An Optical Fiber Sensor Based on Fluorescence Lifetime for the Determination of Sulfate Ions

Abstract

A new optical fiber sensor based on the fluorescence lifetime was prepared for specific detection of sulfate ion concentration, where 1,1′-(anthracene-9,10-diylbis(methylene))bis(3-(dodecylcarbamoyl)pyridin-1-ium) acted as the sulfate fluorescent probe. The probe was immobilized in a porous cellulose acetate membrane to form the sensitive membrane by the immersion precipitation method, and polyethylene glycol 400 acted as a porogen. The sensing principle was proven, as a sulfate ion could form a complex with the probe through a hydrogen bond, which led to structural changes and fluorescence for the probe. The signals of the fluorescence lifetime data were collected by the lock-in amplifier and converted into the phase delay to realize the detection of sulfate ions. Based on the phase-modulated fluorometry, the relationship between the phase delay of the probe and the sulfate ion concentration was described in the range from 2 to 10 mM. The specificity and response time of this optical fiber sensor were also researched.

Full text

sensors
Letter
An Optical Fiber Sensor Based on Fluorescence Lifetime for the
Determination of Sulfate Ions
Liyun Ding 1, * , Panfeng Gong 1, Bing Xu 1and Qingjun Ding 2


Citation: Ding, L.; Gong, P.; Xu, B.;
Ding, Q. An Optical Fiber Sensor
Based on Fluorescence Lifetime for
the Determination of Sulfate Ions.
Sensors 2021,21, 954. https://
doi.org/10.3390/s21030954
Academic Editor: Richard
B. Thompson
Received: 9 December 2020
Accepted: 27 January 2021
Published: 1 February 2021
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4.0/).
1National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology,
Wuhan 430070, China; 278242@whut.edu.cn (P.G.); xb2016@whut.edu.cn (B.X.)
2School of Materials, Science and Engineering, Wuhan University of Technology, Wuhan 430070, China;
dingqj@whut.edu.cn
*Correspondence: dlyw@whut.edu.cn; Tel.: +86-181-6400-2219
Abstract:
A new optical fiber sensor based on the fluorescence lifetime was prepared for spe-
cific detection of sulfate ion concentration, where 1,1
0
-(anthracene-9,10-diylbis(methylene))bis(3-
(dodecylcarbamoyl)pyridin-1-ium) acted as the sulfate fluorescent probe. The probe was immobi-
lized in a porous cellulose acetate membrane to form the sensitive membrane by the immersion
precipitation method, and polyethylene glycol 400 acted as a porogen. The sensing principle was
proven, as a sulfate ion could form a complex with the probe through a hydrogen bond, which led to
structural changes and fluorescence for the probe. The signals of the fluorescence lifetime data were
collected by the lock-in amplifier and converted into the phase delay to realize the detection of sulfate
ions. Based on the phase-modulated fluorometry, the relationship between the phase delay of the
probe and the sulfate ion concentration was described in the range from 2 to 10 mM. The specificity
and response time of this optical fiber sensor were also researched.
Keywords:
optical fiber sensor; sulfate optical detection; fluorescence enhancement; phase-modulation flu-
orometry
1. Introduction
Sulfate concentration as an evaluation index of crystalline corrosivity has been the
focus of research in the field of concrete erosion [
1
]. There are many reasons for the
sulfate ion concentration in water to increase, such as the dissolution of gypsum and
other sulfate deposits, sulfite and thiosulfate oxidation in the air, and the discharge of
domestic sewage and industrial wastewater. Sulfate ions can react with cement hydration
products to precipitate into expansive crystals, which bring about expansion, cracking, and
strength loss in concrete structures [
2
–
4
]. The construction life of concrete structures will be
seriously affected, and there is a hidden danger of causing major accidents [
5
–
7
]. Therefore,
the monitoring of sulfate ion concentration in a concrete environment has important
engineering and economic significance for the early warning of durability problems of
concrete structures.
As an ion probe, special modified gold nanoparticles can be used to detect sulfate
ions specifically. They can detect the different concentrations of sulfate ions through the
aggregation degree of gold nanoparticles [
8
] and the fluorescence intensity of a complex [
9
].
Raman spectroscopy has also been reported for the determination of sulfate ions dissolved
in pore water of sediments [
10
], and it realized the diurnal variability change monitoring
of the SO
42−
intensity of offshore seawater [
11
]. Other techniques have been adopted to
determine sulfate concentration, such as titration [
12
,
13
], atomic absorption spectroscopy
(AAS) [
14
,
15
], ion chromatography (IC) [
16
,
17
], spectrophotometry [
18
], atomic fluores-
cence spectroscopy (AFS) [
19
], and inductively coupled plasma atomic emission spec-
troscopy (ICP-AES) [
20
,
21
]. Optical fiber sensors have many advantages compared with
Sensors 2021,21, 954. https://doi.org/10.3390/s21030954 https://www.mdpi.com/journal/sensors

