Enhanced luminescence for detection of small molecules based on doped lanthanide compounds with a dinuclear double-stranded helicate structure

Article abstract of DOI:10.1039/c9nj04104d

A novel family of lanthanide materials with a dinuclear double-stranded helicate structure Ln₂L₂(DBM)₄(OAc)₂ [Ln = Sm (1), Eu (2) Gd (3) and Yb (4)] constructed from β-diketonate (DBM = dibenzoylmethanide) and an auxiliary ligand Schiff base (H₂L = N,N′-bis(salicylidene)butane-1,6-diamine) was investigated. The structures of isostructural complexes 1-4 were determined by single-crystal and powder X-ray crystallography. The optical properties of these compounds could be enhanced by doping, allowing for doped materials with tunable visible and near-infrared emission. Subsequently, a multifunctional sensor was employed to detect CH₃COOH, as well as the poisonous ions Al³⁺ and Cr₂O₇^2-. The notable quenching effect and luminescence-based color change showed that doped materials are promising fluorescence probes for the identification and detection of specific anions, cations and organic small molecules.

Full text of DOI:10.1039/c9nj04104d

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Enhanced luminescence for detection of small
molecules based on doped lanthanide compounds
with a dinuclear double-stranded helicate
structure†
Cite this: New J. Chem., 2019,
43, 16706
Yuan-Yi Xu,
Ou Sun, Yue Qi, Bo-Yu Xie and Ting Gao
*
A novel family of lanthanide materials with a dinuclear double-stranded helicate structure Ln
DBM) (OAc) [Ln = Sm (1), Eu (2) Gd (3) and Yb (4)] constructed from b-diketonate (DBM =
dibenzoylmethanide) and an auxiliary ligand Schiff base (H
2 2
L -
(
4
2
0
L = N,N -bis(salicylidene)butane-1,6-diamine)
2
was investigated. The structures of isostructural complexes 1–4 were determined by single-crystal and
powder X-ray crystallography. The optical properties of these compounds could be enhanced by
Received 7th August 2019,
Accepted 1st October 2019
doping, allowing for doped materials with tunable visible and near-infrared emission. Subsequently, a
3+
3
multifunctional sensor was employed to detect CH COOH, as well as the poisonous ions Al and
2ꢀ
Cr O
2 7
. The notable quenching effect and luminescence-based color change showed that doped
DOI: 10.1039/c9nj04104d
materials are promising fluorescence probes for the identification and detection of specific anions,
cations and organic small molecules.
rsc.li/njc
transfer and improve the quantum yields of the corresponding
complexes. However, usually solvent molecules are involved in
Introduction
6
In recent years, lanthanide complexes have attracted scientists’ the coordination, which may result in luminescence quenching.
attention on account of their outstanding luminescence in As a consequence, an auxiliary aromatic ligand is selected to
1
functional materials. In particular, lanthanides with sharp prevent the solvent molecules from coordinating with lanthanide
emission bandwidths, relatively long lifetimes and large Stokes ions because it tends to absorb energy and transfer it efficiently to
shifts have revealed new possibilities for a broad range of metal ions, potentially enhancing the luminescence properties.
applications originating from unique 4f electronic configurations Notably, another strategy to enhance luminescence properties is
2
3+
3+
3+
in optical properties, such as bioanalysis, biological imaging,
doping La , Gd or Y into their lanthanide analogs in the
3
4
5
7
fluorescent probes, diode displays, and telecommunications.
appropriate range. The 4f electronic shell is in a stable state of
However, most f - f transitions are forbidden according to fullness, half-fullness or vacancy, so the lowest excited level is
the Laporte rule. Hence, direct excitation of lanthanide ions is considerably higher than the triplet level of the ligand, Thus, the
3+
ineffective owing to their much lower molar absorption coeffi- ligand is scarcely able to transfer energy to Ln . In this way, the
3+
cients. This limitation can be overcome with the help of suitable luminous Ln can acquire much more energy from the ligand, so
3+
organic ligands. They have a significant impact on Ln as the intensity can be improved greatly. This phenomenon of energy
efficient sensitizers via an ‘antenna effect’. This effect implies transfer towards only one nucleus is called the ‘concentration
that excitation of the organic chromophore allows adequate effect’. Conversely, the great variation in symmetry of the crystal
p - p* transitions by electromagnetic radiation, followed by structure of doping compounds probably adds to this effect.
transfer of the corresponding energy to the accepting levels of Obviously, the doping method can be employed in the synthesis,
lanthanide ions. Chromophore ligands originating from con- which not only endows lanthanide luminescent compounds with
jugated aromatic rings and double-bond groups will be better tunable optical properties, but also provides a brilliant strategy to
choices because conjugated ligands are beneficial to energy strengthen luminescence efficiency.
