Angewandte
Chemie
particle affinity is apparently weaker and a low density of
individually dispersed colloids is observed (1.7 particles per
mm2)with unspecific adsorption in the nonirradiated areas
(1.3 particles per mm2). In this case the adsorption process
might be driven primarily by polar and hydrogen-bonding
interactions between the OH and COOH groups.
Keywords: monolayers · photolithography · silanes ·
surface chemistry
.
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Surprisingly, on COO surfaces from exposed layers 3 and
4 a relatively strong colloid adsorption (4.1/1.4 particles per
mm2 for irradiated/nonirradiated 3; 8.5/1.2 particles per mm2
for 4)and substantial particle clustering is found (seen at
higher magnification in the SEM image, Figure 3d). This is in
contrast to initial expectations, since the carboxy functions on
the particles and at the substrate surface should show
electrostatic repulsion. Indeed, no particle adsorption is
achieved when pure water is used instead of buffer solution
as the suspending medium; this indicates the important role of
charge screening by the salt. Colloidal adsorption onto a
mixed layer of 1 and 4 after successive irradiation through a
striped mask, first at 254 nm (CO-Bzn deprotection)and
secondly with the mask rotated by 908 at 411 nm (NH-Nvoc
deprotection), leads to the checkered pattern shown in
Figure 3e, in which the particle density depends on the
activated functional groups and the irradiation dose (highest
particle density in cross regions).
Other methods for particle assembly have been
reported,[19] and one prominent example of orthogonal
particle deposition is based on DNA-assisted specific recog-
nition between DNA-modified spots on a substrate and
DNA-labeled particles.[20] This method was demonstrated
successfully for two different kinds of particles,[20b] but it
requires specific DNA labeling of both the substrate and the
particles. The advantage of the method presented here is the
the fact that a simple silanization process introduces the
mixed protected functionalities and patterning is achieved by
means of standard photolithographic irradiation; the particles
need not be specifically modified with complementary
recognition elements (besides the functional surface groups
introduced during particle synthesis).
In conclusion, the new photosensitive silanes presented
here can be used for direct monolayer lithography and the
introduction of functional surface groups, which is not
possible directly by silanization (OH and COOH functions
are incompatible with the triethoxysilane anchor group).
Complex combinations of different functional and protecting
groups can thus be achieved by simultaneous coadsorption of
the corresponding silane mixtures and orthogonal activation,
as demonstrated here for NH-Nvoc 1 and CO-Bzn 4. Specific
colloid assembly onto the photoactivated regions is possible
and mediated by the free surface functionalities. Further
experiments are currently directed to the selective immobi-
lization of DNA fragments onto the photopatterned mixed
layers (relevant for biochip applications)and the extension of
the Bzn group to other functionalities.
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[14] For molecule 3 the nitroveratryl group is directly linked to the
carboxylic acid function through an ester linkage (without a
oxycarbonyl moiety), but for simplicity it is referred to as CO-
Nvoc.
[15] See the Supporting Information for details.
[16] In analogy to the irradiated silane layers, nonirradiated refer-
ence substrates were also washed to identify and account for any
intensity loss due to simple removal of the silane layer by
washing. All absorbances remained constant after this proce-
dure, corroborating the high stability of these silane layers.
[17] During irradiation a thin film of methanol was sandwiched
between the substrate and a covering quartz plate to allow
dissolution of the generated protecting-group fragments. In the
case of the Nvoc-protected amine 1, semicarbazide hydrochlo-
ride (55 mm solution in methanol)was added as a carbonyl
scavenger to capture the photogenerated benzaldehyde and
prevent potential imine formation with the free amino surface
(Scheme 1).
Received: January 11, 2005
Revised: April 11, 2005
Published online: July 1, 2005
Angew. Chem. Int. Ed. 2005, 44, 4707 –4712
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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