Activatable molecular rotor based on bithiophene quinolinlium toward viscosity detection in liquid

doi:10.21203/rs.3.rs-1497018/v1

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Activatable molecular rotor based on bithiophene quinolinlium toward viscosity detection in liquid

https://doi.org/10.21203/rs.3.rs-1497018/v1

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Noninvasive and effective viscosity inspection method during the liquid deterioration process is urgent to be developed, since viscosity is one of the most important physical parameter toward liquid. The method with rapid detection, high sensitivity, cheap equipment, and fast results output is obliged to be exploited. In this study, one kind of viscosity-sensitive molecular rotor (BTPEQ) with large Stokes shift (187 nm), and long emission wavelength (648 nm) has been developed. The rotor comprised bithiophene donor and quinolinium acceptor, and displayed typical twisted intramolecular-charge transfer (TICT) feature, with good photostability, selectivity, universality in various commercial liquids. With the aid of BTPEQ, the thickening effects of liquid thickeners can be captured. More importantly, BTPEQ was explored to visualize the viscosity variations in the liquids at different metamorphic stages, and found that the microenvironment viscosity level is highly dependent on the liquid food metamorphic period. It is noting that this approach can facilitate the continued perfection of fluorescent analytical methods toward food quality and safety inspection.

Molecular rotor

Bithiophene quinolinlium

Fluorescent technique

Viscosity detection

Liquid safety

Liquid safety is a major public health issue around the world, the corruption usually occurs during the long-term transportation and storage procedure [1–3]. Various food-borne illnesses (including the kidney, liver failure or even death) may be caused by liquid deterioration, the inspection has become urgent [4,5]. Generally, the rapidly growth of bacteria, mold and yeast may lead to obvious changes in the microenvironment during this deterioration process [6]. Viscosity, one kind of representative physical parameter in liquid food, acts as a typical marker during the deterioration process, which is closely related to the spoilage extent of liquids [7,8]. So far, many measuring methods for viscosity inspection have been developed, such as damping vibration viscometer, capillary viscometer, rotational viscometer, rheometer and so on [9–12]. However, these traditional methods are developed for macroscopic viscosity detection, which requires large amount of test samples, complex pretreatment process, longer test time, and strongly depends on the device [13]. Thereby, a method with rapid detection, high sensitivity, and fast results output, especially toward microenvironment viscosity determination is urgent to be exploited.

Among the multiple approaches, fluorescent method has become one of the attractive techniques, due to its simple preparation procedure, highly visualized results, non-invasive testing process, less sample requirement, high specificity, etc. [14–16]. To date, many organic fluorophores have been designed as the molecular rotors for viscosity sensing, such as the mitochondrial viscosity tracking [17], lysosomal viscosity investigation [18], and other physiological viscosity change determination [18]. Most of the reported probes have been developed in the biological system, as collected in Table S1. And several kinds of rotors have been designed for environment viscosity detection [19], and chemical analysis program [19]. However, most of these fluorophores are suffering from short Stokes shift and short emission wavelength, which may be harmful toward fluorescence signal release. In fact, several kinds of probes have initially been applied to identify the indicators from multiple aspects, such as the toxic volatile amines [20], allergens determination [21], metal cations and anions [22], and so on. Whereas, few organic fluorophores are designed from the viscosity aspect, thus, the functional molecular rotors with large Stokes shift, long emission wavelength for food metabolic process tracking are still lacking.

In this study, a typical molecular rotor BTPEQ was constructed by the thiophene electron donor (D) group and quinolinium electron acceptor (A) group, linked with conjugated thiophene structure, which formed into the flexible D-π-A chemical stucture. Such molecular rotor usually exhibits quenching state in low-viscous medium, due to the free intramolecular rotation, distortion, and vibration. When in high-viscous medium, upon intramolecular motions are inhibited, leads to a turn-on signal. With the microscopic viscosity increased, the fluorescence intensity enhanced at 648 nm gradually, which confirmed its sensing capacity toward viscosity. Meanwhile, it displayed a large Stokes shift not only in low viscous aqueous media, but also in high viscous glycerol. Moreover, this rotor performed good adaptability, photostability in various commercial liquids, excellent specificity in various analytes. The thickening effect can be specifically identified with different mass contents of food thickeners. More importantly, we found that the rotor BTPEQ can be served as a powerful molecular tool to track the viscosity changes during the deterioration process, the spoiled liquids can be discriminated from the fresh ones through the florescent method. We believe that our trial can promote the fluorescence analytical technique for liquid safety inspection and quality control determination.

