Functionalized calix[4]arene as a colorimetric dual sensor for Cu(ii) and cysteine in aqueous media: experimental and computational study

Abstract

A family of calix[4]arenes with variation in steric crowding at the upper rim and incorporating pyridine and amide moieties at two of the four OH groups located in the lower rim have been synthesized and their performance as sensors for metal ions has been investigated. Two of the compounds exhibited selective interaction with Cu²⁺ ions with sharp colour change, observed by the naked eye. Spectroscopic and computational studies revealed that two of the compounds form 1 : 2 complexes, in which one of the Cu²⁺ ions interacts strongly with the phenolic OH, resulting in a colour change due to strong absorption (ICT) in the visible region. For the third compound, the Cu²⁺ ions coordinate with the pyridine and amide nitrogen atoms, and the Cu²⁺ ion could not enter inside to interact with the OH group due to restricted flexibility at the lower rim because of the bulky groups at the upper rim. The Cu²⁺ complexes thus formed have been used for sensing of amino acids and interestingly, one of the complexes exhibits interactions selectively with cysteine out of eighteen amino acids tested with a sharp colour change in aqueous media. The formation of the Cu²⁺–cysteine complex is confirmed from mass data, and the surface morphology of the complex before and after the addition of cysteine is investigated by SEM study.