Fluorescent probes are convenient, show a rapid response,
8
Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of
have high sensitivity and are inexpensive. In consideration of
the satisfactory photo-physical properties of Ln , scholars have
Chemistry and Materials Science, Heilongjiang University, No. 74, Xuefu Road,
Nangang District, Harbin 150080, P. R. China. E-mail: gaotingmail@sina.cn
3+
taken advantage of dozens of lanthanide compounds for
†
Electronic supplementary information (ESI) available. CCDC 1883718. For ESI
9
sensing of anions, cations and small molecules. It seems that
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9nj04104d
the wide variety of luminescent lanthanide compounds and
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their inherent synthetic multifunctionality could make them Elemental analyses of C, H and N were done by using a
ideal for applications based on molecular recognition. Com- PerkinElmer 2400 analyzer. UV-vis spectra in CH OH were
3
pared with traditional fluorescence probes that identify a single recorded on a Shimadzu UV2240 spectrophotometer. IR spectra
substance, multifunctional fluorescence probes have a convenient, were obtained on a PerkinElmer 60000 spectrophotometer
ꢀ
1
clear and uniform synthetic method, are environmentally friendly, using KBr disks in the range 4000–450 cm . Fluorescence
and do not consume many chemical resources. The principle of spectra for complexes were recorded on a FLSP920 spectro-
sensing is based on fluorescence quenching, enhancement of photometer equipped with a xenon lamp and quartz carrier.
luminescence, or luminescent color changes. The latter is much Luminescence lifetimes were recorded on a single photon-
more novel and simple for identification of which inorganic ions or counting spectrometer with a microsecond pulse lamp as the
organic molecules are present in drinking water because it can be excitation. The overall photoluminescence quantum yields of
seen directly in contrast with the other two effects. Molecular europium and samarium complexes were measured in an
recognition is based principally on the interaction between host integrating sphere. Powder X-ray diffraction (PXRD) data were
materials and guest molecules, which is dependent upon the recorded on a Rigaku D/Max-3B X-ray diffractometer with
solvent effect, hydrogen-bonding capacity, p–p stacking and Cu/Ka as the radiation source (l = 0.15406 nm) in the angular
10
electrostatic interaction. Lanthanide compounds with high coor- range y = 5–501 at room temperature. The crystal data and
dination numbers and modes may serve as promising lumines- structure-refinement details for complex 2 are displayed in
11
cence probes for the recognition of small molecules. With the Table 1.
rapid development of industry, environmental pollution, such as
Synthesis of complexes 1–4
Synthesis of [Eu (DBM) (OAc) ] (2). All complexes were
4 2
poisonous ions (heavy metals and anions), are being discharged
increasingly from industrial facilities, which can lead to harmful
L
2
2
12
effects on human health. Therefore, it is vital to identify and synthesized using the same approach. The synthesis of complex
monitor these hazardous chemical substances. In view of their 2 is detailed herein. First, 5 mL of a methanol solution of
intense luminescent signals and visible luminescent colors, lantha- Cd(OAc)
nide compounds could be excellent candidates as luminescent (0.081 g, 0.25 mmol) in 4 mL of dichloromethane. After refluxing
sensing materials for monitoring environmental pollution.
for 1 h, the solution became yellow. The mixture was filtered and
Previously, our research team we looked forward to exploit- the precipitate was removed from the solution. The filtrate was
ing the excellent optical properties of lanthanide compounds. diffused slowly by a methanol solution (8 mL) of Eu(DBM)
2 2
(0.067 g, 0.25 mmol) was added to a solution of H L
1
3
3 2
ꢁ2H O
As a result, a series of lanthanide complexes with a dinuclear (0.218 g, 0.25 mmol) and allowed to stand undisturbed at
double-stranded helicate structure built by b-diketonate (DBM = room temperature for 2 weeks. Yellow diamond crystals of 2 were
dibenzoylmethanide) and an auxiliary ligand Schiff base (H
2
L = obtained in 50.0% yield (0.183 g). Elemental analysis (%) calcd for
0
N,N -bis(salicylidene)butane-1,6-diamine) were designed and
synthesized. Yang and colleagues recently reported initial
studies focusing on luminescence sensing with lanthanide-
Table 1 Crystallographic data for complex 2
1
4
Crystal data
2
based clusters formed by salen-type Schiff base ligands.
