when compared with graphene: singlelayer MoS 2 exhibits a direct electronic bandgap of ≈1.8 eV and optical absorption rate of ≈5%, which is quite different with the zero bandgap and ≈2.3% absorption rate for monolayer graphene. [3] The synthesis of 2D MoS 2 can be classified into top-down method (mechanical exfoliation, liquid exfoliation, electrochemical exfoliation, etc.) and down-top method (the chemical vapor deposition growth). [4] Particularly, the intriguing optical properties, including f luorescence quenching and photoluminescence abilities, enable MoS 2 to play an important role in the optical biosensing field. [5] For example, a fluorescence quenching system of aptamer-CdTequantum dots (QDs)-MoS 2 nanosheets has been developed to detect ochratoxin A (OTA, a typical mycotoxin). [6] The originally fluorescence-quenched QDs-conjugated aptamers, which are assembled on MoS 2 , again exhibit fluorescence in the presence of OTA, and the intensity is dependent on the concentration of OTA. A detection limit of 1.0 ng mL −1 and a dynamic range over 1.0-1000 ng mL −1 were obtained. On the other hand, based on the photoluminescence property of MoS 2 and the fact that its intensity is influenced by the extrinsic charge doping, a graphene/MoS 2 heterostructure has been proposed for ultrasensitive detection of DNA hybridization (as low as 1 × 10 −18 m).[7] However, the both sensing schematics mentioned above lack the ability of label-free or real-time detection, limiting the applications of MoS 2 in the field of optical sensing.As a label-free and real-time optical sensing technology, surface plasmon resonance (SPR) sensing has attracted tremendous researches, and seen spectacular progresses over the past Benefiting from the unique properties of MoS 2 nanosheets including high electron mobility, quantum confinement, nanoscale thickness, etc., an effective way is proposed and demonstrated to enhance the refractive index sensitivity of surface plasmon resonance (SPR) sensors, which is strongly desired all the time in the field of biochemical sensing. The SPR sensors are modified by the physical deposition of MoS 2 nanosheets, and the sensitivity dependence on the number of deposition cycles is investigated experimentally. It is found that the sensitivity first increases and then declines with the increase of the number of deposition cycles, meaning an optimal thickness thus existing. By depositing MoS 2 nanosheets for two cycles, the maximal sensitivity of 2793.5 nm RIU −1 (RIU: refractive index unit) can be achieved, which shows an enhancement of 30.67% compared with the case without any modification. Taking into account the evanescent field intensity and the propagation length, the experimental results can be well analyzed and explained. Simulation results show that the increase of MoS 2 overlayers can enhance the intensity of electrical field penetrating into the analyte solution while reducing the propagation length, which collectively results in the nonmonotonic change of the sensitivity depending on deposition cy...
