A highly selective fluorescence sensor for Tin (Sn4+) and its application in imaging live cells

Article abstract of DOI:10.1039/c2ob25895a

A naphthalimide-rhodamine B derivative was synthesized as a fluorescence turn-ON chemodosimeter for Sn⁴⁺. A colour change and marked enhancement of fluorescence was found in the presence of Sn⁴⁺, Cu²⁺ and Cr³⁺ due to the ring open reaction of rhodamine and a fluorescence resonance energy transfer process. Addition of the strong chelating agent ethylenediaminetetraacetic acid disodium salt (EDTA) partly released the cation from the complex with Sn⁴⁺ and restored the yellow fluorescence. In addition, the compound can be used as a fluorescent probe for Sn⁴⁺ in biological systems and may act as a tool with which to study the physiological functions of tin or pathogenesis in the human body.

Full text of DOI:10.1039/c2ob25895a

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Cite this: Org. Biomol. Chem., 2012, 10, 6740
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PAPER
A highly selective fluorescence sensor for Tin (Sn4+) and its application in
imaging live cells†
Qi Wang, Chunyan Li, Ying Zou, Haoxuan Wang, Tao Yi* and Chunhui Huang
Received 10th May 2012, Accepted 21st June 2012
DOI: 10.1039/c2ob25895a
A naphthalimide–rhodamine B derivative was synthesized as a fluorescence turn-ON chemodosimeter
for Sn4+. A colour change and marked enhancement of fluorescence was found in the presence of Sn4+,
Cu2+ and Cr3+ due to the ring open reaction of rhodamine and a fluorescence resonance energy transfer
process. Addition of the strong chelating agent ethylenediaminetetraacetic acid disodium salt (EDTA)
partly released the cation from the complex with Sn4+ and restored the yellow fluorescence. In addition,
the compound can be used as a fluorescent probe for Sn4+ in biological systems and may act as a tool
with which to study the physiological functions of tin or pathogenesis in the human body.
pathogenesis of tin in the human body. To our knowledge,
Introduction
however, there is no report of a fluorescent probe used as a selec-
tive Sn4+ sensor.
Tin is an essential trace mineral for humans and is found in the
greatest amounts in the adrenal glands, liver, brain, spleen and
thyroid gland.1 A deficiency of tin may result in poor growth
and hearing loss and there is some evidence that tin is involved
in growth factors and cancer prevention. Some toxic effects of
tin have been reported, with symptoms limited mostly to gastro-
intestinal complaints such as nausea, abdominal pain and vomi-
ting.1c However, we have little other knowledge of the role of tin
in human metabolism due to the lack of effective tools with
which to study the mechanisms.
In recent years, significant emphasis has been placed on the
development of new, highly selective fluorescent sensors of
metal cations because of their potential applications in biochem-
istry and environmental research.2 Many kinds of signaling
mechanisms have been proposed and utilized for optical detec-
tion of metal ions, including photo-induced electron/energy
transfer (PET),3 intramolecular charge transfer (ICT),4 fluore-
scence resonance energy transfer (FRET),5 and so on. Some of
those sensors can also be applied in fluorescence bioimaging,6
which causes little cell damage and is highly sensitive with high-
speed spatial analysis of living cells.7,8 Specifically, FRET
imaging that affords simultaneous recording of two emission
intensities at different wavelengths in the presence and absence
of analytes has provided a facile method for visualizing complex
biological processes at the molecular level.9 This technique
appears to be suited to the study of physiological functions or
Rhodamine B is widely used as a fluorescent probe for the
detection of cysteine10 and metal ions,11 including Cu2+,12
Cr3+,13 Fe3+,14 and Hg2+,15 due to the ring opening reaction of
rhodamine. Naphthalimides are classic fluorescent dyes whose
electronic absorption and emission depend on the properties of
the molecular structure and surrounding medium.16 Herein, the
two fluorochromes are combined into a new fluorescent probe 1
(Scheme 1), in which rhodamine B selectively senses cations
with high quantum yields and good photostability,15b and
naphthalimide can be used as a second fluorescent chemo-
dosimeter for FRET process.6,11 This naphthalimide–rhodamine
B derivative can selectively detect Sn4+ from other metal ions by
the use of ethylenediaminetetraacetic acid (EDTA) as a second
ligand, and thus be used as a Sn4+ sensor in living cells.
