Theoretical analysis of gold-deposited optical fiber sensor and characterization of the gold film.

Gold-deposited optical fiber sensors with film thicknesses from 30 to 60 nm were prepared, and the responses to a wide range of a refractivity (1.33-1.54 refractive index (RI) units) were investigated both experimentally and theoretically. The response curve of the sensor has two minima in the refractivity range from 1.33 to 1.44 and at 1.462 RI units. The former minimum is due to surface plasmon resonance (SPR) in the thin gold film, and shifts to a lower refractivity as the film becomes thicker. The response curves of the sensors with film thicknesses of 45 and 60 nm agreed well with those calculated from SPR theoretical equations. Morphology observations of the surfaces of deposited gold films on glass by atomic force microscopy (AFM) and a variation in resistance of the films with various thicknesses show the structure of the gold films. We concluded that the thin deposited gold films have many defects, and that the core of the gold-deposited optical fiber leaks light through the defects to the sample solution with the same refractivity (1.462 RI units) as that of the core.     

基于金银合金薄膜的高灵敏度宽光谱表面等离子体共振成像传感器

报道了一种基于金银合金薄膜的宽光谱表面等离子体共振成像(SPRI)传感器,该传感器能够对吸附在薄膜局部或整个表面上的生化分子进行原位定量检测,而且与常规的金膜SPRI传感器相比,检测成本更低,检测灵敏度更高。利用质量比1:1的金银合金溅射靶在玻璃基板上淀积了厚约50 nm的均匀的金银合金薄膜。利用实验室自制的Krestchmann结构多功能平台在不同入射角下测试了金银合金薄膜被纯水覆盖后的SPR光谱和SPR彩色图像。基于色相算法计算获得了每个SPR彩像的二维色相分布及其平均色相,从而使得宽光谱SPRI传感器能够利用平均色相作为灵敏度参数进行定量检测。实验确定了平均色相对溶液折射率(RI)变化和分子吸附最为敏感的光谱区间是595–610 nm之间。在这个窄光谱范围内,平均色相与共振波长呈线性关系,其斜率为?hue/?λR=7.52 nm~(-1),这意味着基于色相的RI灵敏度是基于共振波长的RI灵敏度的7.52倍,这一结论已被实验证明。将SPRI传感器的起始共振波长设定在色相敏感光谱区间内之后,实验测得基于色相的RI灵敏度为S=29879 RIU~(-1),比在相同条件下测得的金膜SPRI的灵敏度高8倍。利用时间分辨宽光谱SPRI方法实时监测了牛血清白蛋白(BSA)分子在金银合金薄膜表面的非特异性吸附,从实验测得的平均色相随时间的变化曲线可知BSA吸附达到平衡所需时间约15 min。研究结果表明,基于金银合金薄膜的SPRI传感器具有动态定量检测蛋白质分子吸附过程的功能。     

Preparation and characterization of thin films of SiO(x) on gold substrates for surface plasmon resonance studies

We report here on the fabrication and characterization of stable thin films of amorphous silica (SiO(x)) deposited on glass slides coated with a 5 nm adhesion layer of titanium and 50 nm of gold, using the plasma-enhanced chemical vapor deposition (PECVD) technique. The resulting surfaces were characterized using atomic force microscopy (AFM), ellipsometry, contact angle measurements, and surface plasmon resonance (SPR). AFM analysis indicates that homogeneous films of silica with low roughness were formed on the gold surface. The deposited silica films showed excellent stability in different solvents and in piranha solution. There was no significant variation in the thickness or in the SPR signal after these harsh treatments. The Au/SiO(x) interfaces were investigated for their potential applications as new surface plasmon resonance sensor chips. Silica films with thicknesses up to 40 nm allowed visualization of the surface plasmon effect, while thicker films resulted in the loss of the SPR characteristics. SPR allowed further the determination of the silica thickness and was compared to ellipsometric results. Chemical treatment of the SiO(x) film with piranha solution led to the generation of silanol surface groups that have been coupled with a trichlorosilane.     

