Fiber Optic pH Sensor Research Articles

Optical Fiber pH Sensors Based on PAni-Coated Microstructured Optical Fibers

Optical Fiber pH Sensors Based on PAni-Coated Microstructured Optical Fibers

Temperature-compensated high-sensitivity refractive index, liquid level and PH sensing realization enabled by a small-period microfiber long period grating cascaded with microfiber Bragg grating

Conventional reported optical fiber sensors could not have both good temperature-compensated characteristic and high PH sensing sensitivity. In this study, we proposed and fabricated a small-period microfiber long period grating (SP-mFLPG) cascaded with a line-by-line microfiber Bragg grating (mFBG) by using femtosecond (fs) laser directly writing technique for high-sensitivity temperature-compensated sensing refractive index(RI), liquid level(LL), PH value. To be the different from traditional LPG devices, the temperature sensitivities of SP-mFLPG was measured as −8.61 pm/℃ owing to the strong coupling occurred between the fundamental mode of TE11 mode and the higher-order mode of TE111, which could be compensated with the mFBG device with a temperature sensitivity of 7.85 pm/℃. On the basis of this temperature-insensitive sensing strategy, our proposed SP-mFLPG sensor can sense RI, LL and pH with high sensitivities of 866.343 nm/RIU, 7.224 nm/mm, −5.70 nm/PH, respectively. Our obtained PH sensitivity is much higher than numerous reported state-of-the-art optical fiber PH sensors. Thus, our proposed optical microfiber device can be found potential applications in real-time measuring some important biochemical reaction procedures.

PH sensor based on tilted fiber Bragg grating surface plasmon resonance with a polyaniline reaction deposition film layer.

In this paper, we propose a surface plasmon resonance (SPR) fiber-optic pH sensor combined with a tilted fiber Bragg grating (TFBG) by continuously coating gold and polyaniline (PANI) onto the surface of a TFBG. The micron-scale thickness polyaniline film provides the sensor with good sensitivity, and it achieves accurate measurement of pH values ranging from 2 to 12 by utilizing the pH-responsive mechanism of PANI and the surface plasmon resonance characteristics. Experimental results show that within the 2-12 pH range, the sensitivity of the TFBG surface plasmon resonance pH sensor based on PANI coating is 0.50335 nm/pH, and results demonstrate, a linear correlation coefficient between wavelength and pH value reaching 0.96614. This indicates significant potential for future engineering applications in real-world pH measurement using this sensor.

Optical pH Sensor Based on a Long-Period Fiber Grating Coated with a Polymeric Layer-by-Layer Electrostatic Self-Assembled Nanofilm.

An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92-9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry-Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

Ratiometric fiber optic fluorescent pH sensor for hydroxide diffusion measurements in concrete

This article delineates the development of an advanced ratiometric pH optode designed for protracted monitoring of pH dynamics in cementitious materials—a crucial aspect of structural management. The optode features a dual-layer Hydromed™ D4 polymer pellet incorporating a pH-sensitive Naphth-AlkyneOMe fluorescent probe (pKa ≈ 12) and CdSe quantum dots as an internal reference. The polymer matrix, optimized with diamond powder and carbon black, enhances the signal while minimizing light artifacts. Demonstrating a robust response to pH fluctuations, the optode, utilizing ratiometric measurements, enables steadfast monitoring for up to 75 days.The experimental setup includes a Delrin® sample compartment housing a glass slide with the optode connected to an optical fiber via an SMA connector. HydroMed™ D4 films, forming the pH-sensitive layer, are deposited on the glass slide and secured. The optode configuration is fine-tuned to optimize signal intensity by reducing its length. For optode-detector coupling, a 1000 µm core diameter optical fiber links the optode, and a 405 nm LED illuminates the sensing film. The emission signal is collected through the optical fiber, and signals are processed using computer-controlled lock-in amplifiers.In the investigation of hydroxide ion diffusion through a concrete sample, the optode's efficacy was validated, aligning with pH electrode measurements. Addressing long-term stability, a degradation model uncovered leisurely kinetics with commendable stability. Slow degradation kinetics, with a reaction rate coefficient of 1.6 × 10⁻⁷ s⁻¹ and a half-life of 1180 h, exceeded expectations.This pH optode, showcasing an accurate and resilient response to pH variations within the 10 to 12.5 range, stands as a promising tool for monitoring pH dynamics in low-pH cementitious materials. The incorporation of a degradation model underscores fidelity over prolonged durations and offers a blueprint for enduring surveillance of concrete structures.

