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.

Tunable localized surface plasmon resonances in MoO_(3-x)-TiO_(2) nanocomposites with enhanced catalytic activity for CO_(2) photoreduction under visible light

The photocatalytic reduction of CO2 with H2O to fuels and chemicals using solar energy is one of the most attractive but highly difficult routes.Thus far,only a very limited number of photocatalysts has been reported to be capable of catalyzing the photocatalytic reduction of CO2 under visible light.The utilization of the localized surface plasmon resonance(LSPR)phenomenon is an attractive strategy for developing visible-light photocatalysts.Herein,we have succeeded in synthesizing plasmonic MoO3?x-TiO2 nanocomposites with tunable LSPR by a simple solvothermal method.The well-structured nanocomposite containing two-dimensional(2D)molybdenum oxide(MoO3?x)nanosheets and one-dimensional(1D)titanium oxide nanotubes(TiO2-NT)showed LSPR absorption band in the visible-light region,and the incorporation of TiO2-NT significantly enhanced the LSPR absorption band.The MoO3?x-TiO2-NT nanocomposite is promising for application in the photocatalytic reduction of CO2 with H2O under visible light irradiation.

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.

Photothermal properties of Cu@polycarbonate composites with strong localized surface plasmon resonances

Copper is relatively low cost and highly abundant compared with the well-studied noble metals such as gold and silver.However,the poor plasmonic and high susceptibility towards oxidation limit the study of its optical properties and applications as well.Herein,copper nanoparticles@polycarbonate(Cu@PC)composites were prepared by using a facile one-step solvothermal method.The Cu@PC composites have strong localized surface plasmon resonances(LSPR)due to that the PC shell can induce the particles to form many-particles system with different particle numbers,which not only lead to overlap and hybridize of the LSPR modes,but also shift the LSPR away from the interband transitions,and the PC layer also prevents the oxidation of Cu nanoparticles.The photothermal conversion efficiency of Cu@PC composites reaches 41.1%under 808 nm continuous wave(CW)laser irradiation which is higher than previously reported Cu nanomaterials that have been reported.Meanwhile,the composites also have high photothermal stability.Moreover,interfacial evaporator is prepared by assembling the Cu@PC composites on scouring sponge as light absorption layer which has>92.8%absorption in entire solar spectrum range.Its seawater evaporation rate is 3.177 kg·m^(-2)·h^(-1)with a E_(evaporator)/E_(water)of 5.2.The high evaporation rate interfacial evaporator with low cost,simple,and scalable approach shows great application value in the field of photothermal evaporation.

Fiber cladding dual channel surface plasmon resonance sensor based on S-type fiber

Fiber cladding surface plasmon resonance(SPR)sensors have few structures,and a clad SPR sensor based on S-type fiber is proposed in this paper.This new type of fiber cladding SPR sensor was formed by electrofusing an S-shaped structure on the fiber to couple the light in the fiber core to the cladding.In this paper,the effects of fiber parameters on the performance of the sensor were studied by simulation and experiment.Based on the conclusion that the smaller the core diameter is,the closer the working band of the SPR resonance is to long wavelengths,and that the geometric characteristics mean that a multimode fiber can receive the fiber cladding light from a small core diameter few-mode fiber,a dual channel SPR sensor with a double S-type fiber cascade was proposed.In the refractive index detection range of 1.333–1.385refractive index units(RIU),the resonant working band of channel I is 627.66 nm–759.78 nm,with an average sensitivity of 2540.77 nm/RIU,and the resonant working band of channel II is 518.24 nm–658.2 nm,with an average sensitivity of2691.54 nm/RIU.The processing method for the S-type fiber cladding SPR sensor is simple,effectively solving the problem of this type of SPR sensor structure and the difficult realization of a dual channel.The sensor is expected to be used in the fields of medical treatment and biological analysis.

Dual-channel fiber-optic surface plasmon resonance sensor with cascaded coaxial dual-waveguide D-type structure and microsphere structure

To address the restriction of fiber-optic surface plasmon resonance(SPR) sensors in the field of multi-sample detection, a novel dual-channel fiber-optic SPR sensor based on the cascade of coaxial dual-waveguide D-type structure and microsphere structure is proposed in this paper. The fiber sidepolishing technique converts the coaxial dual-waveguide fiber into a D-type one, and the evanescent wave in the ring core leaks, generating a D-type sensing region;the fiber optic fused ball push technology converts the coaxial dual waveguides into microspheres, and the stimulated cladding mode evanescent wave leaks, producing the microsphere sensing region. By injecting light into the coaxial dual-waveguide middle core alone, the sensor can realize single-stage sensing in the microsphere sensing area;it can also realize dual-channel sensing in the D-type sensing area and microsphere sensing area by injecting light into the ring core. The refractive index measurement ranges for the two channels are 1.333–1.365 and 1.375–1.405, respectively, with detection sensitivities of 981.56 nm/RIU and 4138 nm/RIU. The sensor combines wavelength division multiplexing and space division multiplexing technologies, presenting a novel research concept for multi-channel fiber SPR sensors.

