Cascade OFF-ON-OFF fluorescent probe: Dual detection of trivalent ions and phosphate ions

Article abstract of DOI:10.1039/c4ra03339f

A new rhodamine B-based fluorescent probe was developed for the selective cascade signaling of trivalent cations (Fe³⁺, Al³⁺, Cr³⁺) and phosphate anion (PO₄^3-). Non-fluorescent rhodamine derivatives can selectively detect trivalent cations over some other metal ions in CH₃CN-Tris buffer (1/1, v/v, pH 7.0) solutions, leading to prominent fluorescence OFF-ON switching. The obtained probe-cation complex can subsequently serve as a sensitive and selective chemosensor for PO₄^3-, exhibiting complete signal quenching (fluorescence ON-OFF switching).

Full text of DOI:10.1039/c4ra03339f

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Cascade OFF–ON–OFF fluorescent probe: dual
detection of trivalent ions and phosphate ions†
Cite this: RSC Adv., 2014, 4, 29479
ab
a
b
b
b
*
*
Tianqi Liu,‡ Haiyang Liu, Liqiang Gu and Youwei Yao
Xuejuan Wan,‡
A new rhodamine B-based fluorescent probe was developed for the selective cascade signaling of trivalent
cations (Fe3+, Al3+, Cr3+) and phosphate anion (PO43ꢀ). Non-fluorescent rhodamine derivatives can
selectively detect trivalent cations over some other metal ions in CH3CN–Tris buffer (1/1, v/v, pH 7.0)
solutions, leading to prominent fluorescence OFF–ON switching. The obtained probe–cation complex
can subsequently serve as a sensitive and selective chemosensor for PO43ꢀ, exhibiting complete signal
quenching (fluorescence ON–OFF switching).
Received 13th April 2014
Accepted 16th June 2014
DOI: 10.1039/c4ra03339f
www.rsc.org/advances
biological processes. Human serum contains 0.80–1.45 mM
phosphate but higher phosphate levels are directly connected to
cardiovascular disease and acute renal failure.31
Introduction
The uorogenic sensing of different metal ions has become
particularly attractive because of its simplicity, high sensitivity,
and instantaneous response.1–6 Among them, triple-charged
metal cation detection is of signicant importance because of
its crucial inuence in a wide range of environmental and
human health areas.7–17 For example, Fe3+ ion is an essential
element in the human body and acts as a cofactor in several
enzymatic reactions;18–20 whereas the widely existing Al3+ ion in
the environment is considered to be toxic in biological activi-
ties, and responsible for the Parkinson's disease;21,22 for Cr3+,
which is also an effective nutrient for the human body, a devi-
ation of its concentration from normal levels can increase the
risk of diabetes and cardiovascular diseases.40 Simultaneously,
the detection of anions through uorescence emission readouts
has also attracted signicant attention considering the recog-
nized biological roles played by anions.23 However, it is difficult
to recognize anions in water or in a water mixture media
compared with most of the metal cations because anions
usually experience competition from the solvent for sensor
binding sites.24 Nevertheless, a metal complex may resolve this
problem via a simple displacement approach, causing a signal
change in the UV-vis or uorescence spectrum.25–28 Phosphate
anions (PO43ꢀ), both inorganic and organic, have a unique
position in nature because they participate in almost all the
metabolic processes.29,30 Moreover, phosphorylated biomole-
cules play an important role in signal transduction for various
Up till now, even though a large number of uorescence
chemosensors for divalent metal ions and phosphate anions
have been reported in the literature, relatively few studies have
been devoted to the development of uorescence sensors which
are sensitive to trivalent ions41–43 and phosphate ions.39 In view
of achieving a rapid qualitative analysis of the pollutants, it is
highly desirable to design a probe which is capable of simul-
taneous discrimination of trivalent ions and phosphate ions
from other ions.
Herein, we report a simple cascade “OFF–ON–OFF” uores-
cent chemosensor for trivalent and phosphate ions based on the
equilibrium between spirolactam (non-uorescent) to the ring-
open amide (uorescent) form of rhodamine chromophore.4,9,32
In general, the chemosensor showed a specic selectivity toward
trivalent ions over other common metal ions, leading to prom-
inent uorescence OFF–ON switching. The resultant sensor–
3ꢀ
cation complex could selectively recognize PO4
over other
anions, exhibiting complete ON–OFF signal quenching.