Lanthanide luminescent materials can demonstrate a wide CCDC
1883718
C H94Eu N O
104 2 4 16
1959.75
293
Empirical formula
variety of enhanced visible and near-infrared (NIR) optical
properties using doping methods. In addition, a multifunctional
Formula weight
Temperature (K)
luminescent probe has been shown to have an extremely selective Wavelength (Å)
0.71073
Orthorhombic
Fddd
21.821(4)
25.478(5)
40.301(8)
90
3+
2ꢀ
Crystal system
space group
a (Å)
and rapid response to Al , Cr
neously compared with other cations, anions and organic small
2
O
7
3
and CH COOH simulta-
molecules, which could be distinguished readily by the naked eye b (Å)
c (Å)
a (1)
b (1)
under UV-light irradiation. Notably, this was the first example of a
3+
luminescent sensor for Al and CH COOH based on a lanthanide
3
90
compound that employed luminescence-based color changes.
g (1)3
90
22 406(8)
1.162
1.166
8000
V (Å )
ꢀ
3
Calculated density (Mg m
Absorption coefficient (mm
F(000)
y range
Limiting indices
)
1
ꢀ
)
Experimental section
Materials and instrumentation
3.18 to 25
ꢀ25 r h r 24
ꢀ
30 r k r 19
All procedures were carried out in the atmosphere. The lantha-
nide precursors Ln(DBM)3 (Ln = Sm, Eu, Gd and Yb) were
synthesized according to a procedure described previously.
ꢀ47 r l r 26
Reflections collected
Completeness to theta = 27.481
Data/restraints/parameters
12 532
99.3%
4918/96/286
1.119
1
5
2
Schiff base ligands (H L) were obtained from the reaction Goodness-of-fit on F
2
Final R indices [I 4 2s(I)]
R
1
= 0.0686
= 0.2067
= 0.1034
2
= 0.2282
between 1,6-hexylenediamine and salicylaldehyde according
wR
R
1
wR
2
1
6
to a procedure reported previously. All other reagents were
purchased commercially and used without further purification.
R indices (all data)
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C
104 2 4
H94Eu N O16 (1959.75): C, 63.74; H, 4.83; N, 2.86. Found: C,
ꢀ
1
63.69; H, 4.88; N, 2.90; IR (KBr, cm ): 3057 (w), 2846 (s), 2765 (w),
2592 (w), 1737 (vs), 1684 (vs), 1553 (s), 1391 (vs), 1311 (s), 1218 (vs),
1149 (m), 1098 (m), 937 (vs), 863 (w), 715 (m), 673 (w), 587 (m).
UV-vis [MeOH, l]: 210, 252, 352 nm.
Synthesis of [Sm (DBM) (OAc)
Elemental analysis (%) calcd for C104
3.77; H, 4.80; N, 2.86. Found: C, 63.80; H, 4.86; N, 2.97; IR
2
L
2
4
2
] (1). Yield: 0.179 g (48.9%).
16 (1956.54): C,
H
2 4
94Sm N O
6
(
ꢀ
1
KBr, cm ): 3056 (w), 2844 (s), 2767 (w), 2593(w), 1736 (vs),
685 (vs), 1556 (s), 1390 (vs), 1312 (s), 1219 (vs), 1150 (m), 1099
m), 937 (vs), 861 (w), 717 (m), 671 (w), 588 (m). UV-vis [MeOH,
l]: 210, 252, 352 nm.
Fig. 1 (a) The crystal structure of 2 (all hydrogen atoms have been
omitted for clarity). Color code: dark green (Eu), blue (N), red (O), gray (C).
