A fluorescent probe for the determination of Ce4+ in aqueous media

Article abstract of DOI:10.1016/j.dyepig.2011.07.001

A rhodamine-based probe bearing an N,N-dimethylaniline unit was developed as a fluorescent chemodosimeter for Ce⁴⁺ in aqueous media. Importantly, the sensor can selectively respond to Ce⁴⁺ over other commonly coexistent metal ions (s

Full text of DOI:10.1016/j.dyepig.2011.07.001

Dyes and Pigments 96 (2013) 770e773
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journal homepage: www.elsevier.com/locate/dyepig
A fluorescent probe for the determination of Ce4þ in aqueous media
,
b
a,
Liang Huang a, Yong Liu b, Cai Ma a, Pinxian Xi a , Wei Kou , Zhengzhi Zeng
*
*
a Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry and College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
b Medical College of Northwest University for Nationalities, Lanzhou 730030, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A rhodamine-based probe bearing an N,N-dimethylaniline unit was developed as a fluorescent che-
modosimeter for Ce4þ in aqueous media. Importantly, the sensor can selectively respond to Ce4þ over
Received 11 April 2011
Received in revised form
30 June 2011
Accepted 3 July 2011
Available online 12 July 2011
other commonly coexistent metal ions (such as La3þ, Ce3þ, Pr3þ, Nd3þ, Sm3þ, Eu3þ, Gd3þ, Er3þ, Tb3þ
,
Ho3þ, Tm3þ, Yb3þ, Lu3þ, Y3þ) in aqueous media with a rapid response time. The system, which utilizes an
irreversible Ce4þ-promoted oxidation reaction, responds instantaneously at room temperature with
linear proportionality to the amount of Ce4þ
.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Chemodosimeter
Selectivity
Ce4þ
Fluorescence
Lanthanide ions
Oxidation
1. Introduction
It is known that CAN (Ceric ammonium nitrate) promotes an
irreversible oxidative cyclization of the originally N-acylhydrazones
into 1,3,4-oxadiazoles in mild conditions [5], (Scheme 1) and
rhodamines undergo a great fluorescence enhancement via their
structure change from spirocyclic (nonfluorescent and colorless) to
ring-open (fluorescent and pink) states induced by specific chem-
ical environments at room temperature [6]. We envisioned that this
irreversible N-acylhydrazone reaction could be incorporated into
the rhodamine hydrazone system and become involved in the
conversion from the nonfluorescent spirocyclic form to the fluo-
rescent ring-opened one.
Recently, the design and synthesis of fluorescent sensors for
metal ions has attracted considerable attention. A large amount of
work has been undertaken on the development on the develop-
ment of fluorescent probes for alkali/alkaline earth/transition metal
ions due to their importance in many biological and environmental
processes [1]. In this regard, reports on the sensing and detection of
different lanthanide ions are relatively scarce despite of their well-
known involvement in biological/therapeutic/imaging processes
and catalytic reactions [2,3]. Cerium ion has an ionic radius similar
to that of a calcium ion and which generally has a higher affinity for
Ca2þ binding sites present in biological molecules because of its
higher charge. Moreover, Ce4þ represents the most notable oxidant
among lanthanide ions which could react with various biomole-
cules including proteins, fatty acids, RNA and DNA. While Ce4þ has
strong antibacterial properties, excess amounts of Ce4þ can also
cause serious damage to biological systems [4]. To the best of our
knowledge, no work on fluorescent probe for Ce4þ has previously
been reported. Therefore, the development of highly selective and
sensitive turn-on fluorescent probes for monitoring Ce4þ still
remains a significant challenge.
2. Experiment procedure
1H and 13C NMR spectra were taken on a Varian mercury-400
spectrometer with TMS as an internal standard and CDCl3, DMSO as
solvent. Absorption spectrawere determined on a Varian UV-Cary100
spectrophotometer. Fluorescence spectra measurements were per-
formed on a Hitachi F-4500 spectrofluorimeter. All pH measurements
were made with a pH-10C digital pH meter. HRMS were determined
on a Bruker Daltonics APEXII 47e FT-ICR spectrometer.
All the materials for synthesis were purchased from commercial
suppliers and used without further purification. Methanol for
spectra detection was HPLC reagent without fluorescent impurities.
Stock solutions of the metal ions (2.5 ꢀ 10ꢁ3 mol Lꢁ1) were
* Corresponding authors. Tel./fax: þ86 931 8912582.
E-mail addresses: xipx@lzu.edu.cn (P. Xi), zengzhzh@yahoo.com.cn (Z. Zeng).
prepared in deionized water.
A
stock solution of L1
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2011.07.001

