Design and synthesis of two new terbium and europium complex-based luminescent probes for the selective detection of zinc ions

Article abstract of DOI:10.1002/bio.3883

Zinc plays a key role in many physiological processes and has implications for the environment. Consequently, detection of chelatable zinc ion (Zn²⁺) has attracted widespread interest from the research community. Lanthanide-based luminescent probes offer particular advantages, such as high water solubility, long luminescence lifetimes and a large Stokes’ shift, over common organic dye-based fluorescent sensors. Here, we report the synthesis of terbium and europium complex-based probes, Tb-1 and Eu-1, for sensitive and selective detection of Zn²⁺ in water. These probes featured the incorporation of bis(2-pyridylmethyl)]amine (DPA) receptor for Zn²⁺ chelation and the 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) ring to chelate lanthanide (Ln³⁺). Tb-1 and Eu-1 displayed high selectivity for Zn²⁺ ions over a wide range of competing ions, with limits of detection of 0.50 ± 0.1 μM and 1.5 ± 0.01 μM, respectively. Density functional theory simulations were in good agreement with experimental observations, displaying high Zn²⁺ selectivity compared with most competing ions. In the competing ions experiments, the luminescence response of Tb-1 and Eu-1 was moderately quenched by some ions such as Cu²⁺, this was linked to the comparable binding abilities of these ions for the receptor of the probe.

Full text of DOI:10.1002/bio.3883

Received: 28 March 2020
DOI: 10.1002/bio.3883
Revised: 6 May 2020
Accepted: 17 May 2020
R E S E A R C H A R T I C L E
Design and synthesis of two new terbium and europium
complex-based luminescent probes for the selective detection
of zinc ions
Abdul Waheed1
| Safwat Abdel-Azeim2 | Nisar Ullah1
| Sulayman A. Oladepo1
1Chemistry Department, King Fahd University
of Petroleum and Minerals, Dhahran, Saudi
Arabia
Abstract
Zinc plays a key role in many physiological processes and has implications for the
environment. Consequently, detection of chelatable zinc ion (Zn2+) has attracted
widespread interest from the research community. Lanthanide-based luminescent
probes offer particular advantages, such as high water solubility, long luminescence
lifetimes and a large Stokes’ shift, over common organic dye-based fluorescent sen-
sors. Here, we report the synthesis of terbium and europium complex-based probes,
Tb-1 and Eu-1, for sensitive and selective detection of Zn2+ in water. These probes
featured the incorporation of bis(2-pyridylmethyl)]amine (DPA) receptor for Zn2+
chelation and the 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A)
ring to chelate lanthanide (Ln3+). Tb-1 and Eu-1 displayed high selectivity for Zn2+
ions over a wide range of competing ions, with limits of detection of 0.50 0.1 μM
and 1.5 0.01 μM, respectively. Density functional theory simulations were in good
agreement with experimental observations, displaying high Zn2+ selectivity compared
with most competing ions. In the competing ions experiments, the luminescence
response of Tb-1 and Eu-1 was moderately quenched by some ions such as Cu2+, this
was linked to the comparable binding abilities of these ions for the receptor of the
probe.
2Center of Integrative Petroleum Research,
College of Petroleum Engineering and
Geosciences (CPG), King Fahd University of
Petroleum and Minerals, Dhahran, Saudi
Arabia
Correspondence
Nisar Ullah, Chemistry Department, King Fahd
University of Petroleum and Minerals.
Dhahran, Saudi Arabia.
Email: nullah@kfupm.edu.sa
Funding information
King Fahd University of Petroleum and
Minerals, Grant/Award Number: DF181029
K E Y W O R D S
europium complex, luminescent probe, terbium complex, zinc chemosensor, zinc ions
1
|
INTRODUCTION
due to the modulatory role of zinc in neurotransmission, the pathol-
ogy of disorders such as Parkinson’s disease, epilepsy, Alzheimer’s dis-
Zinc is an essential metal that plays some fundamental structural
and/or functional roles in the human body.[1] As the second most
abundant metal after iron,[2] zinc exists in its Zn2+ form in all tissues
and fluids. In multicellular organisms, zinc is predominately found
inside cells, therefore zinc is involved in structural, catalytic, and regu-
latory roles at the cellular level.[3] Zinc is considered to be nontoxic,
but its deficiency or its excess in the body can be harmful, causing
clinical conditions and adverse consequences to human health.[4] For
example, in patients with breast cancer, zinc concentrations in serum
and hair sample were found to be lower than normal.[5] In addition,
ease and cerebral ischaemia arises from disruption of Zn2+
homeostasis.[6] Imbalance of Zn2+ in the human body can result in
type 1 or type 2 diabetes.[7] Finally, excess zinc in the environment
results in decreased microbial activity, which ultimately leads to
decreased soil fertility.[8] Zinc is discharged into the environment from
industries that make alloys and protecting steel, batteries, and oxide
fillers for corrosion.[9]
In the light of the above, it is highly desirable to develop
water-soluble chemosensors that could be used to detect Zn2+ ions
for use in environmental monitoring and biological systems. Such
Luminescence. 2020;1–10.
