SELECTED SUMMARY
nano-FTIR Enables Scientific
Breakthroughs Worldwide
Novel applications of nano-FTIR to
biological, polymer and 2D materials.
Sergiu Amarie, PhD
INTRODUCTION
Nano-Fourier transform infrared (nano-FTIR) is an optical
Applications Scientist
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technique that combines scattering-type scanning nearfield optical microscopy (s-SNOM) with FTIR spectroscopy.
This technique offers the spatial resolution of atomic-force
microscopy (AFM) plus the analytical power of IR spectroscopy
at the nano-scale. Thus, nano-FTIR has the potential to
advance scientific research across fields such as science,
engineering, and industrial research, which include 2D
Tobias Gokus, PhD
Applications Scientist
neaspec designs
materials, plasmonics, polymers, materials and life sciences,
and semiconductor research.
This paper highlights several scientific breakthroughs of nanoFTIR users in diverse fields of biologics, polymer science, and
2D materials.
Stefan Mastel, PhD
Sales Applications Scientist
neaspec designs
WHAT IS NANO-FTIR?
Nano-FTIR offers some distinct advantages over conventional FTIR. For instance, if one were to
compare the spectra of an insulin/tobacco mosaic virus (TMV) mixture with that of pure insulin
using conventional FTIR technology, the instrument would be unable to distinguish between
the two, as will be explored later in this paper (see FIGURE 1). It is impossible to identify a single
virus that is tens of nanometers in size with a light that has a wavelength 1,000 times larger (~10
μm).
Nano-FTIR technology,however,does allow such imaging and spectroscopy of nano-sized objects,
even when using such a large IR light. Nano-FTIR overcomes the diffraction limit because of how
an IR laser-beam interacts with the AFM tip.
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When the tip is illuminated with a broadband
Germany, conducted a study in which TMV was
infrared laser beam, its geometrical shape forces
mixed with insulin fibrils.1 The two materials
the charges to accumulate at the cone apex,
have different secondary structures that should
which leads to enhanced field concentration.
be visible in the infrared. The insulin spectrum
This concentration occurs at a scale that is
has a peak at 1634 cm -1 with a small shoulder
comparable with the radius of the tip apex.
at 1660 cm -1. In contrast, the virus spectra
Illuminated AFM tips generate a nano-focus at
with predominant a-helix conformation
their apex, revealing only the objects below the
peaks at 1660 cm -1. The researchers obtained
tip and allowing one to conduct IR microscopy
IR absorbance images at those two peak
and spectroscopy at the nanoscale (FIGURE 2).
locations (1634 to map the β-sheet and 1660 for
the α-helical conformation).
USE OF NANO-FTIR IN BIOLOGICS
Nano-FTIR is very useful for studying biologics
With conventional FTIR (FIGURE 1A), the
such as the secondary structure of proteins
insulin and the mixture of insulin/TMV are
and enabling label-free studies of nanoscale
barely visible. This is noteworthy because the
biological systems. For example, researchers
virus clearly has a ratio above 1.5and the insulin
have used nano-FTIR spectroscopy to
has a ratio below 1.The substrate has a neutral
differentiate the secondary structure of viruses
ratio of around 1.
differences between the spectra of the pure
with predominant α-helix conformation (e.g.,
virus membrane) and the β-sheet protein
Meanwhile, FIGURE 1B shows the results when
arrangements of insulin or prions.
using the nano-FTIR technique. The two peak
maxima are clearly discernible. In a next step,
Research groups from nanoGune, in San
the two different peak maxima can be mapped
Sebastian, Spain, and Freie University in Berlin,
to illustrate the distributions.
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elucidate the nuclear organization of white
blood cells.2 Unlike other techniques, nanoFTIR successfully differentiated DNA spectra
from proteins without labeling.
In summary, due to its high spatial resolution
and high detection sensitivity,nano-FTIR
provides a tool to study biological systems where
Another interesting application is for research
the dimensions of interest are typically below the
on amyloid diseases such as Alzheimer’s
diffraction limit—even particles as small 10nm.
disease or mad cow disease, which cause
characteristic protein aggregation in the brain.
Here, scientists from Paderborn, Germany,
MATERIALS APPLICATIONS
Polymer brushes are thin, functional surface
used nano-FTIR spectroscopy to study the
coatings composed of densely packed linear
formation of amyloid fibrils and differentiate
polymer chains that stretch away from the
between the secondary structure of the
surface to which they are attached. Depending
α-helical conformation and the β-sheet protein
on the chemical functionalization of the
arrangement of amyloid fibrils. Nano-FTIR was
molecular chains end groups as well as the
able to quantify the α-helix and β-sheet of a
conformation and orientation of these chains,
single protein on the 10-nm scale, well below
polymer brushes can act as repellents or
the diffraction limit of conventional FTIR.
attractors (see FIGURE 3).
