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Sensing ppm level ammonia and ppb level acetic acid in the gas phase by common black film with a fluorescent pH probe Jingni Fu, and Luning Zhang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Analytical Chemistry

Sensing ppm level ammonia and ppb level acetic acid in the gas phase by common black film with a fluorescent pH probe Jingni Fu, Luning Zhang* School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China. ABSTRACT: Relying on the nanometer-thick water core and large surface area-to-volume ratio (~2×108 m-1) of common black film (CBF), we are able to use a pH-sensitive dye (carboxy-seminaphthorhodafluor-1, SNARF-1) to detect ammonia and acetic acid gas adsorption into the CBF, with the limit of detection reaching 0.8 ppm for NH3 gas and 3 ppb for CH3COOH gas in the air. Data analysis reveals that fluorescence signal change is linearly proportional to the gas concentration up to 15 ppm and 65 ppb for NH3 and CH3COOH, respectively.

Introduction Systematic studies of various properties of soap films were first conducted by the Belgian scientist Joseph Plateau.1 Plateau studied the influence of various external factors (e.g. the stream of air, evaporation, etc.) on soap film stability and formulated Plateau’s laws of soap film structure.2 In addition to curved soap film, a flat film can be created by pulling a metal frame from an aqueous surfactant solution. As the water in the flat soap film drains out, the thickness of the film reaches an equilibrium value. The film can be either a common black film (CBF) with thickness of 10-100 nm or a Newton black film (NBF) with thickness of 5-10 nm.3 Among the various properties of soap film, transfer of gas through a soap film has been thoroughly studied and is found to be dependent on the gas permeability of two surfactant monolayers at the surfaces and the aqueous core in the center.4 Various methods such as stationary bubble method, gas tracer technique have been used to measure the permeability of gas through soap films.5-8 Quoc et al.8 investigated the permeability of Newton black films, using the pH changes and mass spectrometry signal from reactive gas (NH3) and inert gas (argon). Farajzadeh et al.9 studied the effect of surfactant type on black film permeability and found that the trend of permeability is: cationic > anionic > nonionic. Continuous and real time monitoring of gas has been the focus of both industry and laboratory research.10,11 Absorption, fluorescence, conductivity have been successfully utilized for the detection of gas components.12-16 Fluorescence sensing has many advantages such as flexible dye choice, high sensitivity, non-intrusive and in-situ.17 Mills and Yusufu18 reported the detection of gaseous and dissolved CO2 with a pH-sensitive fluorescent dye embedded in polymer film. The detection range of the dye-based CO2 sensors depends on various factors, including the pKa of the dye and the properties of the materials hosting the dye.18 Tavoli and Alizadeh19 used Eriochrome Cyanine R in polypyrrole film for optical ammonia gas sensing. Since soap films have large surface area-to-volume ratio and can be easily renewed, it can be used to efficiently collect water-soluble gases.20 Dasgupta’s group21-23 developed soap filmbased assays for gas sensing. They successfully detected ammonia and hydrogen peroxide on soap films by measuring the