Experiment
General instruments for characterization
1H NMR and 13C NMR spectra were recorded on a Varian
Mercury Plus 400 spectrometer with tetramethylsilane as the
internal standard. The element analysis was performed on a
VarioEL III O-Element Analyzer system. Electrospray ionization
mass spectra (ESI-MS) were measured on a Micromass LCTTM
system. Melting points were determined on a hot-plate melting
point apparatus XT4-100A and uncorrected. UV-Vis spectra
were recorded on a Shimadzu UV-2250 spectrophotometer.
Fluorescence spectra were recorded on an Edinburgh FLS-920
spectrophotometer. All pH measurements were made with a
Model PHS-2F meter.
Department of Chemistry, Fudan University, 220 Handan Road,
Shanghai, China. E-mail: yitao@fudan.edu.cn; Fax: +86-21-55664621;
Tel: +86-21-55664330
†Electronic supplementary information (ESI) available: Details of sup-
plementary spectra and images. See DOI: 10.1039/c2ob25895a
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8.50 (d, 1H, J = 12 Hz), 8.46 (d, 1H, J = 12 Hz), 7.68 (t, 1H,
J = 12 Hz), 7.14 (d, 1H, J = 16 Hz), 4.17 (t, 2H, J = 7 Hz),
3.12 (s, 6H), 2.36 (t, 2H, J = 12 Hz), 1.75–1.63 (m, 4H), 1.43
(m, 2H), 1.30 (m, 12H); 13C NMR (100 MHz, CDCl3), δ (ppm)
164.6, 164.1, 132.6, 131.1, 125.3, 1240.9, 123.2, 113.4, 44.8,
40.3, 33.7, 29.4, 29.3, 29.2, 29.0, 28.9, 28.1, 27.0, 24.6.
MS (EI) calculated for C25H32N2O4: 424.24; found: 425.24
[M + H]+. Anal. calcd for C25H32N2O4: C 70.73, H 7.60,
N 6.60; found: C 70.69, H 7.61, N 6.58.
Rhodamine B hydrazide (4)
To a 100 mL flask, rhodamine B (1.8 g, 3.8 mmol) was dis-
solved in 50 mL ethanol. 3.0 mL (excess) hydrazine hydrate
(85%) was then added dropwise with vigorous stirring at room
temperature. The mixture was heated to reflux in an oil bath for
2 hours. The solution changed from dark purple to light orange.
Then the mixture was cooled and solvent was removed under
reduced pressure. HCl (1 M, about 70 mL) was added to the
solid to generate a clear red solution. After that, NaOH (1 M,
about 70 mL) was added slowly with stirring until the pH of the
solution reached 9. The resulting precipitate was filtrated and
washed 3 times with 15 mL water. After drying over P4O10
in vacuo, rhodamine B hydrazide was obtained as a pink solid
Scheme 1 Chemical structure and synthetic routine of 1.
1
(1.4 g, 81%). M.p. 176–177 °C. H NMR (400 MHz, CDCl3),
Synthesis of the sensor
δ (ppm) 1.16 (t, 12H, J = 7.0 Hz), 3.32 (q, 8H, J = 7.0 Hz),
3.61 (bs, 2H), 6.28 (q, J = 2.4 Hz), 6.41(d, 2H, J = 2.4 Hz), 6.46
(d, 2H, J = 8.8 Hz), 7.09–7.11 (m, 1H), 7.43–7.45 (m, 1H),
7.92–7-94 (m, 1H,). 13C NMR (100 MHz, CDCl3), δ (ppm):
12.84, 44.59, 66.11, 98.21, 104.83, 108.25, 123.18, 124.04,
128.29, 128.31, 130.26, 132.70, 149.09, 151.78, 154.07, 166.32.
