Comparative examination of the stability of semiconductor quantum dots in various biochemical buffers

Article abstract of DOI:10.1021/jp056371p

Due to their greater photostability compared to established organic fluorescence markers, semiconductor quantum dots provide an attractive alternative for the biolabeling of living cells. On the basis of a comparative investigation using differently sized thiol-stabilized CdTe nanocrystals in a variety of commonly used biological buffers, a method is developed to quantify the stability of such a multicomponent system. Above a certain critical size, the intensity of the photoluminescence of the nanocrystals is found to diminish with pseudo-zero-order kinetics, whereas for specific combinations of particle size, ligand, and buffer there appears to be no decay below this critical particle size, pointing out the necessity for thorough investigations of this kind in the view of prospect applications of semiconductor nanocrystals in the area of biolabeling.

Full text of DOI:10.1021/jp056371p

1959
2006, 110, 1959-1963
Published on Web 01/13/2006
Comparative Examination of the Stability of Semiconductor Quantum Dots in Various
Biochemical Buffers
Klaus Boldt,† Oliver T. Bruns,‡ Nikolai Gaponik,§ and Alexander Eychmu1ller*,§
Institute of Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, 20146 Hamburg, Germany,
Department of Biochemistry and Molecular Biology II: Molecular Cell Biology, UniVersity Medical Center
Hamburg-Eppendorf, Martinistraâe 52, 20246 Hamburg, Germany, and Institute of Physical Chemistry and
Electrochemistry, TU Dresden, Bergstraâe 66b, 01062 Dresden, Germany
ReceiVed: NoVember 4, 2005; In Final Form: December 22, 2005
Due to their greater photostability compared to established organic fluorescence markers, semiconductor
quantum dots provide an attractive alternative for the biolabeling of living cells. On the basis of a comparative
investigation using differently sized thiol-stabilized CdTe nanocrystals in a variety of commonly used biological
buffers, a method is developed to quantify the stability of such a multicomponent system. Above a certain
critical size, the intensity of the photoluminescence of the nanocrystals is found to diminish with pseudo-
zero-order kinetics, whereas for specific combinations of particle size, ligand, and buffer there appears to be
no decay below this critical particle size, pointing out the necessity for thorough investigations of this kind
in the view of prospect applications of semiconductor nanocrystals in the area of biolabeling.
Introduction
cence depends on the concentration, pH value, concentration
of salts in solution, and time is essential. Since CdTe QDs may
Due to their tuneable luminescence wavelength and high
photostability compared to organic dyes, II-VI semiconductor
quantum dots (QDs)1 are attractive candidates for fluorescence
markers in biolabeling. They are small enough to be incorporated
by cells and even seem to be transferred into cells on their own
just by adding a solution of QDs. Due to the size quantization
effect, only one material is necessary to mark different proteins
or cell compartments simply by employing different size
fractions of one particle synthesis.
Apart from the organometallic synthesis of II-VI semicon-
ductor QDs based on the thermolysis of precursors in highly
coordinating, nonpolar solvents (trioctylphosphine/trioctylphos-
phineoxide, TOP/TOPO), a synthetic route using water as a
solvent and thiols as ligands has been established.2 QDs prepared
by this method incorporate the sulfur of the thiol groups as part
of their crystalline structure. When using Se or Te as the
chalcogenide, one can speak of a core/shell nanocrystal (NC)
containing a metal sulfide shell.3 To keep the particles in
solution, the ligands have polar moieties such as carboxylic
groups. This facilitates the interest in such QDs for biological
applications, as they are water-soluble as formed and can be
transferred into living cells without prior ligand exchange.
Furthermore, these groups may be functionalized by coupling
with proteins for labeling. Successful conjugation of semicon-
ductor QDs to proteins such as bovine serum albumin (BSA)
and IgG has already been reported.4,5 To use QDs for labeling
in a living organism, knowledge of how the particle’s lumines-
slowly release the toxic Cd2+ and Te2- ions into the solution,
the particles must be as inert as possible for any in vitro
application. As Parak et al. have shown,6,7 the toxicity of CdTe
QDs not only depends on the concentration of free Cd2+ ions
but also depends on whether the particles are ingested by a cell
and where they are stored. The release of Cd2+ from the
particles’ surface can be reduced by employing core/shell
particles or coating of the particles with silica or a polymer.
Also, the photoluminescence (PL) of the QD must not be
influenced by desorption of the ligands, destruction of the NC,
or creation of traps for radiationless recombination of an excited
electron/hole pair.
