Determination of Gaseous Sulfur Dioxide and Its Derivatives via ...

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 9381

dx.doi.org/10.1021/ac503214v | Anal. Chem. 2014, 86, 9381−9385

Analytical Chemistry

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 9382



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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 9383



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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.

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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).

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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. 9384



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