Tuning Dynamic Range and Sensitivity of White-Light, Multimode, Fiber-Optic Surface Plasmon Resonance Sensors

Optical fiber biosensors toward in vivo detection

This review takes stock of the various optical fiber-based biosensors that could be used for in vivo applications. We discuss the characteristics that biosensors must have to be suitable for such applications and the corresponding transduction modes. In particular, we focus on optical fiber biosensors based on fluorescence, evanescent wave, plasmonics, interferometry, and Raman phenomenon. The operational principles, implemented solutions, and performances are described and debated. The different sensing configurations, such as the side- and tip-based fiber biosensors, are illustrated, and their adaptation for in vivo measurements is discussed. The required implementation of multiplexed biosensing on optical fibers is shown. In particular, the use of multi-fiber assemblies, one of the most optimal configurations for multiplexed detection, is discussed. Different possibilities for multiple localized functionalizations on optical fibers are presented. A final section is devoted to the practical in vivo use of fiber-based biosensors, covering regulatory, sterilization, and packaging aspects. Finally, the trends and required improvements in this promising and emerging field are analyzed and discussed.

Plasmonic fiber-optic sensing system for in situ monitoring the capacitance and temperature of supercapacitors

Compared with ex situ measurement, the in situ measurement is more suitable for inspecting complex electrochemical reactions and improving the intelligent energy storage management. However, most of the in situ investigation instruments are bulky and expensive. Here we demonstrate a miniaturized, portable, and low-cost fiber-optic sensing system for in situ monitoring the capacitance and temperature. It can help evaluate the self-discharge rate in supercapacitors (SCs). The fiber-optic sensing system with two probes are implanted inside the SCs to monitor the capacitance and temperature, respectively. The dual fiber-optic probes can work independently and avoid cross-interference through structure design. The fiber-optic localized surface plasmon resonance (LSPR) probe near the electrode surface can detect the capacitance in real-time by monitoring ion aggregation on the opposite electrode. The fiber-optic surface plasmon resonance (SPR) probe encapsulated in the thermosensitive liquid can independently detect the temperature change. The measurement uncertainties of the two sensing probes are 5.6 mF and 0.08 ℃, respectively. The proposed tiny and flexible fiber-optic sensing system provides a promising method for in situ monitoring the critical parameters. It is also a powerful tool for investigating electrochemical reactions in various energy storage devices.

HER2 breast cancer biomarker detection using a sandwich optical fiber assay

Optical fiber-based surface plasmon resonance (OF-SPR) sensors have demonstrated high versatility and performances over the last years, which propelled the technique to the heart of numerous and original biosensing concepts. In this work, we contribute to this effort and present our recent findings about the detection of breast cancer HER2 biomarkers through OF-SPR optrodes. 1 cm-long sections of 400 μm core-diameter optical fibers were covered with a sputtered gold film, yielding enhanced sensitivity to surface refractive index changes. Studying the impacts of the gold film thickness on the plasmonic spectral response, we improved the quality and reproducibility of the sensors. These achievements were correlated in two ways, using both the central wavelengths of the plasmon resonance and its influence on the bulk refractive index sensitivity. Our dataset was fed by additional biosensing experiments with a direct and indirect approach, relying on aptamers and antibodies specifically implemented in a sandwich layout. HER2 biomarkers were specifically detected at 0.6 μg/mL (5.16 nM) in label-free while the amplification with HER2-antibodies provided a nearly hundredfold signal magnification, reaching 9.3 ng/mL (77.4 pM). We believe that these results harbinger the way for their further use in biomedical samples.

Hydrogen peroxide and glucose concentration measurement using optical fiber grating sensors with corrodible plasmonic nanocoatings

We propose and demonstrate hydrogen peroxide (H₂O₂) and glucose concentration measurements using a plasmonic optical fiber sensor. The sensor utilizes a tilted fiber Bragg grating (TFBG) written in standard single mode communication fiber. The fiber is over coated with an nm-scale film of silver that supports surface plasmon resonances (SPRs). Such a tilted grating SPR structure provides a high density of narrow spectral resonances (Q-factor about 10⁵) that overlap with the broader absorption band of the surface plasmon waves in the silver film, thereby providing an accurate tool to measure small shifts of the plasmon resonance frequencies. The H₂O₂ to be detected acts as an oxidant to etch the silver film, which has the effect of gradually decreasing the SPR attenuation. The etching rate of the silver film shows a clear relationship with the H₂O₂ concentration so that monitoring the progressively increasing attenuation of a selected surface plasmon resonance over a few minutes enables us to measure the H₂O₂ concentration with a limit of detection of 0.2 μM. Furthermore, the proposed method can be applied to the determination of glucose in human serum for a concentration range from 0 to 12 mM (within the physiological range of 3-8 mM) by monitoring the H₂O₂ produced by an enzymatic oxidation process. The sensor does not require accurate temperature control because of the inherent temperature insensitivity of TFBG devices referenced to the core mode resonance. A gold mirror coated on the fiber allows the sensor to work in reflection, which will facilitate the integration of the sensor with a hypodermic needle for in vitro measurements. The present study shows that Ag-coated TFBG-SPR can be applied as a promising type of sensing probe for optical detection of H₂O₂ and glucose detection in human serum.

