Colorimetric and Fluorescent Dual Mode Sensing of Alcoholic

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Colorimetric and Fluorescent Dual Mode Sensing of Alcoholic Strength in Spirit Samples with Stimuli-Responsive Infinite Coordination Polymers Jingjing Deng, Wenjie Ma, Ping Yu, and Lanqun Mao* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: This study demonstrates a new strategy for colorimetric and fluorescent dual mode sensing of alcoholic strength (AS) in spirit samples based on stimuli-responsive infinite coordination polymers (ICPs). The ICP supramolecular network is prepared with 1,4-bis(imidazol-1-ylmethyl)benzene (bix) as the ligand and Zn2+ as the central metal ion in ethanol, in which rhodamine B (RhB) is encapsulated through self-adaptive chemistry. In pure ethanol solvent, the asformed RhB/Zn(bix) is well dispersed and quite stable. However, the addition of water into the ethanol dispersion of RhB/Zn(bix) destroys Zn(bix) network structure, resulting in the release of RhB from ICP into the solvent. As a consequence, the solvent displays the color of released RhB and, at the meantime, turns on the fluorescence of RhB, which constitutes a new mechanism for colorimetric and fluorescent dual mode sensing of AS in commercial spirit samples. With the method developed here, we could distinguish the AS of different commercial spirit samples by the naked eye within a wide linear range from 20 to 100% vol and by monitoring the increase of fluorescent intensity of the released RhB. This study not only offers a new method for on-spot visible detection of AS in commercial spirit samples, but also provides a strategy for designing dual mode sensing mechanisms for different analytical purposes based on novel stimuli-responsive materials.

W

evaluating of unrecorded alcohols in the cases of anticounterfeiting of some alcohols, and in time, controlling of alcohol consumption to well avoid the traffic accident and alcoholic intoxication. To meet the requirements mentioned above, optical probes with colorimetric and fluorescent dual modes are highly desired because they not only enable the visualization of the target simply with bare eyes, but also validate a quantitative fluorescence assay in a simple and rapid feature.20−26 In this study, we demonstrate a colorimetric and fluorescent dual mode for simple and rapid AS sensing in real spirit samples. The rationale for our assay is essentially based on the excellent properties of infinite coordination polymers (ICPs) in terms of their solvent-dependent stability and capability for encapsulating functional components through an adaptive self-assembly chemistry. In order to validate the AS sensing even with bare eyes, rhodamine B (RhB) is encapsulated into the ICP supramolecular network formed with 1,4-bis(imidazol-1ylmethyl)benzene (bix) as a ligand and Zn2+ as a central metal ion to generate RhB-encapsulated Zn(bix) (i.e., RhB/ Zn(bix); Scheme 1). In pure alcohol, RhB/Zn(bix) is quite stable with less fluorescent emission. However, the presence of water into the

ith the development of social economy and the improvement of living standard, the production of alcoholic beverage plays more and more significant roles in the world.1−3 Nowadays, increasing alcohol supply in a developing society is usually assumed to have positive effects on economic development, while this has to be recognized to have negative effects on public health and order issues.4 Generally, the alcohol content in the alcoholic beverages refers to as the “Alcoholic Strength (AS)”, which, by law in the European Union (EU), is expressed as the volume percentage of ethanol at 20 °C, and such a system has been widely used in Asia and EU.5,6 As one of the most important parameters in spirits analysis, AS reflects the quality of alcoholic beverages in the production process and has been used to judge the standard for inebriety that may cause some kind of trauma and even traffic accident.7−9 Great concern in AS detection also closely relates to the fact that this value should have to be precisely controlled during a continuous production process to warrant the product quality.3,10 Moreover, different AS values in the spirit market correspond to different prices.11−13 While some methods such as chromatography and near-infrared assays have been used for AS detection, these methods unfortunately require sophisticated instrumentation, time-consuming process, and experts to run the systems.14−19 These features eventually invalidate the existing methods for rapid and on-spot AS detection, even though on-spot AS detection remains very essential for quick labeling control of spirits, process monitoring of fermentation, © XXXX American Chemical Society

