A selective and sensitive near-infrared fluorescent probe for real-time detection of Cu(i)

Article abstract of DOI:10.1039/d1ra00725d

The disruption of copper homeostasis (Cu+/Cu2+) may cause neurodegenerative disorders. Thus, the need for understanding the role of Cu+ in physiological and pathological processes prompted the development of improved methods of Cu+ analysis. Herein, a new near-infrared (NIR) fluorescent turn-on probe (NPCu) for the detection of Cu+ was developed based on a Cu+-mediated benzylic ether bond cleavage mechanism. The probe showed high selectivity and sensitivity toward Cu+, and was successfully applied for bioimaging of Cu+ in living cells. This journal is

Full text of DOI:10.1039/d1ra00725d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A selective and sensitive near-infrared fluorescent
probe for real-time detection of Cu(I)†
Cite this: RSC Adv., 2021, 11, 14824
Yiqing Liu, ‡a Ting Kang,‡b Qian He,‡c Yuefu Hu,a Zeping Zuo,b Zhihua Cao,b
b
b
a
* *
Bowen Ke,
Weiyi Zhang and Qingrong Qi
The disruption of copper homeostasis (Cu+/Cu2+) may cause neurodegenerative disorders. Thus, the need
for understanding the role of Cu+ in physiological and pathological processes prompted the development
of improved methods of Cu+ analysis. Herein, a new near-infrared (NIR) fluorescent turn-on probe (NPCu)
for the detection of Cu+ was developed based on a Cu+-mediated benzylic ether bond cleavage
mechanism. The probe showed high selectivity and sensitivity toward Cu+, and was successfully applied
for bioimaging of Cu+ in living cells.
Received 27th January 2021
Accepted 26th March 2021
DOI: 10.1039/d1ra00725d
rsc.li/rsc-advances
expensive instrumentation. More importantly, these methods
Introduction
cannot provide the real-time visualization of labile Cu+ in situ.
Optical imaging techniques are non-invasive, highly sensitive,
easily handled, and suitable for detecting analytes in biological
system. In the past decade, a number of uorescent probes have
been designed for copper detection.26–30 Among this collection,
NIR uorescent probes stand out due to their unique properties
of high penetration through tissues,31–34 low auto-uorescence
and less photodamage.35,36 T. Govindarajua et al. reported on
the development of a NIR uorescent probe with a tripicolyl-
amine (N4) functionality for the detection of intracellular Cu+.37
B. R. Cho et al. and W. Wan et al. developed probes with the
recognition group being bis(2-((2-(ethylthio)ethyl)-thio)ethyl)
amine (BETA) containing electron rich S atoms.38,39 Although
these probes offer a promising strategy for intravital non-
invasive quantitative imaging, they cannot completely
As an essential trace transition metal element, found in both
the oxidized Cu2+ and reduced Cu+ states in living organisms,
copper is considered a vital redox-active cofactor for various
cytosolic, mitochondrial and vesicular oxygen-processing
enzymes, including cytochrome-c-oxidase,1,2 copper/zinc
superoxide dismutase3 and metallothionein.4 The dispropor-
tionation of Cu+ in cells could produce reactive oxygen species
(ROS), leading to oxidative damage of proteins, nucleic acids
and lipids.5–7 There is a growing body of evidence to suggest that
the imbalance of Cu+ may cause neurodegenerative disorders,
such as prion, Parkinson's, Alzheimer's, Menkes, and Wilson's
diseases and amyotrophic lateral sclerosis.8–11 In addition,
copper was also demonstrated to play a critical role in other
diseases including urinary tract infection12 and infertility.13 It is
of signicance, therefore, to establish an effective and reliable
strategy to monitor Cu+ in complex environmental settings and
biological systems. The involvement of Cu+ in physiological and
pathological processes promoted the development of methods
in Cu+ analysis. There are existing techniques such as electro-
chemistry,14–17 chromatography,18–21 pulse polarography,22 vol-
tammetry23 and atomic absorption spectrophotometry (AAS),24,25
but these generally require sophisticated procedures and
discriminate Cu+ from other interfering cations, such as Cu2+
,
Co2+ or Hg2+. Developing a highly selective tool to detect Cu+,
especially to visualize dynamic process of Cu+ in living system,
is a challenging endeavour.
Herein, we developed a new near-infrared (NIR) uorescent
turn-on probe (NPCu) for the detection of Cu+, based on the
mechanism of a Cu(I)-mediated benzylic ether bond cleavage.
