FRET-based ratiometric fluorescent detection of arginine in mitochondrion with a hybrid nanoprobe

Article abstract of DOI:10.1016/j.cclet.2019.07.047

A ratiometric fluorescent hybrid nanoprobe CDs-1 for arginine (Arg), exhibiting high sensitivity (the limit of detection, LOD, being 6.5 × 10^?8 mol/L) and excellent selectivity and anti-interference ability, was fabricated through fluorescence resonance energy transfer (FRET) and the electrostatic attraction between positively-charged hemicyanine molecules and negatively-charged carbon dots (CDs). Arg can be quantitatively detected in the concentration range from 6.0 × 10^?5 mol/L to 2.7 × 10^?4 mol/L. Further, due to its ability to target mitochondrion and low cytotoxicity, intracellular Arg was successfully tracked through ratiometric fluorescence imaging.

Full text of DOI:10.1016/j.cclet.2019.07.047

Accepted Manuscript
Title: FRET-based ratiometric fluorescent detection of
arginine in mitochondrion with a hybrid nanoprobe
Authors: Yueyue Li, Yanan Ban, Ruihui Wang, Zheng Wang,
Zhanxian Li, Chenjie Fang, Mingming Yu
PII:
DOI:
Reference:
S1001-8417(19)30451-6
https://doi.org/10.1016/j.cclet.2019.07.047
CCLET 5130
To appear in:
Chinese Chemical Letters
Accepted date:
23 July 2019
Please cite this article as: Li Y, Ban Y, Wang R, Wang Z, Li Z, Fang C, Yu M, FRET-based
ratiometric fluorescent detection of arginine in mitochondrion with a hybrid nanoprobe,
Chinese Chemical Letters (2019), https://doi.org/10.1016/j.cclet.2019.07.047
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Communication
FRET-based ratiometric fluorescent detection of arginine in mitochondrion with a
hybrid nanoprobe
Yueyue Lia, Yanan Bana, Ruihui Wangb, Zheng Wangb, Zhanxian Lia,*, Chenjie Fangb, Mingming Yua,*
a College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
b School of Pharmaceutical Sciences, Capital Medical University, Beijing 100069, China
* Corresponding authors.
E-mail addresses: lizx@zzu.edu.cn (Z. Li), yumm@zzu.edu.cn (M. Yu).
ARTICLE INFO
Article history:
Received
Received in revised form
Accepted
Available online
Graphical Abstract
As an important amino acid, Arg participates in the synthesis of many materials such as agmatine, cretine, and has a significant part in
many life activities, such as cell division and wound healing. Here, a ratiometric fluorescent hybrid nanoprobe CDs-1 for arginine (Arg)
was prepared by the supramolecular assembly between hemicyanine molecules and carbon dots (CDs) to modulate fluorescence
resonance energy transfer (FRET) process, exhibits high sensitivity and excellent selectivity and anti-interference ability. Further, due to
its ability to target mitochondrion and low cytotoxicity, intracellular Arg was successfully tracked through ratiometric fluorescence
imaging.
ABSTRACT
A ratiometric fluorescent hybrid nanoprobe CDs-1 for arginine (Arg), exhibiting high sensitivity (the limit of detection, LOD, being 6.5 × 10−8
mol/L) and excellent selectivity and anti-interference ability, was fabricated through fluorescence resonance energy transfer (FRET) and the
electrostatic attraction between positively-charged hemicyanine molecules and negatively-charged carbon dots (CDs). Arg can be quantitatively
detected in the concentration range from 6.0 × 10−5 mol/L to 2.7 × 10−4 mol/L. Further, due to its ability to target mitochondrion and low cytotoxicity,
intracellular Arg was successfully tracked through ratiometric fluorescence imaging.
Keywords:
FRET
Ratiometric
Arg
Mitochondrion
Nanoprobe
Cell imaging
As an important amino acid, Arg participates in the synthesis of many materials such as agmatine, cretine, and has a significant part
in many life activities, such as cell division and wound healing [1,2]. Consequently, high-sensitivity, high-selectivity detection of Arg
has long attracted the attention of scientists. Selective detection of a specific amino acid without interference from other amino acids,
however, remains a great challenge, and only a few fluorescent probes are available for Arg detection [3–7].
———

