Traditional Surface Plasmon Resonance Sensors Research Articles

Concept, simulation, and fabrication of inverted grating structures for surface plasmon resonance sensors

Abstract. Surface plasmon resonance (SPR) sensors offer the possibility of label-free analysis of biosamples. The long-term stability of standard approaches is limited due to degradation of the grating upon contact with the analyte, and strategies to improve the performance in this regard include the use of so-called inverted configurations. By exciting surface plasmons from the back side of the grating, this design overcomes limitations of traditional grating-based SPR sensors caused by direct contact with the analyte medium and offers new design possibilities for implementing microfluidic analytical systems. Here a simulation for optimizing design parameters and a corresponding microfabrication technology to create the inverted grating are presented. An experimental evaluation of surface plasmon excitation and sensitivity enhancement demonstrates the feasibility of the approach. While the observed coupling effect of inverted grating structures is not as strong as the conventional configuration, it offers benefits like preventing surface oxidation, enabling repeated usage and new concepts for biosample processing.

Experimental study of SPR sensor performance enhancement by metal oxides

With the widespread application of fiber surface plasmon resonance (SPR) sensors in the field of biochemical sensing, especially in the detection of large molecules such as proteins, the sensitivity of traditional fiber SPR sensors has made it difficult to meet the requirements of detection accuracy in practical applications. This article proposes and proves an effective method for selecting the optimal metal oxide material to improve the sensing performance of SPR sensors. Metal oxides (TiO2 and ZnO) can improve the sensitivity by enhancing the evanescent field of fiber SPR phenomenon. Through simulation, it was found that when the metal oxide film of the composite film sensor maintains the same thickness, the metal oxide with a high effective refractive index (RI) has a better improvement effect on the sensitivity of the SPR sensor. Experimental results show that the maximum RI sensitivity of TiO2 composite film sensors and ZnO composite film sensors in the RI range of 1.333–1.420 is 15,200 nm/RIU and 13,600 nm/RIU, respectively. Compared to silver-based sensors, RI sensitivity has increased by 16.92 % and 4.62 %, respectively. To further verify the enhancement effect of different metal oxides on fiber sensors, another layer of PDMS film is deposited on TiO2 and ZnO films. Within the temperature sensing range, the maximum temperature sensitivity of these two composite film temperature sensors is 4.9 nm/°C and 4.3 nm/°C, respectively. Overall, metal oxides with high RI can improve the sensing performance of fiber optic sensors.

Giant and controllable Goos–Hänchen shift of a reflective beam off a hyperbolic metasurface of polar crystals

We conduct a theoretical analysis of the massive and tunable Goos–Hänchen (GH) shift on a polar crystal covered with periodical black phosphorus (BP)-patches in the THz range. The surface plasmon phonon polaritons (SPPPs), which are coupled by the surface phonon polaritons (SPhPs) and surface plasmon polaritons (SPPs), can greatly increase GH shifts. Based on the in-plane anisotropy of BP, two typical metasurface models are designed and investigated. An enormous GH shift of about −7565.58λ 0 is achieved by adjusting the physical parameters of the BP-patches. In the designed metasurface structure, the maximum sensitivity accompanying large GH shifts can reach about 6.43 × 108 λ 0/RIU, which is extremely sensitive to the size, carrier density, and layer number of BP. Compared with a traditional surface plasmon resonance sensor, the sensitivity is increased by at least two orders of magnitude. We believe that investigating metasurface-based SPPPs sensors could lead to high-sensitivity biochemical detection applications.

