A new dual fluorogenic and chromogenic "turn-on" chemosensor for Cu2+/F- ions

Article abstract of DOI:10.1016/j.saa.2015.06.078

Turn "off-on" chemosensor 2-(-2-((3′,6′-bis(diethylamino)-3-oxospiro[isoindoline-1,9′-xanthen]-2-yl)imino)ethylidene)-N-phenylhydrazine-1-carbothioamide (RBS) was designed and synthesized. Using the naked eye, RBS showed favorable observation characterist

Full text of DOI:10.1016/j.saa.2015.06.078

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new dual fluorogenic and chromogenic ‘‘turn-on’’ chemosensor for
Cu2+/Fꢀ ions
b
Hyungwook Yu a,1, Jae-Young Lee a, Satheshkumar Angupillai a,1, Sheng Wang b, , Shuhang Feng ,
⇑
Shinya Matsumoto c, Young-A Son a,
⇑
a Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, South Korea
b School of Chemistry Science & Technology, Zhanjiang Normal University, Development Center for New Material Engineering & Technology in Universities of Guangdong,
Zhanjiang 524048, PR China
c Graduate School of Environment and Information Sciences Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢁ
We report a rhodamine appended
thiourea based chemosensor for
detection of biologically important
Cu2+ and Fꢀ ions.
ꢁ
ꢁ
RBS shows fluorescence activity
towards Cu2+ ion.
RBS sensing mode with Cu2+ and Fꢀ
ion was confirmed by 1H NMR
titration experiment.
a r t i c l e i n f o
a b s t r a c t
Turn ‘‘off–on’’ chemosensor 2-(-2-((30,60-bis(diethylamino)-3-oxospiro[isoindoline-1,90-xanthen]-2-
yl)imino)ethylidene)-N-phenylhydrazine-1-carbothioamide (RBS) was designed and synthesized. Using
the naked eye, RBS showed favorable observation characteristics with both Cu2+ and Fꢀ ions. The various
modes of sensitivity shown by RBS toward the Cu2+ and Fꢀ ions were investigated by spectral techniques,
including UV–Vis, fluorescence and 1H NMR spectroscopy. The Job’s plot indicated the formation of 1:1
complex between RBS and Cu2+/Fꢀ. The binding constant of the RBS-guestꢀ complexes were found to
be 1.3 ꢂ 104 and 6.2 ꢂ 103 Mꢀ1 for the RBS–Cu2+ and RBS–Fꢀ, respectively.
Article history:
Received 2 May 2015
Received in revised form 20 June 2015
Accepted 23 June 2015
Available online 23 June 2015
Keywords:
Rhodamine B
Thiourea
Ó 2015 Elsevier B.V. All rights reserved.
Copper
Fluoride
Dual sensor
Introduction
The invention of chemosensors for recognizing cations, anion
and neutral species has been attracted much dynamic in molecular
⇑
Corresponding authors at: Department of Advanced Organic Materials Engi-
neering, Chungnam National University, Daejeon 305-764, South Korea (Y.A. Son).
School of Chemistry Science & Technology, Zhanjiang Normal University, Zhanjiang
524048, PR China (S. Wang).
recognition study and supramolecular chemistry [1] due to their
main role in biological systems and high toxicity nature.
Currently, single chemosensors for multiple analytes have been
paid much attention by analysts because of its faster response
and potential cost reduction [2–4]. Up to date, enormous amount
of works have been employed for the design of fluorescent
E-mail addresses: wang_sheng@ymail.com (S. Wang), yason@cnu.ac.kr
(Young-A Son).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.saa.2015.06.078
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
49
chemosensor for ions and neutral analytes [5–8]. Rhodamine
derivatives are excellent chromophores/fluorophores and have
attracted considerable interest due to their very good photophysi-
cal properties [9], such as long absorption and emission
wavelengths, large extinction coefficients and high fluorescence
quantum yields. Rhodamine derivatives with spirolactam struc-
tures are non-fluorescent; however, opening of the spirolactam
ring results in a strong fluorescence emission [10–12]. The rho-
damine framework is particularly suited for constructing OFF–ON
fluorescent or colorimetric chemosensors [13,14] due to its special
structural property. Several rhodamine-modified chemosensors
have recently been developed for the detection of heavy and tran-
sition metal ions because of the widespread use of these metal ions
and because of their harmful impacts on the environment and
human health [15].
quenching of the fluorescence emission upon the addition of the
metal due to its paramagnetic nature [30–33]. Only few
a
chemosensors in which the binding of Cu2+ leads to an increase
in the fluorescence intensity have been found, which are desirable
for analytical purposes by the fluorescence enhancement [34,35].
