Letter
pubs.acs.org/ac
Determination of Gaseous Sulfur Dioxide and Its Derivatives via
Fluorescence Enhancement Based on Cyanine Dye Functionalized
Carbon Nanodots
Mingtai Sun,†,∥ Huan Yu,†,∥ Kui Zhang,† Yajiao Zhang,†,§ Yehan Yan,†,§ Dejian Huang,*,‡
and Suhua Wang*,†,§
†
Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, People’s Republic of China
Food Science and Technology Programme, Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543
§
Department of Chemistry, University of Science & Technology of China, Hefei, Anhui 230026, People’s Republic of China
‡
S Supporting Information
*
ABSTRACT: The development of convenient methods for sulfur
dioxide and its derivatives analysis is critically important because
SO2 causes worldwide serious environmental problems and human
diseases. In this work, we show an unprecedented example of an
energy-transfer-based fluorescence nanoprobe for selective and
quantitative detection of SO2, through molecular engineering of the
fluorescent carbon nanodots by a cyanine dye which have a unique
reactivity to bisulfite, achieving a detection limit of 1.8 μM with a
linear relationship (R2 = 0.9987). The specific detection was not
interfered with other potential coexisted species. In addition, the probe is demonstrated for the determination of SO2 gas in
aqueous solution as well as for visually monitoring of SO2 gas in air. This nanomaterial based probe is easily prepared, fast
responding, and thus potentially attractive for extensive application for the determination of SO2 and other similar air pollutants.
S
its hydrated derivatives are usually identified as the concentration index of SO2. Most of these methods were based on the
nucleophilic reaction to the aldehyde11−17 or selective
deprotection of levulinate18−20 as well as the addition reaction
to the unsaturated bond,21−25 which is advantageous in terms
of high sensitivity and quick response.
Carbon nanodots (CDs) have superior properties such as
chemical inertness, easy preparation, and environmental
friendliness and can be molecularly engineered as chemosensors through grafting on the surface of CDs, a layer of
organic molecules that are responsive toward analytes.26−29
Herein, we show an unprecedented example of an energytransfer-based photoluminescence (PL) nanoprobe for selective
and quantitative detection of SO2, through molecular engineering of the superior optical properties of inorganic CDs with a
cyanine dye which have a unique reactivity to bisulfite. The
cyanine dye was synthesized and documented as a fast response
colorimetric probe for bisulfite from yellow-green to colorless
by 1,4-addition reaction. We took advantage of this optical
phenomenon to assemble a functional Cy-CDs probe for
sensing sulfur dioxide through a fluorescence “switching on”
mechanism. Amine-coated CDs emitting blue fluorescence was
selected as the fluorescence nanomaterial based on the fact that
ulfur dioxide (SO2) is a main atmospheric pollutant in
ambient air which is produced from the burning of fossil
fuels and the smelting of mineral ores that contain sulfur.1 SO2
can react with other gases and particles in the air to produce
sulfate aerosols, which can be inhaled by people, and thus lead
to increased respiratory disease (such as chronic bronchitis,
asthma, and emphysema) and aggravate existing heart disease.2
SO2 dissolves easily in water to form acid, then bisulfite and
sulfite. These species can be further oxidized to form sulfuric
acid, a major component of acid rain, to cause serious
environmental problem especially for lakes, streams, and
forests. In addition, the toxicity of SO2 can be mainly affected
by its derivatives bisulfite (HSO3−) and sulfite (SO32−) (3:1 M/
M, in neutral fluid), which are usually used as additive in food
in our daily life.3 Hence, the development of convenient
methods for sulfur dioxide and its derivatives analysis is
important for environmental security and human being health.
However, as a colorless and strong pungent odor gas, its rapid,
sensitive, and selective determination still remains a challenge.
