A highly sensitive and selective fluorescent chemosensor for Pb 2+ ions in an aqueous solution

Article abstract of DOI:10.1039/c2dt32531d

A new fluorescent sensor based on the BODIPY fluorophore and the polyamide receptor for Pb²⁺ was designed and synthesized. The sensor is highly selective for Pb²⁺ over relevant competing metal ions, and sensitive to ppb levels of Pb<

Full text of DOI:10.1039/c2dt32531d

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A highly sensitive and selective fluorescent
chemosensor for Pb2+ ions in an aqueous solution†
Cite this: DOI: 10.1039/c2dt32531d
Jing Liu,a Kai Wu,a Sha Li,a Tao Song,a Yifeng Han*a and Xin Li*b
Received 23rd October 2012,
Accepted 14th November 2012
A new fluorescent sensor based on the BODIPY fluorophore and the polyamide receptor for Pb2+ was
designed and synthesized. The sensor is highly selective for Pb2+ over relevant competing metal ions, and
sensitive to ppb levels of Pb2+. It features the most sensitive probe to date for Pb2+ ions in water.
DOI: 10.1039/c2dt32531d
www.rsc.org/dalton
present the synthesis and properties of a new Pb2+-ion specific
fluorescent chemosensor Leadsensor-1 (LS1) that has visible
Introduction
Lead, which is widely distributed in the environment such as wavelength excitation and emission profiles, excellent selecti-
the air, soil, and water due to its use in batteries, gasoline, and vity for Pb2+ ions over relevant competing metal ions, a 19-fold
pigments,1 is one of the most ubiquitous and poisonous heavy turn-on response, and high sensitivity to ppb levels of Pb2+ in
metals.2 Lead poisoning may induce anemia, neurological water.
damage, physical growth impairments, nerve disorders,
In 2006, Qian et al.14 disclosed that polyamide receptor
memory loss, kidney disorders, and reduced IQ.3 Despite (MR) based fluorescent molecule sensors featured high water
efforts to reduce global emissions, lead poisoning remains one solubility, unique Hg2+ ion selectivity, and significant signal
of the top environmental health problems.4 Due to the toxicity response upon Hg2+ ion complexation. We noticed that one of
of Pb2+, the determination of lead in biological and environ- the fluorescent sensors S3 also has slight fluorescence
mental samples is crucial both to the monitoring of environ- enhancements by Pb2+ ions (I/I0 = 1.4) in neat water solu-
mental pollution and to the diagnosis of clinical disorders. tion.14c Inspired by this, we designed a new fluorescent sensor
However, standard techniques, such as atomic absorption for Pb2+ (LS1) by introducing a diethanolamine group to the
spectrometry,5 inductively coupled plasma mass spec- polyamide receptor (Scheme 1). We reasoned that the introduc-
trometry,6 and anodic stripping voltammetry,7 often require tion of the diethanolamine group would potentially enhance
expensive and sophisticated instrumentation, and/or sample the selectivity of the sensor to Pb2+ ions while retaining its
preparation, and are therefore not suitable for real-time and water solubility because the diethanolamine group has been
in situ analysis.
used as a receptor for Pb2+ in acetonitrile which shows selectiv-
Optical sensors involving fluoroionophores are becoming ity but is limited by the relatively low sensitivity and quenching
popular because of their ease of application in solution as well in aqueous media.13j
as their high sensitivity to and selectivity for trace analytes
Herein, the preparation and fluorescence responses of the
with spatial and temporal resolution.8 In the past several new Pb2+-ion-responsive chemosensor LS1 are reported below.
years, considerable efforts have been made to develop fluor-
escent chemosensors for Pb2+ ions based on peptide,9
protein,10 DNAzyme,11 polymer,12 and small-molecule
scaffolds.13 However, most of them are limited by interfering
background fluorescence, nonspecific quenching from com-
peting metal ions, and/or incompatibility with water. We now
aThe Key Laboratory of Advanced Textile Materials and Manufacturing Technology,
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, China.
