Fluorescence turn-on of salicylaldimine ligands by co-ordination with magnesium and amines

Article abstract of DOI:10.1039/c8nj03285h

A new fluorescence enhancement mechanism based on Schiff base ligand and magnesium complexation is established. The co-ordination number of the Schiff base ligand-magnesium complex (2:1) can be tuned to 1:1 in the presence of amines to induce significant

Full text of DOI:10.1039/c8nj03285h

NJC
View Article Online
View Journal | View Issue
LETTER
Fluorescence turn-on of salicylaldimine ligands
by co-ordination with magnesium and amines†
Cite this: New J. Chem., 2018,
42, 18513
Zideng Gao,a Haojie Feng,a Shunyi Wang,a Yuanfang Huang,a Xueqin Ren,a
a
Long Pang*b and Shuwen Hu
*
Received 28th August 2018,
Accepted 12th October 2018
DOI: 10.1039/c8nj03285h
rsc.li/njc
A new fluorescence enhancement mechanism based on Schiff base been an abundance of research studying the effects of magne-
ligand and magnesium complexation is established. The co-ordination sium ions in clinical medicine, nutrition and physiology.16
number of the Schiff base ligand–magnesium complex (2 : 1) can be Hence, considerable efforts have been undertaken to develop
tuned to 1 : 1 in the presence of amines to induce significant fluores- fluorescent chemosensors for magnesium.17–19 However, the
cence enhancement. Using this approach, a novel and easily available majority of fluorescent probe/chemosensor research has
fluorescent chemosensor was designed, which showed highly selective focused on signaling magnesium by ligand molecule design.
recognition toward magnesium and amines in ethanol.
Few cases are related to achieving fluorescence detection of
magnesium by complexation of imides and magnesium. It is
Schiff base ligands are called ‘‘privileged ligands’’ because they generally understood that the chelating groups CQN and CQO
are easily prepared by the condensation of aldehydes and have a high affinity to transition and post-transition metal
amines and are well known to be effective in stabilizing many cations, but possess a low binding affinity toward alkali metal
different metals in various oxidation states.1 The effectiveness and alkaline earth metal cations because of their differing
of Schiff base–metal complexes enables their use across a electronic structures.14 Additionally, few studies have reported
diverse range of fields, from medicine2,3 to catalysis4,5 and how to activate the fluorescence of complexes resulting from
fluorescent probes/chemosensors.6–9 Fluorescent probes/ changes to the coordination number.
chemosensors have been widely studied in selectively detecting
Herein, we observe the fluorescence properties from a
guest ions and are of particular interest because of their high complex formed via ligand–Mg(II) chelating at a 1 : 1 stoichio-
selectivity, sensitivity, and simplicity. A number of signaling metric ratio20 in the absence of imides, while salicylaldimine
mechanisms have been developed and applied targeting the ligands formed a 2 : 1 complex21,22 with metal ions in most
optical detection of different species. Signaling mechanisms cases. The Salen ligands were prepared by condensation of
include the following: photo-induced electron/energy transfer diamine and two equivalents of salicylaldehyde, which can
(PET),10 metal–ligand charge transfer (MLCT),11 intramolecular directly chelate with Mg(II) at a 1 : 1 stoichiometric ratio.1 There
charge transfer (ICT),12 excimer/exciplex formation, excited- are Schiff base ligands that can also form a 1 : 1 complex with
state intra/intermolecular proton transfer (ESPT),13 and CQN Mg(II) by co-ordinating with N,N-dimethylformamide.9 Ligand L
isomerization.14 Imines are almost non-fluorescent, partly (Scheme 1) is a well-known chelation-enhanced fluorescent
because of the isomerization of the CQN bond in the excited chemosensor for Zn(II) and forms a 2 : 1 complex with Mg(II).
state and partly because ESPT contains the phenolic protons of With this in mind, we speculate that the ESPT and CQN
the salicylic amide moiety.
isomerization are inhibited upon co-ordination of L with
Magnesium is one of the most abundant elements in both Mg(II) and amines, thus resulting in a change to the complex
living organisms and the inorganic environment.15 There has stoichiometry.