Numerical simulation of a truncated cladding negative curvature fiber sensor based on the surface plasmon resonance effect

A refractive index(RI)sensor based on the surface plasmon resonance effect is proposed using a truncated cladding negative curvature fiber(TC-NCF).The influences of the TC-NCF structure parameters on the sensing performances are investigated and compared with the traditional NCF.The simulation results show that the proposed TC-NCF RI sensor has an ultra-wide detection range from 1.16 to 1.43.The maximum wavelength sensitivity reaches 12400 nm/RIU,and the corresponding R^(2)of the polynomial fitting equation is 0.9999.The maximum and minimum resolutions are 2.56×10^(-5)and 8.06×10^(-6),respectively.In addition,the maximum amplitude sensitivity can reach-379.1 RIU^(-1)when the RI is chosen as 1.43.The proposed TC-NCF RI sensor could be useful in biochemical medicine,environmental monitoring,and food safety.

Photocatalytic seawater splitting by 2D heterostructure of ZnIn_(2)S_(4)/WO_(3) decorated with plasmonic Au for hydrogen evolution under visible light

Photocatalytic H_(2) evolution from seawater splitting presents a promising approach to tackle the fossil energy crisis and mitigate carbon emission due to the abundant source of seawater and sunlight on the earth.However,the development of efficient photocatalysts for seawater splitting remains a formidable challenge.Herein,a 2D/2D ZnIn_(2)S_(4)/WO_(3)(ZIS/WO_(3))heterojunction nanostructure is fabricated to efficiently separate the photoinduced carriers by steering electron transfer from the conduction band minimum of WO_(3) to the valence band maximum of ZIS via constructing internal electric field.Subsequently,plasmonic Au nanoparticles(NPs)as a novel photosensitizer and a reduction cocatalyst are anchored on ZIS/WO_(3) surface to further enhance the optical absorption of ZIS/WO_(3) heterojunction and accelerate the catalytic conversion.The obtained Au/ZIS/WO_(3) photocatalyst exhibits an outstanding H_(2) evolution rate of 2610.6 or 3566.3μmol g^(-1)h~(-1)from seawater splitting under visible or full-spectrum light irradiation,respectively.These rates represent an impressive increase of approximately 7.3-and 6,6-fold compared to those of ZIS under the illumination of the same light source.The unique 2D/2D structure,internal electric field,and plasmonic metal modification together boost the photocatalytic H_(2) evolution rate of Au/ZIS/WO_(3),making it even comparable to H_(2) evolution from pure water splitting.The present work sheds light on the development of efficient photocatalysts for seawater splitting.

Rapid and fine tailoring longitudinal surface plasmon resonances of gold nanorods by end-selective oxidation

Facile achievement of gold nanorods(AuNRs) with controllable longitudinal surface plasmon resonance(LSPR) is of great importance for their applications in various fields.The LSPR of AuNRs is sensitive to their aspect ratio,which is still hard to be precisely tuned by direct synthesis.In this work,we report a simple approach for end-selective etching of AuNRs by a rapid oxidation process with Au(Ⅲ) in cetyltrimethylammonium bromide(CTAB) solution at a mild temperature.The LSPR wavelength and the length of AuNRs blue shifted linearly as a function of the amount of Au(Ⅲ),while the diameter of Au NRs remained nearly constant.The oxidative rate is tempe rature dependent,and the oxidative process for a desired LSPR can be accomplished within 15 min at 60℃.Further investigations indicated that Br determine the occurrence of the oxidation between AuNRs and Au(Ⅲ),and a small amount of surfactant chain(CTA+) is crucial for stabilizing AuNRs.This method presents a quick but robust strategy for acquiring AuNRs with an arbitrary intermediate LSPR wavelength using the same starting AuNRs,and can be a powerful tool for subsequent applications.

Surface plasmon resonance-induced visible-light photocatalytic performance of silver/silver molybdate composites

