Rhodamine-derived Schiff base for the selective determination of mercuric ions in water media

Article abstract of DOI:10.1016/j.saa.2010.12.010

A new rhodamine-derived Schiff base (RS) was synthesized and its sensing property to metal ions was investigated by UV/vis and fluorescence spectroscopies. Addition of Hg²⁺ ions to the aqueous solution of RS gave a visual color change as well a

Full text of DOI:10.1016/j.saa.2010.12.010

Spectrochimica Acta Part A 78 (2011) 753–756
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Rhodamine-derived Schiff base for the selective determination of mercuric ions
in water media
Duong Tuan Quanga,∗, Jia-Sheng Wub,∗, Nguyen Dinh Luyena, Tran Duonga, Nguyen Dang Dana,
Nguyen Chi Baoa, Phan Tu Quyc
a Department of Chemistry, Hue University, Hue 8454, Viet Nam
b Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
c Tay Nguyen University, Buon Ma Thuot 84050, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2010
Accepted 6 December 2010
A new rhodamine-derived Schiff base (RS) was synthesized and its sensing property to metal ions was
investigated by UV/vis and fluorescence spectroscopies. Addition of Hg2+ ions to the aqueous solution of
RS gave a visual color change as well as significantly fluorescent enhancement, while other ions includ-
ing Pb2+, Cd2+, Cr3+, Zn2+, Cu2+, Fe2+, Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+ ions did not induce any distinct
color/spectral changes, which constituted a Hg2+-selective fluorescent OFF–ON chemosensor. The Hg2+
-
Keywords:
Rhodamine 6G
Mercuric determination
Fluorescent OFF–ON
Chemosensor
induced ring-opening of spirolactam of rhodamine in RS resulted in the dual chromo- and fluorogenic
observation.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ions as well as the destabilization of non-emissive n–␲* excited
states [7]. Consequently, a large number of papers involving fluo-
Mercury, one of the most prevalent toxic metal elements in the
environment, can easilypassthroughbiologicalmembranes such as
skin, respiratory, and gastrointestinal tissues [1]. When absorbed
in the human body, mercury causes damage to the central ner-
vous and endocrine system. A variety of symptoms are observed
upon mercuric exposure including digestive, kidney, and especially
neurological diseases [2]. Concerns over toxic exposures to mer-
cury provide motivation to explore new methods for monitoring
Hg2+ ions from biological and environment samples. Several meth-
ods, including atomic absorption spectroscopy, inductively coupled
plasma atomic emission spectrometry, electrochemical sensoring,
and the use of piezoelectric quartz crystals, make it possible to
detect low limits of Hg2+ ions [3–6]. However, these methods
require expensive equipment and involve time-consuming and
laborious procedures that can be carried out only by trained profes-
sionals. Alternatively, analytical techniques based on fluorescence
detection are very popular because fluorescence measurements are
usually very sensitive (parts per billion/trillion), easy to perform,
and inexpensive [7–11]. Furthermore, the photophysical proper-
ties of a fluorophore can be easily tuned using a range of routes:
charge-, electron-, energy-transfer, the influence of the heavy metal
rescent chemosensors have been published [7–9,12–15]. However,
many of these systems displayed shortcomings in practical use,
such as the lack of aqueous solubility, cross-sensitivities toward
other metal ions, short emission wavelength, narrow pH span, and
delayed response, etc. Accordingly, developing new and practical
sensing systems for Hg2+ ions is still a challenge.
Rhodamine-based fluorescent chemosensors have been
received considerable attention for the sensing of heavy metal
ions in recent years because of their particular structural prop-
erties [16,17]. As well known, the rhodamine with spirolactam
structure is non-fluorescent, whereas ring-opening of the spiro-
lactam gives rise to a strong fluorescence emission. This property
provides an ideal mode to construct OFF–ON fluorescent switch
sensors. Moreover, they have
a longer emission wavelength
(about 550 nm), which is often preferred to serve as reporting
groups for analyte to avoid the influence of the background
fluorescence (below 500 nm) [18,19]. Actually, to date, many
spirolactam-based fluorescent chemosensors have been devel-
oped for Hg2+ ions, in which rhodamine–hydrazine structure is
widely applied for the ring-opening of spirolactam [8,10,20–23],
while rhodamine–ethylenediamine structure was still relatively
scarce [24,25].
