A novel O-phenanthroline-based bis(half-salamo)-like chemical sensor: For rapid and efficient continuous recognition of Cu2+, HPO42? and H2PO4?

Article abstract of DOI:10.1016/j.ica.2020.120098

A novel O-phenanthroline-based bis(half-salamo)-like fluorescent chemical sensor L_POV with two N₃O coordination sites was synthesized. These N₃O coordination sites were used for binding to specific metal ions. The sensor L

Full text of DOI:10.1016/j.ica.2020.120098

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Research paper
A novel O-phenanthroline-based bis(half-salamo)-like chemical sensor: For
-
rapid and efficient continuous recognition of Cu2+, HPO4 2- and H2PO4
Ruo-Nan Bian, Xin Xu, Tao Feng, Wen-Kui Dong
PII:
DOI:
Reference:
S0020-1693(20)31298-6
https://doi.org/10.1016/j.ica.2020.120098
ICA 120098
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Inorganica Chimica Acta
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Revised Date:
Accepted Date:
3 August 2020
12 October 2020
20 October 2020
Please cite this article as: R-N. Bian, X. Xu, T. Feng, W-K. Dong, A novel O-phenanthroline-based bis(half-
2-
-
salamo)-like chemical sensor: For rapid and efficient continuous recognition of Cu2+, HPO4 and H2PO4 ,
Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.120098
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1 A novel O-phenanthroline-based bis(half-salamo)-like
2 chemical sensor: For rapid and efficient continuous
2-
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3 recognition of Cu2+, HPO4 and H2PO4
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Ruo-Nan Bian, Xin Xu, Tao Feng, Wen-Kui Dong
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School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China
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A B S T R A C T
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A novel O-phenanthroline-based bis(half-salamo)-like fluorescent chemical sensor
LPOV with two N3O coordination sites was synthesized. These N3O coordination sites
were used for binding to specific metal ions. The sensor Lpov can detect Cu2+ by the
phenomenon of fluorescence quenching and the Lpov-3Cu2+ complex can detect
2-
-
HPO4 and H2PO4 within the detection range. By combining and calculating the
experimental results, the recognition mechanism of the sensor for the corresponding
ion detection is proposed and the DFT calculation is carried out. The LOD = 7.22×10-
6 M, LOQ = 2.41×10-5 M and the binding constant Ka (Ka = 2.29×1013 M-1) of Lpov
were calculated. The order of the binding constant is as high as 1013, which indicates
that the Lpov-3Cu2+ complex generated by the reaction of the fluorescent chemical
sensor Lpov with Cu2+ is extremely stable.
Keywords: O-phenanthroline; bis(half-salamo)-like sensor; hydrogen phosphate;
DFT calculation; water quality monitoring1
25 1. Introduction
∗ Corresponding author. Fax: +86 931 4938703.
E-mail addresses: dongwk@126.com (W.-K. Dong).
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The world is enriched by ions, but excess ions can also be a burden on the
environment and human life. For example, the addition of copper (II) ions to
swimming pools can have a sterilizing effect and copper is a component of many
important enzymes in the body that can promote the absorption and utilization of iron
can maintain the function of the central nervous system. However, when excess
copper remains in the body, it tends to put a strain on the body's organs, especially the
liver and gall bladder [1-8]. When there are problems with these two organs,
maintaining the body's metabolism can lead to disorders, cirrhosis, ascites or even
worse [9-13]. Phosphates are important in inorganic chemistry, biochemistry, and
biogeochemistry, but in excess, they can trigger red tide. They contaminate the soil
and further affect the crops, putting human health at risk [14-16]. Fluorescent
chemical sensors have become popular in recent years in the field of ion detection
because they can be used to measure ion rapidly and accurately through changes in
fluorescence intensity. The convenient detection of the corresponding ions has
attracted increased attention [17-23]. O-phenanthroline combines with metal ions to
form chelates easily, and the products are more stable [24-29].
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Since O-phenanthroline and its derivatives can be used as a kind of common
excellent fluorescent chemical sensors for subsequent fluorescence detection work,
here, we have introduced O-phenanthroline into the design of half-salamo-like probe
molecules. It is the first time to synthesize half-salamo-like probe molecules with O-
phenanthroline, unprecedented. Two N and one O atoms from one half-salamo-like
unit and one N atom on O-phenanthroline can form together one N3O coordination
sites, and used as binding sites for identifying specific metal ions. Half-salamo-based
compounds provide a more stable and flexible sensor structure due to the introduction
of oxygen atoms that are more electronegative compared to half-salen-based
compounds [30-35]. By introducing different groups, the solubility of the ligands in
water system can be adjusted, achieve the purpose of detecting metal ions in
environmental water samples.
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2. Experimental
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2.1. Materials and measurements
Please refer to the Supporting information for all experimental instruments and
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raw materials.
