CHEF induced highly selective and sensitive turn-on fluorogenic and colorimetric sensor for Fe3+

Article abstract of DOI:10.1039/c3dt51974k

A new fluorescent probe was synthesized from rhodamine-6G and 6-(hydroxymethyl) picolinohydrazide for the sensing of Fe³⁺ in an aqueous environment. The structure of the sensor was confirmed through its single crystal X-ray study. It exhibits a high specificity and sensitivity towards Fe³⁺ over other interfering heavy and transition metal ions (HTM). The turn-on greenish-yellow fluorescence and a colorimetric change from colourless to pink were observed upon addition of Fe³⁺ which evokes almost 116- and 23-fold enhancement in absorbance and emission intensity respectively in an acetonitrile-water (1: 1, v/v, 25 °C) solution at a neutral pH (pH = 7.2). The Fe³⁺ promoted ring opening of the spirolactam framework to the open chain amide platform of the sensor is responsible for its visible colour change and turn-on fluorescence activity. It also exhibits an excellent performance in the dip stick method. Moreover the limit of detection of the probe is in the 10^-8 M range.

Full text of DOI:10.1039/c3dt51974k

Dalton
Transactions
View Article Online
View Journal
PAPER
CHEF induced highly selective and sensitive turn-on
fluorogenic and colorimetric sensor for Fe3+†
Cite this: DOI: 10.1039/c3dt51974k
Shyamaprosad Goswami,*a Sangita Das,a Krishnendu Aich,a Deblina Sarkar,b
Tapan Kumar Mondal,b Ching Kheng Quahc and Hoong-Kun Func,d
A new fluorescent probe was synthesized from rhodamine-6G and 6-(hydroxymethyl) picolinohydrazide
for the sensing of Fe3+ in an aqueous environment. The structure of the sensor was confirmed through
its single crystal X-ray study. It exhibits a high specificity and sensitivity towards Fe3+ over other interfering
heavy and transition metal ions (HTM). The turn-on greenish-yellow fluorescence and a colorimetric
change from colourless to pink were observed upon addition of Fe3+ which evokes almost 116- and
23-fold enhancement in absorbance and emission intensity respectively in an acetonitrile–water (1 : 1,
v/v, 25 °C) solution at a neutral pH (pH = 7.2). The Fe3+ promoted ring opening of the spirolactam frame-
work to the open chain amide platform of the sensor is responsible for its visible colour change and
turn-on fluorescence activity. It also exhibits an excellent performance in the “dip stick” method. More-
over the limit of detection of the probe is in the 10−8 M range.
Received 20th July 2013,
Accepted 12th August 2013
DOI: 10.1039/c3dt51974k
www.rsc.org/dalton
determination of iron is in great demand. It is necessary
to design simple, highly sensitive and selective chemosensors
Introduction
Iron, one of the most essential trace elements in biological for Fe3+ detection and establish a method for the determi-
systems, plays a major role in the function of haemoglobin nation of trace amounts of Fe3+ ions. There are many reports
and various enzymes in the human body and plays crucial of fluorescent receptors to detect heavy and transition metal
roles in cellular metabolism,1,2 and both its deficiency and (HTM) ions, due to the attributes of fluorescence detection,
excess can induce a variety of diseases,3 with iron storage and such as simplicity, high sensitivity, low cost and real time
balance being tightly regulated in an organism.4 Iron being monitoring.7
both useful and cytotoxic,5 its deficiency throughout the devel-
In this regard, a rhodamine based probe is highly signifi-
opmental phases may lead to permanent loss of motor skills.6 cant as it has high water solubility and gives a pink colour
Accretion of iron in the central nervous system has been along with turn-on fluorescence upon binding with metal ions
involved in a number of diseases such as Parkinson’s, Hunt- in aqueous medium.8 In continuation of our research work on
ington’s and Alzheimer’s disease, associated with an increased the development of ‘switching on’ fluorescence sensors9 for
quantity of iron.5a Keeping in view the roles played by iron in biologically important substrates, herein we report the design,
day-to-day life, the development of techniques for selective synthesis, and photophysical properties of a new rhodamine
6G-pyridine based probe (RHP). The experimental studies
revealed that the probe is highly selective and sensitive
aDepartment of Chemistry, Bengal Engg. and Science University, Shibpur,
towards Fe3+ ions in an aqueous environment exhibiting a
Howrah-711 103, India. E-mail: spgoswamical@yahoo.com; Fax: +91 33 2668 2916;
large CHEF (chelation enhanced fluorescence) effect along
with color change detectable by the naked eye.