Simple method for self-referenced and lable-free biosensing by using a capillary sensing element

We demonstrated a simple method for self-reference and label free biosensing based on a capillary sensing element and common optoelectronic devices. The capillary sensing element is illuminated by a light-emitting diode (LED) light source and detected by a webcam. Part of gold film that deposited on the tubing wall is functionalized to carry on the biological information in the excited SPR modes. The end face of the capillary was monitored and separate regions of interest (ROIs) were selected as the measurement channel and the reference channel. In the ROIs, the biological information can be accurately extracted from the image by simple image processing. Moreover, temperature fluctuation, bulk RI fluctuation, light source fluctuation and other factors can be effectively compensated during detection. Our biosensing device has a sensitivity of 1145%/RIU and a resolution better than 5.287 × 10^-4 RIU, considering a 0.79% noise level. We apply it for concanavalin A (Con A) biological measurement, which has an approximately linear response to the specific analyte concentration. This simple method provides a new approach for multichannel SPR sensing and reference-compensated calibration of SPR signal for label-free detection.

Localized Surface Plasmon Resonance (LSPR)-Coupled Fiber-Optic Nanoprobe for the Detection of Protein Biomarkers

Here is presented a miniaturized, fiber-optic (FO) nanoprobe biosensor based on the localized surface plasmon resonance (LSPR) at the reusable dielectric-metallic hybrid interface with a robust, gold nano-disk array at the fiber end facet. The nanodisk array is directly fabricated using electron beam lithography (EBL) and metal lift-off process. The free prostate-specific antigen (f-PSA) has been detected with a mouse anti-human prostate-specific antigen (PSA) monoclonal antibody (mAb) as a specific receptor linked with a self-assembled monolayer (SAM) at the LSPR-FO facet surfaces. Experimental investigation and data analysis found near field refractive index (RI) sensitivity at ~226 nm/RIU with the LSPR-FO nanoprobe, and demonstrated the lowest limit of detection (LOD) at 100 fg/mL (~3 fM) of f-PSA in PBS solutions. The SAM shows insignificant nonspecific binding to the target biomarkers in the solution. The control experimentation using 5 mg/mL bovine serum albumin in PBS and nonspecific surface test shows the excellent specificity and selectivity in the detection of f-PSA in PBS. These results indicate important progress toward a miniaturized, multifunctional fiber-optic technology that integrates informational communication and sensing function for developing a high-performance, label-free, point-of-care (POC) device.

Dual-channel fiber surface plasmon resonance biological sensor based on a hybrid interrogation of intensity and wavelength modulation

We investigate an intensity and wavelength modulation combined plasmon resonance-based fiber-optic sensor technology. Composed of gold nanoparticles (GNPs) and sandwich configuration of Au/indium tin oxide (ITO)/Au film, two sensing regions are fabricated separately along with unclad portions of the fiber-optic probe. It can simultaneously monitor both the light intensity from the Au NP channel and the wavelength from the Au/ITO/Au film channel with a single detector. As the refractive index (RI) of the external environment changes, the transmission intensity and resonance wavelength in the two channels are modified, which provides an interrogation of intensity and wavelength modulation. The sandwich film structure is formed using magnetron sputtering technology, and the GNPs functioning as localized surface plasmon resonators are coated on a multimode optical fiber via the layer-by-layer method. The experimental results reveal that the RI sensitivities of the two sensing channels are 334.1% RIU?1 and 1963.2??nm/RIU, respectively. Based on the above sensing design, we conduct real-time and label-free monitoring of IgG/anti-IgG and Con A/RNase B biomolecular interaction. The resonant dips excited by different sensing modes make it more attractive as a multichannel surface plasmon resonance analysis technology, which is valuable in biological and life sciences research and rapid diagnostics.

