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Article

An Efficient pH Detector for Water Contamination Based on Mach–Zehnder Interferometer Application

by
Mario Angel Rico-Mendez
1,†,
Romeo Selvas
1,*,†,
Oxana V. Kharissova
1,†,
Daniel Toral-Acosta
1,†,
Norma Patricia Puente-Ramirez
2,†,
Ricardo Chapa-Garcia
1,† and
Abraham Antonio Gonzalez-Roque
1,†
1
Facultad de Ciencias Físico-Matemáticas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza 66455, Mexico
2
Facultad de Ingenieria Mecanica y Electrica, Universidad Autónoma de Nuevo León, San Nicolás de los Garza 66455, Mexico
*
Author to whom correspondence should be addressed.
†
Current address: Av. Universidad S/N, Cd. Universitaria, San Nicolás de los Garza 66455, México.
Sci 2024, 6(4), 80; https://doi.org/10.3390/sci6040080
Submission received: 9 October 2024 / Revised: 20 November 2024 / Accepted: 26 November 2024 / Published: 2 December 2024
Browse Figures
Figure 1
<p>In the geometric structure of the sensor, the regions labelled with A, B, and C are 1 mm of length, corresponding to the DT, UT, and WT, respectively, and region B has a diameter of 40 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m.</p> "> Figure 2
<p>Transmission spectrum of the Mach–Zehnder Interferometer (MZI), observed in the absence of an external surrounding medium.</p> "> Figure 3
<p>Simulated evolution of the input electromagnetic field E is highlighted in the taper regions of the optical fiber.</p> "> Figure 4
<p>Simulated transmission spectrum of an MZI featuring two tapers, each with a transition length of 1 mm, a waist length of 40 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m, and a separation of 10 mm between the tapers.</p> "> Figure 5
<p>Setup for pH optical fiber sensor.</p> "> Figure 6
<p>Stability of the output power of pH 4.0 in a period of 50 min; the axis output power was adjusted (shift) for better data appreciation.</p> "> Figure 7
<p>Comparative of the interference spectrum to dip centered at 1559 nm of transmission of MZFI for each pH sample.</p> "> Figure 8
<p>Output power shifting of the central response from the sensor when submerged in buffer pH values of 4 to 10 in a time of 0–50 min.</p> "> Figure 9
<p>Wavelength shifting of the central response from the sensor as a function of the time in a range of 50 min.</p> ">
Versions Notes

Abstract

:
This paper presents a pH sensor with a Mach–Zehnder Interferometer (MZI) that operates in solutions of 4.0, 7.0, and 10.0. The sensor device consists of two tapered sections with dimensions of 1 mm/1 mm/1 mm for down-taper, waist-length, and up-taper, respectively, with a separation of 10 mm. The diameter of the waist is 40  μ m. This work includes the experimental evaluation of an MZI fiber optic pH sensor at 1559 nm, where 1559 nm represents a specific wavelength chosen for its optimal sensitivity in evaluating the sensor pH detection performance. It is not the central wavelength of the optical fiber, but one of the minimal values selected to enhance the interaction between the evanescent field and the sample, ensuring the reliable detection of pH variations. These sensor dimensions and the functionalized solution of multi-walled carbon nanotubes (MWCNTs) increase the detection of pH in dyes used in the textile industry. Alizarin is a strong anionic red dye that is part of the anthraquinone dye group. The experimental results demonstrated effective detection of pH levels in water contamination involving dye. This development could resolve the problem with Alizarin. The simple fabrication, low cost, and stability of the optical response make this sensor relevant for pH measurements in water contamination.

