Molecular engineering and biomedical applications of ultra-sensitive fluorescent probe for Ag+

Article abstract of DOI:10.1016/j.cclet.2021.03.076

A series of probes KJ-x (x = 1?3) with carbon chains of different lengths based on the matrix of rhodamine B were engineered to detect Ag⁺ in aqueous solution in this work. Among them, KJ-1 is selected as the best option after in vitro investigation in view of its most sensitive and rapid response to Ag⁺, whose possible sensing mechanism is studied by experimental investigation and theoretical calculation. To identify the practical application of the probe, the detection of Ag⁺ in nonantibiotic fungicide Silver&Health and differentiation between normal hepatocytes and hepatoma cells using confocal imaging was conducted.

Full text of DOI:10.1016/j.cclet.2021.03.076

G Model
CCLET-6272; No. of Pages 5
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Molecular engineering and biomedical applications of ultra-sensitive
fluorescent probe for Ag+
a,1
a,1
a
a
a
b,
Jianfei Kan , Xiaonan Zhou , Yanyan Sun , Liyuan Sun , Hao Chu , Zhaosheng Qian *,
a,
Jin Zhou *
a
School of Pharmacy, Maternal and Child Health Hospital of Weifang Medical University, Weifang Medical University, Weifang 261053, China
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of probes KJ-x (x = 1À3) with carbon chains of different lengths based on the matrix of rhodamine
Received 22 February 2021
Received in revised form 29 March 2021
Accepted 30 March 2021
Available online xxx
+
B were engineered to detect Ag in aqueous solution in this work. Among them, KJ-1 is selected as the
+
best option after in vitro investigation in view of its most sensitive and rapid response to Ag , whose
possible sensing mechanism is studied by experimental investigation and theoretical calculation. To
+
identify the practical application of the probe, the detection of Ag in nonantibiotic fungicide
Silver&Health and differentiation between normal hepatocytes and hepatoma cells using confocal
imaging was conducted.
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Fluorescent probe
+
Ag
Detection
Imaging
Theoretical calculation
Cell typing
Spirolactam
Noble metal silver (Ag) and its derivatives play a significant part
in the long history and modern civilization development of human
beings [1]. For most people, Ag is known in real life for its rarity and
attractive surface gloss appearance, which is fit for the production
of jewellery, coins and silverware [2–4]. Owing to its germicidal
characteristic of inactivating microorganisms such as gram-
positive bacteria, gram-negative bacteria and fungi, Ag ions are
also used as antibacterial drug for dental, vaginal or nasal
sterilization and disinfection [5–7]. In recent years, Ag is more
and more widely used in biomedicine, drug delivery systems and
biosensors [8–11]. On the other hand, its abuse especially
industrial waste from the electronic and photographic imaging
industry would cause a potential threat to human health because it
could inactivate sulfhydryl enzymes and combine with imidazole,
amine and carboxyl groups of a wide range of metabolites, and
sediments induced damage on skin, kidney and cornea [16–19].
Thus, the development of a rapid, selective and sensitive method
+
for the detection of Ag has drawn intense interest, which would
further reveal its physiological function and is of considerable
importance for human health.
Conventional methods, such as atomic absorption or emission
spectroscopy, inductively coupled plasma mass spectrometry and
anodic stripping voltammetry, can be used to monitor trace levels
of silver ions, but they require complex and time-consuming
pretreatment and expensive apparatus [20–22]. Fluorescent probe
is a simple, fast, highly sensitive and low cost method, which has
real-time imaging ability to track and detect ions in biological
systems in real time [23–31]. Many quantum dots based
fluorescent probes have been developed for detection of silver
ions in water, food or in vivo [32–37], but most of the probes work
with relatively long response times and turn-on response type, and
are susceptible to interference from other heavy metal ions such as
2
+
2+
replace Ca and Zn in hydroxyapatite which is an essential part
of bone [12–15]. As a result, the excessive Ag would lead to the
occurrence of various diseases such as growth retardation, anemia,
heart enlargement, neurodegenerative diseases, and insoluble
2
+
2+
3+
Hg , Cu or Fe , which are not fit for the selective, sensitive and
rapid sensing. The development of fluorescent probes based on
small molecules may provide the new option to solve the above
issues.
