A novel turn-on fluorescent probe for Fe3+ in aqueous media based on CN isomerization

Article abstract of DOI:10.1039/c2nj40767a

A new type of fluorescent probe (L) for Fe³⁺ based on CN isomerization was synthesized. It shows fluorescence enhancement (turn-on) response for Fe³⁺ in aqueous media (v(CH₃CN):v(H ₂O) = 1:9). The complexation mode and the corresponding fluorescent enhancement mechanism were elucidated by IR spectra, transient spectra, Job plot, and DFT calculations.

Full text of DOI:10.1039/c2nj40767a

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LETTER
A novel ‘‘turn-on’’ fluorescent probe for Fe3+ in aqueous media based on
Q
C N isomerizationw
Huajie Xu,a Zhaodi Liu,*a Liangquan Sheng,a Mingming Chen,ab Deqian Huang,a
Hong Zhang,a Chongfu Songa and Shuisheng Chena
Received (in Gainesville, FL, USA) 29th August 2012, Accepted 26th October 2012
DOI: 10.1039/c2nj40767a
1
A new type of fluorescent probe (L) for Fe3+ based on C
N
was confirmed by X-ray crystallography, H NMR, 13C NMR
and MS data. The details of the synthesis are described in the
ESI.w This probe exhibits a fluorescence turn-on signal toward
Fe3+ in CH3CN–H2O buffer solution with high sensitivity and
selectivity compared with other metal ions examined.
Q
isomerization was synthesized. It shows fluorescence enhancement
(turn-on) response for Fe3+ in aqueous media (v(CH3CN) :
v(H2O) = 1 : 9). The complexation mode and the corresponding
fluorescent enhancement mechanism were elucidated by IR spectra,
transient spectra, Job plot, and DFT calculations.
The single-crystal of compound L was obtained from acetonitrile
solution by slow evaporation at ambient temperature. The crystal
structure with atom-numbering is shown in Fig. 1. The chromone-
ring and the N2, N3/C11–C13 ring are nearly coplanar with a
dihedral angle of 4.3 (2)1, the semirigid structure with the potential
binding sites (O2, N1, O3) may create suitable space to better
match the corresponding ions.
Fe3+ is the most abundant transition metal in cellular systems
and is of outstanding importance due to its biological functions,
including essential roles in oxygen uptake, oxygen metabolism,
electron transfer, and transcriptional regulation.1 Fe3+ deficiency
and overload can induce various diseases, such as Parkinson’s
disease, Alzheimer’s disease and cancer.2 Therefore, development
of a Fe3+-amplified fluorescent probe, as an efficient and
economic method for the analysis of Fe3+, is very important in
biological systems and environmental science. However, the
paramagnetic nature of Fe3+ always leads to the fluorescent
quenching,3 so the probes of Fe3+-amplified emission in aqueous
media are very rare examples,4 most of which are based on
opening rhodamine derivatives spiro ring via ion binding.
Therefore, design of a new type of Fe3+-amplified fluorescent
probe is still a challenge.
The UV-vis titration of the Fe3+ was conducted by using 20 mM
of L in CH3CN–water (1/9, v/v). As shown in Fig. S1 (ESIw), free L
reveals three absorption bands at 238, 284 and 334 nm
(molar extinction coefficient e238 = 3.7 Â 104 MÀ1 cmÀ1, e284
=
3.1 Â 104 MÀ1 cmÀ1 and e334 = 3.3 Â 104 MÀ1 cmÀ1), upon
addition of Fe3+, the band at 334 nm gradually weakens and a new
band appears with an isosbestic point at 372 nm, indicating
formation of a new complex between L and Fe3+
.
The acid titration control experiments revealed that no
significant fluorescence response of L was observed in the
pH range from 6.0 to 10.0, suggesting that L was insensitive to
the pH under neutral or alkali conditions (Fig. S2, ESIw).