In this work, buffers commonly used for the indirect
immunofluorescence labeling of cells were tested.8 To do so,
several fractions of the size-selective precipitation of thioglycolic
acid (TGA)- and mercaptopropionic acid (MPA)-stabilized
particles were mixed with different buffers of a defined
concentration. The particle PL decay was observed over 1 week
by emission spectroscopy.
Experimental Section
Synthesis of Thiol-Capped CdTe Nanocrystals.2 A 3.06
mmol portion of the stabilizer and 0.985 g (2.35 mmol) of Cd-
(ClO4)2‚6H2O was dissolved in 125 mL of deionized water. The
pH was adjusted to a value of 12 using a 1 M solution of NaOH.
The solution was stirred and deaerated by bubbling nitrogen
through it for 30 min. Under stirring, H2Te gas generated by
adding 10 mL of 0.5 M H2SO4 to 0.2 g (0.46 mmol) of Al2Te3
was passed through the solution with a slow stream of nitrogen.
Depending on the stabilizer, a red or orange solution of CdTe
precursors was formed. Afterward, the solution was refluxed
* Corresponding
author.
E-mail:
alexander.eychmueller@
chemie.tu-dresden.de.
† University of Hamburg.
‡ University Medical Center Hamburg-Eppendorf.
§ TU Dresden.
10.1021/jp056371p CCC: $33.50 © 2006 American Chemical Society

1960 J. Phys. Chem. B, Vol. 110, No. 5, 2006
Letters
under open-air conditions for several hours, until the particles
had grown to the desired size. The progress of the reaction was
followed by absorption spectroscopy.
Size-Selective Precipitation of CdTe QDs. The colloidal
solution of CdTe QDs was evaporated until the first particles
precipitated. A 500 µL portion of 2-propanol was then added
to the solution. The resulting precipitate was removed by
centrifugation for 5 min at 4500 rpm and redissolved in water.
The procedure was repeated with a stepwise increase of the
amount of 2-propanol until most of the CdTe was precipitated.
Chemicals and Methods. Milli-Q water (Millipore) was used
as bidistilled water. All chemicals were purchased from Merck
and used as received. Dulbecco’s Modified Eagle Medium
(DMEM) (1X) liquid (low glucose) containing 1000 mg/L
D-glucose and L-glutamine and 110 mg/L sodium pyruvate from
Invitrogen was used as the cell medium.
For analysis of the QDs, a Cary 100 absorption spectrometer
(Varian) and a Fluoro-Log emission spectrometer (Instruments
SA) were employed. The wellplates for the stability assay were
measured with a Cary Eclipse emission spectrometer (Varian).
Results and Discussion
To quantify the stability of the CdTe QDs in aqueous buffers,
all variables must be collected and a method must be found
with which the variation of all pertinent variables can easily be
measured without changing the measurement. The independent
variables in a system containing nanoparticles and a buffer are
the following: the type of the QDs, the ligands, the size of the
QDs, the concentration of the QDs, the type of buffer, the
concentration of the buffer, the pH value, and time. The
dependent variables are the following: the intensity of the PL,
the full width at half-maximum of the emission signal, and the
wavelength at the emission maximum. In this work, the particle
size and the type of ligands and buffer were varied and the
dependent variables were measured over a time span of 5 days.
The particles employed were CdTe QDs with TGA or MPA as
ligands after application of size-selective precipitation.
Figure 1. (a) True color photography of an 8 × 12 wellplate with
TGA-capped CdTe QDs under a UV lamp after 1 day of incubation.
The particle size decreases from top to bottom and lies between 3.46
and 2.42 nm. The buffers used are, from left to right, Milli-Q H2O, pH
5.0 citrate/NaOH, MES, MOPS (3-(N-morpholino)propansulfonic acid),
PIPES (piperazine-1,4-bis(2-ethanesulfonic acid)), 500 mM Tris/HCl,
1 M Tris/HCl, pH 6.8 PBS, pH 8.0 PBS, pH 8.0 Tris, pH 9.0 B(OH)3/
KCl/NaOH, and pH 11.0 B(OH)3/KCl/NaOH. (b) Fluorescence spectra
of fraction TGA 5 (3.22 nm) after 20 h in the named buffers.
To obtain a comparable signal, the amount of particles which
were added to 100 µL of buffer was calculated to result in an
optical density (OD) of 0.2 at the first absorption maximum. A
varying concentration of QDs was accepted in favor of a
constant OD, although a lower concentration of QDs results in
diffusion of ligands from the particle surface and therefore a
decreased stability.3 In a living cell, it is desirable to maintain
the concentration of CdTe as low as possible without a decrease
of the particles’ stability. The parameters are therefore similar
to the parameters that were expected in biolabeling.