Single-crystal sapphire-fiber optic sensors based on surface plasmon resonance spectroscopy for in situ monitoring

Single-crystal sapphire-fiber optic sensors based on surface plasmon resonance (SPR) for refractive index (RI) measurements of aqueous and hydrothermal water solutions are described. Accurate measurement of RIs is essential to efficient operation and control of broad range of engineering processes. Some of these processes are carried out with harsh environments, such as high-temperature, high pressure, and chemical corrosion. These extreme physical conditions are proving a limiting factor in application of the conventional silica-based optical sensors. Single-crystal sapphire is an ideal material for sensor applications, where reliable performance is required in the extreme environment conditions. With regard to the liquid species detection, most applications of SPR sensors are designed to function near the refractive index of water (1.3330 RI). The RI changes of aqueous solution can be easily monitored by silica-fiber (RI, 1.4601 at 550 nm) based SPR sensor. However, the sapphire waveguide has a prohibitively high RI (1.7708 at 546 nm) for unmodified monitoring of the RI changes of aqueous solutions. For that purpose, a practical SPR probe geometry has been applied to the ability to tune the SPR coupling wavelength/angle pair with sapphire-fiber based SPR probe.     

纳米多孔金薄膜的表面等离子体共振传感特性

对淀积在玻璃衬底上厚度约60 nm的金银合金溅射薄膜进行硝酸腐蚀脱银处理, 得到纳米多孔金薄膜. 利用自建的波长检测型表面等离子体共振(SPR)传感装置研究了腐蚀时间对纳米多孔金薄膜SPR特性的影响, 结果发现纳米多孔金薄膜与水溶液接触后在400-900 nm光谱范围内不具有SPR效应, 而当薄膜置于空气中时会产生明显的传播等离子体共振吸收峰, 其共振波长随腐蚀时间增加逐渐红移. 纳米多孔金薄膜在空气气氛中的SPR效应使其能够用于原位监测气相分子在孔内的吸附, 还可对在液相中吸附的生化分子进行离位测试. 本文对L-谷胱甘肽、L-半胱氨酸、2-氨基乙硫醇三种含巯基的生化小分子在纳米多孔金薄膜内的吸附进行了离位分析, 结果表明与传统的致密金薄膜SPR芯片比较, 纳米多孔金薄膜对这些分子显示出更高的灵敏度和更低的检测下限, 这归功于多孔金的大比表面积使其能够吸附大量的生化小分子. 实验还对乙醇蒸气在纳米多孔金薄膜内的吸附进行了原位监测, 发现吸附平衡所用时间较长, 约为160 min.     

Fiber-optic surface plasmon resonance for vapor phase analyses

Fiber-optic sensors based on surface plasmon resonance (SPR) for direct refractive index (RI) measurements of samples with the RI between 1.00 and 1.30 are described. Most applications of SPR sensors are designed to function near the refractive index of water (1.3330 RI). The RI changes of aqueous solution (RI, ca. 1.34) can easily be monitored by silica-fiber (RI, 1.4601 at 550 nm) based SPR sensor. With regard to gas species detection, the fiber-optic SPR sensor must be modified for sensitivity to changes in refractive index near 1.0008 (i.e., RI of air). However, the silica waveguide has a prohibitively high RI for unmodified monitoring of the RI changes of gas. The silica-fiber based SPR probe design presented here is based upon the modification of the probe geometry to the ability to tune the SPR coupling wavelength/angle pair. In this study, the tapered silica-based fiber SPR sensors are shown to directly determine the RI changes of gas species and the density change of dry air.     

