Surface functionalization for self-referencing surface plasmon resonance (SPR) biosensors by multi-step self-assembly

Recently, a novel SPR sensor with on-chip referencing has been realized. In this sensor, one-half of the gold sensing surface is covered with a high refractive index overlayer of tantalum pentoxide (Ta₂O₅). When polychromatic beam illuminates the sensing surface, surface plasmon resonance in the areas with and without the overlayer occur at different wavelengths. Therefore, the reflected light exhibits two dips associated with SPRs in those two areas. When functionalized properly, one of the areas can be used as a specific sensing channel for detection of specific bio-interactions and the other can act as a reference channel for compensation for background refractive index fluctuations. In this paper we present a new functionalization approach for these mixed architecture chips. The gold side of the chip is functionalized with a mixed self-assembled monolayer of polyethylene oxide (PEO) and biotin terminated (BAT) thiols whereas the Ta₂O₅ side is coated with PEO terminated silanes. The PEO terminated thiols and silanes serve as a protein resistant background, while the biotin terminated thiols are used to bind streptavidin, which in turn immobilizes biotinylated antibodies. Hence, the gold side of the chip is used for the binding and detection of target analytes and the Ta₂O₅ side functions as a reference channel that monitors bulk refractive index changes and temperature drift. We have studied human chorionic gonadotropin (hCG) as a model system, currently detecting down to 5 ng/ml. In addition, we demonstrate the power of the on-chip reference channel for compensating for refractive index changes and eliminating false alarms.     

Effect of fiber core dopant concentration on the performance of surface plasmon resonance-based fiber optic sensor

In the present work we study the influence of varying concentrations of different fiber core dopants on the sensor performance in terms of sensitivity, signal to noise ratio (SNR) and operating range of surface plasmon resonance (SPR)-based fiber optic sensors. We consider the use of different concentrations of GeO₂, B₂O₃ and mixture of both in the core of plastic cladded silica (PCS) optical fiber. The study is further extended to consider the performance of quenched fiber. We show how sensitivity and signal to noise ratio of surface-plasmon-based fiber optic sensors can be optimized by wisely choosing their fiber core doping.     

Manufacture of robust surface plasmon resonance fiber optic based dip-probes

Surface plasmon resonance (SPR) spectroscopy has been conducted on both prism and fiber optic (FO) based sensors for several years. This technique measures the refractive index (RI) of a solution or layer adsorbed to a thin (50 nm) Au layer on the sensor substrate. To date a succinct set of protocols have not been published regarding the optimization of fiber-based SPR dip-probe sensors. Such sensors would allow application of SPR to a wider variety of applications. This paper focuses on consideration of the choice of fiber, isolation of the mirror from the sensing area, and orientation of the probes in the metal layer sputter deposition chamber in the manufacture of SPR dip-probes for reproducibility and robustness. Optimization of the process yields sensors with a batch to batch reproducibility as low as 0.5 nm in the location of the SPR spectral minima. Further study of RI measurements by the same probe over 2 months show these SPR dip-probes have a long shelf-life. A selection of probes was exposed to various solutions to monitor their drift. The data shows the probes’ response indicated a lowering of the RI measured over a period of 3 or 7 days depending on the probe type. Evidence of surface porosity and damage upon exposure to hydrothermal water seems to indicate these sensors are prone to chemical attack. Further research is needed to characterize this attack and allow creation of more robust sensors.     

Phase-sensitive spatially-modulated surface plasmon resonance polarimetry for detection of biomolecular interactions

We here describe a scheme of spatially modulated surface plasmon resonance (SPR) polarimetry that enables to combine ultra-high phase sensitivity with good signal-to-noise background. The proposed approach uses spatial modulation of s-polarized component by birefringent elements and the extraction of phase-polarization information by Fourier-transform methods. This scheme was tested for monitoring the interactions between an antibody and its biological partner. Our experimental data, collected by amplitude-sensitive and phase-sensitive polarimetry demonstrate that the latter scheme provides at least one order of magnitude improvement in terms of detection limit.     