Tel: +91 33 2668 2961-3
bDepartment of Chemistry, Jadavpur University, Kolkata-700032, India.
E-mail: tkmondal@chemistry.jdvu.ac.in
cX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia,
11800 USM, Penang, Malaysia
dDepartment of Pharmaceutical Chemistry, College of Pharmacy, King Saud
University, Riyadh 11451, Saudi Arabia. E-mail: hfun.c@ksu.edu.sa;
Fax: (+966) 146-76220; Tel: (+966) 146-77335
Experimental
General
†Electronic supplementary information (ESI) available: Job plot, association con-
Unless otherwise mentioned, materials were obtained from
commercial suppliers and were used without further purifi-
cation. Thin layer chromatography (TLC) was carried out using
Merck 60 F254 plates with a thickness of 0.25 mm. Melting
points were determined using a hot-plate melting point
stant determination, detection limit determination, 1H NMR, 13C NMR, ESI MS
spectroscopy, fluorescence titration spectra of the receptor with different metal
ions, UV-vis titration spectra of the receptor with different metal ions, X-ray data,
computational data. CCDC 935238. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt51974k
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.

View Article Online
Paper
Dalton Transactions
1
apparatus in an open mouth capillary and are uncorrected. H of the reaction, monitored by TLC, the solvent was evaporated
and 13C NMR spectra of RHP were recorded using JEOL by a rota-evaporator and water was added to it and extracted
1
400 MHz and 100 MHz instruments respectively and H NMR with CHCl3 (3 × 100 ml). The combined organic solvents were
spectra of compound C were recorded using a Brucker dried over anhydrous sodium sulphate and evaporated to get
300 MHz instrument. For NMR spectra, CDCl3 was used as a the crude product. The crude product was purified by column
solvent using TMS as an internal standard. Chemical shifts are chromatography using 3% methanol in chloroform yielding
1
1
expressed in δ units and H–1H and H–C coupling constants the receptor as a brown coloured solid (0.45 g, yield = 77%).
in Hz. IR spectra were recorded on a JASCO FT/IR-460 plus
spectrometer using KBr discs. Fluorescence spectra were
Mp = 260 °C–262 °C.
1H NMR (400 MHz, CDCl3): δ 1.30 (t, J = 8, 6H), 1.97 (s, 6H),
recorded using a PTI spectrophotometer and UV-vis titration 3.17 (q, J = 14.4, 4H), 4.64 (s, 2H), 6.32 (s, 2H), 6.56 (s, 2H),
experiments were performed using
spectrophotometer.
a
JASCO UV-V530 7.12 (q, J = 8, 1H), 7.34 (d, J = 7.6, 1H), 7.52 (m, 2H), 7.72 (t, J =
7.8, 1H), 7.90 (d, J = 7.6, 1H), 8.01 (m, 2H), 8.90 (s, 1H).
13C NMR (100 MHz, CDCl3): δ 14.82, 16.89, 38.58, 64.48,
General method of UV-vis and fluorescence titration
66.61, 96.75, 105.18, 118.08, 121.86, 123.67, 123.72, 124.28,
UV-vis method. For UV-vis titration we used the solution of 129.06, 129.28, 133.36, 137.91, 147.67, 148.18, 151.91, 152.31,
the host on the order of 2 × 10−5 M. The solution was prepared 158.50, 161.97, 165.49.
in CH3CN–H2O (1 : 1, v/v, 25 °C) HEPES buffer. The solution of
TOF MS (ESI, positive): calcd for C33H33N5O4 [M + H]+ (m/z):
the guest cations using their perchlorate salts on the order of 564.6626; found: 564.3770.
