1. Introduction

Accurate and localized pH monitoring is indispensable in biological systems, primarily due to the significant implications slight pH variations hold in health and disease. This need is particularly underscored by its pivotal role in the development of implantable medical devices and advanced drug delivery systems, where accurate pH detection within confined spaces is imperative [1,2]. These systems, operating within the human body, depend on pH-sensitive mechanisms for controlled drug release and real-time physiological feedback. Furthermore, understanding the pH levels of human cells holds significant importance [3]. For instance, in the cerebrospinal fluid, a mere alteration from its typical pH range of 7.3 to 7.4 can hint at serious health anomalies like meningitis or brain abscesses [4]. Similarly, regions like intracellular organelles or arterioles, sized approximately 10-25 micrometers, are highly susceptible to metabolic dysfunctions and disorders like acid reflux, where even minute pH discrepancies can be consequential [5]. Such nuanced measurements necessitate advanced sensors that can delve into these minuscule regions without disruption. Conventional sensors, lacking this precision, might miss these pH variations, resulting in missed diagnoses or inaccurate treatments. A pH sensor that aligns with these stringent requirements, both in terms of sensitivity and size, is thus pivotal, promising improved and precise diagnostic capabilities, which in turn could revolutionize treatment protocols and patient outcomes.

Fiber optic sensors (FOS) are devices that use optical fibers to detect physical or chemical changes in their environment. FOS often operates by utilizing the principle of total internal reflection within an optical fiber. Light is guided along the fiber's core through multiple reflections at the core-cladding interface. Perturbations in the surrounding environment affect the guided light's propagation characteristics, which are detected by monitoring changes in light intensity, phase, wavelength, or polarization at the sensor's output. FOS offers benefits such as immunity to electromagnetic interference, durability, and remote sensing capabilities, finding applications in diverse fields, such as structural health monitoring, environmental monitoring, industrial process control, and medical diagnostics [6–10]. Optical fiber pH probes have been developed based on inherent advantages such as high sensitivity, immunity to interference, multiplexing, and miniaturization of optical fibers. These probes leverage fiber-optic platforms that are equipped with pH-sensitive coatings or indicators, thereby enabling the attainment of remote and minimally invasive pH measurements. Among these approaches, the surface plasmon resonance (SPR) technique stands out, relying on the modulation of SPR angle or resonance wavelength to detect pH-induced refractive index changes in the vicinity of the fiber surface [11]. This technique offers label-free and real-time monitoring with high sensitivity, making it suitable for biomolecular interactions and environmental monitoring. However, the SPR signal can be influenced by variations in surface conditions and temperature, necessitating careful calibration and control. Besides that, absorption-based optical fiber pH sensors were well developed and investigated in previous studies [12,13]. These sensors measure the change in light absorption of a pH-sensitive indicator dye coated on the optical fiber. The dye's absorption characteristics shift with pH changes. Absorption-based sensors are simple, cost-effective, and easily integrated into optical fiber systems. They are less susceptible to fluorescence interference. However, these sensors might have limitations in sensitivity compared to fluorescence-based sensors. They are also affected by factors like dye stability and environmental conditions. Another approach involves the incorporation of fluorescent pH indicators onto the optical fiber, allowing for pH-dependent emission intensity or wavelength shifts [14–16]. This technique enables near non-invasive and localized pH mapping, making it particularly valuable for biological and medical applications. Nonetheless, concerns regarding photobleaching, dye leakage, and environmental factors affecting indicator stability warrant consistent recalibration and optimization. To address this challenge, researchers have explored various approaches. Some have attempted to use optical fibers in combination with optical lenses to focus light and thereby enhance light collection efficiency [17,18]. However, the optical lenses assembled in this manner often lack the required durability for long-term applications. In response, other researchers have focused on designing optical lenses that are integrated with the optical fibers themselves [19]. Nonetheless, a limitation arises in this case, as the size of the resulting ball lenses is too large for practical use in biochemical environments. Hence, the miniature and compact fiber-optic pH probe, offering both excellent signal stability and efficient signal collection, holds considerable promise and significant advantages for enabling real-time in vivo monitoring in the field of biochemistry.

