Diffraction-based tracking of surface plasmon resonance enhanced transmission through a gold-coated grating

Surface plasmon resonance enhanced transmission through metal-coated nanostructures represents a highly sensitive yet simple method for quantitative measurement of surface processes and is particularly useful in the development of thin film and adsorption sensors. Diffraction-induced surface plasmon excitation can produce enhanced transmission at select regions of the visible spectrum, and wavelength shifts associated with these transmission peaks can be used to track adsorption processes and film formation. In this report, we describe a simple optical microscope-based method for monitoring the first-order diffracted peaks associated with enhanced transmission through a gold-coated diffraction grating. A Bertrand lens is used to focus the grating's diffraction image onto a CCD camera, and the spatial position of the diffracted peaks can be readily transformed into a spectral signature of the transmitted light without the use of a spectrometer. The surface plasmon peaks appear as a region of enhanced transmission when the sample is illuminated with p-polarized light, and the peak position reflects the local dielectric properties of the metal interface, including the presence of thin films. The ability to track the position of the plasmon peak and, thus, measure film thickness is demonstrated using the diffracted peaks for samples possessing thin films of silicon oxide. The experimental results are then compared with calculations of optical diffraction through a model, film-coated grating using the rigorously coupled wave analysis simulation method.     

Surface plasmon enhanced diffraction for label-free biosensing

Surface plasmon enhanced evanescent field at a (noble) metal/dielectric interface can be employed to enhance the diffraction efficiency of surface grating structure composed of biomolecules. Based on a Kretschmann configuration, we realized a diffraction biosensor to monitor the dynamic interaction of biological molecules in a label-free way. It was demonstrated by the binding of an anti-biotin antibody to the biotin-functionalized region of a periodically patterned surface, which generated significant optical contrast to diffract the surface plasmon field. With the aid of the synchronic surface plasmon resonance signal, a quadratic dependence of diffraction signal on the amount of bound antibody was found, which coincides with the theoretical expectation. Time-dependent measurements were conducted to estimate the density of biotin thiols on the functional region.     

Nanoparticle-enhanced diffraction gratings for ultrasensitive surface plasmon biosensing

Ultrasensitive surface bioaffinity sensors are created by the adsorption of gold nanoparticles onto gold diffraction gratings. An enhanced diffraction obtained in a surface plasmon resonance geometry is observed due to the optical coupling of the planar surface plasmons in the grating to the localized surface plasmons in the gold nanoparticles. As a first example, these nanoparticle grating biosensors are employed to detect unmodified DNA at a concentration of 10 fM.     

自组装Ag纳米颗粒的表面等离子体共振及其对CdSe量子点的荧光增强效应

采用柠檬酸三钠还原硝酸银的方法制备了Ag纳米粒子溶胶,利用静电自组装技术制得单分散Ag纳米颗粒薄膜.研究了该Ag纳米颗粒薄膜的光学特性,通过改变反应物浓度和后期的热退火温度,有效调控了Ag纳米颗粒尺度和其在外场作用下产生的表面等离子体共振(SPR)特性.将所制备的Ag纳米颗粒薄膜与CdSe量子点耦合,利用SPR对荧光的...     

Surface plasmon resonance near-infrared spectroscopy

Near-infrared (NIR) spectroscopy is ill-suited to microanalysis because of its low absorptivity. We have developed a highly sensitive detection method for NIR spectroscopy based on absorption-sensitive surface plasmon resonance (SPR). The newly named SPR-NIR spectroscopy, which may open the way for NIR spectroscopy in microanalysis and surface science, is realized by an attachment of the Kretschmann configuration equipped with a mechanism for fine angular adjustment of incident light. The angular sweep of incident light enables us to make a tuning of a SPR peak for an absorption band of sample medium. From the dependences of wavelength, incident angle, and thickness of a gold film on the intensity of the SPR peak, it has been found that the absorbance can be enhanced by approximately 100 times compared with the absorbance obtained without the gold film under optimum conditions. This article reports the details of the experimental setup and the characteristics of absorption-sensitive SPR in the NIR region, together with some experimental results obtained by using it.     

White LED light coupling into light guides with diffraction gratings

Radial diffractive gratings are used to couple light of a white LED into a light guide. Theoretical coupling efficiencies are evaluated with rigorous diffraction theory in a pure conical mounting. It is shown that when the refractive index of the grating increases from 1.46 to 2.05 the incoupling efficiency increases from 42% to 63%. Also, with the increasing refractive index the incoupling efficiency is shown to become more nearly uniform over the visible spectrum. Experimental results for the incoupled efficiencies and the color coordinates of the incoupled spectra are introduced for refractive indices n=1.46 and n=1.56.     

