Real-Time Observation of Nanotube Disassembly
Prof. Dr. Ben L. Feringa and Dr. Wesley R. Browne from the University of Groningen,
the Netherlands, are using synthetic chemistry to create new light-responsive nanoscale
structures that could one day find use in applications such as smart materials and drug
delivery. The photoreactivity of the structures being developed places the Andor Revolution
DSD confocal microscope as a key asset in studying the dynamic properties of the materials
in real time.
Scientists in the field of nanotechnology strive to create smart
nano and microscale structures, incorporating switch and motor
units that mimic Nature’s fascinating mechanical cellular systems.
In the research groups of Feringa and Browne, molecular motors
and switches are built into new smart materials through self-
assembly and give these materials dynamic properties that respond
to external stimuli. Recently the Groningen team created micron-
long nanotubes that are tens of nanometers in diameter using a
functional amphiphilic building block. The amphiphile is highly
fluorescent in the blue region, but at the same time undergoes rapid
photochemical conversion to a green fluorescent structure that cannot
form the nanotubes. Hence with light the nanotubes can be forced to
disassemble (Figure 1).
Dr. Browne says that their work builds on that of other research
groups, in particular that of Takuzo Aida’s group at the University of
Tokyo, which has developed several beautiful examples of nanotube-
forming molecular systems. However, by building a reactive element
into their amphiphile, the Groningen team’s new nanotube systems
exhibit unprecedented multifunctionality. “Not only can they be
studied by fluorescence microscopy, but we can use light to control
the stability and structure of the tubes, ultimately allowing us to
trigger the tubes’ disintegration in a controlled manner to form new
types of structures,” he says.
Following the UV-triggered disassembly of the nanotubes in real
time at room temperature was singularly the biggest challenge
faced by the team because conventional point-scanning, PMT-based
confocal systems could provide images but could not provide the
time resolution needed. The rapid switching between excitation/
emission wavelengths allowed for by the DSD together with the
speed of image acquisition offered by the Clara CCD camera was
critical.
“In this case the ‘switching’ between the blue tube-forming
fluorescence and green tube-disrupting fluorescence is such that we
need to be able to rapidly switch between excitation and emission
wavelengths to capture the entire image simultaneously,” Dr.
Browne says. “This is not practical with PMT-based systems that use
scanning to produce an image and require much higher total light
intensities.”
The researchers used the DSD Revolution confocal microscope
to monitor a cross-section of a nanotube during irradiation. When
exposed to ultraviolet light, they could see the blue fluorescence
decreasing and the green fluorescence increasing in intensity. As
green fluorescence increased, they observed matching structural
changes within the tube until it eventually disassembled.
“Furthermore with the DSD we are able to collect a widefield
and confocal image with the one system,” says Dr. Browne. In
widefield mode the DSD can use external excitation sources so it
is not restricted to the excitation wavelengths provided for by the
interchangeable filter sets.
The researchers also used the DSD microscope to follow controlled
disassembly of the nanotube/vesicle system, which could be
accomplished by varying the intensity or wavelength of light (Figure
2.). They found, for example, that irradiation at 365 nm using a
UV lamp held above the sample instead of 390 nm light from the
DSD’s filtered superbright white light source allowed them to
slow the disassembly process. “The precise software control of the
illumination intensity that is possible with the DSD and the flexibility
and speed of switching between excitation wavelengths was central
to discovering the functionality of the nanotubes,” Dr. Browne says.
For Prof. Feringa’s and Dr. Browne’s research teams it was important
that the DSD could be connected to the side port of any microscope.
The researchers use it on a set-up that can also perform widefield
imaging and Raman spectroscopy. “The DSD Revolution Confocal
system is an ideal workhorse instrument, and a number of projects
make use of it,” Dr. Browne says. “The key benefit is that at a
relatively low cost we have access to a powerful microscopy system
that allows optical, widefield, and confocal fluorescence, and together
with a Shamrock303 spectrograph and a spectroscopy camera on the
second port of the microscope we are able to obtain emission spectra
of the fluorophores and carry out Raman microspectroscopy at the
flick of a switch on the one sample without any changes in sample
position.”
In addition, the DSD Revolution confocal microscope uses a
bright white light source, which eliminates the expense and safety
precautions of working with lasers. “In the future we can easily
change the system to a different excitation emission combination -
something that would be prohibitively expensive with lasers,” Dr.
Browne says.
Research Paper:
Light–induced disassembly of self-assembled
vesicle-capped nanotubes observed in real time, Nature
Nanotechnology (2011), doi:10.1038/nnano.2011.120.