Visual detection of viscosity through activatable molecular rotor with aggregation-induced emission

Article abstract of DOI:10.1016/j.saa.2021.120016

Food safety has become one of the urgent affairs in the global public health studies, and irregular viscosity is closely associated with the food spoilage extent. In this study, one kind of activatable molecular rotor (TPA-PBZ) based on triphenylamine derivates has been synthesized via the Schiff base condensation reaction. This rotor is comprised by donor-accepter conjugated structure, with aggregation induced-emission feature and a large Stokes shift of 160 nm in water. The rotation of aromatic rings in TPA-PBZ is restricted in high-viscosity microenvironment, with the gradually increasing fluorescence emission signal at 568 nm. Significantly, this rotor TPA-PBZ has successfully been applied not only in the determination of thickening effects of food gum, but also in the detection of viscosity enhancement during the liquid food spoilage process. This molecular rotor can be utilized as an intelligent monitor platform for food quality and safety inspection in viscosity-related conditions.

Full text of DOI:10.1016/j.saa.2021.120016

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Visual detection of viscosity through activatable molecular rotor with
aggregation-induced emission
a
c
a
a
a
a,
Lingfeng Xu a,b, , Kui Wu , Runlin Han , Yan Sui , Chunfang Huang , Wei Huang , Limin Liu
⇑
⇑
a School of Chemistry and Chemical Engineering, Jinggangshan University, Ji’an, Jiangxi 343009, China
b State Key Laboratory of Luminescent Materials & Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, College of Materials Science
& Engineering, South China University of Technology, Guangzhou 510640, China
c School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An activatable molecular rotor was developed for liquid food spoilage extent inspection.
ꢀ
An activatable molecular rotor TPA-
PBZ for viscosity detection was
synthesized.
ꢀ
TPA-PBZ exhibits aggregation
induced-emission feature with a large
Stokes shift.
ꢀ
ꢀ
TPA-PBZ showed the potential to
visualize the food thicken effect.
TPA-PBZ is capable of tracking the
viscosity change during food spoilage
process.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 March 2021
Received in revised form 20 May 2021
Accepted 23 May 2021
Available online 25 May 2021
Food safety has become one of the urgent affairs in the global public health studies, and irregular viscos-
ity is closely associated with the food spoilage extent. In this study, one kind of activatable molecular
rotor (TPA-PBZ) based on triphenylamine derivates has been synthesized via the Schiff base condensation
reaction. This rotor is comprised by donor-accepter conjugated structure, with aggregation induced-
emission feature and a large Stokes shift of 160 nm in water. The rotation of aromatic rings in TPA-PBZ
is restricted in high-viscosity microenvironment, with the gradually increasing fluorescence emission sig-
nal at 568 nm. Significantly, this rotor TPA-PBZ has successfully been applied not only in the determina-
tion of thickening effects of food gum, but also in the detection of viscosity enhancement during the
liquid food spoilage process. This molecular rotor can be utilized as an intelligent monitor platform for
food quality and safety inspection in viscosity-related conditions.
Keywords:
Food spoilage
Activatable molecular rotor
Aggregation-induced emission
Fluorescence
Ó 2021 Elsevier B.V. All rights reserved.
Viscosity-dependence
1. Introduction
become the main agenda towards general public [1-3]. During
the long-term transportation of food, various factors may change
As the global demand for food supplement and internal trade is
rising up amazingly, the food safety and nutrition sustainable has
the food quality, especially food spoilage event [4]. Since foodborne
disease negatively influence the consumers, intelligent inspection
strategy towards food spoilage extent is essential [5,6]. In fact,
the viscosity, one kind of important microenvironment parameter,
can be acted as the key role during the deterioration process, since
the detection of the viscosity increment can be directly related to
⇑
Corresponding authors at: School of Chemistry and Chemical Engineering,
Jinggangshan University, Ji’an, Jiangxi 343009, China (L. Xu).
E-mail addresses: mslfxu@mail.scut.edu.cn (L. Xu), llm24@126.com (L. Liu).
https://doi.org/10.1016/j.saa.2021.120016
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
the spoilage extent evaluation [7,8]. And consequently, visualiza-
tion detection is realized. Nowadays, various methods have been
developed for viscosity determination, including the capillary vis-
cometer, damping vibration viscometer, falling ball viscometer,
rotating viscometer, and so on [9-12]. Whereas, these methods
are usually explored for macroscopic detection of viscosity, rela-
tively large amounts of samples were required, with the defects
of time-consuming and complex pretreatment processes [13].
