Harmonic Generation up to Fifth Order from Al/Au/CuS Nanoparticle Films

Dual heterostructures integrating noble-metal and copper chalcogenide nanoparticles have attracted a great deal of attention in nonlinear optics, because coupling of their localized surface plasmon resonances (LSPRs) substantially enhances light-matter interactions through local-field effects. Previously, enhanced cascaded third-harmonic generation was demonstrated in Au/CuS heterostructures mediated by harmonically coupled surface plasmon resonances. This suggests a promising approach for extending nonlinear enhancement to higher harmonics by adding an additional nanoparticulate material with higher-frequency harmonic resonances to the hybrid films. Here we report the first observation of enhanced cascaded fourth- and fifth-harmonic generation in Al/Au/CuS driven by coupled LSPRs at the fundamental (1050 nm), second harmonic (525 nm), and third harmonic (350 nm) of the pump frequency. An analytical model based on incoherent dipole-dipole interactions among plasmonic nanoparticles accounts for the observed enhancements. The results suggest a novel design for efficiently generating higher harmonics in resonant plasmonic structures by means of multiple sum-frequency cascades.

Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

In Operando Photoswitching of Cu Oxidation States in Cu-Based Plasmonic Heterogeneous Photocatalysis for Efficient H2 Evolution

Metal nanoparticles (NP) supported on TiO₂ are known to be efficient photocatalysts for solar-to-chemical energy conversion. While TiO₂ decorated with copper NPs has the potential to become an attractive system, the poor oxidative stability of Cu severely limits its applicability. In this work, we demonstrate that, when Cu NPs supported on TiO₂ nanobelts (NBs) are engaged in the photocatalytic generation of H₂ from water under light illumination, Cu is not only oxidized in CuO but also dissolved under the form of Cu⁺/Cu²⁺ ions, leading to a continuous reconstruction of nanoparticles via Ostwald ripening. By nanoencapsulating the CuO_x (Cu/CuO/Cu₂O) NPs by a few layers of carbon supported on TiO₂ (TC@C), Ostwald ripening can be suppressed. Simultaneously, the resulting CuO_x@C NPs are photoreduced under light illumination to generate Cu@C NPs. This photoswitching strategy allows the preparation of a Cu plasmonic photocatalyst with enhanced activity for H₂ production. Remarkably, the photocatalyst is even active when illuminated with visible light, indicating a clear plasmonic enhancement of photocatalytic activity from the surface plasmonic resonance (SPR) effect of Cu NPs. Three-dimensional electromagnetic wave-frequency domain (3D-EWFD) simulations were conducted to confirm the SPR enhancement. This advance bodes for the development of scalable multifunctional Cu-based plasmonic photocatalysts for solar energy transfer.

Monocrystalline Labeling Enables Stable Plasmonic Enhancement for Isolation-Free Extracellular Vesicle Analysis

Sensitive detection of extracellular vesicles (EVs) as emerging biomarkers has shown great promises for disease diagnosis. Plasmonic metal nanostructures conjugated with molecules that bind specific biomarker targets are widely used for EVs sensing but involve tradeoffs between particle-size-dependent signal intensity and conjugation efficiency. One solution to this problem would be to induce nucleation on nanoparticles that have successfully bound a target biomarker to permit in situ nanoparticle growth for signal amplification, but approaches that are evaluated to date require harsh conditions or lack nucleation specificity, prohibiting their effective use with most biological specimens. This study describes a one-step in situ strategy to induce monocrystalline copper shell growth on gold nanorod probes without decreasing signal by disrupting probe-target interactions or lipid bilayer integrity to enable EV biomarker detections. This approach increases the detected nanoparticle signal about two orders of magnitude after a 10 min copper nanoshell growth reaction. This has significant implications for improved disease detection, as indicated by the ability of a novel immunoassay using this approach to detect low abundance EVs carrying a pathogen-derived biomarker, after their direct capture from serum, to facilitate the diagnosis of tuberculosis cases in a diagnostically challenging pediatric cohort.

Optomechanical Hot-Spots in Metallic Nanorod-Polymer Nanocomposites

Plasmonic coupling between adjacent metallic nanoparticles can be exploited for acousto-plasmonics, single-molecule sensing, and photochemistry. Light absorption or electron probes can be used to study plasmons and their interactions, but their use is challenging for disordered systems and colloids dispersed in insulating matrices. Here, we investigate the effect of plasmonic coupling on optomechanics with Brillouin light spectroscopy (BLS) in a prototypical metal-polymer nanocomposite, gold nanorods (Au NRs) in polyvinyl alcohol. The intensity of the light inelastically scattered on thermal phonons captured by BLS is strongly affected by the wavelength of the probing light. When light is resonant with the transverse plasmons, BLS reveals mostly the normal vibrational modes of single NRs. For lower energy off-resonant light, BLS is dominated by coupled bending modes of NR dimers. The experimental results, supported by optomechanical calculations, document plasmonically enhanced BLS and reveal energy-dependent confinement of coupled plasmons close to the tips of NR dimers, generating BLS hot-spots. Our work establishes BLS as an optomechanical probe of plasmons and promotes nanorod-soft matter nanocomposites for acousto-plasmonic applications.

