DNA-templated assembly of nanoscale architectures for

next-generation electronic devices

Sarmiza Elena Stanca,aRamon Eritja

band Donald Fitzmaurice*

a

Received 15th June 2005, Accepted 14th July 2005First published as an Advance Article on the web 7th September 2005DOI: 10.1039/b508471g

We report the assembly and structural characterization of a Y-shaped DNA template

incorporating a central biotin moiety. We also report that this template may be used to

assemble nanoscale architectures, which demonstrate the potential of this and related

approaches to the fabrication of next-generation electronic devices. Of particular

significance is the finding that it is possible to selectively metallize the above DNA

template to obtain a three-electrode configuration. Also of particular significance is the

finding that a biotin modified nanoparticle will recognize and bind selectively the central

biotin moiety of the same template, once functionalized by the protein streptavidin.

1. Introduction

The demand for integrated circuits that will allow information be processed at even faster speedsremains undiminished. This is despite the fact that, as a result of miniaturization, the density of thewires and switches that comprise such circuits has doubled every eighteen months giving rise toMoore’s Law.1 While it appears certain that Moore’s Law will hold true until 2016, it is not certainthat it will hold true thereafter for two reasons.2

The first reason is that to build smaller wires and switches using established fabrication andmaterials technologies will require major scientific and technological advances. Specifically, it willrequire the development at great cost of new light sources and process tools; new mask and resistmaterials; and new high and low dielectric constant materials. The second reason is that as wiresand switches become smaller the materials of which they are composed no longer exhibit bulkproperties, but exhibit properties dominated by confinement and surface effects. As a consequencethese wires and switches may exhibit novel characteristics. In other contexts this will represent anopportunity, in this context it will represent a major challenge.There have been two principal responses of the related scientific and engineering communities.

The first response has been to develop alternative fabrication and materials technologies. Thesecond response has been to propose new integrated circuit architectures that can accommodate oreven exploit the novel characteristics exhibited by these smaller wires and switches.When contemplating alternative fabrication technologies, one is attracted to the self-assembly in

solution and self-organization at a conventionally patterned silicon wafer substrate of nanoscalewires and switches. When contemplating alternative materials technologies, one is attracted to theuse of biological molecules as templates and nanoparticles as building blocks.3,4 It is noted, thatthere have been a number of recent reports that have demonstrated the potential of these andrelated approaches.5–9

It is in this context that we have recently reported the DNA templated assembly of a protein-functionalized 10 nm gap electrode from suitably modified gold nanoparticles on a silicon wafer

aDepartment of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland.E-mail: donald.fitzmaurice@ucd.ie; Fax: þ353 1 716 2127

b Instituto de Biologia Molecular de Barcelona, Consejo Superior de Investigaciones Cientıficas, Jordi Girona18-26, E-08034, Barcelona, Spain

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substrate.10 We also reported that the above protein-functionalized electrode is recognized andbound selectively by a suitably modified gold nanoparticle, which is localized in the 10 nm gap.Outlined here is the previously reported assembly and structural characterization of a Y-shaped

DNA template incorporating a central biotin moiety. Also outlined here, is the previously reporteduse of this template to assemble model nanoparticle architectures. Of significance was the findingthat a biotin modified nanoparticle will recognize and bind selectively the central located biotinmoiety previously functionalized by the protein streptavidin.11

Detailed here is the previously unreported use of the above Y-shaped DNA template to assemblenanoscale architectures, which demonstrate the potential of this and related approaches to thefabrication of next-generation electronic devices. Of particular significance is the finding that it ispossible to selectively metallize the above DNA template to obtain a three-electrode configuration.Also of particular significance is the finding that a biotin modified nanoparticle will recognize andbind selectively the central biotin moiety of the same template, once functionalized by the proteinstreptavidin.

