Electrochemical detection of cystatin C by oriented antibody immobilization on streptococcal protein G–modified ZIF-8-Cu_1−xNi_x(OH)₂@Cu core-shell nanostructured electrode

Herein, we prepared a novel nanostructure involving Cu shell on Zeolitic imidazolate frameworks (ZIF-8) and Cu_1−xNi_x(OH)₂ composite (ZIF-8-Cu_1−xNi_x(OH)₂@Cu) combining sol-gel and co-precipitation method. The morphology, stoichiometry, and structure of the nanocomposite were elucidated by various physicochemical analyses. A poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coated indium tin oxide (ITO) was modified with the synthesized ZIF-8-Cu_1−xNi_x(OH)₂@Cu nanoparticles (NPs) to obtain an efficient electrode for further antibody immobilization. The ZIF-8-Cu_1−xNi_x(OH)₂@Cu/PEDOT:PSS/ITO was applied for the detection of cystatin C, a promising biomarker of chronic kidney disease (CKD). The electrode was functionalized by streptococcal protein G (SPG) to bind the Fc region of anti-cystatin C in an oriented manner. The synergistic catalytic activities, high surface coverage, enhanced electroactive sites, and excellent redox properties of the proposed electrode lead to excellent electrochemical sensing. The proposed sensor obtained a much lower detection limit (33 pg/mL) for a linear range of 0.1 ng/mL to 1,000 ng/mL with high selectivity, stability, and reproducibility compared with bare ZIF-8/PEDOT:PSS/ITO-based immunosensor. The clinical feasibility of the sensor was confirmed by measuring the human serum in the presence of different concentrations of cystatin C. This work demonstrates a new and facile approach to fabricating a metal-organic framework (MOF) –based nanoimmunosensor for cystatin C, which has significant importance in diagnosing the renal failure.