The preparation of novel triphenylamine-based AIE-effect fluorescent probe for selectively detecting mercury(ii) ion in aqueous solution

Article abstract of DOI:10.1039/d1nj00270h

A TPA-ME fluorescent probe, based on triphenylamine derivative TPA-CHO, has been designed, synthesized and characterized by different spectroscopic methods successfully. TPA-ME and TPA-CHO present similar aggregation-induced emission (AIE) effects under s

Full text of DOI:10.1039/d1nj00270h

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The preparation of novel triphenylamine-based
AIE-effect fluorescent probe for selectively
detecting mercury(II) ion in aqueous solution†
ab
Cite this: New J. Chem., 2021,
45, 5049
Lian Liu,a Jie Ma,
Honggao Yanga
*
Jiamin Pan,a Denghui Li,a Huiling Wanga and
A TPA-ME fluorescent probe, based on triphenylamine derivative TPA-CHO, has been designed, synthesized
and characterized by different spectroscopic methods successfully. TPA-ME and TPA-CHO present similar
aggregation-induced emission (AIE) effects under same conditions. Due to the deprotection reaction of
mercaptal promoted by mercury ions (Hg2+), TPA-ME exhibits excellent selectivity and anti-inference
properties over other tested ions in the DMF/H2O (fw 70%, pH 6.8) medium. Upon increasing Hg2+, the
fluorescence of the TPA-ME probe shows an obvious red-shift from blue to green. The fluorescence
Received 17th January 2021,
Accepted 15th February 2021
intensity has a good linearity toward Hg2+, with the limit of detection (LOD) below 1.23 ꢀ 10ꢁ7 mol Lꢁ1
.
A theoretical calculation was carried out to understand the difference between TPA-ME and TPA-CHO, and
experimental results have been met with simulation calculations. Along with the result of the fluorescence
change, Hg2+ can be easily tested through a conveniently prepared test paper.
DOI: 10.1039/d1nj00270h
rsc.li/njc
Recently, some kinds of fluorescent probes have been developed
for detecting mercury, based on specificity coordination or
1. Introduction
Mercury, also known as the incarnation of metals in alchemy reactions, in water and in vivo.9 Attributed to the specific
during middle ages, is a highly hazardous and ubiquitous heavy recognition group, the reaction-type probes can display excellent
metal of global pollution. Human activities, including oil sensitivity and selectivity, such as mercury-included facile
refining, mining, and the burning of fossil fuels, are the main desulfurization–lactonization reaction,10–12 displacement reac-
causes of mercury spread.1 Not only do aquatic food chains tion of arylboronic acid,13,14 mercuration of dichlorofluorescein
intentionally increase exposure to Hg2+ in human beings,2 but derivatives,15 mercury-promoted hydrolysis for olefin and
inorganic mercury also eventually generates highly toxic organic alkynes,16 and others. In 2001, Tang et al. found that some
mercury through high bioaccumulation and the metabolism of organic molecules have an aggregation-induced emission (AIE)
organisms.3,4 Hg2+ has such a horrible effect to our endocrine effect.17 That means that these amazing ones can oppose the
system, nervous system and other systems even at an extremely aggregation-caused quenching (ACQ) effect,18 which is widely
low level of concentration.5 Recently, various analytical media found in traditional fluorescent materials, wherein the increase
have been developed in leaps and bounds, such as the electro- of the aggregation of the material will lead to the decrease or
chemical test,6 optical test,7 and hyperspectral remote sensing.8 disappearance of its fluorescence intensity. Since then, many
Although many of these methods are reliable and mature, they kinds of molecules with the AIE effect have been synthesized; for
still have disadvantages, including expensive, complex pretreat- instance, tetraphenylethylene derivatives,19,20 tetraphenylsilane
ment, and reliance on large instruments. To meet the modern derivatives,21 and part of the polycyclic aromatic hydro-
technological development, it is necessary to establish a reli- carbons.22,23 By these means, the fluorescent probes for Hg2+
able detection method for Hg2+ with fast response, low-cost, detection with various fluorescent properties have been obtained,
convenience and efficiency, especially via fluorescent probes.
and their application range can be further expanded.24,25 The
triphenylamine-based fluorescent probe has received widespread
attention due to its different properties, which could be affected by
the disperse systems and/or functional groups.26 Its derivatives
presented a completely different fluorescent phenomenon when
the groups change from aromatic hydrocarbons and heterocycles to
electron-withdrawing groups27 and tautomeric systems.28 Some of
the triphenylamine derivatives exhibit the ACQ effect,29 while the
a College of Science, University of Shanghai for Science and Technology, Shanghai,
200093, P. R. China. E-mail: majie0203ch@hotmail.com, majie@usst.edu.cn
b Department of Chemistry, University of Kentucky, KY, 40506, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00270h
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others display the AIE effect at high concentrations.30 The mercury dissolved into deionized water. Third, 0.1 mL TPA-ME stock
ion is a desulfurizer from the mercaptal group to aldehyde group in solution and 2.9 mL DMF were dropped into the 10 ml volumetric
aqueous solution. Fluorescent molecules containing mercaptal flask, then several ion stock solutions were added into the flask.
