Yong Zhang talks with
ScienceWatch.com and answers a few questions about
this month's New Hot Paper in the field of Materials
Science. The author has also sent along images of his
Article Title: Multicolor Core/Shell-Structured
Upconversion Fluorescent Nanoparticles
Authors: Li, ZQ;Zhang, Y;Jiang, S
Journal: ADVAN MATER
Year: DEC 17 2008
* Natl Univ Singapore, Div Bioengn, Singapore 117574,
* Natl Univ Singapore, Div Bioengn, Singapore 117574,
(addresses have been truncated.)
Why do you think your paper is highly
This paper reports on the synthesis of near-infrared (NIR)-to-visible (VIS)
upconversion fluorescent nanoparticles and their use in biological
applications. Although upconversion fluorescent nanoparticles present a
relatively new area of research, the wide attention that this paper has
received reflects upon the increasing interest that it has spurred amongst
the scientific community.
I think this paper is highly cited because it touches on a subject that is
of wide-ranging relevance and which is also being actively pursued by
various researchers of diverse backgrounds, beginning from lead-user
material scientists to end-user biologists.
Indeed, it has aroused much interest, as it outlines the development and
application of a fluorescent material that has the potential to overcome
major challenges which other fluorophores have faced during the past
several decades, while also specifically addressing issues that used to be
technologically challenging in making upconversion fluorescent
nanoparticles with well-controlled sizes, shapes, surfaces, and multicolor
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The paper described a new method to synthesize core-shell structured
upconversion nanoparticles with multicolor emission. The traditional route
of synthesizing these upconversion nanocrystals is normally done in organic
solvents or at high temperatures.
Although some chelating agents, such as ethylenediaminetetraacetic acid
(EDTA), have been employed to control the growth of the nanocrystals, these
resultant nanocrystals can only be dispersed in some organic solvents to
form colloidal solution after sonication.
Upconversion Flourescence Imaging
View/download two accompanying slides and
In this paper, we have developed a facile and user-friendly method to
produce uniform hexagonal-phase nanocrystals with strong upconversion
fluorescence at low temperature and silica-coated nanoparticles with
core-shell structure and having multicolor upconversion emission.
Would you summarize the significance of your paper
in layman's terms?
Today's commonly used fluorescent probes are mainly based on downconversion
materials requiring excitation by short-wavelength light (e.g. organic
dyes, quantum dots, and green fluorescent proteins).
Their use remained problematic due to several drawbacks, such as high rate
constants for photobleaching, short penetration depth, and strong
autofluorescence background, all of which decreases the sensitivity and
efficiency of detecting the fluorescent signal and thus, consequently,
hampers their use for more varied applications.
On the other hand, NIR-to-VIS upconversion nanomaterials dismissed the need
for such excitation by short-wavelength light. Rather, they can convert NIR
light to VIS light upon NIR light excitation.
Since all biomolecules absorb minimally in the NIR window, this will solve
current problems associated with the use of illumination by
short-wavelength light, such as autofluorescence background, low light
penetration depth, and photodamage to biological specimens. These
upconversion materials also show negligible photobleaching.
Endowed with all these favorable properties, upconversion fluorescent
materials show a great potential for use in diverse bio-applications,
including, but not limited to, deep-tissue imaging, detection of low
abundant sub-cellular components, gene delivery and tracking,
photo-controllable gene expression, and targeted photodynamic therapy
It is also anticipated to leave room for applications in many other
research areas, examples of which include solid-state lasers, flat-panel
displays, solar cells, and so forth.
How did you become involved in this research, and
were there any problems along the way?
My involvement in nanoparticle research began during my postdoctoral
fellowship at the University of Washington in Seattle, where I was then
focusing mainly on magnetic nanoparticles. Later on, when I moved on to
take a faculty position in the Division of Bioengineering at National
University of Singapore, I found research on quantum dots to be quite
However, after spending two to three years on doing research on quantum
dots, I encountered many problems with quantum dots that have yet to be
resolved. Not only do we face problems with synthesizing these materials,
but their intrinsic properties, such as cytotoxicity, has also posed a
limitation on their potential applications.
"I think this paper is highly cited because it touches
on a subject that is of wide-ranging relevance and which is
also being actively pursued by various researchers of
diverse backgrounds, beginning from lead-user material
scientists to end-user biologists."
This had thus sparked in me an urge to explore other new materials that
could possibly replace the caveats faced in using quantum dots. It was then
that I found lanthanide-based upconversion nanoparticles.
Due to their unique optical properties, I instinctively knew that this is
the material which I had been looking for all along. So that was how we
embarked on our research in upconversion nanoparticles.
At the beginning, we focused on the development of new methods for
synthesizing this material. By using existing methods reported in the
literature, only upconversion nanoparticles that were not biocompatible and
not dispersible in water could be produced, and as such their use for
bio-related applications was limited.
Furthermore, the particles were too large in size, unstable in solution,
and their surface was non-functionalized, thus making conjugation of
biomolecules impossible. There was no commercial equipment that could be
used to capture images of the upconversion fluorescence in biological
Today, we have resolved these issues by developing a facile synthesis
method to produce biocompatible upconversion nanoparticles with a suitable
surface and good dispersibility in water for use in bioimaging,
biodetection, photodynamic therapies of viruses and cancer,
photo-controllable gene expression, and possibly even more. We have also
set up a custom-fitted confocal microscope and animal imaging system and
are proud to be pioneering in this area as well.
Where do you see your research leading in the
I can foresee that upconversion nanoparticles will become the next
generation of ideal fluorophores. Their potential for use in biological and
even clinical applications is wide-ranging, from biolabeling and bioimaging
to photodynamic therapy and photo-controllable gene expression, and perhaps
even in photo-induced micropatterning.
As such, there is huge potential for their commercialization. As part of
our continuous efforts in producing nanoparticles with improved qualities,
development of a new synthesis route to produce nanocrystals with reduced
size and high upconversion efficiency is already underway.
Do you foresee any social or political
implications for your research?
Though still a far-fetched idea, the potential of upconversion
nanoparticles for use in clinical settings, such as photodynamic therapies
against viruses and cancer, can possibly provide an alternative and better
treatment modality to overcome the flaws of our existing ones.
Yong Zhang, Ph.D.
Division of Bioengineering
National University of Singapore