Dual–channel colorimetric fluorescent probe for determination of hydrazine and mercury ion

Article abstract of DOI:10.1016/j.saa.2021.119868

Hydrazine and mercury (Hg) poisoning represented a serious hazard to human health. So, developing method to detect and recognize them is highly desirable. Here, we prepared a multifunctional colorimetric and fluorescent probe (PI-Rh) consisting of a phena

Full text of DOI:10.1016/j.saa.2021.119868

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Dual–channel colorimetric fluorescent probe for determination of
hydrazine and mercury ion
⇑
Xiaofeng Yang , Yiming Ding, Yexin Li, Mei Yan, Yu Cui, Guoxin Sun
School of Chemistry and Chemical Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, Shandong, 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
ꢀ
A multifunctional colorimetric and
fluorescent probe PI-Rh has been
developed.
ꢀ
Discriminative detection of hydrazine
and mercury ion with differential
colorimetric and fluorescence
outputs.
ꢀ
ꢀ
High sensitivity, superior selectivity,
and low detection limit for hydrazine
and mercury ion.
Its superiority in practical
applications to sense hydrazine and
mercury ion.
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrazine and mercury (Hg) poisoning represented a serious hazard to human health. So, developing
method to detect and recognize them is highly desirable. Here, we prepared a multifunctional colorimet-
ric and fluorescent probe (PI-Rh) consisting of a phenanthroimidazole (PI) dye conjugated with a
Rhodamine (Rh) group for the effective recognition of hydrazine and Hg2+, induvidually and collectively,
with different colorimetric and fluorescence outputs. Probe PI-Rh displays low detection limits measured
Received 21 November 2020
Received in revised form 4 April 2021
Accepted 21 April 2021
Available online 24 April 2021
to be 0.0632 lM (~2 ppb) and 0.0101 l
M (~2 ppb) respectively for hydrazine and Hg2+ with high selec-
Keywords:
Chemosensor
Hydrazine
Mercury ion
Discriminative
Practicality
tivity and excellent sensitivity. Moreover, the experimental results indicated that the superiority of this
probe lied in its wide applications, for example, successful response in real water, and soil analysis.
Interestingly, an visual, rapid, and real–time detection of gaseous hydrazine can be realized with
0.2793
lM detection limit using the facile PI-Rh–impregnated test paper.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
and its aqueous solutions have great toxicity to humans and ani-
mals [6], and could cause adverse effects on liver, kidney, lung,
central nervous system, respiratory tract and other systems [7,8].
Recently, the U.S. Environmental Protection Agency (EPA) identi-
fied hydrazine as a potential carcinogen, and the threshold limit
value (TLV) for hydrazine in drinking water was recommended to
be as low as 10 ppb [9]. Therefore, it is of great significance to
develop an effective method to detect trace hydrazine hydrate.
Several traditional analytical methods for hydrazine detection
are chromatography [10–13], electrochemistry [14,15], titrimetry
[16,17], surface-enhanced Raman spectroscopy [18], and
Hydrazine (N2H4) is flammable and explosive, and is usually
served as a propellant in rocket propulsion systems and missiles
[1]. It is also a strong reducing agent, which can remove oxygen
in the thermal circulation system and achieve the effect of inhibit-
ing metal corrosion [2]. In addition, it is also an important chemical
reagent in chemical industry [3], pharmaceutical industry [4] and
pesticide industry [5]. However, research suggested that hydrazine
⇑
Corresponding authors.
E-mail address: chm_yangxf@ujn.edu.cn (X. Yang).
colorimetric-/fluorometric-analysis
[19–63].
Among
them,
https://doi.org/10.1016/j.saa.2021.119868
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
fluorescence detection has attracted much attention owing to its
high efficiency, real-time, on-line analysis and simplicity. Hydra-
zine, as a strong nucleophilic reagent, could selectively react with
some structure of functional groups, such as aromatic aldehydes
[19–21], malononitrile [22–31], ethyl cyanoacetate [32], ester
[33–50], phthalimide [51–58], trifluoroacetylacetonate [59],
conjugated-1,3-diketo [60], chalcone [61,62]. Inspired by the spe-
cial nucleophilic characteristics of hydrazine, herein, we conceived
a new compound PI-Rh based on a nonfluorescent Rhodamine (Rh)
with ethylene diamine as a linker with phenanthroimidazole (PI)
benzaldehyde (Scheme 1A). This work confirmed that hydrazine
could react with the imine bond of compound PI-Rh resulting in
yellow–green fluorescence blue-shifting cyan fluorescence. The
hydrogen bond fromed between the compound PI-Rh and hydra-
zine played a very important role in such response strategy, there-
fore, this probe should expect to work well without disruption
from other nucleophile reagents except for hydroxylamine. Its
sensing property and response mechanism were investigated in
detailed. All analysis and application results showed that not only
the aqueous solution of PI-Rh could used as an effective probe for
detetion hydrazine in pure water, real water samples and in soil,
but also PI-Rh was successfully fabricated into paper-based test
stripes for visual, rapid, sensitive, and real-time detection of gas-
eous hydrazine with a low detection limit.
