This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 138.253.100.121

This content was downloaded on 15/11/2013 at 15:36

Please note that terms and conditions apply.

Self-assembling of large ordered DNA arrays using superhydrophobic patterned surfaces

View the table of contents for this issue, or go to the journal homepage for more

2013 Nanotechnology 24 495302

(http://iopscience.iop.org/0957-4484/24/49/495302)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 24 (2013) 495302 (7pp) doi:10.1088/0957-4484/24/49/495302

Self-assembling of large ordered DNAarrays using superhydrophobic patternedsurfacesG Ciasca1, L Businaro2, M Papi1, A Notargiacomo2, M Chiarpotto1,A De Ninno2, V Palmieri1, S Carta2, E Giovine2, A Gerardino2 andM De Spirito1

1 Istituto di Fisica, Universita Cattolica SC, L.go Francesco Vito 1 I-00168, Roma, Italy2 Istituto di Fotonica e Nanotecnologie—CNR, Via Cineto Romano 42, Roma, I-00156, Italy

E-mail: m.papi@rm.unicatt.it

Received 13 August 2013, in final form 22 September 2013Published 14 November 2013Online at stacks.iop.org/Nano/24/495302

AbstractIn this paper we present a simple and robust method to realize highly ordered arrays ofstretched and suspended DNA molecules over the millimeter length scale. To this end we usedan ad hoc designed superhydrophobic surface made of high aspect-ratio silicon pillars, wherewe deposited a droplet containing genomic DNA. A precise positioning of DNA strands wasachieved by shaping the silicon pillars so that sharpened features resembling tips wereincluded. Such features allowed us to accurately control the droplet de-wetting dynamics,pinning DNA strands in a well-defined position above pillars. The proposed technique has thepotential to positively impact on the development of novel DNA chips for genetic analysis.

(Some figures may appear in colour only in the online journal)

1. Introduction

Stretching DNA molecules at the molecular level is apromising tool for a wide range of genetic studies hardlyaccessible by other techniques, such as the detection andprecise measurement of micro-deletions and micro-insertionsin genes [1]. In this context, the fabrication of large orderedstretched DNA arrays will open the way to the realizationof highly automated analysis, providing the basis for thenext-generation DNA chips [2].

Beside DNA chips, practical interest in patterning DNA isalso related to its self-assembly properties which allow one toobtain ad hoc shaped DNA superstructures with a bottom-upapproach [3–5]. Thanks to its high aspect ratio, DNA isalso an attractive candidate for the realization of highlyaligned 1D nanostructures showing the potential to addresschallenges in electronics [6, 7] and in magnetic [8] andchemical sensing [9]. For these purposes, the development ofnovel strategies for the precise positioning of DNA moleculesin highly ordered structures is of paramount importance.

Superhydrophobic patterned surfaces have recentlyattracted much attention due to their biological applicationswhich include, to name a few, protein concentration [10–12],the development of wet sample handling techniques for x-raymeasurements [12, 13] and the search for and validation ofnovel biomarkers for cancer detection [14, 15].

Recently, the pioneering studies of Su et al havedemonstrated the possibility to realize large ordered arrays ofsuspended nanowires made of small organic compounds usingsuperhydrophobic patterned surfaces [15, 16].

In this paper we provide a simple and robust method torealize highly ordered arrays of suspended and stretched DNAstrands. To this end, we deposited a small sample solutiondroplet containing genomic DNA on a superhydrophobicpatterned surface wetted in the Cassie–Baxter state. Thispatterned surface—designed to contain ad hoc shaped pillarswith sharpened features—allowed us to control the dropletde-wetting dynamics and, therefore, to achieve a precisepositioning of each DNA strand over the millimeter lengthscale.

