1. Review
10.1586/14789450.4.4.565 © 2007 Future Drugs Ltd ISSN 1478-9450 565www.future-drugs.com
Nanobiotechnology: quantum dots
in bioimaging
Yong Zhang†
, Noritada Kaji, Manabu Tokeshi and Yoshinobu Baba
†
Author for correspondence
Graduate School of Pharmaceutical
Sciences, University of Tokushima,
1-78, Shomachi, Tokushima
770-8505; Nagoya University,
Department of Applied Chemistry,
Graduate School of Engineering,
Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan
Tel.: +81 527 894 666
Fax: +81 527 894 666
yl7173@gmail.com
KEYWORDS:
biological application,
modification, nano, quantum
dot, synthesis, toxicity
Many biological systems, including protein complexes, are natural nanostructures. To
better understand these structures and to monitor them in real time, it is becoming
increasingly important to develop nanometer-scale signaling markers. Single-molecule
methods will play a major role in elucidating the role of all proteins and their mutual
interactions in a given organism. Fluorescent semiconductor nanocrystals, known as
quantum dots, have several advantages of optical and chemical features over the
traditional fluorescent labels. These features make them desirable for long-term stability
and simultaneous detection of multiple signals. Here, we review current approaches to
developing a biological application for quantum dots.
Expert Rev. Proteomics 4(4), 565–572 (2007)
With the completion of the Human Genome
Project and the cataloging of all gene
sequences [1], biological and biomedical inves-
tigations are now focusing on how the tens or
hundreds of thousands of proteins in a single
cell function and interact with each other.
Proteomics evokes the set of all protein iso-
forms and modifications, the interactions
between them and the structural description
of proteins and their higher-order complexes
[2]. By studying global patterns of protein con-
tent and activity and how these change during
development or in response to disease, pro-
teomics research is poised to boost our under-
standing of systems-level cellular behavior.
Clinical research also hopes to benefit from
proteomics by both the identification of new
drug targets and the development of new
diagnostic markers.
The scanning probe microscopies have limi-
tations in observing intracellular structures
with high selectivity and following the
dynamic behavior of these structures. Fluores-
cence is a widely used tool to address this
problem. However, conventional organic
fluorophores have two significant limitations:
they can not fluoresce continuously for long
periods and they are not optimized for multi-
color applications. The latter limitation stems
from two factors: each fluorophore can be
optimally excited only by the light of a defined
wavelength (which usually makes it necessary
to use as many excitation sources as types of
fluorophore) and each fluorophore has a rela-
tively broad emission spectrum (which often
causes the signals from different fluorophores
to overlap) [3].
Nanoparticles are microscopic particles with
at least one dimension less than 100 nm.
They are usually made of materials such as
metals, dielectrics and semiconductors. Great
scientific interest is focused on nanoparticles
as they are, effectively, a bridge between bulk
materials and molecular structures. A quan-
tum dot (QD) is made of a semiconductor
and has a discrete, quantized energy spectrum.
Properly modified QDs were largely used for
single-molecule probing. Compared with con-
ventional fluorophores, the nanocrystals have
a narrow, size-tunable, symmetric emission
spectrum and are photochemically stable
(FIGURE 1) [4,5].
Synthesis of quantum dots
The most common QD system is a CdSe/ZnS
core/shell semiconductor nanocrystal system.
The surface-to-volume ratio of CdSe cores is
very high. There are many vacancies and trap
sites on the surface, such that the fluorescence
spectrum of bare QDs has a broad tail due to
CONTENTS
Synthesis of quantum dots
Modification of
quantum dots
Biological application of
quantum dots
Concern regarding the
toxicity of quantum dots
Conclusion
Expert commentary
Five-year view
Financial disclosure
Key issues
References
Affiliations
For reprint orders, please contact reprints@future-drugs.com

2. Zhang, Kaji, Tokeshi & Baba
566 Expert Rev. Proteomics 4(4), (2007)
surface traps. In order to enhance the fluorescence efficiency,
ZnS was directly grown onto CdSe cores to passivate the sur-
face. The fluorescence efficiency of the ZnS-capped CdSe clus-
ters was dramatically enhanced [6,7]. A two-step procedure is
involved in the synthesis of CdSe/ZnS core/shell nanocrystals.
