LIGHT

Proteins that glow when they bind

Genetically encoded sensors are proteins built into neurons that brighten when they bind a specific transmitter. iGluSnFR detects glutamate, and sensors such as dLight and GRAB-DA detect dopamine. A microscope reads the glow, with speeds of milliseconds to seconds and cell-level detail.

The catch is that the cells must carry the sensor gene, so this is mainly a research tool.

Light is selective and fast, and it sees where the molecule is.

CHARGE

A transistor that feels binding

A transistor sensor, as in the BioFET course, has receptors on its gate. When a charged molecule binds, the gate's potential shifts and the current changes.

The same idea as a BioFET, applied to a neurotransmitter.

THE SALT PROBLEM

Debye screening

Problem In body-like salt, the Debye length is under about a nanometer, so charge farther from the surface is screened

Size Neurotransmitters are small, but receptors are large enough to hold them beyond that distance

Fixes Short receptors such as aptamers, polymer layers, and organic electrochemical transistors

Sensing a small charge in a salty fluid takes clever engineering.

WHAT'S NEXT

Where the field is heading

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Arrays

Flexible probes with many sensors measure several molecules and places at once

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Closed loop

Sensors that read a neurotransmitter and trigger stimulation or drug release

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Multimodal

Combining electrical, chemical, and optical recording

The goal is to watch many messengers at once, in real time.

BEFORE YOU BELIEVE IT

Questions to ask of any sensor claim

Which molecule? And is it distinguished from look-alikes such as ascorbic acid and metabolites

How fast? Time resolution that matches the event of interest

Where? In a dish, or in a living brain

How calibrated? Whether the numbers are absolute or relative

A measurement is only as good as its selectivity, speed, and calibration.

UNIT 17 STUDY COMPLETE

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Ready for the Fab Challenge?

You've finished the course. You can now weigh a neurotransmitter-sensing claim by asking what it measures, how fast, and how selectively.