2 × 10−4 M was prepared in deionized water using HEPES
buffer at pH = 7.2. Different concentrations of the host and
increasing concentration of cations were prepared separately.
The spectra of these solutions were recorded by means of the
UV-vis method.
Results and discussion
The synthetic route of RHP is shown in Scheme 1. Compound
C was treated with rhodamine 6G to afford the desired
receptor.
Fluorescence method. For the fluorescence titration, the
solution of the receptor (2 μM) was prepared in CH3CN–
H2O (1 : 1, v/v, 25 °C) using HEPES buffer. The solutions of the
guest cations using their perchlorate salts in the order of 2 ×
10−5 M were prepared in deionised water. Here also, various
concentrations of the guest and increasing concentration of
metal ions were prepared and the spectra were recorded by
means of a fluorescence method.
The objective of preparation of such kinds of CHEF
induced rhodamine 6G based probes is to get a beautiful spec-
trum both in emission and in absorbance, which was due to
the change in the photophysical properties upon binding with
cations. Now the cation binding properties of RHP were
observed by employing aqueous solutions of different cations
as their perchlorate salts (Fig. 1). The receptor itself does not
show any colour in CH3CN–H2O (1 : 1, v/v, 25 °C, pH = 7.2),
which proves that the structure predominantly exists in its
expected spirolactam framework.
In order to perform a UV-vis absorption experiment, solu-
tions of different cations were added separately to the prepared
solution of RHP. Only Fe3+ was found to perturb the electronic
behaviour of RHP, which showed an enhancement of absorp-
tion, and no other metals of interest showed any kind of sig-
nificant effect on the absorption spectrum of RHP (20 µM, in
CH3CN–H2O, 1 : 1, v/v, pH = 7.2). As shown in Fig. 2, upon
addition of Fe3+ to the solution of RHP, a new peak arose at
Synthetic method for the preparation of the receptor. Inter-
mediate compounds A and B were prepared according to a lit-
erature procedure.10
Synthesis of compound C. To a stirred solution of com-
pound B (0.5 g, 2.99 mmol) in methanol (20 ml) excess hydra-
zine hydrate was added. The reaction mixture was refluxed
until the TLC showed complete consumption of the reactant
(about 1 h). After completion of the reaction, the solvent was
evaporated under reduced pressure and the resulting slurry
was poured into ice water. The white precipitate that appeared
was filtered and dried under vacuum to afford compound C as
a white crystalline solid (0.4 g, 2.39 mmol, yield = 80%). The
compound was pure enough to use for the next step without
further purification.
Mp = 172 °C.
1H NMR (300 MHz, d6 DMSO): δ 4.605 (t, J = 6.2 Hz, 4H),
5.433 (t, J = 5.81 Hz, 1H), 7.565 (d, J = 7.8 Hz, 1H), 7.836 (d, J =
7.5 Hz, 1H), 7.949 (t, J = 7.8 Hz, 1H), 9.922 (s, 1H).
TOF MS (ESI, positive): calcd for C7H10N3O2 [M + H]+ (m/z):
168.0769; found: 168.0767.
Synthesis of the receptor (RHP). A mixture of compound C
(0.5 g, 2.99 mmol) and rhodamine 6G (1.4 g, 2.92 mmol) was
taken in 30 ml of methanol and 2 ml of triethylamine was
added to it. The resulting solution was stirred for 12 hours
Scheme 1 Synthesis of the receptor (RHP). Reagents and conditions: (i) NaBH4,
CH3OH–CHCl3 (7 : 3, v/v), rt, 2 h. (ii) Hydrazine hydrate, CH3OH, reflux, 1 h.
