Triazole-imidazole (TA-IM) derivatives as ultrafast fluorescent probes for selective Ag+ detection

Article abstract of DOI:10.1039/c8ob02482k

1,2,3-Triazole-imidazole derivatives (TA-IM) were prepared as fluorescent probes for silver ion detection. The design principle is the incorporation of an intramolecular H-bond between the imidazole and triazole moiety that enables a co-planar conformation to achieve fluorescence emission in the UV-blue range. Screening of different metal ions revealed excellent binding affinity of this new class of compounds toward silver ions in aqueous solution. The novel probe provided ultrafast detection (<30 s) even for a very low concentration of silver ions (in the nM range) with good linear correlation, making it a practical sensor for detection of silver ions.

Full text of DOI:10.1039/c8ob02482k

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Triazole-imidazole (TA-IM) derivatives as ultrafast
fluorescent probes for selective Ag+ detection†
Cite this: Org. Biomol. Chem., 2018,
16, 7801
Qi Lai,a Qing Liu,a Ying He,b Kai Zhao,a Chiyu Wei,b Lukasz Wojtas,b
Received 5th October 2018,
Accepted 8th October 2018
Xiaodong Shi
*
a,b and Zhiguang Song*a
DOI: 10.1039/c8ob02482k
rsc.li/obc
1,2,3-Triazole-imidazole derivatives (TA-IM) were prepared as new class of fluorescent active compounds with excellent
fluorescent probes for silver ion detection. The design principle is selectivity toward Ag+ (over 20 other tested cations) in aqueous
the incorporation of an intramolecular H-bond between the imid- media. These new fluorescent probes also gave high sensitivity
azole and triazole moiety that enables a co-planar conformation with a linear concentration–emission correlation. Moreover,
to achieve fluorescence emission in the UV-blue range. Screening compared with other reported Ag+ detection methods, TA-IM
of different metal ions revealed excellent binding affinity of this demonstrated an ultrafast response time of less than 30
new class of compounds toward silver ions in aqueous solution. seconds upon coordination. All these features make TA-IM a
The novel probe provided ultrafast detection (<30 s) even for a very new practical sensor for Ag+ detection.
low concentration of silver ions (in the nM range) with good linear
Our interest in developing triazole derivatives as metal
correlation, making it a practical sensor for detection of silver ions. cation sensors originated from the conformational analysis of
the N-2-aryl triazole (NAT) fluorophore. As shown in
Fluorescence active molecules are of great importance in
Scheme 1B, we have previously demonstrated that 4-pyridyl tri-
azole 1a showed good fluorescence emission (Φ = 0.28), while
the 2-pyridyl isomer 1b was fluorescence-inactive (no emis-
sion). The X-ray crystal structure of 1b revealed a twisted con-
chemical,1 biological2 and medicinal research.3 A new class of
small organic molecules with good fluorescence emission
could offer opportunities for interesting applications.4 Over
the past decade, our group has been working on the develop-
formation between the two aromatic rings, likely due to repul-
ment of functional 1,2,3-triazole (TA) compounds as ligands
sion between the lone pair electrons of nitrogen.8 Based on
for metal coordination.5 A series of triazole ligands have been
this analysis, we postulated that metal coordination between
identified to promote metal-catalyzed reactions.6 Upon syn-
the two nitrogen atoms in 1b could force the two aromatic
rings to adopt a co-planar conformation and, therefore, may
“turn on” the fluorescence emission upon coordination. To
thesis of triazole derivatives,7 we found that N-2 aryl triazole
(NAT) showed strong emission in the UV-blue range (λmax
between 380 nm and 430 nm). In contrast, the N-1 isomer
confirm this idea, we performed the coordination experiments
exhibited almost no emission (Φ < 0.02). According to our
of 1b (TA-Py) with various metal cations (Cu2+, Ag+, Pd2+, Pt2+,
structural analysis, strong fluorescence emission can be attrib-
uted to the co-planar conformation between the triazole ring
and N-2 aryl groups (Scheme 1A).8
Based on these unique features,9 we wondered whether
1,2,3-triazole can be developed as a novel fluorescent probe for
metal cation detection upon coordination. Herein, we report
imidazole substituted 1,2,3-triazole (TA-IM) derivatives as a
aState Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, Jilin 13002, China. E-mail: szg@jlu.edu.cn
bThe Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue,
Tampa, Florida 33620, United States. E-mail: xmshi@usf.edu
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data, NMR spectra and X-ray data. CCDC 1835133(5a),
1835134(5b), 1835139(5d), 1835140(5i) and 1835141(5k). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c8ob02482k
Scheme 1 Design of triazole-based fluorescent probes.
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Scheme 2 Revised sensing strategy with the TA-IM ligand.