Hence, the method with easy operation, rapid detection, with high
sensitivity, and especially, towards microscopic viscosity detection
is obliged to be exploited [14].
probes with aggregation induced emission (AIE) can display strong
signal even in the aggregation state, is suitable to be applied in the
poor solvent [45-50]. To satisfy the request of drinking liquid qual-
ity control, the AIEgens have initially applied to monitor various
indicators from multiple aspects, including the determination of
acetaldehyde [51], toxic organic pollutants [52], toxic volatile ami-
nes [53], metal cations [54-56], anions [57,58], even the gram bac-
teria [59,60], etc. Particularly, Wu et al. have tried to achieved the
application of AIE probe for sensing of triacylglycerol polymers in
the frying oil, this AIEgen probe was designed to detect the viscos-
ity response during the frying times enhancement of oil, which dis-
played a rapid screening pathway towards food hazards degree
[61]. Roche et al. explored two kinds of spectroscopic probes that
were used to expose the molecular level changes in hydration shell
water interactions, the major focus was on the use of pyranine
probe as a function of added osmolytes detection, this model can
be accounted for the preferential hydration and water/osmolyte-
mediated conformational changes [62]. Du et al. designed a lipo-
philic azo dye which was approved to monitor the micro-
viscosity of oil confined in colloidal fat crystal networks, and the
fluorescence intensity in the degree of oil confinement were deter-
mined [63]. Whereas, the fluorescence-based rotors with large
Stokes shift and AIE feature for fluid viscosity measuring, especially
for liquid food spoilage tracking and food safety inspection are still
lacking.
To date, fluorescence imaging technology has become increas-
ingly attractive, and has become one of qualitative pattern towards
viscosity sensing, which can be employed in the microscopic vis-
cosity determination attracted the interests of viscosity detection,
due to the beneficial advantages such as noninvasive, excellent
selectivity, in situ detection, and so on [15-18]. In fact, several
kinds of fluorescence-based dyes have been developed for environ-
ment viscosity detection [19,20], physiological viscosity change
process tracking [21-24], even the exosome size determination
[25,26], etc. However, most of the existing fluorescence-based dyes
for viscosity detection have been explored in the biological system,
especially for living cells imaging, as presented in Table S1 (in the
supplementary data). These dyes not only displayed short Stokes
shift, but also were hydrophobic and preferred to be aggregated
in aqueous media, which may be harmful towards fluorescence
signal releasing [27]. Usually, an intramolecular hydrogen bonding
interaction between a hydrogen bond donor (AOH) and a hydrogen
bond acceptor (@NA and C@O) is favorable to build the excited
state intramolecular proton transfer (ESIPT) event [28-31]. Indeed,
ESIPT is a unique four-level photochemical process, with the elec-
tronic ground state of ESIPT fluorophores typically existing in an
enol (E) form. Upon photoexcitation, an extremely fast enol to keto
phototautomerization (kESIPT > 1012 sÀ1) event takes place, with the
excited state enol form (E*) rapidly converting to its excited keto
form (K*). After decaying radiatively back to its electronic ground
state, a reverse proton transfer (RPT) takes place to produce the
original E form [32]. Herein, the phenolic hydroxyl group in the
para-position with the Schiff base was designed in the structure,
and the major role of this ESIPT effect is to enlarge the extension
of the conjugated structure and enhance the Stokes shift, which
will be better for fluorescence performance. Moreover, the normal
fluorescence band is generated by a locally excited (LE) state while
the anomalous band corresponds to emission from an intramolec-
ular charge transfer (ICT) state, many theories such as the wagged
ICT (WICT), rehybridized ICT (RICT), planar ICT (PICT), and twisted
ICT (TICT), have been applied to explain this process. The WICT was
proposed by the Schuddeboom et al. [33], which suggests that the
N atom in the amino group of the electron donor will transform the
sp2 hybrid planar configuration to the sp3 hybrid conical configura-
tion. This model is the least supported. Second, the RICT theoretical
research is proposed by Sobolewski et al. [34], considered that the
ICT reaction occurs during the transformation of the geometry of