2. Experimental

2.1 Preparation of gold nanoparticles

An aqueous dispersion of citrate-stabilized 5 nm diameter gold nanoparticles was prepared asdescribed in detail elsewhere using reagents supplied by Aldrich.12 The optical density of the abovedispersion (0.53 au at 520 nm) was used to estimate the nanoparticle concentration (3.4 � 1014

particles dm�3).An aqueous dispersion of citrate-stabilized 16 nm diameter gold nanoparticles was prepared as

described in detail elsewhere using reagents supplied by Aldrich.13 A volume of this dispersion wassubsequently modified by adsorption of a sub-monolayer of a biotin derivative at the surface of thegold nanoparticles also as described in detail elsewhere. The optical density of the above dispersion(0.49 au at 520 nm) was used to estimate the nanoparticle concentration (3.6 � 1012 particles dm�3).Immediately prior to use an aliquot (20 mL) of the above dispersion was diluted in distilleddeionized water (2 mL) yielding the working nanoparticle concentration (3.6� 1010 particles dm�3).Biotin-labelled and albumin-passivated 5 nm diameter gold nanoparticles were purchased from

Aldrich. The as-received nanoparticles were dispersed in a buffered water–glycerol mixture (1 : 1 byvol., pH 7.4, sodium phosphate). The optical density of the above dispersion (5.3 au at 520 nm) wasused to estimate the nanoparticle concentration (4.0 � 1015 particles dm�3). Immediately prior touse an aliquot (2 mL) of the above dispersion was diluted in distilled deionized water (2 mL) yieldingthe working nanoparticle concentration (4.0 � 1012 particles dm�3).A stable aqueous dispersion of 4-(dimethylamino)pyridine (DMAP) stabilized gold nanoparticles

was prepared as described in detail elsewhere.14 Briefly, DMAP was transferred from a chloro-formic phase to an aqueous phase containing hydrochloroauric acid. Subsequent reduction, by theaddition of sodium borohydride, led to nanoparticle formation. The nanoparticles were charac-terised by TEM and were found to have an average diameter of 3.6 nm and polydispersity of 1.10.The nanoparticle concentration of the as prepared dispersion was 17.6 mM. Prior to use, 10 mL ofthe above dispersion was diluted in 1 mL of water yielding a nanoparticle concentration of 176 nMat pH 8.0.

2.2 Synthesis of oligonucleotides

The oligonucleotides comprising the DNA template shown in Scheme 1 were synthesised using anApplied Biosystems 392 DNA synthesizer.The oligonucleotide, which acted as the anchoring element (50-thiolhexyl-CGA GTC ATT GAG

TCA TCG AG-30) was prepared on 1 mmol scale using controlled-pore glass (CPG) supports(Applied Biosystems) and standard phosphoramidites (Applied Biosystems) as described in detailelsewhere.15 The above oligonucleotide was deprotected with concentrated ammonia (0.05 M DTT,16 h, 55 1C) to obtain the thiol and desalted on a NAP-10 column.The oligonucleotides, which acted as the two complementary extension elements (50-CTA CGT

CGC TGA CTA CCT GCG TAG GTC CCT AGA TGG CTA ACT CGG TGC ATC GCT CACTGG ATA CAT CAG TCC ATG AAT GAC TCG ATG ACT CAA TGA CTC G-30 and 50-TCA

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TTC ATG GAC TGA TGT ATC CAG TGA GCG ATG CAC CGA GTT AGC CAT CTA GGGACC TAC GCA GGT AGT CAG CGA CGT AGG CAT TGA GCG ATG CAG GCA G-30),were prepared on 0.2 mmol scale using polystyrene (LV200) supports (Applied Biosystems) andstandard and reversed phosphoramidites (Applied Biosystems and Cruachem) as described in detailelsewhere.15