To this end, a series of probes KJ-x (x = 1À3) (Scheme 1) with
*
Corresponding authors.
E-mail addresses: qianzhaosheng@zjnu.cn (Z. Qian), zhoujin@wfmc.edu.cn
carbon chains of different lengths on the matrix of rhodamine B
+
were designed and synthesized to detect Ag in aqueous solution in
(
J. Zhou).
1
this work. Among them, KJ-1 is selected as the best option after in
These authors contributed to this work equally.
https://doi.org/10.1016/j.cclet.2021.03.076
001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
1
Please cite this article as: J. Kan, X. Zhou, Y. Sun et al., Molecular engineering and biomedical applications of ultra-sensitive fluorescent probe
+
for Ag , Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2021.03.076

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J. Kan, X. Zhou, Y. Sun et al.
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Scheme 1. Synthesis of KJ-x.
vitro investigation in view of its most sensitive and rapid response
to Ag , whose possible sensing mechanism is studied by
amide ring. We tested the fluorescence response of the probes
+
+
towards 0,10 and 30
mmol/L Ag as time went by. The performance
experimental investigation and theoretical calculation. To identify
the practical application of the probe, the detection of Ag in
is shown in Fig. S10 (Supporting information), where it is noted
that the reaction between 2 carbons contained KJ-1 and Ag+ of
different concentrations could reach equilibrium almost instan-
taneously and the probe itself kept steady under the 60 min
detection. While it takes much more time for KJ-2 and KJ-3 to
achieve the reaction balance under the same test conditions. In
addition, as the increase of carbon chain length, the obtained
fluorescence after reaction becomes weaker and weaker, and the
two probes themselves would fluoresce as time flies, which
indicates their poor stability under physiological conditions. Based
on the above results, KJ-1 was selected as the best candidate in the
next test.
+
nonantibiotic fungicide Silver&Health (an active silver ion
antibacterial solution, used for the treatment of acute rhinitis,
sinusitis, acute pharyngitis and oral ulcer) and differentiation
between normal hepatocytes and hepatoma cells using confocal
imaging were conducted.
As shown in Scheme 1, to prepare the desired compounds,
rhodamine B acyl chloride intermediate KJ-0 was synthesized first
and then reacted with bromoalkylamine of different carbon chain
1
lengths. The resulting probe structure was well characterized by H
13
and C NMR and high-resolution mass spectra (Figs. S1–S9 in
Supporting information), and the formation of probes was
demonstrated.
The study of pH and temperature on the fluorescence intensity
+
of probe KJ-1 in presence of Ag suggests that the probe could work
+
The response of KJ-1 towards Ag in aqueous solution (pH 7.4)
efficiently under the physiological conditions (Figs. S11A and B in
was tested first after 30 min reaction. As shown in Fig. 1A, KJ-1
Supporting information). Under the optimal conditions, the
itself has almost no absorption (less than 0.003) around 555 nm.