groups are most likely to be used to detect or identify mercury ions. Finally, the 10 ml constant volume probe detecting system was
Based on the above, we successfully designed and synthe- obtained by adding 0.01 mol Lꢁ1 of a phosphate buffer solution
sized a triphenylamine-based fluorescent molecule, 40-(1,3- (PBS) with given pH at 6.8. The fluorescence emission spectra of
dithiolan-2-yl)-N,N-diphenyl-[1,10-biphenyl]-4-amine (TPA-ME), all detection systems were recorded in the wavelength range from
applied as the reaction-type fluorescent probe for detecting 300 to 800 nm under 277 nm excitation.
the mercury ion. The TPA-ME is synthesized via two steps. The
fluorophore intermediate, 40-(diphenylamino)-[1,10-biphenyl]-4-
carbaldehyde (TPA-CHO), is prepared through the Suzuki–
Miyaura reaction. Then, TPA-ME is obtained via thioacetal
reaction at room temperature. The AIE properties of TPA-ME
were studied in a dimethyl formamide (DMF)/H2O (PBS buffer,
pH 6.8) mixed medium. The Hg2+ caused an obvious fluore-
scent red shift of the TPA-ME probe from blue to green, and the
fluorescent intensity of the probe presents a good linear
relationship with the concentration of Hg2+. The TPA-ME probe
also shows excellent selectivity over a variety of cations and
anions. The sensing mechanism of the TPA-ME probe was
investigated with the analysis of the fluorescent spectra of
TPA-CHO/TPA-ME under the same conditions by studying the
Job’s plot, LC-MS spectra of the reaction mixture of TPA-ME
and Hg2+, and the simulation calculations based on the density
functional theory (DFT). It reveals that the change of the
molecular structure causes the enhancement of the ICT effect,
which leads to the decrease of the excitation energy and the red
shift of the absorption wavelength. Based on this obvious
fluorescence change, test papers for the detection of Hg2+ have
been prepared. After that, this may provide a new idea for the
synthesis of reaction-type fluorescent probes.
2.3 Computational details
The construct optical properties of the TPA-ME and TPA-CHO
molecules were simulated by density functional theory (DFT)
and time-dependent density functional theory (TD-DFT) carried
out in the Gaussian 09 software package. The geometries of
TPA-ME and TPA-CHO were optimized by the B3LYP/6-31g(d)
level in the DFT framework. Their transition energies were
calculated with the B3LYP/6-31g(d) level of the time-dependent
density functional theory.
2.4. Synthesis of TPA-ME
Preparation the fluorophore intermediate, TPA-CHO. The
fluorophore intermediate, 40-(diphenylamino)-[1,10-biphenyl]-4-
carbaldehyde (TPA-CHO), was prepared through the Suzuki–
Miyaura reaction. According to the literature method,31 the typical
procedures are presented in Scheme 1, and briefly described as
follows: firstly, 4-bromotriphenylamine (1.29 g, 3.98 mmol) and
(4-formylphenyl) boronic acid (0.66 g, 4.38 mmol) were fully
dissolved into 40 mL dry tetrahydrofuran (THF), and then 10 mL
K2CO3 (9.95 mmol) aqueous solution was added. After the mixture
was stirred for 30 minutes at room temperature, 150 mg of
Pd(PPh3)4 was added into the system, and the mixed system was
heated to continually reflux for 16 h under nitrogen. Finally, when
the system was cooled to room temperature, the reaction was
quenched by adding 30 mL of water, and the product was extracted
with CH2Cl2 (3 ꢀ 50 mL). The crude TPA-CHO product was
obtained when CH2Cl2 was removed from the organic phase by
a rotary evaporation device. The crude product was further purified
by silica gel column with petroleum ether/CH2Cl2 (2 : 1) as the
eluent. TPA-CHO was yielded as a yellow-green amorphous solid
2. Material and methods
2.1 Materials and apparatus
All raw materials and solvents, including 4-bromotriphenylamine,
(4-formylphenyl) boronic acid, Pd(PPh3)4, enthronement, Hg(NO3)2
and various other salts, were purchased through commercial
channels, unless otherwise mentioned.
1HNMR and 13CNMR spectra were recorded by a nuclear
magnetic resonance spectrometer (AV-400, Bruker, Germany).
The UV-Vis spectra and fluorescence spectra were monitored
via a UV-Vis spectrophotometer (UV-3900, Hitachi, Japan) and
fluorescence spectrometer (FL-7000, Hitachi, Japan). The LC-MS
data of the sample were obtained by a liquid chromatography-mass
spectrometer (LCMS-2020, Shimadzu Co., Japan). The fluorescence
phenomena of the samples were primarily detected by a three-
purpose ultraviolet spectrometer (ZF-I, Sinopharm Group, China).