Experimental
Materials and methods
Rhodamine B was obtained from Aladdin. Phthalic anhydride
and hydrazine hydrate were purchased from Sinopharm
Chemical Reagent. The solution of metal ions was prepared
from their chloride salts of analytical grade. The solvents were
used as-received without further purication. Deionized water
was used throughout the experiment.
aShenzhen Key Laboratory of Special Functional Materials, College of Materials
Science and Engineering, Shenzhen University, Shenzhen 518060, PR China. E-mail:
wanxj@szu.edu.cn
bAdvanced Materials Institute, Graduate School at Shenzhen, Tsinghua University,
Shenzhen 518055, PR China. E-mail: yaoyw@sz.tsinghua.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03339f
UV-vis spectra were recorded using a SCINCO S-4100. A
HORIBA Jobin Yvon SPEX spectrouorimeter was used for
uorescence measurements. The excitation wavelength was 520
nm and the spectra were recorded in the range of 540–700 nm.
1
‡ These authors contributed equally to the work.
Both H NMR (400 MHz) and 13C NMR spectra (100 MHz) of
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addition of 8 equiv. chloride salts of a wide range of cations in
CH3CN–Tris-buffer (1/1, v/v, pH 7.0) media, including Na+, K+,
Mg2+, Cu2+, Hg2+, Cd2+, Fe2+, Pb2+, Zn2+, Mn2+, Ca2+, Fe3+, Al3+
and Cr3+. The uorescence emission spectra of L were measured
in 1.0 cm quartz cells, and the excitation and emission slits were
set to 2.0 and 2.0 nm, respectively. As shown in Fig. 1, the sensor
itself was almost non-uorescent due to its ring-closed spi-
rolactam structure. Aer the addition of Fe3+, probe L exhibited
a color change from colorless to pink as well as a bright red
uorescence. A strong emission band centered at 586 nm was
observed with an extreme uorescence enhancement compared
to the metal-free L. This result indicated that the probe L
exhibited a high selectivity toward Fe3+ over the monovalent and
divalent cations in CH3CN–Tris-buffer (1/1, v/v, pH 7.0) media.
To further explore the selectivity of the sensor, competitive
ions were rst added to the detection solution, and then Fe3+
ions were added aer 20 minutes. As shown in Fig. 2, most of
the detection systems exhibited minimum interference in the
detection of Fe3+; Al3+ and Cr3+ (80 mM each) also did not induce
Scheme 1 Schematic illustration of the synthesis of an RhB-based
fluorescence probe L.
the compounds were recorded on a Brurker Dpx spectrometer
in CDCl3 with tetramethylsilane as the internal standard.
The high-resolution mass spectra were recorded on a Bruker
APEX IV.
any obvious interference in the uorescence sensing of Fe3+
,
indicating that L has a relatively higher binding affinity to Fe3+
than Al3+ and Cr3+. In addition, the association constants were
6.13 ꢁ 104 for L–Fe3+, 3.14 ꢁ 104 for L–Al3+ and 2.26 ꢁ 104 for L–
Cr3+, according to the reported work,44,45 respectively. A similar
study was carried out with Al3+ and Cr3+, and there were no
obvious interferences in the presence of Na+, K+, Mg2+, Cu2+,
Synthetic methodology
Compound L was synthesized in two steps, as shown in Scheme
1. Rhodamine B hydrazide was rst synthesized following liter-
ature procedures.32 Phthalic anhydride (100 mg, 0.67 mmol) was
added to a solution of rhodamine B hydrazide (300 mg,
0.66 mmol), which was dissolved in 40 mL anhydrous toluene.
