A ratiometric CdSe/ZnS nanocrystal pH sensor

Article abstract of DOI:10.1021/ja0618999

The development of a reversible chemical sensor based on a CdSe/ZnS nanocrystal (NC) is described. Signal transduction is accomplished by fluorescence resonance energy transfer (FRET) between the NC and a fluorescent pH-sensitive squaraine dye attached to

Full text of DOI:10.1021/ja0618999

Published on Web 09/26/2006
A Ratiometric CdSe/ZnS Nanocrystal pH Sensor
Preston T. Snee,† Rebecca C. Somers, Gautham Nair, John P. Zimmer, Moungi G. Bawendi,* and
Daniel G. Nocera*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4306
Received March 20, 2006; E-mail: mgb@mit.edu; nocera@mit.edu
Semiconductor nanocrystals (NCs) serve as useful fluorescent
labels owing to their photostability, continuous absorption spectra,
and efficient, narrow, and tunable emission.1-3 These properties
of NCs have been exploited for applications in biological imaging4
and in single particle tracking studies.5 Whereas NCs are useful in
identifying position in a microenvironment, their intrinsic insensitiv-
ity to the presence of most biological or chemical agents renders them
of limited utility as sensing probes of that microenvironment. An
important step in this direction has been the recent development of
water-soluble CdSe NCs with the ability to sense target analytes
irreversibly using fluorescence resonant energy transfer (FRET) as
a signal transduction mechanism.6-8 These previous studies, how-
ever, do not demonstrate reversible and ratiometric chemical sensing
using fluorescent NCs. We now show that a reversible NC-based
fluorescent sensor can be designed by conjugating a dye with an
equilibrium response to an analyte to the surface of a CdSe/ZnS NC.
We show that properly controlling energy transfer between the NC
and dye engenders a general method for the development of ratio-
metric NC sensors, thus providing a means for detecting analytes
with high precision, irrespective of changes in excitation intensity,
wavelength, or collection efficiency. Our constructs thus remove
several difficulties encountered with dye-only-based ratiometric
nanoscopic sensing systems.9
dye conjugation method may be applied to water-soluble NCs that
have been cap exchanged with thiol-functionalized poly(ethylene
glycol)s.14 The samples can be easily purified with dialysis filters
to ensure that unreacted dyes are removed from the sample. The
complete procedures for the synthesis of CdSe/ZnS NCs and
squaraine dye and their subsequent conjugation are given in the
Supporting Information (SI).
The sensing action of the NC-squaraine conjugate is imparted
by modulation of the FRET efficiency arising from the engineered
overlap of the pH-sensitive dye absorption spectrum with the (pH-
insensitive) quantum dot emission (Figure S1 of SI). Owing to the
pH dependence of the dye absorption spectrum, the spectral overlap
between the dye absorption and the NC emission increases as the
pH is lowered. This spectral overlap integral J is directly related
to the critical FRET distance, Ro, which in turn affects the energy
transfer efficiency E(Ro) as shown in eq 1:
6
Ro
J(pH) ) dλꢀ (λ)d(λ)λ4, R
J1/6, E(Ro) )
(1)
∫
pH
o
Ro6 + R6
where λ is wavelength, ꢀpH(λ) is the pH-dependent molar absorp-
tivity, d(λ) is the normalized NC donor emission, and R is the
donor-acceptor distance. The inset of Figure S1 shows a calculation
of Ro as a function of pH; details of the FRET calculation are
included in the SI. At high pH, FRET should be inefficient as the
spectral overlap is small. Hence, the emission spectrum should be
dominated by luminescence from the NC. As the pH is lowered,
Ro grows larger and the FRET efficiency increases owing to a
strengthening of the dye absorption cross section ꢀpH(λ). The NC
emission should then be quenched as a result of energy transfer to
the dye, which will in turn become more emissive.
High quality CdSe NCs overcoated with ZnS and capped with
trioctylphosphine oxide ligands10,11 were encapsulated by a hydro-
phobically modified poly(acrylic acid) (Figure 1).12 The squaraine
The experimental results shown in Figure 2 are consistent with
the calculated trend in Ro and expected FRET efficiency. The inset
shows the response of the absorption profile of the NC-dye
conjugate above and below the pKa (∼8.5) of the dye. As observed
for the free dye in homogeneous solution (Figure S1), the dye
absorption band is suppressed under basic conditions. Consequently,
energy transfer from the NC to the dye is inefficient, and the
emission spectrum is dominated by the NC at 613 nm. As pH is
lowered, the absorption cross section of the dye is increased, and
FRET from the NC to the dye becomes more efficient; emission
from the NC-dye conjugate is now dominated by that of the dye
at 650 nm. The largest changes occur near the pKa of the dye. The
