Investigation Of The Stability Of Supramotecular Systems Using AFM-based Single Molecule Force Spectroscopy

Supramolecular assemblies refers to a well-organized entity of building blocksheld together by intermolecular interactions. Understanding the nature ofsupramolecular assembly and the dynamic assembly process as well as the effects ofenvironment conditions on the stability of the supramolecular system atsingle-molecule level will shed light on the rational design/fabrication of novelsupramolecular structures and materials. In this thesis, atomic force microscopy(AFM)-based single molecule force spectroscopy (SMFS) was employed to study theassembly mechanism and the effects of environmental conditions on the strength ofindividual interactions in several supramolecular systems.In chapter1, the working principle of both AFM and SMFS are presented,including the functions of the instrument components, the acquisition and analysis ofthe force curve, the criteria for the identification of a single molecule stretching andthe measurement of the strength of individual interactions. Recent progresses inSMFS study of supramolecular systems have also been reviewed.In chapter2, the breaking/forming mechanism and the effect of externalconditions on the stability of individual thiol–gold contacts were studied. The bindingmodes between thiols and gold shift from a coordinate bond to a covalent one with theincrease in pH and interaction time. Individual Au–Au bonds around Au-S bindingsites will be broken under pulling. The strength of thiol–gold contacts formed onoxidized Au surfaces is larger than that formed on reduced Au surfaces at the same pHvalue for all cases. Surprisingly, the stability of Au-S contacts for isolated thiols issignificantly larger than that in SAMs.In chapter3, the effect of external conditions on the stability of individualmonosulfide–gold interactions formed both by isolated individual monosulfides and in SAMs on gold surface have been investigated. The results show that the strength ofthe individual monosulfide–gold contact formed on oxidized Au surfaces is largerthan that formed on reduced Au surfaces for all cases. Besides, solvent quality greatlyinfluence the strength of the individual monosulfide–gold contacts. The rupture forceobtained from isolated individual monosulfide–gold contacts is smaller than that inSAMs.In chapter4, the underlying mechanisms of the conformational transition and thestability of collapsed structures of single PNIPAM chains were investigated. Thecharacteristic smooth and sawtooth pattern force curves corresponding to the pullingof the random coiled structure and collapsed globule structure were obtained beforeand after the phase transition, respectively. Multiple folding structures existed withinthe collapsed globules. In addition, the driving forces for the phase transition ofPNIPAM include hydrophobic interaction and hydrogen binding.In chapter5, the underlying mechanism of the salt-induced LCST variation ofsingle PNIPAM chains was studied. A consecutive two-step collapsing process of theconformational transition of a single PNIPAM chain (i.e., low temperature and hightemperature phase transition) was detected for the first time. More compact collapsedstructures has formed in the high-temperature collapsing step.