Sensors 2021,21, 954 2 of 13
those detection methods: energy savings, high sensitivity, strong resistance to electromag-
netic interference, enabling continuous remote monitoring, compactness, flexible shape,
and real-time detection capability [
22
–
25
]. Combining sensitive material with a fiber optic
sensor is a significant research direction which could prepare a sulfate fiber sensor, which
has great potential for practical detection.
For ion detection, most of the receptors belong to the off type (fluorescence quench-
ing), which means that the fluorescence emission intensity decreases when the analyte
is combined. Although the off type of detection is widely used, fluorescence enhance-
ment (on) is preferable to quenching (off) because it reduces the chance of false positives,
is more suitable for multiplexing, and could use multiple detectors simultaneously to
produce unique responses to different analytes [
26
,
27
]. A substance that can specifically
bind sulfate ions and produce a fluorescent reaction is a superior fluorescent probe whose
name is 1,1
0
-(anthracene-9,10-diylbis(methylene))bis(3-(dodecylcarbamoyl)pyridin-1-ium)
(AMDP) [
28
]. The probe AMDP can combine with sulfate ions through hydrogen bonds
to form a fluorescent complex and realize the detection of sulfate ion concentration based
on the fluorescence enhancement effect. According to the molecular orbital theory [
29
],
the reduced fluorescence effect of anthracene moiety in the probe AMDP is due to elec-
tron transfer between the anthracene moiety and the pyridine ring, which means that
the photo-induced electron transfer (PET) effect is generated and prevents the genera-
tion of fluorescence. The sulfate group can bind to the two pyridine rings on the probe
AMDP through hydrogen bonding, which invalidates the PET effect in the complex and
restores fluorescence [30,31].
Fluorescence lifetime refers to the average residence time of molecules in the ex-
cited state before returning to the ground state after being excited by light pulse. The
fluorescence lifetime is generally absolute and only related to the microenvironment of
the fluorophore. It has been reported that fiber-optic sensors exist based on fluorescence
lifetime to detect temperature [
32
], strain [
33
], and Fe
3+
ion concentration [
34
] in solution.
As it effectively avoids the inherent characteristics of fluorophore photobleaching, spectral
drift, and inappropriate excitation light, the optical fiber sensor based on fluorescence
lifetime detection has high-precision and stable detection performance.
In this paper, a novel method based on fluorescence lifetime for detecting sulfate ions
was proposed by combining a sulfate-sensitive fluorescent cellulose acetate (CA) membrane
with an optical fiber sensor. To immobilize the probe AMDP, the immersion precipitation
method was adopted to prepare the sulfate-sensitive CA membrane. Polyethylene glycol
(PEG) 400 was used as porogen to improve the porosity of the sensitive membrane and
enhance the detection ability. The lock-in amplifier was used in the optical fiber sensor to
collect and convert the fluorescence lifetime signal into a phase delay, which avoids the
photobleaching effect to improve the anti-interference and detection capability of the sensor.
Based on the principle of the phase-modulation fluorometry, the relationship between
phase delay and sulfate ion concentration was researched. The sensing characteristics
of the optical fiber sensor were further studied, including detection range, sensitivity,
repeatability, and selectivity.
2. Experimental Methods
2.1. Materials and Apparatus
All chemicals used were of analytical-reagent grade. Cellulose acetate, dimethyl sulfox-
ide (DMSO), polyethylene glycol 400, sodium phosphate dibasic, sodium bromide, sodium
iodide, sodium chloride, sodium carbonate, sodium persulfate, sodium thiosulfate, anhy-
drous sodium sulfate, sodium nitrate, magnesium chloride hexahydrate, calcium chloride,
and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd (Wuhan,
China). 1,1
0
-(anthracene-9,10-diylbis(methylene))bis(3-(dodecylcarbamoyl)pyridin-1-ium)
was procured from Heowns Biochem Technologies LLC (Tianjin, China).
The Fourier-transform infrared spectroscopy (FT-IR) spectrum of the fluorescent
complex was obtained by a Nexus (Nexus-470, Thermo Nicolet, Thermo Fisher Scientific,

Sensors 2021,21, 954 3 of 13
Waltham, MA, USA) intelligent Fourier transform infrared spectrometer. The scanning
electron microscope (SEM) photograph of the sensitive membrane was obtained using
a scanning electron microscope (JSM-7500F, Jeol, Japan). A lock-in amplifier (SR-830,
Stanford Research Systems, Sunnyvale, CA , USA) was used to convert the fluorescent
signal into phase shift information. Ultraviolet–visible (UV–Vis) adsorption spectrum and
fluorescence spectrum were obtained from a UV–Vis spectrometer (UV-2450, Shimadzu,
Japan) and fluorescence spectrophotometer (F-4500, Hitachi, Japan), respectively.
2.2. Fluorescence Probe Detection of AMDP
First, 0.1 mM of standard fluorescent indicator solution was prepared by dissolving
1.07 mg fluorescent probe AMDP in 10 mL DMSO. Then, 0.1 mL of indicator solution was
added into 3 mL of 1 mM sodium sulfate DMSO–H
2
O (3:7) mixed solution to detect the
properties of the fluorescent complex. The optimal excitation wavelength and emission
wavelength of the fluorescent complex were found by fluorescence spectra, and further
discussed according to the UV–Vis adsorption spectra. The detection ability of AMDP
for sulfate ion in different pH values was also studied. In addition, the luminescence
time, fluorescence lifetime, complexation ratio, and photobleaching effect of the fluorescent
complex were also investigated.
2.3. Preparation of Sensitive Membrane
Cellulose acetate sensitive membrane was prepared by immersion precipitation
method [
35
–
37
]. DMSO was used as a solvent; 14 wt% cellulose acetate and 10 wt%
PEG 400 were added to DMSO and stirred at room temperature for 6 h to dissolve. Then,
1.0
×
10
−4
M fluorescent probe AMDP was added and stirred for 2 h to dissolve. Ultrapure
water was used as a gel bath. The 3 mL casting solution was tiled in a petri dish with
a diameter of 60 mm, and then immersed in gel bath to form a membrane. After 2 h of
membrane formation, the membrane was taken out from ultrapure water and dried at
30 °C
temperature. Then, the sensitive membrane was characterized by scanning electron
microscope and infrared spectrometer. Leakage experiment was conducted to verify the
reliability of the sensitive membrane. The sensitive membrane was immersed in
10 mM
sulfate ion solution to restore fluorescence and keep for 96 h. During this period, the
absorption spectra of the solution into which the sensitive membrane was immersed
were studied.
2.4. Sulfate Ion Detection with the Optical Fiber Sensor
The optical fiber sensor monitored the sulfate ion concentration by detecting the
change of fluorescence lifetime, which avoided photobleaching and undesirable light-
source interference, thereby improving the reliability and accuracy of detection. The lock-in
amplifier was adopted to collect fluorescence phase change data, and the schematic illus-
tration of the optical experimental platform is shown in Figure 1. The bifurcated fiber
bundle consisted of 37 identical plastic optical fibers, with a core diameter of 0.25 mm and
a fiber cladding of 30
µ
m. The sensitive membrane was excited by an LED light source
with emission wavelength of 410 nm at 40 kHz frequency through the bifurcated fiber. The
fluorescence signal was collected by an optical fiber contact (FC) optical connector, modu-
lated by the lock-in amplifier, and then transmitted to the computer for further processing
and recording. The sensitive membrane was cut into a small wafer of appropriate size
and placed in the metal probe for detection. After each concentration of SO
42−
ions, the
membrane needed to be replaced with a new one in the sensor. The sensitive membrane
was fastened in the sensor probe through a metal fixing nut, and the diagram is shown
in Figure 1.