ESI mass calculated for C28H32N4O2: 456.25, found: 456.3.
Anal. calcd for C28H32N4O2: C 73.66, H 7.06, N 12.27; found:
C 73.32, H 7.09, N 12.27.
The synthesis of compound 1 is shown in Scheme 1. Compound
2 has been reported in our previous work17 and checked by
1H NMR, 13C NMR and ESI-MS. The N,N-dimethylamino
group was bonded to the naphthalic group of 2 using CuI as the
catalyst. The methyl ester was cleaved by LiOH to yield 3.17 The
procedure for the synthesis of rhodamine B hydrazide 4 is
described in the literature.12 Compounds 3 and 4 were coupled
using (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexa-
fluorophosphate (PyBOP) in dichloromethane as the coupling
reagent to yield compound 1. The structure of 1 was confirmed
1
by H NMR, 13C NMR spectroscopy, and MALDI-TOF mass
Compound 1
spectrometry. The details of the synthesis were as follows.
A mixture of 3 (0.3 g, 0.07 mmol, 1 eq), PyBOP (0.35 g,
0.08 mmol, 1.1 eq) and N-methylmorpholine (0.6 mL) was
stirred in dichloromethane (50 mL) at room temperature for
3 hours. 4 (0.31 g, 0.07 mmol, 1 eq) was added and the resulting
solution was stirred overnight. After TLC showed completely
disappearance of 3, the solvent was evaporated and the crude
product was further purified by flash chromatography using ethyl
acetate–dichloromethane (1 : 3, v/v) as an eluent and yielding a
yellow solid (0.31 g, 46%). M.p. 156–159 °C; 1H NMR
(400 MHz, CDCl3), δ (ppm) 8.58 (d, 1H, J = 12 Hz), 8.49 (d,
1H, J = 12 Hz), 8.44 (d, 1H, J = 12 Hz), 7.93 (t, 1H, J = 12 Hz),
7.66 (t, 1H, J = 16 Hz), 7.45 (t, 2H, J = 6 Hz), 7.11 (t, 1H, J =
8 Hz), 6.45 (d, 2H, J = 8 Hz), 6.42 (d, 2H, J = 8 Hz), 6.28 (d,
2H, J = 8 Hz), 4.15 (t, 2H, J = 7 Hz), 3.61 (t, 2H), 3.35 (q, 8H,
J = 12 Hz), 3.12 (s, 6H), 2.44 (m, 2H), 1.75–1.63 (m, 2H),
1.41–1.27 (m, 12H), 1.17 (m, 12H); 13C NMR (100 MHz,
CDCl3), δ (ppm) 166.6, 153.8, 151.6, 148.9, 132.4, 130.9,
130.5, 128.0, 124.9, 123.8, 122.9, 113.3, 108.0, 104.6, 98.0,
67.0, 46.3, 46.2, 44.7, 44.3, 29.4, 29.3, 26.5, 26.4, 26.3, 26.2,
12.6. MALDI-TOF mass calculated for C53H62ClN6NaO5:
N-(11-Carboxydecyl)-4-dimethylamino-1,8-naphthalimide (3)
To 250 mL flask, compound 2 (0.8 g, 1.7 mmol), K2CO3 (0.5 g,
3.6 mmol) and CuI (trace) was dissolved in 50 mL dry DMF.
The mixture was refluxed under a nitrogen atmosphere for
12 hours. Then the mixture was cooled and solvent was removed
under reduced pressure. The product was purified by column
chromatograph using ethyl acetate–petroleum ether (1 : 3, v/v) as
an eluent to give a yellow solid (0.55 g, 78%). The obtained
product was dissolved in a mixture of 100 mL THF–H2O (1 : 1,
v/v), then LiOH (0.5 g) was added to the solution and stirred at
RT for 48 hours. The solvent was evaporated and the white solid
was neutralized with HCl (1 M). The product was then extracted
with dichloromethane (50 × 3 mL) from the solution. The crude
product was purified by flash chromatography using ethyl
acetate–petroleum ether (2 : 3, v/v) as an eluent. The product was
obtained as a yellow solid, yield 0.53 g, 96%. M.p. 73–75 °C;
1H NMR (400 MHz, CDCl3), δ (ppm) 8.59 (d, 1H, J = 12 Hz),
This journal is © The Royal Society of Chemistry 2012
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921.53; found: 922.45 [M + H]+. Anal. calcd for C53H62N6O5:
C 73.75, H 7.24, N 9.74; found: C 73.65, H 7.28, N 9.69.
