Utilization of photo-luminescent technique toward viscosity detection in the liquid food system with triphenylamine-michaelitic acid molecular sensor

Abstract

A noninvasive and effective viscosity inspection method is expected to ease the burden of continued increased health problems caused by liquid food safety. In this study, we proposed the viscosity of the liquid food micro-environment as a marker and further developed a versatile optical sensor, DPTMDD, for monitoring liquid food micro-environmental viscosity alterations. This sensor was strategically constructed by the triphenylamine-thiophene derivate and michaelitic acid, rotatable conjugate structure was utilized as the recognition site. The molecular sensor was synthesized in a one-step facile way, and DPTMDD displayed a longer emission wavelength (592 nm), low detection limit (1.419 cP), and larger Stokes shift (193.7 nm in glycerol and 177.8 nm in water) with narrower energy band, endowing the sensor with the capacity of achieving high signal-to-noise ratio imaging. Meanwhile, DPTMDD exhibits high adaptability, selectivity, sensitivity, and good photo-stability in various liquid foods, bright fluorescent signal (37.5-fold) of DPTMDD is specifically activated in the high viscosity media. Thickening efficiencies can be identified as well. More importantly, the viscosity fluctuations during the metamorphic stages of liquid foods are also screened through in situ monitoring. We expected that this unique strategy will reinvigorate the continued perfection of liquid food safety investigation systems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05699-y.

Introduction

Liquid food safety is one of the leading causes of global health threats now, which constitutes an enormous burden on society in more and less economically developed countries alike (Chen et al. 2015; Guo et al. 2018; Nsor-Atindana et al. 2018). Although the cases of health problems caused by liquid food safety are growing globally, a substantial portion of liquid food safety is expected to be prevented through effective monitoring for timely intervention. Commonly, commercial liquid food is composed of various nutritious additives, such as cations, anions, amino acids, small-molecular-weight glucose, vitamin, etc., and these basic components play key roles in maintaining homeostasis in liquid foods (Mäkelä et al. 2020; Fu et al. 2021). Meanwhile, the growth of bacteria, mold and yeast depend on the aforementioned nutrients, which can cause obvious changes in the microenvironment (Morreale et al. 2018). As a vital important physical marker, viscosity plays an indispensable role in the food deterioration process (Zou et al. 2019; Pal et al. 2021). To date, viscosity has been detected through the traditional viscometer, such as damping vibration viscometer, capillary viscometer, rotational viscometer, rheometer and so on (Arora et al. 2011; Kweku Amagloh et al. 2013; Yang et al. 2022). These methods are still suffering from several inevitable shortcomings, such as complex pretreatment processes, longer test time, large volume of samples, destructiveness to the sample and strongly depends on the device (Ma et al. 2020). Importantly, these methods cannot measure the microscopic viscosity at a molecular level (Liu et al. 2020; Ludwanowski et al. 2021). These shortcomings limit wide application in many fields.

Fluorescence imaging is a powerful photo-luminescent technique for dynamically monitoring various markers in liquids with merits of high sensitivity, real-time, and in situ detection (Han and Heinonen 2021). Especially, tedious pretreatments, derivatization procedures, and device dependence can be avoided (Tang et al. 2019). Up to now, most of the probes developed for liquid foods deterioration tracking were based on multiple chemical indicators, such as the toxic volatile amines (Pal et al. 2021), allergens determination (Yin et al. 2021), metal cations and anions (Li et al. 2016), and so on. When the recognition group is combined with the analyte, the chemical structure is induced to transform, which will result in the color variation, the shift of spectrum, or the enhancement/weakening of fluorescence intensity. From another perspective, the fluorescence technique can also be employed for non-invasive feedback of the environment viscosity changes during the deterioration process, great facilitating significant advances in liquid foods imaging from the physical perspective. Currently, several kinds of sensors have been developed for viscosity determination, and the potential applications of molecular sensors in food science and engineering have gained much attention. For example, PicoGreen and SYBR Green have been utilized in the collagen solutions, which responded to the bulk viscosity of these solutions of macromolecules effectively (Dragan et al. 2014). A lipophilic azo dye, Citrus Red 2, which is also approved in food applications, was used to monitor the micro-viscosity of oil confined in colloidal fat crystal networks (Du et al. 2014), which the fluorescence intensity of the dye increased when the degree of oil confinement enhanced. Another fluorescent probe, Thioflavin T has been used to monitor the formation of colloidal structures, which shows a progressive increase in fluorescence intensity with an increase in surfactant concentration up to the critical micelle concentration (Kumar et al 2008). Local environmental constraints associated with Thioflavin T binding to surfactant micelles were deemed responsible for the increase in intensity. The molecular sensor approach deserves special attention for viscosity detection, in contrast, functional molecular sensors with longer emission wavelength, and large Stokes shift are still lacking. Based on this, engineering a new molecular sensor is highly favorable for achieving the inspection target.

