Voltammetric detection of vitamin D employing Au-MoS₂ hybrid as immunosensing platform

A voltammetric immunosensor based on molybdenum sulphide (MoS₂) and gold nanoparticles (Au NPs) for the determination of 25-hydroxy vitamin D₃ (25(OH)D₃) is reported. Anti-vit D (Ab-25(OH)D₃) was immobilized onto the cysteamine-modified MoS₂ and Au NPs which were deposited onto a fluoride tin oxide (FTO) electrode (Ab/Cys/Au/MoS₂/FTO). The MoS₂ sheets were prepared by hydrothermal method followed by an in situ growth of Au film onto the MoS₂/FTO surface. Self-assembled monolayer (SAM) of cysteamine was synthesized onto the Au/MoS₂/FTO which acts as a linker to covalently bind Ab-25(OH)D₃. The Ab-25(OH)D₃-immobilized Cys/Au/MoS₂/FTO was used to detect 25(OH)D₃ using differential pulse voltammetry. The electrochemical system provided an anodic peak current at a potential of +0.21 V vs. Ag/AgCl (satd. KCl) of ferricyanide/ferrocyanide redox couple. The detection principle relies on the inhibition of electron transfer at the electrode surface owing to the hindrance caused by the formation of immune complex between Ab-25(OH)D₃ and 25(OH)D₃. The immunosensor shows linear response from 1 pg mL^-1 to 100 ng mL^-1 25(OH)D₃ and a sensitivity of 189 μA [log (pg mL^-1)]^-1 cm^-2 along with a low limit of detection (LOD) of 0.38 pg mL^-1. The immunosensor is highly selective towards 25(OH)D₃ and presented a long shelf life of 28 days. Also, the immunosensor exhibits satisfactory performance towards spiked human serum samples with recovery between 95.1 and 102% (RSD 1.15-3.22%).