Dual-mode SPR sensing with surface and bulk refractive index decoupling via hybrid gap plasmonic architecture

A fundamental challenge in surface plasmon resonance (SPR) sensing arises from the mutual interference between surface refractive index and bulk refractive index responses. In practice, strong plasmonic excitation and reliable bulk refractive index referencing are rarely compatible within a single sensing configuration. To address this long-standing limitation, we develop a high-performance tilted fiber Bragg grating (TFBG)-SPR sensing platform based on a hybrid gap-plasmonic architecture. By limiting the gold film thickness to 30 nm, evanescent field tunneling is retained, allowing the cutoff cladding mode to be excited and employed as an intrinsic bulk refractive index reference. The accompanying loss of plasmonic confinement associated with such thin coatings is compensated by the introduction of gold nanoparticles (AuNP) together with a thickness-tunable two-dimensional metal–organic framework (MOF) spacer. This combination promotes strong hybridization between localized and propagating plasmonic modes, creating nanoscale gaps that concentrate electromagnetic energy at the sensing interface. As a consequence, the SPR quality is substantially improved, while interference from the surrounding bulk medium is effectively suppressed. With this architecture, the platform achieves an amplitude sensitivity of 3925 dB/RIU, corresponding to a 2.28-fold enhancement relative to conventional gold-coated TFBG, while preserving stable self-referencing capability. The robustness of the dual-mode self-decoupling mechanism is further demonstrated through selective detection of Hg 2+ ions, in which weak surface-binding signals are reliably resolved in real time against background bulk fluctuations. These results establish a general and practical strategy for reconciling high sensitivity with robust surface and bulk signal decoupling in fiber-optic plasmonic sensing platforms.