The reaction mixture was heated under microwave irradiation for
30 minutes and allowed to cool to room temperature. Aer that,
the solvent was removed under reduced pressure, and the crude
product was puried with a silica gel column using a mixture of
petroleum ether and ethyl acetate (2 : 1, v/v) as an eluent to afford
a yellow solid in a 30% yield. 1H NMR (Fig. S1†) (400 MHz, CDCl3)
d 8.02 (d, J ¼ 7.4 Hz, 1H), 7.75–7.65 (m, 4H), 7.62 (t, J ¼ 7.2 Hz,
1H), 7.56 (t, J ¼ 7.2 Hz, 1H), 7.25 (d, J ¼ 7.5 Hz, 1H), 6.83 (d, J ¼
8.9 Hz, 2H), 6.40 (d, J ¼ 6.4 Hz, 2H), 6.26 (d, J ¼ 2.3 Hz, 2H), 3.42–
3.26 (m, 8H), 1.16 (t, J ¼ 7.0 Hz, 12H). 13C NMR (Fig. S2†) (100
MHz, CDCl3) d (ppm) 164.07, 153.68, 153.06, 149.47, 148.82,
133.97, 133.40, 130.88, 130.09, 128.32, 124.45, 123.64, 123.40,
107.52, 104.23, 97.01, 73.37, 44.18, 12.47. HRMS (ESI) (Fig. S3†):
calcd for C36H35N4O4 [M + H+]+: 587.2658; found: 587.2652.
Hg2+, Cd2+, Fe2+, Pb2+, Zn2+, Mn2+ and Ca2+
.
The sensing property of L was investigated in detail utilizing
the Fe3+ cation. Fig. 3 displays the absorption spectrum changes
of L upon Fe3+ addition. Sensor L exhibited almost no absorp-
tion peak in the visible wavelength due to the spirolactam form
of molecule L. However, a new band centered at about 562 nm
emerged upon the gradual addition of Fe3+, indicating the
sensor L–Fe3+ complex formation and Fe3+-induced spirolactam
ring-opening processes. Moreover, the titration solution dis-
played a characteristic color change of rhodamine derivatives
from colorless to pink, indicating that probe L could serve as a
“naked-eye” indicator for Fe3+ in CH3CN–Tris-buffer (1/1, v/v)
media. With increasing amounts of Fe3+, the absorbance
increased proportionally and levelled off when the
Results and discussion
The equilibrium between the non-uorescent spirolactam form
and the uorescent ring-open amide form of rhodamine
derivatives has been proven to be an efficient platform for the
construction of uorescent sensors for numerous heavy metal
cations (such as Hg2+, Pb2+ and Cu2+) due to its large absorption
coefficient and high uorescence quantum yield.33–35 In the
current work, this typical ion-recognition mechanism was
utilized for the successive detection of trivalent ions and then
phosphate ions via uorescence OFF–ON–OFF switching.
Fluorescence OFF–ON sensing for Fe3+, Al3+ and Cr3+
The detection selectivity of probe L (10 mM) toward Fe3+ ions was
Fig. 1 Fluorescence spectra of L (10 mM) in the presence of various
metal ions (80 mM each) in CH3CN–Tris-buffer (1/1, v/v, pH 7.0), lex
¼
investigated by monitoring the uorescence changes upon the 520 nm.
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Fig. 2 Change in fluorescence intensity of L (10 mM) in various
mixtures of metal ions (the concentration of Fe3+ and other metal ions
were all 80 mM).
Fig. 4 Fluorescence intensities recorded for L (10 mM, CH3CN–H2O
1 : 1, v/v) at various pH values in the (a) absence and (b) presence of 8
equiv. Fe3+ (lex ¼ 520 nm, lem ¼ 586 nm).
concentration of L was maintained at 10 mM, while the
concentration of Fe3+ was varied between 0 and 250 mM. The
uorescence spectra were recorded at an excitation wavelength
of 520 nm and emission wavelength of 540–700 nm. For free L,
no obvious characteristic emission of rhodamine derivatives
was observed. With increasing concentrations of Fe3+, the
uorometric titration reaction curve showed a steady and
smooth enhancement, which was used as the basis of Fe3+
-
sensing. The recognition interaction was completed immedi-
ately aer the addition of Fe3+, i.e., within 1 min. When the
concentration of Fe3+ was greater than 200 mM, the uorescence
intensity did not increase any further and a plateau was
reached. The inset in Fig. 5 shows the dependence of uores-
cence intensity at 586 nm on Fe3+ concentration. Plotting uo-
rescence intensity versus Fe3+ concentration (1–4 equiv.)