The bridging–recognition element (30-CGT AAC TCG CTA CGT CCG TC-50-heg-bio-bpp-[heg-50-CTG CCT GCA TCG CTC AAT GC-30]2 was prepared on 0.2 mmol scale using polystyrene(LV200) supports (Applied Biosystems) and standard and reversed phosphoramidites (AppliedBiosystems and Cruachem) as described in detail elsewhere.16 Here heg denotes hexaethyleneglycol,bpp denotes –[PO3O(CH2)4CONHCH2]CH2OPO3 and bio denotes biotin–tetraethyleneglycol.Following preparation of the above oligonucleotides, they were released form their supports by

treatment with aqueous ammonia (32% by vol., 55 1C, 16 h). The resulting ammonia solutions wereconcentrated to dryness and the product purified by reverse-phase HPLC. The above oligonucleo-tides were prepared with the last DMT group at the 50 end (DMT-on protocol) to facilitate reverse-phase purification.The conditions for HPLC purification are outlined below. All purified products presented a

major peak, which was collected. Solvent A: 5% acetonitrile in 100 mM triethylammonium acetate(pH 6.5). Solvent B: 70% acetonitrile in 100 mM triethylammonium acetate (pH 6.5). Columns:PRP-1 (Hamilton), 250 � 10 mm. Flow rate: 3 mL min�1. A 30 min linear gradient from 10–80% B(DMT on), or a 30 min linear gradient from 0 to 50% B (DMT off). Yield: 10–20 OD at 260 nmafter HPLC purification (200 nmol).It should be noted that all the above oligonucleotides were analysed using a PCR primer design

programme and the probabilities for formation of unwanted assemblies (hairpins etc.) were foundto be small.

2.3 Streptavidin preparation and characterization

Streptavidin (Aldrich, lyophilized powder, essentially salt free, approx.14 unit mg�1, proteinmolecular weight approximately 60 kDa) was dissolved as received in distilled deionized water(1.0 mg mL�1). The activity of the resulting solution was determined by a spectrophotometrictitration (1.15 � 10�6 mol dm�3 streptavidin equivalents).17

2.4 DNA templated assembly of model nanoscale architectures

A solution of bridging–recognition elements and a three-fold excess of pairs of complementaryextension elements was hybridized by heating to 70 1C for 5 min and cooling to room temperatureduring 15 h in buffered aqueous solution (pH 7.4, phosphate, 0.3 M NaCl, 0.01% by wt sodium

Scheme 1 DNA template incorporating anchoring, extension and bridging–recognition elements.

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azide). The resulting DNA template, prepared on 1.0 mm scale and shown in Scheme 1, was purifiedby gel electrophoresis and redisolved in TBE (0.5 M) at a nominal concentration of nmol dm�3.Anchoring elements were adsorbed at the surface of citrate stabilized gold nanoparticles. Briefly,

an aliquot (840 mL, 5� 10�6 mol dm�3) of an aqueous solution of the anchoring elements was addedto an aliquot (2.000 mL, 3.42 � 1014 particles dm�3) of an aqueous dispersion of nanoparticles(5 nm). This implies an approximately ten-fold excess of anchoring elements, assuming that eachanchoring element occupies an area of 20 A2 on the surface of a gold nanoparticle and thatapproximately 400 oligomers are adsorbed at the surface of each nanoparticle.18 An aliquot of anaqueous solution of NaCl (160 mL, 937 mmol dm�3) was added to yield the desired finalconcentration of salt (50 mmol dm�3). The resulting dispersion was stored in the fridge untilrequired for use.The above DNA template solution (10 mL, 30 nmol dm�3) and the anchoring element modified

gold nanoparticle dispersion (2.372 mL, 2.28 � 1014 particles dm�3) were combined. An aliquot ofan aqueous NaCl solution (343 mL, 397 mmol dm�3) was added to yield the desired finalconcentration (4.97 � 10�9 mol dm�3). The resulting dispersion was allowed stand overnight andstored in the fridge until required for use.The templated assembly was immobilized on carbon-coated copper TEM grid or on a silicon