10957-4484/13/495302+07$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 495302 G Ciasca et al

2. Materials and methods

2.1. DNA isolation

Genomic DNA isolation was performed according to thestandard phenol/chloroform protocol. To a 1 ml blood sample(EDTA, heparin, citrate), 0.8 ml of 1X SSC buffer wasadded and mixed. The sample was centrifuged for 1 minat 12 000 rpm and the top portion of the supernatant wasdiscarded. Then 1 ml of 1X SSC buffer was added, thesample was vortexed, centrifuged as above for 1 min, and thesupernatant was removed. 375 µl of 0.2 M NaOAc was addedto the pellet and vortexed briefly. Then 25 µl of 10% SDS and5 µl of proteinase K (20 mg ml−1 in H2O) (Sigma P-0390)were added, vortexed briefly and incubated for 4 h at 55 ◦C.120 µl of phenol/chloroform/isoamyl alcohol was added andvortexed for 30 s. The sample was centrifuged for 2 minat 12 000 rpm in a microcentrifuge tube. The aqueous layerwas removed and transferred to a new 1.5 ml microcentrifugetube, and 1 ml of cold 100% ethanol was added, mixed,and incubated for 15 min at −20 ◦C. Then the samples werecentrifuged for 2 min at 12 000 rpm in a microcentrifuge. Thesupernatant was decanted and drained. 180 µl of 10:1 TEbuffer was added to the sample, vortexed, and the sample wasincubated at 55 ◦C for 10 min.

20 µl of sodium acetate (2 M) was added and mixed tothe sample. Then 500 µl of cold 100% ethanol was addedand centrifuged for 1 min at 12 000 rpm in a microcentrifuge.The supernatant was decanted and the pellet was rinsedwith 1 ml of 80% ethanol. After 1 min of centrifugation at12 000 rpm in a microcentrifuge, the supernatant was decantedand the pellet was dried. The pellet was resuspended byadding 200 µl of 10:1 TE buffer. After incubation overnightat 55 ◦C, the sample was vortexed periodically to dissolvethe genomic DNA; the DNA concentration was assayed byspectrophotometer (260 nm) and was 150 ng ml−1.

2.2. Fabrication of superhydrophobic surfaces

Superhydrophobic surfaces were fabricated directly onsilicon substrate using electron beam lithography (VistecEPBG-5HR, acceleration voltage: 100 keV) and inductivecoupled plasma (ICP) Si etching.

Different pillar geometries, namely square-shaped pillarsand tooth-shaped pillars, were tested in order to provide theoptimal design in terms of DNA strand positioning. For theperiodic array of square-shaped pillars, a 1.2 µm thick layerof Shipley UVIII positive tone electronic resist was spun onthe silicon top layer, exposed with a dose of 25 µC cm−2

and developed. A 30 nm thick chromium (Cr) film was thendeposited by an e-gun assisted evaporator (Balzer 510) andlifted off to define the device pattern: an array of pillars(5 µm side; 14 µm pitch). The device pattern was thentransferred to the substrate by means of a two-step ICPSi etching (first step: protection, pressure: 100 mTorr, Ar:30 sccm, C4F8: 187 sccm, ICP: 600 W, t: 2 s; second step:etching, pressure: 100 mTorr, Ar: 100 sccm, SF6: 90 sccm,cathode bias: 70 V, ICP: 500 W, t: 10 s). For the tooth-shaped

pillars a similar procedure was adopted. A 400 nm thicklayer of polymethyl methacrylate (PMMA) was spun on asilicon substrate, exposed with a dose of 600 µC cm−2 anddeveloped. A 30 nm thick Cr film was then deposited byan e-gun assisted evaporator (Balzer 510) and lifted off. Thedevice pattern was transferred to the substrate by the same ICPetching step.

After cleaning in piranha solution (H2SO4/H2O2 = 3:1),the microstructured Si wafers were silanized to impart thesuperhydrophobic behavior with 10% trimethylchlorosilane(TMCS, (CH3)3SiCl, ≥99.0% FluKa, purchased by SigmaAldrich (CAS number 75-77-4)), in toluene in N2 environ-ment for 48 h, followed by a rinse in methanol and n-heptane.

2.3. Design of a superhydrophobic surface for the realizationof suspended ordered DNA arrays

A rough surface can be wetted in two modes, the Wenzelmode and the Cassie–Baxter mode [10]. In the first case, theliquid wets the entirety of the rough surface, thus increasingthe interfacial contact area. Conversely, in the Cassie–Baxtermode, the liquid bridges between surface features withoutpenetrating the spaces separating them (figure 1(a)). Theapparent contact angle of the drop in the first configurationis given by the Wenzel formula,

cos θw= r cos θe, (1)

where θw is the predicted Wenzel contact angle and r is theWenzel roughness factor, which is defined as the ratio betweenthe actual wetted surface and the flat geometrical surface.