E-mail: hanyf@zstu.edu.cn; Tel: +86-751-86843550
bZJU-ENS Joint Laboratory of Medicinal Chemistry, College of Pharmaceutical
Sciences, Zhejiang University, Hangzhou, 310058, China.
E-mail: lixin81@zju.edu.cn; Tel: +86-751-88981051
†Electronic supplementary information (ESI) available: Fluorescence properties
of LS2, and copies of NMR and MS spectra. See DOI: 10.1039/c2dt32531d
Scheme 1 LS1 design inspired by previous work.
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complexation. The enhancement of fluorescence intensity of
Results and discussion
LS1 is linear to the corresponding lg[Pb2+] (0.05–1.25 μM)
LS1 has been synthesized as shown in Scheme 2. Alkylation of (R2 = 0.99294) (Fig. 2b). The association constants K11 and K21
1 with ethyl bromoacetate affords tetraester 2 in 67% yield. were determined by a nonlinear least-squares analysis of fluor-
Vilsmeier formylation of 2 using POCl3–DMF followed by basic escence intensity versus Pb2+ ion concentration to be 5.3 × 108
workup generates aldehyde 3 in 40% yield. BODIPY 4 is and 8.0 × 106, respectively (ESI†).15
obtained in a one-pot, three-step procedure via condensation
Moreover, the selectivity of LS1 toward different metal ions
of 3 with 2,4-dimethylpyrrole, followed by DDQ oxidation and was examined. Much to our delight, the turn-on response of
boron insertion with BF3·OEt2 (20% overall yield for three LS1 is highly specific for Pb2+ and no obvious change of fluor-
steps). Ester ammonolysis of 4 gives LS1 (5) in 23% yield.
escent emission was observed when it is treated with Hg2+,
Spectroscopic measurements for LS1 were performed in Ag+, Zn2+, Sn4+, Mn2+, Cu2+, Co2+, Ca2+, Fe3+, Na+, Ni2+, Cd2+,
neutral aqueous media (0.1 M PBS buffer, pH 7.2) for LS1 is Cr3+, Bi3+, Fe2+, and Mg2+ (Fig. 3a). It should be mentioned
inert to pH in the range of 6.5–8.0 (Fig. 1). LS1 alone shows a that LS1 still responds to Pb2+ sensitively even in the presence
visible absorption band centered at 497 nm (ε = 4.0 × 104 M−1 of other relevant competing ions (Fig. 3b).
cm−1) and is almost nonfluorescent (λem = 510 nm, Φ = 0.006),
Furthermore, binding analysis using the method of con-
indicative of efficient photoinduced electron transfer (PET) tinuous variations (Job’s plot) establishes that a 1 : 2 LS1 : Pb2+
quenching of the excited BODIPY fluorophore by the electron- complex is responsible for the observed fluorescence enhance-
donating polyamide receptor.14c
ment (Fig. 4).
However, addition of 2 equiv. of Pb2+ triggers a ca. 19-fold
To clarify the actual polyamide–Pb2+ interactions, LS2 and
(Φ = 0.112, Fig. 2a and Table S1, ESI†) increase in integrated LS3 bearing three and two amide arms into the polyamide
emission for LS1 with the same absorption (λabs = 496 nm) receptors were synthesized (Scheme 2). The results showed
and emission maxima (λem = 510 nm), indicating that the PET that LS3 is unable to form a complex with Pb2+ ions while
quenching pathway is efficiently blocked by Pb2+ ion LS2 forms a 1 : 1 complex with Pb2+ ions (K11 = 2.8 × 103)
Scheme 2 Synthesis of LS1, LS2, and LS3: (a) ethyl bromoacetate–KI–DIPEA, CH3CN, reflux, 8 h; (b) POCl3–DMF, 2 h; (c) (1) 2,4-dimethylpyrrole–TFA, CH2Cl2, rt,
10 h; (2) DDQ, rt, 4 h; (3) NEt3–BF3·OEt2, rt, 2 h; (d) diethanolamine, CH3CN, reflux, 24 h.
