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Abstract
In this work, authors have developed a portable, sensitive, and quick-response fiber optic sensor that is capable of detection of Aflatoxins B1 (AFB1) quantitatively and qualitatively. Using multi-mode fiber (MMF) and multi-core fiber (MCF), the MMF-MCF-MCF-MMF fiber structure based on symmetric transverse offset splicing and waist-expanded taper is fabricated. The evanescent waves are enhanced to form a strong evanescent field by etching the fiber surface with hydrofluoric acid. To successfully excite the localized surface plasmon resonance phenomenon, gold nanoparticles are deposited on the optical fiber probe's surface. Further, to modify the fiber optic probes, Niobium carbide (Nb2CTx) MXene and AFB1 antibodies are functionalized. Nb2CTx MXene is employed to strengthen the biocompatibility of the sensor and increase the specific surface area of the fiber probe, while AFB1 antibody is used to identify AFB1 micro-biomolecules in a specific manner. The reproducibility, reusability, stability, and selectivity of the proposed fiber probe are tested and validated using various concentration of AFB1 solutions. Finally, the linear range, sensitivity, and limit of detection of the sensing probe are determined as 0 - 1000 nM, 11.7 nm/µM, and 26.41 nM, respectively. The sensor offers an indispensable technique, low-cost solution and portability for AFB1-specific detection in agricultural products and their byproducts with its novel optical fiber structure and superior detecting capability. It is also useful for marine species like fish and consequently affecting health of human body.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mycotoxins are subordinate metabolite produced by fungi that can cause cancer, teratogenicity and induce gene mutation to living organisms. Mycotoxins are very common in our daily life and mycotoxin exist in several forms [1]. Among them, the most common mycotoxins are aflatoxins, zearalenone (ZEN), ochratoxin and fumonisin. Mycotoxins pose a great threat to organisms in nature because of their compatibility, camouflage and diversity. The aflatoxins are considered as the highly toxic mycotoxins and can exist in B1, B2, G1, G2, M1 and M2 forms. Among aflatoxins family, aflatoxins B1 (AFB1) is the most carcinogenic and toxic aflatoxins [2]. AFB1 generally presents in a variety of agricultural products, including peanuts, corn, soybeans, and certain allied products that are especially kept in high temperature, humidity and other complex environments [3]. To avoid excessive humans and animals exposure to AFB1 environment, the World Health Organization (WHO) set a limit of 48 nM for AFB1 in food, while the maximum level of AFB1 in Chinese National Food Safety standards is 64 nM [4,5]. In recent years, AFB1 has become increasingly harmful to humans and other organisms, where the quantitative and qualitative analysis of AFB1 in the environment has significant implications for humans and organisms. Some researchers have showed that aquafeeds with AFB1 impaired growth, alter metabolism, tissue integrity, and transcriptomic responses [6]. Fish from aquaculture are cultivated using different feedstuffs and raw materials of plant origin and plant products, which can be contaminated by mycotoxins like AFB1 [7,8]. The liver is the primary target organ when animals are exposed to AFB1-contaminated feed and in some cases in muscles [8–10]. In European Legislation 574/2011/EC, the maximum level of AFB1 in complementary and completed feed is 10 µg/kg, except for dairy and young animals for which maximum levels are 5 µg/kg [9,11].
So far, researchers have used several methods to detect AFB1, for example, Karaman et al. designed an electrochemical AFB1 immunosensor based on gold nanoparticles (AuNPs)-decorated porous graphene nanoribbons and silver nano-cube-incorporated MoS2 nanosheets [12]. Yu et al. used chemiluminescent enzyme immunoassay to determine the concentration of AFB1 in agricultural products [13]. Traditional methods of AFB1 detection like chromatography, spectroscopy and immunoassay faces several disadvantages including complexity, time-consuming low sensitivity and reliability [14]. In addition, some researchers use the surface plasmon resonance (SPR) method, which is less flexible and more expensive than the localized surface plasmon resonance (LSPR) method. LSPR-based biosensors serve a portable, label-free, sensitive, fast-response, and cost-effective approach for detecting biomolecular interactions at the nanoscale [15]. Therefore, the LSPR method is suitable for measuring the concentration of AFB1 in the analytes.
Metal nanoparticles (MNPs) have specific electronic structures due to quantum size effects, that give them distinct physical and chemical qualities from their bulk or molecular counterparts. When the diameter of the noble MNPs is dramatically smaller than the incident light’s wavelength, the incident light interacts with the MNPs that leads to the collective coherent vibration of the conducting electrons on the surface of the NPs under the electromagnetic field, this is called the LSPR phenomenon. The LSPR phenomenon is sensitive to slight changes in ambient refractive index (RI) caused by the interaction of biomolecules on the sensing region [16,17]. The MNPs commonly used to excite LSPR are gold (Au) and silver (Ag) because of their stable physical and chemical properties. In fact, the resonance wavelength of the LSPR is highly dependent on the size, shape, and local permittivity of the MNPs [18]. The diameter of MNPs need to be greatly smaller than the incident light’s wavelength in order to excite LSPR phenomenon effectively.