wileyonlinelibrary.com/journal/bio
© 2020 John Wiley & Sons, Ltd.
1

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WAHEED ET AL.
sensors would enable zinc biochemistry to be deciphered, which, in
turn, is crucial for understanding its physiological and pathological
roles. However, Zn2+ is spectroscopically and magnetically inactive
due to its d[10] electron configuration, which makes its detection
complicated. Increasing levels of Zn2+ in water bodies due to the
abundant use of zinc in industry has led to the development of
numerous analytical methods for detection of Zn2+ including atomic
absorption spectroscopy (AAS), inductively coupled plasma mass
spectrometry (ICP-MS), X-ray fluorescence (XRF), atomic emission
spectroscopy (AES) and electrochemical techniques. Nevertheless,
these techniques require expensive instrumentation, have time-
consuming procedures and involve laborious sample preparation.[10]
In recent years, organic dye-based fluorescent probes due to their
high sensitivity and operational simplicity for real-time analysis
have gained significant attention for detection and quantification of
Zn2+ ions . However, these probes have short fluorescence life-
times, and show interference due to autofluorescence from the
biological matrix, undergo photobleaching, and have small Stokes’
shifts, all of which may lead to experimental errors caused by exci-
tation and light scattering.[11] In contrast, lanthanide (Ln) complex-
based luminescent probes exhibit sharp line-like emission bands
and long luminescence lifetimes.[12] Therefore, fast-decaying back-
ground signals (scattering or autofluorescence) in the nanosecond
range can be discriminated from Ln luminescence (in the microsec-
ond or millisecond range). In addition, Ln complexes generally pro-
vide tunable colours, large Stokes’ shifts, and high water
solubility.[13] Consequently, several Ln-based probes for detection
of Zn2+ ions have been studied.[14,15]
are the most frequently used chelating ligands as they form Ln
complexes with enhanced kinetic inertness and high thermody-
namic stability.[21–23]
In light of the above, we synthesized luminescent Ln-based
probes Tb-1 and Eu-1 (Figure 1) and evaluated them for selective
detection of Zn2+ ions in water. The design of these probes was
based on incorporating bis(2-pyridylmethyl)]amine (DPA) receptor
for Zn2+ chelation, and the DO3A ring to chelate Ln3+. The recep-
tor and fluorescence emission resource (DO3A ring) were con-
nected via a phenyl chromophore, which contained both π– π* and
n– π* transitions. The design of these probes was expected to
allow six-membered metal–ligand complexes in the amide tautomer
form (Figure 1). According to the ligand design concepts,[24,25] six-
membered coordination rings would increase selectivity for smaller
metal ions such as Zn2+, over larger metal ions such as Cd2+. This
in turn could result in high affinity and selectivity for Zn2+ in a
competitive medium over several other biologically or environmen-
tally important cations.
2
| EXPERIMENTAL
2.1 | Materials and instrumentation
Unless otherwise noted, reagents and solvents were used as received,
without further purification, from commercial suppliers. 1H-NMR and
13C-NMR spectra were obtained on a JEOL spectrometer (JNM-LA
500 MHz, JEOL USA Inc.), whereas infrared spectra were obtained on
a Perkin-Elmer spectrometer (16F PC FTIR, Perkin-Elmer Inc., USA).
