Engineered CdS@ZIF67 nanozyme-assisted detection of vitamin D deficiency

Peroxidases are natural enzymes commonly employed in developing sensitive immunoassays; however, their limited stability has sparked growing interest in enzyme-mimics. Zeolitic imidazolate frameworks (ZIFs), a subclass of metal-organic frameworks (MOFs), are promising alternatives due to their ease of synthesis, tunable porosity, multiple active sites, and robust catalytic properties. In the current work, we synthesized and characterized CdS@ZIF67, which is a nanocomposite of CdS quantum dots and ZIF-67, exhibiting peroxidase-like activity. Catalytic performance was then examined using Michaelis-Menten kinetics, with estimated key parameters (k_cat, K_m, and catalytic efficiency k_cat/K_m) benchmarked against natural Horseradish Peroxidase (HRP) enzyme. It showed an increase in catalytic efficiency of the order 10³ as compared to ZIF 67, and a comparable catalytic efficiency with that of HRP. To demonstrate its biosensing ability, we further conjugated it to anti-vitamin D3 antibodies and implemented this for sensing vitamin D3 molecules in a competitive dipstick format. Here, the nanozymes catalyzed a clear colorimetric response for clinically relevant concentrations of vitamin D3 in the presence of TMB and H₂O₂, with a limit of detection (LOD) of 26.98 ng/ml, and a LOD of 24.93 ng/ml in spiked serum samples. Our assays could reliably distinguish between deficiency and sufficiency of vitamin D3. This study provides valuable insights into nanozyme design and mimetic function, emphasizing the promising role of CdS@ZIF67 nanozymes as substitutes for natural enzymes in biosensing applications.