A novel ruthenium (II) polypyridyl complex bearing 1,8-naphthyridine as a high selectivity and sensitivity fluorescent chemosensor for Cu2+ and Fe3+ ions

Article abstract of DOI:10.3390/molecules24224032

A novel ruthenium(II) polypyridyl complex bearing 1,8-naphthyridine was successfully designed and synthesized. This complex was fully characterized by EI-HRMS, NMR, and elemental analyses. The recognition properties of the complex for various metal ions were investigated. The results suggested that the complex displayed high selectivity and sensitivity for Cu²⁺ and Fe³⁺ ions with good anti-interference in the CH3CN/H2O (1:1, v/v) solution. The fluorescent chemosensor showed obvious fluorescence quenching when the Cu²⁺ and Fe³⁺ ions were added. The detection limits of Cu²⁺ and Fe³⁺ were 39.9 nmol/L and 6.68 nmol/L, respectively. This study suggested that this Ru(II) polypyridyl complex can be used as a high selectivity and sensitivity fluorescent chemosensor for Cu²⁺ and Fe³⁺ ions.

Full text of DOI:10.3390/molecules24224032

molecules
Article
A Novel Ruthenium(II) Polypyridyl Complex Bearing
1,8-Naphthyridine as a High Selectivity and
Sensitivity Fluorescent Chemosensor for Cu2+ and
Fe3+ Ions
Chixian He 1,2 , Shiwen Yu 2, Shuye Ma 3, Zining Liu 1, Lifeng Yao 2, Feixiang Cheng 1,2,* and
Pinhua Liu 2
1
Center for Yunnan-Guizhou Plateau Chemical Functional Materials and Pollution Control,
Qujing Normal University, Qujing 655011, China; cxhe@mail.qjnu.edu.cn (C.H.); LZning151@163.com (Z.L.)
College of Chemistry and Environmental Science, Qujing Normal University, Qujing 655011, China;
2
zlq9941@163.com (S.Y.); leoyao1982@163.com (L.Y.); lph13099867071@sina.com (P.L.)
Department of Medicine, Qujing Qilin Vocational and Technical School, Qujing 655000, China;
3
msy_1009@163.com
*
Correspondence: chengfx2019@163.com; Tel.: +86-0874-099-8658
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Victor Mamane and John S. Fossey
Received: 26 September 2019; Accepted: 31 October 2019; Published: 7 November 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: A novel ruthenium(II) polypyridyl complex bearing 1,8-naphthyridine was successfully
designed and synthesized. This complex was fully characterized by EI-HRMS, NMR, and elemental
analyses. The recognition properties of the complex for various metal ions were investigated.
The results suggested that the complex displayed high selectivity and sensitivity for Cu2+ and Fe3+
ions with good anti-interference in the CH3CN/H2O (1:1, v/v) solution. The fluorescent chemosensor
showed obvious fluorescence quenching when the Cu2+ and Fe3+ ions were added. The detection
limits of Cu2+ and Fe3+ were 39.9 nmol/L and 6.68 nmol/L, respectively. This study suggested that this
Ru(II) polypyridyl complex can be used as a high selectivity and sensitivity fluorescent chemosensor
for Cu2+ and Fe3+ ions.
Keywords: Ru(II) polypyridyl complex; 1,8-naphthyridine; fluorescence; chemosensor; Cu2+; Fe3+
1. Introduction
With the development of agriculture and industry, the application of heavy and transition metals
becomes more and more prevalent, which leads to these kinds of metal ions entering ecological and
environmental systems. These metal ions have posed significant damage to living organisms even at
per million concentrations due to their toxicity, accumulation, and low rate of clearance, especially to
the health of human beings [
element in the organism. It is involved in several biological processes, such as oxygen transport,
cellular respiration, enzymatic catalysis regulation of transcription, DNA repair [ ], and so on.