In this work, we have developed a viscosity-sensitive optical sensor, DPTMDD, as a powerful tool for liquid foods viscosity fluctuation tracking with non-invasive, in situ procedures in liquid foods. Based on the configuration of triphenylamine and michaelitic acid groups, and thiophene bridge, a flexible D–π–A chemical structure was formed. Longer emission wavelength, large Stokes shift, with narrower energy band, which endows DPTMDD with the capacity of sensing the viscosity in a higher signal-to-noise ratio model. We hypothesized that DPTMDD would be able to undergo intramolecular donor–acceptor twisting around the single bond connecting the triphenylamine and michaelitic acid, and the conversion between the locally excited state and stabilized ICT state would hold great promise in viscosity sensing (Wen et al. 2021). In the high-viscous medium, intramolecular motions are inhibited, leading to a turn-on signal, as displayed in Scheme 1. Moreover, the capacity of DPTMDD to evaluate the deterioration extent with viscosity as a robust marker was tested. It can precisely pinpoint the spoilage stage by tracking viscosity fluctuations through the fluorescent technique, demonstrating its capacity for screening of liquid foods deterioration process. Hence, we believe that our trial can promote the fluorescence analytical technique for current liquid food safety inspection.

Scheme 1.

The viscosity sensing mechanism of the molecular sensor DPTMDD toward liquid foods metamorphic process

Materials and methods

Chemicals and instruments

Detailed synthesis chemicals and instruments have been illustrated in the Supporting Information (SI).

Synthesis of 5-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-2,2-dimethyl-1,3-dioxane-4,6-dione (Molecular sensor DPTMDD)

2,2-Dimethyl-1,3-dioxane-4,6-dione (144.0 mg, 1.0 mM) was dissolved in ethanol, the 5-(4-(diphenylamino)phenyl)thiophene-2-carbaldehyde (443.88 mg, 1.25 mM) was mixed as well. The mixture was stirred under room temperature in the N₂ atmosphere, and then the reaction system was refluxed at 78 °C overnight. During the reaction process, pyridine (80 μL, 1 mM) was injected dropwise by syringe. Afterwards, the solvent was removed and extracted in the DCM/de-ionized water, the crude product was purified through the silica-gel column chromatography using DCM/ethyl acetate (v/v = 1/1), and the molecular sensor DPTMDD as a bright tangerine powder was obtained (408.9 mg, 85%). ¹H NMR (400 MHz, DMSO-d₆) δ 8.68 (d, J = 12.4 Hz, 1H), 8.36 (s, 1H), 7.93 (d, J = 15.1 Hz, 1H), 7.81 (d, J = 10.1 Hz, 2H), 7.74–7.63 (m, 4H), 7.53–7.27 (m, 8H), 1.65 (s, 6H). ¹³C NMR (101 MHz, DMSO-d₆) δ 162.06, 150.04, 146.47, 143.23, 137.33, 131.66, 130.74, 128.42, 127.18, 126.06, 124.49, 123.06, 121.47, 120.99, 105.51, 29.16. MS (ESI): m/z 481.13478 [M]⁺, calcd for C₂₉H₂₃NO₄S 481.17455.