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Fig. 1 (a) Effect of the pH on the fluorescence emission of LS1 (0.05 μM) in a buffer solution. (b) Effect of the pH on the fluorescence emission of LS1–Pb2+
complex (2.0 equiv. of Pb2+) in a buffer solution.
Fig. 2 (a) Fluorescence emission spectra (λex = 470 nm) of LS1 (0.05 μM) in
Fig. 3 (a) Fluorescence responses of LS1 (0.25 μM) with 2.0 equiv. of metal
neutral aqueous media (0.1 M PBS buffer, pH 7.2) upon the addition of Pb2+
(0–2.5 μM). Inset: binding isotherm between LS1 and Pb2+ with emission inten-
sity at 510 nm. (b) Fluorescence intensity of LS1 at 510 nm as a function of
lg[Pb2+] (0.05–1.25 μM) under the same condition as the Pb2+ titration.
ions in 0.1 M PBS (pH 7.2). Metal ions include Pb2+, Hg2+, Ag+, Zn2+, Sn4+, Mn2+
,
Cu2+, Co2+, Ca2+, Fe3+, Na+, Ni2+, Cd2+, Cr3+, Bi3+, Fe2+, and Mg2+. λex = 470 nm.
(b) The fluorescence intensity of LS1 (0.25 μM) at 510 nm with 40 equiv. of the
competing metal ions, followed by 2.0 equiv. of Pb2+
.
localization of the aniline nitrogen lone-pair electrons by the
Pb2+ ions. The two single peaks assigned for the eight Hd
protons showed a slight upfield shift and split into two peaks,
indicating the electron shielding effect by the amide groups in
the complexation form. Accordingly, the structure of the LS1–
Pb2+ complex has been proposed (Fig. 6).
(Fig. S1–S3, ESI†), indicating that the amide arms play a key
role in the stability and binding mode of coordination com-
plexes. We recorded the 1H NMR spectra of LS1 and its
complex with Pb2+ in D2O (Fig. 5). Three aromatic protons
attributed to the polyamide receptor were distinctly downfield
shifted from δ 6.50–6.94 ppm to 7.52–7.88 ppm due to the
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Owing to its excellent optical properties, LS1 is sensitive
enough to detect environmentally relevant concentrations of
Conclusion
Pb2+ ions in an aqueous solution. As shown in Fig. 2b, In conclusion, we have designed and synthesized a new fluor-
addition of 15 ppb of Pb2+ ions, the maximum US EPA limit escent sensor (LS1) for Pb2+ ions in water. LS1 is a unique
for allowable levels of lead in drinking water, to a 0.05 μM solu- Pb2+-responsive small-molecule indicator that features visible
tion of LS1 affords a 276% emission increase.16 Furthermore, wavelength excitation and emission profiles, a selective turn-
the detection limit was calculated to be 1.34 × 10−8 M accord- on response to Pb2+ compared to relevant competing metal
ing to a reported method,17 which indicates that LS1 features ions, a 19-fold turn-on response, and sensitivity to ppb levels
the most sensitive probe to date for Pb2+ ions in water.
of Pb2+ ions in water. Titration experiments show that LS1 can
be used to detect changes of Pb2+ levels in drinking water,
illustrating its feasibility as a versatile tool to monitor Pb2+
widely in the environment. In addition, comparison of the
Fig. 4 Job’s plot showing the 1 : 2 binding of LS1 with Pb2+. The total concen-
tration of the sensor and Pb2+ is 0.5 μM in neutral aqueous media (0.1 M PBS
buffer, pH 7.2).
Fig. 6 A proposed structure of LS1–Pb2+ complex.
Fig. 5 1H NMR spectra of LS1 and LS1 + Pb2+ complex.
