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 Ce 3+ and H 2 PO 4 − sequentially in the aqueous medium. The detection process has two steps. The interaction of CAL and Ce 3+ in the first step with a noticeable color shift from purple to blue. Further, the reaction between CAL‐Ce 3+ and H 2 PO 4 − 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 H 2 PO 4 − removes Ce 3+ and restores CAL content. The results showed that CAL and CAL‐Ce 3+ had a strong binding affinity and adequate detection limits for the Ce 3+ and H 2 PO 4 − , respectively. For Ce 3+ , the linear range and detection limit are 1.20 × 10 −6 ‐1.02 × 10 −4 and 2.0 × 10 −7 molL −1 , while for H 2 PO 4 − , 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 Ce 3+ and H 2 PO 4 − inputs are added in proper order, the receptor may also function as a molecular “keypad lock.”