Fiber Optic pH Sensor Research Articles

Characterization of a fast response fiber-optic pH sensor and illustration in a biological application.

Optical, and especially fiber-optic techniques for the sensing of pH have become very attractive and considerable research progress in this field has been made over recent years. The determination of the value of pH across a broad range of applications today, important for areas of study such as life sciences, environmental monitoring, manufacturing industry and widely in biological research is now accessible from such optical sensors. The need for such technology arises because familiar, commercial sensors are often limited in terms of their response time and the presence of drift, all of which emphasize the value of newer and rapidly developing technologies such as fiber-optic sensors, to address these wider applications. As a result, a new compact sensor design has been developed, designed around a specially-formed fiber-optic tip, coated with a pH-sensitive dye, and importantly covalently linked to a hydrogel matrix to provide high stability. The sensor developed was designed to have a very fast response time (to 90% of saturation, Δt90) of <5 s and a sensing uncertainty of ∼±0.04 pH units. Given the covalently bonded nature of the dye, the problem of leaching of the indicator dye is reduced, creating a probe which has been shown to be very stable over many days of use. Illustrating this through extended continuous use, over ∼12 h at pH 7, this stability was confirmed showing a drift of <0.05 pH h-1. In order to give an illustration of the value of the probe in an important biological application, the monitoring of pH levels between pH 7 to pH 8 in an AMES' medium, a substance which is important to maintain the metabolism of retinal cells is shown and the results as well as temperature stability of the probe discussed.

Hydrogel based Fabry-Pérot cavity for a pH sensor.

A simple, reliable, and quick reactive Fabry-Pérot (FP) structure-based fiber optic pH sensor is presented. The pH-sensitive hydrogel and single-mode fiber (SMF) are placed inside a fused silica capillary to form the FP cavity. The gel thickness is characterized by the spin coating method with respect to different spin speeds. The proposed sensor shows a pH sensitivity of 0.30 nm/pH along with a fast response time of 15 s to 20 s for different pH solvents in the acidic range. Also, the temperature sensitivity of the FPI sensor is found to be -0.56 nm/°C.

Self-referencing optical fiber pH sensor for marine microenvironments

This study presents the development of an optical fiber pH sensor based on evanescent wave absorbance for continuous pH measurements in marine microenvironments. The sensing layer consists of an optimized sol-gel matrix of tetraethoxysilane and dimethyldiethoxysilane, which substantially improves the entrapment efficiency of the pH indicator meta-cresol purple, leading to a long useable lifetime. The optical fiber pH sensor conforms to the Global Ocean Acidification Observing Network “weather” measurement quality guideline with precision of approximately 0.02 pH units, has a dynamic pHT range of 7.4–9.7 in seawater, a response time of 2.5–6.5 min and a useable lifetime of 7 days. The optical fiber pH sensor has additional advantages of being self-referencing, without the need of an external sensor reference, having a simple fabrication method and basic spectrometer instrumentation. The suitability of the optical fiber pH sensor was demonstrated in real-time measurements of the ecologically significant green seaweed Ulva sp. The optical fiber pH sensor monitored pH variations due to metabolic activity over 7 days within the seaweed canopy and 4 days within the diffusion boundary layer interface, demonstrating the suitability for measurements in marine microenvironments.

Fiber-Optic SPR pH Sensor Based on MMF–NCF–MMF Structure and Self-Assembled Nanofilm

A fiber-optic pH sensor, based on layer-by-layer (LBL) self-assembly and surface plasmon resonance (SPR) technology, is proposed and experimentally confirmed. This article proposes poly (diallyldimethylammonium chloride) (PDDA) as the pretreatment solution for the surface of the gold film and the highly sensitive polyacrylic acid/chitosan (PAA/CS) self-assembled nanomembrane as the pH-sensitive film. It has been experimentally verified that the sensor can detect pH from 3.18 to 11.84 and it exhibits a high average sensitivity of 32.31 nm/pH in this range. In addition, experiments have confirmed that the sensor has attractive stability, repeatability, and hysteresis characteristics.