Materials and methods

Thiophen-2-ylboronic acid, 5-bromothiophene-2-carbaldehyde, 1-ethyl-4-methylquinolin-1-ium, palladium acetate (Pd(OAc)₂), tetrabutylammonium bromide (TBAB), pyridine, potassium carbonate (K₂CO₃), sodium sulphate (Na₂SO₄), glycerol, ethanol, methanol, dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), toluene, tetrahydrofuran (THF), dichloromethane (DCM), petroleum ether, acetonitrile, and various metal salts such as the potassium nitrate, sodium chloride, calcium chloride, magnesium sulphate, etc., were purchased from Aladdin Reagent (Shanghai) Co., Ltd. Common food additives including the glucose (Glu), sodium benzoate (SB), trisodium citrate dehydrate (TCD), vitamin C (VC), sodium carboxymethyl cellulose, pectin, xanthan gum were obtained from Energy Chemical Technology (Shanghai) Co., Ltd. The amino acid glutathione (GSH) was obtained from Sigma-Aldrich (Merck Life Sciences Co., Ltd., Shanghai, China). All the chemical reagents used in this work were of analytical grade. Triple distilled water was used in the experiments.

Nuclear magnetic resonance (NMR) spectra were obtained with Bruker AVANCE III HD 600 NMR Spectrometer. High resolution mass spectra were performed through Aglient 7250 & JEOL-JMS-T100LP AccuTOF mass spectrometer. Fluorescence spectra were measured by the Hitachi F-7000 fluorescence spectrophotometer. Absorption spectra were recorded on Hitachi U-3010 UV-vis spectrophotometer. The viscosity was determined by the rotating viscometers (DV2T, Brookfield, AMETEK Corp., USA).

Synthetic procedures

Detailed synthesis route of the rotor BTPEQ are exactly showed in the Scheme S1, and corresponding test results are presented in Fig. S1-S8.

Optical properties investigation

The stock solution was prepared at first, rotor BTPEQ was dissolved in DMSO with the concentration of 1 mM, and stored at lower temperature before test. In a typical viscosity determination test, the stock solution was diluted to 10μM, the glycerol-water system was established by mixing different proportions of glycerol with water from 0% to 99%. The photostability was studied in various commercial liquids and wide pH range (from 3.0 to 9.0). The anti-interference capability of rotor was investigated by adding various potential analytes in the distilled water, such as the common metal ions, amino acids, food additives, and glycerol. Each interfering test was operated with the mixture system. Upon test solutions were shaken well before leading to the spectra test, and the absorption spectra were described from 400 nm to 700 nm, the emission spectra were recorded from 520 nm to 850 nm, the excitation wavelength was 488 nm.

Measurement of thickening effect

Common food thickeners including the sodium carboxymethyl cellulose, pectin and xanthan gum with different mass concentrations (from 1 g/kg to 5 g/kg) were added into the distilled water with ultrasonic dispersion and the thickening solutions were prepared. Before the test, bubbles in the solution were eliminated, and the stock solution of rotor BTPEQ was diluted as 10 μM. The excitation wavelength was 488 nm, and emission spectra was recorded from 520 nm to 850 nm.

Deterioration investigation

For deterioration process tracking, commercial beverages such as the cranberry tea, and mango juice were stored under ambient temperature (25 °C) and lower temperature (5 °C), respectively. The fluorescence spectra were recorded at different time intervals in 6 days, and corresponding viscosities were determined as well. A linear equation was established between the fluorescence intensity changes and viscosity variations, (F_n-F₀)/F₀ ~ (η_n-η₀)/η₀, F₀ and F_n were displayed as the fluorescence intensity of liquids at day 0 and day n, η_n and η₀ were defined as the viscosity of liquids at day 0 and day n, 0<n<7. The excitation wavelength was 488 nm.