fluorometric and electrochemical signal. Kanyanee et al.14 monitored the conductance of a spherical soap film (bubble) with spherical cap electrodes and detected sub-ppm levels of SO2 after reaction with hydrogen peroxide in the soap bubble. Oudenhoven et al.24 demonstrated the electrochemical detection of ammonia using an ionic liquid film-based sensor, which could sense 1-10 ppm ammonia in ambient conditions. In this study, we focus on the detection of two odorous and harmful gases, ammonia and acetic acid by an assay based on common black film. Exposure to different concentration of ammonia vapor may cause obstruction of breathing, even to edema and severe damage of the mucosa membranes of the respiratory tract with possible fatal results.25 As for acetic acid, it is known to be corrosive to metals and may become a health hazard to the human body, causing irritation to the eyes, nose and skin when in contact with the concentrated liquid or vapor.26,27 The U.S. National Institute of Occupational Safety and Health (NIOSH) recommends an exposure limit of 25 ppm (18 µg/L) for ammonia and 10 ppm (25 µg/L) for acetic acid as time-weighted average values.28 Many types of ammonia sensors have been reported in the literature, such as metal-oxide, catalytic reaction on metal, conductive polymer and optical detectors.29-33 Imawan et al.30 modified MoO3 thin films using Ti-overlayers to sense ammonia gas by measuring the DC electrical resistance change. Winquist et al.31 fabricated an ammonia sensor based on palladium MOS field-effect capacitor. The voltage drop of the sensor could be used to monitor ammonia as low as 1 ppm.31 Mirica et al.32 used conductance change of dispersed single-walled carbon nanotubes on paper to detect ammonia gas at sub-ppm level. Strobl et al.33 presented fiber-optic ammonia sensors based on aza-BODIPY dyes which could detect dissolved ammonia from 10 mg/L to 1 µg/L. Acetic acid can be detected by HPLC, GC-MS, electrochemistry and FT-IR technology.34-36 Ueta et al.35 reported acetic acid sensing by gas chromatograph. The limit of detection was 60 ng L−1 (24 ppb) with 100 mL of sample volume, and 10 ng L−1 (4 ppb) with 600 mL of sample volume.35 Cheng et al.36 fabricated porous coral-like SnO2, which could sense a lower limit of 10 ppm acetic acid through the change of electrical resistance.

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There is an on-going quest for assays that are operationally simple, requires small sample volume, and has low detection limit to sense of ammonia and acetic acid at ambient condition. In this paper, we present a sensing method based on two unique properties of the CBFs: large surface area for relatively fast gas uptake, and nanometer-scale water core for relatively large pH change. We use carboxy-seminaphthorhodafluor-1 (SNARF-1) in CBFs as the pH indicator. This fluorescent dye exhibits ratiometric behavior with its acid peaks/base peaks ratio related to the pH. After describing the details of experimental methods and data analysis, we present a series of fluorescence spectra of the SNARF-1 dye in CBFs upon sensing different concentrations of ammonia and acetic acid gas. Data shows that the change in fluorescence signal is linearly proportional to the gas concentration up to 15 ppm and 65 ppb, with the detection limit of 0.8 ppm and 3 ppb for ammonia and acetic acid, respectively. Experimental section Material. Cetyltrimethylammonium bromide (CTAB), ammonia solution (25%) and acetic acid (99.7%) are from Aladdin Chemicals. Sulfurous acid (99%) is purchased from Adamas reagent co.,ltd. Solutions with different concentrations of ammonia, acetic acid and sulfurous acid are prepared by volumetric dilution using high purity water (resistivity ∼18.2 MΩ cm−1). The concentration of CTAB in all aqueous solutions is 10-2 M. The dye carboxy-seminaphthorhodafluor-1 (SNARF-1) is from Molecular Probes Inc. in solid form and is dissolved volumetrically into the CTAB solutions to reach 10-4 M. All samples are prepared and kept at 25 °C during measurement. Instrument. A square-shaped stainless steel frame (laser-cut from plate) with a side length of 10 mm is used to lift a vertical soap film from the CTAB (10-2 M) micellar solution containing SNARF-1 (10-4 M). CTAB is used because of its excellent gas permeability.9 After the soap film is completely drawn out of the bulk solution, it is put inside a quartz cuvette (4 cm × 4 cm × 4 cm) with some bulk solution at the bottom to maintain equilibrium of gas phase vapor pressure, as illustrated in Figure 1a. The top region of the soap film loses its interference pattern and turns to black first, then the black region extends to the whole film gradually. Finally, the film reaches equilibrium and becomes a CBF after about 20 minutes. Inside the cuvette, the CBFs exhibit good stability for hours during the fluorescence measurement. In order to determine the thickness of the CBF, FT-IR absorption is used by measuring the absorbance in the OH stretching region (3000-3600 cm-1). The FT-IR spectrum of the CBF is shown in Figure S-1. Conversion of the absorbance at 3400 cm-1 into water film thickness is based on the study of K. Liao et al.37 The corresponding thickness of the water core is 23 ± 1 nm. We use a custombuilt fluorescence spectrometer with a 473 nm laser for excitation. The laser focuses on the CBF at 45 degree incident angle through a 20 cm focal length quartz lens. The fluorescence emitted by SNARF-1 within the CBF is focused by a quartz lens onto the entrance slit of a spectrograph (Andor Shamrock SR-303i-B) equipped with a cooled CCD detector at -60 °C. Since the micellar solution with SNARF-1 and CTAB is mildly acidic (pH = 5.0), the solution is suitable for ammonia sensing. For acetic acid, we adjust the bulk solution to pH = 8.0 prior to sensing.