wafer and exposed to an aliquot (10 mL) of the diluted stock dispersion (10 fmol dm�3) ofstreptavidin (4 h), which is known to have a high affinity for biotin.19 After washing, the templatedassembly, now incorporating a streptavidin at the biotin site, it was exposed to an aliquot (10 mL) ofthe stock dispersions of 5 nm or 16 nm diameter biotin-modified gold nanoparticles (4 h).Alternatively, the templated assembly was immobilized on carbon-coated copper TEM grid or on

a silicon wafer and the gold nanoparticles enlarged by electroless deposition using GoldEnhance(Nanoprobes) for 1 min. The same assembly was then exposed to an aliquot (10 mL) of the dilutedstock dispersion (10 fmol dm�3) of streptavidin (4 h). The templated assembly, now incorporating astreptavidin at the biotin site, was subsequently exposed to an aliquot (10 mL) of the stockdispersion 5 nm diameter biotin-modified gold nanoparticles (4 h).

2.5 DNA templated assembly of single-electron transistor architecture

A solution of bridging–recognition elements and a three-fold excess of pairs of complementaryextension elements and anchoring elements was hybridized by heating to 70 1C during 5 min andcooling to room temperature during 15 h in buffered aqueous solution (pH 7.4, phosphate, 0.3 MNaCl, 0.01% by wt sodium azide). The resulting DNA template, prepared on 1.0 mm scale, waspurified by gel electrophoresis and redisolved in TBE (0.5 M) at a nominal concentration of 30 nmoldm�3. An aliquot of an aqueous NaCl solution (343 mL, 397 mmol dm�3) was added to the aboveDNA template solution (10 mL, 30 nmol dm�3) to yield the desired final concentration (4.97 � 10�9

mol dm�3). The resulting dispersion was allowed stand overnight and stored in the fridge untilrequired for use.Samples were prepared by placing a drop of the assembled and purified DNA template (10 mL of

the 5 nM stock solution) on a polylysine modified carbon coated copper grid. Any excess solutionwas removed from the grid by wicking after 2 min. The above grid was placed on a filter paper,washed with a drop (20 mL) of distilled-deionised water (18.2 MO cm) and dried in air.The DNA template immobilized on the carbon-coated copper grid was partially metallized with

DMAP-modified gold nanoparticles. A drop of the DMAP-modified gold nanoparticle dispersion(10 mL of the 176 nM stock solution at pH 8.0) was deposited on the carbon-coated grid bearing thetemplate. The excess gold nanoparticle dispersion was removed by wicking after 1 min. The abovegrid was placed on a filter paper, washed with a drop (20 mL) of distilled-deionised water (18.2 MOcm) and dried in air.The DMAP-modified gold nanoparticles were displaced from the biotin moiety located in the

middle of the thiol-DNA-biotin template by adsorption of a streptavidin. The carbon coated coppergrid bearing the partially metallized template was placed on a clean silicon wafer. A drop (20 ml) ofthe streptavidin dispersion (0.01 mM in protein stock solution) was deposited on the grid. Thesilicon wafer, bearing the TEM grid, was then placed in a closed container for 4 h in order to slowsolvent evaporation of the streptavidin dispersion. The TEM grid was placed on a filter paper,

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washed with a drop (20 mL) of distilled deionised water (18.2 MO cm) and dried in air. Thestreptavidin bound to the centrally located biotin moiety was visualized using a uranyl aciacetate.To complete metallization of the DNA, Goldenhancet solution was used in order to enlarge and

enjoin the DMAP-modified gold nanoparticles. The enhancement procedure followed was thatgiven by Nanoprobest (Cat. No. 2113). An incubation time of 1 min was employed. TEM grid wasplaced on a filter paper, washed with a drop (20 mL) of distilled-deionised water (18.2 MO cm) anddried in air.Finally, the metallized thiol-DNA-biotin template was exposed to a dispersion of biotin-modified

gold nanoparticles. As before the TEM grid was placed on a clean silicon wafer and a drop (20 mL)of the biotin modified gold nanoparticle dispersion (16 nm diameter, 0.36 nM in particle) was placedon the grid, completely wetting it. The silicon wafer with the carbon grid was placed in a closedcontainer in order to slow solvent evaporation. After 4 h the TEM grid was placed on a filter paper,washed with a drop (20 mL) of distilled-deionised water (18.2 MO cm) and dried in air.