A U-Shape Fibre-Optic pH Sensor Based on Hydrogen Bonding of Ethyl Cellulose With a Sol-Gel Matrix

Development of a biocompatible pH sensor is of importance in biomedical applications, particularly for in vivo measurement, providing necessary information for clinical diagnosis and treatment such as chronic wounds and foetal acidosis. Traditional pH-indicator based optical sensors have problems of dye-leaching and photobleaching that restrict their uses in long-term monitoring. In this work, a dye-free fibre optic pH sensor is proposed consisting of a U-shape multimode optical fibre coated with a hybrid organic-inorganic composite film. The film is formed by cross-linking ethyl cellulose with a silica matrix at an optimised ethyl cellulose/silica molar ratio of 0.0065 via weakly interacted hydrogen bonding. This bonding is affected by hydrogen concentration (i.e., pH) in a solution resulting in a morphological change of the polymer aggregation presented in the silica matrix leading to refractive index change of the film. The developed sensor shows a reversible response to pH from 4.5 to 12.5 and exhibits linear correlation between transmitted light power and pH with a limit of agreement (LoA) between the sensor and a commercial probe of ±0.2 pH. For a clinically important range of pH values between 6 and 8 the LoA is ±0.1 pH. The sensor has low cross-sensitivity to temperature as the maximum interpreted pH change attributed to the power change is 0.12 pH when the temperature changes from 21 °C to 39 °C. To demonstrate biomedical relevance, the sensor is used to monitor pH of human serum. An in-house cytotoxicity assay is conducted with mouse fibroblast cell revealing that the film is not cytotoxic.

Fast responsive anatase nanoparticles coated fiber optic pH sensor

The sol-gel method is employed to synthesize the phenol red encapsulated anatase nanoparticles (PR/ANPs) matrix at low temperature (60 °C). Field emission scanning electron microscopy/Energy Dispersive X-Ray (FESEM/EDX) analysis shows porous nanostructures, and uniform elemental distribution. Atomic Force Microscopy exhibited that after phenol red (PR) encapsulation, average surface roughness (Ra) is increased up to ∼ 20 nm than pure anatase matrix Ra ∼14 nm. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy analysis confirmed the presence of bond ∼ 1054 cm−1 which is indicative of good PR encapsulation within the anatase matrix. X-ray diffraction analysis (XRD) shows intense peak corresponding to phenol along with anatase peaks suggested the interaction of Ti and O atoms with PR species. High resolution transmission electron micrographs (HR-TEM) analysis exhibited crystalline anatase structure before and after encapsulation along (101) direction. The average particle size is observed 5 nm for ANPs matrix which reduced to 3 nm after encapsulation. Observed high surface area 218 m2/g after encapsulation is helpful to enhance the activity of the sensing device. Thermogravimetric analysis (TGA) shows that after 422 °C, no mass weight loss is observed which confirmed the thermal stability of synthesized matrices. After encapsulation, observed low refractive index 1.8 at 550 nm indicates the porous nature of the matrix. The fiber optic pH sensor shows a linear response over the dynamic pH range 1–12, sensitivity is calculated as 1.83 counts/pH at 454 nm with determination coefficient (R2) ∼ 0.97. The proposed sensing device has fast response time 1.09s–0.42s within acidic-basic media, good reversibility, and validity. Experimental findings suggested that synthesized material has great potential in photonic devices/applications.

A hydrogel-based optical fibre fluorescent pH sensor for observing lung tumor tissue acidity

Technologies for measuring physiological parameters in vivo offer the possibility of the detection of disease and its progression due to the resulting changes in tissue pH, or temperature, etc.. Here, a compact hydrogel-based optical fibre pH sensor was fabricated, in which polymer microarrays were utilized for the high-throughput discovery of an optimal matrix for pH indicator immobilization. The fabricated hydrogel-based probe responded rapidly to pH changes and demonstrated a good linear correlation within the physiological pH range (from 5.5 to 8.0) with a precision of 0.10 pH units. This miniature probe was validated by measuring pH across a whole ovine lung and allowed discrimination of tumorous and normal tissue, thus offering the potential for the rapid and accurate observation of tissue pH changes.

