A Film-based Fluorescent Sensor for Monitoring Ethanol-Water

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A Film-based Fluorescent Sensor for Monitoring Ethanol-Water Mixture Composition via Vapor Sampling Rongrong Huang, Ke Liu, Huijing Liu, Gang Wang, Taihong Liu, Rong Miao, Haonan Peng, and Yu Fang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04897 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Analytical Chemistry

A Film-based Fluorescent Sensor for Monitoring Ethanol-Water Mixture Composition via Vapor Sampling Rongrong Huang, Ke Liu, Huijing Liu, Gang Wang, Taihong Liu, Rong Miao, Haonan Peng and Yu Fang Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China *Corresponding author: [email protected]; [email protected]

ABSTRACT: In situ, on line, non-contact and fast monitoring of compositions of ethanol- water mixtures via vapor phase sampling remains challenge for years. In this work, we report for the first time of a film-based fluorescent sensor showing unprecedented discriminative ability to the compositions of ethanol-water mixtures. Importantly, ethanol content in the mixtures could vary from 0 to 100% (v/v), the response time is less than 2 s, and the sensing is fully reversible. More importantly, the monitoring was performed via vapor phase sampling, avoiding sample contamination. The principle behind is ascribed to the big difference of the fluorescent quantum yield of the sensing unit, a newly designed and synthesized mono-substituted fluorescent o-carborane derivative (ZPCarb), in the two solvents. In addition, the sensor as developed was successfully used for the determination of ethanol contents in four commercial liquors, suggesting its potential applications in quality control of beverages, monitoring fermentation processes, etc. in a quantitative way. Keywords: Film-based fluorescent sensor, Ethanol-water mixture, Fluorescent o-carborane derivative, Vapor sensing

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INTRODUCTION Motivated by the drastic demands in optoelectronic devices,1, 2 chemical sensors,3-7 bio-imaging,8 and multi-stimuli responsive materials,9-11 etc., development of π-conjugated fluorophores with efficient luminescence in solid state have attracted great attention during the last few decades. However, for traditional polycyclic aromatics, one annoying problem is aggregation-caused quenching (ACQ) effect,12-14 which could seriously limit their applications in solid state. Moreover, fluorescent films made from them lack porosity, a pivotal factor affecting fast and reversible sensing owing to poor mass transfer of the analyte molecules within the adlayer.15-17 Therefore, it is intriguing to develop fluorophores with high fluorescent quantum yield in solid state from both fundamental and practical viewpoints.18,

19

For this

reason, one of the possible strategies is to develop fluorophores of non-planar structures,20, 21 which is believed to form films with rich porosities since the structure will screen π-π stacking and the relevant ACQ effect. o-Carborane is a boron cluster possessing a typical three-dimensional (3D) polyhedral structure,22 and moreover, it exhibits outstanding thermal stability,23 unique electronic properties24,

25

and is ease to be modified.26 Accordingly,

o-carborane could be taken as an ideal building block to develop fluorophores with non-planar structures so that gaps among fluorescent molecules would be formed to function as molecular channels for the analytes. To demonstrate the strategy as described, an example fluorescent o-carborane derivative (ZPCarb) was designed and prepared (Scheme 1). The fluorescence was introduced via introduction of a structure of ZPE, which is a benzene derivative. To enhance the fluorescent efficiency and the solubility of the structure, azetidine27,

28

and two ester structures were introduced onto the benzene ring (Scheme 1). UV-Vis absorption and fluorescence studies revealed that the created ZPCarb shows impressive solid state emission, and the film fabricated from the compound exhibits remarkable, nearly linear response to the composition of ethanol-water mixtures within the whole concentration range. Importantly, the sensing could be performed 2 / 18


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via vapor sampling. Based upon the discoveries, a conceptual fluorescent film sensor was developed and successfully used for non-contact determination of four commercial liquors with different ethanol contents. This paper reports the details.

Scheme 1. The synthetic route for ZPCarb.

RESULTS AND DISCUSSION Absorption and Emission in Solution. The photophysical properties of the as-prepared ZPE and ZPCarb were firstly investigated in dichloromethane. As shown in Figure 1, the UV-Vis spectra of the two compounds are characterized by two absorption bands, of which the only difference is blue shift of the o-carborane modified one (375 nm  367 nm; 296 nm  280 nm) probably due to its strong electron withdrawing property.29 In addition, the molar absorption coefficient increased significantly with introduction of the o-carborane unit (12985 M-1 cm-124854 M-1 cm-1).

