On-chip metal/polypyrrole quasi-reference electrodes for robust ISFET operation

Area of Science:

• Materials Science
• Electrochemistry
• Microfabrication

Background:

• Ion-sensitive field-effect transistors (ISFETs) require stable reference electrodes for operation.
• Conventional reference electrodes are often too large and fragile for microscale applications.
• Existing solid-state micro-reference electrodes face challenges with robustness, cost, and integration.

Purpose of the Study:

• To develop a simple, robust, and cost-effective on-chip quasi-reference electrode for ISFETs.
• To investigate the electrodeposition of polypyrrole on micro-patterned metal leads for this purpose.
• To evaluate the performance and compatibility of these novel micro-reference electrodes.

Main Methods:

• Fabrication of micro-patterned metal leads on a substrate.
• Electrodeposition of polypyrrole onto the metal leads via cyclic voltammetry.
• Characterization of electrode performance using open circuit potential measurements.
• Testing compatibility with complementary metal-oxide-semiconductor (CMOS) processes using nickel leads.
• Integration and operation of ISFETs biased with the fabricated polypyrrole electrodes.

Main Results:

• Polypyrrole quasi-reference electrodes exhibited good stability (±1 mV) and low drift (∼1 mV h⁻¹).
• The electrodes showed a reduced pH response (5 mV per pH).
• Nickel was identified as a compatible metal for polypyrrole deposition in standard CMOS processes.
• ISFETs operated with polypyrrole electrodes demonstrated pH sensitivity and resolution comparable to conventional reference electrodes.

Conclusions:

• Polypyrrole electrodeposition provides a simple and robust method for creating on-chip quasi-reference electrodes.
• These micro-reference electrodes are compatible with standard CMOS fabrication processes.
• The developed polypyrrole microelectrodes enable sensitive and stable operation of ISFET sensors, suitable for miniaturized devices.