Fluorescence enhancement of molecules inside a gold nanomatryoshka

Nanooptics of Plasmonic Nanomatryoshkas: Shrinking the Size of a Core-Shell Junction to Subnanometer

Quantum effects in plasmonic systems play an important role in defining the optical response of structures with subnanometer gaps. Electron tunneling across the gaps can occur, altering both the far-field optical response and the near-field confinement and enhancement. In this study, we experimentally and theoretically investigate plasmon coupling in gold "nanomatryoshka" (NM) nanoparticles with different core-shell separations. Plasmon coupling effects between the core and the shell become significant when their separation decreases to 15 nm. When their separation decreases to below 1 nm, the near- and far-field properties can no longer be described by classical approaches but require the inclusion of quantum mechanical effects such as electron transport through the self-assembled monolayer of molecular junction. In addition, surface-enhanced Raman scattering measurements indicate strong electron-transport induced charge transfer across the molecular junction. Our quantum modeling provides an estimate for the AC conductances of molecules in the junction. The insights acquired from this work pave the way for the development of novel quantum plasmonic devices and substrates for surface-enhanced Raman scattering.

Gold Nanoframes by Nonepitaxial Growth of Au on AgI Nanocrystals for Surface-Enhanced Raman Spectroscopy

Plasmonic noble metal nanoparticles with defined interior nanogaps are of great significance to surface-enhanced Raman spectroscopy (SERS) applications owing to the presence of intraparticle hotspots. In this contribution, we discovered site-selective nonepitaxial growth of Au on nonmetallic AgI nanocrystals, and on the basis of this observation, we designed an unconventional route to synthesize monometallic Au nanoframes that possess ∼7 nm of interior nanogaps and ∼23 nm of overall size by templating of small AgI nanocrystals. Chemical bonding between Au and the iodide-rich surface of the AgI nanocrystals was proposed to play a critical role in the nonepitaxial growth of the Au nanoframes against the AgI nanocrystals. The Au nanoframes obtained from this synthesis showed superior SERS activity in detecting molecules of interest in low concentrations owing to the presence of intraparticle hotspots in additional to the interparticle ones, benchmarking against Au nanospheres. This intriguing synthesis may open up new opportunities toward a variety of noble metal/semiconductor nanoconjugates for a broad range of applications such as synergistic catalysis.

Fabrication and robotization of ultrasensitive plasmonic nanosensors for molecule detection with Raman scattering

In this work, we introduce the history and mechanisms of surface enhanced Raman scattering (SERS), discuss various techniques for fabrication of state-of-the-art SERS substrates, and review recent work on robotizing plasmonic nanoparticles, especially, the efforts we made on fabrication, characterization, and robotization of Raman nanosensors by design. Our nanosensors, consisting of tri-layer nanocapsule structures, are ultrasensitive, well reproducible, and can be robotized by either electric or magnetic tweezers. Three applications using such SERS nanosensors were demonstrated, including location predictable detection, single-cell bioanalysis, and tunable molecule release and monitoring. The integration of SERS and nanoelectromechanical system (NEMS) devices is innovative in both device concept and fabrication, and could potentially inspire a new device scheme for various bio-relevant applications.

Quantum effects in the optical response of extended plasmonic gaps: validation of the quantum corrected model in core-shell nanomatryushkas

Electron tunneling through narrow gaps between metal nanoparticles can strongly affect the plasmonic response of the hybrid nanostructure. Although quantum mechanical in nature, this effect can be properly taken into account within a classical framework of Maxwell equations using the so-called Quantum Corrected Model (QCM). We extend previous studies on spherical cluster and cylindrical nanowire dimers where the tunneling current occurs in the extremely localized gap regions, and perform quantum mechanical time dependent density functional theory (TDDFT) calculations of the plasmonic response of cylindrical core-shell nanoparticles (nanomatryushkas). In this axially symmetric situation, the tunneling region extends over the entire gap between the metal core and the metallic shell. For core-shell separations below 0.5 nm, the standard classical calculations fail to describe the plasmonic response of the cylindrical nanomatryushka, while the QCM can reproduce the quantum results. Using the QCM we also retrieve the quantum results for the absorption cross section of the spherical nanomatryushka calculated by V. Kulkarni et al. [Nano Lett. 13, 5873 (2013)]. The comparison between the model and the full quantum calculations establishes the applicability of the QCM for a wider range of geometries that hold tunneling gaps.

Tuning interior nanogaps of double-shelled Au/Ag nanoboxes for surface-enhanced Raman scattering

Double-shelled Au/Ag hollow nanoboxes with precisely controlled interior nanogaps (1 to 16 nm) were synthesized for gap-tunable surface-enhanced Raman scattering (SERS). The double-shelled nanoboxes were prepared via a two-step galvanic replacement reaction approach using Ag nanocubes as the templates, while 4-aminothiolphenol (4-ATP) as SERS probe molecules were loaded between the two shells. More than 10-fold enhancement of SERS is observed from the double-shelled nanoboxes than Ag nanocubes. In addition, the SERS of the double-shelled nanoboxes increase significantly with the decrease of gap size, consistent with the theoretical prediction that smaller gap size induces larger localized electromagnetic enhancement.