2.6 Transmission electron microscopy

Samples were deposited on treated carbon-coated 400 mesh copper grids. These grids hadpreviously been treated by depositing a drop of polylysine solution (5 mg mL�1) on the carbonfilm to increase the hydrophilicty of the surface and to improve DNA binding. All transmissionelectron micrographs (TEMs) were obtained using a JEOL 2000 FX TEMSCAN operating at anacceleration voltage of 80 kV.In order to image the DNA components of the assembled nanoscale architectures, a drop (aprox.

50 mL) of an aqueous uranyl acetate solution (1%) was placed on a Teflon surface.20 A carbon-coated copper grid was immersed sample-side down in the above drop (20 s) and any excess solutionremoved by wicking using filter paper. Finally, the grid was washed thoroughly with distilled-deionised water and dried in air.

2.7 Atomic force microscopy

Samples were deposited on n-type silicon wafers (2–4 O cm). These wafers had previously beenheated to 350 1C in air for 4 h, cleaned using a piranha solution (70% H2SO4, 30% H2O2) andstored under ethanol until required for use. Immediately prior to use they were cleaned again usingoxygen plasma (30 W, 50 Hz, 0.2–0.8 mbar O2) for 5 min. All atomic force micrographs (AFMs)were obtained using a Digital Instrument’s Multimodet Nanoscope III instrument equipped withtapping mode etched silicon tips (Olympus Model OTESPA). Images were processed usingNanoscope Image Software (Ver. 4.23r6).

2.8 UV-visible spectroscopy

UV-visible spectra were recorded using a HP8452A diode array spectrophotometer and quartzcuvettes (1 cm).

3. Results and discussion

The three-branched DNA template shown in Scheme 1 is assembled from three oligonucleotidesreferred to as anchoring elements (20 bp), which have been modified to incorporate a thiol group atthe terminal 50-position. It is also assembled from a complex Y-shaped oligonucleotide, referred toas a bridging–recognition element (3 � 20 bp), which consists of three identical oligomers connectedby a hexaethyleneglycol linker. Between each anchoring element and each branch of the bridging–recognition element there are two complementary oligonucleotides collectively referred to asextension elements (2 � 100 bp), which determine the length of the DNA backbone comprisingeach arm of the Y-shaped template. In this case the length of each arm is expected to beapproximately 45 nm.The template was first assembled and then deposited on a carbon-coated copper grid or on a

silicon wafer for imaging by TEM and AFM, respectively. The template deposited on the coppergrid was stained using an uranyl acetate solution. The resulting images, shown in Fig. 1, clearlyshow a Y-shaped DNA template with each branch the expected 45 nm in length. It should be noted

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that because the persistence length of double-stranded greater than 45 nm the arm of the Y-shapedtemplate are generally straight.21 It should also be noted, however, that because the bridgingelement is flexible,16 the relative orientations of the branches comprising the DNA template are notfixed and do vary from template to template. Parenthetically, some flexibility is not necessarily adisadvantage (for entropic reasons) when considering the subsequent organization of thesetemplates at patterned substrates.Shown in Fig. 2a is a TEM image of the nanoscale architecture depicted in Scheme 2, consisting