Sensitive monitoring of refractive index by surface plasmon resonance (SPR) with a gold α-iron (III) oxide thin film

In order to solve the problem that the optical fiber surface plasmon resonance (SPR) sensor is insufficiently sensitive for biomolecule detection, a novel sensing probe was proposed and designed, where the semiconductor material α-Fe2O3 was introduced for the first time. The α-Fe2O3 with high refractive index enhances the electric field and evanescent field depth at the interface between the α-Fe2O3 film and the ambient medium, resulting a sensitive performance. The characteristics of the sensors with gold film and α-Fe2O3 film at different thicknesses were analyzed and compared by finite element method. The thicker the α-Fe2O3 film, the higher the sensitivity. When the thickness of gold nanofilm is 50 nm and the thickness of α-Fe2O3 nanofilm is 20 nm, the highest figure of merit (FOM) is 50.98 RIU−1, corresponding to a sensitivity of 4800 nm/RIU, which is 2.16 times more sensitive than the traditional SPR sensor. In addition, three α-Fe2O3 film thickness-assisted SPR sensors were successfully prepared with the highest sensitivity of 4173.1 nm/RIU, which is nearly two times higher than without the α-Fe2O3 film. The enhancement after introducing α-Fe2O3 nanofilms and its promising application in the field of biosensing were confirmed through experiments and simulations.

The High-Sensitivity Phase Interrogation of Bloch Surface Wave Sensor Based on 1-D Photonic Crystal

The Bloch surface wave (BSW) is excited in a 1-D photonic crystal and supports a rather steep resonance dip, much sharper than that of traditional surface plasmon resonance (SPR). A highly sensitive sensor based on BSW was fabricated by electron beam evaporation, the structure was characterized by scanning electron microscopy (SEM) and ellipsometry spectrometer, and the sensitivity in wavelength and phase interrogation was achieved experimentally by an optical system based on a Mach–Zehnder interferometer. The wavelength sensitivity and the phase sensitivity can reach 219 nm/RIU and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.16\times10$ </tex-math></inline-formula> 4 deg/RIU, respectively. After comparison, we demonstrate that the phase interrogation has a higher sensitivity than the wavelength interrogation and is more suitable for BSW sensors. Experimental results show that the detection limit of the sensor is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.62\times10$ </tex-math></inline-formula> −7 RIU in phase interrogation, which proves that the sensor could become a valuable and promising tool to replace traditional SPR sensors for the application of ultralow concentration detection. This sensor, only composed of TiO2 and SiO2 materials, has the advantages of mechanical and chemical stability, easy fabrication, and low cost, which can be used as an alternative to traditional SPR sensors in the fields of biomedicine, chemical analysis, and environmental monitoring.

High-Sensitivity Triple-Channel Optical Fiber Surface Plasmon Resonance Sensor

Optical fiber surface plasmon resonance (SPR) sensors are widely used for refractive index (RI) sensing, thanks to their high sensitivity. However, with the increasing demand for multiple analytes detection and sensor integration, a traditional single-channel optical fiber SPR sensor is outdated. We propose and demonstrate a novel triple-channel optical fiber SPR sensor with high sensitivity. The sensor is fabricated with side-polished single-mode fiber (SMF), where the sensing film consists of a layer of gold nanofilm and a layer of TiO2 nanofilm. The resonance wavelength range of the SPR sensing zone can be adjusted significantly by changing the TiO2 nanofilm thickness. Experimentally, by cascading three sensing zones whose TiO2 nanofilm thicknesses are 10, 30, and 60 nm, respectively, a triple-channel SPR sensor is realized. The average sensitivities of three sensing channels reach up to 2637.8, 3962.2, and 6097.3 nm/RIU in the RI range of 1.333–1.370 RIU, which is superior to most current multichannel SPR sensors. Besides, three sensing channels are independent without crosstalk. The proposed triple-channel SPR sensor has great potential in biological and chemical sensing fields, especially in multiple analytes detection applications.

Tunable fano resonance-enhanced surface plasmon biosensor based on MXene/MoS2 heterostructure

Two-dimensional materials have unique physical properties such as high electrical conductivity, excellent light absorption, and hydrophilicity, thus have been widely used as responsive materials for surface plasmon effect sensors in recent years. Here, we propose a Kretschmann configuration-based MXenes/MoS2 heterostructure film material as an improved sensing layer for surface plasmon resonance (SPR) sensors and introduce an optical waveguide layer into the nanostructure to achieve a remarkable Fano resonance (FR). By separately adjusting the thickness and refractive index of the coupling dielectric and optical waveguide layers, the effective coupling between the surface plasmon polarization and the planar optical waveguide mode is achieved, which significantly improves the sensor's response performance to biomolecules and achieves high sensitivity (S, 86.13°/RIU) and better figure of merit (FOM, 281.74 RIU−1) in the visible red range (around 632.8 nm). Benefiting from the light absorption gain of the heterostructure and the FR effect, the FOM of the MXene/MoS2 Fano resonance sensor (FRS) is improved by 7.1 times compared to the traditional SPR sensor based on noble metal Au (50 nm thickness) thin film.