In the first step, CdSe core particles are formed. The core parti-
cles are then overcoated by ZnS in the second step. Tempera-
ture plays a critical factor in controlling the procedure in both
steps. The synthesis is based on Ostwald ripening, where fewer
and larger crystals, which have smaller surface-to-volume ratios
compared with small particles, form the solid, thus making the
entire system more stable [6,8].
QD that emits blue light is elusive owing to a lack of appro-
priate core-shell materials. Although blue emission can be
obtained from CdSe particles, the size of CdSe particles is lim-
ited within 2 nm. The small size makes synthesis and other
operations difficult. Steckel et al. developed a synthesis
method that was more facile to operate. They substituted CdS
(4.7 ± 0.4–5.2 ± 0.4 nm) for CdSe core. Blue light-emitting
(CdS)ZnS core-shell nanocrystals (460–480 nm) showed
quantum efficiencies in the range of 20–30% [9].
Bailey et al. reported a procedure for preparing large quanti-
ties of alloyed semiconductor quantum dots (CdSeTe) for con-
tinuous tuning of quantum confinement without changing
the particle size. Their results demonstrated that, besides par-
ticle size, composition and internal structure of QDs were
available for tuning the optical and electronic properties of
alloyed semiconductor quantum dots [10]. Zhong et al. discov-
ered that the composition-tunable emission across the visible
spectrum could be formed over the composition of
ZnxCd1-xSe nanocrystals (the emission wavelength blue shifts
gradually with the increase in Zn content). The high lumines-
cence efficiency and stability of the resulting alloy nanocrystals
were attributed to the larger particle size, higher crystallinity,
higher covalency, lower interdiffusion and spatial composition
fluctuation [11]. They also successfully synthesized high-quality
alloyed ZnxCd1-xS nanocrystals with high luminescent quan-
tum yields and extremely narrow emission spectral widths of
14–18 nm. The obtained narrow spectral width stems from the
uniform size and shape distribution, the high composition
homogeneity and the relatively large particle radius [12].
A wavelength range of particular interest for biomedical
imaging is the near-infrared (NIR) between 800 and 900 nm,
where absorption in tissue is minimal. Kim et al. developed
QDs with a core/shell/shell structure consisting of an alloy core
of InAs1-xPx, an intermediate shell of InP and an outer shell of
ZnSe. Alloyed core dots of InAs1-xPx show tunable emission in
the NIR region and the InP shell leads to a red shift and an
increase in the quantum yield [13].
Modification of quantum dots
Hydrophobic surface of QDs leads to aggregation and non-
specific adsorption, which hiders their application as biolabels.
To make QDs water soluble, their surface species were
exchanged with polar species.
Figure 1. Spectrum comparison between fluorescein (A) and a typical
quantum dot (QD) (B). QDs have a much broader adsorption spectrum (dashed
line) than fluorescein. The emission spectrum (solid line) of QDs is relatively
narrow and symmetrical compared with that of fluorescein (adapted from
Figure 1 of [4]). Size-tunable QDs are shown in (C). The average sizes of the QDs
are 4.2 (0.5), 4.4 (0.4), 5.6 (0.4) and 6.4 (0.4) nm for green, yellow, red and dark
red, respectively. Adapted from [18] with permission from the American
Association for the Advancement of Science.
Normalizedintensity
1.0
0.8
0.6
0.4
0.2
0.0
400 500 600 700
Normalizedintensity
1.0
0.8
0.6
0.4
0.2
0.0
400 500 600 700
Wavelength (nm)
Wavelength (nm)
Intensity(arb.units)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
480 520 560 600 640 680 720
B
C
A

3. Nanobiotechnology: quantum dots in bioimaging
www.future-drugs.com 567
Chan et al. modified the surface of CdSe/ZnS QDs with
mercaptoacetic acid through the binding of the mercapto
group to a Zn atom. The carboxylic acid group rendered the
QDs water soluble. The free carboxyl group was also available
for covalent coupling to various biomolecules by crosslinking
to reactive amine groups [14]. Mattoussi et al. developed a strat-
egy based on self assembly utilizing electrostatic attractions
between negatively charged alkyl-COOH-capped CdSe/ZnS
QDs and specific proteins consisting of positively charged
attachment domains. The alkyl-COOH groups permitted dis-
persion of QDs. The specific protein was in charge of fusing
with desired biologically relevant domains (FIGURE 2) [15].