Ni2+ etc.). Unfortunately, no fluorescence emission change was
observed in all cases (see details in ESI, Fig. S1†). In spite of
the possible co-planar conformation upon chelation, the
results suggested that NAT’s excited state was nevertheless
quenched due to potential electron and/or energy transfer
upon cation binding.10 As a result, the initial turn-on sensor
hypothesis did not work, and we turned to design a turn-off
sensor with a similar framework.
Fig. 1 General synthetic route for TA-IM ligands.
With the assumption that fluorescence quenching occurs
upon metal coordination for TA-Py ligand 1b, we sought out to
develop new triazole ligands with both good fluorescence
emission and metal binding ability to achieve metal sensing
(Scheme 2A).11 As discussed above, no fluorescence emission
was observed for the TA-Py ligand due to the nitrogen lone
pair electron repulsion (Scheme 2B). To address this problem
while keeping the bi-dentate binding nature, we proposed a
new ligand system as triazole-imidazole (TA-IM). The key of
our design is the incorporation of an intramolecular H-bond
to avoid lone pair electron repulsion so as to enhance the for-
mation of a co-planar conformation for effective fluorescence
emission while maintaining the bi-dentate nitrogen binding
mode. As shown in Fig. 1A, a group of TA-IM compounds was
successfully prepared. The structures of these TA-IM com-
pounds are summarized in Fig. 1B.
Fig. 2 Fluorescence emission of some TA-IM ligands. Fluorescence
emission of compounds 5a–6c. Concentration: 20 μmol L−1 in EtOH.
in 5a. This suggested that 5g favored the co-planar confor-
The X-ray crystal structures of TA-IM were successfully mation. Switching imidazole to phenyl (6a), thiazole (6b) and
obtained with several TA-IM compounds (5a, 5b, 5d, 5i and pyridine (6c) shut down the fluorescence emission almost
5k), which verified the proposed N-2 isomer conformation. completely, which highlights the crucial role of the intra-
Moreover, as highlighted in Fig. 1B, compared with the C-5 molecular H-bond in achieving the fluorescence-active TA-
phenyl ring, the C-4 imidazole ring had a much smaller di- ligand. Importantly, the fluorescence intensity of 5a in solu-
hedral angle (18.2°) with the triazole ring, which suggested the tion remained the same even after ten months (Fig. S6†),
formation of an intramolecular H-bond. With the TA-IM suc- suggesting the excellent stability of the new TA-IM fluorophore.
cessfully prepared and the N-2-isomer identified, we evaluated The detailed excitation and emission information of all TA-IM
their fluorescence emission to verify our initial design that an substrates 5 is summarized in Table 1.
intramolecular H-bond plays a crucial role in providing confor-
As shown in Table 1, all tested TA-IM 5 exhibited strong UV-
mational control for effective fluorescence emission. The emis- blue fluorescence emission with λmax between 330 nm and
sion spectra of several representative TA-derivatives including 380 nm. The electron donating group on imidazole (5b)
TA-IM are shown in Fig. 2
caused an emission red-shift while the electron withdrawing
As expected, TA-IM 5a and 5g displayed strong fluorescence group resulted in a slight blue-shift (5d). No significant elec-
emission with quantum yields (ΦPL) of 0.77 and 0.98 respect- tronic effect was observed on the phenyl substitution (5e vs.
ively. The stronger emission obtained with 5g over 5a is likely 5f). A large red-shift was obtained with conjugated N-2 aryl
due to the reduced steric hindrance of C-5-H in 5g over C-5-Ph substrates (5i, 5j and 5k), similar to the previously reported
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Table 1 Optical data of TA-IM ligandsa
Stokes
shift
(nm)
Emission
Ligand λmax (nm)
Excitation
λmax (nm)
ΦPL
(%)
Intensity
(a. u.)
5a
5b
5c
5d
5e
5f
5g
5h
5i
290
310
309
293
290
293
296
292
291
309
309
317
—
342
364
346
329
352
349
328
341
368
378
380
370
—
52
54
37
36
62
56
32
49
77
69
71
53
—
—
77
64
45
41
64
86
98
72
93
66
54
—
—
—
3837
2318
3154
2062
1836
2935
7246
3576
2780
2532
1717
258
Fig. 4 (A) Job’s plot: total concentration of 5a a and Ag+ was
10 μmol L−1. (B) ESI-MS of the [(5a)2-Ag]+ ion. aEtOH : HEPES, v : v = 1 : 99.
5j
5k
6a
6b
6c
ment.13 This could cause severe harm to the environment14
and human health.15 Hence, establishment of an efficient and
reliable sensor for the Ag+ detection is in great demand.