Thus, the probe was expected to work well under physiological
conditions. Simultaneously, considering the hydrolysis property
of Fe3+, pH 7.0 was chosen as the optimal test condition.7
The fluorescence titration of the probe (20 mM) was examined
in acetonitrile and acetate buffer solution (v/v = 1/9, 10 mM,
pH = 7.0). As shown in Fig. 2, obviously, the free L is non-
fluorescent because the CQN isomerization is the predominant
decay process in the excited states5 but the fluorescence intensity
Recently, CQN isomerization as a signaling mechanism has
been actively developed to design fluorescent probes.5 Since
CQN isomerization is the predominant decay process of excited
states, compounds containing unbridged CQN bonds are
usually non-fluorescent. However, when metal cations were
added to the group, the CQN isomerization may be inhibited,
thereby leading to strong fluorescence emission. Following this
strategy, a number of excellent CQN isomerization-based
probes have been exploited for sensing various metal cations
À
(e.g. Zn2+, Cd2+, Cu2+, Mg2+) as well as ClOÀ, HSO3
,
Cys and Hcy.6 Herein we reported a Fe3+-amplified fluorescent
probe based on CQN isomerization. The structure of probe L
a Department of Chemistry, Fuyang Normal College, Fuyang, China.
E-mail: zhaodi_liu@163.com; Fax: +86-558-2593836;
Tel: +86-558-2593836
b Department of Chemistry, Anhui University, Hefei, China
w Electronic supplementary information (ESI) available: Synthesis,
characterization and spectroscopic and binding studies of compounds
L and L–Fe3+. CCDC 894138 for L. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2nj40737a
Fig. 1 View of the structure of L with displacement atomic ellipsoids
drawn at the 30% probability level.
c
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Fig. 3 Plot of emission intensities vs. [Fe3+] ([L] = 20 mM, [Fe3+] =
0–90 mM).
Fig. 2 Fluorescence titration spectra of L (20 mM) in CH3CN–water
(1/9, v/v, pH = 7.0) solution upon the addition of Fe3+, from bottom
to top: 0–40 equiv. Fe3+ (lex = 372 nm). Inset: changes in fluorescence
a physiological application point of view, the interference from
other metal ions should be taken into account. Hence, the
fluorescence change in the L–Fe3+ ion reaction in the presence
of other competitive metal ions was investigated as illustrated
in Fig. 4. No change in fluorescence emission was observed
after adding Na+ and K+ (both in 100 mM), and the data also
intensity at 420 nm against concentration of Fe3+
.
(lem = 420 nm) increases significantly upon addition of Fe3+
.
Upon increasing the concentration of Fe3+ up to 0.8 mM (40
equivalents), the solution showed an impressive fluorescence
enhancement with a quantum yield of 0.32 (quinine sulfate in
0.5 M H2SO4 as a standard, Ff = 0.55) and an approximately
16-fold enhancement in the fluorescence intensity at 420 nm.
These spectral properties indicate that its complexation with
Fe3+ ions restricts the rotation of CQN bonds and results in
the suppression of CQN isomerization so that its fluorescence
is drastically increased. In order to obtain insight into the CQN
isomerization-based decay process, the fluorescence lifetime (t)
was measured by single photon counting and it shows a good
single-exponential decay. The lifetime of free L is 0.69 ns and
the radiative decay rate constant (kr) and nonradiative decay
rate constant (knr) are calculated to be about 2.9 Â 107 sÀ1 and
show that Co2+, Cd2+, Mn2+, Hg2+, Ca2+, Mg2+, Ni2+
,
Ba2+, Sr2+, Pb2+, Li+ and Ag+(all in 0.8 mM) caused
1.4 Â 109
s
À1, indicating that the nonradiative decay is the
predominant process in the excited states of L. When Fe3+ was
added, the lifetime of L increases to 0.88 ns, which was longer
than that of free L, and the radiative and nonradiative decay
rate constants changed to 3.6 Â 108 sÀ1 and 7.7 Â 108 sÀ1
,
respectively; both radiative and nonradiative decay processes
became comparative so that the fluorescence was increased.6h
The stoichiometry of the L–Fe3+complex was determined using
Job’s plot, which indicated the formation of a 1 : 1 complex
(Fig. S3, ESIw). By nonlinear least-squares fitting of fluorometric
titration data based on the 1 : 1 binding model, the association
constant was calculated to be 1.07 Â 104 MÀ1.8 To further explore
the complexation model, IR spectrometry was used to investigate
the free ligand L and its Fe3+-complex (Fig. S4, ESIw). One can
observe a shift of the stretch vibration absorption of the CQN
band from 1631 cmÀ1 (free L) to 1616 cmÀ1 (L–Fe3+), and the
stretch vibration absorption of the carbonyl group of L
(1661 cmÀ1) almost disappeared in the Fe3+-complex. So these
data indicate that all the carbonyl groups and the CQN bond took
Fig. 4 (a) Fluorescence spectra of L (20 mM) upon the addition of
various metal ions in CH3CN–water (1/9, v/v, pH = 7.0) solution with
part in coordination with Fe3+
.