When the organic ligand shell of QDs was modified by lig-
and exchange with thiols, the quantum yield (QY) would
diminish. Encapsulating QDs and their initial ligands with
macromolecules could preserve QY, but resulted in a bulky
size that was not desired. Kim et al. developed oligomeric
phosphine ligands to passivate QDs. The thin organic shells
avoided bulky size and retained high QY [16].
Pinaud et al. found a naturally evolved interaction between
organic and inorganic. Based on this interesting finding, they
designed synthetic α-peptides resembling phytochelatins.
These α-peptides could naturally bind on the surface of QDs
and could make QDs buffer soluble,
biocompatible and photostable [17].
Although priming the QD surface with
a thiolated molecule that has a free car-
boxyl group could make QDs soluble, the
bond is dynamic, leading to the low sta-
bility of QDs in water. Gerion et al.
masked QDs with a robust silica shell, the
procedure yielded nanocrystals encapsu-
lated in a silica shell of about 2–5 nm,
functionalized with thiols and/or amines
on the surface. The silica-coated QDs
showed a greater stability in biological
buffers compared with nanoparticles
primed with thiolated molecules [18].
Although polar species could be
exchanged on the surface to make the
QDs water soluble, both monolayers
and multilayers suffered disadvantages,
such as poor stability, long, difficult
coating processes, nonspecific adsorp-
tion and aggregation. Dubertret et al.
discovered that CdSe/ZnS QDs could
be encapsulated in the hydrophobic core
of a micelle composed of a mixture of
N-poly(ethylene glycol) phosphati-
dylethanolamine and phosphatidyl-
choline without any surface modifica-
tion. Transmission electron microscopy
images of QD–micelles were fairly mon-
odisperse, indicating low aggregation.
The fluorescence signal-to-background
ratio of QD–micelles was greater than 150, compared with
approximately four for silica-coated QDs, owing to the low
nonspecific adsorption [19].
With bioconjungation, QDs could target the desired pro-
teins. A three-layer strategy is often adopted: primary anti-
body, followed by biotinylated secondary antibody, followed
by streptavidin–QD. The size of this QD complex (∼50 nm)
can affect membrane protein trafficking and can reduce
accessibility to crowded locations in cells. Howarth et al.
developed a method to target QDs to cell surface proteins
that eliminated the bulky antibodies and provided a stable
linkage between the QD and the protein of interest. Mam-
malian cell-surface proteins tagged with a 15 amino acid
acceptor peptide could be biotinylated by biotin ligase added
to the medium, while endogenous proteins remained
unmodified. The biotin group then served as a handle for
targeting streptavidin-conjugated QDs [20]. The method was
demonstrated in targeting QDs to surface proteins of HeLa
cells (FIGURE 2). Gao et al. developed a class of polymer-encap-
sulated and bioconjugated QD probes for cancer targeting
and imaging in vivo. CdSe/ZnS QDs were encapsulated with
a triblock copolymer, multiple poly(ethylene glycol) molecules
and affinity ligands. The structural design avoided particle
Figure 2. (A) CdSe–ZnS core-shell nanoparticle with dihydrolipoic acid surface capping groups. (B) S–S linked
MBP–zb homodimer and detail showing nucleotide and primary amino acid sequences of the C-terminal basic
leucine zipper interaction domain. The poly-Asn flexible linker is boxed with dashed lines, unique engineered
cysteine is double boxed and lysine residues contributing to the net positive charge of the leucine zipper are
single boxed.
MBP-zb: Maltose-binding protein-basic leucine zipper.
Reproduced from [15] with permission from the American Association for the Advancement of Science.
