Variable wavelength surface plasmon resonance (SPR) in biosensing

In this study, we fabricated a novel variable wavelength surface plasmon resonance (SPR) sensor, which detects resonance conditions such as a maximum attenuation wavelength, measuring change of microscopic refractive index. Such a change was measured to detect a salmonella antigen–antibody reaction and a penicillinase–penicillin reaction. Our experiments were performed after immobilizing a salmonella antibody on the sensor chip. We measured the shift in resonant wavelength during the antigen–antibody reaction for 30 min by injecting 5 × 10⁷ cells/ml concentration of salmonella antigen solution into the sample chamber. Also, after immobilizing penicillinase on the sensor chip, we measured the shift in resonant wavelength during the reaction. Penicillin solution at 10 mM was injected into the sample chamber. The shift of resonant wavelength for each experiment was measured using a white light source, multimode optical fiber, a part of sensor chip and an optical spectrum analyzer.As a result, the resonant wavelength shifted about 0.26 nm/min owing to the salmonella antibody–antigen reaction. Thus, we could detect the change in wavelength (0.8 nm/min) through the interaction of penicillin and penicillinase for 15 min using variable wavelength SPR sensor.     

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators ^1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides ³, asymmetric split-ring resonators ⁴, and photonic band gap structures integrated into parallel-plate waveguides ⁵. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.The sensor design we use here is based on a simple parallel-plate waveguide ^6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE₁) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove ^6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index ⁹.Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves ¹⁰. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

Highly Sensitive SPR Sensor Based on D-shaped Photonic Crystal Fiber Coated with Indium Tin Oxide at Near-Infrared Wavelength

A surface plasmon resonance (SPR) sensor based on D-shaped photonic crystal fiber (PCF) coated with indium tin oxide (ITO) film is proposed and numerically investigated. Thanks to the adjustable complex refractive index of ITO, the sensor can be operated in the near-infrared (NIR) region. The wavelength sensitivity, amplitude sensitivity, and phase sensitivity are investigated with different fiber structure parameters. Simulation results show that ～6000 nm/refractive index unit (RIU), ～148/RIU, and ～1.2?×?10⁶ deg/RIU/cm sensitivity can be achieved for wavelength interrogation, amplitude interrogation, and phase interrogation, respectively, when the environment refractive index varies between 1.30 and 1.31. It is noted that the wavelength sensitivity and phase sensitivity are more pronounced with larger refractive index. The proposed SPR sensor can be used in various applications, including medicine, environment, and large-scale targets detection.     

Integrated Tunable Optofluidics Optical Filter Based on MIM Side-Coupled-Cavity Waveguide

A tunable wavelength filter based on plasmonic metal?Cdielectric?Cmetal waveguide with optofluidics pump system has been proposed and numerically investigated. The finite difference time domain method with perfectly matched layer-absorbing boundary condition is adopted to simulate and study their properties. An analytical solution to the resonant condition of the structure is derived by means of the cavity theory. It is found that the resonant wavelength of the filter is easily tuned in a broadband by manipulating the fluid filled in the cavity. Both analytical and simulative results reveal that the resonant wavelengths are proportional to the volume and refractive index of liquid in the cavity and are related to the structure of the filter. The resonant wavelengths of this structure can be changed from 1,106 to around 1,800?nm in this paper. The waveguide filter may become a choice for the design of devices in highly integrated optical circuits.     

Surface Phonon Resonance Assisted Refractive Index Sensing in Mid-Infrared Wavelength Range

We propose a highly sensitive refractive index sensor based on the surface phonon resonance (SPhR) in the mid-IR spectral range. Surface phonon polaritons (SPhPs) are formed on polar dielectrics such as SiC in mid-IR wavelength range and can be excited with the help of a metallic grating at specific wavelength termed as resonance wavelength. The resonance wavelength of SPhP is significantly affected by the refractive index of the analyte medium placed over the grating. This forms the basis of a refractive index sensor. We have numerically evaluated the performance of such an SPhP-based refractive index sensor by using rigorous coupled wave analysis (RCWA) in terms of sensitivity, detection accuracy, and quality factor. The quality factor and detection accuracy of the sensor formed on SiC substrate are found to be 225.1 RIU^–1 (inverse of refractive index unit) and 6.75, respectively. We have also extended the study for other polar dielectric substrates cBN and GaN and observed considerable enhancement in the performance of the sensor for GaN. The values of quality factor and detection accuracy could be increased to 361.2 RIU^–1 and 10.84, respectively, by using GaN substrate. The proposed sensor finds applications in refractive index sensing of liquids and biomolecules having refractive index in the range 1.33–1.36.

