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Abstract
Ti3C2Tx MXene, as a representative two-dimensional nanomaterial, has been recently receiving attention for constructing high-performance sensors. Herein, a square coreless fiber functionalized with Ti3C2Tx MXene layer is proposed and experimentally demonstrated for highly sensitive refractometric measurement. The refractometric sensor is designed by chemically depositing Ti3C2Tx film on the square coreless fiber, in which Ti3C2Tx film is employed for enhancing the hydrophilicity and promoting the adsorption capacity of molecules. Compared with pristine square coreless fiber, the sensitivity of the refractometric sensor is improved by more than 12% for liquids refractive index (RI) around 1.333. Moreover, the Ti3C2Tx modified square coreless fiber exhibits compact dimension, easy integration, low sample consumption, and good flexibility for enabling the accurate discriminating of small-scale RI changes of analytes. Our work provides a promising and effective platform for general ultra-low concentration analytical detection, which could be extended for biochemical sensing, photocatalysts, and photovoltaic applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Liquid refractometric sensing as a label-free and simple-operation method plays a crucial role in chemical analysis, biological detection, and environmental evaluation [1–6]. Especially in the field of biosensing, where biological reaction process generally induces to a slight change in refractive index (RI) of analytes, highly sensitive refractometric sensors are always preferable and desirable. Nowadays, it is highly demanded for developing sensitivity-enhanced refractometric sensing technology in the RI range of 1.33-1.34 where bioassays are typically performed [7]. In general, many efforts have been made to enhance the perception of the surrounding environment, further promoting the rapid development of liquid refractometric sensors. For instance, the electrochemical sensor could improve the sensing sensitivity of molecules by integrating with functional materials [8,9]. The efficient mechano-based transductive method exhibit excellent biosensing properties, as well as ultrasensitivity [10]. The biosensor also can combine with surface plasmon resonance (SPR) technique to realize highly sensitive and specific diagnosis of viral variants [11]. These sensitive-enhanced biosensor possess the advantages of high performance, which makes them a potential candidate for biological detection. However, it also suffers from the disadvantages of label fluorescent detection and complicated operation for electrochemical sensing. The SPR biosensor often requires a relatively complex deposition process, which limits its practical application. To date, a variety of fiber-type sensing technologies have been proposed for accurate refractometric sensing of liquid or gaseous samples which include tapered microfibers [12–15], chemical-etching fibers [16–19], microstructured optical fibers [20–22], side-polished fibers [23–25], heterocore misaligned fibers [26,27]. These reports show excellent sensing performance and considerable efficiency in analysis fields. However, with the limitation of the numerical aperture of the optical fibers, the sensing performance of the fiber-optic devices can be improved to a limited extent by modulating the fiber structure solely.
With the rapid development of nanotechnology, two-dimensional MXenes pave a new way to solve this challenge. Because of their exceptional properties such as large specific surface area, good biocompatibility, excellent adsorption, and strong hydrophilicity, two-dimensional MXenes have received great attentions particularly to photovoltaics [28], catalysis [29], sensors [30–32], and optoelectronic devices [33,34]. The general formula of MXenes is expressed as Mn + 1XnTx, where M represents an early transition metal, X refers to carbon or nitrogen, and T indicates the surface terminations [35]. The few-layer Ti3C2Tx Mxene with uniform thickness and fewer defects, as one of the most promising two-dimensional materials, has unique optoelectronic properties and structural advantages. Because of its specific accordion-like structures, the large surface area makes Ti3C2Tx MXene easier to fully contact with analytical samples. More importantly, there are abundant functional groups in MXenes layers for interacting with liquid molecules, such as −OH, =O, or −F, which offer a nature ultra-permeable channel for liquid molecules and promote rapid interaction between liquid molecules and MXenes layers. Additionally, it is easy to modulate the optoelectrical performance, biochemical functions, mechanical stability, and other properties through surface modification. Therefore, MXenes and MXene-integrated hybrid composites have been considered as a fascinate choice for sensing materials. Hence, the fibers integrating with MXenes functional layers not only provides a unique opportunity to evaluate the effectiveness and performance of sensitivity-enhanced materials, but also enable the realization of an excellent and balanced fiber-optic device.
