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Optimal Interparticle Gap for Ultrahigh Field Enhancement by LSP Excitation via ESPs and Confirmation Using SERS

We have predicted extremely high electromagnetic hot spots using the extended–localized coupled surface plasmon resonance configuration. With this unique configuration, we found that an array of particles shows the critical importance of the interparticle gap on the enhancement factor, which was confirmed experimentally using surface-enhanced Raman scattering (SERS). The extended plasmon wave excited in the Kretschmann–Raether configuration propagates on the silver film surface and couples with the gold nanoparticles dispersed on top through excitation of the localized plasmons. A monomolecular layer of 4-aminothiophenol sandwiched between the metal film and the nanoparticles showed an SERS enhancement factor of the order of 10¹⁰ per molecule in the hot spots. The configuration was optimized, both by simulations and experiments, with respect to the size of the nanoparticles and the interparticle distances. It is demonstrated that the ultrahigh SERS enhancement does occur only when the extended surface plasmon is coupled to the localized surface plasmon at an optimized interparticle gap. Further, highly sensitive detection of glycerol in ethanol is demonstrated using the optimum structure with a detection limit on the order of 10^–12 to the weight percentage of ethanol, which is equivalent to detection of a few molecules. This ultrahigh enhancement is useful in realizing various highly sensitive biosensors when strong enhancement is required as well as in highly efficient optoelectronic and energy devices

Bioinspired Interfacial Chelating-like Reinforcement Strategy toward Mechanically Enhanced Lamellar Materials

Many biological organisms usually derived from the ordered assembly of heterogeneous, hierarchical inorganic/organic constituents exhibit outstanding mechanical integration, but have proven to be difficult to produce the combination of excellent mechanical properties, such as strength, toughness, and light weight, by merely mimicking their component and structural characteristics. Herein, inspired by biologically strong chelating interactions of phytic acid (PA) or IP6 in many biomaterials, we present a biologically interfacial chelating-like reinforcement (BICR) strategy for fabrication of a highly dense ordered “brick-and-mortar” microstructure by incorporating tiny amounts of a natural chelating agent (<i>e</i>.<i>g</i>., PA) into the interface or the interlamination of a material (<i>e</i>.<i>g</i>., graphene oxide (GO)), which shows joint improvement in hardness (∼41.0%), strength (∼124.1%), maximum Young’s modulus (∼134.7%), and toughness (∼118.5%) in the natural environment. Besides, for different composite matrix systems and artificial chelating agents, the BICR strategy has been proven successful for greatly enhancing their mechanical properties, which is superior to many previous reinforcing approaches. This point can be mainly attributed to the stronger noncovalent cross-linking interactions such as dense hydrogen bonds between the richer phosphate (hydroxyl) groups on its cyclohexanehexol ring and active sites of GO, giving rise to the larger energy dissipation at its hybrid interfaces. It is also simple and environmentally friendly for further scale-up fabrication and can be readily extended to other material systems, which opens an advanced reinforcement route to construct structural materials with high mechanical performance in an efficient way for practical applications

Bioinspired Interfacial Chelating-like Reinforcement Strategy toward Mechanically Enhanced Lamellar Materials

Many biological organisms usually derived from the ordered assembly of heterogeneous, hierarchical inorganic/organic constituents exhibit outstanding mechanical integration, but have proven to be difficult to produce the combination of excellent mechanical properties, such as strength, toughness, and light weight, by merely mimicking their component and structural characteristics. Herein, inspired by biologically strong chelating interactions of phytic acid (PA) or IP6 in many biomaterials, we present a biologically interfacial chelating-like reinforcement (BICR) strategy for fabrication of a highly dense ordered “brick-and-mortar” microstructure by incorporating tiny amounts of a natural chelating agent (<i>e</i>.<i>g</i>., PA) into the interface or the interlamination of a material (<i>e</i>.<i>g</i>., graphene oxide (GO)), which shows joint improvement in hardness (∼41.0%), strength (∼124.1%), maximum Young’s modulus (∼134.7%), and toughness (∼118.5%) in the natural environment. Besides, for different composite matrix systems and artificial chelating agents, the BICR strategy has been proven successful for greatly enhancing their mechanical properties, which is superior to many previous reinforcing approaches. This point can be mainly attributed to the stronger noncovalent cross-linking interactions such as dense hydrogen bonds between the richer phosphate (hydroxyl) groups on its cyclohexanehexol ring and active sites of GO, giving rise to the larger energy dissipation at its hybrid interfaces. It is also simple and environmentally friendly for further scale-up fabrication and can be readily extended to other material systems, which opens an advanced reinforcement route to construct structural materials with high mechanical performance in an efficient way for practical applications

