Novel fluorescent probe bearing triarylimidazole and pyridine moieties for the rapid and naked-eye recognition of Cu2+

Article abstract of DOI:10.1016/j.tetlet.2017.11.059

A new fluorescent probe (TPIP) bearing triarylimidazole and pyridine moieties was synthesized and applied to the detection of Cu²⁺ with high sensitivity and selectivity. Upon the addition of Cu²⁺, the probe displayed an apparent dual

Full text of DOI:10.1016/j.tetlet.2017.11.059

Tetrahedron Letters xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel fluorescent probe bearing triarylimidazole and pyridine moieties
for the rapid and naked-eye recognition of Cu2+
⇑
⇑
Shuzhi Liu, Yijiang Liu , Hua Pan, Hongbiao Chen, Huaming Li
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new fluorescent probe (TPIP) bearing triarylimidazole and pyridine moieties was synthesized and
applied to the detection of Cu2+ with high sensitivity and selectivity. Upon the addition of Cu2+, the probe
displayed an apparent dual-channel signal change of the UV–Vis absorption and fluorescence spectra, and
the obvious color change from bright blue to colorless under a UV lamp was discernable to the naked eye.
The sensing mechanism of the probe towards Cu2+ was verified to be via complexation, and the binding
reaction was rapidly complete within 30 s. Good linearity was observed between the probe and Cu2+, and
the detection limit was calculated to be 1.96 Â 10À8 M. The reversibility of the probe was easily achieved
by adding EDTA, which released the free probe with over 95% fluorescence recovery. Furthermore, the
recognition of Cu2+ on TLC plates was realized, indicating the potential utility of the probe.
Ó 2017 Elsevier Ltd. All rights reserved.
Received 28 September 2017
Revised 28 November 2017
Accepted 30 November 2017
Available online xxxx
Keywords:
Fluorescent probe
Triarylimidazole
Cu2+detection
High sensitivity
Reversibility
Introduction
electrochemical methods due to the merits of simple operation,
low cost, high sensitivity and real-time analysis.13–16 The two main
The design and synthesis of small molecule fluorescent probes
for the detection of various metal ions has attracted considerable
attention in the fields of chemical, biological and environmental
analysis.1–3 Among metal ions, the recognition of Cu2+ has received
considerable interest in the past decades due to its crucial psycho-
logical function in the human body.4–6 As the third most important
trace element (besides Fe2+ and Zn2+), Cu2+ participates in the cat-
alytic reactions of a variety of metalloenzymes, which are involved
in the processes of cell respiration, electron transfer and oxidation,
neurotransmitter biosynthesis and degradation, and signal trans-
duction.7,8 Consequently, the disturbance of cellular homeostasis
of Cu2+ may result in serious diseases. For example, deficient
Cu2+ intake increases the risk of leucoderma, arthritis and anemia,
while excessive Cu2+ levels can cause neurodegenerative diseases
such as Alzheimer’s and Parkinson’s, genetic disorders including
Menkes and Wilson’s diseases, and even influence tumour
growth.9–12 The extensive use and the easy diffusion of Cu2+
increase the likelihood of Cu2+-related contamination. Therefore,
the recognition of trace amounts of Cu2+ has attracted tremendous
attention in recent years.
strategies for fluorescent Cu2+ probes are Cu2+-promoted ‘‘reactive”
probes and ‘‘complexation” probes. The ‘‘reactive” probes are noted
for their high selectivity, but the reaction processes are time-con-
suming and usually suffer from strict reaction conditions.17–20
The ‘‘complexation” probes containing N, O, and S atoms in their
molecular structures usually display fluorescent quenching behav-
ior, and are reversible with the addition of Cu2+ complexing
ligands, such as EDTA and S2À 21,22
For example, Gupta and co-
.
workers reported an imidazoazine-based fluorescent probe, which
could be regenerated with 90% fluorescence recovery upon the
addition of EDTA, and retaining the same level of efficiency in
the reused probe.23 Additionally, Meng and co-workers developed
an ‘‘off-on” fluorescent probe based on a rhodamine B derivative,
where the addition of EDTA could interact with Cu2+ thus releasing
the probe with an obvious color change in sunlight.24 Furthermore,
fluorescein-, benzoyl hydrazone-, and benzimidazole-based fluo-
rescent probes have also been applied for the reversible detection
of Cu2+ 25–27 Therefore, the development of novel fluorescent Cu2+
.
probes with high sensitivity and reversibility is still highly
desirable.