Received: April 29, 2015 Accepted: June 4, 2015

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DOI: 10.1021/acs.analchem.5b01617 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry Scheme 1. Schematic Illustration of Colorimetric and Fluorescent Dual Mode for AS Assay with RhB/Zn(bix) ICPa

a

(A) Formation of Zn(bix) ICP and RhB/Zn(bix) ICP. (B) Water-triggered release of RhB from the Zn(bix) ICP.

crystalline bix dehydrate. RhB/Zn(bix) infinite coordination polymer (ICP) was synthesized with the procedures reported previously.28 In a typical synthesis, an aqueous solution of Zn(NO3)2·6H2O (5 mL, 1 M) was added into an ethanolic solution of bix ligand (25 mL, 200 mM) containing RhB (0.25 mM) under vigorous stirring at room temperature to produce pink colloid. After 5 min, ethanol (50 mL) was added into the reaction mixture to stabilize the polymer. The resulting product was purified by centrifugation and washed several times with ethanol until the supernatant became clear. The processes were repeated until no obvious fluorescent signal was detected in supernatant of the solution. Alcoholic Strength Assay. It is known that potable spirits mostly consist of ethanol and water with the trace flavor-active components to make soft and harmonious liquor body.3,29,30 In this case, spirit could be regarded as one kind of ethanol−water mixture and AS can thus be expressed in percent by water volume:

dispersion of RhB/Zn(bix) in ethanol clearly leads to the destruction of RhB/Zn(bix) structure, resulting in the release of RhB from the network into solution. The visualization and fluorescent assay of AS, which is actually based on the visualization and detection of water content in spirits, can thus be simply accomplished with the changes in the color and the fluorescent emission of the RhB/Zn(bix) dispersion, respectively, caused by the water-triggered release of RhB from the Zn(bix) ICP network. The method demonstrated here is rapid and technically simple and thus holds a good promise for onspot analysis of AS of spirits in various research and industrial fields.



EXPERIMENTAL SECTION Reagents and Apparatus. All reagents were purchased from Aladdin and without further purification unless stated otherwise. Milli-Q water (Millipore, Bedford, MA) was used in the study. Unless noted otherwise, the experiments were carried out at room temperature. Scanning Electron Microscopy (SEM) images were performed by S-4300 and S-4800 (Hitachi, Japan), and Confocal laser scanning microscopy (CLSM) images were performed on an Olympus FV-IX81 CLSM and a Leica TCS SP confocal system (Leica, Germany). Fluorescent spectra were recorded on a HITACHI F-4600 fluorescence spectrophotometer. Photographs were taken by a Cannon IXUS 951S digital camera. Water content was determined with a volumetric ZKF-1 Karl Fischer titrator (Shanghai, China) with Karl Fischer reagent as the titrant and with anhydrous methanol as the solvent. Synthesis of RhB/Zn(bix) Infinite Coordination Polymer. 1,4-Bis(imidazol-1-ylmethyl)benzene (bix) was synthesized as reported previously.27 Briefly, a solution containing imidazole (928 mM) and α,α′-dichloro-p-xylene (89.2 mM) in methanol (50 mL) was refluxed for 18 h. Removal of methanol by evaporation gave a yellow syrup that was recrystallized from an aqueous solution of K2CO3 (100 mL, 444 mM) to yield

AS% =

Vethanol × 100% V

(1)

In the ethanol−water mixture, due to different intermolecular force between water and alcohol, V ≠ Vethanol + VH2O. But, when VH2O ≪ Vethanol V ≅ VH2O + Vethanol

(2)

Thus, the water content in alcoholic beverages could be expressed as H 2O% =

VH2O VH2O + Vethanol

× 100% (3)

By using water content as the indicator, the formula of AS could be changed into the following form: B

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Figure 1. (A) SEM image and photographs (inset) of Zn(bix) particles. (Inset) Size distribution histograms of Zn(bix) particles. (B) SEM image and photographs (inset) of RhB/Zn(bix) particles. (Inset) Size distribution histograms of RhB/Zn(bix) particles.