Metal ion sensing procedures
The solutions of the metal ions (2.5 mM) were prepared in de-
ionized water. A stock solution of 1 (1 mM) was prepared in
ethanol and was then diluted to 20 μM with ethanol–water (2 : 1,
v/v) for spectral measurement. In titration experiments, each
time a 2.5 mL solution of 1 (20 μM) was filled in a quartz
optical cell of 1 cm optical path length, and the Sn4+ stock solu-
tion was added into the quartz optical cell gradually by using a
micro-pipette. Spectral data were recorded at 5 min after the
addition. In selectivity experiments, the test samples were pre-
pared by placing appropriate amounts of metal ion stock into
2.5 mL solution of 1 (20 μM).
Fig. 1 Change in color and fluorescence of 1 (20 μM) (a) upon
addition of Sn4+, Cu2+, Cr3+ (200 μM, respectively) and (b) by further
addition of EDTA (1 mM) to different solvents.
For fluorescence measurements, excitation was provided at
420 nm, and emission was collected from 435 to 750 nm. The
binding constant was calculated from the emission intensity –
titration curve according to the following equation.18
0
IF0=ðIF ꢀ IF Þ ¼ ½a=ðb ꢀ aÞꢁ½ð1=KS½MꢁÞ þ 1ꢁ
where IF0 is the emission intensity of 1 at 580 nm, IF is the emis-
sion intensity of 1 at 580 nm upon addition of different amount
of Sn(IV). [M] stands for the concentration of Sn(IV); a and b are
constants. The association constant values KS is given by the
ratio intercept/slope.
Cell culture
The HeLa cell line was provided by Institute of Biochemistry
and Cell Biology (China). Cells were grown in MEM (Modified
Eagle’s Medium) supplemented with 10% FBS (Fetal Bovine
Serum) and 5% CO2 at 25 °C. Cells (5 × 108 L−1) were plated
on 18 mm glass coverslips and allowed to adhere for 24 hours.
Experiments to assess Sn(IV) uptake were performed in the same
media supplemented with 50 μM SnCl4 for 30 min.
Fluorescence imaging
Confocal fluorescence imaging was performed with an
OLYMPUS IX81 laser scanning microscope and a 60× oil-
immersion objective lens. Excitation of 1-loaded cells at 543 nm
was carried out with a semiconductor laser, and emission was
collected at 560–600 nm (single channel). The data for ratio
fluorescence imaging were analyzed using software package pro-
vided by OLYMPUS instruments. Immediately before the
experiments, cells were washed with PBS buffer and then incu-
bated with 50 μM 1 in PBS for 30 min at 25 °C. Cell imaging
was then carried out after washing cells with PBS.
Fig. 2 Absorption (a) and fluorescence emission (b, λex = 420 nm)
changes of 1 (20 μM) in ethanol–Hepes (2 : 1, v/v, pH 7.2) by addition
of 200 μM different metal ions (Ag+, Ca2+, Cd2+, Cr3+, Cu2+, Fe2+
,
Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Pb2+, Sn4+, Zn2+).