Theoretical modeling and investigations of lossy mode resonance prism sensor based on TiO2 film.

The optical sensor based on lossy mode resonance can overcome the limitations of traditional surface plasmon resonance sensors and work under TE and TM polarized light. In this paper, an LMR sensor theoretical model with the configuration of prism/matching layer/lossy layer/sensing layer is proposed, which is based on the principle of attenuated total reflection. By using TiO2 film as the lossy layer and LiF film as the matching layer, the resonance signal under angle interrogation is effectively improved. One of the advantages of the proposed sensor is that the detection range and detection accuracy are dynamically adjustable, which provides additional degrees of freedom in the design and use of the device. The structural parameters (film thickness, layer refractive index) affecting the resonance signal have been investigated based on the electric field distribution at resonance and the coupled mode theory. The LMR signal under TE and TM polarization can be switched by changing the thickness ratio of the matching layer and the lossy layer. All possible combinations of film thicknesses are given as a reference for the design of the LMR prism sensor based on TiO2 film. Under proper thickness combination, the proposed sensor is capable of detecting the medium with refractive index ranging from 1.32 ∼ 1.47, with a sensitivity range of 34 ∼ 148 °/RIU under angle interrogation and a maximum value of 192 RIU-1 for FOM under TM polarization. We hope these investigations can prove the advantages of LMR prism sensors and provide guidance for the experimental implementation of LMR prism sensors in the future.

Numerical Study to Enhance the Sensitivity of a Surface Plasmon Resonance Sensor with BlueP/WS2-Covered Al2O3-Nickel Nanofilms.

In the traditional surface plasmon resonance sensor, the sensitivity is calculated by the usage of angular interrogation. The proposed surface plasmon resonance (SPR) sensor uses a diamagnetic material (Al2O3), nickel (Ni), and two-dimensional (2D) BlueP/WS2 (blue phosphorous-tungsten di-sulfide). The Al2O3 sheet is sandwiched between silver (Ag) and nickel (Ni) films in the Kretschmann configuration. A mathematical simulation is performed to improve the sensitivity of an SPR sensor in the visible region at a frequency of 633 nm. The simulation results show that an upgraded sensitivity of 332°/RIU is achieved for the metallic arrangement consisting of 17 nm of Al2O3 and 4 nm of Ni in thickness for analyte refractive indices ranging from 1.330 to 1.335. The thickness variation of the layers plays a curial role in enhancing the performance of the SPR sensor. The thickness variation of the proposed configuration containing 20 nm of Al2O3 and 1 nm of Ni with a monolayer of 2D material BlueP/WS2 enhances the sensitivity to as high as 374°/RIU. Furthermore, it is found that the sensitivity can be altered and managed by means of altering the film portions of Ni and Al2O3

A novel doublet-based surface plasmon resonance biosensor via a digital Gaussian filter method

In this paper, a digital Gaussian filter coupled surface plasmon resonance (SPR) sensor with a doublet was proposed to enhance the sensing performance of conventional SPR sensor. The shift of central angle calculated from doublet was employed instead of the shift of resonance angle obtained from SPR curve. Expected angle and FWHM (full width at half maximum) are two parameters for the post-processing Gaussian model. The refractive index (RI) sensitivity has a symmetry relationship with expected angle and the reversed maximum sensitivity is usually larger than that of forward maximum. The reversed maximum sensitivity increases first and then decreases when increasing FWHM from 0.2 to 20 deg. Further, a dynamic and adjustable Gaussian model was employed to satisfy the large sensing range need. The RI sensitivity as high as 254.21 deg/RIU was obtained, which is nearly 2-fold higher than that of traditional gold-based SPR sensor, that is 131.58 deg/RIU. And thus, the RI resolution was calculated to be 7.16 × 10−6 RIU based on three-fold standard deviation theory. The LOD for rabbit IgG as low as 9.58 ng/mL was achieved and it is nearly 3-fold lower than that of traditional gold-based SPR biosensor, that is 30.64 ng/mL.