2.2 Spectroscopic detections
The concentration of TPA-ME was set at 1 ꢀ 10ꢁ5 mol Lꢁ1 in the
typical fluorescence detection system. It was prepared by the
followed procedures. First, TPA-ME was dispersed into DMF
to prepare 1 ꢀ 10ꢁ3 mol Lꢁ1 stock solution. Second, a series of
1 ꢀ 10ꢁ3 mol Lꢁ1 ionic stock solutions were obtained via ions
Scheme 1 Synthetic route for TPA-ME.
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1
(0.92 g, 66%). H NMR (400 MHz, chloroform-d) d 10.15 (s, 1H), 39.80. MS: C27H23NS2 [M + H]+ calculated value: 426.61,
8.04 (d, J = 8.1 Hz, 2H), 7.85 (d, J = 8.0 Hz, 2H), 7.64 (d, J = 8.5 Hz, detected value: 426.1 (Fig. S4–S6, ESI†).
2H), 7.40 (q, J = 6.7, 5.7 Hz, 6H), 7.28 (s, 4H), 7.19 (t, J = 7.2 Hz, 2H).
13C NMR (101 MHz, chloroform-d) d 191.99, 148.55, 147.46, 146.74,
134.80, 132.90, 130.47, 129.54, 128.15, 127.02, 125.01, 123.61,
3. Results and discussion
3.1 Photophysical properties of TPA-CHO and TPA-ME
123.23. MS: C25H19NO [M + 1]+ calculated value 350.43, detected
value 350.3 (Fig. S1–S3, ESI†).
Preparation of the target substance, TPA-ME. The target Since TPA-CHO can be reobtained from TPA-ME through the
substance, 40-(1,3-dithiolan-2-yl)-N,N-diphenyl-[1,10-biphenyl]- mercaptal deprotection reaction promoted by Hg2+, we investi-
4-amine (TPA-ME) was synthesized by a convenient thialdehyde gated the optical properties of TPA-CHO and TPA-ME in the same
reaction, a common aldehyde protection reaction.32 The reac- solvents. Five common solvents were tested with different pola-
tion process is presented in Scheme 1, and the typical synthetic rities in order to select a suitable disperser, and their relative
procedure is as follows: TPA-CHO (0.15 g, 0.43 mmol) and 1,2- images obtained in sunlight or UV-light are shown in Fig. 1. In
ethanedithiol (44 mL, 0.52 mmol) are dissolved in 10 mL Fig. 1a, in the sunlight, the color of TPA-CHO changes from
trichloromethane (CHCl3), before iodide is added. The mixture colorless in THF to cyan-yellow in DMF, DMSO, MeCN, MeOH.
is stirred continuously for 30 minutes by magnetic stirrer at However, under 365 nm UV-light, the fluorescence properties of
room temperature, and then 10 mL Na2S2O3 (0.1 mol Lꢁ1) is TPA-CHO in the above solvents showed a significant difference:
quickly added to the system for quenching the reaction. The TPA-CHO exhibits a strong cyan-green fluorescence in THF
reaction solution is extracted with CHCl3 (3 ꢀ 30 mL). After solvent, medium strong yellow or orange fluorescence in DMSO
that, the organic phase only remained in the end. The crude and DMF, weak yellow fluorescence in MeCN, and very weak blue
TPA-ME product is obtained when CHCl3 is removed from the yellow fluorescence in MeOH. However, TPA-ME appears color-
organic phase by a rotary evaporation device. The crude product less under sunlight and blue fluorescence under 365 nm UV-light,
is further purified by silica gel column with petroleum ether/ and hardly changes with a different solvent (Fig. 1b). Fig. 1c
CH2Cl2 (3 : 1) as the eluent. TPA-ME is yielded as a white further presents that the fluorescence properties of TPA-CHO
1
amorphous solid. (0.12 g, 66%) H NMR (400 MHz, DMSO-d6) change with the above five kinds of solvents. It clearly exhibits
d 7.57 (d, J = 8.5 Hz, 6H), 7.31 (t, J = 7.9 Hz, 4H), 7.05 (q, J = 8.7 that the fluorescent band of TPA-CHO may shift beyond 100 nm
Hz, 8H), 5.77 (s, 1H), 3.60–3.45 (m, 2H), 3.40–3.34 (m, 2H). 13
C
with the change of solvent. Meanwhile, only a slight red-shift
NMR (101 MHz, DMSO-d6) d 147.00, 146.80, 139.47, 139.14, phenomenon appeared in TPA-ME (Fig. 1d). These results should
133.50, 129.59, 128.40, 127.58, 126.13, 124.16, 123.25, 54.72, be attributed to the difference of the molecular polarity between
Fig. 1 The photographs of TPA-CHO (a) and TPA-ME (b) in different solvents under 365 nm UV-light and sunlight, respectively. The fluorescence
emission spectra of TPA-CHO (1 ꢀ 10ꢁ3 mol Lꢁ1, (c) and TPA-ME (1 ꢀ 10ꢁ3 mol Lꢁ1, (d), measured at their maximum excitation wavelength.