Measurements of optical properties

The molecular sensor DPTMDD stock solution (1 mM) was prepared and stored under a lower temperature. During the test, the concentration of DPTMDD was 10 μM. The viscosity was determined in a 3 mL solution, which consisted of various volume percentages of glycerol (from 0 to 99%) and distilled water, and optical measurements were operated. Corresponding fluorescent spectra and viscosity values were recorded and measured by the spectrometer and viscometer.

Various common solvents, such as methanol, DMSO, DMF, etc. were selected with different polarities. Before the test, DPTMDD was added to upon solvents. In a typical specificity test, the solutions with various potential interfering analytes were prepared with distilled water, and DPTMDD was added as well. In the temperature effect experiment, the glycerol was stored under different temperatures, including the normal body temperature (37 °C), fresh-maintenance temperature (~ 5 °C) and common room temperature (25 °C). In all measurements, the excitation wavelength was set as 410 nm.

Common food thickeners including sodium carboxymethyl cellulose, pectin and xanthan gum with different mass concentrations (from 1 to 5 g/kg) were added into the distilled water and corresponding thickening solutions were prepared. Before the test, bubbles in the solution were eliminated with ultrasonic dispersion. The excitation wavelength was 410 nm, and emission spectra were recorded from 450 to 780 nm.

Viscosity tracking in the spoilage process

Two kinds of commercial liquid foods were purchased from the local supermarket, including red pomelo juice and grape juice. These two kinds of beverages were stored under ambient temperature and fresh-maintenance temperature for over 7 days, respectively. Emission spectra were recorded at different time intervals during the storage timeline. One equation among the fluorescence intensity and viscosity has been established: (F_n − F₀)/F₀ ~ (η_n − η₀)/η₀, in which η_n and η₀ were defined as the viscosity of liquid foods at day 0 and day n (0 < n < 8), F₀ and F_n were displayed as the fluorescence intensity of liquid foods at day 0 and day n (0 < n < 8).

Results and discussion

Molecular sensor design and synthesis

Currently, various reported probes have been developed for biomarkers determination in the biological system (several presentative studies have been collected in Table S1, see supplementary data), such as mitochondrial viscosity tracking (Li et al. 2020), lysosomal viscosity investigation (Cui et al. 2021), and other physiological viscosity determination (Li et al. 2019). However, most of these fluorophores are suffering from short Stokes shift and emission wavelength, which may be harmful to fluorescence signal release. More importantly, most of these prepared sensors are relying on the complex design and preparation process, and relatively application fields are limited in the biological system. Nowadays, ingeniously designing a powerful optical sensor for viscosity monitoring in various liquid foods is highly desirable. The predominant challenge to constructing such a fluorescent sensor is to control its photo-luminescent behavior in the viscous systems. The designed sensor should exhibit stronger fluorescence in higher viscosity media, thus the signal intensity can be discriminated from the lower ones. In this case, the viscosity enhancement can be highlighted. Another problem is to avoid auto-fluorescence. Thus, to avoid possible misleading results and spontaneous fluorescence signals of various food additives, longer emission wavelength and large Stokes shift are in highly demand. In this design strategy, the molecular sensor DPTMDD was readily prepared in a one-step facile preparation process without a complex preparation process and a large amount of reagent consumption, as detailed in Fig. 1a. Herein, triphenylamine (TPA) was adopted as the donor group due to its good electron-donating and transporting capabilities (Feng et al. 2020), and thiophene was utilized to enlarge the conjugation structure. The michaelitic acid was introduced as the acceptor through the Knoevenagel reaction because of its good electron-withdrawing ability. A typical D–π–A structure can be found. The chemical structure and relative molecular mass information have been detected through the Nuclear magnetic resonance (NMR) and High-resolution mass spectra (HR-MS), as presented in Figs. S1–S3. Rationally, in terms of complex application attempts in the living system, a simple application model in the liquid system has been made herein. We hypothesized that the rotatable parts can rotate freely in a low-viscosity micro-environment, and will undergo an ICT mechanism from the TPA moiety (D) to the michaelitic acid (A). After absorbing a photon, it first reaches a planar excited state by charge transfer and then reaches a twisted excited state by intramolecular rotation of the michaelitic acid, resulting in the non-radiative relaxation pathway. Thus, in lower-viscosity media, weaker fluorescence can be found. While in high-viscosity media, the rotation will be restricted, the energy consumption pathway through the non-radiative method was reduced, and an obvious turn-on signal was released. As a consequence, molecular sensor DPTMDD may be able to discriminate viscosity changes in a liquid system.

