Ultrasensitive nanosensors based on localized surface plasmon resonances:From theory to applications

The subwavelength confinement feature of localized surface plasmon resonance（LSPR） allows plasmonic nanostructures to be functionalized as powerful platforms for detecting various molecular analytes as well as weak processes with nanoscale spatial resolution. One of the main goals of this field of research is to lower the absolute limit-of-detection（LOD）of LSPR-based sensors. This involves the improvement of（i） the figure-of-merit associated with structural parameters such as the size, shape and interparticle arrangement and,（ii） the spectral resolution. The latter involves advanced target identification and noise reduction techniques. By highlighting the strategies for improving the LOD, this review introduces the fundamental principles and recent progress of LSPR sensing based on different schemes including 1） refractometric sensing realized by observing target-induced refractive index changes, 2） plasmon rulers based on target-induced relative displacement of coupled plasmonic structures, 3） other relevant LSPR-based sensing schemes including chiral plasmonics,nanoparticle growth, and optomechanics. The ultimate LOD and the future trends of these LSPR-based sensing are also discussed.

Manipulated localized surface plasmon resonances in silver nanoshells coated with a spherical anisotropic layer

The influences of the anisotropy of the outer spherically anisotropic （SA） layer on the far-field spectra and near- field enhancements of the silver nanoshells are investigated by using a modified Mie scattering theory. It is found that with the increase of the anisotropic value of the SA layer, the dipole resonance wavelength of the silver nanoshell first increases and then decreases, while the local field factor （LFF） reduces. With the decrease of SA layer thickness, the dipole wavelength of the silver nanoshell shows a distinct blue-shift. When the SA layer becomes very thin, the modulations of the anisotropy of the SA layer on the plasmon resonance energy and the near-field enhancement are weakened. We further find that the smaller anisotropic value of the SA layer is helpful for obtaining the larger near-field enhancement in the Ag nanoshell. The geometric average of the dielectric components of the SA layer has a stronger effect on the plasmon resonance energy of the silver nanoshell than on the near-field enhancement.

Effects of thickness & shape on localized surface plasmon resonance of sexfoil nanoparticles

Localized surface plasmon （LSPR） resonance and sensing properties of a novel nanostructure （sexfoil nanoparticle） are studied using the finite-difference time-domain method. For the sandwich sexfoil nanoparticle, the calculated extinction spectrum shows that with the thickness of the dielectric layer increasing, long-wavelength peaks blueshift, while short- wavelength peaks redshift. Strong near-field coupling of the upper and lower metal layers leads to electric and magnetic field resonances; as the thickness increases, the electric field resonance gradually increases, while the magnetic field resonance decreases. The obtained refractive index sensitivity and figure of merit are 332 nm/RIU and 3.91 RIU^-1, respectively. In order to obtain better sensing ability, we further research the LSPR character of monolayer Ag sexfoil nanoparticle. After a series of trials to optimize the thickness and shape, the refractive index sensitivity approximates 668 nm/RIU, and the greatest figure of merit value comes to 14.8 RIU^-1.

Toward transparent projection display:recent progress in frequency‐selective scattering of RGB light based on metallic nanoparticle’s localized surface plasmon resonance

A transparent display simultaneously enables visualization of the images displayed on it as well as the view behind it,and therefore can be applied to,for instance,augmented reality(AR),virtual reality(VR),and head up display(HUD).Many solutions have been proposed for this purpose.Recently,the idea of frequency-selective scattering of red,green and blue light while transmitting visible light of other colours to achieve transparent projection display has been proposed,by taking advantage of metallic nanoparticle’s localized surface plasmon resonance(LSPR).In this article,a review of the recent progress of frequency-selective scattering of red,green and blue light that are based on metallic nanoparticle’s LSPR is presented.A discussion of method for choosing appropriate metal(s)is first given,followed by the definition of a figure of merit used to quantify the performance of a designed nanoparticle structure.Selective scattering of various nanostructures,including sphere-shaped nanoparticles,ellipsoidal nanoparticles,super-sphere core-shell nanoparticles,metallic nanocubes,and metallic nanoparticles combined with gain materials,are discussed in detail.Each nanostructure has its own advantages and disadvantages,but the combination of the metallic nanoparticle with gain materials is a more promising way since it has the potential to generate ultra-sharp scattering peaks(i.e.,high frequency-selectivity).

