We propose and demonstrate a fibre surface plasmon resonance (SPR) sensor based on a microsphere structure. By employing the microsphere fibre, the cladding mode of the fibre is excited and provides an evanescent field for the SPR sensing structure. The relationship between the diameter of the microsphere, the diameter of the fibre core, the types of fibres, and the sensing performance of the sensing probe are studied experimentally. Based on the experimental results, optimum parameters are obtained that result in the sensitivity of refractive index testing reaching 3570.46 nm/RIU, much higher than that of other common multimode SPR sensors. Furthermore, to detect Hg2+ in solution, we chemically modified the sensing film of the sensor with 1, 6-hexanedithiol. The resultant sensor can detect a 5 pM Hg2+ solution target, which demonstrates the sensor's low chemical detection limit. The proposed fibre microsphere SPR sensor has a simple and stable structure which greatly increases evanescent field leakage. When the sensing film is functionalised accordingly, the sensor can be used in food security, medicine, biochemistry, etc.
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