Bioinspired Interfacial Chelating-like Reinforcement Strategy toward Mechanically Enhanced Lamellar Materials

Many biological organisms usually derived from the ordered assembly of heterogeneous, hierarchical inorganic/organic constituents exhibit outstanding mechanical integration, but have proven to be difficult to produce the combination of excellent mechanical properties, such as strength, toughness, and light weight, by merely mimicking their component and structural characteristics. Herein, inspired by biologically strong chelating interactions of phytic acid (PA) or IP6 in many biomaterials, we present a biologically interfacial chelating-like reinforcement (BICR) strategy for fabrication of a highly dense ordered “brick-and-mortar” microstructure by incorporating tiny amounts of a natural chelating agent (<i>e</i>.<i>g</i>., PA) into the interface or the interlamination of a material (<i>e</i>.<i>g</i>., graphene oxide (GO)), which shows joint improvement in hardness (∼41.0%), strength (∼124.1%), maximum Young’s modulus (∼134.7%), and toughness (∼118.5%) in the natural environment. Besides, for different composite matrix systems and artificial chelating agents, the BICR strategy has been proven successful for greatly enhancing their mechanical properties, which is superior to many previous reinforcing approaches. This point can be mainly attributed to the stronger noncovalent cross-linking interactions such as dense hydrogen bonds between the richer phosphate (hydroxyl) groups on its cyclohexanehexol ring and active sites of GO, giving rise to the larger energy dissipation at its hybrid interfaces. It is also simple and environmentally friendly for further scale-up fabrication and can be readily extended to other material systems, which opens an advanced reinforcement route to construct structural materials with high mechanical performance in an efficient way for practical applications

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)

Plasmon-Modulated Photoluminescence of Single Gold Nanobeams

In this work, we investigate the modulation of the photoluminescence (PL) of a single Au nanobeam (NB) by the surface plasmons of a Ag nanowire (NW) and the gap plasmons between the two nanostructures. By changing the polarization of the laser that excites the nanostructure and controlling the separation distance <i>d</i> between the two nanostructures, we found that the transverse surface plasmon resonance of the Ag NW enhanced the PL (at 520 nm) of the Au NB with a maximum effect at <i>d</i> = 7 nm. The PL enhancement (at 520 nm) was quenched and a new PL peak was observed at a longer wavelength for <i>d</i> < 7 nm. The PL quenching effect could be understood by the quadrupole-like plasmonic resonance between the Ag NW and the Au NB and be qualitatively explained by the mode dispersion as a function of <i>d</i> obtained using the transfer matrix transmittance calculation. FDTD simulations show that the new PL peak at a longer wavelength is caused by the waveguide-mode gap plasmons between the Au NB and the Ag NW

Realizing a Record Photothermal Conversion Efficiency of Spiky Gold Nanoparticles in the Second Near-Infrared Window by Structure-Based Rational Design

The current technical dilemma for gold nanoparticles as photothermal (PT) transducers in cancer therapy is that strong absorption in the second near-infrared (NIR) window is accompanied by strong scattering of the NIR light, which then overrides the absorption, thus significantly weakening the light-to-heat conversion efficiency. Here we successfully prepared spiky gold nanoparticles (spiky Au NPs) with a controlled number of spikes, designed according to our simulations and experimentally verified. Their overall sizes and the numbers, lengths, and widths of the spikes were judiciously adjusted to locate their surface plasmon resonance peaks in the second NIR window and also to achieve a higher absorption-to-extinction ratio. As a result, the spiky Au NPs with optimal size and 6 spikes exhibited a record light-to-heat conversion efficiency (78.8%) under irradiation by 980 nm light. After surface PEGylation and conjugation with a lactoferrin (LF) ligand on the resulting spiky Au NPs, they <i>in vivo</i> displayed long circulation time (blood circulation half-life of ∼300 min) and high tumor accumulation due to their larger surface-to-volume ratio. Therefore, spiky Au NPs allowed complete ablation of tumors without recurrence merely after 3 min of light irradiation at 980 nm, opening up promising prospects of cancer photothermal therapy