Coated colloid probe

Hilal et al. [30] used AFM, in conjunction with a colloid probe, coated colloid probe, and cell probe techniques, to measure directly the adhesive force between two different UF membranes and a polystyrene sphere (diameter 11 jim), protein bovine serum albumin (BSA), and a yeast cell. These two membranes were ES 404 and XP 117 mentioned above (Table 7.1). The experiments were performed in 10 M NaCl solution. It was reported that the adhesive force of the polystyrene, the protein, and the cell system on the ES 404 membrane was greater than that on the XP 117 membrane. The relatively high affinity of protein for synthetic membrane surfaces was also observed. 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

For the capsule measurements, glass substrates coated with polyethyleneimine were used on which the PSS-terminated capsules showed adhesion. The capsule force deformation characteristics of individual capsules were obtained by placing a capsule under the colloidal probe and pressing onto it. Special care was taken to press onto the pole of the capsule as described in more detail in [13]. Reference curves on hard substrates were obtained before and after the experiments to determine the inverse optical lever sensitivity (InvoLS) and to derive the absolute deformation of the capsules. In rare cases when the InvoLS was found to change during the measurement, the measured data was discarded. 

The work of Larson et al. (62) represented the first detailed study to show agreement between AFM-derived diffuse layer potentials and ((-potentials obtained from traditional electrokinetic techniques. The AFM experimental data was satisfactorily fitted to the theory of McCormack et al. (46). The fitting parameters used, silica and alumina zeta-potentials, were independently determined for the same surfaces used in the AFM study using electrophoretic and streaming-potential measurements, respectively. This same system was later used by another research group (63). Hartley and coworkers 63 also compared dissimilar surface interactions with electrokinetic measurements, namely between a silica probe interacting with a polylysine coated mica flat (see Section III.B.). It is also possible to conduct measurements between a colloid probe and a metal or semiconductor surface whose electrochemical properties are controlled by the experimenter 164-66). In Ref. 64 Raiteri et al. studied the interactions between... 

Patscheider, J. Nanocomposite hard coatings for wear protection. Mater. Res. Bull. 2003,28,180-183. Zauscher, S. Klingenberg, D.J. Friction between cellulose surfaces measured with colloidal probe microscopy. Colloids Surf 2001, A178, 213-229. Kopta, S. Salmeron, M. The atomic scale origin of wear on mica and its contribution to friction. J. Chem. Phys. 2000, 113 (18), 8249-8252. 

Another technique that was used to estimate the solvent content and the number of solvent molecules per EG monomers of PLL- -PEG coatings was recently developed by Pasche et al. and involves coUoidal-probe APM surface force measurements. The main assumption made in this technique is that the unperturbed PEG layer is compressed by the colloidal probe from a fully solvated state to a solvent-free, dry state. Thus, the decrease in the layer thickness upon compression is likely to reflect the amount of solvent absorbed within the polymer brush. The results of that study are in reasonable agreement with the findings of the present work. 

Table 1. Average Values ( Standard Deviation) for PLL(20)-g[3.5]-PEG(5), Adsorbed on a Silica Substrate Surface, of the Wet Mass Density, wei, the Dry Mass Density, mdry, the Solvation, W, the Number of Solvent Molecules per EG Monomer Averaged over the Cross Section of the PEG Brush, A mig, and the Coefficient of Friction for Symmetric (Silicon Wafer and Colloidal Probe Coated with PLL-g-PEG), and Asymmetric (Silicon Wafer Coated with PLL-g-PEG,...

The friction force measurements were performed in two different configurations asymmetric, where only the fiat substrate surface was coated by PLL-g-PEG and the colloidal probe of the LPM remains bare, and... 

