Highly selective and sensitive fluorescent turn-on chemosensor for AI 3+ based on a novel photoinduced electron transfer approach

Article abstract of DOI:10.1021/ol202054v

A photoinduced electron transfer (PET)-based chemosensor possessing dual PET processes by simultaneously introducing both nitrogen and sulfur donors was achieved. The fluorescence signal of the free chemosensor is in a normal-off state due to the sulfur d

Full text of DOI:10.1021/ol202054v

ORGANIC
LETTERS
2011
Vol. 13, No. 19
5274–5277
Highly Selective and Sensitive
Fluorescent Turn-on Chemosensor for
Al3þ Based on a Novel Photoinduced
Electron Transfer Approach
Yan Lu,† Shanshan Huang,† Yiyang Liu,† Song He,† Liancheng Zhao,†,‡ and
Xianshun Zeng*,†
School of Materials Science & Engineering, Tianjin University of Technology,
Tianjin 300384, China, and School of Materials Science and Engineering,
Institute of Information Functional Materials & Devices, Harbin Institute of Technology,
Harbin 150001, China
xshzeng@tjut.edu.cn
Received August 7, 2011
ABSTRACT
A photoinduced electron transfer (PET)-based chemosensor possessing dual PET processes by simultaneously introducing both nitrogen and
sulfur donors was achieved. The fluorescence signal of the free chemosensor is in a normal-off state due to the sulfur donor being insensitive to
environmental pH stimuli. As a result, the device can be used over a wide pH span of 3ꢀ11. Upon binding Al3þ, a significant fluorescence
enhancement with a turn-on ratio over 110-fold was triggered by the inhibition of PET processes from both the sulfur and the nitrogen donors to
the fluorophore.
The heightened concern for environmentally and biolo-
gically relevant species such as Al3þ, Pb2þ, Hg2þ, Cu2þ
and Fꢀ has stimulated active research on the potential
impact of their toxic effects. The molecular design and
synthesis of chemosensors to detect such species is cur-
rently of great interest.1 In this regard, chemosensors with
highselectivityand sensitivitytothese species are especially
important.2 Al3þ which widely exists in the environment
due to acidic rain and human activities is considered toxic
in biological activities.3 The incremental increase of Al3þ
concentrations in the environment is detrimental to grow-
ing plants.4 Excessive exposure of the human body to Al3þ
leads to a wide range of diseases, such as Alzheimer’s disease,
osteoporosis, etc.5 According to a WHO report, the average
daily human intake of aluminum is approximately 3ꢀ10 mg
per day.6 Due to the potential impact of Al3þ ions on human
,
† Tianjin University of Technology.
‡ Harbin Institute of Technology.
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2011, 40, 3416. (b) Xu, Z.; Chen, X.; Kim, H. N.; Yoon, J. Chem. Soc.
Rev. 2010, 39, 127. (c) He, Q.; Miller, E. W.; Wong, A. P.; Chang, C. J.
J. Am. Chem. Soc. 2006, 128, 9316. (d) Huang, S.; He, S.; Lu, Y.; Wei, F.;
Zeng, X.; Zhao, L. Chem. Commun. 2011, 47, 2408. (e) Li, Y.; He, S.; Lu,
Y.; Zeng, X. Org. Biomol. Chem. 2011, 9, 2606.
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Jr.; Cullum, U. X., Jr.; McCall, J. T.; Ellis, H. A. Kidney Int. 1980, 18,
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r
10.1021/ol202054v
Published on Web 09/14/2011
2011 American Chemical Society

health and the environment, highly selective and sensitive
chemosensors for Al3þ are hence highly demanded.
hydroxyl of 1,2-dihydroxyanthraquinone serves as an
ionophore by providing a coordinating site. The free
chemosensor L shows no fluorescence emission over a very
wide pH span (pH 1ꢀ12). Upon binding of a Al3þ cation, a
significant fluorescence enhancement with turn-on ratios
over 110-fold was achieved.