Surface plasmon resonance radiation from germanium

It is well known that when certain reflection gratings which are coated with different metal or dielectric-metal layers are illuminated by p-polarized light surface plasmon resonance radiation appears in the diffraction spectra in the form of polarization anomalies (Wood's anomalies). Surface plasmons can be excited in this way if the real part ε₁ of the dielectric constant of the metal layer has a value less than -1. This technique has been extended to semiconductors in the present work by coating a grating with Ge and investigating the diffracted light in the 4–6 eV region, where ε₁ for Ge is less than -1. Experimental results are presented which show that certain peaks in the observed spectra may be due to the expected resonance radiation.     

Surface plasmon resonance assay for chloramphenicol

We report a rapid and ultrasensitive surface plasmon resonance (SPR) assay of chloramphenicol (CAP) by using large gold nanoparticles (40 nm) for signal enhancement on a mixed self-assembled monolayer (mSAM) sensor surface. After immobilization of the target antibiotic CAP through its ovalbumin (OVA) conjugates with an oligoethylene glycol (OEG) linker on the mSAM surface, sequential binding of anti-CAP antibody and IgG/nanogold (40 nm) onto the sensor surface afforded a rapid (<10 min) and ultrasensitive assay format for CAP. A limit of detection (LOD) for CAP as low as 0.74 fg/mL was achieved in aqueous buffer, and the linear working range was between 1-1000 fg/mL. While the LOD of CAP in a honey spiked-specimen is 17.5 fg/mL, the detection range is 80-5000 fg/mL. The mSAM sensor surface was also shown to be highly stable with over 400 binding/regeneration cycles performed.     

Surface plasmon resonance imaging for biosensing

Surface plasmon resonance imaging (SPRI) is a useful tool for the study of surface biomolecular interactions allowing for label-free detection and elegant instrumentation. SPRI imaging system is described in this review with an emphasis on recent applications with examples of different biological interactions and high throughput analysis. Signal amplification in SPRI using nanoparticle and waveguide-based optical coupling is introduced. Finally the detection sensitivity of the SPRI system is examined in terms of other competitive methods.     

Surface plasmon resonance of two-segmented Au-Cu nanorods

Two-segmented gold-copper nanorods were electrodeposited inside the pores of polycarbonate track-etched membranes from two separate solutions. The PCT membranes were dissolved in dichloromethane (CH(2)Cl(2)) and the solvent was replaced by methanol solution. Optical absorption spectra of two-segmented Au-Cu nanorods dispersed in methanol showed two peaks which were related to the transverse mode of copper and the longitudinal mode of gold. By increasing the length of the gold segment, when the total length of both metals was fixed at 1?μm, the copper and gold peaks shifted to the blue and red wavelengths, respectively. We observed that the wavelengths of the extinction peaks are not in good agreement with the expected value obtained from calculations using the nominal dielectric constant of the medium. Therefore, we suggest the effective medium dielectric constant (ε(m)(eff)) for Cu and Au segments by comparing the experimental data and calculation results. The values of ε(m)(eff) decrease by increasing the gold length.     

Effective grating theory for resonance domain surface-relief diffraction gratings

An effective grating model, which generalizes effective-medium theory to the case of resonance domain surface-relief gratings, is presented. In addition to the zero order, it takes into account the first diffraction order, which obeys the Bragg condition. Modeling the surface-relief grating as an effective grating with two diffraction orders provides closed-form analytical relationships between efficiency and grating parameters. The aspect ratio, the grating period, and the required incidence angle that would lead to high diffraction efficiencies are predicted for TE and TM polarization and verified by rigorous numerical calculations.     

Surface plasmon resonance/mass spectrometry interface

A strategy for combining surface plasmon resonance (SPR) biomolecular interaction analysis, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is reported. Both techniques are highly complementary but need separate optimization to improve their individual specificity and sensitivity. Sensor surfaces that are optimal for kinetic analysis are not well suited for MALDI-MS and vice versa. In addition, the transfer of analyte from SPR to MS is crucial and often accompanied by sample loss. To address both of these points, a bifunctional SPR fluid cell was constructed where optimized surfaces can be used for binding studies and MS simultaneously with regard to the special need of each technique. The setup guarantees that the SPR and the loading experiment for MS are performed at identical conditions. A removable pin carries the affinity-surface-bound analyte to the mass spectrometer so that handling is minimized, avoiding analyte elution. Functionalized transfer pins can also be used independently of SPR for microaffinity capture-MS.     