Plasmonic Chiral Metasurface-Induced Upconverted Circularly Polarized Luminescence from Achiral Upconversion Nanoparticles

Chirality induction, transfer, and manipulation have aroused great interest in achiral nanomaterials. Here, we demonstrate strong upconverted circularly polarized luminescence from achiral core-shell upconversion nanoparticles (UCNPs) via a plasmonic chiral metasurface-induced optical chirality transfer. The Yb³⁺-sensitized core-shell UCNPs with good dispersity exhibit intense upconversion luminescence of Tm³⁺ and Nd³⁺ through the energy transfer process. By spin-coating the core-shell UCNPs on this chiral metasurface, strong enhancement and circular polarization modulation of upconversion luminescence can be achieved due to resonant coupling between surface plasmons and upconversion nanoparticles. In the UCNPs-on-metasurface composite, a significant upconversion luminescence enhancement can be achieved with a maximum enhancement factor of 32.63 at 878 nm and an overall enhancement factor of 11.61. The luminescence dissymmetry factor of the induced upconverted circularly polarized luminescence can reach 0.95 at the emission wavelength of 895 nm. The UCNPs-on-metasurface composite yields efficient modulation for the emission intensity and polarization of UCNPs, paving new pathways to many potential applications in imaging, sensing, and anticounterfeiting fields.

Optoelectronics of Atomic Metal-Semiconductor Interfaces in Tin-Intercalated MoS2

Metal-semiconductor interfaces are ubiquitous in modern electronics. These quantum-confined interfaces allow for the formation of atomically thin polarizable metals and feature rich optical and optoelectronic phenomena, including plasmon-induced hot-electron transfer from metal to semiconductors. Here, we report on the metal-semiconductor interface formed during the intercalation of zero-valent atomic layers of tin (Sn) between layers of MoS₂, a van der Waals layered material. We demonstrate that Sn interaction leads to the emergence of gap states within the MoS₂ band gap and to corresponding plasmonic features between 1 and 2 eV (0.6-1.2 μm). The observed stimulation of the photoconductivity, as well as the extension of the spectral response from the visible regime toward the mid-infrared suggests that hot-carrier generation and internal photoemission take place.

Two-Photon-Excited Single-Molecule Fluorescence Enhanced by Gold Nanorod Dimers

We demonstrate two-photon-excited single-molecule fluorescence enhancement by single end-to-end self-assembled gold nanorod dimers. We employed biotinylated streptavidin as the molecular linker, which connected two gold nanorods in end-to-end fashion. The typical size of streptavidin of around 5 nm separates the gold nanorods with gaps suitable for the access of fresh dyes in aqueous solution, yet small enough to give very high two-photon fluorescence enhancement. Simulations show that enhancements of more than 7 orders of magnitude can be achieved for two-photon-excited fluorescence in the plasmonic hot spots. With such high enhancements, we successfully detect two-photon-excited fluorescence for a common organic dye (ATTO 610) at the single-molecule, single-nanoparticle level.

A Super-Boosted Hybrid Plasmonic Upconversion Process for Photodetection at 1550 nm Wavelength

A super-boosted hybrid plasmonic upconversion (UC) architecture comprising a hierarchical plasmonic upconversion (HPU) film and a polymeric microlens array (MLA) film is proposed for efficient photodetection at a wavelength of 1550 nm. Plasmonic metasurfaces and Au core-satellite nanoassembly (CSNA) films can strongly induce a more effective plasmonic effect by providing numerous hot spots in an intense local electromagnetic field up to wavelengths exceeding 1550 nm. Hence, significant UC emission enhancement is realized via the amplified plasmonic coupling of an HPU film comprising an Au CSNA and UC nanoparticles. Furthermore, an MLA polymer film is synergistically coupled with the HPU film, thereby focusing the incident near-infrared light in the micrometer region, including the plasmonic nanostructure area. Consequently, the plasmonic effect super-boosted by microfocusing the incident light, significantly lowers the detectable power limit of a device, resulting in superior sensitivity and responsivity at weak excitation powers. Finally, a triple-cation perovskite-based photodetector coupled with the hybrid plasmonic UC film exhibits the excellent values of responsivity and detectivity of 9.80 A W^-1 and 8.22 × 10¹² Jones at a weak power density of ≈0.03 mW cm^-2 , respectively, demonstrating that the device performance is enhanced by more than 10⁴ magnitudes over a reference sample.

Plasmonic Enhancement of Two-Photon Excited Luminescence of Gold Nanoclusters

Plasmonic-enhanced luminescence of single molecules enables imaging and detection of low quantities of fluorophores, down to individual molecules. In this work, we present two-photon excited luminescence of single gold nanoclusters, Au₁₈(SG)₁₄, in close proximity to bare gold nanorods (AuNRs). We observed 25-times enhanced emission of gold nanoclusters (AuNCs) in near infrared region, which was mainly attributed to the resonant excitation of localized surface plasmon resonance (LSPR) of AuNRs and spectral overlap of LSPR band with photoluminescence of AuNCs. This work is an initial step in application of combined nanoparticles: gold nanorods and ultrasmall nanoclusters in a wide range of multiphoton imaging and biosensing applications.

