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A liquid-gate sensor for Alzheimer’s research

Diagram of a liquid gate device for measuring pH.

Diagram of the liquid-gate transistor device and measurement process (detailed in the notes below).

August 2019 • NIST • • Tagged: Diagrams


Per-panel caption of the measurement process:

  1. Overall view of the transistor section of the sensor, including the liquid layer. The amount of current that flows through the liquid between the source and drain is determined by the voltage of the top gate. Ionic fluids such as saltwater are good conductors of electricity.

  2. The sensor has a second, bottom gate (VB) in addition to the top gate (VT). The bottom gate voltage can be adjusted up or down, affecting the amount of current flow across the top layer.

  3. Outside the transistor, the Cdk5 enzyme is placed in a test tube along with ATP (adenosine triphosphate, the universal energy currency of life) and a substrate (biological target) of Cdk5. As the enzyme does its work, it throws off positively charged protons, changing the electrical state of the solution.

  4. The test tube solution is connected electrically to the transistor top gate (VT). Because of the build-up of positive charges in the solution, the connection changes the voltage at the top gate. The more protons in the solution, the more current moves across between the source and drain.

  5. An automated feedback system continuously alters the voltage of the bottom gate (VB) so that the current flow on the top of the transistor remains constant. The bottom-gate voltage changes thus provide a measure of the amount of Cdk5 in the solution.

  6. Trend of pH and voltage over time.