Dual-Lifetime Referencing (t-DLR) Optical Fiber Fluorescent pH Sensor for Microenvironments.

The pH behavior in the μm to cm thick diffusion boundary layer (DBL) surrounding many aquatic species is dependent on light-controlled metabolic activities. This DBL microenvironment exhibits different pH behavior to bulk seawater, which can reduce the exposure of calcifying species to ocean acidification conditions. A low-cost time-domain dual-lifetime referencing (t-DLR) interrogation system and an optical fiber fluorescent pH sensor were developed for pH measurements in the DBL interface. The pH sensor utilized dual-layer sol-gel coatings of pH-sensitive iminocoumarin and pH-insensitive Ru(dpp)3-PAN. The sensor has a dynamic range of 7.41 (±0.20) to 9.42 ± 0.23 pH units (95% CI, T = 20 °C, S = 35), a response time (t90) of 29 to 100 s, and minimal salinity dependency. The pH sensor has a precision of approximately 0.02 pHT units, which meets the Global Ocean Acidification Observing Network (GOA-ON) "weather" measurement quality guideline. The suitability of the t-DLR optical fiber pH sensor was demonstrated through real-time measurements in the DBL of green seaweed Ulva sp. This research highlights the practicability of optical fiber pH sensors by demonstrating real-time pH measurements of metabolic-induced pH changes.

Optical fiber pH sensor based on a multimode interference device with polymer overlay.

An optical fiber pH sensor based on a multimode interference structure is presented. The sensitive element is a piece of no-core fiber (NCF) coated with a mixture of polyallylamine hydrochloride and polyacrylic acid by a modified layer-by-layer (LbL) self-assembly method. It is experimentally shown that by reducing the diameter of the NCF by chemical etching, the sensitivity is increased from -0.31n m/p H to -2n m/p H. The sensor exhibits a high linear response of 0.997 over a pH range from 5 to 11.3 with a rapid response time lower than 1s.

PANI-ZnO clad modified multimode optical fiber pH sensor based on EWA

In the present investigation, an intrinsic intensity modulated optical fiber pH sensor is developed and experimentally tested at room temperature. To fabricate the sensor probe, original cladding of multimode fiber was replaced by a pH-sensitive polyaniline-zinc oxide (PANI-ZnO) nanocomposite matrix whose absorbance property and refractive index (RI) are pH dependent. The synthesized PANI, ZnO, and incorporation of ZnO into PANI was confirmed and analyzed by some characterization techniques. It was observed that the ZnO composition into PANI not only enhances the materials's optical activity, porosity and hence sensitivity of the sensor but also boosts the mechanical strength of the coating layer over silica surface. Moreover, high index ZnO material is used to enhance the evanescent field towards the sensing region, which can help in better interaction of light from the outer medium. The pH sensing principle depends upon evanescent wave absorption (EWA) due to swelling/shrinkage of PANI and hence modification in volume and RI of the layer leads to change in transmitted optical power. The average sensitivity of proposed sensor was found to be 2.46 μW/pH for a wide pH range of 2–10 unit. The sensor also possesses excellent stability, fast response time, low hysteresis, good repeatability and durability characteristics. Results suggest that this sensor can be productively utilized in biomedical and chemical sensing applications.

Solvent-assisted titania nanoparticles based fiber optic pH sensor: structural, optical, and sensing characteristics

Solvent-assisted titania nanoparticles based fiber optic pH sensor: structural, optical, and sensing characteristics

Au and zincite assisted silica-titania nanocomposite coated fiber optic pH sensor: Structural thermal, and optical characteristics

Accurate pH evaluation is required in different fields including food science, agriculture, environmental monitoring, and medical research. The low-temperature sol-gel method is employed to synthesize the Au and zincite-assisted silica-titania nanocomposites for the pH assessment. A mixture of indicator dyes (phenol red, creosol red, phenolphthalein, and bromophenol blue) is encapsulated in mesoporous nanocomposites and coated on fiber optic. Crack-free, dense, and porous surface with low roughness and high thermal stability (≤500 °C) is observed for all three nanocomposites. Low roughness 4 nm, low refractive index 1.5, and high surface area 333 m2/g DST nanocomposite exhibited sensitivity 28.3 counts/pH which is greater than 17.5 counts/pH (DAuST) and 14 counts/pH (DZST). The fiber pH devices possessed reversible, stable, and fast response 0.19 s into pH 12 (DST), DZST ∼ 0.18 s, and DZST ∼ 0.22 s, without any leaching.

