Michael Geisler

Single molecule force measurements delineate salt, pH and surface effects on biopolymer adhesion

In this paper we probe the influence of surface properties, pH and salt on the adhesion of recombinant spider silk proteins onto solid substrates with single molecule force spectroscopy. A single engineered spider silk protein (monomeric C(16) or dimeric (QAQ)(8)NR3) is covalently bound with one end to an AFM tip, which assures long-time measurements for hours with one and the same protein. The tip with the protein is brought into contact with various substrates at various buffer conditions and then retracted to desorb the protein. We observe a linear dependence of the adhesion force on the concentration of three selected salts (NaCl, NaH(2)PO(4) and NaI) and a Hofmeister series both for anions and cations. As expected, the more hydrophobic C(16) shows a higher adhesion force than (QAQ)(8)NR3, and the adhesion force rises with the hydrophobicity of the substrate. Unexpected is the magnitude of the dependences--we never observe a change of more than 30%, suggesting a surprisingly well-regulated balance between dispersive forces, water-structure-induced forces as well as co-solute-induced forces in biopolymer adhesion.

Aging of Hydrogenated and Oxidized Diamond

2010 WILEY-VCH Verlag Gm Effective control of the surface properties of devices becomes more and more important to ensure their prolonged performance, as the size of devices diminishes to the nanoscale. At the nanoscale interfacial properties can differ from the macroscopic properties including conductivity, electron affinity, surface potential, and reactivity. Here, we present a single-molecule sensor to detect molecular adsorption free energies in liquid under variable conditions. Utilizing atomic force microscopy (AFM), this approach provides nanoscale precision and excellent sensitivity to surface properties. This is demonstrated by the investigation of the aging of a diamond that was chemically modified to control its conductivity, surface potential, and wettability. Diamond exhibits extraordinary mechanical properties and is chemically inert as well as biocompatible. It is, therefore, suitable for biomedical applications such as cell growth support and as a refinement of medical indwelling devices. In addition to the exceptional material characteristics, hydrogenated diamond exhibits p-type surface conductivity, whereas oxidized diamond is insulating. These electronic properties have advanced the further development of diamond-based sensor devices, such as electrochemical sensors, biosensors, and ion-sensitive field effect transistors (ISFETs) in bioelectronics. The application of diamond-based devices that take advantage of the unique surface properties of diamond, be it in air, in aqueous solution, or even in blood, necessitates stable surface conditions and interfacial properties in order to ensure the long lasting functionality of the device. Only the limited chemical stability under physiological conditions has hampered the implementation of silicon and other semiconductive materials into similar devices. Implanted medical devices should stay intact for months or even years without the need for removal due to restricted biocompatibility and foreign body reactions. We utilize single-molecule force spectroscopy (SMFS) in order to determine the adsorption free energy of poly(allylamine) (PAAm) on diamond surfaces of different termination and different age (Fig. 1A). In contrast to macroscopic setups, which suffer from bubbles and cavitation effects hampering equilibration, our single-molecule approach allows for equilibration to be achieved on the experimental time scale. The forced desorption under equilibrium conditions is defined by a velocity-independent plateau of constant force over a separation defined by the polymer contour length and, thus, allows for a direct determination of the adsorption free energy per unit length from the area under the plateau curve (Fig. 1B). The distribution of plateau forces for Hand O-terminated diamond are given in Figure 1C. The general observed trend of measuring higher adsorption forces and therefore higher free energies on freshly prepared, hydrophobic diamond compared to hydrophilic diamond is reversed after two months and can be restored after new termination. These adsorption free energies on both kinds of terminations mirror the change of the contact angle, thus wettability (Fig. 1D). Moreover, there is a relation between the adsorption free energies and the surface energy of diamond, as revealed by contact-angle measurements with varying reference liquids on the same surfaces of different termination and age (Table 1). Higher surface energies correspond to lower adsorption free energies per monomer. We analyzed the time-dependent change of themeasured static contact angle on hydrogenated (black) and oxidized (blue) diamond in more detail over a time period of several hundred days (Fig. 2A). The aging effect on the wettability of both diamond surfaces starts immediately after the surface treatment and saturates after the first 50 days. Both surfaces were stored under vacuum, except for the time span of the SMFS measurements that were performed in electrolyte solution. The hydrophobicity of the H-terminated surface decreased nearly exponentially during half a year. 260 days after the initial termination of the substrate the termination was renewed, which resulted in a rise of the contact angle to almost the starting level followed by a similar exponential decay over time. This behavior is likewise observed for the O-terminated diamond but with opposite sign. After half a year, they approach the same value although the contact angle initially differed by more than 708. Surface potential scans of the patterned C H/C O surface over a period of nearly one year corroborate our findings. As depicted in Figure 2B, the decay of the surface potential difference is similar to that of the contact angles. The early surface-potential difference of 290mV is even inverted after 300 days of storage. Figure 2C illustrates the aging effect on the surface contact potential of both terminations. The sharp contrast of the dark C O and the bright C H regions is blurred with time. The final potential difference of 20mV barely reproduces the surface pattern. Considering the origin of this aging process, likewise observed in SMFS and contact-angle measurements, one can think of two possible mechanisms, which can act on both surfaces, namely (i) contaminations that cover the surface (such a molecule-thick adsorbate layer was previously verified on hydrogenated diamond) and (ii) a conversion of the artificial surface termination that has been predicted theoretically. Figure 3A shows a typical tapping-mode AFM image of an O-terminated diamond surface after 50 days. For all other images in Figure 3 possible persistent pollutants, which were still present