Sensors 2021,21, 954 4 of 13
Sensors 2021, 21, x FOR PEER REVIEW 4 of 13
Figure 1. Schematic diagram of the optical fiber sensor.
The sensitive membrane was immersed in the solutions of the different sulfate ion con-
centrations, and SO42− combined with the fluorescent probe AMDP on the membrane to
form a fluorescent complex to emit fluorescence. The relationship between fluorescence in-
tensity, fluorescence lifetime, and concentration of SO42− could be described by Equation (1)
[34]:





(1)
where I is the fluorescence intensity and τ is the fluorescence lifetime of the sensitive
membrane. In the formula, subscripts 1 and 2 mean data of the sensitive CA membrane
with the different SO42− ion concentrations.
The fluorescence was emitted by the sensitive membrane via a 410 nm wavelength
LED light source at 40 kHz frequency (f), which could be converted into phase shift infor-
mation (φ) by lock-in amplifier. The following equation shows the relationship between
phase delay (Δφ) and fluorescence lifetime [38,39]:
  
(2)
the following equation can be obtained by combining Equations (1) and (2):




(3)
According to Equation (3), we can determine the change of fluorescence lifetime by
the change of phase delay.
The prepared sensitive membrane was cut into prototype sheets of uniform size for
the detection process. The assembled optical probe was immersed in the DMSO-H2O (3:7)
solution to detect its sulfate concentration to obtain the standard concentration curve and
formula. According to the environmental requirements of real detection [4], the concen-
trations of sulfate in the solution were 0, 2, 4, 6, 8, and 10 mM.
There are many other anions in the concrete environment, so to detect the specificity
of the optical fiber sensor, the experiment proceeded with the anions of HPO42−, Br−, Cl−,
CO32−, I−, S2O32−, SO32−, and NO3− under the same conditions as sulfate.
3. Results and Discussion
3.1. Characterization of the CA Membrane
SEM images were taken in order to explain the effect of PEG 400 on the morphology
of the sensitive CA membrane. Figure 2a depicts SEM surface section image of sensitive
membrane produced without PEG, and the surface micromorphology shows many micro-
sized holes and pits. These holes and pits of the surface are not connected with the interior,
as shown in the cross-section of Figure 2b. As the solvent of casting solution, DMSO was
Figure 1. Schematic diagram of the optical fiber sensor.
The sensitive membrane was immersed in the solutions of the different sulfate ion
concentrations, and SO
42−
combined with the fluorescent probe AMDP on the membrane
to form a fluorescent complex to emit fluorescence. The relationship between fluores-
cence intensity, fluorescence lifetime, and concentration of SO
42−
could be described by
Equation (1) [34]:
I1
I2
=τ1
τ2
, (1)
where Iis the fluorescence intensity and
τ
is the fluorescence lifetime of the sensitive
membrane. In the formula, subscripts 1 and 2 mean data of the sensitive CA membrane
with the different SO42−ion concentrations.
The fluorescence was emitted by the sensitive membrane via a 410 nm wavelength LED
light source at 40 kHz frequency (f), which could be converted into phase shift information
(
ϕ
) by lock-in amplifier. The following equation shows the relationship between phase
delay (∆ϕ) and fluorescence lifetime [38,39]:
tan∆ϕ=2πfτ, (2)
the following equation can be obtained by combining Equations (1) and (2):
tan∆ϕ1
tan∆ϕ2
=τ1
τ2
(3)
According to Equation (3), we can determine the change of fluorescence lifetime by
the change of phase delay.
The prepared sensitive membrane was cut into prototype sheets of uniform size for
the detection process. The assembled optical probe was immersed in the DMSO-H
2
O
(3:7) solution to detect its sulfate concentration to obtain the standard concentration curve
and formula. According to the environmental requirements of real detection [
4
], the
concentrations of sulfate in the solution were 0, 2, 4, 6, 8, and 10 mM.
There are many other anions in the concrete environment, so to detect the specificity
of the optical fiber sensor, the experiment proceeded with the anions of HPO
42−
, Br
−
, Cl
−
,
CO32−, I−, S2O32−, SO32−, and NO3−under the same conditions as sulfate.
3. Results and Discussion
3.1. Characterization of the CA Membrane
SEM images were taken in order to explain the effect of PEG 400 on the morphology
of the sensitive CA membrane. Figure 2a depicts SEM surface section image of sensitive
membrane produced without PEG, and the surface micromorphology shows many micro-
sized holes and pits. These holes and pits of the surface are not connected with the interior,
as shown in the cross-section of Figure 2b. As the solvent of casting solution, DMSO was

