THE PROBLEM

Sticky surfaces

Everything sticks to something

Proteins and other molecules stick to surfaces through weak, non-specific forces. In a real sample, thousands of species could land on the sensor and add charge that is not your target. That false signal can swamp a real one.

A sensor is only as good as its ability to ignore the rest.

BLOCKING

Fill the gaps

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BSA

Bovine serum albumin coats bare spots with a harmless protein

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Antifouling polymers

PEG or OEG chains form a hydrated brush that resists proteins

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Wash buffers

A mild detergent such as Tween-20 rinses off loosely stuck molecules

Blocking happens after the receptors are attached and before the sample arrives.

CONTROLS

Ways to expose false signals

Blank sample Buffer alone should give no threshold shift

Wrong target An unrelated molecule should give no shift

No-receptor twin A FET with no receptors, like the REFET, shows the non-specific signal alone

Difference Signal minus twin isolates the specific binding

Controls turn "it responded" into "it responded to the target".

SIGNAL VS BACKGROUND

How selective is the surface?

specific ÷ non-specific

A useful summary is the response to the target divided by the response to a wrong or blank sample. A ratio near 1 means the surface cannot tell them apart. Well-blocked surfaces push that ratio as high as they can.

Blocking and washing raise the ratio.

REAL SAMPLES

Serum is harder than buffer

More species, more background

Blood serum carries roughly 60 to 80 mg of protein per mL, mostly albumin, so it is much harder on a surface than clean buffer. BioFETs are often shown in buffer first and then tested in diluted or processed serum.

A good result in buffer does not guarantee one in a real sample.

UNIT 17 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered how to keep everything except the target off the surface.