Polymer Adhesion at the Solid–Liquid Interface Probed by a Single–Molecule Force Sensor

A method based on atomic force microscopy is used to delineate the properties that determine single-molecule adhesion onto solid substrates in aqueous environment. Hydrophobicity as well as electrical properties of the substrate and the polymer are varied. In addition, the influence of the solvent composition, in particular the effect of ions, on the molecular adhesion at the solid-liquid interface is studied. Surprisingly, the polymer and surface-related properties account for only small changes in adhesion force, while dissolved ions show a much larger effect. These results point towards the energy of solvation as the most important contribution to adhesion for a wide variety of polymers and substrate materials.

Forced desorption of polymers from interfaces

In the past decade it has become possible to directly measure the adsorption force of a polymer in contact with a solid surface using single-molecule force spectroscopy. A plateau force in the force–extension curve is often observed in systems of physisorbed or noncovalently bonded polymers. If a molecule is pulled quickly compared to internal relaxation, then nonequilibrium effects can be observed. Here we investigate these effects using statistical mechanical models and experiments with a spider silk polypeptide. We present evidence that most experiments showing plateau forces are done out of equilibrium. We find that the dominant nonequilibrium effect is that the detachment height hmax(v) increases with pulling speed v. Based on a nonequilibrium model within a master-equation approach, we show the sigmoidal dependence of the detachment height on the pulling speed of the cantilever, agreeing with experimental data on a spider silk polypeptide. We also show that the slope with which the plateau forces detach is given by the cantilever force constant in both theory and experiment.

Controlling the structure of proteins at surfaces

With the help of single molecule force spectroscopy and molecular dynamics simulations, we determine the surface-induced structure of a single engineered spider silk protein. An amyloid like structure is induced in the vicinity of a surface with high surface energy and can be prohibited in the presence of a hydrophobic surface. The derived molecular energy landscapes highlight the role of single silk protein structure for the macroscopic toughness of spider silk.

Measuring the interaction between ions, biopolymers and interfaces--one polymer at a time.

Atomic force microscopy (AFM) based single polymer force spectroscopy allows to detect the interaction (energy) between single polymers and interfaces in aqueous environment. We use this method to delineate the effect of ions, pH, co-solutes and temperature on the adhesion of biopolymers onto solid substrates.