Molecular rotor design and synthesis

The molecular rotor BTPEQ was synthesized in two steps readily, as shown in Scheme S1. In the first step, thiophen-2-ylboronic acid and 5-bromothiophene-2-carbaldehyde were combined through the Suzuki coupling reaction, and formed as bithiophene. Afterward, the bithiophene and quinolinium were condensed into the final molecular rotor BTPEQ through the Knoevenagel condensation reaction. The chemical structure and corresponding mass spectra were presented in Fig. S1-S8. In this study, rationally designing a powerful rotor for viscosity inspection in liquids is desirable, thiophene derivative is acted as the typical electron donor (D) group, quinolinium is displayed as the electron acceptor (A), which are formed into a traditional twisted intramolecular-charge transfer (TICT) system. The thiophene herein can enlarge the conjugation degree in a further step, which may render the emission spectra red-shifting, the background fluorescence signal noise caused by the food additives may be avoided [23]. As exhibited in Scheme 1, we expected that the viscosity sensing capability was constructed on the basis of intramolecular rotation, after absorbing the photon, the conversion between the planar excited state and twisted excited state occurred. It is anticipated that when in low viscous media, the rotor BTPEQ can rotate freely, resulting in a weaker emissive fluorescence signal, due to the rapid consumption of excited energy via the non-radiative pathway. In comparison, the rotation of rotor can be inhibited when in a high viscous media, the energy was consumed in a radiative pathway, stronger fluorescence signal was released [24]. Thus, this rotor BTPEQ may be able to discriminate the viscosity variations through the fluorescence signal.

Optical properties towards viscosity

At first, the optical responsibility of the rotor BTPEQ toward viscosity was investigated. As seen from Fig. 1a, in the emission spectra, the fluorescence signal increased obviously in the glycerol, and weaker fluorescence signal was found in low viscous water. This phenomenon may be attributed to the rotor suppression effect by the higher microscopic viscosity. And the absorption peak was around 461 nm when in low viscous water, whereas the absorption wavelength was slightly red-shifting toward 488 nm in high viscous glycerol. This phenonemon may be attributed to the parallel stacking of molecular rotor in high viscous media, by which the conjugation was enlarged (Fig. 1b) [25]. In detail, as displayed in Fig. 1c and Fig. 1d, the fluorescence intensity was gradually enhanced upon increasing the content of glycerol (volume fraction from 0%-99%). The rotatable parts in the molecular rotor were inhibited to some extent in this procedure, leading to a radiative pathway to consume the excited energy [26]. Moreover, a good linear relationship was found among the logarithmic function of fluorescence intensity (log I₆₄₈) and viscosity value (log η) by fitting the Förster-Hoffmann equation [27], where R²=0.99, x=0.46. Besides, the Stokes shift in high viscous glycerol and low viscous water were determined as 160 nm and 187 nm respectively, as shown in Fig. 1e and Fig. 1f. In comparation with the developed rotors as listed in Table S1, the Stokes shift is larger enough to avoid the interference of excitation light signal, the signal-to-noise ratio can be enhanced, and better for application.

The viscosity can be affected by temperature, since the viscosity may be enhanced when liquid food stored in lower temperature, whereas higher temperature can cause the viscosity decreased. Subsequently, the fluorescence intensities of glycerol under different temperatures were investigated. As shown in Fig. S9, the fluorescence spectra of the glycerol stored under different temperature were recorded. The fluorescence intensity was found to be increased when the glycerol test system stored in the lower temperature. By contrast, the fluorescence signal became weaker when the glycerol stored under higher temperature. On the other hand, the viscosity was determined by the viscometer as well, and similar result was found. Overall, the results demonstrated the superior viscosity detection capability of the rotor BTPEQ.

Adaptability, selectivity, and photostability

Variety of food related additives are contained in the liquid, and the complex microenvironment is formed. At first, absorption and emission spectra in various solvents with different polarity were studied. Herein, eight kinds of common solvents with different dielectric constant (ε) were selected, such as the toluene, DCM, THF, acetonitrile, DMF, methanol, DMSO, water, glycerol. As shown in Fig. 2a, the absorption peak in glycerol was slightly red-shifting by contrast to other absorption peaks in the low viscous media. This finding may be attributed to the parallel stacking in high viscous microenvironment, leading to the conjugation enlarged [28]. On the other side, as shown in Fig. 2b, strong fluorescence signal was occurred in the high viscous glycerol, whereas, the fluorescence signal was extremely weaker in other solvents. This result may be ascribed to the intramolecular free rotation and consumed the excited energy in non-radiative pathway. Detailed photophysical properties of the rotor BTPEQ in these solvents were collected in Table S2.