Sensors 2021,21, 954 5 of 13
gradually replaced by ultrapure water when the casting solution was immersed in a gel
bath, which led to cellulose acetate gradually precipitating on the membrane. At the same
time, PEG 400 rapidly moved from the casting solution to the ultrapure water because of
its good compatibility with ultrapure water, resulting in many pore channels left in the
membrane. The addition of the porogen, PEG 400, obviously promoted the formation
of pores on the surface of the membrane and made a three-dimensional pore structure
form in the membrane, as shown in Figure 2c,d. When the sensor with a porous, sensitive
CA membrane was used to detect sulfate solutions, the surface and internal pores of the
membrane allowed the sulfate ions in solution to fully contact AMDP, which promoted the
enhancement of the fluorescence of the sensitive membrane and thus the detection of the
sensor. As shown in Figure 2c, the SEM image was processed, and the results showed that
the average pore size of CA membrane was 0.574 µm.
Sensors 2021, 21, x FOR PEER REVIEW 5 of 13
gradually replaced by ultrapure water when the casting solution was immersed in a gel
bath, which led to cellulose acetate gradually precipitating on the membrane. At the same
time, PEG 400 rapidly moved from the casting solution to the ultrapure water because of
its good compatibility with ultrapure water, resulting in many pore channels left in the
membrane. The addition of the porogen, PEG 400, obviously promoted the formation of
pores on the surface of the membrane and made a three-dimensional pore structure form
in the membrane, as shown in Figure 2c,d. When the sensor with a porous, sensitive CA
membrane was used to detect sulfate solutions, the surface and internal pores of the mem-
brane allowed the sulfate ions in solution to fully contact AMDP, which promoted the
enhancement of the fluorescence of the sensitive membrane and thus the detection of the
sensor. As shown in Figure 2c, the SEM image was processed, and the results showed that
the average pore size of CA membrane was 0.574 μm.
(a)
(b)
(c)
(d)
Figure 2. SEM images of sensitive CA membrane. Surface section (a) and cross-section (b) of the
membrane without PEG 400; surface section (c) and cross-section (d) of the membrane with PEG 400.
The chemical structure of the fluorescent probe AMDP is shown in Figure 3a [28] and
the FT-IR spectra of the CA membrane with and without the addition of AMDP are shown
in Figure 3b. The CA membrane with AMDP has a series of characteristic peaks by com-
parison, such as 3089, 2928, 2856, 1664, 1589, 1501 cm−1. The absorption peak at 3089 cm−1
is the C–H stretching vibration peak of the aromatic hydrocarbon ring; the peaks at 1589
and 1501 cm−1 are both stretching vibration peaks of the aromatic hydrocarbon ring skel-
eton; the peaks at 2928 and 2856 cm−1 are respectively the C–H antisymmetrical and sym-
metrical stretching [40] vibration peaks of the alkane; the peak at 1664 cm−1 is the C=O
stretching vibration peak of the amide [41]. Within the fingerprint region, the absorption
peaks 845 and 675 cm−1 are the out-of-plane deformation vibration absorption peaks of C–
H. Therefore, the AMDP was successfully fixed into the CA membrane by the immersion
precipitation method and the composition structure of the probe was not changed.
Figure 2.
SEM images of sensitive CA membrane. Surface section (
a
) and cross-section (
b
) of the membrane without PEG
400; surface section (c) and cross-section (d) of the membrane with PEG 400.
The chemical structure of the fluorescent probe AMDP is shown in Figure 3a [
28
]
and the FT-IR spectra of the CA membrane with and without the addition of AMDP are
shown in Figure 3b. The CA membrane with AMDP has a series of characteristic peaks
by comparison, such as 3089, 2928, 2856, 1664, 1589, 1501 cm
−1
. The absorption peak at
3089 cm−1
is the C–H stretching vibration peak of the aromatic hydrocarbon ring; the peaks
at 1589 and 1501 cm
−1
are both stretching vibration peaks of the aromatic hydrocarbon ring
skeleton; the peaks at 2928 and 2856 cm
−1
are respectively the C–H antisymmetrical and
symmetrical stretching [
40
] vibration peaks of the alkane; the peak at 1664 cm
−1
is the C=O
stretching vibration peak of the amide [
41
]. Within the fingerprint region, the absorption
peaks 845 and 675 cm
−1
are the out-of-plane deformation vibration absorption peaks of

