Fiber Optic pH Sensor Research Articles

A wide range and highly sensitive optical fiber pH sensor using polyacrylamide hydrogel

Abstract In the present study we report the fabrication and characterization of no-core fiber sensor (NCFS) using smart hydrogel coating for pH measurement. The no-core fiber (NCF) is stubbed between two single-mode fibers with SMA connector before immobilizing of smart hydrogel. The wavelength interrogation technique is used to calculate the sensitivity of the proposed sensor. The result shows a high sensitivity of 1.94 nm/pH for a wide range of pH values varied from 3 to 10 with a good linear response. In addition to high sensitivity, the fabricated sensor provides a fast response time with a good stability, repeatability and reproducibility.

Fabrication and characterization of down-tapered optical fiber pH sensor using sol-gel method

In the present paper the fabrication and characterization of down-tapered optical fiber based pH sensor have been reported. The sensing probe is formed by fusing single mode fibers (SMF) to make it down-tapered which is spliced with multi-mode fiber (MMF). The sensing region is immobilized with sol-gel prepared by mixture of tetraethyl orthosilicate with three different pH indicators on to cladding of middle SMF. The proposed sensor utilizes intensity interrogation method and operates in low and high pH values. The maximum sensitivity of 0.49dBm/pH is obtained with a good linear response for wide range of pH value varied from 4 to 13. In addition to high sensitivity, this sensor has additional advantages of good stability, fast response time and capable of using in remote sensing.

Polyaniline-coated tilted fiber Bragg gratings for pH sensing

In this paper, we report an optical fiber pH sensor based on a polyaniline coating deposited on the surface of a tilted fiber Bragg grating. The pH-sensitive film was synthesized by in situ chemical oxidative polymerization keeping track of the deposition time in order to optimize the sensor response. As a result, the sensor reacts to pH changes in the range of 2–12 with a fast response and its sensitivity is directly related to the film thickness. The main advantages of this PAni-TFBG pH sensor are its biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features.

Fluorescence Ratiometric Optical Broad Range pH Sensor Based on CdSe/ZnS Quantum Dots and O170 Embedded in Ethyl Cellulose Matrix

This paper presents a ratiometric optical broad range pH sensor that comprises an optical fiber coated at one end with CdSe/ZnS quantum dots (QDs) and Oxazine 170 perchlorate (O170) embedded in ethyl cellulose (EC) matrix. The feasibility of coating an optical fiber with the sensing film to fabricate a ratiometric fiber optic broad range pH sensor is investigated. Using an LED with a central wavelength 405 nm as an excitation light source, the CdSe/ZnS QDs shows that the fluorescence intensity at 575 nm decreases with increasing pH and O170 shows the fluorescence intensity at 655 nm increases with increasing pH. The ratio of fluorescence intensities at 575 and 655 nm has a linear relationship with pH value in the 1.6–13.2 range. The ratiometric sensing approach presented in this study has the advantage of suppressing spurious fluctuations in the intensity of the excitation source and optical transmission properties of the optical fiber.

No-core fiber-based highly sensitive optical fiber pH sensor.

The present work describes the fabrication and characterization of an optical fiber pH sensor using a sol–gel technique. The sensing head configuration is incorporated using a short section of no-core fiber, coated with tetraethyl orthosilicate and spliced at the end of a single mode fiber with a bulge. Different types of indicators (bromophenol blue, cresol red, and chlorophenol red) were used to achieve a wide pH range from 2 to 13. High sensitivities of the fabricated device were found to be 1.02 and ? 0.93 ?? nm / pH for acidic and alkaline solutions, respectively. From the characterization results, it was noted that there is an impact of ionic strength and an effect of the temperature of liquid on the response characteristic, which is an advantage of the existing device over the other pH sensors. The fabricated sensor exhibited good reflection spectrum, indicating a blueshift in resonance wavelength for alkaline solutions and a redshift for acidic solutions.