Upconversion lanthanide nanomaterials: basics introduction, synthesis approaches, mechanism and application in photodetector and photovoltaic devices

Upconversion (UC) of lanthanide-doped nanostructure has the unique ability to convert low energy infrared (IR) light to high energy photons, which has significant potential for energy conversion applications. This review concisely discusses the basic concepts and fundamental theories of lanthanide nanostructures, synthesis techniques, and enhancement methods of upconversion for photovoltaic and for near-infrared (NIR) photodetector (PD) application. In addition, a few examples of lanthanide-doped nanostructures with improved performance were discussed, with particular emphasis on upconversion emission enhancement using coupling plasmon. The use of UC materials has been shown to significantly improve the NIR light-harvesting properties of photovoltaic devices and photocatalytic materials. However, the inefficiency of UC emission also prompted the need for additional modification of the optical properties of UC material. This improvement entailed the proper selection of the host matrix and optimization of the sensitizer and activator concentrations, followed by subjecting the UC material to surface-passivation, plasmonic enhancement, or doping. As expected, improving the optical properties of UC materials can lead to enhanced efficiency of PDs and photovoltaic devices.

A Flexible Plasmonic-Membrane-Enhanced Broadband Tin-Based Perovskite Photodetector

Lead-free perovskite quantum dots (QDs) have been widely investigated for optoelectronic devices because of their excellent electrical and optical properties. However, optoelectronic devices based on such lead-free perovskites still have much lower performance than those made of Pb-based counterparts. Herein, we developed a lead-free photodetector with an enhanced broadband spectral response ranging from 300 to 630 nm. By balancing plasmonic near-field enhancement and surface energy quenching through precisely controlling the thickness of Al₂O₃ spacer between the CsSnBr₃ QDs and silver nanoparticle membrane, the photodetector with 5 nm thick Al₂O₃ experiences a maximum photocurrent enhancement of 6.5-fold at 410 nm, with a responsivity of 62.3 mA/W and detectivity of 4.27 × 10¹¹ Jones. Moreover, its photocurrent shows a negligible decrease after 100 cycles of bending, which is ascribed to the tension-offset induced by the self-assembled nanoparticle membrane. The proposed plasmonic membrane enhancement provides a great potential for high-performance perovskite optoelectronic devices.

Plasmonic Enhancement of Chiroptical Property in Enantiomers Using a Helical Array of Magnetoplasmonic Nanoparticles for Ultrasensitive Chiral Recognition

Recognition of enantiomeric molecules is essential in pharmaceutical and biomedical applications. In this Article, a novel approach is introduced to monitor chiral molecules via a helical magnetic field (hB), where chiral-inactive magnetoplasmonic nanoparticles (MagPlas NPs, Ag@Fe₃O₄ core-shell NPs) are assembled into helical nanochain structures to be chiral-active. An in-house generator of hB-induced chiral NP assembly, that is, a plasmonic chirality enhancer (PCE), is newly fabricated to enhance the circular dichroism (CD) signals from chiral plasmonic interaction of the helical nanochain assembly with circularly polarized light, reaching a limit of detection (LOD) of 10^-10 M, a 1000-fold enhancement as compared to that of conventional CD spectrometry. These enhancements were successfully observed from enantiomeric molecules, oligomers, polymers, and drugs. Computational simulation studies also proved that total chiroptical properties of helical plasmonic chains could be readily changed by modifying the chiral structure of the analytes. The proposed PCE has the potential to be used as an advanced tool for qualitative and quantitative recognition of chiral materials, enabling further application in pharmaceutical and biomedical sensing and imaging.

Fabrication of low cost highly structured silver capped aluminium nanorods as SERS substrate for the detection of biological pathogens

We report the fabrication of low cost highly structured silver (Ag) capped aluminium (Al) nanorods (NRs) as surface enhanced Raman spectroscopy (SERS) substrate utilising the glancing angle deposition technique. The nano-capping of silver onto the Al NRs can concentrate the local electric field within the minimal volume that can serve as hotspots. The average size of the Ag nanocaps was 50 nm. The newly proposed nanoporous Ag capped Al NRs as SERS substrate could detect the Raman signal of rhodamine 6G (R6G) up to 10^-15molar concentration. The significant enhancement in the Raman signal of 10⁷was achieved for Ag capped Al NRs considering R6G as a probe molecule. Using the developed SERS substrate, we recorded Raman spectra forEscherichia colibacteria with its concentration varying from 10⁸colony forming units per ml (CFU ml^-1) up to 10²CFU ml^-1. All the reported Raman spectra were acquired by a portable handheld Raman spectrometer. Hence, this newly proposed low cost, effective SERS substrate can be used commercially for the onsite detection of clinical pathogens. The 3D finite difference time domain simulation model was performed for Ag capped Al nanostructure to understand the generation of hotspots. The simulated results show excellent agreement with the experimental results. We fabricated uncapped Ag nanorods of similar dimensions and performed the experimental measurements and simulations for comparison. We found a significant enhancement in Ag capped Al NRs compared to the long Ag NRs. The description of the Raman signal enhancement has been elaborated.

Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2

Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light-matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to ∼150 nm) enhancement of optical nonlinearity in monolayer MoS₂ with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS₂-plasmonic structure is in the order of 10^-17 m²/V², far superior (∼10-100-times larger) to the widely used conventional bulk materials (e.g., LiNbO₃, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.

Dark-Field Imaging of Cation Exchange Synthesis of Cu2-xS@Au2S@Au Nanoplates toward the Plasmonic Enhanced Hydrogen Evolution Reaction

The development of novel electrocatalysts, especially Pt-free electrocatalysts, is of great significance for evolving hydrogen fuel cells. Two-dimensional materials have many advantages, such as large specific surface area, abundant active edges, and adjustable electronic structure, which provide broad prospects for studying high-performance electrocatalysts. In this paper, Cu_2-xS@Au₂S@Au nanoplates (NPs) were synthesized by cation exchange, which showed good catalytic performance toward the hydrogen evolution reaction (HER). Dark-field microscopy can help observe the process of cation exchange in real time to precisely control the synthesis of the composite materials. The synthesized Cu_2-xS@Au₂S@Au nanoplates (NPs) exhibited greatly enhanced plasmonic emission, resulting in accelerated chemical conversion and improved HER efficiency. Under 532 nm laser excitation, the overpotential of the HER shifted from 152 to 96 mV at a current density of -10 mA cm^-2. The plasmonic nanocatalysts show exciting prospects in the field of new energy resources.

An Ultrasensitive Gateless Photodetector Based on the 2D Bilayer MoS2-1D Si Nanowire-0D Ag Nanoparticle Hybrid Structure

Atomically thin transition metal dichalcogenides (TMDC) have received much attention due to their wide variety of optical and electronic properties. Among various TMDC materials, molybdenum disulfide (MoS₂) has been intensely studied owing to its potential applications in nanoelectronics and optoelectronics. However, two-dimensional MoS₂ photodetectors suffer from low responsivity due to low optical cross section. Combining MoS₂ with plasmonic nanostructures can drastically increase scattering cross section and enhance local light-matter interaction. Moreover, suspended MoS₂ has been shown to exhibit higher photoluminescence intensity and strong photogating effect, which can be employed in photodetectors. Herein, we propose an approach to utilize plasmonic nanostructures and physical suspension for 2D MoS₂ photosensing enhancement by hybridizing 2D bilayer MoS₂, 1D silicon nanowires, and 0D silver nanoparticles. The hybrid structure shows a gateless responsivity of 402.4 A/W at a wavelength of 532 nm, which represents the highest value among the ever reported gateless plasmonic MoS₂ photodetector. The great responsivity and large active area results in an exceptional detectivity of 2.34 × 10¹² Jones. This study provides a new approach for designing high-performance 2D TMDC-based optoelectronic devices.

Efficient Generation of Two-Photon Excited Phosphorescence from Molecules in Plasmonic Nanocavities

Nonlinear molecular interactions with optical fields produce intriguing optical phenomena and applications ranging from color generation to biomedical imaging and sensing. The nonlinear cross-section of dielectric materials is low and therefore for effective utilisation, the optical fields need to be amplified. Here, we demonstrate that two-photon absorption can be enhanced by 10⁸ inside individual plasmonic nanocavities containing emitters sandwiched between a gold nanoparticle and a gold film. This enhancement results from the high field strengths confined in the nanogap, thus enhancing nonlinear interactions with the emitters. We further investigate the parameters that determine the enhancement including the cavity spectral position and excitation wavelength. Moreover, the Purcell effect drastically reduces the emission lifetime from 520 ns to <200 ps, turning inefficient phosphorescent emitters into an ultrafast light source. Our results provide an understanding of enhanced two-photon-excited emission, allowing for optimization of efficient nonlinear light-matter interactions at the nanoscale.

Hierarchically Ordered Silicon Metastructures from Improved Self-Assembly-Based Nanosphere Lithography

We developed an improved self-assembly method to obtain a large-area, high-quality templated monolayer mask using the polystyrene spheres. On the basis of the templated mask, hierarchically ordered Si metastructures with different nanosteps are fabricated using cyclic inductively coupled plasma etching technique. By evaporating a thin gold capping layer on these Si metastructures, their optical properties are comparatively studied using the surface-enhanced Raman scattering spectroscopy. Our proposed technique is highly promising for fabricating a variety of periodic three-dimensional hierarchically ordered metastructures, which could be further utilized for applications in SERS-based biosensors, optical absorbers, metamaterial/metasurface devices, etc.