1–4
]. Among these metal ions, Fe3+ is the most abundant essential trace
5–8
Similarly, Cu2+ is another indispensable element in the human body after Fe3+. It is required for a
number of physiological activities of the organism including enzymatic catalysis, nerve conduction,
and hematopoiesis. Either the deficiency or excess of Fe3+ and Cu2+ can disturb the balance of cellular
systems and metabolism. For example, exposure to high levels of Fe3+ and Cu2+ can cause many
diseases, such as Menkes disease, Alzheimer’s disease, Wilson disease, gastrointestinal disorders,
kidney damage, hepatitis, cancer, and neurodegenerative disease [
recognition of Fe3+ and Cu2+ have very important practical significance. The development of single
molecular light-up probes for Fe3+ and Cu2+ has received increasing attention in recent years [12
13].
9–11]. Therefore, the detection and
,
Molecules 2019, 24, 4032; doi:10.3390/molecules24224032
www.mdpi.com/journal/molecules

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Wang et al. reported a rhodamine B hydrazine derivative that had a sufficiently satisfactory selective
response to Fe3+ and Cu2+
[
14]. Ajayakumar et al. designed and synthesized a new Rubp-Ptz in
2+
which the phenothiazine moiety was covalently linked to one of the bipyridine units of Ru(bpy)3
.
2+
Excitation of Ru(bpy)3 led to electron transfer from the phenothiazine moiety to the metal to
ligand charge transfer (MLCT ) excited state of Ru(bpy)32+, which resulted in efficient quenching
of the luminescence. The phenothiazine moiety of Rubp-Ptz can be oxidized to a stable entity by
2+
Cu2+. The new product is incapable of electron donation to the MLCT excited state of Ru(bpy)3
.
2+
The emission of the Ru(bpy)3 moiety is restored [15]. Qian et al. synthesized five coumarin
derivatives, in which two compounds exhibited selectivity towards Cu2+
[16]. Wang et al. reported
a terthiophene-derived colorimetric and fluorescent dual-channel sensor terthiophene-phenylamine
TTA, which showed a significant fluorescence “turn-on” response to Fe3+ and an obvious fluorescence
“turn-off” response to Cu2+ 17]. Although a large number of luminescence probes for Fe3+ and
[
Cu2+ have been reported, most of them have disadvantages such as low sensitivity, poor selectivity,
poor water solubility, and so on. Hence, the design and development of highly selective, sensitive,
and water soluble chemosensors are very important.
Ru(II) complexes and in particular Ru(II) polypyridyl complexes have received much attention
due to their potential application in the field of solar energy conversion [18
water-oxidation catalyst [25 26], and luminescence sensing [27 28]. Similarly, they are applied in
32] owing to their low cytotoxicity, redox stability,
–22], photocatalysis [23,24],
,
,
photodynamic therapy [29] and bio-probes [30
–
and water solubility.
Significant research has focused on the derivatives of 1,8-naphthyridine due to their interesting
complexation properties and application. They can be used in coordination chemistry as monodentate
and bidentate ligands [33]. The derivatives of 1,8-naphthyridine have been used as molecular sensor
for transition and heavy metals, carboxylic acids, and guanine [34]. 1,8-Naphthyridine derivatives are
remarkable fluorescent markers for nucleic acids [35,36].
Taking advantage of the excellent luminescence properties of Ru(II) polypyridyl complexes
and the coordination properties of 1,8-naphthyridine derivatives, we envision that the Ru(II)
polypyridyl complexes bearing 1,8-naphthyridine derivatives can also be used as molecular probes.
For the aforementioned reason, we propose to synthesize a Ru(II) polypyridyl complex containing
1,8-naphthyridine derivatives. It can be used as a luminescence probe for Fe3+ and Cu2+ ions.
2. Experiment
2.1. Materials
All the solvents and reagents (analytical grade and spectroscopic grade) were purchased from
reagent companies and used without further purification. 2,7-Dimethly-1,8-naphthyridine [37,38] and
cis-Ru(bpy)2Cl2·2H2O [39] were obtained according to the literature procedures.
2.2. Instrumentation
All of these compounds were characterized by 1H-NMR, MS, and elementary analysis.