The use of fluorescent probes has stood out from the traditional
methods of atomic absorption spectrometry (AAS), inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) and
Triarylimidazole derivatives are well-known fluorophores
owing to their extended conjugated-system structure, high molar
absorption coefficient and good photochemical properties, and
have been applied as fluorescent probes.28,29 Recently, we reported
a novel compound containing the triarylimidazole chromophore
(TPI-H) to sequentially detect Cu2+ and S2À with a detection limit
⇑
Corresponding authors.
E-mail addresses: liuyijiang84@163.com (Y. Liu), lihuaming@xtu.edu.cn (H. Li).
https://doi.org/10.1016/j.tetlet.2017.11.059
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu S., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.059

2
S. Liu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
at the nanomole level.30 Meanwhile, an azoaniline-arylimidazole
dyad was synthesized to detect Cu2+ with high sensitivity based
on the Cu2+-catalyzed oxidative cyclization.31 Given the good per-
formance of triarylimidazole-based fluorescent probes, herein, we
report the synthesis of a new triarylimidazole-pyridine based fluo-
rescent probe (TPIP), which can be applied to detect Cu2+ with high
sensitivity and selectivity, rapid response and a visible color
change based on the complexation mechanism. The probe could
also be easily regenerated by adding EDTA with excellent fluores-
cence recovery.
Results and discussion
Synthesis and characterization of TPIP
The probe (TPIP) was synthesized via a two-step route as shown
in Scheme 1. First, 4-(4,5-diphenyl-1H-imidazole-2-yl)benzalde-
hyde (1) was synthesized according to a reported literature proce-
dure.32 Next, an efficient one-pot dehydrogenative cross-coupling
between compound 1 and 2-aminopyridine proceeded under CuI
catalysis in DMF, affording the target molecule TPIP in 70% yield
as a pale-yellow solid. The molecular structure of TPIP was charac-
terized by 1H NMR (ESI, Fig. S1), 13C NMR (ESI, Fig. S2) and MALDI-
TOF mass spectrum (ESI, Fig. S3). The probe was soluble in
common organic solvents, and the solvent mixture CHCl3/CH3OH
(8/2, v/v) was selected for further experiments.
Absorption and fluorescence response of TPIP towards Cu2+
The UV–Vis absorption titration of the probe with various
amounts of Cu2+ was initially investigated. As shown in Fig. 1a,
the probe in CHCl3/CH3OH (20 lM, 8/2, v/v) showed two absorp-
tion bands centered at 290 nm and 345 nm, which might be attrib-
uted to the corresponding pyridine moiety and triarylimidazole
moiety. The absorbance of both peaks gradually decreased upon
the addition of Cu2+, indicating that the energy levels of the mole-
cule were changed due to coordination between TPIP and Cu2+. The
absorption profile remained unchanged when 0.5 equivalents of
Cu2+ was introduced, suggesting a 2:1 complex between TPIP and
Cu2+ was achieved. The linear response of TPIP as a function of
Cu2+ concentration was observed in the range of 0–10
lM (inset
Fig. 1a). The association constant (Ka) determined from the UV–
Vis titration was calculated to be 8.5 Â 10À4. Upon excitation at
280 nm, the free TPIP displayed a distinct emission peak centered
at 436 nm. The fluorescent intensity at 436 nm gradually
decreased with increasing amounts of Cu2+, which quenched
98.2% of the fluorescence and stabilized upon the addition of 0.5
equivalents of Cu2+ (Fig. 1b). The continuous color change of the
TPIP solution from bright blue to colorless under a UV lamp was
observable to the naked eye (inset Fig. 1c). It is clearly indicated
that the fluorescent intensity is linear to the Cu2+ amount in the
Fig. 1. (a) Absorption spectra and (b) fluorescence spectra (kex, 280 nm) of a TPIP
solution (20 mM, CHCl3/MeOH, 8/2, v/v) responding to various concentrations of
Cu2+ (0, 0.05, 0.10, 0.15, 0.20, 0. 25, 0.30, 0.35, 0.40, 0.45 and 0.50 eq.), inset (a) Plot
of TPIP absorbance at A345 as a function of Cu2+ concentration; (c) Plot of TPIP
fluorescent intensity at F436 as a function of Cu2+ concentration, inset photographs
of the TPIP solutions with various amount of Cu2+ under a UV lamp.
range of 0–10 lM with a correlated coefficient of 0.995, which
facilitated the quantitative detection of Cu2+ (Fig. 1c). The detec-
tion limit was calculated to be 1.96 Â 10À8 M based on the equa-
tion DL = 3
r/S, which was far lower than the permissive level of
M) assigned by the U.S. Environmental Protection
Cu2+ (20
l
Selectivity and competitiveness of TPIP towards Cu2+
Agency (EPA).
