A highly sensitive and selective turn-on fluorescent probe for Pb(II) ions based on a coumarin-quinoline platform

Article abstract of DOI:10.1039/c6ra19734e

A new fluorescent probe T1, which contained a coumarin fluorophore with a triazole substituted 8-hydroxyquinoline (8-HQ) receptor and a Schiff base spacer, was rationally designed and synthesized. It displayed a high affinity towards Pb²⁺ ions with a dissociation constant (K_d) of 0.1 μM and possessed a high selectivity for Pb²⁺ ions. A visually detectable colour change from colourless to yellow and a 30-fold fluorescence enhancement were observed upon addition of Pb²⁺. Experiments also indicated that a 1 : 1 stoichiometry of T1 with Pb²⁺ inhibited the photo-induced electron transfer (PET) process, resulting in the increase of fluorescence intensity.

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A Highly Sensitive and Selective Turn-on Fluorescent
Probe for Pb(II) ion based on Coumarin-quinoline
Platform
Cite this: DOI: 10.1039/x0xx00000x
Guangfu Wu a, b, Mingxin Li a, Jieji Zhu a, King Wai chiu Lai b, Qingxiao Tong a,*
Feng Lu a,*
,
Received 00th **** 2016,
Accepted 00th **** 2016
DOI: 10.1039/x0xx00000x
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used in fluorescent probes presenting great advantages in bioꢀ
imaging. [10] But it had some intrinsic disadvantages such as short
emission wavelength, low fluorescence quantum yield, poor stability
and solvent polarityꢀdependent emission. Recently 8ꢀHQ was
introduced into fluorescent probes acting as a metal ion chelating
agent for metal ions detection [11], but most of these probes were
using 2ꢀ/7ꢀ modified 8ꢀHQ as the receptors. 8ꢀsubstituted 8ꢀHQ was
rarely involved in fluorescent probes construction because the
alkylation of 8ꢀOH in 8ꢀHQ might sacrifice efficient ligating
hydroxyl group. Previously, our group reported two fluorescent
probes for cadmium ion [12] recognition in which 8ꢀOH moiety was
substituted by 2ꢀmethylpyridine. Although both of these two probes
could specifically recognize cadmium ions, the low affinity might
limit their practical applications.
A new fluorescent probe T1, which contained coumarin
fluorophore with a triazole substituted 8-hydroxyquinoline
(8-HQ) receptor and a Schiff base spacer, was rationally
designed and synthesized. It displayed a high affinity towards
Pb2+ ions with a dissociation constant (Kd) of 0.1 µM and
possessed a high selectivity for Pb2+ ions. A visually detectable
colour change from colourless to yellow and 30-fold
fluorescence enhancement were observed upon addition of
Pb2+. Experiments also indicated that a 1:1 stoichiometry of
T1 with Pb2+ inhibited photo-induced electron transfer (PET)
process, resulting in the increase of fluorescence intensity.
Lead ions, extremely toxic heavy metal ions, could cause great
threat to human health because they can enter into the human body
and accumulate through the food chain, and they are difficult to be
excreted. [1ꢀ2] Exposure to even small amounts of lead ions can
cause health problems, such as digestive, neurological and
cardiovascular disorders. For children, it can bring some particular
problems including slowed motor responses, decreased IQs, and
hypertension. [3] The safety concentration of lead ions in aquatic
organisms is 0.16 mg/L and the normal level in human body should
be below 0.1 mg/L. Fluorescent probes are becoming popular
because of their ease of application in solution as well as their high
sensitivity and selectivity for trace metal ions, [4] anions [5] and
small molecules. [6] However, Pb2+, as well as Cu2+ ions and Hg2+
ions, is known as a kind of inherent fluorescence quenching metal
ion, [7ꢀ8] so few studies were available for “OFFꢀON” fluorescent
probes which can recognize Pb2+ ions [9] with high selectivity and
sensitivity.
Motivated by these studies, we used triazole to substitute 8ꢀOH in
8ꢀHQ as the receptor. Schiff base structure was built between 2ꢀ
formyl substituted 8ꢀHQ and 3ꢀamino coumarin. Trizole substituted
8ꢀHQ could furnish an extra binding site, and might inhibit the
possible excitedꢀstate intramolecular proton transfer (ESIPT) event.
8ꢀHQ derivative was connected with coumarin by C=N structure.
Coumarin dye was employed as fluorophore due to its outstanding
photophysical properties and easy modification [13]. The C=N
structure might provide another binding state, which could improve
the selectivity. As expected, the probe T1 presented excellent
selectivity and sensitivity towards Pb2+ that caused an “OFFꢀON”
response with a 30ꢀfold fluorescence enhancement.