The 1H-NMR and 13C-NMR experiments were taken using a Mercury Plus 400 MHZ spectrometer
(Bruker, Karlsruhe, Germany). The chemical shifts (δ, ppm) were acquired relative to tetramethylsilane
(TMS). ESI-MS spectra were recorded by using the Bruker amaZon SL mass spectrometer (Bruker,
Karlsruhe, Germany). ESI-HRMS spectra were taken on a Bruker Daltonics APEXII47e mass
spectrometer (Bruker, Karlsruhe, Germany). The C, H, and N microanalyses were carried out
on a Perkin-Elmer 240C analytical instrument (PerkinElmer, Norwalk, CT, USA). FTIR spectra were
taken in the range of 4000–400 cm−1 on a Thermo Nicolet AVATAR 360 FTIR spectrometer (Thermo
Fisher Scientific, Waltham, MA, USA) using KBr pellets. Electronic absorption spectra and emission
spectra were taken on a Varian Cary-100 UV-Visible spectrophotometer (Varian, Palo Alto, CA, USA)
and a Hitachi F-4600 fluorescence spectrophotometer (Hitachi, Tokyo, Japan), respectively, using a

Molecules 2019, 24, 4032
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10 mm path length colorimetric ware. All theoretical calculations were taken using the Gaussian
09 program package in this study (Gaussian, Wallingford, CT, USA). The structure of complex
[{Ru(bpy)2}2(µ2-H2L)](PF6)4 was optimized using the Becke-3-Lee-Yang-Parr in conjunction with the
6-311+G* basis set for C, N, and H atoms and the LANL2DZ inclusion effective core potential (ECP)
for the ruthenium atom [40]. All the optimized stationary points were identified as minima (zero
imaginary frequencies) via the vibrational analyses at the same level.
2.3. Synthesis of 1,8-Naphthyridine-2,7-Dicarbaldehyde
1,8-Naphthyridine-2,7-dicarbaldehyde was synthesized with a minor change according to the
literature [37,41]. A mixture of selenium dioxide (1.11 g, 10 mmol) in anhydrous 1,4-dioxane (10 mL)
was heated to 110 °C under a nitrogen atmosphere, then 2,7-dimethly-1,8-naphthyridine was added
(316 mg, 2.0 mmol). Stirring was continued for two hours and hot filtered. Chloroform (25 mL) was
added to the filtrate and washed with water (3
5% sodium bicarbonate (aq) (25 mL), dried over anhydrous Na2SO4, and concentrated in vacuo.
×
10 mL). The combined organic was washed with
This residue was redissolved in hot ethyl acetate (25 mL), hot filtered, giving a pale brown powder.
1
Yield: 214 mg (58%). H-NMR (400 MHz, DMSO-d6)
δ
= 8.28 (d, J = 8.0 Hz, 2H), 8.52 (d, J = 8.0 Hz,
2H), 10.40 (s, 2H).
2.4. Synthesis of Compound L
A mixture of 5-amino-1,10-phenanthroline (585 mg, 3.0 mmol) and 1,8-naphthyridine-2,7-dicarbaldehyde
(186 mg, 1.0 mmol) in anhydrous EtOH (80 mL) was heated at 80 °C, then 10 drops of HOAc were added.
The mixture was refluxed for 24 h. A suspension was obtained. The yellow precipitate was collected
by filtration, washed with hot EtOH, and dried in vacuo. Yield: 463 mg (88%) of a yellow solid.
1H-NMR (400 MHz, DMSO-d6)
δ = 7.79–7.91 (m, 6H), 8.55–8.57 (m, 2H), 8.76–8.79 (m, 6H), 8.96–8.99
(m, 2H), 9.13–9.17 (m, 4H), 9.24–9.26 (m, 2H). ESI-MS m/z = 540.65 [M + H]+, 562.21 [M + Na]+.
Theoretical exact mass: m/z = 540.18 [M + H]+, 563.17 [M + Na]+. Found: C, 74.4; H, 3.8; N, 20.8.
Calculated for C34H20N8: C, 74.5; H, 3.7; N, 20.7%.
