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Langmuir 2008, 24, 10390-10394
Liquid Crystal Alignment on a Chiral Surface: Interfacial Interaction with Sheared DNA Films M. Nakata,†,§ G. Zanchetta,‡ M. Buscaglia,‡ T. Bellini,*,‡ and N. A. Clark*,† Department of Physics and Liquid Crystal Materials Research Center, UniVersity of Colorado, Boulder, Colorado 80309-0390, and Dipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, UniVersita` di Milano, Milano, Italy ReceiVed February 28, 2008. ReVised Manuscript ReceiVed April 15, 2008 We explore the alignment of various achiral liquid crystals on films of aligned double-stranded helical DNA. In all cases and both for the nematic and smectic A phases, we find a distinctly chiral interfacial structure, with the mean orientation of the liquid crystal in contact with the DNA-treated surfaces chirally rotated through a substantial angle with respect to the mean DNA orientation. This rotation originates in the chirality of double-stranded DNA and depends on the liquid crystal molecular structure. We dicuss the role of dipolar and hydrophobic coupling in determining the observed orientation.
Introduction
Experiments and Results
Liquid crystals (LC) are characterized by fluidity and structural anisotropy, a combination of properties that produces their facile response to interfacial forces and enables a variety of techniques for producing ordered LC monodomains. One of the most effective and technologically important alignment methods is the use of spin or dip-coated thin polymer films, rendered anisotropic by rubbing1 or shear alignment.2,3 For a wide variety of films, the nematic LC phase of rod-shaped molecules typically aligns at the surface in a monodomain of orientation of the director, n, the mean molecular long axis. If both the polymer and LC molecules are achiral or chiral but racemic, then the LC/interface system is mirror symmetric about the s-p plane, containing the surface normal p and the rubbing or shearing direction s, and hence n orients into the s-p plane. In the case that the molecular components of either or both the polymer and the LC are chiral, the mirror symmetry of the surface interaction is lost, and n must rotate about p (i.e., out of the s-p plane). To our knowledge such a chiral surface orientation has not been directly observed. Here we explore the alignment of achiral LCs of various chemical structure on films of aligned double-stranded (i.e., helical) DNA. In all cases and both for the nematic (N) and smectic A (SmA) LC phases, we find a distinctly chiral interfacial structure, with n at the surface oriented at an angle with respect to the shearing direction. Such an angle is different for the various LC compounds, indicating that the DNA-LC coupling is governed not only by the steric constraints at the interface between the two molecular species but also by the detailed electric coupling between the undissociated phosphate groups and the dipoles and polarizabilities of the LC compounds.
Experiments were carried out using cells made from glass microscope slides spaced by 2 to 8 µm, with the LC in the gap between. The inner surfaces of these glass substrates were either both coated with DNA or one of them was treated differently to promote degenerate planar or homeotropic alignment. Once assembled, the cells were filled with LC by capillarity in the isotropic phase. The DNA surface treatments were made from calf thymus DNA sodium salt (Sigma/Aldrich D1501), first solubilized in distilled water and later concentrated to form gel-like suspensions at concentration c ≈ 200-300 mg/mL. (For comparison, the concentration of dehydrated crystalline DNA is c ≈ 1800 mg/mL.) A drop of such a suspension was placed on a substrate and covered with a second plate. The two plates were slid past each other along a given direction with a last pass that left a unidirectionally sheared, nearly dry DNA film on the substrate. Drying was completed by holding the DNA films at about 60 °C for a few hours. This procedure forms DNA films of thickness dDNA , 1 µm, which do not have significant birefringence. The alignment of DNA was probed by doping the DNA solution with ethidium bromide (EtBr), a disk-shaped fluorescent molecule known to intercalate efficiently between the paired nitrogen bases of double-stranded DNA. Such a strong orientational coupling of EtBr with the double helix is easily exploited to detect the mean orientation of DNA in the sheared films. The result is shown in Figure 1a, where the fluorescence emission (λ > 570 nm) intensity of a film excited by UV polarized light was measured as a function of the orientation of the polarizer, expressed through the angle φ with respect to the shearing direction s. This experiment shows maximum fluorescence for φ ) 90°, indicating that the DNA is indeed aligned in the direction of shear. The dependence of the polarized fluorescence signal versus φ is in agreement with that reported in other works.4 The alignment of DNA can also be probed in thicker films of dried DNA film obtained by increasing the amount of DNA initially trapped between the sheared plates. For thickness dDNA ≈ 1 µm, the film birefringence could be seen in depolarized transmitted light microscopy (DTLM, Figure 1b,c), indicating optic axes respectively parallel and normal to the shear direction, s, as indicated by the good quality of the extinction between crossed polarizers for these orientations. Measurements of the birefringence give an optical refractive index anisotropy of sign ∆n ) n| - n⊥ < 0 (n for polarization | s < n for polarization ⊥ s) and magnitude ∆n
* Corresponding authors. E-mail:
[email protected]; noel.clark@ colorado.edu. † University of Colorado. ‡ Universita` di Milano. § Deceased. (1) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, Germay, 2003. (2) Pidduck, A. J.; Haslam, S. D.; Bryan-Brown, G. P.; Bannister, R.; Kitely, I. D. Appl. Phys. Lett. 1997, 71, 2907–2909. (3) Syed, I. M.; Carbone, G.; Rosenblatt, C.; Wen, B. J. Appl. Phys. 2005, 98, 034303.