On the other hand, chemosensors for anions have received contin-
uous attention as anions play important role in many fields. Among
the anions, the detection of fluoride in chemical and physiological
systems are of great interest because of their role in dental health,
treatment of osteoporosis, association with hydrolysis of the nerve
gas sarin [36–40]. However, excess fluoride can lead to fluorosis,
which is a type of fluoride toxicity that generally manifests itself
clinically in terms of increases in bone density. The diversity of
its function makes the detection of fluoride anion important. The
receptors containing pyrrole, amide, urea, imine, urea and thiourea
groups act as both cation and anion sensors [41–45].
In human body, Cu2+ is the third most abundance essential ele-
ment (behind Fe2+ and Zn2+) and also Cu2+ plays a critical role as a
catalytic co-factor for a variety of metalloenzymes, including
superoxide dismutase, cytochrome c oxidase, tyrosinase and so
on [16]. However, excess intake of Cu2+ can produce oxidative
stress and disorders associated with neurodegenerative diseases,
such as Alzheimer’s disease, Wilson’s disease, and Menke’s disease,
most likely because of its involvement in the production of reactive
oxygen species [17]. Especially, the average concentration of blood
The anion can be recognized either by H-bonding or by the
deprotonation of protons on the receptor–NH in organic solvents.
Very few authors have been developed single chemosensor for
multi analyte, for example, Kim and co-workers [46] reported
bifunctional fluorescent Calix [4] arene chemosensor for both
Pb2+ and Fꢀ. Yang et al. [47] designed N-butyl-1,8-naphthalimide
derivative for Cu2+ and Fꢀ ion. The results of this study indicated
that a hydrazine NH was involved in hydrogen bonding and was
subsequently deprotonated. Udhayakumari et al. [48] have
reported that thiourea based dual-mode chemosensor for
both Cu2+ and Fꢀ ion. A survey of literature reveals that the
thiourea/imine appended chemosensors were act as dual sensor
for both Cu2+ and Fꢀ. However, those chemosensors are still rarely
applied in neutral systems due to the strong hydration ability of
Cu2+ in aqueous solution. Most of these chemosensors only func-
tion well in organic solutions and show fluorescence quenching
in the presence of an analyte due to the absence of a strong
fluorophore. These limitations reduce the sensitivity and restrict
the application of these chemosensors to environmental and bio-
logical samples. Therefore, it is important to develop some rapid
and ‘‘simple-to-use’’ fluorescent chemosensors with Cu2+ induced
‘‘turn-on’’ fluorescence signal, in neutral aqueous buffer containing
organic cosolvent or in neutral pure water. To keep this point in
our mind, herein a new thiourea appended rhodamine based dual
chemosensor (RBS) was successfully designed and synthesized
(Scheme 1), for sensing of both Cu2+ and Fꢀ ion.
copper in the normal group is 100–150 l
g/L [18]. Notably, Cu2+ can
be toxic to biological systems when levels of Cu2+ ions exceed cel-
lular needs, due to its capability of displacing other metal ions
which act as cofactor in enzyme-catalyzed reactions [19]. Due to
the diverse nature of Cu2+, the various methods have been devel-
oped to detecting the Cu2+, including atomic absorption spectrom-
etry [20], inductively coupled plasma mass spectroscopy (ICPMS)
[21], inductively coupled plasma-atomic emission spectrometry
(ICP-AES) [22], and voltammetry [23]. However, these methods
require expensive equipment and involve time-consuming and
laborious procedures that can be carried out only by trained pro-
fessionals. Consequently, research has devoted to the development
of new colorimetric/fluorescent chemosensors for the detection of
Cu2+ ions with enough selectivity [24,25]. To date, fluorescent
probes for copper ion have been extensively explored owing to
its biological significance. Even though great achievements have
been obtained in the field of colorimetric and/or fluorescent
chemosensors for Cu2+ [26–29], most of these sensors cause
Scheme 1. Synthetic procedure for chemosensor RBS.