Dual-truncated-cone structure for quasi-distributed multichannel fiber surface plasmon resonance sensor

We propose and demonstrate an effective method to adjust the dynamic range of a fiber surface plasmon resonance (SPR) sensor by introducing a multimode fiber-sensing probe with a dual-truncated-cone (DTC) structure. When the grind angle of the DTC structure increases, the dynamic range redshifts. Based on this result, we fabricate a quasi-distributed two-channel multimode fiber SPR sensor by cascaded-connecting a DTC-sensing probe of 14° grind angle and a traditional transmitted multimode fiber (TMF)-sensing probe in the same fiber. The corresponding sensitivities of two sensing probes are 3423.08 nm/RIU and 2288.46 nm/RIU. By using this quasi-distributed multichannel fiber SPR-sensing approach, we may improve the detecting accuracy by extracting, calibrating, and compensating for the signals caused by the nonspecific bindings, other physical absorptions, and temperature changes in detecting samples, truly achieving dynamic detection in real-time. The excellence of this multichannel fiber SPR sensor is that the sensitivity of each subchannel-sensing probe stays unreduced after it is cascaded-connected in the main-channel fiber; the sensor is based on the multimode fiber, which is inexpensive, accessible, and convenient to be universalized in applications.

Curvature effects on flexible surface plasmon resonance biosensing: segmented-wave analysis

We investigate the effect of surface curvature on characteristics of flexible surface plasmon resonance biosensors. For simplified analysis, segmentation-based approximation of curved substrates has been conducted in a range of curvature radius |r| > 225 μm in the parallel and perpendicular light incidence with respect to the surface. The results suggest that resonance characteristics in general broaden with increased curvature due to larger momentum dispersion, the effect of which appears more prominent and direct in the parallel light incidence. Resonance shifts as a result of biosensing, such as DNA immobilization and hybridization, overall decrease with curvature and perpendicular incidence is more robust with a curvature change. The approach was extended to multi-curvature structure and finds significant fluctuation of resonance shift for parallel light incidence. The study can be of profound importance for plasmonic devices using flexible substrates and in fiber-based in vivo applications.

Distributed fiber surface plasmon resonance sensor based on the incident angle adjusting method

We propose and demonstrate a distributed surface plasmon resonance (SPR) fiber sensor based on a novel, simple, and effective incident angle adjusting method. For normal fiber SPR sensors, it is hard to realize distributed sensing because it is hard to produce two dynamic ranges (resonance wavebands) with a great difference. The dynamic range depends on the incident angle, and therefore, we propose an incident angle adjusting method that is implemented by grinding an eccentric-core fiber to different angles, which helps to produce different SPR wavebands with great difference, thus realizing distributed sensing. In our two cascaded distributed configuration, with the refractive index range of 1.333-1.385, the fiber grind angles are 9° and 17°, the testing wavelength ranges are 613-760 nm and 745-944 nm, and the average testing sensitivities are 2826 nm/RIU and 4738 nm/RIU, respectively. Larger resonance wavelengths are associated with larger testing sensitivities. This distributed fiber sensor has important significance in the fields of multichannel liquid refractive indices and temperature self-reference measurements.

Compact distributed fiber SPR sensor based on TDM and WDM technology

By using a twin-core fiber (TCF), we propose and demonstrate a novel distributed SPR sensor, which employs both the time division multiplexing (TDM) technology and the wavelength division multiplexing (WDM) technology together. The proposed sensor has two sensing passages with four sensing channels (and there are two sensing channels in each sensing passage). We employ the TDM technology to realize the two passage distributed sensing, which are parallel-connection; and we employ the WDM technology to realize the distributed sensing of two channels in a sensing passage, which are series-connected. In order to realize the TDM technology, we employ a two-core fiber, which has two cores in a same cladding, being equal to dividing the traditional single core into two independent sensing zones; in order to realize the WDM technology, we employ a fiber end polishing-connecting method to adjust the resonance angle/wavelength to realize the dynamic range separation. This twin-passage four-channel twin-core fiber SPR sensor is suitable for applying in fields of the multi-channel liquid refractive index and temperature self-reference measurement.

Surface Plasmon Resonance Biosensor Based on Smart Phone Platforms

We demonstrate a fiber optic surface plasmon resonance (SPR) biosensor based on smart phone platforms. The light-weight optical components and sensing element are connected by optical fibers on a phone case. This SPR adaptor can be conveniently installed or removed from smart phones. The measurement, control and reference channels are illuminated by the light entering the lead-in fibers from the phone’s LED flash, while the light from the end faces of the lead-out fibers is detected by the phone’s camera. The SPR-sensing element is fabricated by a light-guiding silica capillary that is stripped off its cladding and coated with 50-nm gold film. Utilizing a smart application to extract the light intensity information from the camera images, the light intensities of each channel are recorded every 0.5 s with refractive index (RI) changes. The performance of the smart phone-based SPR platform for accurate and repeatable measurements was evaluated by detecting different concentrations of antibody binding to a functionalized sensing element, and the experiment results were validated through contrast experiments with a commercial SPR instrument. This cost-effective and portable SPR biosensor based on smart phones has many applications, such as medicine, health and environmental monitoring.