The measurement of a full wellplate took 20 min, which is
the limiting factor in the determination of the accuracy of the
time. On the day the buffers were added to the QDs, the
wellplates were measured three times: immediately after the
experiment was started and again after 2 and 5 h. After that,
the luminescence was measured every 24 h.
used biochemical buffers (Figure 1). For extreme pH values,
buffers sold by Merck were used instead of biochemical
buffers: a citrate/NaOH buffer for pH 5.0 and B(OH)3/KCl/
NaOH buffers for pH 9.0 and 11.0. The buffers were systemati-
cally tested for the application of indirect immunofluorescence
labeling with CdTe nanocrystals. In such an experiment, cells
are fixed with 4% paraformaldehyde (PFA) in phosphate-
buffered saline (PBS) for 30 min. Their membranes are made
permeable using PBS/glycine/saponine for 30 min at 37 °C after
which free binding spaces are blocked using BSA for 30 min.
After each step, the cells are washed with PBS. The fluorescence
marker, linked covalently to a specific antibody, is then
transferred into the cells over a period of 45 min after which
the cells are placed under a microscope where the location of
the marker shows the location of the specific antigen. Often, a
secondary antibody that binds to an antigen-specific primary
antibody is used, because this strategy simplifies the experiment
by requiring only one type of marker-linked antibody. Other
buffers tested were cell medium, morpholinethansulfonic acid
(MES), and a tris(2-hydroxyethyl)-amine (Tris) buffer (pH 8.0,
200 mM).
The particle size was determined from the first absorption
maximum using an empirical formula derived for particles
synthesized via an organometallic route by Peng et al.:9
D ) (9.8127 × 10-7)λ3 - (1.7147 × 10-3)λ2 +
(1.0064)λ - (194.84)
Usually, organic markers have very little photostability and
bleach in the order of minutes under illumination. Semiconductor
QDs are more robust and show longer durability which makes
them an interesting alternative.10
where D is the diameter of the particles and λ is the wavelength
of the first electronic transition.
The buffers were chosen to represent a wide spectrum of the
pH scale between 5.0 and 11.0 and to test the most frequently
Large semiconductor QDs have a greater resistance to
photobleaching than smaller ones. Regarding the stability toward

Letters
J. Phys. Chem. B, Vol. 110, No. 5, 2006 1961
Figure 3. Relative intensity of all TGA-capped fractions in PBS (pH
7.4) with respect to time. The intensity at t ) 0 h was set to 100%.
Figure 2. Fractions TGA 1 (a) and TGA 6 (b) in PBS (pH 7.4),
observed over a time span of 1 week.
their chemical environment, the reverse tendency was observed.
The smaller the QDs, the more stable they are under acidic
conditions and low buffer concentrations. In acidic citrate/NaOH
buffer (pH 5.0), the fluorescence of red TGA-capped QDs drops
down to 5% of the original PL after a few minutes, while, for
the green and blue species, the intensity remaining was about
25%. The reason for this behavior could be the clusterlike, more
defined structure of very small nanocrystals.11-14 The surface
structure of very small particles and hence the occurrence of
charge carrier traps strongly depends on the size and the
symmetry of the nanoobjects.
Figure 4. Temporal decrease in intensity for fraction TGA 1. Zero-
order kinetics is recognized in many cases.
TABLE 1: Size and OD of the Particles Used for the Testa
size
color
V3mL
OD
V100µL
name
(nm)
(PL UV)
(µL)
(a.u.)
(µL)
TGA 1
TGA 2
TGA 3
TGA 4
TGA 5
TGA 6
TGA 7
TGA 8
MPA A
MPA B
MPA C
MPA D
MPA E
MPA F
MPA G
3.46
3.44
3.40
3.36
3.22
3.06
2.83
2.42
∼3.6
∼3.6
∼3.6
∼3.6
∼3.6
3.57
3.53
red
red
red
orange
yellow
green
green
light blue
red
red
red
red
red
red
red
20
20
20
20
30
40
50
60
90
30
100
60
120
200
200
0.05
0.16
0.12
0.08
0.05
0.03
0.02
0.01
0.09
0.10
0.03
0.05
0.05
0.04
0.05
2.4
0.75
0.99
1.5
3.6
8.0
The TGA 6 fraction, for example, remains stable in PBS for
1 week, while the emission of fraction TGA 1 decreases notably
over the same time span (Figure 2). These results can be applied
to other buffers. Figure 3 shows the observed temporal evolution
of the relative intensity maxima of the TGA-capped fractions
in PBS. The maximum at 0 h after adding the buffers was set
to 100%. For the larger nanocrystals, a decrease in intensity
over time is evident. The tendency to degrade diminishes with
decreasing particle size, until the size has reached 3.06 nm
beyond which no further changes in intensity are observed. This
behavior can also be seen in most of the other buffers. The slope
of the intensity change with time, ∂I/∂t (cf. Figure 4), and the
critical size, Dk, below which no change in intensity can be
determined, depend on the buffer and, in case of the slope, on
the particle size. With these two values, namely, ∂I/∂t which is
proportional to the particles’ rate of decay and the critical size,
Dk, a means to characterize a specific particle/buffer mixture is
found.