50
H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
chemosensor RBS (160 mg, yield: 50.0%). 1H-NMR (300 MHz,
Experimental section
Materials and instruments
DMSO-d6): d (ppm): 1.10 (t, 12H, J = 6.9 Hz), 3.35–3.33 (q, 8H,
ACH2), 6.45–6.34 (m, 6H), 7.07 (t, 1H, J = 7.2 Hz), 7.17 (t, 1H,
J = 7.5 Hz), 7.35 (t, 2H, J = 7.5 Hz), 7.61–7.48 (m, 5H), 7.90 (d, 1H,
J = 7.2 Hz), 8.77 (d, 1H, J = 8.4 Hz), 10.09 (s, 1H), 11.92 (s, 1H);
FAB mass: (m/z) Calc. 645.0 Found: 646 [M + H+]; EA: Found: C,
68.54; H, 6.04; N, 15.01; S, 4.63. Calc. For C37H39N7O2S: C, 68.81;
H, 6.09; N, 15.18; O, 4.95; S, 4.97.
All reagents and rhodamine B dye were purchased from Aldrich
Chemical Co. Ltd. and Alfa Aesar Chemical Co. Ltd. The commer-
cially available Tris–HCl buffer solution (pH = 7.2) were purchased
from Aldrich. Cations Cu2+, Hg2+, Ag+, Co2+, Pb2+, Zn2+, Fe3+, Fe2+, and
Mg2+ (as perchlorate salts) were also purchased from Aldrich.
Anion stock solutions were prepared from tetrabutyl ammonium
salts, all obtained from Aldrich. All solvents were analytical grade
and used without further purification. Nuclear magnetic resonance
(1H-NMR) spectra were recorded in DMSO-d6 on Brucker-300 MHz
spectrometer with TMS as internal standard. Agilent 8453 UV–Vis
spectrophotometer was used to record UV–Visible spectra using
quartz cell with 1 cm path length. Fluorescence spectra were
measured on a Shimadzu RF-5301 PC fluorescence spectropho-
tometer equipped with a xenon discharge lamp, 1 cm quartz cell
at slit width Ex: 10 nm; Em: 10 nm. Elemental analyses were
recorded on a Flash EA 1112 analyzer. Mass spectra were recorded
on a JEOL MStation [JMS-700].
Results and discussion
Naked eye detection
The colorimetric/fluorescent recognition property of chemosen-
sor RBS (1 ꢂ 10ꢀ5 M in CH3CN) toward various cations (1 ꢂ 10ꢀ5
M
in CH3CN:10 mM Tris–HCl, (1:1) pH = 7.2, as perchlorate salts) and
anions (1 ꢂ 10ꢀ5 M in CH3CN as tetra butyl ammonium salt) were
investigated using daylight and irradiation with
a UV lamp
(at 360 nm). As shown in Fig. 1, the color of RBS solution changed
from colorless to dark pink in the presence of Cu2+. Besides, the
addition of Co2+ influenced the tiny color changes in RBS solution.
The fluorescence color of RBS changed from blue to bright yellow
after the addition of Cu2+ ions. However, the color remained
unchanged after the addition of the other investigated cations such
as Mg2+, Fe2+, Hg2+, Co2+, Zn2+, Pb2+, Ag+ and Fe3+. In naked-eye
experiments, RBS turned from colorless to yellow in the presence
of 1 equivalent of tetrabutyl ammonium fluoride (Fig. 2). The visual
color change implies the deprotonation of the receptor unit
(thiourea). However, the addition of other anions such as Clꢀ,
Brꢀ, Iꢀ, AcOꢀ, H2POꢀ4 , HSO4ꢀ and ClOꢀ4 even in excess was found to
be insensitive. This observation indicated that RBS was act as a
dual chemosensor for both Cu2+ and Fꢀ ion.
Synthesis
Rhodamine B hydrazone and compound (1) were synthesized
according to previously reported methods [49,50] and confirmed
by 1H NMR, elemental analysis (EA) and mass spectrometry.