Fig. 1.

a Synthesis route for the molecular sensor DPTMDD. b Fluorescence spectra of the sensor DPTMDD in water and glycerol. c Absorption spectra of the sensor DPTMDD in water and glycerol. d Fluorescence emissive spectra of the sensor DPTMDD in a water-glycerol mixture with the fraction of glycerol (f_g) from 0 to 99%. e A linear relationship between log I₅₉₂ and log η. λ_ex = 410 nm, the concentration of DPTMDD is 10 μM

Optical properties toward viscosity

At first, the spectral properties of the molecular sensor DPTMDD were recorded. In Fig. 1b, an obvious fluorescence signal was observed in glycerol, and a weaker fluorescence signal was found in the water. In the absorption spectra (Fig. 1c), the maximum absorption peak in the glycerol (412.2 nm) was slightly red-shifted when in water (406.6 nm). The parallel stacking of the sensor may afford this, thus the conjugation was enlarged and red shifting in the wavelength was found. Furthermore, fluorescence’s responsibility for viscosity changes has been investigated in detail. In Fig. 1d, the fluorescence intensity was gradually enhanced with the increasing content of glycerol, and 37.5-fold enhancement can be found in the glycerol system, the fluorescence intensity reached the maximum. Based on the viscosity titration studies, a good linear relationship between the log I and log η was observed (the Förster–Hoffmann equation, see supplementary data), where the viscosity sensitivity coefficient was 0.51, as displayed in Fig. 1e. The results demonstrated that the molecular sensor DPTMDD can efficiently respond to the viscosity changes. Additionally, the density functional theory calculation based on DFT/B3LYP/6-31G toward DPTMDD was performed using Gaussian 09 software. In Fig. S4, the highest occupied molecular orbital is on the triphenylamine moiety, while the lowest unoccupied molecular orbital localizes on the michaelitic acid. Energy gaps at different rotation states were calculated to be 0.9387 eV and 0.0010 eV for dihedral angles of 0° and 90° between triphenylamine and michaelitic acid, respectively. The photo-excitation from the S₀ to S₁ state of DPTMDD involves a substantial TICT effect from the triphenylamine moiety to the michaelitic acid unit with the π-bridge thiophene group.

Subsequently, six kinds of representative solvents with different polarities were selected to test the solvatochromism of the DPTMDD, typical red shifting phenomenon can be found in both normalized fluorescence and absorption spectra (in Fig. S5). It can be attributed to the stabilization of ICT occurring between the donor and acceptor groups. On the other hand, the detection limit for viscosity investigation was determined as 1.419 cP, in Fig. S6. Afterward, the viscosity of glycerol stored under different temperatures have been investigated as well, the fluorescence intensities were recorded. As shown in Fig. S7, fluorescence intensity was found to be decreased when the glycerol was stored at higher temperature, while the fluorescence signal became stronger when the glycerol was stored at lower temperature. It may be attributed to the restriction of rotation in high viscous media to some extent. Overall, the results demonstrated the superior viscosity detection capability of the sensor DPTMDD. In the meantime, the sensor DPTMDD has been utilized to determine the viscosity in various commercial liquids, as shown in Fig. S8. Nine kinds of commercial liquids were selected, including water, grape juice, litchi juice, red pomelo juice, milk, kiwi juice, jasmine juice, watermelon juice and edible oil. Diverse fluorescence intensities were found, which indicated the viscosities of these liquid foods were different. These data were collected in Table S2. Moreover, the viscosities of the liquids have been determined by the viscometer and confirmed upon test results as well, as shown in Table S3. Both results indicated that the viscosity can be evaluated through the fluorescent technique sensitively.

