3D Nanotube Field-Effect Biosensor: Threshold-Voltage Shift Simulator

Target & Receptor

Target concentration 100 nM

Electrolyte & Bias

Ionic strength 150 mM

Gate bias V_gs -0.10 V

Simulated speed 1.0x

Readouts

Coverage θ

0 %

Debye length λ_D

–

ΔV_th shift

0.0 mV

Drain current I_ds

–

How real nanotube biosensors work

A carbon-nanotube channel bridges source and drain, functionalized with receptors and gated through the surrounding electrolyte. Each bound target adds surface charge σ, screened by mobile ions over the Debye length — the translucent shell hugging the tube is exactly that screening layer:

λ_D [nm] ≈ 0.304 / √(I [mol/L])
ΔΨ = σ_bio · λ_D / (ε_r ε₀)        (double-layer capacitor)
ΔV_th = ΔΨ,   I_ds = ½ k′ (V_th − V_gs)²

Every one of the 36 receptor sites along the tube independently binds and unbinds using real Langmuir kinetics (dθ/dt = k_on·C·(1−θ) − k_off·θ), so the equilibrium coverage is exactly C/(C+K_d) for whatever concentration and affinity you set. Raise ionic strength and watch the screening shell visibly thin — the same real effect that limits how far an FET biosensor can "feel" a charged biomarker through physiological saline. The tube is modeled as p-type: negatively-charged targets binding at physiological pH shift V_th positive, turning the channel further on and raising the drain current — the sensor's actual electronic output.

Channel: single carbon nanotube (liquid-gated)

drag = orbit camera

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