Investigation of cellulose systems in closer detail requires the choice of representative cellulose model surfaces for the experiment. A spin-coated cellulose surface on mica was the first model surface used in studies of forces in papermaking systems (70). This work was followed by other SFA studies using Langmuir-Blodgett (LB) films of cellulose (11-14). These films are noticeably smoother and more stable than spin-coated surfaces. In studies using the atomic force microscope (AFM) colloidal probe technique (Ducker et al. (75)), interaction forces were measured either between two cellulose beads (16,17) or between cellulose beads and spin-coated cellulose surfaces (18,19). 

The approach described above is illustrated by the work of Matzelle et al. (2002). Force distance curves were obtained for the indentation of a colloidal probe in PNiPAAm coatings immersed in water above and below the phase transition temperature (Figure 6.8). The calculation of Young s modulus using the Hertz model revealed a variation of more than two orders of magnitude between the swollen and the collapsed state at 10 C and 35 C, respectively. 

Beyond the elastic modulus, SFM-based colloidal probe measuronents were successfully applied to study interaction forces between thermo-responsive polymers and protein-coated surfaces. This was shown by Cole, Voelcker, Thissen, Horn, and Griesser (2010) with bovine serum albumin-coated probes on PNiPAAm surfaces. Furthermore, time-dependent colloidal probe measuronents on thermo-responsive polymer coatings... 

Cole, M. A., Voelcker, N. H., Thissen, H., Horn, R. G., Griesser, H. J. (2010). Colloid probe AFM study of thermal coUapse and protein interactions of poly(A-isopropylacrylamide) coatings. Soft Matter, 6, 2657-2667. 

It was observed that the adsorbed layers lowered the attraction between the surface and the hydrophilic probe. Friction between the coated surfaces and the colloidal probe also remained low, indicating that adsorbed micellar layers were mobile and had lubricating properties. 

The rheology of networks of cross-linked and bundled F-actin has been shown to exhibit exceptional elastic behaviour that reflects the mechanical properties of individual filaments. The local viscoelasticity has been investigated by microrheology techniques using embedded colloidal probe particles and optical interferometry, or magnetic tweezers. The elastic modulus is a strong function of the actin concentration, thus cross-link density. Actin can be cross-linked to form a network coating the surface of a vesicle. The viscoelastic and deformation properties of actin-coated vesicles has also been examined. 

Fig. 6 Force curves measured with a Colloidal Probe AFM. The outer surfaces (Sdicon colloidal probe and Silicon wafer) were coated with different numbers of polyfethylene imine) (PEI, poly-cation)/poly (styrene sulfonate) (PSS, polyanion) double layers. The solid lines correspond to fits by the Eq. 1. Due to sake of clarity the curves are shifted in vertical direction. The polyanion (PSS) was always the outermost layer The abscissa corresponds to the distance between colloidal probe (microsphere) and the Silicon wafer. The graph is taken from [41]...

One year later Mohideen and coworkers [115] used the colloid probe AFM technique to probe the Casimir force between a metalcoated planar surface and a metal coated sphere. After several improvements [135], they could achieve a measurement over the range of distance starting from 62 to 400 nm with an experimental error ofless than 1% at the lowest distance (Figure 2.10). This level of precision allows a quantitative comparison with theory. In this case, the influences of surface roughness, finite temperature, and finite conductivity of the metal have been taken into account. 

However, the colloid probe technique is not limited to particles. For biological studies, strategies to attach single spores [264], bacteria-coated beads [265], or single cells [266, 267] have been developed. To study forces in emulsions [268] or flotation cells such as oil drops [269, 270, 696] and bubbles have been attached to cantilevers [271]. 

A limitation of the colloid probe technique, however, is the minimum particle size that can be reproducibly attached by using optical microscopy. For spheres smaller than 1 J.m, it becomes difficult to correctly position the particle at the very end of the cantilever to avoid touching the substrate with the edge of the cantilever. In this respect, the name colloid probe is somewhat misleading since colloidal particles are usually smaller than 1 pm. Recently, there have been attempts to attach nanoparticles to the end of AFM tips either by wet chemistry [272] or by epoxy-coating of tips and dipping them into a powder [273]. 

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