Determination of the formation constant for the inclusion complex between Lanthanide ions and Dansyl chloride derivative by fluorescence spectroscopy: Theoretical and experimental investigation

Article abstract of DOI:10.1016/j.saa.2009.06.016

In this paper, a sensitive, easy, efficient, and suitable method for the calculation of K_f values of complexation between one derivative of Dansyl chloride [5-(dimethylamino) naphthalene-1-sulfonyl 4-phenylsemicarbazide] (DMNP) and Lanthanide(I

Full text of DOI:10.1016/j.saa.2009.06.016

Spectrochimica Acta Part A 74 (2009) 253–258
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Determination of the formation constant for the inclusion complex between
Lanthanide ions and Dansyl chloride derivative by fluorescence spectroscopy:
Theoretical and experimental investigation
Siavash Riahia,b,∗, Mohammad Reza Ganjalib,c, Maryam Haririb,
Shaghayegh Abdolahzadehd, Parviz Norouzib
a Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, P.O. Box 14399-57131, Tehran, Iran
b Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, P.O. Box 14155-6455, Tehran, Iran
c Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran
d Faculty of Chemistry, Mazandaran University, Babolsar, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a sensitive, easy, efficient, and suitable method for the calculation of Kf values of
complexation between one derivative of Dansyl chloride [5-(dimethylamino) naphthalene-1-sulfonyl 4-
phenylsemicarbazide] (DMNP) and Lanthanide(III) (Ln) ions is proposed, using both spectrofluorometric
and spectrophotometric methods. Determination of Kf showed that DMNP was mostly selective towards
the erbium (III) ion. The validity of the method was also confirmed calculating the Stern–Volmer fluo-
rescence quenching constants (Ksv) that resulted in the same consequence, obtained by calculating the
Kf of complexation values. In addition, the UV–vis spectroscopy was applied for the determination of Kf
only for the Ln ions that had interactions with DMNP. Finally, the DFT studies were done on Er3+ and the
DMNP complex for distinguishing the active sites and estimating the pair wise interaction energy. It can
be concluded that this derivative of Dansyl chloride with inherent high fluorescence intensity is a suitable
reagent for the selective determination of the Er3+ ion which can be used in constructing selective Er3+
Received 4 December 2008
Received in revised form 23 May 2009
Accepted 7 June 2009
Keywords:
Dansyl
Lanthanides
Formation constant
Chemometrics
Fluorescence quenching
Stern–Volmer
sensors.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
electronic transitions through an intersystem energy transfer pro-
cess [12–14]. The ligand is able to enhance the Ln fluorescence
Recently, the properties of Lanthanide organic complexes have
been extensively studied [1–3]. These complexes play an impor-
tant role in different areas of chemistry, e.g. as catalysts in organic
syntheses, as shift reagents in nuclear magnetic resonance spec-
troscopy and for light converters [4]. Luminescent Lanthanide
complexes [5–7] have attracted vast areas of research from both
material and biological sciences [8,9] mainly due to their very nar-
row emission bands and Large Stokes shifts, etc [10]. The probes
based on erbium ion are of special interest because erbium organic
complexes have attracted much attention for their potential appli-
cations in polymeric and organic–inorganic derived systems [11].
The absorption and emission of Ln ions originates from f–f tran-
sitions which are forbidden by spectral selection rules. In practice
this means that both absorption and emission are very weak. There-
fore, using appropriate ligands can lead to enhancement of the f–f
intensity for three reasons: (1) it increases the quantum yield of
emission, (2) it can absorb light efficiently and transfer absorbed
energy onto the complexed Ln ion and (3) the ligand provides a
means to add a reactive functional group to link the Ln complex to
a target molecule. Complexes of Ln ions with organic ligands have
also been doped in polymers for optical amplification [15,16].
The biological properties of the Ln ions, primarily based on their
similarity to calcium, have been the basis for research into potential
therapeutic applications of Lns since the early part of the twentieth
century [17–19].
In the last few years, many various derivatives of Dansyl chloride
(5-dimethylamino-1-naphthalenesulfonyl chloride) as a complex-
ing agent have been extensively used in different studies [20–24].
Dansyl and its derivatives can be applied as synthetic receptors to
selectively bind a wide variety of guest molecules/ions forming the
host–guest complexes [25,26].
Dansyl chloride is a yellow crystalline powder that reacts
violently with water. Materials hydrolyze in contact with mois-
ture/water releasing toxic and corrosive fumes of hydrogen chloride
and aqueous hydrochloric acid. Hydrochloric acid solutions react
with most metals forming flammable hydrogen gas. This complex-
∗
Corresponding author at: Institute of Petroleum Engineering, Faculty of Engi-
neering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran.
Tel.: +98 21 61114714; fax: +98 21 88632976.
E-mail address: riahisv@khayam.ut.ac.ir (S. Riahi).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.06.016