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TPA-CHO and TPA-ME, and different interactions between TPA- of methanol. In the MeCN solvent, TPA-CHO may strongly
CHO and solvents. The UV-Vis absorption spectroscopies of the interact with MeCN because of its small molecular size and
two fluorophores in these solvents are shown in Fig. S8 (ESI†). It the presence of the -CN group, causing a fluorescent red-shift
indicates that the solvent polarity has little effect on the UV-Vis with weak fluorescence intensity.33 The environmental factor
absorption of TPA-ME, for they have similar absorption bands in decreases the excitation energies and increases the molecular
the UV region with the maximum absorption peak at about dipole moment, resulting in a red shift.34 However, the fluor-
340 nm. However, the absorption spectra of TPA-CHO are more escent properties of the TPA-ME molecule are less affected by
affected by the solvents. This is because they have two similar the solvent, except for its intensity compared with that of
absorption bands in the range of 270–320 nm and 325–400, but TPA-CHO. This result should be attributed to the mercaptal
the maximum absorption peaks slightly vary with the type of group, which destroys the formation of a large p-conjugation
solvents.
and significantly decreases the polarity of the molecule. It not
These distinct differences between TPA-CHO and TPA-ME only leads to the interaction between the solvent and TPA-CHO,
can be attributed to the aldehyde group in TPA-CHO, which but also results in the lower fluorescent red shift of TPA-ME in
brings about a large rigid plane by the large p-conjugation of different solvents with various polarities.
the molecule with strong polarity. The aldehyde group attracts
Although TPA-CHO and TPA-ME have good solubility in the
the electronic cloud within the molecule, and generates the ICT above organic solvents, they are almost insoluble in water. The
effect due to the strong electron-withdrawing ability. Thus, the effect of the water content (fw) was investigated based on their
fluorescent band of TPA-CHO is red-shifted by about 100 nm fluorescent properties in DMF/H2O (pH 6.8) medium at the
compared to that of TPA-ME. Although the order of polarity of same concentration of 1 ꢀ 10ꢁ5 mol Lꢁ1. As shown in Fig. 2a,
the five solvents is as follows: DMSO 4 MeOH 4 DMF 4 all samples with various fw values were a colorless and trans-
MeCN 4 THF, they may exhibit different types of combinations parent solution in sunlight, but they presented diverse colors
with TPA-CHO, resulting in their fluorescence properties not from yellow to violet to green under 365 nm UV-light. The
being consistent with the polarity of the solvents. In the DMSO, fluorescence spectra of TPA-CHO are shown in Fig. 2c with
DMF and THF solvents, the interaction between TPA-CHO and diverse water content under 277 nm UV-light, and their relative
the solvent increases with the polarity of solvents, leading to intensity changes are shown in Fig. S7(a) (ESI†). Compared to
the fluorescence red-shift and the decrease of its intensity. the yellow fluorescence at 561 nm of TPA-CHO in pure DMF,
Compared with the THF solvent, in the methanol solvent, the fluorescent band is quickly shifted to 428 nm when a slight
TPA-CHO may react with methanol to form a hemiacetal amount of water is added to the system, and with its intensity
structure. This leads to the destruction of the large conjugated gradually decaying with the fw value increasing from 0 to 60%.
system, the weakening of the ICT effect of the aldehyde group, However, when the water content is beyond 60%, a new green
and the fluorescence blue-shift phenomenon. The weakest fluorescent band appears around 482 nm, and its fluorescent
fluorescence intensity is attributed to the strong solvent effect intensity was enhanced with the increase of the fw value until it
Fig. 2 The photographs of TPA-CHO (a) and TPA-ME (b) in various DMF/H2O (pH 6.8) media under 365 nm UV-light irradiation, respectively.
The fluorescence emission spectra of TPA-CHO (c) and TPA-ME (d) excited by 277 nm.
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reaches 95%. These results are consistent with the photographs are listed in Fig. 3b. In general, the fluorescence intensity of the
listed in Fig. 2a. Compared with the pure DMF solvent, the H2O system decreases with the increase of the mercury ion concen-
molecules may strongly act with TPA-CHO by hydrogen bond or tration. In addition, the fluorescence intensity at 462 nm has a
hemiacetal structure, similar with TPA-CHO dispersed into the reliable linear relationship with the mercury ion concentration
MeOH solvent, leading to the blue-shift of the fluorescence within a certain concentration range from 1 ꢀ 10ꢁ6 mol Lꢁ1 to
emission band of TPA-CHO. However, when the water content 8 ꢀ 10ꢁ6 mol Lꢁ1 (0.1–0.8 equiv.).
