Differential Fresnel-reflection-based fiber biochemical sensor with temperature self-compensation for high-resolution measurement of Cd²⁺ concentration in solution

A fiber biochemical sensor system was developed to measure cadmium (Cd²⁺) solution based on differential temperature self-compensation, which exhibits high sensitivity in detecting Cd²⁺. The device only consists of a light source, three couplers, two photodetectors, and two fiber sensing ends. Utilizing the efficient adsorption of cadmium ion by allylthiourea, the sensing head was prepared. The estimated concentration resolution of 4.9 × 10⁻⁷ mol/L and the standard deviation of concentration of 4.6 × 10⁻⁶ mol/L in large temperature range are achieved. The measured data is fitted to a second-order polynomial equation with good agreement. Moreover, the differential technique of the sensor can effectively eliminate concentration errors resulting from temperature fluctuations and environment effects, and thus real-time monitoring can be obtained.