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The one-nanometre problem
Graphene FET biosensor · CRP detection
A graphene field-effect transistor can feel the charge of a single protein binding to its surface. In blood, though, the ions in solution hide that charge within about a nanometre, and an antibody is ten times taller than that.
FIG. 1: Debye screening · interactive
Watch the charge disappear
Ions crowd around any charge in solution and cancel its field over the Debye length, λ_D ≈ 0.304 / √I nm. At physiological ionic strength that’s under a nanometre. Lower the salt or shorten the receptor and see how much of the target’s charge still reaches the graphene.
Shading shows how much of a charge’s field survives at each height.
Simplified model: a 1:1 electrolyte at 25 °C, with the field decaying as exp(−d / λ_D). Real interfaces add the Stern layer, receptor charge and Donnan effects in polymer coatings.
FIG. 2: Where my sensor fits
Measuring CRP on graphene
My current device is a label-free graphene FET for C-reactive protein, a marker of inflammation and cardiac risk. It runs on a 28-channel Graphenea mGFET chip read out through an NI-DAQ, and it tracks binding as reference-subtracted resistance shifts. Every design choice on it is a negotiation with the two limits above: how far the charge sits from the channel, and what else in the sample sticks to the surface.
Working on FET biosensors or Debye-length workarounds? I’d love to compare notes.
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