A novel rhodamine-based fluorescent and colorimetric "off-on" chemosensor and investigation of the recognizing behavior towards Fe 3+

Article abstract of DOI:10.1016/j.dyepig.2011.09.014

A novel rhodamine based probe has been synthesized as an "off-on" chemosensor for Fe³⁺. Upon coordination with Fe³⁺, the probe displayed good brightness and fluorescent enhancement. A linear relationship was observed to exist between the relative fluorescence intensity of this probe and the concentration of Fe³⁺ in the range of 5 μM-20 μM with a detection limit of 5 μM. It offered highly sensitive toward Fe³⁺ over other ions. The recognizing behaviors toward Fe ³⁺ have been investigated both experimentally and computationally. It can be expected that Fe³⁺ coordinated with the N atom of thiazole moiety in the probe accompanied by the transferring of electrons of the phenylthiazole resulted in the opening of the spiro-ring.

Full text of DOI:10.1016/j.dyepig.2011.09.014

Dyes and Pigments 92 (2012) 1337e1343
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel rhodamine-based fluorescent and colorimetric “offeon” chemosensor
and investigation of the recognizing behavior towards Fe3þ
,
,
*
Mengyao She a, Zheng Yang a, Bing Yin a c, Jin Zhang a, Jia Gu a, Wenting Yin a, Jianli Li a
,
Guifang Zhao b, Zhen Shi a
a Ministry of Education Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an,
Shaanxi 710069, PR China
b College of Life Sciences, Northwest University, Xi’an, Shaanxi 710069, PR China
c College of Chemistry, Beijing Normal University, Beijing 100875, PR China
a r t i c l e i n f o
a b s t r a c t
A novel rhodamine based probe has been synthesized as an “offeon” chemosensor for Fe3þ. Upon
coordination with Fe3þ, the probe displayed good brightness and fluorescent enhancement. A linear
relationship was observed to exist between the relative fluorescence intensity of this probe and the
Article history:
Received 14 August 2011
Received in revised form
17 September 2011
Accepted 20 September 2011
Available online 29 September 2011
concentration of Fe3þ in the range of 5
mMe20 mM with a detection limit of 5 mM. It offered highly
sensitive toward Fe3þ over other ions. The recognizing behaviors toward Fe3þ have been investigated
both experimentally and computationally. It can be expected that Fe3þ coordinated with the N atom of
thiazole moiety in the probe accompanied by the transferring of electrons of the phenylthiazole resulted
in the opening of the spiro-ring.
Keywords:
Rhodamine
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorescent chemosensor
Theoretical computation
Mechanism
Fe3þ
X-ray crystallography
1. Introduction
during this past century with identification of Fe3þ as a body
constituent and realization of the relationship between adequate
The development of artificial chemosensors for selective and
sensitive quantification of environmentally and biologically
important ionic species in solution, especially for heavy and tran-
sition metal ions, has attracted a great deal of attention [1e3]. As
one of the most essential trace elements in biological systems, Fe3þ
performs a major role in the growth and development of living
systems as well as in many biochemical processes at the cellular
level [4,5]. Numerous enzymes use Fe3þ as a catalyst for oxygen
metabolism, detective transfer as well as DNA and RNA synthesis
[6,7]. High levels of Fe3þ within the body have been associated with
increasing incidence of certain cancers and dysfunction of certain
organs, such as heart, pancreas and liver [8e11]. Some recent
research results suggested that Fe3þ could also be involved in the
underlying mechanisms of many neurodegenerative diseases, such
as Parkinson’s disease and Alzheimer’s disease [12e14]. The
essential role of iron in human and animal health became apparent
Fe3þ intake and prevention of certain diseases [15e18].
Although significant contributions to the development of
spectroscopic sensing for Fe3þ have been made over the last few
decades, there have been relatively few fluorescent chemosensors
for Fe3þ due to the fluorescent quenching of the paramagnetic Fe3þ
[19e22]. Recently, Lee et al. [23] have developed a Fe3þ-selective
chemosensor FS1 which could demonstrate sensitive and selective
detection of intracellular Fe3þ in hepatocytes. Another rhodamine
6G derivative reported by Wang et al. [24] could detect Fe3þ among
various metal ions in water with the detection limit reaching as low
as 2 ppb. At present, there is an intense demand for new efficient
Fe3þ chemosensors. Works related to this area are of great chal-
lenge and interest.