15
36
6.14
1.82
21.43
7.35
14.4
30.8
25.0
a Due to the large full width at half-maximum of the first absorption
maximum, the size of the MPA-capped QDs could only be ap-
proximately determined.
The slope (Figure 4) is linear in buffers which do not
aggressively destroy the QDs. This is not trivial, since one could
expect more complex kinetics for this reaction, which includes
desorption of ligands from the particle surface, dissolving or
aggregation of QDs, and adsorption of other molecules. Table
2 summarizes the experimental results including ∂I/∂t, Dk, and

1962 J. Phys. Chem. B, Vol. 110, No. 5, 2006
Letters
TABLE 2: Buffers Employed in the Studies
pH
slope of intensity,a
critical size,b
red shift,c
buffer
value
∂I/∂t (%/h)
Dk (nm)
∆λ (nm)
comment
Millipore Milli-Q H2O
citrate/NaOH
B(OH)3/KCl/NaOH
B(OH)3/KCl/NaOH
MES
MOPS
PIPES
500 mM Tris/HCl
1 M Tris/HCl
PBS
7.0
5.0
9.0
11.0
6.5
6.5
7.0
7.4
7.4
7.4
7.4
-0.94
2.42
-2.1
d
-2.1
0.0
19.9
d
d
d
d
0.0
0.0
e
destroyed all samples at once
-0.39
-0.39
e
e
e
-0.72
e
-1.07
e
3.44
3.36
<2.42
2.42
<2.42
2.83
2.42
3.06
2.42
no linear ∂I/∂t
red shift of ∼24 nm
red shift of ∼24 nm
destroyed most of the samples
PBS/glycine/saponin
saponin is an extract from the bark of the
South American soaptree
2% BSA in PBS/Gly/saponin
4% PFA in PBS
MOWIOL
medium
glycine in H2O
Tris
7.4
7.4
e
e
e
e
<2.42
-4.0
18.0
22.0
32.9
4.0
3.06
low intensity, QDs > Dk destroyed
nonaqeous medium
complex mixture
7.5 mg/mL Gly
200 mM
7.1
5.9
8.0
<2.42
<2.42
3.44
-0.72
-0.33
0.0
a Taken from fraction TGA 1. b For TGA-capped QDs. c For fraction TGA 6 after 2 h of incubation. d No data. e Decay not of pseudo-zero-order
kinetics.
∆λ, the observed red shift of the emission (see below) in all of
the buffers used in this study.
To derive a model for a reaction of pseudo-zero order,
strongly bound ligands on the particle surface are assumed.
Desorption is slow, and adsorption practically does not occur,
since the concentration of free ligands in solution is very low.
On the contrary, the concentrations of buffer ions are high,
namely, between 50 mM and 1 M. Adsorption and desorption
of ions at the surface are fast processes due to the weak,
unspecific bonding between ions and QDs. This results in a
constant, low concentration of free coordination places at the
surface. Quenching of the QD luminescence can be an effect
of oxidation by dissolved oxygen. The multiplate was kept under
open-air conditions, resulting in a concentration of oxygen in
solution which is also assumed to be constant. Following this
explanation, the kinetics do not depend on any concentration
that was varied in the experiment.
Indeed, the kinetics appear to be more complex in the first
hours after the addition of the buffers. However, since the slope
is constant over several days, it can be used to quantify the
stability.
Peng et al. have proposed zero-order kinetics to explain the
photocatalytic oxidation of thiols to disulfides on the surface
of CdSe with regard to the concentration of free thiols in
solution.15 They proposed three processes contributing to the
decay of the QDs: photocatalytic oxidation of the ligands,
photocatalytic oxidation of the CdSe core, and precipitation of
the QDs. Water-insoluble disulfides which might be made up
from MPA will form micelle-like structures around a nanocrystal
until the core has shrunk to the point that it can no longer
stabilize the micelle. After a micelle has broken down or when
the disulfides are soluble in water, CdTe will precipitate. In
contrast to Peng’s results, especially the small particles seem
to be stable toward oxidation. Keeping the samples in the dark
also leads to the conclusion that photooxidation is not necessarily
taken into account. Figure 5 summarizes the processes taking
place in the degradation mechanism of semiconductor QDs in
liquid environments.