Mercury (Hg) is widely used in metallurgy, instrument and
lamp manufacturing, however, its opening and smelting processes
often cause environmental pollution. Organic mercury and inor-
ganic mercury are formed after the oxidation of metal mercury,
and some of them will enter the human body through the enrich-
ment of food chain [64]. The most horrible thing is that accumula-
tion of mercury ions (Hg2+) could damage the central nervous
system in organisms and cause various cognitive, kidney failure,
brain damage, and motor disorders [65]. Therefore, it is very
important to detect the mercury concentration in clinical diagnosis
and environment quickly and efficiently. In recent years, a large
number of fluorescent probes based on ring-opening of spirocyclic
xanthene and related derivatives for detection Hg2+ have been
developed [66–80]. However, such type probes would be com-
monly interfered by other competing metal ions and no
satisfactory selectivity would be obtained. To overcome these
obstacles, some novel reaction-based sensing strategies relying
on a specific chemical reaction to Hg2+ have been developed, such
as a specific Hg2+-promoted cyclization of an aryl vinyl ether [81–
85] and the desulfurization reaction promoted by Hg2+ (including
the elimination reaction [86,87], the hydrolysis [88–93], ring open-
ing reaction [94]). Although great progresses have been achieved in
this regard, it is still significant to design new probes with high
efficiency for Hg2+ detection.
Herein, we pregnanted with a new ‘‘reactive” probe PI-Rh for
response to Hg2+ (Scheme 1B). We proposed that PI-Rh based on
the hydrolysis of an imine group was specifically promoted by
Hg2+. Additionally, the proposed hydrolysates were compound
PI-1 that possessed two different emission channels (blue and
green lights), and compound Rh-2 that possessed a red fluorescene
(Fig. 1B). Thus, the imine group of probe PI-Rh was expected to
undergo a Hg2+-promoted hydrolysis to produce the final mixture
of aldehye PI-1 and Rhodamine B derivative Rh-2, which emited
an approximate organic white fluorescence with three different
channels, blue, green and red. As expected, probe PI-Rh could be
applied to effectively detect Hg2+ in both colorimetric and ratio-
metric mode. Moreover, it could be successfully used for analyzing
Hg2+ in real water samples, and the facile PI-Rh-impregnated test
paper could be also developed as an effective method of naked-eye
detection of mercury ion.
2. Results and discussion
2.1. Design strategy of the compound PI-Rh
The synthetic routes of compound PI-Rh, the PI derivatives (PI-
0, PI-1, and PI-2), and the Rh derivatives (Rh-1 and Rh-2) were
illustrated in Scheme 2. Compounds PI-0 and PI-1 were prepared
according to our group0s previous work [95]. Compound PI-2 was
synthesized by a simple condensation of compound PI-1 and
hydrazine hydrate. Compound Rh-1 was prepared according to
previous work [75]. Compound Rh-1 had undergone the HgCl2-
promoted nucleophilic addition by H2O to generate Rh-2. Finally,
Scheme 1. Design and sensing mechanism of the new hydrazine and Hg2+ probe PI-Rh.
2

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 1. UV–vis absorption (A) and emission spectra (B) of compound PI-Rh (10 lM), PI-0 (10 lM), PI-1 (10 lM), PI-2 (10 lM), Rh-0 (10 lM), Rh-1(10 lM), and Rh-2 (10 lM)
in DMSO–PBS buffer (2:8, v/v, pH 7.4) solution, respectively. Excitation wavelength: 370 nm.
Scheme 2. Synthetic routes for compounds PI-0, PI-1, PI-2, Rh-1, Rh-2 and PI-Rh, (i) CH3COONH4, CH3COOH, 100 °C, 1 h; (ii) EtOH, reflux, 12 h; (iii) EtOH–DMF, r.t., 4 h; (iv)
DMF, reflux, 12 h; (v) HgCl2, DMSO–d6/D2O, r.t.
the target compound PI-Rh was easily synthesized by condensa-
tion of compounds PI-1 and Rh-1 in refluxed DMF solution. The
chemical structures of these compounds were confirmed with 1H
NMR, 13C NMR, and high resolution mass spectroscopy (HRMS).
Corresponding analytical characterization data were provided in
the supporting information.
the binding of the HgCl2 with compound PI-Rh could induce the
ring-opening of the spirocyclic PI-Rh, and promote the hydrolysis
of the imine group produce the corresponding mixture of aldehye
PI-1 and Rhodamine B derivative Rh-2 with an approximate
organic white fluorescenceas shown in Scheme 2.
Multifunctional fluorescent probe PI-Rh we designed was based
on the following considerations: a PI fluorophore was chosen
because its absorption and emission spectra were completely sep-
arated (Fig. 1), and its large Stokes shift could remove the endoge-
nous autofluorescence interference satisfactorily [96]. The first
reason we selected Rhodamine B as the other fluorophore was that
the hydrogen bonds formed between the spirolactam of PI-Rh and
hydrazine could activate imine unit to improve its electrophilic
ability, and favor for the next chemoslective nucleophilic addition
and the subsequent hydrolysable of the imine linkage as shown in
Scheme 1. Thus, compound PI-Rh could be used as a new ratiomet-
ric probe for fluorescence detection of hydrazine. The second rea-
son we selected Rhodamine B as the other fluorophore was that
2.2. Spectroscopic analysis
2.2.1. Photophysical characterization of PI-Rh
With these compounds in hand, we began to study their optical
properties in DMSO–PBS buffer (2:8, v/v, pH 7.4) solution and the
corresponding photophysical data were summarized in Table 1.
Compound PI-Rh had two major absorption bands centered at
323 nm (belonged to the Rh moiety), and at 374 nm (belonged
to the PI moiety) (Fig. 1A, blank line). The UV–vis absorption spec-
tral bands for the model reagent Rh-1 (Fig. 1A, wine line) and PI-1
confirmed this assignment (Fig. 1A, blue line).