2.5. Synthesis of H2L
L (324 mg, 0.60 mmol) was dissolved in anhydrous CHCl3-EtOH (200 mL, 1:1, v/v), in which
NaBH4 (228 mg, 6.0 mmol) was added. The solvent was stirred at room temperature for 72 h,
then washed with distilled water (50 mL
CH2Cl2, then the combined organic layers were dried over anhydrous Na2SO4. The organic solvent
was evaporated in vacuo, then the crude product was purified by column chromatography on SiO2
×
3). The combined aqueous layers were extracted with
1
(eluent: v(CH2Cl2)/v(EtOH ) = 10:1), giving a yellow solid. Yield: 130 mg (40%). H-NMR (400 MHz,
DMSO-d6)
δ = 5.03 (s, 4H), 6.7 (s, 2H), 7.37 (s, 2H), 7.47 (dd, J = 4.0, 8.0 Hz, 2H), 7.73–7.78 (m,
4H), 7.95 (d, J = 8.0 Hz, 2H), 8.21 (d, J = 8.0 Hz, 2H), 8.78 (d, J = 4.0, 2H), 8.90 (d, J = 8.0 Hz, 2H),
9.16 (d, J = 4.0 Hz, 2H). ESI-MS m/z = 544.64 [M + H]+. Theoretical exact mass: m/z = 544.21 [M + H]+
.
Found: C, 75.1; H, 5.3; N,19.6. Calculated for C34H24N8: C, 75.0; H, 4.4; N, 20.6%.
2.6. Synthesis of [{Ru(bpy)2}2(µ2-H2L)](PF6)4
Under a nitrogen atmosphere, a mixture of cis-Ru(bpy)2Cl2 2H2O (207 mg, 0.40 mmol) and H2L
·
(107 mg, 0.20 mmol) in 100 mL ethylene glycol in a three necked flask was heated to 150 °C for 12 h
to give a deep red solution. The solvent was evaporated in vacuo. The crude product was purified
twice by column chromatography on Al2O3, first using CH3CN and EtOH (8:1, v/v) as the eluant,
then using EtOH to obtain the complex [{Ru(bpy)2}2(µ2-H2L)]Cl4. This complex was dissolved in a
minimum amount of water, then the saturated aqueous NH4PF6 was added. The product was collected
by vacuum filtration and recrystallized from an acetone/diethyl ether mixture, giving a deep red
1
powder. Yield: 117 mg (30%). H-NMR (400 MHz, DMSO-d6)
δ = 4.98 (d, J = 4.0 Hz, 4H), 6.94 (s, 2H),
7.37 (t, J = 8.0 Hz, 4H), 7.53–7.63 (m, 12H), 7.75 (d, J = 8.0 Hz, 2H), 7.80 (d, J = 4.0 Hz, 2H), 7.85 (d, J =

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4.0 Hz, 2H), 7.90–7.94 (m, 2H), 8.10–8.14 (m, 6H), 8.18–8.24 (m, 6H), 8.31 (s, 2H), 8.43 (d, J = 8.0 Hz, 2H)
,
8.82–8.90 (m, 8H), 9.11 (d, J = 8.0 Hz, 2H). 13C-NMR (400 MHz, DMSO-d6) δ = 49.6, 124.4, 124.9, 125.7,
126.7, 128.3, 131.9, 132.9, 134.2, 138.2, 138.4, 138.8, 141.5, 147.2, 148.2, 151.7, 151.9, 152.0, 152.5, 157.0, 157.1,
157.3, 163.4. ESI-HRMS: m/z = 343.0735 [M
−
4PF6]4+
,
505.7527 [M − 3PF6]3+
,
831.1112 [M – 2PF6]2+
,
,
1087.1864 [M – PF6]+. Theoretical exact mass for C74H56F24N16P4Ru2: m/z = 343.0737 [M − 4PF6]4+
505.7535 [M
−
3PF6]3+, 831.1132 [M
−
2PF6]2+, 1086.1885 [M
−
PF6]+. Found: C, 65.1; H, 4.0; N, 16.7.
Calculated for C74H56F24N16P4Ru2: C, 64.81; H, 4.1; N, 16.34%. IR (KBr, cm−1) 3445br 1624m, 1528m,
1465m, 1434m, 1312w, 1241w, 1169w, 1047w, 837s, 764m, 728w, 558m.