This manuscript presents the development of a miniaturized pH sensor utilizing an assembly-free ball lens integrated onto a TMMF. The initial steps involve tapering a MMF with a core diameter of 62.5 µm and cladding diameter of 125 µm. This process creates an ultra-thin needle-like structure, measuring 12.5 µm in cross-sectional diameter and 3 mm in taper length. A subsequent meticulous fabrication of a ball lens with a diameter of 20 µm is performed at the taper's apex. This structure is then coated using the dip-coating technique, involving a composite mixture of epoxy and the pH-sensitive dye, 2′,7′-bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), encasing the tapered ball lens region for pH detection. Computational simulations and numerical analyses evaluate six distinct lens configurations, elucidating optical behaviors. The proposed pH sensor's efficacy is assessed in a complex liquid medium covering a pH range from 6 to 8, demonstrating a rapid 0.5 second response time. The evaluation reveals exceptional sensitivity, quantified at 1001.57 fluorescent intensity counts per pH unit. Comparative analysis with a pH sensor based on a single TMMF embedded with fluorescence dye highlights significant signal enhancement achieved by the proposed system, almost doubling the fluorescence signal magnitude. This enhancement signifies improved signal collection efficiency that addresses challenges in contemporary miniaturized pH probes, crucially advancing signal-to-noise ratios. Moreover, the tapered optical fiber with assembly-free ball lens architecture's compact and robust nature holds promise across biochemistry and materials science applications. The integration of microfluidics, organ chips, and FOS represents a powerful approach for in vitro studies of complex and heterogeneous tumor microenvironments (TME) [20]. This innovative combination allows precise control of fluid and cell flow within confined spaces, mimicking the physiological conditions of different organs or tissues. FOS have the capability to measure different parameters. These include pH, oxygen levels, and concentrations of specific molecules. When integrated into organ chips, FOS enables real-time monitoring and response studies within the TME. This approach holds the potential to revolutionize our understanding of TME dynamics over time, interactions between different cell types, and responses to therapeutic agents. By providing a platform for detailed TME analysis, it may lead to the development of more effective cancer treatments, as illustrated in the context of brain tumor research.

2. Theory and fabrication of a ball lens structure

2.1 Ball lens theory

A ball lens is a spherical optical element that manipulates light propagation based on principles of refraction, focusing, and curved surface geometry. When light passes through a ball lens, its path is altered due to refractive index changes at the lens-medium interfaces. This refraction occurs as light moves from the surrounding medium into the lens and then back out again. The curved lens surface induces curvature-dependent changes in the direction of incident light rays, with a notable focusing effect due to the convex lens surface. This focusing behavior is similar to conventional lenses, concentrating light into a focal point, determined by the lens's curvature [21]. The ball lens's ability to converge or diverge light depends on its curvature and the refractive indices of the involved media [21]. This property has applications in laser beam manipulation, microscopy, and fiber optics. Total internal reflection occurs when the angle of light incidence exceeds a critical threshold, causing light to be reflected back into the lens medium instead of emerging into the external medium. This property is crucial for guiding light within optical fibers.

Illustrated in Fig. 1 is a comprehensive diagram delineating the pivotal physical and optical attributes inherent to a ball lens configuration. The focal point is the location on the optical axis where light intensity is at its peak. The principal planes have been designated to coincide with the central point of the spherical lens. In accordance with the paraxial approximation, the determination of significant parameters such as the effective focal length (EFL) and the back focal length (BFL) is facilitated by the following expressions [21–24]:

 

Fig. 1. Optical pathway associated with the ball lens structure. Geometric optical descriptors pertinent to a ball lens are enumerated, encompassing parameters such as effective focal length (EFL), back focal length (BFL), the diameter of the input source (denoted as “d”), index of refraction of ball lens (n), and the diameter of the spherical lens (“D”).

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This research is based on a theoretical framework. It focuses on using a ball lens structure positioned at the end of a fiber taper. The purpose is to concentrate incident light into the thin dye layer present on the hemispherical surface of the ball lens to amplify the fluorescence signal generation. In accordance with the parameters established in this research, the refractive index attributed to the multimode fused silica fiber stands at 1.48. Three distinct ball lens structures, measuring 15, 20, and 25 µm in diameter, were employed to determine the BFL. The resultant calculations demonstrate that the BFL values for these respective ball lens diameters are 4.06 µm, 5.42 µm, and 6.77 µm.