Surface-plasmon-resonance-enhanced cavity ring-down detection

The cavity ring-down technique is used to probe the absolute optical response of the localized surface plasmon resonance (SPR) of a gold nanoparticle distribution to adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) from the gas phase. Extended Mie theory for a coated sphere with a particle-size-dependent dielectric function is used to elucidate size-dispersion effects, the size-dependence of the SPR sensitivity to adsorption, and the kinetics of adsorption. An approximate Gaussian distribution of nanospheres with a mean diameter of 4.5 nm and a standard deviation of 1.1 nm, as determined by atomic force microscopy, is provided by the intrinsic granularity of an ultrathin, gold film, having a nominal thickness of approximately 0.18 nm. The cavity ring-down measurements employ a linear resonator with an intracavity flow cell, which is formed by a pair of ultrasmooth, fused-silica optical flats at Brewster's angle, where the Au film is present on a single flat. The total system intrinsic loss is dominated by the film extinction, while the angled flats alone contribute only approximately 5 x 10(-5)/flat to the total loss. Based on a relative ring-down time precision of 0.1% for ensembles averages of 25 laser shots from a pulsed optical parametric oscillator, the minimum detectable concentrations of PCE and TCE obtained by probing the SPR response are found to be 2 and 7 x 10(-8) mol/L, respectively, based on a 30 s integration time.     

A surface plasmon resonance probe without optical fibers as a portable sensing device

A surface plasmon resonance (SPR) sensor integrating a small sensor probe, a laser emission diode, a photo detector, and a polarizer was developed as a portable sensing device. The sensor probe was made with a glass cylinder, 50 mm long and 1.5 mm in diameter, that was connected directly to a beam splitter without optical fibers. The SPR spectrum obtained with this probe system showed a 10% reflectivity minimum at 690 nm. Shifts of the SPR spectrum induced by refractive index (RI) changes in the sample were measured by detecting the reflection light intensity at 670 nm. When the sensitivity was compared using a BIAcore™ SPR instrument, the lowest sensor response of 1 mV observed with the SPR probe system coincided with 1.4 × 10⁻⁶ of the RI changes. The RI resolution of the SPR probe was estimated with experimentally evaluated noise on the signal, and, consequently, it was concluded that the RI resolution was 1.2 × 10⁻⁵. Moreover, immunoreaction was demonstrated with adsorbed bovine serum albumin (BSA) and anti-BSA antibody as an analyte. As a result, 50 ng mL⁻¹ of the lower detection limit was estimated.     

用表面等离子体子共振传感器测定人心肌肌钙蛋白I的研究

采用自组装表面等离子体子共振(SPR)传感装置,固定入射角,以波长为变量,以CCD为检测系统,用对金和抗体均有较强吸附作用的葡萄球菌A蛋白作为基底膜,观测了人心肌肌钙蛋白I的抗体和抗原之间免疫反应的动力学过程,并进行了人心肌肌钙蛋白I的定量测定.结果表明,人心肌肌钙蛋白I的浓度在5.0～50μg/L范围内与传感器的响应值呈线性关系.     

Surface plasmon resonance and electrochemistry for detection of small molecules using catalyzed deposition of metal ions on gold substrate

Small molecules are difficult to detect by conventional surface plasmon resonance (SPR) spectroscopy due to the fact that the changes in the refractive index resulted from the binding process of small biomolecules are quite small. Here, we report a simple and effective method to detect small biomolecule using SPR spectroscopy and electrochemistry by catalyzed deposition of metal ions on SPR gold film. As an example, the ascorbic acid-mediated deposition of Ag on gold film was monitored by in situ SPR spectrum. The deposition of Ag atom on gold film resulted in an obvious decrease of depth in SPR angular scan curves of reflectance intensity and minimum reflectivity angle. The depth change of the SPR reflectance intensity and minimum reflectivity angle curves mainly relied on the amount of Ag atom deposited on gold film that can be controlled by the concentration of ascorbic acid. By monitoring the deposition of Ag atom on gold film, ascorbic acid was detected in the concentration range of 2 × 10⁻⁵ M to 1 ×  10⁻³ M. After each of detections, the SPR sensor surface was completely regenerated by a potential step that stripped off the Ag atom. Furthermore, the regeneration process of the sensor surface provides the feasibility for detecting the concentration of ascorbic acid by electrochemical method.     

