Manipulating the Surface Chemistry of Quantum Dots for Sensitive

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Manipulating Surface Chemistry of Quantum Dots for Sensitive Ratiometric Fluorescence Detection of Sulfur Dioxide Huihui Li,†,‡ Houjuan Zhu,†,‡ Mingtai Sun,*,† Yehan Yan,†,‡ Kui Zhang,† Dejian Huang,§ and Suhua Wang*,†,‡ †

‡

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui, 230031, China

Department of Chemistry, University of Science & Technology of China, Hefei, Anhui, 230026, China §

Food Science and Technology Programme, Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore

ABSTRACT: Herein, we report a novel approach to the rapid visual detection of gaseous sulfur dioxide (SO2) by manipulating the surface chemistry of 3-aminopropyltriethoxysilane (APTS) modified quantum dots (QDs) using fluorescent coumarin-3-carboxylic acid (CCA) for specific reaction to SO2. The CCA molecules are attached to the surface aminogroups of the QDs through electrostatic attraction, thus the fluorescence of CCA is greatly suppressed due to the formation of ion-pair complex between the ATPS modified QDs and CCA. Such an interaction is vulnerable to SO2 since SO2 can readily react with surface aminogroups to form strong charge1

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transfer complexes, and subsequently release the strongly fluorescent CCA molecules. The mechanism has been carefully verified through a series of control experiments. Upon exposure to different amounts of SO2, the fluorescence color of the nanoparticle-based sensor displays continuously changes from red to blue. Most importantly, the approach owns high selectivity for SO2 and tolerance of interference, which enables the sensor to detect SO2 in a practical application. Using this fluorescence-based sensing method, we have achieved a visual detection limit of 6 ppb for gaseous SO2.

KEYWORDS: sulfur dioxide, quantum dots, ratiometric fluorescence, turn-on INTRODUCTION Sulfur dioxide (SO2) is one of the harmful important pollutant in air, and it can cause acidification of soils, lakes and streams and subsequently damage trees, crops, buildings and monuments when it rapidly oxidizes to SO2 in the presence of hydrogen peroxide (H2O2), (sun-) light, or ozone.1,2 It can spontaneously react with water to form sulfuric acid.2 Furthermore, excessive SO2 in air can cause the breathing difficulty with asthmatic wheezing, respiratory illness, chest tightness, nausea, alterations in the lungs defenses and the aggravation of existing cardiovascular disease.3,4 With the growing concern over its major health risk and eco system, there is an immediate demand for development of simple, reliable and convenient methods for selective and sensitive determination of SO2 in various scenarios. Currently, several conventional techniques have been employed for the monitoring of SO2 level, such as titrimetry,5 spectrophotometry,6 polarographic methods,7 physical or chemical adsorption method,8 electrochemical sensors,9 flow injection,10 chemiluminescence,11 and ion chromatography methods.12 However, there are intrinsic limitations for these methods because 2


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they are often expensive, time-consuming, or requires complicated instrumentation. Recently, Guo’s group have synthesized metalloporphyrin-based conjugated microporous polymers (CMPs) nanoparticles for colorimetric detection of gaseous SO2.6 Zhang et al. have reported that graphene oxide (GO) nanosheets derived from chemically prepared GO act as a promising material for SO2 sensing.13 Quantum dots (QDs) have been documented for superior fluorescent properties, however, there is few report on using QDs for the detection of SO2. Fluorescent nanomaterials such as QDs and GO have recently attracted increasing attention due to their superior spectroscopic properties and hence the wide variety of potential applications.14 For example, fluorescent QDs have been widely used for cell imaging and labeling after functionalized modification with biomolecules.15,16 As a counterpart of organic dyes, the fluorescent QDs have narrow and symmetric emission bands, which make it more suitable as fluorescence probes.17-19 However, the analytical performance of such QDs based fluorescence probes is greatly dependent on the chemistry properties of the QDs surface. Therefore, it is very significant to judiciously manipulate the surface chemistry of QDs for extensive exploration of specific potential applications. Herein, we demonstrate that the QDs are firstly encapsulated in silica shells for stabilization, which are subsequently functionalized with 3-aminopropyltriethoxysilane (APTS) for surface attachment of coumarin-3-carboxylic acid (CCA), an organic dye. The red emissive QDs were chosen as the acceptor because they exhibit broad excitation spectra, feasibility of surface grafting for chemical recognition, and red fluorescence color which can be identified by naked eyes easily; while CCA was used as the fluorescence reporter because CCA molecular structure contains carboxyl group which can react with amino group of APTS easily and may form a charge-transfer complex, and it emits blue fluorescence which makes the target probe possible to


