C O M M U N I C A T I O N S
be noted that the conformation of azobenzenes on the nanotubes
returned to the trans form after the nanotube solution was kept in
the dark for 48 h at 4 °C, probed by Raman microscopy. These
spectroscopic results support that the attachment/detachment of
azobenzene nanotubes on the R-CD SAMs was photochemically
controlled by the conformations of azobenzenes on the nanotubes,
as is illustrated in Figure 1.
In conclusion, this study demonstrates that the azobenzene
nanotubes could recognize and attach onto well-defined regions of
thiolated R-CD SAMs via host-guest molecular recognition. The
binding between the azobenzene nanotubes and the R-CD SAMs
was controlled by UV irradiation. Some of the nanotubes were
capable of interconnecting two regions of R-CD SAMs on the Au
substrate. This smart building block may be applied to build
photoactive nanometer-scale mechanical switches in electronics.
Figure 3. (a) UV-vis spectra of the azobenzene nanotube/R-CD solution
before (upper trace) and after (bottom trace) UV irradiation. (b) Raman
spectra of the azobenzene nanotubes (bottom trace), the azobenzene
nanotubes in the R-CD solution (middle trace), and the azobenzene
nanotubes in the R-CD solution after UV irradiation (upper trace).
Acknowledgment. This work was supported by the NSF-
CARRER (EIA-0133493), the NSF-NER (ECS-0103430), and the
U.S. Department of Energy (DE-FG-02-01ER45935).
solution was kept in the dark for 24 h. Therefore, the reversibility
of the nanotube immobilization can be controlled by UV irradiation.
Although less than 20% of nanotubes formed the bridge configu-
ration between two R-CD SAMs due to the aggregation, we are
currently limiting the azobenzene immobilization at the ends of
nanotubes13 and shrinking the patterned Au areas to increase the
number of nanotubes interconnecting multiple R-CD SAMs/Au
electrodes for electronics and sensor applications.
Supporting Information Available: Procedures to form azoben-
zene nanotubes, R-CD SAMs, azobenzene nanotube assembly on the
R-CD SAMs; Raman instrumentation; magnified Figure 3b and
additional Raman spectra of the neat nanotube and the neat HABA
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
To confirm that the attachment/detachment of azobenzene
nanotubes on the R-CD SAMs is controlled by the conformation
change of azobenzenes on the nanotubes, UV-vis spectra and
Raman spectra of the azobenzene nanotubes were analyzed in the
R-CD solution before and after UV irradiation. Figure 3a (upper
trace) is a UV-vis spectrum of the azobenzene nanotube solution
(250 µL) mixed with 1.0 mM R-CD in 500 µL of DMSO/H2O
(1:1, v/v) solution before UV irradiation. Appearance of a peak at
340 nm indicates that the R-CD molecules bound the trans-
azobenzenes on the nanotubes.14 When this solution was irradiated
by UV light for 3 h, the 340 nm peak in the UV-vis spectrum
disappeared, as is shown in Figure 3a (bottom trace). This spectral
change suggests that the azobenzenes were detached from the R-CD
molecules via the photoisomerization of azobenzenes to the cis
form.14 Further investigation of the conformation change was
examined by Raman microscopy. When the azobenzenes were
immobilized on the nanotubes, characteristic -NdN- and Ph-
Nd peaks of the azobenzene in the trans conformation at 1406
and 1470 cm-1 were observed in the Raman spectrum (bottom trace
in Figure 3b).12 When this nanotube solution was mixed with the
R-CD solution, the Raman spectrum of the resulting solution
(middle trace in Figure 3b) agrees with the combined spectrum of
the R-CD and the azobenzene15 (Supporting Information). This
spectral change indicates that the R-CD molecules were complexed
with the azobenzenes on the nanotubes. When this R-CD-
azobenzene nanotube complex was irradiated by the UV light (360
nm) for 3 h, the resulting Raman spectrum (top trace in Figure 3b)
was consistent with the azobenzene nanotube spectrum (bottom
trace in Figure 3b) except for peaks at 1501 and 1531 cm-1. These
peaks are assigned as the -NdN- and Ph-Nd vibrations of the
cis-azobenzene, which were shifted from the 1406 and 1470 cm-1
peaks of the trans form in Figure 3b (bottom trace).15 Therefore,
this spectral change means that the azobenzenes were isomerized
to the cis form after UV irradiation and the R-CD molecules were
removed from the surfaces of the azobenzene nanotubes. It should
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