Protein Based Localized Surface Plasmon Resonance Gas Sensing

We apply the localized surface plasrnon resonance （LSPR） of gold nanoparticles （GNPs） covalently coupled with cytochrorne c （cyt c） to create a nanobiosensor for detecting hydrogen sulfide （H2S） in the range of 15 lOOppb. Monolayer formation of GNPs on glass surface functionalized with 3-aminopropyltrirnethoxysilane （APTMS） is performed for fabricating a chip-based format of the optical transducer. By chemical introduction of short-chain thiol derivatives on cyt c protein shell via its lysine residues, a very fast self-assembled rnonolayer （SAM） of cyt c is formed on the GNPs. Significant shifts in the LSPR peak （△λLSPR） are observed by reacting H2S with cyt c. Results show a linear relationship between △λLSPR and H2S concentration. Furthermore, shifts in the LSPR peak are reversible and the peak positions return to their pre-exposure values once the H2S is removed. The experirnental results strongly indicate that the protein based LSPR chip can be successfully used as a simple, fast, sensitive and quantitative sensor for H2S detection.

Review of Fiber-Optic Localized Surface Plasmon Resonance Sensors:Geometries,Fabrication Technologies,and Bio-Applications

Localized surface plasmon resonance(LSPR)biosensors,which enable nanoscale confinement and manipulation of light,offer the enhanced sensitivity and electromagnetic energy localization.The integration of LSPR with the fiber-optic technology has led to the development of compact and versatile sensors for miniaturization and remote sensing.This comprehensive review explores various sensor configurations,fiber types,and geometric shapes,highlighting their benefits in terms of sensitivity,integration,and performance improvement.Fabrication techniques such as focused non-chemical bonding strategies and self-assembly of nanoparticles are discussed,providing control over nanostructure morphology and enhancing sensor performance.Bio-applications of fiber-optic LSPR(FOLSPR)sensors are detailed,specifically in biomolecular interactions and analysis of proteins,pathogens and cells,nucleic acids(DNA and RNA),and other small molecules(organic compounds and heavy metal ions).Surface modification and detection schemes are emphasized for their potential for label-free and real-time biosensing.The challenges and prospects of FOLSPR sensors are addressed,including the developments in sensitivity,fabrication techniques,and measurement reliability.Integration with emerging technologies such as nanomaterials is highlighted as a promising direction for future research.Overall,this review provides insights into the advancements and potential applications of FOLSPR sensors,paving the way for sensitive and versatile optical biosensing platforms in various fields.

Photocurrent improvement of an ultra-thin silicon solar cell using the localized surface plasmonic effect of clustering nanoparticles

The cluster-shaped plasmonic nanostructures are used to manage the incident light inside an ultra-thin silicon solar cell.Here we simulate spherical,conical,pyramidal,and cylindrical nanoparticles in a form of a cluster at the rear side of a thin silicon cell,using the finite difference time domain(FDTD)method.By calculating the optical absorption and hence the photocurrent,it is shown that the clustering of nanoparticles significantly improves them.The photocurrent enhancement is the result of the plasmonic effects of clustering the nanoparticles.For comparison,first a cell with a single nanoparticle at the rear side is evaluated.Then four smaller nanoparticles are put around it to make a cluster.The photocurrents of 20.478 mA/cm2,23.186 mA/cm2,21.427 mA/cm2,and 21.243 mA/cm2 are obtained for the cells using clustering conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.These values are 13.987 mA/cm2,16.901 mA/cm2,16.507 mA/cm2,17.926 mA/cm2 for the cell with one conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.Therefore,clustering can significantly improve the photocurrents.Finally,the distribution of the electric field and the generation rate for the proposed structures are calculated.

Deep-ultraviolet surface plasmon resonance of Al and Al_(core)/Al_2O_(3shell) nanosphere dimers for surface-enhanced spectroscopy

The localized surface plasmon resonance properties of Al and Alcore/Al2O3shell nanosphere dimers with Al and Al core nanosphere radii of 20 nm and Al2O3 shell of 2 nm in the deep-ultraviolet region have been studied using the finite difference time domain method. The extinction spectra and the electric field distribution profiles of the two dimers for various gap distances between two individual nanospheres are compared with those of the corresponding monomers to reveal the extent of plasmon coupling. It is found that with the interparticle distance decreasing, a strong plasmon coupling between two Al or Alcore/Al2O3shell nanospheres is observed accompanied by a significant red shift in the extinction spectra at the parallel polarization direction of the incident light related to the dimer axis, while for the case of the perpendicular polarization direction, a weak plasmon coupling arises characterized by a slight blue shift in the extinction spectra. The electric field distribution profiles show that benefiting from the dielectric Al2O3 shell, the gap distance of Alcore/Al2O3shell nanosphere dimers can be tailored to 〈 1 nm scale and results in a very high electric field enhancement. The estimated surface-enhanced Raman scattering enhancement factors suggests that the Alcore/Al2O3shell nanosphere dimers with the gap of 〈 1 nm gave rise to an enhancement as high as 8.1 × 10^7 for interparticle gap = 0.5 nm. Our studies reveal that the Alcore/Al2O3shell nanosphere dimers may be promising substrates for surface-enhanced spectroscopy in the deep-ultraviolet region.