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get continuous color changes due to the variations of the dual emission intensity ratios. Such a hybrid system exhibits independently dual-emission bands from QDs and CCA. Furthermore, the hybrid system possesses a specific chemistry property to recognize SO2 due to the formation of stable 1:1 charge-transfer complex between the aminogroup and SO2.20 We thus employ such a hybrid system to design a fluorescence method for sensitive detection of SO2. EXPERIMENTAL SECTION Materials. The chemicals and solvents were obtained from commercial sources (Sigma Aldrich or Aladdin) and were used directly without further purification unless specified. Ultrapure water (18.2 MΩ·cm) was provided by a Millipore water purification system. Anhydrous ethanol was obtained by distillation at 110 oC after addition of moderate CaO solid powder. Trichloromethane (CHCl3) was dried by adding some anhydrous sodium sulfate. SO2 gas was obtained by injecting 200 µL concentrated H2SO4 (98%) using syringe into a 50 mL round glass bottle which contains 250 mg Na2SO3 solid. The SO2 stock solution in CHCl3 was prepared by injecting repeatedly freshly prepared SO2 gas into a small glass bottle which contains 10 mL of CHCl3, after that placing the glass bottle which contains SO2 (CHCl3) solution in ice bath in order to slow down the volatilizing speed of SO2 gas. Exact SO2 concentrations were determined by measuring the UV absorbance at λ = 288 nm (ε = 365 M-1·cm-1) and applying the Beer-Lambert law immediately before the experiment (The absorption spectrum of SO2 in CHCl3 was present in Figure S1, Supporting Information). For the sensing experiments, we diluted the SO2 stock solution using CHCl3 to obtain a 1 mM SO2 (CHCl3) solution. Nitrogen dioxide (NO2) was prepared by diluting the pure NO2 gas to the target concentration in a 50 mL flask. CO gas was prepared from the reaction of HCOOH with concentrated H2SO4 in a 50 mL flask. H2S gas was prepared from the reaction of Na2S with 3M H2SO4 in a 50 mL flask. NH3 gas


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was prepared from the reaction of NH4Cl with Ca(OH)2 in a 50 mL flask. CO2 gas was prepared from the reaction of CaCO3 with 2 M HCl. All glassware were cleaned with ultrapure water and then dried before use. Instrumentation. UV-vis spectroscopic studies were carried out on a Shimadzu UV-2550 spectrometer. Fluorescence spectra were recorded on a Perkin-Elmer LS55 with slit width of 10.0 nm for both excitation and emission using a 340 nm excitation wavelength and a 600 nm/min scan rate. The color changes were observed under a UV lamp (λex = 312 nm). Photographs were taken using a canon 350D digital camera. The FT-IR spectra were obtained with a Thermo Scientific Nicolet iS10 spectrometer. The transmission electron microscopy (TEM) images were recorded on a JEOL 2010 transmission electron microscope. Preparation

of

CdTe

QDs

embedding

in

silica

nanoparticles

and

surface

functionalization. The red emissive CdTe QDs were synthesized in the aqueous phase by a previous method.14 The red emissive 628QDs (λem = 628 nm) embedded silica nanoparticles were prepared according to the method with minor modification.18 Typically, 40 mL of ethanol, 15 mL of ultrapure water, and 5 mL of red

628

QDs solution were mixed in a 100 mL one-necked

flask and stirred for 10 min at room temperature. After the flask was covered with aluminum foil, 20 µL of trimethoxysilane (MPS) was introduced, and the resultant solution was stirred for 10 h. Then, 1.5 mL of tetraethylorthosilicate (TEOS) was added dropwise into the solution, followed by adding 1.5 mL of ammonium hydroxide. The mixture was allowed to react for 12 hours. Then 200 µL of APTS (the volume of APTS is optimized before) was added into the mixture under vigorous stirring for the surface functionalization with amino groups. After 12 h of reaction, the functionalized silica nanoparticles were separated by centrifugation and the precipitate was redispersed in 10 mL of ethanol for further use.