Compound PI-Rh emitted yellow–green fluorescence. Its maxi-
mum emission band was at 540 nm (kex = 370 nm) (Fig. 1B, black
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X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Table 1
DMSO–PBS buffer (2:8, v/v, pH 7.4) solution at room temperature
was recorded and plotted against time (Fig. S15). The value of
Photophysical characterizations of PI–derivatives, Rh–derivatives, and PI-Rh.
Compound
kabs (nm)a
kem (nm)b
I445/I542 enhanced significantly within 1 min, and a plateau was
reached nearly in 30 min after addition of 200 M hydrazine,
l
PI-0
315
392
demonstrating that PI-Rh was a fast responsive probe to hydra-
zine. Under identical conditions, the free PI-Rh exhibited almost
no fluorescent changes. Next, the effect of pH on the fluorescence
emission pattern of PI-Rh in the presence or absence of hydrazine
in DMSO–PBS buffer (2:8, v/v, pH 7.4) solution at room tempera-
ture was evaluated using acid–base titration experiments. As
shown in Fig. S16, PI-Rh was stable within pH range from 5.1 to
10.2. When PI-Rh was incubated with hydrazine, the fluorescence
intensity at 445 nm hardly changed with the change of pH value, in
the range of 5.1–10.2. These experimental results demonstrated
that PI-Rh showed a good response to hydrazine in the pH range
of 5.1–10.2, which could be used in environment and biology.
PI-1
PI-2
Rh-0
Rh-1
Rh-2
PI-Rh
374
361
555
321
555
323, 374
450, 540
445
586
436 (very weak)
586
540
a
The major absorption band of dye.
The major emission band of dye.
b
line), which was ascribed to the PI moiety. Compared its emission
spectra with that for the model reagent PI-1 (Fig. 1B, blue line), a
similar emission band was observed. As anticipated, under the
excitation of 370 nm, the characteristic emission band of the
ring-opening of the Rhodamine could not be detected because of
its closed–spirolactam moiety in compound PI-Rh.
Next, the photostability of compound PI-Rh in DMSO–PBS buf-
fer (2:8, v/v, pH 7.4) solution was tested. Results showed that the
initial fluorescence intensity of compound PI-Rh could be main-
tained above 98% when exposed to a Xe lamp (150 W) for 1 h
(Fig. S14). These results indicated that compound PI-Rh had a good
photostability and could be used in biological research.
2.2.2.2. Proposed mechanism and confirmation of the product struc-
tures. We proposed the response mechanism of PI-Rh and hydra-
zine as depicted in Scheme 3. Firstly, the hydrogen bonds
between the spirolactam of PI-Rh and hydrazine was formed,
which could activate imine unit to improve its electrophilic ability
and favor for the next chemoslective nucleophilic addition. Next,
another hydrazine molecule attacked the activated C@N double
bond to form the intermediate II, and followed by hydrolysis of
the corresponding CAN linkage to generate intermediates III and
IV respectively. III was the complex of Rh-1 and hydrazine, and
removal of one molecule of water from IV to produce compound
PI-2. Thus, hydrazine played an important role in the fracture of
C@N double bond in PI-Rh, which was both a reactant and a
catalyst.
To clarify the above work mechanism of PI-Rh for hydrazine,
relevant experimental data of HRMS, UV–vis absorption, fluores-
cence, and NMR spectra (1H NMR and 13C NMR) of the reaction
mixture of PI-Rh with hydrazine were compared with those of
PI-Rh, Rh-1 and PI-2. As shown in Fig. S17, after mixing compound
PI-Rh and excessive hydrazine for 30 min at ambient temperature,
two peaks at m/z value of 337.1445 and 485.2916 corresponding to
the products [PI-2+H]+ and [Rh-1+H]+ were observed, and there
was no peak of [PI-Rh+2DMF+H2O+K+]+ (m/z 991.4633). This result
indicated that PI-Rh was completely converted to PI-2 and Rh-1
by reaction with excess hydrazine. Compared the UV–vis absorp-
tion and fluorescence spectra with those for the model regents
PI-2 and Rh-1 (Fig. S18), we could find that the reaction products
of PI-Rh with excess hydrazine in DMSO–PBS buffer (2:8, v/v, pH
7.4) solution at ambient temperature were PI-2 and Rh-1.
2.2.2. Sensing properties of compound PI-Rh for hydrazine
2.2.2.1. Spectroscopic response toward hydrazine. First, the spectral
response of PI-Rh toward hydrazine was investigated upon titra-
tion of PI-Rh with various concentration of hydrazine in DMSO–
PBS buffer (2:8, v/v, pH 7.4) solution (Fig. 2A). Upon titration with
hydrazine (0–20 equiv.), the two major absorption bands at
323 nm and 374 nm of PI-Rh gradually blue sifted to about
320 nm and 371 nm that belonged to the PI fluorophore, and no
absorption band above 500 nm was observed. These results indi-
cated that compound PI-Rh in the presence of excess hydrazine
presented the PI fluorophore and the closed-spirolactam form of
the Rh fluorophore.
Subsequently, we investigated the fluorescence spectra change
of PI-Rh after addition of hydrazine under the same conditions.
As shown in Fig. 2B, the free compound PI-Rh emitted yellow–
green fluorescence with a maximum emission band at 540 nm.
However, when hydrazine was gradually added to the solution of
PI-Rh, the fluorescence intensity at around 540 nm decreased.