Known methods for aluminum detection, such as gra-
phite furnace atomic absorption spectrometry and induc-
tively coupled plasma atomic emission spectrometry, are
generally expensive and time-consuming in practice. Com-
paratively, optical detection, particularly fluorescence
methods, shows unique potentialfor high sensitivity. Com-
pared to transition metals, the detection of Al3þ has always
been problematic due to the lack of spectroscopic char-
acteristics and poor coordination ability.7 To the best of
our knowledge, only a few fluorescent chemosensors have
been reported for detection of Al3þ with moderate success
todate.3b,8 The majorityofthereported Al3þ sensors, how-
ever, have limitations such as tedious synthetic efforts8bꢀg
and/or lack of practical applicability in aqueous solut-
ions.8aꢀf Thus, it is still highly desirable to develop new or
improved methods for the selective evaluation of Al3þ ions
in aqueous environments.
Scheme 1. Structure and the Synthesis of the Chemosensor L
The photoinduced electron transfer (PET) process has
been widely exploited for the sensing of anions as well as
for cations.9 Generally, PET sensors based on nitrogen
donors are highly sensitive to environmental pH stimuli,
because the degree of nitrogen protonation is strongly pH
dependent.9 This pH sensitivityisunfavorable toobtaining
reproduciable and reliable signals for most PET sensors
used in a wide pH span. To obtain highly sensitive Al3þ
chemosensors in a wide pH span, novel chemosensors with
dual PET processes are desirable by simultaneously intro-
ducing a nitrogen donor10 and pH inert donors to ensure
the PET processes are always ‘ON’ in both acidic and basic
media. This design enables the fluorescence signal of the
free chemosensor to be always ‘OFF’, leading to remark-
able signal turn-on ratios over a wide pH span before and
after binding of a guest species.
Figure 1. UVꢀvis spectra of L (50 μM) with increasing amounts
of Al(NO3)3 (0ꢀ2.5 equiv). Inset: absorbance of L at 505 nm as a
function of CAl3þ/CL.
Based on this idea, we report a successful PET chemo-
sensor L by using 1,2-dihydroxyanthraquinone as the
fluorophore and a conjugated S2N podand moiety as the
chelating unit. The S2N podand moiety together with the
The synthesis of L is shown in Scheme 1. 1,2-Dihydrox-
yanthraquinone reacted with N,N-bis(2-phenylthioethyl)-
amine in the presence of paraformaldehyde in acetonitrile
yielded L in 53% yield. The structure of L was confirmed
by its NMR, MS spectra and elemental analysis (Figures
S1ꢀS4).
(6) (a) Barcelo, J.; Poschenrieder, C. Environ. Exp. Bot. 2002, 48, 75.
(b) Krejpcio, Z.; Wojciak, R. W. Pol. J. Environ. Stud. 2002, 11, 251.
(7) Soroka, K.; Vithanage, R. S.; Phillips, D. A.; Walker, B.; Dasgupta,
P. K. Anal. Chem. 1987, 59, 629.
The spectroscopic properties of the chemosensor L were
investigated in an ethanolꢀwater (1:1, v/v) solution. As
illustrated in Figure 1, the free chemosensor L exhibited a
maximal absorption at 446 nm (ε = 4.24 ꢁ 103 Mꢀ1 cmꢀ1).
Upon addition ofAl3þ ions (0ꢀ2.5 equiv), the absorbances
at 395 and 600 nm decreased gradually while the absor-
bance at 446 nm remained constant along with the increase
of the Al3þ concentration. In contrast, a significant in-
crease was observed for the absorption band at 504 nm.