afforded a good linear relationship (R ¼ 0.9844) (Fig. S7†). The
detection limit of Fe3+ was calculated from the equation DL ¼
(3Sb1 ꢀ I)/S instead of DL ¼ 3Sb1/S (ref. 37 and 38) because the
intercept is not negligible, where Sb1 is the standard deviation of
the blank solution, I is the intercept and S is the slope of the
Fig. 3 UV-vis spectra and (inset) absorbance changes (564 nm)
recorded for L (25 mM) in CH3CN–Tris (1/1, v/v) with various amounts of
Fe3+ ions.
concentration of Fe3+ reached 250 mM. When UV-vis titration
measurements were carried out with Al3+ (Fig. S4†) and Cr3+
(Fig. S5†), similar changes were found to those of Fe3+, i.e.,
absorbance increased at rst and then levelled off.
Appropriate pH conditions for the successful operation of
the uorescence sensing were evaluated. Without the addition
of the Fe3+ ion, the ring opening of the RhB-based sensor L
occurred under acidic conditions (pH < 5.0) due to protonation,
while no uorescence change was observed with pH values over
5 (Fig. 4). The gradual addition of Fe3+, however, led to an
obvious uorescence enhancement over a wide pH range from
5.0 to 8.0, which was attributed to the similar opening of the
spirolactam structure.36 Because the most remarkable Fe3+
-
induced OFF–ON uorescence changes occurred under the
physiological pH window, all the uorescence measurements
were conducted at pH 7.0. Similar phenomenon can also be
observed in the uorescence sensing of Al3+ and Cr3+, as shown
in Fig. S6.†
The relative affinities of Fe3+ toward sensor L were evaluated
from uorescence spectroscopic titration experiments in
CH3CN–Tris-buffer (1/1, v/v) medium, as shown in Fig. 5. The
Fig. 5 Fluorescence titrations of L (10 mM) with Fe3+ ions in CH3CN–
Tris (1/1, v/v, pH 7.0). Inset: fluorescence emission intensity changes
with increasing Fe3+ ions (lex ¼ 520 nm, lem ¼ 586 nm).
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calibration curve. The detection limit of L for Fe3+ was 1.10 ꢁ
10ꢀ5 M. And the detection limits of L were 3.20 ꢁ 10ꢀ7
M
(Fig. S8 and S9†) for Al3+ and 2.55 ꢁ 10ꢀ5 M for Cr3+ (Fig S10
and S11†).
To determine the interaction stoichiometry between sensor L
and Fe3+, Job's method was employed using an absorbance
intensity at 562 nm as a function of molar fraction of L because
the total concentration of L and Fe3+ ion was located at 50 mM.
The maximum absorbance was observed when the molar frac-
tion of L reached 0.50 (Fig. 6), which was indicative of a 1 : 1
stoichiometry complexation between L and Fe3+. This result was
further conrmed by high-resolution mass spectrum (HR-MS
Fig. S12†), in which the peak at m/z 786.0831 (calcd ¼ 786.0811)
corresponding to [L + Fe3+ + 2H+ + 4Clꢀ]+ was clearly observed
when 5 equiv. of FeCl3 was added to probe L. Similarly, the
stoichiometric ratios were 1 : 1 for Al3+ (Fig. S13†) and Cr3+
(Fig. S14†). As inferred by FTIR (Fig. S15†), the structure of the
L–M3+ complex was proposed, as shown in Fig. S16.† When
probe L was treated with Fe3+, the characteristic amide carbonyl
absorption peak at 1631 cmꢀ1 shied to 1608 cmꢀ1, indicating
that the amide carbonyl O of rhodamine B unit was actually
involved in the recognition of Fe3+, which also agreed with the
previously reported work.46,47
3ꢀ
Fluorescence ON–OFF sensing for PO4
Interestingly, when the L–Fe3+ complex was treated with a
sodium salt of PO43ꢀ, the color of the solution changed from
pink to colorless, and the emission was completely quenched
within 1 min (Fig. 7a), while other anions, such as Clꢀ, Brꢀ, Iꢀ,
SO42ꢀ, SO32ꢀ, HPO42ꢀ, H2PO4ꢀ, NO3ꢀ, did not show any inter-
ference to the detection. This observation suggested that the
Fig. 7 (a) Fluorescence spectra obtained for L (10 mM) in the presence
of Fe3+ (80 mM) in CH3CN–Tris (1/1, v/v) 10 min after the addition of
various anions (80 mM each), lex ¼ 520 nm. (b) Fluorescence emission
spectra and (inset) variation of fluorescence intensity recorded for L
(10 mM) with 8 equiv. Fe3+ ions upon gradual addition of PO43ꢀ (0–30
equiv. for Fe3+) in CH3CN–Tris (1/1, v/v). The spectra were obtained 10
min after PO43ꢀ addition (lex ¼ 520 nm, lem ¼ 586 nm).
