Lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) as ratiometric primary sensor toward Ag⁺ and the complex of Ag⁺ as secondary sensor toward cys: experimental, computational, and microscopy studies and INHIBIT logic gate properties of L

Abstract

A structurally characterized lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) exhibits high selectivity toward Ag⁺ by forming a 1:1 complex, among nine other biologically important metal ions, viz., Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Zn²⁺, as studied by fluorescence, absorption, and ¹H NMR spectroscopy. The 1:1 complex formed between L and Ag⁺ has been further proven on the basis of ESI mass spectrometry and has been shown to have an association constant, K_a, of 11117 ± 190 M^-1 based on fluorescence data. L acts as a primary ratiometric sensor toward Ag⁺ by switch-on fluorescence and exhibits a lowest detectable concentration of 450 ppb. DFT computational studies carried out in mimicking the formation of a 1:1 complex between L and Ag⁺ resulted in a tetrahedral complex wherein the nitrogens of all four pyridyl moieties present on both arms are being coordinated. Whereas these pyridyls are located farther apart in the crystal structure, appropriate dihedral changes are induced in the arms in the presence of silver ion in order to form a coordination complex. Even the nanostructural features obtained in TEM clearly differentiates L from its Ag⁺ complex. The in situ prepared silver complex of L detects Cys ratiometrically among the naturally occurring amino acids to a lowest concentration of 514 ppb by releasing L from the complex followed by formation of the cysteine complex of Ag⁺. These were demonstrated on the basis of emission, absorption, ¹H NMR, and ESI mass spectra. The INH logic gate has also been generated by choosing Ag⁺ and Cys as input and by monitoring the output signal at 445 nm that originates from the excimer emission of L in the presence of Ag⁺. Thus L is a potential primary sensor toward Ag⁺ and is a secondary sensor toward Cys.