Optical fiber surface plasmon resonance (SPR) sensors combine the advantages of optical fiber technology and SPR effect, offering small size, high sensitivity, fast response, and anti-electromagnetic interference capabilities. These features make them widely studied in biological sensing, chemical sensing, and environmental monitoring. However, when applied to biomolecular detection, further enhancement in refractive index sensitivity is still required. With advances in materials science, various two-dimensional (2D) nanomaterials, such as graphene and black phosphorus, have been increasingly used to improve the performance of biosensors. MXene, a relatively new member of the 2D nanomaterials family, is expressed as M_n+1X_nT_x(n=1-4), where “M” is a transition metal, “X” is C or N, and “T” represents surface terminations such as —O, —F, or —OH groups. Due to its excellent optical response, tunable band gap, and electronic properties, MXene holds promise for enhancing the performance of coreless optical fiber SPR sensors. In this paper, we focus on leveraging Ti₃C₂-MXene to improve refractive index sensitivity, making the sensors more suitable for applications requiring high sensitivity, such as biological sensing.

The proposed sensor adopts a multimode-coreless-multimode (MCM) fiber structure. Using COMSOL software, simulations are conducted to optimize the gold film thickness, coreless fiber length, and Ti₃C₂-MXene layer thickness. The simulation results reveal the optimal parameters: a 10 mm coreless fiber length, a 50 nm thick gold film sputtered on the fiber surface, and three layers of Ti₃C₂-MXene coating. The fabrication process involves attaching a negligible-thickness Ti₃C₂-MXene layer to the coreless fiber to fix the gold film, sputtering a thin gold layer to excite the SPR effect, and subsequently applying the Ti₃C₂-MXene coating to further enhance the SPR effect. The refractive index sensitivity is assessed by immersing the sensor in NaCl solutions with varying refractive indexes and recording the corresponding resonance wavelength.

The surface morphology of the gold film and Ti₃C₂-MXene coatings is characterized using field emission scanning electron microscopy (FESEM, ZEISS SIGMA HD) (Fig. 5). Energy spectrum analysis confirms the effective fixation of Ti₃C₂-MXene on the sensor surface (Fig. 6). The experimental results demonstrate that introducing Ti₃C₂-MXene significantly enhances the sensor’s refractive index sensitivity. Without the MXene layer, the refractive index sensitivity is 2326.42 nm/RIU within the refractive index range of 1.3330 to 1.3660. After applying three layers of Ti₃C₂-MXene, the sensitivity increases to 4361.04 nm/RIU, representing an 87.5% improvement. A comparison of experimental results with theoretical predictions shows high consistency, validating the sensor’s performance. However, the study also highlights that while the MXene layer improves sensitivity, it broadens spectral bandwidth, reducing the sensor’s quality factor. Table 1 compares the performance of the proposed MCM fiber SPR sensor with other fiber-based SPR sensors, showing that the Ti₃C₂-MXene and MMF-CLF-MMF combination achieves superior sensitivity (4361.04 nm/RIU) and an enhancement (87.5%) exceeding other designs. In addition, the MCM fiber structure avoids mechanical strength reduction associated with grinding sensors into D-shapes or etching their cladding.

In this paper, we propose an MCM fiber SPR sensor utilizing the sensitization effects of Ti₃C₂-MXene. The principle of refractive index sensing and the influence of structural parameters are analyzed through simulations and experiments. The optimal sensor parameters are determined as a 50 nm gold film, 10 mm CLF fiber length, and three layers of Ti₃C₂-MXene. The experimental results indicate that within the refractive index range of 1.3330?1.3660, the refractive index sensitivity of the sensor increases from 2326.42 to 4361.04 nm/RIU, an 87.5% improvement compared to that of the MCM fiber SPR sensor without Ti₃C₂-MXene. The proposed MCM fiber SPR sensor offers a simple structure, low fabrication difficulty, reduced cost, and high sensitivity, making it promising for biological detection applications.