Novel silver/silver molybdate（Ag/Ag2MoO4） composites with surface plasmon resonance（SPR）-enhanced photocatalytic performance were successfully fabricated via a facile one-pot hydrothermal route with the presence of sodium dodecyl sulfate（SDS） in this study.The as prepared silver/silver molybdate（Ag/Ag2MoO4） composites were systematically characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM） and ultraviolet-visible diffuse reflectance absorption spectroscopy（DRS） in order to investigate their crystal structure,morphology and optical property as well.The photocatalytic activities of the composites were subsequently evaluated by their ability to degrade rhodamine B（RhB） under visible-light irradiation.Varies of controlled experiments were then carefully operated to gain a deep insight into the assembling of Ag/Ag2MoO4composites.It was found that preparation conditions such as pH,reaction time,and the amount of surfactant played important roles in the formation of composites with octahedral microstructures.And the composite obtained at 160 ℃ using 0.5 g of sodium dodecyl sulfate exhibited the highest photocatalytic performance under visible-light irradiation.Capture experiments were also conducted to clarify the function of different active species generated on the surface of Ag/Ag2MoO4during the photocatalytic process,in which both holes and ·OH radicals were found to play crucial role in photocatalytic removal of RhB under visible light irradiation.A possible photocatalytic mechanism of Ag/Ag2MoO4 was finally proposed on the basis of all the results to explain the higher photocatalytic activity of the octahedral Ag/Ag2MoO4 composites.It was inferred that the photoinduced ＂hot＂ electrons can quickly transfer from the Ag NPs to the conduction band of Ag2MoO4 and react with oxygen and H2O to generate a large quality of active radicals such as ·OH and ·O2^- because of the SPR effects.Besides,this SPR effects of Ag nanoparticles deposited on the surface of Ag2MoO4 can not only dramatically amplify its light absorption,especially in the visible region,but also promote the separation of photoexcited electron-hole pairs and effectively decrease electron-hole recombination.

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.

Using a Surface Plasmon Resonance Biosensor for Rapid Detection of Salmonella Typhimurium in Chicken Carcass

Chicken is one of the most popular meat products in the world. Salmonella Typhimurium is a common foodbome pathogens associated with the processing of poultry. An optical Surface Plasmon Resonance （SPR） biosensor was sensitive to the presence of Salmonella Typhimurium in chicken carcass. The Spreeta biosensor kits were used to detect Salmonella Typhimurium on chicken carcass successfully. A taste sensor like electronic tongue or biosensors was used to basically ＂taste＂ the object and differentiated one object from the other with different taste sensor signatures. The surface plasmon resonance biosensor has potential for use in rapid, real-time detection and identification of bacteria, and to study the interaction of organisms with dif- ferent antisera or other molecular species. The selectivity of the SPR biosensor was assayed using a series of antibody con- centrations and dilution series of the organism. The SPR biosensor showed promising to detect the existence of Salmonella Typhimurium at 1 x 106 CFU/ml. Initial results show that the SPR biosensor has the potential for its application in pathogenic bacteria monitoring. However, more tests need to be done to confirm the detection limitation.

Analysis of 17β-Estadiol from Sewage in Coastal Marine Environment by Surface Plasmon Resonance Technique

In this study, surface plasmon resonance （SPR） for monitoring 17β-eatradiol （E2） was developed. The small molecule E2 was immobilized on a CM5 sensor chip for an indirect competitive immunoassay to detect E2. The SPR response bahed on the antigen-antibody reaction was measured by injecting the sample solution into the flow system. The limitation of detection was 0.445 μg/L. The developed SPE-SPR system was applied to analyze the seawater samples. Recovery of E2 was 91.6%-93. 1%. Relative standard deviations（RSD） for the E2 assay were between 10.9%-15.1% （n = 3）. The range of determination of E2 samples from the sewage in the coastal marine environ-ment was between ND（lower than detection limit） and ca. 11.78 ng/L.

Surface plasmon resonance analysis to evaluate the importance of heparin sulfate groups＇ binding with human aFGF and bFGF

Human acidic and basic fibroblast growth factors (aFGF and bFGF) are classic and well characterized members of the heparin binding growth factor family. Heparin is generally thought to play an extremely important role in regulating aFGF and bFGF bioactivities through its strong binding with them. In order to unravel the mechanism of the interactions between heparin and FGFs, and evaluate the importance of heparin sulfate groups' binding with FGFs, surface plasmon resonance analyses were performed using IAsys Cuvettes System. Heparin and its regioselectively desulfated derivatives were immobilized on the cuvettes. aFGF and bFGF solutions with different concentrations were pipetted into the cuvettes and the progress of the interaction was monitored in real\|time by Windows based software, yielding kinetic and equilibrium constants for these interactions. In addition, in order to reduce the delicate difference among the cuvettes, inhibition analyses of mixture of FGFs and immobilized native heparin by modified heparins were also done. The data from these two methods were similar, indicating that all sulfate groups at 2 O, 6 O and N in heparin were required for the binding to aFGF; and that their contribution to the binding was in the order 2 O, N and 6 O sulfate group. In contrast, definite contribution of the 6 O sulfate group to the binding with bFGF was most apparent, while the other two sulfate groups appeared to be necessary in the order 2 O and N sulfate group. These methods established here can be used for analysing the effect of sulfate groups in heparin on the binding with other human FGF members or other heparin binding proteins.