Selective enhancement of green upconversion luminescence of Er-Yb:NaYF4 by surface plasmon resonance of W18O49 nanoflowers and applications in temperature sensing

The W（18）O（49） nanoflowers with a diameter of 500 nm are prepared by a facile hydrothermal method. The Er-Yb：NaYF4 nanoparticles are adsorbed on the petals（the position of the strongest local electric field on W（18）O（49） nanoflowers）.With a 976 nm laser diode（LD） as an excitation source, the selectively green upconversion luminescence（UCL） is observed to be enhanced by two orders of magnitude in Er-Yb： NaYF4/W（18）O（49） nanoflowers heterostructures. It suggests that the near infrared（NIR）-excited localized surface plasmon resonance（LSPR） of W（18）O（49） is primarily responsible for the enhanced UCL, which could be partly reabsorbed by the W（18）O（49）, thus leading to the selective enhancement of green UCL for the Er-Yb： NaYF4. The fluorescence intensity ratio is investigated as a function of temperature based on the intense green UCL, which indicates that Er-Yb： NaYF4/W（18）O（49） nanoflower heterostructures have good potential for developing into temperature sensors.

Modulating carrier density of the(Ag)_(x)(MoO_(3))_(y)system to enhance SERS:Localized surface plasmon resonance contribution

Semiconductors typically exhibit long-wavelength LSPR absorption in the infrared region due to lower carrier density.Tuning the carrier density of semiconductors and blue-shifting their LSPR absorption to the visible and near-infrared region has always been a great challenge.Herein,we discussed how the controllable carrier of(Ag)x(MoO_(3))y composite influences the LSPR based on SERS test and UVeViseNIR absorption spectra.We were surprised to find that the LSPR absorption wavelength can be easily tuned from 950 to 735 nm by changing the sputtering power of MoO_(3)of the(Ag)x(MoO_(3))_(y)composite.This shows that LSPR can be precisely adjusted by increasing the semiconductor content and even the carrier density.In addition,the carrier density was measured by Hall effect to investigate the SERS intensity change caused by electromagnetic(EM)enhancement,and obtain the relationship between the two.The findings of this work provide an idea for tunable LSPR and the research of EM contributions to SERS.

High-order plasmon resonances in an Ag/Al_2O_3 core/shell nanorice

Using numerical simulation, we investigate the high-order plasmon resonances in individual nanostructures of an Ag nanorice core surrounded by an Al2O3 shell. The peak positions of localized surface plasmon resonances （LSPRs） are red-shifted exponentially with the increase of the dielectric shell thickness. This is due to the exponential decay of electromagnetic field intensity in the direction perpendicular to the interface. This exponential red-shift depends on the wavelength of the resonance peak instead of the resonance order. In addition, we find that the LSPRs in an Ag nanorice of 60-nm width can be perfectly described by a single linear function. These features make nanorice an ideal platform for sensing applications.

Fano-like resonance characteristics of asymmetric Fe_2O_3@Au core/shell nanorice dimer

A geometrical configuration of Fe2O3/Au core-shell nanorice dimer is proposed and its multipolar plasmon Fano- like resonance characteristics are theoretically investigated by generalizing the plasmon hybridization model of individual nanorice to the bright and dark modes of the nanorice dimer. Under the irradiation of polarization light, the extinction spectra of the nanorice dimer are numerically simulated by using the finite element method （FEM）. Our studies show that the Fano-like resonance of the nanorice dimer results in an asymmetric line shape of the Fano dip in the extinction spectrum which can be controlled by varying the structure parameters of the nanorice dimer. Meanwhile, there is a giant field enhancement at the gap between the two nanorices on account of the plasmonic coupling in the nanorice dimer. The aforementioned two characteristics of the nanorice dimer are useful for plasmon-induced transparency and localized surface plasmon resonance sensors.

