Single molecule dynamics on hydrophobic self-assembled monolayers.

Area of Science:

• Surface science
• Physical chemistry
• Biophysics

Background:

• Understanding molecular interactions at hydrophobic interfaces is crucial for biological and separation processes.
• Macroscopic measurements often fail to capture molecular-level dynamics.
• Hydrophobic solid-aqueous interfaces are relevant in various scientific fields.

Purpose of the Study:

• To directly observe and characterize the dynamic behavior of individual molecules at hydrophobic interfaces.
• To investigate how varying surface properties (alkyl chain length) affect adsorbate interactions.
• To bridge the gap between macroscopic observations and molecular-level phenomena.

Main Methods:

• Utilized total internal reflection fluorescence microscopy (TIR-FM) for single-molecule imaging.
• Employed fluorescently labeled dodecanoic acid as probe molecules.
• Studied interactions with self-assembled monolayers (SAMs) of n-alkyltriethoxysilanes with chain lengths from n=4 to n=18.

Main Results:

• Observed at least two distinct surface residence times and diffusive modes, indicating multiple adsorbed populations.
• Mean surface residence time increased, and mobility decreased with increasing SAM chain length.
• The shift towards longer residence times and slower diffusion on longer chains was primarily due to an increased fraction of molecules in long-lived, slow-moving states, not altered dynamics within individual populations.

Conclusions:

• Hydrophobic surface interactions are heterogeneous, involving multiple molecular populations with distinct dynamics.
• Increasing surface alkyl chain length significantly alters adsorbate behavior by increasing the proportion of molecules in long-lived, less mobile states.
• Direct single-molecule imaging provides critical insights into interfacial phenomena that are not apparent from ensemble-averaged measurements.