Ag-TiO2薄膜表面等离子体共振光谱特性研究

以拓宽Ag膜波长调制型表面等离子体共振(SPR)传感器的工作波长区间,将TiO2膜沉积于Ag膜表面制成Ag-TiO2复合薄膜.利用数值计算方法对不同厚度Ag膜和As-TiO2复合膜波长调制型SPR光谱特性进行仿真研究.仿真结果表明:共振吸收峰显著依赖于Ag膜厚度;当保持Ag-TiO2复合膜厚度60 nm时,复合薄膜SPR波长随TiO2厚度的增大而向长波长方向移动,发现红移;与60 nm厚Ag膜共振波长相比,12 nm厚TiO2与48 rnm厚Ag组成的复合膜共振波长红移超过200 nm.     

表面等离子共振传感器的研究进展

表面等离子体是在金属和电介质交界面上所形成的电荷层,在电磁波的激励下表面等离子体发生共振现象,影响电磁波的传播。根据这一原理制作的表面等离子共振传感器可对化学和生物等量进行探测。介绍了表面等离子共振传感器的工作原理和最新研究进展。由于具有体积小、准确度高、抗电磁干扰能力强和可用于遥测等优点,SPR传感器将有广泛的应用前景。     

Enhanced surface plasmon resonance detection of DNA hybridization based on ZnO nanorod arrays

We demonstrated an enhanced surface plasmon resonance detection incorporating ZnO nanorod arrays (NRAs) built on a thin gold film. ZnO NRAs were fabricated by wet chemical growth method and used for the detection of DNA hybridization. Experimental results exhibited that ZnO NRAs provided a notable sensitivity improvement by more than 3 times, which is attributed to an increase in the surface reaction area. The measured sensitivity enhancement matched well with the numerical analyses based on the effective medium theory. Our approach is intended to show the feasibility and extend the applicability of the ZnO-based SPR biosensor to diverse biomolecular binding events.     

Weak signal detection: Condition for noise induced enhancement

For the detection of a weak known signal in additive white noise, a generalized correlation detector is considered. In the case of a large number of measurements, an asymptotic efficacy is analytically computed as a general measure of detection performance. The derivative of the efficacy with respect to the noise level is also analytically computed. Positivity of this derivative is the condition for enhancement of the detection performance by increasing the level of noise. The behavior of this derivative is analyzed in various important situations, especially showing when noise-enhanced detection is feasible and when it is not.     

Surface plasmon resonance immunosensor using Au nanoparticle for detection of TNT

In order to develop the fully integrated portable surface plasmon resonance (SPR) system for detection of explosives, the amplification strategy of SPR signal was investigated. Indirect competitive inhibition method allowed the middle-sized SPR sensor to detect trinitrotoluene (TNT) at ppt level. However, this enhanced SPR signal was not high enough to detect TNT at ppt level by a miniaturized SPR sensor. Therefore, localized surface plasmon resonance (LSPR) effect using Au nanoparticle as further signal amplification approach was used. The amplification method of indirect competitive inhibition and LSPR were combined together for fabrication of the immunosurface using Au nanoparticle. TNT detectable range of this immunosurface was from 10 ppt (10 pg/ml) to 100 ppb (100 ng/ml), which was almost comparable to that without Au nanoparticle. The observed resonance angle change due to binding monoclonal TNT antibody (M-TNT Ab) with the immunosurface modified with Au nanoparticle was amplified to four times higher than that in absence of Au nanoparticle.     

Detection of EGFR on living human gastric cancer BGC823 cells using surface plasmon resonance phase sensing

Label-free and real-time information acquisition of molecular phenotype and its function on living cells plays a significant role in disease diagnosis and drug development. In this paper, SPR phase sensing was applied to monitor the interactions between EGFR antibody, EGFR1, and membrane proteins EGFR on living human gastric cancer BGC823 cells. When 50 μg/mL EGFR1 was added onto the fixed cells chip and the living cells chip, a significant difference in the binding amount could be observed from the immunofluorescence images. Quantitative results were obtained by following SPR detection, which were 722 RU and 438 RU, respectively. On the same living cells chip, SPR detection also showed markedly different results of cellular responses when it was stimulated by EGFR1 at different concentrations, such as adhesion and/or morphology variation, revealing the EGFR1's cytotoxic effect on the BGC823 cells. The results demonstrate SPR phase sensing is capable of real-time detection of molecular interactions and cellular responses on living cells, and suggest that further studies on the mechanism and the technique may allow SPR sensing become a powerful tool not only for the basic research of cell biology, but also for medical diagnosis and drug development.     