Sensitivity Enhancement of Surface Plasmon Resonance (SPR) Sensor Assisted by BlueP/MoS2 Based Composite Heterostructure

This paper addresses a very high sensitivity of surface plasmon resonance (SPR) sensor assisted by novel BlueP/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based composite heterostructure based on angular interrogation technique. Research on SPR techniques based on angular interrogation system is attracted a lot in recent years due to ease of implementation, robust, reliable and compatibility. These SPR sensors have wide applications in the field of biosensing, biochemical, gas detection, and biological science to mention a few. By these SPR techniques, performance parameters like figure of merit (F.O.M.), detection accuracy (D.A.), and most importantly sensitivity have been tremendously increased. In recent past 2D materials have attracted a massive response in SPR biosensors. Hence in this paper, 2D material like blue Phosphorene/molybdenum disulfide (i.e., BlueP/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) hybrid nanostructure as an integrating layer with the analyte is suggested to improve the sensitivity substantially of SPR sensor. In this connection performance of multilayer structure in conjunction with CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> prism, Ag metal layer, Black Phosphorus, BlueP/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has been analyzed in this paper. The maximum sensitivity of the order of 458°/RIU has been obtained in this proposed scheme attributed to BlueP/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based composite heterostructure at the visible wavelength of 662 nm. The proposed design analysis results in a significant improvement in sensitivity compared to traditional and graphene-based SPR sensors.

Facile layered structure of sensing chip of a prism–based surface plasmon resonance device

Surface plasmon resonance (SPR) is a popular optical phenomenon for sensing application owing to its capability for label-free measurement which offers very high resolution and sensitivity. The traditional prism based Kretschmann configuration employs a sensing chip consists of a glass substrate deposited with 50 nm thick gold/silver layer. The parameters of the figure-of-merit (FOM) of an SPR sensor are a quality factor, signal-to-noise ratio, and the strength of the plasmonic field over the gold surface. In this article, we present a simulation work on a facile SPR sensor chip that yields a high FOM as compared to the traditional SPR sensor chip. We introduced a metallic intermediate layer (IML) in the traditional SPR chip and optimized the performance with respect to the material and thickness of IML. We optimized the thickness of the glass substrate such that the plasmonic field generated in the IML can couple with the outer gold film (OGF). Such coupling enhances the overall plasmonic field over the OGF by 112%.

A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis

In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high refractive index silicon layer enhances the electric field on the surface of the sensor, so that the sensor has a better overall performance in terms of sensitivity and figure of merit (FOM). Compared with ordinary dielectrics, 2D materials have excellent photoelectric properties, such as larger specific surface area, higher carrier density and stronger adsorption capacity, which improve the detection ability of the sensor. The sensitivity of the optimized sensor achieves 297.2°/RIU, 274°/RIU and 246°/RIU when the silicon layer is covered with graphene, MXene (Ti3T2Cx) and MoS2, respectively. Compared with the traditional SPR sensor, the sensitivity of the structure has been significantly improved, and its excellent performance has broad application prospects in biosensing and other fields.