Procedure. For each analyte, we perform two types of experiments which are called “bulk sensing” and “gas sensing”. In bulk sensing, we volumetrically mix solution with dissolved analyte into the micellar solution containing SNARF-1 and CTAB to prepare a bulk sample solution. We then form CBF using this bulk solution for fluorescence measurement. In this way, we establish a relationship between the known analyte concentration in CBF and the corresponding fluorescence intensity change. In the gas sensing, we directly sense the gas phase analyte utilizing the gas uptake property of CBFs. In these experiments, together with a freshly prepared CBF that contains SNARF-1, we put aqueous solution containing dissolved ammonia or acetic acid with known vapor composition at the bottom of the quartz cuvette. The cuvette is tightly covered by a plastic lid to ensure vapor phase equilibrium. In the gas sensing scheme, we cannot directly measure the precise concentration of ammonia or acetic acid dissolved in the CBFs from the gas phase. We need to derive the amount of analyte inside CBF by comparison with the bulk sensing results. The assumption is that the concentration of ammonia (or acetic acid) in the CBFs is the same if the spectral change is the same in the two sensing schemes, no matter whether the analyte is mixed directly into the bulk in the liquid phase or adsorbed into the CBF from the gas phase. Data analysis. Figure 1b shows the chemical structure and acid-base equilibrium of SNARF-1. In aqueous solution, the pKa of SNARF-1 is 7.5, and the fluorescence emission maxima of the acid and conjugate base are at 580 nm and 640 nm, respectively. The profile of the bands and the shoulder peaks in the spectra indicate that the SNARF-1 molecules are most likely partitioned at different locations within the CBF. Because the environment of the dye is complex, we find that four Gaussian peaks are needed to correctly evaluate the contributions of the acid and base forms in the fluorescence signal. The spectrum of 10-5 M ammonia (bulk sensing) can be fitted by two acidic peaks and two basic peaks, as shown in Figure 1c. The green lines and blue lines correspond to the acid form and the base peaks. The red line is the overall fitting curve which closely resembles the black experimental curve. The relative contribution of each peak to the total intensity is calculated by the peak areas. The first acid peak centers at 545-552 nm with a peak width decreases as ammonia concentration increases. The second acid peak is at 600-602 nm with a relative constant width of 50 nm. The third peak is of the base and centers at 653-666 nm. The fourth peak is also from the base and centers at 713-722 nm, with a constant width of about 50 nm. More spectral fitting details are shown in Figure S-2 and S-3. The ratio between acid form and base form of SNARF-1 is denoted as Acid/Base. Without the addition of ammonia or acetic acid, the ratio is called the blank, which is (Acid/Base)blank. Upon reaction with the analyte, a change of the Acid/Base ratio occurs which is ∆(Acid/Base). It is defined as the following: ∆(Acid/Base) = Acid/Base - (Acid/Base)blank. Thus, the value ∆(Acid/Base) corresponds to the amount of ammonia and acetic acid in the CBF compared with the blank. For ammonia assay, Acid/Base decreases with increasing ammonia concentration, hence the value of ∆(Acid/Base) is always negative. While in the sensing of acetic acid, the ratio Acid/Base increases with increasing acetic acid concentration, hence the value of ∆(Acid/Base) is positive. For ease of under-