Smart design of fiber optic surfaces for improved plasmonic biosensing

Although the phenomenon of surface plasmon resonance (SPR) is known for more than a century now, traditional prism-based SPR platforms have hardly escaped the research laboratories despite being recognized for the sensitive and specific performance. Significant efforts have been made over the last years to overcome their existing limitations by coupling the SPR phenomenon to the fiber optic (FO) technology. While this platform has been promoted as cost-effective and simpler alternative capable of handling label-free bioassays, quantification and real-time monitoring of biomolecular interactions, examples of its applicability in sensing and biosensing remain to date very limited. The FO-SPR system is still in development and requires further advancements for reaching the stability and sensitivity of the benchmark SPR systems. Among existing strategies for device improvement, those based on modifying the FO tips using nanomaterials are mostly studied. These small-scale objects provide a wide range of possibilities for alternating the architecture of the FO sensitive zone, enabling also unique effects such as localized SPR (LSPR). This mini-review summarizes the latest innovations in the fabrication procedures which use nanoparticles or other nanomaterials, aiming at FO-SPR technology performance improvements, as well as addition of new device features and functionalities.

Surface plasmon resonance properties of silver nanoparticle 2D sheets on metal gratings

Grating-coupled propagating surface plasmons associated with silver-nanoparticle 2D crystalline sheets exhibit sensitive plasmonic resonance tuning. Multilayered silver-nanoparticle 2D crystalline sheets are fabricated on gold or silver grating surfaces by the Langmuir- Blodgett method. We show that the deposition of Ag crystalline nanosheets on Au or Ag grating surfaces causes a drastic change in propagating surface plasmon resonance (SPR) both in angle measurements at fixed wavelengths and in fixed incident-angle mode by irradiation of white light. The dielectric constant of the multilayered silver nanosheet is estimated by a rigorous coupled-wave analysis. We find that the dielectric constant drastically increases as the number of silver-nanosheet layers increases. The experimentally obtained SP dispersions of Ag crystalline nanosheets on Au and Ag gratings are compared with the calculated SP dispersion curves. The drastic change in the surface plasmon resonance caused by the deposition of Ag-nanoparticle 2D crystalline sheets on metal grating surfaces suggests the potential for applications in highly sensitive sensors or for plasmonic devices requiring greatly enhanced electric fields.

Evanescent wave assisted nanomaterial coating

In this work we present a novel nanomaterial coating technique using evanescent wave (EW). The gradient force in the EW is used as an optical tweezer for tweezing and self-assembling nanoparticles on the source of EW. As a proof of the concept, we have used a laser coupled etched multimode optical fiber, which generates EW for the EW assisted coating. The section-wise etched multimode optical fiber is horizontally and superficially dipped into a silver/gold nanoparticles solution while the laser is switched on. The fiber is left until the solution recedes due to evaporation leaving the fiber in air. The coating time usually takes 40-50 min at room temperature. The scanning electron microscope image shows uniform and thin coating of self-assembled nanoparticles due to EW around the etched section. A coating thickness <200 nm is achieved. The technique could be useful for making surface-plasmon-resonance-based optical fiber probes and other plasmonic circuits.

Fiber-optic waveguide coupled surface plasmon resonance sensor

A novel approach to give an excellent tunability and self-referencing capability was presented by applying a concept of waveguide coupled surface plasmon resonance mode to a fiber-optic sensor. The presence of dielectric waveguide sandwiched between two metal layers made it possible to precisely tune the resonance wavelength in a broad range from visible to infrared region and to generate multiple modes which may be selectively used for suitable applications. Our approach also verified the potential capability of self-referencing based on a remarkable difference in sensitivity between the plasmonic and waveguide modes excited by p- and s-polarized lights, respectively, without using an additional reference channel. Experimental measurement carried out on sucrose solutions with varying concentration demonstrated the feasibility of our approach.