To identify the best TA-IM in Ag+ sensing, comparisons of
the quenching efficiency among all the prepared TA-IMs (5a–
5k) were performed. Compound 5a was identified as the
optimal ligand with the highest sensitivity (92%) in response
to Ag+ binding (Fig. 3B). To explore Ag+/TA-IM coordination,
standard titration was performed (Job’ method, Fig. 4A). A 2 : 1
ligand/cation ratio was revealed. ESI-MS further confirmed the
formation of a [L2Ag]+ complex with the detection of m/z at
809.10 as the major complex peak (Fig. 4B, see the detailed MS
spectra in Fig. S9†).
47
31
—
—
a Fluorescence emission of compounds 5a–6c. Concentration:
20 μmol L−1 in EtOH.
N-2-aryl triazole (NAT) system. Compared with NAT, TA-IM pos-
sessed similar UV-blue emission and comparable fluorescence
intensity/efficiency, suggesting a similar planar intramolecular
charge transfer (PICT) mechanism as proposed. With the fluo-
rescence-active ligand available, we tested whether metal
cation coordination may influence TA-IM fluorescence
emission.
As shown in Fig. 3A, treating a solution of 5a with various
metal cations (24 cations tested) showed almost no fluo-
rescence change except for the Ag+ cation. Notably, this highly
selective Ag+ induced fluorescence quenching is very efficient
(92%) under mild conditions (room temperature).
Impressively, this Ag+ sensing was very robust, showing almost
no influence of other cations and anions (no significant
change while combining Ag+ with more than 30 cations and
anions, see details in Fig. S7 and S8†). This result is interesting
and suggests that TA-IM can be developed as a potential fluo-
rescent sensor toward Ag+ detection. Since silver and silver
ion-containing materials have been increasingly used in
industry,12 a large amount of non-biodegradable silver ions
from industrial wastes is being discharged into the environ-
Based on the Benesi–Hildebrand equation, the binding
association constant Ka was determined as 1.01 × 107 M−2
,
suggesting a strong coordination between the ligand and Ag+
(Fig. S10†). The fluorescence lifetime of 5a was monitored
(Fig. S11†), showing no changes in the absence and presence
of Ag+. This result suggested that the observed silver cation
sensing was attributed to static quenching as proposed.16
One important feature of any cation fluorescent probe is
the response time. As shown in Fig. 5A, the solution fluo-
rescence intensity decreased immediately upon treating with
silver cations and reached equilibrium within 20 seconds. To
the best of our knowledge, this sensing response rate is faster
than any previously reported Ag+ fluorescent probes.17
Moreover, the FL emission intensity of 5a solution remained
stable even after 120 min irradiation treatment with a xenon
lamp (Fig. S12†). Detection within the pH range from 4.0 to
9.0 was performed with no clear drop of sensitivity and stabi-
Fig. 3 (A) The high selectivity of TA-IM for the detection of Ag+. (B)
Sensitivity of all TA-IM (5a–5k) in response to silver cations.
Concentration: 5a–5k, 2 μmol L−1; metal ions, 2 μmol L−1; EtOH : HEPES,
v : v = 1 : 99. The blue bar is the normalized FL intensity of 5a–6d; the
blank bar is the FL intensity of 5a–6d after treating with Ag+; the red
number is the quenching ratio of FL.
Fig. 5 (A) Time dependent titration of 5a (2 μmol L−1
)
with Ag+
(2 μmol L−1); (B) linear correlation of [Ag]+ and FL emission.
EtOH : HEPES, v : v = 1 : 99.
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lity (Fig. S13†). All these features (ultrafast response time,
excellent stability and a wide pH operating range in aqueous
medium) make TA-IM a promising fluorescent sensor for Ag+
detection.
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Finally, to establish the quantitative detection curve, titra-
tion of Ag+ with the TA-IM ligand was performed (Fig. 5B). At a
concentration from 1.0 × 10−7 to 1.0 × 10−6 mol L−1, a good
linear correlation between the fluorescence intensity and Ag+
concentration was obtained with R2 = 0.9948. The detection
limit (LOD) was calculated to be 9.4 nmol L−1 (based on S/N =
3, n = 20). To the best of our knowledge, this is much lower
than the previously reported fluorescent probes (see the
detailed comparison of TA-IM with the reported Ag+ sensors in
ESI, Table S5†).
Conclusions
In summary, we have successfully developed triazole-imidazole
ligands as highly selective fluorescent probes for detection of
silver ions. The introduction of an intramolecular H-bond
allows TA-IM to exhibit strong emission in both organic and
aqueous solutions. This sensor can be used in aqueous media
with an ultrafast response time (<30 s), which highlights the
practical advantages of this new cation probe and the impor-
tance of these triazole-based fluorescence compounds in
chemical and material research.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to the NSF (CHE-1665122), the NIH
(1R01GM120240-01), the NSFC (21629201) and the Development
Project of the Pharmaceutical Industry of Jilin Province
(20150311070YY; 20170307024YY) for financial support.
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