Herein, we reported a rhodamine–ethylenediamine derived
Schiff base (RS) for the selective determination of Hg2+ ions
in aqueous solution (Scheme 1). Rhodamine–ethylenediamine
was selected as a good binding framework for Hg2+ ions, and
∗
Corresponding authors. Tel.: +84 54 3826359.
E-mail addresses: duongtuanquang@dhsphue.edu.vn (D.T. Quang),
wujs@mail.ipc.ac.cn (J.-S. Wu).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.12.010

754
D.T. Quang et al. / Spectrochimica Acta Part A 78 (2011) 753–756
O
N
NH2
NH2CH2CH2NH2
Ethanol, Reflux
CO2C2H5
N
H
O
N
N
H
N
H
O
N
HO
O
N
HO
N
O
N
Ethanol, Reflux
N
H
O
N
H
RS
Scheme 1. Synthetic route of chemosensor RS.
the attached 4-diethylaminosalicylaldehyde provided additional
binding groups to induce the ring-opening of spirolactam upon
complexation with Hg2+ ions. As expected, upon addition of Hg2+
ions, the aqueous solution of RS gave rise to an obviously enhanced
fluorescence as well as visual change from colorless to pink, indi-
cating that Hg2+ did induce the ring-opening of spirolactam in RS.
was purified by recrystallization from absolute ethanol to give
514 mg of RS (white crystal) in 81.2% yield. 1H NMR (CDCl3) ı 7.92
(d, 1H), 7.66 (s, 1H), 7.46 (s, 2H), 7.06 (m, 1H), 6.93 (m, 1H), 6.36 (s,
2H), 6.17 (m, 3H), 3.56–3.48 (m, 4H), 3.38–3.36 (m, 4H), 3.22 (m,
4H), 1.88 (s, 6H), 1.34–1.31 (t, 6H), 1.19 (t, 6H). FAB-MS (M+H+):
m/z = 632.
2. Experimental
2.3. General procedures of spectral detection
2.1. Instruments and reagents
A 1.0 × 10−3 M stock solution of RS was prepared in C2H5OH
and diluted to 1.0 × 10−5 M in 1:4 EtOH/H2O solution (v/v). The
metal ion stock solutions are dissolved in deionized water with
a concentration of 1.0 × 10−3 M for the spectral analysis. Each time
a 2 mL solution of RS was filled in a quartz cell of 1 cm optical path
length, and different stock solutions of metal ions were added into
the quartz cell gradually by using a micro-pipette. The volume of
anionic stock solution added was less than 100 ␮L with the purpose
of keeping the total volume of testing solution without obvious
change. Excitation wavelength was 500 nm and the temperature is
20 ◦C.
All UV–vis spectra and fluorescence spectra were recorded in
S-3100 spectrophotometer and Hitachi F-4500 fluorescence spec-
trometer, respectively. 1H NMR spectra were recorded at 400 MHz,
Bruker-400 instrument. Mass spectra were recorded at Finnigan
4021C MS-spectrometer.
Rhodamine
diethylaminosalicylaldehyde were purchased from Aldrich and
used without further purification. All cationic compounds of Pb2+
6G,
ethylenediamine,
and
4-
,
Cd2+, Hg2+, Cr3+, Zn2+, Cu2+, Fe2+, Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+
ions (perchlorate or chloride), were purchased from Aldrich and
used as received. Ethanol for spectral detection was HPLC reagent
without fluorescent impurity and H2O was deionized water. All
solvents were analytical reagents.
3. Results and discussion
Fig. 1 shows spectral changes of RS in C2H5OH/H2O solution
(1/4, v/v) upon addition of various competitive metal ions, such as
Hg2+, Pb2+, Cd2+, Cr3+, Zn2+, Cu2+, Fe2+, Co3+, Ni2+, Ca2+, Mg2+, K+ and
Na+ ions. From UV/vis spectra of RS (5 ␮M) (Fig. 1A), we can clearly
observe a new absorption band centered at 530 nm in the presence
of 5 equiv. of Hg2+ ions. In contrast, other metal ions do not lead to
any distinct spectral changes. On the other hand, fluorescence spec-
tra (Fig. 1B) also show a similar result, which is well consistent with
that of UV/vis spectra. Addition of only 5 equiv. Hg2+ ion results in
an obviously enhanced fluorescence peaked at 556 nm (OFF–ON),
2.2. Synthesis
2.2.1. Synthesis of N-(rhodamine-6G)lactam–ethylenediamine
The preparation of N-(rhodamine-6G)lactam–ethylenediamine
was based on the reported procedure [10]. Rhodamine 6G (958 mg,
2 mmol) was dissolved in 20 mL of hot ethanol, followed by addi-
tion of ethylenediamine (1 mL, 15 mmol). The reaction mixture was
refluxedfor 4 htill thefluorescenceofthesolution was disappeared.