基于表面等离子体共振传感器的尿微量白蛋白检测

检测尿微量白蛋白(MAU)可对轻微肾脏损害进行早期诊断,也可对高血压患者可能发生的心脑血管事件进行预测。实验将抗体通过巯基自组装固定于Au膜表面,研制了一种快速检测MAU的表面等离子体共振(SPR)生物传感器。结果表明:直接检测法可以检测到0.3μg/L的MAU,而纳米Au放大法检测限可以低至0.03μg/L。     

Sensitivity optimization of surface plasmon sensors for detection of intermediate layers

A sensitivity optimization method for the detection of an intermediate layer using a surface plasmon sensor is presented. The dependence of detection sensitivity on distance from the metal surface and the average refractive index over the sensing region are considered. Based on the calculated results, the sensitivity for detecting an intermediate layer in a multilayered sample is determined by varying the refractive index and thickness of each layer. It is shown that, in particular cases, controlling the refractive index and thickness increases the detection sensitivity. The proposed method is useful for designing multilayered samples.     

A multichannel surface plasmon resonance sensor using a new spectral readout system without moving optics

Surface plasmon resonance (SPR) sensors with spectral interrogation provide a high refractive index resolution, a large dynamic range and a fixed optical detection module. In this work, we propose a new multichannel spectral detection unit that uses only one spectrometer to measure the reflection spectrum from multiple sensing spots serially without any mechanical movement. This spectral detection unit is designed based on a spatial light modulator (SLM) configured as a programmable optical aperture for the spectrometer. To demonstrate this concept, a five-channel laboratory SPR prototype was built based on the proposed multichannel detection unit, and we evaluated the device's sensitivity and resolution using a refractive index test. Refractive index resolution of 1.4 × 10⁻⁶ refractive index units (RIU) can be reached using the five-channel prototype. This sensor is suitable for low-cost multichannel biosensing applications that do not contain fast kinetics.     

Searching for optimal sensitivity of single-mode D-type optical fiber sensor in the phase measurement

To improve the sensitivity of a single-mode D-type optical fiber sensor, we selected a D-type optical fiber sensor with 4 mm long and 4 μm core thickness made of a single-mode fiber, a Au-coating on the sensor with a thickness range of 15–32 nm, a light wavelength of 632.8 nm, and an incident angle of 86.5–89.5° for different refractive index (1.33–1.40) sensing. These simulations are based on the surface plasmon resonance (SPR) theory using the phase method which shows that the sensitivity is proportional to the refractive index, Au film thickness and lower incident angle on the sensing interface. The sensitivity is higher than 4000 (degree/RIU), and the resolution is better than 2.5 × 10⁻⁶(RIU) as the minimum phase variation is 0.01°. This device is used to detect the refractive index or gas or liquid concentration in real-time. The proposed sensor is small, simple, inexpensive, and provides an in vivo test.     

Sensitivity of surface plasmon resonance sensors based on metallic columnar thin films in the spectral and angular interrogations

Surface Plasmon Resonance (SPR) from metallic Columnar Thin Films (CTFs) of porosity as high as 0.5 was experimentally and theoretically investigated. The CTF layers were prepared by the Glancing Angle Deposition (GLAD) method. The SPR features were investigated in both the angular and the spectral modes. In the angular interrogation, increasing the porosity causes broadening to the dip width, shift to larger resonance angles, and increase of the sensitivity to analyte refractive index (RI) changes by about threefold compared with closed metal films. In the spectral interrogation, on the other hand, the resonance wavelengths are red-shifted for porous films; hence their spectral sensitivities are higher than those of closed films under the same experimental conditions. Nevertheless, the sensitivity behavior versus the resonance wavelength is similar to that of SPR sensors based on dense film layers. The shapes of the nanostructures constituting the CTF are described as ellipsoidal inclusions in which the effective permittivity dyadic of the composite material is calculated using the Bruggeman formalism with exact depolarization dyadics. The correlation between the sensitivity enhancement and the electromagnetic field intensity at the interface between the metallic film and the analyte was examined. Electromagnetic fields analyses were performed using the general 4 × 4 propagation matrices of general homogenous biaxial layers.     