3. Result and Discussion
3.1. Synthesis
A novel dinuclear Ru(II) polypyridyl complex containing 1,8-naphthyridine was first designed
and synthesized. The target complex was prepared in four steps (Scheme 1). The starting material
2,7-methyl-1,8-naphthyridine was synthesized according to the literature [37,38]. In the first step,
2,7-methyl-1,8-naphthyridine was oxidized to dialdehyde by the selenium dioxide. The Schiff base was
obtained in good yield with the reaction of dialdehyde and 5-amino-1,10-phenanthroline. The Schiff
base was then reduced by sodium borohydride to give the ligand H2L. All the organic compounds were
purified by column chromatography. cis-[Ru(bpy)2Cl2] H2O reacted with the ligand H2L to give the
·
complex [{Ru(bpy)2}2(µ2-H2L)]Cl4. Then, the Cl− ion of [{Ru(bpy)2}2(µ2-H2L)]Cl4 was exchanged by
the PF6− of saturated aqueous NH4PF6 to obtain [{Ru(bpy)2}2(µ2-H2L)](PF6)4. The 1H-NMR, 13C-NMR,
and MS spectra of the complex were consistent with the expected structure.
Scheme 1. Synthesis of the ligand H2L and Ru(II) complex.
3.2. Computational Studies
The simulations of the HOMO and LUMO electron density distribution in the frontier molecular
orbitals of the complex are shown in Figure 1. It was observed that the LUMO and LUMO + 1
were distributed mostly on the 2,20-bipyridine and 1,10-phenanthroline unit and that the LUMO + 2
was localized primarily on the two 2,20-bipyridine units, but the HOMO orbital had amplitudes
on the 1,10-phenanthroline unit in one of the Ru centers. The electron distribution of HOMO and
LUMO provided strong evidence of the photoinduced electron transfer (PET) process in the new
complexes [42–44].

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Figure 1. Electron distribution and energy diagram of HOMO and LUMO orbitals for the complex
[{Ru(bpy)2}2(µ2-H2L)]4+.
3.3. Selectivity of Complex to Metal Ions
The selectivity of the complex to metal ions was evaluated with fluorescence experiments.
The fluorescence experiments were recorded with a concentration of 10−5 mol/L in the presence of
tetrabutyl perchlorate amine (TBAP, 0.1 mol/L) in CH3CN/H2O (1:1, v/v) at room temperature, then 5.0
equivalents of nitrate or perchlorate salts of metal ions (Ag+, Ca2+, Co2+, Ba2+, Cr3+, Cd2+, Ni2+, Li+,
Na+, Mg2+, Zn2+, Mn2+, Fe3+, Fe2+, Pb2+, Hg2+, and Cu2+) were added to the solution, respectively.
The fluorescence properties are recorded and compared in Figure 2. As shown in Figure 2, the complex
showed a strong fluorescence at 612 nm. However, the fluorescence of the solution was quenched to
48 a.u. and 23 a.u., respectively, when Cu2+ and Fe3+ were added. The quenching of the fluorescence
intensities with Pb2+ and Hg2+ was small. All other metal ions tested did not cause any significant
change to the luminescence intensity of the solution. The phenomena of the fluorescence indicated
that the complex had a selectivity for Cu2+ and Fe3+ ions. Therefore, it can be used as a “turn-off”
fluorescent chemosensor for Cu2+ and Fe3+ in the CH3CN/H2O media. This dramatic quenching of the
initial luminescence of RuL induced by Cu2+ and Fe3+ was due to the reverse photoinduced electron
transfer (PET) from the 2,20-bipyridine ruthenium (II) moieties to the 1,8-naphthyridine nitrogen atom
because of the decrease in the electron density upon Cu2+ and Fe3+ complexation [45]. In order to
further evaluate the selectivity of [{Ru(bpy)2}2(µ2-H2L)](PF6)4 for detecting Cu2+ and Fe3+, competitive
experiments were carried out at room temperature. Five equivalents of Cu2+ and Fe3+ were added to
the solution of the complex (CH3CN/H2O = 1:1, v/v), respectively. Then, the other metal ions (Ag+,
Ca2+, Co2+, Ba2+, Cr3+, Cd2+, Ni2+, Li+, Na+, Mg2+, Zn2+, Mn2+, Fe2+, Pb2+, Hg2+) were added to
the mixed solution, respectively (Figure 3; Figure 4). As shown in Figures 3 and 4, the fluorescence
of the complex with Cu2+ had no obvious influence when the other metal ions existed in the system.
