Silicon Nanowire FET Biosensor

Sample Buffer

Ionic strength (salt) 150 mM

Target concentration 1.0 nM

Receptor / Linker

Sensor Readout

initializing…

Debye length λ_D

— nm

Surface coverage θ

Sensor response ΔG/G

0.0%

Bound / receptors

0 / 0

How it works

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.

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.

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)              (K_d = 1 nM here)
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.
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.

This screening trade-off is exactly why real nanowire-FET diagnostics either dilute the sample into low-ionic-strength buffer before measuring, or switch to short aptamer probes instead of full-size antibodies.

Free biomarker (diffusing) Bound biomarker Nanowire channel

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