AC Kelvin Probe Force Microscopy Enables Charge Mapping in Water.

Area of Science:

• Surface Science
• Electrochemistry
• Nanotechnology

Background:

• Mapping charged chemical groups at solid-liquid interfaces is crucial for understanding colloidal systems and biomolecular interactions.
• Conventional surface charge measurement techniques lack spatial resolution or are incompatible with aqueous environments, such as Kelvin-probe force microscopy (KPFM) due to its DC bias voltage requirement.

Purpose of the Study:

• To develop and demonstrate a method for mapping surface potentials of charged chemical groups in aqueous solutions with high spatial resolution.
• To showcase the application of AC Kelvin probe force microscopy (AC-KPFM) for analyzing charged functional groups relevant to biomolecules.

Main Methods:

• Utilized AC Kelvin probe force microscopy (AC-KPFM), replacing the DC bias with a high-frequency AC voltage to enable measurements in aqueous media.
• Employed micropatterned alkanethiol layers functionalized with ionizable amino- and carboxy-groups to represent biomolecular charged sites.
• Investigated the reversible alteration of surface group charge by adjusting solution pH.

Main Results:

• Successfully mapped the surface potential of spatially defined charged groups (amino- and carboxy-) in aqueous solution using AC-KPFM.
• Demonstrated the electrostatic nature of the measured signal through reversible charge modulation via pH changes.
• Explored the influence of the electric double layer (EDL) on the AC-KPFM measurements.
• Showcased the directed deposition of oppositely charged nanoparticles onto patterned surfaces.

Conclusions:

• AC-KPFM is a viable technique for high-resolution surface potential mapping of charged groups in aqueous solutions.
• This method provides insights into the behavior of charged groups relevant to biomolecules and interfacial phenomena.
• The technique allows for controlled manipulation of nanoparticle deposition based on surface charge patterns.