The selectivity of a fluorescent probe is an important property.
Thus, the selective coordination between TPIP and Cu2+ was studied
by fluorescence spectroscopy with various metal ions, including
Zn2+, Na+, K+, Ni2+, Mn2+, Ca2+, Co2+, Pb2+, Ba2+, Hg2+, Mg2+, Fe3+
,
Fe2+ and Cu2+. As shown in Fig. 2a, no obvious fluorescence quench-
ing effect was observed when 50
M of Zn2+, Na+, K+, Ni2+, Mn2+, Ca2+
Scheme 1. Synthetic route towards TPIP: (a) CH3COONH4, CH3COOH, reflux, 4 h,
81%; (b) CuI (10 mol%), DMF, 80 °C, 24 h, 70%.
l
,
Please cite this article in press as: Liu S., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.059

S. Liu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Fig. 3. Reaction-time profile of TPIP (20
of Cu2+ (10
M).
lM, CHCl3/MeOH, 8/2, v/v) in the presence
l
fluorescence spectra. As shown in Fig. 3, the fluorescent intensity
of TPIP at 436 nm was quickly quenched (within 30 s) upon the
addition of Cu2+, reaching a minimum intensity that remained
unchanged even though the reaction time was extended to 300 s.
This phenomenon indicated that the binding reaction between
TPIP and Cu2+ was extremely rapid and complete within 30 s under
the experimental conditions. Therefore, the probe could potentially
be used as an effective method for real-time analysis.
Fig. 2. (a) Fluorescence spectra (kex, 280 nm) of a TPIP solution (20 mM, CHCl3/
MeOH, 8/2, v/v) upon the addition of other metal ions (each 50 mM) and Cu2+ (10
mM), inset photographs of the mixtures under a UV lamp; (b) Fluorescence response
of a TPIP solution (20 mM, CHCl3/MeOH, 8/2, v/v) with other metal ions (red bar,
each 50 mM) and in the presence of other metal ions (50 mM) together with Cu2+ (10
mM) (black bar).
Co2+, Pb2+, Ba2+, Hg2+, Mg2+, Fe3+ and Fe2+ were introduced to the TPIP
solution, and the solution color remained bright blue under a UV
lamp (inset Fig. 2a). In contrast, the addition of 10 l
M of Cu2+
induced almost complete fluorescence quenching, and the bright
blue solution changed to almost colorless. Furthermore, the compet-
itiveness of TPIP binding with Cu2+ was investigated in the presence
of various other metal ions. The fluorescent intensity was recorded
when Cu2+ was sequentially added to various solutions of TPIP and
a metal ion. The results showed that the manner of the fluorescence
quenching of the mixture was the same as Cu2+, demonstrating that
the quenching effect of Cu2+ to TPIP was not affected by competitive
metal ions (Fig. 2b). These results clearly demonstrated the high
selectivity and competitiveness of TPIP towards Cu2+. Furthermore,
the influence of anionic counterions on the fluorescence response of
TPIP towards Cu2+ was measured. The variation of fluorescent inten-
sity (F-F0) of TPIP upon the addition of different copper salts, such as
Cu(CH3COO)2, CuSO4, Cu(NO3)2 and CuCl2, is shown in Fig. S4. Simi-
lar fluorescence quenching behavior was observed using different
copper salts, suggesting a negligible effect of the counter anion on
the detection of Cu2+ using the probe.
Response time of TPIP towards Cu2+
The rapid response of a probe to an analyte is a crucial factor to
evaluate the performance. In this study, the binding kinetics of
TPIP towards Cu2+ was investigated using time-dependent
Fig. 4. (a) Job’s fluorescence titration plot of a TPIP solution with Cu2+ in CHCl3/
MeOH (8/2, v/v); (b) MALDI-TOF mass spectrum of TPIP (black line) and TPIP-Cu
(blue line); (c) Proposed binding mode between TPIP and Cu2+
.
Please cite this article in press as: Liu S., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.059

4
S. Liu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
Proposed mechanism of TPIP binding towards Cu2+
agent. As shown in Fig. 5, upon the addition of EDTA (0.5 eq.), over
95% fluorescence was recovered. Meanwhile, the fluorescence color
of the solution underwent a distinct change from colorless to
bright blue (inset Fig. 5). Additionally, the isolated product from
the reaction of TPIP-Cu and EDTA was collected and purified by col-
umn chromatography, which was confirmed to be free TPIP by 1H
NMR spectroscopy (ESI, Fig. S6). These results clearly suggested
that the probe was released from the complex, indicating that it
is recyclable and reusable.
