Tip size of ion-exchanger based K+-selective microelectrodes. I. Effects on selectivity

Area of Science:

• Electrochemistry
• Analytical Chemistry
• Materials Science

Background:

• Ion-selective microelectrodes (ISMs) are crucial for measuring ion concentrations in biological and chemical systems.
• Understanding factors influencing ISM selectivity is vital for accurate measurements.
• Previous studies have focused on ionophore composition, but the role of physical dimensions is less explored.

Purpose of the Study:

• To investigate the impact of tip diameter on the ion selectivity of potassium-selective microelectrodes (K+ ISMs).
• To compare selectivity differences between K+ ISMs fabricated using different glass configurations.
• To explore potential explanations for observed tip size-dependent selectivity.

Main Methods:

• Fabrication of double-barreled K+ ISMs with varying tip diameters (0.5-6 micron).
• Testing selectivity of K+ ISMs against various ions, including potassium (K+), ammonium (NH4+), tetramethylammonium, tetraethylammonium, and choline.
• Utilizing two distinct glass configurations: "figure-eight glass" and thick septum theta glass.

Main Results:

• Selectivity of K+ ISMs was dependent on both tip size and barrel configuration.
• Figure-eight K+ ISMs exhibited sensitivity to K+ and NH4+ across all tested tip sizes.
• K+ ISMs with tip diameters < 1.5 micron were not sensitive to quaternary amines, unlike those with diameters >= 1.5 micron.
• No tip size-dependent selectivity was observed in K+ ISMs made from thick septum theta glass.

Conclusions:

• Tip diameter is a critical factor influencing the ion selectivity of certain K+ ISMs.
• The "figure-eight glass" configuration shows a pronounced tip size effect on quaternary amine sensing.
• The underlying mechanism for tip size-dependent selectivity remains undetermined, warranting further theoretical and experimental investigation.