The apparent contact angle of the drop in the latterconfiguration can be expressed by the Cassie–Baxter formula,

cos θCB= f1 cos θe

− f2, (2)

where θe and θCB are, respectively, the measured contactangle on a smooth surface and the predicted Cassie–Baxtercontact angle on the corresponding rough surface, while f1 isthe total area of solid under the drop per unit projected areaand f2 is defined in an analogous way, with material 2 as air(θ2 = 180◦). An explicit expression for the parameters f1, f2and r is given in figure 1(a) for square pillars, which is one ofthe relevant configurations in this paper. However, an explicitexpression for other geometries is given in [10].

By equating the right-hand sides of equations (1)and (2), a general stability criterion to determine whetherthe Cassie–Baxter or Wenzel mode is favorable can bederived [10],

cos θCR =−f2

r − f1. (3)

According to equation (3), if the critical intrinsic contactangle (θCR) is less than θe, Cassie–Baxter wetting is favorable;otherwise the Wenzel mode is favorable.

In figure 1(b) we show the predicted contact angle for aperiodic array of square pillars (figure 1(c)) as a function ofboth the aspect ratio and the post spacing-to-width ratio. Thecolor scale indicates—according to equation (3)—whether thecalculation has been performed with the Cassie–Baxter (gray)

2

Nanotechnology 24 (2013) 495302 G Ciasca et al

Figure 1. (a) Schematic view of a rough textured surface wetted in the Wenzel state (left) and in the Cassie–Baxter state (left); (b) predictedcontact angle for a periodic array of square pillars as a function of the aspect ratio (h/a) and the post spacing-to-width ratio (b/a). The colorscale indicates whether the calculation has been performed with the Cassie–Baxter (gray) or Wenzel equation (white). The red filled dothighlights the set of parameters adopted in this work. (c) Two 5 µl drops deposited on a flat hydrophobic silicon surface (on the left) and onthe superhydrophobic patterned surface (on the right).

or Wenzel equation (white). Calculations do not take intoaccount the contribution of capillary pressure which inhibitsthe Wenzel-to-Cassie transition giving rise to a metastableCassie–Baxter state [17].

We realized 22 µm height, 5 µm wide and 9 µm spacedpillars, which correspond to an aspect ratio of 4.4 and aspacing-to-width ratio of 1.8. As shown in figure 1(b) (redspot), using this set of parameters, the patterned surfacesare wetted in the Cassie–Baxter mode, which is preferablefor our purpose to the Wenzel mode, because it allowsthe self-assembling of suspended structures between pillars[14, 15]. In figure 1(c) we show two 5 µl drops deposited onflat silicon (on the left) and on the superhydrophobic patternedsurface (on the right). The latter displays a contact angle ofabout 147◦ ± 5◦, which is consistent with the theoretical valuecalculated in figure 1(b) within two standard deviations.

3. Results

3.1. Large ordered array of suspended DNA filaments

Figure 2(a) shows stretched DNA molecules obtainedby the deposition of a 5 µl genomic DNA solutionon a superhydrophobic patterned surface wetted in theCassie–Baxter mode.

One can observe a few sparse oriented DNA filamentsprecisely positioned on the top edges between adjacent pillars.Stable and suspended DNA strands are obtained thanks to the

de-wetting dynamics induced by the micro-patterned surface,which does not allow the deposited droplet to penetratethe spaces between pillars. Under evaporation conditions,indeed, the retracting drop edge at the three phase contactline (i.e. at the water/air/silicon interface) stretches the DNAfilaments along the de-wetting direction. The same strategyhas been recently used by Gentile et al to obtain thinand stable suspended DNA filaments, which are particularlysuited to acquire high resolution TEM images of thin DNAbundles [18, 19].

Figure 2(b) shows stretched DNA molecules obtained bythe deposition of a 5 µl genomic DNA droplet on a tiltedsuperhydrophobic surface. A tilting angle of 15◦, which is farbelow the rolling-off angle of our superhydrophobic surface,has been adopted. As recently demonstrated in the work ofSu et al focused on small organic molecules [15], in thiscondition the sample solution droplet is pinned to the surfaceand, during its shrinking, slowly slips downwards allowing thedeposition of highly oriented 1D structures.