HRMS (ESI) data were obtained on an ABSciex instrument (Germany)
(50 kV) and XPS analysis was performed on a Thermos Scientific
Escalab 250Xi spectrometer, equipped with an Al Kα (1486.6 eV)
monochromated X-ray source with 5 × 10−10 mbar base pressure in
Parker and colleagues reported 1,4,7,10-tetraazacyclododecane-
1,4,7,10-triacetic acid (DOTA) complexes for Tb3+ and Eu3+. Upon
binding with Zn2+, the luminescence of these complexes increased
to 42% and 26%, respectively.[16] However, selective competition
of these probes was not studied comprehensively. In another
report, Tuck and colleagues documented the synthesis and detec-
tion of Zn2+ by Tb-5 and Eu-6. These probes, upon complexing
with Zn2+
, gave luminescence enhancements of 2.1-fold and
3.4-fold, respectively. However, the selectivity performance of
these probes turned out to be poor, in particular for Cd2+, Fe3+
and Hg2+, and resulted in an increase in basal luminescence of the
probes.[17] Despite impressive progress, the existing probes have
numerous limitations such as selectivity, sensitivity, the ability to
cover large zinc concentration ranges, and optical compatibility
with biological samples.[18,19]
Stability of the Ln complex in solution is vital to suppress
deactivation of Ln3+ excited states nonradiatively and to avoid the
risk of toxicity resulting from free Ln3+. Similarly, the denticity or
coordination number of the chelating ligand is important to protect
the metal centre from solvent coordination. A low coordination
number for the chelating ligand would allow coordination of sol-
vent molecules, such as water, which in turn would lead to non-
radiative deactivation of excited Ln3+ by nonradiative vibrational
energy transfer to high-frequency O−H oscillators.[20] DOTA and
FIGURE 1 (a) Chemical structures of Tb-1 and Eu-1. (b) Expected
high affinity and selectivity for Zn2+
1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane
(DO3A)

WAHEED ET AL.
3
the analysis chamber. A low energy electron flood gun was used for
surface charge compensation. Spectrometer energy was calibrated by
fixing Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 peaks at binding energies of
932.6, 368.2 and 83.9 eV, respectively. Density functional theory
(DFT) calculation details along with output files are given in
Supporting Information (Computational Details). Luminescence stud-
124.83, 123.64, 60.6, 52.9, 36.6. Anal. Calcd for C17H21N3O2: C,
66.40, H, 6.32, N, 15.49. Found: C, 66.28, H, 6.45, N, 15.57.
2.2.3 | Synthesis of tert-butyl 2,20,200-
(10-(4-nitrobenzoyl)-1,4,7,10-tetraazacyclododecane-
1,4,7-triyl)triacetate (6)
ies were performed on
a
fluorescence spectrophotometer
(Edinburgh). Luminescence data were recorded in aqueous solution at
pH 7, using 20 mM HEPES buffer. Similarly, absorption spectra for
Tb-1 and Eu-1 (Figure S1) were recorded on a GENESYS 10S UV–vis
spectrophotometer.
Et3N (1.08 ml, 7.80 mmol) was added to a solution of DO3AtBu amine
5 (1.00 g, 1.94 mmol) in CH2Cl2 (15 ml) at 0ꢀC under a nitrogen atmo-
sphere. This mixture was then added dropwise to an already cold solu-
tion of 4-nitrobenzoyl chloride (0.72 g, 3.89 mmol) in THF (15 ml) at
0ꢀC. The reaction mixture was stirred overnight at room temperature.
After completion of the reaction (TLC analysis), solvents were evapo-
rated under reduced pressure and the residue dissolved in CH2Cl2
(20 ml) and then washed with water (3 × 10 ml). The organic layer
was dried over Na2SO4 and evaporated under vacuum. Column chro-
matography on silica column, followed by elution with methanol–
dichloromethane (0:100 to 5:100) gave compound 6 as a yellow oil
(1.18 g, 92%). Spectroscopic data matched previously known
materials.[26]
2.2 | Synthesis of compounds
2.2.1 | Synthesis of ethyl 3-(bis (pyridin-2-ylmethyl)
amino)propanoate (3)
K2CO3 (5.41 g, 39.1 mmol) was added to a solution of ethyl
3-aminopropionate 2 (1.0 g, 6.53 mmol) in anhydrous acetonitrile
(80 ml) at room temperature and stirred for 15 min, then
2-(bromomethyl)pyridine hydrobromide (4.12 g, 16.2 mmol) was
added and the mixture was refluxed for 6 h. After completion of the
reaction (TLC analysis), the solvent was removed under reduced pres-
sure. A dark brown oily residue was resolved using a silica column,
and eluting with hexane-ethyl acetate (50:50 to 10:90) to produce
compound 3 as a light brown oil (2.2 g, 86%). IR (KBr): 3009, 2953,
2.2.4 | Synthesis of tert-butyl 2,20,200-
(10-(4-aminobenzoyl)-1,4,7,10-tetraazacyclododecane-
1,4,7-triyl)triacetate (7)
2816, 1729, 1567, 1435, 1316, 1047, 763 cm−1
.