Finally,researchers from the University of
Nano-FTIR is ideal for studying nano-scale
Manitoba in Canada applied nano-FTIR to
domains formation, molecular orientation, and
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molecule identification inside a polymer brush
film across applications such as biosensing,
shows a representative, high-resolution
nano-FTIR imaging dataset. FIGURE 4A depicts
biomedicine, and tissue and transplant
the AFM height and reveals nonuniform
engineering. An example can be seen with
polymer brush coverage consisting of ~60 nm
polyethylene glycol (PEO), a polymer brush
wide hills and valleys as well as some larger
material that can be attached to a substrate
agglomerates. Recorded simultaneously,
surface directly or with an intermediate linker
FIGURE 4B
shows the nano-FTIR reflectivity and
layer. An ordered, upright standing PEO polymer
FIGURE 4 C
shows the nano-FTIR absorption
brush film can be formed through a thermal
tenet of the PEO film, revealing a spatially
grafting process. Unfortunately, this process
different optical contrast for the hill and valley
does not always yield a uniform polymer brush
domain. Comparing the nano-FTIR absorption
film. Improving the grafting process and
and AFM topography images of the same area,
improving the coating functionality requires
one will see that the nano-FTIR absorption is
a better understanding of the nano-scale
stronger at the hills.
distribution of the polymer conformations and
their orientation in the polymer brush film.
To learn more about the origin of the
absorption contrast, nano-FTIR absorption
FIGURE 4
shows the first step of acquiring this
information using nano-FTIR imaging. FIGURE 4
spectra were measured at the hill (green
spectra in FIGURE 4D ) and in the neighboring
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valley (blue spectra in FIGURE 4D). Interestingly,
conformations can also be perpendicular to the
both nano-FTIR spectra show the characteristic
surface or parallel to the surface. In the analysis,
absorption bands of P EO. While the entire
a model function is generated to represent
sample is covered by PEO, there is an obvious
these four conformations. Using this result, a
difference in the width, shape, and magnitude
least square fit is applied to the spectra. The
of two absorption bands. The difference in
end result provides more information on the
the absorption strength for these two bands
orientations and conformations.
suggest different molecular chain orientation
and confirmation of the PEO molecular chains
FIGURE 5B
in the two different domains.
composed of about 80% of B conformation
shows that the hill locations are
arranged perpendicular to the surface. The
Conformational analysis measured by
remaining 20% appears to be mostly the B
performing a least-square fit of the as-
conformation parallel to the surface. For the
measured nano-FTIR absorption spectra
valleys, FIGURE 5C suggests there is no clearly
provides more information about the
dominant conformer and the chains do not
distribution of different orientations and the
have a preferential orientation. This clearly
conformations (see FIGURE 5). The polymer
indicates the utility of the nano-FTIR technique
brush molecule can have one of two low-
to understanding domains and conformations
energy conformations (FIGURE 5A). These two
in polymers at nano-scale spatial resolution.
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Another application where nano-FTIR is useful
component of interest, 967 cm -1 for styrene
for chemical composition mapping of complex
butadiene (SBR) (FIGURE 6B ), a map of the
polymer nano-composites. Car tires are an
SBR distribution was obtained. Choosing
example of a complex composite material.
characteristic wavelengths for silicon dioxide
Understanding the distribution of various
(1120 cm-1, FIGURE 6C) and natural rubber (1465
compounds in a material leads to a greater
cm-1, FIGURE 6D) provides maps of the localized
understanding of the mechanical properties
chemical distributions. When the three maps
and performance.
are overlaid in an RBG image (FIGURE 6E), we
can visualize the distribution of the individual
From an AFM topography image, we can see
components and more importantly the mixing
different domains in a tire sample (FIGURE 6A).
of the components as indicated by the light
But this conventional AFM image does not
blue and blue-green. From this compositional
have the chemical sensitivity for differentiating
image, we can also derive information about
between and characterizing the different
the particle diameters and the spacing of the
compounds. Nano-FTIR-based chemical
particles and domains in the sample. These
mapping, however, can provide this information
parameters are all important to understand
because it uses monochromatic IR light with a
the performance of the material.
wavelength that matches a specific absorption
path. It is unique for each compound.
2D MATERIALS
The final application of nano-FTIR is in a
In this example, by selecting the nano-
field that has been emerging rapidly—the
FTIR excitation wavelength to match the
manipulation of polaritons in Van der Waals
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materials. Here, the experimental detection
Antennas placed on the surface of 2D materials
mechanism is very different than in the
can also launch the polaritons. In the final
organic materials discussed previously.
example6, an antenna is placed on the surface
of a 2D material and using the nano-FTIR
A metal AFM tip is place on a graphene
technology, one can see the relevant optical
nano-ribbon and is illuminated with IR laser
properties of the device, something that is not
radiation. The tip acts as an antenna and
visible by standard AFM. In this case, there is a
generates a nano-scale hot spot at the tip’s
clear map of the electric field resonance from
apex. This nano-focus provides high k-vectors
the antenna and one can also visualize that
due to the large field confinement and
the antenna launches graphene plasmons
launches plasmon polaritons in the graphene
(see FIGURE 8). This provides information on the
ribbon. The graphene plasmons travel in
optical behavior of novel devices to contribute
concentric circles away from the tip and are
to the design and performance evaluation of
reflected at the edge. The reflected plasmon
these devices.
wave travels back toward the tip. Right below
the tip, the plasmon waves interfere with
The final 2D material example is in
the excited light at the apex. This modifies
molybdenum trioxide (MoO3).7 In this example,
the scattered light at the tip and is detected
we see nano-edges that were purposely
in the far field, so one can detect the cell’s
fabricated into the MoO3 layers. These
interference of these traveling or propagating
layers provide the high k-vectors to launch
graphene plasmon (FIGURE 7 ).
propagating polaritons but they are different
4, 5
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than the graphene example. The twisted bilayer
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CONCLUSION
The research presented here demonstrates the
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