Optical fiber-coupled Kretschmann SPR sensor with re-attachable gold nano-thin film sensing chip.

We designed and demonstrated a portable and reusable surface plasmon resonance (SPR) sensor based on an optical fiber-coupled Kretschmann configuration with a variable detection limit enabled by the re-attachable gold nano-thin film. The prism angle of SPR has been optimized to 63.5 degrees to enable the SPR sensor to operate in the near-infrared band. We highlight the effects of the chromium to gold film thickness ratio and the prism angle on the SPR characteristics to improve sensitivity. Both simulations and experimental results reveal that the narrowest FWHM and deepest amplitude of the SPR curve were achieved when the chromium to gold film thickness ratio was set to 2 nm/40 nm. Furthermore, the refractive index (RI) sensitivity of the sensor was simulated within the RI detection range of 1.3235-1.3290, and with the optimized SPR sensor, we obtained a record high RI sensitivity of 34,888 nm/RIU. The capability to detect variations in saline and sucrose concentration in aqueous form has been experimentally demonstrated, revealing a sensitivity of 455 nm/M and 3,056 nm/M over concentration ranges of 0.10 to 0.55 M and 0.018 M to 0.053 M, respectively. The results show that the proposed optical fiber-coupled SPR sensor with an optimized sensing chip offers high sensitivity and is promising for a broad range of applications, including medical diagnostics, agriculture, food safety, and environmental monitoring. The sensor's advantages include miniaturization and flexibility, highlighting its potential for broad and versatile use.