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.

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...

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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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