Tunable multiple plasmon resonances and local field enhancement of nanocrescent/nanoring structure

According to the plasmon hybridization theory, the plasmon resonance characteristics of the gold nanocrescent/nanoring(NCNR) structure are systematically investigated by the finite element method. It is found that the extinction spectra of NCNR structure exhibit multiple plasmon resonance peaks, which could be attributed to the result of the plasmon couplings between the multipolar plasmon modes of nanocrescent and the dipolar, quadrupolar, hexapolar, octupolar,decapolar plasmon modes of nanoring. By changing the geometric parameters, the intense and separate multiple plasmon resonance peaks are obtained and can be tuned in a wide wavelength range. It is further found that the plasmon coupling induces giant multipole electric field enhancements around the tips of the nanocrescent. The tunable and intense multiple plasmon resonances of NCNR structure may provide effective applications in multiplex biological sensing.

Design and fabrication of structural color by local surface plasmonic meta-molecules

In this paper, we propose a new form of nanostructures with Al film deposited on a patterned dielectric material for generating structural color, which is induced by local surface plasmonic resonant(LSPR) absorption in sub-wavelengthindented hole/ring arrays. Unlike other reported results obtained by using focus ion beam(FIB) to create metallic nanostructures, the nano-sized hole/ring arrays in Al film in this work are replicated by high resolution electron beam lithography(EBL) combined with self-aligned metallization. Clear structural color is observed and systematically studied by numerical simulations as well as optical characterizations. The central color is strongly related to the geometric size, which provides us with good opportunities to dye the colorless Al surface by controlling the hole/ring dimensions(both diameter and radius), and to open up broad applications in display, jewelry decoration, green production of packing papers, security code,and counterfeits prevention.

Self-Supporting Nanoporous Copper Film with High Porosity and Broadband Light Absorption for Efficient Solar Steam Generation

Solar steam generation(SSG)is a potential technology for freshwater production,which is expected to address the global water shortage problem.Some noble metals with good photothermal conversion performance have received wide concerns in SSG,while high cost limits their practical applications for water purification.Herein,a self-supporting nanoporous copper(NP-Cu)film was fabricated by one-step dealloying of a specially designed Al_(98)Cu_(2)precursor with a dilute solid solution structure.In-situ and ex-situ characterizations were performed to reveal the phase and microstructure evolutions during dealloying.The NP-Cu film shows a unique three-dimensional bicontinuous ligament-channel structure with high porosity(94.8%),multi scale-channels and nanoscale ligaments(24.2±4.4nm),leading to its strong broadband absorption over the 200–2500 nm wavelength More importantly,the NP-Cu film exhibits excellent SSG performance with high evaporation rate,superior efficiency and good stability.The strong desalination ability of NP-Cu also manifests its potential applications in seawater desalination.The related mechanism has been rationalized based upon the nanoporous network,localized surface plasmon resonance effect and hydrophilicity.

Improving the UV-light stability of silicon heterojunction solar cells through plasmon-enhanced luminescence downshifting of YVO_(4):Eu^(3+),Bi^(3+)nanophosphors decorated with Ag nanoparticles

The ultraviolet(UV)light stability of silicon heterojunction(SHJ)solar cells should be addressed before large-scale production and applications.Introducing downshifting(DS)nanophosphors on top of solar cells that can convert UV light to visible light may reduce UV-induced degradation(UVID)without sacrificing the power conversion efficiency(PCE).Herein,a novel composite DS nanomaterial composed of YVO_(4):Eu^(3+),Bi^(3+)nanoparticles(NPs)and AgNPs was synthesized and introduced onto the incident light side of industrial SHJ solar cells to achieve UV shielding.The YVO_(4):Eu^(3+),Bi^(3+)NPs and Ag NPs were synthesized via a sol-gel method and a wet chemical reduction method,respectively.Then,a composite structure of the YVO_(4):Eu^(3+),Bi^(3+)NPs decorated with Ag NPs was synthesized by an ultrasonic method.The emission intensities of the YVO_(4):Eu^(3+),Bi^(3+)nanophosphors were significantly enhanced upon decoration with an appropriate amount of~20 nm Ag NPs due to the localized surface plasmon resonance(LSPR)effect.Upon the introduction of LSPR-enhanced downshifting,the SHJ solar cells exhibited an~0.54%relative decrease in PCE degradation under UV irradiation with a cumulative dose of 45 k W h compared to their counterparts,suggesting excellent potential for application in UV-light stability enhancement of solar cells or modules.