Improved mesostructure by incorporating surfactant on thin film to develop an advanced optical fiber pH sensor with a temperature cross sensitivity feature

A new optical fiber pH sensor based on bromothymol blue (BTB) thin film with temperature cross-sensitivity has been proposed in this paper. The BTB thin film was prepared by depositing a thin layer of a solution containing tetraethyl orthosilicate (TEOS) and a BTB pH indicator in the presence of surfactants, namely cetyltrimethyl ammonium bromide (C19H42BrN, CTAB), by sol–gel technology on an unclad multimode fiber (MMF) surface. The number of layers and the deposition length of the thin film were varied, and the power transmission versus pH variation was studied. The concentration of the surfactant was increased to understand the effect of increasing porosity in the sol–gel matrix to achieve improved pH sensitivity. A straightforward way to utilize the temperature cross-sensitivity feature of the optical fiber pH-sensitive device has been introduced to develop a high sensitivity temperature sensor. A sensitivity of 79.96 nW pH−1 was obtained by a 20-layer thin-film coated sensor in the pH range of 3–12.

Study on Optical Characteristics of pH Indicators in the Immobilized Film for Fiber-Optic pH Sensor

Study on Optical Characteristics of pH Indicators in the Immobilized Film for Fiber-Optic pH Sensor

Tunable optical fiber pH sensors based on TE and TM Lossy Mode Resonances (LMRs)

In this paper, a high sensitivity optical fiber pH sensor based on lossy mode resonance (LMR) generated with transversal electric (TE) and transversal magnetic (TM) polarized light is presented. Poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) polymeric thin-films fabricated onto side-polished D-shaped optical fibers are used as pH-sensitive as well as LMR supporting coatings. The adequate selection of the pH-sensitive coating fabrication parameters enables to obtain devices working at different pH regions. Two devices have been obtained in order to measure pH from 4.0 to 5.0 and from 7.0 to 8.0 respectively. Obtained optical fiber pH sensors based on TE and TM LMRs showed a maximum sensitivity of 69nm/pH, which means a resolution of ∼0.0008pH units by using a conventional communications Optical Spectrum Analyzer (OSA) with 0.06nm resolution.

Optical fiber resonance-based pH sensors using gold nanoparticles into polymeric layer-by-layer coatings

The development of new nanocoatings onto optical fiber core is a hot topic within the optical fiber devices. The possibility of fabricating hybrid nanocoatings based on inorganic (gold nanoparticles, AuNPs) and organic materials (polymeric structure) can be performed using the layer-by-layer embedding deposition technique. The deposition of a nanostructure coating onto an optical fiber core has been performed in order to obtain optical fiber resonance-based pH sensors. The incorporation of gold nanoparticles (AuNPs) into polymeric thin films has been confirmed by atomic force microscopy, scanning electron microscopy and UV---Vis spectroscopy. In addition, two electromagnetic resonances known as localized surface plasmon resonance (LSPR) or lossy mode resonance (LMR), can be generated as a function of the resultant thickness coating. In this work, the fabrication of a dual LSPR-LMR optical fiber pH sensor is presented where the LSPR is used as a reference signal and the LMR is used as a sensing band due to the great difference in their corresponding sensitivities to pH changes of the surrounding medium. It has been demonstrated that LMR improves the sensitivity of the LSPR band in more than one hundred times. The device shows a high sensitivity, fast response time and large dynamical range of 134.7 nm from pH 4.0 to pH 6.0.

Intrinsic Fiber Optic pH Sensor for Measurement of pH Values in the Range of 0.5–6

The development, design, and evaluation of an intrinsic optical fiber pH sensor for low pH values (in the range of 0.5–6) based on fluorescence from a novel coumarin dye, which is covalently immobilized onto the end surface of an optical fiber is described. The sensor provides a rapid response (of approximately 25 s) over this pH range of 0.5–6.0, showing with very good stability over a period of several months. The sensor has also demonstrated insensitivity to ionic strength and excellent photostability through a series of laboratory tests.