To further clarify the binding mechanism, Job’s titration, mass
spectrometry and FT-IR spectroscopy were performed, while the
NMR for the complexation was not available due to the paramag-
netic property of Cu2+ 33
Firstly, the Job’s titration was carried
.
out by maintaining the total concentration of TPIP and Cu2+ at
40 mM and changing the molar ratio of [Cu2+]/[TPIP] in CHCl3/
MeOH (8/2, v/v) through fluorescence spectroscopy. The fluores-
cent intensity of TPIP showed the minimum intensity at 436 nm
when the molar fraction of Cu2+ was close to 33%, which indicated
that a 2:1 stoichiometry was possible for the binding mode of TPIP
and Cu2+ (Fig. 4a). The MALDI-TOF mass spectrum of the isolated
complex also supported the 2:1 stoichiometry with an m/z peak
at 859.060 equal to [2TPIP + Cu2+ À 2H] (Fig. 4b). Furthermore,
the FT-IR spectra of TPIP and TPIP-Cu were measured in KBr disks
and the corresponding curves are shown in Fig. S5. The distinct
characteristic peak of an amide carbonyl located at 1651 cmÀ1 in
TPIP disappeared, and a new peak appeared at 1617 cmÀ1. This
suggested that the amide carbonyl was involved in the coordina-
tion with Cu2+. Therefore, the plausible binding mode between
TPIP and Cu2+ was proposed as shown in Fig. 4c.
Practical application of TPIP
Based on the distinct fluorescence color change of the probe
solution upon the addition of Cu2+, TLC plates coated with TPIP
was proposed to detect Cu2+. As shown in Fig. 6, TPIP coated TLC
plates displayed blue fluorescence. When the TPIP coated TLC
plates were further immersed into different concentrations of
Cu2+ solution, gradual fluorescence color changes were observed
under a UV lamp. However, no color changes were observed when
TPIP coated TLC plates were further immersed in other common
metal ions solutions, such as Pb2+, Hg2+, Ca2+, K+ and Na+, and still
exhibited blue fluorescence. Additionally, when different concen-
trations of Cu2+ aqueous media and other common metal ions
aqueous media were used, the TPIP coated TLC plates exhibited
similar color changes as mentioned above (ESI, Fig. S7). This phe-
nomenon indicates that the probe has potential utility in the detec-
tion of Cu2+ in the solid state, which represents more portable and
convenient detection conditions.
Reversibility of TPIP
For practical applications, the reversibility of the probe is a sig-
nificant factor. To examine whether the probe is recyclable and
reusable, the sodium salt of ethylendiaminetetraacetic acid (EDTA)
was added to the TPIP-Cu complex solution as a sequestering
Conclusion
In summary, a fluorescent probe bearing a triarylimidazole moi-
ety and a pyridine moiety was synthesized and characterized by 1H
NMR and 13C NMR spectroscopy as well as MALDI-TOF mass spec-
trometry. The probe was applied to monitor Cu2+ in CHCl3/MeOH
(8/2, v/v), which displayed rapid response and remarkable fluores-
cent quenching upon binding with Cu2+, with a distinct color
change from bright blue to colorless which was discernable to
the naked eye. The variation of fluorescent intensity was linear
with Cu2+, and the detection limit was 1.96 Â 10À8 M. The 2:1
binding mode between TPIP and Cu2+ was verified by Job’s fluores-
cence titration, MALDI-TOF mass spectroscopy and FT-IR spec-
troscopy. The binding reaction between TPIP and Cu2+ was
extremely rapid and complete within 30 s, indicating significant
potential for real-time analysis. The investigation on the reversibil-
ity of the probe demonstrated that it was simple to regenerate the
probe by the addition of EDTA, which recovered 95% fluorescence.
Furthermore, the recognition of Cu2+ on TLC plates was also suc-
cessfully realized, suggesting the practical application of the TPIP
probe.
Fig. 5. Fluorescence spectra of the probe, TPIP + Cu2+ and TPIP + Cu2+ + EDTA in
CHCl3/MeOH (8/2, v/v), inset corresponding photographs of the solutions under a
UV lamp.
Fig. 6. (a) Photographs of TPIP coated TLC plates immersed in different concentrations of Cu2+; (b) Photographs of the TLC plates immersed in TPIP solution in the presence of
other metal ions (Pb2+, Hg2+, Ca2+, K2+ and Na2+).