Fiber optic-based sensors have gained popularity due to their high sensitivity, robust anti-electromagnetic interference, and lightweight [19]. By modifying the inherent transmission mode of total internal reflection, light can penetrate the interface between the fiber core and cladding and escape with some light waves. Evanescent waves (EWs) are waves whose amplitude diminishes exponentially along the interface's vertical direction. The fundamental purpose of a fiber-based sensor is to receive more EWs, as they are required to stimulate the LSPR phenomena. The LSPR sensors are particularly sensitive to variations in the RI of their surroundings. As the RI of the environment fluctuates, the peak resonance wavelength shifts in LSPR spectra. Concentration of a molecule is determined by its wavelength shift.
At present, the primary consideration of SPR/LSPR sensor is to couple the transmission light of the fiber core into the cladding. Researchers have used a variety of geometric fiber structures, such as tapered, U-shaped, and D-shaped [20]. Esposito et al. [21] proposed an optical fiber biosensor for unlabeled detection of vitamin D by targeting the major circulating form of vitamin D3 in the human body, namely 25-hydroxyvitamin D3(25(OH)D3). The performance of the biosensor was evaluated in the complex medium containing interfering proteins with satisfactory results. Wei et al. [22] fabricated an optical fiber sensor by studying the number, depth and period of V-groove using CO2 laser. This sensor that can be utilized to detect strain and has good sensing performance. In recent years, researchers have become increasingly interested in fiber optic sensors based on long-period gratings (LPGs). In addition, LPGs fiber optic sensors have been developed, laying a crucial foundation for the advancement of sensing field technology [23]. Similarly, Esposito et al. developed a LPGs-based fiber sensor utilizing double clad fiber with W-shaped refractive index distribution in order to analyze surrounding refractive index (SRI) sensitivity. The experimental results show that the sensitivity gain of SRI is 40 to 60, and the maximum sensitivity is about 420 nm/RIU when water is used as the surrounding material [24]. Recently, LSPR sensor based on taper-in-taper fiber structure was used to detect alanine transaminase (ALT). ALT sensor used AuNPs, molybdenum dioxide NPs (MoS2-NPs) and cerium oxide NPs (CeO2-NPs) to improve the performance of sensor [25]. Finally, satisfactory results were obtained especially, the limit of detection (LoD) and sensitivity. Similarly, Xu et al. developed a sensor based on LSPR to detect the concentration of ZEN in food. The experimental findings indicate that the sensor has a potential role in ZEN monitoring [26]. Kumar et al. developed the LSPR biosensors using MoS2-NPs and AuNPs-functionalized optical fiber sensing probe for efficient detection of Shigella bacteria [27]. In order to enhance the LSPR effect, researchers proposed a novel fiber structure based on the fusion of tapered fiber and MMF. The method of this sensor provided a new idea for the development of LSPR-based optical fiber sensor [28].
Researchers are also widely using the two-dimensional (2D) MNPs for development of optical fiber-based biosensors. MXene is an emerging 2D material and its common forms are Mn + 1XnTx, where M refers to early transition metals and X represents C or N, and Tx indicates surface functional groups, such as, -O, -F, -OH, -Cl [29,30]. In general, MXene is synthesized by etching using hydrofluoric (HF) acid. The different functional groups of MXene depend on the type of chemical environment [31]. Due to its distinctive characteristics, such as excellent optical properties, large specific surface area, biocompatibility, high conductivity, highly effective electromagnetic interference screening and high energy capacity, it is of great value in various research fields, especially, in sensing application [32,33]. Li et al. designed and developed a flexible pressure sensor based on a Ti3AlC2 MXene textile network that can monitor physiological data of the human body in real time, including wrist pulse, speech recognition, and finger movement. The sensor proves to have potential application value in wearable sensor field [34]. As niobium carbide (Nb2CTx), MXene has a potential future in agricultural and industrial monitoring. MXene has a flaky structure, a large specific surface area, and an abundance of hydrophilic surface groups. For example, Bi et al. developed a highly sensitive Nb2CTx MXene-functionalized tapered fiber-based humidity sensor that introduces a technology in the field of humidity sensing [35]. Li et al. developed a sensor based on Nb2CTx tapered micro-fiber structure to detect hemoglobin concentration in a wide range. The experimental results show the sensitivity of 7.581 nm/(g·dL-1) and the LoD of 0.0026 g/dL for the sensor. It demonstrates that the probe can meet the needs for human hemoglobin level detection [30].