Synthesis of chemosensor 2-(-2-((3’,6’-bis(diethylamino)-3-
oxospiro[isoindoline-1,9’-xanthen]-2-yl)imino)ethylidene)-N-
phenylhydrazine-1-carbothioamide (RBS) dye
Compound 1 (248 mg, 0.5 mmol) in 5 mL ethanol was added to
a stirred solution of 4-phenylthiosemicarbazide (84 mg, 0.5 mmol)
in 5 mL ethanol at room temperature. The stirred reaction mixture
was heated to reflux for 5 h and monitored by TLC. After cooling to
room temperature, the formed precipitate was washed with cold
ethanol and dried under vacuum to obtain crude RBS. The crude
product was purified by recrystallization from ethanol to give
Spectra recognition of Cu2+ by RBS
The sensing ability of RBS (1 ꢂ 10ꢀ5 M) towards relevance metal
cations such as Mg2+, Fe2+, Co2+, Zn2+, Pb2+, Hg2+, Ag+, Fe3+ and Cu2+
as its perchlorate salts was monitored by the UV–Vis and fluores-
cence spectroscopic techniques. As shown in Fig. 3a, UV–Vis
Fig. 1. Photographs of chemosensor RBS (1 ꢂ 10ꢀ5) in CH3CN at 25 °C, in the presence of different metal ions (in CH3CN:Tris–HCl (10 mM, pH = 7.2) 1 ꢂ 10ꢀ5 M) under (TOP)
visible light and (Bottom) UV light.

H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
51
Fig. 2. Photograph of chemosensor RBS (1 ꢂ 10ꢀ5) in CH3CN at 25 °C, in the presence of different anions (1 ꢂ 10ꢀ5 M) in CH3CN.
absorbance spectrum of RBS (1.0 ꢂ 10ꢀ5 M) exhibited no absor-
bance over 500 nm. Addition of 2.0 equivalent Cu2+ into solution
immediately resulted in a significant enhancement of absorbance
at about 560 nm as well as the color of solution changes from col-
orless to red. In contrast, under the same condition other cations
lead to much smaller spectral changes (Co2+), or almost no spectral
changes (Mg2+, Fe2+, Zn2+, Pb2+, Hg2+, Ag+ and Fe3+). This result was
clearly consistent with Irving–William series. Cu2+ has a specific
high thermodynamic affinity for typical N-donor ligands and fast
metal-to ligand binding kinetics. On the other hand, fluorescence
spectra (Fig. 3b) also show a similar result, which is consistent with
that of UV–Vis spectra. Addition of 2.0 equivalent of Cu2+ ion
results in an obviously enhanced fluorescence at 582 nm
(OFF–ON), while other ions induce much smaller (Co2+) or no
fluorescence enhancement (Mg2+, Fe2+, Zn2+, Pb2+, Hg2+, Ag+, Fe3+
and Ag+).
The UV–Vis titration of the Cu2+ was conducted by using
1 ꢂ 10ꢀ5 M of RBS in CH3CN solution. As depicted in Fig. 4a, upon
the addition of incremental amount of Cu2+ (in CH3CN-Tris–HCl)
to RBS solution, the new absorption peak evidently increased at
560 nm, which induced a clear and gradually color change from
colorless to pink (Fig. S1). This indicated the opening of the
rhodamine-spirolactam ring [51]. To determine the stoichiometry
of the RBS–Cu2+ complex, Job’s method for absorbance measure-
ment was applied [52]. Keeping the sum of the initial concentra-
tion of Cu2+ and RBS at 5 ꢂ 10ꢀ5 M, the molar ratio of Cu2+ was
varied from 0.1 to 0.9. A plot of absorbance versus the mole frac-
tion of Cu2+ was provided in Fig. S2. It showed that the maximum
0.75
a
Cu2+
a
0.50
0.4
0.25
RBS+ other
0.2
metal ions
Co2+
0.00
400
500
600
700
0.0
400
500
600
700
Wavelength (nm)
Wavelength (nm)
400
350
300
250
200
150
100
50
180
b
Cu2+
b
150
120
90
60
30
0
RBS+other metal ions
Co2+
0
500
550
600
650
700
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a) Absorbance spectra of RBS (1 ꢂ 10ꢀ5 M) in CH3CN in the presence of
different metal ions (2 equiv.) in CH3CN-Tris–HCl (10 mM, pH = 7.2); (b)
Fluorescence spectra of RBS (1 ꢂ 10ꢀ5 M) in the presence of different metal ions
(2 equiv.) in CH3CN-Tris–HCl, (10 mM, pH = 7.0) excited at 560 nm.