of fluorescence spectroscopy to include the related heavy, tran-
sition and main group metal ions. A solution of 1 (20 μM) in
optimized ethanol/4-(2-hydroxyethyl)piperidine-1-ethanesulfonic
acid (Hepes) buffer (2 : 1, v/v, pH 7.2) is pale yellow and emits
yellow fluorescent light, indicating that the spirocyclic form of
rhodamine B is retained (Fig. 1a). The weak absorption band at
∼418 nm is ascribed to a 1,8-naphthalimide chromophore
(Fig. 2a). The wavelength of the related emission is 523 nm
(Fig. 2b). Compound 1 has little variation of absorption or
Results and discussion
Metal ion response
High-level selectivity is of paramount importance for an excel-
lent chemosensor.19 In the present work, we extended our
studies of the selective coordination of cations with 1 by means
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fluorescence upon the addition of excess metal ions, such as
Na+, K+, Mg2+, Ca2+, Li+, Fe2+, Cd2+, Ag+, Mn2+, Hg2+, Zn2+
and Pb2+ (see Fig. S1, ESI†). However, within 1 min following
the addition of Cu2+, Cr3+ and Sn4+ there was a marked change
of color from light yellow to red with a maximum absorption
wavelength of 555 nm (Fig. 1a and 2a). The fluorescent intensity
of 1 + M (where M is Sn4+, Cu2+ or Cr3+) was enhanced and the
emission color was changed from light yellow (523 nm) to
orange (580 nm) (Fig. 2b). However, when EDTA was added to
a solution of 1 + M, complex 1 + Sn4+ was partly restored to its
original color with the orange fluorescent fading to yellow
within 30 min. The red color and orange fluorescence of
1 + Cu2+ and 1 + Cr3+ were practically unchanged (Fig. 1b).
Thus, this system could be used for the real-time monitoring of
Sn4+ in cells and organisms in vivo.
An optimized ethanol–Hepes buffer (2 : 1, v/v, pH 7.2) solu-
tion of 1 (20 μM) was used for titration experiments. The absor-
bance at 418 nm of solutions of 1 was little changed following
addition of Sn4+ (0–10 eq); however, a new absorption peak
appeared at ∼555 nm (Fig. 3a), indicating the formation of the
ring open amide form of 1 as a result of cation binding. The
detection limit of this chemosensor for Sn4+ was estimated to be
1.1 × 10−5 M by absorption spectral change upon addition of
Sn4+ (see Fig. S2, ESI†).
Moreover, when excited at 420 nm, addition of Sn4+ to a solu-
tion of 1 caused marked enhancement of the emission band at
580 nm and the intensity of the fluorescent peak at 523 nm
gradually decreased. This is due to the FRET process from
1,8-naphthalimide to the open form of rhodamine (Fig. 3b).
With the addition of 10 eq Sn4+, the ratio of emission intensity
of rhodamine B and 1,8-naphthalimide at 580 nm and 523 nm
(F580/F523) varied from 0.72 to 2.46, corresponding to a 3.4-fold
enhancement. A non-linear fit of the fluorescence titration curve
exhibited a 1 : 1 stoichiometry for Sn4+ and 1, with an associa-
tion constant Ka = (4.73 0.05) × 103 M−1 (see Fig. S3 in
ESI†).20
The efficiency of the energy transfer (E) was investigated
according to Förster theory.21 The results are shown in Fig. 4.
Before the addition of Sn4+, the fluorescence lifetime of 1 in
solution (2.0 × 10−5 M) demonstrated a single exponential decay
with a lifetime of τD = 4.71 ns and this emission decay becomes
faster with an addition of Sn4+ (see Fig. S6 in ESI†). The value
of E is also enhanced with increasing of Sn4+. For example, with
the addition of equivalent of Sn4+ increase from 0.25 to 4.0 eq in
dilute solution of 1, the energy transfer efficiency from naphtha-
limide to rhodamine is increased from 0.4 to 21.6%.
