Single Molecule Force Spectroscopy Reveals the pH-Dependent Mechanical Strength of the Alkynyl-Gold Bond

Alkynyl groups are widely used as the gold-binding motifs for many applications, including the stabilization of gold nanoparticles, surface protection and surface chemical conjugation. However, at the fundamental level, the strength and binding mechanism of the alkynyl-gold bonds were yet to be revealed. Here, we use atomic force microscopy (AFM) based single molecule force spectroscopy to directly quantify the mechanical strength of the alkynyl-gold bond one bond at a time. We find that alkynyl-gold bonds are relatively strong with rupture forces in a broad range from 200 pN to 400 pN, which are stronger than many coordination bonds studied so far but weaker than a thiol-gold bond under similar experimental conditions. More interestingly, we find that the dissociation forces of alkynyl-gold bonds depend on the pH of the solution, suggesting a heterolytic rupture mechanism. The average rupture forces decrease from ∼270 pN at pH 7.4 to ∼100 pN at pH 3.4. Moreover, we find that the solvent conditions can also significantly affect the dissociation of alkynyl-gold bonds, indicating the existence of different binding modes and different dissociation pathways. Together, our results provide direct quantification of the mechanical strength of alkynyl-gold bonds and reveal the complex bond dissociation mechanism. This study may shed new light on the modulation of the stabilities of alkynyl-gold bonds at the bulk level for various biomedical and nanotechnological applications.