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structure of LS1 to that of S3, which is a chemosensor for Hg2+ dichloromethane–methanol–triethylamine (10 : 3 : 1, v/v/v) as
ions, reveals that the slight difference in appended amide the eluant to give 21.0 mg (23%) of LS1 as a red solid; 1H NMR
arms of a polyamide receptor can thoroughly change the recog- (400 MHz, D2O) δ 6.82 (brs, 1H), 6.69 (s, 1H), 6.34 (brs, 1H),
nition preference of sensors. We anticipate that the experimen- 5.98 (s, 1H), 5.97 (s, 1H), 4.49 (s, 4H), 4.45 (s, 4H), 3.56 (m,
tal results of this study should find utility in the future design 16H), 3.37 (m, 16H), 2.30 (s, 6H), 1.28 (s, 6H); 13C NMR
of metal-ion sensors in water for a variety of chemical and bio- (100 MHz, D2O) δ 172.74, 172.58, 155.46, 144.61, 142.69,
logical applications.
141.82, 141.75, 131.35, 127.34, 121.39, 121.20, 121.01, 119.69,
59.19, 59.13, 59.06, 53.95, 51.42, 50.14, 49.85, 48.48, 48.33,
13.78; HRMS (ESI) Calcd for ([M + H])+, 935.4861; Found,
935.4880.
The procedures of the synthesis of LS2 and LS3 were the
same as LS1.
Experimental section
Materials and measurements
All the solvents were of analytical grade. NMR experiments
were carried out on a Bruker AV-400 NMR spectrometer with
chemical shifts reported in ppm (in D2O, CDCl3, and CD3OD).
Mass spectra were measured on an Agilent 1290 LC-MS spec-
trometer. All pH measurements were made with a Sartorius
basic pH-Meter PB-10. Fluorescence spectra were determined
Compound 6. Compound 6 was synthesized according to
the literature.14c 1H NMR (400 MHz, CDCl3) δ 9.67 (s, 1H), 7.32
(d, J = 8.0 Hz, 1H), 7.19 (s, 1H), 6.73 (d, J = 8.0 Hz, 1H), 4.58
(s, 2H), 4.26–4.07 (m, 10H), 1.19 (t, J = 6.8 Hz, 9H).
1
Compound 7. Red solid (13%); H NMR (400 MHz, CDCl3)
δ 6.96 (d, J = 8.0 Hz, 1H), 6.81 (dd, J = 8.0, 1.6 Hz, 1H), 6.65 (d,
J = 1.6 Hz, 1H), 5.97 (s, 2H), 4.62 (s, 2H), 4.30–4.12 (m, 10H),
2.54 (s, 6H), 1.44 (s, 6H), 1.26 (d, J = 7.2 Hz, 9H).
on
a PerkinElmer LS55 Fluorescence spectrophotometer.
Absorption spectra were collected on a Shimadzu UV 2501(PC)
S UV-Visible spectrophotometer. The excitation and emission
widths for LS1–LS3 were all 5 nm.
Compound 8 (LS2). Red solid (33%); 1H NMR (400 MHz,
CD3OD) δ 7.32 (d, J = 8.4 Hz, 1H), 6.97 (s, 1H), 6.84 (d, J =
8.4 Hz, 1H), 6.05 (s, 2H), 4.27 (s, 4H), 3.70 (m, 13H), 3.53 (m,
13H), 2.47 (s, 6H), 1.49 (s, 6H); 13C NMR (100 MHz, CD3OD)
Synthesis
Compound 3. Compound 3 was synthesized according to δ 173.32, 170.98, 156.55, 152.46, 144.88, 142.04, 132.85,
the literature.14c 1H NMR (400 MHz, CDCl3) δ 9.83 (s, 1H), 7.60 122.53, 122.11, 114.02, 60.81, 60.54, 60.14, 56.07, 51.53, 50.96,
(d, J = 1.2 Hz, 1H), 7.49 (dd, J = 8.4, 1.2 Hz, 1H), 7.10 (d, J = 50.21, 49.76, 14.88; HRMS (ESI) Calcd for ([M + H])+, 791.3963;
