Facile synthesis of rhodamine-based highly sensitive and fast responsive colorimetric and off-on fluorescent reversible chemosensors for Hg2+: Preparation of a fluorescent thin film sensor

Article abstract of DOI:10.1039/c1dt11307k

Fluorescence-active chemosensors (L1-L4), comprising a rhodamine scaffold and a pseudo azacrown cation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg²⁺. An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide form of the rhodamine moiety, which was induced by the interactions between Hg²⁺ and the chemosensor. Upon the addition of Hg²⁺, an overall emission change of 350-fold was observed, and the selectivity was calculated to be 300 times higher than Cu²⁺ for receptors L2-L4. A polymeric thin film can be obtained by doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can be used to detect Hg ²⁺ with high sensitivity and can be recovered using diluted NaOH. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11307k

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PAPER
Facile synthesis of rhodamine-based highly sensitive and fast responsive
colorimetric and off-on fluorescent reversible chemosensors for Hg2+:
preparation of a fluorescent thin film sensor†
Chatthai Kaewtong,*a Banchob Wanno,a Yuwaporn Uppa,b Nongnit Morakot,a Buncha Pulpokac and
Thawatchai Tuntulanic
Received 9th July 2011, Accepted 15th September 2011
DOI: 10.1039/c1dt11307k
Fluorescence-active chemosensors (L1–L4), comprising a rhodamine scaffold and a pseudo azacrown
cation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg2+.
An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide form
of the rhodamine moiety, which was induced by the interactions between Hg2+ and the chemosensor.
Upon the addition of Hg2+, an overall emission change of 350-fold was observed, and the selectivity was
calculated to be 300 times higher than Cu2+ for receptors L2–L4. A polymeric thin film can be obtained
by doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can be
used to detect Hg2+ with high sensitivity and can be recovered using diluted NaOH.
Pb2+ and Cu2+ ions, which induced much smaller color/spectral
changes.2b Moreover, in 2008, Li and colleagues7 demonstrated
Introduction
the use of Hg2+-selective fluorescence probes to construct a
combinational logic circuit. However, the increasing absorbance
of Cu2+ (50-fold) and Zn2+ (30-fold) at 553 nm were also detected.
Later, Das and co-workers reported a rhodamine-based reversible
chemosensor that could bind Hg2+ and Cu2+ in aqueous methanol
solution with detectable changes in colour. Previous studies have
shown that rhodamine derivatives have been used in synthesizing
fluorescent sensors, but most of them have problems in selectivity
between Hg2+ and Cu2+. Even though complicated molecules
were synthesized, the results were exactly the same. Therefore,
they were not suitable for integration into real applications.
Recently, we demonstrated that the amide group connecting to
rhodamine could preferably bind metal cations.8 It is well known
that the azacrown, one of the most extensively studied ligands,
can coordinate strongly with Cu2+. However, a pseudo-azacrown
from non-cyclic polyamines can probably regulate the binding
properties towards Cu2+ and Hg2+ by varying chain lengths and
donor atoms. We envision that ethylene polyamine groups can act
as pseudo azacrowns and can coordinate transition metal ions to a
different extent. Herein, we describe a simple synthetic procedure
to obtain sensors for Hg2+ from rhodamine derivatives containing
ethylene polyamine groups possessing different donor atoms and
chain lengths. The sensors were expected to have high sensitivity
and selectivity for possible applications as thin film sensors.
Rhodamine is a dye used extensively as a fluorescent signal trans-
ducer due to its excellent photophysical properties, such as long
absorption and emission wavelength, large absorption coefficient,
and high fluorescence quantum yield. Indeed, a longer wavelength
emission (over 550 nm) of this dye was often employed as a sensing
signal to avoid the influence of background fluorescence (below
500 nm).1 Many recent developments have shown that rhodamine
is a promising structural scaffold for the design of selective
chemosensors. The cation-sensing mechanism of these probes is
based on a change from the spirolactam to an open-ring amide, re-
sulting in a magenta-colored, highly fluorescent compound. Based
on this mechanism, some rhodamine-based chemosensors and
chemodosimeters for Hg2+ have been developed.2–6 It was found
that most rhodamine probes met serious interference from other
transition metal ions, especially from Zn2+ and Cu2+, which usually
induced a comparable fluorescence response to that of Hg2+. For
example, a novel tren-based tripodal sensor was synthesized and
found to selectively bind the Hg2+ ion to give a visual color
change, while other ions induced no color/spectral changes except
aSupramolecular Chemistry Research Unit, Department of Chemistry
and Center of Excellence for Innovation in Chemistry, Faculty of
Science, Mahasarakham University, Mahasarakham, 44150, Thailand.