Ultrafast Single-Molecule Fluorescence Measured by Femtosecond Double-Pulse Excitation Photon Antibunching

Most measurements of fluorescence lifetimes on the single-molecule level are carried out using avalanche photon diodes (APDs). These single-photon counters are inherently slow, and their response shows a strong dependence on photon energy, which can make reconvolution of the instrument response function (IRF) challenging. An ultrafast time resolution in single-molecule fluorescence is crucial, e.g., in determining donor lifetimes in donor-acceptor couples which undergo energy transfer, or in plasmonic antenna structures, where the radiative rate and non-radiative rates are enhanced. We introduce a femtosecond double-excitation (FeDEx) photon correlation technique, which measures the degree of photon antibunching as a function of time delay between two excitation pulses. In this boxcar integration, the time resolution of fluorescence transients is limited solely by the laser pulse length and is independent of the detector IRF. The versatility of the technique is demonstrated with a custom-made donor-acceptor complex with one donor and two acceptors and with single dye molecules positioned accurately between two gold nanoparticles using DNA origami. The latter structures show ∼75-fold radiative-rate enhancement and fluorescence lifetimes down to 19 ps, which is measured without the need for any reconvolution. With the potential of measuring subpicosecond fluorescence lifetimes, plasmonic antenna structures can now be optimized further.

Wrapping AgCl Nanostructures with Trimetallic Nanomeshes for Plasmon-Enhanced Catalysis and in Situ SERS Monitoring of Chemical Reactions

Selective chemical control of multiple reactions is incredibly important for the fabrication of sophisticated nanostructures for functional applications. A representative example is the synthesis of plasmonic nanomaterial-silver chloride (AgCl) conjugates, where metal ions should be selectively reduced into metallic nanostructures for plasmon-enhanced catalytic activity, while the reducible AgCl nanomaterials remain intact despite the presence of a chemical reductant. In addition to the selectively controlled reduction, the plasmonic nanostructures should be appropriately designed for the high stability and photoefficiency of catalysts. In this study, we demonstrate how AgCl nanocubes and nanospheres could be comprehensively wrapped by plasmonic three-dimensional nanomesh structures consisting of gold, silver, and palladium by the selective reduction of their ionic precursors while the AgCl nanostructures remain intact. Complete trimetallic wrapping provided the absorption of visible light, while the porosity of the nanomesh structures exposed the photocatalytic AgCl surface to catalyze desired reactions. Platinum in place of palladium was examined to demonstrate the versatility of the wrapping scheme, resulting in an extraordinary catalytic activity. Importantly, the detailed chemical mechanism behind the trimetallic wrapping of the AgCl nanostructures was systematically investigated to understand the roles of each reaction component in controlling the chemical selectivity. The synthesized AgCl-trimetal nanoconjugates excellently exhibit both metal-based and plasmon-enhanced catalytic properties for the removal of environmentally harmful Cr⁶⁺. Moreover, their applications as surface-enhanced Raman-scattering (SERS) probes for the in situ monitoring of catalytic reduction in real-time and as single-nanoparticle SERS probes for molecular detection are thoroughly demonstrated.

Plasmonically Enhanced Upconversion Luminescence via Holographically Formed Silver Nanogratings

Greatly enhanced upconversion luminescence was demonstrated by integrating the core-shell upconversion nanorods with the Ag nanogratings. Both the Ag nanogratings and upconversion nanorods were fabricated/synthesized in a facile, cost-effective, high-throughput way. Experimental results showed that the upconversion luminescence intensity of Er³⁺ in the core-shell upconversion nanorods can be well tuned and enhanced by changing the shell thickness and the period of the Ag nanograting. The underlying physical mechanism for the upconversion luminescence enhancement was attributed to the plasmonically enhanced near infrared broadband absorption of the periodic Ag nanograting and the localized surface plasmon resonance of Ag nanocrystals. The maximum enhanced factors of 523 nm, 544 nm (green emission), and 658 nm (red emission) of Er³⁺ ions excited at 980 nm are 3.8-, 5.5-, and 4.6-folds, respectively. Our fabrication approach and results suggest that such a simple integration is potentially useful for biosensing/imaging and anti-counterfeiting applications.

Theranostic nanobubble encapsulating a plasmon-enhanced upconversion hybrid nanosystem for cancer therapy

Nanobubble (NB), which simultaneously enhances ultrasound (US) images and access therapeutic platforms, is required for future cancer treatment. Methods: We designed a theranostic agent for novel cancer treatment by using an NB-encapsulated hybrid nanosystem that can be monitored by US and fluorescent imaging and activated by near-infrared (NIR) light. The nanosystem was transported to the tumor through the enhanced permeability and retention effect. The hybrid nanosystem comprised upconversion nanoparticle (UCNP) and mesoporous silica-coated gold nanorod (AuNR@mS) with the photosensitizer merocyanine 540 to realize dual phototherapy. Results: With the NIR light-triggered, the luminous intensity of the UCNP was enhanced by doping holmium ion and emitted visible green and red lights at 540 and 660 nm. The high optical density state between the UCNP and AuNR@mS can induce plasmonic enhancement to improve the photothermal and photodynamic effects, resulting in cell death by apoptosis. The nanosystem showed excellent stability to avoid the aggregation of nanoparticles during the treatment. JC-1 dye was used as an indicator of mitochondrial membrane potential to identify the mechanism of cell death. The results of in vitro and in vivo analyses confirmed the curative effect of improved dual phototherapy. Conclusion: We developed and showed the therapeutic functions of a novel nanosystem with the combination of multiple theranostic nanoplatforms that can be triggered and activated by 808 nm NIR laser and US.

