Search Results (226)

Measuring pH is a critical parameter in environmental monitoring, biomedical diagnostics, food safety, and industrial processes. Optical fiber sensors have proven highly effective for pH detection due to their exceptional sensitivity, rapid response, and resistance to electromagnetic interference, making them well suited for real-time monitoring. This review offers a comprehensive analysis of recent advances in optical fiber-based pH sensors, covering key techniques such as fluorescence-based, absorbance-based, evanescent wave, and interferometric methods. Innovations in Fiber Bragg Grating and Surface Plasmon Resonance technologies are also examined. The discussion extends to the impact of pH-sensitive coatings—ranging from nanomaterials and polymeric films to graphene-based compounds—on enhancing sensor performance. Recent advancements have also enabled automation in data analysis and improvements in remote sensing capabilities. The review further compares the economic viability of optical fiber sensors with traditional electrochemical methods, while acknowledging persistent issues such as temperature cross-sensitivity, long-term stability, and fabrication costs. Overall, recent developments have broadened the functionality and application scope of these sensors by improving efficiency, accuracy, and scalability. Future research directions are outlined, including advanced optical interrogation techniques, such as Addressed Fiber Bragg Structures (AFBSs), microwave photonic integration, and optimized material selection. These approaches aim to enhance performance, reduce costs, and enable the broader adoption of optical fiber pH sensors. Full article

This article presents a comprehensive overview of recent advancements in photonic crystal fiber (PCF)-based sensors, with a particular focus on the surface plasmon resonance (SPR) phenomenon for biosensing. With their ability to modify core and cladding structures, PCFs offer exceptional control over light guidance, dispersion management, and light confinement, making them highly suitable for applications in refractive index (RI) sensing, biomedical imaging, and nonlinear optical phenomena such as fiber tapering and supercontinuum generation. SPR is a highly sensitive optical phenomenon, which is widely integrated with PCFs to enhance detection performance through strong plasmonic interactions at metal–dielectric interfaces. The combination of PCF and SPR technologies has led to the development of innovative sensor geometries, including D-shaped fibers, slotted-air-hole structures, and internal external metal coatings, each optimized for specific sensing goals. These PCF-SPR-based sensors have shown promising results in detecting biomolecular targets such as excess cholesterol, glucose, cancer cells, DNA, and proteins. Furthermore, this review provides an in-depth analysis of key design parameters, plasmonic materials, and sensor models used in PCF-SPR configurations, highlighting their comparative performance metrics and application prospects in medical diagnostics, environmental monitoring, and chemical analysis. Thus, an exhaustive analysis of various sensing parameters, plasmonic materials, and sensor models used in PCF-SPR sensors is presented and explored in this article. Full article

To enhance the sensing performance of fiber-optic magnetic field sensors, we explored the design, optimization, and application prospects of a D-type fiber-optic magnetic field sensor. This D-type PCF-SPR sensor is metal coated on one side (the metal used in this study is gold), which serves as the active metal for SPR and enhances structural stability. Magnetic fluid is applied on the outer side of the gold film for SPR magnetic field sensing. Six internal air holes arranged in a hexagonal shape form a central light transmission channel that facilitates the connection between the two modes, which are the sensor’s core mode and SPP mode, respectively. The outer six large air holes and two small air holes are arranged in a circular pattern to form the cladding, which allows for better energy transmission and reduces energy loss in the fiber. In this paper, the finite element method is employed to analyze the transmission performance of the sensor, focusing on the transmission mode. Guidelines for optimizing the PCF-SPR sensor are derived from analyzing the fiber optic sensor’s dispersion curve, the impact of surface plasmon excitation mode, and the core mode energy on sensing performance. After analyzing and optimizing the transmission mode and structural parameters, the optimized sensor achieves a magnetic field sensitivity of 18,500 pm/mT and a resolution of 54 nT. This performance is several orders of magnitude higher than most other sensors in terms of sensitivity and resolution. The SPR-PCF magnetic field sensor offers highly sensitive and accurate magnetic field measurements and shows promising applications in medical and industrial fields. Full article

This paper proposes a high-sensitivity U-shaped optical fiber sensor based on indium tin oxide (ITO) for surface plasmon resonance (SPR) sensing. Finite element simulations reveal that introducing ITO enhances the surface electric field strength by 1.15× compared to conventional designs, directly boosting sensitivity. The U-shaped structure optimizes evanescent wave–metal film interaction, further improving performance. In an external refractive index (RI) range of 1.334–1.374 RIU, the sensor achieves a sensitivity of 4333 nm/RIU (1.85× higher than traditional fiber sensors) and a figure of merit (FOM) of 21.7 RIU⁻¹ (1.68× improvement). Repeatability tests show a low relative standard deviation (RSD) of 0.4236% for RI measurements, with a maximum error of 0.00018 RIU, confirming excellent stability. The ITO coating’s strong adhesion ensures long-term reliability. With its simple structure, ease of fabrication, and superior sensitivity/FOM, this SPR sensor is well-suited for high-precision biochemical detection in intelligent sensing systems. Full article