is beyond 60%, due to the poor dissolubility in water, the TPA-
CHO molecules are aggregated via intermolecular interaction, molecules have the best aggregation to produce the maximum
and results in aggregation-induced fluorescence.
fluorescence intensity. With the addition of Hg2+, the TPA-CHO
Compared to Fig. 2c, it is speculated that the TPA-CHO
For another thing, it was found that the fluorescence properties molecules are formed and tend to pile up with each other due
of TPA-ME also change with the water content, except for the to its large p-conjugation rigid structure. At the low level of Hg2+
change of the color not being more obvious (Fig. 2b). It was (from 0.5 ꢀ 10ꢁ6 mol Lꢁ1 to 1 ꢀ 10ꢁ6 mol Lꢁ1, 0.05 equiv. to
further proved by the fluorescence spectra obtained at 277 nm, 0.1 equiv.), the TPA-CHO molecules are quickly aggregated.
shown in Fig. 2d and Fig. S7(b) (ESI†). The fluorescence intensity They reach the best aggregation for the highest fluorescent
of TPA-ME decays at 425 nm with a fw value change from 0 to intensity at the given 70% water content. After that, with the
50%. When the water content increases from 50% to 80%, the increase of Hg2+, more TPA-CHO molecules are aggregated and
new fluorescence peak of the system presents and reaches the result in the gradual decay of the fluorescence intensity. As the
strongest intensity around 451 nm. Compared to TPA-CHO, TPA- concentration of Hg2+ surpasses 1.0 ꢀ 10ꢁ5 mol Lꢁ1, TPA-ME is
ME does not exhibit a significant fluorescent band shift with the completely converted to TPA-CHO, and its degree of aggrega-
change of the water content. It should be due to the mercaptal tion is not increased, so the fluorescent intensity of the system
group with weak polarity and steric hindrance, which weakly acts is not obviously decay.
with H2O and interacts among the TPA-ME molecules. The
The decrease in the distance between the molecules, and the
above investigation obviously reveals that TPA-CHO and TPA- radiation transition produced by the molecules after they absorb
ME possess the aggregation-induced emission characterization. the photons is disturbed. In this way, most of the energy is
According to the abnormal phenomenon of TPA-CHO in methanol dissipated in the form of non-radiative transitions, which leads
and DMF/H2O medium, it suggests that TPA-CHO can be used as a to a decrease in the fluorescence intensity. In addition, the
potential fluorescent probe for detecting methanol and water at fluorescence intensity at 462 nm showed an excellent linear
given conditions.
relationship with the concentration of Hg2+. According to the
detection limit formula ‘‘DL = 3s/k’’, the detection limit of the
3.2 Fluorescence response to Hg2+
probe TPA-ME for Hg2+ can be calculated as 1.23 ꢀ 10ꢁ7 mol Lꢁ1
,
The fluorescence responsive behavior of TPA-ME toward Hg2+ which is lower than some reported probes.35–37
was investigated in DMF/H2O (fw 70%, pH 6.8) medium. As
3.3 Selectivity and anti-interference capacity of the TPA-ME
probe
shown in Fig. 3, TPA-ME exhibited the unique emission peak at
451 nm. When Hg2+ was added into the system, the fluore-
scence band was obviously red-shifted from blue to green. Selectivity is an important indicator to measure whether the
Meanwhile, the fluorescence intensity changes with Hg2+ fluorescent probes can be applied in actual situations. The
increased from 0.1 to 1.5 equiv., and their relationship curves concentration of all interfering ions is twice (2 ꢀ 10ꢁ5 mol Lꢁ1
)
Fig. 3 (a) Fluorescence emission spectra of TPA-ME excited at 277 nm changed with Hg2+ from 0 to 1.5 ꢀ 10ꢁ5 mol Lꢁ1. Inset: The change of the
fluorescence with the addition of Hg2+ under 365 nm UV-light irradiation. (b) The linear matching curve of the fluorescence intensity with the
concentration of Hg2+ at 462 nm.
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intensity of TPA-ME containing these interfering ions and Hg2+
was also measured in turn. It can be obtained from the data in
Fig. 5 that these different interfering ions do not cause an
obvious interference toward the detection of Hg2+. Compared
with the blank one, the samples with both Hg2+ and different
interfering ions exhibited a red shift from 451 nm to 482 nm,
which is attributed to the reaction between Hg2+ and mercap-
tals. Based on the above two experiments, the probe TPA-ME
has excellent specific detection ability for Hg2+
.
3.4 Mechanism of the sensing process for Hg2+
Mercaptals are commonly used as aldehyde protecting groups
in organic synthesis. Hg2+ is a mercaptal deprotection reagent
because of its strong sulfur–absorbing property. Based on this
characteristic, the reaction mechanism of the probe TPA-ME
with Hg2+ is shown in Scheme 2.