     

All-Optical Tunable Wavelength-Division Multiplexing Based on Colloidal Crystal Coated Silver Film

An all-optical tunable nanoscale wavelength-division multiplexing device is realized theoretically based on a plasmonic microstructure, which is composed of a silver film coated with a monolayer colloidal crystal made of cholesteryl iodide-doped polystyrene. The physical mechanism is attributed to the variation of surface plasmon polariton modes and guided modes caused by pump-laser-induced refractive index change of cholesteryl iodide. An up to 90-nm shift in the resonant wavelength of optical channels can be reached under excitation of a 500?mJ/cm² pump laser. The number of optical channels can be tuned by adjusting the structure parameters of the monolayer colloidal crystal. This may open a new way for the study of integrated photonic devices.     

A MIM Filter Based on a Side-Coupled Crossbeam Square-Ring Resonator

In this paper, a surface plasmon polarition filter based on a side-coupled crossbeam square-ring resonator is presented and the transmission characteristics of the filter are analyzed by using the finite difference time domain method. The simulation results indicate that the proposed resonator supports multiple resonant modes, and these resonant modes can be adjusted all together by varying the length and refractive index of the outer square ring or partially adjusted by changing the width and refractive index of the crossbeam. By adding two coupled waveguides to the structure, we further demonstrate that a multiple wavelength download filter can be achieved via different coupled waveguides. The proposed structure has potential applications in plasmonic integrated circuits.     

Characterisation of a Gold Nanorod Sol–Gel Utilising Inter-particle Coupling to Yield High Refractive Index Sensitivity

The development of gold nanorod plasmonic sol?Cgel polymer is presented and characterised with respect to its sensitivity to refractive index change. Structural characterisation of the polymer was conducted using Raman microscopy and energy dispersion spectroscopy while plasmonic function was investigated using UV/VIS spectroscopy. Refractive index sensitivities utilising the peak wavelength shift of the localised surface plasmon resonance were shown to be of the order of 2,338?nm per refractive index unit; in addition, peak absorbance was considered as an alternative measure. Furthermore, demonstration of sensitivity to biomolecule interaction has been shown as a model study. Strong photoluminescence was observed during Raman studies that restricts the potential use of the polymer as a surface-enhanced Raman substrate.     

Integrated Terahertz Surface Plasmon Resonance on Polyvinylidene Fluoride Layer for the Profiling of Fluid Reflectance Spectra

We design terahertz (THz) surface-plasmon-resonance (SPR) sensors using a ferroelectric polyvinylidene fluoride (PVDF) thin layer for biological sensing. The reflectivity properties based on SPR are described using transfer matrix method (TMM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the reflectivity spectrum. The results reveal that the pronounced SPR resonance peak has quasi-linear relationship with the refractive index variation of the material under investigation. Through analyzing and optimizing the structural parameters of the THz SPR sensor, we achieved the theoretical value of the refractive index detection sensitivity as high as 0.393 THz/unit change of refractive index (RIU) for a 20-μm-thick liquid sample with a 10-μm PVDF layer. This work shows great promise toward realizing a THz SPR sensor with high sensitivity for identifying the signatures of biological fluid sample.     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Surface Plasmon Resonance Based Fiber Optic Ammonia Sensor Utilizing Bromocresol Purple

In this paper, a surface plasmon resonance (SPR) based fiber optic ammonia gas sensor has been designed and fabricated using bromocresol purple (BCP) as sensing element. The sensor works under wavelength modulation scheme. The detection of ammonia gas has been carried out at room temperature. Three different kinds of film coating configurations, namely silver + BCP, gold + BCP, and silver + silicon + BCP on the unclad portion of the fiber have been used for studying the role of each layer. Further, to optimize the performance of the sensor, the films of varying thicknesses were coated using thermal evaporation technique. Experiments have been performed for the ammonia concentrations ranging from 0 to 150 ppm around the probe. To record the SPR spectrum, light from a polychromatic source is launched in the fiber and the spectrum is recorded at the other end of the fiber. The spectrum has a peak at lower wavelength while a dip at the higher wavelength. The dip corresponds to SPR while the peak appears to be due to fluorescence properties of the dye. It has been observed that as the ammonia gas comes in contact of the BCP layer, it changes the refractive index of the BCP dye which, in turn, changes the resonance wavelength. Further, the change in refractive index increases as the concentration of ammonia gas increases up to certain concentration of ammonia after that it saturates. Silicon layer has been shown as a protection layer for silver and gold from oxidation and acts as a tuner of wavelength. The proposed ammonia sensor has small response as well as recovery time.     