L. Huang et al. / Dyes and Pigments 96 (2013) 770e773
771
135.25, 129.70, 127.77, 126.78, 124.43, 124.03, 118.46, 104.76, 95.18,
37.40, 17.03, 14.08.
3. Results and discussions
Scheme 1. Oxidative cyclization of N-acylhydrazones by CAN.
Herein, we report a new rhodamine derivative (L1) bearing an
N,N-dimethylaniline unit, which has been demonstrated to be
a chemodosimeter for Ce4þ. The rhodamine derivative L1 was
prepared in high yield from Rhodamine 6G by using a two-step
procedure. The structure of L1 was confirmed by 1H NMR, 13C
NMR, MS data and X-ray analysis (Figs. S6 and S7). The single crystal
of L1 was grown in dichloromethane and acetonitrile solutions, and
a unique spirolactam structure ring formation was observed (Fig.1).
The solution of L1 in aqueous media is colorless and weakly
fluorescent, indicating that the spirolactam form of L1 exists
predominantly. Spectrophotometric titrations for L1 with varying
pH revealed that L1 retained the spirocyclic form within the pH
range of 5.0e12.0. As we expected, addition of CAN to the solution
of L1 in CH3CN/water (1:1, v/v) at pH 5e10 caused an immediate
and significant enhancement of fluorescence intensity in the 510e
650 nm range (Fig. S2). This observation indicates that the CAN-
induced oxidation of L1 could take place in a wide pH range at
room temperature.
To explore the mechanism, the mixtures of L1 with CAN in
CH3CN/water (1:1, v/v) were detected by the ESI mass spectrum
analysis. The peak at m/z 558.5 corresponding to [RG1þ1]þ was
observed in the ESI mass spectrum of L1 with 20 equiv CAN in the
CH3CN/water (1:1, v/v) solution. The peak at m/z 415.4 assigned to
[RGþ1]þ was also observed under the same condition. After addi-
tion of 40 equiv CAN, L1 almost completely converted to RG
(Figs. S10 and S11). We then attempted to isolate the reaction
product of the probe L1 and Ce4þ ions. Unfortunately only product
RG could be isolated from chromatograph on silica-gel (Figs. S8 and
S9). From the molecular structure, NMR and mass spectral results of
L1 with CAN, it is concluded that the addition of the CAN induced
an oxidative cyclization to form product RG1, and then further
promoted oxidation of the compound to the brilliant pink and
highly fluorescent Rhodamine 6G (RG) in the CH3CN/water (1:1, v/
v) solution (Scheme 3b). This result was also supported by Evans
et al., (Scheme 3a) [7].
(1 ꢀ 10ꢁ3 mol Lꢁ1) was prepared in DMF: CH3CN (1:1 v/v). The
solution of L1 was then diluted to 2 ꢀ 10ꢁ5 mol Lꢁ1 with water/
CH3CN (1:1 v/v). In titration experiments, each time a 2 ꢀ 10ꢁ3
L
solution of L1 (2 ꢀ 10ꢁ5 mol Lꢁ1) was filled in a quartz optical cell of