Thermally and optically functionalized Anatase nano-cavities based fiber optic pH sensor

Owing to the fiber optic sensing applications, the cavity shaped cresol red (CR) immobilized anatase nanoparticles (CR-ANPs) are synthesized by sol-gel route. The homogeneous distribution of CR species within ANPs is confirmed by EDX mapping analysis. The chemical bonding between Ti and functional groups indicated the successful immobilization of CR species in the anatase matrix. Thermally stable CR-ANPs have average surface roughness ∼ 9 nm, a large surface area of 210 m2/g, small pore volume 0.12 cm3/g, and crystallite size 4 nm. The sensing response of the proposed device is found to be 7.05 counts/pH within pH 1-7 at 453 nm with the determination coefficient (R2) ∼ 0.95. Whereas, in basic media (pH 9-12), the sensor response is found to be 5.95 counts/pH. Experimental findings suggested that this proposed material is of pronounced interest because it can be used within extreme acidic and basic media without any deterioration/leaching.

Fiber optic pH sensor using TiO2-SiO2 composite layer with a temperature cross-sensitivity feature

An intensity-modulated fiber optic pH sensor is fabricated and demonstrated in the present work. Three pH indicators, bromothymol blue, cresol red and chlorophenol red along with titania-silica composite are used as pH sensitive layers. The sensing principle is based on evanescent field absorption and then transmitted power loss of light guided within the fiber. The sensor parameters were studied by changing the volume ratio of the composite layer, and a maximum sensitivity of 1.76 μW/pH is obtained for 1:3 (TiO2:SiO2) composite layer with excellent linear response for pH value from 2 to 11. The TiO2 content induces strong interaction between guided light and outer medium and the concentration of SiO2 was increased to improve porosity in the sol-gel matrix to achieve enhanced sensitivity. A straight forward way has been introduced to utilize the temperature cross-sensitivity feature of the optical fiber pH sensor with high temperature sensitivity. In addition to its high sensitivity, the proposed sensor has additional characteristics of fast time response and good stability which are very useful in biomedical, environmental, food processing and industrial applications.

Development and characterization sensitivity of optical fibre pH sensor for industrial application

Optical fibre sensor (optrode) depending on a transmittance was presented as a valuable sensitive sensor for several applications. Thus, the development of a wide-area detection system for continuously monitoring structural changes, together with a capability for non-destructive evaluation and early warning, is an important task the sensitive fibre was preparing by removing the cladding from several places and modified by different chemical solutions. The transmittance spectrum used to measure the pH of distilled water that contains HCL and NaOH in a range between 2 to 12. The special design of this sensor made it a perfect for laboratory and industrial usage, only 25 cm of modified fibre was enough to measure the pH in real time with a sensitivity 0.015-0.2 1/pH. The results showed this sensor can work in different solutions and it gives accurate results at any conditions.

Fibre optic ratiometric fluorescence pH sensor for monitoring corrosion in concrete.

In this communication a novel concept for pH sensing is introduced which is specifically adapted to monitor carbonation induced corrosion in concrete structures. The method is based on a ratiometric measurement principle, exploiting the pH sensitive colour switching of thymol blue in the basic pH regime and the emissive properties of two different (Zn)CdSe/ZnS core shell quantum dots. The transition point of thymol blue in a Hydrogel D4 matrix was determined to be at around pH 11.6, which fits ideally to the intended application. Next to the fundamental spectroscopic characterization of the ratiometric response, a new design for a sensor head, suitable for the incorporation into concrete matrices is presented. Toward this, a manufacturing process was developed which includes the preparation of a double layer of polymers containing either thymol blue or a quantum dot mixture inside a porous ceramic tube. Results of a proof-of-priciple performance test of the sensor head in solutions of different pH and in cement specimens are presented, with encouraging results paving the way for future field tests in concrete.

