Fiber Optic Surface Plasmon Resonance Research Articles

Highly sensitive differential fiber-optic SPR sensor in telecom band.

We proposed a differential fiber-optic SPR remote sensor with ultra-high sensitivity in telecom band. The working band of the sensor is designed as the C-band which is the low loss band of optical fiber communication aiming to improve the sensitivity and enable the capability of remote monitoring. The sensor head is a BK7 prism coated with Au/TiO2 films, enabling two channels for differential intensity interrogation. The intensities of the reflected lights through the channels vary oppositely within the measurement range of refractive index. Due to the sharp dip of angular resonant response in the C-band, the differential signal produces a steep slope as the refractive index of the sample varies, thus higher sensitivity is expected in a narrow measurement range. According to the results, the sensitivity is as high as 456 V/RIUs within the narrow measurement range of 1.3×10-2 RIUs and the resolution reaches to 6×10-6 RIUs. The measurement range can be tuned conveniently by adjusting the thickness of TiO2 film and can be expanded by increasing the number of sensing channels, which provides great convenience for the application of biosensor requiring high sensitivity.

Comparative study of block copolymer-templated localized surface plasmon resonance optical fiber biosensors: CTAB or citrate-stabilized gold nanorods

Block copolymer (BCP) brush-like layer absorbed on optical fibers are shown to address the longstanding challenge of forming dense and well-dispersed gold nanorods (AuNRs) films on highly curved surface, allowing the development of real-time localized surface plasmon resonance (LSPR) biosensors for antigen-antibody interaction. This method requires only solution-based manufacturing processes and a plasma cleaning step to self-assemble AuNRs with various aspect ratios on the optical fibers. Because AuNRs capped with cetyltrimethylammonium bromide (CTAB) have been problematic for biosensing applications due to their cytotoxicity and displacement difficulty in the construction of biosensors, citrate-stabilized AuNRs were prepared from CTAB-stabilized AuNRs via surfactant exchange. For comparison, both a poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) BCP template with CTAB-stabilized AuNRs and a poly(styrene)-b-poly(4-vinyl pyridine) (PS-b-P4VP) BCP template with citrate-stabilized AuNRs were employed for the fabrication of the optical fiber biosensor. The LSPR spectral analysis results demonstrate that the sensitivity of the sensor produced by the PS-b-P4VP templating method was 1.5 times better than that of the sensor produced by the PS-b-PAA templating method, which was due to the improved surface coverage and reduced aggregation of nanoparticles. The detection limit of the PS-b-P4VP-templated sensor for human IgG was 0.6 nM. Furthermore, for citrate-stabilized AuNRs, the PS-b-P4VP templating method also led to a better surface morphology and higher sensitivity as compared to the standard particle absorption methods using alkoxysilanes or polyelectrolytes on identical fibers. The simplicity of the fabrication process and the far superior optical performance of the PS-b-P4VP-templated LSPR optical fiber sensor illustrate the ability to regulate the detection range by tuning the aspect ratios of the AuNRs, and demonstrate its application prospects as a real-time biosensor without cytotoxicity.

Thermo-Optic Property Measurement Using Surface Plasmon Resonance-Based Fiber Optic Sensor

The temperature disturbances greatly influence the performances of microfluidic chips during the working process. It is therefore important to evaluate the thermo-optic properties of microfluidic materials. Here, we measure the thermo-optic property by employing a fiber optic surface plasmon resonance (SPR) sensor combined with a thermocouple. This setup can be applied for embedding in microfluidic materials or directly detecting fluidic samples. Three fiber optic SPR sensors were fabricated with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$400~\mu m$ </tex-math></inline-formula> -bare-core, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$600~\mu m$ </tex-math></inline-formula> -bare-core and mismatched-core structures. The thermo-optic responses of the three sensors were compared to determine the optimum sensor structure. The thermo-optic property of polydimethylsiloxane was then investigated over the full temperature detection range of 25–200 °C. The thermo-optic coefficient of polydimethylsiloxane was determine to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 4.19\times 10^{-4}$ </tex-math></inline-formula> RIU/°C, which agrees well with previous reports. The thermo-optic properties of ethanol and brine were also evaluated to confirm the universality of the proposed measurement method. To our knowledge, this is the first reported detection of thermo-optic properties using a fiber optic SPR sensor.

Co(III)-NTA Mediated Antigen Immobilization on a Fiber Optic-SPR Biosensor for Detection of Autoantibodies in Autoimmune Diseases: Application in Immune-Mediated Thrombotic Thrombocytopenic Purpura.