High-resolution surface plasmon resonance sensors based on a dove prism

Wavelength interrogation surface plasmon resonance (SPR) spectroscopy using a dove prism combines a simple and inexpensive optical design with high-resolution refractive index monitoring and biosensing. A BK7 dove prism inverts an optical image with a total internal reflection angle of 72.8°, an angle active in SPR. Hence, a unique system can accomplish SPR biosensing using wavelength interrogation and also perform SPR imaging. This optical configuration advantageously uses a single axis optical path between each optical component, simplifying the optical design of SPR instruments without compromise of the analytical performance. Fluidics were also incorporated to the instrument design for efficient sample delivery. The SPR instrument is characterized in terms of refractive index (RI) sensitivity, RI resolution, reproducibility, and application for monitoring low concentration biological events. Data analysis methodologies are compared for improved resolution of the measured response. Raw data analyzed using a minimum hunting procedure results in RI resolution in the 10⁻⁶ range, while pre-treating data with singular value decomposition improves the resolution by one order of magnitude. Depending on the spectrophotometer employed, the RI range accessible can be easily tuned; examples with a 550-850 nm and a 550-1100 nm spectrophotometers are shown and results respectively in RI ranges of 1.32-1.39 RIU and 1.32-1.42 RIU. Monitoring of μM concentration of β-lactamase is performed using the wavelength interrogation configuration of the biosensor. Finally, a SPR image of a surface with a water droplet (volume = 500 nL) was obtained using the dove prism SPR with a band pass filter and a CCD camera. SPR using a dove prism configuration combines advantages of portable SPR instruments, SPR imagers and research-grade SPR instruments in a unique platform.     

基于色相算法的表面等离子体共振成像传感器对苯并芘的敏感特性

报道了一种用于原位探测水中苯并芘的彩色表面等离子体共振成像(SPRI)传感器,该传感器既能提供直观的图像信息,又能借助色相算法定量分析待测物质的浓度及其吸附/脱附过程。首先利用自制的波长-图像同步检测型SPR传感器测试了裸金薄膜芯片在不同入射角下的共振波长和共振图像,然后利用色相算法建立了SPR共振波长与图像色相的依赖关系,基于该依赖关系获得了SPR传感器最佳色相灵敏度对应的起始共振波长约为650 nm;另一方面,制备了聚四氟乙烯涂覆的SPR传感芯片,基于聚四氟乙烯膜对水中苯并芘的可逆富集作用实现了苯并芘的原位快速探测。实验取得以下4个结果:(1)在20-100 nmol?L-1浓度范围内彩色SPR图像的平均色相值随着苯并芘浓度的升高线性减小;(2)对100 nmol?L-1的苯并芘的响应和恢复时间分别约为7和5 s;(3)由于聚四氟乙烯膜的厚度大于SPR消逝场穿透深度,检测结果不受溶液折射率影响;(4)在聚四氟乙烯敏感膜厚度较小且不均匀的情况下,传感器容许获取敏感膜的不同厚度区域对苯并芘的色相灵敏度。实验结果有力地证明了这种彩色SPR图像传感器在生化物质检测中具有良好的应用前景。     

Surface plasmon sensor based on the enhanced light transmission through arrays of nanoholes in gold films

Arrays of nanoholes in a gold film were used to monitor the binding of organic and biological molecules to the metallic surface. This technique is particularly sensitive to surface binding events because it is based upon the resonant surface plasmon enhanced transmission through the array of nanoholes. The sensitivity was found to be 400 nm per refractive index unit, which is comparable to other grating-based surface plasmon resonance (SPR) devices. The array of nanoholes is well suited for dense integration in a sensor chip. Furthermore, the optical geometry is collinear, which simplifies the alignment with respect to the traditional Kretschmann (reflection) arrangement for SPR sensing.     