Selected bond lengths (in Å): Eu1–O1, 2.309(5); Eu1–O1a, 2.309(5);
Eu1–O3, 2.369(6); Eu1–O3a, 2.369(6); Eu1–O2, 2.404(5); Eu1–O2a,
1
(
Synthesis of [Gd L (DBM) (OAc) ] (3). Yield: 0.187 g (51.3%). 2.404(5); Eu1–O4, 2.513(6); Eu1–O4a 2.5136. (b) Perspective view of
2
2
4
2
3
+
Elemental analysis (%) calcd for C104
3.34; H, 4.77; N, 2.84. Found: C, 63.40; H, 4.69; N, 2.82; IR
KBr, cm ): 3059 (w), 2844 (s), 2769 (w), 2596 (w), 1737 (vs),
685 (vs), 1551 (s), 1392 (vs), 1311 (s), 1219 (vs), 1152 (m),
100 (m), 938 (vs), 861 (w), 714 (m), 671 (w), 589 (m). UV-vis
MeOH, l]: 210, 252, 352 nm.
2 4
H94Gd N O
16 (1970.38): C, coordination polyhedron for Eu in 2.
6
(
1
1
[
ꢀ
1
an anti-parallel structure in which two backbone phenolato
oxygen atoms linked two europium ions with an Eu1 Eu1a
distance of 11.944 Å. Each Eu exhibited eight-coordinated
environments from two phenolato O atoms from the different
bridging Schiff base ligand, four O atoms from two DBM
molecules, and two O atoms from one bidentate acetate (OAc )
group. The coordination environment of Eu had dodecahedron
geometry (Fig. 1b). The Eu–O distance ranged from 2.309(5) Å to
.513(6) Å. The shortest Eu–O bond involving hydroxyl oxygen
atoms was 2.309(5) Å. The shortest Eu–O bonds involving chelat-
ing b-diketone oxygen atoms were 2.369(6) Å and 2.404(5) Å. The
longest Eu–O bonds involving chelating acetate (OAc ) group
oxygen atoms was 2.513(6). Cd(OAc) afforded acetate groups
which involved bidentate chelate Eu , whereas cadmium ions
did not take part in this coordination.
.
. .
3
+
Synthesis of [Yb L (DBM) (OAc) ] (4). Yield: 0.194 g (53.1%).
2
2
4
2
Elemental analysis (%) calcd for C104H Yb N O (1998.89): C,
2.43; H, 4.70; N, 2.80. Found: C, 62.53; H, 4.65; N, 2.85; IR
KBr, cm ): 3060 (w), 2846 (s), 2771 (w), 2600 (w), 1735 (vs),
687 (vs), 1552 (s), 1396 (vs), 1308 (s), 1216 (vs), 1151 (m), 1101
m), 938 (vs), 865 (w), 717 (m), 672 (w), 595 (m). UV-vis [MeOH, l]:
94
2
4
16
ꢀ
6
(
1
(
3+
ꢀ
1
2
210, 252, 352 nm.
Synthesis of doped materials. For isostructural complexes,
the synthetic methods of Sm Gd , Eu Gd , and Yb Gd
1–x
were the same as those mentioned above but instead using the
corresponding Ln(DBM) O as the starting materials in
stoichiometric ratios. The molar ratios of lanthanide ions in
the resulting mixed-lanthanide complexes were confirmed by
inductively coupled plasma (ICP) spectroscopy (Table S1).
X-ray crystallography. Diffraction-intensity data for the
single crystals of the complexes were collected on a Rigaku
ꢀ
x
1–x
x
1–x
x
2
3
+
3
ꢁ2H
2
Luminescence properties
UV absorption spectra
R-AXIS RAPID imaging-plate X-ray diffractometer at 293 K. The UV-vis absorption spectra of ligand H L, Ln(DBM) , com-
2 3
The structures were solved by direct methods and refined by plexes 1–4 and doped materials are shown in Fig. S2 (ESI†). In
2
the full-matrix least squares method on F using SHELXTL-97. MeOH, H
L exhibited four main absorption bands at ca. 225, 252,
2
All non-hydrogen atoms were refined with anisotropic displace- 321 and 400 nm, which belong to the p–p* transitions of the
ment parameters. CCDC 1883718 for complex 2 contains the Ar–OH chromophores and imine group. For Ln(DBM) , there were
three absorption bands at ca. 226, 254, and 352 nm, which could
be assigned to the intra-ligand p–p* (DBM) transitions. Complexes
3
supplementary crystallographic data for our study.†
1–4 displayed three wide absorption bands located at 203, 252 and
3
02 nm due to the intra-ligand transitions of H L and DBM
2
Results and discussion
Crystal structure
ligands, respectively.