Smartphone-Based Hand-Held Optical Fiber Fluorescence Sensor for On-Site pH Detection

In order to realize remote and on-site pH detection, we design a simple, economical, and hand-held optical fiber pH sensor based on smartphone platforms. All components of the sensor are encapsulated in a 3-D printed shell, including a semiconductor laser for exciting fluorescence signal, a smartphone with a filter for collecting fluorescence signal, a fiber coupler for transmitting light, and batteries for laser power supply. With the combination tapered fiber probe modified with CdSe/ZnS quantum dots, whose fluorescence signal can be captured by the smartphone camera and calculated by a self-developed Android application, the sensor has a minimum sensitivity of 0.5-pH unit and a wide detection range between pH 4 and 12. Moreover, the sensor shows good reusability and stability. Considering its low cost, miniaturization, portability and the capability for remote detection, real-time monitoring, and on-site analysis, the proposed smartphone-based optical fiber pH sensor has great application potential in environmental monitoring and biomedicine.

A Numerical Analysis of Various pH Level for Fiber Optic pH Sensor Based on Bromophenol Blue in Silica

The fabrication and characterization of an optical fiber pH sensor for the detection range 2 – 7 are described. The sensing element was prepared by coating the uncladded middle portion of a multimode plastic clad silica fiber (PCF) with xerogel film of immobilized bromophenol blue (BPB) prepared by a sol-gel process. The exponential decay of the evanescent wave at the core-cladding interface of the multimode fiber was utilized to determine the pH response. The objective of this work is to analyze the measurement of the fiber optic sensing probe towards various pH level. The intensity of the absorbance was analyzed using statistical technique (Statistical Package for the Social Sciences, SPSS) to identify the significant pH range that can be used as a reference in developing the sensor instrumentation in the future. pH2 shows the highest significance among all pH values based on the statistical results using error plots.

Distributed Fiber Optic pH Sensor for the Application in the Wellbore

pH measurement is important in many applications such as environmental science, civil engineering, biotechnology, earth science, and marine sciences. In the wellbore, the pH of the cement pore solution is around 13 at early stage, and the wellbore cement is relatively stable and less permeable above pH 10.5. However, pH drops in the CO2/H2S containing environment making the cement more porous and permeable, which leads to higher risk of wellbore integrity failure. The lower pH also makes the casing steel more susceptible to corrosion. Therefore, real time monitoring of pH in the wellbore is crucial for corrosion detection and cement failure prediction. Wellbores are often at high temperature high pressure (HTHP) conditions (> 149 °C, >1400 psi). Conventionally, pH measurement is made by a glass electrode, which is fragile, prone to drifting, and suffers from alkali interference. Furthermore, it requires a reference electrode, which becomes less stable and less reliable in HTHP conditions, hence there are only limited options available for conventional pH sensors under wellbore conditions. In the contrast, fiber optic pH sensors can offer desirable advantages such as the elimination of electrical connections and wiring and the lack of need for a separate reference electrode, making them inherently more compatible with harsh environment applications. Importantly, they can also be used for remote, distributed, and continuous pH sensing in harsh environment to enable a map of pH throughout a region of interest. Therefore, fiber optic sensors have become a promising solution for pH monitoring in the wellbore. To enable distributed pH sensing with the optical fiber platform, an optical fiber must be integrated with a pH sensitive layer along the fiber, such that the optical properties can be locally altered due to the pH change of the surrounding solution and the change is monitored by a detector in either a transmission or reflection/backscatter configuration. Therefore, the development of a stable, selective, and sensitive pH sensing layer is the critical enabling technology. Sol-gel SiO2 film is one of the most widely used substrates to immobilize pH indicators, such as organic dyes, on the optical fiber surface for pH sensing. SiO2 film has high thermal stability and excellent mechanical strength, which satisfies the HTHP wellbore conditions. The SiO2 thin film can be prepared over a large area at a comparatively low cost. Most investigations use the sol-gel SiO2 film as the substrate for immobilizing pH indicators, but very few studies have reported that the film itself can be used as a pH sensing layer. Using only SiO2 film makes the pH sensor simple to fabricate and free of leaching or photobleaching which are often encountered by the entrapped dye indicators. However, the mechanistic details of the response require elucidation to understand the potential for a selective and sensitive response as a function of pH in the presence of other chemical analytes and variations in temperature and pressure. Here we report recent results on fiber optic pH sensors utilizing porous silica based films and discuss them in the context of fundamental mechanistic considerations as well as their relevance to harsh environment sensing applications. The SiO2 film is coated onto the optical fibers through a sol-gel dip-coating method. The dip-coated fibers are calcined at temperatures of 200-600oC afterwards to obtain a porous SiO2 film of a few hundred nanometers thick. The pH sensing performance is investigated in terms of light transmission to confirm sensitivity and reversibility, and the distributed pH monitoring capability is demonstrated using an optical backscatter reflectometer. The optical fiber coated with a silica-based sol-gel and calcined at 200 °C shows good sensitivity and reversibility in the pH range from 4 to 13, and the sensor is found to be more sensitive in the alkaline region, which indicates that the developed fiber optic pH sensor has potential application in the wellbore pH range.