Multicore fiber (MCF) was originally developed for data transmission, but in recent years, a group of experts have used MCF to the field of optical fiber sensing [36]. MCF is a special fiber with seven cores encased in a single cladding, therefore it provides parallel transmission channels throughout distinct spatial cores of a fiber, so significantly enhancing the transmission capacity of each fiber. Due to its unique characteristics such as multi-channel, compact fabrication, and diminutive size, MCF has generated significant interest in the field of sensing [37]. Singh et al. used etched MCF to develop optical fiber sensors for cancer cell detection [38]. Similarly, the etched MCF-based Mach-Zehnder interferometer (MZI) is utilized to monitor the external RI and temperature [39]. Then, Amorebieta et al. utilized MCF to fabricate a compact vector bending sensor. It can measure the bending angle using MCF-based sensor by concurrently detecting the wavelength shift and the variation in light power [40].
This work aims to design and develop a label-free, fast-response, highly sensitive, and low LoD AFB1 fiber-optic sensor. The MMF-MCF-MMF-MCF optical fiber structure with symmetric lateral offset and over-splicing is fabricated and applied to the LSPR-based AFB1 sensor development. Nb2CTx MXene has a large specific surface area due to its layered structure that provides a larger surface area for AFB1 antibody attachment. In addition, it provides attachment sites for antigen-antibody interactions. Here, AuNPs and Nb2CTx MXene were used on the surface of sensing probes to enhance sensor performance. The AuNPs were used to excite the LSPR phenomenon. The performance of the sensing probes can be enhanced by the combined effects of Nb2CTx MXene, and AuNPs. The proposed optical fiber sensor has the potential application for the detection of AFB1 in agricultural products and their by-products, as well as for the AFB1 detection in aquaculture sectors such as water fish tanks [41–43].
2. Experimental
2.1 Materials
The multi-mode fiber (MMF, 62.5 µm/125 µm) and multi-core fiber (MCF, 6.1 µm/125 µm) used in the experiment were purchased from EB-link Technologies Co., Ltd., China and Fibercore Ltd., UK, respectively. AFB1 and ZEN were purchased from Macklin, China. Aflatoxins B2 (AFB2), Aflatoxins G1(AFG1), and deoxynivalenol (DON) purchased from Aladdin, China. Anti-Aflatoxin B1 antibody was purchased from Sigma-Aldrich, Shanghai. AuNPs was synthesized from tetrachloroauric acid (HAuCl4), sodium citrate (Na3C6H5O7·2H2O) and deionized (DI) water. N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), 11-mercaptoundecanoic acid (MUA) and (3-mercaptopropyl)trimethoxysilane (MPTMS) were utilized for the immobilization of nanomaterials and the functionalization of antibody are purchased form Sigma-Aldrich, Shanghai. N,N-Dimethylformamide (DMF) was used as an excellent solvent for preparation of Nb2CTx MXene solution and purchased form Hushi, Shanghai. Phosphate Buffered Saline (PBS) buffer was used to prepare different concentrations of AFB1 solutions and purchased form Sigma-Aldrich, Shanghai. HF acid was used as an etching agent for removing the cladding from optical fiber structure.
2.2 Instruments and measurements
The instruments used in the experiment of optical fiber probe fabrication include special fusion splicer machine (FSM-100P+, Fujikura, Japan) and combiner manufacturing system (CMS, USA), which were used for fiber splicing and fiber diameter scanning, respectively. The fiber cleaver (CT-32, Fujikura) was used to cut the fiber to obtain flat end face, so that the specialty fiber fusion splicer can successfully splice different parts of the fiber. UV-Visible Spectrophotometer (Hitachi-U-3310) was used to measure the absorption spectra of the AuNPs to determine the morphology of the NPs. Tungsten-Halogen light source (HL-2000-LL, Ocean Optics, USA) can emit a wide spectrum of light and has a long lifetime, was served as an experimental light source. The spectrometer (USB2000+, Ocean Optics, USA) was used to process and record the LSPR spectral data to the computer. High-resolution transmission electron microscope (HR-TEM, Talos L120C, Thermo Fisher Scientific) was used to characterize the overall appearance and particle size of the nanomaterials used in the experiment. Scanning electron microscope (SEM, Gemini, Carl Zeiss Microscopy) scanned the samples by emitting a high-energy electron beam to obtain the surface characteristics of the sample. In the experiment, the nanomaterials functionalized on the surface of the fiber probe and the different splicing structures of the fiber probe were characterized by SEM.