Polyacrylonitrile/noble metal/SiO₂ nanofibers as substrates for the amplified detection of picomolar amounts of metal ions through plasmon-enhanced fluorescence

Electrospun polymer/noble metal hybrid nanofibers have developed rapidly as surface-enhanced Raman scattering (SERS)-active substrates over the last few years. However, polymer/noble metal nanofibers with plasmon-enhanced fluorescence (PEF) activity have received no attention to date. Herein, we show a general and facile approach for the preparation of polyacrylonitrile (PAN)/noble metal/SiO2 nanofibrous mats with PEF activity for the first time by combining electrospinning and controlled silica coatings. These PEF-active nanofibrous mats can selectively improve the fluorescence intensity of conjugated polyelectrolytes (CPEs). Importantly, the CPE solution in the presence of a PAN/noble metal/SiO2 nanofibrous mat showed dramatic fluorescence quenching towards picomolar (pM) amounts of heavy metal ions, while the fluorescence of the CPE solution without the nanofibrous mat had no apparent quenching towards micromolar (μM) amounts of metal ions. The combination of the distance-dependent fluorescence enhancement performance of metal NPs and the ionic characteristics of the CPE solution makes the polymer/noble metal nanofibers promising substrates for greatly improving the detection sensitivity towards metal ions. We believe that this work provides a general strategy for preparing plasmon band-tuned PEF-active substrates with advantages including good selectivity, remarkable sensitivity and recyclability, which make them a preferable choice for practical sensing applications.

Controlled synthesis of multilayered gold nanoshells for enhanced photothermal therapy and SERS detection

It can be streamlined: A facile and controllable approach for the fabrication of core/shell-structured multilayer gold nanoshells with uniform nanosize, monodispersity, and tunable plasmonic properties has been successfully developed by utilizing an organosilica layer as the dielectric spacer layer.

Boosting Fano resonances in single layered concentric core-shell particles

Efficient excitation of Fano resonances in plasmonic systems usually requires complex nano-structure geometries and some degree of symmetry breaking. However, a single-layered concentric core-shell particle presents inherent Fano profiles in the scattering spectra when sphere and cavity modes spectrally overlap. Weak hybridization and suitable choice of core and shell materials gives rise to strong electric dipolar Fano resonances in these systems and retardation effects can result in resonances of higher multipolar order or of magnetic type. Furthermore, suitable tailoring of illumination conditions leads to an enhancement of the Fano resonance by quenching of unwanted electromagnetic modes. Overall, it is shown that single layered core-shell particles can act as robust Fano resonators.

Sensitive and selective plasmon ruler nanosensors for monitoring the apoptotic drug response in leukemia

Caspases are proteases involved in cell death, where caspase-3 is the chief executioner that produces an irreversible cutting event in downstream protein substrates and whose activity is desired in the management of cancer. To determine such activity in clinically relevant samples with high signal-to-noise, plasmon rulers are ideal because they are sensitively affected by their interparticle separation without ambiguity from photobleaching or blinking effects. A plasmon ruler is a noble metal nanoparticle pair, tethered in close proximity to one another via a biomolecule, that acts through dipole-dipole interactions and results in the light scattering to increase exponentially. In contrast, a sharp decrease in intensity is observed when the pair is confronted by a large interparticle distance. To align the mechanism of protease activity with building a sensor that can report a binary signal in the presence or absence of caspase-3, we present a caspase-3 selective plasmon ruler (C3SPR) composed of a pair of Zn0.4Fe2.6O4@SiO2@Au core-shell nanoparticles connected by a caspase-3 cleavage sequence. The dielectric core (Zn0.4Fe2.6O4@SiO2)-shell (Au) geometry provided a brighter scattering intensity versus solid Au nanoparticles, and the magnetic core additionally acted as a purification handle during the plasmon ruler assembly. By monitoring the decrease in light scattering intensity per plasmon ruler, we detected caspase-3 activity at single molecule resolution across a broad dynamic range. This was observed to be as low as 100 fM of recombinant material or 10 ng of total protein from cellular lysate. By thorough analyses of single molecule trajectories, we show caspase-3 activation in a drug-treated chronic myeloid leukemia (K562) cancer system as early as 4 and 8 h with greater sensitivity (2- and 4-fold, respectively) than conventional reagents. This study provides future implications for monitoring caspase-3 as a biomarker and efficacy of drugs.

Distance and plasmon wavelength dependent fluorescence of molecules bound to silica-coated gold nanorods

Plasmonic nanoparticles can strongly interact with adjacent fluorophores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. This dipolar coupling is dependent upon nanoparticle composition, distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore's absorbance/emission and the surface plasmon band of the nanoparticles. In this work, we examine the distance and plasmon wavelength dependent fluorescence of an infrared dye ("IRDye") bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11-26 nm thick. IRDye is covalently attached to the nanoparticle surface via a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed with 17 nm shells when the nanorod plasmon maximum is resonant with IRDye absorption. Time-resolved photoluminescence reveals multiexponential decay and a sharp reduction in fluorescence lifetime with decreasing silica shell thickness and when the plasmon maximum is closer to IRDye absorption/emission. Control experiments are carried out to confirm that the observed changes in fluorescence are due to plasmonic interactions, is simply surface attachment. There is no change in fluorescence intensity or lifetime when IRDye is bound to mesoporous silica nanoparticles. In addition, IRDye loading is limited to maintain a distance between dye molecules on the surface to more than 9 nm, well above the Förster radius. This assures minimal dye-dye interactions on the surface of the nanoparticles.

Nanoparticles heat through light localization

Aqueous solutions containing light-absorbing nanoparticles have recently been shown to produce steam at high efficiencies upon solar illumination, even when the temperature of the bulk fluid volume remains far below its boiling point. Here we show that this phenomenon is due to a collective effect mediated by multiple light scattering from the dispersed nanoparticles. Randomly positioned nanoparticles that both scatter and absorb light are able to concentrate light energy into mesoscale volumes near the illuminated surface of the liquid. The resulting light absorption creates intense localized heating and efficient vaporization of the surrounding liquid. Light trapping-induced localized heating provides the mechanism for low-temperature light-induced steam generation and is consistent with classical heat transfer.