Electrochemical aptamer-based (EAB) sensors enable reagent-free, real-time tracking of drugs and biomarkers, but in complex media like saliva or undiluted whole blood their aptamers quickly foul via protein adsorption and are cut by nucleases, causing signal loss and drift. Size-selective hydrogels can mitigate these issues, yet long-term operation is hard to achieve without severely slowing analyte transport or electron transfer. Here, we demonstrate that a tetra-poly(ethylene glycol) (tetra-PEG) hydrogel coating creates a soft, water-rich network with an ~31 nm mesh that screens out large glycoproteins and cells, curbs drift, and still permits efficient electron transfer and sufficiently fast access for small molecules. In undiluted saliva and bovine blood, this coating lowers electrode fouling and preserves EAB function; for instance, in whole blood at 37 °C over 8 h, hydrogel-protected sensors drift by ~5.7% versus ~25.1% for unprotected controls while maintaining similar signal gain across tested vancomycin levels. Thus, tetra-PEG hydrogels offer a practical balance of antifouling and nuclease resistance with electrochemical compatibility, pointing to durable, in-matrix, and ultimately in vivo, EAB monitoring.