On the other hand, as a simple, efficient and economic method,
fluorescent signaling has been widely used in biological and envi-
ronmental science [25e28]. Owing to their simplicity, low detec-
tion limit, the capability for special recognition and excellent
spectroscopic properties, such as long wavelength excitation and
emission profiles, large extinction coefficient and high fluorescence
quantum yields [29,30], rhodamine dyes appear to be particularly
* Corresponding author. Tel.: þ8629 88302604; fax: þ8629 88302601.
E-mail address: lijianli@nwu.edu.cn (J. Li).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.09.014

1338
M. She et al. / Dyes and Pigments 92 (2012) 1337e1343
attractive for the construction of an OFFeON-type fluorescent
chemosensor. Recently, on the basis of spirolactam (non-fluores-
cent) to ring-open amide (fluorescent) equilibrium of rhodamine,
a number of rhodamine derivatives have been utilized for the
detection of metal ions, such as Cu2þ, Hg2þ, Cr3þ, Pb2þ, Auþ
[31e35]. With these criteria in mind, herein we introduced a phe-
nylthiazole moiety to rhodamine B and obtained a new fluorescent
probe S1 (Scheme 1, synthesis), which can give a highly selective
and rapid spectroscopic response to Fe3þ
.
2. Experimental
2.1. Apparatus and reagents
The fluorescence spectra measurements were performed on
a HITACHI F-4500 fluorescence spectrophotometer. Absorbance
spectra measurements were carried out on a Shimadzu UV-1700
spectrophotometer. Mass spectra were measured with Model
AXIMA-CFRÔ plus MALDI-TOF Mass Spectroscopy. IR spectra were
taken in KBr disks on a Bruker Tensor 27 spectrometer. X-ray crystal
data were collected on Bruker Smart APEX II CCD diffractometer.
NMR spectra were recorded on a Varian INOVA-400 MHz spec-
trometer (at 400 MHz for 1H NMR and 100 MHz for 13C NMR) with
tetramethylsilane (TMS) as internal standard. Rhodamine B, ace-
tophenone, iodine, thiourea, anhydrous FeCl3 and methanol were
all obtained from J&K Scientific Ltd. The solutions of metal ions
were performed from their nitrate and chloride salts. Analytical
thin layer chromatography was performed using Merck 60 GF254
silica gel (precoated sheets, 0.25 mm thick). Silica gel
(0.200e0.500 mm, 60A, J&K Scientific Ltd.) was used for column
chromatography. All the reagents were of analytical-reagent grade.
Double distilled water was used throughout the experiment.
Fig. 1. Molecular structure of probe S1.
61.40; N, 15.86; S, 18.14. 1H NMR (400 MHz, CDCl3, TMS):
d 7.80
(d, J ¼ 8.0 Hz, 2H), 7.41 (t, J ¼ 8.0 Hz, 2H), 7.32 (t, J ¼ 8.0 Hz, 1H), 6.76
(s, 1H), 5.12 (s, 2H).
2.2.2. Synthesis of probe S1
In brief, to a stirred solution of rhodamine hydrochloride (4.78 g,
0.01 mol) in 1,2-dichloroethane (10 mL), 5 mL phosphorus oxy-
chloride was added. The solution was refluxed for 6 h and
concentrated by evaporation. The obtained crude acid chloride was
dissolved in acetonitrile (10 mL),
a solution of 2-amino-4-
2.2. Synthesis routes
phenylthiazole (1.76 g, 0.01 mol), triethylamine (5 mL) in acetoni-
trile (20 mL) was added dropwise in 30 min. After refluxing for 4 h,
the solvent was removed under reduce pressure to give violet oil.
Water was then added to the mixture and the aqueous was
extracted with dichloromethane (15 mL ꢁ 3). The organic layer was
washed with water and dried over anhydrous MgSO4 and filtered.