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Analytical Chemistry standing, we use the absolute value │∆(Acid/Base)│ for both analytes in order to show the positive correlation between the spectral change and the analyte concentration. Results and Discussion Ammonia sensing. Many people have measured the vaporliquid equilibrium data for ammonia.38-40 In this paper, we refer to Lv B’s41 empirical formula to estimate the NH3 vapor pressure above different concentration ammonia solution. At 25 °C, the calculation formula is shown in equation (1). ln 2.4868 ln 0.0713 (1) In this equation, pNH3 (mmHg) is the partial pressure of NH3, and MNH3 is the molar concentration of ammonia in the liquid phase. In our experiment, the concentration of ammonia is lower than 2×10-3 M，therefore 0.0713 in equation (1) can be approximated as zero. We thus obtain a linear relation between the concentration of ammonia in the aqueous phase and its partial pressure in the gas phase under equilibrium conditions, which can be understood by Henry’s law.

The Henry’s law constant is thus 0.62 mol m-3 Pa-1 by the calculation above, which is consistent with the literature value of 0.60 mol m-3 Pa-1 measured by Clegg and Brimblecombe38. The equilibrium concentration of gaseous NH3 can be adjusted by changing the concentration of the aqueous ammonia solution. The fluorescence of CBFs is measured after 5 min, allowing the concentration of NH3 in gas phase and in CBFs to reach equilibrium. Figure 2a illustrates the fluorescence spectra in bulk sensing as the concentration of ammonia in the bulk increases from 0 to 1.5 mM. The gas sensing spectra are shown in Figure 2b as the concentration of NH3 (gas) increases from 0 to 40 ppm. As the concentration of ammonia rises, the acid peaks of SNARF-1 decrease and the base peaks increase. In both cases, the │∆(Acid/Base)│approaches a maximum value of 2.20 ± 0.06 and stops changing, even with increasing amount of ammonia in the system. The │∆(Acid/Base)│ in the bulk sensing and gas sensing experiments are plotted against the ammonia concentration in Figures 2c and 2d. At 0.32 mM ammonia concentration, the fluorescence in bulk sensing is saturated. This is probably because SNARF-1 cannot respond to further pH changes above pH = 9.0, which is an intrinsic property of the dye. If we prepare bulk solutions with pH larger than 9.0 and form CBFs for measurement, we find that the Acid/Base of SNARF-1 remains almost constant. In the gas sensing, the fluorescent signal becomes unchangeable at above 20 ppm The spectral change │∆(Acid/Base)│ is linearly proportional to ammonia concentration up to 0.20 mM in bulk sensing and 15 ppm in gas sensing. The fitting results are plotted as insets in Figures 2c and 2d. The two linear fitting functions are described by equation (2) and (3). │∆Acid/Base │ 10.9449"NH% aq. , )* 0.0205 (2)

Figure 1. (a) Schematic of cuvette with CBF and analyte. (b) Chemical structures of protonated (acid form) and deprotonated (base form) SNARF-1 dye. (c) A typical fitting curve of SNARF1 in CBF with added 10-5 M ammonia in the bulk sensing scheme.

│∆Acid/Base │ 0.1326"NH% gas , )* 0.0967 (3) [NH3 (aq.)] means the concentration of ammonia in the CBF in bulk sensing experiments. A reasonable assumption is that a certain │∆(Acid/Base)│ value corresponds to a specific ammonia concentration in the CBF, regardless of the exact route of introducing the ammonia in the system. A side-by-side comparison of bulk sensing and gas sensing is needed to know the assay’s response. We can establish a relationship of the ammonia gas phase concentration and the ammonia concentration in CBF by associating equations (2) and (3), which gives: "NH% aq. , )-. )/% * 0.12"NH% gas , 01* 7.0 2 10/% (4) Equation (4) shows the relationship between the concentrations of ammonia (mol m-3) dissolved in CBFs and its gas phase partial pressure (Pa). It highly resembles the Henry's law. Compared with the constant of 0.60 mol m-3 Pa-1 for ammonia in Henry’s law, equation (4) indicates that CBF surrounded by vapor is drastically different from a solution in equilibrium with its vapor. For example, with 99 ppm (10 Pa) of ammonia in the air, the corresponding bulk ammonia solution has a concentration of 0.60 mM, while the CBF assay after ammonia uptake would have a concentration of just 0.13 mM. In other words, higher gas phase ammonia concentration is needed to reach same concentration for CBF than for regular water. If we assume that the CBF behaves like a homogeneous phase, then gas molecules will encounter a combined resistance of surfactant monolayers and the aqueous core.8 The gas permeability reduction through the CBF is the result of the resistance effect.6