The reaction was cooled to room temperature, and the precipitate
was collected and washed with absolute ethanol for three times.
Crude product was purified by recrystallization from acetonitrile to
give 824 mg of N-(rhodamine-6G)lactam–ethylenediamine (white
solid) in 90.3% yield. 1H NMR (CDCl3): ı 7.95 (d, 1H), 7.47 (t, 2H),
7.05 (d, 1H), 6.34 (s, 2H), 6.23 (s, 2H), 3.50 (t, 2H), 3.24 (t, 4H), 2.39
(t, 2H), 1.90 (s, 6H), 1.36 (t, 6H). FAB-MS (M+H+): m/z = 457.
while other metal ions including Pb2+, Cd2+, Cr3+, Zn2+, Cu2+, Fe2+
,
Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+ ions do not give rise to any fluo-
rescence increases. Further experiments for Hg2+-selective sensing
were performed using 5 ␮M of RS in aqueous solution in the pres-
ence of multifarious ions including Pb2+, Cd2+, Cr3+, Zn2+, Cu2+, Fe2+
,
Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+ ions (1 equiv., respectively). Upon
addition of Hg2+ ions, the solution above still displays a distinctly
enhanced fluorescence. Both UV/vis and fluorescence results indi-
cate that RS shows a good selectivity and sensitivity toward Hg2+
ions over other competitive ions.
Fig. 2 shows a spectral variation of RS upon the gradual addi-
tion of Hg(ClO4)2. The UV–vis titration spectra of Hg2+ ions was
conducted using 5 ␮M of RS in ethanol aqueous solution at pH
∼7. Upon the addition of increasing concentrations of the Hg2+ ion
2.2.2. Synthesis of chemosensor RS
A
portion of N-(rhodamine-6G)lactam–ethylenediamine
(456 mg, 1.0 mmol) and 4-diethylaminosalicylaldehyde (212 mg,
1.1 mmol) were combined in absolute ethanol (30 mL). The reac-
tion solution was refluxed for 6 h under N2 atmosphere and stirred
for another 2 h at room temperature to form precipitate. The solid
was filtrated, washed with ethanol for three times. Crude product

D.T. Quang et al. / Spectrochimica Acta Part A 78 (2011) 753–756
755
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
500
A
B
Free RS
400
2+
Hg
2+ 2+ 3+ 2+ 2+
Other ions including Pb , Cd , Cr , Cu , Zn
,
2+ 3+ 2+ 2+
2+
+
+
300
200
100
0
Fe , Co , Ni , Ca , Mg , K , Na , etc
2+
2+ 3+
2+
2+ 2+
Pb , Cd , Cr , Cu , Zn , Fe
,
2+
Hg
3+ 2+ 2+ 2+
+
+
Co , Ni , Ca , Mg , K , Na , etc
Free RS
350
400
450
500
550
600
520
540
560
580
600
620
640
Wavelength (nm)
Wavelength (nm)
Fig. 1. (A) UV–vis spectra and (B) fluorescence spectra of RS (5 ␮M) in 1:4 C2H5OH/H2O (v/v) at pH ∼7 upon addition of different metal ions including Hg2+, Pb2+, Cd2+, Cr3+
,
Zn2+, Cu2+, Fe2+, Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+ ions (25 ␮M, respectively) with an excitation wavelength at 500 nm.
500
600
500
400
300
200
100
0
B
A
1.6
400
1.4
2+
Hg
2+
Hg
1.2
1.0
0.8
0.6
0.4
0.2
0.0
300
200
100
0
Free RS
0.0 0.2 0.4 0.6 0.8 1.0
2+
[RS1] / [RS1]+ Hg
2+
Hg
Free RS1
520
540 560
580 600 620
640
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. (A) UV–vis titration spectra and (B) fluorescence titration spectra of RS (5 ␮M) in C2H5OH/H2O solution (1/4, v/v) at pH ∼7 upon gradual addition of Hg(ClO4)2 with
an excitation wavelength at 500 nm (Hg2+ concentrations: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, and 25 ␮M).