2.2.1. Synthesis of 2-amino-4-phenylthiazole
12.0 g (0.1 mol) of acetophenone, 25.4 g (0.1 mol) of iodine and
15.2 g (0.2 mol) of thiourea were well crushed in crucible. The
mixture was taken in 250 mL round bottom flask and heated at
110 ꢀC for 24 h. A reaction mixture was cooled to room temperature
and diluted with 100 mL of water and extracted with ether to
remove the unreacted iodine and acetophenone. Excess of ether
was distilled off. Residue was dissolved in boiling water and filtered
off the hot solution. It was allowed to stand for 30 min. Make the
reaction mixture alkaline (up to pH 8e9) using ammonium
hydroxide solution. The solid obtained was filtered and washed
successively with water (2 ꢁ 150 mL). The separated solid was
crystallized by aqueous ethanol (1:1) to give 12.0 g solid, yield: 68%.
mp: 147e148 ꢀC (Reported [36] mp: 147 ꢀC). Anal. Calcd for
C9H8N2S: H, 4.58; C, 61.34; N, 15.90; S, 18.19. Found: H, 4.60; C,
Cl
N
O
N
N
N
O
N
POCl3, ClCH2CH2Cl
COCl
S
COOH
O
O
N
H2N
H3C
NH2
O
S
N
I2
NH3
H2N
N
N
S
Fig. 2. Changes in UVeVis absorption spectra of S1 (20 mM) in methanol solution with
various amounts of Fe3þ (0e1.5 equiv). Inset: (a). Absorbance at 558 nm of S1 as
a function of Fe3þ concentration; (b). Job’s plot of S1 and Fe3þ. The total concentration
S1
Scheme 1. Synthesis of rhodamine based probe S1.
of S1 and Fe3þ was 100
mM. The absorbance was measured at 558 nm.

M. She et al. / Dyes and Pigments 92 (2012) 1337e1343
1339
The product was purified by column chromatography on silica gel
(eluent: CH2Cl2) to give 3.12 g yellow powder, yield: 52%. mp:
269e270 ꢀC. Anal. Calcd for C37H36N4O2S: H, 6.04; C, 73.97; N, 9.33;
S, 5.34. Found: H, 6.03; C, 73.77; N, 9.35; S, 5.35. 1H NMR (400 MHz,
CDCl3, TMS):
d
8.072 (d, J ¼ 7.2 Hz, 1H), 7.865 (d, J ¼ 6.8 Hz, 2H),
7.590 (q, J ¼ 8 Hz, 2H), 7.374 (t, J ¼ 6.8 Hz, 2H), 7.284e7.250 (m, 2H),
7.046 (s, 1H), 6.474 (d, J ¼ 3.6 Hz, 2H), 6.410 (d, J ¼ 8.8 Hz, 2H), 6.164
(d, J ¼ 8.8 Hz, 2H), 3.286 (q, J ¼ 8.4 Hz, 8H), 1.112 (t, J ¼ 7.2 Hz, 12H).
13C NMR (100 MHz, CDCl3, TMS):
d 12.59, 44.30, 67.74, 76.74, 77.06,
77.36, 97.45, 106.34, 106.73, 107.16, 123.31, 124.87, 126.09, 127.29,
128.19, 128.23, 128.64, 129.52, 134.16, 134.84, 148.73, 150.10, 153.36,
154.25, 154.28, 166.39.
2.3. Crystal growth and conditions
Yellow single crystal of S1 was obtained at room temperature
from the mixed solvents of acetonitrile-dichloromethane-dioxane
(5:3:2, v/v/v) solution by slow evaporation, and then mounted on
the goniometer of single crystal diffractometer. The crystal data has
Fig. 3. Fluorescence spectra changes of S1 (20 mM) in methanol solution upon addition
been collected at 296 K by using Mo K
a
radiation (
l
¼ 0.71073 Å)
of Fe3þ (0e1.5 equiv), lex ¼ 558 nm. Inset: Changes in the emission intensity at 580 nm.
the scan mode and collected for
q
range of 1.78e25.10ꢀ by using
4/u
Lorentz and polarization effect (SADABS). The structure was solved
using the direct method and refined by full-matrix least-squares
fitting on F2 by SHELX-97. Crystal data for S1: Crystal size:
0.33 ꢁ 0.27 ꢁ 0.15 mm, monoclinic, space group P21/n. a ¼
12.0495(17),
b
b
¼
18.109(3),
c
¼
14.957(2),
a
¼
g
¼
90ꢀ,
¼ 99.638(2)ꢀ, V ¼ 3217.6(8) Å3, Z ¼ 4, 15,989 reflections collected,
5718 unique (Rint ¼ 0.0494). Final residual for 402 parameters and
5718 reflections with I > 2
GOF ¼ 1.27.