Figure 2. (A) Solid-state fluorescence of RhB/Zn(bix) particles at different excitation wavelength of λex = 355 nm (black curve) and λex = 545 nm (red curve). (B) CLSM images of encapsulation of RhB in RhB/Zn(bix) particles. Scale bar, 5 μm. (B1) Confocal fluorescence and (B2) bright-field images of RhB/Zn(bix) particles, and (B3) the overlay images of (B1) and (B2) (λex = 559 nm). (C) CLSM fluorescence profile and CLSM image (inset) of RhB/Zn(bix) particle.

AS% =

Vethanol × 100% Vethanol + VH2O

⎛ ⎞ VH2O ⎟ × 100% = ⎜⎜1 − VH2O + Vethanol ⎟⎠ ⎝

ethanol. The mixtures were ultrasonicated for 1 min and then centrifuged. After that, the upper supernatant of the mixtures were used for fluorescent measurements. Karl Fischer measurements were simultaneously conducted as a reference method to determine water content. For AS detection in real samples, 100 μL of commercially available spirits were added into the alcoholic dispersion of RhB/Zn(bix) (2 mg/mL, 1.9 mL), and the mixtures were first ultrasonicated for 1 min and then centrifuged. The upper supernatant was taken out and determined by fluorescent spectrophotometer. Karl Fischer measurements were also conducted as a reference method to determine water content.

(4)

Prior to the colorimetric and fluorescent dual mode detection of AS of spirits, 100 μL of the artificial samples (Vethanol/(Vwater + Vethanol) with different ethanol concentrations ranging from 20% vol to 100% vol) were added into RhB/Zn(bix) particles (3.8 mg) dispersed in ethanol solution (1.9 mL). The artificial samples were prepared by blending ethanol and water with different volume ratio, for example, 90% vol sample was prepared by mixing 90 μL of ethanol and 10 μL of water and the final volume of the mixture was adjusted to be 100 μL with



RESULTS AND DISCUSSION As mentioned above, Zn(bix) ICP supramolecular network was prepared by adding an aqueous solution of Zn(NO3)2·6H2O to C