Luminescence spectra
The PXRD patterns of complexes 1–4 were isostructural with the
simulated patterns from their respective single-crystal X-ray Doping is used widely in the synthesis of high-performance
data (Fig. S3a, ESI†). Taking 2 as an example, the unit cell of lanthanide luminescent materials. Any one series of compounds
complex 2 crystallized in the orthorhombic space group Fddd. will be isostructural, so enhancement of lanthanide-based lumines-
The perspective view of the molecular structure of 2 is shown in cence using a doping will improve optical properties such as
Fig. 1. Crystallographic details are provided in Table 1. Complex emission intensity, lifetime and quantum efficiency. As shown in
2
was a double-stranded dinuclear helicate. It contained two Fig. 2, this method was indeed efficient: a decrease of emission
3
+
3+
Schiff base ligands, two independent Eu , four DBM mole- from the ligand and growth of Sm typical luminescence upon
cules, two bidentate acetate (OAc ) groups without solvent Gd addition was observed. Also, partially substituted gadolinium
molecules (Fig. 1a). Two flexible Schiff base ligands displayed compounds were synthesized, and were denoted as Ln Gd1ꢀxDBM
ꢀ
3+
x
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Fig. 3 Emission spectra of Eu
The inset shows the corresponding transition intensities of the doped
x
Gd1ꢀxDBM solid samples, lex = 348 nm.
Fig. 2 Emission spectra of Sm Gd1ꢀxDBM solid samples, lex = 328 nm.
The inset shows the corresponding transition intensities of the doped
x
compounds as a factor of the doping concentration x.
compounds as a factor of the doping concentration x.
17
complex 2. Compared with complex 2, the emission intensity of
(
0 r x r 1, Ln = Sm, Eu and Yb). The luminescence intensity ratio,
5
7
the peak at 616 nm ( D - F ) increased by 1.6-times. The lifetime
0
2
3+
lifetime and quantum yield of Ln in doped materials is displayed
in Table S2 and Fig. S5–S7 (ESI†).
The luminescence spectra of samarium and europium
complexes in the solid state were obtained at room tempera-
ture. For doped complex Sm0.4Gd0.6DBM, when excited at
of Eu0.22Gd0.78DBM in the solid state was t = 765.8 ms. The
fluorescence quantum yield (jem) for Eu0.22Gd0.78DBM was 78.2%.
The luminescence spectra of complex Yb0.1Gd0.9DBM in the
solid state were obtained at 193 K. The NIR emission from the
doped materials is displayed in Fig. 4. When excited at 296 nm
3
28 nm (Fig. S4, ESI†), the emission spectra displayed a weak,
3+
(
Fig. S4, ESI†), the NIR emission bands of Yb were detected
broad emission band at 420–550 nm, which was assigned to the
p–p* electronic transitions from the ligand. There were four
typical emission bands of Sm located at 562 nm ( G5/2 - H5/2),
2
2
and assigned at nearby 970 nm ( F - F7/2), alongside the
5
/2
other broad bands centered at B1020 nm. In early spectro-
3+
4
6
3+
scopic studies on Yb b-diketonate compounds, it was implied
4
6
4
6
4
6
00 ( G5/2 - H7/2), 645 nm ( G5/2 - H9/2) and 704 nm ( G5/2 -
that the structure could lead to crystal field splitting in the
6
H11/2), with the main emission band being at 645 nm (Fig. 2). The
18
emission spectrum and, in the meantime, similar splitting in
emission spectra indicated dual luminescence, with a wide band
covering the blue-green region from the ligand and narrow bands
originating from samarium. These data suggested that the
ligand could only partly sensitize luminescence of Sm and
that the ligand incompletely transferred the absorbed energy to
13
the Schiff base–ytterbium complex also took place. Similarly,
2
2
3+
the emission intensity of F5/2 - F7/2 for Yb increased by
.35-times compared with complex 4. The lifetime of Yb0.1Gd0.9DBM
in the solid state was t = 31.37 ms.