1H-NMR (500 MHz,
Reduced iron powder (0.46 g, 8.2 mmol), NH4Cl (0.30 g, 5.6 mmol)
and glacial acetic acid (8.0 ml) were added to a solution of compound
6 (1.0 g, 1.58 mmol) in a mixture of EtOH and H2O (2.5:1, 60 ml) and
the suspension was sonicated at 50ꢀC for 8 h. After completion of the
reaction (TLC analysis), the solid residue was filtered off, and washed
with CHCl3 (10 ml). The filtrate was added to CHCl3 (20 ml) and
washed with 2 M KOH (3 × 30 ml). The organic layer was dried over
Na2SO4 and evaporated under reduced pressure. The crude residue
was passed through a plug of a silica column to produce compound
7 as a yellow oil (0.68 g, 71%). Spectroscopic data matched previously
known materials.[26]
CDCl3): δ 8.39 (2H, d, J = 4.60 Hz, Ar-H), 7.53 (2H, t, J = 7.33 Hz, Ar-
H), 7.38 (2H, d, J = 7.63 Hz, Ar-H), 7.02 (2H, t, J = 6.10 Hz, Ar-H), 3.98
(2H, q, J = 7.02 Hz, CH2), 3.74 (4H, s, 2 x CH2), 2.84 (2H, t,
J = 6.86 Hz, CH2), 2.45 (2H, t, J = 7.0 Hz, CH2), 1.09 (3H, t, J = 7.0 Hz,
CH3). 13C-NMR (125.7 MHz, CDCl3): δ 171.98, 159.03, 148.56,
135.97, 122.55, 121.62, 59.89, 59.73, 49.55, 32.35, 13.80. Anal.
Calcd for C17H21N3O2: C, 68.20, H, 7.07, N, 14.04. Found: C,
68.10, H, 7.14, N, 13.95.
2.2.2 | Synthesis of 3-(bis (pyridin-2-ylmethyl)amino)
propanoic acid (4)
2.2.5 | Synthesis of tert-butyl 2,20,200-(10-(4-(3-(bis
(pyridin-2-ylmethyl)amino)propanamido)benzoyl)-
1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetate
(8)
1 M sodium hydroxide (13 ml, 13.0 mmol) was added to a solution of
ester 3 (0.75 g, 2.52 mmol) in a mixture of methanol and water (1:2,
15 ml) at room temperature and the reaction was stirred overnight;
solvents were evaporated under reduced pressure. Column chroma-
tography of the brown oily material, followed by elution with
methanol–chloroform (5:95 to 10:90) yielded the desired 4 as a light
brown oil (0.53 g, 78%). IR (KBr): 3365, 3001, 2948, 2801, 1714,
HOBT (0.04 g, 0.29 mmol) and Et3N (0.13 ml, 0.94 mmol) were added
sequentially to a solution of 4 (0.07 g, 0.26 mmol) in DMF (8 ml) and
the mixture was stirred at 0ꢀC for 30 min. Next, a solution of
7 (0.15 g, 0.23 mmol) in DMF (2 ml) and EDCI (0.06 g, 0.31 mmol)
were added to the reaction and stirring was continued for 48 h at
room temperature. Upon completion of the reaction (TLC analysis),
H2O (15 ml) was added and the product extracted with ethyl acetate
(3 × 20 ml). The organic layer was washed with brine (3 × 10 ml), dried
1560, 1430, 1292, 1027, 761 cm−1 1H-NMR (500 MHz, CD3OD): δ
.
(ppm) 8.39 (2H, d, J = 4.58, Ar-H), 7.76 (2H, t, J = 7.62, Ar-H), 7.58
(2H, d, J = 7.63 Hz, Ar-H), 7.24 (2H, t, J = 5.80 Hz, Ar-H), 3.78 (4H, s,
2 x CH2), 2.85 (2H, t, J = 7.32 Hz, CH2), 2.43 (2H, t, J = 7.36 Hz, CH2).
13C-NMR (125.7 MHz, CD3OD): δ 181.19, 160.28, 149.20, 138.64,

4
WAHEED ET AL.
over Na2SO4, and evaporated under reduced pressure. Column chro-
matography of the brown oily material, followed by elution with
methanol–chloroform (0:100 to 10:100) gave the product as a light
brown oil (0.14 g, 71%). IR (KBr): 3471, 3006, 2960, 2825, 1751,
centrifugation to produce Tb-1 (0.021 g, 60%) and Eu-1 (0.019 g,
55%) as a white solid. Tb-1: HRMS (ESI+): calcd for C36H43N8O8Tb
[M + H]+: 874.2457, found: 875.2516. Eu-1: HRMS (ESI+): calcd for
C
36H43EuN8O8 [M + H]+: 868.2416. Found: 869.2472 (Figures S5
1469, 1631, 1470, 1122, 923, 840, 752 cm−1 1H-NMR (500 MHz,
.
and S6).