Figure 2. The fluorescence spectra of CBFs containing 10-2 M CTAB and 10-4 M SNARF- for the sensing of (a) NH3 in bulk sensing, and for (b) NH3 gas sensing. The fluorescence spectral change │∆(Acid/Base)│ is derived for (c) bulk sensing (solid red dots), and (d) ammonia gas sensing (square open dots). Insets show the linear fitting function for bulk sensing and gas sensing.

We want to point out that in the gas sensing scheme, gas uptake process is not to be regarded as equilibrium between gas phase ammonia and CBF. It is essentially a one-way process, which means that ammonia goes inside CBF’s water core, but does not come out as a typical volatile solute from a solutionair interface. In one set of experiments (shown in Figure S-4), we put a CBF previously saturated in an environment with 4 ppm ammonia into a cuvette with 2 ppm ammonia. The fluorescence spectra clearly indicate that the CBF does not show any obvious change upon switching its gas phase environment. Once the ammonia molecules are inside CBF they do not escape into the gas phase. According to IUPAC recommendations, the limit of detection (LOD) is calculated using the formula42: LOD = 3δ / k, where δ represents the standard deviation for the Acid/Base ratio in blank experiments and k is the slope of the linear fitting curve in the insets of Figures 2c and 2d. The LOD of ammonia bulk sensing and gas sensing are calculated to be 0.01 mM and 0.8 ppm, respectively. The gas sensing LOD of our method is lower than the reported metaloxide, catalytic metal and conducting polymer sensor systems.29-32 The linear response range has an upper limit of 15 ppm, which is within the 25 ppm limit set by NIOSH.28 Acetic acid sensing. The molar fraction of the acetic acid partial pressure in the gas phase has an approximate linear relation with it aqueous solution concentration as determined by Henry’s law.43 Sander et al.44,45 in the NASA Panel for Data Evaluation had evaluated and compiled sets of chemical kinetics data. In this paper, we adopt their acetic acid vapor–liquid equilibrium data with a Henry’s law constant of 40.0 mol m-3

Pa-1 at 298.15 K. In the gas sensing scheme, the CH3COOH gas partial pressure is adjusted by changing the concentration of the aqueous acetic acid solution. In both the bulk sensing and gas sensing experiments, the pH of the bulk micellar solution is adjusted to 8.0 using NaOH prior to contact with acetic acid. The fluorescence spectra in bulk sensing with different concentrations of CH3COOH are shown in Figure 3a. At pH = 8.0, the base peaks and acid peaks have similar intensity initially. As more and more acetic acid is being added into the bulk solution, the corresponding spectrum begins to have more contribution from the acid forms near 540-600 nm, with decreasing contribution from the base form at 660-720 nm. When the concentration of acetic acid is above 0.11 mM, no more spectral change can be seen. Figure 3b shows the gas sensing fluorescence spectra with 0130 ppb of CH3COOH in the gas phase. When the gas phase partial pressure of CH3COOH reaches about 90 ppb, the spectrum stops showing any change. In Figure 3c and its inset, the │∆(Acid/Base)│ is plotted against the acetic acid concentration in bulk sensing. One can see that the spectral change is linearly proportional to concentration up to 0.11 mM. Figure 3d and the inset show the plot of │∆(Acid/Base)│ versus acetic acid gas concentration up to 130 ppb. We find that below 65 ppb, a linear response can be established between the spectral change and the gas concentration. We can derive the following linear relationship functions between │∆(Acid/Base)│and the analyte concentration for bulk sensing (5) and gas sensing (6):


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Analytical Chemistry

Figure 3. The fluorescence results of CBFs containing 10-2 M CTAB and 10-4 M SNARF-1 for (a) CH3COOH bulk sensing, and for (b) CH3COOH gas sensing. The │∆(Acid/Base)│ values derived from fluorescence spectra for (c) bulk sensing (solid green dots), and (d) ammonia gas sensing (square open dots). Insets show the linear fitting function for bulk sensing and gas sensing.

