Probe-Type Optical Fiber Refractometer Based on Surface Plasmon Resonance Enhanced With Gold Nanoparticles of Optimized Size and Coverage

This article investigates the effect of gold film thickness and spherical gold nanoparticles (AuNPs, of different sizes and at different coverage rates) on the performance of optical fiber surface plasmon resonance (SPR) sensors based on multimode fiber–single-mode fiber (MMF–SMF) structure. Simulations and experiments showed that sensors with gold film thickness around 50 nm have higher performance when AuNPs are not immobilized. AuNPs in the 28–58 nm diameter range offered superior sensitization and can improve the figure of merit (FOM), but it required precise control of AuNPs fixation time, especially for AuNPs with a diameter over 48 nm. As the coverage of AuNPs increased, the sensitivity of the sensor increased first in the reverse direction and then in the positive direction. The refractive index (RI) sensitivity can reach 4234 nm/RIU with the FOM of 12.5 RIU $^{-{1}}$ in the experiment, which are a 2.75-fold increase and a 1.74-fold increase compared to before the AuNPs were fixed (1541 nm/RIU and 7.18 RIU $^{-{1}}$ ). Our work is of great significance for improving the sensitivity of fiber optic SPR biosensors.