(4) Uy, J. L.; Asbury, C. L.; Petersen, T. W.; van den Engh, G. Cytometry 2004, 61A, 18–25.
10.1021/la800639x CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
Liquid Crystal Alignment on a Chiral Surface
Figure 1. (a) Intensity of fluorescence emission (λ > 570 nm) of ethidium bromide as a function of the orientation φ of the polarization of the excitation light with respect to the shearing direction s. (b and c) Depolarized transmitted light microscopy images of a ∼1-µm-thick shearaligned dried DNA film with the shear direction either aligned parallel to the polarizer (b) or at 45° with respect to it (c), showing that the optical axes are normal and parallel to s. The direction of the chain axes, pictured by a drawing of the DNA helix, is established both by fluorescence and by birefringence measurements.
≈ -0.05, comparable with that of neat DNA,5 confirming the alignment quality and that the DNA chains are parallel to the shear direction, as generally found in sheared samples.6 Experimental cells were fabricated in which at least one of the glass plates was coated with a sheared DNA film. Some cells were made with DNA on one plate whereas the opposite surface was treated to favor homeotropic alignment by spin coating a polyimide solution (Nissan Chemical Industries SE1211). In other cells, the DNA film was combined with an opposite surface treated with silane surface-coating agent GLYMO 3-(glycidoxypropyl trimethoxysilane), prepared as described by Dozov et al.7 The GLYMO surface gives LC (planar) alignment (i.e., has n parallel to the surface), but with an azimuthal energy that is nearly degenerate (i.e., makes a planar, orientationally slippery surface). GLYMO films were made by dip coating onto clean glass from a solution of 90% isopropyl alcohol and 10% of a 1:20 GLYMO/water mixture. Experiments were also carried out with the DNA aligning film on indium/tin oxide (ITO)-coated plates, where the ITO was lithographically patterned into electrodes, enabling the selective application of an (5) Samoc, A.; Miniewicz, A.; Samoc, M.; Grote, J. G. J. Appl. Polym. Sci. 2007, 105, 236–245. (6) Simonson, T.; Kubista, M. Biopolymers 1993, 33, 1225–1235. (7) Dozov, I.; Stoenescu, D. N.; Lamarque-Forget, S.; Martinot-Lagarde, Ph.; Polossat, E. Appl. Phys. Lett. 2000, 77, 4124–4126. (8) Shadt, M.; Helfrich, W. Appl. Phys. Lett. 1971, 18, 127–128. (9) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 19, 6436–6442.