Integration of fiber optic-particle plasmon resonance biosensor with microfluidic chip

This article reports the integration of the fiber optic-particle plasmon resonance (FO-PPR) biosensor with a microfluidic chip to reduce response time and improve detection limit. The microfluidic chip made of poly(methyl methacrylate) had a flow-channel of dimensions 4.0 cm × 900 μm × 900 μm. A partially unclad optical fiber with gold or silver nanoparticles on the core surface was placed within the flow-channel, where the volume of the flow space was about 14 μL. Results using sucrose solutions of various refractive indexes show that the refractive index resolution improves by 2.4-fold in the microfluidic system. The microfluidic chip is capable of delivering a precise amount of biological samples to the detection area without sample dilution. Several receptor/analyte pairs were chosen to examine the biosensing capability of the integrated platform: biotin/streptavidin, biotin/anti-biotin, DNP/anti-DNP, OVA/anti-OVA, and anti-MMP-3/MMP-3. Results show that the response time to achieve equilibrium can be shortened from several thousand seconds in a conventional liquid cell to several hundred seconds in a microfluidic flow-cell. In addition, the detection limit also improves by about one order of magnitude. Furthermore, the normalization by using the relative change of transmission response as the sensor output alleviate the demand on precise optical alignment, resulting in reasonably good chip-to-chip measurement reproducibility.

Lab-on-a-chip based immunosensor principles and technologies for the detection of cardiac biomarkers: a review

This review examines the current state of the art lab-on-a-chip and microfluidic based biosensor technologies used in the detection of cardiac biomarkers. The determination and quantification of blood based, cardiac biomarkers are crucial in the triage and management of a range of cardiac related conditions, where time delay has a major impact on short and longer-term outcomes of a patient. The design and manufacturing of biomarker detection systems are multi-disciplinary in nature and require researchers to have knowledge of both life sciences and engineering for the full potential of this field to be realised. This review will therefore provide a comprehensive overview of chip based immunosensing technology as applied to cardiac biomarker detection, while discussing the potential suitability and limitations of each configuration for incorporation within a clinical diagnostics device suitable for point-of-care applications.

Sensor properties and surface characterization of silver-deposited SPR optical fibers

The response curves and sensor properties of silver-deposited optical fibers with Ag film thicknesses of 20.0 - 80.0 nm based on surface plasmon resonance (SPR) were investigated. The response of the Ag-deposited optical fiber sensor depends on the thickness of the Ag film. The Ag-deposited optical fiber sensors show higher responses than those deposited with Au. The reflection properties of Ag films with thicknesses of 30.5 - 70.2 nm due to the SPR phenomenon were also measured and considered. The surfaces of these Ag films consist of various spherical grains with diameters of 30 - 90 nm and the surface height distribution is almost random, having a value of more than 8 nm. X-ray photoelectron spectroscopy (XPS) showed the presence of very thin (0.3 nm) native oxide layers on the Ag films. The Ag-deposited optical fiber sensor exhibited no change in the sensor properties following prolonged use for 4 months. The response curves of the Ag-deposited optical fiber sensors by use of SPR theoretical equations were calculated and compared with those obtained by experimentation.

Design and optimization of a novel surface plasmon resonance biosensor based on Otto configuration

A theoretical study on a novel planar waveguide surface Plasmon Biosensor is presented in this paper. The proposed biosensor has a configuration similar to the Otto excitation mechanism for surface Plasmon polaritons. The performance of the device with respect to key system parameters such as gap-width and device length is investigated using an eigenmode solver with perfectly matched layers (PML). Device resolution of 2.3 x 10(-6) RIU has been demonstrated for an aqueous analyte.

Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions

In this paper we present a fiber optic surface plasmon resonance (SPR) sensor as a reusable, cost-effective and label free biosensor for measuring DNA hybridization and DNA-protein interactions. This is the first paper that combines the concept of a fiber-based SPR system with DNA aptamer bioreceptors. The fibers were sputtered with a 50nm gold layer which was then covered with a protein repulsive self-assembled monolayer of mixed polyethylene glycol (PEG). Streptavidin was attached to the PEG's carboxyl groups to serve as a versatile binding element for biotinylated ssDNA. The ssDNA coated SPR fibers were first evaluated as a nucleic acid biosensor through a DNA-DNA hybridization assay for a random 37-mer ssDNA. This single stranded DNA showed a 15 nucleotides overlap with the receptor ssDNA on the SPR fiber. A linear calibration curve was observed in 0.5-5 microM range. A negative control test did not reveal any significant non-specific binding, and the biosensor was easily regenerated. In a second assay the fiber optic SPR biosensor was functionalized with ssDNA aptamers against human immunoglobulin E. Limits of detection (2nM) and quantification (6nM) in the low nanomolar range were observed. The presented biosensor was not only useful for DNA and protein quantification purposes, but also to reveal the binding kinetics occurring at the sensor surface. The dissociation constant between aptamer and hIgE was equal to 30.9+/-2.9nM. The observed kinetics fully comply with most data from the literature and were also confirmed by own control measurements.

