A silicon-nanowire field-effect-transistor (FET) biosensor is a label-free electrical diagnostic: the nanowire itself is the transistor channel, receptors (antibodies or short aptamers) are grafted on its surface, and a charged biomarker binding a receptor acts like a tiny extra gate voltage that shifts the channel's conductance — no fluorescent tag, no optics. This 2D view is a side cross-section of the sample chamber: the wire runs along the bottom, receptor stalks reach up into the fluid, and biomarker molecules diffuse above.
The catch is Debye screening. In an electrolyte, mobile ions form a cloud around any fixed charge that cancels its field beyond a characteristic distance, the Debye length:
λ_D ≈ 0.304 / √I(M) nm (I = ionic strength, monovalent salt, room T)
A bound molecule sitting a distance d above the nanowire surface (set by its receptor's linker length) contributes to the measured signal only in proportion to exp(−d/λ_D). In physiological buffer (≈150 mM, λ_D ≈ 0.8 nm) a bulky antibody-bound antigen sitting 12 nm out is almost completely screened; the same charge on a 2 nm aptamer, or in a diluted low-salt buffer, is barely attenuated at all. The teal band hugging the wire in the main view is exactly this shell, redrawn every frame at its current λ_D.
Binding itself follows Langmuir kinetics — receptors fill toward an equilibrium coverage set by target concentration C and dissociation constant K_d:
θ_eq = C / (C + K_d)
ΔG/G ∝ θ · exp(−d_link / λ_D)
- Ionic strength — sets λ_D; drag it down to see a low-salt buffer un-screen the same bound charge.
- Target concentration — drives Langmuir binding toward θ_eq; watch coverage rise and plateau, never overshoot.
- Dissociation constant K_d — a tighter-binding probe (lower K_d) reaches the same coverage at a much lower target concentration; this is what "assay sensitivity" means in practice.
- Antibody vs. aptamer — swaps the linker length d_link (12 nm vs 2 nm), the single biggest lever on real-world FET-biosensor sensitivity.
- Flush Buffer — instantly unbinds every receptor, as a real assay wash step would, so you can watch θ climb from zero again.
The small chart bottom-right plots the same exp(−d/λ_D) screening curve against distance, with your current linker length marked — a direct picture of why physiological salt kills label-free electrical diagnostics. Drag the main view to pan along the wire, or scroll to zoom.