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Preparation of nanohybrid through hybridization of CCA with APTS-CdTe@SiO2 QDs. Nanohybrid was prepared according to the following procedure. Briefly, 1.0 mL of APTSCdTe@SiO2 QDs solution and 600 µL of CCA solution (32.6 mM) were mixed in a 25 mL flask and stirred at room temperature. After 3 h of reaction, the products were centrifuged and the precipitate was redispersed in 2.0 mL of ethanol for future use while the supernatant was discarded. The fluorescent CCA was attached on the surface of APTS-CdTe@SiO2 through electrostatic attraction between the amino groups and carboxylic groups to generate a nanohybrid fluorescence probe (CCA-APTS-CdTe@SiO2 QDs). Detection of SO2 using the nanohybrid. The nanohybrid fluorescence probe was dissolved in ethanol and showed a dual-emission at 425 nm and 628 nm from the CCA and APTSCdTe@SiO2 QDs under the 340 nm excitation. To evaluate the fluorescence response of the nanohybrid to SO2, different concentrations of SO2 solutions were added into the nanohybrid solutions followed by monitoring the fluorescence intensities. The experiments were replicated three times and the average was obtained for each concentration. RESULTS AND DISCUSSION Scheme 1. Schematic Illustration of Nanohybrid Fluorescence Probe Structure and Visual Detection Principle for SO2


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The synthesis procedure and the sensing principle of the nanohybrid were shown in Scheme 1. First, the red

628

QDs are fully wrapped by silica shell to get the

628

QDs-embedded silica

nanoparticles. Then, the surface of the silica particles was further functionalized with APTS which could act as reactive sites for SO2 anchoring CCA molecules. The APTS-CdTe@SiO2 QDs showed a maximum emission at 628 nm (Figure 1c, the orange line) and exhibited a strong red fluorescence under a 312 nm UV lamp. The CCA molecules were attached on the

628

QDs-

embedded silica particle surface through the formation of ion-pair complexes to get the nanohybrid. The fluorescence intensity of CCA was greatly suppressed due to deprotonation by aminogroups of surface APTS. This has been confirmed by control experiments (Figure S2 and S3, Supporting Information). The ratiometric probe (Figure 1b, the red line) displayed wellresolved dual emission bands at 425 and 628 nm under a single wavelength excitation at 340 nm. The obtained hybrid probe could easily be redispersed in ethanol, and red fluorescence could be observed under a 312 nm UV lamp. The fluorescence spectra of blue emissive CCA, the ratiometric probe, and red emissive APTS-CdTe@SiO2 QDs solutions are shown in the upper inset of Figure 1. Clearly, significantly different fluorescence colors could be observed among the three systems under a UV lamp (λex = 312 nm) illumination.


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Figure 1. Fluorescence emission spectra of (a) blue emissive CCA, (b) nanohybrid fluorescence probe and (c) orange emissive APTS-CdTe@SiO2 QDs. The inset photos show the corresponding fluorescence colors under a UV lamp (λex = 312 nm). To obtain the architecture properties of the as-prepared nanohybrid, the sizes and morphologies of red MPA coated CdTe QDs, APTS modified QDs@SiO2 nanoparticles were firstly studied with TEM. As can be seen in the TEM images, CdTe QDs and the as-prepared APTS modified QDs@SiO2 nanoparticles have an average size of about 4 nm and 65 nm (Figure S4 of the Supporting Information), respectively. These results suggested that CdTe QDs has been encased in the silica nanoparticle. The silica shell coating on the red

628

QDs not only

improved the QDs’ photostability and chemical stability but also prevented direct interaction between

628

QDs and SO2, thus providing a reliable reference signal in a dual emission

fluorescent system for ratiometric detection of SO2. The FT-IR spectrum of the ratiometric probe nanoparticles was also investigated (Figure S5 of the Supporting Information). As shown in Figure S5, the broad band at 3403 cm-1 is assigned to OH vibration stems from the water molecules. The weak peak at 1732 cm-1 can be attributed to the C=O stretching vibration peak which stems from CCA molecule, and bands at 1626 cm-1, 1596 cm-1, 1451 cm-1, 1395 cm-1 belong to the vibration absorption peaks of the aromatic ring skeleton of CCA. The strong band at 1092 cm-1 belongs to antisymmetric stretching vibration absorption peak of Si-O-Si, while the band at 811 cm-1 is attributed to symmetric stretching vibration absorption peak of Si-O. The band at 470 cm−1 can be ascribed to the bending vibrational frequencies of Si-O. From what has been discussed above, it is suggested that these bands should be a result of CCA molecules which are attached at the surface of APTS modified CdTe@SiO2 QDs.