A Bio-Compatible Fiber Optic pH Sensor Based on a Thin Core Interferometric Technique

There is an increasing demand for compact, reliable and versatile sensor concepts for pH-level monitoring within several industrial, chemical as well as bio-medical applications. Many pH sensors concepts have been proposed, however, there is still a need for improved sensor solutions with respect to reliability, durability and miniaturization but also for multiparameter sensing. Here we present a conceptual verification, which includes theoretical simulations as well as experimental evaluation of a fiber optic pH-sensor based on a bio-compatible pH sensitive material not previously used in this context. The fiber optic sensor is based on a Mach-Zehnder interferometric technique, where the pH sensitive material is coated on a short, typically 20-25 mm thin core fiber spliced between two standard single mode fibers. The working principle of the sensor is simulated by using COMSOL Multiphysics. The simulations are used as a guideline for the construction of the sensors that have been experimentally evaluated in different liquids with pH ranging from 1.95 to 11.89. The results are promising, showing the potential for the development of bio-compatible fiber optic pH sensor with short response time, high sensitivity and broad measurement range. The developed sensor concept can find future use in many medical- or bio-chemical applications as well as in environmental monitoring of large areas. Challenges encountered during the sensor development due to variation in the design parameters are discussed.

Highly sensitive surface plasmon resonance based fiber optic pH sensor utilizing rGO-Pani nanocomposite prepared by in situ method

Reduced graphene oxide-polyaniline (rGO-Pani) nanocomposite based highly sensitive fiber optic pH sensor utilizing SPR technique is fabricated and characterized. The rGO-Pani nanocomposite is successfully synthesized using in-situ method. The sensing probe is fabricated by coating a thin film of silver over the unclad core of the optical fiber which is further coated by a film of rGO-Pani nanocomposite. The preparation of rGO, Pani and rGO-Pani nanocomposite are confirmed using different characterization tools such as X-ray diffraction (XRD), Raman and Fourier-transform infrared spectroscopy (FTIR). The morphology of the rGO-Pani nanocomposite is characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The operating pH sensing range of the present sensor is checked by performing experiments on samples of pH lying from 2.4 to 11.35. The sensor's performance is excellent for low and high pH and it possesses the maximum sensitivity of 75.09 nm/pH at pH 11.35. The high sensitivity of the sensor is because of the change in the optical band gap of the rGO-Pani nanocomposite at the low and high pH values of the samples which is confirmed by using Tau plot. The sensor has many advantages such as compact size, low cost, high sensitivity, stability, repeatability and capability of remote sensing.

