Acid-base chemistry at the single ion limit

Area of Science:

• Electrochemistry
• Nanotechnology
• Analytical Chemistry
• Physical Chemistry

Background:

• Understanding and controlling ion behavior at the single-ion level is crucial for advancing nanoscale devices and chemical sensing.
• Electrochemical techniques offer precise control over ion concentrations, but achieving single-ion sensitivity in detection remains a challenge.

Purpose of the Study:

• To develop and validate a method for manipulating and optically detecting single hydrogen ions (H+) in nanopores.
• To demonstrate the capability of electrochemical zero-mode waveguides (E-ZMWs) for ultrasensitive acid-base experiments.

Main Methods:

• Utilized electrochemical zero-mode waveguides (E-ZMWs) with volumes of approximately 6 aL.
• Employed a pH-responsive fluorophore (fluorescein) to optically monitor changes in local pH.
• Applied electrochemical potentials to deplete or accumulate hydrogen ions (H+) within the nanopores.

Main Results:

• Demonstrated the depletion of hydrogen ions (H+) from an average of 3.6 H+/nanopore to 0.36 H+/nanopore by applying a negative potential.
• Observed clear, potential-dependent changes in fluorescein emission intensity, confirming sensitivity to single H+ manipulations.
• Identified the nucleation of H2 nanobubbles at high overpotentials, validated by calculations and control experiments.

Conclusions:

• The developed electrochemical approach enables precise manipulation and optical detection of ion populations at the single-ion limit.
• This technique enhances fundamental understanding of electrochemical processes in confined volumes.
• Opens avenues for ultrasensitive ion sensing applications leveraging optical detection of H+.