Forces between single pairs of negatively charged micrometer-sized colloids in aqueous solutions of monovalent, divalent, or trivalent counter-ions at varying concentrations have been measured by employing optical tweezers. The experimental data have been analyzed by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and a numerical solution of the Poisson-Boltzmann (PB) equation. With monovalent counterions, the data are well described by the DLVO and PB theories, suggesting that the DLVO theory is adequate to describe the colloidal forces at these conditions. At higher counter-ion valence, the approximations within the two theories become evident.

Optical tweezers are employed to measure the forces of interaction between single DNA-grafted colloids. Parameters to be varied are the length of the DNA, the grafting density, and the ion concentration of the surrounding medium. From the measured force-separation dependence an interaction length at a given force is deduced. It shows in the mushroom regime a scaling with the grafting density which levels off for brushes. For the latter the transition from an osmotic to a salted brush can be traced in detail by varying the ion concentration in accordance with mean field theories.

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Optical tweezers are employed to measure the forces of interaction within a single pair of DNAgrafted colloids in dependence of the molecular weight of the DNA-chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short-range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance dependence is analyzed by means of a theoretical treatment based on the compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.PACS numbers: 82.35. Rs, 82.70.Dd, 87.80.Cc Surface treatment of colloidal particles and the ensuing manipulation and control of their interaction properties is a topic of high and lasting interest, on the grounds of both technological relevance and fundamental importance. On the first count, the main issue pertains to the fact that surface treatment is necessary to achieve colloidal stabilization by inducing thereby a repulsive force that acts against the ubiquitous dispersion attractions between the colloids. Charge stabilization and steric stabilization, the latter being caused by grafted polymer chains, are the two most common mechanisms, whereas grafting of polyelectrolyte (PE) chains on a colloid provides a natural combination of both and results to an electrosteric repulsion. On the second count, surface treatment by polymer grafting provides the possibility to tune the effective colloid interaction by 'dressing' the hard sphere potential with a soft tail, whose range, strength and overall functional form can be controlled by changing the properties of the polymer brush, e.g., its grafting density, height or charge. Systems interacting by a combination of a hard sphere potential and a subsequent short-range repulsion show a tremendous variety in their equilibrium [1,2,3,4] and dynamical [5,6,7] properties.Considerable work has been carried out in the study of the so-called osmotic PE-brushes [8, 9, 10], which result for high surface grafting densities and are characterized by the fact that they spherically condense the vast majority of the counterions released by the chains. These, in turn, bring about an entropic effective force between the brushes, which has been quantitatively analyzed for PE-brushes [11] and stars [12]. On the other hand, little is known for the opposite case of low surface grafting density, for which the theoretical considerations that lead to the interaction between osmotic brushes break down. In this Letter, we investigate by a combination of sensitive and accurate experiments and theoretical analysis the effective forces between spherical DNA brushes and establish a novel mechanism of interaction between those, which results from the mutual compression of PEchains of the colloids against the surface of each other. The quantitative characteristics of the resulting forces are vastly different from those between osmotic brushes.The experimental inves...

Optical tweezers are microscopic tools with extraordinary precision in the determination of the position (±2 nm) of a colloid (diameter: ∼2.0 μm) in 3D-space and in the measurement of small forces in the range between 0.1 and 100 pN (pN=10−12 N). Experiments are reported in which single double-stranded (ds)-DNA chains of different length [2,000 base pairs (bp), 3,000, 4,000, and 6,000 bp] are spanned between two colloidal particles by use of appropriate molecular linkers. For the forces applied (≤40 pN) a fully reversible and well reproducible force–extension dependence is found. The data can be well described by both the worm-like chain model or by an approach developed by R. G. Winkler. For the resulting persistence length, a pronounced dependence on the ionic concentration in the surrounding medium is found.

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Optical tweezers are experimental tools with extraordinary resolution in positioning (± 1 nm) a micron-sized colloid and in the measurement of forces (± 50 fN) acting on it-without any mechanical contact. This enables one to carry out a multitude of novel experiments in nano- and microfluidics, of which the following will be presented in this review: (i) forces within single pairs of colloids in media of varying concentration and valency of the surrounding ionic solution, (ii) measurements of the electrophoretic mobility of single colloids in different solvents (concentration, valency of the ionic solution and pH), (iii) similar experiments as in (i) with DNA-grafted colloids, (iv) the nonlinear response of single DNA-grafted colloids in shear flow and (v) the drag force on single colloids pulled through a polymer solution. The experiments will be described in detail and their analysis discussed.

The kinetics of binding for the histone-like protein TmHU (from Thermotoga maritima) to DNA is analyzed on a single molecule level by use of optical tweezers. For the reaction rate a pronounced concentration-dependence is found with an "all or nothing"-limit which suggests the cooperative nature of the binding-reaction. By analyzing the statistics of mechanically induced dissociation-events of TmHU from DNA multiple reaction sites are observed to become more likely with increasing TmHU concentration. This is interpreted as a hint for a secondary organizational level of the TmHU/DNA complex. The reaction rate of TmHU binding to DNA is remarkably higher than that of the HU protein from Escherichia coli which will be discussed.