s(I): R1 ¼ 0.0584, wR2 ¼ 0.1516 and
2.4. UVevis and fluorescence spectra measurements
2.4.1. S1 stock solution
S1 stock solution (500 mM): in a 25 mL volumetric flask, 0.0939 g
S1 probe was dissolved in acetone and then diluted to the mark
with acetone. To a 50 mL volumetric tube, 4.00 mL of the solution
was added and diluted to 50 mL with methanol.
2.4.2. Fe3þ stock solution
Fe3þ stock solution (5.00 mM): in a 25 mL volumetric flask,
20.27 mg anhydrous FeCl3 was dissolved in methanol and then
diluted to the mark with methanol. The other metal ions were
prepared as 5.00 mM in methanol solution.
Fig. 4. Fluorescence intensity changes of S1 (20
m
M) upon the addition of each equiv of
ethylenediamine with the presence of Fe3þ (20
mM) in methanol solution.
Fig. 5. Fluorescence (a) and absorption (b) spectra of S1 (20
m
M) in methanol solution upon addition of various metal ions (20
mM), lex ¼ 558 nm.

1340
M. She et al. / Dyes and Pigments 92 (2012) 1337e1343
Fig. 6. Fluorescent and color changes of S1 (20
m
M) upon the addition various metal ions (20
m
M). 1: blank; 2: Liþ; 3: Naþ; 4: Kþ; 5: Ba2þ; 6: Ca2þ; 7: Cd2þ; 8: Agþ; 9: Mg2þ; 10: Co2þ
;
11: Mn2þ; 12: Zn2þ; 13: Pb2þ; 14: Hg2þ; 15: Ni2þ; 16: Cuþ; 17: Cu2þ; 18: Fe2þ; 19: Fe3þ
.
Fig. 7. Fluorescence intensity (a) and absorption (b) changes of S1 (20
m
M) upon the addition of various metal ions (20
m
M) in and without the presence of Fe3þ (20
m
M) in methanol
solution. Black bars represent the fluorescence response of S1 to the metal ions of interest. 1: blank; 2: Liþ; 3: Naþ; 4: Kþ; 5: Ba2þ; 6: Ca2þ; 7: Cd2þ; 8: Agþ; 9: Mg2þ; 10: Co2þ; 11:
Mn2þ; 12: Zn2þ; 13: Pb2þ; 14: Hg2þ; 15: Ni2þ; 16: Cuþ; 17: Cu2þ; 18: Fe2þ; 19: Fe3þ. The chromatic bars represent the subsequent addition of 20 M Fe3þ to the above solutions.
m
2.4.3. Procedure for colorimetric method
The ORTEP structure of S1 was shown in Fig. 1. The single crystal
structure of S1 clearly revealed the unique spirolactam ring
formation. In this probe, the bond lengths are in the normal range,
such as N2eC9 (1.3844 Å), N2eC17 (1.512 Å), N2eC10 (1.3844 Å),
C17eC29 (1.5034 Å), C17eC18 (1.5204 Å), C26eN4 (1.3724 Å) and
C21eN3 (1.3874 Å) et al. In addition, the dihedral angle between
the thiazole ring and xanthene ring plane is 89.67ꢀ. There also exist
two kinds of hydrogen bonding interaction in the complex, that is,
For colorimetric determination of Fe3þ, 1.00 mL of 500
mM S1
stock solution and different concentration of Fe3þ to a 25 mL
volumetric tube, and then diluted to the mark with methanol, then
the absorbance was recorded at 558 nm.
2.4.4. Procedure for fluorimetric method
To a 25 mL volumetric tube, 1.00 mL of 500 mM S1 stock solution,
different concentration of Fe3þ were added and the mixture was
diluted to 25 mL with methanol. Then, 3.0 mL each solution was
transferred to a 1 cm cuvette and the fluorescence intensity of the
above solution was recorded at 580 nm with excitation wavelength
set at 558 nm. The excitation and emission wavelength bandpasses
were both set at 5.0 nm.
