Design and Characterization of a Dual-channel Cascaded No-core Fiber SPR Sensor Based on MXene

Surface Plasmon Resonance （SPR） sensing technology detects changes in environmental parameters， such as Refractive Index （RI）， by exciting surface plasmon waves through the interaction between incident light and free electrons on a metal surface. Fiber-optic SPR sensors have been widely applied in medical diagnostics， food safety， and environmental monitoring due to their compact size， low cost， and strong resistance to electromagnetic interference. However， conventional single-channel SPR sensors contain only one sensing region， which limits their capability for simultaneous multi-parameter detection in complex environments. Meanwhile， the emerging two-dimensional material Ti3C2-MXene （hereafter referred to as MXene）， featuring a high specific surface area and anisotropic electrical properties， has demonstrated great potential for enhancing sensor sensitivity and enabling accurate detection of trace analytes.Leveraging the properties of MXene， in this study， a dual-channel No-core Fiber （NCF） SPR sensor based on MXene is proposed and optimized to address the demand for high-sensitivity and multi-parameter detection in complex environments. The sensor is constructed by cascading a Multimode Fiber （MMF） with two segments of NCFs. Channel 1 is coated with a gold film， while channel 2 is functionalized with a composite film of gold-cysteamine-MXene. Owing to the different dielectric constants of the two sensing coatings， SPR is excited at distinct spectral regions， enabling dual-channel RI sensing. The optimal structural parameters， including a 50 nm gold layer， NCF lengths of 5 mm （channel 1） and 12 mm （channel 2）， and MXene film thickness， are determined via the commercial finite element method software COMSOL Multiphysics.In the experiment， a gold film with a thickness of approximately 50 nm is deposited on channel 1 using an ionsputter coater （SBC-12）， with a sputtering time of 310 s and a current of 6 mA. MXene nanosheets are subsequently immobilized onto the gold surface of channel 2 via electrostatic self-assembly. Specifically， channel 2 is immersed in a 4 mg/mL cysteamine solution for 30 minutes， during which the thiol groups of cysteamine formed Au-S bonds with the gold film， while the ammonium ions render the surface positively charged. Thereafter， channel 2 is immersed in a 5 mg/mL monolayer-dispersed MXene solution for 2 hours. The negatively charged —F and —O terminal groups on the MXene surface interacts electrostatically with the positively charged ammonium groups， resulting in the uniform deposition of a monolayer MXene film on the fiber-optic surface. Surface characterization using Scanning Electron Microscope （SEM） and Energy Dispersive Spectroscopy （EDS） confirms the successful immobilization of gold， cysteamine， and MXene. The sensor's RI sensitivity is evaluated by monitoring resonance shifts in NaCl solutions with varying RI. Simulation and experimental results consistently demonstrate that increasing MXene thickness improves RI sensitivity and expands the dual-peak separation. This improvement is attributed to the low carrier concentration and high mobility of MXene， which modulate the SPR resonance from the visible to the near-infrared region and enhance the interfacial electric field.Within the RI range of 1.333 to 1.366， as the number of MXene layers deposited on the surface of channel 2 increases from 0 to 5， the resonance wavelength of channel 2 exhibits a red shift， which is consistent with the simulation results. When the MXene reaches three layers， the resonance peaks of the two channels become completely separated. The RI sensitivity of channel 1 remains stable between 1 850 and 1 900 nm/RIU， with its Full Width At Half Maximum （FWHM） and Figure Of Merit （FOM） showing minimal variation. In contrast， the resonance wavelength of channel 2 （SPR-2） red-shifts from 604.15 nm to 781.28 nm， and its RI sensitivity increases from 2 016.31 nm/RIU to 4 209.16 nm/RIU， representing an improvement of 108.75%. Meanwhile， the resonance wavelength undergoes a red-shift from 604.15 nm to 781.28 nm. However， excessive MXene thickness results in a deterioration in the FOM. To achieve a balance between sensitivity and FOM， a four-layer MXene structure is selected for channel 2. After cascading with channel 1， the dual-channel SPR sensor exhibits optimal performance， achieving a dual-peak separation of 125.35 nm. The RI sensitivities of channel 1 and channel 2 are 1 894.27 nm/RIU and 4 001.48 nm/RIU， respectively， with corresponding FOM values of 16.86 RIU-1 and 32.99 RIU-1. Compared with the pure gold film structure， the RI sensitivity of channel 2 is increased by 98.45%.The dual-parameter detection capability is validated by immersing the two sensing regions in solutions with different RIs. The results demonstrates that both channels responded independently to external RI changes. In the independent response tests， channel 1 and channel 2 achieve RI sensitivities of 1 826.25 nm/RIU and 4 317.96 nm/RIU， respectively， confirming the sensor's potential for simultaneous detection of different analytes. To evaluate the stability of the sensor， both sensing regions are simultaneously immersed in pure water and monitored continuously for 60 minutes. The maximum resonance wavelength fluctuations are only ±0.17 nm for channel 1 and ±0.21 nm for channel 2， indicating the excellent stability of the sensor. Compared with existing dual-channel SPR sensors， the proposed sensor features a simplified structure， ease of fabrication， and low cost， while delivering significantly enhanced RI sensitivity. These advantages make it a promising candidate for multi-parameter biosensing and environmental monitoring applications.