│∆Acid/Base │ 23.8707"CH% COOH aq. , )* 5 0.0609

(5)

│∆Acid/Base │ 0.0357"CH% COOH gas , 6* 5 0.0789 (6) [CH3COOH (aq.)] represents the concentration of acetic acid in the CBF in bulk sensing experiments. Along the same line of reasoning as in ammonia, we may assume that │∆(Acid/Base)│ only depends on the actual amount of acetic acid inside the CBF, independent of the prior route of introducing the analyte. Obviously we have to make a second assumption: when forming CBF from a bulk analyte solution, both the CBF and the solution have the same analyte concentration. By associating equations (5) and (6) we then establish a relationship of gas phase acetic acid pressure and the concentration inside CBF, as in equation (7): "CH% COOH aq. , )-. )/% * 14.8"CH% COOH gas , 01* 5 7.5 2 10/7 (7) The slope is about 3/8 of the Henry’s law constant for acetic acid aqueous solution (40.0 mol m-3 Pa-1). It means if we have 0.1 mM (0.1 mol m-3) of acetic acid adsorbed into CBF from gas phase, then the partial pressure of acetic acid in the air is about 6.8×10-3 Pa, which is about 67 ppb. In contrast, when there is 0.1 mM of acetic acid solution in equilibrium with its vapor, the partial pressure of acetic acid is 2.5×10-3 Pa, which is about 25 ppb. This means that gas analyte uptake by CBF is different from a typical solution interface for gas uptake. Just like the case in ammonia, acetic acid in the gas phase experiences resistance when approaching the surfactant monolayers before being dissolved in the nanoscale water core.8 The CBF’s surfactant monolayers reduce the gas permeability.

One may compare equations (4) and (7) along with the original Henry’s law. The ratios of slope versus Henry's law constant for ammonia and acetic acid are 0.2 and 0.375, respectively. It shows that the acetic acid results are closer to the original Henry's law of aqueous solution than the ammonia results. The difference may be understood as the hindrance of acetic acid gas uptake into the CBFs is less than that of ammonia gas. This hindrance may originate from the gas solubility difference of the analytes in the aqueous phase. The solubility of acetic acid in water is greater than that of ammonia. A positive correlation between gas permeability and gas solubility in foam films has also been reported.46 Based on the standard deviation of our measurements, the limits of detection for CH3COOH gas sensing and bulk sensing are 3 ppb and 0.005 mM by calculation. The detection limit using our method is lower than the reported metal-oxide sensor system36 and similar to the gas chromatograph method.35 The successful implementation of CBF for the detection of ammonia and acetic acid points to future assays using CBF for gas analyte sensing based on various principles. Furthermore, acidic gas released from a real sample, fermented glutinous rice (a common kitchen ingredient in Asia) is studied using gas sensing scheme with results in Figure S-5. Before we use the assay to detect the headspace acidic gas, we first filter, dilute and centrifuge the fermented glutinous rice sample. The final supernatant (pH=4.1) is put in the cuvette together with CBF for gas sensing. We find that the concentration of volatile acid (mainly acetic acid) in the gas phase is about 60±2 ppb. This gives a derived acetic acid concentration of about 0.18±0.01 % (weight. %) in the initial sample.