3ꢀ
probe L could not only act as a sensor for Fe3+ but also for PO4
in a successive manner. Once the Fe3+ ion interacted with the
sensor L, the spirolactum ring was opened, yielding a high
3ꢀ
uorescence emission. However, when treated with the PO4
sensing capability of the obtained complex was further inves-
tigated in detail with uorescence titration analysis (Fig. 7b).
The titration curve showed an excellent linear decrease, and
about 1.0 equiv. of PO43ꢀ (compared with Fe3+) was required to
obtain absolute uorescence quenching. The detection limit
was calculated to be 0.20 mM (Fig. S17†) according to the uo-
rescence titration curve, which reveals a high sensitivity for the
analysis of phosphate ion using the L-Fe3+ complex. The Al3+
anion, the metal ion was abstracted and the spirolactum ring
closed, leading to the absence of uorescence. The PO43ꢀ anion-
3ꢀ
complex can also serve as a sensor for PO4 via a simple
3ꢀ
displacement approach because of the strong affinity of PO4
to Al3+. Moreover, the cascade uorescence OFF–ON–OFF
response can be observed with the alternate addition of Fe3+
cation and PO43ꢀ anion into the detection solution (Fig. S18†),
suggesting that the obtained uorescence probe L can respond
to both the cation and anion even when they co-exist in the
solution. EDTA was further used to investigate the reversibility
of the Fe3+ sensing process (Fig. S19†). The results indicate that
EDTA could also cause an “ON–OFF” response of the L–Fe3+
complex via chelation. Aer the addition of excess Fe3+ to the
detection solution, the recovered uorescence emission can be
clearly observed, suggesting that the probe exhibits good
detection reversibility.
Fig. 6 Job's plot obtained for the determination of binding stoichi-
ometry between L and Fe3+ in CH3CN–Tris (1/1, v/v). The total
concentration of L and Fe3+ was maintained at 50 mM. Absorbance was
measured at 564 nm.
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´
16 P. Calvo, J. L. Barrio-Manso, F. C. Garcıa, J. L. Pablos,
Conclusions
´
T. Torroba and J. M. Garcıa, Polym. Chem., 2013, 4, 4256–
4264.
In summary, we report the synthesis and characterization of a
new probe for the cascade uorogenic detection of trivalent ions
and phosphate ions. The probe L showed excellent “OFF–ON”
uorescence signals with high sensitivity and selectivity in the
presence of trivalent ions, whereas it remained silent in the
presence of monovalent and divalent cations such as Na+, K+,
Mg2+, Cu2+, Hg2+, Cd2+, Fe2+, Pb2+, Zn2+, Mn2+, Ca2+. A successive
“ON–OFF” uorescence switching was then observed in the
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Acknowledgements
The nancial support from National Natural Scientic Foun-
dation of China (NNSFC) Project (21204042), China Post-
doctoral Science Foundation (2013T60099), National High
Technology Research and Development Program of China (863
Program, 2012AA030312), Fundamental Research Project of
Shenzhen (JCYJ20120830152316443) and Technology Innova-
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