Our results demonstrate that the NPCu probe can effectively
distinguish Cu+ from most interfering cations in vitro and in
vivo.
aKey Laboratory of Drug-Targeting and Drug Delivery System of the Education
Ministry, Sichuan Engineering Laboratory for Plant-Sourced Drug, SichuanResearch
Center for Drug Precision Industrial Technology, West China School of Pharmacy
Sichuan University, Chengdu, 610041, P. R. China. E-mail: qiqinrong@scu.edu.cn
bDepartment of Anaesthesiology, West China Hospital, Sichuan University, China.
E-mail: Zhangweyi@xchscu.cn
Experimental
Materials and instruments
All reagents were purchased from commercial suppliers and
used without further purication (Adamas-beta® or Lab
Network), unless otherwise noted. All chemicals and solvents
were used without purication unless otherwise noted. Column
chromatography was carried out on silica gel (200–300 mesh)
cDepartment of Emergency, West China Hospital, Sichuan University, Chengdu
610000, Sichuan, China
† Electronic supplementary information (ESI) available: Detailed synthesis,
characterization (NMR, MS, etc.) of the probes. See DOI: 10.1039/d1ra00725d
‡ These authors contributed equally.
14824 | RSC Adv., 2021, 11, 14824–14828
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
¼ 5.6 Hz, 1H), 5.55 (s, 2H), 4.83 (t, J ¼ 7.2 Hz, 2H), 4.75 (t, J ¼
7.2 Hz, 2H), 4.58 (s, 1H), 3.87 (s, 2H), 3.81 (s, 2H), 3.54 (d, J ¼
6.0 Hz, 2H), 2.61 (t, J ¼ 6.4 Hz, 2H), 1.95–1.82 (m, 10H), 1.75 (s,
6H), 1.05 (t, J ¼ 7.2 Hz, 3H), 0.97 (t, J ¼ 7.2 Hz, 3H).13C NMR (100
MHz, DMSO-d6) d: 182.51, 182.36, 162.29, 160.47, 159.96,
154.79, 153.02, 149.17, 147.45, 144.40, 144.22, 141.29, 141.27,
138.12, 137.84, 136.99, 134.02, 130.24, 129.93, 129.75, 129.62,
128.51, 124.09, 123.61, 123.58, 123.06, 122.68, 122.58, 121.18,
116.17, 115.87, 115.08, 114.99, 112.56, 72.39, 63.25, 60.64,
59.59, 56.62, 52.77, 52.75, 48.80, 48.34, 26.56, 26.37, 22.41,
22.36, 11.26, 11.23. HRMS (ESI) m/z: calcd for [C51H59N5O]2+
[M]2+: 386.7335; found 386.6984.
Measurement of absorption and uorescence
For UV/vis and uorescence titrations stock solution of NPCu
probe were prepared (c ¼ 10 mM) in 25 mM PBS buffer (pH ¼
7.2). The solutions of cations were freshly prepared in PBS
buffer solution. Working solutions of the probe and metal ions
were prepared from the stock solutions. Excitation and emis-
sion spectra were carried out at 560 nm and 710 nm for NPCu
with 4 nm slit widths. All buffers for pH titration were prepared
in deionized water and adjusted to a suitable pH with HCl or
NaOH aqueous solutions.
Scheme 1 Synthesis procedure of probe NPCu.
using an eluent of ethyl acetate and cyclohexane,
dichloromethane/methanol, or ethyl acetate/methanol. TLC
analyses were conducted on silica gel plates and visualized
using UV (ZF-2 UV254). Mass spectrometry analyses were per-
formed on an API 4000 (ESI-HRMS). NMR spectra were recorded
Selectivity of probes to Cu(I)
Each metal cation was diluted with PBS buffer solution to
a 1 mM working solution for use, and mixed with 2 mL of NPCu
solution (100 mM) in equal volume; in experiment 2, each metal
cation was mixed with Cu(I), and each was congured. A mixed
solution of 1 mM ion concentration was mixed with 2 mL of
NPCu solution (50 mM) in equal volume. The samples from both
experiments were incubated for 1 hour in a dark shaker under
1
at H (400 MHz or 600 MHz) and 13C (100 MHz) on a Bruker
instrument. Chemical shis (d values) and coupling constants (J
values) are given in ppm and hertz respectively. Melting points
were determined on a Mel-Temp apparatus and were not cor-
rected. UV/vis spectra were recorded on a UV-1800 (240V)
spectrophotometer and uorescence spectra were recorded on
a FL3C-iHR320 spectrouorometer (HORIBA Instrument Inc.)
