Characterizations UV-vis spectral studies
The absorption of 8-HQC-2TCH towards different metal cations was investigated by adding of Al3+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+ in DMSO as their metal salts. To being with, the 8-HQC-2TCH was characterized by UV-Visible absorption Spectroscopy and the observed results are shown in (Fig. 4A). The UV-visible spectrum displays 8-HQC-2TCH absorption band 300 nm could attribute to π- π* transitions. The Mg2+ ion was significant to be a new absorption band at 430 nm. Quantitatively shifted in direction to check the sensitivity of the 8-HQC-2TCH towards Mg2+ ion, the sensitivity investigates were also performed by the addition of several concentrations of Mg2+ ions. Absorption bands at 430 nm are slowly increased with may be due to the formation of the complex (Fig. 4B).
The absorption spectrum of 8-HQC-2TCH was recorded in the range of 0-60 mM of Mg2+and observed the detection limit 3.6 x10-7 M for Mg2+ ion by using the absorbance titration data. The competitive studies of HQC-2TCH for Mg2+ in the presence of other metal ions revealed that Al3+, Cd2+, Hg2+ Cr3+, Fe3+, Mn2+, Co2+, Ni2+ and Cu2+ had negligible interference. But no turn-on fluorescence was observed for Mg2+ in the presence of Transition metal ions, showed a strong influence on the Mg2+ selectivity that could be due to the stronger coordination with HQC-2TCH (see Figure. S5-6).
Fluorescence analysis of 8-HQC-PTH detection of Mg2+
The fluorescence spectra of compound 8-HQC-2TCH with its metal ions showed (Fig.7A). The emission band obtained from 8-HQC-2TCH was quite significant in the presence of Mg2+ ion. 8-HQC-2TCH showed fluorescence intensity at 480 nm and the new band appeared at 600 nm while upon addition of Mg2+ ion, confirm that the fluorescence intensity displays Mg2+ ions induced fluorescence changes, while other metal ions did not fluorescence changes, which clearly indicate, that the 8-HQC-2TCH has good selectivity towards Mg2+ ion over the additional competing cations.
Fluorescence spectroscopic titrations were also carried out under similar conditions to obtain a better insight into the response of 8-HQC-2PTCH towards Mg2+ ions. As shown in (Fig. 7B),when excited at 340 nm, the fluorescence band at 480 nm decreased and a band at 600 nm increased which provides ratiometric fluorescence spectra, and isoemittive point at 550 nm, when the gradually increasing the concentration of Mg2+ ion from 0 to 70 mM. Moreover, the quenching efficiency was analysed by following Stern-Volmer Eqn 2:
F0/F = Ksv [Q] + 1……… (eqn. 2)
Where F0 and F are the intensities of 8-HQC-2TCH before and also after the addition of Mg2+, and [Q] is the molar concentration of the Mg2+ and Ksv is the quenching constant. The detection limit to be found 4.9 x 10-8 M for 8- HQC-2TCH. The binding constant 4.3 x10-3 M-1 accrues for the completed was (Fig. 8A-8B).
Competitive of 8-HQC-PTH
The fluorescence response of the sensing system towards Mg2+ ion in the presence of other metal ions was studied and the data were shown in (Fig. 9). The orang bars represent the fluorescence intensity of 8-HQC-2TCH with metal ions and the Cyan bar represents the fluorescence intensity of 8-HQC-2TCH + Mg2+ with other competing cations. The presence of the selected metal ion does not interfere with Mg2+ complex with the 8-HQC-2TCH, indicates that negligible interference effects on Mg2+ sensing were absent. The sensor 8- HQC-2TCH was effective to form a 1:1 complex, are confirmed by Job’s plot analysis. This binding stoichiometric was further confirmed by ESI-MS (Fig. 10-11).
Intramolecular charge transition (ICT) mechanism has been proposed in this study under investigational conditions. The ICT from the electron-rich quinoline to hydrazide takes place (Scheme 2), when Mg2+ ion added, the charge transfer was arrested due to the strong complexation of Mg2+ with 8- HQC-2TCH. Though four coordination of Mg2+ ion with the hydroxyl of quinoline ring, the nitrogen of quinoline moiety (N), imine nitrogen (C=N) and the nitrogen (-NH) of hydrazide moiety was proposed. In addition, it is well known that Mg2+ ion (a Soft acid), differently interacts with the nitrogen, and hydroxyl group according to Pearson's HSAB theory. The modifications in the fluorescence spectrum, quenching intensity atb480 nm the new band at 600 nm were of Mg2+ ion, obviously justified.
Mechanism of Sensing Fluorometric Mg2+ ion
The proposed geometry of the probe (8-HQC-2TCH) was optimized using DFT-B3LYP 6-31G level using Gaussian 09 package. Binding sites of complex with Mg2+, experimental finding supports Job's plot. DFT calculations, shows that the quinoline unit act as HOMO whereas thiophene unit act as LUMO of 8-HQC-PTH, indicating that the intramolecular charge transfer (ICT) slightly takes place from the quinoline unit to hydrazide unit.
The ICT is inhibited resulting in remarkable fluorescence quenching. For the Mg2+ complex the HOMO act as thiophene part and the electron density are found to spread over whole moiety at LUMO, which indicates restriction of the ICT process and fluorescence quenching is occurs. In addition, the energy gap between the HOMMO and LUMO of the chemosensor is higher than it’s complex, this is clearly indicate that strong chelation occurred between the Mg2+ and chemosensor (Fig. 12).
Table 1. Comparison table for various probes for the detection of Mg2+.
S.N
|
8-HQC-2TCH
|
Metal
|
Detect limit
|
Application
|
Ref
|
1
|
Isatin-3-(7-Methoxychromone-3-methylidene) hydrazone
|
Mg2+
|
5.16 x 10-7 M
|
Fluorescent “turn-on” sensor
|
45
|
2
|
4-hydroxy-3-((2-hydroxy-5-methylphenyl)diazenyl)-2H-chromen-2-one
|
Mg2+ &
F-
|
2.4 x 10-8 M
&
2.3 x 10-8 M
|
Fluorescent “turn-on-off” sensor
|
46
|
3
|
N-(1H-benzo[d]imidazol-2-yl)quinoline-2-carboxamide (QLBM)
|
Cu2+, Fe3+
|
1.35× 10−7 M
&
1.24 × 10−7 M
|
Fluorescent sensor and Live cell imaging
|
48
|
4
|
N-(quinolin-8-yl)-2-(quinolin-8-yloxy) acetamide
|
Zn2+ & Cd2+
|
1.8 × 10-4 M
&
1.9 x 10-7 M
|
Fluorescent “turn-on” sensor
|
47
|
5
|
2-Hydroxyquinoline-3-carbaldehyde
|
Zn2+ & Ni2+
|
7.2 x 10-8 M
&
3.3 x 10-7 M
|
Live cell imaging on
lung cancer cell line
|
49
|
6
|
(8-HQC-PTH)
|
Mg2+
|
4.9 x 10-8 M
|
Ratiometric Fluorescent sensor
|
This work
|