Sol–gel based fiber optic pH nanosensor: Structural and sensing properties

Evanescent wave absorption based PCS (plastic clad silica) fiber optic pH sensor is constructed by encapsulation of indicator dyes in the heterogeneous inorganic hybrid silica–titania nanomatrix in the presence of cetyl trimethyl ammonium bromide (CTAB) surfactant by sol–gel method. A nanoporous multilayers (triple layer and six layers) of silica–titania hybrid is deposited on 5cm uncladed middle portion of the fiber by dip-coating sol–gel technique. The CTAB surfactant expressively improved the host matrix structure and enhanced its porosity. Thermally stable, adhesive and smooth thin nanocladding has been examined using microscopic and thermal analysis. Indicators encapsulated nanomatrices are highly sensitive to pH and show optical output signals in terms of intensity. Therefore, it provides confirmation that interactions between the host matrix and pH-indicator molecules are in good assistance. Furthermore, experimental findings reveal that the sensing species are probably better encapsulated in the matrix and show no leaching of sensing molecules. The prepared nanosensors exhibit a much faster response time because of the material to be sensed diffuses faster into the triple-layer sensors <2s. These pH sensors are found to have 60 days (2 months) age stability and high reproducibility. A repeatable response over a wide range of pH values between 3 and 11 is obtained.

Fabrication of Optical Fiber pH Sensor Based on Near-Infrared (NIR) Quantum Dots

Fabrication of Optical Fiber pH Sensor Based on Near-Infrared (NIR) Quantum Dots

Synthesis and characterization of multilayered sol–gel based plastic-clad fiber optic pH sensor

The fabrication and characterization of an optical fiber pH sensor for the detection range 3–11 are described. The sensing element consists of a combination of indicators encapsulated in mesoporous surfactant based hybrid matrix, which are deposited onto an uncladed plastic optical fiber. Stable, dense and uniform sol–gel cladding with more than 90% transparency in the visible range are observed. Roughness value 9.7–6.57nm is obtained by AFM. Film thickness, refractive index, and porosity are found to vary with number of coatings, 70% porosity is determined with 116nm thickness. The response of the sensor with third layer is observed in acid and basic medium.

Fabrication of the optical fiber pH sensor based on CdSe/ZnS quantum dot.

During the past decade, there has been a lot of research on the application of quantum dots (QDs) in photonics and biotechnology. QDs are semiconductor nanocrystals which show many peculiar optical and electrical properties due to their quantum confinement effects. In this work, the correlation of the fluorescence intensity of QDs and the pH under ambient temperature was studied. An optical fiber pH sensor was prepared using CdSe/ZnS carboxyl quantum dots as sensing media. QDs with different sizes and emission colors (green, yellow, and red) were immobilized on the surface of the optical fiber through EDC/sulfo-NHS coupling reaction. The optical fiber was immersed in water, and the photoluminescence (PL) spectra of QDs were obtained at ambient temperature under various pH conditions. The photoluminescence (PL) intensity of QDs increased linearly with the increase of pH, and among the QDs, the red emitting QDs show the largest change in PL intensity with pH. This shows that QDs can be used as pH sensing media and optical fiber pH probe can be prepared.

Double-pass Mach-Zehnder fiber interferometer pH sensor.

A biocompatible fiber-optic pH sensor based on a unique double-pass Mach-Zehnder interferometer is proposed. pH responsive poly(2-hydroxyethyl methacrylate-co-2-(dimethylamino)ethyl methacrylate) hydrogel coating on the fiber swells/deswells in response to local pH, leading to refractive index changes that manifest as shifting of interference dips in the optical spectrum. The pH sensor is tested in spiked phosphate buffer saline and demonstrates high sensitivity of 1.71 nm/pH, pH 0.004 limit of detection with good responsiveness, repeatability, and stability. The proposed sensor has been successfully applied in monitoring the media pH in cell culture experiments to investigate the relationship between pH and cancer cell growth.

Development of a polymer optical fiber pH sensor for on-body monitoring application