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The stability of the as-prepared ratiometric probe against time was systematically investigated by flashing UV light through the ratiometric probe solution. Firstly, we recorded the fluorescence intensities of the probe when it was consecutively illuminated at 340 nm (5 min for each time) and found the fluorescence of the ratiometric probe was stable within 1 h; secondly, we then recorded its fluorescence intensities after the ratiometric probe was placed for 1h, 2h and one month, respectively. The relevant results are shown in Figure S6. From these results, it can be seen that the as-prepared ratiometric probe was stable for at least one month. To optimize the sensing conditions, an experiment on the time course of the ratiometric probe to gaseous SO2 was first conducted before detection (Figure S7 of the Supporting Information). From the results, it can be seen that the fluorescence of the ratiometric probe was enhanced rapidly after a certain amount of SO2 added in 2 minutes and after that the fluorescence intensity remained unchanged. So we can conclude that the response time of the probe for SO2 was as fast as two minutes and the fluorescence intensities of the probe recorded at two minutes after adding SO2 should be reliable. In addition, further experimental results showed that the color change with SO2 exposure can last for 6 hours under ambient conditions (Figure S8). The dose response of the ratiometric probe to SO2 was examined. Before exposure to SO2, the ratiometric probe emitted two weak well-resolved emission peaks centered at 425 and 628 nm under a single wavelength excitation, which were ascribed to the fluorescence of CCA and APTS-CdTe@SiO2 QDs, respectively. Upon addition of SO2, the fluorescence intensity at 425 nm of the CCA was gradually enhanced, whereas the intensity at 628 nm of the APTSCdTe@SiO2 QDs remained unchanged, as shown in Figure 2. Owing to the changes in the intensity ratio of the two emission wavelengths, the fluorescence colors of the ratiometric probe solution changed continuously. Clearly, a slight increase of the emission intensity at 425 nm


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could result in distinct color changes from the original background as demonstrated in the inset of Figure 2. Therefore, the visual detection of SO2 by the naked eyes under a UV lamp was feasible. As can be seen from Figure 3, the ratio of the fluorescence intensities were closely related to the added amounts of SO2, which could be used for the quantification of SO2 with a correlation coefficient of 0.993, and the detection limit of 0.33 µM for SO2 was calculated using the equations: LOD = 3 σ/k, where σ was the standard deviation of a blank and k was the slope of the calibration line.

Figure 2. The fluorescence colors and the corresponding fluorescence spectra of the ratiometric probe solution (0.091 g/L) upon exposure to different concentrations of SO2. The concentrations of SO2 from left to right were 0, 3, 6, 9, 12, 15, 18, 21, and 24 µM, respectively.


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Figure 3. Plot of fluorescence enhancement of the ratiometric probe as a function of the SO2 concentration. (I425/I628)0 and (I425/I628) were the ratio of the fluorescence intensity of the ratiometric probe in the absence and presence of different concentrations of SO2, respectively. The reaction mechanism of the nanohybrid system for detection of SO2 can be ascribed to the initial formation of stable charge-transfer complexes between SO2 and the surface amino groups, and subsequent releasing of highly fluorescent CCA due to protonation. This can be evidenced by a separate experiment that addition of SO2 into weakly fluorescent deprotonated CCA in alkaline solution could restore the fluorescence of CCA (Figure S9 and S10, Supporting Information), since the pKa of SO2 (1.85) was much lower than that of CCA (4.56).21 Further control experiments showed that SO2 had no effect on the fluorescence of protonated CCA and APTS-CdTe@SiO2 QDs solution (Figure S11 and S12, Supporting Information). Furthermore, a control blank experiment of CHCl3 on the fluorescence of the atiometric probe was conducted (Figure S13). The results indicated that CHCl3 itself did not activate the fluorescence of the ratiometric probe. From what has been discussed above, we can conclude that SO2 induces the decomposition of ion-pair complexes between CCA and APTS-CdTe@SiO2 QDs and sequentially restores the fluorescence of CCA. In addition, we studied the UV-vis spectra of SO2 with different amounts of APTS. As shown in Figure 4, the absorption spectra of SO2 (0.64 mM) showed a broad peak near λmax = 288 nm in CHCl3. The gradual addition of APTS to SO2 led the peak near 288 nm shift to lower wavelength (~ 275 nm). The blue shift could be an evidence for the formation of a charge-transfer complex between SO2 and the amino groups of APTS. It is documented that the lowest unoccupied orbital in SO2 is a hybridized pπ-dπ antibonding orbital.22 So SO2 would act virtually as an σ acceptor to form a charge-transfer complex, which was stabilized by a bond formation between SO2 and