This work presents a novel and comprehensive framework for optimizing fiber optic evanescent wave (FOEW) localized surface plasmon resonance (LSPR) sensors by investigating the unique interaction between evanescent waves and plasmonic nanoparticles. Unlike propagating light, the evanescent wave is a localized, non-propagating field that interacts exclusively with absorbing media near the fiber surface. This characteristic highlights the importance of prioritizing nanoparticle absorption over total extinction in FOEW sensor design. The optical response of silver nanoparticles was modeled across a size range of 10–100 nm, showing that absorption increases with particle number. Among the sizes tested, 30 nm silver nanoparticles exhibited the highest absorption efficiency, which was confirmed experimentally. An analytical adsorption kinetics model based on diffusion transport further predicted that smaller nanoparticles yield higher surface coverage, a result validated through atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. Refractive index (RI) sensitivity tests conducted on sensors fabricated with 10 nm, 20 nm, and 30 nm silver nanoparticles revealed that while smaller nanoparticles produced higher initial absorption due to greater surface density, the 30 nm particles ultimately provided superior RI sensitivity due to their enhanced absorption efficiency. These findings underscore the significance of absorption-centered nanoparticle design in maximizing FOEW LSPR sensor performance. Full article

This research presents a novel square-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor, designed using the external metal deposition (EMD) technique, for highly sensitive refractive index (RI) sensing applications. The proposed sensor operates effectively over an RI range of 1.33 to 1.37 and supports both x- polarized and y-polarized modes. It achieves a wavelength sensitivity of 15,800 nm/RIU and 14,300 nm/RIU, and amplitude sensitivities of 11,584 RIU⁻¹ and 11,007 RIU⁻¹, respectively, for the x-pol. and y-pol. The sensor also reports a resolution in the order of 10⁻⁶ RIU and a strong linearity of R² ≈ 0.97 for both polarization modes, indicating its potential for precision detection in complex sensing environments. Beyond the sensor’s structural and performance innovations, this work also explores the future integration of artificial intelligence (AI) into PCF-SPR sensor design. AI techniques such as machine learning and deep learning offer new pathways for sensor calibration, material optimization, and real-time adaptability, significantly enhancing sensor performance and reliability. The convergence of AI with photonic sensing not only opens doors to smart, self-calibrating platforms but also establishes a foundation for next-generation sensors capable of operating in dynamic and remote applications. Full article

This study proposes the development of a dual-wavelength optical fiber sensor (DWOFS) that integrates two optical fiber structures in a multimode transmission line to measure the refractive index and temperature of a liquid concurrently. One structure is based on a refractive index sensor that utilizes surface plasmon resonance, comprising a 5 mm long single-mode fiber (SMF) section coated with chromium/gold (Cr/Au) films. The secondary structure employs a multimode interferometer with a 29 mm long no-core fiber (NCF) section covered with a thick layer of polydimethylsiloxane (PDMS) to measure temperature. The measurements obtained reveal two distinct drops in the transmission spectrum at approximately 600 nm and 1550 nm, respectively, enabling precise measurement of the two parameters. The sensor demonstrates a high degree of sensitivity to both refractive index and temperature, spanning the visible ($2770.30\mathrm{n}\mathrm{m}/\mathrm{R}\mathrm{I}\mathrm{U}$) and infrared $(-0.178\mathrm{n}\mathrm{m}/°\mathrm{C}$) regions of the spectra, respectively. Furthermore, the thermo-optical coefficient for water $({-0.9928\times 10}^{-4}\mathrm{R}\mathrm{I}\mathrm{U}/°\mathrm{C})$ was estimated. The proposed sensor offers a compact solution for the simultaneous measurement of refractive index and temperature in liquid samples for a variety of applications, including biological, environmental, and healthcare research. Full article

