A novel Schiff base ligand HL2 was synthesized as a colorimetric sensor for Ni2+ and CN− /OAC− ions. The binding constants (Ka) calculated using the Benesi-Hildebrand analysis of the UV–vis titration data for Ni2+ and CN− /OAC− ions were determined to be 2.88 × 1010, 7.03 × 104 and 3.44 × 103 M− 1, respectively. The stoichiometry ratio between receptor HL2 and Ni2+ was verified to be 2:1. The experimental results were confirmed by Density Functional Theory (DFT) studies using the B3LYP level and 6-31+g (d) basis set. The absorbance outputs signals were measured at 370 nm (Out1) and 500 nm (Out 2) based on three chemical inputs CN− (Inc), OAC− (InO) and HSO4 − (InH) in eight different combinations. The combinatorial circuit was designed by three logic gates (OR, NOT and AND) using different output signals towards CN− (Inc), OAC− (InO) and HSO4 − (InH). The absorbance output signals for four chemical inputs, Including Ni2+ (In Ni2+), CN− (Inc), OAC− (InO) and HSO4 − (InH) in sixteen different combinations, were successfully recorded at 500 nm (out1) and 425 nm (out2). Furthermore, two chemical inputs (Ni2+ and CN− or Ni2+ and OAC− ) and one output (absorbance at 420 nm) can be used to construct a molecular keypad lock.
