The inspiration comes directly from living cells. Biological ion channels are not rigid sieves; they are dynamic molecular machines that couple their local electrostatic environment to the hydration state of the interface, achieving selectivity and transport rates that approach the diffusion limit. Potassium channels, for example, use coordinated amino acid residues to strip the hydration shell from ions as they pass. Synthetic solid-state nanochannels have long tried to imitate this behavior by decorating rigid channel walls with stimuli-responsive molecules, converting molecular recognition events directly into measurable ionic currents. The problem has always been that these functionalizations are essentially two-dimensional, painting the sensing chemistry onto a flat surface where physiological salt can neutralize it.

The new platform, developed by Liu Shi, Genxi Li and colleagues, flips that geometry. Instead of modifying the channel surface, the team assembled a peptide-DNA hybrid hydrogel asymmetrically on the exterior of an anodic aluminum oxide membrane, creating a volumetric functional zone roughly 25 to 30 micrometers thick that acts as an active ion trap. The hydrogel is superhydrophilic and densely charged, so it pre-concentrates bulk ions through macroscopic Donnan enrichment while simultaneously generating a substantial Donnan potential. Because this charge reservoir occupies a volume orders of magnitude larger than the compressed Debye length, it keeps working even in the harshest ionic environments the body can supply.

The molecular architecture of the gate is a piece of careful supramolecular engineering. Single-stranded linker DNA is covalently anchored to the nanochannel surface, then a bridging strand and polyacrylamide conjugates are hybridized stepwise into the network. At the heart of the assembly sits a peptide-DNA complex cross-linker, synthesized through orthogonal thiol-maleimide coupling and strain-promoted alkyne-azide cycloaddition. The peptide backbone physically tethers the polymer-nucleic acid strands in close proximity, and this localized proximity effect entropically stabilizes the short DNA duplexes that hold the whole gel together. The elegance of the design is that the same tether that builds the network becomes its Achilles heel: a specific enzyme can cut it, and the entire structure falls apart on command.

That enzyme is Granzyme B, a serine protease released by cytotoxic T lymphocytes and a pivotal biomarker for gauging whether lung cancer patients are responding to immune checkpoint blockade therapy. When Granzyme B cleaves the peptide cross-linkers, the hydrogel undergoes a macroscopic gel-to-sol phase transition. Rheological measurements captured the collapse dramatically, with the storage modulus falling below the loss modulus as the elastic network vanished. Confocal microscopy showed fluorescently labeled gels fading after enzymatic treatment, while spectroscopy confirmed the loss of amide and hydrogen-bonding signatures. The gate does not merely weaken; it switches off.

The shutdown is synergistic, and that synergy is where the signal amplification comes from. As the gel disassembles, its volumetric charge reservoir dissipates, with the effective surface charge density dropping from -5.26 to -0.92 millivolts per square centimeter, an 82.5 percent loss in charge capacity. At the same time, the interface loses its superhydrophilicity, with the water contact angle rising from about 7.6 degrees to 39.0 degrees. Finite-element simulations governed by the Poisson-Nernst-Planck equations, fed with these experimentally measured parameters, showed the cation enrichment zone vanishing and the energy barrier for ion entry soaring. In a two-compartment electrochemical cell, the device swung from a high-current ON state of roughly 17.8 microamperes to an OFF state of about 3.07 microamperes, a gating ratio of approximately 5.8.

The comparison with a conventional two-dimensional control is what makes the result compelling. A monolayer of the same peptide chemistry on the channel surface showed only minor changes in wettability and charge after cleavage, and its sensing performance suffered accordingly, with a constrained dynamic range and reduced linearity. More tellingly, when the researchers varied the background ionic strength from 10 to 200 millimolar, the 2D device’s gating ratio collapsed sharply around 50 millimolar, exactly where Debye screening takes over. The 3D hydrogel platform, by contrast, maintained robust signal modulation all the way to 200 millimolar. In a physiological 150 millimolar buffer, the device still achieved a limit of detection of 133.60 femtomolar, while in low-salt conditions it reached an extraordinary 0.830 femtomolar across a range spanning six orders of magnitude.

The modularity of the building blocks opened a further, almost playful dimension: molecular computing. By incorporating cross-linkers responsive to both Granzyme B and matrix metalloproteinase 2 into a single hydrogel, the team built a cascaded logic circuit with two parallel INHIBIT sub-circuits feeding an OR gate. Each enzyme can independently trigger the phase disassembly and generate a current response, but a specific inhibitor for each enzyme vetoes the process. The device executed a full truth table of Boolean operations on biochemical inputs, supported by molecular docking simulations of the enzyme-inhibitor pairs. It is a small demonstration, but it points toward sensors that do not merely detect a molecule but compute a diagnosis from a panel of interacting biological signals.

Robustness in messy biological fluids proved equally impressive. The intact hydrogel doubles as a physical shield against non-specific fouling, and the sensor held its performance in 10 percent fetal bovine serum and 10 percent human serum, showing strict selectivity against a panel of interfering proteins. The device survived five regeneration cycles with consistent current switching, showed negligible degradation over 15 days of storage, and achieved batch-to-batch reproducibility with a relative standard deviation of just 1.62 percent. These are the unglamorous metrics that decide whether a laboratory curiosity can ever become a clinical instrument, and the platform passed them.

The clinical proof of concept is the most striking part of the story. Working with serum samples from healthy donors and lung cancer patients collected before and after immune checkpoint blockade therapy, under a protocol approved by the Medical Ethics Committee of Shanghai Pulmonary Hospital, the team measured Granzyme B directly in unpurified specimens. Healthy individuals averaged around 0.8 picomolar, while pre-treatment patients showed suppressed levels of roughly 0.3 to 0.5 picomolar, reflecting the immunosuppressive microenvironment of advanced malignancies. After immunotherapy, concentrations surged to 2.0 to 2.5 picomolar, signaling restored T-cell cytotoxicity. Benchmarked against a commercial ELISA, the sensor achieved a correlation coefficient of 0.962, a negligible mean bias of 0.0423 picomolar by Bland-Altman analysis, and an area under the ROC curve of 0.980 for distinguishing healthy from pre-treatment patients, with near-perfect discrimination of therapeutic response. The researchers acknowledge that translation will require packing the electrochemistry into automated microfluidic cassettes and expanding the logic gates to wider biomarker panels, but the core demonstration stands: by thinking in three dimensions, a dissolving gel has done what flat chemistry could not, carrying ultrasensitive nanochannel sensing out of the buffer and into the blood.

Subject of Research: A three-dimensional hydrogel soft-gating strategy for solid-state nanochannel biosensors that circumvents Debye screening to enable ultrasensitive detection of Granzyme B for immunotherapy monitoring.

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

Article References: Shi, L., Zhang, Z., Li, B., Gao, Y., Zhang, R., Mu, Z., Ni, J., Bo, B., & Li, G. (2026). Hydrogel Phase Transition‐Driven Soft Gating Circumvents Debye Screening for Advanced Biosensing. Advanced Science, Article e78049. https://doi.org/10.1002/advs.78049

Image Credits: AI Generated

DOI: 10.1002/advs.78049

Keywords: nanochannel biosensing, hydrogel, Debye screening, Donnan enrichment, Granzyme B, peptide-DNA hybrid, iontronics, immune checkpoint blockade, lung cancer, biocomputing, phase transition, clinical diagnostics