The presence of three clear isosbestic points at 446, 579,
650 nm implies the conversion of the free chemosensor L to
the only Al3þ complex. Moreover, the absorbances at 395,
504, 600 nm remained constant in the presence of more
than 1 equiv of Al3þ ions, indicating the formation of a 1:1
complex between L and the Al3þ ion. This is in perfect
agreement with a 1:1 stoichiometry for the Al3þ complex
determined by Job’s plot yielded from UVꢀvis absorption
(8) (a) Kim, S. H.; Choi, H. S.; Kim, J.; Lee, S. J.; Quang, D. T.; Kim,
J. S. Org. Lett. 2010, 12, 560. (b) Maity, D.; Govindaraju, T. Chem.
Commun. 2010, 46, 4499. (c) Lin, W.; Yuan, L.; Feng, J. Eur. J. Org.
Chem. 2008, 3821. (d) Othman, A. B.; Lee, J. W.; Huh, Y. D.; Abidi, R.;
Kim, J. S.; Vicens, J. Tetrahedron 2007, 63, 10793. (e) Wang, Y.-W.; Yu,
M.-X.; Yu, Y.-H.; Bai, Z.-P.; Shen, Z.; Li, F.-Y.; You, X.-Z. Tetrahe-
dron Lett. 2009, 50, 6169. (f) Maity, D.; Govindaraju, T. Inorg. Chem.
2010, 49, 7229. (g) Arduini, M.; Felluga, F.; Mancin, F.; Rossi, P.;
Tecilla, P.; Tonellato, U.; Valentinuzzi, N. Chem. Commun. 2003, 1606.
(h) Upadhyay, K. K.; Kumar, A. Org. Biomol. Chem. 2010, 8, 4892.
ꢀ
(i) Jeanson, A.; Bereau, V. Inorg. Chem. Commun. 2006, 9, 13. (j) Ng,
S. M.; Narayanaswamy, R. Anal. Bioanal. Chem. 2006, 386, 1235.
(k) Zhao, Y.; Lin, Z.; Liao, H.; Duan, C.; Meng, Q.-J. Inorg. Chem.
Commun. 2006, 9, 966. (l) Wang, L.; Qin, W.; Tang, X.; Dou, W.; Liu,
W.; Teng, Q.; Yao, X. Org. Biomol. Chem. 2010, 8, 3751. (m) Kim, H. J.;
Kim, S. H.; Quang, D. T.; Kim, J. H.; Suh, I. H.; Kim, J. S. Bull. Korean
Chem. Soc. 2007, 28, 811.
(9) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997,
97, 1515.
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D. Inorg. Chem. 1995, 34, 3581.
Org. Lett., Vol. 13, No. 19, 2011
5275

(Figure S5). High-resolution mass spectra (HRMS) pro-
vided additional evidence for the formation of a 1:1
complex of L Al3þ (Figure S6).
3
Figure 3. Fluorescence spectra of L (50 μM) in the presence of
several relevant metal ions (8 equiv for each metal ion). Inset:
histogram representing the fluorescence enhancement and
quenching of L in the present of metal ions. λex = 460 nm.
Figure 2. Fluorescent titration spectra of L (50 μM) in the
presence of different concentrations of Al(NO3)3. Inset: the
fluorescence at 603 nm of L as a function of the Al3þ concentra-
tion. λex = 460 nm.
NH4þ, Ni2þ, and Zn2þ has no obvious effect on the fluor-
escence emission, whereas Pb2þ responded with a slight
increase in the fluorescent intensity at 626 nm. In stark
contrast, the addition of Al3þ resulted in a significant
enhancement of the emission intensity positioned at 603 nm.
Thus, L can function as a highly selective fluorescence
Figure 2 shows the fluorescence spectrum of the free
chemosensor L and those in the presence of an incremental
amount of Al3þ in a H2Oꢀethanol solution (1:1, v/v). The
fluorometric titration reaction curve showed a steady and
smooth increase with the increase of the Al3þ concentration.