1 cm optical path length, and the Ce4þ stock solution was added into
the quartz optical cell gradually by using a micro-pippet. Spectral
data were recorded at 2 min after the addition. In selectivity
experiments, the test samples were prepared by placing appropriate
amounts of metal ion stock into
2 mL solution of L1
(2 ꢀ 10ꢁ5 mol Lꢁ1). For fluorescence measurements, excitation was
provided at 500 nm, and emissionwas collected from 510 to 650 nm.
2.1. Synthesis of compound L1 and RG
Compound 2 was synthesized by an improved method accord-
ing to the literature [1g].
Compound L1. Rhodamine 6G hydrazide (0.214 g 0.5 mmol) and
4-(dimethylamino)benzaldehyde (0.075 g, 0.5 mmol) were mixed
in boiling ethanol (60 mL) with 3 drops of acetic acid (Scheme 2).
After heating under reflux for 16 h, the reaction mixture was cooled
and half of the volume of the solvent was removed under reduced
pressure. The yellow precipitate was collected by filtration, washed
with ethanol/ether (1:1) and dried over P2O5 under vacuum. Yield:
67%. 1H NMR (CDCl3, 200 MHz)
d (ppm): 8.27 (s, 1H), 7.98e8.02 (t,
1H), 7.38e7.45 (t, 4H), 7.01e7.05 (p, 1H), 6.52e6.57 (d, 2H, J ¼ 8.6),
6.36e6.38 (d, 4H, J ¼ 4.2), 3.14e3.24 (p, 4H, J ¼ 7.2), 2.91 (s, 6H),1.84
(s, 6H), 1.27e1.34 (t, 6H, J ¼ 7 Hz). 13C NMR (CDCl3, 50 MHz)
d
(ppm): 164.78, 152.35,151.43,151.10,147.53,147.39,132.96,129.01,
128.93, 127.69, 123.46, 123.16, 123.06, 117.89, 111.42, 106.55, 96.71,
65.65, 40.17, 38.32, 16.64, 14.73. ESI-MS m/z ¼ 560.3 [M þ H]þ, calc.
for C35H37N5O2 ¼ 559.29. Elemental Analysis. Found (calculated)
for C35H37N5O2 (%): C, 74.75 (75.11); H, 6.58 (6.66); N, 12.38 (12.51).
Compound RG. To a solution of L1 (34 mg, 0.06 mmol) in CH3CN
(30 mL)/DMF (2 mL) was added CAN (187 mg, 0.36 mmol). Then the
reaction mixture was stirred at room temperature for 3 h. After the
solvent was evaporated under reduced pressure, the crude product
was column chromatographed on silica-gel (CH2Cl2/MeOH ¼ 5:1, v/
v) to give the 18 mg of rhodamine 19 (72%) as a red solid. 1H NMR
The UVevis absorption spectrum of L1 (10 mM) in CH3CN/water
(1:1, v/v) exhibited only a very weak band above 500 nm, which are
attributed to the trace ring-opened form of molecules of L1. The
(400 MHz, DMSO-d6)
d
(ppm): 7.95e7.97 (d, 1H, J ¼ 7.6 Hz), 7.66e
7.77 (m, 2H), 7.17e7.19 (d, 1H, J ¼ 7.6 Hz), 6.21e6.32 (ss, 4H), 3.12e
3.19 (m, 4H), 1.89 (s, 6H), 1.18e1.22 (t, 6H, J ¼ 7.2 Hz), 13C NMR
(DMSO-d6, 100 MHz)
d (ppm): 169.05, 152.76, 150.95, 148.55,
Fig. 1. ORTEP diagram of the compound L1 (50% probability level for the thermal
Scheme 2. Synthesis of compound L1.
ellipsoids).