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S. Riahi et al. / Spectrochimica Acta Part A 74 (2009) 253–258
(dimethylamino) naphthalene-1-sulfonyl-4-phenylsemicarbazide
is as follows: the N-phenylhydrazinecarboxamide (2 mmol, 0.302 g)
was solved in acetone and three drops of triethylamine was
added. After that 5-(dimethylamino) naphthalene-1-sulfonyl
chloride (2 mmol, 0.468 g) was added to the solution of N-
phenylhydrazinecarboxamide at room temperature. Then, the solid
product was crystallized in ethanol.
In order to confirm the (DMNP) structure, the relative NMR spec-
trum is attached. 1H NMR (90 MHz, CDCl3): ꢀH 3.02 (6H, S, NMe2),
7.00 (1H, t, J = 7.5 Hz, CH), 7.23 (2H, dd, J = 7.5, 7.8 Hz, 2CH), 7.35 (2H,
d, J = 7.6 Hz, 2CH), 7.40 (1H, d, J = 6.4 Hz, CH), 7.56 (1H, dd, J = 6.4,
8.4 Hz, CH), 7.80 (1H, dd, J = 6.6, 8.5 Hz, CH), 8.12 (1H, d, J = 8.5 Hz,
CH), 8.22 (1H, d, J = 6.6 Hz, CH), 8.33–8.40 (3H, br, 3NH). 13C NMR
(400 MHz, CDCl3): ꢀC 39.84 (NMe2), 116.21, 118.33, 123.60, 124.72
(4CH), 125.91 (C), 126.05, 127.77, 128.70, 131.63, 133.14 (5CH),
134.41, 139.95, 154.39 (3C), 152.55 (C O).
Fig. 1. Structure of 5-(dimethylamino) naphthalene-1-sulfonyl-4-phenylsemi-
carbazide.
2.2. Sample treatment
ing agent is incompatible with amines, strong oxidizing agents,
and strong bases. As its hazardous decomposition products we
can refer to hydrogen chloride, nitrogen oxides, carbon monoxide,
oxides of sulfur, carbon dioxide and chlorine. Dansyl chloride can
also be dangerous to respiratory system, eyes, skin, and mucous
membranes. Dansyl chloride derivatives are fluorometric detection
reagents emitted by UV light, and can be extensively used as a fluo-
rescent label in immunofluorescence methods as well as in yielding
fluorescent N-terminal amino acids and peptide derivatives. So, in
order to decrease the risk of generating contaminant and hazardous
agents, we used a non-polluting derivative of Dansyl (DMNP) (Fig. 1)
that had a good fluorescence property, to investigate the interac-
tions between this ligand and all Ln ions. This was done by studying
the fluorescence quenchings and UV–vis red shifts caused by titring
the ligand with the Ln ions.
For the purpose of verifying these complexations, it is essential
that we find each ligand–Ln’s formation constant in the acetonitrile
media and compare them with one another to see with which Ln
ion, DMNP interacts most selectively.
Although DMNP was demonstrated to have interactions with
five Lanthanide ions [Ln = La (III), Gd (III), Tb (III), Dy (III) and Er
(III)], gathering information released by calculating each Ln–ligand
formation constant, showed that this complexing agent interacts
selectively with Er (III) ion. This result was confirmed assembling
the UV–vis spectra of titring DMNP with each of the five men-
tioned Ln ions, and also by calculating the fluorescence quenching
constants (KStern–Volmer) values.
For the fluorescence and UV–vis studies, solutions of
DMNP were: 5 × 10−6 mol L−1 and 5 × 10−5 mol L−1 respec-
tively. The Ln ions standard solutions (1.0 × 10−4 mol L−1) and
(1.0 × 10−3 mol L−1
) were also prepared for fluorescence and
UV–vis verifications in acetonitrile, respectively.
2.3. Apparatus and software
The fluorescence study was performed using a Perkin-Elmer
LS50 spectrofluorimeter. The software used to record fluorescence
spectrum was FL-winlab. Experiments were carried out at ambi-
ent temperature (25 ◦C) with magnetic stirring in the fluorimeter
cell. The excitation wavelength was 325 nm in all cases with an
excitation and emission band pass (slit) of 8 nm and scan rate of
1500 nm/min. The solutions were placed in a 1 cm path-length
quartz cell for the fluorescence measurements. No explicit correc-
tion of our spectra for the instrument response was performed
because in our experiments only relative changes in the fluores-
cence intensity of the Ln ions as a function of certain experimental
conditions were required.
The UV–vis study was carried out with a Perkin Elmer Lambda
800 spectrophotometer (slit width of 1 nm and scan rate of
400 nm/min) using 1.00 cm quartz cell. Spectra were acquired over
the wavelength range of 200–800 nm against a blank (the acetoni-
trile solution).
All the spectra are given to DATAN 3.1, which is a program used
in this study to determine the Kf of complexation.
In order to calculate the formation constants of the complexa-
tion, the fluorescence spectra of the titration of DMNP with all Ln
ions, and also the UV–vis spectra of the titration of this ligand with
the five Ln ions DMNP interacted with, were applied.
3. Results and discussions
In recent years, the DFT method was applied in different
branches of chemistry [27–36].
3.1. Intrinsic Dansyl fluorescence
In the present paper, we used the recently introduced approx-
imate DFT method, DFTB (density functional tight-binding),
extended by the empirical London dispersion energy term, which
is accurate and reliable for computational studies [37].
As Dansyl derivatives have widespread applications as fluores-
cent ligands, in this research it was decided to investigate the
interaction of DMNP with all Ln ions, and also to determine the
formation constant of the complex (Kf) being formed using fluores-
cence spectroscopy.
2. Experimental
The type of interaction between DMNP and Lanthanide ions
is charge–dipole. Due to the existence of three oxygen and three
nitrogen atoms in the structure of DMNP as donor atoms (hard and
intermediate properties) and relatively intermediate properties of
Lanthanide ions, there exists a charge–dipole interaction. Due to
the semi-cavity of DMNP and size of Lanthanide ions, a range of
formation constant will be obtained.
2.1. Reagents
All the chemicals used were of analytical reagent grade and were
used without further purification. Acetonitrile and nitric acid were
obtained from Merck.
Ln (III) nitrates were prepared by dissolving Ln oxides in a
nitric acid aqueous solution. The procedure for synthesizing of 5-
The intrinsic fluorescence of DMNP was obtained at 435 nm
when excited at 325 nm. A quantitative analysis of the potential