Direct evidence for surface plasmon-mediated enhanced light transmission through metallic nanohole arrays

This paper provides direct evidence for the role of surface plasmons in the enhanced optical transmission of light through metallic nanoscale hole arrays. Near-field optical images directly confirmed the presence of surface plasmons on gold nanohole arrays with interhole spacings larger than the surface plasmon wavelength. A simple interference model provides an intuitive explanation of the two types of fringe wavelengths observed in the near-field optical images. Far-field spectroscopy revealed a surface plasmon band that contributed a factor > 8 to the transmission enhancement. Furthermore, silicon nanohole arrays did not exhibit any features in the near-field, which demonstrates that metallic materials are necessary for enhanced light transmission through nanohole arrays.     

采用虚拟仪器技术的表面等离子体共振传感器

针对表面等离子体共振(SPR)传感器对精度和数据处理能力的要求,利用虚拟仪器技术,自行设计了一套基于角度扫描的Kretschmann结构SPR测试系统。为了提高折射率的测量分辨力,系统采用了高精度步进电机控制的旋转平台。软件中使用巴特沃思低通滤波器消除出射光干涉噪声引起的高频干扰。同时,提出了一种双棱镜自适应结构对出射光路进行实时调整,实现角度扫描过程中光探测器固定不动。实验测得空气和蒸馏水样品的折射率分别为1.00293和1.33432,结果与理论值基本吻合,且具有良好的重复性和达到10-5RIU(RefractiveIndexUnit)的分辨力。     

Surface plasmon resonance in nanocrystalline gold-copper alloy films

Nanocrystalline Au(x)Cu(1-x) films were synthesized by depositing Cu/Au/Cu multilayer in nanocrystalline thin film form with requisite thickness of individual layers onto fused silica substrates by high pressure sputtering technique. The absorbance spectra showed only one surface plasmon peak for all the compositions with the exception that the peak position did not indicate gradual shift as gold concentration was increased. Peak position for the two compositions corresponding to the two superlattice structures, AuCu3 and AuCu, deviated significantly from linear variation. The experimental results have been discussed in light of the existing Mie theory and the Core-shell model.     

Surface plasmon resonance detection for capillary electrophoresis separations

A miniaturized surface plasmon resonance sensor has been used as an on-line detector for capillary electrophoresis separations. The capillary was modified slightly to shield the sensor electronics from the high voltages applied during the separation. A three-component mixture of high refractive index materials was separated and detected at the millimolar level by an untreated gold-sensing surface. A simple protein immobilization procedure was used to functionalize the surface for selective protein detection. A hybrid buffer system was developed, in which both the deposition of immobilized protein layers and the electrophoretic delivery of protein analytes were optimized. The detection system has a reproducibility of 15%, a dynamic range of 3 orders of magnitude, and a detection limit for IgG of 2 fmol.     

Grating-coupled transmission-type surface plasmon resonance sensors based on dielectric and metallic gratings

We investigated grating-coupled transmission-type surface plasmon resonance (SPR) for sensing applications. In the transmission-type SPR structure, propagating surface plasmons are outcoupled to radiation modes by dielectric and metallic gratings on a metal film. The results calculated in air and water suggest that the proposed structures present extremely linear sensing characteristics. In terms of a figure of merit, a metallic grating-based structure performs 5.4 and 3.7 times better than that of a dielectric grating in air and water, respectively.     

Surface plasmon polaritons on narrow-ridged short-pitch metal gratings in the conical mount

Recent investigations into high-aspect-ratio short-pitch metal grating structures have shown that it is possible to excite surface plasmon polaritons (SPPs) even in the zero-order region of the spectrum. The predominant reason this is possible is that extremely large bandgaps occur in the SPP dispersion curves, which are caused by the large depths, and heights, of the structures. The form of the resultant dispersion curves has also been found to be highly dependent on the shape of the grating profile. We present an extension to a previously published paper that described the nature of the SPPs excited on narrow-ridged short-pitch metal gratings in the classical mount by considering the case in which the radiation is incident at nonzero azimuthal angles (the conical mount). In particular, we consider the case of 90 degrees and 45 degrees azimuthal angles and discuss the coupling to the SPP modes and the way in which polarization conversion is evident on such structures.