772
L. Huang et al. / Dyes and Pigments 96 (2013) 770e773
Scheme 3. CAN-Induced Oxidation of Fluorescent Probe L1.
characteristic peak for C7-atom at
d
¼ 65.65 ppm in the 13C NMR
spectrum of L1 also confirms this conclusion (Fig. S7). Upon the
addition of up to 40 equiv of CAN, a new strong absorption band
centered at 528 nm was formed and led to the color change from
colorless to pink. This indicated that the opened-ring form of L1
became the main species in the examined solution. Other rare earth
ions, such as La3þ, Ce3þ, Pr3þ, Nd3þ, Sm3þ, Eu3þ, Gd3þ, Er3þ, Tb3þ
,
Ho3þ, Tm3þ, Yb3þ, Lu3þ, Y3þ, did not show any significant color and
spectral change under identical conditions (Fig. 2). The emission
spectra were also recorded under the same conditions. On addition
and gradual increase of [CAN] to the nonfluorescent CH3CN/water
(1:1, v/v) solution of L1, a significant enhancement in fluorescence
intensity at 554 nm was observed following excitation at 500 nm,
and the emission quantum yield (
F) was found to be 0.25 (Fig. 3).
From the Ce4þ concentration-dependent fluorescence changes, the
Fig. 3. (a) Absorption spectra of L1 (10
mM) in CH3CN/water (1:1, v/v) in the presence
of different amounts of CAN (0e40 equiv). Inset: absorbance at 529 nm as a function of
CAN concentration. (b) Fluorescence spectra of L1 (10
upon addition of CAN (0e40 equiv). (lex ¼ 500 nm).
mM) in CH3CN/water (1:1, v/v)
detection limit of L1 for the determination of Ce4þ was estimated to
be 8 ꢀ 10ꢁ5 M in CH3CN/water (1:1, v/v) solution.
To eliminate the influence of the acidic NH4NO3, fluorescence
titration of L1 (10 mM) with NH4NO3 was investigated in CH3CN/
water (1:1, v/v) solution. As shown in Fig. S5, the solution didn’t
display any fluorescence change upon addition of 40 equiv NH4NO3.
Therefore, the fluorescence changes of L1 with CAN are mainly
attributed to Ce4þ ion.
An important feature of the chemodosimeter is its high selec-
tivity toward the analyte over other competitive species. The
fluorescence responses of L1 (10
examined in CH3CN/water (1:1, v/v) solution. Upon addition of 40
equiv metal ions (La3þ, Ce3þ, Pr3þ, Nd3þ, Sm3þ, Eu3þ, Gd3þ, Er3þ
mM) to other rare earth ions were
,
Tb3þ, Ho3þ, Tm3þ, Yb3þ, Lu3þ, Y3þ), no significant fluorescence
intensity changes were observed (Fig. S3). Only the Ce4þ led to
a prominent enhancement in fluorescence intensity, and the fluo-
rescence intensity changes caused by the addition of Ce4þ were not
influenced by the presence of other rare earth ions (Fig. 4). All of
these results suggest the high selectivity of L1 for Ce4þ over other
competing cations in CH3CN/water (1:1, v/v) solution.
As a sensor demonstration of a real-time determination was
necessary. We next investigated the time evolution of the
responses of L1 (10
CH3CN/water (1:1, v/v) solution. We found that the interaction of L1
Fig. 2. Changes in the absorption spectra of L1 (10
metal ions (400 M) in CH3CN/water (1:1, v/v). Bottom: Photos of color changes of L1
(10 M) upon addition of 400 M different metal ions in CH3CN/water (1:1, v/v)
solution.
mM) in the presence of different
m
m
M) in the presence of 40 equiv of Ce4þ in
m
m

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Fig. 4. Fluorescence intensities of 10
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Acknowledgments
This work was supported by the National Natural Scientific
Foundation of China (No. 21201092), the Fundamental Research
Funds for the Central Universities (Lzujbky-2012-65) and (Lzujbky-
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reduced glutathione in biological fluids by Ce(IV)equinine chemiluminescence.
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Appendix A. Supplementary material
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.dyepig.2011.07.001.
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