Meanwhile, a new peak at 445 nm that belonged to the PI fluo-
rophore appeared and increased. No opened–Rhodamine–based
emission band was observed indicating that compound PI-Rh in
the presence of hydrazine presented the closed–spirolactam form
of the Rh fluorophore, which was consistent with the UV–vis
absorption observation. An obvious isosbestic point at 511 nm
indicated that the interaction between PI-Rh and hydrazine was
the only reaction process (Scheme 1). When adding 20 equiv.
hydrazine, the fluorescence intensity at 445 nm reached saturation
(Fig. 2C). At this time, an obvious emission color change could be
easily observed. This obvious distinguishable fluorescence
response enabled PI-Rh to visually and ratiometrically determine
hydrazine. Fig. 2D showed that the fluorescence intensity ratio
(I445/I542) was linear correlated with the concentrations of hydra-
Next, the work mechanism was further supported by 1H NMR
and 13C NMR spectra titration analysis in detail (Fig. 3). As illus-
trated in Fig. 3A, the proton signal (Ha) at 8.08 ppm of PI-Rh corre-
sponded to the ACH@NA group. After mixing compound PI-Rh
and excess hydrazine for 30 min, the proton signal of Ha disap-
peared completely and a new signal at 7.78 corresponding to the
ACH@NANH2 group (Hb) of PI-2 appeared. Moreover, the other
aromatic and alkyl proton signals created from the reaction of
PI-Rh and hydrazine were also corresponding to those of PI-2
and Rh-1. For the 13CNMR spectra, in Fig. 3B, after reaction with
hydrazine, the carbon signals at 161.84 ppm (Ca) corresponded to
the ACH@NA group and at 58.82 ppm (C1) adjacent to ACH@NA
group of PI-Rh disappeared completely. Meanwhile, a new carbon
signal at 161.00 ppm corresponding to the carbon signal of Cb of PI-
2 appeared. Furthermore, the characteristic signals at 167.54 ppm
(Cc) and 64.62 ppm (C2) for the closed–spirolactam form of the Rh
fluorophore did not disappear, which indicated that the closed–
spirolactam moiety in RDM was not opened before and after reac-
tion. Additionally, the other carbon signals in the product were cor-
responding to those of PI-2 and Rh-1. These above NMR data
zine over a concentration range of 0–120
lM (Y = 0.15637 + 431
06.61521 ꢁ X, R2 = 0.9946). This result indicated compound PI-
Rh was suitable for quantitative detection of hydrazine. The detec-
tion limit of PI-Rh was estimated to be 0.0632
was lower than the concentration level (10 ppb) set by U.S. EPA [9].
To evaluate the response speed of PI-Rh for hydrazine, the I445
I542 changes of PI-Rh in the presence or absence of hydrazine in
lM (~2 ppb), which
/
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 2. Absorption (A) and emission spectra (B) changes s of PI-Rh (10
lM) with adding different equivalent of hydrazine (0–20 equiv.) in DMSO–PBS buffer (2:8, v/v, pH 7.4)
solution at ambient temperature. (C) The fluorescence intensity ratio (I445/I542) versus concentration of hydrazine. (D) Plot the fluorescence intensity ratios (I445/I542) against
concentration of hydrazine. Excitation wavelength is 370 nm.
effectively validated our proposed work mechanism of probe PI-Rh
to hydrazine (Schemes 1 and 4).
absorption intensity ratio (A563/A373) was linear correlated with
the concentration of Hg2+ over a wide range of 0–100
lM (Y = 0.
01004 + 32304.0913 ꢁ X, R2 = 0.9989). This result indicated that
PI-Rh was suitable for quantitative detection of Hg2+ (Fig. 5B).
The detection limit of PI-Rh for Hg2+ was estimated to be
2.2.3. Sensing properties of compound PI-Rh for Hg2+
2.2.3.1. Spectroscopic response toward Hg2+. Next, we investigated
the capability of PI-Rh for sensing Hg2+ in DMSO–PBS buffer (2:8,
v/v, pH 7.4) solution (Fig. 4). When an increasing amount of Hg2+
was added to the solution of PI-Rh, the fluorescence intensity at
around 540 nm decreased, while, a broad emission band at about
420 nm belonged to the PI fluorophore gradually increased
(Fig. 4A). Meanwhile, a new emission band at about 585 nm
belonged to the opened–Rh fluorophore became more and more
obvious. These results indicated that compound PI-Rh in the pres-
ence of excess Hg2+ presented the PI fluorophore and the opened–
spirolactam form of the Rh fluorophore. Additionally, we were sur-
prised to find that the color of the solution emitted near white flu-
orescence. The CIE chromaticity coordinates of the reaction
mixture of PI-Rh with Hg2+ was (0.382, 0.348), which was very
close to that of pure white light (0.330 0.050, 0.330 0.050)
(Fig. 4B). As far as we know, this was the first reported probe to
sense Hg2+ aqueous solution with a white fluorescence.
0.0101
lM (~2 ppb), which was within the concentration level
(~2 ppb) set by of U.S. EPA.
Next, we examine the response time of PI-Rh with Hg2+ in
DMSO–PBS buffer (2:8, v/v, pH 7.4) solution at room temperature.
The absorption band at 563 nm promptly increased, and a plateau
was reached within minutes after the addition of Hg2+, indicating
this response was very fast (Fig. S19). Besides, we investigated
the applicable pH range of PI-Rh by UV–vis absorption spectra.