Up until now, only a few works have been reported based on
colorimetry for the detection of gaseous SO2,4−7 and these
sensors mostly exhibit low sensitivity, which limit their practical
application. Fluorescence probes have the potential to provide a
practical method for trace analytes due to their advantages for
the real time and space detection with high sensitivity.8−10
Several optical sensors have been developed based on chemical
reaction between various dyes and the derivatives of SO2, since
© 2014 American Chemical Society
Received: August 27, 2014
Accepted: September 22, 2014
Published: September 22, 2014
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Letter
to colorless was obtained as the concentration of HSO3−
increased, which could be used for colorimetric detection
(Figure S4 in the Supporting Information). The optical
phenomenon encourages us to develop a fluorescence turn
on mechanism based nanoprobe for visual detection of HSO3−,
because the fluorescence method generally has higher
sensitivity than colorimetry. First, we investigated the
absorption spectra of Cy in the presence and absence of
HSO3− and the emission spectrum of carbon nanodots in
aqueous media. The carbon nanodots have emission maxima at
450 nm, which is partially overlapped with the absorption
spectrum of Cy, while the absorbance of Cy greatly decreased
after reaction with HSO3− (Figure S5 in the Supporting
Information). Such a characteristic is favorable for constructing
a fluorescence turn-on system based on the FRET between
CDs and Cy. Most importantly, the fluorescence of CDs was
not affected by HSO3− even at the high concentration. To this
end, the probe Cy-CDs was then prepared via the condensation
reaction between the amine group on the surface of carbon
nanodots and the carboxyl group of cyanine dye in the presence
of EDC/NHS (Scheme 1).
As shown in Figure 1, the amine coated CDs show a
fluorescence maximum at 450 nm and exhibit a strong blue
the cyanine dye containing carboxyl group can be easily
covalently linked to the surface of the CDs by condensation
reaction to facilitate the fluorescence resonance energy transfer
(FRET), since the absorption spectrum of cyanine dye is
overlapped favorable with the emission spectrum of CDs.
Comparing with those analytical methods for gaseous SO2
based on colorimetry,4−7 the probe exhibits higher sensitivity
and selectivity. Taking advantage of easy preparation, high
water solubility, and fast response, the as-prepared nanoprobe
Cy-CDs was demonstrated for sensing SO2 derivatives as well
as SO2 gas sample in aqueous solutions and also has been
assembled on a test paper for the on-site visual detection of SO2
gas in air.
Our design strategy to the functionalized carbon nanodots
based probe for SO2 detection was illustrated in Scheme 1. We
Scheme 1. Preparation of the Nanoprobe Cy-CDs and
Schematic Illustration of Fluorescence Turn-On Detection
of SO2
reasoned that the detection mechanism relied on a fluorescence
resonance energy transfer process. It has been demonstrated
that HSO3− or SO32− could add very rapidly and quantitatively
to α,β-unsaturated compounds such as cyanine dyes by
nucleophilic addition in aqueous solution.30,31 Thus, a cyanine
dye (Cy) containing unsaturated bond was synthesized via a
modifying procedure from that of the literature.32 Because
water solubility is preferred for dyes used in environmental and
practical applications,33 the compound 1 containing a
sulfonated heterocycle was used to synthesize the target
cyanine product. Combination of compound 1 and 4formylbenzoic acid in refluxed EtOH in the presence of
piperdine resulted in the target probe Cy as green solid in high
yield. The structure of Cy was characterized by ESI-MS (Figure
S1 in the Supporting Information). The probe has great water
solubility and is resistant to photobleaching. The probe
dissolves in PB buffer easily and shows very weak fluorescence
with emission maxima at 475 nm. As expected, a significant
hypochromatic shift of fluorescence spectra was obtained upon
addition of bisulfite to the solution of probe Cy (Figure S2 in
the Supporting Information). However, the low quantum yield
made it not a suitable fluorescent probe for bisulfite analysis in
assay conditions. Fortunately, the reaction of probe Cy with
bisulfite resulted in two new absorption bands at 234 and 278
nm, accompanied by gradually decreasing of the original
absorption peak at 392 nm, leading to the formation of an
isosbestic point at 314 nm (Figure S3 in the Supporting
Information). A distinct color change of Cy solution from green
Figure 1. Fluorescence emission spectra of (a) amine coated CDs, (b)
the functionalized nanoprobe, and (c) functionalized nanoprobe in the
presence of HSO3−. The inset photos show the corresponding
fluorescence colors under a 365 nm UV lamp, respectively.
fluorescence under a 365 nm UV lamp. While the obtained
naonoprobe dispersed well in water or phosphate buffer
solution with a very weak green fluorescence, this suggests
that the fluorescence of amine coated carbon nanodots has
been quenched due to the FRET between CDs and the cyanine
dye. To the solution of Cy-CDs solution was added NaHSO3,
and the fluorescence intensity of CDs was restored about 65%.