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tion of the acid peaks and base peaks of SNARF-1, we find that for both ammonia and acetic acid, a linear relationship can be observed for │∆(Acid/Base)│ and the analyte gas phase concentration, down to limits of detection of 0.8 ppm and 3 ppb, for ammonia and acetic acid, respectively. The linear response range for NH3 (gas) is 0.8-15 ppm and 3-65 ppb for CH3COOH (gas). The validated sensing mechanism based on common black film is of low cost, easy to regenerate, and has low detection limit. In the near future, we envision that various CBFs containing different indicators may be used to sense various gases via different mechanisms. It will not only sense gases by using pH change, but also by using oxidation, reduction, complexation reactions and other types of chemical reactions in the nanometer water core within the CBF. Figure 4. The fluorescence results of CBFs containing 10-2 M CTAB and 10-4 M SNARF-1 for 0 ppb (red line) and 65 ppb (black line) sulfur dioxide in the gas sensing scheme. The blue line is the difference spectrum between the two tests.

Selectivity According to the literature40,44,45, the Henry's coefficients of some common colorless acid gases such as sulfur dioxide (SO2), hydrogen sulfide (H2S) and carbon dioxide (CO2) are 1.3×10-2, 1.0×10-3 and 3.3×10-4 mol m-3 Pa-1, respectively. Compared with 40.0 mol m-3 Pa-1 for acetic acid, the values above are very small. According to Henry’s law, C=H×P, in which C is the concentration of gaseous solute dissolved in water, P is the partial pressure of the corresponding gas in gas phase, and H is the Henry's coefficient. If C is the same (that is, the dissolved gaseous analyte concentration in the CBF is the same), then one finds that the larger H is, the smaller P is. In our experiments, for gas analyte with a higher Henry's coefficient, the detection limit of its gaseous concentration is smaller. In other words, larger H value means easier detection of the gas. From the above sections, we know that CBF’s water core has its own formula similar to Henry’s law, but with reduced Henry’s constant, due to resistance experienced by the gas molecules through CBF6,8. Nevertheless, the above postulation applies to these modified H coefficients. We use SO2 (highest Henry’ law constant among the three gases) to verify our hypothesis through experiments. As Figure 4 shows, the red line and the black line represent the fluorescence of CBFs in air containing 0 and 65 ppb of SO2 concentrations, respectively. The blue line represents the difference between the two spectra. On one hand, 65 ppb of acetic acid is near its upper dynamic range, beyond which signal starts to saturate (see Figure 3d). On the other hand, 65 ppb of SO2 in gas sensing does not change the fluorescence spectra at all. We speculate that in an acidic gas mixture, our CBF-based assay may maintain selectivity towards gases with larger Henry’s coefficients. Strictly speaking, this is not due to chemical reaction specificity, rather, it is determined by the physical process of gas dissolution. Conclusions The large surface area-to-volume ratio (~2×108 m-1) is a big advantage of using CBFs for gases absorption and sensing. We show that a pH-sensitive dye SNARF-1 in the CBF exhibits significant fluorescence signal change upon exposure to different concentration of NH3 (gas) and CH3COOH (gas) at room temperature. By global fitting of the spectra and calcula-

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S-1. A typical FT-IR spectrum of the CBF containing 10-2 M CTAB and 10-4 M SNARF-1. Figure S-2. The individual fitting peaks for assaying (a) NH3 (aq.), (b) NH3 (gas). The experimental spectra and the fitting results of (c) NH3 (aq.), and (d) NH3 (gas). Figure S-3. The individual fitting peaks for the detection of (a) CH3COOH (aq.), (b) CH3COOH (gas). The experimental spectra plotted alongside the fitting results for (c) CH3COOH (aq.), and (d) CH3COOH (gas). Figure S-4. Comparison of the fluorescence spectra of CBF stabilized with 4 ppm of ammonia in the gas phase, and after putting the stabilized CBF into an ambient with ammonia gas concentration of 2 ppm. Figure S-5. CBF gas sensing results (a) without and (b) with the the supernatant of filtered fermented glutinous rice as the sample in the cuvette. Fitted peaks are shown in (b) for easy comparison with the results in Figure S-3.

Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work is funded in part by: Fundamental Research Funds for the Central Universities (Tongji 1380219126), Shanghai Science and Technology Commission (14DZ2261100) through Shanghai Key Laboratory of Chemical Assessment and Sustainability, Ministry of Science and Technology (China, No. 2012YQ22011303), and National Science Foundation (21275107).

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