Figure 1. UV-Vis absorption spectra of ZPCarb and its parent compound ZPE in DCM (50 μM). To reveal if there is any ACQ effect of the non-planar fluorophore as developed, 3 / 18



concentration effect study was conducted. The result is depicted in Figure 2. Clearly, the intensity of the fluorescence emission of the solution under study increased as increasing the compound concentration, confirming the expectation that non-planar structure effectively screens the dense aggregation and the relevant ACQ effect. In contrast, for ZPE, the emission intensity increased at low concentrations but decreased upon increasing the concentration further, indicating a typical ACQ effect (Figure 2).

Figure 2. Fluorescence emission spectra of (a) ZPE (λex = 380 nm) and (b) ZPCarb (λex = 370 nm) with different concentrations in dioxane. Inset: plots of emission intensity versus concentration and fluorescence images in dioxane at a concentration of 0.1 mM. The photos were taken under UV light (365 nm) illumination. Solvent Polarity Dependence. To have a better understanding of the electronic structure of ZPCarb in the ground and excited state, their UV-Vis absorption and fluorescence emission spectra in solvents of different polarities were examined. The results are depicted in Figure S-1. Clearly, both the profile and position of the absorption and the emission of the compound show little dependence upon the solvent change in spite of its typical donor-acceptor (D-A) structure, suggesting no significant intro-molecular charge transfer (ICT)30 expected. The intensities of the emissions, however, changed significantly upon the solvent variation (Figure S-2), and in particular, introduction of water into ethanol solution of the compound showed significant effect upon the fluorescence emission (Figure S-3). In order to shed light on the possible mechanism why the o-carborane derivative showed no solvatochromic property, both the ground-state and excited-state dipole moments of the fluorophore in vacuum and in a high-polar solvent, DMSO, were 4 / 18


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calculated using a method of CAM-B3LYP/6-31G (d). As listed in Table S-1, the difference between the dipole moments of the two states () for ZPCarb is only 2.07 Debye, a result very close to 3.00 Debye which is a result from experimental measurement (Table S-1, Figure S-4), explaining why the fluorescence emission is not sensitive to the solvent polarity change.31,

32

Further examination of the

differences in the electron distribution of the compound at the two states demonstrates that o-carborane almost shows no effect upon the electron distribution as the HOMOs and LUMOs are mainly localized within the ZPE part in both the ground and excited state (Figure S-5). This result also explains why the profile of the absorption spectrum of ZPE is almost the same to that of ZPCarb. Table 1 Photophysical characteristics of ZPCarb in solution and film states: solvent polarity parameter ET(30), molar absorption coefficient (ε), maximum absorption wavelength (λabs) and emission wavelength (λem), Stokes shifts (ν), fluorescence quantum yield (Φf) in different solvents. The concentrations are 5 μM. Excitation wavelengths employed are 370 nm. The film was prepared on filter paper by dip-coating method. Solvent

ET (30)

abs (nm)

HEX

31.0

279, 367

TOL

33.9

DCM

em

f

f (ns)

kr (MHz)

453

0.62

11.60

53

280, 367

468

0.57

11.74

48

41.1

282, 367

468

0.83

12.96

64

THF

37.4

283, 367

468

0.73

10.84

67

DMSO

45.0

284, 367

471

0.11

2.66

41

EtOH

51.9

281, 367

471

0.53

9.56

55

H2O

63.1

280, 369

462

0.03

11.83

3

Film

-

274, 369

460

0.33

12.96

25

(nm)