1. Introduction

Fiber optic sensors have attracted attention for chemical and biochemical monitoring and applications over the years [1,2] due to features such as their high-resolution detection, ability to operate in different environments, immunity to electromagnetic noise, small size, and high sensitivity. Researchers have successfully used optical fiber sensors to detect parameters such as humidity, salinity, ammonia gas, and temperature. Ongoing research focuses on enhancing sensitivity, improving resolution, reducing costs, and refining the fabrication techniques of these sensors. Given these unique characteristics of optical fiber sensors, they can be used as chemical sensors for the detection of water contaminants [3].
The growing demand for color has led to the use of dyes in various applications, including paper, textiles, cosmetics, and other industrial applications. In addition, the dyeing process generates a significant amount of wastewater, which contributes to the pollution of rivers [4] and lakes with residual dyes. Exposure to these dyes can cause skin irritation, inflammation, allergic reactions, and in severe cases, carcinogenic poisoning [5], affecting human health.
The study of the environment is crucial to maintain the proper functioning of various human activities, ranging from well-being to complex industrial processes [6,7]. Therefore, the accurate monitoring and control of critical ecosystem parameters are required to treat resources efficiently, safely, and effectively in settings such as the oil and natural gas industries, among others. Because of this, unexpected changes that threaten the environment [8] or human health must be closely monitored.
Recent monitoring studies have focused on the analysis of the chemical and physical properties of the environment [9]. Moreover, the pH value is one of the most important physiological parameters. Every organism made up of cells is exposed to different levels of acidity, and only one pH level is the most appropriate to maintain its normal biological function. Due to these acidity levels, water is considered suitable for human consumption. Electrochemical sensors have been developed as pH monitoring sensors utilizing various examination or detection principles [10]. These sensors typically consist of electrodes coated with a catalyst that promotes the oxidation or reduction of chemical species; this is a disadvantage due to its low average useful life. Instead, in [11], a down-tapered detector is proposed using an optical fiber formed by fusing a single-mode fiber (SMF) and a multi-mode fiber (MMF), with a sensitivity of 0.49 dBm/pH; however, this process requires a lot of time and high costs to manufacture. Other electronic detectors, such as electromechanical and thermoelectric detectors, have also been studied as monitoring sensors in harsh environments. These devices have a limited service life in extreme environments due to drag failures, plastic deformation, corrosion, and electrical interference [12].
In addition, their light weight, small size, and ease of use and handling, due to their ability to bend without significant signal loss, make them essential for monitoring with detection schemes. The optical fibers provide a convenient method for creating optical sensors by directing light from the sensing region, known as the intrinsic or extrinsic sensor. Furthermore, optical fibers are biocompatible. Fiber optic sensors offer several advantages over conventional sensor techniques [13,14,15].
The Mach–Zehnder Interferometer (MZI) was selected as the foundation for pH detection due to its well-known advantages. The MZI offers high sensitivity to refractive index changes, which makes it highly suitable for detecting small pH variations. Its compact design allows for easy integration into fiber optic systems, supporting portable and remote sensing applications [16,17].
In this study, an MZI-type device with a dual concatenated tapered geometry is presented. The dimensions of the three regions are as follows: The first region tapers, narrowing in the x-y plane from a diameter of 125  μ m to 40  μ m; the second region maintains constant at 40  μ m in the x-y plane; and the third expands in the x-y plane from a diameter of 40  μ m to 125  μ m. The mentioned regions have lengths of 1 mm each along the z axis, and the separation of these two conical regions was set at 10 mm. It should be noted that the proposed sensor is practically unaffected by changes in ambient temperature within the experimental range of 25 °C to 32 °C, and the temperature did not adversely affect the results. The manufacturing process of this sensor demonstrated repeatability and consistency; this resulted in devices that exhibited identical spectra responses. The proposed sensor exhibits minimal drift and does not require frequent recalibration, which is typical for conventional pH meters on glass electrodes.
The organization of the paper is as follows. In the Materials and Methods Section, there is a description of the type of sensor used, the experimental methodology, and the numerical method employed, specifically, the finite-difference time-domain (FDTD) technique. The Experimental Results Section presents the results for pH values of 4, 7, and 10, including the spectral response of each. A measurement was performed over 3000 s, with intervals of 600 s, to detect pH in water contaminated with Alizarin Red S, supported by carbon nanotubes with OH groups. Additionally, the wavelength shift over the 3000 s is analyzed. Finally, in the Discussion Section, we propose the use of more pH samples to further assess the sensitivity of the device and to evaluate the effect of temperature changes.