Autoantibodies are key biomarkers in clinical diagnosis of autoimmune diseases routinely detected by enzyme-linked immunosorbent assays (ELISAs). However, the complexity of these assays is limiting their use in routine diagnostics. Fiber optic-surface plasmon resonance (FO-SPR) can overcome these limitations, but improved surface chemistries are still needed to guarantee detection of autoantibodies in complex matrices. In this paper, we describe the development of an FO-SPR immunoassay for the detection of autoantibodies in plasma samples from immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients. Hereto, hexahistidine-tagged recombinant ADAMTS13 (rADAMTS13-His6) was immobilized on nitrilotriacetic acid (NTA)-coated FO probes chelated by cobalt (Co(III)) and exposed to anti-ADAMTS13 autoantibodies. Initial studies were performed to optimize rADAMTS13-His6 immobilization and to confirm the specificity of the immunoassay for detection of anti-ADAMTS13 autoantibodies with FO-SPR. The performance of the immunoassay was then evaluated by comparing Co(III)- and nickel (Ni(II))-NTA stabilized surfaces, confirming the stable immobilization of the antigen in Co(III)-NTA-functionalized FO probes. A calibration curve was prepared with a dilution series of a cloned human anti-ADAMTS13 autoantibody in ADAMTS13-depleted plasma resulting in an average interassay coefficient of variation of 7.1% and a limit of detection of 0.24 ng/mL. Finally, the FO-SPR immunoassay was validated using seven iTTP patient plasma samples, resulting in an excellent correlation with an in-house-developed ELISA (r = 0.973). In summary, the specificity and high sensitivity in combination with a short time-to-result (2.5 h compared to 4-5 h for a regular ELISA) make the FO-SPR immunoassay a powerful assay for routine diagnosis of iTTP and with extension for any other autoimmune disease.

Performance analysis of optical fiber surface plasmon resonance sensor in single-mode operation region

Abstract A new approach for studying the performance of single-mode optical fiber surface plasmon resonance sensor for refractive index sensing is presented. Intensity interrogation technique is used to observe and examine the effect of changing the value of the operating wavelength, within the single-mode operation region, on sensor performance. Effect of changing metal thickness, which is a critical parameter of the sensor structure, is also investigated. The results show that the selected operating wavelength within the mentioned region plays a very important role and has a significant impact on the sensor sensitivity. As the operating wavelength increases beyond the cut-off wavelength, above which the first higher-order mode disappears, the sensitivity increases while the refractive index range, for which the sensitivity is above its half maximum value, becomes narrower. In the proposed sensor structure, for a metal thickness of 25 nm, the maximum sensitivity reaches 2554 RIU−1 for analyte refractive index of 1.439 at a wavelength of 1500 nm. This sensitivity corresponds to a resolution of order 106 RIU by assuming 1% minimum reflectivity variation. The operating wavelength should be carefully selected and changed according to the refractive index values intended to be measured to ensure high sensor sensitivity at these values. The analysis provided in this paper enables to select and change the desired range of the sensed refractive index using low cost technique by just changing the operating wavelength in the single-mode operation region.

Fabrication and Measurement of Fiber Optic Surface Plasmon Resonance Sensor Based on Polymer Microtip and Gold Nanoparticles Composite

Fiber optic surface plasmon resonance (FO SPR) sensor offers attractive advantages over the conventional prism-based SPR platforms, such as simplicity, cost-effectiveness, and miniaturization. To improve the performance of the existing FO SPR sensor which has been fabricated by depositing a gold thin film on the flat fiber cross-section or side surface, we propose to fabricate the polymer microtip array on optical fiber using simple cost-effective soft and imprint lithography because SPR instruments mainly depend on the structural aspect of the thin metal plasmonic film deposited on the optical fiber substrate. The fabricated microstructures are also combined with the gold nanoparticles to expand the sensing area. The proposed sensor displayed 175% improved sensitivity with linear response according to refractive index changes, when developed FO SPR sensor and conventional FO SPR sensor without microtip array were applied to refractive index measurement, respectively. Based on the obtained results, the proposed FO SPR sensor can ultimately be used in various applications, ranging from food safety, environmental monitoring, and biomarker analysis.

