Versatile sensing platform of an innovative copper oxide-assisted Cu-phenolic coordination nanosheet-mediated fluorophore-tagged GT-rich SSA-based fluorescence ON–OFF biosensor for the subsequent detection of Cd2+ and S2− ions

Abstract

Increased levels of toxic metal/non-metal ions cadmium (Cd²⁺) and sulfide (S²⁻) in the environment can be detrimental to human health. Given the circumstances, the detection and measurement of Cd²⁺ and S²⁻ have become progressively important to ensure human safety. Herein, we report the fluorophore (FAM)-tagged guanine–thymine-rich single-strand aptamer (FAM tagged GT-rich SSA) mediated Cu-PCNS (copper phenolic coordination nanosheets)-based bio-sensor nano-probe, which is suitable for sensing Cd²⁺ and S²⁻ ions because of its unique affinity. The as-synthesized Cu-PCNS were strongly adsorbed on the FAM-tagged GT-rich SSA, which quenched the FAM emission to give the non-emissive GT-rich SSA@Cu-PCNS (the OFF state). In the presence of Cd²⁺ ions in the quenched system, the fluorescence intensity increased (the ON state). This is due to the creation of a rigid hairpin-like duplex structure that featured the G–Cd²⁺–T motif. Furthermore, when the S²⁻ ion was added to the above sensing system, is strongly coordinated with the Cd²⁺ ions to forms cadmium sulfide (CdS). In that instant, the free FAM-tagged GT-rich SSA is again adsorbed onto the Cu-PCNS and the FAM emission is decreased markedly (the OFF state). Therefore, the FAM-tagged GT-rich SSA@Cu-PCNS bio-sensing system senses Cd²⁺ and S²⁻ successively via a fluorescence “ON–OFF” state mechanism. The proposed biosensor demonstrates good selectivity and high sensitivity with low detection limits of 0.3 nM (Cd²⁺) and 3.3 nM (S²⁻). Moreover, for the first time, this research was effectively applied to sense Cd²⁺ and S²⁻ in lake, tap, and river water samples.