2. Materials and Methods

The MZI operates based on interference between two light modes, one through the core and the other through the cladding of the fiber. The interference pattern depends on the optical path difference (OPD) between these two modes. When the fiber is exposed to different pH values, the refractive index of the surrounding medium changes as a result of the varying levels of protonation or deprotonation. This, in turn, alters the OPD and changes the interference pattern. The shift in the interference spectrum corresponds to changes in the wavelength at which constructive or destructive interference occurs, allowing for the detection of pH values.
A method of manufacturing fiber optics sensors involves thinning a relatively small section of optical fiber. This sensor provides access to the evanescent wave (EW) as it propagates through the conical (thinned) region, allowing interaction with the surrounding environment. It also allows for the measurement of the refractive index (RI) or other variable chemical composition [18]. In research papers, pH detection in tangible biological samples has been proposed using an internally developed fiber optic pH sensor [19,20].
The device proposed for the experiments carried out through this research employs a configuration constructed using the tapered method, which consists of two sections with a separation of 10 mm between them and a specific waist diameter. The optical fiber used for this work was a single-mode SMF-28e (Corning, New York, USA) with core and cladding diameters of 8  μ m and 125  μ m, respectively. The tapers were produced using a Vytran® GPX 3400 glass processor and a Vytran V4 heating element. In this processor, the two taper shapes were made in the optical fiber. The transition sections in the structure are called the down taper (DT) and the up taper (UT), of 1 mm in length each, and the waist (WT) section, of 40  μ m in diameter, as can be seen in Figure 1. The presence of these taper sections in the optical fiber allows for increased attenuation. It is because of this that it is vitally important to secure these lengths between the two taper sections.
During the fabrication process, these parameters are controlled through precise tapering techniques using the GPX 3400 Vytran automatic glass processing system, which enables the controlled heating and pulling of the optical fiber to achieve the desired taper geometry. Real-time monitoring of the tapering process ensures consistency and accuracy in dimensions. These parameters were selected on the basis of previous experimental studies, thereby increasing the sensitivity to refractive index alterations attributed to variations in pH. The four essential fabrication parameters affecting the background loss introduced by the tapering process of the optical fiber have been identified as follows: The starting power (SP) refers to the initial power applied for filament heating at the commencement of the tapering process; the pull velocity (PV) refers to the rate at which the fiber is pulled through the filament heat source during both the initial and final phases of the tapering process.
The pull delay (PD) is the heating time before the tapering process starts; and filament delta (FD) is the decrease in filament power throughout the process of waist fabrication since a smaller volume is being heated. The determination of the necessary four input parameters is achieved by employing typical fabrication values, which are informed by the experiential knowledge gained through the use of the Vytran glass. Typical values for tapering a 125  μ m diameter fiber are SP  = 42  W, PV = 1.0 mm/s, PD = 1.0 ms, and FD = −10%. An additional optimization step was not necessary in the fabrication process, as the chosen dimensions have demonstrated efficacy in detecting pH variations during our experimental procedures.
A fiber optic sensor with the characteristics described above is proposed for pH detection. The sensor presents interference patterns with three peaks, exhibiting an extinction ratio of 16.7 dB in the spectrum at wavelengths between 1520 and 1600 nm, as shown in Figure 2. This spectrum, without pH solution, does not show a complete sinusoidal wavefront due to the pair of optical paths formed by the structure of MZI, resulting in intermodal interference between the core and the cladding [21,22,23].
The proposed structure is sufficient for the fundamental mode of light from the source to propagate and excite higher-order propagation modes. This structure allowed for interaction with the surrounding medium, allowing measurement of parameters such as refractive index (RI) or buffer pH.
Due to the phase change between the effective refractive indices of the tapered sections, waist diameter, and surrounding medium, the interference depends mainly on the degree of interaction between the fundamental core mode and the cladding modes.
The light source propagates through the proposed structure and interacts with the downsizing transition section of the taper segment. The fundamental and higher order modes undergo different optical paths when propagated at the waist until they reach the magnifying transition section of the taper part. Subsequently, within the section waist of uniform diameter that is ideally invariant along its length, the fundamental and higher order modes travel independently, each obtaining a distinct phase shift due to their varying effective refractive indices.
This device maintains the phase difference along the 10 mm separation region between the two tapered sections. This effect is possible because a border is formed between the core and the cladding in the thinnest section, in this case, the air. Once the light reaches the second tapered section, it propagates like in the first section of the structure. Additionally, the light traveling through the core and the cladding experiences destructive interference, resulting in three dips, commonly referred to as notches [24,25,26], with a fluctuation of 16.7 dBm at the wavelengths of 1537.6 nm, 1561.5 nm, and 1583.62 nm in the transmission spectrum.
These fringes in the transmission spectrum are very useful for monitoring phase changes as pH values increase or decrease. This is possible because the interference spectrum shows changes with different pH samples. The following expression offers a simplified way to explain the modal interference in the MZI [27]:
I o u t = I n 1 + I n 2 + 2 ( I n 1 I n 2 ) c o s θ ,
Equation (1) describes the modal interference in the MZI, where the output light intensity  ( I o u t )  results from interference between the light intensities  ( I n 1 )  and  ( I n 2 )  from the core and cladding. Therefore, when light enters the MZI sensor, interference between the fiber propagation modes occurs as it travels through the first taper and converges in the second. Consequently, a phase change exists between the modes traveling in the optical paths formed by the core and the cladding [28], as determined in Equation (2).
Δ ϕ = 2 π λ ( Δ η e f f L )
where  λ  is the central wavelength of the light source, and L is the uniform length of the waist; the expression  n e f f  is the difference between the effective refractive indices of the core and the cladding of the SMF [29].
Δ η e f f = η e f f c o r e − η e f f c l a d d i n g
Therefore, the transmission signal shows notches in wavelengths [30] described by Equation (4).
λ m = ( Δ η e f f ) L m
The parameter  Δ η e f f  will change if the RI of the solution being measured differs;  λ m  describes the change of order m of the interference spectrum given by Equation (5) [31].
Δ λ m = ( Δ η e f f + Δ n ) L m − Δ η e f f L m = Δ L m
where  Δ n  is the change in the refractive index; consequently, Equation (5) is described as the variation of the transmitted signal as a function of  Δ n  when the length L between the optical paths of the sensor is constant.
The light source and the geometry of the proposed sensor were simulated using the numerical tool called the finite-difference time-domain (FDTD), because the method uses the full-wave solution of the Maxwell equation in discretized space. This method proposes the numerical calculation algorithm for solving the electric field and magnetic field. The equations for the electric field and magnetic field are given by the following updated equations.
H y | i , j + 1 / 2 n + 1 / 2 = H y | i , j − 1 / 2 n − 1 / 2 − Δ t μ 0 E z | i , j + 1 n − E z | i , j n Δ H x | i , j + 1 / 2 n + 1 / 2 = H x | i , j − 1 / 2 n − 1 / 2 − Δ t μ 0 E z | i , j + 1 n − E z | i , j n Δ E z | i , j n + 1 = E z | i , j n + Δ t ϵ 0 H x | i + 1 / 2 , j n + 1 / 2 − H x | i − 1 / 2 , j n + 1 / 2 Δ − H y | i , j + 1 / 2 n + 1 / 2 − H y | i , j − 1 / 2 n + 1 / 2 Δ
Here, the subscript is the spatial position, and the superscript denotes the time steps. The calculations were performed by providing input parameters such as the refractive indices of the sensor, its proposed geometry, and the wavelength of the light source. The parameter used in the FDTD calculations of this taper MZI was the length of 18 mm. This length refers to the totality of the two concatenated tapers separated 10 mm from each other, in addition to the regions DT, WT, and UT with a length of 1 mm, respectively. For the two tapers, to the ends of the start and end of the proposed sensor, a length of 1 mm was added for the entry and exit of the light source. The light source used was a sine wave as follows:  E s r c = E 0 s i n ( 2 π s f r e c ( i ) t 0 ) . The wavelength range of the source is 1400 to 1600 nm. The refractive indices used for the construction of the geometry of sensor are 1.47 in the core and 1.46 in the coating.
Figure 3 shows the cross section of the plane x-z perpendicular to the direction of propagation z of the geometric structure of the sensor, where the different interference regions of the electromagnetic field E are highlighted in the thinning regions of the optical fiber. These regions are crucial to understanding how light propagates through the fiber and how various modes interfere with each other.
Figure 3 additionally shows the intensity profile of the radial distribution as a function of the z-axis of propagation. At z = 1.1 mm, the intensity is maximal, corresponding to the region where the electric field intersects the optical fiber. At z = 3.5 mm, it is discernible that propagation occurs solely in the fundamental mode, attributed to the dimensions of the beam waist. Subsequently, as propagation continues at z = 6 and 10 mm, the radial profile broadens, indicating propagation through the core and cladding without attenuation. Upon reaching the second concatenated waist, the fundamental mode persists, albeit with diminished intensity. The radial profile of the resultant spectrum is observed at z = 18 mm for a wavelength of 1559 nm.
This was performed using the FDTD algorithm. According to the FDTD simulation results, the transmission loss within the optical fiber is minimal and is, thus, considered negligible. This implies that the energy loss as the light propagates through this section is so small that it does not significantly affect the overall performance or results of the simulation.
The interaction that occurs between the fundamental mode and a cladding mode along the entire geometry of the MZI between the two tapers separated by L = 10 mm is also shown. The total length of sensor is 18 mm, and 1 mm was added at the beginning and end of the simulation; in these initial and final sections, the electric field propagates freely through the solution used for the simulation, in this case, pH 7.
Figure 4 shows the spectrum generated by the proposed geometry, scanning the signal from the source so that a response is obtained for each wavelength in the range of 1500 to 1600 nm.
These spectra correspond to the sensor with a surrounding medium with a refractive index of 1.35093 corresponding to pH 4, the sensor with a surrounding medium such as water with a refractive index of 1.3333 and a pH of 7, and the spectrum with a surrounding medium such as soap with a refractive index of 1.502 and a pH of 10. The spectrum obtained numerically using the FDTD corresponding to pH 4 (citrus) has dips at 1559.93 and 1580.23 nm, the spectrum obtained numerically corresponding to pH 7 (water) has dips at 1568.07 and 1588.03 nm, and the spectrum obtained numerically corresponding to pH 10 (soap) has dips at 1560.15 nm and 1580.23 nm.
Furthermore, the resulting spectrum corresponding to pH 4 has a dip at 1559.93 nm. pH 7 has dips at 1568.07, and pH 10 has dips at 1560.15 nm. Hence, a shift in wavelength of 7.92 nm and 7.91 nm is observed when the refractive index changes due to pH values.