波长检测型表面等离子体共振传感器对硫堇电聚合成膜的原位分析

利用波长检测型表面等离子体共振(SPR)传感器对硫堇在金膜表面的电化学聚合成膜过程进行了跟踪分析.结果表明,在固定入射角下SPR共振波长随循环伏安扫描周数的增加而线性红移,表明聚硫堇膜的生长是匀速的;扫描100周后共振波长红移总量为96.6 nm.对该实验结果进行理论拟合,得出聚硫堇膜的厚度约为71 nm.聚硫堇膜在酸性缓冲液中的电化学活性很高,其电化学反应过程受扩散控制,在一个完整的循环伏安扫描过程中SPR共振波长的变化完全可逆,说明聚硫堇膜的氧化反应和还原反应是一对可逆过程.与还原态聚硫堇膜相比,氧化态聚硫堇膜对应的SPR共振波长较大,说明氧化态聚硫堇膜折射率高.     

Application of a surface plasmon resonance sensor to analyses of amine compounds with the use of a polymer film and an acid-base reaction.

A surface plasmon resonance (SPR) sensor was applied to analyses of some amine compounds (n-butylamine, isobutylamine, aniline, and N,N-dimethylaniline) by using a polymer film and an acid-base reaction in it. Poly(acrylamide) (PAA) was adopted as the polymer film and was immobilized on an Au film to prepare a sensor chip. Pivalic acid was entered into the PAA film as an acid. The PAA film with a thickness of 50 nm gave the highest sensitivity to the SPR sensor. Although water was better concerning the sensitivity for the SPR sensor as the solvent, ethanol was adopted because it dissolves well all of the amine compounds used. The Au film coated with the PAA film gave higher sensitivity for analyses of n-butylamine and isobutylamine, and lower sensitivity for analyses of aniline and N,N-dimethylaniline than an Au film without the PAA film. The PAA film containing pivalic acid gave 4-5 orders of magnitude higher sensitivity to the SPR sensor for analyses of all the amine compounds due to the reaction between pivalic acid and these amine compounds.     

Electrochemical Surface Plasmon Resonance Spectroscopy at Bilayered Silver/Gold Films

Bilayered silver/gold films (gold deposited on top of the silver film) were used as substrates for electrochemical surface plasmon resonance spectroscopy (EC-SPR). EC-SPR responses of electrochemical deposition/stripping of copper and redox-induced conformation changes of cytochrome c immobilized onto self-assembled monolayers preformed at these substrates were measured. Influence of the Ag layer thickness and the double-layer capacitance on the EC-SPR behavior was investigated. The results demonstrated that the bilayered Ag/Au metal films produce a sharper SPR dip profile than pure Au films and retain the high chemical stability of Au films. Contrary to the result by the Fresnel calculation that predicts a greater fraction of Ag in the bilayered film should result in a greater signal-to-noise ratio, the EC-SPR sensitivity is dependent on both the Ag/Au thickness ratio and the chemical modification of the surface. Factors affecting the overall SPR sensitivity at the bilayered films, such as the film morphology, potential-induced excess surface charges, and the adsorbate layer were investigated. Forming a compact adsorbate layer at the bilayered film diminishes the effect of potential-induce excess surface charges on the SPR signal and improves the overall EC-SPR sensitivity. For the case of redox-induced conformation changes of cytochrome c, the SPR signal obtained at the bilayered silver/gold film is 2.7 times as high as that at a pure gold film.     

Au nanoparticles prepared by physical method on Si and sapphire substrates for biosensor applications

Gold nanoparticles heavily functionalized with oligonucleotides have been used in a variety of DNA detection methods. The optical properties of three-dimensional aggregates of Au nanoparticles in solution or deposited onto suitable surfaces have been analyzed to detect hybridization processes of specific DNA sequences as possible alternatives to fluorescent labeling methods. This paper reports on the preparation of gold nanoparticles directly deposited onto the surface of silicon (Si) and sapphire (Al2O3) substrates by a physical methodology, consisting in the thermal evaporation of a thin Au film and its successive annealing. The method guarantees the preparation of monodispersed single-crystal Au nanoparticles with a strong surface plasmon resonance (SPR) peak centered at about 540 nm. We show that the changes of SPR excitation before and after DNA functionalization and subsequent hybridization of Au nanoparticles immobilized onto Si and Al2O3 substrates can be exploited to fabricate specific biosensors devices in solid phase.     