under reflux under a nitrogen atmosphere. Upon completion (iii) CH3OH, Et3N, reflux, 12 h.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
17 times greater than that of Cu2+. This phenomenon indicates
that the sensor can be employed conveniently for Fe3+ detec-
tion by simple visual inspection. So the selectivity of RHP
towards Fe3+ is thus well proven. A comparative view of absor-
bance of RHP at 530 nm upon addition of different co-existing
metal ions (2 equivalents) is shown in Fig. 3. In order to quan-
tify the stoichiometry of the complex of RHP and Fe3+, a Job’s
plot analysis (method of continuous variation) was carried out
in which the change in absorbance at 530 nm was plotted
against molar fractions of RHP and Fe3+ under the conditions
of an invariant total concentration. The maximum point
appears at a mole fraction of 0.5, which corresponds to a 1 : 1
complex formation of RHP and Fe3+ (ESI†). The association
constant for the metal ion complexation process of the recep-
tor with Fe3+ was estimated to be 1.04 × 104 M from a Benesi–
Hildebrand plot12 using data obtained from the UV-vis titra-
tion (ESI†). The stoichiometry of such complexation fits nicely
with a 1 : 1 (host–guest) binding model.
Fig. 1 A photograph of RHP (20 μM), showing visible color changes in the
presence of Fe3+ and other stated cations (2 equivalents each).
The fluorogenic response of RHP was investigated by moni-
toring the fluorescence behaviour upon addition of several
metal ions such as Fe3+, Na+, K+, Mg2+, Mn2+, Fe2+, Ni2+, Al3+,
Co2+, Zn2+, Cu2+, Cd2+, In3+, Hg2+, Pb2+ and Cr3+ in CH3CN–
H2O (1 : 1, v/v, pH = 7.2) as their perchlorate salts. The free
receptor (2 µM) exhibits a very weak emission band at 550 nm
upon excitation at 505 nm. With the addition of Fe3+, there
arises a remarkable enhancement of fluorescence with an
emission band at 550 nm, which is accompanied by the
opening of the spirolactam ring of the receptor upon chelation
with Fe3+ (Scheme 2). Notably, the addition of other co-existing
metal ions, even in excess amount, caused insignificant
changes in the emission intensity of the receptor.
Thus the competitive free cations would have an insignifi-
cant influence on RHP and the sensor could be used poten-
tially for fluorogenic detection of Fe3+ without any
interference. The fluorescence intensity of RHP maintained a
good linear relationship with [Fe3+]/μM with a good R2 value of
0.998. On continued addition of Fe3+ (0–1.2 equiv.) in the RHP
solution, a large enhancement in intensity was observed at
550 nm (23-fold). So this sensor can be specifically used for
Fig. 2 Changes in absorption spectra of RHP (20 μM) upon gradual addition of
Fe3+ (0 to 1.5 equivalents). Inset: the linear response curve of the absorbance of
RHP at 530 nm depending on Fe3+ concentration.
530 nm and became intense with increasing concentration of
Fe3+. On addition of 2 equivalents of Fe3+ the absorbance
intensity of RHP at 530 nm increased by almost 116 times
(from 0.007899 to 0.918944 at 530 nm). During the addition of
Fe3+ the absorption intensity exhibits a beautiful linear curve
of the fitness relationship with Fe3+ concentration (0 to 19.6 µM)
with a good R2 value of 0.9994. The visual colour change (pink
red) was observed during the addition of Fe3+ due to complexa-
tion of RHP with Fe3+ which was accompanied by the opening
of the spirolactam ring of RHP. The detection limit of the
sensor for Fe3+ was determined from the absorption spectral
change, upon addition of Fe3+, to be 5.55 × 10−7 M, using the
equation DL = K × Sb1/S, where K = 3, Sb1 is the standard devi-
ation of the blank solution and S is the slope of the calibration
curve11 (ESI†).
The selectivity and interference are the two very important
parameters to evaluate the performance of a receptor. Now the
sensitivity and selectivity of RHP towards Fe3+ were examined
by employing different HTM (Cd2+, Co2+, Cr3+, Cu2+, Fe2+,
Hg2+, K+, Mg2+, Na+, Mn2+, Ni+2, Al3+, Pb2+, In3+ and Zn2+) ions
in the RHP solution. RHP shows an extraordinary selective
enhancement of absorbance towards Fe3+ in a CH3CN–H2O
(1 : 1) solution at 530 nm, whereas no noticeable change was
observed upon addition of other competing metal ions except
Cu2+. Upon addition of 2.0 equivalents of Cu2+, the absorbance
of the sensor at 530 nm was increased by 7-fold, whereas, it
increases by 116-fold in the presence of Fe3+, which is almost
Fig. 3 A comparative view of the absorbance of RHP at 530 nm upon addition
of different metal ions (2 equivalents).
