Influence of ions on the surface plasmon resonance spectrum of a fiber optic refractive index sensor

Surface plasmon resonance (SPR) spectra of a fiber optic SPR probe coated with thin gold layer have been studied experimentally for four ionic (NaCl, KCl, Na₂SO₄ and MgCl₂) and two non-ionic (sucrose and urea) liquid samples of different refractive indices. For the same value of refractive index, the response curves for ionic and non-ionic liquid samples have been found to differ, implying the influence of ions on the resonance wavelength of the SPR spectrum. It is observed that the resonance wavelength, for a given refractive index of the liquid, is higher for ionic sample than that in the case of non-ionic one. This extra increase in resonance wavelength is attributed to the interaction of ions with the free electrons of the metal film. The study finds importance in situations where ions get generated during the processes. In such studies, the effect of ions on SPR spectrum must be incorporated otherwise the SPR spectrum may get wrongly interpreted.     

Fast surface plasmon resonance imaging sensor using Radon transform

Surface plasmon resonance (SPR) imaging technique is proposed in which a diverging laser beam at given frequency was used to illuminate the entire sensor surface in Kretschmann-Raether configuration. A pattern of dark intensity line on bright background is observed corresponding to the SPR dip at an angular range depending on the refractive index of the adjacent analyte and monitored by a two-dimensional CCD detector. A novel Radon transform based detection algorithm for the SPR line pattern is proposed, which is non sensitive to the laser speckle noise and improves the accuracy.     

Use of a low refractive index prism in surface plasmon resonance biosensing

A way to improve the angular sensitivity of surface plasmon resonance (SPR) biosensor by tuning the resonance position to a higher incident angle region using a lower refractive index glass prism is described in this paper. A novel effective 3-layer (E3L) model is described to transform a multiple layer biosensor configuration in context of SPR condition. The E3L model supports the use of a low refractive index prism for biosensing. The performance of the sensor in immunosensing is checked for two glass prisms of different refractive index materials. The experimental results showed an enhancement in the amount of resonance angle shift of the immunosensor for the lower refractive index glass prism.     

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.     

Absorption-based fiber optic surface plasmon resonance sensor: a theoretical evaluation

In this paper, an optical absorption based fiber optic surface plasmon resonance (SPR) sensor has been studied theoretically. The theoretical treatment is based on Kretschmann’s SPR theory and the Lorentz model that expresses a damped harmonic oscillator is included in the treatment for optical absorption in the sensing layer. The optical source considered is an un-polarized collimated beam. The light is coupled to the fiber using a microscope objective that focuses the beam at the center of the input face of the fiber. The effects of the parameters related to the sensing region, the light source and the optical fiber on the sensitivity and the operating range of the SPR sensor have been studied with the help of numerical calculations and computer simulations. It has been found that the excitation frequency in absorption-based fiber optic SPR sensor is an important parameter. The sensitivity is better for the lower off-resonance excitation frequency. The sensitivity and the operating range of the sensor are better for large value of the core diameter. The optimization of numerical aperture of the fiber, film thickness and the length of the sensing region is required to achieve the maximum sensitivity. Further, the increase in the extinction coefficient of the sample increases the sensitivity of the sensor while the decrease in the width of its absorption spectrum increases the sensitivity. The sensitivity and the operating range of the sensor are better for small values of the refractive index of the absorbing sample.     

Highly accurate and sensitive surface plasmon resonance sensor based on channel photonic crystal waveguides

A highly sensitive surface plasmon resonance (SPR) sensor based on channel photonic crystal waveguide (PCW) is proposed. The PCW is based on widely used lithographic and nano-fabrication compatible materials like TiO₂ and SiO₂. Gold has been used as a SPR active metal. By rigorously optimizing the different waveguide parameters, we have shown that there is significant transfer of modal power around phase-matching or resonance wavelength which has been utilized to design a compact and highly sensitive sensor for lab on chip. The ultra narrow width (∼765 pm for an interaction length of 10 mm) of surface plasmon resonance curve and sensitivity as high as 7500 nm-RIU⁻¹ will open a new window for bio-chemical sensing applications.     

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.     

Measurement of metal nanolayers optical parameters using surface plasmon resonance method

The results of experimental definition of optical parameters of silver nanolayers by the method of surface plasmon resonance (SPR) are described. The goniometric installation for measurement of angular dependences of reflection of light by layered nanosructures at the attenuated internal reflection (angular spectrum of reflection) is designed. Calculation of optical parameters of nanolayers is executed taking into account the form of a curve of tin angular spectrum of reflection. The text was submitted by the authors in English.     