This investigation presents an overview of various optical biosensors utilized for the detection of cancer cells. It covers a comprehensive range of technologies, including surface plasmon resonance $\left(SPR\right)$ sensors, which exploit changes in refractive index $\left(RI\right)$ at the sensor surface to detect biomolecular interactions. Localized surface plasmon resonance $\left(LSPR\right)$ sensors offer high sensitivity and versatility in detecting cancer biomarkers. Colorimetric sensors, based on color changes induced via specific biochemical reactions, provide a cost-effective and simple approach to cancer detection. Sensors based on fluorescence work using the light emitted from fluorescent molecules detect cancer-specific targets with specificity and high sensitivity. Photonics and waveguide sensors utilize optical waveguides to detect changes in light propagation, offering real-time and label-free detection of cancer biomarkers. Raman spectroscopy-based sensors utilize surface-enhanced Raman scattering $\left(SERS\right)$to provide molecular fingerprint information for cancer diagnosis. Lastly, fiber optic sensors offer flexibility and miniaturization, making them suitable for in vivo and point-of-care applications in cancer detection. This study provides insights into the principles, applications, and advancements of these optical biosensors in cancer diagnostics, highlighting their potential in improving early detection and patient outcomes. Full article

This research introduces a biosensor utilizing surface plasmon resonance in a photonic crystal fiber (PCF) configuration. PCF uses fused silica as the base material, with a layer of gold placed over the U-channels in the cross-section of the fiber to create a surface plasmon resonance. There are three different sizes of internal fiber optic air hole diameters, with a larger channel circle below the u-channel for the formation of an energy leakage window. COMSOL software 6.0 assisted us in tuning the fiber optic structure and performance for the study, and the structural parameters analyzed mainly include the channel circle diameter, the channel circle spacing, the profundity measurement of the polished layer, and the nanoscale size variation of metal films. The results of the simulation study show that the optical fiber sensor achieves refractive index (RI) responsiveness across the 1.30 to 1.41 range, and in the RI interval of 1.40 to 1.41, the sensor exhibits the largest resonance peak shift, and its highest sensitivity reaches 10,200 nm/RIU, and the smallest full width at half peak (FWHM) corresponds to the RI of 1.34 with a value of 4.8 nm, and the highest figure of merit (FOM) corresponds to the RI of 1.34 with a value of 895.83 (1/RIU). COMSOL 6.0 simulation software, was used to simulate the changes in blood refractive index corresponding to different glucose concentrations, and the detection performance of the sensor for different concentrations of glucose was tested. Then, the results show that the glucose concentration in 75 mg/dL–175 mg/dL with RI detection sensitivity is 3750 nm/RIU, where the maximum refractive index sensitivity is 5455 nm/RIU. It shows that the sensor can be applied in the field of biomedical applications, with its convenience, fast response, and high sensitivity, it has great potential and development prospect in the market. Full article

A high-sensitivity surface plasmon resonance (SPR) dual-parameter sensor based on photonic crystal fiber (PCF) is proposed for simultaneous measurement of magnetic field and temperature. The grooves on the right and upper sides of the PCF, serving as distinct detection channels, are filled with magnetic fluid and polydimethylsiloxane, respectively, enabling high-sensitivity detection of magnetic field and temperature. The structure parameters and sensing characteristics of the proposed sensor are investigated based on the finite element method. Numerical results indicate, within the wavelength range of 850–1050 nm, that the sensor achieves a high magnetic field sensitivity of 86 pm/Gs under x-polarization in the range of 100–600 Gs, and exhibits a temperature sensitivity of −2.63 nm/°C under y-polarization within the temperature range of 20–40 °C. Furthermore, the detection precision and applicability of the sensor in actual measurement applications could be further enhanced in the future by introducing surface-enhanced Raman scattering technology. Full article

A plastic optical fiber (POF)-based device for biosensing strategies has been developed to monitor several protein–protein interactions at ultra-low concentrations without functionalization processes, exploiting plasmonic phenomena. In this work, novel tests were applied to different kinds of analyte–receptor interactions, such as interleukins, where the bioreceptor’s (protein antibody) molecular weight is roughly ten times that of the analyte (protein interleukin), while intracellular bioreceptors and small molecules at low molecular weight interactions have already been demonstrated via the same point-of-care test (POCT). The POCT was implemented by a white light source and a spectrometer connected via two POF-based chips connected in series: an innovative microcuvette chip and a D-shaped POF surface plasmon resonance (SPR) probe. In particular, the POF microcuvette chip was achieved by drilling three micro holes in the core of a modified POF. Instead of performing a functionalization step, the micro holes were filled with a specific receptor solution for the analyte (one microliter at the femtomolar level), which selectively captured the target (e.g., cytokine) when the samples were dropped over the filled micro holes (twenty microliters at the attomolar level). Three interleukins, IL-1β, IL-17A, and IL-18, were detected in the attomolar concentrations range by monitoring the resonance wavelength shift over time due to the cytokine/antibody (protein–protein) interaction. The POF-based device was proven to be effective for detecting several interleukins at the attomolar level in a few minutes and without functionalization processes. Full article