Surface-plasmon resonance sensor using silica-core Bragg fiber

A silica-core Bragg fiber surface-plasmon resonance (SPR) sensor with high design flexibility has been developed for refractive index measurement. In this geometry, an HE11-like Bragg fiber core mode is designed to excite the surface plasmonic wave. It is shown that reduction of core size enables measurement of refractive index significantly lower than that of the silica core. This makes the device useful for biosensing or chemical sensing applications handling aqueous samples. The proposed sensor is fabricated using a unique rf-sputtering method, and SPR-induced extinction of more than 20 dB is observed.

Improved sensitivity and stability of amperometric enzyme microbiosensors by covalent attachment to gold electrodes

Covalent attachment of glucose oxidase to a pre-activated 16-mercaptohexadecanoic acid at a gold ultramicroelectrode surface improves sensitivity, stability, and reproducibility of enzyme-based amperometric microbiosensors. Self-assembled monolayers of the N-hydroxysuccinimide ester of 16-mercaptohexandecanoic acid (NHS-MHA) at gold electrodes enable spontaneous covalent linking of glucose oxidase to the gold surface of ultramicroelectrodes. By self-assembling NHS-MHA for 30 min, approximately 93% of the electrode surface is covered, thereby maximizing both the number of attachment sites for glucose oxidase, and sufficient diffusion of hydrogen peroxide to the gold electrode. The glucose oxidase reaction with NHS-MHA was optimized at pH of 6.5, and at a temperature of 43 degrees C, resulting in a surface concentration of 6.8+/-0.6 x 10(11) enzymemoleculescm(-2). Thus obtained amperometric microbiosensors were calibrated in the range of 1-10mM providing excellent correlation with the theoretical prediction of the microbiosensor response. The reported sensitivity of these microbiosensors documents an improvement by one order of magnitude compared to other approaches for covalent enzyme attachment. This is attributed to the NHS-MHA layer spacing the enzymatic recognition interface further from the electrode surface, thereby minimizing quenching of the enzyme activity.

Modeling the response function of dual-enzyme microbiosensors

A general theoretical model for competitive dual-enzyme microbiosensors based on self-assembled monolayers (SAM) is presented. The model is derived for amperometric dual-enzyme ATP sensors and provides excellent agreement with experimental ATP measurements at 25 microm diameter microelectrodes. In this model, the statistical probability of a glucose molecule in competition between two enzymes, glucose oxidase (GOD)/hexokinase (HEX), at the ATP sensor surface is combined with the enzymatic reaction rate. Thereby, a simple model predicting the sensor signal for varying surface concentrations of GOD and HEX, glucose concentration, and ATP concentration is obtained. Excellent agreement of the predicted current signal with experimentally obtained sensor signals was achieved at ATP concentrations between 10 and 300 microM in a buffer containing glucose at physiologically relevant levels. Consequently, the development time for new dual-enzyme biosensors can be reduced, and an analytical model for the sensor response function is provided facilitating the calibration of enzymatic biosensors.

Plasmon resonance microsensor for droplet analysis

Microscale fiber tip sensors based on the plasmon resonance are reported. The fabrication process derived from our previous approach for manufacturing near-field scanning optical microscopy probes has been optimized for sensing applications. A typical tip sensor is a tapered fiber 400 microm in length, coated with a nanoporous thin silver film. The miniaturized geometry of the sensor allows detection in a single droplet of liquid solution (approximately 20 microl). The tip sensor is sensitive for refractive indices between 1.33 and 1.40 with a sensitivity of at least 3 x 10(-4) refractive index unit (RIU)/nm. The Raman scattering enhancement through these microsensors demonstrates the important role played by the localized plasmon resonance. The sensors' linear response covers a large region, interesting for biosensing in aqueous environments such as biomedical applications.