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the nitrogen atom of the amino groups of APTS. The observed blue shift of the absorption band from 288 nm to 275 nm in CHCl3 clearly suggested that SO2 interacted with the amino groups of APTS and formed charge-transfer complexes. Therefore, it is rational to speculate that the fluorescence enhancement of the ratiometric probe by SO2 is ascribed to the interaction with APTS, and hence releasing the fluorescent CCA molecules from the CCA-APTS-CdTe@SiO2 QDs ion-pair complexes.

Figure 4. UV-vis absorption spectral responses of SO2 (0.64 mM) to different amounts of APTS (0-0.64 mM) in chloroform. The selectivity of the ratiometric probe for SO2 was carefully examined among various relevant gaseous samples including SO2, CO, NO2, CO2, NH3, and H2S (Figure 5). It could be seen that when the same concentrations of CO, CO2, NO2, H2S, and NO2 were injected into probe solution, no obvious fluorescence enhancement at 425 nm was observed. However, the fluorescence intensity was increased remarkably upon addition of the same concentration of SO2.


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Figure 5. (a) Selective fluorescence responses of the CCA-APTS-CdTe@SiO2 QDs ratiometric probe to different gas species. (b) Fluorescence images of ratiometric probe solution after exposure to different gas species under a 312 nm UV lamp. The final concentrations of SO2, CO, NO2, CO2, NH3 and H2S were 1.8 ppm. We then further demonstrated that the probe can be used for the visual monitoring of gaseous SO2. First, several of different concentration levels of SO2 gas samples were prepared by diluting pure SO2 gas with nitrogen in 100 mL round-bottom flask. Second, a 2 mL gas sample containing a certain concentration level of SO2 was drawn into a plastic syringe and then was slowly bubbled into 2 mL of ratiometric probe solution (0.273 g/L) by hand. These solutions were then illuminated under a 312 nm UV lamp and subsequently the fluorescence images were taken. Clearly, as shown in Figure 6, the one exposed to 6 ppb of gaseous SO2 led to distinguishable color change from red to blue (Figure 6c), so the visual detection limit of 6 ppb was thus estimated as the least concentration of SO2 capable of producing blue fluorescence that


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could be noted by independent observers. The results suggested that such a method can be used for the visual monitoring of gaseous SO2.

Figure 6. Visual detection of gaseous SO2. The SO2 concentrations of 0, 2, 4, 6 and 8 ppb were used. The photos were taken in the test cuvettes which were injected into gas samples with different concentrations of SO2. The photos were obtained under a 312 nm UV lamp in the dark. CONCLUSIONS We have developed a fluorescence turn-on nanohybrid capable of monitoring gaseous SO2 using QDs as the fluorophore and the surface grafting APTS as the recognition site for SO2 molecules. The fluorescence of the probe is initially weak as a result of the formation of ion-pair complexes between CCA and APTS-CdTe@SiO2 QDs, it is then turned on by SO2, which reacts with the surface APTS and thus leads to synchronous dissociation of ion-pair complexes. This method is demonstrated to be sensitively to detect SO2 and affords a limit of visual detection of 6 ppb for gaseous SO2. ASSOCIATED CONTENT Supporting Information The photostability and characterization of nanohybrid, fluorescence spectra of CCA in different systems containing APTS, APTS-CdTe@SiO2, NaOH, or SO2. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (21205120, 21302187, 21475134, and 91439101). REFERENCES (1) Shankaran, D. R.; Iimura, K. I.; Kato, T. Electrochemical Sensor for Sulfite and Sulfur Dioxide Based on 3-Aminopropyltrimethoxysilane Derived Sol-Gel Composite Electrode. Electroanal. 2004, 16, 556-562. (2) Albrecht, M.; Gossage, R. A.; Lutz, M.; Spek, A. L.; Koten, G. V. Diagnostic Organometallic and Metallodendritic Materials for SO2 Gas Detection: Reversible Binding of Sulfur Dioxide to Arylplatinum(ii ) Complexes. Chem. Eur. J. 2000, 6, 1431-1445. (3) Sun, Y. Q.; Liu, J.; Zhang, J. Y.; Yang, T.; Guo, W. Fluorescent Probe for Biological Gas SO2 Derivatives Bisulfite and Sulfite. Chem. Commun. 2013, 49, 2637-2639. (4) Sun, M. T.; Yu, H.; Zhang, K.; Zhang, Y. J.; Yan, Y. H.; Huang, D. J. Determination of Gaseous Sulfur Dioxide and Its Derivatives via Fluorescence Enhancement Based on Cyanine Dye Functionalized Carbon Nanodots. Anal. Chem. 2014, 86, 9381-9385.