Figure 5. The proposed mechanism for a zero-order reaction is based
on a constant, low number of free binding sites on the surface of the
particles.
in alkaline borate buffers, the difference in the slope seems to
be not or only slightly dependent on the pH value. Henglein et
al. reported on an “activation” of the luminescence of CdS
nanocrystals at pH 10.0 and an excess of free Cd2+ ions,
probably by passivation of the surface by cadmium hydroxide.18
Here, borate was used, in the form of Na2B4O7, to set the pH
value. This results in conditions which are comparable to the
above-mentioned parameters.
Until now, the derived values that characterize the particles’
stability ignore the wavelength of the emission maximum. In
several buffers, profound red and blue shifts were observed.
Since photometric measurement at a constant wavelength can
be disturbed by this, the red shift, ∆λ, must be regarded as a
third characteristic value to describe the stability of QDs in a
buffer.
It is noted that in alkaline buffers the intensity seems to
increase over the first 5 h. A dependency of the PL quantum
yield (QY) on the pH value is known from the literature.2,16,17
The highest PL QY was reported to occur at pH 4.5, which
could not be reproduced using the above parameters. At least
The blue shift (∆λ < 0) can be the result of a partial
destruction of the nanocrystal when ions are removed from the
surface. In this case, an increase of the concentration of toxic

Letters
J. Phys. Chem. B, Vol. 110, No. 5, 2006 1963
Cd2+ and Te2- ions has to be taken into account for applications
involving living cells. This explanation is supported by the fact
that a blue shift is registered when particles are diluted with
distilled water. This phenomenon has already been described
earlier.3 A blue shift was also measured when proteins were
present in the solution (2% BSA in PBS/glycine/saponine). Both
cadmium and tellurium ions are known to react with disulfide
bridges in proteins.
as well as the interaction of thiol-capped particles with proteins,
regarding the desired application in protein labeling.
By using only three values, ∂I/∂t, Dk, and ∆λ, a large number
of dependent variables are reduced to a few parameters which
may then be used to quantify the stability of a particular particle/
buffer combination. The buffer concentration is not a variable
that requires a large range, since biological application calls
for specific, physiological concentrations.
Two models may be used to explain the red shift: First, it is
possible that the particles agglomerate due to the loss of
stabilizers. In this case, CdTe bulk material will precipitate and
a decrease of the PL intensity and scattering and a larger full
width at half-maximum should be observed. The second
possibility is the occurrence of Ostwald ripening. This process
requires the surface of the QDs to be dynamic and will be
strongly dependent on the temperature.
When using MPA instead of TGA as ligands, the results were
similar, although slightly larger particles were used in the case
of MPA as the stabilizer: buffers that adversely affect TGA-
capped QDs also decrease the PL intensity of MPA-capped
particles. Conversely, buffers that stabilize TGA-capped QDs
also stabilize MPA-capped ones. As with TGA as a ligand,
MES, PFA, MOWIOL (a mixture of polyvinyl alcohols), and
cell medium also result in a large red shift. Only with MPA-
capped particles do the alkaline borate buffers seem to be less
stabilizing than PBS and Tris buffer.
Acknowledgment. The authors are grateful to Dirk Dorfs,
Nadja Bigall, and Dr. Stephen Hickey for fruitful discussions
and acknowledge financial support by the DIP D-3.1 “Functional
Nanoparticle Architectures” and EU NoE PHOREMOST.
References and Notes
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Summary and Outlook
The results demonstrate the possibility of the use of water-
soluble, thiol-capped QDs for immunofluorescence labeling
employing the commonly used buffers. The nanocrystals endure
both frequently used buffers PBS and Tris. High pH values and
moderate buffer concentrations stabilize the QDs, while acidic
media and high dilution extinguish the PL completely. Even
under neutral conditions, the particles keep their photolumines-
cence for days. The only problematic solutions found are
aggressive substances such as PFA and the cell medium, the
latter being a mixture of so many substances that the reason
for the instability of the particles cannot be easily determined.
Measurement with a varying concentration of free ligands to
verify the assumptions made to explain the kinetics are in
progress. As higher concentrations of free ligands should result
in a higher stability of the nanocrystals, experiments are in
progress to verify the assumptions made above in order to
explain the kinetics observed. Further investigations will also
focus on measuring the change in absorption and light scattering
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