electron acceptor. However, there exists a high energy barrier
between the LE state and RICT state [35], and this model has been
denied by the Femtosecond instantaneous infrared spectroscopy
[36]. Third, in the PICT model is supported mainly by the fact that
ICT emission is also observed in rigidized systems that can hardly
abandon the planar conformation [37], and its application has been
limited [38]. In comparison, the TICT model proposed by Grabow-
ski and co-workers [39], is supported by large amount of experi-
ments, and has become the most convincing physical model for
the interpretation of dual fluorescence phenomenon [40]. Based
on several previous studies [41-44], the TICT mechanism can be
afforded to explain the photophysics of typical triphenylamine
based fluorescent probe herein. Interestingly, the fluorescent
Herein, we designed an activatable molecular rotor with AIE
characteristic for tracking food spoilage process via responding
towards viscosity. The rotor TPA-PBZ is composed by the electron
donor (D, triphenylamine) and the electron acceptor (A, benzoni-
trile), which formed into the typical D-A type compound through
the single bond. This rotor TPA-PBZ possesses the AIE feature, a
large Stokes shift and a ‘turn-on’ signal towards viscosity incre-
ment. In most cases, the aromatic ring in the triphenylamine, and
the benzene ring (p-bridge), benzonitrile single bond can rotate
freely without any inhibition, the radiative pathways can be chan-
ged [64-66]. With the microenvironmental viscosity enhanced, the
signal was occurred at 568 nm, exhibited a 35.7-fold fluorescence
enhancement at the maximum. Moreover, this rotor performed
excellent anti-interference capability towards viscosity change,
good photo-stability in commercial beverages with the existence
of various food additives, stable fluorescence signal in common
range of pH values. The emission properties of the rotor TPA-PBZ
in various polarity solvents were investigated as well, its universal-
ity has been demonstrated towards complex medium. The temper-
ature factor is considered, corresponding fluorescent spectra have
been recorded. What’s more, the molecular rotor TPA-PBZ was
applied to visualize the thickening effects of the food thickeners
and the viscosity in various liquid food. Moreover, this rotor was
successfully employed to detect the viscosity variation during the
deterioration process as well, for food quality and safety
inspection.
2. Experimental
2.1. Synthesis of the molecular rotor TPA-PBZ
The synthesis route and detailed preparation procedure for the
molecular rotor TPA-PBZ were shown in Scheme S1 and experi-
mental section (see supplementary data), and the corresponding
structural characterizations were described in Fig. S1–S4.
2.2. Spectroscopic properties
Similar as previous reports [67,68], the stock solutions of poten-
tial substances (including NaCl, CaCl2, KNO3, MgSO4, beet molasses,
2

L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
sodium benzoate, glucose, Ser, GSH, Cys, Hcy, Arg, glycerol) were
prepared in the distilled water, respectively. And the molecular
rotor TPA-PBZ was dissolved in DMSO to obtain the stock solution,
further diluted with distilled water and led to a final concentration
anism was achieved as displayed in Scheme 1, the rotatable parts
including the benzene rings and conjugated structure can rotate
freely in low viscous media, led to a non-radiative quenching of
excited state in a mechanically consumption way, while the rota-
tion was restricted in a highly viscous environment, the fluores-
cence signal was restored and dramatically enhanced via the
radiative transition way [69]. It is anticipated that the fluorescence
intensity was enhanced as the viscosity increased, which can be
acted as the viscosity inspection tool via the fluorescent method.
of 10 lM. The selectivity experiments were conducted by mixing
the molecular rotor dilution and upon various substances solutions
(1% DMSO included). The viscosity determination test was oper-
ated in the glycerol-water system, with the volume percent of glyc-
erol ranging from 0% to 99%. The AIE feature of molecular rotor
TPA-PBZ was investigated in the THF/water mixture, different
water fractions (fw) from 0% to 90% was contained. The absorption
spectra were recorded within the wavelength range of 300 nm to
720 nm, and the emission spectra were recorded from 425 nm to
750 nm, under the excitation of 408 nm. All the addition of test
solution was adjusted to 3 mL in total.