A turn-on ratio over 110-fold was triggered with the addition
16 equiv of Al3þ. The inset in Figure 2 shows the dependence
of the emission intensity ratio (Fi/F0) at 603 nm on the Al3þ
concentration. The chemosensor exhibited an efficient fluor-
escence response. An over 35-fold increase in fluorescence
intensity was observed with the addition of 1 equiv of Al3þ
ions. The association constant (Ka) of L with Al3þ was
8.84 ꢁ 103 Mꢀ1 (R = 0.9934), as obtained by nonlinear
chemosensor for Al3þ
.
To explore the possibility of using L as a practical ion-
selective fluorescent chemosensor for Al3þ, competition
experiments were carried out, in which L (50 μM) was
first mixed with 10 equiv of various metal ions including
Ca2þ, Cd2þ, Co2þ, Cr3þ, Kþ, Mg2þ, Naþ, NH4þ, Ni2þ
,
and Zn2þ, followed by adding 10 equiv of Al3þ. Fluores-
cence emission spectroscopy was used to monitor the
competition events. As can be seen from Figure 4, in the
presence of Ca2þ, Cd2þ, Co2þ, Cr3þ, Kþ, Mg2þ, Naþ,
NH4þ, Ni2þ, Zn2þ, the emission spectra were almost
identical to that obtained in the presence of Al3þ alone.
In the case of Agþ, Cu2þ, Fe3þ, Pb2þ, and Hg2þ, the emis-
sion intensities diminished to a different extent to that
obtained in the presence of Al3þ alone, but they still had a
sufficient turn-on ratio for the detection of Al3þ. There-
fore, L was shown to be a promising selective fluorescent
sensor for Al3þ in the presence of most competing metal
ions.
least-squares analysis (Figure S7). From the changes in Al3þ
-
dependent fluorescence intensity (Figure S8), the detection
limit was estimated to be 5.0 ꢁ 10ꢀ7 M, indicating that the
limit of detection of L to Al3þ met the limit for drinking
water according to the China EPA standard (0.05 mg Lꢀ1
,
1.85 mM). Sufficient fluorescence enhancement was ob-
served for L after the addition of Al3þ in the presence of
different counteranions (Clꢀ, Brꢀ, Iꢀ, CO32ꢀ, SO4
,
2ꢀ
PO42ꢀ, ClO4ꢀ, NO2ꢀ, PF6ꢀ, AcOꢀ, and BF4ꢀ) (Figure S9),
suggesting that the chemosensor L can be used as a
selective fluorescent sensor for Al3þ in the presence of a
wide range of the environmentally relevant anions.
Reversibility is a prerequisite in developing novel che-
mosensorsfor practical application. The reversibilityofthe
recognition process of L was performed by adding an Al3þ
bonding agent, Na2EDTA. The addition of Na2EDTA to
a mixture of L and Al3þ resulted in diminution of the
fluorescence intensity at 603 nm, which indicated the
regeneration of the free chemosensor L. The fluorescence
was recovered by the addition of Al3þ again (Figure S10).
Such reversibility and regeneration are important for the
fabrication of devices to sense the Al3þ ion.
For practical applications, the proper pH condition of
the chemosensor L was evaluated. The inset of Scheme 2
shows the dependence of the fluorescence intensity of L on
pH values with and without the addition of Al3þ in EtOH/
H2O (1:1, v/v). As expected, the fluorescence of unbonded
L was almost 100% quenched over a wide pH span of
1ꢀ12. A PET sensor based on nitrogen donors was highly
sensitive to environmental pH stimuli due mainly to the
protonation degree of the nitrogen being strongly depen-
dent on environmental pH.9 The quenching of fluores-
cence of L even at low pH conditions indicated that the
lone electron pairs of the sulfur donors played an impor-
tant role in the modulation of the PET processes11
(Scheme 2). In neutral and acidic media, although the
The response of L to other metal ions was also studied.
