Coumarin Grafted Polyethylene Matrix as Colorimetric and Fluorescent Chemosensor for Metal Ions

Abstract

This mechanism is based on the ability of the lone pair of electrons of tertiary amines to quench the emission of most of the known aromatic chromophores. After protonation of the amine moiety or coordination with a metal ion, the energy of its lone pair of electrons is lowered, leading to a visible increase in the fluorescence emission of the chromophore. [5,6] Coumarin based small molecules that are able to function as colorimetric and fluorescent chemosensors for anions and metal ions have been recently reported. [7-11] The advantages of using coumarin as chromophore are the high photostability, the structural diversity, and the low toxicity of its naturally occurring derivatives. [12,13] A physical immobilization of the chemosensors on a solid support is highly desirable for practical applications. [14] The advantage of such solid supported sensors is that they can be used directly as portable device in the field. Moreover, since they are easy to recover and easy to purify by washing, they can be reused several times. Monomeric or polymeric water soluble compounds have been covalently linked to different types of substrate such as quartz, metal oxide films, and silica based nanoparticles in self-assembled monolayer. [15,16] Sensors for nitroaromatic compounds [17] and metal ions [18-21] have been fabricated into devices capable of combining different outputs. [22] In addition, several types of insoluble polymers as well as hydrogels have been developed as chemosensors for metal ions such as mercury, copper, and iron. [23-26] The modification of polymeric surfaces through grafting is an efficient technique to introduce chemosensor moieties on the surface of a polymeric device. Different types of monomers can be covalently attached to the surface of polymer matrices, modifying the chemical properties of the existing polymer. Several methodologies are applied for the grafting of polymer surfaces, such as chemical or photochemical means, radiation techniques, and enzymatic reactions. [27] The use of gamma radiation leads to the formation of radicals on the polymeric surface that are able to react with vinyl monomers such as acryloyl chloride (AC). [28,29] Further chemical reactions with different types of monomers allow the preparation of polymers for different kind of applications. [30,31] The advantage of the grafting methodology is that the resulting polymers combine A coumarin-based chemosensor is synthesized and grafted onto a polyethylene (PE) surface by means of gamma radiation. The obtained polymeric film results to work as test-paper-like colorimetric sensor for the detection of Cu 2+ ions. After immersion of the grafted PE film into a solution of Cu 2+ in chloroform/methanol (8/2), a color change from colorless to green is observed by naked eye together with a quenching of the fluorescence emission. Control experiment with methoxy-grafted PE films shows no response after the immersion in the same Cu 2+ solution. The polymeric sensor can be washed and reused over several cycles for the detection of Cu 2+...