2.2 Ball lens fabrication

The selection of a MMF as the foundation for the proposed sensor design is underpinned by the advantageous feature of a larger core size in comparison to single-mode fiber (SMF). This bigger core can handle more light modes or paths, making it a smart pick for our sensor. The core and cladding diameter of the MMF are 62.5 µm and 125 µm, respectively, providing a large mode area for efficient light coupling. The fabrication process for the sensor involves multiple steps that aim to achieve high precision and reproducibility. A demonstration of the proposed sensor fabrication setup can be found in Fig. 2. The process involves the use of GPX-3400 and LDC-401 from Thorlabs to taper and cleave the fiber, as illustrated in Fig. 2(a). After removing the coating of the MMF using optical strippers, the non-coated region is localized between the fiber holder blocks FHB-L and FHB-R for the tapering process, as depicted in Fig. 2(b). The tapering process is performed using the GPX-3400. After tapering, the tapered fiber is moved to the LDC-401 for cleaving processing, which ensures a clean and precise cleave with minimal damage to the fiber. Once cleaved, the fiber is stabilized with FHB-R and put back into the GPX-3400 station for lens fabrication. The settled parameters for lens fabrication, such as diameter, hot push, and filament, directly influence the size and type of lens formed and, in turn, affect the performance of the sensor. An example of a tapering region of 3 mm long TMMF with a 20 µm diameter ball lens apparatus was successfully fabricated, as shown in Fig. 2(c).

 

Fig. 2. Photograph of the sensor fabrication setup, along with pertinent microscope images. (a) The GPX-3400 and LDC-401 systems have been configured to cater to the demands of tapering, cleaving, and the fabrication of ball lenses. (b) A magnified view of the GPX-3400 workstation offers insight into the integrated components configured for the sequential sensor fabrication processes. (c) The manufacturing process produced a tapered region 3 mm long (TMMF) with a 20 µm diameter ball lens at the distal end.

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In order to conduct a comprehensive comparative analysis of light propagation through various lens apparatus, the current study employed six distinct fiber structures that were fabricated based on the MMF. This approach was undertaken to garner a deeper understanding of the intricate nuances governing light propagation phenomena. Leveraging the lens theory expounded earlier, it is well-established that the focal length of light is intricately intertwined with both the curvature angle and dimensional attributes of the lens. Consequently, in pursuit of this understanding, manipulation of the ball lens diameter was executed via precise adjustments to the hot-push distance and filament power. This parameter control yielded a spectrum of five diverse configurations, encompassing convex lenses and ball lenses. The first fiber structure was a TMMF with a cleaved endface, as shown in Fig. 3(a). Then, the experimental fabrication sequence commenced with the production of two convex lenses, distinguished by cross-sectional diameters of 12.5 µm and 17.5 µm, as shown in Fig. 3(b) and (c). Subsequently, the manufacturing process extended to encompass the production of three distinct ball lenses, each characterized by varying diameters ranging from 15 to 25 µm, as shown in Fig. 3(d) to (f).

 

Fig. 3. Microscope images of six fiber structures. (a) A TMMF exhibiting an end face-cleaved design and possessing a cross-sectional diameter of 12.5 µm. (b) A TMMF incorporating a convex lens, characterized by a cross-sectional diameter of 12.5 µm. (c) An analogous TMMF configuration, featuring a convex lens, albeit with an enlarged cross-sectional diameter of 17.5 µm. (d) The amalgamation of a ball lens structure onto a TMMF possessing a diameter of 15 µm. (e) The implementation of a corresponding ball lens structure onto a TMMF, wherein the diameter has been expanded to 20 µm. (f) A similar deployment of a ball lens structure onto a TMMF with an augmented diameter of 25 µm.