Ser Ser Ser Asn Asn Asn Asn Asn Asn Asn Asn Asn Leu Gly Ile
TCG AGC TCG AAC AAC AAC AAC AAT AAC AAT AAC AAC CTC GGG ATC
Glu Gly Arg Cys Gly Gly Ser Ala Gln Leu Lys Lys Lys Leu Gln
GAG GGA AGG TGC GGT GGC TCA GCT CAG TTG AAA AAG AAA TTG CAA
Leu Lys Lys Lys Leu Ala Gln Gly Gly Asp ***
CTC AAG AAG AAA CTC GCC CAG GGT GGG GAT TAA TCT AGA GTC GAC
Xbal
Ala Leu Lys Lys Lys Asn Ala Gln Leu Lys Trp Lys Leu Gln Ala
GCA CTG AAG AAA AAG AAC GCT CAG CTG AAG TGG AAA CTT CAA GCC
Flexible peptide linker
Interchain dislufide bond
Basic leucine zipper
MBP
COO–
MBP
+ + +
+
+
+
S
CdSe
ZnS
HS SH
O–
O
HS
HS
O
SHHS
O–
O
SH
SH
O–
O
B
A
S
O–
COO–

4. Zhang, Kaji, Tokeshi & Baba
568 Expert Rev. Proteomics 4(4), (2007)
aggregation and fluorescence loss in physiological buffers and
in live animals. Tumor targeting was achieved by bioconjuga-
tion with an antibody. Combined with wavelength-resolved
imaging, the QD probes allowed sensitive and multicolor
imaging of cancer cells in living animals [21]. Wu et al. coated
CdSe/ZnS nanocrystals with a neutralized amphiphilic poly-
mer. The surface was then coupled to streptavidin or IgG. The
QDs successfully labeled a specific cellular target (i.e., labeling
the breast cancer marker Her2 on the surface of fixed and live
cancer cells, staining actin and microtubule fibers in the cyto-
plasm and detecting nuclear antigens inside the nucleus) [22].
Vu et al. conjugated the peptide ligand βNGF to the QD sur-
face. These βNGF–QDs activated TrkA receptors and initiated
neuronal differentiation in PC12 cells [23].
Biological application of quantum dots
Organic fluorophores have limitations for multicolor imag-
ing, since they require distinct excitation wavelengths and
their broad emission regions overlap with each other. By con-
trast, QDs can be excited by a wide spectrum of single and
multiphoton excitation light and have narrow emission spec-
tra. Voura et al. delivered dihydroxylipoic acid-capped QDs
into cells by Lipofectamine™ 2000 to study extravasation
in vivo. Five different populations of cells were simultaneously
identified [24].
Monitoring the interactions of multiple proteins or cells
within an organism is valuable when trying to understand the
complexity and dynamics of biological interactions. Organic
fluorophores are subject to photobleaching for this aim.
Jaiswal et al. developed an approach to conjugate CdSe/ZnS
QDs capped with dihydrolipoic acid ligands to positively
charged desired proteins. These QDs were demonstrated to be
suitable for simultaneous tracking of multiple proteins and live
cells for long periods [25].
Multiphoton microscopy is a primary fluorescence imaging
technique in thick specimens. Compared with conventional
fluorophores, the cross-sections of QDs were higher by three
orders of magnitude. Therefore, use of QDs may enable
imaging at greater depths than standard fluorophores do.
Larson et al. compared QDs with conventional methods by
injecting 70 kDa fluorescein isothiocyanate dextran at its sol-
ubility limit. An image acquired at the same depth with five-
times as much power shows considerably less detail. Thus,
QDs could be bright specific labels useful for tracking cells
deep within tissue or for detecting low concentrations of
antigens [26].
Targeting the nanoparticles to specific tissues and cell types
is important to realize disease sensing and drug delivery. Aker-
man et al. coated CdSe/ZnS core shell QDs with a site-recog-
nizing peptide. The peptide-coated QDs were then injected
into the tail vein of a mouse to investigate their homing abili-
ties. The modified QDs exactly found their targets in the rele-
vant vascular site [27]. Lidke et al. employed a QD–streptavidin
conjugate for in vivo studies of transduction. The measure-
ments revealed a new insight into processes and interactions
that could previously only be studied on fixed cells or by bio-
chemical fractionation. They discovered that erbB2, but not
erbB3, heterodimerized with erbB1 after EGF stimulation,
thereby modulating EGF-induced signaling [28]. Gac et al.
attached biotinylated annexin V on QD–streptavidin conju-
gates for studying the apoptosis process. The time lapse of
QDs and standard organic dyes was investigated. They discov-
ered that either FITC- or Alexa Fluor 647-annexin V conju-
gates photobleached within 25 min, while QDs stained the
cells well for several hours. Photostability of QDs enabled the
visualization of the fast event occurring at the membrane of
apoptotic cells. However, such events would be missed with
organic dyes [29].
Protein transduction domains (PTDs) are capable of trans-
ducing cargo across the plasma membrane. Whilst the size of
QDs falls well within the range of cargoes; based on these
considerations, Lagerholm et al. utilized a nine residue biotin-
ylated L-arginine peptide as PTD for intracellular delivery of
QDs. The cell uptake efficiency of QDs was greater by a mag-
nitude of almost two, as compared with incubation with bare
QDs. Images of transmission electron microscopy showed
that QDs were concentrated in endosomes and lysosomes.