Absorption and emission were measured using a 1 cm path
length quartz cells. Fluorescence imaging was carried out with
Nikon Ti-E laser scanning microscope that excitation at 560 nm
under a 40ꢀ objective.Water used for this study was puried
with a Milli-Q ltration system (Scheme 1).
ꢁ
a 37 C constant temperature shaker to determine the uores-
cence intensity of each ion solution at an excitation wavelength
of 560 nm and an emission wavelength of 710 nm. The molar
absorption coefficients of probe NPCu without Cu+ was calcu-
lated as 3 ¼ 1.76 ꢀ 104 Mꢂ1 cmꢂ1 while with Cu+ was 0.76 ꢀ
104 Mꢂ1 cmꢂ1. NPCu aer Cu+ treatment exhibited a high
quantum yield (F ¼ 0.39). The quantum yields of NPCu was
0.024 in aqueous medium when excited at the lmax (560 nm, 3 ¼
0.76 ꢀ 104 Mꢂ1 cmꢂ1).
Synthesis of the NPCu probe
To a mixture of compound 5 (297.5 mg, 0.7 mmol) and 3-1
(517.7 mg, 1.8 mmol) in 4 mL of anhydrous ethanol was added
sodium acetate (119.6 mg, 1.46 mmol) in 2 mL of ethanol under
Cell cytotoxicity and imaging
a nitrogen atmosphere. The reaction mixture was reuxed at Human alveolar epithelial A549 cells were grown in RPMI-1640
80 ꢁC for 1 h under N2 protection. When completed, the solvent medium, containing 10% fetal bovine serum (FBS) and 1%
was removed under reduced pressure and the residue was penicillin-streptomycin, at 37 ꢁC in a humidied incubator with
puried by silica gel column chromatography (MeOH/DCM an atmosphere of 5% CO2/95% air. One day before the experi-
from 0 to 10%) to yield a deep yellow solid, NPCu (150.0 mg, ment, a 6-well culture plate was inoculated with logarithmic
21.9% yield). 1H NMR (600 MHz, DMSO-d6) d: 9.41 (s, 1H), 8.62 growth phase cells. The cells in the experimental group were
(dd, J ¼ 16.4, 8.5 Hz, 2H), 8.59–8.54 (m, 1H), 8.46 (d, J ¼ 5.2 Hz, incubated with Cu+ (50 mM) for 6 h. In the control group, only
1H), 8.15 (d, J ¼ 16.4 Hz, 1H), 8.04–7.97 (m, 2H), 7.93 (dd, J ¼ medium was added. Aer that, cells were incubated with NPCu
ꢁ
9.2, 5.6 Hz, 3H), 7.90–7.86 (m, 1H), 7.78–7.72 (m, 1H), 7.70–7.62 (10 mm) at 37 C for 1 h. Finally, an inverted microscope was
(m, 5H), 7.62 (d, J ¼ 8.8 Hz, 1H), 7.56 (d, J ¼ 7.6 Hz, 2H), 7.23 (t, J used to observe the cells with DIC and Cy-3 uorescence lters.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 14824–14828 | 14825

View Article Online
RSC Advances
Paper
Fig. 3 Change of fluorescent intensity: NPCu (50 mM) reacted with
Cu+ (1000 mM) at various time points. lex/lem ¼ 560/710 nm, 25 mM
PBS (pH 7.2) containing 2 mM GSH.
Fig. 1 UV-vis absorption spectra (A) and 3D fluorescence spectra (B
and C) of NPCu (50 mM) incubated without or with Cu+ (500 mM) in
25 mM PBS buffer (pH 7.2) containing 2 mM GSH.
detection limit for Cu+ was 9.1 ꢀ 10ꢂ5 M (S/N ¼ 3). It was shown
that the uorescence intensity of the probe progressively
increased with increasing cocentration of Cu+.