Based on this hypothesis, we investigated the complexation
of Mg(II) with Schiff base ligands in the presence of amines. The
a College of Resources and Environmental Sciences, China Agricultural University,
Beijing 100193, China. E-mail: shuwenhu@cau.edu.cn; Fax: +86 010 62731016;
synthesis of ligand L is shown in Scheme 1. 3-Aminopropyl-
Tel: +86 010 62731255
b College of Chemistry and Chemical Engineering, Qingdao University, Qingdao,
triethoxysilane (APTES) and salicylal were conjugated via
a condensation reaction23 to obtain ligand L. Mg(II) and
Shandong, 266000, China. E-mail: panglongstuta@163.com
tris(hydroxymethyl)aminomethane (Tris) powder were directly
added into the reaction system. The structures of L, L + Mg(II),
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj03285h
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 New J. Chem., 2018, 42, 18513--18516 | 18513

View Article Online
Letter
NJC
Binding analysis using the method of continuous variations
(Job plot)24 provided evidence of a 2 : 1 stoichiometric ratio of
L : Mg(II) for the L–Mg(II) complex in ethanol and a 1 : 1 stoichio-
metric ratio of a L : Mg(II) for the L–Mg(II) complex in the
presence of Tris (Scheme 1). Fig. 2 shows an increase in the
fluorescence intensity of both L in ethanol and in a Tris solution
as a function of Mg(II) concentration. However, changes to
the fluorescence of L were more remarkable in the presence of
Tris. The limit of detection (LOD) of Mg(II) was 0.221 mM at a
signal-to-noise ratio of 3, which was calculated using the IUPAC
recommended methodology.25,26 The stability constant between
L and Mg(II) in a 5 mM Tris solution was calculated to be
1.196 (Æ0.08) Â 104 MÀ1 by the linear Benesi–Hildebrand
expression27 according to the fluorescence titration data in
Fig. 2B having a high correlation factor (R = 0.992), which further
supported that the complex was formed at a 1 : 1 stoichiometric
ratio. Therefore, the coordination number of L and Mg(II)
changed (2: 1 to 1 : 1) by the complexation of Mg(II) and
Tris, which subsequently enhanced the fluorescence of L.
Furthermore, changes to the coordination states of L and
Mg(II) in the presence of Tris can also be observed by FT-IR,
1H NMR and 13CNMR (Fig. S3 and S4, ESI†). L, Mg(II) and Tris
complexes could not be observed by ESI-MS (Fig. S5, ESI†)
because of a low binding affinity (Ka = 1.196 (Æ0.08) Â
104 MÀ1); however, ESI-MS, FT-IR and 1H NMR data suggest that
no reaction occurred between L and Tris. The effect of pH on the
emission intensity of the L–Mg(II) complex (Fig. S6, ESI†) shows
that the fluorescence was highest at pH 7. The fluorescence
intensity of L + Mg(II) + Tris remained almost unchanged under
continuous irradiation at intervals of 10 min for 120 min (Fig. S7,
ESI†). This implies that L + Mg(II) + Tris possessed excellent long
term stability. Furthermore, three additional salicylaldimine
ligand–Mg(II) complexes (Fig. S8, ESI†) were synthesized, with
two complexes showing fluorescence enhancement (Table S1,
ESI†) upon addition of Tris, while the third formed a 1 : 1
complex with Mg(II).
Scheme 1 Synthesis of the Schiff base ligand (L).