Nevertheless, due to their many advantages such as simplicity, high sensitivity, and capacity for in vivo, in situ, real-time
monitoring, fluorescent probes have become a comprehensive and sensitive screening tool for medical, environmental, and biological
analyses [8–16]. As a result, numerous fluorescent probes have been developed for a variety of species including metal ions [17–22],
anions [23–25], pH [26,27], environmental pollutants [28,29], and biological molecules, such as glutathione (GSH), cysteine (Cys),
and carboxylesterase [30–34].
As it is well known, ratiometric fluorescence detection can eliminate restrictions from various analyte-independent factors and
therefore provide superior precision and sensitivity than that with off-on or on-off fluorescent probes. Some methods have been used
for the development of ratiometric fluorescent probes, and the synthesized probes have been reported to detect the substrates in various
human diseases; however, only two examples of ratiometric fluorescent probes for Arg have been reported [35,36].
Due to their small size, good photostability, and λex-dependent photoluminescence (PL) emission, carbon nanodots (CDs) as
fluorescent probes may potentially exhibit good properties [37–39]. Although there have been several studies on the application of CDs
as fluorescent probes/sensors in bioimaging [40–49], there have been no reports of CDs fluorescent probes for Arg.
Previously, we synthesized a hemicyanine-based lysosome-targeted near-infrared (NIR) ratiometric fluorescent probe, exhibiting
high sensitivity, excellent selectivity and anti-interference against various species [35]. From this work, a mitochondrion-targeted Arg-
sensitive fluorescent compound 1 was synthesized (Scheme S1 in Supporting information) [50]. Amino- and carboxyl-coated CDs
were prepared by utilizing citric acid and ethylenediamine [51,52]. Through the electrostatic attraction between compound 1 and CDs,
a FRET-based ratiometric fluorescent probe CDs-1 was constructed toward Arg. Further studies indicated that CDs-1 has low
cytotoxicity and can recognize intracellular Arg through ratiometric fluorescence imaging.
Scheme 1. Schematic illustration of the FRET-based ratiometric nanoprobe for Arg synthesized from hemicyanine dye-modified CDs.
Compound 1 was prepared via four steps starting from 8-hydroxy-2-methylquinoline. Compound 4 (yield, 47.3%) was obtained by
reacting 8-hydroxy-2-methylquinoline and acryloyl chloride for 12 h at room temperature. Product 3 (yield, 71.6%) was given by
oxidizing compound 4 with SeO2 at 101 oC. Compound 2 (yield, 52.6%) was given from the reaction of 1,1,2-
trimethylbenzo[e]indolenine and CH3I. Compound 1 was obtained in a yield of 45.1% by the Knoevenagel condensation reaction of
compound 3 with compound 2.
Compounds 2 (0.7025 g, 1.0 mmol) and 3 (0.5542 g, 1.0 mmol), and 15 mL ethanol were mixed and refluxed at 78 oC for 12 h. After
cooled to 20 oC, the product 1 was obtained as brown solid (0.4854 g, 43.3%) after filtration.
Characterization of 1: HRMS (EI) m/z: Calculated for C29H25N2O2 [M-I], 433.1918; found, 433.1839. 1H NMR (400 MHz, DMSO):
δ 8.74 (d, 1H), 8.62 (d, 1H), 8.52 (d,1H), 8.36 (t, 3H), 8.32 (d, 1H), 8.28 (d, 1H), 8.15 (d, 1H), 8.11 (m, 1H), 7.87 (t, 1H),7.80 (m, 3H),
6.78 (d, 2H), 6.37 (t, 1H), 4.26 (s, 3H), and 2.05 (s, 6H). 13C NMR (100 MHz, DMSO): δ 164.85, 152.04, 148.59, 147.77, 138.48,
131.62, 130.54, 129.09, 128.17, 128.14, 126.62, 125.25, 123.97, 123.06, 117.11, 114.01, 54.73, 36.00, and 25.09.
The fluorescent CDs were prepared using a hydrothermal method [53–58]. Fig. S1 (Supporting information) presents the TEM
image of the CDs. The diameter of the CDs was about 4.19 ± 0.25 nm (Fig. S2 in Supporting information). XPS was employed to
examine the surface states and compositions of CDs [59,60]. As can be seen in Fig. S3 (Supporting information), the representative
XPS peaks at 285.76, 398.60, and 528.03 eV correspond to the elements of C 1s, N 1s, and O 1s, respectively. For the high-resolution
C 1s region (Fig. S4 in Supporting information), there are five peaks observed at 284.6, 285.0, 286.7, 288.0 and 289.0 eV, which are
attributed to the C-C, C-N, C-O, C=O and O-C=O groups, respectively. And three peaks are emerged at 399.0, 400.1 and 400.9 eV
(Fig. S5 in Supporting information), which are attributed to the N-H, C-N and graphite N groups, respectively. The O 1s spectrum
shows two peaks at 531.3 and 532.9 eV, which are assigned to the C=O and C-OH/C-O-C groups, respectively (Fig. S6 in Supporting
information). By comparing the FTIR spectrum of the CDs, organic molecule 1, and CDs-1 (Fig. S7 in Supporting information), it is
obvious that the CDs-1 was successfully assembled. The zeta potentials of the CDs and the hybrid nanoprobe CDs-1 were also
measured (Fig. S8 in Supporting information). The increase in the zeta potential of the CDs from −1.57 mV to 0.35 mV can be ascribed
to the decreasing surface electricity of the CDs after its assembly with compound 1.
Compound 1 was synthesized according to Scheme S1. 1H NMR, 13C NMR and HR-MS confirmed the molecular structure of
compound 1. The CDs were prepared using a hydrothermal method, and the nanoprobe CDs-1 was obtained by interaction between the
CDs and compound 1. The CDs were characterized by TEM, diameter distribution, XPS, FTIR, and zeta potential measurements.