E-mail: kchatthai@gmail.com; Fax: 66 0437 54246; Tel: 66 0437 54246
bDepartment of Chemistry, Faculty of Engineering, Rajamangala University
of Technology Isan Khon Kaen Campus, Khonkaen, 40000, Thailand;
Fax: 66 0437 54246; Tel: 66 0433 36371
cSupramolecular Chemistry Research Unit, Department of Chemistry,
Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand;
Fax: 66 0221 87598; Tel: 66 0221 87643
Results and discussion
As illustrated in Scheme 1, L1–L4 were easily synthesized in
good yields by condensation reactions of rhodamine B with
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt11307k
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fluorescence changes of L2 upon the addition of Hg2+ are shown
in Fig. 2.
Scheme 1 Synthetic pathways of L1–L4.
ethylenediamine, diethylenetriamine, triethylenetetraamine and
tetraethylenepentaamine under N2 at reflux for 3 d to afford
L1, L2, L3 and L4, respectively. Their molecular structures were
confirmed by IR, MS and NMR spectroscopy and they were
designed to chelate with metal ions via their carbonyl O and amine
N atoms. In a similar manner to other rhodamine derivatives,
ligands L1–L4 are colorless and fluorescence inactive in solution
in either low or high concentration (such as L1 in Fig. S1†),
indicating that the spirolactam form predominantly exists. The
fluorescence intensity changes of L1–L4 were monitored upon
adding metal ions to determine the cation binding abilities. Fig.
1a shows fluorescence spectra of ligand L2 in the presence and
absence of 5 equiv. of cations. A high-intensity fluorescence band
at 580 nm was observed upon addition of Hg2+ into the solution
of ligand L2. High fluorescent responses were also observed in
the case of ligands L3 and L4, whereas the ligand L1 showed a
slight response in the presence of Hg2+ (Fig. 1b). The color and
Fig. 2 Color changes (A, B) and fluorescence changes (C, D) of L2
(10 mM) in the presence of 5 equiv. of Hg2+. (A, C) L2 only, (B, D) L2
+ Hg2+
.
To further study the interactions between L2 and the Hg2+ ion,
1H NMR experiments were carried out in CD3CN, and the spectra
are depicted in Fig. 3. For free L2 the chemical shift of the amine
NH is around 4.20 ppm, whereas in the presence of 5 equiv. of
Hg2+ ions it was broader and had shifted downfield (5.17 ppm). In
addition, the chemical shifts of all the protons also showed distinct
downfield changes, especially the aromatic protons. The 1H NMR
results firmly supported that the pseudo-azacrown ether and the
carbonyl group of spirolactam were involved in the Hg2+ binding,
thus inducing a ring-opening form of the spirolactam in L2.
Fig. 3 1H NMR spectra of L2 (10 mM) in CD3CN in (a) the presence
and (b) the absence of 5 equiv. of Hg2+
.
As expected, the addition of Hg2+ to the solution of all ligands
in either CH3CN or DMSO caused a significant enhancement
of absorbance intensity in the 500–650 nm range immediately,
as a result of the Hg2+-induced ring opening of the spirolactam
form. Moreover, UV-vis data were employed to determine the
stoichiometry and stability constants for the complexes using the
SIRKO program. According to the observed absorption spectra
changes (Fig. 4a), the association constants of all ligands were
obtained and summarized in Table 1. The Hg2+ preferred to bind
with L2 selectively (log b = 3.58, Table 1) over other ligands
by forming a 1 : 1 complex, which was supported by the mole
ratio plot (inset, Fig. 4a). The results were also consistent with
the selectivity from fluorescent titration. Upon the addition of
5 equiv. of Hg2+, there were changes in the fluorescence spectra
of all ligands. Continuous florescence enhancements at 580 nm
Fig. 1 (a) Fluorescence spectral changes of L2 after the addition of
5 equiv. of various cations. (b) Fluorescence responses of L1, L2, L3 and
L4 with 5 equiv. of various cations (10 mM of receptor in 0.01 mol L-1 of
TBAPF6 in MeCN).