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previously, for the Zn(bix) particles, the metal center Zn2+ coordinates with N atom of bix, and interacts with O atom of the nitrate ions weakly, resulting in the formation of the polymer of formula [Zn(bix)(NO3)2].28,34 The presence of two strong peaks at 1240 and 1530 cm−1 in the spectrum confirms the coordination of bridging bix ligand to Zn2+ (black curve). After interaction with RhB to form the RhB/Zn(bix) particles, the absorption of Zn(bix) at 3428 and 1338 cm−1 shifts slightly to 3446 and 1384 cm−1, respectively, demonstrating that the encapsulation of RhB had little effect on the coordination mode between Zn2+ and bix. The minor changes in the wavenumber could be attributed to weak interactions between RhB and Zn(bix) through self-assembly process. All these results demonstrate that, by taking advantages of the encapsulation properties of ICPs, we were able to simply prepare functional RhB/Zn(bix) ICP that well combines the inherent properties of both Zn(bix) ICP network (solventdependent stability) and RhB (optical properties). The bifunctionalities of RhB/Zn(bix) ICP substantially enable its application as a probe for colorimetric and fluorescent dual mode detection of AS. We then investigated the different stability of RhB/Zn(bix) particles dispersed in ethanol and water. As depicted in Figure 4A, RhB/Zn(bix) particles were stable in ethanol and the asformed dispersion exist as a pink colloid (Figure 4A, inset) with the fluorescent spectrum characteristic of that of RhB/Zn(bix) particles (Figure 4C, black curve). This, along with the SEM image of the spherical particles strongly suggests that RhB/ Zn(bix) particles were stable in ethanol. Moreover, the stability of the as-synthesized ICP nanoparticles was well preserved in ethanol for at least one month, as judged from both the color and the SEM image of the ICP (data are not shown). In contrast, when RhB/Zn(bix) particles were dispersed into water, the pink colloid of RhB/Zn(bix) clearly turns into a clear pink solution (Figure 4B, inset), and the fluorescence intensity of the supernatant increases (Figure 4C, red curve). Meanwhile, SEM image showed that the structure of spherical RhB/ Zn(bix) particles were destroyed (Figure 4B), suggesting that RhB/Zn(bix) particles were unstable in water. To further verify the different stability of RhB/Zn(bix) particles in ethanol and water, RhB/Zn(bix) particles were first dispersed into ethanol containing low content of water solution (the volume ratio of water to ethanol was lower than 5%) and then dried quickly by the cold blower. As could be seen from the SEM image (Figure 5A), with the addition of water, the RhB/Zn(bix) particles become collapsed and some of the particles aggregate. The CLSM image also shows that the RhB/ Zn(bix) particles changed from smooth spheres to irregular morphology (Figure 5B), and the RhB/Zn(bix) network become rough with the release of rhodamine B from the Zn(bix) network (Figure 5C). The water-triggered collapse of the Zn(bix) network could be elucidated with the reasons listed below, (1) direct attack of water on Zn(bix) networks, in which a water O atom replaces one of the coordinating ICP O atom (O atom in the nitrate ions or RhB, which may coordinate with Zn2+ weakly), (2) the hydrogen bonding between H atom in water molecule and N atom in bix ligand, and (3) a hydrogen-bonded network of water molecules tethered to one or more RhB/Zn(bix).35−37 All these reactions may occur even at low water content solution (vol ≤ 5%), as shown in Figure 5, leading to the distortion of the framework structure and thereby the release of entrapped RhB completely. The different stability of Zn(bix)

an ethanolic solution of bix (in a 1:1 molecular ratio) with vigorous stirring at room temperature. A white precipitate of Zn(bix) was dispersed in ethanol to form a suspension (Figure 1A, inset). Scanning electron microscopy (SEM) shows that the Zn(bix) ICP nanoparticles are regular sphere with a diameter of 539 ± 5 nm (Figure 1A, inset). To enable the colorimetric and fluorescent dual mode for AS assay, RhB was employed as guest molecule to be encapsulated into the Zn(bix) ICP network to form RhB/Zn(bix) because it emits orange fluorescence upon excitation at 559 nm with high quantum efficiency and presents a pink color in solution (Figure 1B, inset). As shown in Figure 1B, the synthetic RhB/Zn(bix) particles have almost the same shape and size (diameter of 543 ± 14 nm) as Zn(bix) particles, suggesting that the encapsulation of RhB does not change the structure of Zn(bix) particles. To verify the encapsulation of RhB into the Zn(bix) particles, solid-state fluorescence spectra, confocal laser scanning microscope (CLSM) image and Fourier transfer infrared (FT-IR) spectra of RhB/Zn(bix) particles were investigated. Since pure Zn(bix) shows blue luminescence when being excited at 355 nm, broadband fluorescence spectra were measured upon solidstate fluorescence.28 As shown in Figure 2A, the fluorescence emission spectrum of the RhB/Zn(bix) (black curve) shows the contributions from the emission of both Zn(bix) network (432 nm, black curve) and RhB(605 nm, red curve) in solid state. Meanwhile, the RhB/Zn(bix) emits fluorescence upon excitation at 559 nm as characterized by CLSM (Figure 2B1), suggesting that RhB is encapsulated into the Zn(bix) network during the formation of the ICP network. Furthermore, fluorescence profile of RhB/Zn(bix) clearly demonstrates that the lights were emitted from the inside, rather than from the surface of the Zn(bix) particles (Figure 2C). By combining solid-state fluorescence and CLSM image results, one may conclude that RhB was spontaneously encapsulated into the interior of Zn(bix) ICP network through the adaptive self-assembly process. FT-IR spectra of free RhB, RhB/Zn(bix) and Zn(bix) were displayed in Figure 3. For RhB

Figure 3. FT-IR spectra of RhB, Zn(bix)/RhB particles, and Zn(bix) particles.