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Figure 2. DTLM images between crossed polarizers of (a-d) nematic and (e, f) smectic A 8CB (a, b) between a sheared DNA film and a homeotropically aligning surface and (c-f) between a sheared DNA and a GLYMO film. (a, c, e) Pictures taken with polarization parallel to the shear direction (φ ) 0) indicate the uniform alignment of nDNA, the LC director on the DNA surface, in a γ * 0 direction. (b, d, f) Rotation of the cell gives the extinction between crossed polarizer and the analyzer when the DNA shear direction is oriented to φ ≈ 50° relative to the polarization direction, showing that the director field is uniform and γ ≈ -40°. The angle γ is the clockwise orientation of the liquid crystal optical axis with respect to the sheared DNA layer when the DNA is facing the viewer, as indicated by the sketch of the substrate.
electric field to the LC. LC orientation in the filled cells was evaluated using DTLM. The cells were filled with various commercial liquid crystals, purchased from Merck: 4-cyano-4′-octylcycanobiphenyl (8CB), 4cyano-4′-hexylbiphenyl (6CB), methoxybenzilidene butylanaline (MBBA), and 4-transbutyl-4-cyano-4-heptyl-bicyclohexane (CCN47). All of these compounds feature a nematic phase close to room temperature in which the experiments have been performed. In the case of 8CB, some of the observations were made in the smectic A phase, found by lowering the temperature from the N phase. In DNA/Nissan-SE1211 cells, DTLM showed hybrid LC alignment, with the director parallel to the sheared DNA layer and perpendicular to the Nissan-SE1211 layer. However, the plane containing the director does not coincide with the shear direction, as shown for 8CB in Figure 2a. The cell shows uniform birefringence color, indicating homogeneous alignment by the DNA over the whole sample area, and exhibits good extinction between crossed polarizers (Figure 2b) as the cell is reoriented through angle φ (between shear direction and polarizer orientation) for either positive or negative φ values, depending on the compound and on the cell. Birefringence measurements performed with a compensator enabled the determination of the direction of maximum refractive index, which was taken as a measurement of the LC director orientation nDNA at the LC/DNA interface. To describe the LC orientation at the aligned
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Table 1. Direction of alignment γ of the nematic director on the sheared DNA layer for the different LC compounds used in the experiments. Errors in γ are estimated from the standard deviations obtained from four different samples. We also indicate the permanent electric dipolar moment µ of each molecular species together with the dielectric anisotropy ∆E and the birefringence ∆n of each compound in the nematic phase. Superscripts are reference numbers. LC compound
γ
6CB 8CB CCN47 MBBA
-53 ( 6° -41 ( 10° -38 ( 8° 75 ( 8°
µ (D) 19
3.2 3.119 3.723 3.225
∆
∆n 20
9.8 6.222 -8.024 -0.526
0.14721 0.15321 0.0324 0.18427
V/µm was applied, excellent extinction was produced (Figure 3b), which is indicative of field-induced homeotropic orientation of the LC (i.e., director n becomes parallel to p, as expected for nematics with positive dielectric anisotropy). The bright lines are disclination defect lines marking the reverse twist domains where n is brought parallel to p by the field but through opposite rotations of the director from the initial planar state. This behavior is the same as that of “twisted nematic (TN)” LC devices made by cross rubbing of achiral polymers in which the starting (E ) 0) configuration has n parallel to the plates but precesses about p as one passes from one cell plate to the other.8 Having determined the DNA anchoring direction nDNA, we could easily obtain extinction in the parallel shear DNA/DNA cells by suitably orienting the cell and uncrossing the polarizer and analyzer. This is illustrated in Figure 3c, where the polarizer is rotated to be parallel to nDNA at the DNA surface for the entering light, and the analyzer is oriented to be normal to nDNA at the exiting surface. In traversing the cell, the polarization follows the twisting n of the LC and is extinguished by the analyzer, as in the TN electrooptic effect.8 This shows that the azimuthal chiral surface anchoring of the DNA films is sufficiently strong to maintain the ∼80°/5 µm director twist in the nematic phase. Upon cooling to the SmA, however, the texture breaks up into irregular domains (Figure 3d). In each domain, the orientation of n is uniform, not twisted, and equal to the nDNA of either one plate or the other, and the surfaces are no longer able to impose twist on the smectic but force it to adopt either one surface-preferred orientation or the other.
Discussion
Figure 3. DTLM images of nematic 8CB aligned between DNA films sheared in the same direction on glass substrates coated with a patterned indium tin oxide film to enable electric field application normal to the plates in some areas. (a) With no field applied, the nematic phase gives significant light transmission between crossed polarizer and analyzer. (b) With voltage applied to the electroded area (right-hand side of the image), the field reorients n to be normal to the plates, giving extinction away from the reverse-tilt domain boundaries (thin bright lines). (c) Extinction can also be obtained for E ) 0 by properly uncrossing the polarizer and analyzer. (d) As the temperature is lowered in the smectic A phase, the cell breaks into uniform domains in which either the top or the bottom anchoring prevails.