3.2. Optical properties of the rotor TPA-PBZ
Based on the original design intention of the rotor TPA-PBZ, the
fluorescence response towards the viscosity detection was investi-
gated at first. As shown in Fig. 1a, low fluorescence signal was
found in distilled water, which was attributed to the free rotation
of aromatic rings and led to a radiative decay pathway. In compar-
ison, an obvious fluorescence intensity enhancement at 568 nm
was observed in the glycerol system, since the rotation process
of aromatic rings was hindered and the radiative pathway was
restored, led to the fluorescence signal occurred. Meanwhile, the
absorption spectra were investigated (as displayed in Fig. 1b), an
absorption peak at 408 nm was exhibited when in the distilled
water. In a contrast, the dominant absorption peak was red shifted
towards 428 nm in glycerol, may be attributed to the parallel
stacking of the rotor molecule TPA-PBZ and thus, the conjugation
degree was enlarged [70,71]. Moreover, the Stokes shift was about
160 nm in distilled water, while the Stokes shift was about 140 nm
in high viscous glycerol system (as shown in Fig. S5 and Fig. S6),
which would be helpful to avoid the interference of the back-
ground light noise caused by the various additives in liquid food.
We further tested the fluorescence response performance of
molecular rotor TPA-PBZ in different volume ratios of water-
glycerol mixture system in detail. As presented in Fig. 2a, the fluo-
rescence intensity at 568 nm was gradually increased while the
volume percent of glycerol was enhanced from 0% (1.0 cP) to 99%
(956.0 cP), and it was notably that the fluorescence intensity
reached the maximum value (35.7-fold). This phenomenon was
attributed to the suppression effect of the rotor TPA-PBZ in high
viscous glycerol solution, thus the radiative decay pathway was
opened and released the stronger fluorescent signal. From the
quantification perspective, a linear relationship (R2 = 0.99, the
slope was 0.50, shown in Fig. 2b) among the fluorescence intensity
2.3. Fluorescence measurements of thickening effect and commercial
beverages
For thickening effect test, the food thickeners including the
sodium carboxymethyl cellulose, arabic gum, and xanthan gum
with various mass concentrations (from 1 g/kg to 10 g/kg) were
prepared with distilled water to achieve the multiple viscosity
microenvironments. The molecular rotor TPA-PBZ stock solution
was diluted into upon solutions with the final concentration of
rotor was 10 lM. The excitation wavelength was 408 nm, and
the fluorescence spectra were recorded from 425 nm to 750 nm.
Meanwhile, six kinds of common liquid food including the white
spirit, pure milk, green tea, edible vinegar, fresh broth and lemon
juice were purchased from the supermarket, the viscosity of these
commercial beverages were determined as well.
2.4. Fluorescence investigation in food spoilage process
Two kinds of commercial liquid food such as fresh broth and
lemon juice were stored under room temperature (about 25 °C)
or fresh retain layer (around 4 °C), respectively. The fluorescence
intensity along with the viscosity variation was fitted into the lin-
ear equation: (Fn-F0)/F0 ꢁ (g0
-g0)/g0, among this equation, F0 and
Fn were represented as the fluorescence intensity of liquid food at
day 0 and day n (0 < n < 10), g0 and g0 were displayed as the vis-
cosity of liquid food at day 0 and day n (0 < n < 10).
(log I568) and the viscosity value (log
g) was established, based on
3. Results and discussion
the Förster-Hoffmann equation (as shown in equ (1) in the supple-
mentary data) [72,73]. The results showed that the rotor TPA-PBZ
was potentially to work as an activatable viscosity detection tool
in liquid food.
