A mong the variety of roles for nanopores in biology, an important one is enabling polymer transport, for example in gene transfer between bacteria 1 and transport of RNA through the nuclear membrane 2 . Recently, this has inspired the use of protein 3-5 and solid-state 6-10 nanopores as single-molecule sensors for the detection and structural analysis of DNA and RNA by voltage-driven translocation. The magnitude of the force involved is of fundamental importance in understanding and exploiting this translocation mechanism, yet so far it has remained unknown. Here, we demonstrate the first measurements of the force on a single DNA molecule in a solid-state nanopore by combining optical tweezers 11 with ionic-current detection. The opposing force exerted by the optical tweezers can be used to slow down and even arrest the translocation of the DNA molecules. We obtain a value of 0.24 ± 0.02 pN mV −1 for the force on a single DNA molecule, independent of salt concentration from 0.02 to 1 M KCl. This force corresponds to an effective charge of 0.50 ± 0.05 electrons per base pair equivalent to a 75% reduction of the bare DNA charge.It is possible to manipulate DNA molecules using electric fields because DNA is negatively charged in solution. Confining an electrical field to a nanopore enables the study of voltagedriven DNA translocation where the force is only applied to the few monomers that are inserted in the nanopore. We can calculate the electrical force F el on the DNA in the nanopore as F el = (q eff (z)/a)E(z)dz, where q eff is the effective charge of a DNA base pair, E(z) is the position-dependent electrical field in our system, a is the distance between two base pairs, and the integral is taken along the DNA contour. Assuming that q eff is identical for every base pair leads to F el = (q eff /a) E(z)dz = q eff V /a, with V the applied potential across the nanopore. The simplicity of this formula stems from the translational invariance of our system, in which the contour length of the DNA exceeds the length of the nanopore.

Carbon nanotube transistors have outstanding potential for electronic detection of biomolecules in solution. The physical mechanism underlying sensing however remains controversial, which hampers full exploitation of these promising nanosensors. Previously suggested mechanisms are electrostatic gating, changes in gate coupling, carrier mobility changes, and Schottky barrier effects. We argue that each mechanism has its characteristic effect on the liquid gate potential dependence of the device conductance. By studying both the electron and hole conduction, the sensing mechanisms can be unambiguously identified. From extensive protein-adsorption experiments on such devices, we find that electrostatic gating and Schottky barrier effects are the two relevant mechanisms, with electrostatic gating being most reproducible. If the contact region is passivated, sensing is shown to be dominated by electrostatic gating, which demonstrates that the sensitive part of a nanotube transistor is not limited to the contact region, as previously suggested. Such a layout provides a reliable platform for biosensing with nanotubes.

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We report charge inversion, the sign reversal of the effective surface charge in the presence of multivalent counterions, for the biologically relevant regimes of divalent ions and mixtures of monovalent and multivalent ions. Using streaming currents, the pressure-driven transport of countercharges in the diffuse layer, we find that charge inversion occurs in rectangular silica nanochannels at high concentrations of divalent ions. Strong monovalent screening is found to cancel charge inversion, restoring the original surface charge polarity. An analytical model based on ion correlations successfully describes our observations. DOI: 10.1103/PhysRevLett.96.224502 PACS numbers: 47.57.jd, 66.90.+r, 68.08.ÿp Screening by counterions is of fundamental importance in mediating electrostatic interactions in liquids. For multivalent counterions (Z ions, where Z is the ion valency including the sign), a counterintuitive phenomenon is observed: Screening not only reduces the effective surface charge, but it can also actually cause it to flip sign. This socalled charge inversion (CI) has been proposed to be biologically relevant in, e.g., DNA condensation, viral packaging, and drug delivery [1]. CI is not explained by conventional mean-field theories of screening. Recently, an analytical model was proposed that assumes that Z ions form a two-dimensional strongly correlated liquid (SCL) at charged surfaces [2]. This effect is particularly strong for high Z, and was confirmed experimentally for Z 3 and 4 [3]. Experimental evidence has remained inconclusive for the cases Z 2 and mixtures of Z ions with monovalent ions [4], both of which are biologically relevant given that K , Na , and Mg 2 are the most abundant cations in the cell. The main difficulty is that existing experimental probes become unreliable at high concentrations (*10 mM): Electrophoretic mobility measurements suffer from increasingly low signal to noise at higher salt, whereas surface force measurements are complicated by short-range forces.In this Letter, we investigate CI in individual silica nanochannels at high ionic strength by employing streaming currents as a new method. A streaming current is an ionic current that results from the pressure-driven transport of counterions in the diffuse part of the double layer [5], as illustrated in Fig. 1(b). The Stern layer, where the SCL is formed, is generally accepted to be immobile [6]. Consequently, streaming currents provide a direct measurement of the effective surface charge at the diffuse layer boundary. The well-defined rectangular channel geometry allows for straightforward interpretation. Contrary to other methods, streaming currents remain a reliable probe of the surface charge at high salt, up to 1 M in our experiments. We report unambiguous CI by divalent ions at concentrations above 400 mM. Additionally, we resolve the effect of screening by monovalent salt. We find that monovalent ions reduce CI by high-Z ions, and even cancel CI entirely at sufficiently high monovalent ion concentrations. We succ...