However, Ca2+, Cr3+, and Zn2+ had a small influence, and the other ions had no obvious influence in
the Fe3+ ion experiments. Therefore, the complex can be used as an excellent fluorescent chemosensor
for detecting Cu2+ and Fe3+ ions [14,46].

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Ag+, Ca2+, Co2+, Ba2+, Cr3+,Cd2+,
Ni2+, Li+, Na+, Mg2+, Zn2+, Mn2+
1200
[{Ru(bpy)2}2(μ2-H2L)]4+
Fe2+
1000
800
600
400
200
0
Pd2+
Hg2+
Cu2+
Fe3+
500
550
600
650
700
750
800
wavelength/nm
Figure 2. Fluorescence spectra of the complex [{Ru(bpy)2}2(µ2-H2L)]4+ (10−5 mol/L in CH3CN/H2O =
1:1, v/v, excitation wavelength = 450 nm) upon the addition of 5.0 equivalents of various metal ions at
room temperature.
Complex + other metal ions + Cu2+
Complex + other metal ions
Complex + Cu2+
1200
1000
800
600
400
200
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Figure 3. Changes in the fluorescence intensity of the complex [{Ru(bpy)2}2(µ2-H2L)]4+ (10−5 mol/L in
CH3CN/H2O = 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Cu2+ in the presence of
other metal ions. The red bars represent the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ upon the
addition of 5.0 equiv. of Cu2+ in the presence of: 1 Cd2+, 2 Pb2+, 3 Ca2+, 4 Mg2+, 5 Zn2+, 6 Hg2+, 7 Fe2+
8 Ba2+, 9 Ni2+, 10 Li+, 11 Mn2+, 12 Cr3+, 13 Na+, 14 Co2+, 15 Ag+, 16 Fe3+; the green bars represent
the changes in the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ upon the addition of other metal
ions, respectively. The blue bar represents the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ when
5.0 equiv. of Cu2+ were added.
,

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Complex + other metal ions + Fe3+
Complex + other metal ions
Complex + Fe3+
1200
1000
800
600
400
200
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Figure 4. Changes in the fluorescence intensity of the complex [{Ru(bpy)2}2(µ2-H2L)]4+ (10−5 mol/L in
CH3CN /H2O = 1:1, v/v, excitation wavelength = 450 nm) toward Fe3+ in the presence of other metal
ions. The red bars represent the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ when 5.0 equiv. of
Fe3+ were added in the presence of: 1 Ag+, 2 Ba2+, 3 Ca2+, 4 Cd2+, 5 Co2+, 6 Cr3+, 7 Fe2+, 8 Hg2+, 9 Li+,
10 Mg2+, 11 Mn2+, 12 Na+, 13 Ni2+, 14 Pb2+, 15 Zn2+, 16 Cu2+; the green bars represent the changes
in fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ when other metal ions were added, respectively.
The blue bar represents the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)]4+ when 5.0 equiv. of Fe3+
were added.
