A simple, sensitive, and selective colorimetric chemosensor for sequential determination of dual analytes, Ce3+, H2PO4−, and its applications in the logic gate construction and the system of keypad lock

Sodium (E)-3-hydroxy-4-((2-hydroxynaphthalen-1-yl)diazenyl)naphthalene-1-sulfonate (Calcon, CAL), is employed for the first time as a reversible colorimetric chemosensor to detect and measure Ce³⁺ and H₂PO₄⁻ sequentially in the aqueous medium. The detection process has two steps. The interaction of CAL and Ce³⁺ in the first step with a noticeable color shift from purple to blue. Further, the reaction between CAL-Ce³⁺ and H₂PO₄⁻ shows a clear color return from blue back to initial purple similar to that for CAL. The UV–visible examination of this color return reveals that H₂PO₄⁻ removes Ce³⁺ and restores CAL content. The results showed that CAL and CAL-Ce³⁺ had a strong binding affinity and adequate detection limits for the Ce³⁺ and H₂PO₄⁻, respectively. For Ce³⁺, the linear range and detection limit are 1.20 × 10⁻⁶-1.02 × 10⁻⁴ and 2.0 × 10⁻⁷molL⁻¹, while for H₂PO₄⁻, they are 2.0 × 10⁻⁷-8.7 × 10⁻⁶ and 3.0 × 10⁻⁸ molL⁻¹. Finally, we measured these two ions in real samples using this methodology. Furthermore, the examination of the suggested receptor's logical behavior revealed that it can operate as a colorimetric chemosensor of the INHIBIT type, receiving chemical inputs and producing a UV–visible absorbance signal as the output. If Ce³⁺ and H₂PO₄⁻ inputs are added in proper order, the receptor may also function as a molecular “keypad lock.”