Direct electronical readout of surface plasmon resonance biosensor enabled by on-fiber Graphene/PMMA photodetector

Surface plasmon resonance (SPR) optical fiber sensors are appealing for biomolecular detection due to their inherent characteristics such as flexibility, real-time performance, and high sensitivity. Concurrently, incorporating SPR sensors into wearable devices has emerged as a significant strategy. However, the majority of traditional SPR optical fiber sensors utilize spectrometers for optical readout, which leads to a relatively bulky overall size of the sensing system. Herein, we present the first optical fiber device capable of conducting sensitive SPR measurements and providing direct electronical readout. This has been achieved by integrating a hyperbolic-metamaterial SPR (HMM-SPR) sensor with an on-fiber graphene/PMMA photodetector (oFGPD). The HMM, composed of three pairs of Au/ZrO₂, has been employed to develop highly sensitive SPR sensors. The oFGPD, which was constructed by transferring a single layer of graphene onto a tapered fiber region and subsequently covering it with a PMMA protecting film, achieved a high responsivity of 3.42 × 10⁶ A W^-1 (at 14.07 pW) and a rapid response time of approximately 90 ms at 1550 nm. More significantly, we have incorporated an SPR sensor based on a side-polished fiber (SPF) into the oFGPD, enabling an electronical readout technique for environmental refractive index (RI) SPR signals in a broad potential spectral range, from visible to near-infrared, all within a more compact device. This integration has been successfully validated in the detection of urea and glucose concentrations in artificial perspiration. This approach provides a novel direction for SPR sensor detection and establishes a solid foundation for their application in wearable technology.