Chemiresistive Nanowire Gas Sensor

Target Gas Exposure

Gas type

Concentration 10 ppb

Nanowire sensor (r ≈ 15 nm)

Coverage θ

–

Resistance

–

ΔR / R₀

–

Adsorbed sites

–

Bulk sensor (r ≈ 1.5 mm)

Coverage θ

–

Resistance

–

ΔR / R₀

–

S/V ratio

–

ΔR/R₀ vs. time

Nanowire Bulk block

Both sensors sit in the same gas, so both reach the same fractional surface coverage θ. Only the nanowire's huge surface-to-volume ratio turns that coverage into a measurable resistance swing.

How it works

A metal-oxide nanowire (e.g. SnO₂) or carbon nanotube bridging two electrodes carries current through a body only a few atomic layers thick. Target gas molecules drifting through the chamber randomly collide with the wire's surface and adsorb onto lattice sites; each adsorption event donates or withdraws a conduction electron, shrinking or widening the wire's depletion layer and shifting its resistance.

theta_eq(C) = C / (C + C_half)      (Langmuir isotherm, same for both sensors)
dR/R0        = alpha * theta * (S/V) / (S/V)_ref

Coverage θ — the fraction of surface adsorption sites currently occupied, set purely by gas concentration and identical for the nanowire and the bulk block since both bathe in the same atmosphere.
Surface-to-volume ratio — a cylinder's S/V scales as 2/r. Shrinking the radius from millimetres to tens of nanometres inflates S/V by five orders of magnitude, so the very same adsorption events that are noise on a bulk sensor dominate the entire cross-section of a nanowire.
Gas polarity — a reducing gas (CO) donates electrons and lowers resistance on an n-type oxide; an oxidizing gas (NO₂) withdraws electrons and raises it. Flip the dropdown to see the resistance trace invert.

This is the physical reason nanoscale chemiresistors can flag gas concentrations in the parts-per-billion range that a macroscopic sensor of the same material would never register above its own readout noise.

Electrode Nanowire Bulk block Gas molecule

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