Subsurface Imaging of Functionalized and Polymer-Grafted Graphene Oxide

We investigate the surface and subsurface morphology of stearylamine-modified graphene oxide sheets and polystyrene-grafted functionalized graphene oxide sheets through atomic force microscopy (AFM) operated in multi-set point intermittent contact (MUSIC) mode. This allows a depth-resolved mapping of the nanomechanical properties of the top surface layer of the functionalized graphene oxide sheets. On the surface of stearylamine-functionalized graphene oxide sheets, we can distinguish areas of hydrophilic graphene oxide from hydrophobic areas functionalized with stearylamine. We find that every sheet of graphene oxide is functionalized with stearylamine on both sides of the sheet. The exposure of polystyrene-grafted functionalized graphene oxide to chloroform vapor during the AFM measurement causes a selective swelling and a softening of the polystyrene envelope. The depth-resolved mapping of the tip–sample interaction allows the shape of the folded and wrinkled graphene oxide sheets within the polystyrene envelope to be imaged; furthermore, it allows the thickness of the swollen polystyrene envelope to be measured. This yields the swelling degree, the grafting density, and the average chain conformation of the grafted polystyrene chains, which we find to be in the brush regime. Our work demonstrates a versatile methodology for imaging and characterizing functionalized and polymer-grafted two-dimensional materials on the nanometer scale

Monitoring Demineralization and Subsequent Remineralization of Human Teeth at the Dentin–Enamel Junction with Atomic Force Microscopy

Using atomic force microscopy, we monitored the nanoscale surface morphology of human teeth at the dentin–enamel junction after performing successive demineralization steps with an acidic soft drink. Subsequently, we studied the remineralization process with a paste containing calcium and phosphate ions. Repeated atomic force microscopy imaging of the same sample areas on the sample allowed us to draw detailed conclusions regarding the specific mechanism of the demineralization process and the subsequent remineralization process. The about 1-μm-deep grooves that are caused by the demineralization process were preferentially filled with deposited nanoparticles, leading to smoother enamel and dentine surfaces after 90 min exposure to the remineralizing agent. The deposited material is found to homogeneously cover the enamel and dentine surfaces in the same manner. The temporal evolution of the surface roughness indicates that the remineralization caused by the repair paste proceeds in two distinct successive phases

Monitoring Nanoscale Deformations in a Drawn Polymer Melt with Single-Molecule Fluorescence Polarization Microscopy

Elongating a polymer melt causes polymer segments to align and polymer coils to deform along the drawing direction. Despite the importance of this molecular response for understanding the viscoelastic properties and relaxation behavior of polymeric materials, studies on the single-molecule level are rare and were not performed in real time. Here we use single-molecule fluorescence polarization microscopy for monitoring the position and orientation of single fluorescent perylene diimide molecules embedded in a free-standing thin film of a polymethyl acrylate (PMA) melt with a time resolution of 500 ms during the film drawing and the subsequent stress relaxation period. The orientation distribution of the perylene diimide molecules is quantitatively described with a model of rod-like objects embedded in a uniaxially elongated matrix. The orientation of the fluorescent probe molecules is directly coupled to the local deformation of the PMA melt, which we derive from the distances between individual dye molecules. In turn, the fluorescence polarization monitors the shape deformation of the polymer coils on a length scale of 5 nm. During stress relaxation, the coil shape relaxes four times more slowly than the mechanical stress. This shows that stress relaxation involves processes on length scales smaller than a polymer coil. Our work demonstrates how optical spectroscopy and microscopy can be used to study the coupling of individual fluorescent probe molecules to their embedding polymeric matrix and to an external mechanical stimulus on the single-molecule level

Multi-Set Point Intermittent Contact (MUSIC) Mode Atomic Force Microscopy of Oligothiophene Fibrils

We developed MUSIC-mode atomic force microscopy (AFM) to emulate intermittent contact mode AFM without a feedback loop and in the absence of lateral forces. This single-pass approach is based on maps of amplitude-phase-distance curves and allows the height and phase images to be simultaneously obtained for almost any amplitude set point. This is advantageous for determining the shape and nanomechanical properties of very soft and fragile samples. As an example, we studied supramolecular aggregates of oligothiophenes, which form ≈15 nm wide fibrils with a rigid core and a soft shell