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Following the design and development of the TMMF apparatus with a ball lens configuration, we opted for a dip coating technique. This method was chosen for applying a pH-sensitive dye material onto the surface of the ball lens structure. The goal is to functionalize the apparatus specifically for pH detection applications. The TMMF was positioned in a vertical orientation upon an optical fiber holder, as illustrated in the left corner of Fig. 4(a). To ensure the placement of the optical fiber, a dual-microscope arrangement was integrated into the system. This configuration enabled real-time visualization and precise localization of the optical fiber and dye-coated region in both the X and Y dimensions. The experimental setup featured a programmable stage holder equipped with a glass slide. This glass slide was systematically controlled by an automated stage controller situated in the right corner of Fig. 4(a), thereby facilitating controlled and consistent sample positioning. To mitigate the potential issue of photobleaching, preliminary experiments were conducted to identify the optimal concentration of BCECF for this study. The BCECF powder was first dissolved in DMSO solvent. After careful consideration, a final BCECF dye material concentration of ∼10−20µg/mL of DMSO was selected as it strikes an effective balance between achieving robust pH sensing and minimizing the risk of photobleaching. Based on the preliminary experiment results, it became evident that the fluorescence intensity is solely influenced by the concentration of the dye material and not by the amount of epoxy. To ensure thorough integration and proper mixing of the fluorescent dye within the epoxy gel, a mixing volume ratio of epoxy gel to fluorescent dye at 1:1 was chosen. This strategic choice is aimed at facilitating the comprehensive mixing of the fluorescent dye with the epoxy gel, contributing to the overall performance and reliability of our pH sensor. Subsequently, a small droplet comprising a well-calibrated mixture of epoxy and fluorescent dye was carefully deposited onto the designated area of the glass slide, as captured in Fig. 4(b). After the fabrication process, the optical fiber was placed onto the optical fiber holder. It was then slowly immersed into a droplet containing a precise mixture of epoxy and fluorescence dye. This procedure assured an accurate and controlled introduction of the functional materials to the optical fiber, thus establishing a foundation for the subsequent pH detection mechanism.

 

Fig. 4. The experimental setup and image for the dip-coated TMMF featuring a ball lens structure. (a) In the experimental configuration aimed at optical fiber dip coating, a specialized 3D movement stage was employed to facilitate precise manipulation. The TMMF apparatus was deliberately oriented in a vertical manner, affixed to the 3D stage, and positioned perpendicular to a delicate glass slide. (b) The glass slide accommodated a confined droplet comprising a composite of epoxy and dye materials. The dip coating operation was orchestrated to achieve a controlled and uniform deposition, resulting in the establishment of a micron layer atop the TMMF structure.

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3. Light propagation simulation

