C O M M U N I C A T I O N S
Figure 3. (a) Emission of a fluorescein/CdS/ZnS NP results in a ratiometric
fluorescence sensor for pH. An isosbestic point is observed at ∼490 nm.
(b) Cartoon of the coupled construct. (c) Ratio of the fluorescein (520 nm)
to CdS/ZnS (455 nm) emission as a function of pH.
Figure 4. (a) Absorption spectra of water solubilized Fe2O3 NP blank and
fluorescein conjugates as a function of pH (blank ) pH 8). (b) NP/dye
conjugate absorption spectra after blank subtraction displays fluorescein-
like character as a function of pH. (c) Photograph of a blank Fe2O3 NP
sample and fluorescein conjugates as a function of pH under room
light conditions and (d) under UV illumination. (e) Emission spectra of the
Fe2O3 NP blank and fluorescein conjugates as a function of pH (blank )
pH 8).
contributes to the NP/dye interaction beyond thiol-maleimide
chemical coupling; we thus performed all further studies using a
more polar functional dye.
To accurately characterize the coupling efficiency we next turned
to a more polar fluorescein maleimide dye which we conjugated
to RAFT amphiphilic polymer coated CdS/ZnS NPs in pH 6 water.
We also sought to examine whether our coupling strategy could
create a ratiometric NP/dye pH sensor similar to a previous report.9
First, CdS/ZnS NPs10 were water solubilized with RAFT amphi-
philic PAA and were exposed to fluorescein maleimide overnight.
After excess dye was removed, we found that the conjugation was
highly efficient with 30 ( 10% of the fluorescein maleimide
reacting with the NP materials. Next, we examined the optical
properties as a function of solution pH as shown in Figure 3. The
increase in the dye absoprtion with increasing pH quenches the
CdS/ZnS emission with concomitant fluorescein brightening as
discussed previously.9 A clear isosbestic point is observed which
is indicative of chemical (pH) sensing via modulation of FRET
energy transfer.9
We next solubilized magnetic Fe2O3 NPs11 into pH 6 water using
the RAFT amphiphilic polymers and conjugated the materials to
pH sensitive fluorescein maleimide dye. After the mixture stirred
for 1 day, we removed the excess dye and measured the NP/dye
conjugate’s absorption and emission spectra as a function of pH.
As shown in Figure 4, the absorption spectra of the coupled
constructs reveal the existence of fluorescein-like absorptions upon
subtraction of the blank oxide NPs. These materials also demon-
strate green fluorescein emission upon excitation of the dye. After
correcting for the NP absorption we found that the dye is quenched
by ∼45% relative to unbound fluorescein. We also confirmed that
the fluorescein is sensitive to the local environment as the dye
brightens considerably in basic solution. Consequently, the use of
RAFT synthesized amphiphilic polymers represents a novel way
to create a variety of functional nanoscopic material systems such
as magnetic fluorescent nanoparticle chemical sensors.
and removes the necessisity for secondary coupling agents that may
cause NP precipitation.4a
Acknowledgment. This work was supported by funding from
the University of Illinois at Chicago and a grant from Motorola
via the University of Illinois Manufacturing Research Center. Also
we thank Wenhao Liu for helpful discussions and Hongyan Shen
for assistance with the manuscript.
Supporting Information Available: All chemicals, procedures,
processing methods as well as characterization, including kinetic data,
GPC traces, quantum yields, FRET efficiencies, and MALDI-TOF
spectra. This material is available free of charge via the Internet at
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