Utilization of albumin-based sensor chips for the detection of metal content and characterization of metal–protein interaction by surface plasmon resonance

We report here the use of albumin-based biosensor chips for the determination of metal content and characterization of metal–protein interaction by surface plasmon resonance. Bovine serum albumin was immobilized onto a carboxymethylated dextran matrix and used for metal detection. The temperature for the analysis was defined and the highest interaction was observed at 25 °C. The albumin sensor chip binds cadmium, zinc or nickel in a concentration-dependent manner, but not magnesium, manganese and calcium. The optimal buffer condition used for the analysis contains 0.01 M HEPES, pH 7.4, 1 mM NaCl and 0.005% Tween-20. Using this condition, a linear calibration curve within the range of 10⁻⁸ to 10⁻⁴ M can be established for the metals. However, a dramatic increase in binding capacity was observed when metal concentration was higher than 10⁻⁴ M and reached a plateau at 10⁻² M. The detection limit for Cd can reach as low as 1 ppb. When measuring a solution containing two species of metal ions with the albumin chip, an additive effect was observed for Ni and Zn. However, 20–30% reduction in resonance response was found upon mixing Cd with Zn or Ni. These observations are consistent with the binding characteristics of albumin. The feasibility of measuring serum metal content by the albumin chip was examined. A linear calibration curve can be established if the samples are boiled and passed through a gel filtration column. The binding affinity of metal with albumin can also be achieved by using the sensor chip. The binding affinity follows the order of Ni > Zn > Cd. These results indicate that the albumin-based sensor chip is useful not only in the quantification of metal content, but also in the characterization of the biochemical properties of albumin.     

Surface plasmon resonance detection of small molecule using split aptamer fragments

It was difficult to detect small molecules directly using conventional surface plasmon resonance (SPR) biosensors since the changes of refractive index, which was resulted by binding small molecules, were usually small. In this paper, split aptamer fragments were used for the construction of SPR biosensor to determine small molecule such as adenosine with high sensitivity. An aptamer for adenosine was designed to be two flexible ssDNA pieces, one was tethered on Au film and the other was modified on Au nanoparticles (AuNPs). In the presence of adenosine, two ssDNA pieces reassembled into the intact aptamer structure and the AuNPs-labeled adenosine-aptamer complex was formed on the Au film. Then, the resonance wavelength shift was enhanced obviously, due to the electronic coupling between the localized plasmon of AuNPs and the surface plasmon wave associated with Au film. The results confirmed that this biosensor could detect adenosine with high sensitivity and selectivity. The limitation of detection (LOD) of this SPR biosensor was ca. 1.5 pM, which was an approximately ca. 2-3 order of magnitude lower than that of those SPR biosensors which utilized competitive methods.     

Comparison of surface plasmon resonance spectroscopy and quartz crystal microbalance for human IgE quantification

Surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) have been known independently as surface sensitive analytical devices capable of label-free and in situ bioassays. In this study a SPR device and a 10 MHz QCM sensor are employed for the study of human IgE and anti-human IgE-binding reactions upon immobilizing the latter on the gold electrodes. The SPR and QCM response curves to the antibody immobilization and antigen binding are similar in shape but different in time scale, reflecting different resonation principles. Through optimization of the anti-human IgE coating, both the SPR and QCM sensors could detect IgE in a linear range from 5 to 300 IU/ml. Although the intrinsic sensitivity of the SPR device is five times of the 10 MHz QCM, the IgE detection sensitivity of the two methods is, however, different only in a factor of 2. The acceptable QCM sensitivity for the IgE detection is attributed to the fact that QCM measures the sum of molar mass of a protein layer and the entrapped water. Although both the devices use open, stand still liquid cell, and all the measurements are performed at room temperature, the SPR reproducibility and reliability are better than QCM, as the QCM frequency is more sensitive to temperature fluctuations, press changes and mechanical disturbances.