Sensors 2021,21, 954 6 of 13
C–H. Therefore, the AMDP was successfully fixed into the CA membrane by the immersion
precipitation method and the composition structure of the probe was not changed.
Sensors 2021, 21, x FOR PEER REVIEW 6 of 13
(a)
(b)
Figure 3. The structure of the fluorescent probe AMDP, (a) and the FT-IR contrast spectra of the
CA membrane with and without the probe (b).
3.2. Properties of the Fluorescent Complex
In order to verify that the probe AMDP can combine with sulfate ions to produce a
fluorescent complex, we dropped the prepared the AMDP solution into a sodium sulfate
solution, and obtained its excitation (EX) and emission (EM) spectra by fluorescence spec-
trometer. The EM spectrum of the mixed solution showed a separate fluorescence peak,
which represented the successful generation and fluorescent emission of the AMDP–SO42-
complex. The UV–Vis absorption spectra of the AMDP–SO42− complex showed that there
were several characteristic absorption peaks of anthracene group in the range from 300 to
400 nm. Additionally, the peak values of the EX spectrum of the complex corresponded to
the characteristic absorption peaks of anthracene group, which proved that the fluorescent
cluster of the AMDP–SO42− complex is the anthracene group, as shown in Figure 4a,b. When
the AMDP is bonded with sulfate ion via a hydrogen bond, the change of internal structure
leads to the disappearance of the PET effect, which is also proved by the recovery of fluo-
rescence of the anthracene group.
According to Equation (1), it is known that the increase of fluorescence lifetime is
proportional to the increase of fluorescence intensity under ideal conditions, which indi-
cates that the higher the fluorescence intensity is, the more significant the detection of
fluorescence life will be. In order to improve the detection accuracy of fluorescence life-
time, the excitation wavelength of LED used in the sensor should be the optimal excitation
wavelength of fluorescent complex. Figure 4c represents that the excitation wavelength of
LED at 390 nm can achieve the best fluorescence lifetime detection effect.
In addition, the luminescence effect of the fluorescent complex was different under
the different pH value environments. With the increase of OH− ion concentration, the abil-
ity of the AMDP probe to bind sulfate ions with hydrogen bonds is inhibited, leading to a
rapid decline in the detection ability under the pH environment close to 13. The cement
starts facing corrosion issues when the pH value is higher than 10.5 [42], and the probe
could work well in this environment, as shown in Figure 4d.
Figure 3.
The structure of the fluorescent probe AMDP, (
a
) and the FT-IR contrast spectra of the CA
membrane with and without the probe (b).
3.2. Properties of the Fluorescent Complex
In order to verify that the probe AMDP can combine with sulfate ions to produce a
fluorescent complex, we dropped the prepared the AMDP solution into a sodium sulfate
solution, and obtained its excitation (EX) and emission (EM) spectra by fluorescence
spectrometer. The EM spectrum of the mixed solution showed a separate fluorescence peak,
which represented the successful generation and fluorescent emission of the AMDP–SO
42−
complex. The UV–Vis absorption spectra of the AMDP–SO
42−
complex showed that there
were several characteristic absorption peaks of anthracene group in the range from 300 to
400 nm. Additionally, the peak values of the EX spectrum of the complex corresponded to
the characteristic absorption peaks of anthracene group, which proved that the fluorescent
cluster of the AMDP–SO
42−
complex is the anthracene group, as shown in Figure 4a,b.
When the AMDP is bonded with sulfate ion via a hydrogen bond, the change of internal
structure leads to the disappearance of the PET effect, which is also proved by the recovery
of fluorescence of the anthracene group.
According to Equation (1), it is known that the increase of fluorescence lifetime
is proportional to the increase of fluorescence intensity under ideal conditions, which
indicates that the higher the fluorescence intensity is, the more significant the detection of
fluorescence life will be. In order to improve the detection accuracy of fluorescence lifetime,
the excitation wavelength of LED used in the sensor should be the optimal excitation
wavelength of fluorescent complex. Figure 4c represents that the excitation wavelength of
LED at 390 nm can achieve the best fluorescence lifetime detection effect.
In addition, the luminescence effect of the fluorescent complex was different under the
different pH value environments. With the increase of OH
−
ion concentration, the ability
of the AMDP probe to bind sulfate ions with hydrogen bonds is inhibited, leading to a
rapid decline in the detection ability under the pH environment close to 13. The cement
starts facing corrosion issues when the pH value is higher than 10.5 [
42
], and the probe
could work well in this environment, as shown in Figure 4d.

Sensors 2021,21, 954 7 of 13
Sensors 2021, 21, x FOR PEER REVIEW 7 of 13
(a)
(b)
(c)
(d)
Figure 4. The fluorescence spectrum of the fluorescence intensity at 535 nm at different excitation
wavelengths and the emission fluorescence spectrum at an excitation wavelength of 390 nm (a).
Absorption spectrum of the AMDP–SO42− fluorescent complex (b). The fluorescence spectrum of the
fluorescent complex at excited wavelengths from 370 to 440 nm (c) and the intensity of the fluores-
cent complex’s fluorescence under different pH conditions (d).
3.3. Response of the Fluorescent Probe when Detecting Sulfate ions
There are three stages in the formation of the fluorescent complex by the reaction of
the probe AMDP with a sulfate ion. The first 15 min are the first stage of the reaction. The
first step is the rapid reaction of the probe AMDP with sulfate radicals to form fluorescent
complexes; the fluorescence intensity of solution increases rapidly as a consequence. With
the eventual decrease in reaction rate, the fluorescence intensity of the solution stabilizes
at a plateau stage. The fluorescence intensity of the complex is stable from 15 to 25 min.
When the reaction is completed, the fluorescence intensity of the fluorescent complex in
the solution fluctuates for a period of time, and finally weakens to a very low fluorescence
level. The fluorescence intensity of the complex at 25 min was used as the reference stand-
ard for measurement. The fluorescence intensity time curve of the fluorescent complex
formed by the probe AMDP and sulfate ion is shown in Figure 5, and the dividing points
of three stages are 15 min and 25 min, respectively.
Figure 4.
The fluorescence spectrum of the fluorescence intensity at 535 nm at different excitation wavelengths and the
emission fluorescence spectrum at an excitation wavelength of 390 nm (
a
). Absorption spectrum of the AMDP–SO
42−
fluorescent complex (
b
). The fluorescence spectrum of the fluorescent complex at excited wavelengths from 370 to 440 nm
(c) and the intensity of the fluorescent complex’s fluorescence under different pH conditions (d).
3.3. Response of the Fluorescent Probe when Detecting Sulfate Ions
There are three stages in the formation of the fluorescent complex by the reaction of
the probe AMDP with a sulfate ion. The first 15 min are the first stage of the reaction. The
first step is the rapid reaction of the probe AMDP with sulfate radicals to form fluorescent
complexes; the fluorescence intensity of solution increases rapidly as a consequence. With
the eventual decrease in reaction rate, the fluorescence intensity of the solution stabilizes
at a plateau stage. The fluorescence intensity of the complex is stable from 15 to 25 min.
When the reaction is completed, the fluorescence intensity of the fluorescent complex in the
solution fluctuates for a period of time, and finally weakens to a very low fluorescence level.
The fluorescence intensity of the complex at 25 min was used as the reference standard for
measurement. The fluorescence intensity time curve of the fluorescent complex formed
by the probe AMDP and sulfate ion is shown in Figure 5, and the dividing points of three
stages are 15 min and 25 min, respectively.

