We use confocal fluorescence microscopy to study the conformation of single DNA molecules endtethered to a solid substrate. The segment distribution z measured for chains with contour lengths 15:4 m L 59:4 m as a function of the distance from the substrate z can be scaled onto a master curve depending only on the scaled distance z=R g , in quantitative agreement with theoretical predictions for end-tethered polymers in a good solvent. The scaling of the radius of gyration R g L 0:57 0:05 shows the presence of excluded-volume interactions between the charged DNA segments. Independent measurements of R g from end-segment distributions are in good agreement with values obtained from the segment distributions and provide evidence that the radius of gyration of end-tethered chains in a good solvent is identical to that of the free chain.PACS numbers: 61.25. Hq, 82.35.Gh, 82.35.Rs, 83.80.Rs Polymers tethered to solid substrates have widespread technical applications, such as the stabilization of colloidal suspensions against coagulation, the protection of biosensors against unspecific binding, and for lubrication and adhesion [1][2][3][4]. The presence of an impenetrable substrate profoundly affects the conformation and segment distribution of a polymer chain with N segments that is attached to it by one of its ends at a tether surface density . When the interaction between monomers and substrate is repulsive, scaling theory for isolated end-tethered polymers predicts a depletion zone near the surface characterized by a segment distribution increasing as z z with the distance z from the substrate; here, is related to the critical (Flory) exponent by 1 ÿ = which in a good solvent ( 0:588 [5]) takes the value 0:7 [6]. At larger distances, the influence of the wall should become weaker, and the segment distribution can be expected to approach a maximum value R g N=R g given by the average segment density within the polymer layer whose height is approximately given by the radius of gyration R g of the free, unperturbed chain [6]. The slow increase of the segment distribution close to the substrate, together with its fast decay for distances z R g predicted by renormalization-group (RG) calculations [7] and computer simulations [8], suggests that the segment distribution of an end-tethered polymer in the low-density limit R 2 g 1 is strongly asymmetric, resembling the shape of a mushroom.Although this ''mushroom'' conformation of an isolated end-tethered polymer represents the simplest situation of a broken symmetry for polymer statistics, detailed experimental tests of the theoretical predictions for z in the low-density limit are lacking. Neutron reflectivity measurements on end-adsorbed diblock copolymers confirmed the existence of a depletion layer close to the surface and the RG prediction for the second moment of the segment distribution hz 2 i 2:16R 2 g [9,10]. However, the determination of the segment distribution z and radius of gyration R g of polymer mushrooms using neutron reflectivity is difficult since thi...

We introduce a method for reversibly orienting long-chain DNA on solid hydrophilic substrates without a fluid meniscus. End-tethered λ-DNA mushrooms are elongated by a hydrodynamic flow in the presence of trivalent cations, resulting in electrostatic adsorption of the extended DNA to the surface. By complexation of the cations the part of the DNA which is unspecifically bound to the surface desorbs quantitatively, and the mushroom conformation is restored. With the use of multiple deposition-combing steps, combined with a final desorption step, tethering densities higher than attainable with single deposition steps can be obtained.