S. Riahi et al. / Spectrochimica Acta Part A 74 (2009) 253–258
255
interaction between Ln ions and DMNP was performed on flu-
orometric titration. A 3 mL solution containing 5 × 10−6 mol L−1
of DMNP was titrated by successive additions of Ln solutions of
1 × 10−4 mol L−1 (to give a final concentration of 10−5 mol L−1), and
the fluorescence intensity was measured.
The elements of the Ln series display different properties (size,
charge densities, hydration energy) in comparison with the other
elements of the periodic table. The effective ionic radii of the triva-
lent Ln cations typically decrease in the order of atomic numbers.
However, no direct correlation between the ionic radii and the for-
mation constant is observed.
Fig. 2a–e shows the spectra for the titration of DMNP by five
Lanthanide ions (Ln = Er3+, Dy3+, Gd3+, Tb3+, La3+), respectively.
The DMNP instinct fluorescence intensity is about 900 when
excited at 325 nm, and the emission wavelength was observed
at 435 nm. The fluorescence intensity was quenched by titrating
DMNP with the five Ln ions. As can be seen in these plots, the high-
3.2. Lanthanides fluorescence
The Lanthanides are unique fluorescent metals. Their absorp-
tion coefficients are low, and their emission ratios are slow. As an
outcome, they are not usually directly excited. In fact, they can be
excited through chelated organic or bio-organic molecules [38,39].
Fig. 2. The fluorescence spectra of the titration of 5 × 10−6 mol L−1 of DMNP by 10−4 mol L−1 solutions of Ln ions at 25 ◦C excited at 325 nm. (a) Er3, (b) Dy3+, (c) Gd3+, (d) Tb3+
,
(e) La3+
.