This method revealed that uptake efficiency of QDs could be
dramatically improved in coding cells [30].
The emission properties of QDs could be tuned to emit into
the NIR region in contrast to the visible emission of the most
conventional photosensitizers. Since there is minimal light
scattering and absorption in the NIR region of the spectrum,
light of low intensity can be used to penetrate tissue to depths
of several centimeters, thereby allowing access to deep-seated
tumors. In addition, their large transition dipole moment led
to strong absorption, making them potential candidates for
application in photodynamic processes. Bakalova et al. thus
reported an exploitation of QDs energy-transfer properties to
give a therapeutic effect (FIGURE 3) [31].
Conventional NIR fluorophores, such as IRDye78-CA,
dissolved in serum or aqueous buffer rapidly photobleach.
Kim et al. prepared NIR CdTe(CdSe) core(shell) type II
QDs for sentinel lymph node (SLN) mapping. When incu-
bating these QDs in 100% serum at 37°C for more than
30 min, fluorescence emission decreased by only 10%. The
relatively stable QD system provided the surgeon with direct
visual guidance throughout the SLN mapping procedure
(FIGURE 4) [32].
Compared with gold nanoparticles (40 nm) or latex spheres
(500 nm), QDs could easily access single molecules and help
us to understand the dynamics of cellular organization.
Dahan et al. used single-QD tracking to study the rapid lat-
eral dynamics of Gly receptors. According to their observa-
tion, the receptors were classified as synaptic, perisynaptic
and extrasynaptic with distinct diffusion properties [33].
Phagokinetic track is a rapid and automatic method for stud-
ying cell motility. The previously used marker in this method
was an Au particle that had many limitations. For example,
the large Au particles could not stick well to the substrate and

5. Nanobiotechnology: quantum dots in bioimaging
www.future-drugs.com 569
Figure 3. Cancer therapy on the dot? (A) Photodynamic processes involved in photodynamic therapy. (B) Possible mechanisms for induction of photodynamic
processes by quantum dots. Reproduced with permission from Macmillan Publishers Ltd: Nat. Biotechnol. [31], © (2004).
ROS: Reactive oxygen species.
Figure 4. NIR QD sentinel lymph node mapping in the mouse and pig. (A) Images of mouse injected intradermally with 10 pmol of NIR QDs in the left paw.
Left: preinjection NIR autofluorescence image; middle: 5 min post-injection white light color video image; right: 5 min post-injection NIR fluorescence image. An
arrow indicates the putative axillary sentinel lymph node. Fluorescence images have identical exposure times and normalization. (B) Images of the mouse 5 min
after reinjection with 1% isosulfan blue and exposure of the actual sentinel lymph node (left: color video; right: NIR fluorescence images). Isosulfan blue and NIR
QDs were localized in the same lymph node (arrows). (C) Images of the surgical field in a pig injected intradermally with 400 pmol of NIR QDs in the right groin.
Four time points are shown from top to bottom: before injection (autofluorescence), 30 s after injection, 4 min after injection and during image-guided resection.
For each time point, color video (left), NIR fluorescence (middle) and color–NIR merge (right) images are shown. Fluorescence images have identical exposure times
and normalization. To create the merged image, the NIR fluorescence image was pseudocolored lime green and superimposed on the color video image. The
position of a nipple (N) is indicated. Reproduced with permission from Macmillan Publishers Ltd: Nat. Biotechnol. [32], © (2004).
NIR: Near infrared; QD: Quantum dot.
