End Group Properties of Thiols Affecting the Self-Assembly Mechanism at Gold Nanoparticles Film As Evidenced by Water Infrared Probe

Water infrared probe has been employed for in situ monitoring of the detailed self-assembly processes of four thiol molecules with different end groups (-CH3, -NH2, -COOH, and -OH) on gold nanoparticles (Au NPs) film in aqueous solution. Based on the change of water IR signal, the significant influence of end group properties on the kinetics and thermodynamics of thiols self-assembly can be estimated. It is found that the assembly kinetics of thiols decreases with the increase of the hydrophobicity of the end groups. In addition, the charges carried by the end groups (-COOH and -NH2 terminated thiols) will also slow down the self-assembly kinetics owing to the electrostatic repulsions. However, the isothermal adsorption is only affected by the wettability of the end groups of thiols. The higher hydrophilicity of the end groups results in larger equilibrium constant of the self-assembly process. Results show that water infrared probe offers an additional approach to the monitoring of thiols self-assembly processes with higher sensitivity and more detailed information as compared to traditional molecule fingerprints.