2.5. Computation details
Density functional theory (DFT) [37] calculations were per-
formed at B3LYP [38,39] level with Guassian 03 [40] Program. Fe
element was described with TZVP basis set [41], Cl element was
described with 6-311þþG** basis set [42], S, N and O elements
were described with 6-31þþG* basis set [42] and 6-31G** [42]
basis set was used for C and H elements. Default type of grids in
Guassian 03 Program for the numerical integration in DFT calcu-
lations was utilized.
3. Results and discussion
3.1. Synthesis and structure characterization
As shown in Scheme 1, S1 was facilely synthesized from
rhodamine B by an uncomplicated reaction with a yield of 52%. The
structure was confirmed by IR, 1H NMR, 13C NMR spectra and X-ray
crystallography (the spectra are shown in Supporting information).
Fig. 8. IR of S1 and S1 þ FeCl3.

M. She et al. / Dyes and Pigments 92 (2012) 1337e1343
1341
changed back to colorless, which implied the reversible binding
between S1 and Fe3þ
.
For an excellent chemosensor, high selectivity is a matter of
necessity. Related metal ions, including Liþ, Naþ, Kþ, Ba2þ, Ca2þ
Cd2þ, Mg2þ, Co2þ, Mn2þ, Zn2þ, Pb2þ, Ni2þ, Hg2þ, Agþ, Cuþ, Cu2þ
,
,
Fe2þand Cr3þ, were used to evaluate the metal ion binding prop-
erties of S1 in methanol solution by fluorescence spectroscopy and
UVeVis spectroscopy. Among the various metal ions, both the
absorption and the fluorescence emission spectra showed a note-
worthy high selectivity to Fe3þ with respect to the large color
changes and fluorescent enhancement. Other metal ions developed
no significant absorption and fluorescence intensity changes
(Figs. 5 and 6). The competition experiment, which was carried out
by adding Fe3þ to S1 solution in the presence of other metal ions,
showed that the Fe3þ-induced fluorescent response was not
interfered by the commonly coexistent ions. The result suggested
that probe S1 showed a remarkable selectivity toward Fe3þ over
other competitive ions (Fig. 7).
Scheme 2. Proposed mechanism for the fluorescent changes of S1 upon the addition
of Fe3þ
.
C5eH5$$$O2 (H5$$$O2, 2.713 Å) and C19eH19$$$O1 (H19$$$O1,
2.614 Å), which can help the molecule extend and stabilize the
three dimensional supramolecular structure.
3.2. Spectral characteristics
Absorption spectra of S1 were performed in methanol solution
(Fig. 2). The free probe of S1 was colorless and scarcely showed
absorption at 558 nm. Upon binding with Fe3þ, the absorption peak
intensity increased dramatically, coupled with a clear color change
from colorless to pink. This implied that S1 allowed naked-eye
3.3. Theoretical computation and mechanism
In the IR spectra (Fig. 8), the peak at 1703 cmꢂ1, corresponding to
the characteristic carbonyl absorption of S1, did not drastically shift
to the lower frequency upon addition of 1.0 equiv of FeCl3, This
supported that the amide carbonyl oxygen was actually not
involved in the coordination.
detection for Fe3þ
. Absorption titrations showed a typical
sigmoidal curve and the increase was saturated with more than
1.0 equiv of Fe3þ (Fig. 2(a), inset). Job’s plot for the absorbance was
applied to study the binding mode of S1 and Fe3þ. The association
constant was estimated [43,44] to be 6.56 ꢁ 104 Mꢂ1 from the
absorption titrations experiments based on the basis of nonlinear
fitting of the titration curve assuming the 1:1 binding model
(Fig. 2(b), inset).
In order to obtain a deep understanding of the new probe S1,
DFT calculations of S1 and S1eFe3þ complex were performed. Fig. 8
showed the optimized geometries of S1 and S1eFe3þ complex, of
which the local minimum character was confirmed by the nonex-
istence of imaginary frequency [42]. The bond length of N4eC26
changed from 1.387 Å (S1) to 1.359 Å (S1eFe3þ), while the length of
C17eC29 changed from 1.516 Å (S1) to 1.409 Å (S1eFe3þ), indi-
cating the existence of the quinoid structure in S1eFe3þ (Fig. 8).