CDCl3): δ 10.93 (1H, s, broad), 8.42 (2H, d, J = 5.1 Hz, Ar-H), 7.75 (1H,
d, J = 7.1 Hz, Ar-H), 7.50 (3H, d, J = 7.32 Hz, Ar-H), 7.22 (3H, t,
J = 7.93 Hz, Ar-H), 7.02–7.07 (3H, br. M, Ar-H), 3.82–3.2.55 (30H, m),
1.38–1.34 (27H, m). 13C-NMR (125.7 MHz, CDCl3): δ 170.79, 170.70,
157.75, 149.88, 149.34, 149.11, 149.07, 136.80, 136.72, 127.43,
123.49, 119.76, 59.73, 53.02, 51.98, 51.22, 50.49, 50.05, 34.40,
28.20, 28.14. Anal. Calcd for C48H70N8O8: C, 64.99, H, 7.95, N,
12.63. Found: C, 64.58, H, 8.05, N, 12.38.
3
| RESULTS AND DISCUSSION
3.1 | Design and synthesis of Tb-1 and Eu-1
Design of luminescent lanthanide-based probes was based on cova-
lently attaching an appropriate antenna, phenyl chromophore that
contained both π– π* and n– π* transitions, and incorporation of a
DO3A ligand to chelate the lanthanide ion. DO3A could provide ther-
modynamic stability and kinetic inertness to the lanthanide chelate.[27]
The design of these probes would permit six-membered metal–ligand
complexes in the imidic acid tautomer form (Figure 1), which in turn
would allow enhanced selectivity for Zn2+ over larger metal ions such
as Cd2+. The synthesis of luminescent probes Tb-1 and Eu-1 necessi-
tated the preparation of intermediate 4, which in turn was synthesized
by alkylation of ethyl 3-aminopropanoate 2 with 2-(bromomethyl)pyri-
dine followed by hydrolysis of the ester function under basic condi-
tions (Scheme 1).
2.2.6 | (2,20,200-(10-(4-(3-(bis (pyridin-2-ylmethyl)
amino)propanamido)benzoyl)-1,4,7,10 tetraazacyclo-
dodecane-1,4,7-triyl)triacetic acid) (9)
Compound 8 (0.1 g, 0.11 mmol) was added to an ice-cold solution of
mixture of TFA (3 ml) and H2O (1 ml) and the reaction mixture was
stirred for 72 h at 60ꢀC. After completion of the reaction (TLC analy-
sis), the volatiles were removed under reduced pressure and then co-
concentrated with methanol (3 × 3 ml). Titration with diethyl ether
produced a trifluoroacetic acid salt of the desired 9 as a light brown
solid [0.06 g, 49%, based on tris(trifluoroacetic acid) salt]. IR (KBr):
3436, 3096, 2925, 2852, 1737, 1631, 1597, 1473, 1048,
Similarly, synthesis of the DO3A-based ligand 9 was accom-
plished as depicted in Scheme 2. Acylation of DO3A-tris-tert-butyl
ester 5 with 4-nitrobenzoyl chloride gave compound 6, which was
transformed to intermediate 7 by reducing the nitro function of
the former with Fe in acetic acid. Similarly, amidation of
846, 763 cm−1 1H-NMR (500 MHz, D2O): δ 8.52 (2H, d, J = 4.90 Hz,
.
Ar-H), 8.22 (2H, t, J = 8.24 Hz, Ar-H), 7.80–7-7.67 (4H, m, Ar-H),
7.40–7.26 (2H, m, Ar-H), 4.20 (4H, s), 3.57–2.52 (26H, m). 13C-NMR
(125.7 MHz, D2O): δ 175.18, 165.60, 165.32, 155.27, 151.82, 147.65,
147.25, 145.57, 145.39, 128.22, 123.11, 58.94, 53.60, 53.23, 52.97,
52.81, 52.66, 52.56, 36.16. HRMS (ESI+): calcd for C36H46N8O8
[M + H]+: 719.3439. Found: 719.3517 (Figure S4).
intermediate
7 with acid 4 under standard conditions allowed
access to intermediate 8, which was then transformed to the
desired ligand 9 by the action of trifluoroacetic acid in water.