Fig. 4. (a) Absorption spectra of RBS (1 ꢂ 10ꢀ5 M) upon addition of increased
amount of Cu2+ (0–1 ꢂ 10ꢀ5 M) in CH3CN-Tris–HCl media (10 mM, pH 7.2); (b) The
fluorescent intensity of RBS (1 ꢂ 10ꢀ5 M) upon addition of increased amount of Cu2+
in CH3CN-Tris HCl media (10 mM, pH = 7.0).

52
H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
of the curve was appear at of 0.5 mol fraction, indicating a 1:1 sto-
ichiometry of the Cu2+ to RBS in the complex [52]. From the
increase in the absorption intensity at 560 nm the binding constant
of the RBS–Cu2+ complex was calculated using the following
Benesi–Hildebrand equation [53].
which is attributed to the opening of the spirolactam ring of rho-
damine to an amide form [51]. The solution showed an intense
orange fluorescence (Fig. S4), with an approximately 150-fold
enhancement in the fluorescence intensity at 582 nm.
Competitive experiments
1=ðA ꢀ A0Þ ¼ 1=KaðAmax ꢀ A0Þð1=½Cu2þꢃÞ þ 1=ðAmax ꢀ A0Þ
Possible interference by other metal ions was evaluated
through competition experiments. The changes in the RBS absor-
bance were measured after treatment with 2.0 equivalents of
Cu2+ in the presence of potentially interfering metal ions, including
Mg2+, Fe2+, Hg2+, Co2+, Zn2+, Pb2+, Ag+ and Fe3+ (Fig. 5). As part of the
competition experiments, the fluorescence properties of RBS along
with other metal ions (above-mentioned) were measured, showing
that the increase of fluorescence intensity resulting from the
addition of the Cu2+ was not influenced by the addition of excess
metal ions. It is gratifying to note that all the tested cations have
no obvious influence on the chemosensor RBS function.
where, A0 is absorption intensity in the absence of Cu2+, A is the
absorption intensity at various Cu2+ ion concentration, Amax is
the absorption intensity at maximum concentration of Cu2+, Ka is
the binding constant for the RBS with Cu2+ ion. In the present
investigation, a plot of 1/(A ꢀ A0) versus 1/[Cu2+] is linear (Fig. S3).
The association constants thus computed was 1.3 ꢂ 104 Mꢀ1 for
the RBS–Cu2+ complex.
To estimate the sensing ability of RBS, the fluorescence titration
experiment has also been carried out. For free RBS there was no
emission peak at 582 nm when excited at 560 nm. Whereas, the
addition of incremental amount of Cu2+ ion to RBS solution the
new emission peak was slowly enhanced at 582 nm (Fig. 4b),
Spectra recognition of Fꢀ by RBS
The interaction of chemosensor RBS with the anions was inves-
tigated through UV–Vis spectra in CH3CN solution. In the absence
1.0
0.8
0.6
RBS+Otheranions
RBS
0.4
RBS+F-
0.2
0.0
300
400
500
600
Wavelength (nm)
Fig. 6. Absorbance spectra of RBS (1 ꢂ 10ꢀ5 M) in CH3CN in the presence of
different anions (1.0 equiv.).
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
Fig. 5. (a) Absorbance intensity of RBS (1 ꢂ 10ꢀ5 M) in the presence of Cu2+ (2.0
equiv.) and additional various ions (2.0 equiv.) in CH3CN-TrisHCl media (10 mM,
pH = 7.2) (b) Fluorescence response of RBS (1 ꢂ 10ꢀ5 M) in the presence of Cu2+ (2.0
equiv.) and additional various metal ions (2.0 equiv) (CH3CN-Tris–HCl, 10 mM,
pH = 7.0).