(blue curve), the peak at 1590 cm−1 was ascribed to the CC bonds in benzene ring, the peak at 1412 cm−1 was ascribed to the bending vibration of CH2, and the absorption at 1179, 1342, and 1645 cm−1 were assigned to the stretching vibration of C−O−C, C−N, and CN+, respectively.31−33 After being encapsulated in Zn(bix) network, the characteristic absorption peaks of RhB were not observed (red curve). As reported D

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Figure 4. SEM images and photographs (inset) of RhB/Zn(bix) particles after the particles were first dispersed in ethanol (A) and water (B). (C) Fluorescence spectra and photographs (inset) of the supernatant of the RhB/Zn(bix) particles dispersed in ethanol (black curve) and water (red curve) after centrifugation.

Figure 5. (A) SEM image of RhB/Zn(bix) after the particles were first dispersed into ethanol solution containing low content of water (vol ≤5%) and then dried quickly by a blower. B) High-solution confocal fluorescence (B1) and bright-field (B2) images of the morphology of RhB/Zn(bix) particles dispersed into ethanol solution containing low content of water (vol ≤ 5%); (B3) Overlay image of (B1) and (B2). λex = 559 nm. Scale bar, 5 μm. (C) Confocal fluorescence (C1) and bright-field (C2) images of RhB release from RhB/Zn(bix) particles when dispersed into ethanol solution containing low content of water (vol ≤ 5%); the overlay image of (C1) and (C2) is shown in (C3); λex = 559 nm. Scale bar, 20 μm.

particles in ethanol and water, combined with the excellent optical property of RhB, validates the colorimetric and fluorescent dual mode detection strategy for AS assay in a technically simple and time-saving manner. The colorimetric and fluorescent dual mode for water content determination and thereby for AS assay was conducted by dispersing RhB/Zn(bix) particles into water/ethanol mixture with different water concentrations (Vwater/(Vwater + Vethanol) ranging from 0.05 to 5.0%. As depicted in Figure 6A, with increasing the content of water in ethanol, the color of the resulting supernatant gradually turns from colorless to pink and, in the meantime, the fluorescence intensity increases (Figure 6A, inset). The fluorescent intensity increases with the water content and shows a linear response to water content within a content range from 0.05 to 5.0% (I = 30.190 + 102.052 (water content), R = 0.9987) with a detection limit of 0.015% (S/N = 3; Figure 6B). This result suggests an increasing release of RhB from the Zn(bix) network with increasing the content of water in ethanol solution, demonstrating that the water

content could be simply visualized with bare eyes through the color change of the supernatant (i.e., from colorless to pink) and quantified with the fluorescence response. For practical determination of AS in real samples, 100 μL of the real (or artificial) samples with different AS values were added into the dispersion of RhB/Zn(bix) (3.8 mg) in ethanol (1.9 mL). This dilution applied for the real samples essentially validates the uses of eq 4 to calculated the AS values and enables the AS values of the real samples fall into the linear range of our method. As typically shown in Figure 7, the increase of AS leads to change of the color of the supernatant from pink to colorless and the linear decrease of the fluorescence intensity within the range from 20% vol to 100% vol, within which the AS of commercial samples falls, validating the method developed in this study for dual mode sensing of AS of real samples. Note that there was no remarkable fluorescent change in the spectra during the repetitive ultrasonication and centrifugation processes applied to the ethanol dispersion of RhB/Zn(bix) E

DOI: 10.1021/acs.analchem.5b01617 Anal. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (A) Typical fluorescent responses of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed into water/ethanol mixture with different water concentrations (Vwater /(Vwater + Vethanol) of (from lower to upper) 0, 0.05, 0.15, 0.25, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0%. Inset, photographs of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed into water/ethanol mixture with water concentration of 0 and 5.0%. (B) Plot of fluorescence intensity against H2O (vol %).

