Light-driven molecular shuttles modified on silicon nanowires

Article abstract of DOI:10.1039/c1cc16339f

Immobilization of light-driven molecular shuttles onto the surface of the silicon nanowires (SiNWs) was realized. The α-cyclodextrins as the shuttles could be reversibly translocated along the thread by the optical stimuli. Such SiNWs-based molecular shut

Full text of DOI:10.1039/c1cc16339f

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Light-driven molecular shuttles modified on silicon nanowiresw
Taiping Zhang,ab Lixuan Mu,a Guangwei Shea and Wensheng Shi*a
Received 12th October 2011, Accepted 27th October 2011
DOI: 10.1039/c1cc16339f
Immobilization of light-driven molecular shuttles onto the surface
of the silicon nanowires (SiNWs) was realized. The a-cyclodextrins
as the shuttles could be reversibly translocated along the thread
by the optical stimuli. Such SiNWs-based molecular shuttles
also exhibited sequential logic with optical stimuli as the input
and fluorescence as the output.
The SiNWs were prepared by a typical chemical vapor deposition
method.9 The as-prepared SiNWs have a crystalline Si core of
about 8 nm in diameter and a silica shell of about 2 nm in
thickness, as shown in Fig. S2w. The synthesis of the (N,N-2-
((4-formyl-biphenyl)–(4-methyl-phenyl)–diazene) dansylamide (S4)
and the modification on the surface of the SiNWs are described
in detail in the Supporting Informationw. The 3-aminopropyl-
triethoxylsilane (APTES)-modified SiNWs are abbreviated as
APTES–SiNWs.10 The final products were named S4–CD–SiNWs.
For comparison, a parallel product without a-CD was synthesized
and abbreviated as S4–SiNWs. The modifications on the SiNWs
were characterized by the X-ray photoelectron spectroscopy (XPS),
the results of which are shown in Fig. S3aw. It was found that no
nitrogen was detected from the bare SiNWs, while the
APTES–SiNWs contained 3.05% nitrogen. The results indicate that
the APTES were modified onto the surface of the SiNWs.9 Reactions
between the APTES–SiNWs and the S4 or S4–CD simultaneously
were verified by a further increase of the nitrogen content and a
variation of the sulfur content (Fig. S3bw). The relative content of the
oxygen in the S4–SiNWs and S4–CD–SiNWs was also compared.
The oxygen content in the S4–SiNWs was 26.41%, while that in the
S4–CD–SiNWs was 29.85% (Fig. S3cw). The additional oxygen was
attributed to the a-CD, suggesting that the a-CD was locked onto
the SiNWs. Furthermore, infrared spectroscopy (IR) was carried out
(Fig. S4w). It was found that the characteristic peak of aldehydes at
1685 cmÀ1 appeared in the IR spectrum of S4, but not in that of
S4–CD–SiNWs, which means that all of the aldehydes were coupled.
Combining the synthesis route and above characterizations, the
construction of the molecular shuttle could be fully confirmed. The
S4–CD–SiNWs consist of the fluorescence unit, thread, a-CD and
SiNWs. The a-CD was interlocked between the two stoppers, i.e. the
fluorescent unit and SiNWs.
Inspired by biological motors, including Adenosine Triphos-
phate (ATP) synthase—a molecular rotary motor participating
in the synthesis and hydrolysis of ATP,1 various molecular
machines, including walkers,2 elevators,3 rotors4 and shuttles5
have been fabricated and exciting results have been achieved.