In order to simulate the propagation of light through the proposed six fiber structures, COMSOL Multiphysics 6.0 software was employed. The simulation model was constructed within the software framework, in accordance with the experimental arrangement delineated in the study. The simulation was centered on a conventional MMF with a core diameter measuring 62.5 µm and a cladding diameter of 125 µm. The previous section informed the tapering of the MMF down to a minute region featuring a cross-sectional dimension of 12.5 µm. The crux of the simulation investigation lies in comprehending the intricate dynamics of light propagation consequent to this tapering segment. This segment focuses on studying six distinct fiber structures, each designed to elucidate the nuances of light propagation under varying conditions. To inaugurate the exploration, the initial simulation scenario involves a taper MMF devoid of any additional lens structure, as depicted in Fig. 5(a). In this depiction, it is evident that light dispersion occurs immediately after traversing the tapered region, attributed to a larger numerical aperture (NA). The energy bar portrayal on the right side signifies that the maximum photon energy converges onto a focal point situated approximately 15 µm away from the fiber tip. Two convex lenses were produced at the tip of the optical fiber taper, as depicted in Fig. 5(b) and (c). It's important to note that the convex lens with a cross-sectional diameter of 12.5 µm does not exhibit a focal spot. This suggests the possibility of total internal reflection, which hinders light from exiting the lens. Additionally, the smaller diameter of the lens introduces significant diffraction effects. As the lens size decreases, diffraction becomes more pronounced, resulting in broader and less well-defined focal spots. Upon increasing the cross-sectional diameter of the convex lens to 17.5 µm, the focal point almost reaches the periphery of the lens structure's endface, which is not optimal for effectively enhancing the fluorescence signal. Subsequently, the fourth structure is examined, incorporating a diminutive ball lens with a diameter of 15 µm positioned at the tip of the fiber taper. The simulation outcomes, illustrated in Fig. 5(d), reveal a departure from the prior dispersion behavior. Instead of dispersing directly, the light attains focus after passing through the ball lens, subsequently dispersing. The change in the light's focal length arises due to the curvature inherent to the ball lens configuration. Evidently, the focal length materializes at about 3 µm from the terminal face of the ball lens. To further control the focal length and enhance focusing capacity, the curvature of the ball lens is manipulated, advocating the employment of a larger ball lens. This results in the fifth simulated structure wherein a more substantial ball lens with a diameter of 20 µm is fabricated at the fiber taper tip. Figure 5(e) visually encapsulates the outcomes of this simulation, manifesting a marked increase in the light's focal length, now positioned approximately 5 µm distant from the terminus of the ball lens. Furthermore, a larger ball lens with a diameter of 25 µm was fabricated at the fiber taper tip, as shown in Fig. 5 (f). The focal length increases about 7 µm from the terminal face of the ball lens. This controllable focal distance exhibits pronounced potential for channeling light onto the outer coated fluorescence dye, a critical maneuver to facilitate robust light scattering and subsequently amplify fluorescence signal generation. The simulations offer a detailed analysis of light propagation in various fiber structures, shedding light on the effective use of ball lenses for focal length manipulation and light distribution optimization. By comparing the results from simulations and the theoretical predictions (using Eqs. (1) and (2)), we observed a consistent pattern in light focal lengths for all three ball lens structures. For efficient light focusing into a 5-micron-thick layer, it's crucial to consider the thickness of the layer and the back focal length of the ball lens. Extensive experiments revealed that a fluorescence layer of 5 µm thickness pairs optimally with a ball lens of 20 µm diameter. This combination is ideal for pH sensor applications, as it ensures maximum incident light focus onto the fluorescence dye layer, thereby significantly enhancing the generation of the fluorescence signal.

 

Fig. 5. Simulation outcomes pertaining to ray tracing were obtained for six distinct optical fiber structures. The various configurations examined in this study included: (a) a TMMF with an end face-cleaved design and a cross-sectional diameter of 12.5 µm; (b) a TMMF featuring a convex lens, where the cross-sectional diameter was 12.5 µm; (c) a similar TMMF with a convex lens, but with a larger cross-sectional diameter of 17.5 µm; (d) integration of a ball lens structure onto a TMMF with a diameter of 15 µm; (e) implementation of a corresponding ball lens structure onto a TMMF, this time expanded to a diameter of 20 µm; (f) similar implementation of a ball lens structure on a TMMF with an even larger diameter of 25 µm.

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4. Experimental results

To attain an optimal excitation wavelength for the fluorescence dye molecules, the pH probe's excitation regimen was undertaken via a solid-state 488 nm laser source [15]. The QEPro spectrometer from Ocean Optics, was employed to capture reflected fluorescent signals in this study. The multimode optical coupler, as depicted in Fig. 6, connects the sensor, laser, and spectrometer together. The upper microscope image in Fig. 6 reveals the initial design of the ball lens structure, featuring a diameter of 20 µm before dip coating. In contrast, the middle image shows the sensor post-dip coating with a cross-sectional diameter of 25 µm. An approximately 5-µm-thick layer was observed at the end of the sensor. As the photons emitted by the 488 nm laser source traverse the fluorescent dye material, their initial cerulean hue transforms into a verdant spectrum, characteristic of fluorescence generation that is shown in the bottom image.

 

Fig. 6. Schematic optical system sensing diagram based on the miniaturized pH sensor with assembly-free ball lens structure. A 50:50 multimode coupler was employed to connect the sensing arm to the laser and spectrometer. The 62.5 µm MMF was tapered down to an ultra-thin optical needle with a cross-section of 12.5 µm. Two microscope images demonstrate the sensor size and structure before and after dip coating.