Fluorescence Quantum Yields in Different States. The fluorescence quantum yields (Φf) of ZPCarb in different solvents and film state were determined, and the results are shown in Table 1. As seen, with exception of DMSO and water, of the other solvents studied, the values are all greater than 50%, demonstrating that ZPCarb is an efficient fluorophore. In particular, its fluorescent quantum yield in film state reaches 33%, indicating again that the ACQ is effectively avoided. In 5 / 18



contrast, the parent compound ZPE displayed a fluorescence quantum yield of 12% in THF, but decreased to 4% in film state, revealing that introduction of 3D structure is an effective way to screen ACQ effect owing to the steric hindrance. However, as other known fluorophores, this fluorophore also emits with highly different efficiencies in different solvents. Performance of ZPCarb-based Film Sensor. As known, monitoring ethanol, especially ethanol content in ethanol-water mixtures, is of vital importance since it is extensively related with brewing industry, perfume production, biochemical research and clinic diagnosis, etc. During the past two decades, on-going efforts have been made to develop new sensors and instrumental methods to monitor ethanol33-35 or to discriminate ethanol from other kinds of polar solvents36, 37 such as methanol38 and water.39-41 Notably, standard techniques for ethanol detection involving IR42 or GC43 have been well established. However, these systems have some significant disadvantages such as irreversible detection and the requirement of special or costly devices. Therefore, fluorescence-based sensors have attracted increasing attention owing to their high sensitivity, ease of operation and low cost.44, 45 For example, Ariga et al. constructed an alcohol sensor using the solid-state fluorescence emission of terphenyl-ol (TPhOH) derivatives. Detection of ethanol contents in alcohol beverages with a detection limit ∼5% (v/v) was demonstrated using carbonate-treated TPhOHs. Moreover, when a nanofibrous polymer was used as the scaffold matrix, 8% (v/v) ethanol in air could be determined.46 In another work, Niu et al. developed an europium chelate based fluorescent-fiber sensor for detecting water content in organic solvents, and the fluorescence intensity responded to water content with a linear relationship.47 For ethanol-water mixtures, volume fraction of water ranged from 0% to 10% (v/v) could be detected. However, to the best of our knowledge, there is still no any report on the fluorescence determination of ethanol-water composition, especially within the whole composition range, via vapor sampling. Considering the great dependence of the fluorescence emission efficiency of ZPCarb on the composition of ethanol-water mixtures, it was expected that the 6 / 18


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compound may find uses in the determination of the composition of the mixtures. Accordingly, a fluorescent film was fabricated using ZPCarb as a sensing unit. This is because compared with solution sensing, film-based sensing possesses a number of advantages. For example, they are easier to be made into devices, they can be used for both vapor sensing and solution sensing provided there is no fluorophore leaking, and determination could be performed with no contamination.

Figure 3. A typical group of responses of the ZPCarb-based film device to the vapors in the headspace of water-ethanol mixtures of different compositions (water content varies from 0% to 90%, v/v, λex/λem= 370 nm/460 nm, 293 K). Inset: the fluorescence intensity as a function of volume fraction of water in the binary solvent. Each dada point was the mean of 30 replicates. The error bars are the standard deviation. The non-linear responses of the film sensor to the composition of the binary solvent could be a result of its non-ideal nature.48 To examine the possibility, the photochemical stability of the ZPCarb-based film was examined in both dry state and saturated ethanol vapor surrounded state. The results are provided in Figure S-6. It is seen that 3.5 h continuous illumination resulted in almost no decrease in the fluorescence emission intensity of the dry film, and for the film in saturated ethanol vapors the decrease is less than 3.7%, demonstrating its usability for practical uses. Meanwhile, the emission spectra (Figure S-7) of the film before and after three-day sensing test (about 600 times) were collected and there is almost no change of their intensity and profile. Thermo-stability is also crucial for a sensor. Herein, accelerated ageing (thermo-oxidative) test of the film was performed, and the result is provided in Figure S-8. It is shown that there is no obvious change after 5-month exposure to air at ambient condition away from 7 / 18



light. Therefore, long-term stability of the film-based sensing device can be ensured. The film was further used for monitoring, via vapor sampling, the ethanol-water mixture solvents with ethanol content varying from 0 to 100% (v/v). The measurement of each ratio was repeated for 30 times and Figure 3 displays the results from three repetitive measurements. Luckily, a nearly linear response covering whole composition range was obtained, of which the coefficient of determination (R2) is greater than 97%. Moreover, the response is less than 2 s and the film recovers within 6 s (Figure S-9).