A pH sensor based on polymethylmethacrylate (PMMA) optical fibers has been developed for on-body monitoring in biological fluids. Detection relies on evanescent wave absorption in a thin film deposited on the fiber core. The sensitive film was prepared using sol–gel technology from a mixture of tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) or phenyltriethoxysilane (PTES), forming a porous hybrid organic–inorganic layer upon drying. The so-called ORMOSILs (organically modified silicate) were doped with bromophenol blue (BB) and the formulation was optimized to suppress dye leaching by varying the fraction of organic precursor, the sol aging time and the drying conditions. The molecular structure of the films was examined by Fourier-transformed infrared (FTIR) spectroscopy, and their optical responses characterized with UV–VIS absorption spectroscopy. Homogeneous crack-free films were obtained with an optimal MTES content of 50%. Leaching from these films was greatly reduced, with more than 80% of dyes remaining after 2 months in phosphate buffered saline (PBS) buffer. The entrapped indicator showed pronounced increase of apparent pKa and a broader response range compared to the free form in solution. The sensor demonstrated good sensitivity from pH 3 to 9, with an optical signal variation of 3.5dB. It showed sensitivity to ionic strength variations, but not to temperature in the range between 20 and 50°C. Reversible pH monitoring between pH 5 and 8 with a precision of 0.2 pH unit was demonstrated in human serum as a model biological fluid, without degradation of the sensing fiber over 24h. This simple and low cost device will be particularly valuable to support diagnosis and the evaluation of therapies for wound healing.

A fiber-optic pH sensor based on relative Fresnel reflection technique and biocompatible coating

A biocompatible fiber-optic pH sensor based on Fresnel reflection technique and a sensing coating is presented. Sodium alginate and polyethylenimine are alternatively deposited on the sensing fiber end to form the sensing coating via a layer-by-layer electrostatic self-assembly technique. An optical switch is added to the measurement system for the convenience of fast calibration. A linear, monotonic and fast response in a large pH range (from pH5.87 to pH10.55) is obtained with the resolution of 0.01pH unit. The sensitivity of the pH sensor is 0.018R.I.U/pH. It is not influenced by fluctuations of light source.

Fluorescence based fibre optic pH sensor for the pH 10–13 range suitable for corrosion monitoring in concrete structures

The design, development and evaluation of an optical fibre pH sensor for monitoring pH in the alkaline region are discussed in detail in this paper. The design of this specific pH sensor is based on the pH induced change in fluorescence intensity of a coumarin imidazole dye which is covalently attached to a polymer network and then fixed to the distal end of an optical fibre. The sensor provides a response over a pH range of 10.0–13.2 with an acceptable response rate of around 50min, having shown a very good stability over a period of longer than 20 months thus far. The sensor has also demonstrated little cross-sensitivity to ionic strength (IS) and also excellent photostability through a series of laboratory tests. These features make this type of sensor potentially well suited for in situ long term monitoring of pH in concrete structures, to enhance structural monitoring in the civil engineering sector.

Fiber-optic pH sensor based on sol-gel film immobilized with neutral red

In this study, we developed a fiber-optic pH sensor based on a sol-gel film immobilized with neutral red (NR). A solgel film was prepared by mixing tetramethylorthosilicate (TMOS), trimethoxymethylsilane (MTMS), ethanol (EtOH), distilled water (H2O), and NR powder. Accordingly, the thin pH sol-gel film was fabricated through a sol-gel process with a dip-coating method. The thickness and diameter of the fabricated pH sol-gel film are 0.11 and 0.6 mm, respectively. We measured the optical absorbance and the light intensity with the spectra of reflected light, which change with the color variation of the pH sol-gel film in the fiber-optic sensing probe. From the experimental results, we demonstrated that the proposed fiber-optic pH sensor has good reversibility, reproducibility, and a fast response time, in which the optical properties of the NR-based pH sol-gel film change with the pH value.

Fiber Optic pH Sensor with Self-Assembled Polymer Multilayer Nanocoatings

A fiber-optic pH sensor based on a tilted fiber Bragg grating (TFBG) with electrostatic self-assembly multilayer sensing film is presented. The pH sensitive polymeric film, poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) was deposited on the circumference of the TFBG with the layer-by-layer (LbL) electrostatic self-assembly technique. The PDDA/PAA film exhibits a reduction in refractive index by swelling in different pH solutions. This effect results in wavelength shifts and transmission changes in the spectrum of the TFBG. The peak amplitude of the dominant spectral fringes over a certain window of the transmission spectrum, obtained by FFT analysis, has a near-linear pH sensitivity of 117 arbitrary unit (a.u.)/pH unit and an accuracy of ±1 a.u. (in the range of pH 4.66 to pH 6.02). The thickness and surface morphology of the sensing multilayer film were characterized to investigate their effects on the sensor's performance. The dynamic response of the sensor also has been studied (10 s rise time and 18 s fall time for a sensor with six bilayers of PDDA/PAA).