(0–25 ␮M), a new absorption band centered at 530 nm appeared
with increasing intensity, which induced a clear color change from
colorless to pink (Fig. 2A). On the other hand, for the fluorescence
titration spectra of RS, in the presence of Hg2+ ions there was
also a new emissive peak at 556 nm (Fig. 2B), which was in good
consistency with the results of UV–vis spectra. Both UV–vis and
fluorescence data lead to a significant fluorescence OFF–ON sig-
nal. From the molecular structure and spectral results of RS, it
is probably concluded that the addition of Hg2+ ions induced its
complexation with carbonyl group in spirolactam, N atom in Schiff
base, and hydroxyl in salicylaldehyde. As a result, a ring-opening of
the spirolactam in rhodamine framework took place, observed by
the distinct color change and fluorescence OFF–ON, as depicted in
Scheme 2.
Job’s plot analysis (inset of Fig. 2B) indicates probe RS and Hg2+
form a 1:1 complex in aqueous solution. On the basis of 1:1 stoi-
chiometry, we can calculate the stability constant of the complex
through a nonlinear curve fitness as following expression [6]:
ꢀ
Ylim − Y0
CM
CL
1
KsCL
Y = Y0
+
1 +
+
2
ꢅ
ꢁ
ꢄ
1/2
ꢂ
ꢃ
2
CM
1
CM
CL
−
1 +
+
− 4
CL
KsCL
H2O
O
H
O
HO
O
Hg2+
N
N
N
N
N
H
N
Hg2+
H2O
N
H
O
N
H
N
H
O
N
RS
RS-Hg2+
Red, Strongly fluorescent
Colorless, Non-fluorescent
Scheme 2. The proposed mechanism for Hg2+-induced ring-opening of spirolactam in RS with fluorescence OFF–ON.

756
D.T. Quang et al. / Spectrochimica Acta Part A 78 (2011) 753–756
there was an obvious fluorescence OFF–ON change between pH 6
350
300
250
200
150
100
50
and 10. Thus, chemosensor RS can detect Hg2+ ions with a wide pH
span (6–10) because in this region RS with Hg2+ induces a remark-
able fluorescence OFF–ON, whereas RS without Hg2+ does not lead
to such change.
4. Conclusions
Linear Regression for Data1_B:
Y = 11.35 + 32.10 * [ Hg
R = 0.9943
2+]
In conclusion, we report a new rhodamine-derived Schiff base
(RS) serving as a sensitive chemosensor, which can show a good
selectivity toward Hg2+ over other competitive ions in aqueous
solution. Hg2+ ions can cause the solution of RS to be subject to
an obvious color change and fluorescence OFF–ON, which is arisen
from the ring-opening of spirolactam. RS can determine Hg2+ ions
with a good linear relationship between the concentration range
from 0.5 to 10 ␮M. Two good features of this system, a remarkably
high selectivity toward Hg2+ ions over miscellaneous competitive
ions and a wide pH span (6–10), make it promising to determine
Hg2+ ions for the practical analysis.
0
0
2
4
6
8
10
2+
[ Hg ] / uM
Fig. 3. Variation of the fluorescence intensity at 552 nm of RS (5 ␮M) in C2H5OH/H2O
solution (1/4, v/v) at pH ∼7 vs the concentration of Hg2+ ions.
800
Acknowledgment
% ( RS1 only)
This work was supported by the National Foundation for Sci-
ence and Technology development of Vietnam – NAFOSTED (No.
104.07.17.09) (DTQ).
% ( RS1 + 5 eq. Hg2+)
600
400
200
0
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.12.010.
References
[1] J. Gutknecht, J. Membr. Biol. 61 (1981) 61.
[2] T.W. Clarkson, L. Magos, G.J. Myers, N. Engl. J. Med. 349 (2003) 1731.
[3] O.T. Butler, J.M. Cook, C.F. Harrington, S.J. Hill, J. Rieuwerts, D.L. Miles, J. Anal.
At. Spectrom. 21 (2006) 217.
[4] Y. Li, C. Chen, B. Li, J. Sun, J. Wang, Y. Gao, Y. Zhao, Z. Chai, J. Anal. At. Spectrom.