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In pursuit of a comparison encompassing the performance metrics of the proposed pH sensor, an investigation was undertaken involving both the designed TMMF configurations—with and without an integrated ball lens. Each aperture underwent a uniform fiber dip-coating regimen, adhering to the protocol in the preceding section. After the fluorophore infusion via dip coating, both configurations were subjected to signal reflection capture, as illustrated in Fig. 7(a). In the case of the TMMF lacking a ball lens, the light path exhibits dispersion. Conversely, when a ball lens is introduced to the TMMF a focal light path emerges. This focused trajectory targets the coated fluorescence layer, thereby resulting in an amplified phenomenon of fluorescence scattering. Evidently manifest within the recorded data, a conspicuous divergence surfaces between the two configurations. The tapered MMF augmented with a ball lens structure evinces an almost twofold elevation in fluorescent intensity at 555.5 nm relative to its counterpart lacking the ball lens adjunct, as shown in Fig. 7(b). This observation stands as a testament to the instrumental role played by the ball lens configuration in confining and amplifying light within the dye-coated domain. Consequently, the enhanced fluorescence signal generation substantiates the pronounced advantage conferred by the ball lens architecture. This enhancement overcomes challenges seen in current miniaturized pH probes. It focuses on improving signal collection efficiency and enhancing signal-to-noise ratios.

 

Fig. 7. Schematic diagram of light propagation and fluorescence emitted signal comparison of TMMF with and without ball lens structure. (a) The configuration without a ball lens in the TMMF exhibited a pattern of dispersed light propagation. However, the introduction of a ball lens into the TMMF transformed the light path into a focused trajectory, precisely converging onto the coated fluorescence layer. This adjustment notably intensified the phenomenon of fluorescence scattering. (b) The tapered MMF augmented with a ball lens structure evinces an almost twofold elevation in fluorescent intensity relative to its counterpart lacking the ball lens adjunct. “FL” is an abbreviation of fluorescence.

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Subsequently, the pH quantification protocol was implemented utilizing the proposed probe configuration. A series of standardized phosphate buffer solutions (PBS), concocted to span the pH continuum from 6 to 8, were prepared, each bearing incremental increments of 0.25 pH units, as depicted in Fig. 8. The observation ensued that as the pH quantum elevated, a concomitant augmentation in fluorescent spectral peak intensities was manifestly discerned at the spectral at 555.5 nm. Throughout the entire experiment, a fast response time of 0.5 seconds was observed for each measurement. By comparing the buffer pH values with their corresponding fluorescent intensity, a linear correlation was observed with a 0.993 R-square value, as exhibited in Fig. 9. A replicative approach was employed, involving the repetition of pH measurements for each buffer solution on nine separate occasions, with the only variation being in pH values. Ten measurements were conducted for each buffer solution. The precision of these measurements is illustrated in Fig. 9. The error bars were incorporated to represent minimal measurement variability, providing additional confirmation of the precision of the experimental results. This reinforces the assertion of precise and dependable pH determination facilitated by the developed sensing probe. To augment the sensor's sensitivity, an escalated concentration of the pH-sensitive dye was introduced, positing the potential for intensified photon emission. However, a caveat emerged in the form of plausible self-quenching and dye-leaching phenomena resulting from excessive dye molecules. This underscores the requisite balance between heightened sensitivity and compromised performance. Alternatively, a prudent modulation of laser power emerges as a viable strategy for enhancing µ-probe performance. It is prudent to acknowledge the intricacies surrounding this approach, including considerations of potential thermal effects and photobleaching, which necessitate cautious mitigation in future endeavors.

 

Fig. 8. Fluorescence spectra of pH measurements under different pH conditions. The fluorescence spectra were systematically recorded while subjecting the system to diverse pH solutions spanning the range of 6 to 8. Evident from the recorded data is a discernible trend: an observable increase in fluorescence intensity as a corollary of escalating pH values. This trend signifies a pronounced relationship between pH and fluorescence response, elucidating the system's responsiveness to changes in pH conditions.