Quantification of cytokines involved in wound healing using surface plasmon resonance

Sensing of three cytokines related to chronic wound healing, interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha), with detection limits at or below 1 ng/mL in buffered saline solution and spiked cell culture medium (CCM) has been achieved. Fiber-optic surface plasmon resonance (SPR) sensors are coated with an antibody binding layer and antibodies specific to the cytokine of interest are covalently attached to this layer. To achieve such detection limits in a complex medium such as CCM, total protein content of 4 mg/mL, the use of a novel N-hydroxysuccinimide ester of 16-mercaptohexadecanoic acid (NHS-MHA) is necessary. A comparison of the detection limits for IL-6 using currently widely used CM-dextran and NHS-MHA shows an improvement by a factor of 3 using NHS-MHA. The detection limits for the monitoring of cytokines in spiked saline solutions and CCM were similar for TNF-alpha and slightly higher for IL-1 and IL-6. The detection of each cytokine in the presence of interfering agents resulted in concentration prediction well within the error of calibration. The SPR sensors are stable in CCM after 20 min of pretreatment in CCM, minimizing the reliance on a reference sensor to quantify the cytokines in complex media. This technique enables a major advancement in the field of real-time monitoring of biologically relevant molecules in complex biological fluids.

Side-polished multimode fiber biosensor based on surface plasmon resonance with halogen light

A side-polished multimode fiber sensor based on surface plasmon resonance (SPR) as the transducing element with a halogen light source is proposed. The SPR fiber sensor is side polished until half the core is closed and coated with a 37 nm gold thin film by dc sputtering. The SPR curve on the optical spectrum is described by an optical spectrum analyzer and can sense a range of widths in wavelengths of SPR effects. The measurement system using the halogen light source is constructed for several real-time detections that are carried out for the measurement of the index liquid detections for the sensitivity analysis. The sensing fiber is demonstrated with a series of refractive index (RI) liquids and set for several experiments, including the stability, repeatability, and resolution calibration. The results for the halogen light source with the resolution of the measurement based on wavelength interrogation were 3 x 10(-6) refractive index units (RIUs). The SPR dip shifted in wavelength is used as a measure of the RI change at a surface, and this RI change varies directly with the number of biomolecules at the surface. The SPR dip shift in wavelength, which was hybridized at 0.1 microM of the target DNA to the probe DNA, was 8.66 nm. The all-fiber multimode SPR sensor, which has the advantages of being low cost, being disposable, having high stability and linearity, being free of labeling, and having potential for real-time detection, permit the sensor and system to be used in biochemical sensing and environmental monitoring.

Quantitative Measurement of Cardiac Markers in Undiluted Serum

Two mycocardial infarction biomarkers, myoglobin (MG) and cardiac troponin I (cTnI), were quantified at biological levels and in undiluted serum without sample pretreatment using surface plasmon resonance (SPR) sensors. To achieve detection of biomarkers in undiluted serum (72 mg/mL total protein concentration), minimization of the nonspecific signal from the serum protein was achieved by immobilizing the antibody for the biomarkers on an N-hydroxysuccinimide activated 16-mercaptohexadecanoic acid self-assembled monolayer. This monolayer reduces the nonspecific signal from serum proteins in such a manner that short exposure of the sensor to serum prior to analysis prevents any further nonspecific adsorption during analysis. Thus, sensing of MG and cTnI was achieved on the basis of the difference between signals from the active sensor and a reference sensor that captured background interference. This resulted in direct measurement of these biomarkers in undiluted serum. Detection limits for both markers were below 1 ng/mL, which is below the threshold needed to detect myocardial infarction. Detecting biomarkers in the low ng/mL range without signal amplification in such a complex matrix as serum corresponds to a selectivity of 108. The root-mean-square-error (RMSE) of calibration was below 2 ng/mL.

Reduction of nonspecific protein binding on surface plasmon resonance biosensors

Reduction of the nonspecific serum protein adsorption on a gold surface to levels low enough to allow the detection of biomarkers in complex media has been achieved using the N-hydroxysuccinimide (NHS) ester of 16-mercaptohexadecanoic acid. Carboxymethylated dextran (CM dextran), which is widely used, nonspecifically adsorbs enough proteins to mask the signal from target biomarkers in complex solutions such as serum or blood. The use of short-chain thiols greatly reduces the amount of nonspecific protein adsorption. Mixed layers of 11-mercaptoundecanoic acid or the NHS ester of 11-mercaptoundecanoic acid mixed layers with either 11-mercaptoundecanol or undecanethiol, and 16-mercaptohexadecanoic acid or the NHS ester of 16-mercaptohexadecanoic acid with hexadecanethiol, were also investigated for nonspecific protein binding properties as well as for biomarker signal response. The NHS ester of 16-mercaptohexadecanoic acid exhibits the largest signal for the biomarker myoglobin (including CM dextran) while offering a significantly diminished amount of nonspecific binding. The sensor has also been shown to detect interleukin-6 in cell culture media containing protein concentrations of at least 4 mg/mL.