21 (2006) 94.
2
4
6
8
10
12
14
pH value
Fig. 4. Fluorescence intensity (552 nm) of RS (5 ␮M) in the absence and presence of
5 equiv Hg2+ ions in C2H5OH/H2O solution (1/4, v/v) with different pH conditions.
[5] M. Leermakers, W. Baeyens, P. Quevauviller, M. Horvat, Trends Anal. Chem. 24
(2005) 383.
[6] B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-VCH,
Weinheim, Germany, 2002.
[7] J.S. Kim, D.T. Quang, Chem. Rev. 107 (2007) 3780.
[8] D.T. Quang, J.S. Kim, Chem. Rev. 110 (2010) 6280.
[9] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515.
[10] J.S. Wu, I.C. Hwang, K.S. Kim, J.S. Kim, Org. Lett. 9 (2007) 907.
[11] M.H. Lee, J.S. Wu, J.W. Lee, J.H. Jung, J.S. Kim, Org. Lett. 9 (2007) 2501.
[12] Y.M. So, R.C. Na, H.K. Young, S.K. Chang, J. Org. Chem. 69 (2004) 181.
[13] A.B. Descalzo, R. Martinez-Manez, R. Radeglia, K. Rurack, J. Soto, J. Am. Chem.
Soc. 125 (2003) 3418.
[14] K. Rurack, U. Resch-Genger, J.L. Bricks, M. Spieles, Chem. Commun. 2 (2000)
2103.
[15] G. Hennrich, W. Walther, U. Resch-Genger, H. Sonnenschein, Inorg. Chem. 40
(2001) 641.
[16] S. Bencic-Nagale, T. Sternfeld, D.R. Walt, J. Am. Chem. Soc. 128 (2006)
5041.
[17] L.F. Mottram, S. Boonyarattanakalin, R.E. Kovel, B.R. Peterson, Org. Lett. 8 (2006)
581.
[18] A. Minta, R.Y. Tsien, J. Biol. Chem. 264 (1989) 19449.
[19] E. Kimura, T. Koike, Chem. Soc. Rev. 27 (1998) 179.
[20] E.M. Nolan, S.J. Lippard, Chem. Rev. 108 (2008) 3443.
[21] Y.K. Yang, K.J. Yook, J. Tae, J. Am. Chem. Soc. 127 (2005) 16760.
[22] H. Zheng, Z.H. Qian, L. Xu, F.F. Yuan, L.D. Lan, J.G. Xu, Org. Lett. 8 (2006) 859.
[23] H. Yang, Z. Zhou, K. Huang, M. Yu, F. Li, T. Yi, C. Huang, Org. Lett. 9 (2007) 4729.
[24] J.H. Soh, K.M.K. Swamy, S.K. Kim, S. Kim, S.H. Lee, J. Yoon, Tetrahedron Lett. 48
(2007) 5966.
where Y and Y0 are the fluorescence intensity at 552 nm of the aque-
ous solution of RS in the presence and absence of Hg2+ ions; CM and
CL are the concentrations of Hg2+ ions and RS; Ks is the stability con-
stant of the complex RS/Hg2+. According to fluorescence titration
data, the stability constant between RS and Hg2+ was determined
to be 3.5 × 106 with a good relationship (R = 0.995).
The high sensitivity of RS for Hg2+ ions might be used to gener-
ate a calibration curve for quantitative measurement of Hg2+ ions
in aqueous solution. By adding Hg2+ ions with different concen-
trations ranged from 0 to 10 ␮M, the fluorescence intensity of RS
(5 ␮M) at 552 nm was recorded to generate a calibration curve.
As shown in Fig. 3, when Hg2+ concentration added was from 0.5
to 10 ␮M, an almost perfect linearity (I552 = 11.35 + 32.10 × [Hg2+],
R = 0.9943) was found between fluorescence intensity of RS and
Hg2+ concentration, indicating a linear detection range for Hg2+
determination.
For practical application, the proper pH environment of this new
chemosensor is also evaluated. Fig. 4 shows that for free RS, at acidic
conditions (pH < 6), the ring-opening of rhodamine framework took
place due to the strong protonation. When pH > 6, no significant
ring-opening was observed. However, in the presence of Hg2+ ions,
[25] L. Zhang, J. Fan, X. Peng, Spectrochim. Acta A 73 (2009) 398.