Plasmonic Coupling Effects on the Refractive Index Sensitivities of Plane Au-Nanosphere-Cluster Sensors

Plasmonic coupling effects (between neighboring components) are able to red shift the peak wavelengths of dipolar-localized surface plasmon resonances (LSPRs) and increase the corresponding refractive index sensitivity of nanoparticle sensors. The coupling effects on plane Au-nanosphere-cluster (including nanosphere dimer, trimer, pentamer, and heptamer) sensors are numerically investigated by finite element method (FEM). We found that the coupling does not violate the quadratic response characteristics of LSPR peak wavelengths, hence the linear responses of the sensitivities to the bulk refractive index of Au cluster sensors. Yet, for nanosphere dimer sensors, they contribute to the exponential decrease of sensitivities with their gap distances, which follow the universal plasmon ruler behavior. The amplitude of their fractional sensitivity shift is revealed to be bulk refractive index independent, which is different from that of their fractional LSPR peak wavelength shift. These are analytically explained well in terms of an effective nanoparticle model. The present work also gives an upper sensitivity limit for Au nanosphere dimer systems and provides a method to estimate the interparticle separation between the two component nanospheres of the dimer.

     

Design of a High-Resolution Metal–Insulator–Metal Plasmonic Refractive Index Sensor Based on a Ring-Shaped Si Resonator

In this paper, a high-resolution refractive index sensor is proposed based on a novel metal–insulator–metal plasmonic topology. The structure is based on a Si nano-ring located inside a circular cavity. It acts as an optical notch filter with a quality factor equal to 269. The proposed filter topology is numerically simulated using the finite difference time domain method. It is shown that the proposed filter can also act as a refractive index sensor with a sensitivity of 636 nm/RIU and a fairly high figure of merit (FoM) equal to 211.3 RIU⁻¹. It is shown that the sensor can easily detect a refractive index change of ± 0.001 for dielectrics whose refractive index is between 1 and 1.2. For the refractive index range of 1.33 to 1.52, the maximum FoM of the sensor is 191 RIU⁻¹. The simplicity of the design and its high resolution are the two main features of the proposed sensor which make it a good candidate for biomedical applications.

     

Two-dimensional PtSe2 Theoretically Enhanced Goos-Hänchen Shift Sensitive Plasmonic Biosensors

Platinum diselenide (PtSe₂), an emerging two-dimensional transition metal dichalcogenide, exhibits thickness-dependent refractive index, and hence, intriguing optical properties. Here, we employ it as a plasmonic sensing substrate to achieve significant enhancement in Goos-Hänchen shift sensitivity. Through systematic optimization of all parameters, four optimum sensing configurations have been achieved at different wavelengths ranging from visible to near-infrared region, where the Goos-Hänchen shift sensitivity receives four times enhancement in comparison with the conventional bare gold sensing substrate. There is a linear range of Goos-Hänchen shift with the tiny change of refractive index for each optimal configuration. The detection limit of the refractive index change can be as low as 5 × 10⁻⁷ RIU which is estimated to be lower by 2 orders of magnitude, and the corresponding sensitivity of biomolecules has a 1000-fold increment compared with that of bare gold-based sensors.

     

Application of chromogenic reagents in surface plasmon resonance (SPR)

In this paper, a new simple approach for sensitivity optimization in surface plasmon resonance (SPR) chemosensors based on colorimetric ligands is presented. A new design of SPR sensor with tunable analytical wavelength (lambda(SPR)) was constructed for this purpose, to perform studies on the ligand absorbance spectra related sensitivity enhancement. Unlike commercial SPR sensors which operate at one lambda(SPR), the new device can be used for sensitivity analysis at selected lambda(SPR) in the range 550-750 nm, offering the possibility to identify the highest sensitivity lambda(SPR) in regard to the spectral changes of the selected ligand. Measurements can be easily done in ligand bulk solutions without immobilization steps. Sensitivity enhancement analysis and optimization of lambda(SPR) on chromogenic reagents with hypsochromic shift in their absorption spectra are demonstrated in this contribution. Optimal selection of analytical wavelength, set at the absorbance peak of chromogenic reagent Eriochrome Black T (EBT) was observed to result in up to two times increased SPR sensitivity to Cd(2+) compared to wavelengths selected in other parts of the ligand absorbance spectra, with a limit of detection (LOD) 0.2 ppm. The sensitivity enhancement at optimal lambda(SPR) was observed to be related to increased refractive index (n), drop in extinction coefficient (alpha) and simultaneous hypsochromic shift of the EBT absorbance spectra causing the lambda(SPR) to match the absorbance peak shoulder.     