Second Harmonic Generation for Moisture Monitoring in Dimethoxyethane at a Gold-Solvent Interface Using Plasmonic Structures

Second harmonic generation (SHG) is forbidden from most bulk metals because metals are characterized by centrosymmetric symmetry. Adsorption or desorption of molecules at the metal interface can break the symmetry and lead to SHG responses. Yet, the response is relatively low, and minute changes occurring at the interface, especially at solid/liquid interfaces, like in battery electrodes are difficult to assess. Herein, we use a plasmonic structure milled in a gold electrode to increase the overall SHG signal from the interface and gain information about small changes occurring at the interface. Using a specific homebuilt cell, we monitor changes at the liquid/electrode interface. Specifically, traces of water in dimethoxyethane (DME) have been detected following changes in the SHG responses from the plasmonic structures. We propose that by plasmonic structures this technique can be used for assessing minute changes occurring at solid/liquid interfaces such as battery electrodes.

DNA-Mediated Self-Assembly of Plasmonic Antennas with a Single Quantum Dot in the Hot Spot

DNA self-assembly is a powerful tool to arrange optically active components with high accuracy in a large parallel manner. A facile approach to assemble plasmonic antennas consisting of two metallic nanoparticles (40 nm) with a single colloidal quantum dot positioned at the hot spot is presented here. The design approach is based on DNA complementarity, stoichiometry, and steric hindrance principles. Since no intermediate molecules other than short DNA strands are required, the structures possess a very small gap (≈ 5 nm) which is desired to achieve high Purcell factors and plasmonic enhancement. As a proof-of-concept, the fluorescence emission from antennas assembled with both conventional and ultrasmooth spherical gold particles is measured. An increase in fluorescence is obtained, up to ≈30-fold, compared to quantum dots without antenna.

Plasmonics with Metallic Nanowires

The purpose of this review is to introduce and present the concept of metallic nanowires as building-blocks of plasmonically active structures. In addition to concise description of both the basic physical properties associated with the electron oscillations as well as energy propagation in metallic nanostructures, and methods of fabrication of metallic nanowires, we will demonstrate several key ideas that involve interactions between plasmon excitations and electronic states in surrounding molecules or other emitters. Particular emphasis will be placed on the effects that involve not only plasmonic enhancement or quenching of fluorescence, but also propagation of energy on lengths that exceed the wavelength of light.

Optical Properties of Submillimeter Silver Nanowires Synthesized Using the Hydrothermal Method

We report on the synthesis of long silver nanowires using the hydrothermal method, with H₂O₂ as the reducing agent. Our approach yields nanowires with an average diameter and length of about 100 nm and 160 µm, respectively, reaching the maximum length of 800 µm. Scanning electron microscopy (SEM) measurements revealed the presence of a thick, inhomogeneous poly(vinylpyrrolidone) (PVP) layer covering the nanowires, which with time becomes much more uniform, leading to well-defined extinction peaks in the ultraviolet-visible (UV-Vis) spectra. This change in morphology is evidenced also by the fluorescence enhancement behavior probed using protein complexes. Wide-field and confocal fluorescence microscopy measurements demonstrate strong, 10-fold enhancement of the protein emission intensity, accompanied by a reduction of the fluorescence decay time. In addition, for the aged, one-month-old nanowires, the uniformity of the intensity profile along them was substantially improved as compared with the as-synthesized ones. The results point towards the importance of the morphology of plasmonically active silver nanowires when considering their application in enhancing optical properties or achieving energy propagation over submillimeter distances.

Total Aqueous Synthesis of Au@Cu2- x S Core-Shell Nanoparticles for In Vitro and In Vivo SERS/PA Imaging-Guided Photothermal Cancer Therapy

Both accurate tumor navigation and nanostructures with high photothermal (PT) conversion efficiency are important but remain challenging to achieve in current biomedical applications. This study reports an anion exchange-based facile and green approach for synthesizing Au@Cu_{2- x} S core-shell nanoparticles (NPs) in an aqueous system. In addition to the PT effect of the suggested NPs, the surface-enhanced Raman scattering (SERS) is also significantly improved due to the tailored localized surface plasmon resonance coupling between the Au metal core and the Cu_{2- x} S semiconductor shell. Using an epitaxial strategy, Au@Cu₂ O NPs are first obtained by the in situ reduction of cupric hydroxide on a cresyl violet acetate-coated Au core; then, Au@Cu_{2- x} S NPs are obtained via anion exchange between the S^2- and Cu₂ O shell. Both the Cu/S atomic ratio and the Cu_{2- x} S shell thickness can be adjusted conveniently. Hence, the ideal integration of the plasmonic Au core and Cu_{2- x} S shell into a single unit is conducive not only to highly efficient PT conversion but also to the construction of a SERS-based navigator. This new type of SERS-guided NP, with enhanced photoacoustic signals, is an important candidate for both accurate tumor navigation and nondestructive PT treatment guided in vivo by two modes of optical imaging.