Meanwhile, the solution changes from weak green fluorescence
color to bright blue under a UV lamp which can be seen clearly
with the naked eye (inset of Figure 1).
For practical application, the pH effect on the fluorescence of
Cy-CDs in the absence and presence of HSO3− was examined
(Figure S6 in the Supporting Information). The variation of pH
does not significantly affect the fluorescence of the nanoprobe
in the absence of bisulfite, in spite of a little increase of
fluorescence intensity in basic conditions (pH 8−10). However,
the fluorescence of the probe can be turned on completely by
HSO3−/SO32− at pH values higher than the pKa2 (7.2) of
H2SO3, whereas the fluorescence turn on efficiency decreases
gradually as the pH value decreased lower than the pKa2 (pH
4−7). The results suggest that it is HSO3− or SO32−, not the
H2SO3, that enhance the fluorescence of Cy-CDs. Therefore, it
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is appropriate to carry all the experiments in neutral conditions
in PB buffer with pH 7.0, and under this condition the target
species bisufite has sufficient concentration.
To explore its reaction behavior with HSO3−, the probe CyCDs was treated with HSO3− at ambient temperature in an
aqueous environment. The fluorescence response time of the
Cy-CDs probe to various concentrations of HSO3− were first
investigated prior to the sensitivity study. The results showed
that the fluorescence of the probe was turned on immediately
in 4 min after the addition of HSO3− and remained unchanged
with a further increase of reaction time (Figure S7 in the
Supporting Information), indicating that it was very fast to
reach an equilibrium in the interaction between HSO3− and the
probe. The fluorescence intensity of Cy-CDs dispersed in PB
7.0 was increased with the addition amount of HSO3− with a
good linear relationship (R2 = 0.9987), which can be used for
the quantification of HSO3− (Figure 2). Quantitative analysis of
Figure 3. Selectivity of the assembled nanoprobe for HSO3− in the
presence of other common species in PB 7.0. The concentrations of
HSO3− and other species were 100 μM and 200 μM, respectively. F
and F0 are the fluorescence intensity of Cy-CDs with and without
HSO3−, respectively.
rate is very slow, and thus the probe Cy-CDs exhibits good
selectivity toward HSO3− in aqueous solution and could be
used for the determination of bisulfite in aqueous solution.
Because of the fast conversation between SO2 and its
derivatives in aqueous solution, it is rational to assume that the
nanoprobe was potential for the detection of gaseous SO2 in air
by simple and proper handling. To demonstrate the application,
various concentrations of SO2 gas in air were first prepared and
syringed to the Cy-CDs solution in PB 7.0, respectively. The
dissolved SO2 was then detected using the same procedure for
HSO3−. It was found that the fluorescence intensity of Cy-CDs
at 450 nm was gradually turned on with the increased amount
of SO2 gas from 0 to 80 ppm (Figure S8 in the Supporting
Information). The switch on effect was comparative with that
of adding NaHSO3, indicating that the probe can also be used
for the determination of gaseous SO2 in aqueous solution. Next,
some control experiments were undertaken to evaluate the
selectivity of this method. The gas containing various possible
species was syringed into the Cy-CDs solution according to the
procedure of adding SO2 gas, then the fluorescence spectra was
recorded with the excitation wavelength at 365 nm. As shown
in Figure 4a, no apparent spectral changes were obtained after
bubbling 100 ppm of other gas species to the Cy-CDs solution;
however, the fluorescence intensity increased remarkably upon
addition of 50 ppm of SO2. Accordingly, the fluorescence color
of the probe solution stay weak green except that the one of
adding SO2 turned strong blue (Figure 4b, upper panel).
Meanwhile, absorption color change was only obtained for the
solution added SO2 gas from yellow green to colorless (Figure
4b, bottom panel). These results suggested that the probe can
be used for the visual detection of SO2 gas easily by the naked
eye based on fluorometry as well as colorimetry.
Moreover, we further demonstrated that the functionalized
carbon nanodots can be used for the direct detection of SO2 gas
in air. A paper sensor was prepared by dropping the solution of
Cy-CDs to the test strip, on which yellow green spots can be
distinguished clearly. Then the test paper was exposed to SO2
atmosphere (50 ppm) in an airtight vial. The spots on the test
strip turned colorless immediately, which can be seen easily by
the naked eye (Figure S9 in the Supporting Information).