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Scheme 2 Probable complexation mode of RHP with Fe3+
.
the detection of Fe3+ fluorometrically. The observed fluore-
scence spectral changes of the sensor with increasing amounts
of Fe3+ are shown in Fig. 4.
The selectivity of RHP for Fe3+ over other metal ions is
shown in Fig. 5. From the fluorescence titration experiment,
the detection limit of the sensors was also evaluated and it
was found to be 3.6 × 10−8 M. Among all other common metal
ions only Fe3+ resulted in a pronounced fluorescence enhance-
ment and the association constant (Ka) was determined by the
fluorescence titration method for the sensor (RHP) with Fe3+
and it was found to be 5.9 × 105 M−1 by the Benesi–Hildebrand
equation.
Fig. 5 A comparative view of emission intensities of RHP (2 μM) at 550 nm
upon addition of different metal ions (2 equivalents). Inset: a photograph
showing the fluorescence changes of RHP in the absence and presence of Fe3+
(2 equivalents) after irradiation with UV light.
The interference of other competing metal ions was investi-
gated by the competition experiment which was conducted
with 2.0 equivalents of Fe3+ ions in the presence of other metal
ions in the same concentration, which is demonstrated in the
following Fig. 6. It was observed that the detection of Fe3+ in
the presence of other metal ions was not hampered; that is,
the interference for the detection of Fe3+ was not hampered.
So RHP could be used as a selective and sensitive colorimetric
as well as a fluorometric sensor for Fe3+ ions.
There are a number of sensors which can detect iron only
in the solution phase, which would restrict their sensitivity
under some special circumstances, for example sensing occurs
only through hydration. So in order to investigate a practical
application of this sensor we performed an experiment called
the “dip-stick” method. It is a very simple but very important
Fig. 6 Changes in emission intensity of RHP (2 µM) at 550 nm upon addition
of
λex = 505 nm.
2 2 equivalents of various metal ions.
equivalents of Fe3+ along with
experiment because it gives instant qualitative information
without resorting to the instrumental analysis. In order to
perform this experiment we prepared TLC plates which were
further immersed into the solution of RHP (2 × 10−4 M) in
acetonitrile, and then evaporating the solvent to dryness. We
immersed the TLC plate in a Fe3+ (2 × 10−3 M) solution and
then exposed it to air to evaporate the solvent. The color of the
TLC plates changes from colorless to pink. This experiment
evokes a real time monitoring without using any instrumental
help. Just by naked-eye detection and the use of TLC plates we
can easily investigate a qualitative instant detection of Fe3+
(Fig. 7).
Reversibility of RHP
As is well known, reversibility is an important phenomenon to
obtain an excellent chemical sensor. Thus, the chemical rever-
sibility behavior of RHP was studied to examine the reusability
of the receptor. Now to demonstrate this fact the absorption
and emission titration experiments were conducted using the
Fig. 4 Changes in emission spectra of RHP (2 µM) upon gradual addition of
Fe3+. Inset: the linear response curve of emission intensity of RHP at 550 nm
depending on the concentration of Fe3+
.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Fig. 7 Color changes of RHP on test paper in the absence and presence of Fe3+
under ambient light.
Fe3+ complex of RHP (RHP–Fe3+) with Na2EDTA (ethylenedi-
aminetetraacetic acid disodium salt). From these experiments
it was revealed that the pink colour of RHP–Fe3+ disappeared
with increasing concentration of Na2EDTA and the emission
intensity of the RHP–Fe3+ complex also gradually decreased
(Fig. 8 and 9). It indicates the decomplexation of RHP–Fe3+ as
Na2EDTA strips away Fe3+ from the binding zone.