Lossy mode resonance-based uniform core tapered fiber optic sensor for sensitivity enhancement

A lossy mode resonance (LMR)-supported fiber optic sensor in which a uniform fiber core is placed among two identical tapered regions, is investigated numerically. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) are considered as LMR active materials used to excite several lossy modes and gold and silver are used as surface plasmon resonance (SPR) active materials. In this probe design, a central uniform core coated with ITO/AZO is the active sensing region, whereas tapered regions are meant for bringing the incident angle close to the critical angle. The sensitivity of the present fiber optic bio-sensor is evaluated for first two LMRs utilizing both ITO and AZO separately, along with its variation with the taper ratio (TR). For ITO, the maximum sensitivity values are observed to be 18.425 μm RIU−1 (refractive index unit) and 0.825 μm RIU−1, corresponding to the first and second LMRs, respectively, at a TR of 1.6 and for AZO, equivalent values are 0.79 μm RIU−1 and 0.35 μm RIU−1, respectively, at a TR of 2.0. The results illustrate that the first LMR is more sensitive than the second LMR and the ITO-coated probe possesses greater sensitivity than the AZO-coated probe for both LMRs. Similarly, for the fiber optic SPR sensor, the maximum value of sensitivity is 5.6425 μm RIU−1, in the case of gold and 5.0615 μm RIU−1 in the case of silver, at a TR of 1.6. Hence, the result shows that the sensor with the present fiber optic probe design has around a 3-fold enhancement in sensitivity compared with conventional SPR sensors. This study will have applications in many sensing schemes where the requirement of large sensitivity is vital.

Comparison of measurement protocol for biosensors using fiber optic localized surface plasmon resonance sensor

Abstract In an immunoassay test, which is a representative analytical method for evaluating the performance of a biosensor, the change of the sensor’s output with respect to the continuous change of the measurement target concentration is represented by a graph and defined as the characteristic curve of the sensor. In most studies using biosensors (disposable or not disposable), the characteristic curve of the sensor is obtained by accumulating and successively changing the concentration of the target using a single sensor (consecutive measurement using a single sensor: CS protocol). However, the CS protocol has an inherent error because the accumulated measurement results for a single sensor that continuously varies with the concentration of the target material are considered as the sensor’s output at each concentration level. In this paper, we obtain the characteristic curve of the sensor by measuring the different concentrations of the target using different sensors (separate measurement using the individual sensor: SI protocol). By comparing this characteristic curve with the characteristic curve obtained using the CS protocol, we prove that the widely used method based on CS protocol is inaccurate as a measurement protocol for biosensors.

Solid-Phase PCR-Amplified DNAzyme Activity for Real-Time FO-SPR Detection of the MCR-2 Gene.

The polymerase chain reaction (PCR) has been the gold standard molecular analysis technique for decades and has seen quite some evolution in terms of reaction components, methodology, and readout mechanisms. Nucleic acid enzymes (NAzymes) have been used to further exploit the applications of PCR, but so far the work was limited to the colorimetric G-quadruplex or fluorescent substrate cleaving NAzymes. In this study, a solid-phase, fiber optic surface plasmon resonance (FO-SPR) technique is presented as an alternative readout for PCR utilizing NAzymes. First, the surface cleavage activity of DNAzyme-extended amplicons (DNAzyme-amps) is established, followed by optimization of the PCR conditions, which are required for compatibility with the FO-SPR system. Next, by integrating the complement of a 10-23 DNAzyme into the primer pair, PCR-amplified DNAzyme-amps were generated, tested, and validated on qPCR for the detection of the antimicrobial resistance gene MCR-2. Once validated, this primer concept was developed as a one-step assay, driven by PCR-amplified DNAzymes, for FO-SPR-based sensitive and specific detection. Using gold nanoparticle labeled RNA-DNA hybrid strands as substrate for the DNAzyme, PCR-amplified DNAzyme-amps generated in the presence of MCR-2 gene were monitored in real-time, which resulted in an experimental limit of detection of 4 × 105 copy numbers or 6.6 fM. In addition, the DNAzyme-based FO-PCR assay was able to discriminate between the MCR-1 and MCR-2 genes, to further prove the specificity of this assay. Henceforth, this DNAzyme-based fiber optic PCR assay provides a universally applicable, real-time system for the detection of virtually any target NA, in a specific and sensitive manner.

Detection of adulteration in pure honey utilizing Ag-graphene oxide coated fiber optic SPR probes.

Fiber optic surface plasmon resonance (SPR) sensor utilizing silver (Ag) and Ag-graphene oxide (GO) is designed and developed for the detection of adulteration of glucose and fructose in pure honey. The concentration range of the two adulterants in pure honey is varied from 4% to 20% with a step change of 4%. The experiments were performed with two different fiber optic probes viz. Probe 1 and Probe 2. Probe 1 is fabricated by coating 50nm Ag film on unclad optical fiber portion and Probe 2 is fabricated by modifying Ag film with GO for sensitivity improvement. The study confirms that using GO modified fiber optic probe, the sensitivity is enhanced to 24% and 37% for glucose and fructose adulterated honey samples respectively. The technique presented in this study is easy, rapid, label free and shows high prospective for the detection of adulterants in pure honey.