As shown in Figure 3, the addition of 8 equiv of Agþ, Ca2þ
,
(11) Gunnlaugsson, T.; Davis, A. P.; O’Brien, J. E.; Glynn, M. Org.
Lett. 2002, 4, 2449.
Cd2þ, Co2þ, Cr3þ, Cu2þ, Fe3þ, Hg2þ, Kþ, Mg2þ, Naþ,
5276
Org. Lett., Vol. 13, No. 19, 2011

Scheme 2. Possible PET Processes of L under Different pH
Conditionsa
Figure 4. Change ratio ((F ꢀ F0)/F0) of fluorescence intensity of
L (50 μM) at 603 nm in various mixtures of metal ions (Al(NO3)3
5 ꢁ 10ꢀ4 mol/L and one other metal ion 5 ꢁ 10ꢀ4 mol/L). (1)
Al3þ;(2) Al3þ þ Agþ;(3) Al3þ þ Ca2þ;(4) Al3þ þ Cd2þ;(5) Al3þ þ
Co2þ; (6) Al3þ þ Cr3þ; (7) Al3þ þ Cu2þ; (8) Al3þ þ Fe3þ; (9)
Al3þ þ Hg2þ; (10) Al3þ þ Kþ; (11) Al3þ þ Mg2þ; (12) Al3þ þ
Naþ; (13) Al3þ þ NH4þ; (14) Al3þ þ Ni2þ; (15) Al3þ þ Pb2þ
;
(16) Al3þ þ Zn2þ
.
a Inset: Effect of pH on the emission intensity of L and its Al3þ
-
PET process from a nitrogen donor to the fluorophore was
blocked by intramolecular hydrogen bonding or by pro-
tonation of the nitrogen donor, the PET derived from the
sulfur donors to the fluorophore is always switched on and
resulted in the fluorescence being 100% quenched (‘L•Hþ’
and ‘L’ in Scheme 2). Under basic conditions, the PET
processes from both the nitrogen donor and the sulfur
donors to the fluorophore were switched on due to the
deprotonation of the phenol hydroxy, leading to the
complete quenching of the fluorescence (‘Lꢀ’ in Scheme 2).
After bonding with Al3þ, the phenol O, N, and S donors
are likely to coordinate to the Al3þ ion. The PET processes
derived from the lone electron pairs of both the nitrogen
and sulfur donors to the fluorophore were fully blocked
and triggered strong fluorescence enhancements over a
wide pH range of 3ꢀ11.
complex at 603 nm in ethanol/H2O (1:1, v/v): red circle, 50 μM L; black
square, 50 μM L þ 5 ꢁ 10ꢀ4 mol/L Al3þ
.
in the presence of Al3þ, allowing its reversible detection in
the presence of a wide range of environmentally relevant
competing metal ions and anions. This strategy may
provide a general way for designing new PET sensors to
detect other environmentally and biologically relevant
species.
Acknowledgment. This work was sponsored by the
NCET-09-0894, NNSFC (20972111, 21074093, 21004044),
SRF for ROCS, SEM, the NSFT (08JCYBJC-26700). and
the State Key Lab. Elemental-Organic Chemistry at Nankai
University (No. 1011).
In conclusion, we have developed a novel chemosensor
L based on multiple PET processes from both the nitrogen
donor and the sulfur donors to the 1,2-dihydroxyanthro-
quinone fluorophore by a facile one-step Mannich reac-
tion. The fluorescence signal of the free chemosensor is
always off due to the sulfur donor being insensitive to
environmental pH stimuli. The device can be used within a
wide pH span of 3ꢀ11. The chemosensor L shows an
excellent turn-on fluorescence signal with high sensitivity
Supporting Information Available. Experimental de-
tails; synthesis of L; the emission spectra of L with Al3þ
salts with different counteranions; Job’s plot; the mea-
surement of the detection limit; the reversible binding
nature of Al3þ with L; 1H and 13C NMR spectra of L or
other electronic format. This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 19, 2011
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