3.4. Sensitivity of Complex to Cu2+ and Fe3+ Ions
Fluorescence titration experiments for the complex with the progressive addition of Cu2+ and
Fe3+ ions were carried out. As shown in Figures 5 and 6, the fluorescence intensity of the complex
gradually decreased at 612 nm with an increasing concentration of Cu2+ and Fe3+ ions from 0 to 1.5
equivalents. The metal ions combined with the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4, which made the
electron transfer process easier than in the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4 [46
–48]. There was a
good linear relationship between the fluorescence intensity and the concentration of Cu2+ ions with
the correlation coefficient of R = 0.9987 when the concentration of Cu2+ varied from 5
to 10−5 mol/L (Figure 7). There was a linear relationship between the fluorescence intensity and the
concentration of Fe3+ ions with the correlation coefficient of R = 0.9501 when the concentration of Fe3+
×
10−7 mol/L
varied from 6.25
×
10−6 mol/L to 8.0
×
10−6 mol/L (Figure 8). Furthermore, the equation: 3
σ/k (where σ
is the standard deviation of blank measurements, k is the slope between intensity vs. the concentration
of ions) was used to obtain the detection limit of Cu2+ and Fe3+ 49]. The detection limits of Cu2+
[48,
and Fe3+ were 39.9 nmol/L and 6.68 nmol/L, which were far lower than the maximum allowable level
of the WHO for drinking water. The results suggested that the complex was potentially applicable for
quantitative analysis of Cu2+ and Fe3+ ions in environmental and biological systems.

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1200
1000
800
600
400
200
0
1000
800
600
400
200
0
0 equiv.
Cu2+
1.6 equiv.
0.0
0.2
0.4
0.6
Cu2+ equiv
0.8
1.0
1.2
1.4
1.6
550
600
650
700
750
800
Wavelength (nm)
Figure 5. Emission spectra of the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (10−5 mol/L in CH3CN/H2O
= 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Cu(ClO4)2 (0–1.0 equiv.) at room
temperature; the plot of emission intensity at 612 nm vs. the added Cu2+ equivalents is shown in
the inset.
1200
1200
0 equiv
1000
800
1000
600
400
1 equiv
200
800
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Fe3+ equiv.
600
400
200
0
500
550
600
650
700
750
800
wavelenght (nm)
Figure 6. Emission spectra of the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (10−5 mol/L in CH3CN/H2O
= 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Fe(ClO4)3 (0–1.6 equiv.) at room
temperature; the plot of emission intensity at 612 nm vs. the added Fe3+ equivalents is shown in
the inset.

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Y = -8.432 x 107X+965.1
R2 = 0.9987
1000
800
600
400
200
0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
0.0
Figure 7. Plot of linear fitting for emission intensity vs. the concentration of Cu2+ (mol/L).
Y=-5.036 x 108X + 4222.9
R2=0.9501
1000
800
600
400
200
6.2x10-6 6.4x10-6 6.6x10-6 6.8x10-6 7.0x10-6 7.2x10-6 7.4x10-6 7.6x10-6 7.8x10-6 8.0x10-6
Figure 8. Plot of linear fitting for emission intensity vs. the concentration of Fe3+ (mol/L).
3.5. Mode of Binding with Cu2+ and Fe3+
In order to explore the binding stoichiometry of Cu2+ and Fe3+ with [{Ru(bpy)2}2(µ2-H2L)](PF6)4,
Job’s plot analyses were used to determine binding stoichiometries. The analysis results suggested that
[{Ru(bpy)2}2(µ2-H2L)](PF6)4 binds with Cu2+ and Fe3+ in 3:2 and 1:1 stoichiometry ratios, respectively
(Figures 9 and 10). The ESI-MS experiments were further carried out by adding 2.0 equivalents of Cu2+
and Fe3+ to the acetonitrile solution of [{Ru(bpy)2}2(µ2-H2L)](PF6)4, respectively (Figures S9 and S10)
.
The peaks at m/z = 424.33 and 646.05 were assigned to [M
−
12PF6
−
4ClO4
−
6H]10+ and
12PF6 4ClO4
[M − 12PF6 − ClO4 − 6H]7+, respectively. The calculated molecular weights of [M
−
−
−
6H]10+ and [M
−
12PF6
−
ClO4
−
6H]7+ were 423.70 and 647.65, respectively. The M was
[({Ru(bpy)2}2(µ2-H2L))3Cu2](PF6)12(ClO4)4 (Figure S11a). The peaks at m/z = 254.40 and 335.41
were attributed to [M
molecular weights of [M
−
4PF6
−
2ClO4]6+ and [M
2ClO4]6+ and [M
−
3PF6
3PF6
−
2ClO4]5+, respectively. The calculated
2ClO4]5+ were 254.53 and 334.43. The M
−
4PF6
−
−
−
was [({Ru(bpy)2}2(µ2-H2L))Fe](PF6)4(ClO4)3 (Figure S11b), respectively. This indicated that the ratios
of [{Ru(bpy)2}2(µ2-H2L)](PF6)4 bound with Cu2+ and Fe3+ were 3:2 and 1:1, respectively. The results
were consistent with Job’s plot analyses.