Plastic Optical Fiber pH Sensor Based on Sol-Gel Film

A plastic optical fiber (POF) pH sensor is developed based on a sol-gel film. A sol-gel film was prepared by mixing tetramethyl orthosilicate (TMOS), trimethoxymethylsilane (MTMS), ethanol, distilled water and Neutral Red (NR) powder. Then, plastic optical fiber is coated by manually depositing the sol-gel film onto the tip of the POF. The sensing probe is dissolved into analytes with different pH levels, such as Coca-Cola (pH 3), tomato sauce (pH 4), black coffee (pH 5), tea (pH 6), distilled water (pH 7), soap (pH 8), baking soda (pH 9) and Clorox bleach(pH 10). The Refractive index will increase when the analytes are more acidic or more alkaline because they are concentrated with hydrogen and hydroxide ions at pH 3 and pH 10. The output power is varied based on the chemical reaction with the pH sol-gel film in the fiber-optic sensing probe. From the experimental results, it is concluded that the sensitivity of coated POF sensor, which is 0.0566 a.u/RIU, is higher than the sensitivity of the uncoated POF, which is 0.0429 a.u/RIU.

Influence of the photopolymerization matrix on the indicator response of optical fiber pH sensors

Optical fiber pH sensors work by observing a change in the indicator’s optical signal caused by variations in pH and these indicators can be immobilized onto the surface of an optical fiber using a polymer matrix. How the composition of the polymer matrix changes pH detection range using the indicator (5(6)-carboxynaphthofluorescein (CNF)) has not been studied. Here we show that the composition of the polymer matrix affects the working pH ranges of optical fiber CNF sensors. We used acrylamide (AAm) or N-isopropylacrylamide (NIPAM) as the backbone monomer, and N, N’-methylenebisacrylamide (BIS) as the crosslinker for the polymer matrix. We found that AAm-based pH sensors showed rapid response over the pH range 6.6 – 8.0, while the dynamic ranges of NIPAM-based sensors shifted to basic pH compared with AAm-based pH sensors. Furthermore, we found that an increased ratio of the backbone monomer, NIPAM, over the crosslinker, BIS, significantly shifted the working range to more basic pH values, covering a pH range of 8.1 – 10.3. Our results demonstrate that the polymer matrix can be a powerful means to control the indicator response of optical pH sensors.

Rapid-Response and Wide-Range pH Sensors Enabled by Self-Assembled Functional PAni/PAA Layer on No-Core Fiber.

The measurement of pH has received great attention in diverse fields, such as clinical diagnostics, environmental protection, and food safety. Optical fiber sensors are widely used for pH sensing because of their great advantages. In this work, an optical fiber pH sensor is fabricated, by combining the merits of the multimode interference configuration and pH-sensitive polyaniline/polyacrylic acid (PAni/PAA) coatings, which was successfully in situ deposited on the no-core fiber (NCF) by the layer-by-layer (LBL) self-assembly method. The sensors' performance was experimentally characterized when used for pH detection. It has a high sensitivity of 0.985 nm/pH and a great linear response in a universal pH range of 2-12. The response time and recovery time is measured to be less than 10 s. In addition, its temperature sensitivity is tested to be about 0.01 nm/°C with a low temperature crosstalk effect, which makes it promising for detecting pH in the liquid phase with temperature variation. The sensors also demonstrated easy fabrication, good stability, and repeatability, which are adapted to pH detection in most practical applications.

Low-cost high-sensitivity pH sensor based on a droplet-shaped single-mode fiber Mach–Zehnder interferometer

In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability.

Influence of the Photopolymerization Matrix on the Indicator Response of Optical Fiber Ph Sensors

Influence of the Photopolymerization Matrix on the Indicator Response of Optical Fiber Ph Sensors

Thermally stable and fast responsive mesoporous cresol red functionalized silica and titania nanomatrices: fiber optic pH sensors

Thermally stable and fast responsive mesoporous cresol red functionalized silica and titania nanomatrices: fiber optic pH sensors

Distributed fiber optic pH sensors using sol-gel silica based sensitive materials

