Fluorescent and pH-responsive diblock copolymer-coated core-shell CdSe/ZnS particles for a color-displaying, ratiometric pH sensor

Article abstract of DOI:10.1039/c1cc13848k

A color distinctive, ratiometric pH sensor was demonstrated using pH responsive and fluorescent (PyMMP-b-P2VP) diblock copolymer coated CdSe/ZnS QDs. Due to the change in the P2VP conformations in response to pH change, the color of QDs in solution changes distinctly from blue to red.

Full text of DOI:10.1039/c1cc13848k

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COMMUNICATION
Fluorescent and pH-responsive diblock copolymer-coated core–shell
CdSe/ZnS particles for a color-displaying, ratiometric pH sensorw
Kwanyeol Paek, Sunhaeng Chung, Chul-Hee Cho and Bumjoon J. Kim*
Received 28th June 2011, Accepted 29th July 2011
DOI: 10.1039/c1cc13848k
A color distinctive, ratiometric pH sensor was demonstrated
using pH responsive and fluorescent (PyMMP-b-P2VP) diblock
copolymer coated CdSe/ZnS QDs. Due to the change in the
P2VP conformations in response to pH change, the color of
QDs in solution changes distinctly from blue to red.
QDs and dyes or unbalanced emission intensities between QDs
and dyes, it has not been possible to effectively monitor the
external changes based on the direct visual observation of
color changes; consequently, an expensive photoluminescence
detector has been required.
In this communication, we demonstrate a simple and robust
route for the fabrication of a ratiometric pH sensor, based on
block copolymer-capped QDs, that exhibits a distinctive color
change in response to a change in pH. Cadmium selenide/zinc
sulfide (CdSe/ZnS) QDs were carefully chosen and coated with
pH responsive and fluorescent poly((1-pyrene)methyl-2-methyl-
2-propenoate))-b-poly(2-vinylpyridine) (PyMMP-b-P2VP) block
Polymer-coated nanoparticles have received considerable
attention because of their potential applications in electronic
and optical devices, as surfactants, and in catalysis.1–5 The
inorganic core of nanoparticles can provide unique optical,
electrical, magnetic, and catalytic properties, and the polymeric
(organic) coating can stabilize the nanoparticles by inhibiting
their aggregation; these polymers can be independently designed
to precisely control the interaction between the nanoparticles and
the outside medium. Thus, the appropriate polymeric coating
allows the polymer-coated nanoparticles to interact favorably
with the polymer and the fluid matrix to produce new morpho-
logies and enhances their bulk properties, e.g., the thermal,
electrical and mechanical properties of the composites.6–10 In
addition, the use of stimuli-responsive polymers allows the
polymer-coated nanoparticles to be used as sensors.11,12
Quantum dots (QDs) have several advantages over the
traditional materials used in sensors, including high resistance
to photobleaching, high quantum yield, and narrow emission
peaks with broad absorption spectra.13 Several research
groups have reported QD-based pH sensors consisting of an
inorganic core and an amphiphilic organic shell, in which the
photoluminescence (PL) intensity of the QDs changed as a
function of pH due to PL quenching between the organic
molecules and QDs.14–18 However, these sensors, which have a
single intensity-based response, have mainly been used as
intracellular sensors and not as ratiometric sensors. To date,
only few studies have reported the development of a QD-based
ratiometric sensor with dual emissions.19,20 However, this
sensor typically requires a multistep synthesis and a specific
organic molecule, which limits the versatility of the approach.
In addition, due to the lack of diversity in the colors emitted by
copolymers. Forster resonance energy transfer (FRET) between
¨
these two distinguishable chromophores of red emitting CdSe/
ZnS cores and blue emitting pyrene shells is governed by the
interspacing between them, which can be controlled through
manipulation of the conformational features of the P2VP
chains that respond to pH changes. Furthermore, the emission
intensity ratio between the QDs and pyrene blocks can easily
be balanced and controlled by simply tuning the number of
repeating pyrene units in the (PyMMP-b-P2VP) chain. Therefore,
our polymer-coated QDs can exhibit not only ratiometric
pH-dependent fluorescence spectra but also a clear color
change from blue to purple to red as a function of pH.