DNA surface, we define γ as the orientation of nDNA with respect to the shear direction s, with positive γ corresponding to a lefthanded rotation about p (i.e., a clockwise rotation when looking directly at the DNA surface, as indicated in Figure 2a). The measured values of γ are given for the different compounds in Table 1. Similar results are obtained with cells prepared with one DNA plate and one GLYMO plate, with a typical result for 8CB in the N phase shown in Figure 2c,d. Here again observations indicate a uniform director field through the cell at this orientation. Tilting the cell through the angle δθ about the unit vector r ) nDNA × p shows that the birefringence is symmetric about δθ ) 0 (i.e., there is no observable pretilt of n out of the surface plane on the unidirectionally sheared DNA film). This overall orientation persists upon cooling the DNA/ GLYMO cell into the SmA phase (Figure 2e,f). Thus, the sheared DNA film produces a distinctly chiral LC orientation at the DNA surface. Additional experiments were carried out on DNA/DNA surface cells with the DNA shearing directions on the two plates parallel or antiparallel. For 8CB in the nematic phase, with crossed polarizer and analyzer these cells transmitted light in DTLM substantially at any orientation of the cells about p, including those where the polarization and shear directions are parallel or perpendicular (Figure 3a), indicating a twisted director orientation field in the LC. The transmitted color and intensity are again uniform. In areas with electrodes, once an alternating current electric field, E, of a few
These observations present clear evidence for a large chiral orientational effect in the anchoring of a typical nematic/SmA LC on dehydrated sheared DNA films. The angle γ characterizing this effect has a strong dependence on the molecular species at play (Table 1), indicating that this chiral anchoring depends on the detailed structural features of the molecules. It is of interest to understand if these observations can be accounted for on the basis of known properties of the interaction of LCs with anisotropic surfaces. LCs generally adopt planar alignment (n parallel to the surface) on polar surfaces, for example, clean glass1 or water.9 Thus, because the surface of a DNA double helix is generally hydrophilic, as evidenced by the aqueous solubility of DNA, the observed planar orientation of the director on a DNA film surface is not unexpected. However, the DNA duplex surface is highly structured, chemically heterogeneous, exhibiting both hydrophobic and hydrophilic regions, and topographically rough on the length scale of the adsorbing LC molecules. Each of these features is characterized by strong inplane anisotropy and hence has the potential to influence LC alignment strongly, making the observed LC orientations difficult to account for in detail. Surface structure, such as the honeycomb lattice of graphite, induces commensurate crystallization of 2D physisorbed monolayers of LC molecules on its surface;10,11 chemical heterogeneity such as patterning into anisotropic domains of hydrophilic and hydrophobic regions induces LC alignment;12 anisotropic surface roughness produced in a variety of ways systematically tends to align LCs such that the LC texture is smoothest, with n parallel to the in-plane direction along which surface contour lines are the smoothest.13 In Figure 4a, we show the basic geometrical features of DNA and of the LC molecules in the experiment. We report here both forms of DNA structure (A-DNA and B-DNA)14–16 because upon (10) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542–545. (11) Smith, D. P. E.; Ho¨rber, H.; Gerber, Ch.; Binnig, G Science 1989, 245, 43–45. (12) Lee, B.-W.; Clark, N. A. Science 2001, 291, 2576–2580. (13) Cull, B.; Shi, Y. S.; Kumar, S.; Shih, R.; Mann, J. Phys. ReV. E 1995, 51, 526–535. (14) Lu, X-J.; Shakked, Z.; Olson, W. K. J. Mol. Biol. 2000, 300, 819–840.