Fig. 4 (a) The photographs of TPA-ME in the presence of 2 ꢀ 10ꢁ5 mol Lꢁ1
of the interfering ions and 1 ꢀ 10ꢁ5 mol Lꢁ1 of Hg2+ in DMF/H2O (fw 70%, pH
6.8) under 365 nm UV-light irradiation. (b) Fluorescence emission spectra of
TPA-ME with different ions excited by 277 nm.
To further verify our conjecture, we compared four fluore-
scence data, including TPA-CHO, TPA-ME, TPA-ME with Hg2+
(1 : 1 equiv.), and TPA-ME with Hg2+ (1 : 1.5 equiv.), in Fig. 6a.
With the addition of Hg2+, the fluorescence emission peak
presents an obvious red-shift from 451 nm to 482 nm. The
fluorescence emission peak of TPA-CHO coincided with the
spectra of TPA-ME with the Hg2+ system. In addition, the Job’s
plot curve assumed the 1 : 1 reacting stoichiometry for TPA-ME
toward Hg2+ (Fig. 6b). This suggests that the mercaptal group in
TPA-ME is hydrolyzed under the influence of Hg2+, and TPA-ME
in the system is gradually reduced and converted to TPA-CHO.
Besides, comparing the retention time of the reaction mixture
(TPA-ME and Hg2+, Fig. S9, ESI†) to pure TPA-CHO and TPA-ME
(Fig. S3 and S6, ESI†), the LC-MS spectra of the reaction mixture
show that TPA-ME turned into TPA-CHO with the addition of
Hg2+, which is consistent with our above conjecture.
that of Hg2+. As shown in Fig. 4a, the change of the fluorescence
emission peak occurs when Hg2+ (1 ꢀ 10ꢁ5 mol Lꢁ1) is added,
while the existence of various interfering ions (Pb2+, Mn2+, Ag+,
Co2+, Ba2+, Ni2+, Zn2+, Cd2+, Cu2+, Ca2+, Cr3+, Fe3+, Fe2+, Mg2+,
2ꢁ
Clꢁ, SO42ꢁ, NO3ꢁ, CO3 and CH3COOꢁ) only exhibit a negligi-
ble response. According to R. G. Pearson’s soft and hard acid–
base theory, Hg2+ and S2ꢁ are soft particles because they have a
lower charge density and a larger ionic radius. Therefore, Hg2+
combines easily with mercaptals,38 and only the fluorescence of
the TPA-ME solution containing Hg2+ has changes from blue to
green (Fig. 4b). These interfering ions cannot induce the depro-
tection reaction of the mercaptal, and the probe TPA-ME has a
good selectivity toward Hg2+.
In actual production applications, the real environment is
often full of various uncertainties. For fluorescent probes, the
anti-interference capacity is another important indicator. The
anti-interference of the probe TPA-ME was carried out upon
the addition of 2.0 equiv. of various interfering ions in
DMF/H2O (fw 70%, pH 6.8). For comparison, the fluorescence
In order to reveal the fluorescence emission of TPA-ME and
TPA-CHO, the relative theoretical calculations were performed via
a Gaussian software package based on the density functional
theory. Their structure and transition energies are first optimized
through the density functional theory (DFT) and time-dependent
Fig. 5 (a) Fluorescence emission spectra of TPA-ME (1 ꢀ 10ꢁ5 mol Lꢁ1) in DMF/H2O (fw 70%, pH 6.8) with the presence of other interfering ions (2 ꢀ 10ꢁ5
mol Lꢁ1) and Hg2+ (1 ꢀ 10ꢁ5 mol Lꢁ1). (b) Fluorescence intensity of TPA-ME (1 ꢀ 10ꢁ5 mol Lꢁ1) in the presence of 2 ꢀ 10ꢁ5 mol Lꢁ1 of other ions at 451 nm
with Hg2+ (2 ꢀ 10ꢁ5 mol Lꢁ1) or not (1. only Hg2+, 2. Pb2+, 3. Mn2+, 4. Ag+, 5. Co2+, 6. Ba2+, 7. Ni2+, 8. Zn2+, 9. Cd2+, 10. Cu2+, 11. Ca2+, 12. Cr3+, 13. Fe3+, 14.
Fe2+, 15. Mg2+, 16. Clꢁ, 17. SO42ꢁ, 18. NO3ꢁ, 19. CO32ꢁand 20. CH3COOꢁ).
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Scheme 2 Sensing process for TPA-ME toward Hg2+
.
