M. Budri, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry390(2020)112298
explore the efficiency of L as potential fluorescent sensor for in-
tracellular detection of Zn2+, bio imaging experiment was performed
[46,64]. Initially, the HeLa cells were treated with 10 μM of L and in-
cubated for 30 min at 25 0C and then corresponding images were re-
corded using fluorescence microscope. The probe L penetrates into the
cells and exhibits a weak fluorescence in live HeLa cells. Later on, HeLa
cells loaded with L were treated with 10 μM Zn2+ and incubated for
another 1 hour to promote the formation of L-Zn2+ complex and re-
sultant images were taken (Fig. 15). Fluorescence imaging experiment
clearly indicate that sensor alone exhibits weak fluorescence but in
presence of Zn2+ displays a distinct strong green turn on florescence
due to the formation of stable L-Zn2+ in live HeLa cells. These results
are in good agreement with those observed earlier in solution studies
and strongly stand as a witness that L could act as a sensor for Zn2+
ions.
3.8. Computational studies
In order to study further on the photophysical properties of L and to
assign experimentally observed spectral changes during host guest in-
teraction, we have undertaken theoretical study. The DFT optimized
structures of L and L-Zn2+ were obtained by employing Density
Functional Theory (DFT/B3LYP) in acetonitrile (Fig. 11). For L, HOMO
(Highest Occupied Molecular Orbital) is mainly populated on the
naphthaldehyde benzene ring along with the coordinating sites
whereas, in LUMO (Lowest Unoccupied Molecular Orbital), electron
density redistributes over benzene rings along with binding atoms. On
the other side, upon introducing Zn2+ which forms L-Zn2+ complex, an
electron density resides over benzene rings along with coordinating
sites and in LUMO most of it is populated over naphthalene rings in
addition to interacting sites.
4. Conclusion
In theoretically calculated absorption spectrum, free L exhibit a
characteristic band at 318 nm (λexp 327 nm) dominated by H-2 L (40.04
% contribution) and H L + 1 (32.81 % contribution), band at 339 nm
(λexp 364 nm) due to H-1 L (90.64 % contribution) and band at 378 nm
(λexp 380 nm) due to H L (95.82 % contribution). Introducing zinc atom
in the framework of L, zinc complex displays a band at 335 nm (λexp
339 nm) dominated by H L + 1
We have developed a novel reversible ‘off-on’ responsive optical
sensor (L) for zinc ion in 20 % aqueous acetonitrile. Sensor (L) responds
selectively for zinc ion over other tested metal ions by enhancing 17
fold high fluorescence intensity. The sensing mechanism of L for zinc
ions was explored using theoretical methods. Jobs experimental results
and ESI-MS studies strongly supports for 1:1 stoichiometry between
host-guest interactions. L and its zinc complex are less toxic and L can
be utilized in live cell imaging of zinc ion in HeLa cells. Finally, we
conclude that, L could act as a promising sensor for zinc ion and con-
tribute to the field of molecular sensor.
(61.23 % contribution) and H-2 L (30.54 % contribution), band at
365 nm (λexp 399 nm) due to H-1 L (96.77 % contribution) and band at
432 nm (λexp 423 nm) due to H L (97.97 % contribution) (Table 2 and
3). Experimentally observed results were well supported by HOMO-
LUMO energy gap (Fig. 12) [57,58]. Furthermore, we have also com-
pared the stabilities of L and L-Zn2+ complex by calculating chemical
Authorship contributions
E
− E
LUMO
hardness (η =
HOMO ) [59,60]. The higher chemical hardness
2
value of L (1.93 eV) compared to L-Zn2+ complex (1.80 eV) indicate its
higher stability.
All persons who meet authorship criteria are listed as authors, and
all authors certify that they have participated sufficiently in the work to
take public responsibility for the content, including participation in the
concept, design, analysis, writing, or revision of the manuscript.
Furthermore, each author certifies that this material or similar material
has not been and will not be submitted to or published in any other
publication before its appearance in the Hong Kong Journal of
Occupational Therapy.
The NBO (Natural Bond orbital) calculations were performed using
NBO 3.1 which is incorporated in Gaussian-09 software, by employing
B3LYP/6-311 G(d) method in order to study the various interactions
between vacant orbitals of one subsystem to the filled orbitals of an-
other. The stabilization energy E(2) for each donor (i) and acceptor (j)
associated with the delocalization i → j is determined by using fol-
lowing equation.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgements
(F )2
i,j
E (2) = ΔEij = q
i (Ej − Ei)
Where, Ei and Ej are orbital energies (diagonal elements), qi is the
donor orbital occupancy and F (i,j) is the off-diagonal NBO Fock matrix
The authors thank USIC, Karnatak University, Dharwad for elec-
tronic spectral analyses and Indian Institute of Science, Bangalore for
recording NMR spectra. One of the authors (Mahantesh B. Budri) is
grateful to UGC for awarding a RFSMS fellowship.
The higher value of E(2) indicates stronger interaction between
electron donors and acceptors. For the ligand (Table 4) the stronger
interactions n1C49→π*(C44-C46), n1C49→π*(C52-C54) and n1N39→
π*(C37-O38) have E(2) values of 67.29, 59.53 and 53.89 kcal/mol re-
spectively. Similarly, in case of complex (Table 5) n1C42→π*(N39-
C40), n3O59→π*(C37-N38) and n1C47→π*(C50-C52) have interaction
energies 119.73, 70.15 and 63.14 kcal/mol respectively. This indicates
the occurrence of charge transfer in the molecule [62].
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
3.9. Cytotoxicity and cell imaging study
References
To evaluate cytotoxic potential of probe on HeLa cells, MTT assay
was performed [63]. The experimental results illustrated that neither L
nor its zinc complex shows severe toxic effect on HeLa cells even at
concentrations as high as 200 μM. At a higher concentration of L
(200 μM) the cell viability was over 65.84 % and at lower concentration
(12.5 μM) it was 97.48 % control (Fig. S16).
The best sensing ability of L for Zn2+ and lower cytotoxic effect of L
encouraged us to go for intracellular detection Zn2+ in HeLa cells. To
11