Detection of Discrete Interactions upon Rupture of Au Microcontacts to Self-Assembled Monolayers Terminated with −S(CO)CH₃ or −SH

Abstract

Pulloff forces were measured under solvent for Au-coated atomic force microscopy (AFM) tips in contact with -S-acetate-, -O-acetate-, -SH-, or -OH-terminated self-assembled monolayers (SAMs). The SAMs were formed by adsorption of ω-functionalized undecylphosphonic acids on metal oxide substrates. In ethanol and hexadecane, the mean force required to rupture Au/S-acetate microcontacts was 7 times greater than the mean force required to break Au/O-acetate contacts, consistent with the known affinity of S-containing functional groups for Au. Further, rupture force histograms for Au/S-acetate microcontacts under ethanol or hexadecane showed 0.1 nN periodicity. Rupture forces for Au/-SH microcontacts were 4 times greater than for Au/-OH microcontacts under ethanol, and the rupture force histograms showed the same 0.1 nN periodicity. We have assigned this 0.1 nN force quantum to rupture of individual chemical bonds and have estimated the bond energy to be on the order of 10 kJ/mol. The specific interaction corresponding to this energy appears to be abstraction of Au atoms from the tip surface upon pulloff. Our ability to detect these discrete interactions was a function of the solvent in which the measurements were made. For example, in water there was no difference in the mean pulloff force for Au/S-acetate and Au/O-acetate contacts and the histograms did not exhibit periodicity. In general, mean rupture forces for tip-SAM microcontacts are strongly solvent-dependent. To observe single bond rupture forces directly, we argue that the tip-substrate interfacial energy must be negatiVe and larger in absolute value than the substrate-solvent and tip-solvent interfacial energies [i.e., |γ substrate-tip | > (γ tip-solvent + γ substrate-solvent )]. Otherwise, nonspecific solvent exclusion effects dominate the microcontact adhesion. These measurements show that, whereas rupture forces for tip-SAM microcontacts are solvent-dependent, these forces can be sensitive, under the right conditions, to fluctuations in the number of discrete chemical interactions.

Quantifying thiol–gold interactions towards the efficient strength control

“…Although the direct contact of thiol-modified AFM tip with gold substrate can speed up the reaction between the thiol and gold (as compared with the reaction in free solution) due to the shorter mass transport time, normally higher indentation force (4200 pN) will not speed up the reaction further, and indentation force that is too high may even destroy the thiol group or the PEG linker. Considering the fact that the time for mass transport in our AFM experiment is nearly zero, the 3.0 s interaction time necessary for strong interaction is the formation of covalent Au-S bonds 25,55 . However, the reaction time for the formation of SAMs can be different.…”

“…Mechanically controlled breakjunction experiments 32,33 that are slightly different from the AFM-based force experiments have demonstrated that the breaking of the molecular junctions takes place at Au-Au bonds near the molecule-electrode contact and produces a similar rupture force of 1.5 nN. However, an AFM-based force experiment conducted by Skulason et al 25 showed a small force value of 0.1 nN, which was also demonstrated to be the abstraction of thiolate-complexed gold atoms from the tip surface. Recently, Venkataraman et al 34 measured the breaking force in thiol junctions and showed a rupture force of B1.0 nN.…”

“…These interactions are unbinding forces that originated from specific interactions in biological systems (that is, the unfolding of proteins, melting of DNA strands, antigen-antibody interactions, ligand-receptor interactions and protein-nucleic acid interactions), entropic elasticity, basic supramolecular interactions (that is, hydrogen bonds, coordinated bonds, p-p interactions, hydrophobic interactions and so on) and even the strength of a single covalent bond 13,[17][18][19][20][21][22][23][24][25][26][27][28] . The investigations of molecular interactions in complicated systems, such as on live bacterial surfaces, in intact virus particle, and in condensed polymer materials, have also been performed successfully via AFM-based SMFS 21,22,29 .…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…The work of Skulason and Frisbie [388] deals with the adhesion between gold and S-acetate-, Oacetate-, SH-and OH-terminated SAMs in different solvents, and the accent is put on the detection of the discrete rupture forces or force quanta (see also [389]). The authors found a force quantum of 0.1 nN in histograms of the adhesion force and assigned it to the rupture of individual chemical bonds.…”

Label-Free Colorimetric Detection of Cadmium Ions in Rice Samples Using Gold Nanoparticles

“…Meanwhile, the binding energy of Cd-S (208.5 ± 20.9 kJ/ mol) is larger than that of Au-S (120 kJ/mol), which also indicates that the Cd(SG) 4 complex is more stable than the Au-SG. 31,32 The spherical shaped (GS) 4 Cd complexes have more net negative charge, thus the (GS) 4 Cd complexes have weaker electrostatic interaction with AuNPs than that of the linear GS-M-SG. Because AuNPs are synthesized by citrate reduction of HAuCl 4 , the surface of AuNPs has negative charges from citrate anions.…”