2.3 Sensing mechanism of the probe
The electric field of the incident light interacts with the free electrons of the MNPs, resulting in the charge separation between the free electrons and the ionic metal nuclei. The Coulomb force between the free electrons as the restoring force, pushes the free electrons in opposite directions, that leads to the collective oscillation of the electrons and stimulate the LSPR phenomena [17]. The schematic of LSPR excitation inside MNPs is shown in Fig. 1(a). The sensor is based on the LSPR phenomenon and is supplemented by MNPs for enhancement the sensing performance of the probe. Evanescent field is crucial to the excitation of LSPR. More EWs are needed if a stronger evanescent field is to be obtained. In order to obtain more EWs, the MMF-MCF-MCF-MMF fiber structure based on symmetric lateral offset and waist-expanded taper is adopted in the experiment. The maximum extinction wavelength of LSPR is particularly sensitive to the dielectric constant of the environment. Because the relationship between the dielectric constant $({{\varepsilon_e}} )$ of the environment and the RI $({{n_e}} )$ of the surrounding environment is ${\varepsilon _e} = {n_e}^2$. In theory, the position of the extinction peak of LSPR will change when the change of RI in the surrounding medium environment is $({\Delta {n_e}} )$, and the wavelength change is $({\Delta \lambda } )$. The wavelength shift is defined as follows [18]:
Here, $(s )$ is the sensitivity constant of the MNPs to the electromagnetic field, $({l\; } )$ is the effective thickness of the adsorption layer, and $({{d_p}} )$ is the EWs’ penetration depth.
The sensor probe is a variant of MZI structure, which can be analyzed according to the principle of MZI. This interferometer has both core mode and cladding mode, and the strength of interference between the two modes may be represented mathematically [44].
where, ${I_{core}}$ and ${I_{clad}}\; $ are the core mode and cladding mode transmitted light intensities, respectively and the phase difference between the two transmission modes is denoted by $(\varphi )$ . The mathematical expression for $(\varphi )$ is shown as:Here $n_{eff}^{co}$ and $n_{eff}^{cl}$ symbolize the effective RI of the core and cladding, respectively, $(\lambda )$ is the wavelength of incident light.
The prominent conceive proposition of core-offset structure is the defective fusion between fibers, either by the change of fiber geometry position or by electrode re-discharge. Due to the offset of the core position, the energy between the core and the cladding can be exchanged while the light is transmitted in the core-offset structure. The core-offset structure is commonly employed as a beam splitter or coupler in sensor probes. The power transmission coefficient equation due to fiber offset is shown as [45]:
In the Eq. (4), the width parameters of the mode field are represented by ${\omega _1}$ and ${\omega _2}$ and ${l_1}$ is the offset value. The peak wavelength of the LSPR spectrum shifts according to variations in RI of the environment. The depth of the EWs penetrating the cladding increases as a result of the fiber probe being etched, that reduces the cladding's capacity to bind the transmitted light in the fiber. The schematic of penetration depth of EWs is shown in Fig. 1(b). At the same time, the mathematical expression for the penetration depth $({{d_p}} )$ can be defined as [46]:
Here, $(\alpha )$ is the incidence angle at the interface between core and cladding.
The schematic of splicing process of two different kind of optical fiber using FSM is shown in Fig. 2(a). The detailed fabrication process of sensor probe is depicted in Fig. 2(b). In the developed sensor structure, there are three joints, one waist expanded core and two transverse offsets. Here, work is divided into three sections. (A) Due to the existence of transverse offset, a portion of the light leaks from the fusion point as it travels from MMF1 to MCF1. Therefore, first offset functions as a beam splitter. The transverse offset splicing produces a diameter mismatch between the MMF and MCF, that stimulates the high-order cladding mode at the splice point [47]. (B) When light reaches the point of waist-expanded taper section, it goes from MCF1 to the MCF2. In this case, MCF2 serves as a coupler. Due to the RI difference between the core and cladding, light is confined in the fiber and propagates ahead along the fiber. The light that escapes from the two junctions that are laterally displaced significantly increases the evanescent field surrounding the fiber. This enhances the fiber probe's sensitivity and other performance factors. (C) As a symmetrical structure, MMF2 also serve as a coupler. As part of the sensing system, the core offset structure functions as a mode converter for energy exchange between the cladding and core modes. Cascaded sensors can theoretically measure several parameters concurrently [45].
Fig. 2. (a) Schematic of fusion splicer machine, (b) fabrication process of MMF-MCF-MCF-MMF sensor probe.
Evanescent field and MNPs are essential elements to stimulate LSPR phenomenon. The plasmon polaritons on the nanoparticle’s substrate are associated with the local surface resonance effect, that shifts the extinction peak and enhances the shift of RI. This coupling property relies on the distance between the NPs pairs [48]. While the surrounding environment's local RI alters due to the interaction of the antibody with AFB1, this will result in a red shift in the LSPR spectrum [49]. The concentration of AFB1 was determined by monitoring the peak resonance wavelength shift of LSPR spectra.