Intersystem
crossing
Chemicalreactions
Fluorescence
Internalconversion
Absorptionoflight
Phosphorescence
S0
S1
S2
S3
3
O2
(or X)
1
O2
(or X*)
Antibody
Cancer
cell
hν
hν
Quantum
dot
hν
ROS
Energy
transfer
Classical
photosensitizer
Cd2+
3
O2
3
O2
3
O2
DNA
degradation
Initiation of apoptosis
1
O2
Cd2+
ROS
1
O2
BA
1 cm
1 cm
Preinjection
autofluorescence
Color video 5 min
post injection
NIR fluorescence
5 min post injection
Color video NIR fluorescence
Color video NIR fluorescence Color-NIR merge
Image-
guided
resection
4min
postinjection
30s
post
injection
Preinjection
(auto-
fluorescence)
B
CA

6. Zhang, Kaji, Tokeshi & Baba
570 Expert Rev. Proteomics 4(4), (2007)
may perturb cell motility. Parak et al. deposited thin layers of
colloidal semiconductor nanocrystals on collagen-coated tissue
culture substrates, followed by seeding of cells. Human mam-
mary epithelial tumor cells (MDA-MB-231) voraciously
engulfed nanocrystals as they migrated and generated a region
free of QDs that revealed their pathways. By contrast, non-
tumor cells (MCF-10 A) appeared to be relatively immotile
depending on the observation that the layer of nanocrystals
was virtually identical to that seen around cells. By comparing
these two cell types, they demonstrated the use of colloidal
QD-based phagokinetic tracking [34].
Pathology scoring is the most widely used method of quan-
titative immunohistochemistry in clinical settings. However,
this method is on a discontinuous scale and the human eye is
not capable of discerning subtle differences in the antigen
expression level. Another method for protein quantification is
the use of fluorescence molecules through acquisition of high-
power images. Low accuracy was suffered due to photo-
bleaching of organic dyes. Ghazani et al. utilized the
QD-based immunoprofiling of proteins in the quantitative
analysis of tissue microarrays. The new method was sensitive,
accurate and on a continuous scale, and was validated in the
analysis of tumor antigens [35].
Reverse-phase protein microarray (RPMA) is a high-through-
put proteomic platform currently being developed for use in
clinical trials. Conventional labeling techniques for RPMA
detection include radioactivity, chromagens and fluorescence.
However, they often have significant limitations in terms of
their sensitivity, dynamic range, durability, speed, safety and
ability to multiplex. Geho et al. demonstrated that the use of
QD conjugated to streptavidin, QD 655 Sav, in a RPMA had
advantages of multiplexed assays, detection of unamplified
signals, expanded dynamic range and robustness [36].
Concern regarding the toxicity of quantum dots
Although QDs have received enormous attention for their
potential applications in biology and medicine, questions con-
cerning their potential cytotoxicity remain unanswered. A key
issue in evaluating the utility of these materials is the assessment
of their potential toxicity – either due to their inherent chemi-
cal composition (e.g., heavy metals) or as a consequence of their
nanoscale properties (e.g., inhalation of particulate carbon
nanotubes). Derfus et al. demonstrated that CdSe-core QDs
were indeed cytotoxic under certain conditions. Specifically,
surface oxidation through a variety of pathways led to the for-
mation of reduced Cd on the QD surface and release of free
cadmium ions, and correlated with cell death. However, the use
of QDs in vivo must be critically examined, as their results sug-
gested Cd release was a possibility over time. Surface coatings
such as ZnS and bovine serum albumin (BSA) were shown to
significantly reduce, but not eliminate, cytotoxicity [37].
Hoshino et al. revealed that the toxicity of QDs in biological
systems was dependent on the surface molecules of the nanoc-
rystal particle instead of core material [38]. Lovric et al. discov-
ered that QD-induced cytotoxicity was in part dependent on
QD size and was characterized by chromatin condensation and
membrane blebbing. BSA–QDs conjugates were significantly
less toxic than free QDs [39].
Conclusion
The emission wavelength desired is available by controlling
particle size, composition and the internal structure of QDs.
Before their biolabel applications, the surface of QDs should
be modified to make them soluble and site targetable. QDs
will complement conventional organic fluorophores for
applications needing better photostability, NIR emission or
single-molecule sensitivity over long time scales. With the
help of QDs, we can better understand the dynamics of
cellular organization.
Expert commentary
QDs have a wide absorption range and relatively narrow emis-
sion spectrum. It is possible to simultaneously probe several
QD-labeled targets with one excitation source. This approach
will provide us with more information to better understand the
dynamics of cellular organization. The higher cross-section of
QDs compared with conventional fluorophores is useful for
detecting low concentrations of antigens. When QDs were
modified by site-recognizing peptides, they could find and pre-
cisely label target proteins. The measurement may reveal new
insight into processes and interactions within cells or tissues.
NIR emission QDs are potential candidates to replace conven-
tional photosensitizers because the light in the NIR region
shows low absorption in tissues. QD-based immunoprofiling
of proteins in microarrays is more sensitive and accurate than
conventional methods.