Figure 7. (A) Photographs of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed in 1.9 mL of ethanol solution after addition of 100 μL of artificial sample with different AS vaues (Vethanol/(Vwater + Vethanol) of 20, 30, 40, 50, 60, 70, 80, 90, and 100% vol. (B) Photographs of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed in 1.9 mL of ethanol solution after addition of 100 μL of artificial sample with different AS values (Vethanol/(Vwater + Vethanol) of 20, 30, 40, 50, 60, 70, 80, 90, and 100% vol) excited by a 365 nm UV lamp. (C) Fluorescence emission spectra of the supernatants described in (A). (D) Calibration plot of fluorescent intensity against AS.

comparison, we have also detected the AS values of the real spirit samples with Karl Fischer method.38,39 As displayed in Figure 8 and Table 1, the results obtained with our assay correlate well with those obtained with Karl Fischer method and certified by the suppliers, again validating the colorimetric and fluorescent dual mode sensing system demonstrated here for effectively quantifying AS in real samples with the stimuliresponsive RhB/Zn(bix) particles as the optical probe.

particles, suggesting RhB/Zn(bix) was quite stable and robust (data not shown). To further demonstrate the validity of our colorimetric and fluorescent dual mode detection method for quick AS assay of spirit samples, three commercially available Chinese spirits with different AS values were used as the samples in this study. As shown in Figure 8, the AS values of three spirits were easily distinguished by the direct colorimetric visualization with bare eyes and quantified with fluorescence spectrometry. For F

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Figure 8. (A) Photographs of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed in 1.9 mL of ethanol solution after addition of 100 μL of commercial spirit samples with different AS (Vethanol/(Vwater + Vethanol): 56, 46, and 38% vol). (B) Photographs of the supernatant of RhB/Zn(bix) particles (3.8 mg) dispersed in 1.9 mL of ethanol solution after addition of 100 μL of artificial sample with different AS (Vethanol/(Vwater + Vethanol): 56, 46, and 38% vol) excited by a 365 nm UV lamp. (C) Fluorescence emission spectra of the supernatants described in (A). (D) The Karl Fischer titration method of AS in commercial spirit samples.



Table 1. AS Sensing for Commercial Spirit Samplesa

a

AS labeling by suppliers (%vol)

AS using Karl Fisher assay (%vol)

AS using colorimetric assay (%vol)

56 46 38

56.61 44.78 38.37

56.63 ± 0.88 (n = 3) 46.16 ± 0.92 (n = 3) 37.05 ± 0.76 (n = 3)

Corresponding Author

*Fax: +86-10-62559373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



n = number of each sample tested.



AUTHOR INFORMATION

REFERENCES

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CONCLUSIONS

In summary, by using stimuli-responsive RhB/Zn(bix) particles and taking advantage of its unique optical properties and different stabilities in ethanol and water, we have successfully developed a simple, rapid assay for on-spot alcoholic strength determination of Chinese commercial spirit samples by both colorimetric and fluorescent channels. This assay has a number of advantages over conventional systems in high sensitivity, in low cost and in the simplicity of the instrumentation and operation. These striking properties substantially make this colorimetric assay more convenient and more readily adopted for on-pot alcoholic strength determination than the existing systems, and it should thus find broad application in spirit market and potentially offers a new analytical platform for in time controlling of alcohol consumption to avoid negative effects on public health and social order issues. G

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H

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