Development of a high-performance optical sensor for sensitive detection of cobalt ions in pharmaceutical, food,...

A re-generable optical chemical sensing film was created using a modified chitosan film that incorporates immobilized 4-(thiazol-2-yldiazenyl) benzene-1,3-diol (TDBD) for the detection of Co²⁺ in acidic aqueous solutions. Upon exposure to Co²⁺, the film's color shifted from yellowish green to red by forming a complex between Co²⁺ and TDBD. The sensor's complex was measured at 574 nm, a wavelength where the sensing membrane exhibited minimal background interference. The film exhibited its highest responsiveness to cobalt ions at pH 5.0. Two sample volumes were analysed: 2.5 mL with a Co²⁺ concentration range of 8.0-140 ng/mL, and 250 mL with a concentration range of 2.4-15.2 ng/mL. Both sample sizes produced linear calibration curves, with detection limits of 2.5 and 0.7 ng/mL, respectively. The relative standard deviation was 1.35 % for six separate films in a 100 ng/mL Co²⁺ solution, and 0.87 % for six individual films in a 10 ng/mL solution using 2.5 and 250 ng/mL, respectively. The sensing films demonstrated good stability over 30 days and were successfully used to determine Co²⁺ in pharmaceutical, food, environmental, and biological samples, yielding satisfactory results compared to the ICP-AES method.