The peak at 555 nm in the absorption spectrum of 1 + Sn4+
was decreased dramatically following further addition of 20 eq
EDTA (Fig. 3c). The lowering of the emission curve indicated
that EDTA had extracted the Sn4+ from the complex and partly
recovered the naphthalimide fluorescence at 523 nm. This can be
explained by the difference between the association constants for
EDTA-Sn and 1-Sn. KEDTA-Sn (3.16 × 1034 M−1 22
is much
)
larger than K1-Sn (4.7 × 103 M−1); therefore, Sn4+ acts as a
trigger for fluorescence switch-ON and EDTA acts as a trigger
for fluorescence switch-OFF. The ratio of the emission intensity
of rhodamine B to that of 1,8-naphthalimide at 580 nm and
523 nm (F580/F523) was decreased from 2.43 to 1.3 (Fig. 3d). As
compared with Sn4+, the K1-M (M = Cr3+, Cu2+) is larger, even
Fig. 3 Absorption (a, c) and fluorescence (b, d) spectra of 1 (20 μM)
in ethanol–Hepes (2 : 1, v/v) upon addition of 0–10 eq of Sn4+ (a, b),
followed by addition of 0–20 eq of EDTA (c, d); measurements were
made at 5 min intervals (λex = 420 nm).
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Scheme 2 The Sn4+-sensing mechanism of compound 1.
Fig. 4 Fluorescence lifetime and energy transfer (ET) efficiency of 1
on addition of different amounts of Sn4+ (0–4.0 eq) in ethanol–Hepes
(2 : 1, v/v, pH 7.2) ([1] = 2.0 × 10−5 M).
Table 1 Association constants of M (M = Sn4+, Cr3+, Cu2+) with 1
and EDTA
M
Sn4+
Cr3+
Cu2+
K1-M
(4.73 0.05) ×
103 a
(2.32 0.01) ×
104 a
(1.94 0.03) ×
105 a
KEDTA-M
KEDTA-M/K1-M
3.16 × 1034 b
6.7 × 1030
2.5 × 1023 c
1.08 × 1019
6.3 × 1018 c
3.3 × 1013
Fig. 5 The ratiometric fluorescence responses (F580/F523) of 1 (20 μM)
with various metal ions (x-axis) in ethanol–Hepes (2 : 1, v/v). The diag-
onal bars represent the addition of 200 μM various metal ions to 1. Inset
show competition experiment in which white bars represent the addition
of an excess of the appropriate metal ion (2 mM for all cations) to a
20 μM solution of 1, black bars represent the subsequent addition of
200 μM Sn4+ to the solution of 1–13; metal ions: 1, Ag+; 2, Ca2+; 3,
Cd2+; 4, Fe2+; 5, Hg2+; 6, K+; 7, Li+; 8, Mg2+; 9, Mn2+; 10, Na+; 11,
Ni2+; 12, Pb2+; 13, Zn2+; 14, Cr3+; 15, Sn4+; and 16, Cu2+).
a K1-M are calculated with the method in Fig. S3 and S4 in ESI.† b Data
from ref. 22. c Data from ref. 23.
though the difference is not so drastic (K1-Cr = (2.32 0.01) ×
104; K1-Cu = (1.94 0.03) × 105, see Fig. S4 and S5, ESI†). The
most important point is that the KEDTA-M of Sn4+ is 1012–1016
order larger that of Cr3+ and Cu2+ due to the higher positive
charge (Table 1). Thus, the ratio of KEDTA-M/K1-M (M = Cr3+,
Cu2+) is much smaller than KEDTA-Sn/K1-Sn. This explained why
the reverse processes of Cr3+, Cu2+ are difficult than that of
Sn4+. Addition of 50 eq EDTA to 1-Cu and 1-Cr for 50 min only
resulted 9% and 26% of the decrease on the emission intensity
of 580 nm (see Fig. S7 and S8, ESI†).
spirolactam ring and exhibits fluorescence emission. This also
proves that the sensing mechanism of 1 with Sn4+ is a process of
complexation.7a,13
The proposed binding mode was further verified by the detec-
tion of a peak at m/z 526.3599 (calc. 526.2) corresponding to
[1 + SnCl2]2+ by ESI-MS (see Fig. S9, ESI†). Thus, we propose
that compound 1 coordinates with Sn4+ with 1 : 1 stoichiometry;
the two carbonyl O atoms are most likely to be the binding sites
for Sn4+ on 1 (Scheme 2).