An enlarged detail of the DNA array (figure 3(a))shows a remarkable periodicity of the filaments, which aresystematically anchored to the same pillar positions. Thisphenomenon is due to the pillar shape, designed to providesharpened protrusions, resembling tips. Recent experimentalresults have shown that the self-assembling of highly ordered1D structures on superhydrophobic surfaces is driven bythe formation of liquid bridges between pillars, whosestability depends on the balance between capillary forces

3

Nanotechnology 24 (2013) 495302 G Ciasca et al

Figure 2. (a) Stretched DNA strands obtained by the deposition of a 5 µl genomic DNA solution on a superhydrophobic patterned surfacewetted in the Cassie–Baxter mode (tilting angle 0◦). The scale bar corresponds to a length of 5 µm. (b) A large ordered array of stretchedand suspended DNA filaments obtained by the deposition of a 5 µl genomic DNA droplet on a tilted superhydrophobic surface (tilting angleof 15◦). The scale bar corresponds to a length of 10 µm.

Figure 3. (a) Enlarged detail of the DNA array shown infigure 2(b). The scale bar corresponds to a length of 5 µm.A remarkable periodicity of the filaments, which are systematicallyanchored to the same pillar positions, can be clearly observed.(b) Top view SEM micrograph showing an enlarged DNA strandwith a diameter of about 35 ± 5 nm. The SEM micrograph wasacquired after the deposition of a 6 nm thin Cr layer in order toavoid charging effects and to protect the DNA filament from e-beamdamage.

and structural cohesive forces in highly viscous liquids. Theformer usually tend to break up the filaments, whereas thelatter favor their formation. At the tip side, the cohesiveforce contribution increases significantly with respect to thecapillary force, guiding the formation of strictly unidirectionalfilaments [15, 16].

The DNA strands in figures 2 and 3 show a regularshape and a fairly monodisperse diameter distribution. Infigure 3(b) we report a typical top view SEM micrographshowing an enlarged DNA strand with a uniform diameteralong the longitudinal axis of about 35 ± 5 nm. The SEMmicrograph was acquired after the deposition of a 6 nm thinCr layer in order to avoid charging effects and to protectthe DNA filament from e-beam damage. Therefore, assuminga uniform Cr coating, the DNA strand diameter measures23 ± 5 nm. The measured size clearly indicates that thesuspended filament is a bundle of several DNA strands.As recently demonstrated by molecular dynamics (MD)

Figure 4. (a) A large ordered array of stretched and suspendedDNA filaments obtained by the deposition of a 5 µl genomic DNAdroplet on a tilted superhydrophobic surface made of tooth-shapedpillars. The scale bar corresponds to a length of 10 µm.(b), (c) SEM micrographs of two enlarged details of suspended andstretched DNA bundles on tooth-shaped pillars. The scale barscorrespond to lengths of 10 µm and 5 µm, respectively.

simulation investigating the aggregation behavior of DNAfilaments under controlled evaporation conditions, capillaryforces are responsible for pushing together filaments formingDNA bundles [19]. The equilibrium separation distance ofdouble helix filaments, as provided by MD simulation, isabout 2.5 nm, suggesting that our DNA bundles could containseveral tens of DNA strands (up to 60).

The effect of the pillar shape has been further analyzed infigure 4(a), where we show an ordered array of DNA strandsrealized on a microstructured surface made of tooth-shapedpillars. The same deposition method as described abovehas been adopted. In this case also, most of the DNA

4

Nanotechnology 24 (2013) 495302 G Ciasca et al

Figure 5. (a) SEM micrograph acquired in the most external regionof the DNA deposit, showing stretched and suspended DNAfilaments between the two halves of the same pillar. The scale barcorresponds to a length of 10 µm. (b) enlarged detail of a thin DNAbundle.