of three 5 nm diameter gold nanoparticles attached to the thiol groups incorporated in the anchoringelements. It also consists of a central 5 nm diameter biotin-modified gold nanoparticle, which boundto the streptavidin protein incorporated in to the bridging–recognition element of the template. Ofparticular significance is the finding that a biotin modified nanoparticle will recognize and bindselectively the central biotin moiety, once functionalized by the protein streptavidin. The corre-sponding AFM image, shown in Fig. 2b, clearly shows four nanoparticles arranged as expected. Itshould be noted that the apparent diameters of these nanoparticles are larger than 5 nm due to thefinite dimensions of the probe tip.Clearly, the nanoscale architecture shown in Scheme 2 has been assembled using the template

shown in Scheme 1. It is estimated that the overall efficiency of the assembly process, for a 1 mMscale preparation, is again 25% based on the assumptions that the assembly of the three terminalparticles is as efficient as above and that the recognition directed assembly of the fourth particle isan efficient process. Building on this capability, a number of variations on this basic architecture

Fig. 2 (a) TEM image of the nanoscale architecture in Scheme 2 incorporating three citrate-stabilized goldnanoparticles (5 nm) and a central biotin-modified and albumin-passivated gold nanoparticle (5 nm),immobilized on a polylysine-modified carbon-coated copper grid. (b) AFM image of the nanoscale architecturein (a), immobilized on a plasma-treated silicon wafer.

Fig. 1 (a) TEM image of the DNA template shown in Scheme 1, immobilized on a polylysine modified carbon-coated copper grid and stained with uranyl acetate 1%. (b) AFM image of the DNA Template in (a) immobilized(unstained) on a plasma-treated silicon wafer.

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have been assembled using the same template, by using different nanoparticles and by enlargingsome of the nanoparticles by electroless deposition.Shown in Fig. 3 are a number of related nanoscale architectures, which have been assembled

using the DNA template shown in Scheme 1. The principal difference in the case of the nanoscalearchitecture shown in Scheme 3 and Fig. 3a, is that the biotin labeled gold nanoparticle thatrecognizes and binds selectively the streptavidin is 16 nm in diameter. In the case of the nanoparticlearchitecture shown in Scheme 4 and Fig. 3b, the principal difference is that the 5 nm diameter goldnanoparticles linked to the anchoring elements are enlarged by electroless deposition. Of particularsignificance is the similarity of the nanoparticle architecture in Fig. 3b to a three electrode singleparticle device, a key milestone in the bottom-up assembly and organization of nanoscale electronicdevices.

Fig. 3 (a) TEM image of the nanoscale architecture in Scheme 3 incorporating three citrate-stabilized goldnanoparticles (5 nm) and a central biotin-modified gold nanoparticle (16 nm), immobilized on a polylysine-modified carbon-coated copper grid. (b) TEM image of the nanoscale architecture in Scheme 4 incorporatingthree gold-enhanced citrate-stabilized gold nanoparticles (initially 5 nm) and a central biotin-modified andalbumin-passivated gold nanoparticle (5 nm), immobilized on a polylysine-modified carbon-coated copper grid.

Scheme 2 Nanoscale architecture assembled by coupling three citrate-stabilized gold nanoparticles (5 nm) tothe anchoring elements and by binding one biotin-modified and albumin-passivated gold nanoparticle (5 nm) tothe biotin–streptavidin-modified bridging–recognition Element of the DNA Template in Scheme 1.

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Accordingly, we have used the DNA template shown in Scheme 1 to template the assembly ofDMAP stabilized gold nanoparticles. These nanoparticles, which are positively charged, areselectively adsorbed at the negatively charged backbone of the DNA template.22 As a consequence,they organize in the Y-shaped pattern shown in Fig. 4a. Electroless deposition of gold enlarges andenjoins these nanoparticles as shown in Fig. 4b to yield a Y-shaped configuration of goldnanowires.22

If the partially metallized DNA templates shown in Fig. 4a is exposed to a dispersion ofstreptavidin, this tetrameric protein recognizes and binds selectively the biotin moiety incorporatedin the bridging recognition element. As a consequence the gold nanoparticles adsorbed at the

Scheme 3 Nanoscale architecture formed by coupling three citrate-stabilized gold nanoparticles (5 nm) to theanchoring elements and by binding one citrate-stabilized and biotin-modified gold nanoparticle (16 nm) to thebiotin–streptavidin-modified bridging–recognition element of the DNA template in Scheme 1.