Highly sensitive fiber optic surface plasmon resonance sensor employing 2D nanomaterials

Optical fiber-based surface plasmon resonance sensors using, silver layer (Ag), platinum (Pt), and indium tin oxide in addition to 2D nanomaterials such as graphene are presented in this research. In terms of sensitivity (S) and figure of merit (FoM), the performance analysis of the proposed and configured sensor has been demonstrated. The proposed sensors are based on the scheme of Kretschmann for obtaining the transmitted power using the transfer matrix method and the equations of Fresnel. With theoretical and numerical studies, the reported sensor exhibits a good wavelength sensitivity which is improved to the maximum value of 4150 nm/RIU. Full width half maximum values are minimized to be 59 nm and the FoM is optimized to be 70 RIU−1 which gives better sensing properties in comparison with other published articles.

Simulation and analysis of 2D material/metal carbide based fiber optic SPR probe for ultrasensitive cortisol detection

Cortisol, a steroid hormone, plays a vital part in regulating various activities such as metabolism and immune response. It is a well-known stress biomarker and should be measured with as high precision as possible. In this work, a non-invasive fiber optic surface plasmon resonance (FOSPR) based sensor probe is proposed for detection of salivary cortisol at the w avelength (λ) of 830 nm. Proposed sensor design consists of chalcogenide glass core, polymer clad, silver (Ag) layer with two-dimensional (2D) materials such as conventional (graphene, WS2 and MoS2) and transition metal carbides (MXenes) based layered materials considered one at a time. Angular and intensity interrogation methods have been envisaged for simulation and analysis purpose. The sensing performance analysis is carried out in terms of figure of merit (FOM) and combined performance factor (CPF). A comprehensive analysis of all the probe variants reveals that among all the sensor design variants, the Ti3C2O2-based probe is able to provide superior overall sensing performance including a detection limit as fine as 15.7 fg/mL, which is finer compared to existing cortisol sensors based on diverse techniques.

Beating the shot-noise limit with optical fiber quantum sensors for salinity measurement

An ultra-resolution optical salinity quantum sensor depended on the tapered hetero-core structure coated with a gold film (around 50 nm) is proposed and demonstrated in this paper. Single photons which work as an input source are used for exciting surface plasmon polaritons at the interface between the metal and the seawater. We exploit the statistical analysis method to get the transmission spectroscopy and quantify its estimation error. Experiment results show that the detected estimation errors of the proposed quantum sensor are completely in agreement with the quantum theory. Though various external interference and experimental defects exist, the error bars are beyond the standard quantum limit. The salinity sensitivity of this proposed sensor is about 0.02204/‰. The resolution of this proposed sensor is 0.0016‰, which is one order of magnitude higher than that of traditional optical fiber surface plasmon resonance (SPR) sensors. The resolution and the sensitivity of the quantum sensor can be further enhanced by modifying the metal coating and the structural parameters. This work realizes the combination of optical fiber sensing technology and quantum plasmonic technology, which will pave the way for the development of high-resolution and high-sensitivity sensor.

Theoretical and experimental characterization of a salinity and temperature sensor employing optical fiber surface plasmon resonance (SPR)

A reflective seawater salinity and temperature sensor based on surface plasmon resonance (SPR) effect with multimode single-mode (MS) fiber structure has been proposed and demonstrated. Firstly, the sensing mechanism of the optical fiber SPR sensor was analyzed. Next, the influence of the type of metal, the thickness of the metal film, and the length of sensing area upon the SPR spectrum were simulated and compared. Based on these measurements, a reflective and highly-sensitive refractive index sensor was fabricated and experimentally demonstrated. Finally, a dual-parameter SPR sensor for the simultaneous measurement of seawater salinity and temperature was proposed by coating polydimethylsiloxane (PDMS) on half of the sensor as a temperature sensitive material, whose sensitivities were demonstrated to be 0.31 nm/‰ and -2.02 nm/°C, respectively. The proposed sensor not only resolves the cross-sensitivity problem between salinity and temperature, but also provides a simple structure with low cost and high sensitivity.