Most of these molecular machines are operated by a chemical
or optical stimulation. Compared with a chemical driving
force,6 light signals as stimuli could operate the molecular
machine without direct touch. Moving a macrocycle as a molecular
shuttle back and forth between two or more stations in
response to external optical stimuli has been realized in
solution.7 However, such molecular shuttles are disordered,
which might cause the molecular shuttles to not be rationally
controlled. As a precondition for their practical application,
molecular shuttles are required to be immobilized onto the
surface of the carriers.8 In this study, in order to meet the
demand of miniaturizing molecular shuttles in microenviron-
ments and integrating more molecular machines into a chip in
the future, silicon nanowires (SiNWs) were selected as the
carrier due to their good compatibility with the prevalent
Si-based integration technology. By covalently attaching
the N,N-2-((4-formyl-biphenyl)–(4-methyl-phenyl)–diazene)
dansylamide (S4, Scheme 1) with a-cyclodextrins (a-CDs)
onto the surface of the SiNWs, SiNWs-based light-responsive
molecular shuttles were realized. The present molecular shuttles
consisted of four components; the SiNWs stopper, dansyl-
amide stopper (fluorescence unit), molecular track and the
a-CD (Fig. S1w). The a-CD was locked between the SiNWs
stopper and the dansylamide stopper. The translocation of the
a-CD was triggered by the external optical stimuli.
When the a-CD is located near to the fluorescent unit, the
vibrations and rotations of the bonds in the dansylamide unit
would be weaken, which results in an increase of fluorescence
emission from the dansylamide.7e Conversely, the fluorescence
emission of the dansylamide would be restrained when the a-CD
a Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China.
E-mail: shiws@mail.ipc.ac.cn; Fax: 86-10-82543513;
Tel: 86-10-82543513
b Graduate University of Chinese Academy of Sciences, Beijing,
100039, China
w Electronic supplementary information (ESI) available: See DOI:
10.1039/c1cc16339f
Scheme 1 The structure of S4.
c
452 Chem. Commun., 2012, 48, 452–454
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moves away from the fluorescent unit. Thus, there is a correlation
between the fluorescence intensities of the dansylamide and the
locations of the a-CD. Accordingly, the movement of the a-CD
on the SiNWs-based molecular shuttles can be monitored by the
change in the fluorescence intensity. It was observed that the
fluorescence intensity of the S4–CD–SiNWs excited by 365 nm in
5% ethanol aqueous solution could increase to 274% of the
initial value (Fig. S5aw) after the system was irradiated continu-
ously with the UV light of 254 nm for 10 min. Further durative
irradiation with the visible light of 420 nm for 10 min can
increase the fluorescence intensity to 513% of the initial value.
Then, the fluorescence intensity can be regressed to 308% of the
initial value if the system is further irradiated with UV light of
365 nm for 10 min. Moreover, it is worthwhile to notice that the
fluorescence intensity cannot be increased by the durative irra-
diation of UV light of 365 nm or visible light of 420 nm, unless
the system was previously irradiated by UV light of 254 nm.
However, it was found that, after a durative irradiation with UV
light of 254 nm, alternate irradiation with light of 420 nm and
365 nm can switch the fluorescence intensities of the system
(Fig. 1). Furthermore, it was observed that the fluorescence
intensity of the system cannot reach the initial value once it
has been irradiated by UV light of 254 nm, only when the system
is placed in the dark again for more than 24 h. The parallel
experiment showed that no significant fluorescence changes were
observed from the S4–SiNWs without the a-CD when it was
separately or combinatorially irradiated by 254 nm, 420 nm, or
365 nm lights (Fig. S5bw). It indicates that the existence and
position of the a-CD determine the changes in the fluorescence
intensities. These phenomena demonstrate that sequentially
irradiating the system with light of 254 nm and 420 nm can
gradually drive the a-CD closer to the dansylamide part. Further
irradiation with light of 365 nm will drive the a-CD away from
the dansylamide part. After the system was irradiated with the
light of 254 nm, alternate illumination with light of 420 nm and
365 nm can reversibly drive the a-CD to move between the two
stoppers, i.e. the fluorescent unit and SiNWs. Thus, light-driven
molecular shuttles were realized on SiNWs.
can be determined that the a-CD was originally located at the
carbon–nitrogen double bond site due to the thermal stability
of the a-CD and carbon–nitrogen double bond. Illumination
by 254 nm UV light can isomerize the carbon–nitrogen double
bond from state 1 (ECQN, ENQN) to state 2 (ZCQN, ZNQN).11
These isomerizations will drive the a-CD to the biphenyl unit,
accompanied by the fluorescence increase of the dansylamide.