3.1. Synthesis and viscosity detection mechanism of the rotor TPA-PBZ
This activatable molecular rotor TPA-PBZ was readily synthe-
sized within two steps, as shown in Scheme S1. In detail, the Com-
pound 1 was prepared through the Suzuki coupling reaction
procedure under the existence of catalyst Pd(OAc)2, and the final
molecular rotor TPA-PBZ was obtained via the Schiff base dehydra-
tion condensation reaction. The chemical structure and detailed
relative molecular mass of Compound 1 and rotor TPA-PBZ were
defined by 1H NMR and high-resolution mass spectroscopy (HR-
MS), as presented in Fig. S1–S4. Two moieties, including the triph-
enylamine donor and benzonitrile acceptor, and were hosted in the
chemical structure of TPA-PBZ, connected with the Schiff base, and
formed into a typical intramolecular proton transfer system. The
introduction of hydroxyl group was expected to improve the
electron-donating capacity, and there existed an ESIPT effect
between the hydroxyl group and Schiff base (which can enlarge
the extension of the conjugated structure and enhance the Stokes
shift), would be better for fluorescence performance improvement
of the rotor TPZ-PBZ. We envisioned the viscosity detection mech-
Then, the AIE property of the rotor TPA-PBZ was investigated, as
shown in Fig. S6. Similar to previous test procedure [49,61], this
investigation was taken in the THF/water mixture system with var-
ious water fractions (fw), from 0% to 90%. Since THF was a good sol-
vent while water was one kind of poor solvent towards TPA-PBZ,
the fluorescence intensity was rather weak when the fw was below
40%, while the emissive signal was increased gradually with the
further enhancement of fw over 40%. Significantly fluorescence sig-
nal was occurred when the fw reached 90%. Moreover, in situ for-
mation of this probe aggregates was measured through dynamic
light scattering (DLS) characterization (displayed in Fig. S7), the
diameter was estimated to be around 100 nm. And corresponding
quantum yield in ethanol and in glycerol, absolute quantum yield
in the solid-state were determined as well, the results were dis-
played in Table S2. A typical AIE feature was observed in aqueous
state.
Next, the emission properties of the rotor TPA-PBZ in various
polarity solvent was studied, since the medium in liquid food
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L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
Scheme 1. The sensing mechanism of the rotor TPA-PBZ towards viscosity.
Fig. 1. (a) Fluorescence spectra of the rotor TPA-PBZ (10 lM) in the water and glycerol. (b) Absorption spectra of the rotor TPA-PBZ (10 lM).
Fig. 2. (a) Fluorescence spectra of the rotor TPA-PBZ (10 lM) with increased glycerol volume ratios (0%-99%). (b) The linear relationship among the log I568 and log g.
was quite complex. The experiment was conducted in different
polarities of common solvents (such as ethanol, DMSO, toluene,
etc.), and the corresponding absorption spectra and emission spec-
tra were studied. As shown in Fig. S8a, the absorption peak was
around 400 nm in most of the usual solvents, while the peak was
red shifted towards 430 nm in the glycerol. This phenomenon
may be ascribed to a more parallel stacking structure which led
to the conjugation enlargement in high viscous environment
[74]. Moreover, the emission intensity in most of the solvents were
extremely weak, while the strong fluorescence signal was observed
in the glycerol system, as presented in Fig. S8b. The photo-physical
properties of the TPA-PBZ in various solvents with different dielec-
tric constant
e and corresponding viscosity were collected in
Table S3 in detail. On the other hand, corresponding diameter of
probe aggregates in the glycerol was determined by DLS, which
was found to be 168 nm, as presented in Fig. S9. These results indi-
cated that the influence of solvent polarity towards the rotor TPA-
PBZ was negligible, and the aggregation extent was higher than
4

L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
that in deionized water. This rotor was suitable to serve as the flu-
orescent tool towards viscosity inspection.
intensities were extremely lower when the inorganic salts, food
additives, amino acids were coexisting in the solution. However,
the fluorescence intensity increased obviously when the rotor
TPA-PBZ was in the higher viscous glycerol. These results sug-
gested that the molecular rotor TPA-PBZ was specific towards vis-
cosity, and potential food additives cannot affect the viscosity
determination. Overall, the molecular rotor TPA-PBZ was consid-
ered to be appropriately as the viscosity identification tool.
It was well known that the viscosity can be affected by temper-
ature, since the low temperature cold chain storage technology
was existed for quality assurance in food production system and
food supply chains [75]. Also, lower temperature may cause the
viscosity enhanced, thus may be helpful to improve the texture,
taste and flavor, while higher temperature may do favor to increase
the flowability during the food processing program [2]. Then the
fluorescence emission spectra were investigated under different
temperature in the glycerol system, as displayed in Fig. 3. It can
be found that the fluorescence intensity was increased while the
glycerol was stored under 4 °C, indicated that the viscosity was
enhanced. By comparison, the fluorescence intensity was reduced
with the temperature increased towards 37 °C, led to the viscosity
decreased. What’s more, the viscosity at different temperature was
determined by viscometer. When the glycerol placed under 37 °C,
the viscosity was about 368 cP, and with the temperature
decreased towards 4 °C, the viscosity was increased to 1450 cP.