2.4 Fabrication of sensor probe
In the experiment, the MMF-MCF-MCF-MMF fiber structure based on symmetric lateral offset and waist-expanded taper is proposed. MMF and MCF are used to fabricate this structure, it is a kind of in-line MZI structure. Seven core-MCF comprises six cores stacked in a hexagonal arrangement, with one core in the center [50], consequently, it has lower transmission loss than SMF, and more durability qualities, and superior sensitivity [51–53]. First, it is necessary to configure the fiber fusion operations on the FSM platform; different fusion interface structures require different configurations. In order to complete the fabrication of the optical fiber probe, it is necessary to establish distinct techniques to meet the various fabrication criteria. The primary problem for the transverse offset fusion point is the offset value. The optimization of offset value is discussed in the following section. In general, normal fuse mode cannot set the offset value, so the attenuating fusion switch must be engaged prior to adjusting the offset in lateral offset fuse mode. The main discharge power is adjusted to +30 bits and the main discharge time is set to 780 ms. The re-discharge power is +20 bits, the re-discharge time and the re-discharge start time are 350 ms and 800 ms, respectively. In order to obtain a prearranged transverse offset and a sufficiently small splice loss at the fiber splice point, four times of re-discharge process were carried out after the main fusion process.
Then, the coating layers of the MMF and the MCF were peeled off by using fiber stripper, and the end faces of the above-mentioned fiber were cut flat by using the fiber cleaver. In order to obtain MMF-MCF structure, FSM was used to fuse the fiber with flat end face. Next, repeat the above steps to get the same lateral offset structure. Thus, a symmetric two-segment transverse offset structure has been obtained. Finally, the two-segment structures were fused by over-fusing to obtain the MMF-MCF-MCF-MMF fiber probe based on symmetrical transverse offset and waist-expanded taper.
2.5 Synthesis of nanomaterials
AuNPs and Nb2CTx MXene are used for the purpose of exciting the LSPR effect and improving the sensing performance of optical fiber probe. The conventional Turkevich approach was used to synthesize AuNPs [54]. The size of AuNPs obtained by this method is around 10 nm, that is noticeably smaller than the light wavelength transmitted via the optical fiber, that can be utilized effectively to excite the LSPR phenomenon. The Turkevich methodology is a well-established method for preparing AuNPs, it used sodium citrate as a reducing agent to reduce tetrachloroauric acid to prepare the AuNPs solution. 2 mg of Nb2CTx MXene was dissolved into 10 mL of DMF and sonication was performed for one hour using an ultrasonic machine for the preparation of Nb2CTx MXene dispersion solution [55]. The Nb2CTx MXene dispersion solution prepared were retained for subsequent experimental use.
2.6 Nanomaterials and antibody functionalization over sensing probe
The fabricated optical fiber probes were cleaned using acetone for 20 minutes in order to eliminate the organic contamination from the surface of optical fiber probes. Then, Piranha solution (30% H2O2 was added to concentrated sulfuric acid (H2SO4) at a volume ratio of 3:7) was used to form hydroxyl functional groups on the surface of the fiber, which can help the nanomaterials more easily attached to the probes. After 30 minutes of hydrolysis with Piranha solution, the fiber optic probes were dried. Thereafter, used the MPTMS, which has a methoxy group and its hydrolysate is silicon alcohol. The -SH group is formed by the interaction of methoxy group and surface adsorbed silicon alcohol. The reaction between -OH and -SH groups produces strong hydrogen bond [56]. Because of the existence of sulfur atom, the -SH group exhibits the property of electron donor [57]. This plays an important role in the bonding of Au-atoms [58]. Then, MPTMS was used as coupling agent to modify the optical fiber probes for 12 hours for immobilizing the AuNPs. The next step was to immobilize the optical fiber probes with AuNPs, which is relatively simple due to the previous series of steps.
AuNPs and Nb2CTx MXene are immobilized on the surface of optical fibers using dip-coating process [59]. The synthesized AuNPs were immobilized on the sensing probes for 48 hours. Thereafter, prior to Nb2CTx MXene immobilization, the probe was rinsed with ethanol to remove the unbound AuNPs and dried with N2 gas. The Nb2CTx MXene solution dispersed in the DMF was used for the next step of the modification of the fiber probes, which can provide sufficient sites for the functionalization of the antibody and the reaction of the antibodies with the AFB1.
The theory of amino conjugation is applied to the functionalization of antibody, that binds AFB1 antibody with nanomaterials by covalent bond. The fiber probe was carboxylated by soaking with ethanolic MUA solution (0.5 mM) for 5 hours. Prior to antibody functionalization, EDC (200 mM) and NHS (50 mM) were used for 30 min to activate carboxyl groups on the fiber surface to form amine-active lipids. This lipid can form a strong covalent bond with AFB1 antibody to achieve the purpose of antibody functionalization. Finally, the fiber probe should be functionalized with AFB1 antibodies to enable its particular detection capability. The antibody was functionalized by dipping the fiber probes into the prepared antibody solution for 12 hours. In this process, the fabrication process for optical fiber probes is completed. Next, optical fiber sensors were utilized to measure the varied concentrations of AFB1 solutions. The schematic of the nanoparticles-immobilization and antibody functionalization process is shown in Fig. 3.
