Angewandte
Communications
Chemie
version of HDS under UV irradiation is 3.4-fold faster than
that without UV irradiation; in CDCl3 and D2O/DCl
(0.72 wt% DCl), the reduction reaction only occurred under
UV irradiation. These results imply that this reaction is
dominated by the thiyl-centered process in less polar solvents
and/or acidic solutions.
These primary results prompted us to explore this UV-DR
reaction on the surface. We tested the reduction of CED-
modified films and determined spectrophotometrically the
conversion of disulfides. The results are summarized in the
Supporting Information, Table S2. In DMF, the conversion of
disulfide was 34% within 2 min of UV irradiation (260 nm,
10 mWcmÀ2), almost ten-fold higher than that without UV
irradiation. In CHCl3, 95% of disulfides have been reduced
after 5 min of UV irradiation, while the conversion is only
4.3% without UV irradiation. This large difference in
reaction rates is important for achieving temporal and spatial
control for surface modification. We further characterized
this UV-DR reaction on the hydrophobic DBS-modified
surface using contact angle measurements. As shown in
Figure 2B, the static WCA decreased from 118 Æ 28 to 54 Æ 28,
which corresponds to the original thiol-modified surface, after
2 min of UV irradiation (260 nm, 10 mWcmÀ2). This indicates
that the hydrophobic butyl groups have been removed by the
UV-DR reaction. In contrast, no change of the static WCA
was detected after 2 min without UV irradiation, which is
consistent with the observed slow kinetics of this reaction in
organic solvents without any catalyst (Supporting Informa-
tion, Table S1). Further study of the surface modification
process shows that the UV-DR reaction is completed in less
than 2 min and also proceeds effectively under 260 and
365 nm light (1.6 mWcmÀ2, Figure 2B).The Supporting Infor-
mation Figures S13 and S14 show that both UV-DF and UV-
DR reactions proceed even without the photoinitiator at
254 nm, albeit slower than with the initiator.
To demonstrate the possibility for photopatterning using
the UV-DR, the (FITC-disulfide)-functionalized film was
subjected to UV irradiation (260 nm, 10 mWcmÀ2) through
a photomask in the presence of a solution of DTT (20 wt%)
and I2959 (1 wt%) in CHCl3. Patterns with various shapes
could be easily prepared by using different photomasks
(Figure 2C). The photopatterning was also confirmed by the
ToF-SIMS results of a BDS-functionalized film irradiated
with UV through a photomask in the presence of DTT in
CHCl3. Both the high-contrast pattern and the appearance of
thiol groups support the spatially-controlled reduction of
disulfides into thiols (Figure 2D). This strategy also enabled
the fabrication of a wettability gradient on the surface as
shown in the Supporting Information, Figure S16.
Figure 2. A) Representation of the UV-DR reaction on disulfide-modi-
fied surfaces. B) Rate of surface modification by the UV-DR reaction in
DMF. The graph shows the static WCA on BDS-modified surface after
the UV-DR reaction as a function of irradiation time under different UV
radiation sources. C) Fluorescence microscopy image of (FITC-disul-
fide)-modified surfaces before and after patterning by using the UV-DR
reaction through different photomasks. Scale bar=500 mm. D) ToF-
SIMS images of (dibutyl disulfide)-modified surface patterned by using
the UV-DR reaction. Scale bar=500 mm.
tion between hydrophilic and hydrophobic surfaces is very
quick as every step needs only 2 min of UV irradiation. SEM
measurements revealed no change of surface topography
after the UV-DR and UV-DF reactions (Figure 3B,C). This
process can be repeated for at least ten cycles, which
demonstrates good reversibility (Figure 3D). The slight
decrease of static WCA for both surfaces possibly results
from oxidation of thiol groups or disulfide groups over time
and under UV light. After ten cycles, the thiol concentration
on this surface was 1.64 ꢀ 10À9 molmmÀ2, which corresponds
to 85% of intact thiols.
Reversible UV-induced disulfide formation and reduction
offer unique opportunities for dynamic photopatterning.
Additionally, as the disulfide-modified surface produced is
photodynamic itself, it opens the possibility for photodynamic
disulfide exchange using other disulfides.[13] Therefore, we can
use light to direct the attachment, exchange, and detachment
of moieties immobilized through the disulfide bond with
These two photochemical reactions, UV-DF and UV-DR,
allow for reversible transitions between thiol-functionalized
surfaces and disulfide-functionalized surfaces. To evaluate
this reversibility, we sequentially modified a thiol-functional-
ized film through the UV-DF reaction with BDS, followed by
the UV-DR reaction with DTT (Figure 3A). After exchange
with BDS, this hydrophilic surface becomes hydrophobic, as
observed in the increasing static WCA from 54 Æ 28 to 118 Æ
28; subsequent UV-DR reaction restores the original hydro-
philic surface with a static WCA of 54 Æ 28. The transforma-
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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