Medical Design Briefs - April 2022 - 50

'Seeing' Nonuniformities in 2D Materials May Lead to
New Medical Sensors
Detecting nonuniformities
in optical properties opens
the door to new uses.
Penn State University
State College, PA
A novel and better approach at detecting
nonuniformities in the optical properties
of two-dimensional materials could
potentially open the door to new uses for
these materials, such as for drug detection,
according to a team of researchers.
" The Two-Dimensional Crystal Consortium
(2DCC) is a world leader in 2D materials
research, and my lab often works with
the 2DCC doing materials characterization
for novel 2D materials, " says Slava V.
Rotkin, Frontier Professor of Engineering
Science and Mechanics with an appointment
in the Materials Research Institute at
Penn State. " There is a big challenge in
these studies: Frequently, optical properties
of 2D materials are not uniform in
space. Furthermore, they may vary at a very
small spatial scale, down to a single atom. "
Rotkin and other researchers were
able to take one step toward a possible
solution, which was outlined in ACS
Nano. While Rotkin stresses they only
gave a demonstration of the principle in
the study, the solution they propose was
used for van der Waals heterostructures
that could enable sensors made with 2D
materials, materials that are one to a few
atoms thick.
Sensors can be developed that enable
sensing of bio-, chemical, and/or medical
analytes of interest. Analytes are specific
chemicals targeted for measurement
or analysis. A good sensor detects
these analytes with minimal sample
preparation, in an abbreviated time
frame, with low detection limits, and
using samples containing substances
other than the key analyte.
Identifying and understanding variability
of properties in materials could be
extremely important for applications of
2D materials as sensors. The sensor material
typically can only interact with the
analyte at the surface. Thus, the material's
surface is an active area, while material's
volume is not. The larger the ratio of surface
to volume, the lower the fraction of
material which cannot be used. Such
atomically thin materials have the ultimate
surface-to-volume ratio for sensor
50
Cov
A molecule of the cancer medication doxorubicin (foreground), detected using the van der Waals
vertical heterostructure biosensor (background). The background is an actual nanoscale scattering
scanning near-field optical microscopy image (sSNOM) of the heterostructure, and the large triangle
is a single-layer MoS2 island (ca. 3.7 μm wide). The smaller triangle is a partially oxidized MoOS
island, and the whole sample is covered with the monolayer graphene, with several wrinkles clearly
seen in the map. The darker graphene area corresponds to the region of extra charge doping.
(Credit: Elizabeth Flores-Gomez Murray/Jennifer McCann/Slava Rotkin)
use and may possess surface non-uniformities
at the nanometer scale. This
includes atomic impurities, adsorbates,
defects, wrinkles, ruptures, etc. Such features
can modulate the optical properties.
" Despite this being critical for effectiveness
in certain application of 2D
materials, there is currently no truly
effective approach to detect these variabilities, "
Rotkin says. " Due to their
being so tiny, they are undetectable by
optical tools and nonoptical tools cannot
resolve optical contrast. "
The researchers conducted experiments
using a heterostructure material
made of graphene, the 2D material version
of graphite, and the inorganic compound
molybdenum disulfide. The
molybdenum disulfide gives a photoluminescence
signal that detects the
amount of charge transfer between the
graphene and the molybdenum disulfide
layers. Therefore, it can detect
changes due to the bio analyte, which in
this case is the cancer treatment drug
doxorubicin, that can affect the charge.
These changes are also detectable in
graphene via analysis by Raman spectroscopy,
which discovers unique vibrawww.medicaldesignbriefs.com
ToC
tions
in molecules. A Raman microscope
picks up shifts in the frequency of photons
in the laser light beam caused by
these vibrations.
" The two channels together allow a better
calibration of the two signals against
analyte concentration and the type of
analyte, " Rotkin says. " And additionally,
graphene enhances the Raman signal of
the analyte itself to the extent one can
'see' a signal from just a few molecules. "
The researchers used doxorubicin as
their analyte because it is a common
cancer drug used in chemotherapy, and
there is an acute need for biosensors to
detect it to help regulate dosage and
reduce side effects. There are two types
of biosensors that work for this purpose,
label-free biosensors, which can be used
to detect a variety of drugs, and labelbased
biosensors, which can detect only
a specific drug. The researchers used
label-free biosensing in the study.
" The label-based biosensor is like a
lock that can be opened with only one
key, but the label-free biosensor is like a
lock with many different keys, " Rotkin
says. " We did not invent label-free multimodal
biosensing. This approach has
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Medical Design Briefs - April 2022

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