2.7 Preparation of AFB1 analyte samples
AFB1, the most toxic aflatoxins found frequently in daily life. The WHO sets a limit of 48 nM for AFB1 in food. Thus, AFB1 sample solutions of 0 nM, 200 nM, 300 nM, 400 nM, 500 nM, 700 nM, 900 nM, 1000 nM concentrations were prepared separately with 1×PBS buffer solution. The preparation of AFB1 sample solution consists of two steps, first of all, in order to prepare 1000 nM concentration of AFB1 stock solution, 15.6 µg of AFB1 powder was dissolved in 50 mL of 1×PBS buffer solution. After diluting the AFB1 stock solution using buffer solution, various concentrations of AFB1 sample solution are obtained. The performance of the proposed MMF-MCF-MCF-MMF sensor structure with symmetrical transverse offset fusion and waist-expanded taper was evaluated using AFB1 sample solutions with concentrations ranging from 0 to 1000 nM.
2.8 Experimental setup
Experimental setup for detection of the different concentrations of AFB1 solution is shown in the Fig. 4. The tungsten-halogen source with a wavelength range of 200 - 1000 nm was used as the excitation light source for LSPR effect. LSPR spectra were collected by a spectrometer with wavelength range of 200 - 1000 nm, and then the spectra were transmitted to a computer. Then, the computer receives the LSPR spectral information transmitted by the spectrometer, and recorded the spectral information in text format to facilitate the further step of data processing. The optical fiber probe is connected to the optical path by a splicer machine, and the optical fiber probe was fixed by the clamp. By varying the concentrations of the AFB1 sample solution in order to obtain varied spectral data, the AFB1 concentration can be determined. The specific binding of AFB1 and AFB1 antibodies can be seen in the Fig. 4.
3. Results and discussion
3.1 Optimization of sensor probes
The fabrication methods of optical fiber probes have been described in section 2.4. This section mainly introduces the determination and optimization of the parameters of each proportion of the optical fiber sensor. The parameters to be determined include two parts, the first part is the transverse offset and the other part is the fiber length. Because the structure is symmetrical, only half of the structure can be determined to get the whole optical fiber sensor. Generally, the loss of fusible fiber probes includes the loss of fusion and transmission. Splice loss generally refers to the weakening of transmission light caused by the splice point, while transmission loss refers to the weakening of light intensity caused by light in a certain length of optical fiber. To minimize the experimental error, the same number of re-discharge and the same splicing loss are ensured in the splicing process. The FSM real-time images of offset splicing and waist- expanded taper are shown in Figs. 5(a) and (b).
First of all, fiber probes with different lengths of 5-5 mm, 10-10 mm, 15-15 mm, 20-20 mm were fabricated, and the transmitted intensities of the fiber probes with different lengths were measured. The results of the optimization experiment are displayed in the Fig. 6(a). The results depict that the transmitted intensity is lowest when the length is 20-20 mm. In theory, the lower the transmitted intensity, the more EWs are produced, and the LSPR effect can be stimulated more effectively. This length was applied in subsequent experiments. The next experiment was to optimize the lateral offset. The optical fiber probes with different offsets were fabricated and the transmitted spectrum of the optical fiber probes were measured. By comparing the transmitted intensity spectra of sensors with different transverse offset values, it can be concluded which transverse offset value is more suitable for fabricating sensor probes. As shown in Fig. 6(b), sensors with 4 µm, 6 µm, 10 µm, 12 µm, 15 µm, 20 µm, 25 µm of different lateral offsets were fabricated, keeping the length of MMF-MCF to 20-20 mm. The results show that the transmission spectrum intensity is the lowest when the offset value is 12 µm, which indicates that the best sensing performance can be obtained at this time. Therefore, the transverse offset of 12 µm is taken as the offset of the proposed novel optical fiber sensor. After the optimization of the above two steps, the MMF-MCF-MCF-MMF fiber structure based on symmetrical transverse offset splicing and expanding taper was obtained eventually.
Fig. 6. (a) Optimization of MMF-MCF lengths of sensing probe, (b) optimization of sensor probe offset value.
Thereafter, the fabricated optical fiber probes were etched using chemical etching method. First of all, placed the optical fiber probe fabricated in the reaction cell, and dropped an appropriate amount of HF and etched for 20 min. Then, cleaned the fiber probe with deionized water to remove unreacted HF. The diameter of the etched fiber probe was scanned using CMS machine, that is shown in Fig. 7. The results reveal that the diameters of the etched fiber probes are very similar. The diameter of the fiber can reach about 100 µm at the fusion point of the waist-expanded taper, while the diameter of the fiber probe can reach about 87 µm in the non-waist-expanded taper region.
Fig. 7. Diameter scan of etched fiber probes, inset shows the cross-sectional views of seven core fiber before and after 20-minutes etching.