256
S. Riahi et al. / Spectrochimica Acta Part A 74 (2009) 253–258
ratio of 1 to 1. Therefore, the formation constant values (Kf) of the
resulting 1:1 complexes could be evaluated in all cases.
The reasons this technique was used are due to its; simplicity,
ease of accessibility, high fluorescence property of DMNP, distinct
observation of fluorescence quenching in the case of titring DMNP
with the Ln ions it had interaction with, and presence of aromatic
rings in the host molecule structure.
3.3. Calculations of Kf of complexation
3.3.1. Fluorescence spectra
Based on Fig. 2, DMNP, has interactions with five Lanthanide
ions [Ln = La (III), Gd (III), Tb (III), Dy (III) and Er (III)]. In order
to determine which Ln ion this ligand interacts the best with, we
have to calculate the formation constant of the complexations. The
obtained Kf values calculated by fluorescence spectra are listed in
the first row of Table 1. The highest log Kf value was obtained for
Er3+ (7.53) in contrast to that of Sm3+ which is the least (3.84) and
observed order in this table acquired for other Ln ions.
Fig. 3. Maximum fluorescence intensity versus concentration of Er3+
.
3.3.2. UV–vis spectra
The complex formation is further evidenced by UV–vis spectra.
The second row of Table 1 shows the obtained Kf values for the
first five Ln ions complex with DMNP, calculated by UV–vis spectra.
These Ln ions are the cations DMNP interacted with.
As seen, the values calculated with the UV–vis spectra for the
Lns–DMNP complexation formation constant are in satisfactory
agreement with those obtained with the fluorescence spectra.
According to Fig. 2 and Table 1, DMNP has the most interaction and
selectivity to Er3+. This result is further approved by calculating the
Stern–Volmer quenching constants.
3.4. The Stern–Volmer quenching constant
Fig. 4. [Er3+]/[L] mole ratio plot at ꢁem = 435 nm, M/L = 1/1.
The quenching can be mathematically expressed by the
Stern–Volmer Eq. (1), which allows for calculating quenching con-
stants [42].
est amount of fluorescence quenching is concluded when titring
DMNP with Er3+
.
The fluorescence quenching refers to any process decreas-
ing the fluorescence intensity of a given substance. There are
mainly two types of quenching. The collisional quenching, which
results from the collision between fluorophores and a quencher
[40], and the static quenching, that is due to the formation of a
ground-state complex between fluorophores and a quencher [41].
In this case, the formation of complex between the fluorophore
(DMNP) and quenchers (Ln ions) suggests the existence of static
quenching.
F0
= 1 + kqꢂ0[Q] = 1 + Ksv[Q]
(1)
F
where F0 and F are the fluorescence intensities in the absence and
presence of the quencher, kq is the bimolecular quenching constant,
ꢂ0 is the lifetime of the fluorescence in the absence of the quencher
[Q]istheconcentrationofthequencher, andKsv istheStern–Volmer
quenching constant. In the presence of a quencher (Lns), the fluo-
rescence intensity is reduced from F0 to F. The ratio (F0/F) is directly
proportional to the quencher concentration [Q].
The maximum intensity of fluorescence in the quenching region
of DMNP versus concentration of Er3+ is shown in Fig. 3. There is an
exponential decrease in the fluorescence of DMNP with increasing
concentration of the Lanthanides it had interacted with [Ln = La (III),
Gd (III), Tb (III), Dy (III) and Er (III)] in the quenching region. As this
figure shows, the intensity decreases as the concentration of Er3+
Evidently:
Ksv = kqꢂ0
F0
(2)
= 1 + Ksv[Q]
(3)
F
increases up to 0.1 ␮mol L−1
.
According to Eq. (3), a plot of F0/F versus [Q] shows a linear graph
In this study, the ligand to Ln mole ratio was calculated
by fluorescence method. Fig. 4 displays a fluorescence intensity
of 5 × 10−6 mol L−1 DMNP which has been titrated by erbium
(1 × 10−4 mol L−1) versus Er3+/DMNP mole ratio. The resulted data
showed this ratio to be 1:1, which provide evidence that each DMNP
molecule can be saturated with the studied Ln ions in the mole
with an intercept of 1 and a slope of Ksv [43]. A typical plot of F0/F
versusEr3+ concentrationisshowninFig. 5. Theresultingregression
equations for all Ln ions are summarized in Table 2.
As known, Kf is a parameter used in this work to evidence the
quantity of interactions between DMNP and Ln ions, besides Ksv
values show the amount of fluorescence quenching caused by the
Table 1
The obtained Kf values calculated by fluorescence and UV–vis spectra.
Lanthanide Ion (III)
Er
Dy
Gd
Tb
La
Nd
Ce
Pr
Ho
Yb
Eu
Lu
Tm
Sm
Log Kf (FL)
Log Kf (UV)
7.53
7.2
6.02
5.91
5.79
5.53
5.47
5.4
5.37
5.23
4.33
–
4.27
–
4.21
–
4.15
–
4.03
–
4.01
–
3.98
–
3.92
–
3.84
–