Molecules 2019, 24, 4032
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0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
0.0
0.1
0.2
0.3
0.4
2+
0.5
0.6
0.7
0.8
0.9
1.0
2+
X=[[Cu ]/{[Cu ]+[{Ru(bpy) } (µ -H L)](PF ) ]}
2 2 6 4
2
2
Figure 9. Job’s plot in CH3CN/H2O (v/v, 1:1) of the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4 and
Cu2+ binding.
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
0.0
0.1
0.2
0.3
0.4
3+
0.5
0.6
0.7
0.8
0.9
1.0
3+
X=[[Fe ]/{[Fe ]+[{Ru(bpy) } (µ -H L)](PF ) ]}
2 2 6 4
2
2
Figure 10. Jobs plot in CH3CN/H2O (v/v, 1:1) of the complex [{Ru(bpy)2}2(µ2-H2L)]4+ and Fe3+ binding.
4. Conclusions
In summary, a novel fluorescent chemosensor was synthesized from 2,7-dimethyl-1,8-naphthyridine
and 5-amino-1,10-phenanthroline. The 1H-NMR, 13C-NMR, and ESI-HRMS results were consistent
with the expected structures. The recognition behaviors of [{Ru(bpy)2}2(µ2-H2L)](PF6)4 toward the
cation were investigated. The luminescence could be almost quenched by the 5.0 equivalents of
Cu2+ or Fe3+ ions. Further study showed that the complex could be used as a “turn-off” florescent
chemosensor to detect the Cu2+ and Fe3+ ions. The chemosensor showed strong anti-interference,
good selectivity, and high sensitivity. The detection limits of Cu2+ and Fe3+ were 39.9 nmol/L and
6.68 nmol/L, respectively. The ESI-MS experiments indicated the modes of binding to be 3:2 and 1:1,
respectively. The study provided an experimental basis for the development of Cu2+ or Fe3+ ion probes
that have high sensitivity and application prospects.
Supplementary Materials: The following are available online.
1,8-naphthyridine-2,7-dicarbaldehyde (in CDCl3), Figure S2: 1H-NMR spectrum for L (in DMSO-d6 and a
Figure S1: 1H-NMR spectrum for

Molecules 2019, 24, 4032
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small amount of CDCl3), Figure S3: 1H-NMR spectrum for H2L (in DMSO-d6 and a small amount of CDCl3).
Figure S4: 1H-NMR spectrum for [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (in DMSO-d6), Figure S5: 13C-NMR spectrum
for [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (in DMSO-d6), Figure S6: ESI-MS for L, Figure S7: ESI-MS for H2L, Figure S8:
ESI-HRMS for [{Ru(bpy)2}2(µ2-H2L)](PF6)4, Figure S9: ESI-MS for [({Ru(bpy)2}2(µ2-H2L))3Cu2](PF6)12(ClO4)4,
Figure S10: ESI-MS for [({Ru(bpy)2}2(µ2-H2L))Fe](PF6)4(ClO4)3, Figure S11: The “sandwich” of hypothetical
structures of new product of Cu2+ (a) and hypothetical structures of new product of Fe3+ (b).
Author Contributions: Conceptualization, C.H. and F.C.; methodology, C.H., P.L. and Z.L.; software, S.Y., L.Y. and
S.M.; formal analysis, C.H., F.C. and P.L.; writing—original draft preparation, C.H.; writing—review and editing,
C.H. and F.C.; funding acquisition, F.C.
Funding: We thank the application basis research project of Yunnan Province Science and Technology
Department (2016FD079), the Scientific Research Project of Yunnan Province Education Department (2017ZDX147),
and Innovative Research Team (in Science and Technology) in University of Yunnan Province (IRTSTYN) for
providing the financial support.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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