3.2 Characterization of nanomaterials
UV-Vis Spectrophotometer was used to measure the absorption spectra of AuNPs. The approximate size of AuNPs can be obtained from the resonance wavelength. The absorption spectrum of AuNPs is shown in Fig. 8(a), and the absorption peak resonance wavelength is at 520 nm. The results validate that the diameter of AuNPs is about 10 nm. The prepared AuNPs were characterized by HR-TEM as shown in the Fig. 8(b). The particle size of AuNPs were uniform and no aggregation was observed. The histogram based on the diameter of AuNPs measured by TEM shows that the diameter of AuNPs is also around 10 nm, that was depicted in in Fig. 8(c). This proves the rationality of the method of synthesizing AuNPs used in the experiment. Nb2CTx MXene was also characterized using HR-TEM, as shown in Fig. 7(d), from which it is evident that Nb2CTx MXene presents a layered structure. This corresponds to the morphology of Nb2CTx MXene used in our experiment.
Fig. 8. (a) Absorption spectrum of AuNPs, (b) HR-TEM image of synthesized AuNPs, (c) histogram shows diameter distribution of AuNPs, and (d) TEM image of Nb2CTx MXene.
3.3 Characterization of nanomaterial functionalized fiber probe
SEM was used to characterize the optical fiber structure as well as the nanomaterials functionalized on its surface. SEM result clearly showed the nanomaterials coating on the surface of optical fiber structure. The transverse offset splicing points in the fiber optic sensor are shown in Fig. 9(a), from which the offset splicing can be clearly observed. The fusion point of the fiber optic probe at the waist-expanded taper was observed using SEM, that was shown in Fig. 9(b). The fiber-coated AuNPs and AuNPs/Nb2CTx MXene as observed by SEM are shown in Fig. 9(c) and (d), respectively. From Fig. 9(c), it can be clearly observed that AuNPs are uniformly immobilized on the fiber sensor’s surface. Similarly, AuNPs and Nb2CTx MXene can be observed in sequence on the surface of the fiber probe in Fig. 9(d). The layered structure of Nb2CTx MXene can immensely enhances the antibody attachment at the surface.
Fig. 9. SEM images (a) core-offset fusion point of fiber structure, (b) the waist-expanded taper, (c) optical fiber probes coated with AuNPs, and (d) AuNPs/ Nb2CTx MXene-immobilized optical fiber probe.
3.4 Measurement of analytical samples
AFB1 analytical sample solutions of different concentrations were prepared for the characterization of sensor probe. The LSPR spectra were measured from low to high concentrations of AFB1 solutions using the experimental setup shown in Fig. 4. The probes were washed using PBS buffer solution before the measurement of new concentrations to minimize the experimental error. The LSPR resonance condition is largely reliant on the dielectric properties of the surrounding medium, therefore the MNPs are applied to detect RI changes in the surrounding environment [60]. Fig. 10(a) is the LSPR spectra measured at different AFB1 concentrations. The spectra illustrates that when the concentration of AFB1 increases, then red shifts occur in LSPR spectrum. This demonstrates that various concentrations of AFB1 solutions alter the effective RI surrounding the optical fiber sensor, hence producing LSPR spectrum drift. The sensitivity, LoD and linear fitting factor of the sensor can be obtained by measurement and analysis. The linear fitting curve is shown in Fig. 10(b), and the linear fitting curve equation is shown as:
Where $(C )$ is the concentration of AFB1, $(\lambda )$ is the peak resonance wavelength of LSPR spectrum. The linear fitting factor is R2 = 0.898, and the linear detection range of the sensor is 0 - 1000 nM. Similarly, LoD is an essential parameter of sensor probe, that was determined by standard deviation (SD) of the peak wavelength measured at lower concentration level under stable conditions [61]. And the calculated value of SD is 0.103. Thus, the sensitivity of probe is found as 11.7 nm/µM and the LoD is 26.41 nM.3.5 Reproducibility and reusability test
Reproducibility and reusability are prominent indicators of performance measurement of fiber probe. The reproducibility indicates whether two fiber optic probes have the same sensing performance, while the reusability is whether the same experiment result can be obtained by measuring the same concentration repeatedly with the same fiber probe in a certain time.
Two optical fiber probes were used to measure 1000 nM AFB1 solution to test the reproducibility of the sensor. The normalized transmittance spectra of the two probes were recorded in sequence, which is shown in Fig. 11(a) and the two recorded transmittance curves were almost identical. Hence, the sensor probes perform excellent reproducibility. The concentrations of 200 nM and 900 nM were used to test the reusability of the sensor probe, and the experimental results are shown in the Fig. 11(b). The curves of LSPR spectrum were almost identical for 200 nM and 900 nM, respectively. It can be concluded from this experimental result that reusability of sensor is quite good.
3.6 Stability and pH test
Stability is another important performance index of sensor probe. The initial wavelength position of the sensor probe remains stable in a certain period of time represents the superior stability of the sensor. The peak wavelength of LSPR spectra was recorded for 1×PBS using developed fiber-optic probe for 10 times. The results of experiment are illustrated in the Fig. 12(a). After ten times measurements, the peak wavelength has only a small deviation (SD = 0.103) that shows that the stability of the fiber probe is reliable.