On the one hand, under a strong acidic condition (pH < 4), the free
PI-Rh molecule undergone H+–catalyzed ring-opening to generate
a characteristic absorption band at 563 nm. However, at the same
pH, the absorbance intensity at 563 nm of PI-Rh in the presence of
Hg2+ was stronger than those in the absence of Hg2+. This result
clear indicated that the spectra changes of PI-Rh with Hg2+ were
mainly due to Hg2+ ion (Fig. S20). On the other hand, at a pH range
from 4.2 to 10.2, PI-Rh was stable and it could be employed to
detect Hg2+. These experimental results demonstrated that PI-Rh
showed a good UV–vis absorption response toward Hg2+ in the
pH range of 4.2–10.2, which could be used in environment and
biology.
For the UV–vis absorption spectra, when Hg2+ was gradually
added to the solution of PI-Rh, the major band at about 373 nm
of PI-Rh gradually decreased. Meanwhile, a new band centered
at 563 nm that belonged to the opened–spirolactam form of the
Rh fluorophore appeared and increased (Fig. 5A). In addition, there
was an obvious isosbestic point at 490 nm. When 30 equiv. Hg2+
was added, the absorbance at 563 nm reached saturation, and
the color of PI-Rh solution turned from colorless to pink. The
2.2.3.2. Proposed mechanism and confirmation of the product struc-
tures. We proposed the response mechanism of PI-Rh and Hg2+ as
depicted in Scheme 4. Firstly, the binding of the Hg2+ ion with PI-
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X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Scheme 3. Probable sensing mechanism of hydrazine by PI-Rh.
Rh inducing the ring opening of the spirolactam of PI-Rh generated
the complex I, which could activate imine unit to improve its elec-
trophilic ability. Next, a water molecule attacked the activated
C@N double bond to form intermediate III and compound PI-1,
respectively. III lost one molecule HgCl2 to form Rh-2. Thus, HgCl2
played a catalyst role in the chemodosimetric hydrolysis of PI-Rh.
appeared. Moreover, the other aromatic and alkyl proton signals
created from the hydrolysis reaction were also corresponding to
those of PI-1 and Rh-1+HgCl2. For the 13C NMR spectra, in
Fig. 6B, after hydrolysis reaction, carbon signals Ca and C1 disap-
peared completely. Meanwhile,
a
new carbon signal at
192.80 ppm corresponding to the carbon signal of Cb of PI-1
appeared. Furthermore, the characteristic signals at 167.54 ppm
(Cc) and 64.62 ppm (C2) for the spirolactam form of the Rh fluo-
rophore also disappeared completely, which indicated that the
spirolactam moiety in RDM was opened after the Hg2+–induced
hydrolysis of PI-Rh. Additionally, the other carbon signals in the
product were corresponding to those of PI-1 and Rh-1+HgCl2.
These above NMR data effectively validated our proposed work
mechanism that the binding of the HgCl2 with PI-Rh induced the
ring-opening of the spirolactam of PI-Rh, and promoted the
hydrolysis of the imine linkage to generate the corresponding mix-
ture of aldehye PI-1 and Rhodamine B derivative Rh-2 (Schemes 2
and 5).
To further clarify the above work mechanism of PI-Rh for Hg2+
,
relevant experimental data of HRMS, UV–vis absorption, fluores-
cence, and NMR spectra (1H NMR and 13C NMR) of the reaction
mixture of PI-Rh with Hg2+ were compared with those of PI-Rh,
Rh-1 and PI-2. As shown in Fig. S21, after adding 30 equiv. Hg2+
to the solution of PI-Rh for about 0.8 min, we could see a mass
spectral peak at m/z 1061.2995 belonging to [PI-Rh+HgCl2]+. When
the reaction time was about 1.7 min, we could see two mass spec-
tral peaks at m/z 323.1181 and m/z 485.2919 belonging to the
+
products [PI-1+H]+ and [Rh-2+H]+. The peak of [PI-Rh+HgCl2]
(m/z 1061.2995) was not find, which demonstrated that PI-Rh
was completely converted to PI-1 and Rh-2 by reaction with
excess hydrazine within minutes. This result was compatible with
the UV–vis absorption observation (Fig. S19). Compared the UV–
vis absorption and fluorescence spectra with those for the model
regents PI-1 and Rh-2 (Fig. S22), we could find that the reaction
products of PI-Rh with HgCl2 in DMSO–PBS buffer (2:8, v/v, pH
7.4) solution at ambient temperature were PI-1 and Rh-2.
2.3. DFT calculation
Next we used density functional theory (DFT) and TDDFT meth-
ods for study of the spectral transduction of PI-Rh for identifying
hydrazine and Hg2+, respectively. We found that the different spec-
tral changes of PI-Rh in response to hydrazine and Hg2+, could be
predicted by the TDDFT theoretical calculations of the Gaussian 03
program, based on the ground–state geometries of the compounds,
respectively [97].
Next, the work mechanism was further supported by 1H NMR
and 13C NMR spectra titration analysis in detail (Fig. 6). As illus-
trated in Fig. 6A, after reaction with excess HgCl2, the proton signal
of Ha disappeared completely and
a new proton signal at
10.11 ppm corresponding to the aldehyde proton Hb of PI-1
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 3. 1H NMR (A) and 13C NMR (B) spectra of PI-Rh (3.5 mM), PI-2 (3.5 mM), Rh-1 (3.5 mM), and PI-Rh (3.5 mM) after reaction with hydrazine (20 equiv.) in DMSO–d6,
respectively.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Scheme 4. Probable sensing mechanism of Hg2+ by PI-Rh.
Fig. 4. (A) Emission spectra changes of PI-Rh (10
l
M) with adding different equivalent of Hg2+ (0–30 equiv.) in DMSO–PBS buffer (2:8, v/v, pH 7.4) solution at ambient
temperature. (B) The CIE chromaticity coordinates of the reaction mixture of PI-Rh with Hg2+. Excitation wavelength is 370 nm.