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Sensors 2021, 21, x FOR PEER REVIEW 8 of 13
Figure 5. Time–intensity graph of the fluorescent complex.
3.4. The Complexation Ratio and Fluorescence Lifetime of the Fluorescent Complex
The complexation ratio of the fluorescent complex formed by the combination of sul-
fate ions and AMDP was researched by the Job plot method [43,44], as shown in Figure
6a. This result was confirmed by the Job plot method in which the fluorescence intensity
exhibits a maximum at a molar fraction of approximately 0.5, indicating that a 1:1 stoichi-
ometry was the most likely binding mode for AMDP and SO42− ion. The 1:1 binding ratio
of the fluorescent complex was also consistent with the binding pattern mentioned in the
references [26]. Fluorescence lifetime is the essential parameter of matter producing a flu-
orescent signal. It refers to the time required for the fluorescence intensity of matter to
decay to 1/e after being excited, which reflects the average stagnation time of an electron
in an excited state. The fluorescence decay curve of the AMDP–SO42− fluorescent complex
was measured by time-correlated single photon counting (TCSPC), as shown in Figure 6b.
The decay of the fluorescence lifetime of the complex was in accordance with the double
exponential fitting calculation. The fluorescence lifetimes were 23.22 ns and 165.97 ns, and
proportions of preexponential factors of both components were 27% and 73%, respec-
tively. The period of the excitation LED source (40 kHz), 25 μs, was much longer than the
fluorescence lifetime of the fluorescent complex, which means that the phase difference
caused by the change of fluorescence lifetime will not be particularly obvious.
(a)
(b)
Figure 6. Fluorescence intensity diagram of the fluorescent complex with different ratios (a) and
fluorescence decay curve of the fluorescent complex (b).
Figure 5. Time–intensity graph of the fluorescent complex.
3.4. The Complexation Ratio and Fluorescence Lifetime of the Fluorescent Complex
The complexation ratio of the fluorescent complex formed by the combination of
sulfate ions and AMDP was researched by the Job plot method [
43
,
44
], as shown in
Figure 6a. This result was confirmed by the Job plot method in which the fluorescence
intensity exhibits a maximum at a molar fraction of approximately 0.5, indicating that
a 1:1 stoichiometry was the most likely binding mode for AMDP and SO
42−
ion. The
1:1 binding ratio of the fluorescent complex was also consistent with the binding pattern
mentioned in the references [
26
]. Fluorescence lifetime is the essential parameter of matter
producing a fluorescent signal. It refers to the time required for the fluorescence intensity of
matter to decay to 1/e after being excited, which reflects the average stagnation time of an
electron in an excited state. The fluorescence decay curve of the AMDP–SO
42−
fluorescent
complex was measured by time-correlated single photon counting (TCSPC), as shown in
Figure 6b
. The decay of the fluorescence lifetime of the complex was in accordance with
the double exponential fitting calculation. The fluorescence lifetimes were 23.22 ns and
165.97 ns
, and proportions of preexponential factors of both components were 27% and 73%,
respectively. The period of the excitation LED source (40 kHz), 25
µ
s, was much longer than
the fluorescence lifetime of the fluorescent complex, which means that the phase difference
caused by the change of fluorescence lifetime will not be particularly obvious.
Sensors 2021, 21, x FOR PEER REVIEW 8 of 13
Figure 5. Time–intensity graph of the fluorescent complex.
3.4. The Complexation Ratio and Fluorescence Lifetime of the Fluorescent Complex
The complexation ratio of the fluorescent complex formed by the combination of sul-
fate ions and AMDP was researched by the Job plot method [43,44], as shown in Figure
6a. This result was confirmed by the Job plot method in which the fluorescence intensity
exhibits a maximum at a molar fraction of approximately 0.5, indicating that a 1:1 stoichi-
ometry was the most likely binding mode for AMDP and SO42− ion. The 1:1 binding ratio
of the fluorescent complex was also consistent with the binding pattern mentioned in the
references [26]. Fluorescence lifetime is the essential parameter of matter producing a flu-
orescent signal. It refers to the time required for the fluorescence intensity of matter to
decay to 1/e after being excited, which reflects the average stagnation time of an electron
in an excited state. The fluorescence decay curve of the AMDP–SO42− fluorescent complex
was measured by time-correlated single photon counting (TCSPC), as shown in Figure 6b.
The decay of the fluorescence lifetime of the complex was in accordance with the double
exponential fitting calculation. The fluorescence lifetimes were 23.22 ns and 165.97 ns, and
proportions of preexponential factors of both components were 27% and 73%, respec-
tively. The period of the excitation LED source (40 kHz), 25 μs, was much longer than the
fluorescence lifetime of the fluorescent complex, which means that the phase difference
caused by the change of fluorescence lifetime will not be particularly obvious.
(a)
(b)
Figure 6. Fluorescence intensity diagram of the fluorescent complex with different ratios (a) and
fluorescence decay curve of the fluorescent complex (b).
Figure 6.
Fluorescence intensity diagram of the fluorescent complex with different ratios (
a
) and fluorescence decay curve
of the fluorescent complex (b).