The elaborately designed probe T1 was synthesized based on the
route listed in Scheme 1. 8ꢀhydroxyquinolineꢀ2ꢀcarboxaldehyde (8ꢀ
HQC) was obtained from 2ꢀmethylꢀ8ꢀhydroxyquinoline oxidation
product by using SeO2 as oxidizer. The hydroxyl group of 8ꢀHQC
substituted by bromoacetylene gave 8ꢀpAlꢀHQC. Triazole was
successfully introduced after alkynyl reacted with NaN3 based on
‘click reaction’ [14] principle. 8ꢀBnTꢀHQC was obtained after this
step. At last, the reaction between formyl group of 8ꢀBnTꢀHQC and
amino group of coumarin devirative afforded the desired product T1,
We herein presented a novel “OFFꢀON” fluorescent probe T1 for
Pb2+ ions detection. T1 displayed high selectivity for Pb2+ ions over
other metal ions and displayed visually detectable colour change
from colourless to yellow with 30ꢀfold fluorescence enhancement.
Receptor is an important factor when designing new fluorescent
probes. As a fluorophore, 8ꢀhydroxyquinoline (8ꢀHQ) was widely
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and it was well characterized by 1HꢀNMR, MS and IR. Detailed
The fluorescence titration spectra of T1 with Pb2+ were shown in
experimental steps and related characterizations were shown in ESI† Fig. 2. T1 exhibited very weak fluorescence with a low fluorescence
(Fig. S1ꢀS3).
quantum yield (0.0038) using quinine sulfate in 0.5 M H2SO4 as the
reference [15] when excited at 385 nm. It was ascribed to the PET
process from lone electron pair in nitrogen atom of triazole and
quinolone to coumarin, which quenched fluorescence of T1. Upon
addition of Pb2+, T1 showed a turnꢀon response and the emission
band located at 466 nm increased gradually. Titration of T1 with
Pb2+ (0ꢀ2 equiv.) in buffer solution resulted in
a 30ꢀfold
enhancement in fluorescence intensity. The clear “OFFꢀON”
response was attributed to that the complexation of T1 and Pb2+
suppressed the PET process from triazoleꢀquinolone to coumarin, the
electronꢀdonating ability of nitrogen atom was effectively
suppressed, restraining PET process and resulting in a significant
increase in fluorescence emission. Additionally, the addition of Pb2+
made the fluorescence colour clearly change from colourless to blue.
The apparent disassociation constant (Kd) of T1 with Pb2+ was 0.1
µM implied that Pb2+ had a high affinity toward Pb2+. This value is
better than that of most reported fluorescent probes (Table S1). The
reaction of T1 with Pb2+ followed a 1 : 1 stoichiometry, as
confirmed by a Job's plot of the emission spectra as a function of
molar fraction of T1 (Fig. S4) and ESIꢀMS (Fig. S2). The
fluorescence intensity of T1 shows a good linear response to Pb2+ in
the concentration range of 6 to 20 µM, with a regression equation of
I466 = 32.098 × [Pb2+] (µM) ꢀ 44.1 (Fig. S6). The detection limit
(3S/m [16], where S is the standard deviation of 15 blank
measurements, and m is the slope of the linear equation) was
determined to be as low as 33.6 nM, which is comparable with other
groups’ work. [17] Besides, the stability of the imine of T1 in buffer
solution was also investigated. The fluorescent intensity at 466 nm
of T1 and T1ꢀPb2+ barely changed within 3 h, suggesting that the
monitoring system is stable (Figure S7) [18].
Scheme 1 Synthesis route of T1.
As shown in Fig. 1, T1 in aqueous HEPES (CH3CN: H2O = 95:5,
v/v, 10 mM HEPES, pH 7.2) exhibited two main absorption bands
located at 240 nm (ε = 5.3 × 104 Mꢀ1 cmꢀ1) and 323 nm (ε = 3.1 × 104
M
ꢀ1 cmꢀ1), respectively. Both of these two bands were attributed to πꢀ
π* transition resulted from PET (photoꢀinduced electron transfer)
process. [12] Upon addition of Pb2+, the absorbance at 240 nm and
323 nm decreased gradually, along with the emergence of two bands
at 260 nm and 385 nm. The presence of three wellꢀdefined isobestic
points at 250 nm, 307 nm and 340 nm indicated the formation of T1ꢀ
Pb2+ complex. These changes were associated with a visually
detectable colour change from colourless to bright yellow (inset of
Fig. 1).
Fig. 1 UVꢀVis absorption spectral changes of T1 (20 ꢁM) in buffer solution
(CH3CN:H2O = 95:5, v/v, 10 mM HEPES, pH 7.2) upon addition of Pb2+ (0
to 40 ꢁM). Inset: visible colour changes of T1 in the absence (left) and
presence (right) of Pb2+ (2 equiv.).
Fig. 3 Selectivity and competition studies of T1 (20 ꢁM) with 1 eq. Pb2+ in
the presence of 2 equiv. other metal ions (Zn2+, Cd2+, Ag+, Fe2+, Co2+, Ni2+,
Cu2+, K+, Ca2+, Na+, Ba2+, Mg2+, Hg2+) in buffer solution (CH3CN:H2O = 95:5,
v/v, 10 mM HEPES, pH 7.2). Top: Fluorescence changes of T1 in the
presence of various metal ions using a handheld UV lamp.
