Characterizing Chelation at Surfaces by Charge Tunneling.

Area of Science:

• Surface Science and Nanotechnology
• Analytical Chemistry
• Materials Science

Background:

• Self-assembled monolayers (SAMs) are crucial for surface functionalization.
• Characterizing metal-ion incorporation and reaction kinetics in SAMs is challenging.
• Existing methods may lack precision for dynamic surface reactions.

Purpose of the Study:

• To introduce and validate the EGaIn junction as a novel surface analysis technique.
• To measure tunneling current densities through SAMs incorporating transition metal ions.
• To characterize the composition and reversible reaction kinetics of metal-chelates within SAMs.

Main Methods:

• Utilized the EGaIn junction to measure tunneling current densities (J(V)) through functionalized SAMs.
• Employed self-assembled monolayers of HS(CH2)11bpy terminated with bipyridyl groups.
• Incubated SAMs in metal salt solutions and analyzed current-voltage characteristics post-rinsing.

Main Results:

• Established that bound metal ions increase negative bias current density and reduce rectification.
• Demonstrated that current density vs. metal ion concentration follows a sigmoidal curve.
• Inferred kinetic rate constants (k_off, k_on) and dissociation constants (Kd) comparable to XPS.

Conclusions:

• The EGaIn junction technique effectively quantifies surface composition and reversible reaction dynamics.
• Bound metal ions are stable under specific rinsing conditions (τ_rinse ≪ τ_dissociation).
• This method offers a new tool for analyzing surfaces undergoing reversible chemical interactions.