Wavelength (nm)
Fig. 7. Change in UV–Vis spectra for RBS (1.0 ꢂ 10ꢀ5 M) in CH3CN upon the addition
of (0–1 ꢂ 10ꢀ5 M) of fluoride ion.

H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
53
of anions, the maximum absorption wavelength of RBS is at about
346 nm. As shown in Fig. 6, when 1.0 equivalent of TBAF was added
to the solution of RBS (1 ꢂ 10ꢀ5 M), the peak at 346 nm
disappeared and new peak was appear at 430 nm. In the same
conditions, no significant changes happened when 1.0 equivalent
of tetrabutylammonium Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSOꢀ4 and
ClOꢀ4 were added. The results suggest that RBS has a high selectiv-
ity for the fluoride anion over other common anions. To further get
the binding ability of RBS with the fluoride was investigated
through UV–Vis spectrophotometric titration by adding standard
solution of TBAF with different concentration (Fig. 7). As the con-
centration of fluoride ion increased, the absorbance intensity
decreased at 346 nm, and a new band at 430 nm increased. The
isosbestic point at about 380 nm observed in both cases was attrib-
uted to the equilibrium between two species (RBS and RBS–F)
throughout the titration process [54]. The stoichiometry of
RBS–Fꢀ complex was calculated as 1:1 ratio by jobs continuous
variation method (Fig. S2). From the spectrometric titration curve
the association constant (Ka) of RBS to Fꢀ was obtained as
6.2 ꢂ 103 Mꢀ1 by Benesi–Hildebrand equation (Fig. S5).
broadened with very tiny downfield shift from d 10.09 and
11.92 ppm. The signal of the xanthene proton on rhodamine B pro-
tons completely shifted to up field (see Fig. 8), this is indicating the
ring opening of rhodamine ring by Cu2+ coordination (Fig. 8). It
should be noted that the signal of the ANH moiety was well
retained with broadening during the titrations. This suggested that
Cu2+ interacts with ‘S’ and develop the resonating bond between
sulfur and NH group. The shifting of imine peak suggested that
involvement of the imine nitrogen on complex formation. Thus,
according to the obtained results, it is very likely due to the metal
ion-induced ring opening of rhodamine spirolactam, rather than
other possible reactions. The chemosensor RBS is the most likely
to chelate with Cu2+ via nitrogen on the two imine moieties and
oxygen on the spirolactam, as well as sulfur on the thiourea group
(Scheme 2).
Based on the optical and spectrophotometric experiment it is
challenging to predict whether the noticeable changes include
formation of complex between RBS and Fꢀ through hydrogen
bonding or deprotonation of RBS by sufficient basic strength of
Fꢀ molecules. Fig.
M DMSO-d6) titrated with [Bu4N]F. Upon addition
of 0–1.0 equivalent of TBAF, the signal at d 10.09 and 11.92 ppm,
9
displays the 1H NMR spectra of RBS
(1 ꢂ 10ꢀ5
1H NMR titration of RBS with Cu2+ and Fꢀ
assigned to the thiourea NH protons, was broadened gradually
To get the better insight into the binding mode of RBS–Cu2+ and
RBS–Fꢀ in solution, 1H NMR titrations were carried out. On addi-
tion of 1.0 equivalent Cu2+ to the DMSO-d6 solution of RBS, the
peak assigned to the proton of the (CH@N) imines moieties were
broadened and shifted downfield from d 7.90 and 8.76 to 8.3 and
9.86 ppm. Further the peak consistent to thiourea NH has also been
and shifted to downfield, indicating the formation of
a
RBS–Fꢀ hydrogen-bonding complex between RBS and Fꢀ. On
further addition of Fꢀ ions up to 1.5 equivalent, the signal of the
ANH protons disappeared completely, which demonstrated the
deprotonation of the ANH fragment [55,56]. While in the case of
other aromatic and xanthene protons are not disturbed even in
Fig. 8. Change in partial 1H NMR (300 MHz) spectra of RBS with 1.0 equivalent metal ion in DMSO-d6.
Scheme 2. Proposed binding modes for complex RBS–Cu2+ and RBS–Fꢀ.

54
H. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 48–55
Fig. 9. Change in partial 1H NMR (300 MHz) spectra of RBS with various equivalent of Fꢀ ion in DMSO-d6.