Then, the selectivity of rotor BTPEQ to various relevant species were investigated, since multiple additives can enhance the taste, texture, and the nutrition value [29]. Herein, several kinds of representative substrates such as the metal ions, anions, amino acids, etc. were selected. As shown in Fig. 2c and Fig.2d, it can be observed that the fluorescence response signal in various potentially interfering species were exemely weaker, whereas the strong fluorescence signal can be found in the high viscous glycerol. The results suggested that the rotor BTPEQ cannot be interfered by the complex potential additives, the selcctivity was higher.

Afterwards, the photostablity in wide pH range was explored, as shown in Fig. S10. The results indicated that fluorescence signals were relatively stable in the pH range of 3.0-9.0, leading to the feasibility of applications in the complex liquid food conditions. Moreover, the photostability under continuous irradiation for 90 min in various commerical liquids were evaluated, respectively. In Fig. S11, notably, the fluorescence signal intensity remained relatively stable in the test procedure, suggesting the signal releasing procedure cannot be affected by the complex microenvironment under continuous irradiation within a certain time range and confirming its potential applicability in these common liquids. Taken all, the desirable advantages of high adaptability, selectivity and photostability may render the rotor BTPEQ favorable for tracking viscosity variations in complex liquid system.

Thickening effect investigation

For the purpose of enhancing the consistency, homogeneity and texture of liquids, various kinds of food thickeners were applied as the additives quite often [30]. Generally, the viscosity may be enhanced by the thickeners, and thickening effect can be investigated through the fluorescent method. Herein, three kinds of representative thickeners including the sodium carboxymethyl cellulose, pectin and xanthan gum were added into the distilled water, various viscous solutions were prepared. As seen from the Fig. 3a-c, the fluorescence signal were gradually enhanced with the addition amounts of thickeners from 1 g/kg to 5 g/kg. The thickening efficiencies can be determined through the fluorescence technique. As shown in Fig. 3d-f, the thickening efficiency among these thickeners were different, and the linear relationship between fluorescence intensity and mass concentrations of thickeners were fitted. From the quantitative aspect, the xanthan gum displayed the highest thickening efficiency herein, while the sodium carboxymethyl cellulose showed the lowest. The results integrally confirmed that the viscosity variations caused by food thickeners can be determined with this molecular rotor BTPEQ, effectively.

Deterioration process detection

Generally, fresh beverages are perishable at ease, and food quality and safety are urgent to be guaranteed. Thus, the deterioration process inspection is meaningful. Considering that the viscosity-sensitive capability of rotor BTPEQ, nine kinds of commercial beverages were selected, including the water, rose tea, cranberry tea, mango juice, lemon juice, milk, jasmine tea juice, watermelon juice and edible oil. At the beginning, the viscosity of each beverage was investigated through the fluorescence method. As displayed in Fig. S12, it can be observed the fluorescence intensity of each beverage was completely different. This phenomenon may be ascribed to the different viscosity in each beverage. On the other hand, the viscosities of each beverages were measured by the viscometer, and similar results were found, as seen from Table S3. Thus, we can conclude that the viscosity of liquids can be effectively evaluated by the molecular tool BTPEQ.

Then, the viscosity variations during the spoilage process was determined, since viscosity is a robust marker. In detail, two kinds of commercial liquids (cranberry tea and mango juice) were utilized to proceed the metamorphic process. These liquids were stored under the ambient temperature and lower temperature for 8 days, respectively. As displayed in Fig. 4a, the beverages had become turbid and vague gradually when the beverages were stored under ambient temperature with the time extended, even the floating objects can be found after day 5. However, the situation was relatively better when the liquids were stored under lower storage temperature, as shown in Fig. 4b. The viscosity variations during this metamorphic procedure were determined by the fluorescent technique, and the fluorescence signal increased obviously along with the spoilage extent. The fluorescence intensities increased 12.1% and 10.3%, respectively. However, when these beverages stored at lower temperature, the metamorphic progress become slower, few floating objects can be found. Fluorescence signal increased in a limited range, only 6.0% and 5.8%, respectively. Thus, we can draw a conclusion that lower temperature can prolong the storage time and maintain the freshness to some extent, and the molecular rotor BTPEQ can act as a powerful tool toward viscosity determination.