Five-year view
QDs have far from exhausted their biological potential. Mostly
driven by cellular labeling, the effort to enable everyday
research is ongoing. The future work is involved in simultane-
ous tracking of multiple proteins and live cells for long periods
and, therefore, in investigating a range of phenomena in cell
and developmental biology that have been unexplored because
of the lack of suitable fluorescent labels. In addition, an under-
standing of receptor-mediated transduction mechanisms is
essential for rational receptor-targeted therapeutics. Delivery
and targeting of ligand compounds that surpass cell surface
binding and evoke sufficient cellular responses are key require-
ments for designing functional cell probes and delivery devices.
The ligand-conjugated QD will play a key role in this effort in
the near future. Finally, it is of great interest to develop new
QDs deposited in a vertical gradient, which may lead to a 3D
view of extracellular matrix media for depth contrast.
Financial disclosure
The authors have no relevant financial interests, including
employment, consultancies, honoraria, stock ownership or
options, expert testimony, grants or patents received or pending,
or royalties related to this manuscript.

7. Nanobiotechnology: quantum dots in bioimaging
www.future-drugs.com 571
Key issues
• Quantum dots (QDs) were prepared for fluorophores. A narrow, size-tunable, symmetric emission spectrum, photochemical
stability and a continuous excitation spectrum made QDs complementary to conventional fluorophores.
• Different color-emitting QDs could be made through the control of constituent stoichiometries in alloy nanoparticles. The
composition-tunable emission was investigated over the composition of the ZnxCd1-xSe nanocrystals.
• A self-assembly method for conjugating protein molecules to CdSe-ZnS core-shell QDs was described. The conjugation
utilized electrostatic attractions between negatively charged lipoic acid-capped CdSe-ZnS QDs and engineered bifunctional
recombinant proteins, comprising positively charged attachment domains.
• Hydrophobic CdSe/ZnS core/shell nanocrystals were embedded in a siloxane shell. The introduction of functionalized groups
onto the siloxane surface would permit the conjugation of nanocrystals to biological entities.
• QDs were first used as markers for phagokinetic tracks.
• QDs were used for multiphoton imaging in live animals.
• In vivo targeting studies of human prostate cancer growing in nude mice.
• CdSe-core QDs were found cytotoxic in the case of forming reduced Cd. Surface coating could dramatically reduce
the cytotoxicity.
References
Papers of special note have been highlighted as:
• of interest
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Affiliations
• Yong Zhang, PhD
Postdoctoral Researcher, Graduate School of
Pharmaceutical Sciences, University of
Tokushima, 1-78, Shomachi, Tokushima
770-8505; Nagoya University, Department of
Applied Chemistry, Graduate School of
Engineering, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan
Tel.: +81 527 894 666
Fax: +81 527 894 666
yl7173@gmail.com
• Noritada Kaji, PhD
Assistant Professor, Nagoya University,
Department of Applied Chemistry, Graduate
School of Engineering, Furo-cho, Chikusa-ku,
Nagoya 464-8603; MEXT Innovative Research
Center for Preventive Medical Engineering,
Nagoya University, Japan
Tel.: +81 527 895 584
Fax: +81 527 894 666
kaji@apchem.nagoya-u.ac.jp
• Manabu Tokeshi, PhD
Associate Professor, Nagoya University,
Department of Applied Chemistry, Graduate
School of Engineering, Furo-cho, Chikusa-ku,
Nagoya 464-8603; MEXT Innovative Research
Center for Preventive Medical Engineering,
Nagoya University, Japan
Tel.: +81 528 046 209
Fax: +81 527 894 666
tokeshi@apchem.nagoya-u.ac.jp
• Yoshinobu Baba, PhD
Professor, Nagoya University, Department of
Applied Chemistry, Graduate School of
Engineering, Furo-cho, Chikusa-ku, Nagoya
464-8603; Plasma Nanotechnology Research
Center, Nagoya University; MEXT Innovative
Research Center for Preventive Medical
Engineering, Nagoya University, Health
Technology Research Center, National Institute
of Advanced Industrial Science & Technology
(AIST) Hayashi-cho 2217-14,
Takamatsu 761-0395, Japan
Tel.: +81 527 894 664
Fax: +81 527 894 666
babaymtt@apchem.nagoya-u.ac.jp