Extremely High Sensitive Plasmonic Refractive Index Sensors Based on Metallic Grating

This work shows that a grating-based surface plasmon (SP) resonance sensing system can exhibit extremely high sensitivity to detect a small change of refractive index in an analyte. The corresponding sensitivity can be much higher than that of the prism-based systems. Both analytical calculation and rigorous coupled-wave analysis are used to study the angular sensitivity of the system. It is found that the system’s sensitivity can be over 600° per unit index change if (1) first-order diffracted wave is chosen to excite SP mode, (2) large SP resonant angles are used in the operation, and (3) grating filling factor is selected to be varied between 0.3 and 0.7. Furthermore, the sensing system has the best performance for detecting low-index analyte with a small change of refractive index.     

Nanostructure shape effects on response of plasmonic aptamer sensors

A localized surface plasmon resonance (LSPR) sensor surface was fabricated by the deposition of gold nanorods on a glass substrate and subsequent immobilization of the DNA aptamer, which specifically bind to thrombin. This LSPR aptamer sensor showed a response of 6‐nm λ_max shift for protein binding with the detection limit of at least 10 pM, indicating one of the highest sensitivities achieved for thrombin detection by optical extinction LSPR. We also tested the LSPR sensor fabricated using gold bipyramid, which showed higher refractive index sensitivity than the gold nanorods, but the overall response of gold bipyramid sensor appears to be 25% less than that of the gold nanorod substrate, despite the approximately twofold higher refractive index sensitivity. XPS analysis showed that this is due to the low surface density of aptamers on the gold bipyramid compared with gold nanorods. The low surface density of the aptamers on the gold bipyramid surface may be due to the effect of shape of the nanostructure on the kinetics of aptamer monolayer formation. The small size of aptamers relative to other bioreceptors is the key to achieving high sensitivity by biosensors on the basis of LSPR, demonstrated here for protein binding. The generality of aptamer sensors for protein detection using gold nanorod and gold nanobipyramid substrates is anticipated to have a large impact in the important development of sensors toward biomarkers, environmental toxins, and warfare agents. Copyright © 2013 John Wiley & Sons, Ltd.     

A High Performance Plasmonic Sensor Based on Metal-Insulator-Metal Waveguide Coupled with a Double-Cavity Structure

A high performance plasmonic sensor based on a metal-insulator-metal (MIM) waveguide coupled with a double-cavity structure consisting of a side-coupled rectangular cavity and a disk cavity is proposed. The transmission characteristics of the rectangular cavity and disk cavity are analyzed theoretically and the improvements of performance for the double-cavity structure compared with a single cavity are studied. The influence of structural parameters on the transmission spectra and sensing performance are investigated in detail. A sensitivity of 1136 nm/RIU with a high figure of merit of 51,275 can be achieved at the resonant wavelength of 1148.5 nm. Due to the high performance and easy fabrication, the proposed structure may be applied in integrated optical circuits and on-chip nanosensors.     

Sensitivity Analysis of a Bioinspired Refractive Index Based Gas Sensor

It was found out that the change of refractive index of ambient gas can lead to obvious change of the color of Morpho butterfly's wing. Such phenomenon has been employed as a sensing principle for detecting gas. In the present study, Rigorous Coupled-Wave Analysis (RCWA) was described briefly, and the partial derivative of optical reflection efficiency with respect to the refractive index of ambient gas, i.e., sensitivity of the sensor, was derived based on RCWA. A bioinspired grating model was constructed by mimicking the nanostructure on the ground scale of Morpho didius butterfly's wing. The analytical sensitivity was verified and the effect of the grating shape on the reflection spectra and its sensitivity were discussed. The results show that by tuning shape parameters of the grating, we can obtain desired reflection spectra and sensitivity, which can be applied to the design of the bioinspired refractive index based gas sensor.