Adaptability, selectivity, photostability and thickening effect investigation

Next, the adaptability in various media was studied, as displayed in Fig. 2a, b. Eight kinds of common solvents were selected herein, including the glycerol, acetonitrile, DMSO, water, DMF, THF, methanol and EA. It can be found that the fluorescence signals in these common liquids were weak, while the fluorescence intensity in the glycerol was stronger. This finding can be attributed to the free rotation in these solvents, whereas the rotation was inhibited, the radiative decay pathway was restored. A slightly red-shifted phenomenon can be found in the absorption spectra of glycerol when compared to those in other media, which may be attributed to the parallel stacking of molecules, and the conjugation may be enlarged to some extent. Upon results clearly show that the signal cannot be affected by the polarity. Moreover, apparent fluorescent images of the sensor DPTMDD in various solvents were presented in Fig. 2c. Detailed photo-physical properties of the sensor DPTMDD in these solvents were collected in Table S4. Then, the selectivity of DPTMDD was determined. In contrast to the turn-on signal of viscosity enhancement, a weaker fluorescence response could be observed with the addition of selected additives, as displayed in Fig. 3a, b, indicating that the DPTMDD was inert to a variety of liquids-related species. The sensor DPTMDD can selectively respond to the viscosity. Afterwards, the Stokes shift was investigated as well. As presented in Fig. 3c, d, it can be found that.the Stokes shifts not only in the lower viscosity water but also in the higher viscosity glycerol were large, 193.7 nm and 177.8 nm, respectively. A larger Stokes shift can be helpful to enhance the signal-to-noise ratio, the spontaneous fluorescence can be filtered out. In contrast to the reported studies listed in Table S1, the results are better herein. Therefore, sensor DPTMDD was able to discriminate viscosity changes from other potential interferences.

Fig. 2.

a Fluorescence emissive spectra of the sensor DPTMDD in various common solvents with different polarities. b Absorption spectra of the sensor DPTMDD in various common solvents with different polarities. c Fluorescence images of the sensor DPTMDD in various solvents

Fig. 3.

a Selectivity of the rotor DPTMDD (10 μM) toward various liquid foods-related analytes, including blank, Na⁺, K⁺, Mg²⁺, Ca²⁺, NO₃⁻, CO₃²⁻, Cl⁻, D-mannitol, acesulfame, sorbitol, vitamin C, glucose, sodium benzoate (SB), beet molasses (BM), glycerol. b Fluorescence spectra of the rotor DPTMDD in the presence of various analytes in the distilled water and the glycerol. c Stokes shift of the molecular sensor DPTMDD in low viscosity water (containing 1% DMSO). d Stokes shift of the molecular sensor DPTMDD in high viscosity glycerol (containing 1% DMSO)

Subsequently, the pH stability of the molecular sensor was determined over the range of 3.0–10.0, as shown in Fig. S9. Negligible fluorescence intensity fluctuations can be found in upon pH range, indicating the potential application in complex commercial liquid foods. Similarly, the photo-stability in various commercial liquid food was examined under continuous irradiation for 60 min, respectively. In Fig. S10, eight kinds of liquids, including water, grape juice, litchi juice, red pomelo juice, milk, kiwi juice, jasmine juice, watermelon juice and edible oil were selected, after the addition of DPTMDD, more than 98% fluorescence intensity still existed during the test time range, demonstrating the excellent photo-stability of sensor DPTMDD in these common liquid foods.