Fig. 10 Fluorescence response of RHP at 550 nm (20 μM, λex = 505 nm) as a
function of pH in CH3CN–H2O (1 : 1, v/v); the pH is adjusted by using aqueous
solutions of 1 M HCl or 1 M NaOH.
Effect of pH on the optical behavior of RHP
To observe that phenomenon, a stock solution of RHP (20 μM)
was prepared in acetonitrile–water (1 : 1, v/v) and the pH, 1–5
and 5–10.5, was maintained by the addition of a calculated
amount of 1 N HCl and 1 N NaOH solutions respectively, and
the emission spectra were monitored. The pH–emission
plot (Fig. 10) showed insignificant changes in emission
intensity of RHP in the pH range 7–10.5, which indicates that
the ring predominantly exists in its spirolactum framework in
this pH range. But in strong acidic medium (pH > 6) there
arose an enhancement in emission intensity, which suggests
the protonation induced ring opening of the spiro cyclic
framework along with a color change detectable by the naked
eye (pink-red). Thus RHP can be used as a sensor for Fe3+ in
near neutral pH (pH = 7.2).
The rhodamine–pyridine containing fluorescent probes are
known to be sensitive to a change in pH in the acidic medium.
Crystallographic study
Single crystals of the sensor (RHP) suitable for X-ray studies
were obtained by dissolving a powder of the pure compound
in CHCl3–CH3CN (1 : 9, v/v) and slow evaporation of the solu-
tion free from vibrations.
Fig. 8 Absorption titration spectra of RHP-Fe3+ (20 µM) upon increasing the
concentration of the Na2EDTA solution (0 to 10 equivalents).
In the compound RHP (Fig. 11), the hydroxyl group is dis-
ordered over two positions, leading to positional disorder of
the two methylene H atoms, with occupancies of 0.556(16) and
0.444(16). The xanthene ring system is almost planar
(maximum deviation = 0.0336(13) Å at atom O1) and forms
dihedral angles of 89.39(5)° and 83.68(6)° with the essentially
planar isoindoline ring system (maximum deviation
=
0.0587(13) Å at atom N3) and pyridine ring, respectively,
indicating that they are almost perpendicular to each other.
The dihedral angle between the isoindoline ring system and
pyridine ring is 71.44(7)°.
In the crystal, molecules are linked in an inversion dimer
by a pair of intermolecular C–H⋯O hydrogen bonds (ESI,
2
Table 2†), generating an R2 (16) ring motif (Fig. 12). The
Fig. 9 Fluorescence titration spectra of RHP-Fe3+ (20 μM) upon increasing the
concentration of Na2EDTA (0 to 10 equivalents) λex = 505 nm.
dimers are further connected by O–H⋯O and C–H⋯O hydro-
gen bonds into a three-dimensional network.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 13 Optimized structure of RHP by the DFT/B3LYP/6-31G(d) method.
Fig. 11 The structure of RHP, showing displacement ellipsoids drawn at the
30% probability level. Hydrogen atoms are drawn as circles with small radii.
Open bonds show the minor disorder component.
Fig. 14 Optimized structure of RHP-Fe3+ (H atoms are omitted for clarity) by
the DFT/B3LYP/LANL2DZ/6-31G(d) method.
HOMO − 1 → LUMO + 3. The transitions with high oscillator
strength ( f ) values found at 274 and 254 nm correspond to
HOMO → LUMO + 4 and HOMO − 2 → LUMO + 3 transitions
respectively.
Fig. 12 Part of the crystal packing of RHP showing the hydrogen-bonding
Conclusions
network.
In summary we report here the synthesis, characterization and
cation binding properties of a new rhodamine based probe
(RHP) which exhibits an enhanced response both in absorp-
tion and fluorescence intensities towards Fe3+ in aqueous
media at almost neutral pH. Water solubility, reversibility and
high sensitivity (both colorimetric and fluorogenic) make the
probe suitable for use as an excellent chemosensor for Fe3+.
Computational study (DFT)
The optimized structures of RHP and its octahedral Fe3+
complex obtained by the DFT/B3LYP method are shown in
Fig. 13 and 14 respectively. The calculated bond parameters of
RHP are well reproduced by the X-ray data. The TDDFT calcu-
lation on the optimized geometry of RHP has been performed
to understand the electronic transitions in the compound.