Sensors 2021,21, 954 9 of 13
3.5. Reliability of the Sensitive Membrane
As an essential parameter of the sulfate ion optical fiber sensor, the reliability of the
sensitive CA membrane was demonstrated. As shown in Figure 7a, the 10 mM sulfate ion
solution was tested by the sensor and the phase was stable after 25 min, which showed
that the fluorescent complex was successfully synthesized in the sensitive membrane and
the stability was excellent. A leakage experiment was also carried out to ensure that
AMDP would not leak during the monitoring process and contaminate the test solution.
The absorption spectra of the 10 mM sulfate solution into which the sensitive membrane
was immersed from 2 h to 96 h were studied, as shown in Figure 7b. The characteristic
absorption peaks of the fluorescent complex were not observed in the absorption spectra,
which indicated that the immobilization of AMDP in the membrane was successful and
that the fluorescent complex was stable during the measurement process.
Sensors 2021, 21, x FOR PEER REVIEW 9 of 13
3.5. Reliability of the Sensitive Membrane
As an essential parameter of the sulfate ion optical fiber sensor, the reliability of the
sensitive CA membrane was demonstrated. As shown in Figure 7a, the 10 mM sulfate ion
solution was tested by the sensor and the phase was stable after 25 min, which showed
that the fluorescent complex was successfully synthesized in the sensitive membrane and
the stability was excellent. A leakage experiment was also carried out to ensure that
AMDP would not leak during the monitoring process and contaminate the test solution.
The absorption spectra of the 10 mM sulfate solution into which the sensitive membrane
was immersed from 2 h to 96 h were studied, as shown in Figure 7b. The characteristic
absorption peaks of the fluorescent complex were not observed in the absorption spectra,
which indicated that the immobilization of AMDP in the membrane was successful and
that the fluorescent complex was stable during the measurement process.
(a)
(b)
Figure 7. Time-phase diagram of sensitive CA membrane at detecting 10 mM sulfate ion (a) and
the absorption spectra of the fluorescent complex and the sensitive CA membrane immersed for 2,
6, 12, 24, 36, 48, and 96 h (b).
3.6. Detection of Sulfate Ions with the Optical Fiber Sensor
To improve the reliability of detection, it is necessary to replace the old sensitive
membrane before using this sensor to detect the concentration of sulfate ions. When the
optical fiber sensor detected the sulfate ions, the fluorescent phase shift (φ) increased with
the sulfate concentration, which was due to the different degree of fluorescence enhance-
ment produced by the fluorescent probe AMDP and the sulfate concentration, as shown
in Figure 8a. As the sulfate concentration increased from 2 to 10 mM, the fluorescence
intensity and lifetime of the sensitive membrane also increased, and the sensor converted
the fluorescence lifetime into a phase shift to obtain phase delay.
Due to the slight phase delay (Δφ) of the sensitive membrane, tanΔφ could be con-
verted into Δφ during calculation. Therefore, the mechanism of sulfate detection was
based on the fluorescence enhancement effect, which could be described by
Δ𝜑 = Δ𝜑0+ 𝐾[𝑄],
(4)
where K and [Q] are the enhancement constant of the fluorescent complex and the concentra-
tion of SO42−, respectively. In the formula, Δφ0 means the phase delay caused by mechanical
error.
The average of the phase values was collected at the 1st minute and the 25th minute.
The delay phase was obtained by the two average values, which could eliminate mechan-
ical errors and obtain more accurate values. Each concentration of sulfate ions was meas-
ured by the sensor many times, and the corresponding standard deviation of phase dif-
ference value was obtained. The sensor has high detection stability, and the maximum
Figure 7.
Time-phase diagram of sensitive CA membrane at detecting 10 mM sulfate ion (
a
) and the absorption spectra of
the fluorescent complex and the sensitive CA membrane immersed for 2, 6, 12, 24, 36, 48, and 96 h (b).
3.6. Detection of Sulfate Ions with the Optical Fiber Sensor
To improve the reliability of detection, it is necessary to replace the old sensitive
membrane before using this sensor to detect the concentration of sulfate ions. When the
optical fiber sensor detected the sulfate ions, the fluorescent phase shift (
ϕ
) increased
with the sulfate concentration, which was due to the different degree of fluorescence
enhancement produced by the fluorescent probe AMDP and the sulfate concentration,
as shown in Figure 8a. As the sulfate concentration increased from 2 to 10 mM, the
fluorescence intensity and lifetime of the sensitive membrane also increased, and the sensor
converted the fluorescence lifetime into a phase shift to obtain phase delay.
Due to the slight phase delay (
∆ϕ
) of the sensitive membrane, tan
∆ϕ
could be con-
verted into
∆ϕ
during calculation. Therefore, the mechanism of sulfate detection was based
on the fluorescence enhancement effect, which could be described by
∆ϕ=∆ϕ0+K[Q], (4)
where Kand [Q] are the enhancement constant of the fluorescent complex and the con-
centration of SO
42−
, respectively. In the formula,
∆ϕ0
means the phase delay caused by
mechanical error.

Sensors 2021,21, 954 10 of 13
Sensors 2021, 21, x FOR PEER REVIEW 10 of 13
variance and average variance were 0.00955 and 0.006657, respectively. The calibration
experiment of the sensor revealed the relationship between the sulfate concentration and
the phase delay, as shown in Figure 8b. The relationship between the phase delay (Δφ)
and the sulfate concentration followed Equation (4), and a linear relationship equation
Δ𝜑 = 0.0091 + 0.01396[𝑆𝑂4
2− ] (R2= 0.99553) was obtained for a sulfate concentration
range from 2 to 10 mM.
(a)
(b)
Figure 8. Time-phase diagram of the sensitive CA membrane at different sulfate ion concentra-
tions (a) and the relationship between the phase delay (Δφ) and the concentration of SO42− (b).
3.7. Selective Experiment of the Optical Fiber Sensor
According to a previous study [28], it is known that the structure of the fluorescent
probe AMDP has a cavity suitable for forming hydrogen bonds with a sulfate ion, which
means that it can specifically bind with a sulfate ion. We carried out selective detection of
common anions (HPO42−, Br−, Cl−, CO32−, I−, S2O32−, SO32−, NO3−, SO42−) in a concrete environ-
ment. The concentration of chloride ions was 1 M and other ions were 10 mM. The phase
delay of the sensitive membrane (Δφ) did not change significantly except for SO42− in the
presence of each of the above chemical substances, which indicates that the optical fiber
sensor has favorable selectivity for SO42− ions, as shown in Figure 9.
In addition, some typical interference cations in concrete, such as Ca2+ and Mg2+, have
little impact on sulfate detection of the sensor compared with Na+, as shown in Table 1.
Figure 8.
Time-phase diagram of the sensitive CA membrane at different sulfate ion concentrations
(a) and the relationship between the phase delay (∆ϕ) and the concentration of SO42−(b).
The average of the phase values was collected at the 1st minute and the 25th minute.
The delay phase was obtained by the two average values, which could eliminate me-
chanical errors and obtain more accurate values. Each concentration of sulfate ions was
measured by the sensor many times, and the corresponding standard deviation of phase
difference value was obtained. The sensor has high detection stability, and the maximum
variance and average variance were 0.00955 and 0.006657, respectively. The calibration
experiment of the sensor revealed the relationship between the sulfate concentration
and the phase delay, as shown in Figure 8b. The relationship between the phase delay
(
∆ϕ
) and the sulfate concentration followed Equation (4), and a linear relationship equa-
tion
∆ϕ=0.0091 +0.01396hSO2−
4i
(R
2
= 0.99553) was obtained for a sulfate concentration
range from 2 to 10 mM.
3.7. Selective Experiment of the Optical Fiber Sensor
According to a previous study [
28
], it is known that the structure of the fluorescent
probe AMDP has a cavity suitable for forming hydrogen bonds with a sulfate ion, which
means that it can specifically bind with a sulfate ion. We carried out selective detection
of common anions (HPO
42−
, Br
−
, Cl
−
, CO
32−
, I
−
, S
2
O
32−
, SO
32−
, NO
3−
, SO
42−
) in a
concrete environment. The concentration of chloride ions was 1 M and other ions were
10 mM
. The phase delay of the sensitive membrane (
∆ϕ
) did not change significantly