steady-state emission results are confirmed by time-resolved
emission studies of the NC-dye conjugate. As shown in Figure
S3, NC emission of a conjugated sensor has a reduced lifetime as
compared to that of the unconjugated NC. These results establish
that the emission from the NC donor is quenched by the presence
of the organic dye acceptor, as has been previously observed in
other constructs.6,7
Figure 1. A sensor constructed from a colloidal CdSe NC that is overcoated
with an outer layer of ZnS. The native phosphine oxide ligands are
encapsulated with an amphiphilic polymer upon which a pH-sensitive
squaraine dye is conjugated. Upon excitation, the CdSe/ZnS nanocrystal
may either fluoresce or transfer energy to the squaraine dye. The FRET
efficiency is modulated by the environment as the dye’s absorption profile
is a function of pH. Consequently, the ratio of NC to dye emission becomes
a function of environmental variables.
dye13 shown in Figure 1 was linked to the polymer backbone via
ester linkages using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC) as the coupling agent. We have also found that the same
† Present address: Department of Chemistry, The University of Illinois at
Chicago, Chicago, IL 60607.
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J. AM. CHEM. SOC. 2006, 128, 13320-13321
10.1021/ja0618999 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
independent band shape (see inset, Figure 3), a feature that is not
present in single molecular ratiometric sensors.17 Thus, the NC-
dye conjugate does not need to use two independent excitation
sources (or alternatively a single excitation specifically at an
absorptive isosbestic point) for proper function.
In summary, we have developed a new strategy for chemical
and biological sensing by tethering emissive water solubilized NCs
to environmentally sensitive dye molecules. We have observed a
ratiometric response to pH owing to modulation of FRET efficiency
between the emissive NC and dyes conjugated to the NC surface.
The approach is general as a sensing construct because the narrow,
size-tunable emission spectrum of NCs enables them to be FRET
donors that may be easily custom-engineered to match the acceptor
absorption features of a dye conjugated to the surface of the NC.
Taken together with the broad excitation spectrum and photostability
conferred by NCs, the reversible and ratiometric approach presented
here makes NCs versatile agents for chemical and biological
sensing.
Figure 2. The emission profile of a water-soluble (3.2 nm radius) NC-
squaraine dye conjugate changes as a function of pH (red solid line, 6.0;
orange dotted line, 7.0; yellow solid line, 8.0; green dotted line, 9.0; and
blue solid line, 10) with λex ) 380 nm. The normalized spectra show pH
dependence with an isosbestic point appearing at 640 nm. The absorbance
of the squaraine dye is suppressed in the conjugate at basic pHs as shown
in the inset. The overall quantum yield of this construct is 7 ( 1%.
Deconvolution of the absorption spectra into dye and NC components reveals
that the dye to NC ratio is (3.4 ( 0.2):1. Shown in Figure S2 of the SI are
the data from a 7.5:1 dye to NC conjugated sensor.
Acknowledgment. The authors would like to thank Y. Chan
for helpful discussions. This work made use of the MRSEC Shared
Facilities supported by the NSF under Award Number DMR-
0213282, as well as the Harrison Spectroscopy Laboratory (NSF-
011370-CHE). M.G.B. and D.G.N. acknowledge support from a
Collaborative Research in Chemistry Grant by the NSF (NSF-CHE-
020989). M.G.B. also acknowledges support from the NIH funded
MIT-Harvard NanoMedical Consortium (1U54-CA119349, a
Center of Cancer Nanotechnology Excellence), and the ARO
through the Institute for Collaborative Biotechnologies. D.G.N.
acknowledges support from Corning Inc.
Supporting Information Available: Full synthetic procedures and
NMR spectra for the preparation of pH-sensitive squaraine dyes, water
solubilized core shell NCs, and the conjugation of the two. Time-
resolved emission of NC versus NC/squaraine dye conjugates as well
as results from other pH-sensing conjugates. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 3. Sensing of the local pH by the NC/dye construct with variations
in the excitation intensity and local environment. The ratio of NC to dye
emission varies such that pH is determined within 5% when altering the
slit entrance of the Xe lamp excitation of the fluorimeter (red dotted line,
0.5 mm; green solid line, 2.00 mm) and when examining the construct within
a highly scattering media (blue solid line) shown by the picture of the vial.
The inset shows that emission was independent of excitation wavelength
(blue solid line, 380 nm; green solid line, 450 nm; red solid line, 520 nm).
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