Nanopores: Graphene Opens up To

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NANOPORES Graphene opens up to DNA It might be possible to sequence DNA by passing the molecule through a small hole in a sheet of graphene. Zuzanna S. Siwy and Matthew Davenport

he nanopores that are found in the ab c membranes of cells have an important Trole in biology and, along with artificial nanopores fabricated in materials such as silicon nitride, they have also been used to detect single molecules and, in some cases, extract more detailed information such as the size and shape of the molecules1. The capabilities of these systems are influenced by the diameter of the nanopore and also by the thickness of the membrane 5 nm 10 nm 10 nm that contains it. In general, however, these membranes are too thick to be able to probe the local structure of a molecule as it Figure 1 | Images of nanopores sculpted in graphene by the electron beam of a transmission electron passes through the nanopore. To read the microscope and used in the work of a, Golovchenko and colleagues6, b, Dekker and colleagues7 and sequence of nucleotides in a DNA molecule, c, Drndić and colleagues8. The rings surrounding the pore in (c) are indicative of the number of graphene for example, a membrane of sub-nanometre layers that form the membrane. Parts b and c reproduced with permission from: b, ref. 7, © 2010 ACS; thickness is required. c, ref. 8, © 2010 ACS. Graphene is a two-dimensional sheet of carbon atoms arranged in a honeycomb lattice. The material has remarkable The three teams — led by Jene be viable for DNA detection8. Furthermore, mechanical, electronic and optical properties Golovchenko of Harvard, Cees Dekker of the nanopores could differentiate between and, as a result, is at present being investigated Delft and Marija Drndić of UPenn — all DNA that passed through the pore in an for a variety of applications, including flexible address the issue of DNA detection using extended form and that which passed touch-screen devices2–4. This mechanical graphene nanopores. DNA is a long, through in a folded form (Fig. 2c). stability also means that it is possible to use spaghetti-like molecule made up of repeating Despite these exciting developments, an electron beam to sculpt a nanopore in a units called nucleotides. Each nucleotide is the central question remains: is single- suspended sheet of graphene5. Writing in composed of one of four bases — adenine base resolution with a graphene nanopore Nature and Nano Letters, three independent (A), cytosine (C), guanine (G) or thymine feasible? All three groups found that the groups based at Harvard University and (T) — attached to a backbone of sugar and translocation events are too fast to be Massachusetts Institute of Technology6, Delft phosphate groups. Only one nucleotide can resolved by the existing detection electronics. University of Technology7, and the University reside in the ultrathin graphene pore at a However, the Harvard–MIT team went a of Pennsylvania8 now report using graphene time, so it should be possible to sequence by step further and quantified the resolution membranes as nanopore sensors. observing the translocation of consecutive obtainable with a graphene nanopore using The basis of nanopore detection is the nucleotides through the nanopore. numerical simulations6. Golovchenko and resistive pulse technique, or Coulter-counter The Harvard–MIT team used graphene colleagues considered the translocation method, in which a membrane containing a membranes 1 or 2 layers thick6, the Delft of a cylindrical molecule whose diameter single nanopore is sandwiched between two team membranes 1–8 layers thick7 and changed discontinuously from 2.2 nm to reservoirs of electrolytic solution, such as the UPenn team membranes 3–15 layers 2.0 nm (diameters that are consistent with aqueous potassium chloride. Ions are driven thick8, and all report initial DNA detection the dimensions of base pairs in double- through the pore by applying an electric experiments that are an important step stranded DNA). Using a membrane potential difference across the membrane, towards DNA sequencing (Fig. 1). The thickness derived from experiment (0.6 nm), resulting in an ionic current that can be membranes (prepared by chemical vapour as well as a more conservative estimate measured. When the electrolyte contains deposition6,8 or exfoliation from graphite7) of thickness (1.5 nm), they showed that larger charged molecules, such as DNA or each contain single pores with diameters graphene nanopores should inherently proteins, these are also driven through the of 5–25 nm drilled in the graphene sheet possess sub-nanometre resolution. In the pore, causing a transient dip in the ionic with an electron beam. All groups presented case of the 0.6-nm membrane, the resolution current. Thus, each current dip represents a membranes that could detect double- was determined to be 0.35 nm — nearly passage, or translocation, of a biomolecule, stranded DNA molecules with lengths that identical to the length of a single base pair. with the magnitude and duration of the varied from 400 to 48,000 base pairs (Fig. 2). On a more fundamental level, the pulse being indicative of the molecule’s Even membranes with significant variability electrolyte/graphene interface presents an radius and length, respectively. in the baseline current levels were found to intriguing physical system. The Harvard–