Scheme 4 Nanoscale architecture formed by coupling three citrate-stabilized gold nanoparticles (5 nm) to theanchoring elements, by enhancing these nanoparticles using electroless deposition of gold and by binding onebiotin-modified and albumin-passivated gold nanoparticle (5 nm) to the biotin–streptavidin-modified bridging–recognition element of the DNA template in Scheme 1.

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centrally located bridging–recognition element are displaced opening a gap as shown in Fig. 5a.That there is a streptavidin located in this gap is confirmed by staining with uranyl acetate, whichclearly visualizes this protein in Fig. 5b.Taking the nanoscale architecture in Fig. 5b and enhancing and enjoining the gold nanoparticles

by electroless deposition we obtain the nanoscale architecture shown in Fig. 6a. Subsequentexposure to a dispersion of biotin-modified 16 nm diameter gold nanoparticles, results in theseparticles recognizing and binding selectively the streptavidin in the gap. As a consequence therenanoparticles are localized in the gap as shown in Fig. 6b.In short, the DNA template in Scheme 1 has been used to assemble the nanoscale architecture

shown in Scheme 5. This nanoscale architecture is that of a single electron transistor. Specifically, itconsists of three proximal nanowires, corresponding to a source–gate–drain architecture, and of acentrally located nanoparticle.

Fig. 4 (a) TEM image of the DNA template in Scheme 1 that has been recognized and bound selectively byDMAP-stabilized gold nanoparticles (4 nm). (b) As in (a) following enhancement of the gold nanoparticles byelectroless deposition of gold.

Fig. 5 (a) TEM image of the DNA template in Scheme 1 that has been recognized and bound selectively byDMAP-stabilized gold nanoparticles (4 nm) and subsequently treated with a dispersion of streptavidin todisplace the nanoparticles adsorbed at the centrally-located biotin moiety of the bridging–recognition element. (b)As in (a) following staining with uranyl acetate 1% to visualize the bound streptavidin.

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4. Conclusions

Here we have reported the assembly and structural characterization of a Y-shaped DNA templateincorporating a central biotin moiety. We have also reported that this template may be used toassemble nanoscale architectures, which demonstrate the potential of this and related approaches tothe fabrication of next-generation electronic devices. Of particular significance is the finding that ithas been possible to selectively metallize the above DNA template to obtain a three-electrodeconfiguration. Also of particular significance is the finding that a biotin modified nanoparticlerecognizes and binds selectively the central biotin moiety of the same template, once functionalizedby the protein streptavidin.

Fig. 6 (a) TEM image of the DNA template in Scheme 1 that has been recognized and bound selectively byDMAP-stabilized gold nanoparticles (4 nm), subsequently treated with a dispersion of streptavidin to displacethe nanoparticles adsorbed at the centrally-located biotin, and enhanced by electroless deposition of gold toenlarge and enjoin the adsorbed nanoparticles. (b) As in (a) following exposure to a dispersion of biotin-modifiedgold nanoparticles (16 nm) which recognise and bind selectively the centrally-located biotin–streptavidin moietyof the bridging–recognition element to yield the nanoscale architecture in Scheme 5.

Scheme 5 Nanoscale architecture formed by binding of DMAP-stabilized gold nanoparticles (4 nm) to theextension elements, by their enhancement using electroless deposition to form a continuous nanowire and bybinding a citrate-stabilized and biotin-modified gold nanoparticle (16 nm) to the biotin–streptavidin-modifiedbridging–recognition element of the DNA template in Scheme 1.

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Acknowledgements

This study was supported by Science Foundation Ireland through the Centre for Research onAdaptive Nanostructures and Nanodevices.

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