3. Experimental Results

The sample utilized for detection involves the use of a multi-walled carbon nanotube (MWCNT) functionalization in a low quantity of 0.1 g/L added with a solution of Alizarin Red S (ARS) in 10 mL volumes of solution. The optical fiber sensors have the potential to undergo modification through the utilization of functional solutions, such as nanoparticles or nanostructures. With their distinctive structure and properties, carbon nanotubes (CNTs) are extremely attractive for application as a solution functional [32].
The spray pyrolysis method, augmented by Chemical Vapor Deposition (CVD), was employed to synthesize an array of Multi-Walled Carbon Nanotubes (MWCNTs). To introduce carboxyl functional groups (-COOH) on the surface of MWCNTs, the nanotubes were subjected to nitric acid ( HNO 3 ) treatment at 120 °C for one hour. For the generation of functional groups of hydroxyl (-OH) on the MWCNT surfaces, the nanotubes were exposed to hydrogen peroxide ( H 2 O 2 ) at 100 °C for one hour. Subsequently, a tapered fiber optic sensor container was filled with MWCNT-COOH and Alizarin Red S (ARS) at a concentration of 100 mg/L, at various pH levels (pH 4, pH 7, and pH 10).
The samples corresponding to pH values of 4.0, 7.0, and 10.0 were prepared utilizing standard buffer solutions sourced from certified suppliers. These buffers represent acidic, neutral, and basic conditions, which are critical to evaluating sensor performance across a wide pH range. This preparation involved combining the solution of iron salts with a suspension of f-MWCNTs under ultrasonication to achieve a homogeneous distribution of the iron-containing nanomaterial on their surface, followed by the reduction of the iron salts using  N 2 H 4  (hydrazine). Alizarin acted as an interfering dye within the pH buffer solution.
MWCNTs functionalized with hydroxyl groups (MWCNT-COOH) were used in combination with Alizarin Red S (ARS) to enhance the pH detection capability of the fiber optic sensor. MWCNT-COOH increases the surface area available for the adsorption of ARS, which, in turn, amplifies the interaction between the pH sensitive dye and the optical fiber. The changes in refractive index induced by the pH-sensitive ARS were more effectively transmitted to the optical fiber sensor. The experimental setup used in this study is shown in Figure 5, where a light source emitted in the range of 1520–1600 nm with a power of  − 20  dBm was used to transmit light through the arrangement. This improved the sensitivity of sensor to pH variations, as indicated by the change in the interference patterns recorded over time, as shown in Figure 6.
The MS9740A Anritsu optical spectrum analyzer model (OSA) was used to sense transmitted light and analyze its optical transmission characteristics [33]. The MZI sensor used in this study has high sensitivity to fiber torque and was, thus, fixed with two clamps at both ends. The sensor head was then immersed in a container of 20 mL solutions with different buffer pH 4, 7, and 10, all buffers belonging to the Analytyka company, and carefully prepared at an environment temperature of 24 °C.
In this study, we selected three pH values—acidic (pH 4), neutral (pH 7), and basic (pH 10)—as they represent key points across the pH spectrum where significant changes in the response of the sensor can be observed. Acidic buffer at pH 4 was chosen to test the sensor performance in environments with high protonation levels, causing notable changes in the refractive index [34,35,36]. A neutral pH of 7 serves as a critical reference point, facilitating the identification of deviations in both acidic and alkaline environments, and acts as a baseline for sensor calibration [37,38]. The basic buffer at pH 10 was used to evaluate the sensor response in alkaline conditions, where deprotonation occurs, resulting in further refractive index changes. These pH values span a broad range of environments, allowing for a comprehensive demonstration of the sensor’s ability to detect pH variations. In future experiments, additional pH points will be considered to provide a more detailed analysis of sensor performance [39,40].
In order to investigate the response of the interference spectrum to changes in the refractive index (RI) for each buffer pH sample, the light source from OSA conducted interrogations on the samples for each pH for periods of 600 s. To confirm the precision and reliability of the measurements, five measurements were made in a total period of 3000 s. The spectral data acquired at each temporal interval are presented in Figure 6. It is evident that at the initial time point t = 0 s, the transmission spectrum exhibits a lower intensity compared to that at the final time point t = 3000 s. Furthermore, a gradual, near-linear shift in wavelength is discernible over the duration of the observation.
The transmission spectra corresponding to pH measurements exhibit a notable shift at 1559 nm, attributed to variations in the refractive index (RI) of the examined pH values. Measurements were conducted on pH solutions with values of 4, 7, and 10, respectively.