UV-vis and fluorescence experiments (Figs. S9 and S10 in Supporting information) were conducted in a solution of 1 with 0.1 mol/L
Arg (Vwater/VDMSO = 100/1, 3.0 × 10−5 mol/L). The absorption peaks at 283, 375 and 436 nm in the UV-vis spectrum decreased upon
addition of 3.0 × 10−4 mol/L Arg (Fig. S9). Fig. S10 demonstrates the emission spectral change of compound 1 before and after
addition of Arg. With excitation at 360 nm and addition of Arg, the emission intensity at 607 nm disappeared, which may be due to the
reaction between compound 1 and Arg.
As illustrated in Fig. S11 (Supporting information), an emission peak at 440 nm appeared in the fluorescence spectrum of 0.67
μg/mL CDs water solution. After addition of 3.0 × 10−4 mol/L Arg, no obvious emission spectral changes were observed in the
fluorescence spectrum of CDs; indicating Arg does not induce fluorescence changes in CDs.
Changes in the absorption and fluorescence spectra were monitored prior to the preparation of hybrid nanoprobes to investigate the
interaction between CDs and compound 1 in the hybrid system. As shown in Fig. S12 (Supporting information), there was an overlap
between the absorption spectrum of compound 1 (black line in Fig. S12) and the PL spectrum of CDs (green line), which highly
suggests that energy transfer from CDs to compound 1 may occur. However, after the addition of Arg, no energy transfer took place
because no absorption was found in the absorption spectrum of compound 1 (red line in Fig. S12). Along with Fig. S10, a 360 nm laser
can be used to excite the CDs in the hybrid probe. With excitation at 360 nm, compound 1 can absorb the emission of CDs, and a very
strong emission peak was exhibited by the nanohybrid system at 607 nm, whereas a very weak emission peak was exhibited at 440 nm.
After addition of Arg, no emission at 607 nm was observed and a strong emission at 440 nm was detected. Therefore, a mixture
solution containing CDs (0.67 μg/mL) and compound 1 (3.0 × 10−5 mol/L) was prepared.
Fig. 1. (a) Emission spectral change of nanoprobe CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L compound 1) in water-DMSO solution (100:1, v/v) upon
titration of Arg water solution (0–10.0 equiv. to 1) under 360 nm excitation. Reaction time was 12 min. (b) Photographs of hybrid nanoprobe CDs-1 (0.67
μg/mL CDs and 3.0 × 10−5 mol/L compound 1) in water-DMSO solution (100:1, v/v) with different concentrations of Arg (from left to right: 0, 3, 6, 9, 12, 15,
18, 21, 24, 27 (× 10−5 mol/L)) upon 365 nm excitation. (c) Arg titration curve plotted by the ratiometric emission spectra of CDs-1 as linear function of Arg
concentration. (d) Fluorescence change of CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L compound 1) in water-DMSO solution (100:1, v/v) before and after the
addition of Arg with varying pH.
In the following experiments, a water-DMSO solution (100:1, v/v) of nanoprobe CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L
compound 1) and a 0.1 mol/L water solution of Arg were used. The absorption peaks at 374 and 434 nm in the UV-vis spectrum
gradually decreased upon addition of Arg (Fig. S13 in Supporting information). Three clear isosbestic points were observed at 277,
304, and 335 nm, indicating that only one product was generated from the reaction of nanoprobe CDs-1 with Arg. As evidenced by
Fig. S14 (Supporting information), the solution color, originally yellow, became colorless in the presence of different Arg
concentrations. Upon 360 nm excitation and gradual addition of Arg, the characteristic emission band of cyanine centered at 607 nm
decreased with the emission intensity at 440 nm increasing significantly (Fig. 1a). As demonstrated in Fig. 1b, the fluorescence color of
nanoprobe CDs solution changed from orange to blue, which is consistent with the fluorescence spectral change. Fig. 1c suggests a
good linear relationship between the ratio of I440/I607 to Arg concentration, ranging from 6.0 × 10−5 mol/L to 2.7 × 10−4 mol/L; implying
Arg can be quantitatively detected in a wide concentration range via ratiometric fluorescence method. From the linear calibration graph
of the fluorescence titration experiment (Fig. 1c) and based on the signal-to-noise ratio (S/N) = 3 [32], the detection limit of nanoprobe
CDs-1 for Arg was determined to be about 6.5× 10−8 mol/L, which was sufficiently low for Arg detection. Therefore, it can be
concluded that an effective ratiometric fluorescent Arg-probe CDs-1 was made herein with a minimum LOD comparable to those
obtained from other methods (Table S1 in Supporting information).
Although the influence of pH on Arg detection has hardly been reported previously, the pH of the solution was determined to be
critical in the reaction between CDs-1 and Arg. Here, the fluorescence of CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L compound 1)
in water-DMSO solution (100:1, v/v) in the absence and presence of 3.0 × 10−4 mol/L Arg was tested in the pH range of 5.0−11.0.
From Fig. 1d, we can see the CDs-1 itself was stable and its emission spectrum remained unchanged in the pH range of 5.0−11.0.
However, a great fluorescence change was found in the pH range of 5.0–11.0 upon addition of Arg. The fluorescence change of CDs-1
to Arg peaked at about 6.0 and became almost constant in the pH range of 6.0–11.0. Such results imply that Arg can be detected by
CDs-1 in a broad pH range.

