Design of a novel optical sensor for the determination of cadmium based on the immobilization of β-2-hydroxybenzyl-5-chloro-2-hydroxyazastyrene on an agarose membrane

An optode featuring high selectivity was prepared for cadmium detection <i>via</i> chemically binding β-2-hydroxybenzyl-5-chloro-2-hydroxyazastyrene (HCHAS) onto a clear agarose membrane. Colorimetric analysis of complexation between HCHAS and various metal ions at pH 2.75 revealed that the Cd²⁺ complex exhibited a markedly higher stability constant than others. As a result, HCHAS was applied as a suitable ionophore for fabricating a Cd²⁺-selective optode, <i>via</i> chemically anchoring it onto a transparent agarose matrix. The immobilization parameters were optimized and the resulting optode represented a conspicuous color transition from yellow to purple as a result of increasing Cd²⁺ concentrations in a pH 2.75 buffered solution. Detailed investigations were conducted to assess the influence of pH, ionophore loading, ionic strength, stirring conditions, and reaction duration on the optode's performance. A strong linear relationship was observed over the 4.0-140 ng mL^-1 concentration range, with <i>R</i>² values of 0.9882 and 0.9990, respectively. The high sensitivity of the method was demonstrated by the molar absorptivity and Sandell sensitivity, evaluated at 5.11 × 10⁶ L mol^-1 cm^-1 and 0.002 ng cm^-2, respectively. Furthermore, Cd²⁺ detection was not considerably influenced by the existence of various potentially interfering ions at concentrations 500-fold higher. The fabricated optode was effectively utilized for the quantification of cadmium ions in fortified food, environmental and biological specimens.