Highly-sensitive and reflective glucose sensor based on optical fiber surface plasmon resonance

A reflective optical sensor based on surface plasmon resonance (SPR) was proposed to measure the glucose concentration. The gold film was sputtered on the surface of plastic cladding optical fiber to excite SPR and reflect light at the same time, making the SPR sensor highly sensitive to the variation of surrounding refractive index. Then, glucose oxidase (GOD) was adopted as the sensitive film of glucose and covalently bonded on the gold film. The enzymatic reaction between the GOD and the glucose led to the change of refractive index around the optical fiber sensor, and then cause a shift of the SPR spectrum. Therefore, the glucose concentration could be obtained by monitoring the shift of the resonance wavelength. Experimental results showed that the measurement sensitivity of the SPR sensor could reach 85.4 nm/(mg/mL) (for glucose concentration) with good selectivity and stability, which has great potential application in biomedicine and human health monitoring.

The temperature responsive mechanism of fiber surface plasmon resonance sensor

The temperature responsive mechanism of the optical fiber Surface Plasmon Resonance (SPR) sensor is analyzed. It is found that, both the gold film and the fiber core change with increasing temperature and affect the temperature sensitivity of the SPR sensor, but the main factor of the intrinsic temperature sensitivity of the sensor is the temperature dependent SPR response of the gold coating. The gold film plasma frequency plays a critical role in the temperature response mechanism of the sensor. Experimental and simulation results show that the SPR wavelength shift due to the change of the gold film temperature contributes 19.6 % to the total, as the sensor surrounded with ethanol and the environment temperature increases from 0°C to 50°C. The reason is that the plasma frequency of the gold film rises significantly as the external temperature increases. The rising rate of plasma frequency with temperature is related to the morphology and the deposition method of the gold film. This result has provided a novel insight on the physical properties of optical fiber SPR sensors and has improved the capability of SPR sensors for measuring signals in a temperature-changing environment.

Surface plasmon resonance‐based fiber optic sensor prepared from bilayers of indium tin oxide‐indium oxide

AbstractExperimental investigations on fiber optic surface plasmon resonance (SPR) sensor with ITO (indium tin oxide)‐IO (indium oxide) layers are reported. Sensitivity ascends with increase in IO layer thickness till a definite thickness and begins descending with further enhancement in IO layer thickness. Maximum rise in sensitivity is observed for sensor fabricated from 40 nm ITO to 30 nm IO layers.

On the role of different 2D materials/heterostructures in fiber-optic SPR humidity sensor in visible spectral region

In this paper, excellent light harvesting ability of plasmonic structures based on metal-2D material junction has been explored for simulation and analysis of fiber optic relative humidity sensing. The influence of various 2D materials/heterostructure has been analyzed. Angular interrogation and intensity interrogation method have been used. The graphene-based sensor probe shows the highest CPF (171.11 dB/%RH). Further, with intensity interrogation method, the limit of detection (LOD) achievable with graphene-based and WS2-based sensor probes is as fine as 6.8 × 10−4 %RH whereas the LOD for angular interrogation method is 4.1 × 10−3 %RH.

Beta-cyclodextrin based reflective fiber-optic SPR sensor for highly-sensitive detection of cholesterol concentration

Abstract A fiber-optic surface plasmon resonance (SPR) cholesterol sensor is proposed and demonstrated. The sensor adopts a reflective structure and uses a gold film to excite the SPR. Through esterification reaction, the beta-cyclodextrin is fixed on the gold film of the fiber-optic sensor. When the cholesterol concentration changes, the sensitive membrane structure changes and the resonant wavelength of the SPR sensor generates a red shift, so the cholesterol concentration can be measured through monitoring the wavelength shift of the SPR resonance spectrum. Experiment results showed that when the cholesterol concentration was 0 μM–0.05 μM, the shift of the resonance wavelength was linear with the change of cholesterol concentration with high sensitivity of 156.18 nm/μM. The sensor is simple, green and sensitive, and has good potential applications in biomedicine.

Au nanoparticles as label-free competitive reporters for sensitivity enhanced fiber-optic SPR heparin sensor

A label-free Au NPs-enhanced surface plasmon resonance (SPR) sensor was developed for the ultrasensitive detection of heparin based on competitive adsorption behavior of heparin and Au NPs on the poly (dimethyl-diallylammonium chloride) (PDDA)-modified optical fiber surface and the corresponding change in the resonance wavelength of SPR. Due to the high affinity between heparin and PDDA, the present senor shows good analytical performance with respect to heparin detection. Two obvious advantages of the proposed heparin sensor over other reported methods are: its much wider linear concentration range (10−6-10−10 g/mL) and lower limit of detection (0.0257 ng/mL). The analysis of heparin in serum demonstrated that the present sensor exhibited high sensitivity and selectivity. It should be noted that the sensing strategy takes advantage of a portable fiber-optic SPR sensing system and avoids the need for complex processes for labeled-Au NPs, and thus the present sensor promises to be a practical tool for the point-of-care monitoring of heparin.