strands are pinned to triangular shaped protrusions, bothoutside and inside the tooth-shaped pillars. This result furtherconfirms that sharpened microstructured protrusions—almostregardless of their shape—can be an effective tool to preciselylocalize DNA filaments in well-defined positions. Figures 4(b)and (c) show two enlarged details of suspended and stretchedDNA bundles on tooth-shaped pillars. It worth stressing herethat the distance between adjacent pillars ranges between10 and 7 µm, depending on whether one considers thepillar–pillar distance or the tip–tip distance, respectively.The distance between the two halves of the same pillarranges between 4 and 2 µm. One can observe that—in bothcases—a bundle suspended between two different pillars issignificantly thinner than one suspended between the twohalves of the same pillar. This result suggests that theprotrusion distance might play a role in determining thebundle size: the larger the distance between microstructuredprotrusions, the thinner the size of the DNA bundle. However,together with inter-protrusion distances, other factors seemto play a role in determining the bundle size, such as theposition of the filament with the respect to the drop center. Infigure 5(a) an SEM micrograph acquired in the most externalregion of the DNA deposit—i.e. as far as possible from thedrop center—is shown. As can be clearly noticed, very thinDNA bundles are stretched and suspended between the twohalves of the same pillars, whereas DNA bundles betweenadjacent pillars are missing. In figure 5(b) an enlarged detailof a thin DNA bundle, with a diameter of less than 10 nm,is shown. This finding suggests that the larger the distancefrom the drop center the thinner the DNA bundle size. Theabove mentioned results might help in controlling the bundlesize, i.e. the number of DNA filaments per bundle, andtherefore deserve a more in-depth study. This could be highlyinteresting since many applications require the stretching ofsingle DNA filaments [1].

3.2. E-beam manipulation of DNA strands

As stated before, patterning of DNA molecules in orderedarrays has several applications in the field of DNA chips and in

Figure 6. (a) Five typical DNA strands suspended betweenadjacent square pillars. The scale bar corresponds to a length of5 µm. (b) Enlarged detail of the central strand, acquired at the sameacceleration voltage and at the same scanning rate as those used infigure 6(a). (c), (d) SEM micrographs of the same DNA filamentacquired on increasing the absorbed dose. (e) Selective breakage ofthe central filament upon e-beam illumination. The scale barcorresponds to a length of 5 µm.

the realization of novel electronic devices. To this purpose, notonly precise DNA filament positioning, but also the possibilityto remove unwanted DNA strands, could have a potential rolein the design and fabrication of novel devices.

In figure 6(a), five typical non-metallized DNA strandssuspended between adjacent pillars are shown. Figure 6(b)shows an enlarged detail of the central strand, acquired atthe same acceleration voltage and at the same scanningrate. Figure 6(c) shows an SEM micrograph of the sameDNA filament acquired using a lower scan rate, i.e.increasing the absorbed dose by the filament. As can beclearly observed, the suspended DNA strand undergoes anoscillating motion—resembling that of a typical vibratingrope—which increases with increasing e-beam time exposure(figure 6(d)), and, ultimately, leads to breakage of the filament(figure 6(e)). Figure 6 clearly highlights that radiation damageeffects—induced by controlled illumination of DNA strandswith high electron doses—can be tuned to improve the devicedesign, as they allow one to selectively remove DNA strandslocated in well-defined positions, leaving other filamentsintact.

4. Discussion

In this paper we demonstrate the possibility to realize highlyordered arrays of stretched and suspended DNA filamentsusing advanced functional surfaces. Moreover, we achievefine control over the DNA filament position by introducingsharpened features in our microstructured surfaces.

The technique described in this paper has the potentialto provide a tremendous advance when precise positioning of

5

Nanotechnology 24 (2013) 495302 G Ciasca et al

DNA filaments is highly required, e.g. DNA based electronicdevices. In this context, many experiments have beendesigned to probe the electronic transport properties of DNAmolecules. Unfortunately, as highlighted in the recent work ofChepelianskii and co-workers [7], most of these attempts haveled to controversial results mainly because of the interactionbetween the DNA and the substrate. This problem canbe easily overcome by suspending DNA filaments betweenadjacent pillars as described here. Furthermore, the pinningof DNA to small and precisely positioned protrusions couldaid in the integration of functional DNA filaments intodevices, since the integration process usually starts from thefabrication of metallic electrodes followed by the alignmentof DNA above the electrodes.