an excitation at 372 nm. (b) Metal ions selectivity of L (20 mM) (lex
=
We further investigated the ability of the probe to quantify
Fe3+ in water solution. Fig. 3 demonstrates that there is a
good linear dependence of fluorescence intensity on Fe3+
concentration (0–90 mM), and the regression equation is
F420nm = 62.0302 + 4.5389 [Fe3+] (mM), with R2 = 0.9965.
Ion selectivity study was performed in acetonitrile and
acetate buffer solution (v/v = 1/9, 10 mM, pH = 7.0). From
372 nm, lem = 420 nm). The black bars represent the fluorescence
emission of L and various metal ions. The red bars represent the
fluorescence changes that occur upon addition of 40 equiv. Fe3+ to the
solution containing L and other metal ions. 1, blank; 2, Al3+; 3, Cu2+
;
4, Zn2+; 5, Mg2+; 6, Cr3+; 7, Ca2+; 8, Ba2+; 9, Pb2+; 10, Hg2+; 11,
Co2+; 12, Ni2+; 13, Mn2+; 14, Cd2+; 15, Ag+; 16, Li+ (above ions
were all 0.8 mM); 17, K+ (100 mM); 18, Na+ (100 mM); 19, Fe3+
.
c
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Cu2+, Mn2+, Hg2+, Na+, Ca2+, Zn2+, Mg2+, Pb2+, K+,
Co2+, Li+, Cd2+, Cr3+, Al3+, Sr2+, Ba2+, and nitrate salts of
Ag+. Stock solutions of metal ions (20 mM) were prepared in
deionized water.
A stock solution of L (200 mM) was prepared in CH3CN. In
selectivity experiments, the test samples were prepared by
adding appropriate amount of metal ion stock into 3 ml
solution of L (20 mM). Absorbance spectra were measured
using
a Purkinje general UV-1901 spectrophotometer.
Fluorescence spectra measurements were performed on a Cary
1
Eclipse fluorescence spectrophotometer. H NMR, 13C NMR
Fig. 5 B3LYP/6-31G* optimized structure for the L–Fe3+ complexes.
spectra were obtained on a Bruker Avance 400 MHz NMR
spectrometer with tetramethylsilane (TMS) as internal
standard. X-ray data were collected on Bruker Smart APEX
II CCD diffractometer with graphite-monochromated Mo Ka
(l = 0.71073 A) radiation.
negligible response to the fluorescence of L. Of all the tested
metal ions, only the addition of Cu2+, Zn2+ and Al3+ (all in
0.8 mM) gave a limited increase in fluorescence intensity. The
results indicate that L can potentially be used to detect Fe3+ in
environmental and biological systems, even in the presence
of other competitive pollutant cations.