Fig. 6 (a) The fluorescence emission spectra of TPA-ME with a typical concentration of Hg2+ and TPA-CHO in DMF/H2O (fw 70%, pH 6.8), excited by
277 nm UV-light. (b) The Job’s plot between TPA-ME and Hg2+, with a total concentration of ([Hg2+] + [TPA-ME]) = 2 ꢀ 10ꢁ5 mol Lꢁ1
.
density functional theory (TD-DFT) using the Gaussian 09, the TPA-CHO molecules, the aldehyde group presents strong
Revision-D.01 program and visualized using the Gauss view 6.0.16 electron-absorbing ability and larger p-conjugation with the
program.39 Their optimized molecular geometries and transition diphenyl group, causing the electron cloud to shift almost
energies are carried out under the calculation level of B3LYP/ entirely in favor of the aldehyde group, showing the ICT effect. This
6-31g(d). The calculated values are in good agreement with the suggests that the substituents play a critical role in the fluorescence
experimental results shown in Fig. 7. From the calculation of the properties of the molecules under the same fluorophore. Compared
electronic excitation contribution by Multiwfn,40 the charge of with TPA-ME, the fluorescence emission band of TPA-CHO is red-
TPA-CHO and TPA-ME are excited from the ground states shifted due to the strong intramolecular conjugation. It is further
(HOMOs) to the excited states (LUMOs) by suitable UV-light. From proved by the energy gap of their HOMO–LUMO listed in Fig. 7a.
their simulated electronic cloud distribution image, it shows that
According to Koopmans theorem, the HOMO–LUMO gap
their electronic clouds are uniformly distributed throughout can be approximately equivalent to a fundamental gap and
triphenylamine at the ground state. However, in the excited states, optical gap. The energy between HOMO and LUMO can be used
there are significant differences in the distribution of the electron to roughly describe some properties of the organic molecules,
cloud between them. In the TPA-CHO molecule, the electron such as the difference of the reaction activity and excitation
cloud is mainly concentrated on the phenyl-aldehyde group, while energy.41 The HOMO–LUMO gap of TPA-ME and TPA-CHO can
in the TPA-ME molecule, the electron cloud is mainly compiled in be calculated, which are 3.9118 eV and 3.1733 eV, respectively.
the biphenyl group. The main reason for this difference is due to This indicates that the mercaptal and aldehyde group directly
the aldehyde group and mercaptal group. In the TPA-ME molecule, affect the p-conjugation and polarity of the molecule. Com-
mercaptal cannot attract electrons strongly as a weak electron- pared to TPA-ME, TPA-CHO exhibits a significant reduction in
withdrawing group, while the diphenyl group has a strong the HOMO–LUMO gap, leading to a decrease in the excitation
conjugation effect, leading to the low distribution of the electron energy and red shift in the UV-Vis absorption spectrum. It
cloud on the mercaptal group in the excited state. However, in indicates that TPA-CHO has a larger dipole moment due to the
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Fig. 7 (a) HOMO, LUMO, optimized molecular geometries for TPA-ME and TPA-CHO. (b) The UV-Vis absorbance for TPA-ME and TPA-CHO in different
solvents with PBS solution (fw 70%, pH 6.8).
aldehyde group. This result is further indicated by the UV-Vis were observed under the irradiation of 365 nm UV-lamp after
absorption spectra shown in Fig. 7b of different solvent systems they were dried completely. As shown in Fig. 8, the fluorescence
containing 70% water. Due to the strong polar interaction of the blank TPA-ME test sample is blue, similar to TPA-ME in
between the aldehyde group and water, the UV-visible absorp- solution. When Hg2+ is dropped on it, the test paper shows an
tion band of the organic solvent is red-shifted. When the polar obviously yellow-green fluorescence. Meanwhile, the fluorescent
organic solvent and water form a mixed solvent, the interaction color of the test paper does not obviously change upon adding
between the solvent and aldehyde group causes the solvent Zn2+ and Cd2+, although their concentration is twice that of Hg2+.
effect of UV absorption, which leads to the decrease of the These results indicate that the TPA-ME test paper can react with
absorption intensity. Compared with the energy of the ground Hg2+ to form TPA-CHO. The test paper further shows that
state (HOMOs), TPA-ME (ꢁ5.0234 eV) is higher than TPA-CHO TPA-ME and TPA-CHO have different AIE fluorescent properties
(ꢁ5.0941 eV), which suggests that TPA-ME easily converts into due to the difference of the molecular structure. This test paper
TPA-CHO. These results further confirm that TPA-ME may be experiment shows that the probe TPA-ME has the ability to
used as a reaction-type fluorescent probe for Hg2+
.
detect Hg2+ in practice.
3.5 Test paper application
To explore the practical application of the fluorescent testing
paper, the TPA-ME test paper was prepared to detect Hg2+ in
water. The filter paper was first placed on a clean slide, and then
immersed in a storage solution of 1 ꢀ 10ꢁ3 mol Lꢁ1 TPA-ME for
5 minutes. The solvent was removed and dried in an oven. Finally,
a drop of Hg2+ (1 ꢀ 10ꢁ3 mol Lꢁ1) aqueous solution was added
to the dry TPA-ME test paper. For comparison, a drop of Zn2+
(2 ꢀ 10ꢁ3 mol Lꢁ1) and a drop of Cd2+ (2 ꢀ 10ꢁ3 mol Lꢁ1) aqueous
solution were added to other two identical test papers. Samples
Fig. 8 The photographs of the TPA-ME based test paper under sunlight
and 365 nm UV-light irradiation.