Results and discussion
Sensitivity research
We rst evaluated its spectral properties and determined its
responsiveness towards Cu(I). As shown in Fig. 1, the absor-
bance, extraction and emission spectra of NPCu peaked at
415 nm, 430 nm, and 560 nm respectively in 25 mM PBS (pH
7.2) containing 2 mM glutathione (GSH). The addition of Cu+
(10 eq.) induced signicant signal changes in the optical
properties of NPCu solutions. As shown in Fig. 1A, a marked
redshi (ꢃ150 nm) was noted in absorption. Meanwhile, in the
emission spectra, a turn-on response from 560 nm to 710 nm
was noted upon introduction of Cu+ to the probe. Aer the
incubation with Cu+, large Stokes shis (>150 nm) both in the
excitation and emission spectra were observed (Fig. 1B and C).
This is considered a highly desirable feature as it increases the
signal-to-noise ratio.
Fluorescence intensity changes of NPCu with time
To 2 mL of NPCu solution (50 mM) was added the same volume
of Cu+ (10 eq.) in 25 mM PBS buffer (pH 7.2) containing 2 mM
ꢁ
GSH and incubated at 37 C for different lengths of time. The
uorescence intensity was measured at 1, 5, 10, 15, 20, 25, 30,
40, 50, 60, 90, 120, 180, and 240 min (Fig. 1). The correlation
between NPCu responsiveness and different concentrations of
Cu+ was determined in 25 mM PBS (pH 7.2) containing 2 mM
GSH. To the EP tubes (1.5 mL), each containing 500 mL of Cu+
solution at various concentrations from 0 to 1 mM, was added
the same volume of NPCu (250 mM) and incubated at 37 ꢁC for 1
hour. It was found that the colour of the reaction mixture
deepened with increasing Cu+ concentration (Fig. S1†) (Fig. 3).
To investigate whether NPCu had the sensitivity to measure
small uctuations in Cu+ level in aqueous solutions, the probe
(50 mM) was exposed to a range of Cu+ concentrations. The
uorescence intensity at 710 nm was plotted and exhibited good
linear correlation (R2 ¼ 0.995) with Cu(I) levels from 0 to 1000
mM. A typical calibration curve is shown in Fig. 2 (inset). The
Selectivity research
NPCu can be conveniently used as a ‘switch-on’ probe for the
detection of Cu+ without interference from pH-related effects in
physiological environments. As shown in Fig. 4, the specicity
Fig. 4 Competition tests. Fluorescence responses of 50 mM NPCu to
Fig. 2 Titration of NPCu (50 mM) in pH 7.2 buffer with different various metal ions (10 eq.). (A) Addition of competing metal ions to the
concentrations of Cu+ at 0–1000 mM (lex ¼ 560 nm). Inset: linear solution of the receptor. (B) Addition of equal amounts of Cu+ to the
correlation between emission intensity and Cu+ concentration from solution containing the other metal. Excitation was at 560 nm. Spectra
0 to 1 mM.
were acquired in 25 mM PBS (pH 7.2) + 2 mM GSH.
14826 | RSC Adv., 2021, 11, 14824–14828
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Fig.
5
The chemical structure of NPCu and its Cu+ sensing
mechanism.
of NPCu ion detection was tested with various metal ions
including Cu2+, Co2+, Hg2+, and Fe3+. NPCu exhibited
a remarkable 30-fold enhancement in the NIR region (lex ¼ 560
nm) upon addition of Cu+ (10 eq.) aer 2 h, whereas the
responses toward other ions were negligible. The NPCu
exhibited high selectivity for Cu+ over other biologically relevant
interfering metals, including redox-active copper and cobalt
transition metals.
Fig. 7 Confocal microscopy images of A549 cells incubated with 10
mM NPCu for 1 h at 37 ꢁC. Upper: incubated with 50 mM Cu+ prior to
NPCu staining. Lower: incubated with vehicle prior to NPCu staining. (a
and d) Bright field images; (b and e) fluorescence images; (c and f):
overlay of fluorescence and bright field images. Scale bar ¼ 50 mm.
(Fig. 5, black), a Cu+ reactive moiety (Fig. 5, red). NPCu with
a tetradentate ligand N3O moiety, which is rst employed as
a highly selective trigger for the detection of Cu+. Upon binding
to Cu+, the benzylic ether bond (C–O) of NPCu was cleaved, and
cyanine-quinone dye was released from NPCu, leading to
a robust NIR uorescence enhancement. This mechanism was
conrmed by mass spectrometry (Fig. S4†).
We also used orbital theory to better verify our hypothesis.