8

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 5. (A) Absorption spectra changes of PI-Rh (10
l
M) with adding different equivalent of Hg2+ (0–30 equiv.) in DMSO–PBS buffer (2:8, v/v, pH 7.4) solution at ambient
temperature. (B) The absorption intensity ratio (A563/A373) versus concentration of Hg2+. Inset: plot of the absorption intensity ratio (A563/A373) against concentration of Hg2+
.
First, the geometry optimization of PI-Rh, PI, and Rh-1 were
optimized with DFT method, respectively. Then, the UV–vis
absorption of PI-Rh, PI, and Rh-1 were calculated with the TDDFT
method on the basis of the optimized S0 state geometry, respec-
tively. As shown in Fig. 7, for compound PI-Rh, the highest occu-
pied orbital (HOMO) was distributed in the xanthenecore moiety,
while the lowest unoccupied orbital (LUMO) was distributed in
the PI moiety; while for PI-2, the isosurface was mainly localized
within the donor part (PI core) in the HOMO and moved to the
acceptor part (ACHNNH2) in the LUMO; in the case of Rh-1, the
HOMO was completely distributed in the xanthenecore moiety,
and the LUMO was completely distributed in the closed–spirolac-
tam part. The gaps between HOMO and LUMO level of PI-Rh, PI,
and Rh-1 were 3.4526 eV (396 nm), 3.6733 eV (363 nm), and
4.3344 eV (326 nm), respectively, indicating the blue–shift of max-
imum absorption of PI-Rh when reacted with hydrazine hydrate.
The same calculation method was applied for PI-Rh detecting of
Hg2+. As shown in Fig. 8, for PI-1, the isosurface was mainly local-
ized within the PI core (donor part) in the HOMO and moved to the
benzaldehyde group (acceptor part) in the LUMO, which was sim-
ilar to that of PI-2. In the case of Rh-2, the HOMO was completely
distributed in the ethylenediamine moiety, while the LUMO was
almost distributed in the xanthenecore, which was opposite to that
of Rh-2. This wide divergence in the distribution of the isosurface
indicated that the spectral properties of compounds Rh-2 and Rh-1
were quite different. The gaps between HOMO and LUMO level of
PI-1, and Rh-2 were 3.3934 eV (397 nm), and 2.6533 eV (554 nm),
respectively, indicating the red–shift of maximum absorption of
PI-Rh when reacted with HgCl2 aqueous solution.
a visible change in absorbance of PI-Rh; (iii) Addition other ana-
lytes, the UV–vis absorption of PI-Rh remained almost unchanged.
These results indicated that PI-Rh was high selective toward Hg2+
ion over other competitive analytes. For the fluorescence spectra
(Fig. 9B), (i) hydrazine and NH2–OH induced an obvious blue shift,
and hydrazine induced stronger (ca. 2.3 times compared to that
with NH2–OH) the fluorescent intensity of PI-Rh; (ii) Addition
Hg2+ could induce a decrease in fluorescent intensity of PI-Rh;
(iii) Fe3+ and Pd2+ led to an almost complete fluorescence quench-
ing; (iv) the fluorescence spectrum of PI-Rh remained almost
unchanged by addition of the other analytes. Thus, PI-Rh was high
selective toward hydrazine and Hg2+ ion over other competitive
analytes. Also, hydrazine and Hg2+ ion induced obvious fluores-
cence emission changes from yellow-green to cyan and white,
respectively, enabled naked eye detection of hydrazine and Hg2+
ion.
To validate the non–interference of other metal ions and
nucleophilic reagents, competition experiments of PI-Rh were
carried out in the presence of excess concentrations of other
commonly coexisting materials in DMSO–PBS buffer (2:8, v/v,
pH 7.4) solution (Figs. S22 and S23). These results indicated that
PI-Rh could effectively detect N2H4 and Hg2+ even if there were
excess other competitive metal ions and nucleophilic reagents in
the solution.
After establishing the non-interference of metal ions and nucle-
ophilic reagents, it was imperative to estimate the effect of N2H4 in
the detection of Hg2+, and vice versa. Addition N2H4 (30 equiv.) to
the solution of PI-Rh (10 lM), a small blue–shift of the maximum
absorption band and an obvious blue–shift of fluorescence emis-
sion were obtained (Fig. S24). When 30 equiv. of Hg2+ was added
to the solution of PI-Rh, the fluorescence intensity at around
540 nm decreased, while, two broad emission bands at about
420 nm and 585 nm gradually increased. Under comparable condi-
tions, on the one hand, addition of only 30 equiv. of Hg2+ to the
The above theoretically predicted maximum absorption bands
at 396 nm, 363 nm, 326 nm, 397 nm, and 554 nm for PI-Rh, PI-
2, Rh-1, PI-1, and Rh-2, were quite well with the experimental
observation at 374 nm, 361 nm, 321 nm, 374 nm, and 555 nm,
respectively (Table 1, Figs. 1, 2, and 5). And these theoretical calcu-
lation results further verified the experimental results.
solution that obtained from PI-Rh (10 lM) and 30 equiv. of N2H4
developed an absorption band at about 563 nm (Fig. S24A) and
enhanced the emission intensity at about 585 nm (Fig. S24B).