fluorescence spectra intensity of probe KJ-1 increased gradually
+
+
Upon the addition of Ag (20
m
mol/L), the maximum absorption
with the addition of Ag from 0 to 50
m
mol/L (Fig. 1C, Fig. S11C in
peak changes a little at wavelength (to 558 nm) while increases
nearly 20 fold at intensity. Consequently, Ag caused an obvious
Supporting information). The detection limit was calculated to be
as low as 2.1 nmol/L based on 3 /S (where is the standard
+
$
$
color change from almost colorless and transparent to visible pink
under the daylight. As shown by the corresponding fluorescence
spectra in Fig. 1B, the fluorescence signal increases sharply with a
deviation of the only probe contained solution and S is the slope of
the sensitive calibration curve ranging from 6 nmol/L to 100 nmol/
L). In order to examine the selectivity of KJ-1, other heavy metal
2
+
3+
2+
2+
2+
2+
2+
3+
25 fold, which is visually reflected by the right red fluorescence
ions (Hg , Bi , Cu , Pb , Cd , Co , Ni , Cr , 5.0 equiv.) and
2
+
2+
2+
photograph in the inset. Encouraged by the preliminary experi-
mental exploration, we tried to explore the structure-activity
relationship of silver ion reaction with a series of homologous
probes (KJ-1, KJ-2 and KJ-3) in which there are 2, 3 and 4 saturated
carbons between halogen atom bromine and nitrogen on spiro
biologically representative relevant substances (Zn , Mg , Ca ,
ꢀ
–
2
Hcy, Cys, Ala, Leu, Asp, Glu, Arg, Ser, GSH, O , OH) were gauged
(Fig. 1D). The results demonstrate that none of them led to a
+
significant fluorescence enhancement except for Ag , suggesting
its excellent selectivity.
As for the response mechanism, we proposed that probe KJ-1
would be converted to be rhodamine derivative with oxazoline
+
structure after the binding of Ag to the bromine, which promotes
the spirolactam ring opening and the production of oxazoline Rh-
O, which was verified by the high resolution electrospray
ionization mass spectra (HRMS, Fig. S12 in Supporting informa-
tion). We also studied the spectroscopic difference with rhodamine
+
B (RD-B). The maximum absorption peak of Ag added KJ-1
solution is very near to that of the RD-B solution (Fig. S13A in
Supporting information). However, there is a noticeable fluores-
cence emission difference between them. The emission peak
locates at 575 nm, where there is a 5 nm decrease compared to that
of the product Rh-O after probe KJ-1 reaction, further suggesting
that the product is not RD-B (Fig. S13B in Supporting information).
To further understand the reaction mechanism and the difference
of three probes in response to silver ion, DFT calculations were
performed. As shown in Fig. 2A, it is proposed that two elementary
reactions are involved in the response mechanism. In the first
2
elementary reaction, a SN substitution reaction takes place first to
form the cyclized structure and the bromine atom is transformed
to the negative bromide ion, which forms a transient intermediate
with no charge. Then, the silver ion binds with negative bromide
ion to produce AgBr precipitate in the secondary reaction, and such
a precipitation greatly promotes the whole forward reaction. The
energy differences between reactant, intermediate and product for
Fig. 1. (A) Absorption spectra of KJ-1 (10
m
mol/L) before (a) and after (b) reaction
mol/L); the inset shows the solution color changes under visible
color. (B) Corresponding fluorescence spectra ( ex = 558 nm) of (A); the inset herein
illustrate the color changes under 365 nm light. (C) Fluorescence spectra of KJ-1
+
with Ag (20
m
l
+
(
10
of KJ-1 (10
mean Æ standard deviation of three separate measurements.
m
mol/L) reacting with Ag at varied concentrations. (D) Fluorescence responses
mol/L) to various substances (5 equiv.). The results are the
ex/em = 558/580 nm.
m
l
2

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J. Kan, X. Zhou, Y. Sun et al.
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Fig. 2. (A) Response mechanism. Energy profiles of the proposed reaction route (B) and optimized structures (C).