This fact suggested the ring-opening progress in S1 upon coordi-
nation with Fe3þ. In the case of previous works, coordination of
carbonyl oxygen to metal ions leaded to spirocycle opening [46,47].
While this study, it was clear shown that, in the lowest energy
structure of the S1eFe3þ complex, Fe3þ ion was mainly coordinated
with the N on the phenylthiazole moiety of S1 with the average
distance of 1.925 Å which was much shorter than the sum of the
Van der Waals radii of Fe and N (3.35 Å). On the other hand, the
distance between O atom and Fe was 5.300 Å, longer than the sum
of the Van der Waals radii (3.34 Å), indicating that O atom was hard
to coordinate with Fe3þ. It is probable that the positive charge of the
Fe3þ was significantly decreased by coordination with the thiazole
nitrogen which made Fe3þ hard to coordinate with carbonyl
Fig. 3 shows the results of fluorescence titration of S1 upon the
gradual addition of Fe3þ in methanol solution under excitation at
l
¼ 558 nm. No obvious fluorescent emission was observed around
580 nm in the absence of iron because the spirocyclic form of
rhodamine prevailed. Upon the addition of increasing concentra-
tion of the Fe3þ, the intensity increased drastically which was
reasonably assigned to the delocalized xanthene tautomer of the
rhodamine group indicating the ring-opened process of rhodamine
B unit in S1. The increment saturated after adding 1.0 equiv of Fe3þ
(Fig. 3, inset). The fluorescence quantum yield was calculated [45]
as 0.34 by using rhodamine B as a standard. A linear relationship
was observed to exist between the relative fluorescent intensity of
S1 and the concentration of Fe3þ in the range of 5
a detection limit of 5 M. Addition of ethylenediamine to the
mixture of S1 (20 M) decreased the fluorescence
M) and Fe3þ (20
intensity of the solution (Fig. 4) and the color of the mixture
mMe20 mM with
m
m
m
Fig. 9. B3LYP optimized structure of S1(a) and S1eFe3þ(b) complex. Carbon atoms are gray, oxygen atoms are red, nitrogen atoms are blue, sulfur atoms are yellow, chlorine atoms
are green, and iron atom is steelblue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1342
M. She et al. / Dyes and Pigments 92 (2012) 1337e1343
Fig. 10. HOMO and LUMO distributions of S1eFe3þ
.
oxygen. All in all, it can be expected that Fe3þ coordinated with the
N atom of thiazole moiety in S1 accompanied by the transferring of
electrons of the phenylthiazole resulted in the opening of the spiro-
ring (Scheme 2).
The spatial distributions and orbital energies of HOMO and LUMO
of S1eFe3þ were also determined (Fig. 9). It was clearly shown that
the HOMO distribution of the complex was located essentially over
the phenylthiazole moiety, while the LUMO was mainly distributed
over Fe3þ and neighboring atoms. The energy gap between HOMO
and LUMO was computed to be 1.985 eV (Fig. 10).
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In conclusion, a novel rhodamine based “offeon” fluorescent
chemosensor bearing a phenylthiazole moiety has been synthe-
sized for the selective and sensitive detection of Fe3þ. The sensor
showed a remarkable enhancement of the fluorescence intensity
and a clear color change from colorless to pink upon binding with
Fe3þ. The response of the sensor to Fe3þ was unaffected by the
presence of other common coexistent metal ions. Theoretical
studies supported that the Fe3þ binding to N atom of thiazole
moiety in S1 accompanied by the transferring of electrons of the
phenylthiazole drove the structural changes.
Acknowledgments
The project was supported by National Natural Science Foun-
dation of China (No. 20972124), the China Postdoctoral Science
Foundation (No. 20080441180), the Chinese National Science
Foundation for Talent Training (No. J0830417) and the Chinese
National Innovation Experiment Program for University Students
(No. 20101069702). Y.B. wants to express his thanks to Prof. Yuanhe
Huang (College of Chemistry, Beijing Normal University) for his
great help.
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Appendix. Supplementary data
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.dyepig.2011.09.014,
http://www.sciencedirect.com.
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