A Label-Free and Anti-Interference Dual-Channel SPR Fiber Optic Sensor With Self-Compensation for Biomarker Detection

In this work, we report a self-compensating, label-free, and anti-interference surface plasmon resonance (SPR) fiber biosensor based on a cascaded U-shaped multimode fiber and a single-mode fiber and both coated with gold film. In the traditional dual-channel SPR sensor structure, in order to obtain two independent SPR signals on one probe, different metal films need to be coated on two sensing channels. We proposed a U-shaped cascade dual-channel SPR optical fiber sensor, two channels are covered with gold film, and two independent SPR signals can be obtained by controlling the U-bend radius. In order to achieve self-compensating in detection of biomarkers, the U-shaped multimode fiber area of sensing probe is biologically functionalized as a detection channel, and the straight single-mode fiber without functionalized used as a reference channel. During the process of measuring biomarkers concentration, the existence of reference channel can be used to eliminate the effect of impurities' physical adsorption in the detection channel on the result. This sensor was used to detect the IgG concentration in complex samples, achieving the high precision detection with a limit of detection of $0.104\mu $ g/mL. A small-in-size, simple-to-manufacture, dual-channel SPR sensor with self-compensation was proposed and demonstrated in this article which has an excellent application prospect in the clinical monitoring of various biomarkers.

Self-referencing SPR biosensing with an ultralow limit-of-detection using long-wavelength excitation

There is an urgent need of label-free technique with low limit of detection (LOD) for biomedical research and early clinical diagnostics. Surface plasmon resonance (SPR) as a label-free approach has been exploited in DNA determination, but the LOD of most SPR sensors is inferior to labeling approaches. A novel approach for SPR biosensing with low LOD was reported in the present study. SPR for long wavelength excitation is theoretically and experimentally demonstrated to possess a high refractive index sensitivity of 11773.93 nm/RIU due to the long decay length of the electromagnetic field near the metal surface, which is optimized by an order of magnitude compared to traditional SPR sensor. Subsequently, self-referencing sensing strategy was employed to eliminate cross-sensitivity problem and further improve sensing accuracy significantly. Finally, we focused on using this sensor to measure DNA hybridization events using graphene oxide-assisted Au nanoparticle conjugates as signal amplification tags. It achieves a large dynamic range of 10−15-10-11 M and a low LOD of 0.2 fM for target DNA determination. The proposed SPR sensing architecture is technologically simple, highly sensitive and extensively appropriate for existing SPR instrumentation, so it has a great potential for low LOD of various biomolecules.

High-sensitive optoelectronic SPR biosensor based on Fano resonance in the integrated MIM junction and optical layers

Development of the miniaturized surface plasmon resonance (SPR) sensors with high sensitivity is required for biomedical sensing applications. In this regard, elimination of the optical detector and enhancement of sensitivity is an important step towards miniaturization of the traditional Kretschmann SPR sensors. In this study, we propose a compact high-sensitivity SPR biosensor with an electrical response based on Fano resonance in the metal–insulator–metal (MIM) junction integrated with the planar waveguide (PWG). The SPR excitation at the top metal of the MIM junction enhances the photoabsorption and hot electrons generation, which can produce photocurrent. We demonstrate that the coupling between the MIM junction SPR and PWG modes can result in modulation of output photocurrent by variation of analyte refractive index. The response of this sensor was calculated in detail and the results showed that the Fano-like profile appears in the photocurrent curve as well as the reflectivity curve. To evaluate the sensing performance of the proposed structure, the figure of merit (FOM) for the bulk sensitivity is predicated as 3.4×103 RIU−1, which is translated to change in the photocurrent of 33.5 μA by introducing the variation of 0.0001 in the refractive index of the analyte. Our results are highly useful for designing an integrated multi-function detector for future diagnoses and sensing applications.

A D-shaped fiber SPR sensor with a composite nanostructure of MoS2-graphene for glucose detection

Fiber-based techniques make it possible to implant a miniaturized and flexible surface plasmon resonance (SPR) sensor into the human body for glucose detection. However, the miniaturization of fiber SPR sensors results in low sensitivity compared with traditional prism-type SPR sensors due to limited sensing area. In this paper, we proposed a D-shaped fiber SPR sensor with a composite nanostructure of molybdenum disulfide (MoS2)-graphene to improve the sensor sensitivity. Compared with the traditional cylindrical fiber, the planar sensing area on the side-polished fiber makes it easier to modify two-dimensional materials. Chemical vapor deposition (CVD) graphene and CVD MoS2 were modified on the sensor surface to obtain the MoS2-graphene composite nanostructure. π-π stacking interactions were used to modify pyrene-1-boronic acid (PBA) on the graphene. The excellent photoelectric properties of the MoS2-graphene composite nanostructure and the ability of PBA to specifically bind glucose molecules improved the glucose detection performance of the SPR sensor. The results show that specific detection of glucose was realized and that the highest sensitivity was achieved with three-layer MoS2 and monolayer graphene.