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2.2 Synthesis method of Lpov
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The synthetic route to Lpov is shown in Scheme 1.
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Scheme 1 Synthesis route to Lpov
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2.2.1 Synthesis of 1,10-phenanthroline-2,9-dicarbaldehyde
1,10-Phenanthroline-2,9-dicarboxaldehyde was synthesized according to a
previously reported method [36-40]. To a double-mouthed round bottom flask, 30 mL
of a dioxane solution dissolved in selenium dioxide (4.5 g, 40.55 mmol) was added
and placed in a water bath. With a constant-pressure drop funnel, 100 mL of dioxane
solution dissolved in neo-copper reagent (3 g, 14.40 mmol) was added drop by drop to
the double-bottomed flask. After 12h in the water bath reaction, while hot filter out
the tar and selenium collection filtrate, the filtrate was yellow. When the filtrate was
cooled, a large number of yellow precipitates were generated, namely 1,10-
phenanthroline-2,9-dicarboxaldehyde crude product. After filtering out the crude
product, it was collected and dissolved in benzene, to which n-hexane was added for
recrystallization. Finally, the resulting product was filtered and dried to obtain a
golden yellow needle-like solid. Golden yellow needle-like solid 3.1313 g, yield:
81.75%. M.p.: 238-240 °C. Anal. Calcd. for C14H8N2O2 (%): C, 71.18; H, 3.41; N,
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11.86. Found: C, 71.29; H, 3.25; N, 11.62. H NMR (500 MHz, DMSO)  10.36 (d,
J = 0.8 Hz, 1H), 8.80 (dd, J = 8.2, 0.8 Hz, 1H), 8.32 (d, J = 8.2 Hz, 1H), 8.29 (s, 1H).
2.2.2 Synthesis of 2-[O-(1-Ethyloxyamide)]oxime-6-methoxyphenol
2-[O-(1-Ethyloxyamide)]oxime-6-methoxyphenol was prepared according to a
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previously reported method [41-46]. Weighing 1,2-bis(aminooxy)ethane 414.45 mg
(4.5 mmol) in ethanol (20 mL), another 684.67 mg (4.5 mmol) of 3-
methoxysalicylaldehyde was dissolved in ethanol (20 mL). An ethanol solution of 3-
methoxysalicylaldehyde was added drop by drop to an ethanol solution of 1,2-
bis(aminooxy)ethane in a water bath at 55°C, with a controlled drop acceleration of
about 10 s a drop and continuous reaction for 6 h to obtain light yellow ethanol
solution. Decompression distillation removes excess solvent and concentrates the
reaction solution to about 5 mL. The concentrated solution is a bright yellow oily
liquid, further purified by column chromatography to obtain white solid 748.44 mg,
yield: 73.51%; M.p.: 95.5-96.5°C. Anal. Calcd. for C10H14N2O4 (%): C, 53.09; H,
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6.24; N, 12.38. Found: C, 53.32; H, 6.20; N, 12.16. H NMR (500 MHz, CDCl3) 
3.92 (s, 3H), 3.96 (t, J = 4.5 Hz, 2H), 4.38 (t, J = 4.5 Hz, 2H), 5.50 (brs, 2H), 6.82
(dd, J = 7.8, 1.6 Hz, 1H), 6.87 (t, J = 7.8 Hz, 1H), 6.90 (dd, J = 7.8, 1.6 Hz, 1H), 8.24
(s, 1H), 9.88 (s, 1H).
2.2.3 Synthesis of Lpov
Lpov
was
synthesized
using
the
one-pot
method.
2-[O-(1-
Ethyloxyamide)]oxime-6-methoxyphenol (453 mg, 2.0 mmol) and 1,10-
phenanthroline-2,9-dicarboxaldehyde (236 mg, 1.0 mmol) were dissolved in the
ethanol (20 mL), respectively. The reaction was stirred at 55℃ for 6 h. At the end of
the reaction, a large amount of yellowish precipitate is generated at the bottom of the
round bottom flask. The product is filter-dried. Light yellow solid: 517.23 mg. M.p.:
145-148°C. Yield: 79.25%. Anal. calcd. for C34H32N6O8 (%): C, 62.57; H, 4.94; N,
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12.88. Found: C, 62.82; H, 4.75; N, 12.67. H NMR (500 MHz, DMSO) δ 9.41 (s ,
2H), 8.52 (dd, J = 18.5, 15.5 Hz, 6H), 8.19 (d, J = 8.4 Hz, 2H), 8.06 (s, 2H), 7.17 (d, J
= 7.9 Hz, 2H), 7.00 (d, J = 9.1 Hz, 2H), 6.81 (t, J = 8.0 Hz, 2H), 4.55 (dd , J = 5.7, 2.9
Hz, 4H), 4.48 (dd, J = 5.6, 3.0 Hz, 4H), 3.80 (s, 6H).
