Cadmium-ion detection: a comparative study for a SnO2, MoS2, SnO2/MoS2, SnO2-MoS2 sensing membrane combination with a fiber-optic Mach–Zehnder interferometer

A novel fiber-optic Mach–Zehnder interferometer based on S n O 2 , M o S 2 , S n O 2 / M o S 2 , and S n O 2 − M o S 2 sensing film for cadmium-ion ( C d 2 + ) detection is proposed and fabricated. The photonic-crystal fiber (PCF) is sandwiched between the no-core-fiber-1 (NCF1) and no-core-fiber-2 (NCF2), forming the Mach–Zehnder interferometer with the NCF1-PCF-NCF2 structure, which is regarded as the sensing unit. The S n O 2 , M o S 2 , S n O 2 / M o S 2 , and S n O 2 − M o S 2 sensing films are, respectively, coated on the surface of two NCFs’ claddings. The comparative experiment results of the sensors with four membranes ( S n O 2 , M o S 2 , S n O 2 / M o S 2 , S n O 2 − M o S 2 ) show that the sensors have good sensing performance for C d 2 + in the concentration range of 0–100 µM. When these sensing films adsorbed C d 2 + , the monitoring wavelength shows blueshift of 0.6931 nm, 1.0252 nm, 1.9505 nm, respectively, and redshift of 3.0258 nm, and the sensitivities are 6.931 pm/µM, 10.252 pm/µM, 19.505 pm/µM, and 30.258 pm/µM, respectively. The sensor with S n O 2 − M o S 2 bilayer film exhibits the optimal response to C d 2 + with excellent selectivity and stability. The proposed sensor has the advantages of simple structure, easy fabrication, small size, etc., having potential application in the monitoring of C d 2 + in aqueous solution.