Liquid Crystal Alignment on a Chiral Surface
Figure 4. Geometrical description of the molecular species involved in the experiments. (a) Characteristic lengths and angles of the DNA and LC molecules. DNA double helices represented as cylinders and decorated with a line indicating the phosphate chain (beads represent phosphate groups). (b) Drawing of an ∼10-base-pair-long section of duplex DNA and the LC molecules on the same scale, showing the solvent accessibility surface of these molecules. The Huckel charge density is color coded (red positive, blue negative) as computed by ChemBio3D Ultra (CambridgeSoft). In the case of DNA, the exposed O and OH phosphate terminals are pictured as van der Waals spheres, where for each phosphate an arbitrary choice has been made as where to locate the undissociated hydrogen atom. The drawing shows that the DNA film surface is sterically rough for the LC molecules and covered with structured arrays of strong dipolar groups.
dehydration the ordinary B form could, in our sheared layers, have turned into the A form.17 DNA duplexes expose two highly polar (if undissociated) helices of phosphate chains separated by two less polar grooves. The phosphate chains make an angle β with respect to the helix axis, with β ≈ 68° for the A-DNA structure and β ≈ 59° for the B-DNA structure, and hence the directions normal to the grooves are -22° and -31° for the two structures, respectively. On this basis, the results in Table 1 indicate that whereas 6CB, 8CB, and CCN47 align nearly perpendicular to the duplex DNA grooves, MBBA aligns at some small angle away from the groove direction. This MBBA orientation would be that expected if the DNA duplex roughness was the dominant orientational effect. The large density of phosphate groups provides the DNA surface with strong electric dipoles and local electric fields to which LC molecules significantly couple. The planar alignment of LC on DNA is coherent with experiments performed by Shah and Abbott18 showing that both 5CB and MBBA display random planar alignment on surfaces that are rich in carboxylic acid groups that in LC remain largely undissociated. They also found (15) Calladine, C. R.; Drew, H. R.; Luisi, B. F.; Travers, A. A. Understanding DNA: The Molecule and How It Works, 3rd ed.; Elsevier Academic Press: San Diego, CA, 2004. (16) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag: New York, 1984. (17) Franklin, R. E.; Gosling, R. G. Nature 1953, 171, 740–741. (18) Shah, R. R.; Abbott, N. L. J. Phys. Chem. B 2001, 105, 4936–4950.
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that by replacing the acid groups (pKa ∼ 4.7) with the more easily dissociated sodium hydroxylate (pKa ∼ 1) the surface partially ionizes in LC, thus favoring homeotropic alignment in compounds with permanent dipole moments along the geometric axis, such as 5CB. This condition of electric-field-driven crossover between planar and homeotropic alignment is difficult to achieve with DNA because the pKa for phosphate (pKa ∼ 2.1) is larger than that of sodium hydroxylate and because the surface density of exposed phosphate groups in the DNA layer is smaller than that of the closely packed carboxylic groups in ref 18. Overall, the notion that the planar orientation is due to electric coupling between surface dipolar groups and dipolar or polarizable groups in the LC molecules suggests that the diverse azimuthal anchoring found for the various LC compounds mainly depends on the different electric properties of LC molecules. In Table 1 we have also listed some of the electric properties of the LC compounds used in the experiments. As it appears, there are significant differences. All compounds have a strong permanent electric dipole µ, but with different directions with respect to the molecular axis: whereas in the cyanobiphenyls (6CB and 8CB) µ is along the axis, in CCN47 µ is almost perpendicular to it, as indicated by the negative dielectric anisotropy. The dipolar moment of MBBA is instead oriented at some intermediate angle and results in a small dielectric anisotropy. Another crucial difference in the electric structure of the LC molecules in our experiments is apparent in Figure 4b, where we display, with color code (red positive, blue negative), their electric charge density distribution. In the case of 6CB, 8CB and CCN47, µ is strongly localized in the CtN group, whereas the electric dipole of MBBA is spread over the molecule, thus making dipole-dipole interaction involving MBBA weaker and less positionally constraining. 