Various kinds of food thickeners were added quite often to enhance the consistency, homogeneity and texture of liquid foods (Moret-Tatay et al. 2015). From the ingredients lists of the commercial liquid foods, it is known that grape juice and litchi juice contain sodium carboxymethyl cellulose (SCC), red pomelo juice and kiwi juice contain pectin (Pec), watermelon juice and jasmine juice contain xanthan gum (XG). Commonly, the addition amount of SCC is around 0.01–0.5% in commercial liquid foods, the pectin is commonly in the range of 1%-5% in mass, while the xanthan gum is often in the range of 0.01–2% in mass. From a typical lay out simulation, the capability of the sensor DPTMDD for thickening effect determination was investigated. Herein, three kinds of representative thickeners including the SCC, Pec and XG, were selected, and various viscous solutions were prepared with different amounts o additives from 1 to 5 g/kg. As shown in Fig. S11a–c, the fluorescence signal was gradually enhanced with the increased addition amount of thickeners. The fluorescence enhancements among these thickeners were different. After establishing the nearly linear relationship between fluorescence intensity and mass concentrations of thickeners, it was found that xanthan gum had the highest thickening efficiency, while sodium carboxymethyl cellulose showed the lowest, as shown in Fig. S11d–f. The results integrally confirmed that the viscosity variations can be determined by the sensor DPTMDD. With the measured results, the added amounts of thickeners in commercial liquid foods can be calculated, as listed in Table S5. It can be observed that the determined mass concentrations of thickeners in commercial liquid foods are all within the suggested common range. Therefore, the simulation model operated well. It can be concluded that the sensor DPTMDD owned excellent adaptability, selectivity, photo-stability and viscosity tracking capability, which may render DPTMDD favorable for tracking viscosity in complicated liquid food systems.

Deterioration process detection

More importantly, the feasibility of sensor DPTMDD for viscosity inspection during the spoilage process was assessed as well. Two kinds of commercial liquid foods named red pomelo and grape juice were chosen. These liquid foods were stored under the ambient temperature and fresh-maintenance temperature for about one week, respectively. The deterioration processes were recorded by the camera. As displayed in Fig. 4a, b, at the beginning (before day 5), both liquid foods displayed transparent and clear appearances. After day 5 under ambient temperature, the floating objects can be observed in the red pomelo juice and a yellower appearance was found in the grape juice. On the contrary, when these two kinds of liquid foods were stored under the fresh-maintenance temperature, the liquid foods remained relatively clear and displayed a normal appearance even after day 5. Meanwhile, this process was measured through the fluorescent method as well (in Fig. 4c, d). The fluorescence intensities of red pomelo juice and grape juice increased obviously along with the storage time extended, and 7.4% and 13.0% increment was observed when the stored under ambient temperature, respectively. However, fluorescence signal increased in a limited range, only 3.1% and 6.6% enhancement in red pomelo juice and grape juice can be found when these liquid foods are stored under lower temperature, respectively. The deterioration progress became slower. The above-mentioned experiment phenomenon indicated that lower temperature can slow down the corruption rate of liquid foods to some extent, and the molecular sensor DPTMDD can monitor the viscosity fluctuations of liquid foods during the deterioration process.

Fig. 4.

Digital images of the red pomelo juice and grape juice stored under a ambient temperature (25 °C), b lower temperature (5 °C) for varying time (from 0 to 7 days) and corresponding fluorescence spectra of c red pomelo juice and d grape juice at different time intervals. Concentration of DPTMDD = 10 μM, λ_ex = 410 nm

