Development of a FRET aptasensor based on MoS₂-doped Zn-MOF as luminophore for selective detection of cadmium in aqueous solutions

A robust “on–off” fluorescent aptasensor was developed using nanohybrids of molybdenum sulfide (MoS₂) quantum dot (QD)–doped zinc metal–organic frameworks (Zn-MOF) for selective and sensitive detection of cadmium ions (Cd²⁺) in water. This nanohybrid (MoS₂@Zn-MOF), synthesized via “bottle around the ship” methodology, exhibited a high-intensity fluorescence emission centered at 430 nm (λ_Em) (blue) on excitation at 320 nm (λ_Ex). Further, the conjugation of this fluorophore to phosphate-modified cadmium aptamer (Cd-2–2) was achieved through carbodiimide reaction. The hybridization of prepared sensing probe (MoS₂@Zn-MOF/Cd-2–2 aptamer) was done with dabcyl-conjugated complementary DNA (cDNA), acting as energy donor–acceptor pair in the fluorescence resonance energy transfer (FRET) system. This hybridization causes the fluorescence quenching of the nanohybrid. In the presence of Cd²⁺, the aptamer from the fabricated nano-biosensing probe binds to these ions, resulting in release of dabcyl-cDNA oligomer. This release of dabcyl-cDNA oligomer from the sensing probes restores the fluorescence of the nanohybrid. Under optimized conditions (sensing probe/dabcyl-cDNA ratio 1/7, pH 7.4, and temp 28 °C), the sensing probe showed a fast response time of 1 min. The fluorescence intensity of the nanohybrid can be utilized to determine the concentration of Cd²⁺. The proposed aptasensor achieved highly sensitive detection of Cd²⁺ with a limit of detection (LOD) of 0.24 ppb over the range of 1 × 10⁻⁹ to 1 × 10⁻⁴ M along with minimal effects of interferences (e.g., Hg²⁺, Pb²⁺, and Zn²⁺) and good reproducibility. The designed aptasensor based on MoS₂@Zn-MOF nanofluorophore offers a highly sensitive and selective approach for rapid screening of metal ions in aqueous environments. Graphical Abstract: (Figure presented.)