Layered rare-earth hydroxide (LRH, R = Tb, Y) composites with fluorescein: delamination, tunable luminescence and application in chemosensoring for detecting Fe(iii) ions

Abstract

We demonstrate a novel example of tunable luminescence and the application of the delaminated FLN/OS-LRH composites (LRHs are layered rare-earth hydroxides, R = Tb, Y; FLN is the fluorescein named 2-(6-hydroxy-3-oxo-(3H)-xanthen-9-yl)benzoic acid; OS is the anionic surfactant 1-octane sulfonic acid sodium) in detecting Fe(III) ions. The FLN_xOS_1−x species (x = 0.02, 0.05, 0.10, and 0.20) are intercalated into the LTb_yY_1−yH layers (y = 1, 0.9, 0.7, 0.5, 0.3, 0.1 and 0) by ion exchange reactions to yield the composites FLN_xOS_1−x-LTb_yY_1−yH. In the solid state, the LYH composites display green emission (564 nm) arising from the organic FLN, while in LTbH composites, the luminescence of the Tb³⁺ in the layers (545 nm) and the FLN in the interlayers is co-quenched. In the delaminated state in formamide (FM), FLN_xOS_1−x-LTbH composites display green to yellowish-green luminescence (540–574 nm) following the increasing FLN/OS ratio; while the FLN_0.02OS_0.98-LTb_yY_1−yH composites show green emission at ∼540 nm. The fluorescence lifetimes of the composites (4.22–4.63 ns) are comparable to the free FLN-Na, and the quantum yields (31.62–78.70%) of the composites especially that (78.70%) of the FLN_0.02OS_0.98-LYH are much higher than that (28.40%) of free FLN-Na. The recognition ability of the FLN_0.02OS_0.98-LYH composite for metal cations is researched. The delaminated FLN_0.02OS_0.98-LYH colloidal suspension exhibits high selectivity for Fe³⁺ over other ions (Mg²⁺, Al³⁺, Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺, Mn²⁺, Pb²⁺, and Cd²⁺) with fluorescence quenching, which can work as a kind of turn-off fluorescence sensor for the detection of Fe³⁺. The detection limit of Fe³⁺ is determined to be 2.58 × 10⁻⁸ M and the quenching constant (K_sv) is 1.70 × 10³ M⁻¹. This is the first work on LRH materials working as a chemosensor for recognising metal cations. It provides a new approach for the design of LRH materials to be applied in fluorescence chemosensing.