Sensor mechanism
Moreover, the co-existent ions caused negligible interference
of Sn4+ sensing by 1, even when other cations were present at
micromolar levels (Fig. 5). Fluorescence measurements of 1 in
complex with various metal ions revealed excellent selectivity
for Sn4+, Cu2+ and Cr3+. As shown in Fig. 5, alkali and alkaline-
earth metal cations Li+, Na+, K+, Mg2+ and Ca2+ at a 100-fold
excess caused no interference, and transition-metal and heavy-
metal ions Zn2+, Fe2+, Mn2+, Ni2+, Cd2+, Hg2+, Ag+ and Pb2+
gave a weak response. Competition experiments found no
obvious change in F580/F523 for Sn4+ mixed with other metal
ions (200 μM), except Cu2+ and Cr3+, which can be removed
from the complex with 1 by further addition of EDTA (Fig. 5,
inset). The pH dependence of 1 indicated that in the acidic
environment a proton resulted in the ring open form of the
rhodamine B group with increased fluorescent intensity (see
The sensor mechanism is very important for the practical appli-
cation of this probe. However, our efforts on preparation of
single crystal of Sn4+ complex of 1 failed. The probable com-
plexation mechanism of 1 with Sn4+ was validated by spectral
study and ESI-MS. The spectral change underlying Sn4+ sensing
suggests that the mechanism involves opening the spirolactam
ring of rhodamine by Sn4+. The spirocyclic form of rhodamine
B, which is colorless and non-fluorescent, becomes colored and
strongly fluorescent in the open form by reaction with metal
ions. The Sn4+-promoted ring-opening reaction of 1 occurs
instantly upon the addition of Sn4+ owing to the strong
binding of O atoms with high positive charged Sn4+. The chela-
tion of the two carbonyl O atoms in rhodamine B group stabil-
izes the structure of the metal complex. After Sn4+ is removed
by the strong chelator EDTA, compound 1 partly recovers its
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Conclusions
In conclusion, we have synthesized a naphthalimide–rhodamine
B derivative and demonstrated its utility as a fluorescence switch
chemodosimeter that responds stoichiometrically and rapidly to
Cu2+, Cr3+ and Sn4+ in aqueous media. The selective recognition
of Sn4+ is achieved by addition of EDTA to the solution of
1 + M (where M is Cu2+, Cr3+ or Sn4+), whereby 1 partly
recovers its light yellow fluorescence in the complex with Sn4+.
The process involves Sn4+-promoted reversible ring opening via
coordination/disconnection reactions, which we attribute to the
lower affinity between Sn4+ and the rhodamine moiety. With the
help of optical spectra, we can easily identify Sn4+ from the
other cations. To the best of our knowledge, this is the first
example of a fluorescent probe which is sensitive to Sn4+. We
anticipate that this new probe will be of great benefit for studying
the role of Sn4+ in biological systems.
This work was supported by the National Natural Science
Foundation of China (91022021, 30890141 and 21125104), the
National Basic Research Program of China (2009CB930400),
the Program for Innovative Research Team in University
(IRT1117), and the Shanghai Leading Academic Discipline
Project (B108). Qi Wang thanks for the Department of Macro-
molecular Science for MALDI-TOF MS experiment.
Fig. 6 CLSM images of HeLa cells. (a–d) Cells incubated with 5 μM
1 for 20 min, (e–h) followed with 10 μM Sn4+ for 20 min, (i–l) and with
50 μM EDTA for 20 min; emission was collected in green channel at
520 20 nm (a, e and i) and red channel at 580 20 nm (b, f and j); c,
g and k are bright field images and d, h and l are overlay images for a, e
and i, respectively (λex = 405 nm).
Fig. S10, ESI†). However, owing to protonation of the dimethyl
amine group on 1,8-naphthalimide, the emission at 523 nm of 1
was increased with decreased pH, resulting in a comparatively
small change in F580/F523. This phenomenon is quite different
for the reaction of 1 with metal ions. The excellent selectivity of
1 for Sn4+ indicates its utility for a wide range of biological
applications.
Notes and references
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