Table 1
Performances comparison of various colorimetric/ fluorescent chemosensors for Cu2+/Fꢀ ion.
Modes
Fluorophore
Testing media
Sensing ions
Remarks
Ref
UV–Vis, fluorescence, cyclic voltammetry Ferrocene
CH3CN
DMSO
CH3CN
DMSO:HEPES
Cu2+/Fꢀ
Cu2+/Fꢀ
Complex formation/hydrogen bonding 56
Complex formation/hydrogen bonding 57
UV–Vis, fluorescence
UV–Vis
Isoindoline
Dipyrrole
Azoaniline
Co2+, Ni2+, Cu2+, Zn2+/Fꢀ Complex formation/hydrogen bonding 58
UV–Vis, fluorescence
UV–Vis, fluorescence
UV–Vis, fluorescence
UV–Vis, fluorescence
UV–Vis, fluorescence
Cu2+/Fꢀ
Fꢀ/Cu2+
Chemodosimeter
59
Benzthiazole Ethanol/Water
Chemodosimeter/sequential
Chemosensor/sequential
Complex formation/sequential
Chemosensor/hydrogen bonding
60
61
62
This work
Rhodamine
BODIPY
Ethanol/Water Tris-HNO3 Al3+/Fꢀ
CH3CN
CH3CN:Tris HCl pH = 7.0
Cu2+/Cys
Cu2+/Fꢀ
Rhodamine
excess amount of Fꢀ ion. These results clearly indicating the
sensing mode of fluoride ion on RBS is quite different from the
Cu2+ ion. The likely binding modes between RBS and Cu2+, as well
as between RBS and Fꢀ, are shown in Scheme 2.
bearing imines and thiourea moiety that shows dual response for
Cu2+ and Fꢀ ion through different mechanisms. A remarkable
enhancement in the fluorescence intensity of RBS and a noticeable
color change from colorless to pink were observed upon binding
with Cu2+. The response of the RBS to Cu2+ was unaffected by the
presence of other common coexistent metal ions. 1H NMR studies
supported that the Cu2+ binding to N atom of imine and S atom of
thiourea moiety in RBS. The RBS was form 1:1 complexes with Fꢀ
by multiple hydrogen-bonding interactions. Among the relevant
anions, RBS has higher selectivity for only fluoride and leads to a
distinct color change that can be observed by the naked eye.
Experiments with a series of anions suggested that RBS can effi-
ciently and preferentially bind with the fluoride anion, as estab-
lished by the UV–Vis and 1H NMR results.
Method performance comparison
The performance of the proposed dual chemosensor RBS toward
Cu2+/Fꢀ was compared with some reported colorimetric/fluores-
cent chemosensors based on different fluorophores. As shown in
Table 1, rhodamine B structural motif dual sensor for Cu2+ and Fꢀ
is either very rare or nil. All the methods present good selectivity
for Cu2+/Fꢀ with signal enhancement, and most of them can adopt
dual chromo- and fluorogenic changes toward Cu2+ and Fꢀ [57–63],
as well as our proposed RBS. A few of chemosensors possess wide
metal ion sensing ability [59], but some of them need more
rigorous testing media and the biological pH are not investigated
[57–59,63]. Notably, as for the three types of sequential fluorescent
chemosensors based on bezthiazole [61], rhodamine [62] and
BODIPY derivatives [63] as sequential dual-function detection for
Fꢀ/Cu2+ [61], Al3+/Fꢀ [62], and Cu2+/Cys [63], are realized, respec-
tively, however, in these sensors, applications for second target
analytes are not possible in the absence of first target ion. Our
newly developed chemosensor presents a number of attractive
analytical features such as high sensitivity, distinct naked eye color
change and biological pH. The fluorescent chemosensor RBS based
on thiourea modified rhodamine spirolactam derivative is easy to
prepare with low cost and can be used for rapid analysis of Cu2+
and Fꢀ in biological pH. To the best of our knowledge, this is the
first attempt for the Cu2+ and Fꢀ ions dual sensor based on
rhodamine derivatives, which follow different mechanism.
Acknowledgement
This study was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT and Future Planning
(Grant no. 2015021972).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2015.06.078.
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