Rhodamine functionalized mesoporous silica as a chemosensor for the efficient sensing of Al3+, Cr3+ and Fe3+ ions and their removal from aqueous media

Abstract

Rhodamine 6G functionalized mesoporous silica (RFMS) has been used for the selective sensing of Al³⁺, Cr³⁺ and Fe³⁺ ions. RFMS has been characterized via FT-IR spectroscopy, PXRD, N₂-adsorption/desorption, TEM, ¹³C and ²⁹Si solid state NMR, and absorption and fluorescence spectroscopy studies. The amount of organic moieties incorporated into the silica matrix has been ascertained via TGA. RFMS shows no absorption band in the visible region in the absence of any other cations, but in the presence of Al³⁺, Cr³⁺ and Fe³⁺, it exhibits a band at 528 nm. The intensity of this peak increases with the increase in the metal ion concentration. The fluorescence intensity of RFMS at 550 nm is enhanced significantly in the presence of Al³⁺, Cr³⁺ and Fe³⁺ ions in water/ethanol (14 : 1, v/v) (λ_ex = 500 nm). No other metal ion is able to induce any enhancement in the emission intensity. The quantum yield and lifetime of RFMS increase considerably in the presence of these trivalent cations. The naked eye detection of these cations is possible through monitoring the color change of RFMS from colorless to pinkish orange under visible light, while under UV light the color change occurs from bluish to fluorescent greenish. The opening of the spirolactam ring of RFMS is induced by interactions with these cations, which results in the pink color and strong fluorescence. RFMS has been utilized to separate Al³⁺, Cr³⁺ and Fe³⁺ ions from an aqueous medium with very high removal efficiency (∼90%). Its maximum uptake capacities have been determined to be 11.20, 19.72 and 21.55 mg/g for Al³⁺, Cr³⁺ and Fe³⁺ ions, respectively.