A bifunctional thioether linked coumarin appended Calix[4]arene acquires selectivity toward Cu²⁺ sensing on going from solution to SAM on Gold

Abstract

A bifunctional calix[4]arene molecule bearing coumarin moiety on the lower rim and thioether moiety on the upper rim (L₁), has been synthesized and well characterized by ¹H, ¹³C NMR and mass spectrometry. Suitably functionalized coumarin moieties are well suited for selective recognition of various cations and anions. Among the 10 different metal ions studied, only Cu²⁺ and Fe³⁺ exhibit appreciable changes in the absorption spectra owing to the availability of functional moieties present at both the lower as well as the upper rim of free L₁ in acetonitrile solution. To bring better selectivity, we blocked one of these functional moieties by coating on to a surface so that only the other one is exposed to the environment for sensing. Such a study carried out in the present case using the self-assembled monolayer (SAM) of L₁ on Au(111) resulted in selective sensing of Cu²⁺ over several other metal ions as studied by surface plasmon resonance (SPR). The SAM of L₁ on Au(111) was confirmed by different techniques, such as grazing incidence FT-IR, contact angle measurement, cyclic voltammetry and scanning tunneling microscopy. Thus, L₁ is proven to be a suitable sensor for Cu²⁺ when attached to gold surface.