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Table 1 Stability constants (log b)a of 1 : 1 complexes of the ligands with
Hg2+ in 0.01 mol L-1 of TBAPF6 in MeCN by UV-vis methods (T = 25 ◦C)
Ligands
log b
L1
L2
L3
L4
3.00 (0.08)b, 8.91 (0.05)c
3.58 (0.03)b
3.39 (0.08)b
3.24 (0.08)b
a Mean values derived from at least three independent determinations with
the standard deviation sn-1 in parentheses, b 1 : 1 complex (LM), c 2 : 1
complex (L2M).
Fig. 5 Fluorescence enhancement response of L2 (10 mM in 0.01 mol L-1
of TBAPF6 in MeCN) to 5 equiv. of various cations (black bars) and to
the mixture of 0.01 M different metal ions with 67 mM of Hg2+ (gray bars).
Excitation and emission was at 520 and 580 nm, respectively.
of Hg2+ by L2 is hardly affected by these commonly co-existent
ions. The selectivity ratio of L1–L4 to Cu2+ and Hg2+ was also
2+
2+
investigated. The fluorescence intensity ratios (IHg /ICu at 580
nm) of L1–L4 showed the following the order: L2 (460) > L3
(376) > L4 (335) > L1 (36). Furthermore, since the color and
fluorescence of L2 + Hg2+ disappeared immediately when excess
of a diluted solution of NaOH (1 N) was added, the sensing
process was considered to be a reversible rather than ion-catalyzed
reaction.
Recognition mechanism of ligand towards Hg2+: to explain the
recognition abilities of L1–L4 towards Hg2+, the optimized geome-
tries and the HOMO and LUMO energies of the complexes were
determined by the density functional theory (DFT) calculations
at B3LYP/LanL2DZ level.9 The optimized structures of L1–L4
complexes with Hg2+ are illustrated in Fig. 6. The results suggested
that the receptors formed stable complexes with Hg2+ through a
large number of cation-dipole and ion-ion interactions. Further-
more, the negative and high values of calculated complexation
energies found in all complexes indicated that Hg2+ could form
stable complexes with synthesized receptors.10 Interestingly, for the
L1 and Hg2+ complexation system, the calculated complexation
energy was lower for the 2 : 1 stoichiometry than that of 1 : 1
model. This is in good agreement with experimental results that
the association constant for the 2 : 1 model is higher than that
of 1 : 1 model. This implied that the 2 : 1 binding mode model
was more favored than the 1 : 1 model. As shown in Fig. S2,† the
HOMO distributions of L1–L4 are concentrated exclusively on
the thiourea and xanthene moieties, while those of the LUMO are
located at the Hg2+. In the case of L1, an insignificant fluorescence
spectral change upon the addition of Hg2+ ions was observed when
compared with L2–L4. It was assumed that the low sensitivity
of L1 toward Hg2+ was induced by intermolecular fluorescence
self-quenching as shown in Fig. 7. The HOMO mainly locates
on the xanthene ring of rhodamine B, and the LUMO showed
a significant density on the other rhodamine of L1. These results
also showed that the intermolecular distance of the dimeric species
Fig. 4 (a) Absorption spectra of L2 (10 mM) in 0.01 mol L-1 of TBAPF6
in MeCN in the presence of different amounts of Hg2+. Inset: mol ratio
plots by using absorbance at 556 nm as a function of Hg2+ concentration,
indicating a 1 : 1 metal–ligand ratio. (b) Fluorescence spectra of L2 (10 mM)
under the same conditions. Excitation was performed at 520 nm. Inset: mol
ratio plots by using fluorescence at 580 nm, indicating a 1 : 1 metal–ligand
ratio.
were observed (Fig. 4b). The stability constant of ligands with
Hg2+ was calculated according to the 1 : 1 model and thus varies
as follows: L2 (b = 83 000 M-1) > L3 (b = 55 000 M-1) > L4 (b =
37 375 M-1) > L1 (b = 23 111 M-1). This can be explained by the
most suitable coordination sphere of the receptor structure (L2)
to accommodate a Hg2+ ion.