LUMO and HOMO levels of NPCu in the absence or presence of
Cu+ all have obvious delocalization effect on the entire molec-
ular skeleton. As shown in Fig. 6, HOMO and LUMO energies of
NPCu were calculated to be ꢂ3.55 eV and ꢂ1.91 eV while the
energies of NPCu + Cu+ were ꢂ3.37 eV and ꢂ1.02 eV. The energy
gaps (LUMO–HOMO) of NPCu and NPCu + Cu+ are 2.45 eV and
1.64 eV, respectively. It is worth noting that aer reacting with
Cu+, the UV absorption peak of NPCu was observed to have red-
shied. Theoretical predictions were consistent with the spec-
troscopy experimental data, indicating that the proposed reac-
tion mechanisms were reasonable.
Fluorimetric pH titration of NPCu
We further assessed the probe's capability of detecting Cu+ at
different pH's.The pH was adjusted between 4.0 and 10.0 by
adding 1 M HCl, and the uorescence titration was performed
in a quartz colorimetric dish with a path length of 1 cm
(constant temperature set to 25 ꢁC). The absorption spectra
(Fig. S2†) were collected from 200–410 nm and the uorescence
intensity was measured at 710 nm with excitation at 560 nm. In
the absence of Cu+, there was almost no change uorescence
intensity in the pH range of 4–10, which suggested this probe
has considerable stability. Under neutral to mildly alkaline
conditions, NPCu can maintain high responsiveness to Cu+.
Sensing mechanisms
According to the structural characteristics of NPCu and litera-
ture reports,40,41 we speculate a new approach for the detection
of Cu+ in live cells through the development of a NIR probe
(NPCu), which is comprised of two discrete elements: a NIR core
Cytotoxicity and cell imaging
The cytotoxicity of NPCu was evaluated using the methyl-
thiazolyldiphenyltetrazolium (MTT) viability assay. As indicated
in Fig. S4,† A549 cells incubated with NPCu showed little
decrease in cell viability. Viability was >70% at 30 mM NPCu,
which indicates that NPCu is suitable for bioimaging applica-
tions at 10 mM. Next, we tested the utility of this probe for
detecting Cu+ in cultured A549 cells by confocal microscopy.
The results clearly demonstrate that NPCu is permeable to
membrane, and can react with intracellular Cu+ to release near
infrared uorescent Cy-quinone dye in living cells (Fig. 7).
Conclusions
In summary, we have developed a new NIR uorescent turn-on
Cu+ probe based on the cyanine-quinone uorophore and novel
Cu+ receptor N3O-ol skeleton. The probe NPCu exhibited high
Fig. 6 The HOMO and LUMO of NPCu and NPCu + Cu+. Red, blue,
grey and white balls represent O, N, C, and H atoms, respectively.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 14824–14828 | 14827

View Article Online
RSC Advances
Paper
sensitivity, good selectivity, and considerable stability at phys- 14 B. K. Jena and C. R. Raj, Anal. Chem., 2008, 80, 4836.
iological pH. More importantly, NPCu showed excellent 15 M. Verma, A. F. Chaudhry, M. T. Morgan, et al., Org. Biomol.
biocompatibility and high permeability for penetrating cell
membrane and tracking intracellular Cu+. NPCu shows prom- 16 X. Shao, H. Gu, Z. Wang, et al., Anal. Chem., 2012, 85, 418–
ising properties for non-invasive imaging of Cu+ in living cells
425.
Chem., 2010, 8, 363–370.
and provides a useful tool to better understand the contribution 17 J.-H. Luo, D. Cheng, P.-X. Li, et al., Chem. Commun., 2018, 54,
of Cu+ in living systems.
2777–2780.
18 T. Gunnlaugsson, J. P. Leonard and N. S. Murray, Org. Lett.,
2004, 6, 1557.
19 R. Sheng, P. Wang, Y. Gao, et al., Org. Lett., 2008, 10, 5015.
20 G. C. Bandara, C. A. Heist and V. T. Remcho, Anal. Chem.,
2018, 90, 2594–2600.
Conflicts of interest
There are no conicts to declare.
21 Y. Shi, R. Wang, W. Yuan, et al., ACS Appl. Mater. Interfaces,
2018, 10(24), 20377–20386.
22 S. Kocak, O. Tokusoglu and S. Aycan, J. Agric. Food Chem.,
2005, 4, 871–878.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 81773685), the 1.3.5 Project for
Disciplines of Excellence, West China Hospital, Sichuan
University (No. ZYJC18032, and No. ZY2016101), Sichuan
Science and Technology Program (No. 2019JDJQ0004), the
Innovation spark project, Sichuan University (No.