Some selected vertical electronic excitations of RHP are shown
in Table 3 (ESI†). Contour plots of some selected molecular
Acknowledgements
orbitals of RHP are shown in Fig. S12.† The transitions at 375 Authors thank the CSIR and DST, Govt. of India for financial
and 317 nm corresponding to HOMO → LUMO and HOMO → support. S.D., K.A. and D.S. acknowledge the CSIR for provid-
LUMO + 1 transitions are very weak in nature ( f ∼ 0.004). One ing them with fellowships. The authors extend their appreci-
moderately intense transition at 288 nm corresponds to ation to the Deanship of Scientific Research at King Saud
Dalton Trans.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
University for funding the work through the research group
project no. RGP VPP-207. The authors also thank Universiti
Sains Malaysia (USM) for the APEX DE2012 grant (no. 1002/
PFIZIK/910323).
D. K. Epstein and R. P. Allen, Neurology, 2000, 54,
1698.
7 D. Prabhakaran, M. Yuehong, H. Nanjo and H. Matsunaga,
Anal. Chem., 2007, 79, 4056.
8 (a) X. Chen, T. Pradhan, F. Wang, J. S. Kimand and J. Yoon,
Chem. Rev., 2012, 112, 1910; (b) H. N. Kim, M. H. Lee,
H. J. Kim, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2008, 37,
1465.
9 (a) S. Goswami, D. Sen, N. K. Das, H.-K. Fun and C. K.
Quah, Chem. Commun., 2011, 47, 9101; (b) S. Goswami,
K. Aich, S. Das, A. K. Das, A. Manna and S. Halder, Analyst,
2013, 138, 1903; (c) S. Goswami, K. Aich, A. K. Das,
A. Manna and S. Das, RSC Adv., 2013, 3, 2412;
(d) S. Goswami, K. Aich and D. Sen, Chem. Lett., 2012, 863.
10 S. Goswami, D. Sen and N. K. Das, Tetrahedron Lett., 2010,
51, 6707.
Notes and references
1 M. Ghaedi, A. Shokrollahi, R. Mehrnoosh, O. Hossaini and
M. Soylak, Cent. Eur. J. Chem., 2008, 6, 488.
2 S. Bae and J. Tae, Tetrahedron Lett., 2007, 48, 5389.
3 D. Touati, Arch. Biochem. Biophys., 2000, 373, 1.
4 (a) G. Cairo and A. Pietrangelo, Biochem. J., 2000, 352, 24;
(b) E. Beutler, V. Felitti, T. Gelbart and N. Ho, Drug Metab.
Dispos., 2001, 29, 495.
11 (a) M. Shortreed, R. Kopelman, M. Kuhn and B. Hoyland,
Anal. Chem., 1996, 68, 1414; (b) W. Lin, L. Yuan, Z. Cao,
Y. Feng and L. Long, Chem.–Eur. J., 2009, 15, 5096.
12 (a) H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc.,
1949, 71, 2703; (b) D. C. Carter and J. X. Ho, Adv. Protein
Chem., 1994, 45, 153; (c) A. Mallick and N. Chattopadhyay,
Photochem. Photobiol., 2005, 81, 419; (d) I. Ravikumar,
B. N. Ahamed and P. Ghosh, Tetrahedron, 2007, 63, 12940.
5 (a) J. R. Burdo and J. R. Connor, Biometals, 2003, 16, 63;
(b) J. R. Connor, S. L. Menzies, S. M. St. Martin and E. J.
Mufson, J. Neurosci. Res., 1990, 27, 595; (c) R. R. Crichton,
S. Wilmet, R. Legssyer and R. J. Ward, J. Inorg. Biochem.,
2002, 91, 9; (d) R. S. Eisenstein, Annu. Rev. Nutr., 2000, 20,
627.
6 (a) B. T. Felt and B. Lozoff, J. Nutr., 1996, 126, 693;
(b) C. J. Earley, J. R. Connor, J. L. Beard, E. A. Malecki,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.