Mass spectrometry was used to study the recognition mechanism. Since CDs cannot react with Arg, compound 1 was selected to
react with Arg. A peak at 433.1839 corresponding to the species [1-I] was found for compound 1 only. After reaction with Arg, the
peak at 433.1839 disappeared and a new peak at 224.1438 was detected, which indicated a new compound was formed (Scheme S1).
The selectivity of CDs-1 toward Arg was evaluated by testing various species, including nucleophilic biological species (Thr, Val,
Ser, Phe, His, Gly, Pro, IIe, Trp, Ala, Tyr, Glu, Glc, Leu, and H2O2), metal ions (Na+, K+, Mg2+, Ca2+, Mn2+, Ni2+, Cd2+,Cu2+, Pb2+, and
−
−
Ag+), and anions (Cl−, F−, Br−, I−, N3 , SO42−, ClO3 , S2O32−, SO32−, HS−, CH3COO−, and C2O42−). As shown by the red bars in Figs.
S15-S17 (Supporting information), Arg induced a great fluorescence change, whereas, other species did not result in obvious
fluorescence change. While most previous literatures have only studied biological molecules in regards to the selectivity of 1, the
selectivity of nanoprobe CDs-1 was measured among several different species, from which it was implied that the CDs-1 has excellent
selectivity toward Arg and high potential in a complicated practice, such as working in biological surroundings.
The fluorescent response of CDs-1 to Arg was tested in the complicated surroundings mentioned above to further assess its utility as
an Arg-selective fluorescent probe (shown by the green bars of Figs. S15-S17). From the results, it is evident that the tested species do
not interfere with Arg detection, which indicates that the nanoprobe CDs-1 could be an excellent fluorescent probe, possessing strong
anti-interference ability towards Arg.
Time-dependent fluorescence change in probes upon addition of Arg has been discussed previously [4,22]. Reaction kinetics
between probe CDs-1 and Arg in different concentrations were also studied. As shown in the time-dependent fluorescence spectra (Fig.
S18 in Supporting information), the reaction of CDs-1 and Arg was completed within 45 min. When the concentration of Arg was
higher or equal to 1.2 × 10−4 mol/L, Arg was detected within 3 min. The reaction time was longer compared to our previous work [22],
which may be ascribed to the difference in molecular structure and the recognition system.
Fig. S19 (Supporting information) illustrates the photostability experimental results, the emission intensity ratio I440/I607 of the
nanoprobe CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L compound 1) in water-DMSO solution (100:1, v/v) hardly changed with
increasing irradiation time, indicating excellent photostability. As for the thermal stability experiment (Fig. S20 in Supporting
information), the emission intensity ratio I440/I607 of the nanoprobe CDs-1 (0.67 μg/mL CDs and 3.0 × 10−5 mol/L compound 1) in
water-DMSO solution (100:1, v/v) changed significantly (0.60 to 1.28) as the temperature increased from 25 oC to 43 oC, showing its
poor thermal stability. In future work, we will focus on improving the thermal stability of the CDs-1.
Living Hela cells were used to investigate the mitochondrion-targeting ability of CDs-1. The cells were co-stained with CDs-1 (0.67
μg/mL CDs and 6.0 × 10−6 mol/L compound 1, 30 min, 25 oC) and a commercially available mitochondrion-specific dye
(mitochondrion-Tracker) (1.0 × 10−7 mol/L, 30 min) in culture medium. Imaging results show that the green image for the 1 channel
obtained upon 488 nm excitation was almost identical to the red image for the mitochondrion-Tracker channel obtained upon 550 nm
excitation (Fig. S21 in Supporting information), suggesting the ability of CDs-1 to target mitochondrion.
MTT assay was used to measure the cytotoxicity of the probe CDs-1 and evaluate its potential application in live-cell imagining.
When the concentration of CDs was 0.67 μg/mL and the concentration of compound 1 was 7.0 × 10−6 mol/L, the cellular viability was
assessed to be greater than 82% after 4 h, suggesting the probe CDs-1 has low cytotoxicity (Fig. S22 in Supporting information).
A confocal fluorescence microscopy with a 405 nm diode laser was used to investigate the practical applications of CDs-1 in
imaging live Hela cells. As shown in Fig. 2, after incubation of Hela cells with CDs-1 (0.67 μg/mL CDs and 6.0 × 10−6 mol/L
compound 1) for 30 min, strong fluorescence was detected at the red (595 ± 25 nm) channel, whereas weak fluorescence was detected
at the green (525 ± 25 nm) channel, in the cytoplasm, demonstrating that the CDs-1 had good cell permeability. Following addition of
Arg (5.0 × 10−4 mol/L), an obvious rapid decrease in fluorescence at the red channel and an increase in fluorescence at green channel
were rapidly observed, suggesting the reaction of CDs-1 to Arg took place and the intracellular Arg can be detected via ratiometric
fluorescence imagining. Bright-field measurements (Fig. S23 in Supporting information) indicated that the cells remained viable before
and after treatment with Arg throughout the imaging experiments.