Figure 4. Performance stability test of the ZPCarb-based film sensor (λex/λem = 370/460 nm). (a) 50 times replicate test with pure ethanol as a sample (coefficient of variation: 1.33%); (b) Progressive test via increasing and then decreasing ethanol content in the tested ethanol-water mixtures (sampling time for each test is 20 s). Reusability is crucial for practical applications. To assess the property of the as-prepared film, it was first examined with pure ethanol as a sample. After each measurement, the film was purged with air for 1 min. The results are shown in Figure 4a. It is clearly seen that 6 s purging is enough for the film to recover completely. The test was repeated for 50 times, and no significant change was observed, demonstrating the excellent reusability of the film. Moreover, progressive tests were well completed via first increasing and then decreasing ethanol fraction in the ethanol-water mixtures under tests, further indicating its excellent reusability (Figure 4b). Interference-free is another key criterion for a sensing method or sensing device to find real-life application. As known, methanol is highly toxic and its presence in liquor can cause blindness. Considering the similarity of the two alcohols, discrimination of them is of great importance. To realize the discrimination, another fluorescent film was fabricated, of which silica gel plate was chose as a substrate. The sensing results of this film-based device to the two alcohols at room temperature are 8 / 18


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shown in Figure S-11. With reference to the figure, it is seen that the film device shows instantaneous response to the vapors of both alcohols, but the recovery time is different from one to another (Figure S-12). For methanol, the signal recovers within 120 s, but for ethanol it recovers 300 s later. To further evaluate the discriminative ability, ethanol-methanol binary liquids of different methanol contents were employed as the samples. The discriminative tests were re-conducted. The results are depicted in Figure S-13a. It is found that, as expected, the recovery time decreases with increasing methanol content. Considering the complexity of samples to be analyzed in real life, different amount of methanol was added to one kind of commercial liquor, and further tests revealed that addition of methanol significantly reduces the recovery time of the liquor (Figure S-13b). This finding may allow quantitative determination of methanol content in ethanol products via correlation the degree of fluorescence recovery at a given time window to methanol content in the mixtures, of which the details are provided in Figure S-13c and related figure captions. Overall, a challenging task: discrimination, via vapor sampling in particular, of methanol from ethanol in aqueous medium, is achieved by adopting the new film, laying foundation for quality control in fermentation industry and for screening of blue ruin.

Figure 5. ZPCarb-based film device and the relevant sensor. (a) Schematic representation of the home-made conceptual sensor; (b) A picture of the conceptual sensor and the film device as developed. Quality Checking of Commercial Liquors. To further examine the practical applications of the film sensor (Figure 5), four commercial liquors with different ethanol contents were determined, and each test was repeated for 20 times (Figure S-10). As summarized in Table 2, measured ethanol contents with the sensor are in 9 / 18



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good agreement with that on the label of the liquors, suggesting that the as-designed film-based sensor may find applications in beverage and fermentation industries. Table 2 The ethanol content of different commercial liquors measured by the conceptual sensor as developed and its comparison to the value on the label. [a] The ethanol content calculated; [b] The standard derivation of 20 replicates. Commercial Liquor

Ethanol content on the label (%, v/v)

Measured ethanol content[a] (%, v/v)

Standard deviation[b]

Du Kang Chinese white wine

52.00

54.24

0.12

Tomintoul Whisky

40.00

42.47

0.05

Carbernet red wine

12.00

12.08

0.14

TsingTao beer

3.90

2.98

0.03

It is to be noted that careful examination of the plot shown in the inset of Figure 3 reveals that the data from experimental tests are slightly deviated from linearity, which could be a result of the non-ideal nature of the ethanol-water mixture. Therefore, for more accurate detection, corrections can be made via utilization of the modified Raoult’s law (eqs. 1 and 2), the Antoine equation, and the Van Laar model.48 𝑃1 = 𝑃𝑇𝑌1 = 𝛾1𝑋1𝑃𝑠𝑎𝑡 1 (𝑇)

(1)

𝑃2 = 𝑃𝑇𝑌2 = 𝛾2𝑋2𝑃𝑠𝑎𝑡 2 (𝑇)

(2)

𝐵𝑖

ln𝑃𝑠𝑎𝑡 𝑖 (𝑇) = 𝐴𝑖 ― 𝐶𝑖 + 𝑇 (𝑖 = 1, 2)

(3)