A single chip multi-channel surface plasmon resonance imaging system

A single chip, multi-channel surface plasmon resonance (SPR) imaging system has been developed. The equipment has no moving parts and uses a single sensor chip on which multiple channels can be incorporated. A light emitting diode is used as a photon source while a CCD camera forms the detector. The optical configuration has been designed to achieve a uniform illumination of the sample over a fixed area with a range of incident angles. A calibration test using sucrose solutions shows that the sensitivity of the equipment is 4.3×10⁻⁴ refractive index units per pixel line-pair. The use of the system for simultaneous interrogation of different polyelectrolyte thin films, formed by the electrostatic layer-by-layer deposition method, is demonstrated. A reversible pH-dependent response for these organic layers is also reported.     

Sensitivity enhancement of a surface plasmon resonance based biomolecules sensor using graphene and silicon layers

A surface plasmon resonance based biomolecules sensor using silicon and graphene layers coated over the base of the high index prism sputtered with gold has been analyzed. The graphene layer has been used to enhance the adsorption of biomolecules while the addition of silicon layer between gold and graphene increases the sensitivity. The thicknesses of gold and silicon layers along with the number of graphene layers have been optimized to achieve the best performance of the sensor in terms of sensitivity and Full Width at Half Maximum (FWHM). To see the effect of wavelength of the light source, simulations have been carried out for three different wavelengths. The best performance is obtained for 633 nm wavelength with optimized thicknesses of gold and silicon layers as 40 nm and 7 nm respectively while the optimum number of graphene layers is 2. The sensitivity obtained with optimized parameters and additional silicon layer, is more than twice the value reported in the literature.     

Novel, high-quality surface plasmon resonance microscopy

A surface plasmon resonance microscope capable of high-quality speckle-free imaging has been designed that uses a laser as a source. An inexpensive acoustic transducer is used to reduce speckle and other image artifacts arising from the use of illumination from an inexpensive laser pointer. The microscope is described and operation of the system demonstrated.     

A microfluidic based differential plasmon resonance sensor

A new type of differential surface plasmon (SPR) sensor integrated with a microfluidic system is presented. The working principle of the microfluidic device is based on hydrodynamic modulation of two laminar streams inside a microchannel to provide periodic changes of the environment on the SPR sensor. The modulated reflectance is then demodulated using a lock-in amplifier. The presented sensor provides sensitivities of index of refraction about 4 × 10⁻⁸ RIU together with a 4 orders of magnitude dynamic range. This method demonstrates a sensitive detection scheme which could be used for label-free detection.     

Interrogation of surface plasmon resonance sensors using temporally stretched ultrashort optical pulses

We propose a novel technique that has the potential to realize interrogation of surface plasmon resonance (SPR) sensors at very high speed. In contrast to the incoherent light source used in the traditional wavelength interrogation schemes, a broadband coherent laser generating short optical pulses at a high repetition rate is used along with a highly dispersive optical element. The dispersion causes strong broadening of the optical pulses, and the temporal pulse shape could exactly resemble the spectral distribution of the pulses due to the induced linear chirp. Therefore, by measuring the changes in the pulse shapes with a single high-speed photodetector, the spectral response of the SPR sensor can be obtained for each input pulse and the interrogation speed could reach the repetition rate of the pulse train. This could enable SPR measurements at the speed of tens of MHz or higher, which is well beyond that of other current SPR interrogation techniques. We experimentally demonstrate that, by measuring the variations in the pulse shapes of the chirped pulses, sensitive SPR measurements can be made. Implementing this scheme with a femtosecond fiber laser and other fiber optic components also show the potential to realize more compact and integrated SPR systems.     

Surface plasmon resonance sensor based on a single-mode polarization-maintaining optical fiber

A novel wavelength modulation-based fiber-optic surface plasmon resonance (SPR) sensor is reported which utilizes both polarization separation and broad band radiation depolarization in polarization-maintaining fibers to enhance sensor stability. Theoretical analysis of the sensing structure with ideally separated polarizations based on the mode of expansion and propagation method is presented. The effect of polarization cross-coupling was also analyzed in the approximation of an equivalent bulk optic structure. A laboratory prototype of the fiber-optic SPR sensor was characterized in terms of sensitivity and resolution. Experimental results indicate that this fiber-optic SPR sensor is able to resolve refractive index changes as low as 4×10⁻⁶ under moderate fiber deformations.     

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.     

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。     

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.     

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.