Finally, the reaction of ligand
9 with Tb(OTf)3 and Eu(OTf)3,
respectively, led to the synthesis of the desired Tb-1 and Eu-1 in
high yields (Scheme 2).
2.2.7 | Synthesis of Tb-1 and Eu-1
3.2 | Characterization and luminescence properties
of Tb-1
Compound 9 (0.041 g, 0.04 mmol) was dissolved in H2O (0.5 ml) and
pH was adjusted to 8 with 1 M NaOH (aq.). Next, a solution of Ln
(CF3SO3)3 (0.09 mmol) in H2O (0.5 ml) was added dropwise and the
mixture was heated at 70ꢀC for 72 h. The pH was monitored periodi-
cally and maintained at 8 with 1 M NaOH (aq.) as needed. Upon com-
pletion of the reaction, the precipitated Ln(OH)3 was removed by
centrifugation. The filtrate was lyophilized to yield the title com-
pounds as white solids, which were then dissolved in a mixture of
methanol–ethanol (2:1) and excess salts were separated by
The absorption maximum (λmax) of Tb-1 was found to be between
225 nm and 275 nm (Figure S1). Figure 2 represents the steady-state
luminescence emission of Tb-1 in the absence or presence of Zn2+
ions. It is evident that the neat Tb-1 complex is weakly luminescent.
However, upon addition of Zn2+ ions and increasing the concentration
from 0.0 to 2.0 equivalents, the luminescence intensity of Tb-1 was
enhanced to almost five-fold. These results indicated that binding of
SCHEME 1 Synthesis of
intermediate 4

WAHEED ET AL.
5
SCHEME 2 Synthesis of Tb-1 and
Eu-1
the receptor (DPA) to Zn2+ led to inhibition of the photoinduced elec-
tron transfer (PeT) process, which in turn modulated the ‘off–on’ lumi-
nescence behaviour of the Tb-1 complex, therefore affording a
distinct signal for the luminescence sensing of Zn2+. When the recep-
tor is in an unbound state (‘off-state’), the electron pair in the highest
occupied molecular orbital (HOMO) of the tertiary amine nitrogen lies
over the S0 ground state of the antenna. On photoexcitation of the
antenna to the S1 state, PeT from the receptor to the unpaired elec-
tron in the S0 ground state of the antenna occurred. PeT not only
enhanced the nonradiative decay of the photoexcited antenna but
also reduced its sensitizing efficiency, which in turn resulted in
reduced Tb3+ luminescence. Upon binding with Zn2+, the lone-pair
electrons of the receptor became engaged (‘on-state’), which lowered
its HOMO to lower than the S0 ground state of the antenna. This, in
turn, inhibited the PeT process and reclaimed the sensitizing ability of
the antenna that led to enhanced Tb3+ luminescence.[17,28,29] It is per-
tinent to mention that the luminescence pattern for Tb-1 showed four
distinctive intensity bands in the luminescence emission spectra, with
maxima at 489, 548, 586 and 622 nm. These bands were associated
with the transitions from the 5D4 excited state of terbium to the 7F6,
7F5, 7F4, and 7F3 ground states.[30] The band at 548 nm was used for
subsequent analysis.
except for Na1+and Ba2+ for which the luminescence intensity of Tb-1
slightly increased (Figure 3). Moreover, Cu2+ and Fe3+ completely
quenched the luminescence of Tb-1 (Figure 3). However, the addition
of 1 equivalent of Zn2+ to the solutions of these metal ions produced
a marked luminescence response, which suggested that Tb-1 is a
selective chemosensor for Zn2+
.
3.3 | Characterization and luminescence properties
of Eu-1
The absorption spectrum for Eu-1 was quite similar to that of Tb-1
with λmax in the region 225–275 nm. The luminescence pattern for
Eu-1 showed four distinctive bands at 579, 605, 635 and 690 nm,
which correspond to relaxation from 5D0 to 7F0, 7F1 and 7F2 and
7F3 ground states, respectively.[30,32] Luminescence intensity of Eu-1
was found to increase two-fold with the gradual increase in Zn2+
from 0.0 to 2.0 equivalents (Figure 4). Moreover, LOD for Eu-1 was
determined using the above method and was found to be
1.5 0.01 μM.
To determine the effect of various ions on luminescence of Eu-1,
various ions were added to Eu-1 at a metal cation molar ratio of 1:1
(Figure 5). The response of Zn2+ was quite vivid and obvious com-
pared with the response of other metal cations, making the sensor
quite selective for the target metal ion, i.e. Zn2+. However, most ions
caused a decrease in the basal luminescence of the Eu-1, especially
copper and iron ions. In the competing ions experiment, the response
of the sensor was least affected by Na+ and Ba2+, while Cu2+ and Fe3+
resulted in quenching of the Eu-1 response.