1
and L + Mg(II) + Tris were confirmed by H nuclear magnetic
resonance (NMR), 13CNMR, FT-IR and electrospray ionization-
mass spectrometry (ESI-MS) data. Further synthetic details are
given in the ESI.†
All absorption and emission spectral studies were performed
after reactants were mixed for 3 h in pure ethanol at room
temperature. Variations in the absorption and fluorescence spectra
of L upon addition of magnesium with and without Tris are shown
in Fig. 1. The free ligand L and L + Mg were almost nonfluorescent
due to the isomerization of the CQN double bond in the excited
state. However, L, Mg and amines formed a new chelating
structure, which inhibited the CQN isomerization, and fluores-
cence was clicked on. Metal-free L shows maximum absorption at
315 nm and weak fluorescence. The fluorescence quantum yield
(Ff) of L is B0.296% of the standard rhodamine B in ethanol.17
A significant enhancement in absorbance and fluorescence
(17-fold) at 459 nm was observed in the presence of one equivalent
of Mg(II) in ethanol. Furthermore, in the presence of Tris (5 mM),
an additional absorbance peak at a higher wavelength (365 nm)
appeared, indicating that a new complex was formed. Conversely,
the fluorescence of the solution increased dramatically and
the maximum emission wavelength shifted from 459 to 438 nm.
Ff increased to 31% in the presence of Tris. The complexation of L,
Mg(II) and Tris resulted in a significant enhancement (100-fold) of
Ff and a visible bluish emission of the solution was observed.
The fluorescence responses of L to the different metal ions
including Ba(II), Pb(II), Hg(II), Fe(III), Cu(II), Co(II), Cd(II), Zn(II),
Mn(II), Mg(II) and Ca(II) with and without Tris were studied.
As shown in Fig. 3, in the absence of Tris, only the L–Zn(II)
complex showed an obvious fluorescence enhancement.
In the presence of Tris (5 mM), a remarkable fluorescence
Fig. 2 Fluorescence spectra (excitation wavelength: 365 nm) of L (10 mM)
as a function of increasing Mg(II) concentration (0, 2.5, 5, 7.5, 10, 15, 20, 30,
40, 50, 60 mM) in (A) ethanol and (B) ethanol in the presence of 5 mM Tris.
Inset: Linear regression plot of the fluorescence titration of L with Mg(II) in
a 5 mM Tris solution.
Fig. 1 Absorption and emission spectra (excitation wavelength: 365 nm)
of L (1 mM), a L–Mg(II) complex (1 : 1, 1 mM) and a L–Mg(II) complex
(1 : 1, 1 mM) in the presence of five equivalents of tris(hydroxymethyl)-
aminomethane (Tris) in ethanol.
18514 | New J. Chem., 2018, 42, 18513--18516 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Letter
Fig. 3 Fluorescence response of L (20 mM) to different metal ions (0, only
L; 1, Ba(II); 2, Pb(II); 3, Hg(II); 4, Fe(III); 5, Cu(II); 6, Co(II); 7, Cd(II); 8, Zn(II);
9, Mn(II); 10, Mg(II); 11, Ca(II); 20 mM) in ethanol (white) and in a 5 mM Tris
solution (black). Inset: Visual fluorescence emissions of L (20 mM) after the
addition of metal ions (as above) in (a) EtOH and (b) a 5 mM Tris solution
during excitation at 365 nm using a UV lamp at room temperature.
Fig. 5 Fluorescence spectra (excitation wavelength: 365 nm) of a L–Mg(II)
(1 : 1, 100 mM) complex as a function of Tris concentration (0, 10, 20, 40,
60, 80, 100, 200 mM). Inset: Linear regression plot of the fluorescence
titration of Tris with L–Mg(II) (1 : 1, 100 mM) in ethanol.
