Portable pH meter-based competitive immunoassay of E-selectin using urease-encapsulated metal-organic frameworks

E-selectin (CD62E) is an adhesion molecule expressed on the surface of endothelial cells (ECs) and its level increases significantly upon the stimulation of ECs by inflammatory factors. Quantitative analysis of CD62E is of great importance to early diagnosis and treatment of vascular diseases and hypertension. A new method for the determination of CD62E was developed using a portable pH meter in this work. Two nanocomposites, urease-encapsulated and CD62E antibody-linked zeolitic imidazolate framework-8 (U@ZIF-8-Ab) and CD62E protein-coated Fe₃O₄ magnetic beads (MB-CD62E), were prepared successfully. MOF shells of U@ZIF-8-Ab provided an effective protection against inactivation for urease. The reaction product of two nanocomposites, which was collected via magnetic separation, catalyzed the hydrolysis of urea and led to the increase in pH of the substrate solution. CD62E could competitively react with U@ZIF-8-Ab, inhibiting the binding of two nanocompoaites. Due to the reduction of urease in the magnetically separated product, the hydrolysis of urea was limited and the pH increasing extent declined. The pH difference was found to be linearly related to the logarithm of the CD62E concentration from 0.02 to 500 ng mL^-1 with a detection limit of 9.28 pg mL^-1. The developed method displayed some advantages, such as good selectivity, reproducibility and stability, low-dose reagents, and simple measurement procedure. This work provides a valuable sample for immunoassay and biosensing using enzyme-encapsulated metal-organic frameworks and portable pH meter as signal readout.