2
3
+
3
+
Sm in the solid state. After doping, the emission intensity of
4
6
3+
G
5/2 - H9/2 for Sm increased by 1.44-times in contrast with
complex 1. The lifetime of Sm0.4Gd0.6DBM in the solid state
was t = 42.73 ms. The fluorescence quantum yield (jem) for
Sm0.4Gd0.6DBM was 4.2%.
With regard to complex Eu0.22Gd0.78DBM with excitation at
3
+
3
48 nm (Fig. S4, ESI†), four typical emission bands of Eu were
5
7
5
7
observed: 592 nm ( D
0
- F
1
), 616 nm ( D
0
- F
2
), 650 nm
) (Fig. 3). In particular, a
5
7
5
7
(
D
0
- F
3
) and 702 nm ( D
0
- F
4
strong emission band at 616 nm dominated with no emission
from the ligand, implying that the transfer of energy from the
3
+
ligand to Eu was very effective. H
2
L and DBM ligands may
play a significant part in boosting the luminescence efficiency
3
+
of the Eu complex. Therefore, ligands can act as sensitizers
3
+
5
7
for Eu . The stronger electric dipole transition ( D - F ) in
0
2
5
7
contrast to the magnetic dipole transition ( D
the intense red luminescence of Eu . The band of the D
0
- F
1
) triggered
- F
0 1
3+
5
7
Fig. 4 Emission spectra of the Yb
The inset shows the corresponding transition intensities of the doped
ment for Eu , which is consistent with the crystal structure of compounds as a factor of the doping concentration x.
x
Gd1ꢀxDBM solid samples, lex = 296 nm.
transition was indicative of a high symmetry coordination environ-
3+
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A widely acceptable energy-transfer mechanism of luminescent
lanthanide complexes was postulated by Crosby. To guarantee
high-performance energy transfer, the energy-level difference
Paper
19
between the lowest triplet energy level (T ) of the ligand and the
1
3+
lowest excited state level of Ln dominated the luminescence
properties of the complexes. Due to the difficulty in obtaining the
triplet energy level directly, the phosphorescence spectrum of
complex 3 at 77 K was employed to estimate it. The triplet energy
3+
levels of the ligand and the energy gaps DE(T
1
–Ln ) are displayed
in Table S3 (ESI†). The triplet energy level of H
pp* = 20 366) was determined by the top emission peak of the
2
L/DBM (491 nm,
3
corresponding phosphorescence spectra of the gadolinium
3+
complex at 77 K (Fig. S8, ESI†). Their energy gaps, DE(T –Sm ), Fig. 5 Emission spectra of doped complex Eu0.4Gd0.6DBM immersed in
1
3+
3+
ꢀ1
various solvents.
DE(T
1 1
–Eu ) and DE(T –Yb ) were 2242, 2866, 103 66 cm for
complexes 1, 2 and 4, respectively. An energy gap in the range
ꢀ
1
2
500–3200 cm may be suitable for energy transfer from ligand response. As shown in Fig. S10 (ESI†), the luminescence
to lanthanide (Fig. S9, ESI†). As a consequence, the energy transfer intensities of the compounds in emulsion decreased inch-by-
3+
from the ligand to Eu was quite effective and sensitized inch upon addition of acetic acid from 20 mL to 100 mL, which
the luminescence of Eu completely. However, in the case of indicated Eu0.4Gd0.6DBM to be a promising probe for identify-
DE(T –Sm ), the gap was slightly narrower, resulting in an energy ing acetic acid molecules.
3+
3+
1
back-transfer process compared with that observed in europium
3
+
complexes. Meanwhile, the energy levels of Yb were even lower
Cations
3
+
than those of other Ln in the visible region, so ytterbium
complexes showed much weaker luminescence properties
compared with those of Eu or Sm . This phenomenon was
consistent with the theory of phosphorescence measurements.
Aluminium can enter the central nervous system readily and
accumulate in the brain under normal physiological conditions.
Aluminium can harm human brain cells. Therefore, an acceptable
daily intake of aluminium should be determined and ways to
reduce it developed.