Broad-Band High-Sensitivity ZnO Colloidal Quantum Dots/Self-Assembled Au Nanoantennas Heterostructures Photodetectors

Tunable plasmonic resonance induced by the collective oscillation of the electrons on metallic nanostructures can excellently enhance the light response of ZnO films, which provides an effective way to break through the limitation of the performance of ZnO photodetectors. Here, broad-band high-performance ZnO/Au heterostructures photodetectors with various morphologies of self-assembled Au nanoantennas are fabricated via a facile approach under the spin-coated ZnO colloidal quantum dots films. With a systematic control on growth condition, the self-assembled Au nanoantennas undergo a drastic evolution from the corrugated nanomounds to the island-like nanostructures, and the light absorption of the resulting ZnO/Au heterostructures correspondingly exhibits a strongly morphological dependence on the Au nanoantennas. Meanwhile, the photoresponse of the ZnO-based photodetectors is significantly improved throughout a wide spectrum between UV and visible regions owing to the enhanced light absorption induced by the localized surface plasmon resonance. As a result, the optimal switch ratio of the ZnO/Au heterostructures photodetector increases by 1 order to ∼1.13 × 10⁵ than that of the pristine ZnO one because of the obviously increased photocurrent ( I_ph) and comparable dark current, thus leading to ∼9.1 and ∼4.9 times increases in the photoresponsivity and the normalized detectivity. Meanwhile, the significant increases in the I_ph of ∼5.2 and ∼9.7 times are likewise observed with the ZnO/Au heterostructures under 530 nm and white-light illumination. This work can offer a handy and effective approach for the fabrication of ultrasensitive ZnO-based photodetectors within a broad-band wavelength by utilizing the Au plasmonic nanostructures.

A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction

A dual-band self-powered photodetector (SPPD) with high sensitivity is realized by a facile combination of InSe Schottky diode and Au plasmonic nanoparticle (NP) arrays. Comparing with pristine InSe devices, InSe/Au photodetectors possess an additional capability of photodetection in visible to near-infrared (NIR) region. This intriguing phenomenon is attributed to the wavelength selective enhancement of pristine responsivities by hybridized quadrupole plasmons resonance of Au NPs. It is worth pointing out that the maximum of enhancement ratio in responsivity reaches up to ∼1200% at a wavelength of 685 nm. In addition, owing to a large Schottky barrier difference formed between active layer and two asymmetric electrodes, the responsivities of dual-band InSe/Au photodetector could reach up to 369 and 244 mA/W at the wavelength of 365 and 685 nm under zero bias voltage, respectively. This work would provide an additional opportunity for developing multifunctional photodetectors with high performance based on two-dimensional materials, upgrading their capacity of photodetection in a complex environment.

Plasmonic Enhancement of Two-Photon-Excited Luminescence of Single Quantum Dots by Individual Gold Nanorods

Plasmonic enhancement of two-photon-excited fluorescence is not only of fundamental interest but also appealing for many bioimaging and photonic applications. The high peak intensity required for two-photon excitation may cause shape changes in plasmonic nanostructures, as well as transient plasmon broadening. Yet, in this work, we report on strong enhancement of the two-photon-excited photoluminescence of single colloidal quantum dots close to isolated chemically synthesized gold nanorods. Upon resonant excitation of the localized surface plasmon resonance, a gold nanorod can enhance the photoluminescence of a single quantum dot more than 10 000-fold. This strong enhancement arises from the combined effect of local field amplification and the competition between radiative and nonradiative decay rate enhancements, as is confirmed by time-resolved fluorescence measurements and numerical simulations.

Functionalization of nanostructured gold substrates with chiral chromophores for SERS applications: The case of 5-Aza[5]helicene

Nanostructured gold thin films can be fabricated by controlled pulsed laser deposition to get efficient sensors, with uniform morphology and optimized plasmon resonance, to be employed as plasmonic substrates in surface enhanced Raman scattering spectroscopy. By attaching 5-aza[5]helicen-6-yl-6-hexanethiol to such gold nanostructures, used in a previous work for label-free drug sensing with biomedical purposes, we successfully prepared functionalized substrates with remarkable surface enhanced Raman scattering activity. The long-term motivation is to develop probes for drug detection at low concentrations, where sensitivity to specific chiral targets is required.

Noninvasive Cathodoluminescence-Activated Nanoimaging of Dynamic Processes in Liquids

In situ electron microscopy provides remarkably high spatial resolution, yet electron beam irradiation often damages soft materials and perturbs dynamic processes, requiring samples to be very robust. Here, we instead noninvasively image the dynamics of metal and polymer nanoparticles in a liquid environment with subdiffraction resolution using cathodoluminescence-activated imaging by resonant energy transfer (CLAIRE). In CLAIRE, a free-standing scintillator film serves as a nanoscale optical excitation source when excited by a low energy, focused electron beam. We capture the nanoscale dynamics of these particles translating along and desorbing from the scintillator surface and demonstrate 50 ms frame acquisition and a range of imaging of at least 20 nm from the scintillator surface. Furthermore, in contrast with in situ electron microscopy, CLAIRE provides spectral selectivity instead of relying on scattering alone. We also demonstrate through quantitative modeling that the CLAIRE signal from metal nanoparticles is impacted by multiplasmonic mode interferences. Our findings demonstrate that CLAIRE is a promising, noninvasive approach for super-resolution imaging for soft and fluid materials with high spatial and temporal resolution.