In this paper, incorporating MXene property with square coreless fibers, we proposed a highly sensitive refractometric devices based on few-layer Ti3C2Tx Mxene and heterogeneous splicing fibers. Based on the efficient chemical immobilization of the Ti3C2Tx MXene on square coreless fiber surface, the interaction between the Ti3C2Tx film and the propagating light of the square coreless fiber further improves the liquids refractometric sensing sensitivity without destroying the fibers. Furthermore, the transmission spectra characteristics of the square coreless fiber have been theoretically analyzed and experimentally compared with and without Ti3C2Tx Mxene layer. Our proposed Ti3C2Tx-modified square coreless fiber exhibits several desirable advantages, such as high sensitivity, ease of integration and miniaturization, which makes it a powerful lab-around-fiber tools for applications in biosensor and photochemistry analysis.
2. Materials and methods
2.1 Reagents and materials
The experimental regents employ sulfuric acid (H2SO4), hydrogen peroxide (H2O2), (3-aminopropyl)triethoxysilane (APTES), ethanol, deionized water, glycerin and Ti3C2Tx Mxene. The piranha solution containing H2SO4 and H2O2 is mixed in the ratio of 7:3. APTES is dissolved in ethanol (vol ratio, 1:20). Ti3C2Tx Mxene solution (Nanjing MKNANO, Inc) with the concentration of 5 mg/ml is synthesized by ion intercalation method. The glycerin RI solutions ranging from 1.333 to 1.3357 is prepared by utilizing the serial dilution method. All chemicals used in our experiment are of analytical grade without further purification. The single mode fibers (SMFs) are SMF-28 (Corning, Inc) with the core/cladding diameter of 8/125 µm. The square coreless fiber is pure silica with the cross-sectional width size of 90 µm.
2.2 Fiber design and fabrication
Figure 1(a) shows the schematic diagram of the misaligned square coreless fiber, which mainly consists of lead-in SMF, square coreless fiber, and lead-out SMF. Here, the length of the square coreless fiber is 122 µm, respectively. The square coreless fiber is sandwiched by two SMFs with a lateral offset distance of about 45 µm, so that the propagating light can been effectively coupled into the sensing/reference arms and obvious interference fringe can be obtained. Such a miniature square coreless fiber integrative device could facilitate the biosamples analysis with microsolutions. During the fabrication of the misaligned square coreless fiber, a micromachining technique based on ultrasonic vibration cutting and arc discharge fusion are utilized in our experiments, as shown in Fig . 1(b). The detailed microfabrication process is as follows: a section of input SMF is spliced with a square coreless fiber by manual misalignment. Then an ultrasonic transducer transmits high-frequency ultrasonic vibrations to precisely cut the square coreless fiber, preserving only hundred-micron-length square coreless fiber. Finally, the cascaded square coreless fibers are spliced with the output SMF by using the previous misaligned fusion splicing method. In this way, a square fiber integrated micro-device is completely fabricated.
Fig. 1. (a) Schematic diagram of the misaligned square coreless fiber. (b) Fabrication process of the misaligned square coreless fiber.
2.3 Ti3C2Tx deposition on a square coreless fiber surface
In this study, the Ti3C2Tx is chosen to serve as the optical buffer layer, which is deposited on square coreless fiber by chemical bonding technique. Figure 2 schematically illustrates the Ti3C2Tx nanosheets deposition process. First, the square coreless fiber is immersed into the as-prepared piranha solution for 1.5 hours to enrich the fiber surface with a large number of -OH functional groups. Then the hydroxylated square coreless fiber is repeatedly washed with deionized water and naturally dried to remove the residual hydroxyl groups. Next the square coreless fiber is modified by 5% APTES for 30 minutes to deposit positively charged amino groups (-NH3) on the fiber surface. Subsequently, the fiber surface is cleaned several times with ethanol to remove the sialylation compounds without chemical bonding. Finally, 100 µL Ti3C2Tx dispersion solution is carefully dropped onto the -NH2 modified fiber surface until the square coreless fiber is completely covered. The surface of the Ti3C2Tx nanosheets are characterized to be negative charged properties due to the presence of negative charged −OH and = O functional groups distributed on the surface terminals of Ti3C2Tx. Therefore, the Ti3C2Tx MXene is deposited on fiber sensing region through the chemical bonding between the negative charged functional groups of Ti3C2Tx nanosheets and the surface amino-groups of square coreless fiber [36]. After solvent is fully evaporated for about 6 h, the square coreless fiber is completely functionalized with Ti3C2Tx nanosheets. In the next step, the Ti3C2Tx-based fiber element is integrated in sensing system for detecting low-range glycerin RI samples.