Surface plasmon resonance and electrochemistry characterization of layer-by-layer self-assembled DNA and Zr4+ thin films, and their interaction with cytochrome c

Through electrostatic layer-by-layer (LbL) assembly, negatively charged calf thymus double stranded DNA (CTds-DNA), and positively charged Zr⁴⁺ ions were alternately deposited on gold substrate modified with chemisorbed cysteamine. Thus-prepared three-dimensional DNA networks were characterized by surface plasmon resonance (SPR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). SPR spectroscopy indicates that the effective thickness of DNA monolayer in the (DNA/Zr⁴⁺)₁ bilayer was 1.5 ± 0.1 nm, which corresponds to the surface coverage of 79% of its full packed monolayer. At the same time, a linear increase of film thickness with increasing number of layers was also confirmed by SPR characterizations. The data of XPS and IR-RAS show that Zr⁴⁺ ions interact with both the phosphate groups and nitrogenous bases of DNA and load into the framework of DNA. Furthermore, the interactions between this composite film and heme protein cytochrome c (Cyt c) were investigated by SPR spectroscopy and electrochemistry. Compared with the adsorption of Cyt c on DNA monolayer, this composite multilayer film can obviously enhance the amount of immobilized Cyt c confirmed by SPR reflectivity-incident angle (R-θ) curves. Cyclic voltammetry (CV) indicates the Cyt c adsorbed on the composite film is electroactive, and the enhancement of peak current in CV indirectly verifies the increase of the amount of immobilized Cyt c.     

表面接枝法制备睾酮素分子印迹表面等离子体共振传感器

构建了一种选择性检测睾酮素的分子印迹表面等离子体共振( Surface plasmon resonance, SPR)传感器。采用紫外光引发表面接枝技术,在固定引发转移终止剂的SPR芯片表面制备了以睾酮素为模板分子,甲基丙烯酸为功能单体,乙二醇二甲基丙烯酸酯为交联剂的分子印迹膜( Molecularly imprinted film, MIF)。利用SPR在目标共振角处现场监测使MIF聚合成膜过程更易控。偏振调制-红外反射吸收光谱表征证明MIF接枝成功。原子力显微镜结果显示,MIF 表面均匀散布着纳米尺寸的孔穴。利用 SPR 对2.5×10-16~2.5×10-6 mol/L睾酮素进行吸附检测,检出限低至2.5×10-16 mol/L,对低浓度和高浓度的睾酮素吸附分段进行线性拟合,得到拟合线性方程分别为y=19.69+1.21x(R2=0.9913)和y=11.5+0.45x(R2=0.9895);睾酮素类似物雌二醇、雌三醇和黄体酮的吸附实验结果显示,此印迹膜对模板分子有很好的选择性;5次重复洗脱吸附后,MIF仍保持较好的传感性能,说明此传感器具有较高的稳定性和重复利用性;在人工尿液样品中测得的睾酮素回收率为85.2%~92.8%,说明此传感器可以用于实际样品测定。     

Gold films deposited over regular arrays of polystyrene nanospheres as highly effective SERS substrates from visible to NIR

Gold nanostructured films of various thicknesses (15, 30, and 60 nm) are deposited over regular arrays of polystyrene nanospheres in an attempt to evaluate their potential as SERS-active substrates. Atomic force microscopy is used to topographically characterize the substrates as well as to ensure the thickness of the deposited gold films. The optical response of the prepared substrates recommends their use in SERS experiments with multiple laser lines from visible and NIR spectral domains. The assessment of the substrates' SERS activity is performed by using the 532, 633, and 830 nm excitation lines and different average enhancement factor (EF) values are obtained depending on the film thickness and employed laser line. The 60 nm gold nanostructured film generates the greatest local electromagnetic field confinement under NIR excitation and consequently gives rise to maximum SERS enhancement. The large tunability of surface plasmon excitation combined with the advantage of relatively high exhibited average EF values obtained under NIR excitation recommends these substrates as outstanding candidates for upcoming investigations of biological relevant molecules.