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(13) Shen, F. P.; Wang, D.; Liu, R.; Pei, X. F.; Zhang, T.; Jin, J. Edge-Tailored Graphene Oxide Nanosheet-Based Field Effect Transistors for Fast and Reversible Electronic Detection of Sulfur Dioxide. Nanoscale 2013, 5, 537-540. (14) Zhu, H. J.; Zhang, W.; Zhang, K.; Wang, S. H. Dual-Emission of A Fluorescent Graphene Oxide–Quantum Dot Nanohybrid for Sensitive and Selective Visual Sensor Applications Based on Ratiometric Fluorescence. Nanotechnol. 2012, 23, 315502-315510. (15) Zhang, P. F.; Liu, S. H.; Gao, D. Y.; Hu, D. H.; Gong, P.; Sheng, Z. H.; Deng, J. Z.; Ma, Y. F. Click-Functionalized Compact Quantum Dots Protected by Multidentate-Imidazole Ligands: Conjugation-Ready Nanotags for Living-Virus Labeling and Imaging. J. Am. Chem. Soc. 2012, 134, 8388-8391. (16) Medintz, I. L.; Stewart, M. H.; Trammell, S. A.; Susumu, K.; Delehanty, J. B.; Mei, B. C.; Melinger, J. S.; Blanco-Canosa, J. B.; Dawson, P. E.; Mattoussi, H. Quantum-dot/Dopamine Bioconjugates Function as Redox Coupled Assemblies for in Vitro and Intracellular pH Sensing. Nat. Mater. 2010, 9, 676-684. (17) Sun, J.; Yan, Y. H.; Sun, M. T.; Yu, H.; Zhang, K.; Huang, D. J.; Wang, S. H. Fluorescence Turn-On Detection of Gaseous Nitric Oxide Using Ferric Dithiocarbamate Complex Functionalized Quantum Dots. Anal. Chem. 2014, 86, 5628-5632. (18) Yao, J. L.; Zhang, K.; Zhu, H. J.; Ma, F.; Sun, M. T.; Yu, H.; Sun, J.; Wang, S. H. Efficient Ratiometric Fluorescence Probe Based on Dual-Emission Quantum Dots Hybrid for On-Site Determination of Copper Ions. Anal. Chem. 2013, 85, 6461-6468. (19) Yu, Z. Q.; Schmaltz, R. M.; Bozeman, T. C.; Paul, R.; Rishel, M. J.; Tsosie, K. S.; Hecht, S. M. Selective Tumor Cell Targeting by the Disaccharide Moiety of Bleomycin. J. Am. Chem. Soc. 2013, 135, 2883-2886.


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(20) Leontiev, A. V.; Rudkevich, D. M. Revisiting Noncovalent SO2-Amine Chemistry: An Indicator-Displacement Assay for Colorimetric Detection of SO2. J. Am. Chem. Soc. 2005, 127, 14126-14127. (21) Riter, R. E.; Undiks, E. P.; Levinger, N. E. Impact of Counterion on Water Motion in Aerosol OT Reverse Micelles. J. Am. Chem. Soc. 1998, 120, 6062-6067. (22) Grundnes, J.; Christian, S. D. Solvent Effects on Strong Charge-Transfer Complexes. I. Trimethylamine and Sulfur Dioxide in Gas and in Heptane. J. Am. Chem. Soc. 1968, 90, 22392245.

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Manipulating the Surface Chemistry of Quantum Dots for Sensitive

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