A cascade SPR sensor based on Ag/Au coated coreless optical fiber for RI and pH measurement

•A cascade SPR sensor based on Ag/Au-coated coreless fiber for measuring RI and pH values is propsed.•The pH sensitive film was successfully coated on the optical fiber sensing probe by LBL self-assembly operation, and the pH value was detected.•The sensor probe is constructed with Ag and Au plasma materials and cascaded, which successfully broadens the bandwidth of the sensor. Due to the restriction of resonant wavelength, the detection range of traditional two-parameter sensors is greatly limited. To solve this problem, a cascade SPR sensor with Ag/Au coating is proposed to measure refractive index (RI) and pH value. In this paper, coreless optical fiber is used as the sensor probe, and Ag/Au are coated on its surface by a magnetron sputtering method. The two sensing channels of this cascade sensor are independent of each other and have a wide parameter detection range. The effects of sensor length and coating time on the performance of the sensor were investigated, and the optimal sensor length and coating time were determined. The experimental results show that the maximum refractive index sensitivity is 3888.6 nm /RIU in the RI range of 1.333–1.385, and the maximum pH sensitivity is 38.01 nm/pH in the pH range of 3.15–8.86. The sensor has the advantages of strong stability and high integration and has a good application prospect in the fields of biosensing and environmental detection.