Cadmium-ion detection: a comparative study for a SnO₂, MoS₂, SnO₂/MoS₂, SnO₂-MoS₂ sensing membrane combination with a...

A novel fiber-optic Mach-Zehnder interferometer based on SnO₂, MoS₂, SnO₂/MoS₂, and SnO₂-MoS₂ sensing film for cadmium-ion (Cd²⁺) 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 SnO₂, MoS₂, SnO₂/MoS₂, and SnO₂-MoS₂ sensing films are, respectively, coated on the surface of two NCFs' claddings. The comparative experiment results of the sensors with four membranes (SnO₂, MoS₂, SnO₂/MoS₂, SnO₂-MoS₂) show that the sensors have good sensing performance for Cd²⁺ in the concentration range of 0-100 µM. When these sensing films adsorbed Cd²⁺, 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 SnO₂-MoS₂ bilayer film exhibits the optimal response to Cd²⁺ 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 Cd²⁺ in aqueous solution.