Nanostructured materials with localized surface plasmon resonance for photocatalysis

Localized surface plasmon resonance (LSPR) enhanced photocatalysis has fascinated much interest and considerable efforts have been devoted toward the development of plasmonic photocatalysts. In the past decades, noble metal nanoparticles (Au and Ag) with LSPR feature have found wide applications in solar energy conversion. Numerous metal-based photocatalysts have been proposed including metal/semiconductor heterostructures and plasmonic bimetallic or multimetallic nanostructures. However, high cost and scarce reserve of noble metals largely limit their further practical use, which drives the focus gradually shift to low-cost and abundant nonmetallic nanostructures. Recently, various heavily doped semiconductors (such as WO_(3-x), MoO_(3-x), Cu_(2-x)S, TiN) have emerged as potential alternatives to costly noble metals for efficient photocatalysis due to their strong LSPR property in visible-near infrared region. This review starts with a brief introduction to LSPR property and LSPR-enhanced photocatalysis, the following highlights recent advances of plasmonic photocatalysts from noble metal to semiconductor-based plasmonic nanostructures. Their synthesis methods and promising applicability in plasmon-driven photocatalytic reactions such as water splitting, CO_(2) reduction and pollution decomposition are also summarized in details. This review is expected to give guidelines for exploring more efficient plasmonic systems and provide a perspective on development of plasmonic photocatalysis.

Size dependent optical properties of LaB_6 nanoparticles enhanced by localized surface plasmon resonance

Lanthanum hexaboride nanopartieles, with high emission electrons in cathode materials and peculiar blocking near infrared wavelengths, were applied for many aspects. Based on the quasi-static approximation of Mie theory, the size dependent optical prop- erties of LaB6 nanoparticles were researched, such as refractive index n（ω）, extinction coefficient k（ω）, reflectivity R（ω）, absorption coefficient a（ω）, and electron energy loss L（ω）. Due to the localized surface plasmon resonance （LSPR）, the extinction coefficient k（ω） and absorption coefficient a（ω） depended on the size, and the LSPR peaks red-shifted with sizes increased, which was different from that of bulk materials. In addition, electron energy-loss spectrum L（co） showed electrons oscillation reinforced, since electrons absorbed the photon energy and generated resonance. Further, reftectivity R（ω） and refractive index n（ω） indicated that the light in near infrared region could not be propagated on the surface of LaB6 materials, which exhibited metallic behaviors. So the resonance peak of LaB6 nanoparticle was located in near-infrared region, making use of this property for solar control glazing and heat-shielding application.

Biomolecule-templated photochemical synthesis of silver nanoparticles： Multiple readouts of localized surface plasmon resonance for pattern recognition

Silver nanoparticles （AgNPs） with distinct localized surface plasmon resonance （LSPR） absorption spectra can be synthesized using different proteins as templates upon irradiation by light. We utilized the multiple readouts of LSPR signals of AgNPs to construct sensor arrays for pattern recognition of proteins. Room temperature, aqueous solutions, and lack of harsh reducing reagents make the whole process inherently ＂green＂. Meanwhile, the strategy efficiently simplified the process of array-receptor preparation and data acquisition, leading to lower time consumption, sample use, and cost. Furthermore, the system can differentiate proteins using flexible and alterable sensor elements by choosing different combinations of LSPR signals at different wavelengths. The principle of the sensor design can also be further extended to differentiate other biomolecules. The study provides a new method to construct feasible, economical, and general nanoparficle-based sensing arrays for pattern recognition.

Localized surface plasmon resonance improves transdermal photodynamic therapy of hypertrophic scars

Photodynamic therapy(PDT)is an emerging therapeutic strategy for hypertrophic scars(HS),which is heavily dependent on reactive oxygen species(ROS)generation.However,the unsatisfactory delivery and excitation of 5-aminolevulinic acid(ALA,a commercial photosensitizer in dermatology)result in an insufficient ROS generation,and thus limit the clinical application of PDT treating HS(HS-PDT).Consequently,sophisticated transdermal co-delivery nanoethosomes(named A/A-ES)with ALA and Au nanotriangles(AuNTs)in cores are prepared via an in-situ seed-mediated growth method,and then applied to improve HS-PDT through localized surface plasmon resonance(LSPR)-enhanced ROS generation.A/A-ES display a satisfactory performance in co-delivery in HS tissue with sufficient protoporphyrin IX production and LSPR effect in cytoplasm,which is beneficial for ALA excitation as well as ROS generation.In vitrolvivo studies reveal that A/A-ES significantly improve HS-PDT in promoting to fibroblast apoptosis and collagen remodeling through LSPR-enhanced ROS generation.Therefore,this study provides a feasible strategy that integrates transdermal delivery and LSPR to enable the beneficial effects of HS-PDT through boosting the delivery and excitation of ALA.