Superior Plasmonic Photodetectors Based on Au@MoS2 Core-Shell Heterostructures

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS₂ heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS₂ shell, which perfectly realizes the intimate and direct interfacing of Au and MoS₂. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS₂ transistors. The other type of device geometry is a Si-supported Au@MoS₂ heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS₂.

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide-doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF₄ :Yb³⁺ /Er³⁺ /Mn²⁺ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm-diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near-field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time-resolved measurements and finite-difference time-domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.

Plasmon-Enhanced Photodynamic Cancer Therapy by Upconversion Nanoparticles Conjugated with Au Nanorods

Photodynamic therapy (PDT) based on photosensitizers (PSs) constructed with nanomaterials has been widely applied to treat cancer. This therapy is characterized by an improved PS accumulation in tumor regions. However, challenges, such as short penetration depth of light and low extinction coefficient of PSs, limit PDT applications. In this study, a nanocomposite consisting of NaYF₄:Yb/Er upconversion nanoparticles (UCPs) conjugated with gold nanorods (Au NRs) was developed to improve the therapeutic efficiency of PDT. Methylene blue (MB) was embedded in a silica shell for plasmon-enhanced PDT. UCPs served as a light converter from near-infrared (NIR) to visible light to excite MB to generate reactive oxygen species (ROS). Au NRs could effectively enhance upconversion efficiency and ROS content through a localized surface plasmon resonance (SPR) effect. Silica shell thickness was adjusted to investigate the optimized MB loading amount, ROS production capability, and efficient distance for plasmon-enhanced ROS production. The mechanism of plasmon-enhanced PDT was verified by enhancing UC luminescence intensity through the plasmonic field and by increasing the light-harvesting capability and absorption cross section of the system. This process improved the ROS generation by comparing the exchange of Au NRs to Au nanoparticles with different SPR bands. NIR-triggered nanocomposites of UCP@SiO₂:MB-NRs were significantly confirmed by improving ROS generation and further modifying folic acid (FA) to develop an active component targeting OECM-1 oral cancer cells. Consequently, UCP@SiO₂:MB-NRs-FA could highly produce ROS and undergo efficient PDT in vitro and in vivo. The mechanism of PDT treatment by UCP@SiO₂:MB-NRs-FA was evaluated via the cell apoptosis pathway. The proposed process is a promising strategy to enhance ROS production through plasmonic field enhancement and thus achieve high PDT therapeutic efficacy.

Nanoscopic Cellular Imaging: Confinement Broadens Understanding

In recent years, single-molecule fluorescence imaging has been reconciling a fundamental mismatch between optical microscopy and subcellular biophysics. However, the next step in nanoscale imaging in living cells can be accessed only by optical excitation confinement geometries. Here, we review three methods of confinement that can enable nanoscale imaging in living cells: excitation confinement by laser illumination with beam shaping; physical confinement by micron-scale geometries in bacterial cells; and nanoscale confinement by nanophotonics.

Properties of plasmonic arrays produced by pulsed-laser nanostructuring of thin Au films

A brief description of research advances in the area of short-pulse-laser nanostructuring of thin Au films is followed by examples of experimental data and a discussion of our results on the characterization of structural and optical properties of gold nanostructures. These consist of partially spherical or spheroidal nanoparticles (NPs) which have a size distribution (80 ± 42 nm) and self-organization characterized by a short-distance order (length scale ≈140 nm). For the NP shapes produced, an observably broader tuning range (of about 150 nm) of the surface plasmon resonance (SPR) band is obtained by renewal thin film deposition and laser annealing of the NP array. Despite the broadened SPR bands, which indicate damping confirmed by short dephasing times not exceeding 4 fs, the self-organized Au NP structures reveal quite a strong enhancement of the optical signal. This was consistent with the near-field modeling and micro-Raman measurements as well as a test of the electrochemical sensing capability.

Plasmonic enhanced emissions from cubic NaYF(4):Yb: Er/Tm nanophosphors

A metal shell was used in this study to provide significant enhancement of the up-converted emission from cubic NaYF(4) nanoparticles, creating a valuable composite material for labeling in biology and other applications - use of the cubic form of the material obviates the need to undertake a high temperature transformation to the naturally more efficient hexagonal phase. The NaYF(4) matrix contained ytterbium sensitizer and an Erbium (Er) or Thulium (Tm) activator. The particle sizes of the as-synthesized nanoparticles were in the range of 20-40 nm with a gold shell thickness of 4-8 nm. The gold shell was macroscopically amorphous. The synthesis method was based on a citrate chelation. In this approach, we exploited the ability of the citrate ion to act as a reductant and stabilizer. Confining the citrate ion reductant on the nanophosphor surface rather than in the solution was critical to the gold shell formation. The plasmonic shell enhanced the up-conversion emission of Tm from visible and near-infrared regions by up to a factor of 8, in addition to imparting a visible color arising from the plasmon absorption of the gold shell. The up-conversion enhancement observed with Tm and Er were different for similar gold coverages, with local crystal field changes as a possible route to enhance up-conversion emission from high symmetry structural hosts. These novel up-converting nanophosphor particles combine the phosphor and features of a gold shell, providing a unique platform for many biological imaging and labeling applications.