEC) as low as 13.7 μm (∼0.2 mg/dL) over a clinically relevant glucose concentration range of 0.1–20 mm (1.8–360 mg/dL). The highly versatile imprinted hybrid microgels could potentially be used for continuous glucose monitoring in clinical diagnostic and bioprocess applications.

Controlled Dispersion of Silver Nanoparticles into the Bulk of Thermosensitive Polymer Microspheres: Tunable Plasmonic Coupling by Temperature Detected by Surface Enhanced Raman Scattering

By in situ reduction of Ag(+) ions pre-dispersed inside thermosensitive microspheres of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] (P(NIPAM-co-MAA)), a 3D copolymer-supported network of silver nanoparticles is created and extensively characterized by surface-enhanced Raman scattering (SERS). The effective dispersion and the suitable density of the silver nanoparticles in the composite microspheres are demonstrated by the thermal-induced SERS signal and its high reproducibility during thermocycling. When the temperature of the system increases above 32 °C, spatial separation of the silver nanoparticles decreases and the numbers of Ag nanoparticles and P(NIPAM-co-MAA) microspheres under illumination spot increase as a result of the shrinkage of the P(NIPAM-co-MAA) chains, leading to the ramp of the SERS effect. By means of the high reversibility of the thermosensitive phase transition of the P(NIPAM-co-MAA) microspheres, SERS activity of the silver nanoparticle network embedded in the microsphere can be well controlled by thermal-induced variation of special separation.

Tunable plasmonic coupling between silver nano-cubes and silver nano-hole arrays

A quasi-three-dimensional (quasi-3D) system composed of Ag nano-cubes andAg nano-hole arrays was fabricated through a low cost chemical process. Thecoupling of localized surface plasmons (LSPs) in the cube–hole array system hasbeen investigated through surface-enhanced Raman scattering (SERS) fromRhodamine 6G (R6G) molecules. A SERS enhancement factor as large as1.1 × 108 can be achieved due to this plasmonic coupling effect, and is highly sensitive to geometricalparameters, such as cube–hole array distance, hole diameter, inter-hole spacing and Ag filmthickness.

High-Density Silver Nanoparticle Film with Temperature-Controllable Interparticle Spacing for a Tunable Surface Enhanced Raman Scattering Substrate

The formation of high-density silver nanoparticles and a novel method to precisely control the spacing between nanoparticles by temperature are demonstrated for a tunable surface enhanced Raman scattering substrates. The high-density nanoparticle thin film is accomplished by self-assembling through the Langmuir-Blodgett (LB) technique on a water surface and transferring the particle monolayer to a temperature-responsive polymer membrane. The temperature-responsive polymer membrane allows producing a dynamic surface enhanced Raman scattering substrate. The plasmon peak of the silver nanoparticle film red shifts up to 110 nm with increasing temperature. The high-density particle film serves as an excellent substrate for surface-enhanced Raman spectroscopy (SERS), and the scattering signal enhancement factor can be dynamically tuned by the thermally activated SERS substrate. The SERS spectra of Rhodamine 6G on a high-density silver particle film at various temperatures is characterized to demonstrate the tunable plasmon coupling between high-density nanoparticles.