Table 1 Determination results of Mg(II) in human serum samples (unit:
enhancement was observed in the L–Mg(II) complex, yielding
a visible blue emission. The fluorescence spectra of the other
L–metal complexes barely changed. The L–Zn(II) complex
showed a decrease in fluorescence intensity at 449 nm; we
speculate this to be a result of pH increase (5 mM Tris/ethanol
solution B pH 9.53). Therefore, the specific complexation
among L, metal ions, and Tris demonstrated that L has a high
affinity toward Mg(II) in the presence of Tris.
mmol LÀ1
)
Sensor reported
in this work
Method
Mg assay kit AAS
Human serum-pooled 0.876 Æ 0.025 0.968 Æ 0.025 0.868 Æ 0.011
Human serum 1
Human serum 2
0.904 Æ 0.010 0.972 Æ 0.021 0.953 Æ 0.014
0.934 Æ 0.036 1.013 Æ 0.042 0.993 Æ 0.038
Fig. 4 indicates that only amine-containing nitrogenous
compounds can significantly enhance the fluorescence inten-
sity of the L–Mg(II) complex. Amides and nitro compounds
cannot promote L and Mg(II) to generate a 1 : 1 complex.
Fluorescence enhancement is proportional to the quantity
of the free amino group; however, this trend is not observed
for diethylenetriamine because of the influence of stereo-
hindrance. The fluorescence intensity of the L–Mg(II) complex
was studied as a function of Tris concentration (Fig. 5). The
fluorescence intensity of the L–Mg(II) complex increased with
increasing Tris concentration, and showed linear growth at
low Tris concentration (Fig. 5, inset). Furthermore, Ff of L
increased too. Meanwhile, the LOD of Tris was obtained as
0.667 mM at a signal-to-noise ratio of 3. Consequently, the
L–Mg(II) complex has a high affinity toward amines, and can
be used for the quantitative detection of amino groups across a
specific concentration range.
To assess the reliability of the developed approach in serum
samples, the L was used for Mg(II) activity assays of human
serum samples. The results detected by our standard addition
approach are displayed in Table S2 (ESI†). The recoveries varied
from 87% to 107%, and the relative standard deviation (RSD)
was from 3.89% to 22.71%. The result of the actual magnesium
content in the serum is determined as shown in Table 1.
Fluorescence quantitative results are similar to those measured
using a commercialized Mg assay kit and atomic absorption
spectrophotometer (AAS).
In summary, we have reported a novel, easily available
fluorescent chemosensor based on the co-ordination of salicyl-
aldimine with magnesium and amines. It was proved that
fluorescence enhancement was induced by changing the
co-ordination number of the salicylaldimine and magnesium
complex. The fluorescent chemosensor has high sensitivity and
affinity toward magnesium and amines because of the specific
complexation among L, Mg(II) and Tris. We believe that this
study can provide a reference for the design of fluorescent
chemosensors in molecular detection and triggered release, etc.
Fig. 4 Fluorescence intensity change of the L–Mg(II) (1 : 1, 20 mM)
complex upon the addition of various nitrogenous compounds (5 mM)
in ethanol: 0, CK; 1, Tris; 2, 2-aminoethanol; 3, ethylenediamine;
4, diethylenetriamine; 5, acrylamide; 6, methanamide; 7, n,n-dimethyl-
acetamide; 8, nitromethane; 9, p-nitrophenol.
Conflicts of interest
There are no conflicts to declare.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 New J. Chem., 2018, 42, 18513--18516 | 18515

View Article Online
Letter
NJC
12 Z. Xu, X. Qian and J. Cui, Org. Lett., 2005, 7, 3029–3032.
13 P. D. Beer, Acc. Chem. Res., 1998, 31, 71–80.
14 J. Wu, W. Liu, X. Zhuang, F. Wang, P. Wang, S. Tao,
X. Zhang, S. Wu and S. Lee, Org. Lett., 2007, 33–36.
15 B. L. Mordike and T. Ebert, Mater. Sci. Eng., A, 2001, 302,
37–45.