3. Results and discussion
3.1 Surface morphology characterization of Ti3C2Tx-Mxene-based square coreless fiber
The surface morphology of the Ti3C2Tx-Mxene-based square coreless fiber is investigated and characterized by using optical microscopy (Olympus), scanning electron microscopy (SEM: Apreo), and Raman microscope at 638 nm laser excitation (Horiba). Figure 3(a)∼(c) show the side-view microscopic and SEM photographs of square coreless fiber incorporating with Ti3C2Tx layer. It could be seen that Ti3C2Tx film is obviously deposited on the fiber surface. Figure 3 (d) and (e) show the cross-sectional photographs of square coreless fiber incorporating without and with Ti3C2Tx layer, respectively. The bare square coreless fiber maintains a smooth surface, while the surface roughness and thickness of the square coreless fiber obviously increase after Ti3C2Tx deposition. Moreover, as shown in Fig. 3(f), the Raman spectrum of the Ti3C2Tx-Mxene-based square coreless fiber presents five resonance Raman peaks, including 154.8, 211.5, 457.3, 562.1 and 673.4 cm-1, indicating the presence of Ti3C2Tx layer on the fiber surface. The effective deposition of Ti3C2Tx layer ensures enhanced-sensitive detection for the following refractometric sensing applications.
Fig. 3. (a) Microscope image of the Ti3C2Tx-depositedsquare coreless fiber. (b) and (c) side-view SEM images of the depositing Ti3C2Tx surface morphology. (d) and (e) cross-section images of square coreless fiber without and with Ti3C2Tx. (f) Raman spectrum of Ti3C2Tx-based square coreless fiber.
3.2 Low-range glycerin refractometric sensing
The experimental setup for glycerin refractometric sensing is constructed and shown in Fig. 4. The lead-in SMF is connected with a supercontinuum broadband source (SBS: NKT photons) operating in the wavelength range of 1200-1550 nm. The transmission spectra are monitored and collected by connecting the lead-out SMF to an optical spectrum analyzer (OSA: YOKOGAWA, AQ6370B). After that, the Ti3C2Tx-based square coreless fiber is inserted in the sensing system through fusion splicing method. Figure 5 shows the experimental comparison of the square coreless fiber before and after Ti3C2Tx deposition by silanization. When Ti3C2Tx MXene is deposited on the square coreless fiber surface, the resonance dips show obvious red-shift due to that the fiber/MXene composite waveguide would increase the effective RI difference between the sensing arm and reference arm of the square coreless fiber [37,38]. After the Ti3C2Tx MXene are deposited on the square coreless fiber surface, the resonance dips shift toward longer wavelength region. According the mode coupling and interference theory, the dip wavelength λ in the transmission spectrum is determined by the effective refractive index difference Δn between the sensing arm and reference arm of the interferometer, and the interference length L (λ=2ΔnL/(2m + 1)). As Ti3C2Tx MXene are deposited onto the square coreless fiber surface, the overall refractive index of the fiber/MXene composite structure will be increase. The variation of the refractive index of the fiber/MXene will increase the effective refractive index difference Δn, which will correspondingly result in resonance dips shifting toward the longer wavelength region.
Fig. 5. Transmission spectra of the square coreless fiber surface before and after Ti3C2Tx-MXene deposition.
During the low-range glycerin refractometric sensing test, a series of glycerin RI solutions ranging from 1.333 to 1.3357 are added to cover the entire square coreless fiber region. After each liquid sample is tested, the sensing region is repeatedly cleaned with ethanol until there is no residual test liquids. At first, the square coreless fiber without a Ti3C2Tx nanosheet layer is performed for refractometric sensing as a control group. Figure. 6(a) and (b) show the response transmission spectra evolution and the corresponding wavelength shift as functions of the glycerin RI for the three resonance dips. It could be seen that the resonance dips shift toward shorter wavelengths with the increase of glycerin RI. In the very low-range of liquid RI changing from 1.333 to 1.3357, the resonance dip a, b and c exhibit good linear behaviour with sensing sensitivity of 11053.43, 12122.42 and 12711.69 nm/RIU, respectively.