Synthesis of L
The photophysical properties and IR spectrometry revealed
that a 1 : 1 complex was formed between L (binding sites O2,
N1, O3) and Fe3+. More direct evidence was obtained from
the MALDI-TOF mass spectrum of the system L–Fe3+
(Fig. S5, ESIw), and its isotopic distribution pattern suggests
the existence of a [L–Fe3+] species in the gaseous state. To
further verify the configuration of FeLCl3, we carried out the
density functional theory (DFT) calculations with B3LYP by
using the Gaussian03 package.9 The 6-31G* basis sets were
used for the H, C, N, O and Cl atoms, the exception was for
the Fe atom, where the LANL2DZ effective core potential
(ECP) was employed. The optimized configuration is shown in
Fig. 5, which shows that the Fe is six-coordinated with an
octahedral geometry. The tridentate ONO planar ligand L
coordinates the iron(III) center in a meridional fashion. The
three monodentate chloride ligands occupy the remaining
meridional sites. The Fe–O2, Fe–O3 bond lengths are 1.942 A
and 1.996 A, and the Fe–N1 bond length is 2.049 A.
The Fe–Cl2 (axial) and Fe–Cl3 (axial) bond lengths are
2.357(1) and 2.362(1) A longer than the Fe–Cl1 (equatorial)
distance (2.254 A) which is similar in reported FeCl3 complexes.10
These data indicate that L could provide suitable space to better
accommodate iron(III).
A solution of chromone-3-carboxaldehyde (0.34 g, 2 mmol)
and 4-aminoantipyrine (0.40 g, 2 mmol) in 30 ml ethanol was
stirred at 50 1C for 2 h. After completion of the reaction, the
obtained yellow precipitate was filtered and washed several
times with cold ethanol. After drying under reduced pressure,
1
the reaction afforded 0.58 g (79%) a yellow solid. H NMR
(400 MHz, d6-DMSO): d 9.67 (s, 1H), 8.92 (s, 1H), 8.14 (d,
J = 8 Hz, 1H), 7.85 (t, J = 7.2 Hz, 1H), 7.72 (d, J = 8.4 Hz,
1H), 7.52–7.56 (m, 4H), 7.39 (t, J = 6 Hz, 4H), 3.19 (s, 3H),
2.46 (s, 3H). 13C NMR (100 MHz, d-CDCl3): d (ppm) 175.83,
160.52, 156.18, 154.68, 151.71, 150.47, 134.79, 133.65, 129.19,
126.29, 125.49, 124.65, 124.36, 122.18, 118.91, 118.22, 35.87,
10.18. MALDI-TOF MS: ([M + H]+), 360.204.
Crystal structure determination of L
A crystal (0.2 Â 0.2 Â 0.1 mm) was mounted on a Bruker Smart
APEX II CCD equipped with graphite-monochromated Mo
Ka (l = 0.71073 A) radiation. Tmax = 0.991 and Tmin = 0.982.
The relevant crystal data and structural parameters are: fw
=
%
359.38; space group R3 is trigonal; a = b = 34.465(13) A,
c = 7.825(3) A, a = b = 901, g = 1201; V = 8050(5) (A3);
Z = 18; r = 1.334 g cmÀ3; m(Mo Ka) = 0.091 cmÀ1, the
intensities were collected at 296(2) K. 13438 reflections were
measured and after merging (Rint = 0.0215), some 3152 reflections
(of which 2409 were labelled as observed) were used in the
In conclusion, we have developed a new type of ‘‘turn-on’’
fluorescent sensor L based on the chromone fluorophore. It
shows high sensitivity and selectivity toward Fe3+ in aqueous
solution. When L bound with Fe3+, the CQN isomerization
was eliminated, thus, it induced an enhancement in the
fluorescence intensity of L. The present work also provides a
novel concept for design of Fe3+ fluorescent probes. The
complexation mode and the corresponding enhancement
mechanism were elucidated by IR spectrometry, transient
spectra, Job plot and DFT calculations.
P
P
refinement. The final R1 (R1 = ||Fo| À|Fc||/ ||Fo|) value was
P
Acknowledgements
P
0.0415, wR2 ([ (|Fo| À |Fc|)2/ w|Fo|2]1/2) value was 0.1007.
This project is supported by National Natural Science
Foundation of China (Grant No. 20971024), Educational
Commission of Anhui Province of China (KJ2012Z306,
KJ2011A210), and Key Subject in Analytical Chemistry of
Fuyang Teachers College (2010xk7-02).
Experimental section
Notes and references
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