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Fig. 9. Correlation analysis of pH with FL intensity. The investigation centered on the analysis of fluorescence intensity at the specific wavelength of 555.5 nm, subsequently establishing a correlation with the pH value of the testing solution. Employing regression analysis, a linear model was employed to effectively capture this relationship, resulting in a commendable R-squared value of 0.993. Notably, the error bars integrated into the data representation underscore the measurement precision, effectively encapsulating the variability inherent in ten testing instances.

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In pursuit of evaluating the reproducibility and sustained performance of the proposed pH sensor, a continuous experimental regimen was devised. Buffered solutions of PBS were prepared, corresponding to pH values of 6, 7, and 8. The subsequent experimental protocol encompassed a cyclic immersion of the developed pH probe into these buffer solutions, following the sequence of pH 8 → pH 7 → pH 6 → pH 8, and so forth. To facilitate equilibrium attainment, the probe resided within each solution for a standardized duration of 90 seconds during the first three cycles. Additionally, to assess the reproducibility and robustness of the proposed sensor, an extended duration of 120 and 150 seconds was employed for the fourth and fifth cycles. Presented in Fig. 10 is a comprehensive depiction of the pH measurements sustained over a duration of 1800 seconds, designed to effectively showcase the sensor's repeatability and signal stability. The fluctuations in signal intensity throughout the measurement period remained consistently below 3%, thereby demonstrating signal fidelity akin to traditional pH measurement methodologies.

 

Fig. 10. The repeatability and signal stability characteristics of the proposed pH probe were systematically examined through cycling measurements conducted in pH 6 (lower), pH 7 (middle), and pH 8 (upper) buffer solutions over a continuous duration of 1800 seconds.

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Sensitivity plays a crucial role in the effectiveness of a fluorescent sensor, being defined as the slope of the calibration curve approximation. In the case of the prepared pH sensor, it exhibits an impressive sensitivity of 1001.57 counts per pH unit. This sensitivity level surpasses that reported in the literature [25] by a significant factor, being 10 times higher (refer to Table 1 for details). Additionally, when evaluating the sensor's response time in comparison to reported research [26,27], our proposed sensor outperforms others, further solidifying its dominance in terms of both sensitivity and response time.

Table 1. Comparison of performance of proposed sensor and other pH sensors

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5. Conclusions

In summary, this study unveils an advancement in the realm of pH sensing by creating a miniaturized fluorescence-based pH sensor ingeniously constructed via the integration of an assembly-free ball lens onto a TMMF. The preliminary phase of this innovation involved the meticulous tapering of an MMF boasting dimensions of 62.5 µm core diameter and 125 µm cladding diameter, yielding an ultra-thin needle configuration characterized by a cross-sectional diameter of 12.5 µm and taper length of 3 mm, executed with precision by a Glass Processor device. Subsequent to this, a ball lens boasting a diameter of 20 µm was fabricated at the terminal point of the taper. To realize the envisioned optical performance in detecting pH, the dip coating technique was harnessed to envelop the tapered ball lens region in a composite layer comprising epoxy and BCECF pH-sensitive fluorescence dye material. Notably, an approximately 5 µm-thick layer of the pH-sensitive dye material was applied to envelop the ball lens surface. Furthermore, a comparative assessment involving six distinct fiber structures, encompassing the cleaved endface, the convex lens, and ball lens structures, was conducted, systematically scrutinizing light focal lengths and propagation intensities. Complementary numerical and simulation investigations were undertaken, effectively delineating the optical characteristics encompassing light focal distances and spot sizes inherent to the devised lens architecture. Subsequent evaluation of the proposed pH sensor's performance was executed within a liquid medium spanning the pH spectrum from 6 to 8 with a fast response time of 0.5 seconds, yielding an exceptional sensitivity of 1001.57 fluorescent intensity counts per pH unit. Notably, compared with a pH sensor predicated upon a standard TMMF incorporating the fluorescence dye, the proposed sensor yielded a remarkable fluorescence signal amplification of two-fold magnitude. This enhancement emerges as a pivotal solution to the challenges incumbent upon modern miniaturized pH probes, effectively addressing the pressing objectives of heightened signal collection efficiency and augmentation of signal-to-noise ratios—an accomplishment of considerable significance. Moreover, the compactness, resilience, and novelty of the assembly-free ball lens structure underscores this innovation's potential.