These results could be attributed to the Hg2+–selective inducing
the opening of Rh spirolactam form. On the other hand, upon addi-
tion of N2H4 (30 equiv.) to the solution that obtained from PI-Rh
2.4. Selectivity and interference capability
In order to investigate the sensing specificity of PI-Rh for Hg2+
ion and hydrazine, the selectivity experiments were carried out
by using UV–vis absorption and fluorescence spectra. Metal ions
(Ag+, Zn2+, Pd2+, Al3+, Ca2+, Cu2+, Cd2+, Mg2+, Co2+, Fe3+, Cr3+, Li+,
Mn2+, and Ni2+) and nucleophilic reagents (NH2–OH, NH3, Et2NH,
Et3N, nPrNH2, CN–, HS–, HSO3–, Hcy, GSH, and Cys) were investi-
gated. As shown in Fig. 9A, (i) only Hg2+ ion triggered an obvious
red shift of PI-Rh; (ii) Adding hydrazine or NH2–OH could induce
(10
l
M) and 30 equiv. of Hg2+, a new emission band centered at
about 445 nm appeared (Fig. S24B). This result could be attributed
to the N2H4–selective inducing the condensation with PI-1. Thus,
the absorption and fluorescence profiles displayed by PI-Rh to
the addition of N2H4 and Hg2+ appear to permit the selective detec-
tion of N2H4 and Hg2+ in isolation and in combination.
9

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 6. 1H NMR (A) and 13C NMR (B) spectra of PI-Rh (3.5 mM), PI-1 (3.5 mM), Rh-1 (3.5 mM) + HgCl2 (30 equiv), and PI-Rh (3.5 mM) + HgCl2 (30 equiv.) in DMSO d6:
D2O = 8:1, respectively.
10

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 7. Optimized geometries, the potential energy differences (
D
E) of HOMO–LUMO orbitals, and their corresponding predicted major absorption bands (kabs) of PI-Rh, PI-2,
and Rh-1 calculated on the DFT level using a B3LYP/6-311G, respectively.
2.5. Application
naked-eye (Fig. 10). Next, we poured the soil in the absence and
presence of hydrazine hydrate into a glass bottle with an aqueous
solution of PI-Rh (0.1 mM), respectively. After a few minutes, we
were surprised to find out that the fluorescence changes of the
solution containing hydrazine–treated soil were quite obviously
and the response speed was very quickly in all soils (Fig. 10 and
Fig. S27). However, there were no fluorescence changes for the
hydrazine–unpretreated soils, even for 1 day. These results showed
that PI-Rh could be used to detect hydrazine hydrate for environ-
mental analysis.
2.5.1. Application for detection N2H4
2.5.1.1. Performance of PI-Rh in sensing N2H4 in real water sam-
ple. Because hydrazine has been widely used in medicine, agricul-
ture and chemical industry, detection of trace amounts of
hydrazine in actual water is very necessary. Initially, the linear
plotting of the fluorescence intensity ratios (I445/I542) of PI-Rh
(10 mM) versus concentration of hydrazine (0–500 nM) in redis-
tilled water was made (Fig. S26). Next, the actual water samples
(local lake water and tap water) were centrifuged and filtered,
and followed by incubation with PI-Rh (10 mM). No obvious fluo-
rescence changes were observed. Then, the adding an aliquot of
hydrazine to the real water samples determined by PI-Rh was
detailed in Table S1. The recovery rates of hydrazine based on
the linear plotting were found to be 98.1–101.4% for all the spiked
samples, indicating that PI-Rh could reliably and effectively detect
hydrazine quantitatively in realistic water samples.
2.5.1.3. Portable test paper strip for the detection of gaseous N2H4. The
above results demonstrated that PI-Rh showed high selectivity and
responsiveness with a visualization of N2H4 fluorescence detection.
Hence, we tried to develop the feasibility of the portable PI-Rh–
based test paper for N2H4 detection. Soak an appropriate size cellu-
lose filter paper strips into a solution of PI-Rh (1 mM in DMSO) for
30 min at ambient temperature, and dried. As shown in Fig. 11A
and 11C, PI-Rh–impregnated test paper emitted an emission at
~525 nm with bright yellow–green fluorescence. When PI-Rh–im-
pregnated test paper was hung in respective cuvettes containing
an increase concentrations of N2H4 solution (0–1 mM) at ambient
temperature for 30 min, the UV color of the test paper gradually
changed from yellow–green to dark cyan and finally to blue with
an emission at ~400 nm (Fig. 11C). In addition, the response was
2.5.1.2. Detection of N2H4 in soil. To explore the environmental anal-
ysis, we try to apply PI-Rh for detection of hydrazine hydrate in
various soils (field soil, sand soil and clay soil). Firstly, 3 mL of
the aqueous solution of hydrazine (0.5 mM, 2.0 mM) was sprayed
evenly on various soils (~1g), respectively. Obviously, it was diffi-
cult to find out whether hydrazine hydrate existed in soil with
11

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 8. Optimized geometries, the potential energy differences (
D
E) of HOMO–LUMO orbitals, and their corresponding predicted major absorption bands (kabs) of PI-Rh, PI-1,
and Rh-2 calculated on the TDDFT level using a B3LYP/6–311G, respectively.
Fig. 9. Absorption (A) and emission (B) spectra changes of PI-Rh (10 lM) with adding 30 equiv. of metal ion or nucleophilic reagent in DMSO–PBS buffer (2:8, v/v, pH 7.4)
solution at room temperature. Excitation wavelength is 370 nm.
very fast and it was easy to distinguish the UV color change.