Scheme 1 presents a synthetic procedure for preparing
(PyMMP-b-P2VP) coated CdSe/ZnS QDs ((PyMMP-b-P2VP)-
QDs). Red-emitting CdSe/ZnS QDs (1) were synthesized by a
one-pot synthesis method that used oleic acid for stabilization,21
and then P2VP-b-PyMMP diblock copolymers were synthesized
from the QD surface by reversible addition–fragmentation chain
transfer (RAFT) polymerization using the ‘‘grafting-from’’ method.
Department of Chemical and Biomolecular Engineering,
Korea Advanced Institute of Science and Technology (KAIST),
Daejeon 305-701, Korea. E-mail: bumjoonkim@kaist.ac.kr;
Fax: +82-42-350-3910; Tel: +82-42-350-3935
w Electronic supplementary information (ESI) available: Experimental
procedures and additional data (Schemes S1 and S2, and Fig. S1–S5).
See DOI: 10.1039/c1cc13848k
Scheme 1 Synthesis of (PyMMP-b-P2VP)-CdSe/ZnS QDs.
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amount of DMF (10 : 1 v/v) at different pH conditions.
The concentration of (PyMMP-b-P2VP)-QDs in each solution
was maintained at 0.1 mg mlÀ1. The color of the (PyMMP-
b-P2VP)-QD solution dramatically changed from blue to
purple to red as the pH increased. These colors matched well
with the corresponding PL spectra shown in Fig. 2(b). The PL
spectra of the (PyMMP-b-P2VP)-QDs exhibit two emission
peaks due to the PyMMP blocks and the CdSe/ZnS QDs at
470 nm and 630 nm, respectively. The paired pyrenes have
been shown to exhibit a single PL band at 470 nm because of
their excimer formation.24 The PL intensity at 470 nm gradually
decreases as the pH of the solution increases from pH 1 to pH 4.
In contrast, the PL intensity at 630 nm increases as the pH
of the buffer solution increases. At pH 5, the peak reaches
saturation, and the solution exhibits no further change in
color. The color and PL changes as a function of pH can be
correlated to a change in the P2VP chain conformation at
different pH conditions. Because the pKa of P2VP is around 3,25
the P2VP chains were protonated and swelled by water below
pH 3. Under these conditions, the distance between the two
chromophores, i.e., the blue-emitting PyMMP block and the red-
emitting QDs, becomes significantly larger and suppresses the
FRET from the higher energy PyMMP to the lower energy
QDs. The FRET efficiency is inversely proportional to the
sixth power of the FRET donor-to-acceptor separation
distance because of a dipole–dipole coupling mechanism.26
Conversely, the P2VP chains were deprotonated and collapsed
at a higher pH, thus facilitating the FRET from the PyMMP
molecules to the QDs and causing the observed color change
from blue to red. For further quantitative analysis, the PL
intensity ratio at different emission levels of 630 nm and 470 nm
(I630 nm/I470 nm) was plotted as a function of pH, as shown in
Fig. 3(a). Interestingly, the plot of pH versus I630 nm/I470 nm
was almost linear in the pH range of 1–4, indicating that the
response is ratiometric in this range. Subsequently, the plot
became saturated in the pH range of 4–5.
Fig. 1 TEM images of (a) OA-coated CdSe/ZnS, (b) P2VP-coated
CdSe/ZnS, and (c) (PyMMP-b-P2VP) coated CdSe/ZnS. Scale bar is
50 nm.
In this process, oleic acid ligands on the QD surface were first
replaced with the thiol-terminated trithiocarbonate RAFT
chain transfer agent (3) via ligand exchange. Compared to
the previously reported phosphine oxide-functionalized RAFT
chain transfer agent,22 our thiol-terminated RAFT agent can
be synthesized easily using only a two-step reaction and can be
applied to other nanoparticle systems, including gold and
silver, because of strong binding interactions between the thiol
group and the nanoparticle surface. Because the molecular
weight (Mn) of the P2VP block is a critical parameter that
determines the distance between the two chromophores and
the Mn of the PyMMP block can influence the brightness of
blue emitters, the Mn of both the P2VP and PyMMP blocks
were carefully controlled. The total Mn of P2VP-b-PyMMP
was 4.7 kg molÀ1 with a polydispersity index (PDI) of 1.17.