KJ-1, KJ-2 and KJ-3 were calculated with DFT calculations. Fig. 2B
shows that the all the energy gaps are positive for the first step,
suggesting these reactions are unflavored thermodynamically. All
calculated values for the second step of three probes are negative
and no much difference exists among them, indicating that the
second reaction is much favored thermodynamically and the
precipitation reaction plays a key role to promote the whole
reaction. But the energy gap for KJ-1 is the lowest in the first step
among these three probes, clearly indicating that the reaction
starting from KJ-1 is more favorable than that of the others because
the first step is the rate-determining step, which sufficiently
supports the experimental observations that all three probes can
viability of KJ-1 was more than 82% when its concentration is as
high as 50 mmol/L, as shown in Fig. S14 (Supporting information),
suggesting its low cytotoxicity and good biocompatibility. It is
À
widely known that the Cl at physiological concentration (mmol/L
+
level) is prone to cause the precipitation of Ag , which might
compete with probe reaction. To get rid of this concern, we further
carried out an in vitro experiment to study the influence of added
À
anion Cl with considerable concentration (mmol/L level) at pH 7.4
À
PBS solution. As shown in Fig. S15 (Supporting information), Cl
with high physiological concentration (from 1 mmol/L to 5 mmol/
L) causes slight fluorescence decline, which ensures that the probe
can still work effectively in the cells. Subsequently, the laser
confocal fluorescent imaging of probe KJ-1 was carried out. The
time dependence for probe in fluorescence imaging showed that
the probe could enter the cells efficiently and quickly and the probe
incubation time of 20 min could achieve the reaction equilibrium
+
respond to Ag but only KJ-1 has the best response to silver ion. The
optimized structures of reactants and products for KJ-1, KJ-2 and
KJ-3 are shown in Fig. 2C, where it is clearly observed that a
cyclization reaction occurs accompanying the change of bromine
to bromide ion during the substitution reaction. These conclusions
are also consistent with common knowledge that the formation of
five-member ring is most favorable thermodynamically, and well
supports our experimental observations.
+
time when the concentration of Ag is a constant (Fig. S16 in
Supporting information). In the actual test process, in order to
ensure full response, 30 min is adopted. Then the time dependence
+
for Ag is studied. As depicted in Fig. 3A, almost no fluorescence
The accuracy of the detection results directly affects the future
application potential of the probe. Next, Ag was determined in
could be observed in BEL-7402 cells only treated with probe KJ-1.
After BEL-7402 cells had been pretreated with Ag , the noticeable
+
+
commercial nonantibiotic fungicide Silver&Health to explore the
practical application of this method using a standard addition
method. The estimated recoveries (between 94% and 106%) and the
relative standard derivation (RSD, less than 2%, n = 3) are listed in
fluorescence signals could be observed and the signal intensity
+
increased gradually when the concentration of added Ag grew
+
from 1
m
mol/L to 20
m
mol/L. To further confirm the Ag induced
fluorescence response to KJ-1, a classical chelating ligand ethylene
+
Table 1 by adding Ag of different concentrations into spiked
diamine tetraacetic acid (EDTA) was added into the culture
+
Silver&Health solution. Ideal recoveries and low RSD were
obtained for spiking experiments, which indicate that the
medium prior to the before the two reactants KJ-1 and Ag came
into contact. As shown in Fig. 3A-f, EDTA showed an effective
+
+
developed KJ-1 can be employed reliably to detect Ag with great
masking effect for Ag . This phenomenon is consistent with the
potential.
changing trend reflected by flow cytometry which is a high-
throughput detection method (Fig. 3B). Then we carried out the
practical application in Silver&Health imaging in Fig. S17 (Support-
+
In view of the above excellent performance for sensing Ag , we
next further delved into the application of KJ-1 in biological
system. The performance of cytotoxicity test in living BEL-7402
cells (human hepatoma cells) via MTT assay showed that the cell
+
ing information). The drug Silver&Health rich in Ag could also
cause an impressive fluorescence enhancement no matter in the
confocal imaging or flow cytometry.
+
Now that KJ-1 is able to sense the Ag fluctuation induced either
Table 1
+
+
by a standard Ag direct addition or Ag contained drug in cancer
cell BEL-7402 in the above study, it would be of great interest to tell
the difference between the homologous cancer cells and normal
cells from the same species and reveal the potential mechanism
behind it. We might as well choose two cells from human liver,
cancer cells (BEL-7402) and normal cells (L-02), as the experimen-
tal subjects to observe the fluorescence changes under the same
+
5
Detection of Ag in Silver&Health (1 Â10 -fold diluted).