Limit of detection (LOD) was calculated using
a IUPAC
method.[31] By applying the above method, LOD was found to be
0.50 0.1 μM for Tb-1. To investigate the selectivity of the Tb-1 for
Zn2+ over the various metal ions, the effect of addition of various
metal ions on luminescence of Tb-1 was investigated by adding 1.0
equivalent of each metal cation (Figure 3). The effect of various metal
ions on the luminescence was investigated at pH 7.0 (20 mmol
HEPES). Most metal cations did not show any increase in basal lumi-
nescence for Tb-1, instead intensity of luminescence decreased,
XPS analysis of Tb-1 and Eu-1 was performed to confirm the
chemical composition of these complexes (Figure 6). A line shape

6
WAHEED ET AL.
FIGURE 2 Response of Tb-1 to increasing concentrations of Zn2+ (λex = 265 nm, 1 cm quartz cell, pH 7.0, 20 mM HEPES buffer). Inset shows
changes in the luminescence intensity measured at 548 nm
FIGURE 3 Changes in the luminescence of
100 μM Tb-1 (λex = 265 nm, λem = 548 nm, 1 cm
quartz cell, pH 7.0, 20 mM HEPES buffer) upon
addition of 1.0 equivalent of different metal ions
in the presence or absence of Zn2+
analysis with Gaussian fitting was applied to draw plots from the
XPS spectra. A wide scan survey analysis of Tb-1 (Figure 6a) and
Eu-1 (Figure 6b) exhibited peaks at 1240 eV and 1140 eV due to
binding energies by Tb3+ (3d5/2) and Eu3+ (5d3/2), respectively. In
addition, high-resolution XPS analysis showed multiplet structures
in Tb3+ (3d5/2 (Figure S2a)32 and Eu3+ (5d3/2
) ) (Figure S2b)
regions.33 A multiplet structure is indicative of the high binding
energies of Tb3+ and Eu3+ with the ligand. Detailed XPS analysis of
Tb-1 and Eu-1 for the binding energies of nitrogen, oxygen and
carbon is given (Figure S3).

WAHEED ET AL.
7
FIGURE 4 Response of Eu-1 to increasing concentrations of Zn2+ (λex = 285 nm, 1 cm quartz cell, pH 7.0, 20 mM HEPES buffer). Inset shows
changes in the luminescence intensity measured at 610 nm
FIGURE 5 Changes in luminescence of
200 μM Eu-1 (λex = 280 nm, λem = 610 nm, 1 cm
quartz cell, pH 7.0, 20 mM HEPES buffer) upon
addition of 1 equivalent of different metal ions in
the presence or absence of Zn2+
3.4 | Computational studies
only, as the lanthanide part is thermodynamically and kinetically stable
and not involved in metal binding. For this purpose, representative
ions such as Na+, Zn2+, Cu+ and Cu2+ were selected. It is pertinent to
mention that when metal solvation free energies were calculated, the
solvation model based on density (SMD) did not reproduce the
DFT simulations were employed to explain the effect of various ions
on the luminescence of Tb-1 and Eu-1. For ease and simplicity, DFT
calculations were limited to the metal–receptor part of the sensors

8
WAHEED ET AL.
FIGURE 6 X-ray photoelectron spectroscopy (XPS) analysis of (a) Tb-1 and (b) Eu-1 indicating peaks in the region of Tb3+ and Eu3+
FIGURE 7 Optimized structures of metal–receptor complexes for Zn2+, Na+, Cu+, and Cu2+ ions are shown as sticks and ions as balls.
Coordination distances are reported in Å. Two unbound states of the antenna molecule are also shown
experimental values. Therefore, to get reliable metal binding free
energies, atomic radii were optimized in the continuum solvent model
(CSM) by minimizing the difference between the calculated solvation
energy and the experimental counterpart (Table S1). After, minimizing
the error of the metal solvation free energies, optimized parameters
were then used to calculate the solvation free energies of the metal
complexes.34 The optimized structures (Figure 7) adopted a distorted
trigonal bipyramid geometry, except for Cu+, which assumed a square
planar structure. The average metal–ligand distance among Zn2+, Cu+
or Cu2+ did not change significantly (2.10 Å for Zn2+, 2.00 Å for Cu2+
,
TABLE 1
Metal binding free energies of our considered metals to
and 2.01 Å for Cu+). However, Na+ (2.45 Å) displayed a large metal–
ligand distance due to differences in its ionic radii. The Cu+ complex
(square planar) in the Cu2+ electronic state (charge = 2, spin = doublet)
was also re-optimized, which resulted in a trigonal bipyramid geome-
try. This indicates that trigonal and square planar are intrinsic geome-
tries of Cu2+ and Cu+ complexes, respectively, and not due to the
initial structures.