Fiber-optic surface plasmon resonance sensors in the near-infrared spectral region

The sensitivity of fiber-optic surface plasmon resonance (SPR) sensors was improved by a factor of at least thirteen for aqueous solutions by modifying the tip geometry to allow interrogation of the surface plasmon (SP) band in the near-infrared (NIR) region. This was achieved by tuning the angle at the distal end of the SPR sensor to a dual taper of 71 degrees and 19 degrees . Using a low numerical aperture (NA) fiber-optic sensor, NA = 0.12, is necessary to obtain a functional SPR sensor working in the NIR region. Theoretical simulations using the Maxwell equations demonstrated that even higher enhancement is theoretically possible while maintaining a narrow spectral feature upon the excitation of the SP bands on gold surfaces. The manufacture of the SPR sensors yields good agreement between theoretical simulations and experimental observations. To investigate the properties of these fiber-optic SPR-NIR sensors, sucrose solutions ranging from 0 to 15 x 10(-3) in mole fraction were utilized. The increased sensitivity of the fiber-optic SPR sensors, when used to monitor biomarkers, would yield lower detection limits. The smaller sensing area, compared to planar or other fiber-optic SPR sensors, combined with an improvement of the sensitivity, would yield a dramatic reduction of the absolute amount detected by biosensors.

Investigation of dual-channel fiber-optic surface plasmon resonance sensing for biological applications

A dual-channel fiber-optic sensor based on surface plasmon resonance (SPR) for self-referencing refractive-index measurements has been proposed. Most applications of fiber-optic SPR sensors are designed to measure the refractive index of a liquid or gas sample by measuring the signal from a single surface, the sensitivity and stability of which is easily affected by the fluctuation of external environmental conditions. We have designed a dual-channel fiber-optic surface sensor with two independent SPR signals from two areas of the same probe. A prototype sensor was fabricated and characterized. The preliminary experimental results demonstrate the characteristic responses of both SPR signals from two channels that independently correspond to the refractive index changes in the liquid samples with which they are in contact. The design could be extended to a multichannel sensor with further developments. The experimental results confirmed that one channel can be used as a reference sensor that could compensate for unexpected changes in bulk refraction or temperature and develop this sensor as a practicable high-sensitivity biosensing device.

Tapered fiber optic surface plasmon resonance sensor for analyses of vapor and liquid phases

We report a new approach to analyze both vapor and liquid phases by utilizing a tapered fiber optic surface plasmon resonance (SPR) probe. This technique employs a fiber optic SPR probe with a modified geometry to tune the SPR coupling wavelength-angle pair. The observed composite spectrum included two distinct SPR dips associated with surface plasmons excited in the gas and liquid active regions. This sensor is able to detect refractive index changes in both vapor and liquid phases individually by simultaneous monitoring SPR coupling wavelengths from the two sensing surfaces.

Nondestructive monitoring of the photochromic state of dithienylethene monolayers by surface plasmon resonance

The use of surface plasmon resonance (SPR) as a nondestructive, nonerasing readout of the isomerization state of a photochromic dithienylethene covalently linked to a chemically modified gold surface was investigated. Four different binding layers were examined: 11-mercaptoundecanol (MUO), an amine-modified 11-mercaptoundecanol (MUO-NH2), dextran, and an amine-modified dextran. The binding of dithienylethene to the modified gold surface and photoisomerization of the photochrome in the bound state were established by FTIR. Solvent effects were measured for every layer tested using ethanol and hexanes. In general, large, easily measurable SPR signal changes could be detected under conditions where photoisomerization of the dithienylethene photochrome was not quenched by the gold plasmon, establishing SPR as a viable form of readout for potential dithienylethene-based optical data storage or processing devices. Dextran-bound photochrome in ethanol exhibited the largest SPR response upon photoisomerization, but is more prone to time-dependent fluctuations resulting from swelling of the dextran layer (caused by slow diffusion of the solvent) than the other layers. Large responses are also provided by MUO-NH2 and MUO, and the signal is much more stable than that for dextran.

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.

Optical-fiber surface-plasmon resonance sensor with multiple resonance peaks

We report on an optical fiber surface plasmon resonance sensor that exhibits multiple resonance peaks. The sensor is based on a uniform-waist single-mode tapered fiber coated on one side with a thin metal layer. Owing to the asymmetry of the sensor structure, the different hybrid surface plasmon modes supported by the semicircular layer can be excited by the fundamental fiber mode. As a result, the sensor transmission spectrum exhibits several dips that depend on the taper waist diameter. The advantages of a plasmon resonance sensor with multiple dips are discussed.