Hydrothermal synthesis of Ag@Zn-salen MOF nanocomposite <i>via</i> a two-step method for ultrasensitive CA15-3 biosensing in breast cancer diagnostics

Metal-organic frameworks (MOFs) integrated with noble metal nanoparticles offer transformative potential in biosensing, yet bimetal composites face persistent challenges, including inhomogeneous nanoparticle distribution and framework destabilization. Salen-based MOFs, prized for their thermal stability and tunable electronic properties, remain underexplored for bimetal systems despite their catalytic and plasmonic synergies. This study addresses the critical need for a synthesis strategy that ensures the uniform dispersion of plasmonic silver nanoparticles (Ag NPs) within a robust Zn-salen metal-organic framework (MOF) matrix while preserving structural integrity for biomarker detection applications. Conventional one-pot methods for bimetal MOFs often result in Ag NP aggregation (>200 nm) and pore blockage, compromising porosity and active site accessibility-key limitations for biosensor development. A hierarchical Ag@Zn-SalenMOF composite was engineered <i>via</i> a novel two-step hydrothermal approach: (1) Zn-salen framework assembly, followed by (2) DMF-mediated <i>in situ</i> reduction of AgNO₃ to submicron Ag NPs (150-200 nm). The composite was characterized by FE-SEM, HR-TEM, XPS, and XRD, and functionalized with anti-CA15-3 antibodies for biosensor fabrication. The composite exhibited exceptional thermal stability (>300 °C), uniform Ag NP distribution (189 nm avg.), and strong interfacial electronic coupling. The biosensor achieved a CA15-3 detection limit of 0.12 U mL^-1 (Stern-Volmer constant: 0.004 U mL^-1) with 93.3% sensitivity and 91.6% specificity in clinical serum samples (<i>n</i> = 20), outperforming conventional immunoassays. Stability studies confirmed less than 5% signal drift over 2 months, enabled by sodium azide preservation. This work addresses long-standing synthesis challenges in bimetallic metal-organic frameworks (MOFs), providing a scalable platform for ultrasensitive biomarker detection. The Ag@Zn-Salen MOF biosensor's precision, robustness, and clinical validity position it as a transformative tool for breast cancer monitoring and early diagnosis.