Mixed Charge Monolayers Enable Multi-Week Continuous Analyte Monitoring in Whole Serum

Real-time, continuous monitoring of large peptide and protein biomarkers has transformative potential for personalized medicine but is hindered by the rapid degradation of affinity-based sensors in complex biofluids. While physical barriers such as size-exclusion membranes can mitigate biofouling, they also exclude the high-molecular-weight proteins to be measured, creating a fundamental paradox for sensor design. Here, we present a solution to this stability-fouling trade-off: a "mixed charge" monolayer interface that confers multi-week fouling resistance without relying on physical filtration. By incorporating compact amine- and carboxyl-terminations onto 8-carbon alkyl thiolates, we engineered a surface that mimics zwitterionic hydration while preserving the dense molecular packing and intermolecular forces required for long-term monolayer stability. This architecture extends the operational lifetime of electrochemical aptamer sensors to 14 days in whole serum. Sensors targeting the small molecule cortisol retained ∼80% of their initial analyte response signal gain, while those targeting the protein platelet-derived growth factor bb (PDGF-bb) preserved nearly 100% of their analyte response signaling capacity over the two-week period. These findings demonstrate a generalized, membrane-free platform capable of robust, multi-week molecular monitoring of larger peptides and proteins.