The AFB1 solvent used in the experiment was 1×PBS buffer solution. To validate the suitability of PBS solutions as a buffer, acetic acid, anhydrous ethanol and potassium hydroxide were used to prepare the pH values of 3, 6, 10 and 14 solutions. AFB1 analyte was dissolved to different pH value of solutions and prepared the AFB1 of 1000 nM concentration for detection. The experimental results suggest that the highest wavelength shift of LSPR appears when the pH value is 7.4, that is shown in Fig. 12(b). Thus, PBS is an excellent solvent (buffer) for the experiment. The pH of human fluid also lies in this range thus, our proposed sensor is suitable for clinical purpose and also in-line with the sensor usage in aquaculture sector [41].
3.7 Selectivity test
The presence of AFB1 is often accompanied by other types of mycotoxins or biomolecules, such as AFB2, AFG1, ZEN and bovine serum albumin (BSA). It is a significant performance requirement for the sensor to recognize the specificity of the substance to be measured.
The AFB2, AFG1, ZEN, DON, and BSA are commonly present in conjunction with AFB1, thus these substances were used for selective testing. Firstly, peak wavelength with PBS buffer solution (0 nM) was measured using the developed sensor probe. Thereafter, experiments were performed to record the resonance wavelength with 1000 nM of AFB1, AFB2, AFG1, ZEN, DON, and BSA solutions. Then, the peak wavelength shifts of LSPR of different analytes were obtained and shown in Fig. 13. Here, the peak wavelength shift is most noticeable when the test sample is AFB1, while other substances have little or no drift. The reason for high selectivity is due to the AFB1 antibody only has specific recognition to AFB1, but has no specific recognition to other substances. It can be concluded that the sensor probe has excellent specificity recognition ability.
3.8 Evaluation of sensor performance
In this work, LSPR-based optical fiber biosensor is developed for detection of AFB1 solution, and the sensor probe achieved excellent sensing performance. The sensor probe is promising to be applied in the real-time monitoring of AFB1 because of its novelty, portability and high sensitivity. A comparison among the sensitivity, LoD, and linear range of proposed sensor with existing AFB1 sensors is presented in Table 1. Compared with the traditional sensors, the advantages of optical fiber sensor are remarkable, including immediate response, no complex equipment requirement, light-weight and cost-effective.
Table 1. Performance comparison of proposed sensor with existing AFB1 sensors.
4. Conclusion
In this article, a novel fiber sensor probe based on MMF-MCF-MCF-MMF structure with symmetric transverse offset splicing and waist-expanded taper is proposed. The proposed sensor is based on LSPR phenomena, an important factor in LSPR excitation is EWs. In order to obtain more EWs, optical fiber probes are etched by using HF acid. The optical fiber structure serves as a substrate for LSPR, where AuNPs are immobilized on the sensor surface for LSPR excitation. Furthermore, Nb2CTx MXene, a novel 2D layered nanomaterial, is immobilized on the surface of the AuNPs-coated optical fiber probe to enlarge the binding sites on the surface of the optical fiber probe and therefore intensify the sensing performance of sensor. Then, the sensor probe functionalized by using AFB1 antibodies, that distinctly enhanced the specific recognition performance of the sensor. HR-TEM and SEM were used to characterize the novel nanomaterials and nanomaterials-immobilized optical fiber sensor probe. The performance indexes such as reproducibility, reusability, pH test, stability, and selectivity of the presented sensor probe are verified. At last, the linear range, sensitivity and LoD of the sensing probe were obtained as 0 - 1000 nM, 11.7 nm/µM and 26.41 nM, respectively. The results demonstrate that the fiber-optic sensor introduced in this experiment can efficiently determine the concentration of AFB1 in the wide linear range, and has a positive prospect of practical application in detection of AFB1 in human fluid, agricultural products and aquaculture industry.
Funding
Fundação para a Ciência e a Tecnologia (LA/P/0037/202, PTDC/EEI-EEE/0415/2021, UIDB/50025/2020, UIDP/50025/2020, CEECIND/00034/2018, UI/BD/153066/2022); Natural Science Foundation of Shandong Province (ZR2022QF137); Natural Science Foundation of Shandong Province (ZR2020QC061); Liaocheng University (31805180301, 31805180326, 318051901); Special Construction Project Fund for Shandong Province Taishan Mountain Scholars; Double-Hundred Talent Plan of Shandong Province, China.
Acknowledgments
This work was supported by Special Construction Project Fund for Shandong Province Taishan Mountain Scholars, China. C. Marques acknowledges Fundação para a Ciência e a Tecnologia (FCT) through the 2021.00667.CEECIND (iAqua project). This work was also developed within the scope of the projects i3N, LA/P/0037/202, UIDB/50025/2020 & UIDP/50025/2020, and DigiAqua, PTDC/EEI-EEE/0415/2021, financed by national funds through the FCT/MEC.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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