Hydrogel Phase Transition‐Driven Soft Gating Circumvents Debye Screening for Advanced Biosensing

ABSTRACT Solid‐state nanofluidic sensors provide a versatile platform for label‐free molecular diagnostics. However, achieving high sensitivity in complex physiological environments remains severely restricted by the Debye screening effect. Here, we introduce a stimuli‐responsive 3D soft gating strategy based on a hydrogel phase transition to address this limitation. By asymmetrically assembling a peptide‐DNA hybrid hydrogel on the outer surface of nanochannels, we establish a volumetric functional zone governed by macroscopic Donnan equilibrium. Target‐induced structural disassembly of this network triggers a synergistic gating response, simultaneously abolishing Donnan enrichment and decreasing interfacial wettability. This dual‐parameter modulation significantly alters transmembrane ion flow, generating a substantial shift in ionic current. By shifting the dominant physics from 2D surface electrostatics to this 3D volumetric Donnan effect, the platform effectively circumvents Debye screening in high‐ionic‐strength media. Targeting the immune effector Granzyme B (GrzB), the soft gating sensor (SGS) achieves an ultralow detection limit of 0.830 fM. Clinically, the SGS has successfully tracked the longitudinal dynamics of serum GrzB in lung cancer patients undergoing immunotherapy. The high diagnostic accuracy confirms the capability of this SGS to operate directly in unpurified physiological fluids, providing a robust analytical tool for advanced molecular diagnostics.