cluster decorated DNA strands, but only a few aggregated
structures without nanoparticles visible in the AFM phase image
could be found on the mica surfaces. This effect might be caused
by the hydrophobic surface of the alkyne-modified DNA strands.
Fig. 3a shows a representative TEM micrograph of the product
of the ‘click’ reaction. A linear arrangement of nanoparticles
assembled on the DNA template is clearly visualized in these
images. It should be noted that the DNA template as well as the
ligand shell of the nanoparticles is invisible due to a lack of
contrast for organic material in TEM. As can be deduced from this
and further TEM micrographs (see ESI{) of the ‘click’ reaction
product, the one dimensional nanoparticle arrangement exhibits
a nearly equidistant interparticle spacing of approximately 2.8 ¡
0.5 nm. The high density of the particle coverage on the strands
may result from the small distance between the alkyne tags in the
DNA strands, which in the present case is the distance between
two modified thymine bases.
azide derivative. We have demonstrated that these particles can
rapidly be coupled to artificial, alkyne-modified DNA duplexes
using the copper(I)-catalyzed Huisgen cycloaddition reaction (‘click
chemistry’). As a result, we could obtain one-dimensional
nanoparticle arrangements showing dense coverage of the DNA
with particles with highly regular interparticle distances. The
potential for this method is based on the versatility of this coupling
chemistry to bind the nanoparticles to different alkyne-functiona-
lized targets, as well as on the high degree of spatial control for the
immobilization which can be influenced by the defined incorpora-
tion of the alkyne-modified bases in the synthesis of the DNA
duplexes. Furthermore, this approach to covalent immobilization
of metal clusters on a highly programmable template holds great
promise for the development of novel nanoscale electrical circuit
elements.
We thank the Volkswagen-Stiftung and the DFG for generous
financial support. We thank Tanja Ko¨pping for her help in the
preparation of the glutathione bisazide.
Owing to the statistical distribution of the four DNA bases over
the length of the DNA strand and the fact that we replaced all
thymine bases in the strands with the alkyne-modified derivatives,
a frequency of one alkyne group per four bases in the single strand
and one alkyne group per two base pairs in the DNA duplex can
be expected. Thus, we can estimate a distance of approximately
0.68 nm between two adjacent binding sites, which theoretically
provides more than one alkyne tag per particle. The measured
interparticle distance of 2.8 ¡ 0.5 nm (determined from the TEM
micrographs) is presumably affected by the steric hindrance of the
clusters due to their organic ligand shell. In a simple model, the
space required by the ligand shell, i.e. the thickness of the ligand
shell, can be estimated by calculating the binding lengths from the
thiol group to the azide group in the longer chain of the glutathione
molecule. For the fully extended conformation of the ligand, this
calculation results in a value of approximately 1.4 nm (Fig. 3b). If
the DNA template is regarded as a rigid rod in the simplified
model, the average interparticle distance will be twice the thickness
of the ligand shell, approximately 2.8 nm. Though this model is
strongly simplified and disregards the flexible helix structure of the
DNA, repulsion effects or interlocking of the ligand shells of two
adjacent nanoparticles, the theoretical value matches the experi-
mental value observed by TEM extremely well.
Notes and references
1 e.g.: (a) C. M. Niemeyer and U. Simon, Eur. J. Inorg. Chem., 2005,
3641–3655; (b) J. Richter, Phys. E (Amsterdam, Neth.), 2003, 16,
157–173.
2 (a) G. Schmid and U. Simon, Chem. Commun., 2005, 697; (b) S. Semrau,
H. Schoeller and W. Wenzel, Phys. Rev. B: Condens. Matter Mater.
Phys., 2005, 72, 205443.
3 (a) A. P. Alivisatos, K. P. Johnsson, X. Peng, T. E. Wilson,
C. J. Loweth, M. P. Bruchez, Jr. and P. G. Schultz, Nature, 1996,
382(6592), 609; (b) Z. Deng, Y. Tian, S. H. Lee, A. E. Ribbe and
C. Mao, Angew. Chem., 2005, 117(23), 3648, (Angew. Chem., Int. Ed.,
2005, 44, 3582).
4 (a) G. H. Woehrle, M. G. Warner and J. E. Hutchison, Langmuir, 2004,
20(14), 5982; (b) G. Braun, K. Inagaki, R. A. Estabrook, D. K. Wood,
E. Levy, A. N. Cleland, G. F. Strouse and N. O. Reich, Langmuir, 2005,
21, 10699.
5 M. Noyong, K. Gloddek, J. Mayer, T. Weirich and U. Simon, J. Cluster
Sci., 2007, 18(1), 193.
6 R. Huisgen, G. Szeimies and L. Mo¨bius, Chem. Ber., 1967, 100,
2494.
7 (a) J. Gierlich, G. A. Burley, P. M. E. Gramlich, D. M. Hammond and
T. Carell, Org. Lett., 2006, 8, 3639; (b) M. Fischler, U. Simon, H. Nir,
Y. Eichen, G. A. Burley, J. Gierlich, P. M. E. Gramlich and T. Carell,
Small, 2007, 3, 1049.
8 J. L. Brennan, N. S. Hatzakis, T. R. Tshikhudo, N. Dirvianskyte,
V. Razumas, S. Patkar, J. Vind, A. Svendsen, R. J. M. Nolte,
A. E. Rowan and M. Brust, Bioconjugate Chem., 2006, 17(6), 1373.
9 M. Gelinsky, R. Vogler and H. Vahrenkamp, Inorg. Chim. Acta, 2003,
344, 230.
Besides the regular interparticle spacing, the TEM micrographs
reveal that the one dimensional array consists of nanoparticles of a
very homogeneous size of 1.6 nm without the ligand shell.
Assuming that the metal cluster is enclosed by the major groove of
the DNA due to multiple triazole formation, a structure guiding
effect of the DNA on the array is possible, which could explain this
uniform particle size.
10 B. Carboni, A. Benalil and M. Vaultier, J. Org. Chem., 1993, 58, 3736.
11 M. Schulz-Dobrick, K. V. Sarathy and M. Jansen, J. Am. Chem. Soc.,
2005, 127, 12816.
12 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless,
Angew. Chem., 2002, 114, 2708, (Angew. Chem., Int. Ed., 2002, 41, 2596).
13 T. R. Chan, R. Hilgraf, K. B. Sharpless and V. V. Fokin, Org. Lett.,
2004, 6, 2853.
Summarizing, we have reported on the synthesis of a new type
of gold nanoparticle which is functionalized with a glutathione
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 169–171 | 171