700

Chemistry Letters Vol.33, No.6 (2004)

Direct Attachment of Double-stranded DNA to Gold Surface for Preparationof Nano-structured Devices
Jun Sumaoka, Fenggang Pan, Aya Nonaka, Osamu Takeuchi,y Hidemi Shigekawa,y and Makoto KomiyamaResearch Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904
yInstitute of Applied Physics, University of Tsukuba, 1-1-1 Ten-nodai, Tsukuba 305-8573(Received April 9, 2004; CL-040389)

Step (A)Step (B)

DNA-I

DNA-IIS

AFM
Step (E)SSS S S S S SS

S SS S SS S

Au particles

AgNO3AS SS S SS S

Scheme 1. Procedure for the preparation of Au(nanoparticle) /DNA-duplex/Au(plate) composites. The closed hexagonals are
the trityl groups protecting the 50 -end of DNA-II.
porated to the 50 -end of DNA on an automated synthesizer. The
two 20-mer DNAs used in the present study are complementaryeach other: DNA-I, 50 -*TCG CGC TGG CGC GAG CCC CA;
DNA-II, 50 -*TGG GGC TCG CGC CAG CGC GA. The melting
temperature of the duplex between them in PBS(-) buﬀer isabout 80  C, as determined from the absorbance change at
260 nm. The colloidal solutions of gold particles were purchasedfrom Sigma, and the averaged particle size is 5 nm unless noted
otherwise. Water was puriﬁed by a Millipore puriﬁcation system
Milli-Q.
The nano-structured composites were fabricated as shown in
Scheme 1.6 First, a gold plate was cleaned with ethanol and thoroughly rinsed with water. This plate was immediately immersed
in 1.0 mM (M = mol dm3 ) solution of DNA-I in PBS(-) buﬀer(step (A)). After 2 h, the plate was rinsed with water and subsequently immersed for 1 h in 1.0 mM solution of DNA-II which is(B)

0S0−100TO:Au particle(A)∆F / HzOO P OOSSS S SS S

SSH(A)S

HO-C11-SH

S

Step (D)DNA is one of the most promising candidates for building
blocks in bottom-up nanotechnology. Its molecular size, topological speciﬁcities, and self-assembling properties satisfactorily
fulﬁll the requirements for fabrication of various molecular devices. For the communication with external systems, DNA isusually attached to electrodes or nanoparticles as interfaces.1,2
In most cases, a thiol group was bound to DNA via alkyl chain(e.g., hexamethylene), and reacted with gold surface to form
Au–S linkage (see Figure 1B). A number of elegant nano-devices have been obtained by this strategy. However, the alkyl
chains used for this indirect attachment often diminish the transfer of signals from DNA to gold interface (and vice versa),3 andthus their removal is desirable. In this paper, DNA is directly attached to gold plates and gold nanoparticles by using 50 -mercapto-50 -deoxythymidine (*T), as depicted in Figure 1A. Formationof well-deﬁned Au(nanoparticle) /DNA-duplex/Au(plate) composites
is clearly evidenced by quartz-crystalline microbalance (QCM),
scanning tunneling microscopy (STM), and conductive probe
atomic force microscopy (CP-AFM).4 These composites are
considerably more conductive than the corresponding composites involving hexamethylene linker.
The phosphoramidite monomer of thymidine analog *T having 50 -SH was synthesized according to the literature,5 and incor-Step (C)(B)−5−200∆m / ng

To the 50 -ends of two complementary oligonucleotides, 50 mercapto-50 -deoxythymidine was introduced and reacted withthe surface of either gold plate or colloidal gold particle. Welldeﬁned Au(nanoparticle) /DNA-duplex/Au(plate) composite was fabricated in self-assembled monolayer (SAM) of 11-hydroxy-1undecanethiol. The current through this composite was measured by conductive probe atomic force microscopy (CP-AFM).

ST
O

−300
−10O
T0OO P O
O

2500

3000

time / secOT
S

500

SFigure 1. Direct attachment (A) and indirect attachment (B) ofDNA to gold surface.

Figure 2. Change of QCM frequency responding to (A) thebinding of DNA-I to the gold electrode and (B) the hybridizationof 50 -protected DNA-II with the DNA-I bound to the electrode.
Conditions: [DNA-I]0 ¼ [DNA-II]0 ¼ 125 nM in pH 7.4 PBS(-)
buﬀer at 20  C.