Fig. 2. Confocal microscopy images of HeLa cells after addition of CDs-1 (0.67 μg/mL CDs and 6.0 × 10−6 mol/L of compound 1) (a, b, c), and after addition of
CDs-1 (0.67 μg/mL CDs and 6.0 × 10−6 mol/L of compound 1) and Arg (5.0 × 10−4 mol/L) (d, e, f). Excitation wavelengths was 405 nm. Images were collected
at 595 ± 25 nm (a and d), and 525 ± 25 nm (b and e). Ratio images obtained from first and second channels (c and f).
In summary, a ratiometric fluorescence hybrid nanoprobe CDs-1 for Arg, which can quantitatively detect Arg concentrations ranging
from 6.0 × 10−5 mol/L to 2.7 × 10−4 mol/L and has a very low detection limit of 6.5 × 10−8 mol/L, was fabricated and demonstrated.
Further, it could locate mitochondrion and selectively detect intracellular Arg. While the application of the probe CDs-1 is intrinsically
limited by its thermal stability, rapid development in organic synthesis and fabrication of hybrid nonmaterial methods can overcome
this problem.
Acknowledgments
We are grateful to the National Natural Science Foundation of China (Nos. U1704161, U1504203, 21601158) and Zhengzhou
University (No. 32210431) for the financial supports.

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