Selectivity is an important indicator to evaluate a fluorescent
probe. To verify specific recognition to Pb2+ ions, the emission
spectra of T1 response to other heavy, transition, and main group
metal ions were recorded as well. As shown in Fig. 3, addition of
Fig. 2 Fluorescence spectral changes of T1 (20 ꢁM) in buffer solution
(CH3CN:H2O = 95:5, v/v, pH 7.2) upon addition of Pb2+ (0 to 40 ꢁM),
λex=385 nm. Inset: Fluorescence color changes of T1 in the absence (left) and
presence (right) of Pb2+ (2 equiv.) using a handheld UV lamp.
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Pb2+ ions caused remarkable fluorescence changes, whereas other coumarin led to weak fluorescence of T1. Upon binding with Pb2+,
metal ions Zn2+, Cd2+, Hg2+, Ag+, Fe2+, Co2+, Ni2+, Cu2+, K+, Ca2+, PET process was effectively suppressed, causing a considerable
Na+ and Ba2+ exerted no or little change on emission of T1 which increase in fluorescence intensity and realizing an “OFFꢀON”
testified that T1 had a good selectivity towards Pb2+. Moreover, a response toward Pb2+. The colour changes from colourless to bright
competition experiment was also carried out to further explore the yellow in absorption, colourless to blue in emission were visually
utility of T1 as an ionꢀselective probe for Pb2+ ions. As illustrated by detectable.
the red bars in Fig. 3 below, a significant level of fluorescence was
observed for Pb2+ ions as compared with other metal ions. The
coexistent metal ions had a negligible influence on the fluorescence
intensity, whereas the existent Hg2+ ions didn’t cause fluorescence
enhancement. Possibly, it is because Hg2+ possessed a higher affinity
than Pb2+ toward T1, and Hg2+ could quench fluorescence via CHEQ
(chelationꢀenhanced fluorescence quenching) effect which was
observed in other probes. [19] Both C=O in coumarin and triazole
provided binding sites, causing a stronger binding of T1 with Hg2+
[20]. The distinct responses of selectivity and competition studies
Fig. 5 Proposed binding mode of T1 toward Pb2+.
indicated that T1 could be used to distinguish Pb2+ ions over many
other metal ions.
In conclusion, we have designed and synthesized
a new
The binding mode of T1 and Pb2+ was further investigated by 1Hꢀ
NMR in CD3CN. As shown in Fig. 4, the chemical shifts of T1ꢀPb2+
had significant down field shift from 8.39 ppm to 8.82 ppm for Ha
and from 5.08 ppm to 5.20 ppm for Hb, c, whereas other protons
showed no or little changes. Similar phenomena were also found in
other reported probes. [16b] Therefore, we deduced that one nitrogen
atom of triazole close to methylene, oxygen atom, nitrogen atom of
quinolone and nitrogen atom of HC=N structure together
participated in binding with a Pb2+ ion. However, oxygen atom of
carbonyl group in coumarin might be another binding site, and this
hypothesis couldn’t be confirmed from 1HꢀNMR titration results.
Then IR was employed to examine the binding mode (Fig. S2). The
distinct vibration located at ~3260ꢀ3330 cmꢀ1 and ~1713 cmꢀ1 in IR
spectrum of T1 were respectively assigned to CꢀH stretching
vibration of HC=N structure and C=O stretching vibration of
coumarin. When binding with Pb2+ ion, a new significant vibration
located at ~1088 cmꢀ1 appeared which was attributed to C=N
stretching vibration of HC=N structure. It is because C=N was fixed
and showed strong IR signal upon binding with Pb2+ ions. The
vibration wavelength corresponding to C=O stretching vibration kept
stable before and after binding with Pb2+ ions, which indicated that
C=O didn’t provide a Pb2+ binding site. Additionally, the vibration
located at ~3260ꢀ3330 cmꢀ1 exhibited an obvious change. The MS
results further verified the binding ratio of 1:1 between T1 and Pb2+
as mentioned above (Fig. S2).
fluorescence probe for Pb2+ based on a PET mechanism. Probe T1
exhibited high selectivity and sensitivity towards Pb2+ with 30ꢀfold
fluorescence intensity enhancement (OFFꢀON response) because of
the formation of Pb2+ꢀT1 complex with a binding ratio of 1:1. It
could availably distinguish Pb2+ over other metal ions. The binding
event between T1 and Pb2+ resulted in detectable colour change from
colourless to bright yellow in absorption, colourless to blue in
emission. Hence, probe T1 possessed great potential in preliminary
rapid detection of Pb2+.
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (No. 91027041 and 51273108) and the
National Basic Research Program of China (No. 2013CB834803).
Notes and references
a
Department of Chemistry, Shantou University, Guangdong, 515063, P.
R. China. Fax: +86 754 86500648; Tel: +86 754 86502508 E-mail:
qxtong@stu.edu.cn;
b Department of Mechanical and Biomedical Engineering, City University
of Hong Kong, 83 Tat Chee Ave., H.K. SAR, China.
† Electronic Supplementary Information (ESI) available: Detailed
experimental procedures, synthesis and characterization data, additional
spectroscopic data available. See DOI: 10.1039/b000000x/
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