Ag+ Added (nmol/L)
Ag+ Found (nmol/L)
Recovery (%)
RSD (%)
0
18.6
–
–
1
0
28.01 Æ 0.19
50.28 Æ 0.52
69.32 Æ 0.39
94.24
105.64
101.47
1.94
1.72
0.78
3
5
0
0
3

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Chinese Chemical Letters xxx (xxxx) xxx–xxx
Fig. 3. (A) Laser confocal fluorescence microscopic images of BEL-7402 cells under different conditons. (a) Only KJ-1 (10
m
mol/L) 30 min treatment; (b-e) Pretreatment with
+
1
m
mol/L, 5
m
mol/L,10
L Ag first for 30 min, then 20
bar: 30 m; ex = 560 nm,
m
mol/L and 20
mmol/L Ag respectively for 30 min, then cultured with KJ-1 (10
mmol/L) contained medium for 30 min; (f) Pretreatment with 20
mmol/
+
m
mol/L EDTA was added prior to the KJ-1 (10
m
mol/L) culture for 30 min. The directly below the dark field are corresponding DIC images; scale
m
l
l
em = 570–650 nm. (B) Flow cytometric analysis of above respective cells.
+
Fig. 4. Comparation of confocal fluorescence microscopic images between BEL-7402 cells and L-02 cells under different treatment time of Ag . (A) BEL-7402 cells were treated
ꢁ
with medium containing probe (10
m
mol/L) for 30 min, then AgNO
3
solution (10
m
mol/L) at 37 C for 5 min, 10 min, 20 min, and 30 min repectively before imaging. (C) L-02
cells were treated as those in (A). The directly below the dark field are corresponding DIC images; scale bar: 30
corresponding flow cytometric analysis data of (A) and (C) respectively.
mm; lex =560 nm, lem = 570–650 nm. (B) and (D) are
experimental conditions. Both cells were divided into 5 groups and
the difference between the difference between the homologous
cancer cells and normal cells.
+
incubated with 10
0 min respectively before the KJ-1 incubation. From Figs. 4A and
C, we can intuitively see that in the initial stage of reaction time
5 min), the fluorescence signal for BEL-7402 cells is much weaker
mmol/L Ag for 0 (control group), 5, 10, 20 and
3
In conclusion, a rapid fluorescent probe, KJ-1, was constructed
+
to selectively and sensitively detect Ag in nonantibiotic fungicide
(
in a turn-on type fluorescent response. The possible response
than that for L-02 cells. In the meanwhile, compared to the L-02
cells, even in the same kind of cells BEL-7402, the initial signal
intensity is much lower than that when the reaction reaches
equilibrium. Moreover, it is noted that the intensity of BEL-7402
cells is inferior to the signal from the L-02 cells when both kinds of
cells are in the state of reaction equilibrium. The flow cytometry
analysis was performed to further evaluate the difference of these
changes, as shown in Figs. 4B and D, which solidly support the
trend of fluorescence intensity in terms of more quantity.
Considering the differences in physiology and pathology for the
two kinds of cells, the greater signal intensity may be due to the
richer intracellular mercapto proteins in the cancer cells, which is
mechanism based on tandem spiro amide ring opening and the
formation of new ring triggered by Ag to the bromine of the probe
+
is studied by experimental investigation and theoretical calcula-
tion. Also, this probe was successfully applied to distinguish
homologous cancer cells and normal cells. The present protocol
supplies a new pathway for the test of silver related consumer
product and disease diagnosis.
Declaration of competing interest
The authors report no declarations of interest.
Acknowledgments
+
prone to bond the added Ag and causes the probe produce the
weak fluorescence. We infer that this effect is more evident in the
beginning in the cancer cells because the limited silver ions are
effectively adsorbed by sulfhydryl proteins. As the incubation time
is extended, more Ag would enter the cancer cells which
overflows the masking ability of intracellular mercapto proteins.
Therefore, these results suggest that probe KJ-1 is able to indicate
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21705120), the Project of
Shandong Province Higher Educational Outstanding Youth Inno-
vation Team (No. 2019KJM008), the Natural Science Foundation of
Shandong Province, China (No. ZR2017LB016), the Project of
+
4

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Chinese Chemical Letters xxx (xxxx) xxx–xxx
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