In addition, a competition experiment was also carried out
by adding Hg2+ to the solution of L2 in the presence of other
metal ions. Fig. 5 showed the comparison between the fluorescence
response of L2 with each metal ion and the emission that occurred
after the addition of Hg2+. The results indicate that the sensing
˚
was estimated to be about 11 A, which strongly supported the
idea of the self-quenching processes. In addition, an increase in
selectivity was observed on increasing the length of the polyamine
units due to the increased donor strength of the ethereal nitrogen
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high degree of conformational flexibility, which is supported by
the previous reports.11
Preparation of the polymeric thin films: to prepare the polymeric
thin film sensors, poly(methyl methacrylate) PMMA polymer (300
mg) was dissolved in dichloromethane and poured onto a clean
glass surface and doped with the ligand L2 (1.5 mg, 0.5 wt%).
The solvent was evaporated to dryness, and a homogeneous, non-
fluorescent polymer sensor film was obtained. This thin film can be
used for Hg2+ detection. A solution containing Hg2+ in acetonitrile
(1 mM) was sprayed onto the film, and the solvent was evaporated
in air. A strong fluorescent image appeared on the regions exposed
to Hg2+ ions (Fig. 8). For the erasing process, an acetonitrile
solution of NaOH was sprayed onto the thin film. The fluorescent
Hg2+ image disappeared, and the non-fluorescence of the Hg2+-
exposed region of the film was restored. Infrared spectra were also
conducted to confirm the binding of the carbonyl group of L2
with the Hg2+ and the reversible process. It was clearly observed
that upon addition of the Hg2+, the carbonyl stretching band of
L2 at 1741 cm-1 was changed to the lower number (1725 cm-1)
and turned back to 1741 cm-1 after treatment with diluted NaOH
solution and elution through a silica column (Fig. S3, ESI†).
Fig. 8 Fluorescence image of poly(methyl methacrylate) polymer sheets
doped with ligand L2. The polymer film on the glass slide was irradiated
with a hand-held UV lamp at 360 nm.
Conclusions
We synthesized simple, but showing good characteristics, Hg2+
sensors, containing various chain lengths of polyamine units
connecting to the rhodamine building block. UV-vis and fluores-
cence spectrophotometry studies showed that L2 exhibited high
selectivity and sensitivity towards Hg2+ compared to the other
ligands. Common co-existing metal ions displayed insignificant
interference to the detection of Hg2+. Theoretical calculations
showed that a suitable number of N-donor atoms and chain
length in L2 allowed the best accommodation for Hg2+. After
sensing Hg2+, the sensor was found to be recovered by adding
diluted NaOH. Therefore, compound L2 can be used to fabricate
a reversible polymeric thin film sensor for Hg2+.
Fig. 6 The optimized structures of (1 : 1) L1–Hg2+, L1–Hg2+, L1–Hg2+
,
L1–Hg2+ and (2 : 1) L12–Hg2+ complexes, the complexation energies (DE)
are in kcal mol-1).
Experimental
Synthesis of N-(rhodamine B)lactam-derivatives (L1–L4)
The derivatives were synthesized by adapting the synthesis proce-
dure of similar compounds reported in the literature.12 Rhodamine
B (0.20 g, 0.42 mmol) was dissolved in 30 mL of ethanol and
then ethylenediamine, diethylenetriamine, triethylenetetraamine
or tetraethylenepentaamine (0.22 mL, excess) were added dropwise
to the solution and refluxed overnight (24 h) until the solution lost
its red color. The solvent was removed by evaporation. Water
(20 mL) was added to the residue and the solution was extracted
Fig. 7 HOMO and LUMO distributions of L1 calculated by DFT
calculations.
of the azacrown. However, a slight decrease of sensitivity occurred
for both ligands L3 and L4. One reason for this might be their
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with CH2Cl2 (20 mL ¥ 2). The combined organic phase was washed
twice with water and dried over anhydrous Na2SO4. The solvent
was removed by evaporation, and the product was dried in vacuo,
affording a pale-yellow solid of L1–L4, respectively.
allowed to stand at room temperature for 30 min. For fluorescent
measurements, excitation was provided at 520 nm, and emission
was collected from 530 to 700 nm.
Preparation of the polymeric thin films15
L1: (0.17 g, yield 84%). 1H NMR (400 MHz, CDCl3) d 7.86–7.81
(m, 1H, ArH), 7.45–7.32 (m, 2H, ArH), 7.08–7.03 (m, 1H, ArH),
6.42 (s, 1H, ArH), 6.39 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.38–6.21
(m, 2H, ArH), 3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.12 (t, J = 6.8
Hz, 2H, NCH2CH2), 2.23 (t, J = 6.8 Hz, 2H, NCH2CH2NH2), 2.05
(s, 2H, CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3).