• Fiber optic pH sensors demonstrated repeatable and reversible pH sensing responses in the pH range of ∼8–13. • 1.6 um thick SiO 2 and 600 nm thick Au-SiO 2 coatings possessed good pH sensitivity with fast responses. • The developed fiber optic sensors are highly selective to pH compared to ionic strength and refractive index of solutions. • Distributed pH sensing was demonstrated using SiO 2 and Au-SiO 2 coated fiber optic sensors. Fiber optic pH sensors using either silica (SiO 2 ) or gold nanoparticle incorporated silica (Au-SiO 2 ) as the sensitive layers for pH monitoring are presented. A facile sol-gel dip-coating process was utilized to immobilize the SiO 2 based sensitive layers on the coreless fiber. In the high pH range of ∼8−12 simulating the wellbore cement conditions, the transmission spectra at room temperature demonstrated notable sensitivity using the SiO 2 based coating. The sensitivity was enhanced via either increasing the coating thickness or incorporation of Au nanoparticles. The sensitivity was 19.9 T%/pH for the 1.6 μm thick SiO 2 coating and 13.4 T%/pH for the 600 nm thick Au-SiO 2 coating, respectively. And these two sensors revealed good reversibility, and the response times were in the order of 10 s of seconds. No obvious chemical structure changes in SiO 2 thin film were observed through Fourier-transform infrared spectroscopy (FTIR) analysis after the film was exposed to the alkaline media for 30 min. However, the sensing layers were found to become thinner due to corrosion when tested in strong alkaline solutions with the pH up to 14 for 3 days, especially for the pure SiO 2 coating. The incorporation of Au nanoparticles was found to stabilize the pH sensing signals and prolong the lifetime of the pH sensitive layer. Importantly, with the coating of SiO 2 based sensitive materials, spatially distributed pH sensing was enabled. The intensity of the backscattered light increased with the increasing pH values along the coated segments on the fiber optic sensors.

Ex-Vivo Measurement of the pH in Aqueous Humor Samples by a Tapered Fiber-Optic Sensor.

A practical demonstration of pH measurement in real biological samples with an in-house developed fiber-optic pH sensor system is presented. The sensor uses 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) fluorescent dye as the opto-chemical transducer. The dye is immobilized in a hybrid sol-gel matrix at the tip of a tapered optical fiber. We used 405 nm and 450 nm laser diodes for the dye excitation and a photomultiplier tube as a detector. The sensor was used for the measurement of pH in human aqueous humor samples during cataract surgery. Two groups of patients were tested, one underwent conventional phacoemulsification removal of the lens while the other was subjected to femtosecond laser assisted cataract surgery (FLACS). The precision of the measurement was ±0.04 pH units. The average pH of the aqueous humor of patients subjected to FLACS and those subjected to phacoemulsification were 7.24 ± 0.17 and 7.31 ± 0.20 respectively.

Synthesis and characterization of bromophenol blue encapsulated silica and silica-titania nanocomposites for detection of volatile organic vapors

Owing to pH sensing applications in the food industry, bromophenol blue immobilized silica (BPB–S) and silica-titania (BPB-ST) nanocomposites are synthesized by the sol-gel route. Fundamental analysis confirms that both nanomatrices have porous surface and thermal stability at ≥ 400 °C. Evanescent wave absorption-based fiber optic pH sensors are prepared by coating of BPB-S and BPB-ST nanocomposites. when the evanescent field interacts with the sensing coating, absorption varied due to pH change and the output intensity is recorded as a sensor response. The sensitivity of the BPB-S and BPB-ST nanocomposite is calculated as −136.65 counts/pH and - 17.95 counts/pH, respectively. The BPB-ST nano-matrix exhibited a fast response 0.2 s than BPB-S nanocomposite response 0.8 s. The fish's freshness is determined visually by assessing the change in the color against the fish pH. The findings confirm that synthesized nanocomposites have the potential to detect volatile organic vapors in terms of pH.

Mesoporous nanostructures-based fiber optic pH sensors: Synthesis, structure-tailoring, physiochemical and sensing stimuli

Owing to opto-chemical sensing, three matrices i.e., SiO2, TiO2, and SiO2-TiO2 are synthesized and a mixture of four different organic dyes encapsulated in them through the sol-gel method. Thermally stable, large surface area 434 m2/gm, 197 m2/gm, 332 m2/g, and refractive index 1.49, 1.8, 1.5 at 632 nm are observed for d-SNPs (dyes encapsulated silica nanoparticles), d-TNPs (dyes encapsulated titania nanoparticles), and d-STNPs (dyes encapsulated silica-titania nanoparticles), respectively. When the evanescent field interacts with the fiber optic coating, absorption varied due to pH change and the output intensity is recorded as a sensor response in terms of transmission. The prepared sensors exhibited linear response, non-leachability, and stability towards high pH 12. The fast time response is estimated ∼ 0.35 s, 0.33 s, and 0.31 s in pH 12 for d-SNPs, d-TNPs, and d-STNPs, respectively. Therefore, these prepared optical fiber sensing devices have the potential for practical pH sensing applications.