Please cite this article in press as: Liu S., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.059

S. Liu et al. / Tetrahedron Letters xxx (2017) xxx–xxx
11. Muthaup G, Schlicksupp A, Hess L, et al. Science. 1996;271:1406–1409.
5
Acknowledgements
12. Barnham KJ, Masters CL, Bush AI. Nat Rev Drug Discovery. 2004;3:205–214.
13. Yin BC, Zuo P, Huo H, Zhong XH, Ye BC. Anal Biochem. 2010;401:47–52.
This study was financially supported by National Natural
Science Foundation of China (NSFC 51603177), China Postdoctoral
Science Foundation (2017M612571) and the Opening Foundation
of the Key Laboratory of Creative Platform of the Education Depart-
ment of Hunan Province, China.
14. Gonzales APS, Firmino MA, Nomura CS, Rocha FRP, Oliveira PV, Gaubeur I. Anal
Chim Acta. 2009;636:198–204.
15. Becker JS, Matusch A, Depboylu C, Dobrowolska J, Zoriy MV. Anal Chem.
2007;79:6074–6080.
16. Li DX, Sun X, Huang JM, et al. Dyes Pigments. 2016;125:18–191.
17. Li D, Sun X, Wang M, et al. Sensors Actuators B. 2015;220:1006–1016.
18. Long LL, Wu YJ, Wang L, Gong AH, Hu RF, Zhang C. Anal Chim Acta.
2016;908:1–7.
A. Supplementary data
19. Wang Y, Mao PD, Wu WN, et al. Sensors Actuators B. 2017;251:813–820.
20. Huo FJ, Yin CX, Yang YT, Su J, Chao JB, Liu DS. Anal Chem. 2012;84:2219–2223.
21. Li G, Tao FR, Wang H, Li YC, Wang LP. Sensors Actuators B. 2015;211:325–331.
22. Hu Y, Zhang J, Lv YZ, Huang XH, Hu SL. Spectrochim Acta A. 2016;157:164–169.
23. Kumawat LK, Kumar M, Bhatt P, Sharma A, Asif M, Gupta VK. Sensors Actuators
B. 2017;240:365–375.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2017.11.059.
24. Meng XF, Xu YX, Liu JL, Sun LN, Shi LY. Anal Methods. 2016;8:1044–1051.
25. Wu X, Gong XJ, Dong WJ, et al. RSC Adv. 2016;6:59677–59683.
26. Anbu S, Ravishankaran R, Silva MFCG, Karande AA, Pombeiro AJL. Inorg Chem.
2014;53:6655–6664.
27. Tang LJ, Cai MJ, Huang ZL, et al. Sensors Actuators B. 2013;185:188–194.
28. Yagi K, Soong CF, Irie M. J Org Chem. 2001;66:5419–5423.
29. Zhao B, Xu Y, Ma MJ, Kan W, Deng QG. Res Chem Intermed. 2015;41:8031–8040.
30. Wang YP, Qiu DL, Li MN, Liu YJ, Chen HB, Li HM. Spectrochim Acta A.
2017;185:256–262.
References
1. Lee MH, Kim JS, Sessler JL. Chem Soc Rev. 2015;44:4185–4191.
2. Vendrell M, Zhai DT, Er JC, Chang YT. Chem Rev. 2012;112:4391–4420.
3. Su W, Gu B, Hu XJ, et al. Dyes Pigments. 2017;137:293–298.
4. Cotruvo Jr JA, Aron AT, Ramos-Torres KM, Chang CJ. Chem Soc Rev.
2015;44:4400–4414.
5. Liu S, Wang YM, Han J. J Photochem Photobiol C. 2017;32:78–103.
6. Niu CX, Song QW, He G, Na N, Ouyang J. Anal Chem. 2016;88:11062–11069.
7. Culotta VC, Yang M, O’Halloran TV. BBA. 2006;1763:747–758.
8. Uauy R, Olivares M, Gonzalez M. Am J Clin Nutr. 1998;67:952S–959S.
9. Tiffany-Castiglioni E, Hong S, Qian YC. Int J Dev Neurosci. 2011;29:811–818.
10. Rossi L, Lombardo MF, Ciriolo MR, Rotilio G. Neurochem Res. 2004;29:493–504.
31. Wang YP, Liu SZ, Chen HB, Liu YJ, Li HM. Dyes Pigments.. 2017;142:293–299.
32. Zheng W, He XZ, Chen HB, Gao Y, Li HM. Spectrochim Acta A. 2014;124:97–101.
33. Helal A, Rashid MHO, Choi CH, Kim HS. Tetrahedron. 2011;67:2794–2802.
Please cite this article in press as: Liu S., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.11.059