These results suggest that the test strips immobilized with the
Cy-CDs probe can be used for on-site and rapid detection of
SO2 gas.
Figure 2. Fluorescence spectra changes of Cy-CDs in PB 7.0 (50 mM)
upon addition of HSO3− (0−100 μM). Inset shows the plot of
fluorescence of the Cy-CDs as a function of the HSO3− concentration.
F and F0 are the fluorescence intensity of Cy-CDs with and without
HSO3− at 450 nm (λex = 365 nm), respectively.
this approach showed a good limit of detection (LOD) for
HSO3− at 1.8 μM. The fluorescence enhancement of the CDs
could be attributed to the HSO3−-induced nucleophilic addition
to the molecular skeleton, which has been confirmed by Sun et
al.,24,25 who demonstrated that HSO3− broke the structure of
cyanine dye and decreased the absorption at 450 nm. The
obtained nucleophilic addition product results in less spectral
overlap with the emission of the CDs, which is thus expected to
shut off the pathway of FRET from CDs to cyanine dye. As a
result, the fluorescence of the Cy-CDs will be turned on upon
the addition of HSO3−.
We then evaluated the selectivity and interference of Cy-CDs
toward common anion species with relative fluorescence
intensity in aqueous solution (Figure 3). The responses of
the nanoprobe to other species including F−, Cl−, Br−, HCO3−,
SCN−, NO2−, P2O74−, S2O32−, CH3COO−, SO42−, HS−, and the
mercapto-compound GSH (200 μM) were carefully examined
at the same conditions as HSO3− (100 μM). Clearly, only
HSO3− turns on the fluorescence intensity of Cy-CDs.
However, other species showed no apparent fluorescence
enhancement effect including HS−. The good selectivity can be
attributed to the strong nucleophilic attack of bisulfite to the
unsaturated double bond.21 In addition, no apparent interference was obtained in fluorescence intensity of the Cy-CDs
solution in the presence of other potential coexisting species
even at the concentration of 200 μM. These results indicate
that the Cy-CDs do not react with other species or the reaction
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Letter
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
Author Contributions
∥
M.S. and H.Y. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We acknowledge the financial support from the National Basic
Research Program of China (Grant 2011CB933700), Overseas,
Hong Kong & Macao Scholars Collaborated Researching Fund
(Grant 21228702), and the National Natural Science
Foundation of China (Grant Nos. 21302187 and 21205120).
■
Figure 4. (a) Enhancement effect of various gas species on the
fluorescence of Cy-CDs. (b) Color responses of probe Cy-CDs to
different gas species under a 365 nm UV lamp (upper panel) and
daylight (bottom panel) in PB 7.0. The final concentrations of SO2
were 50 ppm, CO, NO2, CO2, NH3, and H2S were 100 ppm,
respectively. N2 was bubbled to the solution of Cy-CDs for 1 min.
To further assess its applicability, the nanoprobe was used to
detect HSO3− in real water samples spiked with different
amounts of HSO3−, including rainwater and lake water. Upon
the addition of these water samples spiked with HSO3−, the
fluorescence intensity of the nanoprobe gradually increased.
The relative standard deviations (RSD) were obtained by
repeating the experiment three times under the same
conditions. The estimated recoveries of the measurements
and the RSD are satisfactory (Table S1 in the Supporting
Information), which indicate the reliability of the nanoprobe for
HSO3− determination in real samples.
In summary, we have developed a highly sensitive and
selective method for the detection of SO2 and its derivatives via
fluorescence enhancement. The method is achieved based on
the fluorescence resonance energy transfer mechanism by
functionalizing a reactive organic molecule (Cy) on the surface
of the carbon nanodots to give a weak fluorescence probe.
Because of the specific reactive response of bisulfite to the
organic molecule, the energy transfer pathway between the
molecule Cy and the nanomaterial CDs was shut down and the
weak fluorescence was enhanced upon addition of bisulfite in
aqueous solution. Thus, the probe has been demonstrated for
the determination of SO2 gas in aqueous solution as well as SO2
gas in air by assembling the probe on a simple test strip. The
probe displays advantages such as being easy-to-make, excellent
fluorescent response, and high selectivity. This method may
provide a new route for sensing other pollutants in a gas
sample.
■
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ASSOCIATED CONTENT
S Supporting Information
*
Details about the experiments, Figures S1−S9, and Table S1.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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