Moreover, as stated above, our technique has the potentialto positively impact on the development of novel toolsfor genetic analysis based on stretched DNA filaments.Nowadays, one of the most powerful techniques for stretchingand visualizing single DNA fibers is molecular combing.In this technique, a chemically modified glass coverslipis dipped into a buffered DNA solution. DNA fibers bindto the chemically modified, hydrophobic surface by oneor both of their extremities. Thus, the coverslip is pulledout with constant speed and the receding water meniscusstretches the anchored DNA molecules, applying a constantperpendicular force to them. The above described proceduregives well aligned filaments along a preferential direction [1].However, the filaments are usually unevenly spaced and arenot uniformly distributed. This lack of a strictly orderedconfiguration of DNA filaments could be a serious drawbackof the combing technology as it hampers the possibilityto perform highly automated analysis of a large numberof DNA fibers, impairing the quality and the reliabilityof DNA combing data [20]. The approach described herecould provide significant advances in this direction. Ourmethod indeed allows one to obtain a highly periodicDNA arrangement which is far more ordered than thatprovided by conventional molecular combing. This orderedconfiguration could open the way to the use of highlyautomated analysis, which is currently a difficult task whenusing conventional combing strategies. However, for thispurpose, scaling of the bundle size to the single filamentlevel would be highly desired [20]. As suggested by figures 4and 5, a possible strategy to further decrease the bundlesize could rely on careful design of the inter-pillar distances.However, this aspect requires a more in-depth study. Itworth stressing here that our method—if combined with theconventional molecular combing technique—could offer onemore advantage: a large ordered array of DNA could berealized using an extremely small amount of sample solution(about 5 µl of genomic DNA), thus greatly reducing the costper analysis.

Besides DNA filaments, stretching and suspendingnano-filaments in a controlled way could provide a powerfultool for many applications, such as atomic force microscopystudy of mechanical properties, especially the bendingproperties of biological fiber, e.g. fibrin, cellulose, elastin orsilk [21–25].

Furthermore, this system could be particularly importantfor the study of the polymerization of proteins, which areusually available in a very limited quantity, and the fiberor network formation of all the mechanosensing molecules,such as the Von Willebrand factor, which undergoes dramaticmodification under shear stress [26, 27].

5. Conclusion

We successfully fabricated a large ordered array of stretchedDNA filaments by using superhydrophobic pillar surfaces.The pillar sizes (width, height and pitch) were designedto wet the surface in the Cassie–Baxter mode. Underthis configuration, the deposited droplet containing genomicDNA bridges between surface features without penetratingthe spaces separating them. Under evaporation conditions,the retracting drop edge stretches the DNA filaments,suspending them above pillars. In addition, the pillarswere shaped to contain sharpened features which pin theDNA strands, allowing us to control their position andorientation with a high level of accuracy. A strictly orderedconfiguration was preserved over a considerable lengthscale by tilting the superhydrophobic surface below itsrolling-off angle. In this condition, the sample solution dropletslowly slips downwards, allowing the formation of aligned,one-dimensional DNA filaments.

Acknowledgments

The authors are indebted to Mr Mario Amici for his technicalassistance. The experimental data reported in this paper wereobtained at the LABCEMI (Laboratorio Centralizzato diMicroscopia, Ottica ed Elettronica) of the Universita Cattolicadel S Cuore di Roma (Italy) (www.rm.unicatt.it/).

Conflict of interestThe authors declare that they have no conflict of interest.

References

[1] Michalet X et al 1997 Dynamic molecular combing:stretching the whole human genome for high-resolutionstudies Science 277 1518–23

[2] Guan J and Lee L J 2005 Generating highly ordered DNAnanostrand arrays Proc. Natl Acad. Sci. 20 18321–5

[3] Yan H, Ha Park S, Finkelstein G, Reif J H and LaBean T H2003 DNA-templated self-assembly of protein arrays andhighly conductive nanowires Science 301 1882–4

[4] Liu W, Zhong H, Wang R and Seeman N C 2011 Crystallinetwo-dimensional DNA-origami arrays Angew. Chem. Int.Edn 50 264–7

[5] Zheng J, Birktoft J J, Chen Y, Wang T, Sha R,Constantinou P E, Ginell S L, Mao C and Seeman N C2009 From molecular to macroscopic via the rationaldesign of a self-assembled 3D DNA crystal Nature461 74–7

[6] Braun E, Eichen Y, Sivan U and Ben-Yoseph G 1998DNA-templated assembly and electrode attachment of aconducting silver wire Nature 391 775–8

[7] Chepelianskii A D, Klinov D, Kasumov A, Gueron S,Pietrement O, Lyonnais S and Bouchiat H 2012 Longrange electronic transport in DNA molecules depositedacross a disconnected array of metallic nanoparticles C. R.Physique 13 967–92

6

Nanotechnology 24 (2013) 495302 G Ciasca et al

[8] Hopkins D S, Pekker D, Goldbart P M and Bezryadin A 2005Quantum interference device made by DNA templating ofsuperconducting nanowires Science 308 1762–5