The increasing concentration of antibiotics in natural water poses a significant threat to society’s sustainable development due to water pollution. Photocatalytic technology is an efficient and environmentally friendly approach to environmental purification, offering great potential for addressing pollution and attracting significant attention from scholars worldwide. TiO₂, as a representative semiconductor photocatalytic material, exhibits strong oxidation ability and excellent biocompatibility. However, its wide band gap and the rapid recombination of photo-generated electron–hole pairs significantly limit its photocatalytic applications. Recent studies indicate that constructing heterojunctions with synergistic plasmonic effects is an effective strategy for developing high-performance photocatalysts. In this study, Bi metal nanoparticles and (BiO)₂CO₃ nanosheets were simultaneously grown on TiO₂ nanofibers via an in situ hydrothermal method, successfully forming a Bi@(BiO)₂CO₃/TiO₂ composite fiber photocatalyst with synergistic plasmonic effects. The surface plasmon resonance (SPR) effect of Bi nanoparticles combined with the (BiO)₂CO₃/TiO₂ heterojunction enhances sunlight absorption, facilitates efficient separation of photo-generated carriers, and significantly strengthens the photo-oxidation and reduction abilities. This system effectively generates abundant hydroxyl (·OH) and superoxide (·O²⁻) radicals under sunlight excitation. Consequently, Bi@(BiO)₂CO₃/TiO₂ exhibited outstanding photocatalytic performance. Under simulated sunlight for 60 min, the photodegradation efficiencies of the quinolone antibiotics lomefloxacin, ciprofloxacin, and norfloxacin reached 93.2%, 97.5%, and 100%, respectively. Bi@(BiO)₂CO₃/TiO₂ also demonstrates excellent stability and reusability. This study represents a significant step toward the application of TiO₂-based photocatalyst materials in environmental purification. Full article

In this paper, we demonstrated a method of filling the air holes of a photonic crystal fiber (PCF), coated with gold film, with magnetic fluid (MF) to enhance the Surface Plasmon Resonance (SPR). The simulation results show that at the wavelength of 1260–1675 nm, the minimum loss coefficient of the y-polarization mode is 4.7 times that before filling with MF, and the x-polarization mode is 0.45 times greater. Then, based on this method, we designed a polarizing filter with a core diameter of 9 µm. The numerical simulation results indicate that it not only maintains the same core diameter as the single-mode fiber, but also has a larger bandwidth and a higher extinction ratio (ER). Additionally, we can optimize its ER at a specific wavelength by adjusting the magnetic field. Full article

In this work, a novel optical–chemical sensor for the detection of per- and polyfluorinated substances (PFASs) in a real scenario is presented. The proposed sensing approach exploits the multimode characteristics of plastic optical fibers (POFs) to achieve unconventional sensors via surface plasmon resonance (SPR) phenomena. The sensor is realized by the coupling of an SPR-POF platform with a novel chemical chip based on different polymeric nanolayers over the core of a D-shaped POF, one made up of an optical adhesive and one of a molecularly imprinted polymer (MIP) for PFAS. The chemical chip is used to launch the light into the SPR D-shaped POF platform, so the interaction between the analyte and the MIP’s sites can be used to modulate the propagated light in the POFs and the SPR phenomena. Selectivity tests and dose–response curves by standard PFOA water solutions were carried out to characterize the detection range sensor response, obtaining a wide PFAS response range, from 1 ppt to 1000 ppt. Then, tests performed on river water samples collected from the Bormida river paved the way for the applicability of the proposed approach to a real scenario. Full article

A multimode fiber-based sensor is proposed and demonstrated for the detection of lead traces in contaminated water. The sensing mechanism involves using a light sheet to excite a specific group of skew rays that optimizes light absorption. The sensing region features an inline microbubble structure that funnels the skew rays into a tight ring, thereby intensifying the evanescent field. The sensitivity is further refined by incorporating gold nanoparticles, which amplify the evanescent field strength through localized surface plasmon resonance. The gold nanoparticles are functionalized with oxalic acid to improve specificity for lead ion detection. Experiment results demonstrated the significantly enhanced absorption sensitivity of the proposed sensing method for large center offsets, achieving a detection limit of 0.1305 ng/mL (the World Health Organization safety limit is 10 ng/mL) for concentrations ranging from 0.1 to 10 ng/mL. Full article