8.4 Hz, 1H), 4.42 (s, 4H), 4.28 (s, 4H), 4.12 (m, 8H), 1.21 (t, J = Found, 791.3968.
7.2 Hz, 12H).
Compound 9. Compound 9 was synthesized according to
Compound 4. 2.6 g of 3 (5.4 mmol) and 1.4 mL (15.9 mmol) the literature.14c 1H NMR (400 MHz, CDCl3) δ 9.78 (s, 1H), 7.75
of 2,4-dimethylpyrrole were dissolved in 30 mL of absolute (d, J = 8.8 Hz, 2H), 6.66 (d, J = 8.8 Hz, 2H), 4.34–4.15 (m, 8H),
dichloromethane under a nitrogen atmosphere, three drops of 1.29 (t, J = 7.2 Hz, 6H).
1
trifluoroacetic acid were added and the solution was stirred at
Compound 10. Red solid (28%); H NMR (400 MHz, CDCl3)
ambient temperature for 10 h. A solution of dichlorodicyano- δ 7.08 (d, J = 8.2 Hz, 2H), 6.71 (d, J = 8.2 Hz, 2H), 5.96 (s, 2H),
benzoquinone (DDQ, 1.2 g, 5.3 mmol) in 10 mL dichloro- 4.30–4.14 (m, 8H), 2.54 (s, 6H), 1.45 (s, 6H), 1.28 (t, J = 7.2 Hz,
methane was added and stirring was continued for 4 h, 6H).
followed by the addition of triethylamine (10.0 mL,
Compound 11 (LS3). Red solid (48%); 1H NMR (400 MHz,
71.9 mmol) and BF3-OEt2 (10.0 mL, 79.6 mmol). After stirring CD3OD) δ 6.93 (d, J = 8.4 Hz, 2H), 6.73 (d, J = 8.4 Hz, 2H), 5.92
for another 2 h, the reaction mixture was washed with 200 mL (s, 2H), 4.40 (s, 4H), 3.66 (m, 8H), 3.58–3.44 (m, 8H), 2.36 (s,
water, extracted with dichloromethane (3 × 100 mL). The 6H), 1.39 (s, 6H); HRMS (ESI) Calcd for ([M + H])+, 630.3274;
extract was dried over sodium sulfate and then concentrated Found, 630.3280.
under vacuum. The product was purified by flash chromato-
graphy using petroleum ether–ethyl acetate (4 : 1, v/v) as the
eluant to give 0.8 g (20%) of 4 as a red solid; 1H NMR
Acknowledgements
(400 MHz, CDCl3) δ 7.12 (d, J = 8.0 Hz, 1H), 6.93 (s, 1H), 6.83
(d, J = 8.0 Hz, 1H), 5.95 (s, 2H), 4.34 (s, 4H), 4.28 (s, 4H),
4.16–4.01 (m, 8H), 2.54 (s, 6H), 1.40 (s, 6H), 1.19 (m, 12H).
Compound 5 (LS1). 68.0 mg of 4 (0.1 mmol) was dissolved
in 5 mL acetonitrile, and 10 mL diethanolamine was added.
The solution was refluxed under nitrogen for 24 h, cooled and
concentrated under vacuum to get rid of the acetonitrile, then
poured into 50 mL saturated brine. The mixture was neutral-
ized with sodium dihydrogen phosphate, and extracted with
dichloromethane (10 × 50 mL). The extract was dried over
sodium sulfate and then concentrated under vacuum. The
product was purified by flash chromatography using
This work was supported by the National Natural Science
Foundation of China (Grant no. 20902082) and the Zhejiang
Provincial Natural Science Foundation of China (Grant no.
Y3100416).
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