2082604401004/056), the Fundamental Research Funds for the
Central Universities, Sichuan university.
¨
23 P. Salaun and V. D. B. Cm, Anal. Chem., 2006, 78, 5052.
24 J. Chen and K. C. Teo, Anal. Chim. Acta, 2001, 450, 215–222.
25 G. Doner and A. Ege, Anal. Chim. Acta, 2005, 547, 14–17.
26 A. F. Chaudhry, M. Verma, M. T. Morgan, et al., J. Am. Chem.
Soc., 2010, 132, 737–747.
27 A. F. Chaudhry, S. Mandal, K. I. Hardcastle, et al., Chem. Sci.,
2011, 2, 1016–1024.
28 M. T. Morgan, A. McCallum and C. J. Fahrni, Chem. Sci.,
2016, 7, 1468–1473.
29 S. Wang, C. Liu, G. Li, et al., ACS Sensors, 2017, 2, 364–370.
30 Y. Ma, Y. Cen, M. Sohail, et al., ACS Appl. Mater. Interfaces,
2017, 9(38), 33011–33019.
31 J. Tang, X. Yang, F. Zhao, et al., Sens. Actuators, B, 2021, 330,
129283.
32 X. Han, X. Yang, Y. Zhang, et al., Sens. Actuators, B, 2020, 321,
128510.
33 J. Tang, Q. Li, Z. Guo, et al., Org. Biomol. Chem., 2019, 17(7),
1875–1880.
34 Z. Yang, X. Fan, X. Liu, et al., Chem. Commun., 2021, 57,
3099–3102.
35 Y. Shiraishi, K. Tanaka and T. Hirai, ACS Appl. Mater.
Interfaces, 2013, 5, 3456–3463.
36 C. Tang, Y. Du, Q. Liang, et al., Mol. Pharm., 2018, 15, 4702–
4709.
37 D. Maity, A. Raj, D. Karthigeyan, et al., Supramol. Chem.,
2015, 27, 589–594.
38 C. S. Lim, J. H. Han, C. W. Kim, et al., Chem. Commun., 2011,
47, 7146–7148.
39 X. Cao, W. Lin and W. Wan, Chem. Commun., 2012, 48, 6247–
6249.
40 N. Karton-Lifshin, E. Segal, L. Omer, et al., J. Am. Chem. Soc.,
2011, 133, 10960–10965.
References
1 A. Boulet, K. E. Vest, M. K. Maynard, et al., J. Biol. Chem.,
2018, 293, 1887–1896.
2 L. Cerqua, V. Morbidoni, M. A. Desbats, et al., Biochim.
Biophys. Acta, Bioenerg., 2018, 1859, 244–252.
3 E. S. Kim, C. S. Lim, H. J. Chun, et al., J. Clin. Pathol., 2012,
65, 882–887.
4 E. A. Ostrakhovitch, Y. P. Song and M. G. Cherian, J. Trace
Elem. Med. Biol., 2016, 35, 18–29.
5 H. Kozlowski, A. Janicka-Klos, J. Brasun, et al., Coord. Chem.
Rev., 2009, 253, 2665–2685.
6 M. L. Giuffrida, E. Rizzarelli, G. A. Tomaselli, et al., Chem.
Commun., 2014, 50, 9835.
7 L. Zhang, J. C. Er, H. Jiang, et al., Chem. Commun., 2016, 52,
9093–9096.
8 A. Langley and C. T. Dameron, Anesthesiol. Res. Pract., 2013,
2013, 1–10.
9 X. Wang, Q. Miao, T. Song, et al., The Analyst, 2014, 139,
3360–3364.
10 B. Sullivan, G. Robison, J. Osborn, et al., Redox Biol., 2016,
11, 231–239.
11 M. C. Heffern, H. M. Park, H. Y. Au-Yeung, et al., Proc. Natl.
Acad. Sci. U. S. A., 2016, 113, 14219–14224.
12 A. N. Hyre, K. Kavanagh, N. D. Kock, et al., Infect. Immun.,
2016, 8, e01041-16.
41 D. Maity, V. Kumar and T. Govindaraju, Org. Lett., 2012, 14,
6008–6011.
13 E. Tvrda, R. Peer, S. C. Sikka, et al., J. Assist. Reprod. Genet.,
2015, 32, 3–16.
14828 | RSC Adv., 2021, 11, 14824–14828
© 2021 The Author(s). Published by the Royal Society of Chemistry