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abThis work on graphene nanopores is Add DNA certain to be the precursor to further studies in a variety of fields. Within the scope of nanopore sensing, there are numerous

1 ns avenues to explore using the graphene 0.2 s itself as an electrode to control the local electric potential and translocation rates, or c to monitor the transverse conductance of individual nucleotides as they pass through a pore9. Previous theoretical work suggests that the latter technique, if used in the proper geometry, could provide DNA sequencing 10 1 ns with a 0% error rate . Combined with recent 2 ms advances in the production of large-area high-quality graphene (sheets with up to a ++ + 30-inch diagonal)4, the results open the door ––– to a host of interesting explorations including the development of new membrane systems Figure 2 | Translocation of DNA through a graphene nanopore. a, Double-stranded DNA threading a for biosensing. ❐ graphene nanopore. Each coloured segment represents a different nucleotide. The software VMD 1.8.7, PyMOL1.2r1, and TubeGen 3.3 was used in preparation of the image. b, Characteristic conductance versus Zuzanna S. Siwy and Matthew Davenport are in the time signals of a graphene nanopore before and after the addition of DNA. c, The conductance signature Department of Physics and Astronomy, University of (top) of various DNA conformations (bottom) as they translocate through a graphene nanopore. Parts b California, Irvine, California 92697, USA. and c reproduced with permission from ref. 7, © 2010 ACS. e-mail: [email protected] References 1. Dekker, C. Nature Nanotech. 2, 209–215 (2007). MIT team observed that although the understood if an ionic-current-based DNA 2. Novoselov, K. S. et al. Science 306, 666–669 (2004). 3. Neto, A. H. C., Guinea, F., Peres, N. M. R., Novoselov, K. S. & ion permeability of an intact graphene sequencing algorithm is to be employed. Geim, A. K. Rev. Mod. Phys. 81, 109–162 (2009). membrane was negligible compared with Perhaps the most striking question 4. Bae, S. et al. Nature Nanotech. 5, 574–578 (2010). that of a membrane that contained a that arises from these reports relates to the 5. Fischbein, M. D. & Drndić, M. Appl. Phys. Lett. nanopore, it was not zero and it was different dimensionality of the graphene pore: the 93, 113107 (2008). 6. Garaj, S. et al. Nature 467, 190–193 (2010). for different cations, which indicates that pore was seen to behave as both a cylinder 7. Schneider, G. F. et al. Nano Lett. electrolyte–graphene interactions need to of finite length (with its conductance being 10, 3163–3167 (2010). be taken into account6. Evidence of DNA– proportional to the square of the diameter)7 8. Merchant, C. A. et al. Nano Lett. 6–8 10, 2915–2921 (2010). graphene interactions was also observed . and as an infinitesimally thin circular aperture 9. Zwolak, M. & Di Ventra, M. Rev. Mod. Phys. 80, 141–165 (2008). These interactions will need to be carefully (with conductance proportional to diameter)6. 10. Postma, H. W. Ch. Nano Lett. 10, 420–425 (2010).

NANOElEcTRONicS Nanoribbons on the edge Arrays of graphene nanoribbons are fabricated on structured silicon carbide substrates using self-organized growth, without lithography and with well-controlled widths. John A. Rogers

raphene is attractive for electronics better switching capabilities, but they are large enough to allow efficient transistor because of its exceptional properties difficult to manufacture owing to the very switching3. However, practical circuits Gand the relative ease with which it can small widths that are required. Writing in require very narrow widths (less than be integrated into transistors and circuits. In Nature Nanotechnology, Michael Sprinkle, ~10 nm), with exceptional uniformity in the the form of large-area planar sheets, it can be Walter de Heer and colleagues at the Georgia lateral dimension, and well-ordered edges. processed using straightforward adaptations Institute of Technology and the Institut Recent reports suggest that such ribbons of methods that are already in widespread Néel in Grenoble report a clever growth can be achieved by ultrasonic tearing and use by the semiconductor industry. However, technique that yields organized, aligned exfoliation of larger sheets4, chemical graphene in its native single-layer form has arrays of narrow ribbons of graphene on ‘unzipping’ of carbon nanotubes5 and zero electronic bandgap, which prevents structured substrates of silicon carbide covalent linking of assemblies of molecular transistor devices made with it from being (SiC), with the potential to enable switching building blocks6. In their present state of turned off completely. This makes graphene properties suitable for broad classes of development, however, these strategies do unusable in power-efficient digital circuits electronic systems2. not offer the levels of engineering control (though it can still be used in radio frequency Early theoretical work suggested that over dimensions or spatial layouts needed electronics)1. Narrow graphene ribbons have graphene ribbons would have bandgaps for realistic applications. Lithographic

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