To consolidate this conclusion, the viscosity in upon liquids were investigated through the viscometer from the quantitative perspective. As seen from Fig. 5a and Fig. 5b, when the beverages stored at ambient temperature, the viscosities of cranberry tea and mango juice increased 23.2% and 21.8%, respectively. On the other hand, the viscosities of these beverages enhanced only 13.3% and 12.6% when stored at the lower storage temperature. The results were consistent with the fluorescence spectra. Notably, a fitting linear relationship can be established between the viscosity increment percentage (η_n-η₀)/η₀×100% and the fluorescence intensity increment percentage (F_n-F₀)/F₀×100%, as displayed in Fig. 5c. Thereby, these results collectively manifested that the micro-environmental viscosity variations during the deterioration process can be visualized by the molecular rotor BTPEQ through the fluorescent technique.

In brief, BTPEQ, an activatable molecular rotor was successfully designed via the incorporation of bithiophene derivatives and quinolinium. Notably, this rotor BTPEQ exhibited an obvious ‘turn-on’ signal from low viscous media to high viscous media. Furthermore, BTPEQ displayed large Stokes shift (187 nm) in low viscous water and 160 nm in high viscous glycerol. High photostability, selectivity, and sensitivity were observed in various media, the results indicate that this rotor BTPEQ can be widely applied in complex microenvironment. Based on these features, this rotor has been utilized to sense the viscosity variations with the addition of various food thickeners with different mass concentration, and the thickening effects were determined. More importantly, the viscosity in commercial beverages at different spoilage stages was detected through the fluorescent technique, and a linear relationship was found among the fluorescence increment percentage and viscosity enhanced degree. We believed that BTPEQ can be served as a multi-functional platform for liquid safety inspection, which can inspire the creation of molecular tools for broad application.

Conflict of interest The authors declare no competing interests.

Ethics Approval Not applicable.

Consent to Participate All the authors approved the participation.

Availability of Data and Materials All data generated or analysed during this study are included in this published article and its supplementary information files. And please contact the corresponding author Lingfeng Xu ([email protected]) if necessary.

Funding Natural Science Foundation of Jiangxi Province (20212BAB214031), Doctoral Research Foundation of Jinggangshan University (JZB2006), Innovation and Entrepreneurship Training Program for College Students of Jiangxi Province (S202110419075), Science and Technology Program of Jiangxi Provincial Education Bureau (GJJ211032), Innovation and Entrepreneurship Training Program for College Students of Jinggangshan University (JDX2021237).

Supplementary Information Supplementary data related to this article can be found in the online version, at https://doi.org/10.1007/xxxxxx-xxxx-xxxxx-x.

Acknowledgements This study was supported by the Natural Science Foundation of Jiangxi Province (20212BAB214031), Doctoral Research Foundation of Jinggangshan University (JZB2006), Innovation and Entrepreneurship Training Program for College Students of Jiangxi Province (S202110419075), Science and Technology Program of Jiangxi Provincial Education Bureau (GJJ211032), Innovation and Entrepreneurship Training Program for College Students of Jinggangshan University (JDX2021237).

Author Contributions Statement Lingfeng Xu: Conceptualization, Writing - original draft, Methodology, Validation, Visualization, Funding acquisition. Jingyi Zhao, Fangzhi Xiong: Formal analysis, Investigation. Yanrong Huang: Resources, Formal analysis, Investigation. Fei Deng: Software. Limin Liu: Supervision, Writing - review & editing.

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No competing interests reported.

Scheme.1.tif
Scheme 1. Viscosity sensing mechanism of the rotor BTPEQ toward liquids metamorphic process.
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Activatable molecular rotor based on bithiophene quinolinlium toward viscosity detection in liquid

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Abstract

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