At the conclusion of each pH measurement value, the optical fiber was meticulously rinsed with deionized water to eliminate any extraneous impurities and residual pH, thereby preventing potential contamination.
The transmission spectra show that each pH value produces a wavelength change after a time delay of 600 s. This change in wavelength is due to the RI of the solution and the pH values, which affect the optical path of the light source as it passes through the sensor head. The transmission spectra of pH 4, pH 7, and pH 10 were compared, and the change in the wavelength displacement was compared depending on the type of pH measured.
Figure 7 shows the bottom line fringe at 1559 nm, which was selected to observe the variations in the spectrum of different pH values. The spectrum of the sensor depends directly on the interaction between the core and cladding modes with the dissolved liquids surrounding it. The change in fringes is caused by the interaction of the fundamental and higher-order modes along the uniform waist, where they converge and change abruptly downwards and upwards. The spectral change between these modes will depend on the phase shift between their different effective refractive indices, mainly on the degree of interaction between the cladding mode and the refractive index of the surrounding solution.
The properties of the solution are altered by the adsorption of Alizarin Red S by the MWCNTs of functionalized -OH groups. The carboxyl groups of MWCNT-COOH can form favorable electrostatic interactions with the positively charged ARS species. This results in efficient adsorption, due to the density of the hydrogen ion in the different pH solutions, which showed a significant decrease in the absorption dip to 1559 nm.
Figure 8 shows the relationship between the fluctuations of the output spectrum and the applied pH value in a period of 50 min, on the first day that the pH 4 was measured, and later, on the consecutive days that the pH 7 and pH 10 were measured. Regarding optical power stability within a measurement time between 0 and 50 min, the results show fluctuations of ≤−3.06 dBm, ≤−1.348 dBm, and ≤−1.491 dBm for buffers pH 4, pH 7, and pH 10, respectively. Figure 9 shows the stability of wavelength in a sample with a fixed pH value. The wavelength displacements are ≤0.52 nm, ≤0.26 nm, and ≤0.26 nm for buffer pH 4, pH 7, and pH 10, respectively. These variations are due to external changes in the arrangement of the fiber optics sensor. The quasi-linear behavior of these fluctuations allows observing a constant change in the RI, in a period of 50 min. Due to the difference in optical path due to the absorption of ARS, generated by the -OH carbon nanotubes diluted in the pH buffers, the alteration in RI will result in a shift in the peak resonance wavelength.
The slopes of the fluctuations over time remain at a considerable distance, which leads to determining that the values of acids such as pH 4 maintain an upward slope, while alkaline substances such as pH 10 maintain a downward slope. Therefore, pH 7, which is neither acidic nor alkaline, shows a quasi-horizontal zero slope.
It is essential to mention that the resolution of OSA was 0.03 nm to determine the detection of pH values. Using functionalized substances such as -OH carbon nanotubes can enhance the electrical and optical properties of fiber-optic sensors. These enhancements can sense these changes in properties between different pH buffers.
To ensure sensor stability, measurements were made for 50 min at 10 min intervals and measurement sessions were carried out; the proposed sensor retains its optical characteristics. The temperature was also considered in our measurement, as a temperature plate controller was located in the sensor head. As a result, the transmission remains constant for environment temperatures ranging from 25 °C to 32 °C. This makes the sensor suitable for continuous buffer pH monitoring in a controlled temperature setting.
The pH and temperature parameters are stringently regulated to mitigate the potential for cross-interference. Through independent quantification, it is determined that each variable remains unaffected by fluctuations in the other. For instance, during pH value assessments, temperature conditions are meticulously maintained and controlled. This methodological approach precludes any inadvertent interactions between the two variables and ensures that the observed responses are precise, reflecting solely the specific effects of the investigated variables.
However, the absorbance spectra of the ARS solution were carefully monitored before and after the introduction of the MWCNTs to determine the sensor sensitivity [41].
The sensor was evaluated at pH levels of 4, 7, and 10, the aforementioned values representing acidic, neutral, and basic environments. Based on the observed spectral shifts and the sensitivity of MZI to refractive index variations, the sensor is reliably estimated to detect pH values within a range of approximately 4 to 10.
In general, the experimental results obtained in this study suggest that the MZI sensor can effectively sense changes in RI due to changes in buffer pH, making it a promising indirect tool for pH sensing applications.