[9] Patolsky F, Zheng G and Lieber C M 2006 Nanowire-basedbiosensors Anal. Chem. 78 4261–9

[10a] Shirtcliffe N J, McHale G, Atherton S and Newton M I 2010An introduction to superhydrophobicity Adv. ColloidInterface Sci. 161 124–38

[10b] Milne A J B and Amirfazli A 2012 The Cassie equation: howit is meant to be used Adv. Colloid Interface Sci. 170 48–55

[11] Ressine A, Auzelyte V, Kristiansson P, Marko-Varga G andLaurell T 2006 Superhydrophobic properties ofnanostructured–microstructured porous silicon forimproved surface-based bioanalysis Nucl. Instrum.Methods Phys. Res. B 249 715–8

[12] Ciasca G et al 2013 Microelectron. Eng. 111 304–9[13] Accardo A, Burghammer M, Di Cola E, Reynolds M,

Di Fabrizio E and Riekel C 2010 Langmuir 26 15057–64[14] Tirinato L et al 2012 SERS analysis on exosomes using

super-hydrophobic surfaces Microelectron. Eng.97 337–40

[15] Su B, Wang S, Ma J, Wu Y, Chen X, Song Y and Jiang L2012 Elaborate positioning of nanowire arrays contributed,by highly adhesive superhydrophobic pillar-structuredsubstrates Adv. Mater. 24 559–64

[16] Su B, Wang S, Wu Y, Chen X, Song Y and Jiang L 2012Small molecular nanowire arrays assisted bysuperhydrophobic pillar-structured surfaces with highAdhesion Adv. Mater. 24 2780–5

[17] Varanasi K K, Deng T, Hsu M F and Bhate N 2008 Design ofsuperhydrophobic surfaces for optimum roll-off anddroplet impact resistace Proc. ASME Int. MechanicalEngineering Congr. and Exposition (Boston, MA,Oct.–Nov.)

[18] De Angelis F et al 2011 Breaking the diffusion limit withsuper-hydrophobic delivery of molecules to plasmonicnanofocusing SERS structures Nature Photon. 5 682–7

[19] Gentile F, Moretti M, Limongi T, Falqui A, Bertoni G,Scarpellini A, Santoriello S, Maragliano L,Proietti Zaccaria R and di Fabrizio E 2012 Direct imagingof DNA fibers: the visage of double helix Nano Lett.12 6453–8

[20] Lebofsky R and Bensimon A 2003 Single DNA moleculeanalysis: applications of molecular combing Brief Funct.Genomic Proteomic. 1 385–96

[21] Piechocka I K, Bacabac R G, Potters M, MacKintosh F C andKoenderink G H 2010 Structural hierarchy governs fibringel mechanics Biophys. J. 9 2281–9

[22] De Spirito M, Arcovito G, Papi M, Rocco M and Ferri F2003 Small- and wide-angle elastic light scattering studyof fibrin structure J. Appl. Crystallogr. 36 636–41

[23] Papi M, Arcovito G, De Spirito M, Amiconi G, Bellelli A andBoumis G 2005 Simultaneous static and dynamic lightscattering approach to the characterization of the differentfibrin gel structures occurring by changing chlorideconcentration Appl. Phys. Lett. 86 183901

[24] Missori M, Mondelli C, De Spirito M, Castellano C,Bicchieri M, Schweins R, Arcovito G, Papi M andCongiu Castellano A 2006 Modifications of themesoscopic structure of cellulose in paper degradationPhys. Rev. Lett. 97 238001

[25] Bracalello A, Santopietro V, Vassalli M, Marletta G,Del Gaudio R, Bochicchio B and Pepe A 2011 Design andproduction of a chimeric resilin-, elastin-, andcollagen-like engineered polypeptide Biomacromolecules12 2957–65

[26] Scaglione G L et al 2013 The type 2B p.R1306W naturalmutation of von Willebrand factor dramatically enhancesthe multimer sensitivity to shear stress J. Thromb.Haemost. 11 1688–98

[27] Papi M, Maulucci G, De Spirito M, Missori M, Arcovito G,Lancellotti S, Di Stasio E, De Cristofaro R and Arcovito A2010 Eur. Biophys. J. 39 1597–603

7