Results shown in Fig. 11B indicated that the PI-Rh–impregnated
test paper and its stimulus response paper had an excellent fluo-
rescent stability in an ambient atmosphere and temperature. The
above satisfactory results enabled PI-Rh to visually and ratiomet-
rically determine N2H4 in PI-Rh–impregnated test paper. The
results were shown in Fig. 11D, the fluorescence intensity ratio
(I445/I542) was linear correlated with the concentrations of hydra-
zine (0.001–0.5 mM) (Y
=
0.01779
+
9244.72801
ꢁ
X,
R2 = 0.99369), indicating the suitability of PI-Rh-impregnated test
paper for semiquantitative detection of N2H4. The calculated detec-
tion limit was estimated to be 0.2793 lM (8.93 ppb).
2.5.2. Application for detection Hg2+
2.5.2.1. Performance of PI-Rh in sensing Hg2+ in real water sam-
ple. The previous experimental results revealed that compound PI-
12

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 10. Photographs of 3 mL PI-Rh solution in deionized water (0.1 mM, 2% DMSO, v/v) after upon addition of various soils. (A) Photographs for field soil. Top: N2H4–
untreated, Middle: moistened with N2H4 (0.5 mM), Bottom: moistened with N2H4 (2.0 mM). (B) Photographs for sand soil. Top: N2H4–untreated, Middle: moistened with
N2H4 (0.5 mM), Bottom: moistened with N2H4 (2.0 mM). (C) Photographs for clay soil. Top: N2H4–untreated, Middle: moistened with N2H4 (0.5 mM), Bottom: moistened with
N2H4 (2.0 mM). (D) Relative fluorescence intensity of solutions as shown in panels (A, B, and C) for the soil moistened with N2H4 (0.5 mM).
Rh could ratiometrically detect Hg2+ in a wide concentration range
using UV–vis absorption well. Thus, the absorption mode was pre-
ferred for examining the practicability of PI-Rh to detect Hg2+ in
real water samples. Initially, the linear plotting of the ratio of the
absorption intensity ratio (A563/A373) versus concentration of
Hg2+ (0–200 nM) in redistilled water was made (Fig. S28). Then,
the adding amount of Hg2+ in tap water and lake water determined
by PI-Rh was detailed in Table S2. The recovery rates of Hg2+ based
on the linear plotting were found to be 96.1–99.3% and
101.2–107.4% in tap water and lake water samples, respectively,
indicating that PI-Rh could reliably and effectively detect Hg2+ in
actual water samples.
2.5.2.2. Portable test paper strip for the detection of Hg2+. The suc-
cessful detection of N2H4 encouraged us to study whether the PI-
Rh–impregnated test paper could be used to detect Hg2+. Soak
the prepared PI-Rh–impregnated test papers into different concen-
trations of Hg2+ solutions for 1 h, and dried. Naked–eye color
changes could be distinguished in a range of 0–200 mM of Hg2+
.
The visual recognition ability for Hg2+ was ca. 5 mM under ambient
13

X. Yang, Y. Ding, Y. Li et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 258 (2021) 119868
Fig. 11. (A) Visualization detection of N2H4 using the test paper supported with PI-Rh. These photos were taken under 365 nm UV light. (B) The stability of fluorescent paper
strips within 60 days, from bottom to top are blank (filter paper), test paper supported with PI-Rh (1 mM), and test paper supported with PI-Rh (1 mM) + N2H4 (0.5 mM),
respectively. (C) Fluorescence spectra of the PI-Rh–impregnated test paper and followed treated by N2H4. (D) Plot of test paper color change (I400/I525) versus concentration of
N2H4.
light, and ca. 0.5 mM under a hand–hold UV lamp, respectively
(Fig. S29). Compared with N2H4, the detection effect of the PI-
Rh–impregnated test paper for Hg2+ was a little bit worse.
could be applied to effectively detect mercury ion in both colori-
metric and ratiometric mode. Moreover, it could be successfully
used for analyzing mercury ion in real water, and the facile PI-
Rh–impregnated test paper was also well exploited for naked–
eye detection of Hg2+
.
3. Conclusions
CRediT authorship contribution statement
In conclusion, a new multifunctional phenanthroimidazole–Rho
damine conjugate probe PI-Rh has been develpoed for the detec-
tion of both hydrazine and mercury ion, induvidually and collec-
tively, with different fluorescence outputs. The sensing
mechanisms of PI-Rh for hydrazine and mercury ion were investi-
gated in detail by HRMS, UV–vis absorption spectroscopy, fluores-
cence spectroscopy, NMR (1H and 13C) titration and control
experiments, respectively. Probe PI-Rh showed good sensitivity
and fast response for hydrazine and mercury ion at ppb levels,
and precluded interference by other competitive ions and demon-
strated high selectivity in solutions with obvious fluorescence
color changes. In addition, the UV–vis absorption changes of PI-
Rh for hydrazine and mercury ion were rationalized by DFT and
TDDFT calculations. All the analysis and application results showed
that not only the aqueous solution of PI-Rh could used as an effec-
tive probe for hydrazine detetion in actual water samples and in
soil, but also PI-Rh was successfully fabricated into paper–based
test stripes for visual, rapid, and real–time detection of gaseous
hydrazine with a low detection limit. Besides, compound PI-Rh
Xiaofeng Yang: Conceptualization, Methodology, Writing -
original draft, Funding acquisition. Yiming Ding: Investigation,
Data curation. Yexin Li: Software, Funding acquisition. Mei Yan:
Validation, Funding acquisition. Yu Cui: Validation. Guoxin Sun:
Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grants 21708013, 21675064, 51473067), the Nat-
ural Science Foundation of Shandong Province of China
14

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response time of colorimetric fluorescent probe, HRMS spectrum
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analysis of the recovery of N2H4 and Hg2+ in actual samples, and PI-
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