Sensors 2021,21, 954 11 of 13
except for SO
42−
in the presence of each of the above chemical substances, which indicates
that the optical fiber sensor has favorable selectivity for SO42−ions, as shown in Figure 9.
Sensors 2021, 21, x FOR PEER REVIEW 11 of 13
Table 1. Influences of different cations on the detection of 10 mM sulfate ion by the sensor.
Cations
Δφ
Difference
Na+
0.1543
Ca2+
0.1544
+0.0001
Mg2+
0.1493
−0.005
Figure 9. Effects of different ions on the phase delay (Δφ) of the sensitive CA membrane.
4. Conclusions
A new optical fiber sensor was prepared for detecting sulfate ion concentration based
on the fluorescence lifetime, where AMDP contained in the sensitive membrane can spe-
cifically combine with a sulfate ion to form a stable fluorescent complex. The sensitive CA
membrane with PEG 400 as the porogen has a porous structure on the surface and inside,
which allows the sulfate ions in the solution to be fully combined with the fluorescent
probe AMDP. The phase delay of fluorescence changed as the sulfate concentration in-
creased from 2 to 10 mM, and the calibration equation of the sensor was 𝛥𝜑 = 0.0091 +
0.01396[𝑆𝑂4
2−] (R2 = 0.99553, under DMSO-H2O (3:7) solution). This optical fiber sensor
provided a promising method with repeatability, a short reaction time, and high selectiv-
ity. Moreover, the sensor has the potential for further research and development, and the
hydrophilicity of the sensor can be improved by modifying the sensitive material to adapt
to the detection environment.
Author Contributions: Conceptualization, P.G. and L.D.; methodology, P.G.; software, P.G. and
B.X.; validation, P.G., L.D.; formal analysis, P.G.; investigation, P.G.; resources, P.G.; data curation,
P.G.; writing—original draft preparation, P.G. and B.X.; writing—review and editing, P.G. and L.D.;
visualization, Q.D.; supervision, L.D.; project administration, L.D. and Q.D.; funding acquisition,
L.D. All authors have read and agreed to the published version of the manuscript.
Funding: This research has received funding from the Natural Science Foundation of China (num-
ber 51878524).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data will be made available on request.
Acknowledgments: We give our thanks for the technology and equipment support from the Mate-
rials Research and Analysis Center of Wuhan University of Technology.
Conflicts of Interest: The authors declare no conflict of interest.
Figure 9. Effects of different ions on the phase delay (∆ϕ) of the sensitive CA membrane.
In addition, some typical interference cations in concrete, such as Ca
2+
and Mg
2+
, have
little impact on sulfate detection of the sensor compared with Na+, as shown in Table 1.
Table 1. Influences of different cations on the detection of 10 mM sulfate ion by the sensor.
Cations ∆ϕDifference
Na+0.1543
Ca2+ 0.1544 +0.0001
Mg2+ 0.1493 −0.005
4. Conclusions
A new optical fiber sensor was prepared for detecting sulfate ion concentration based
on the fluorescence lifetime, where AMDP contained in the sensitive membrane can specif-
ically combine with a sulfate ion to form a stable fluorescent complex. The sensitive CA
membrane with PEG 400 as the porogen has a porous structure on the surface and inside,
which allows the sulfate ions in the solution to be fully combined with the fluorescent probe
AMDP. The phase delay of fluorescence changed as the sulfate concentration increased from
2 to 10 mM, and the calibration equation of the sensor was
∆ϕ=
0.0091
+
0.01396
hSO2−
4i
(R
2
= 0.99553, under DMSO-H
2
O (3:7) solution). This optical fiber sensor provided a
promising method with repeatability, a short reaction time, and high selectivity. Moreover,
the sensor has the potential for further research and development, and the hydrophilic-
ity of the sensor can be improved by modifying the sensitive material to adapt to the
detection environment.
Author Contributions:
Conceptualization, P.G. and L.D.; methodology, P.G.; software, P.G. and
B.X.; validation, P.G., L.D.; formal analysis, P.G.; investigation, P.G.; resources, P.G.; data curation,
P.G.; writing—original draft preparation, P.G. and B.X.; writing—review and editing, P.G. and L.D.;
visualization, Q.D.; supervision, L.D.; project administration, L.D. and Q.D.; funding acquisition, L.D.
All authors have read and agreed to the published version of the manuscript.
Funding:
This research has received funding from the Natural Science Foundation of China (number
51878524).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Sensors 2021,21, 954 12 of 13
Data Availability Statement: Data will be made available on request.
Acknowledgments:
We give our thanks for the technology and equipment support from the Materi-
als Research and Analysis Center of Wuhan University of Technology.
Conflicts of Interest: The authors declare no conflict of interest.
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