21
3
+
3+
Based on this consideration, samples of the complex
Eu0.4Gd0.6DBM were dispersed in different aqueous solutions
Luminescence sensing
ꢀ2
ꢀ1
3 n
containing the same concentration (10 mol L ) of M(NO )
Considering the discussion above, complex Eu0.4Gd0.6DBM
possessed excellent luminescence properties with stronger red
color in the visible region. Thus, it could serve as a potential
fluorescence probe for sensing measurements to detect and
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
(
M = Na , K , Ag , Ca , Ba , Zn , Ni , Cu , Co , Cd , Pd ,
3
+
3+)
Cr , and Al to form Eu0:4Gd0:6DBM@suspensions of cations:
After 2 days, the luminescence intensities of these suspensions
were ready for testing. The corresponding emission spectra
were recorded and compared in Fig. 6, in which various cations
displayed different degrees of impact on luminescence properties.
Most Eu0:4Gd0:6DBM@suspensions retained the typical emission
2
0
identify small molecules, cations and anions.
Solvent molecules
First, to test the sensing applications of Eu0.4Gd0.6DBM in a
solvent environment, the corresponding samples were immersed
in different pure solvents for 2 days to allow them to form stable
emulsions for fluorescence studies. The solvents used were
petroleum ether, n-hexane, tetrahydrofuran (THF), diethyl ether
3+
3+
of Eu , except Eu0:4Gd0:6DBM@Al aqueous solution: The latter
showed blue-green emission concentrated 450–550 nm from the
(
Et O), acetonitrile (CH CN), N,N-dimethylformamide (DMF),
2 3
ethyl alcohol (EtOH), acetone, dichloromethane (CH Cl ), benzal-
2
2
dehyde and acetic acid (CH COOH). The luminescence intensities
3
of complex Eu0.4Gd0.6DBM in the various solvents were recorded.
The luminescence spectra of these suspensions are shown in
3+
Fig. 5. The characteristic emissions of Eu could be observed in
most solvents, except for acetic acid, which exhibited only weak
2
emission from ligand–H L/DBM. The luminescence intensities of
Eu0.4Gd0.6DBM relied exceedingly on solvent molecules but, for
acetic acid, led to a complete quenching effect.
Detailed experiments were conducted to verify quantitative
analyses. Complex Eu0.4Gd0.6DBM immersed in diethyl ether
was treated as the standard emulsion, in which different
Fig. 6 Emission spectra of doped complex Eu0.4Gd0.6DBM immersed in
volumes of acetic acid were added to monitor the emission various cationic aqueous solutions.
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2
H L/DBM ligand that was observable by the naked eye when
irradiated under UV light at 365 nm (Fig. 8). This feature could be
3+
utilized to selectively identify Al through luminescence-based
3+
color change after addition of complex Eu0.4Gd0.6DBM into Al
aqueous solution, which has not been studied previously.
Further detailed experiments were undertaken to examine
3
+
the capacity for detection of Al . Complex Eu0.4Gd0.6DBM was
3
+
immersed in Al aqueous solution of different concentrations
to investigate the emission response. As indicated in Fig. S11
(
ESI†), the blue-green emission intensity of suspensions
3
+
enhanced gradually, and emission of Eu decreased step-by-
step upon increasing Al concentration (20–100 mM). These
data demonstrated that Eu0.4Gd0.6DBM could sense Al ,
thereby showing that complex Eu0.4Gd0.6DBM had high selectivity
for detection of Al . Complex Eu0.4Gd0.6DBM is the first example
3
+
3
+
3+
72ꢀ
irradiated
Fig. 8 Effect of Eu0:4Gd0:6DBM@CH
under UV light at 365 nm:
3
COOH, Al and Cr
2
O
3+
3+
of a luminescent sensor for Al based on Ln-compounds through
luminescence-based color changes.
2
ꢀ
gradually with increasing Cr
2
O
7
concentration (20–100 mM).
These data suggested that doped materials were potential
2
ꢀ
Anions
fluorescence candidates for detection of Cr
2
O
7
.
Hexavalent chromium is poisonous and a strong oxidizing
agent with carcinogenic characteristics. It has been used in
2
2
Cycling experiments
electroplates, chromate pigments and organic synthesis. Accord- Cycling experiments revealed that the doped complex could
ingly, detection of toxic hexavalent chromium has become very be recycled up to three times after detecting CH COOH by
3
important.