This quantitative data further confirmed that the molecular rotor
TPA-PBZ can identify the viscosity changes accurately and highly
effectively. Moreover, the photo-stability under different pH values
of solutions were investigated as well, since various pH values
were occurred in the commercial liquid food. As exhibited in
Fig. S10, the fluorescence intensity was relatively stable during
the pH range of 3.0–9.0. And the photo-stability under continuous
illumination at 365 nm in a certain time range was studied, as this
optical performance was essential in practical application. The
results presented in Fig. S11a showed that fluorescence signal
was substantially stable as time extended, which gave the founda-
tion towards the application of molecular rotor TPA-PBZ in a fur-
ther step.
3.3. Viscosity detection with food thickeners
The food thickeners are usually contained in various commer-
cial liquid food, for the purpose of enhancing the consistency
[77]. The consistency can enhance the taste and appearance, and
the beverage stability and homogeneity were increased as well,
which can make the texture palatable and slippery [78]. In this
case, the solutions with three kinds of food thickeners (sodium car-
boxymethyl cellulose, arabic gum, xanthan gum, with different
additive amounts ranging from 1 g/kg to 10 g/kg) in distilled water
were prepared, and the corresponding viscosity variations were
investigated by the rotor TPA-PBZ. As displayed in Fig. 5a–c, the
fluorescence intensities were increased with the enhanced mass
concentrations of these food thickeners to varying extent. More-
over, we found the linear relationships were existed among the
mass concentrations of food thickeners and fluorescence intensi-
ties, as shown in Fig. 5d–f. The results indicated that significant dif-
ferences of the thickening efficiency were existed in each food
For the purpose of maintaining and improving the flavor, tex-
ture, nutritional value, or other qualities, various food additives
were contained in common liquid food [76]. In order to verify
the specificity of the rotor TPA-PBZ, the anti-interference experi-
ment was performed with several kinds of potential competitive
species including the inorganic salts, food additives, amino acids,
and glycerol. It can be seen from Fig. 4a and b that the fluorescence
Fig. 4. (a) Fluorescence spectra of the rotor TPA-PBZ (10
lM) in the glycerol (99%)
and in the absence or presence of other potential analytes (100
lM) in water
respectively, including the Ca2+, Mg2+, Cu2+, Cd2+, Hg2+, Pb2+, Al3+, Na+, K+, SO24-, Cl-,
NO–3, beet molasses, sodium benzoate, glucose, Ser, GSH, Cys, Hcy, Arg. (b)
Fluorescence intensity of the rotor TPA-PBZ (10 lM) at 568 nm in the glycerol
Fig. 3. Fluorescence spectra of the rotor TPA-PBZ (10
l
M) in glycerol under
(99%) and with the existence of various potential substances (100 lM) in water,
different temperatures.
respectively. kex = 408 nm.
5

L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
Fig. 5. (a) Fluorescence spectra of the rotor TPA-PBZ (10
lM) with the existence of different mass contents of sodium carboxymethyl cellulose in water. (b) Fluorescence
spectra of the rotor TPA-PBZ (10
existence of different mass contents of xanthan gum in water. (d) The fluorescence intensities of the rotor TPA-PBZ (10
contents of sodium carboxymethyl cellulose in water. (e) The fluorescence intensities of the rotor TPA-PBZ (10 M) at 568 nm with the addition of various mass contents of
lM) with the existence of different mass contents of arabic gum in water. (c) Fluorescence spectra of the rotor TPA-PBZ (10
lM) with the
lM) at 568 nm with the addition of various mass
l
arabic gum in water. (f) The fluorescence intensities of the rotor TPA-PBZ (10
lM) at 568 nm with the addition of various mass contents of xanthan gum in water.
thickeners, and the thickening efficiency can be determined by the
slope. Among these three kinds of food thickeners, xanthan gum
owned the highest thickening efficiency, and lowest thickening
efficiency belonged to sodium carboxymethyl cellulose. Based on
this, appropriate food thickeners can be chosen during the food
thickening process, which may be helpful towards thickener selec-
tion in different liquid beverages.
hand, the viscosity of these beverages was measured by the vis-
cometer, it can be found that the viscosity of white spirit, pure
milk, green tea, edible vinegar, fresh broth, and lemon juice was
3.53 cP, 2.34 cP, 12.88 cP, 31.62 cP, 37.50 cP, and 3.39 cP, sepa-
rately. It can be observed the fluorescence intensities were fluctu-
ated with the viscosity, indicated that the viscosity can be detected
by the rotor TPA-PBZ effectively and sensitively in various
beverages.
