Metal Coating of DNA Molecules by Cationic, Metastable Gold Nanoparticles Tetsu Yonezawa, y;yy;# Shin-ya Onoue, y and Nobuo Kimizuka y y Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581 yy PRESTO, Japan Science and Technology Corporation, Hakozaki, Fukuoka 812-8581 (Received August 29, 2002; CL-020738) Metastable cationic gold nanoparticles were newly devel- oped by the use of thiocholine bromide (TCB) as stabilizer. The gold nanoparticles electrostatically bound to DNA molecules and then underwent self-fusion into the wire-like structures. The use of TCB as a labile ligand was essential to achieve non- electrochemical plating of the non-conductive molecules. Metal coating of organic materials has been widely used to manipulate electronic circuits. Evaporation of metals with masks is one of the most fundamental techniques to produce metal wires orcircuits.However,inordertodownsizesuchmetalcircuits,the ability to produce narrower metal wires is required. Fine lithography, for example, is one promising technique. On the other hand, the development of ‘‘bottom-up’’ nanotechnology may replace the conventional ‘‘bulk-down’’ approach. 1;2 Metal nanoparticles are attracting much interest as elements for nanocircuits. Assembling metal nanoparticles into low-dimen- sional arrays or three-dimensional aggregates has been inten- sively studied due to the expected novel properties which cannot be achieved either from bulk metals or metal complexes. 3;4 Electrostatic interactions have been frequently used to accumu- late charged metal nanoparticles on templates. 5{7 However, in manycases,electrostaticrepulsionbetweentheparticlesprevents themfrompackingathighdensity. 5 We have previously reported thatdenselypackedmonolayersofanionicgoldnanoparticlescan be formed on the cationic monolayer of synthetic amphiphiles. 6;7 In addition, when cationic gold nanoparticles were adsorbed on DNA molecules or anionic bilayer membranes, low-dimensional arrays of nanoparticles were obtained. 8;9 Sastry recently reported that the lysine-stabilized gold nanoparticles were also electro- statically adsorbed and accumulated on synthetic DNA molecules. 10 In this study, we describe the formation of wire-like structures templated by !-DNA molecules from cationic, metastable gold nanoparticles. The cationic stabilizer molecule used in this study is thiocholine bromide (TCB: HS-(CH 2 ) 2 - N(CH 3 ) 3 þ Br  ), which was obtained by hydrolysis of commer- cially available acetylthiocholine bromide (Aldrich). The purity of the product was confirmed by FT-IR, 1 H NMR and elemental analysis. The length of the cationic part of TCB(HS-(CH 2 ) 2 - N(CH 3 ) 3 ) is ca. 8.1 # A, which is the smallest quaternary ammonium-terminated thiol molecule available. The gold nanoparticles were prepared by NaBH 4 -reduction of HAuCl 4 in thepresenceofTCBataAu/TCBratioof 1:3 (mol/mol), similar to Brust’s method. 11 The obtained particles could be readily dispersed homogeneously into water even at a high concentration but not in methanol or in other organic solvents. A transmission electron microscopic (TEM) image and the size distribution of the TCB-stabilized gold nanoparticles are shown in Figure 1. The average size is about 2.7 nm, and the particles are fairly mono-dispersed. This is probably due to the efficient complexation between AuCl 4  and TCB during the preparation. Elemental analysis of the TCB-stabilized gold nanoparticles revealed that each particle contains 14.3 wt% of TCB molecules, which indicates that the gold nanoparticle is surrounded by ca. 100 TCB molecules with an occupied surface area of 22.9 # A 2 per molecule (Figure 1c). This occupied area is slightlylargerthanthatofalkylthiolonAu(111)(ca.21.5 # A 2 ) 12 or a longer cationic alkylthiol on gold nanoparticles (21.4 # A 2 ). 8;13 The lower surface density of TCB can be attributed to the electrostaticrepulsionbetweentheTCBmoleculesontheparticle surface. TheaqueousdispersionofTCB-stabilizedgoldnanoparticles and !-DNA in tris buffer (pH = 7.9) were mixed with a final concentration of [Au] = 1:2  10 2 moldm 3 and [!-DNA] = 72 "gdm 3 . The mixed solution was immediately sonicated at 0  C for several seconds in order to obtain homogeneous mixing withoutdecompositionoftheDNAmolecules.Figure 2 showsthe typical TEM images of the gold nanoparticles adsorbed on !- DNA molecules. These images were taken several hours after sample preparation. Most of the particles were aligned one- Figure 1. (a) Transmission electron micrograph and (b) the size distribution of thiocholine bromide (TCB)-stabilized gold nanoparticles. (c)ModelofTCB-stabilizedgoldnanoparticle.Theimagewastakenwith a Hitachi H-7000 at the acceleration voltage of 100kV. The size distribution was determined by measuring more than 300 particles in an arbitrarily chosen area in the enlarged TEM image. The model was illustrated by using the elemental analysis data. 1172 Chemistry Letters 2002 Copyright Ó 2002 The Chemical Society of Japan