S. Riahi et al. / Spectrochimica Acta Part A 74 (2009) 253–258
257
Fig. 7. Plot of Kf values versus interaction energy by DFT method.
ble of reliably predicting ligand selectivity in a variety of cations
have proven to be valuable tools for the advancement of practical
works [28–32].
Fig. 5. The Stern–Volmer plot of the titrations of DMNP with different concentra-
tions (␮ mol L−1) of Er3+
.
Quantum methods for binding of Lanthanide systems have
grown recently [44–46]. But Lanthanum metals are heavy atoms
and application of calculation method for these ions is expensive
and time consuming, so these issues develop no more [46–48].
In this work we applied SBKJC/6-31G* that is high level of theory
and results are very accurate, for investigation of binding energy
with supercomputer. In addition, these calculation methods help
us for tendency of different lanthanums ion to different ligands and
understanding of interaction between them.
In order to have a clear picture about the selectivity of ligand for
various metal ions, in this work, we investigated its binding to Ln
ions by using the Gamess software with SBKJC/6-31G* calculations
[49]. The binding energy (ꢃE) was calculated using Eq. (4) [33,34]:
Table 2
The Stern–Volmer equations for different Lns.
Ln ions
Stern–Volmer equation
F0/F = 1 + Ksv[Q]; Y = F0/F; X = [Q]
R2
Related equation
Er3+
0.9937
0.9914
0.9929
0.9958
0.9903
0.9919
0.9934
0.9973
0.9935
0.9915
0.9943
0.9989
0.9956
0.9906
y = 5,000,000x + 0.6405
y = 3,000,000x + 0.7209
y = 2,000,000x + 1.1915
y = 1,600,000x + 1.1314
y = 1,000,000x + 1.3678
y = 73,881x + 1.0356
y = 43,399x + 1.0144
y = 40,886x + 1.0620
y = 35,731x + 1.0768
y = 29,676x + 1.0743
y = 29,171x + 1.0534
y = 20,738x + 0.9898
y = 19,831x + 1.0485
y = 8146.6x + 0.9968
Dy3+
Gd3+
Tb3+
La3+
Nd3+
Ce3+
Pr3+
Ho3+
Yb3+
Eu3+
Lu3+
Tm3+
Sm3+
ꢃE = ꢃEcomplex − (ꢃEligand − ꢃEcation
)
(4)
where ꢃEcomplex, ꢃEligand and ꢃEcation are the total energies of the
complex, un-complexed ligand and metal ion, respectively. Fig. 7
shows the interaction energy versus Kf for different Ln ions. Inspec-
tion of Fig. 1 reveals that the cation binding energy with ligand
shows a pronounced dependence on the nature of metal ions used.
Thus, based on the above calculation results, DMNP could pos-
sibly be used as a suitable ligand to interact with Er3+. Fig. 8 shows
theoptimizedstructureofDMNPwiththeminimumlevelofenergy.
The heteroatoms in the DMNP structure which are closer to Er3+ ion
are those that had most interaction with this ligand.
Fig. 6. Colleration plot of Kf versus Ksv
.
formation of complex between DMNP and Ln ions. More quantity
of interactions leads to more fluorescence quenching, so further
obtained amount of Kf means further Ksv value. As it is apparent in
Tables 1 and 2, the Ksv values consequence corresponds exactly to
that of the Kf values of the Ln–DMNP complexation. Investigation
of colleration between Kf and Ksv demonstrated that the results are
reliable and Fig. 6 shows the Kf versus Ksv with R2 = 0.955.
4. Computational studies
Many experimental and theoretical investigations have been
carried out for further comprehension of the fundamental inter-
action between metal ions and neutral molecules, also their
relationship to molecular recognition. Computational models capa-
Fig. 8. Optimal conformation of the DMNP with Er (III) (without hydrogen atoms).

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S. Riahi et al. / Spectrochimica Acta Part A 74 (2009) 253–258
5. Conclusion
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