Rapid manufacturing of low-noise membranes for nanopore sensors by trans-chip illumination lithography.

In recent years, the concept of nanopore sensing has matured from a proof-of-principle method to a widespread, versatile technique for the study of biomolecular properties and interactions. While traditional nanopore devices based on a nanopore in a single layer membrane supported on a silicon chip can be rapidly fabricated using standard microfabrication methods, chips with additional insulating layers beyond the membrane region can provide significantly lower noise levels, but at the expense of requiring more costly and time-consuming fabrication steps. Here we present a novel fabrication protocol that overcomes this issue by enabling rapid and reproducible manufacturing of low-noise membranes for nanopore experiments. The fabrication protocol, termed trans-chip illumination lithography, is based on illuminating a membrane-containing wafer from its backside such that a photoresist (applied on the wafer's top side) is exposed exclusively in the membrane regions. Trans-chip illumination lithography permits the local modification of membrane regions and hence the fabrication of nanopore chips containing locally patterned insulating layers. This is achieved while maintaining a well-defined area containing a single thin membrane for nanopore drilling. The trans-chip illumination lithography method achieves this without relying on separate masks, thereby eliminating time-consuming alignment steps as well as the need for a mask aligner. Using the presented approach, we demonstrate rapid and reproducible fabrication of nanopore chips that contain small (12 µm × 12 µm) free-standing silicon nitride membranes surrounded by insulating layers. The electrical noise characteristics of these nanopore chips are shown to be superior to those of simpler designs without insulating layers and comparable in quality to more complex designs that are more challenging to fabricate.

Electromagnetic torque tweezers: a versatile approach for measurement of single-molecule twist and torque.

The well-established single-molecule force-spectroscopy techniques have recently been complemented by methods that can measure torque and twist directly, notably magnetic torque tweezers and the optical torque wrench. A limitation of the current torque measurement schemes is the intrinsic coupling between the force and torque degrees of freedom. Here we present electromagnetic torque tweezers (eMTT) that combine permanent and electromagnets to enable independent control of the force and torsional trap stiffness for sensitive measurements of single molecule torque and twist. Using the eMTT, we demonstrate sensitive torque measurements on tethered DNA molecules from simple tracking of the beads' (x,y)-position, obviating the need for any angular tracking algorithms or markers. Employing the eMTT for high-resolution torque measurements, we experimentally confirm the theoretically predicted torque overshoot at the DNA buckling transition in high salt conditions. We envision that the flexibility and control afforded by the eMTT will enable a range of new torque and twist measurement schemes from single-molecules to living cells.

Distinguishable populations report on the interactions of single DNA molecules with solid-state nanopores.

Solid-state nanopores have received increasing interest over recent years because of their potential for genomic screening and sequencing. In particular, small nanopores (2-5 nm in diameter) allow the detection of local structure along biological molecules, such as proteins bound to DNA or possibly the secondary structure of RNA molecules. In a typical experiment, individual molecules are translocated through a single nanopore, thereby causing a small deviation in the ionic conductance. A correct interpretation of these conductance changes is essential for our understanding of the process of translocation, and for further sophistication of this technique. Here, we present translocation measurements of double-stranded DNA through nanopores down to the diameter of the DNA itself (1.8-7 nm at the narrowest constriction). In contrast to previous findings on such small nanopores, we find that single molecules interacting with these pores can cause three distinct levels of conductance blockades. We attribute the smallest conductance blockades to molecules that briefly skim the nanopore entrance without translocating, the intermediate level of conductance blockade to regular head-to-tail translocations, and the largest conductance blockades to obstruction of the nanopore entrance by one or multiple (duplex) DNA strands. Our measurements are an important step toward understanding the conductance blockade of biomolecules in such small nanopores, which will be essential for future applications involving solid-state nanopores.

Magnetically controlled rotation and torque of uniaxial microactuators for lab-on-a-chip applications.

We demonstrate the controlled rotation and torque generated by uniaxial magnetic microactuators formed by two bound superparamagnetic particles in a fluid. The torque and rotation are precisely controlled by rotating magnetic fields, generated by an external electromagnet or by on-chip current wires. We present the magnetic energy equations and the equations of motion for two-particle microactuators, with contributions from the permanent and induced magnetic moments of the particles. A comparison of theory and experiments allows an estimation of the different moments with accuracy better than 10% across a wide frequency range. At low frequencies and low magnitudes of the applied magnetic field, both the permanent and induced moments of the particles have contributions to the torque. At either high fields or high frequencies, the torque is dominated by the induced moment. The predictability of the torque is highest in the regime of low frequencies and high field, where the torque has a large magnitude and is determined by the magnetic shape anisotropy of the microactuator. A comparison of rotation in bulk fluid and on a chip surface shows an increase of friction by a factor 9 originating from the surface proximity. The detailed understanding of the torque and rotation of two-particle uniaxial magnetic microactuators opens a range of possibilities in lab-on-a-chip applications, such as the actuation of single molecules, fluid mixing in microfluidic chambers, and novel cluster-based assays.