To consolidate this conclusion, the viscosities of these liquid foods were investigated by the viscometer as well. As seen from Fig. 5a, b, red pomelo juice’s and grape juice’s viscosities increased by 16.3% and 22.1% when stored at ambient temperature, respectively. When stored under fresh-maintenance temperature, the viscosities of red pomelo juice and grape juice enhanced by only 8.7% and 12.5%. Both results were consistent. The spoiled process can be identified from the quantity perspective, this target can be achieved by the traditional viscometer and fluorescent method. Traditional viscometer is relying on the mechanical rotating mechanism, while the fluorescent method is relying on sensor sensing in a molecular level. The macroscopic and microscopic viscosity determination methods can be connected through the viscosity value, which may be helpful to measure the spoiled extent of liquid foods. Based on the original intention, we have tried to establish a Mathematical model to connect both the macroscopic and microscopic viscosity values. The extent of macroscopic viscosity increment displayed a similar pattern as the enhancement of viscosity values got from the fluorescent method. Notably, a fitting linear relationship was 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. Based on the fitting results, it can be collectively manifested that the micro-environmental viscosity variations can be visualized by the molecular sensor DPTMDD through the fluorescent technique. Herein, a simple fluorescent approach to determine the spoilage extent during the deterioration process has been provided.

Fig. 5.

Viscosity values of the a red pomelo juice and b grape juice stored at room temperature or lower temperature within 7 days. c Fitting linear relationship among the fluorescence increment percentage and viscosity enhanced degree

Liquid foods may be spoiled during the longer storage and transportation process, and the spoilage extent can be described via the effective and rapid fluorescent method. The most important work is to verify the effectiveness and applicability of the sensor in the detection of viscosity (one kind of physical biomarker) in food stuffs and liquids. Establishing the linear curve or even the test paper based on the spectrophotometer can be utilized to illustrate the semi-quantitative and quantitative relationship between the sensor and the detected object, not only from the chemical analytes but also from the physical environment. In brief, a novel and effective detection tool can be obtained to track the variations at a molecular level through the fluorescent method.

Conclusion

In summary, we have engineered a new viscosity-sensitive optical agent, DPTMDD, as a powerful optical tool for effective viscosity evaluation has been developed. It was strategically designed via the incorporation of triphenylamine-thiophene derivatives and michaelitic acid. DPTMDD spontaneously activates its fluorescent signals in higher-viscosity liquids. DPTMDD exhibits long emission wavelength and large Stokes shift, which allows DPTMDD to indicate the deterioration state of liquids with a high signal-to-noise ratio. With the advantages of high photo-stability, selectivity, and sensitivity in various media, DPTMDD is capable of being applied in a complex microenvironment. With the assistance of fluorescent technique, the novel viscosity-sensitive molecular sensor DPTMDD was available for the quantitative determination of viscosity fluctuations in the liquid foods with non-invasive, in situ procedures, and a linear relationship was found among the fluorescence increment percentage and viscosity enhanced degree. Our experiments can effectively intersect molecular luminescence and fluorescence imaging technology, displaying great advantages for a liquid food safety inspection, which will open a new avenue for broad application in spectroscopy.

Supplementary Information

Below is the link to the electronic supplementary material.

Authors' contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Lingfeng Xu, Hui Peng and Gengxiang Ma. The first draft of the manuscript was written by Lingfeng Xu, and Yanrong Huang. The project was designed by Lingfeng Xu and Yanrong Huang. All authors read and agreed to the published version of the manuscript.

Funding

This study was supported by the Natural Science Foundation of Jiangxi Province (20212BAB214031), Jiangxi Post-doctoral Scientific Research Program (2021KY57), Innovation and Entrepreneurship Training Program for College Students of Jiangxi Province (202210419011), Science and Technology Program of Jiangxi Provincial Education Bureau (GJJ211032, GJJ2209316), Doctoral Research Foundation of Jinggangshan University (JZB2006), Innovation and Entrepreneurship Training Program for College Students of Jinggangshan University (JDX2022150).

Availability of data and materials

Data will be made available on reasonable request.

Declarations

Conflicts of interest

The authors declare no competing financial interest.

Ethics approval

Not applicable.

Consent to participate

All authors agreed to participate in this study.

Consent for publication

All the authors declared for publication in this Journal.