Further irradiation by 420 nm light will lead to the isomerization
of the nitrogen–nitrogen double bond (ZNQN to ENQN) and,
simultaneously, the carbon–nitrogen double bond will not be
excited.11 Such isomerizations can shift the a-CD gradually
close up to the dansylamide group (state 3) and, as a result, the
fluorescence intensities are further enhanced. Illumination
with 365 nm light could relocate a-CD at biphenyl under
the isomerization of the nitrogen–nitrogen double bond from
the state 3 (ZCQN, ENQN) back to state 2 (ZCQN, ZNQN). The
alternate illumination with light of 420 nm and 365 nm can
control the isomerization of the nitrogen–nitrogen double
bond and, consequently, results in the movement of the a-CD
along the S4 chain. Furthermore, the system will be recovered
to initial state 1 (ECQN, ENQN) after being settled in the dark
for more than 24 h (Fig. S6w).
To confirm the mechanism of the a-CD movement, the
1HNMR spectra of the S4–CD and S4 in d-DMSO and CDCl3
were recorded under various conditions. By comparing the
1HNMR spectra of the S4 and S4–CD (Figs S7a and bw), it
can be found that the Hg exhibited a chemical shift by d =
0.02 ppm, Ha and Hb by 0.05, Hc, He, Hf by 0.02 ppm, Hd by
0.16 ppm from the original chemical shift of S4. These results
suggest that the a-CD located at the azobenzene site. Comparing
the 1H NMR spectra of the S4-CD before and after the irradiation
(Figs S7b and cw) of 365 nm light, 0.5 ppm chemical shift for
Hd, 0.03 ppm for Ha and 0.15 for Hb and 0.02 for Hg were
produced. Finally, the chemical shift of the 420 nm-stimulated
S4–CD can be recovered by irradiation of the 420 nm light
The movement of the a-CD within the molecular shuttles
can be understood based on the variation of the configurations
of the S4–CD–SiNWs under the different light illuminations
(Fig. 2). Considering the initial low fluorescence intensity, it
Fig. 1 Fluorescence intensity of S4–CD–SiNWs sequently irradiated
with 254 nm light, 420 nm light and 365 nm light.
Fig. 2 A schematic illustration of the molecular shuttle.
Chem. Commun., 2012, 48, 452–454 453
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corresponds to the threshold value. a, b, c is the sequence of irradiation.
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variation of the a-CD position.7l The above information
further confirmed that the a-CD was first located at the
azobenzene group and then transferred to the biphenyl part
by the optical stimuli, which is in accordance with the analysis
based on the observation of the fluorescence intensities.
Interestingly, it was also found that the fluorescence emission
from the system was related to the order of the irradiation with
254 nm, 420 nm, 365 nm light. However, without the premier
irradiation with 254 nm light, any irradiation cannot alter the
fluorescence intensities. These results indicate that the fluores-
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irradiation, but also relate to the history of the irradiation.
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by defining the 254 nm, 420 nm and 365 nm light as inputs and
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shuttle could mimic the sequential logic in which only the
proper order of inputs can result in the output being 1.
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back and forward at tri-states by the external light stimulations.
The configuration changes between the cis and trans isomers
resulted from light illuminations with different wavelengths
provide the driving force for the a-CD movement. Furthermore,
the molecular shuttle can mimic a sequential logic by defining the
254 nm, 420 nm and 365 nm light as inputs and the fluorescence
of molecular shuttles as the output. This method paves a way to
miniaturize molecular shuttles and could be extended to make
other molecular machines with multiple functions.
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This work was supported by Chinese Academy of Sciences,
NSFC (Grants 10874189, 50902134, 61025003, and 21103211),
National Basic Research Program of China (973 Program)
(Grants 2010CB934103 and 2012CB932400) and Beijing Natural
Science Foundation (Grant 2102043).
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454 Chem. Commun., 2012, 48, 452–454
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