A strategy of screening and binding analysis of bioactive components from traditional Chinese medicine based on surface plasmon resonance biosensor

Elucidating the active components of traditional Chinese medicine(TCM)is essential for understanding the mechanisms of TCM and promote its rational use as well as TCM-derived drug development.Recent studies have shown that surface plasmon resonance(SPR)technology is promising in this field.In the present study,we propose an SPR-based integrated strategy to screen and analyze the major active components of TCM.We used Radix Paeoniae Alba(RPA)as an example to identify the compounds that can account for its anti-inflammatory mechanism via tumor necrosis factor receptor type 1(TNF-R1).First,RPA extraction was analyzed using an SPR-based screening system,and the potential active ingredients were collected,enriched,and identified as paeoniflorin and paeonol.Next,the affinity constants of paeoniflorin and paeonol were determined as 4.9 and 11.8 mM,respectively.Then,SPR-based competition assays and molecular docking were performed to show that the two compounds could compete with tumor necrosis factor-a(TNF-a)while binding to the subdomain 1 site of TNF-R1.Finally,in biological assays,the two compounds suppressed cytotoxicity and apoptosis induced by TNF-a in the L929 cell line.These findings prove that SPR technology is a useful tool for determining the active ingredients of TCM at the molecular level and can be used in various aspects of drug development.The SPR-based integrated strategy is reliable and feasible in TCM studies and will shed light on the elucidation of the pharmacological mechanism of TCM and facilitate its modernization.

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.

Surface plasmon-enhanced dual-band infrared absorber for VO_x-based microbolometer application

We propose a periodic structure as an extra absorption layer（i.e., absorber） based on surface plasmon resonance effects, enhancing dual-band absorption in both middle wavelength infrared（MWIR） and long wavelength infrared（LWIR）regions. Periodic gold disks are selectively patterned onto the top layer of suspended SiN/VO_2/SiN sandwich-structure.We employ the finite element method to model this structure in COMSOL Multiphysics including a proposed method of modulating the absorption peak. Simulation results show that the absorber has two absorption peaks at wavelengths λ =4.8 μm and λ = 9 μm with the absorption magnitudes more than 0.98 and 0.94 in MWIR and LWIR regions, respectively. In addition, the absorber achieves broad spectrum absorption in LWIR region, in the meanwhile, tunable dual-band absorption peaks can be achieved by variable heights of cavity as well as diameters and periodicity of disk. Thus, this designed absorber can be a good candidate for enhancing the performance of dual band uncooled infrared detector, furthermore, the manufacturing process of cavity can be easily simplified so that the reliability of such devices can be improved.

Detection of Penicillin via Surface Plasmon Resonance Biosensor

A method of using Au colloid to capture the decomposed product of penicillin, penicillamine, on a surface plasmon resonance（SPR） biosensor for the quantitative determination of penicillin was developed. Based on the de-composition of penicillin to generate penicillamine and penilloaldehyde, a high sensitive biosensor for detecting penicillin was also developed. In our experiment, it was penicillamine rather than penicillin that has been measured. This is because penicillamine contains a functional group that makes it self-assembling on Au colloid to increase the molecular weight so as to improve the surface plasmon resonance signal. On a UV-Vis spectrophotometer, a high concentration of penicilliamine-Au complex was determined, indicating that penicillamine was already well combined with Au colloid. The method, using the combination of Au colloid with penicillamine, proved to detect penicillin.

Low-cost,high performance surface plasmon resonance-compatible films characterized by the surface plasmon resonance technique

A new analytical method based on the surface plasmon resonance （SPR） technique is presented, with which SPR curves for both wavelength and angular modulations can be obtained simultaneously via only a single scan of the incident angle. Using this method, the SPR responses of TiO2-coated Cu films are characterized in the wavelength range from 600 nm to 900 nm. For the first time, we determine the effective optical constants and the thicknesses of TiO2-coated Cu films using the SPR curves of wavelength modulation. The sensitivities of prism-based SPR refractive index sensors using TiO2-coated Cu films are investigated theoretically for both wavelength and angular modulations, the results show that in the case of sensitivity with wavelength modulation, TiO2-coated Cu films are not as good as the Au film, however, they are more suitable than the Au film for SPR refractive index sensors with angular modulation because a higher sensitivity can be achieved.

Advances in Phase Detection Technique of Surface Plasmon Resonance Biosensor

Surface plasmon resonance (SPR) sensing is an optical method based on evanescent wave.SPR biosensor can detect interaction of label-free biomolecules in real-time.With further development,it can become a research instrument in proteomics.SPR biosensor can be divided intensity measurement and phase measurement,and the latter possesses higher sensitivity than the former one.This paper attempts to summarize the SPR phase detection theory,discuss the major developments,compare the merits and deficiencies of various methods,and look forward to future prospects.