Characterization and Sensing of Inert Gases with a High-Resolution SPR Sensor.

It is generally difficult to characterize inert gases through chemical reactions due to their inert chemical properties. The phase interference-sensing system based on high-resolution surface plasmon resonance (SPR) has an excellent refractive index detection limit. Based on this, this paper presents a simple and workable method for the characterization and detection of inert gases. The phase of light for the present SPR sensor is more sensitive to the change in the external dielectric environment than an amplitude SPR sensor. The limit of detection (LOD) is usually in the order of 10−6 to 10−7 RIU, which is superior to LSPR (Localized Surface Plasmon Resonance) sensors and traditional SPR sensors. The sensor parameters are simulated and optimized. Our simulation shows that a 36 nm-thick gold film is more suitable for the SPR sensing of inert gases. By periodically switching between the two inert gases, helium and argon, the resolution of the system is tested. The SPR sensing system can achieve distinguishable difference signals, enabling a clear distinction and characterization of helium and argon. The doping of argon in helium has a detection limit of 1098 ppm.

High-Sensitivity Detection of the Lung Cancer Biomarker CYFRA21-1 in Serum Samples Using a Carboxyl-MoS2 Functional Film for SPR-Based Immunosensors.

We constructed a novel surface plasmon resonance (SPR) detection assay using carboxyl-functionalized molybdenum disulfide (carboxyl-MoS2) nanocomposites as a signal amplification sensing film for the ultrasensitive detection of the lung cancer-associated biomarker cytokeratin 19 fragment (CYFRA21-1). The experiment succeeded in MoS2 reacted with chloroacetic acid giving carboxyl-MoS2 as the reaction product. The additional shoulder in the C 1s and O 1s peaks of carboxyl-MoS2, which were increased in X-ray photoelectron spectroscopy, confirmed the presence of O–C=O groups on the surface of the carboxyl-MoS2. Compared to MoS2, the experimental results confirmed that carboxyl-modified MoS2 had improved low impedance and low refractive index. The carboxyl-MoS2-based chip had a high affinity, with an SPR angle shift enhanced by 2.6-fold and affinity binding KA enhanced by 15-fold compared to a traditional SPR sensor. The results revealed that the carboxyl-MoS2-based chip had high sensitivity, specificity, and SPR signal affinity, while the CYFRA21-1 assay in spiked clinical serum showed a lower detection limit of 0.05 pg/mL and a wider quantitation range (0.05 pg/mL to 100 ng/mL). The carboxyl-MoS2-based chip detection value was about 104 times more sensitive than the limit of detection of an enzyme-linked immunosorbent assay (ELISA) (0.60 ng/mL). The results showed that the carboxyl-MoS2-based chip had the potential to rapidly assay complex samples including bodily fluids, whole blood, serum, plasma, urine, and saliva in SPR-based immunosensors to diagnose diseases including cancer.

High Sensitivity Fiber Optic SPR Refractive Index Sensor Based on Multimode-No-Core-Multimode Structure

In this paper, a surface plasmon resonance (SPR) fiber sensor based on a multimode-no-core-multimode (MNM) structure for refractive index (RI) measurement is proposed and experimentally investigated. The influence of the diameter and length of the no-core fiber on the performance of the sensor are studied. The results show that the sensitivity increases nonlinearly with RI in the range of 1.3330-1.4102, and a maximum sensitivity of 11792 nm/RIU is achieved. The maximum RI resolution of the sensor is 2.04 $\times 10^{\mathsf {-5}}$ RIU. Compared with traditional SPR sensors, the proposed sensor has the merits of low cost, easy fabrication, high sensitivity, and a wide measuring range. It can be an important tool in many chemical and biological applications.