Angewandte
Chemie
1.0 nm) indicates that the triazole alkyl chain is substantially
tilted relative to the substrate surface.
to increase the wettability and improve the spreading of the
polar ink on the stamp. Directly after oxidation the stamp was
inked with a solution of LRA in ethanol (1 mm), dried with
nitrogen for 1 min, and brought into conformal contact with
azido-terminated SAM 2 on a glass slide for 1 min. The
substrate was vigorously rinsed with MilliQ water, sonicated
in ethanol for 5 min, and dried with nitrogen. It is evident
from the confocal microscopy image in Figure 4 that the 20-
mm dot features of the stamp are reproduced faithfully on the
substrate and that the ink is distributed homogeneously over
the contact area.[16] As fluorescent alkyne LRA is still present
on the surface in spite of extensive rinsing and sonication, any
type of absorption other than covalent immobilization
through triazole formation can be excluded. To scavenge
any adventitious metal-ion contamination, mCP of LRA on
SAM 2 was also performed in the presence of EDTA
(0.05 mm). No change was observed (see the Supporting
Information). However, when mCP of LRA was performed on
an “inert” SAM of n-dodecyltriethoxysilane, no evidence of
immobilization of LRA was found (see Supporting Informa-
tion). We explain the remarkable efficiency of the click
reaction of LRA printed on the azido SAM by the high local
concentration of the polar ink at the surface of the oxidized
PDMS stamp.
It should be emphasized that the [2+3]cycloaddition of
azides and electron-rich acetylenes such as 1-octadecyne is
normally very slow in the absence of a CuI catalyst. Indeed,
Collman et al.[9a] and Lummerstorfer and Hoffmann[10]
reported that no reaction occurs between azido SAMs and
electron-rich acetylenes such as 1-octyne and ethynyl ferro-
cene in the absence of a CuI catalyst.[15] To scavenge any
adventitious metal-ion contamination in solvents, reactants,
PDMS stamps, or substrates, the cycloaddition reaction was
carried out in the presence of ethylenediaminetetraacetic acid
(EDTA, 0.05 mm; see the Supporting Information). No
difference was observed in the reaction of the azido SAM
with 1-octadecyne, either by mCP or from solution in the
presence of EDTA.
Furthermore, to illustrate the power and scope of the
triazole click reaction by mCP, fluorescent alkyne LRA
(lissamine rhodamine with a terminal acetylene unit) was
printed on azido SAM 2 (Figure 4). The synthesis of LRA is
described in the Supporting Information. Prior to printing, the
PDMS stamp was oxidized with UV/ozone plasma for 30 min
In conclusion, click chemistry can be applied to the
microcontact printing of acetylenes onto azido-terminated
SAMs. Synthesis in the nanoscale confinement between a
PDMS stamp and a reactive substrate leads to the desired
product within a short period of time, without a catalyst, and
under mild conditions. We envisage that click chemistry by
mCP can be applied using a wide variety of acetylenes and
immobilized azides, as well as azides and immobilized
acetylenes. In particular, this methodology will be useful for
the directed immobilization of (bio)molecules that are
modified with either acetylene or azide units. Click chemistry
by mCP should serve the development of biological arrays that
can be obtained within a short reaction time, under mild
reaction conditions with no toxic catalyst required, with high
selectivity and quantitative yields, and tolerance for a wide
range of functionally complex substances.
Received: March 20, 2006
Published online: July 12, 2006
Keywords: alkynes · azides · cycloaddition ·
.
microcontact printing · monolayers
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Figure 4. Top: Fluorescence microscopy image (700700 mm) of LRA
printed in 20-mm dots onto azido-terminated SAM 2. Bottom: Sche-
matic representation of the mCP-induced click reaction between the
fluorescent alkyne LRA and the azido-terminated silane SAM on a
glass slide.
Angew. Chem. Int. Ed. 2006, 45, 5292 –5296
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5295