The complex Eu0.4Gd0.6DBM and samples were introduced several times with water. Recovery of quenching was possible
into various Na Y or K Y anion aqueous solutions at the same for three cycles without substantial decay in intensity, suggest-
concentration (10 mol L ): ClO4 , F , Br , I , MnO , HS , ing that this compound retained its stability and reversibility
centrifuging the suspension and washing the precipitate
x
x
ꢀ
2ꢀ
2
ꢀ1
2ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
ꢀ
2ꢀ
2ꢀ
3ꢀ
3+
2ꢀ
NO2 , SO3 , SO4 , CO3 , Cr O
and PO4 . After this during recycling. However, in relation to Al and Cr O7 , the
2
7
2
introduction, suspensions were formed and used for lumines- luminescence of the doped compound did not recover, suggest-
cence studies. The luminescence emission spectra of these ing no recycling.
suspensions are shown in Fig. 7. Most of the anions had a
quenching effect on complex Eu0.4Gd0.6DBM. In particular,
2
ꢀ
quenching by Cr
that complex Eu0.4Gd0.6DBM could recognize Cr
was not observed upon irradiation under UV light (Fig. 8).
To better understand the response of fluorescence of To explore the possible mechanism for quenching effects, PXRD
2
O
7
was very pronounced. These data suggested Possible mechanism for luminescence
2ꢀ
2 7
O
. A red color
sensing
2ꢀ
3+
Eu0:4Gd0:6DBM@Cr O , concentration-dependent luminescence was done. The structures of Eu0:4Gd0:6DBM@CH
COOH, Al and
changed compared with that of Eu0.4Gd0.6DBM (Fig. S13,
in aqueous solutions at ESI†). In relation to CH COOH, a slight difference in PXRD
2
7
3
2ꢀ
experiments were undertaken. Fig. S12 (ESI†) shows the emission Cr O
spectra of Eu0:4Gd0:6DBM@Cr
2 7
2
ꢀ
2
O
7
3
different concentrations. The luminescence intensity decreased patterns could be ascribed to the small molecules attached onto
the surface of doped materials. To investigate the quenching
mechanism, the UV absorption spectra of ligands immersed in
CH COOH were recorded. As shown in Fig. S13b (ESI†), an
3
extensive absorption band ranging from 300 nm to 400 nm was
observed. CH COOH displayed a broader absorption band from
3
300 nm to 360 nm, which covered the range of the ligand,
implying that, in this emulsion, CH COOH could absorb most
3
of the excitation energy and only part of the energy could be
absorbed by the ligand. As a result, only a small amount of energy
3+
could be transferred to Ln . That is, there was competition for the
excitation energy between the ligand and solvent molecules.
3+
Moreover, Eu cannot acquire excitation energy, so slight emission
3+
from the ligand (rather than that of Eu ) was observed.
3
+
With regard to Al , according to the literature, the effect on
luminescent metal complexes caused by metal cations can be
assigned to three factors: (1) the collapse of the structure of
Fig. 7 Emission spectra of complex Eu0.4Gd0.6DBM immersed in various
anionic aqueous solutions.
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excellent energy-transfer efficiency. Furthermore, considering
regulation of this energy-transfer efficiency from ligands to
lanthanide ions, a novel and multifunctional fluorescence probe
was developed that could detect CH COOH, and the toxic ions
3
3+
2ꢀ
Al and Cr O . These luminescent materials showed high
2
7
sensitivity, and they could be promising sensors for anions, cations,
or organic small molecules. Hence, we showed, for the first time,
3+
3
that Al and CH COOH could be sensed by luminescence-based
color change using lanthanide compounds.
Conflicts of interest
3+
Fig. 9 Effect of Al from ligand-to-metal energy transfer.
There are no conflicts to declare.
metal complexes; (2) cation exchange between the central
cations of the metal complexes and target cations; (3) inter-
Acknowledgements
2
3
actions between metal cations and organic ligands. We
wished to determine the possible sensing mechanism by which
This work is financially supported by Heilongjiang Province
(KJCX201811).
3
+
Al enhances the luminescence of ligands yet decreases the
3
+
typical emission of Eu , which indicated that the antenna
affect was weakened. Thus, PXRD was utilized to ascertain if Notes and references
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+
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+
3+
3+
employed. The ratio of Al and Eu /Gd was 4.8 : 5.2 in the
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+
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ꢀ
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ꢀ
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Conclusions
A family of isostructural lanthanide compounds with visible
and NIR properties were explored and enhanced luminescence
was accomplished by doping gadolinium into other Ln analogs
(
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3+
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