4. Discussion

The study demonstrates the effectiveness of a tapered fiber sensor head in detecting various pH values in a solution containing Alizarin red S and hydroxyl-functionalized CNTs (-OH). The sensor shows a monitored behavior of pH fluctuations and wavelength shifts over a period of 50 min, attributed to changes in the refractive index (RI) in different pH buffers.
Furthermore, the device consistently senses Alizarin Red S absorption over time when diluted with multi–walled carbon nanotubes functionalized with -OH groups. This finding is significant for measurements at a wavelength of 1559 nm. The finite-difference time-domain (FDTD) method used in the numerical analysis confirms that the fiber optic sensor can sense changes in the pH value through variations in refractive index at different pH levels.
The refractive index of a solution is directly influenced by its pH level as a result of ionic interactions and alterations in the molecular composition of the medium. In acidic environments, increased protonation leads to a denser ionic environment, which can elevate the refractive index. On the other hand, in basic conditions, deprotonation reduces the ionic density, resulting in a lower refractive index. These variations are essential for the functionality of the MZI sensor, as alterations in the refractive index result in quantifiable changes in the optical path difference of the sensor. While the sensor exhibits sensitivity to pH values between 4 and 10, further calibration is required to assess its performance in more pH ranges.
However, a significant constraint is the sensitivity of the device to temperature variations, which compromises the precision of the pH measurement under ambient conditions. Subsequent investigations should aim to address this constraint to improve the robustness and reliability of sensor in diverse thermal environments.
The sensor operates on the principle of wavelength demodulation, where shifts in the interference spectrum are caused by changes in the optical path difference resulting from pH variations. However, in the experimental analysis, power demodulation was used as a complementary approach. This method involves monitoring changes in power at specific wavelengths, which correlate with changes in interference pattern.
The employment of power demodulation was motivated by its straightforward approach to data acquisition and processing, while sustaining the reliability of pH detection. This methodology offers an alternative perspective on the sensor response, particularly when examining spectral power variations. Both methods confirm the sensitivity to pH changes, and their combined analysis ensures robustness in the interpretation of the results.
In future work, we will address the limitations posed by the current device structure that have made it difficult to perform temperature-controlled experiments. The integration of a thermal plate or other temperature regulation methods will be prioritized to evaluate the performance of the sensor under varying temperature conditions.

5. Conclusions

The tapered fiber sensor head can effectively detect different pH values in a solution of Alizarin red S and -OH carbon nanotubes. The sensor exhibits a monitored behavior of these fluctuations and a wavelength change in a period of 50 min, due to the change in IR in the different pH buffers measured. The device provides a fluctuation of ≤−3.06 dBm, ≤−1.348 dBm, and ≤−1.491 dBm for pH values of 4, 7, and 10, respectively. The fluctuations detected in the numerical analysis, due to the variations of the pH value in the proposed device, were  − 3.53  dBm between pH 4 and pH 7, and 20.56 dBm between pH 7 and pH 10, the latter due to a large change in the refractive index of 0.17.
Additionally, the device can sense a constant absorption of Alizarin Red S over time when diluted with multi-walled carbon nanotubes of -OH groups, making this work a significant contribution to the measurement range at 1559 nm. Numerical analysis using the FDTD method demonstrates that the fiber optic sensor is capable of detecting the change in pH value, due to the change in the refractive index at each pH value.
The device we propose will allow the remote and operational detection of the pH value in drinking water and contaminated water in storage tanks with several sensors in real time. Furthermore, the detector has the advantage of being a compact structure and quick to respond, with simple and economical manufacture, and can be used as a sensor for biochemical solutions with different pH values.