Next, the beverage spoilage process was monitored by the rotor
TPA-PBZ through the viscosity detection, the fresh broth and lemon
juice were chosen as the representative beverages. In detail, these
two kinds of beverages were stored under different temperature
(including the ambient temperature 25 °C and fresh-keeping tem-
perature 4 °C) for 9 days, separately. The visual images of the bev-
erages stored under different temperature were taken, as shown in
Fig. S13 and Fig. S14. It can be seen that these two beverages had
become turbid gradually with the time extended, and corruption
smell had been released in both beverages after being placed at
ambient temperature. However, apparent images of the beverages
were relative transparent when stored under the fresh keeping
temperature, especially within 6 days, and had become mild turbid
for next 3 days. These results revealed that the deterioration extent
was much worse in higher temperature, and lower temperature
can prolong the time required for deterioration. From the quantita-
tive perspective, the viscosity and corresponding fluorescence
intensities were recorded within 9 days at different time intervals.
As presented in Fig. 6a and c, the viscosity of the fresh broth and
3.4. Deterioration process monitoring
Since food trade has expanded throughout the world to meet
the demands around the world, large-scale food processing and
long-distance distribution were required and increased rapidly
[79]. Thus, the food security, especially the sustainable nutrition
maintain has become a common concern [80,81]. Based on this
problem, the deterioration process was essential to be monitored,
as the viscosity was one of the physical parameters, and may be
enhanced during this process. Herein, six kinds of commercial liq-
uid food were chosen, including the white spirit, pure milk, green
tea, edible vinegar, fresh broth and lemon juice. The photostability
of the rotor TPA-PBZ in these beverages were investigated at first,
the results were shown in Fig. S11b. Within 120 min, the fluores-
cence intensities at 568 nm remained relatively stable in both six
beverages, which revealed the potential application universality
in various beverages. Then, the corresponding viscosity of these
beverages were determined through fluorescent method with the
existence of rotor TPA-PBZ, as displayed in Fig. S12. On the other
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
Fig. 6. (a) Viscosity values of the fresh broth which stored at ambient temperature (25 °C) and fresh keeping temperature (4 °C) within 9 days. (b) Fluorescence intensities of
the rotor TPA-PBZ (10 M) at 568 nm in the fresh broth which stored at ambient temperature (25 °C) and fresh keeping temperature (4 °C) within 9 days. (c) Viscosity values
of the lemon juice which stored at ambient temperature (25 °C) and fresh keeping temperature (4 °C) within 9 days. (d) Fluorescence intensities of the rotor TPA-PBZ (10 M)
l
l
at 568 nm in the lemon juice which stored at ambient temperature (25 °C) and fresh keeping temperature (4 °C) within 9 days. (e) The fitting line of the fluorescence intensity
increment percentage versus viscosity value enhancement percentage.
7

L. Xu, K. Wu, R. Han et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120016
lemon juice increased 46.71% and 117.88% at ambient temperature
within 9 days. In contrast, the viscosity of the fresh broth increased
only 14.21% and 31.84% when the beverages were stored in fresh
keeping layer within 9 days. On the other hand, as shown in
Fig. 6b and d, the fluorescence intensity of the fresh broth and
lemon juice increased 21.6% and 48.11% when the beverages were
stored at ambient temperature for 9 days. By comparison, the flu-
orescence intensity enhanced 9.01% and 13.22% when stored under
fresh-keeping environment. It can be found that a similar incre-
ment pattern was existed among the viscosity value and fluores-
cence intensity. Indeed, a linear relationship was found among
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Lingfeng Xu: Methodology, Investigation, Data curation, Writ-
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Runlin Han: Formal analysis, Data curation. Yan Sui: Conceptual-
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Wei Huang: Formal analysis. Limin Liu: Resources, Supervision,
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