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D. Sarkar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 397–401
The selectivity of HL for Ni2+ is studied in the presence of other
metal ions such as Ca2+, Mg2+, Mn2+, Fe3+, Al3+, Cr3+, Co2+, Zn2+
,
Cu2+, Cd2+ and Hg2+. The intensity of the absorption band at 600 nm
due to formation of HL–Ni2+ complex is not at all disturbed due to the
presence of other metal ions simultaneously in the solution (Fig. 5).
Thus HL shows an excellent binding affinity for Ni2+ even in the pres-
ence of other metal ions and HL can detect Ni2+ very rapidly in other
physiological samples where other metal ions like Ca2+, Mg2+, Mn2+
,
Fe3+, Al3+, Cr3+, Co2+, Zn2+, Cu2+, Cd2+ and Hg2+ usually coexist
with the analyte.
Job's plot reveals that the maximum absorption at 600 nm corre-
sponds to mole fraction at 0.5 (Fig. S4), while intensity at 403 nm is
minimized at mole fraction 0.5 (Fig. S5). Thus it clearly indicates that
the complex formation between Ni2+ and HL has the stoichiometric
ratio of 1:1. The mole ratio plot also reflects that the absorption intensity
at 600 nm increases till the mole ratio of the analyte to the receptor HL
reaches ~1.0, thus indicating 1:1 complex formation (Fig. S6).
From UV–Vis spectral change, limit of detection of the chemosensor
for Ni2+, is determined using the equation LOD = K × SD / S, where SD
is the standard deviation of the blank solution and S in the slope of the
calibration curve (Fig. S7). The limit of detection for Ni2+ is 0.14 μM
from UV–Vis spectral titration. This result clearly demonstrates that
the chemosensor is highly efficient in sensing Ni2+ even in very minute
level. The association constant of Ni2+ for the receptor HL is found to be
2.074 × 105 M−1 (Fig. S8).
Fig. 7. Contour plots of HOMO, LUMO and HOMO–LUMO energy gap in HL and HL–Zn2+
complex.
in case of HL–Ni2+ complex, appear at a bit downfield region compared
to that of the free receptor HL (Fig. S3).
3.4. Electronic spectra and DFT calculation
3.3. Cation sensing studies of HL
To get detailed insight into the interaction of HL with Ni2+, the DFT
studies of the proposed square planar nickel complex has been carried
out. In a square planar environment, three coordination sites are satis-
fied by the N, N, O donor of HL, and one site being occupied by a Cl−
(Fig. 6). The calculated N = N(azo) bond distance of the free receptor
corresponding to 1.267 Å has been significantly enhanced to 1.298 Å
in nickel complex, supporting coordination of azo-N to nickel center,
while the C–O bond distance in HL decreased from 1.349 Å to 1.289 Å
due to coordination. The proposed geometry is supported by mass
spectral analysis of HL–Ni2+ complex and NMR studies.
The calculated energy and composition of selected molecular
orbitals of Ni2+ complex are summarized in Table S1. Contour plots of
selected molecular orbitals of HL–Ni2+ complex are given in Fig. S9.
The HOMO–LUMO gap of HL is significantly decreased from 3.13 eV to
2.32 eV in Ni2+ complex and thus influenced the solution spectrum
which is in good agreement with the red shift in solution spectra
(Fig. 7).
3.3.1. UV–Vis study
The binding studies of receptor HL with different metal ions has
been carried out in DMSO–HEPES buffer (1:1, v/v, pH 7.4) solution.
Receptor HL (10 μM) shows an absorbance band at 403 nm. Addition
of NiCl2 (100 μM) solution causes decrease in intensity of the absorption
band at 403 nm and a new absorption band appears at 600 nm with an
isosbestic at around 500 nm (Fig. 1). This indicates the formation of a
complex between the receptor HL and Ni2+. Due to the complexation
with Ni2+, distinct color change occurs from orange yellow to blue
(Fig. 2). Furthermore the sensing ability of HL with Ni2+ at different
pH has also been studied. At lower pH, the receptor HL has no significant
response to Ni2+ in absorption spectroscopy, may be due to protonation
of the receptor HL. The absorbance at 600 nm is in maximum and almost
constant in the pH range of 7.0 to 9.0, above pH 9.0, the absorbance is
gradually decreased (Fig. 3). It indicates that the receptor may be suit-
able for biological applications at the physiological pH. UV–Vis spectrum
of HL is also studied in the presence of other metals i.e. Ca2+, Mg2+
,
The changes in electronic spectrum of HL upon complexation with
Ni2+ have been interpreted by TDDFT calculations. In free receptor HL,
the experimental bands at 403 nm and 282 nm correspond to
HOMO → LUMO and HOMO-5 → LUMO + 1 transitions respectively
(Table 1). In the complex, the low energy band has been observed at
Mn2+, Fe3+, Al3+, Cr3+, Co2+, Zn2+, Cu2+, Cd2+ and Hg2+ but no sig-
nificant changes are observed in either of the cases (Fig. 4). Actually, the
cavity of HL binds selectively with Ni2+ due to the proper size matching
of the analyte with that of the binding site.
Table 1
Vertical electronic transitions in HL calculated by TDDFT/B3LYP/CPCM method using DMSO as solvent.
Eexcitation (eV)
λexcitation (nm)
Osc. strength (f)
Key transitions
Character
λexpt. (nm)
2.7736
4.2158
447.0
294.1
0.2528
0.1762
(97%) HOMO → LUMO
(69%) HOMO-5 → LUMO + 1
π → π
π → π
403
282
⁎
Table 2
Vertical electronic transitions in HL–Ni2+ complex calculated by TDDFT/B3LYP/CPCM method using DMSO as solvent.
E
excitation (eV)
λ
excitation (nm)
Osc. strength (f)
Key transitions
Character
λ
expt. (nm)
1.8642
1.9860
2.2737
3.0231
665.1
624.3
545.3
410.1
0.0289
0.1786
0.0206
0.2022
(83%) HOMO-4 → LUMO + 1
(79%) HOMO → LUMO
(69%) HOMO-5 → LUMO + 1
(87%) HOMO-3 → LUMO
π(L) → dπ(Ni), LMCT
π(L) → π (L), ILCT
644
600
562
393
⁎
dπ(Ni) → dπ(Ni), d-d
⁎
π(L) → π (L), ILCT