THE LAYER CAKE

Each layer has one job

The silane gives the oxide a reactive tail. The linker ties that tail to the receptor. The receptor catches the target. A blocking layer, added after the receptors, covers any bare spots in between.

Every layer has to bond to the one below it and the one above it.

COMMON LINKERS

Ways to tie a receptor down

Glutaraldehyde Joins amines on the surface to amines on the receptor

EDC/NHS Turns a carboxyl group into an amide bond with an amine

Biotin–streptavidin One of the strongest non-covalent bonds in biology

Thiol–maleimide Links a thiol on the receptor to a maleimide on the surface

Pick the linker that matches the groups on both sides.

BIOTIN–STREPTAVIDIN

A near-permanent bond

K_D ≈ 10⁻¹⁴ M

Streptavidin binds biotin with a dissociation constant near 10⁻¹⁴ M, so tightly that it is effectively irreversible on lab timescales. Streptavidin has four binding pockets, so it can bridge a biotin-coated surface to a biotin-labeled receptor.

It is strong, and a standard way to attach biotin-labeled receptors.

ORIENTATION

Random or upright?

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Random

Amine coupling can attach at any lysine, so some receptors face the wrong way

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Oriented

Protein A or G, or a tagged receptor, holds antibodies with their binding sites out

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Why it matters

Fewer active receptors per area means a weaker signal

Upright receptors put more binding sites to work.

DENSITY

Crowding blocks the target

Too dense: targets can't reach

Receptors packed too tightly leave no room for a large target to reach the binding sites. Too sparse, and there are too few receptors to make a measurable charge change. Sensors are tuned between the two, often by diluting the linker with a spacer molecule.

Aim for spaced-out receptors, each with room to bind.

UNIT 16 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the crosslinkers that tie receptors to the surface, and why orientation matters.