MS (MALDI-TOF) calcd for [C30H36N4O2]+: m/z 484.28. Found:
m/z 485.91 [M + H]+.
Polymethylmethacrylate PMMA polymer (300 mg) was dissolved
in dichloromethane, poured onto a clean glass surface and doped
with the ligand L2 (10 mM). The solvent was evaporated to dryness,
and a homogeneous, non-fluorescent polymer sensor film was
obtained. This thin film was used for Hg2+ detection. For the
erasing process, a solution of sodium hydroxide (NaOH) was
sprayed onto the film. The non-fluorescent thin film was restored.
L2: (0.20 g, yield 91%). 1H NMR (400 MHz, CDCl3) d 7.90–7.88
(m, 1H, ArH), 7.44–7.42 (m, 2H, ArH), 7.09–7.07 (m, 1H, ArH),
6.43 (s, 1H, ArH), 6.41 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.28–6.25
(m, 2H, ArH), 3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.26 (t, J =
6.8 Hz, 2H, NCH2CH2), 2.59 (t, J = 6.0 Hz, 2H, NCH2CH2NH),
2.44 -2.38 (m, 4H, NCH2CH2NH), 1.70 (s, 3H, NCH2CH2NH
and CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3). MS
(MALDI-TOF) calcd for [C32H41N5O2]+: m/z 527.33. Found: m/z
528.95 [M + H]+.
Acknowledgements
The authors gratefully acknowledge funding from The Asia Re-
search Center, Chulalongkorn University and Thailand Research
Fund and Commission on Higher Education (RTA5380003) and
Thailand Research Fund Young New Staff (MRG5380167) and
the center of excellence for innovation in chemistry (PERCH-
CIC).
L3: (0.23 g, yield 95%). 1H NMR (400 MHz, CDCl3) d 7.89–7.88
(m, 1H, ArH), 7.48–7.43 (m, 2H, ArH), 7.08 (s, 1H, ArH), 6.43–
6.39 (m, 3H, ArH), 6.37 (s, 1H, ArH), 6.28–6.26 (m, 2H, ArH),
3.32 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.28–2.3 (m, 12H, NCH2CH2,
NCH2CH2NH and NCH2CH2NH), 2.04 (s, 4H, NCH2CH2NH
and CH2CH2NH2) and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3). MS
(MALDI-TOF) calcd for [C34H46N6O2]+: m/z 570.37 Found: m/z
572.02 [M + H]+.
Notes and References
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Bhalla, H. Singh, P. R. Sharma and T. Kau, Org. Lett., 2011, 13, 1422.
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1
L4: (0.22 g, yield 87%). H NMR (400 MHz, CDCl3) d 7.82–
7.81 (m, 1H, ArH), 7.38–7.36 (m, 2H, ArH), 7.15 (s, 1H, ArH),
6.37–6.34 (m, 2H, ArH), 6.30 (s, 2H, ArH), 6.21–6.19 (m, 2H,
ArH), 3.25 (q, J = 6.8 Hz, 8H, NCH2CH3), 3.20–2.1 (m, 21H,
NCH2CH2, NCH2CH2NH, NCH2CH2NH, NCH2CH2NH and
CH2CH2NH2) and 1.09 (t, J = 7.2 Hz, 12H, NCH2CH3). MS
(MALDI-TOF) calcd for [C36H51N7O2]+: m/z 613.41 Found: m/z
615.86 [M + H]+.
Complexation studies of ligands by using UV-vis and fluorescence
titrations
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The complexation abilities of ligands L1–L4 with cations was
◦
investigated by spectrophotometric titration in MeCN at 25 C.
2 mL of the 10 mM L1, L2, L3 or L4 solution was placed in
a spectrophotometric cell (1 cm path length). The solutions of
cations were added successively into the cell from a microburette.
The mixture was stirred for 40 s after each addition and its
spectral variation was recorded. For UV-vis titration, the stability
constants were calculated from spectrometric data using the
program SIRKO.13 For fluorescent titration, the stability constant
for a complex was obtained from a plot of the quantity Io/(Io-I)
vs. 1/[M]. The ratio of intercept/slope gave the stability constant
( Io = fluorescence intensity of free L and I = fluorescence intensity
of the complex).14
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Competition experiments
Hg2+ was added to the solution containing L2 and the other metal
ions of interest. All test solutions were stirred for 1 min and then
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