PANI-PVA composite film coated optical fiber probe as a stable and highly sensitive pH sensor

An intrinsic optical fiber pH sensor coated with polyaniline (PANI)-polyvinyl alcohol (PVA) composite layer is developed and experimentally investigated in this paper. The sensor was fabricated by replacing the polymer cladding of a multimode fiber with pH sensitive PANI-PVA layer whose absorption properties as well as refractive index are pH dependent. PANI was synthesized as stable aqueous dispersion in presence of PVA and the coated film exhibits porous morphology. The experimental results demonstrate high sensitivity of 2.79 μW/pH for 2–9 pH range. The impact of environmental temperature and ionic strength on the sensor performance are studied for the fabricated sensor along with time response and reversibility. Additionally, real sample analysis was done to test for the reliability and chemical stability of the sensor and compared with conventional method. Apart from high sensitivity, wide dynamic range and fast time response, the experimental investigations indicate that the proposed sensor possesses excellent long term stability and durability over multiple measurements.

Broad range and highly sensitive optical pH sensor based on Hierarchical ZnO microflowers over tapered silica fiber

In present work we report the fabrication and characterization of optical fiber pH sensor using smart hydrogel coating over Zinc Oxide (ZnO) microflowers. ZnO microflowers were hydrothermally grown over chemically etched tapered multi-mode fiber (MMF). Mainly, the sensing head contains two layers in which one is ZnO microflower and another is pH sensitive hydrogel layer. The variation in pH of liquid around the sensing probe causes the swelling and shrinkage of hydrogel which leads the change in its refractive index (RI). The work of ZnO microflower is to enhance the evanescent waves that interact with the change in RI of hydrogel which causes the variations of wavelength dips with respect to each pH solutions. The sensitivity was estimated by using wavelength interrogation technique. The ZnO microflower based sensor is highly sensitive for wide range pH values (both high and low range of pH varied from pH = 3 to pH = 7 and pH = 7 to pH = 11, respectively). The sensitivity of ZnO microflower based MMF sensor was found to be 2.59 nm/pH and 0.70 nm/pH for both acidic and basic media, respectively. In addition to high sensitivity, the proposed sensor shows fast response time and good stability and reproducibility.

All-polymer fiber-optic pH sensor.

A novel all-polymer fiber-optic pH sensor using a UV-cured pH-sensitive hydrogel, poly(ethylene glycol) diacrylate (PEGDA), coated on a polymer fiber Bragg grating was developed. The PEGDA increased in volume according to the pH value of the surrounding fluid, which subsequently induced a lateral stress in the polymer fiber Bragg grating. The proposed pH sensor exhibits a pH sensitivity of up to -0.41 nm/pH and a fast response time of 30 s.

Fiber-Optic Biosensor to Detect pH and Glucose

In this paper, we propose fast, highly sensitive, and wide-dynamic-range fiber-optic probe-type biosensors to detect pH and glucose. The operation principle of the proposed biosensors is based on the Fabry-Perot interferometry technique. Three pH-sensitive dyes (methyl orange, methyl red, and thymol blue) and three solvatochromic dyes (Nile red, rhodamine-B, and 4-amino-N-methylphthalimide) were individually mixed with polymers to obtain five pHand three glucose-sensitive sensing membranes. These membranes were subsequently deposited on eight gold nanoparticles coated on fiber-optic probes to obtain five pH and three glucose fiber-optic probe sensors. To ascertain the effectiveness of the sensors, various concentrations of pH buffer solutions and glucose solutions were tested. The proposed fiber-optic pH and glucose probe sensors showed sensitivities of 1.95 nm/pH and 3.25 nm/mM, respectively, with high-sensing stability with the relative standard deviation about 2.5%. The proposed sensors offer a linear sensing ability over wide ranges of pH (2-12) and glucose (1 μM-1 M) with an R 2 value of approximately 0.996. The response and recovery times of the proposed sensors were approximately 8 and 9 s, respectively. Finally in this paper, we compared the performance of the proposed fiber-optic probe sensors with that of other pH and glucose sensors and determined that the proposed sensors achieve better sensing performance.