6CB, 8CB, and MBBA share a large electronic polarizability resulting in a large birefringence, whereas the polarizability of CCN47 is weak because of the lack of double bonds between the carbon atoms and results in a smaller birefringence. The values for refractive indices (not shown) and birefringence indicate that MBBA has the largest polarizability of this set of compounds. On this basis, we give the following interpretation of the observed behavior. Molecules with a localized dipole, such as 8BC and CCN47, experience pinning to the phosphate groups. The polar/nonpolar modulation along the DNA double helix induces an analogous modulation in the LC molecules contacting the DNA surface by favoring the segregation of the aliphatic chains into the grooves, a situation more easily achieved when LC molecules are oriented perpendicular to the grooves. However, because the fit of the LC molecules onto the DNA duplex may not be ideal, for example, because the grooves are shorter than the length of the LC molecules, packing constraints could imply tilting the average molecular axis at some angle with respect to the groove direction. The direction of this tilt is governed by the broken mirror symmetry at the DNA surface and must be a result (19) Raszewski, Z.; Rutkowska, J.; Kedzierski, J.; Zielinski, J.; Perkowski, P.; Piecek, W.; Zmija, J. Mol. Cryst. Liq. Cryst. 1994, 251, 357–365. (20) Dunmur, D. A.; Manterfield, M. R.; Miller, W. H.; Dunleavy, J. K. Mol. Cryst. Liq. Cryst. 1994, 45, 127–144. (21) At T - TNI ) -7 °C. Coles, H. J. In Optics of Thermotropic Liquid Crystals; Elston, S., Sambles R., Eds.; Taylor and Francis: London, 1998; p 82. (22) Ratna, B. R.; Shashidhar, R. Mol. Cryst. Liq. Cryst. 1977, 42, 113–125. (23) Computed with GAMESS. See Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347-1363. (24) Merck, Product Information (1986). (25) Adamski, P. Zesz. Nauk. Politech. Lodz., Fiz. 1994, 13, 17–25. (26) Sprokel, G. J. Mol. Cryst. Liq. Cryst. 1973, 22, 249–260. (27) At T - TNI ) -7 °C. Haller, I.; Huggins, H. A.; Freiser, M. J. Mol. Cryst. Liq. Cryst. 1972, 16, 53–59.
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of the detailed LC-DNA molecular coupling. A different situation may be envisaged for MBBA. The delocalization of the dipole and the strong polarizability of the molecule favor in this case the maximum contact between the dipolar structure of DNA and the central part of the MBBA molecular body, where electrons can accumulate to minimize the interaction energy with the phosphate chain. This is also favored by the fact that the resonant double carbon bonds of MBBA extend over a large fraction of the molecule whereas the terminal aliphatic chains are shortest among the tested LC compounds. Accordingly, MBBA molecules could lay along the phosphate chains, with some minor adjustment to optimize packing. This orientation would also be obtained as a response to groove roughness. Figure 5 shows schematically, but to scale, some of the possibile motifs of the dressing of the B-DNA duplex with LC molecules. These sketches, in which 8CB and MBBA are drawn according to their measured orientations relative to the duplex axis (the shear direction), show how, for example, the MBBA would fit into the DNA major groove and the 8CB molecules would span the major and minor grooves (gray) if the 8CB CN dipoles are on the surface in the vicinity of the phosphate bands (magenta). In conclusion, we have presented evidence of significant chiral orientational effect in the coupling of liquid crystals to a layer of aligned double-stranded DNA. We understand this surface chiral effect to result from the strong sensitivity of the tenuous liquid crystalline order to the presence of geometrical, chemical, and electrical surface structures on the nanoscale. Data reported here support the potential of using liquid crystals as amplifiers of surface properties not only through the homeotropic/planar transition exploited to detect biochemical processes9 but also via azimuthal coupling to the fine ordering features of molecules on the surface. This may open a new possibility for the readout of
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Figure 5. Sketches superimposing possible arrangements of 8CB and MBBA molecules on B-DNA strands. On the DNA, the grooves and polyphosphate chains are indicated by gray and magenta bands, respectively. Outlines of the 8CB and MBBA molecules are drawn to the same scale. The weakly dipolar MBBA orients nearly parallel to the DNA grooves whereas 8CB, which has strong localized dipoles, orients parallel to the surface but nearly normal to the grooves. The orientations of the LC molecular long axes are taken from the data of Table 1.
DNA arrays or for other technologies in which the sensing of duplex DNA is involved. Acknowledgment. This work was supported by a Cariplo Foundation grant (T.B., M.B. and G.Z), NSF grant DMR 0606528 (N.A.C. and M.N.), and NSF MRSEC grant DMR 0213819 (N.A.C.). LA800639X