the receptor part of the Ln complex. Total free energy, gas phase, and
solvation (including ligand strain energy with 1.89 kcal/mol as a
correction added to the free energy to adjust the value from 1.0 atm
to the 1.0 M standard) are reported in kcal/mol
Metal ions
Na(I)
Δ Egas
Δ ESol.
87.1
Δ ETotal
−18.0
−64.1
−74.4
−37.5
−105.1
−414.2
−439.1
−154.9
Zn(II)
350.1
364.7
117.4
The metal binding free energies trend (Table 1 and Figure 8)
suggested favourable binding for the selected metal ions to the recep-
tors of Tb-1 and Eu-1. It is worth mentioning that the current study
Cu(II)
Cu(I)

WAHEED ET AL.
9
FIGURE 8 Total receptor–metal binding free
energies for Na(I), Cu(I), Cu(II), and Zn(II) ions, gas
phase, and solvation contributions are shown
was directed towards relative binding free energy rather than absolute
binding free energy. In fact, metal–ligand binding events possess a
complex potential energy surface, which could not be deduced by sin-
gle configuration calculations. However, the driving force of metal–
receptor binding turned out to be electrostatic interactions and dehy-
dration free energies, which allowed preference towards various metal
bindings to the receptor. Despite favourable binding in the gas phase,
which is mainly electrostatic in nature, metal dehydration free ener-
gies are the main barrier for metal binding. Furthermore, ligand strain
energy (taken as the energy difference between bound and unbound
receptor structures), was found to be 7.5 cal/mol, with changes less
than 0.2 kcal/mol among all the metal ions. In addition, the vibrational
correction did not exceed 0.7 kcal/mol. Based on the relative binding
energies (Table 1), the DPA receptor exhibited selectivity toward Zn2+
over Na+ and Cu+ ions.
such as presence of DPA and amide linkage for binding the target
metal i.e. Zn2+. The sensors had a cyclene analogue, which pos-
sessed carboxylic groups, a perfect ligand for binding Tb3+ and Eu3+
.
Both sensors did not show an appreciable increase in basal lumines-
cence when applied to other competing ions including Na+, K+,
Ca2+, Mg2+ and Ba2+. DFT simulations predicted that the newly syn-
thetized sensors displayed high Zn2+ selectivity over other compet-
ing ions. Moreover, theoretical calculations also suggested the
participation of carbonyl function along with the three nitrogen
atoms of DPA in binding to Zn2+, leading to the formation of a six-
membered ring in coordination with Zn2+ having a slightly distorted
trigonal bipyramidal geometry. Therefore, sensors can be easily uti-
lized for tracking Zn2+ in aqueous medium with high sensitivity and
very low limits of detection.
Cu2+ binding was found to be comparable with that of Zn2+ due
to their similar ionic radii. Significant charge transfer from the receptor
to the metal, as for Cu2+, is the origin of such a strong interaction.
Mullikan charge analysis of the two complexes revealed charges of
0.36e and 0.03e for Zn2+ and Cu2+, respectively (see Table S2). Higher
charge transfer from ligand to Cu2+ was presumably due to unfilled d
orbitals.[35] Nevertheless, the higher selectivity for Zn2+ predicted by
DFT simulation was in complete agreement with the experimental
observation for the competing ions experiments (Figures 3 and 5).
Tb-1, and Eu-1 exhibited high selectivity for Zn2+ over most of
the competing ions in the competition experiment. However, partial
luminescence quenching was observed by addition of some compet-
ing ions such as Cu2+ and Fe3+, and was attributed to their competing
binding ability for the receptor.
ACKNOWLEDGEMENTS
Financial support from the King Fahd University of Petroleum & Min-
erals project # DF181029 is gratefully acknowledged.
ORCID
Abdul Waheed
https://orcid.org/0000-0002-5603-5507
https://orcid.org/0000-0002-8172-2771
Nisar Ullah
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In summary, we successfully synthesized excellent and promising
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Additional supporting information may be found online in the
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How to cite this article: Waheed A, Abdel-Azeim S, Ullah N,
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