Fig. 6. (a) Transmission spectra evolution of the square coreless fiber without depositing under different RI liquids. (b) Resonance wavelength shift as functions of liquid RI for the corresponding dips.
To investigate the contribution of Ti3C2Tx to the sensing performance, we have conducted the glycerin RI sensing experiment for the square coreless fiber incorporating with Ti3C2Tx. As shown in Fig. 7(a), the resonance dips also shift to the shorter wavelengths as the glycerin RI increases from 1.333 to 1.3357. Compared to the control experimental group, the resonance dips show different shift amounts and sensing sensitivities. In order to investigate the sensing performance more directly, the resonant wavelength is plotted as the functions of the glycerin RI for the three resonance dip A, B and C, which show highly linearity with respective sensing sensitivities of 12080.59, 13496.48 and 14265.53 nm/RIU, respectively. In the cases of being depositing with Ti3C2Tx, the sensing sensitivities of the modified square coreless fiber has been significantly improved, which is 12% more than the fiber without Ti3C2Tx deposition. The dominant mechanism could be attributed to the interaction between Ti3C2Tx MXene and square coreless fiber. When the Ti3C2Tx is modified onto the square coreless fiber surface, the propagating light is transferred between two adjacent mediums of hybrid fiber/Ti3C2Tx waveguide and liquid medium. Both the enriched functional groups and the enlarged specific surface area induced by the Ti3C2Tx incorporation enhances the adsorption capacity of glycerin molecules. Thus, the strong light-matter interaction significantly modulates the effective mode RI different between the sensing arm and reference arm of the modified square coreless fiber, resulting in an amplified RI sensing sensitivity. Furthermore, it would be possible to further improve the sensing sensitivity of the square coreless fiber by optimizing the deposited Ti3C2Tx MXene with higher adsorption coefficients.
Fig. 7. (a) Transmission spectra evolution of the square coreless fiber with depositing Ti3C2Tx-Mxene under different RI liquids. (b) Resonance wavelength shift as functions of liquid RI for the corresponding dips.
We have re-conducted our experiment to analyze the detection limit of the sensor. Figure 8 shows the dip wavelength shift as a function of RI with error bars in the low RI range of 1.33. The standard deviation was σ=1.84 nm, and the detection limit (DL) of this device is estimated as DL = 3σ/S = 4 × 10−4 RIU. Moreover, Table 1 is added to compare the sensing performance of our work with some previously reported works. Compared with the previous works in Table 1, it has optimal advantages of simple structure, small size, high integration, low sample consumption, and high sensitivity. Moreover, the composite fiber/MXene device has great potential in the field of biomolecular sensing with higher sensitivity and lower limits of detection.
Table 1. Comparison of the Sensing Performance with Different Sensors
4. Conclusion
In summary, we have demonstrated the enhanced sensing sensitivity of the square coreless fiber by depositing a 2D nanomaterial, Ti3C2Tx MXene nanosheets. The misaligned square coreless fiber is fabricated by ultrasonic vibration cutting technique and further modified with APTES for the covalent immobilization of Ti3C2Tx Mxene. Owing to the Ti3C2Tx functionalization, the modified square coreless fiber exhibits stronger light- analyte interaction and higher molecular adsorption capacity, which significantly improve the detection sensitivity. Experimental results show that the Ti3C2Tx-Mxene-based square coreless fiber achieves an ultra-high RI sensitivity of 14265.53 nm/RIU in the low analyte RI range of 1.333 to 1.3357. Compared to the case of uncoated square coreless fiber, it indicates that the presence of the Ti3C2Tx layer results in an enhancement of 12% in sensing sensitivity. Moreover, the composite fiber/MXene element features high-sensitivity, ease to integration, good compatibility and compact structure, opening new avenues for development of highly sensitive analytical platform for various biomolecules sensing applications.
Funding
National Natural Science Foundation of China (11904262, 62105164, 62275131); Natural Science Foundation of Tianjin City (21JCQNJC00210); Opening Foundation of Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology; Opening Foundation of Tianjin Key Laboratory of Micro-scale Optical Information Science and Technology.
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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