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Paper
6 R. You, P. Li, G. Jing and T. Cui, Ultrasensitive micro ion
selective sensor arrays for multiplex heavy metal ions detec-
tion, Microsyst. Technol., 2019, 25, 845–849, DOI: 10.1007/
s00542-018-4067-z.
7 T. M. de Oliveira, J. Augusto Peres, M. Lurdes Felsner and
K. Cristiane Justi, Direct determination of Pb in raw milk by
graphite furnace atomic absorption spectrometry (GF AAS)
with electrothermal atomization sampling from slurries,
Food Chem., 2017, 229, 721–725, DOI: 10.1016/j.foodchem.
2017.02.143.
8 S. Lu, R. Deng, Y. Liang, L. Xiong, X. Ai and Y. Qin, Remote
Sensing Retrieval of Total Phosphorus in the Pearl River
Channels Based on the GF-1 Remote Sensing Data, Remote
Sens., 2020, 12, 1420, DOI: 10.3390/rs12091420.
9 X. Nan, Y. Huyan, H. Li, S. Sun and Y. Xu, Reaction-based
fluorescent probes for Hg2+, Cu2+ and Fe3+/Fe2+, Coordin, Chem.
Rev., 2021, 426, 213580, DOI: 10.1016/j.ccr.2020.213580.
10 Y. Gao, T. Ma, Z. Ou, W. Cai, G. Yang, Y. Li, M. Xu and Q. Li,
Highly sensitive and selective turn-on fluorescent chemo-
sensors for Hg(2+) based on thioacetal modified pyrene,
Talanta, 2018, 178, 663–669, DOI: 10.1016/j.talanta.2017.
09.089.
11 C. Li, Q. Niu, J. Wang, T. Wei, T. Li, J. Chen, X. Qin and
Q. Yang, Bithiophene-based fluorescent sensor for highly
sensitive and ultrarapid detection of Hg(2+) in water, sea-
food, urine and live cells, Spectrochim. Acta, Part A, 2020,
233, 118208, DOI: 10.1016/j.saa.2020.118208.
12 Y. Yu, C. Liu, B. Tian, X. Cai, H. Zhu, P. Jia, Z. Li, X. Zhang,
W. Sheng and B. Zhu, A novel highly selective ratiometric
fluorescent probe with large emission shift for detecting
mercury ions in living cells and zebrafish, Dyes Pigm., 2020,
177, 108290, DOI: 10.1016/j.dyepig.2020.108290.
4. Conclusions
In summary, based on the fluorescent group from the triphe-
nylamino derivatives and the recognition group of mercaptal,
the TPA-ME molecule was first prepared and applied to detect
Hg2+ in aqueous solution successfully. Through the investiga-
tion of five kinds of organic solvents, a probe solution was
constructed with DMF as the solvent. With the increase of the
water content, the fluorescence of TPA-ME shows the charac-
teristics of the first inhibition and then increased, with a red-
shift of the luminescence band. This indicated that TPA-ME
may exhibit ACQ or AIE characterization in suitable water
content. In the DMF/H2O system with 70% water content, the
TPA-ME probe shows good selectivity, anti-interference and
linear correlation with the LOD at about 1.23 ꢀ 10ꢁ7 mol Lꢁ1
.
Through DFT and TD-DFT simulation calculation, it was
further proved that the aldehyde group and mercaptal group
have an important influence on the properties of the fluore-
scent molecules. The TPA-ME test strip provides a convenient
and qualitative way to detect mercury ions. In the future, we will
apply other groups to modify triphenylamine and prepare more
diverse functional fluorescent materials, further exploring the
fluorescent properties of the TPA-CHO probe for detecting
hydroxyl compounds and their photoluminescence mechanism
of these kinds of fluorescent molecules.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
The authors are thankful to the Natural Science Foundation of
Shanghai (No. 15ZR1428500) for financial support.
13 L. N. Neupane, J. Park, P. K. Mehta, E. T. Oh, H. J. Park and
K. H. Lee, Fast and sensitive fluorescent detection of
inorganic mercury species and methylmercury using a
fluorescent probe based on the displacement reaction of
arylboronic acid with the mercury species, Chem. Commun.,
2020, 56, 2941–2944, DOI: 10.1039/c9cc09240d.
14 A. Chatterjee, M. Banerjee, D. G. Khandare, R. U. Gawas,
S. C. Mascarenhas, A. Ganguly, R. Gupta and H. Joshi,
Aggregation-Induced Emission-Based Chemodosimeter
Approach for Selective Sensing and Imaging of Hg(II) and
Methylmercury Species, Anal. Chem., 2017, 89, 12698–12704,
DOI: 10.1021/acs.analchem.7b02663.
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