16 N. E. L. Saris, E. Mervaala, H. Karppanen, J. A. Khawaja and
A. Lewenstam, Clin. Chim. Acta, 2000, 294, 1–26.
17 D. Ray and P. K. Bharadwaj, Inorg. Chem., 2008, 47,
2252–2254.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (21775163) and the National Key R&D
Program of the Ministry of Science and Technology of China
(2016YFC0501205, 2016YFC0501208 and 2017YFD0200706).
Thanks for the support of the Beijing Key Laboratory of
Farmland Soil Pollution Prevention and Remediation.
Notes and references
18 Y. Suzuki, H. Komatsu, T. Ikeda, N. Saito, S. Araki,
D. Citterio, H. Hisamoto, Y. Kitamura, T. Kubota,
J. Nakagawa, K. Oka and K. Suzuki, Anal. Chem., 2002,
74, 1423–1428.
1 P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410–421.
2 M. S. Alam, J. H. Choi and D. U. Lee, Bioorg. Med. Chem.,
2012, 20, 4103–4108.
3 T. Jeewoth, M. G. Bhowon and H. L. K. Wah, Transition Met.
Chem., 1999, 24, 445–448.
4 R. Ramnauth, S. Al-Juaid, M. Motevalli, B. C. Parkin and
A. C. Sullivan, Inorg. Chem., 2004, 43, 4072–4079.
5 E. S. Marzena Białek and Krystyna Czaja, J. Polym. Sci., Part A:
Polym. Chem., 2008, 47, 565–575.
6 Z. Q. Hu, C. Lin, X. M. Wang, L. Ding, C. L. Cui, S. F. Liu and
H. Y. Lu, Chem. Commun., 2010, 46, 3765–3767.
7 M. Royzen, A. Durandin, V. G. Young, N. E. Geacintov and
J. W. Canary, Tetrahedron, 2006, 3854–3855.
´
´
19 L. F. Capitan-Vallvey, M. D. Fernandez-Ramos, A. Lapresta-
´
´
Fernandez, E. Brunet, J. C. Rodrıguez-Ubis and O. Juanes,
Talanta, 2006, 68, 1663–1670.
20 R. P. Houghton, J. Chem. Soc., 1967, 2030–2032.
21 Y. Zhang, B. Shi, P. Zhang, J. Huo, P. Chen, Q. Lin, J. Liu and
T. Wei, Sci. China: Chem., 2013, 56, 612–618.
22 N. Malumbazo and S. F. Mapolie, J. Mol. Catal. A: Chem.,
2009, 312, 70–77.
23 H. Zhang, P. Zhang, K. Ye, Y. Sun, S. Jiang, Y. Wang and
W. Pang, J. Lumin., 2006, 117, 68–74.
24 W. Jiang and W. Wang, Chem. Commun., 2009, 3913–3915.
25 L. Guo, Y. Xu, A. R. Ferhan, G. Chen and D. H. Kim, J. Am.
Chem. Soc., 2013, 135, 12338–12345.
8 M. Li, X. Zhou, S. Guo and N. Wu, Biosens. Bioelectron., 2013,
43, 69–74.
9 L. Wang, W. Qin, X. Tang, W. Dou and W. Liu, J. Phys. Chem.
A, 2011, 115, 1609–1616.
26 X. Yang, Y. Luo, Y. Zhuo, Y. Feng and S. Zhu, Anal. Chim.
Acta, 2014, 840, 87–92.
27 J. S. Wu, J. H. Zhou, P. F. Wang, X. H. Zhang and S. K. Wu,
Org. Lett., 2005, 7, 2133–2136.
10 S. Kyung and J. Yoon, Chem. Commun., 2002, 770–771.
11 M. J. Kim, K. Rajeshwar, R. Konduri, H. Ye, F. M.
MacDonnell, F. Puntoriero, S. Serroni, S. Campagna,
T. Holder and G. Kinsel, Inorg. Chem., 2002, 41, 2471–2476.
18516 | New J. Chem., 2018, 42, 18513--18516 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018