The Mn and PDI of the P2VP block were 4.0 kg molÀ1 and
1.10, respectively. Therefore, the radius of gyration (Rg) of
P2VP was calculated to be 1.7 nm, approximately corresponding
to the distance between the QDs and the PyMMP block when
P2VP is not swelled by solvent. The areal chain density (S) of
polymers on the QD surface was calculated to be 2.5 chains
per nm2 based on the QD core size, which was determined
using TEM, and the weight fraction of the QD core and
polymer ligands in the (PyMMP-b-P2VP)-QDs, which was
determined using thermogravimetric analysis (TGA).23
Fig. 1 shows representative TEM images of the CdSe/ZnS
QDs before and after polymerization. The core diameters of
three different QDs were all approximately 8.7 nm, indicating
that the particle size was not affected by polymerization. Oleic
acid-stabilized QDs were well dispersed in organic solvents,
such as chloroform, hexane, toluene and THF, but they could
not be dispersed in alcohol. Conversely, the (PyMMP-b-
P2VP)-QDs could easily be dispersed in alcohol. This result
indicated that PyMMP-b-P2VP was successfully polymerized
on the CdSe/ZnS QD surface via the grafting-from method.
Fig. 2 shows photographic images and the PL spectra of
the (PyMMP-b-P2VP)-QDs in buffer solution with a small
A deeper insight into the dramatic change in PL and color
spectra of (PyMMP-b-P2VP)-QDs as a function of pH can be
gleaned by examining the change in particle size via dynamic
light scattering (DLS) measurements (Fig. 3(b)). The hydro-
dynamic sizes of the (PyMMP-b-P2VP)-QD particles at different
pH conditions were measured; the PL change of the particles
was found to correlate to the distance between the PyMMP
and QD core chromophores. The distance can be estimated
by measuring the particle size at a given pH. The particle size
was constant at 12 nm above pH 4, showing good agreement
with the sum of the QD core (B8.7 nm) and twice the Rg of
Fig. 2 (a) Photographic image of (PyMMP-b-P2VP)-QDs solutions
under irradiation at 365 nm using a UV lamp, and (b) PL spectra of
(PyMMP-b-P2VP)-QDs.
Fig. 3 (a) PL intensity changes (I630/I470) as a function of pH, and
(b) hydrodynamic size of (PyMMP-b-P2VP)-QDs as a function of pH,
measured by DLS.
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the P2VP chains (B1.7 nm). Under these pH conditions, the
distance between the PyMMP and QD chromophores is short
enough to induce a strong increase in FRET efficiency from
the PyMMP block to the QDs. As the pH decreases, the
particle size gradually increases from 13 nm (pH 3) to 19 nm
(pH 2) to 36 nm (pH 1). Because the hydrodynamic particle
size at low pH can be overestimated by DLS measurements
due to extensive swelling of the polymers by the solvent,27 the
increase in the particle size is not equal to the increase in the
donor-to-acceptor distance. However, it is worthwhile to
note that the trend of the particle size dependence on the
change in pH is consistent with that of the P2VP conformation
and the PL spectra shown in Fig. 2. As the pH decreases from
pH 4 to pH 1, the P2VP chains were swelled by water,
increasing the particle size and the donor-to-acceptor distance.
The corresponding decrease in the FRET efficiency results in
an increase in the PL intensity of PyMMP, which is a FRET
donor, and a decrease in the PL intensity of the QD, which is a
FRET acceptor. Conversely, the P2VP blocks were fully
deprotonated above pH 4, and the corresponding particle size
was unchanged. As a result, there is no further change in the
PL intensities of the two different chromophores.
In addition, (PyMMP-b-P2VP)-QDs exhibited good stability
under a wide range of pH conditions. At an optimal concen-
tration of QDs, high brightness and sensitivity can also be
obtained. Our approach of diblock copolymer-coated QDs
provided a simple and robust route for the construction of a
ratiometric pH sensor, which can be easily extended to other
types of sensors for the detection of alternate stimuli, such as
temperature and light, using appropriate, responsive polymers.
This research was supported by the Korea Research
Foundation Grant, funded by the Korean Government
(2009-0088551, 2010-0029611, 2010-0011033). The authors
thank Prof. Ryan C. Hayward for useful discussions.
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