The details of the corrections are provided in the fourth part of Supporting Information. From the calculation, the vapor phase composition of the ethanol-water binary liquid system can be correlated with the liquid phase composition, and in this way, plots of the fluorescence intensity change (I/I0) to Y1 (molar fraction of ethanol in the vapor phase) or X1 (molar fraction of ethanol in the liquid phase) can be obtained. The results are listed in Figure S-14. Clearly, a roughly linear plot of I/I0 to Y1, particularly at lower Y1 values, was obtained with a correlation co-efficient of 0.96, suggesting the closer to ideal gas nature of the vapor. In contrast, the plot of I/I0 to X1 is far from linearity, a result of non-ideal nature of the binary liquid. In 10 / 18


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addition, Y1 was also plotted against X1 as depicted in the inset of Figure S-14b. Based upon the plots, fluorescence measurement via vapor sampling can be used for the determination of the composition of an ethanol-water binary liquid.

CONCLUSION A fluorescent o-carborane derivative, ZPCarb, was designed and prepared. The prepared fluorophore is highly fluorescent in both solution and film state owing to its unique non-planar structure. The compound was further used for fabricating a fluorescent film, of which the emission is highly sensitive to the composition of ethanol-water mixtures in vapor state. The response time is less than 2 s, ethanol content in the mixtures could range from 0 to 100%, and the response is fully reversible. Owing to the extraordinary performance, the film was further made into a conceptual sensor. Importantly, the sensor as developed was successfully used for checking the quality of four commercial liquors. We believe that our sensor could be used as a powerful, quantitative tool for quality control of beverage production, in situ monitoring of brewing process, etc.

EXPERIMENTAL SECTION All reagents were purchased from commercial suppliers and used as received unless otherwise noted. Prior to use, toluene and diisopropylamine were purified by distillation. Reactions were conducted under a dry N2 atmosphere using a standard vacuum line technique. Column chromatographic purification was performed with silica gel (300-400 mesh) as the stationary phase. 1H NMR spectra were recorded on Bruker 600 MHz spectrometer in CDCl3 and CD2Cl2 with tetramethylsilane (TMS) as internal standard. Mass spectrometry was collected on a Bruker maxis UHR-TOF mass spectrometer in ESI and APCI positive mode. UV-Visible absorption spectra were recorded on an UV-Visible spectrophotometer (U-3900, Hitachi), and the molar absorption coefficients for the compounds were calculated using the Lambert-Beer law, A = bc, where A is the absorbance,  the molar absorption co-efficient, c the concentration of the compound under study, and b the length of the cell, which is 1 11 / 18



cm in general. Steady state fluorescence measurements were performed at room temperature on a time correlated single photon counting Edinburgh Instruments FLS920 fluorescence spectrometer. All the solvents for fluorescence measurements were freshly distilled prior to use. Absolute fluorescence quantum yields were determined on the same system equipped with an integral sphere and the measured f values may be subject to an error of + 5%. Preparation of the films was performed in the following way. The fluorescent compound, ZPCarb, was dissolved in toluene with a concentration of 5 μM, then 30 μL of the solution as-prepared was dropped onto a substrate (filter paper or silica gel plate) surface, and then the substrate was air-dried at room temperature for more than 2 h. In this way, a fluorescent film was obtained. It is to be noted that in the tests, different substrates, including filter paper (Whatman®, standard number: No GB/T1914-2007), smooth glass plate, and silica gel plate (200-300 mesh, GF254) were examined, but only the filter paper-based fluorescent film showed the properties expected. For discrimination of ethanol and methanol, silica gel plate-based film is preferred. The details are depicted in Supporting Information.

ASSOCIATED CONTENT Supporting Information. Detailed synthesis procedures, characterizations of compounds, theoretical calculation results, sensing methods, calculation of composition fraction in gas phase and additional figures.

AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected]

*E-mail:

[email protected]

ORCID Yu Fang: 0000-0001-8490-8080 Notes 12 / 18


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The authors declare no competing financial interest.

ACKNOWDGEMENTS Funding from the Natural Science Foundation of China (21527802, 21673133, and 21820102005), 111 project (B14041) and Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33) is greatly acknowledged.

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A Film-based Fluorescent Sensor for Monitoring Ethanol-Water

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