Copyright  2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.6 (2004)(A)

701(B)(C)

Figure 3. STM image (A) and CP-AFM image (B) of the Au(nanoparticle) /DNA-duplex/Au(plate) composite. In (C), the current proﬁlealong the line in (B) is shown.
protected at the 50 -end with a trityl group (step (B)). Then, thegold plate was rinsed and kept in 1 mM solution of 11-hydroxy-1-undecanethiol for 2 days to cover the surface with its
SAM (step (C)). The trityl group was removed from DNA-IIby keeping the plate in 1 M AgNO3 solution for 0.5 h (step(D)). The sample was rinsed with water and incubated for 2 daysin a colloidal solution of gold particles (step (E)). Finally, thesample was rinsed again and a CP-AFM measurement was carried out on an SPA300HV unit using Au-coated AFM Probe SIAF01A (from Seiko Instruments Co.).
Completion of the step (A) and the step (B) in Scheme 1 was
conﬁrmed by QCM (Aﬃnix Q system, INITIUM INC). When agold electrode for QCM was soaked in PBS(-) solution of DNA-I(125 nM), the frequency decrease (FA ) was about 175 Hz (thepart A in Figure 2). This change corresponds to the mass increase(m) of 5.3 ng (0.86 pmol of DNA-I). The elecrode was thenrinsed and immersed in a PBS(-) solution of 50 -end protected
DNA-II. As shown in the part B of Figure 2, the frequency further decreased and saturated at about FB ¼ 195 Hz (m ¼5:9 ng). The amount of 50 -protected DNA-II bound to the plateis 0.91 pmol, which is identical with the amount of DNA-I boundin the step (A) within experimental error. Thus, these complementary DNAs almost completely hybridize each other evenon the gold surface. From the values of FA and FB , eachof the double-stranded DNA molecules occupies around
10 nm2 on the surface of the gold electrode.
Figure 3A shows the STM image of Au(nanoparticle) /DNA-duplex/Au(plate) composites thus obtained. The size of bright spots
is in fair agreement with the size of gold particles used (5 nm).
When another colloidal gold solution (the averaged particle size
10 nm) was used, bright spots of the corresponding size were observed as expected. The possibility that these particles are simply
adsorbed on the surface of gold plate (covered with the SAM) isruled out by the following two control experiments. First, nobright spots were observed by STM, when the gold plate was directly (without steps (A) and (B)) immersed in 1 mM solution of
11-hydroxy-1-undecanethiol for 2 days and then was incubatedfor 2 days in a colloidal solution of gold particles. Second, theDNA-I/Au(plate) system, obtained by step (A), never bound thegold particles, when it was directly (without step (B)) incubatedin the colloidal gold solutions. In order to form the present nanostructured composite, (i) covalent binding of DNA-I to the goldplate, (ii) hybridization of DNA-II to the DNA-I bound to thegold plate, and (iii) covalent binding of DNA-II to the gold particle are inevitable. CP-AFM also provided clear images of thegold particles of diameter 5 nm (see Figures 3B and 3C). It isnoteworthy that this Au(nanoparticle) /DNA-duplex/Au(plate) com-posite shows considerable conductivity. When the bias voltage
was 0.05 V, the current through the composite was about 7 nA(20%).7 The current was hardly altered even when the composite was stored under P2 O5 for 2 days.
For the purpose of comparison, similar composite was prepared by indirect attachment of DNA to the surface of gold plateand gold nanoparticle. Commercially available hexamethylenelinker was used for the fabrication (see Figure 1B). However,the current through this composite was around 10-fold smaller
than that observed on the direct attachment. Apparently, the alkyl linker suppresses the conduction at both the DNA/Au(plate)
and the Au(nanoparticle) /DNA junctions.
In conclusion, DNA molecules are directly attached to gold
plates and gold nanoparticles by using 50 -mercapto-50 -deoxythymidine as the terminal nucleoside, and well-deﬁned
Au(nanoparticle) /DNA-duplex/Au(plate) composites are fabricated.
These composites are more conductive than the composites
involving hexamethylene linker, and promising for furtherapplications.
The authors thank Dr. Takao Ishida of National Institute ofAdvanced Industrial Science and Technology for valuablecomments on CP-AFM measurement. This work was partially
supported by a Grant-in-Aid for Scientiﬁc Research from theMinistry of Education, Culture, Sports, Science and Technology,
Japan.
References and Notes
1 a) T. G. Drummond, M. G. Hill, and J. K. Barton, Nat. Biotechnol., 21, 1192 (2003). b) J. J. Gooding, Electroanalysis,
14, 1149 (2002). c) R. Elghanian, J. J. Storhoﬀ, R. C. Mucic,R. L. Letsinger, and C. A. Mirkin, Science, 277, 1078 (1997).
2 Alternatively, the communication is achieved by using
ﬂuorescence and other photochemical means.
3 R. Wirtz, C. Wälti, P. Tosch, and M. Pepper, Langmuir, 20,1527 (2004).
4 The electrical properties of SAM were investigated by use of
CP-AFM; a) X. D. Cui, A. Primak, X. Zarate, J. Tomfohr,O. F. Sankey, A. L. Moore, T. A. Moore, D. Gust, G. Harris,and S. M. Lindsay, Science, 294, 571 (2001). b) D. J. Woldand C. D. Frisbie, J. Am. Chem. Soc., 122, 2970 (2000).
5 B. S. Sproat, B. Beijer, P. Rider, and P. Neuner, Nucleic
Acids Res., 15, 4837 (1987).
6 M. L. Sauthier, R. L. Carroll, C. B. Gorman, and S. Franzen,
Langmuir, 18, 1825 (2002).
7 When the bias voltages were 0.02 and 0.01 V, the currents
were 3 and 1 nA, respectively.

Published on the web (Advance View) May 17, 2004; DOI 10.1246/cl.2004.700