pH-Dependent Properties of Functional Monolayers of Helical Oligoureas

We present the design and characterization of a stimuli-responsive self-assembled monolayer (SAM) of a helical N,N′-linked oligourea onan Au(111) surface. The oligourea features carboxylic acid and amine side chains that confer pH sensitivity. This molecularly engineered monolayer exhibits tunable charge transport properties governed by pH and electrochemical potential. Using electrochemical impedance spectroscopy, we demonstrate that changing the pH alters the internal charge distribution within the SAM, leading to reversible changes in the charge-transfer resistance for both Ru(NH3)63+/Ru(NH3)62+ and Fe(CN)63–/Fe(CN)64– redox couples. In situ polarization modulation infrared reflection–absorption spectroscopy (PM IRRAS) reveals a controllable reorientation of the helical oligoureas under an applied potential, driven by repulsive interactions between the negatively polarized electrode and the dipole of the helix. This reorientation is maximized at neutral pH, where zwitterionic forms of the oligoureas dominate. These results highlight the potential of oligourea-based monolayers as adaptive building blocks for dynamic, switchable, and functional two-dimensional (2D) materials applicable in nanoelectronics and soft nanotechnology.