IEEE Spectrum October, 2016 - 30

of miCe and moleCules: Quantum dots have long been used
to label proteins. The red and green dots in this micrograph of
mouse intestine [top] do just that. (The blue is an organic dye.) Red
and green quantum dots from Nanoco were used to mark polymer
beads, standing in for cells [middle]. Quantum dots have also been
used experimentally in mice to trace flow within the lymphatic
system [bottom]. Panel A is before injection; panels B and C are
5 minutes after injection in the forepaw [arrow]. (In C, the injection
site is blocked from the imager.)
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whaT exacTly are These magical quantum dots? Quantum dots are made up of tiny
chunks of semiconducting material, which typically measure between 2 and 10 nanometers across. Because
they are so small, their electronic and optical properties
are distinct from those normally associated with whatever
material they are made of. They straddle an odd middle
ground between molecules and bulk material, a fact that
becomes most evident when you consider how they fluoresce, absorbing a broad range of wavelengths and then
reemitting that energy as light of a very specific wavelength-
an ultrapure color.
The beauty of quantum dots is that you can tailor their
optical properties simply by changing their size: The
larger the dot, the redder (longer wavelength) its optical
emissions. For most semiconductors, you have to change
their composition to change their optical properties. No
wonder the engineers making displays are keen to take
advantage of quantum dots.
Beginning in the mid-1990s, scientists began devising various ways to produce these tiny crystals in quantity. In 2001,
Nanoco was spun off from Paul O'Brien's research group in
the School of Chemistry at the University of Manchester to
commercialize a process for quantum-dot synthesis called
molecular seeding, which allows very good consistency and
reproducibility. The challenge for us then became making
quantum dots that don't cause any ill effects. You certainly
wouldn't want to use the typical kinds of quantum dots that
have been created in the past: The cadmium they contain is
a huge threat to human health. You wouldn't want to allow
that cadmium to be released into the environment, much
less be injected into a patient.
Anticipating that this issue can be overcome, medical
researchers around the globe have been investigating the
possible uses of quantum dots in the body. Working with
breast-cancer tumors in mice, for example, scientists have
found that, unlike conventional dyes, quantum dots are less
likely to migrate through the lymphatic system beyond the
regional lymph nodes around the tumor.
Soon after Nanoco was established, it began engineering a new type of quantum dot that is free of cadmium and
other toxic heavy metals. Other companies quickly followed
↗ PosT youR CommeNTs at http://spectrum.ieee.org/quantumdot1016

From Top: Thomas DeeriNck aND mark ellismaN/Ncmir; NaNoco TechNologies; marioN helle/plos oNe

the area with either ultraviolet or blue light. The quantum dots would fluoresce brightly and could do so for the
duration of the operation. They could be engineered to
emit light at whatever wavelength the surgeon desired, be
it one that can be seen directly or one that emits in the
infrared, allowing its light to pass through skin and fatty
tissue. The surgeon could take as much time as needed to
determine the locations of sentinel nodes, which would
presumably make such procedures more accurate.

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Table of Contents for the Digital Edition of IEEE Spectrum October, 2016