Author Contributions

Conceptualization, R.S. and O.V.K.; Data curation, M.A.R.-M. and R.S.; Formal analysis, M.A.R.-M. and R.S.; Investigation, M.A.R.-M.; Methodology, M.A.R.-M.; Project administration, R.S. and O.V.K.; Resources, R.S. and O.V.K.; Software, M.A.R.-M.; Supervision, R.S.; Validation, R.S., O.V.K. and D.T.-A.; Visualization, M.A.R.-M., N.P.P.-R., R.C.-G. and A.A.G.-R.; Writing—original draft, M.A.R.-M. and R.S.; Writing—review and editing, R.S., O.V.K. and D.T.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported in part by the scientific and technology research support program (PAICYT) hosted by the Universidad Autónoma de Nuevo Leon (UANL), by project 513-IT-2022.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author, and the dataset was jointly completed by the team, so the data are not publicly available.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. In the geometric structure of the sensor, the regions labelled with A, B, and C are 1 mm of length, corresponding to the DT, UT, and WT, respectively, and region B has a diameter of 40  μ m.
Figure 1. In the geometric structure of the sensor, the regions labelled with A, B, and C are 1 mm of length, corresponding to the DT, UT, and WT, respectively, and region B has a diameter of 40  μ m.
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Figure 2. Transmission spectrum of the Mach–Zehnder Interferometer (MZI), observed in the absence of an external surrounding medium.
Figure 2. Transmission spectrum of the Mach–Zehnder Interferometer (MZI), observed in the absence of an external surrounding medium.
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Figure 3. Simulated evolution of the input electromagnetic field E is highlighted in the taper regions of the optical fiber.
Figure 3. Simulated evolution of the input electromagnetic field E is highlighted in the taper regions of the optical fiber.
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Figure 4. Simulated transmission spectrum of an MZI featuring two tapers, each with a transition length of 1 mm, a waist length of 40  μ m, and a separation of 10 mm between the tapers.
Figure 4. Simulated transmission spectrum of an MZI featuring two tapers, each with a transition length of 1 mm, a waist length of 40  μ m, and a separation of 10 mm between the tapers.
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Figure 5. Setup for pH optical fiber sensor.
Figure 5. Setup for pH optical fiber sensor.
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Figure 6. Stability of the output power of pH 4.0 in a period of 50 min; the axis output power was adjusted (shift) for better data appreciation.
Figure 6. Stability of the output power of pH 4.0 in a period of 50 min; the axis output power was adjusted (shift) for better data appreciation.
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Figure 7. Comparative of the interference spectrum to dip centered at 1559 nm of transmission of MZFI for each pH sample.
Figure 7. Comparative of the interference spectrum to dip centered at 1559 nm of transmission of MZFI for each pH sample.
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Figure 8. Output power shifting of the central response from the sensor when submerged in buffer pH values of 4 to 10 in a time of 0–50 min.
Figure 8. Output power shifting of the central response from the sensor when submerged in buffer pH values of 4 to 10 in a time of 0–50 min.
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Figure 9. Wavelength shifting of the central response from the sensor as a function of the time in a range of 50 min.
Figure 9. Wavelength shifting of the central response from the sensor as a function of the time in a range of 50 min.
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MDPI and ACS Style

Rico-Mendez, M.A.; Selvas, R.; Kharissova, O.V.; Toral-Acosta, D.; Puente-Ramirez, N.P.; Chapa-Garcia, R.; Gonzalez-Roque, A.A. An Efficient pH Detector for Water Contamination Based on Mach–Zehnder Interferometer Application. Sci 2024, 6, 80. https://doi.org/10.3390/sci6040080

AMA Style

Rico-Mendez MA, Selvas R, Kharissova OV, Toral-Acosta D, Puente-Ramirez NP, Chapa-Garcia R, Gonzalez-Roque AA. An Efficient pH Detector for Water Contamination Based on Mach–Zehnder Interferometer Application. Sci. 2024; 6(4):80. https://doi.org/10.3390/sci6040080

Chicago/Turabian Style

Rico-Mendez, Mario Angel, Romeo Selvas, Oxana V. Kharissova, Daniel Toral-Acosta, Norma Patricia Puente-Ramirez, Ricardo Chapa-Garcia, and Abraham Antonio Gonzalez-Roque. 2024. "An Efficient pH Detector for Water Contamination Based on Mach–Zehnder Interferometer Application" Sci 6, no. 4: 80. https://doi.org/10.3390/sci6040080

APA Style

Rico-Mendez, M. A., Selvas, R., Kharissova, O. V., Toral-Acosta, D., Puente-Ramirez, N. P., Chapa-Garcia, R., & Gonzalez-Roque, A. A. (2024). An Efficient pH Detector for Water Contamination Based on Mach–Zehnder Interferometer Application. Sci, 6(4), 80. https://doi.org/10.3390/sci6040080

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