2.3 Experimental methods
2.3.1 Preparation of the required solutions
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The metal cations used in this experiment are all nitrates and the anions are all
sodium salts. Altogether 16 cations and 16 anions were used, respectively Li+, Na+,
Ag+, K+, Cr3+, Fe3+, Ca2+, Zn2+, Mg2+, Sr2+, Ba2+, Cu2+, Ni2+, Cd2+, Co2+ and Al3+,
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respectively. The anions are Cl-, H2PO4 , I-, F-, CN-, CH3COO-, HS-, NO3 , S2-, HCO3 ,
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Br-, P2O7 , CO3 , ClO4 , B4O72- and HPO4 .
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All salts were configured with distilled water at a concentration of 1×10-2 mol/L.
Lpov was used with VDMF:VWater = 9:1 solution was configured at a concentration of
1×10-3 mol/L. The pH of the buffer solution was adjusted using a pHS-45 model
acidimeter.
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2.3.2 Fluorescence spectroscopy experiments
Fresh Tris-HCl buffer solution was used for all the measurements of
fluorescence spectra. The ions were added in an amount 20 times greater than Lpov
and diluted with the buffer solution so that Lpov in the cuvette was mixed with the
ions. The concentration of the solution was 1×10-5 mol/L.
2.3.3 Determination of Job Work Curves
Keeping the total concentrations of Lpov and constant while measuring the
Job work curve, will The concentrations of Lpov and Cu(NO3)·3H2O were adjusted
in the following ratios 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, and 10:0. Configure
the desired solution according to the ratios and stir at room temperature for 4h. The
configured solutions under the same conditions were measured, the fluorescence data
at 382 nm were recorded, and the Job work curves were plotted according to the data
obtained.
2.3.4 ESI-MS spectrum sample
Lpov (0.05 mmol, 32.64 mg) and Cu(NO3)2 (0.25 mmol, 46.89 mg) were
dissolved in trichloromethane solution, stirred for 6h. Evaporated and concentrated to
obtain solid powder, it's the complex Lpov-3Cu2+. Lpov-3Cu2+ (0.03 mmol, 25.24
mg) and NaH2PO4 (0.15 mmol, 17.98 mg) / Na2HPO4 (0.15 mmol, 21.29 mg) were
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dissolved in trichloromethane solution, stirred for 6h. Evaporated and concentrated to
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obtain solid powder, it's the complex [Lpov-Cu2+]-H2PO4 / [Lpov-Cu2+]-HPO4 .
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3. Results and discussion
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3.1 Solubility testing and characterization of Lpov
The solubilities of the sensor in common organic solvents were tested. Lpov
was dissolved in dichloromethane, trichloromethane, DMSO (dimethyl sulfoxide), EA
(ethyl acetate), py (pyridine), THF (tetrahydrofuran) and DMF (N,N-
dimethylformamide) at room temperature. Three methods were used to characterize
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Lpov: H NMR, C NMR and ESI-MS. Through the comprehensive analysis of the
spectrum results (Supporting information), it is determined that the synthesized
powder solid is the target product Lpov.
3.2 Fluorescent chemical behavior of Lpov and the effect of metal ions
Solvent effect is an important factor affecting the photochemical behavior of
fluorescent chemical sensors [47-51]. First, a solution of the same concentration of
Lpov dissolved in different organic solvents was prepared to compare the
fluorescence in different organic solvents behavior. The experimental results are
shown in Fig. 1, from which it can be seen that the Lpov emits the highest
fluorescence intensity in DMF, thus DMF was chosen as a solvent for later probing
the chemical behavior of Lpov fluorescence.
Fig. 1 Effect of the solvent system on the fluorescence intensity of the Lpov
A set of experiments were made to investigate the effect of water content on
the fluorescence of Lpov, in order to detect metal ions in ambient water samples.
Formulated at DMF:H2O ratios of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6 solvent, weighing the
same mass of Lpov and dissolving it in prepared DMF:H2O solutions of different
ratios, it was found that Lpov is not completely soluble in a solution with a ratio of
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4:6, so the fluorescence measurement of that ratio was abandoned. To test the
fluorescence intensity of the same concentration Lpov solution dissolved in another
five proportions, the choice was determined by comparing the peak value DMF: H2O.
The ratio of 9:1 was used as the solvent for the subsequent fluorescence spectrum
experiment. The experimental results are shown in Fig. 2.
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Fig. 2 The effect of water content in solvent (DMF:H2O, v/v) on the fluorescence
intensity of Lpov
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Next, the effect of metal ions on the fluorescence behavior of Lpov was
investigated in Tris-HCl buffer solution (DMF/H2O = 9/1, v/v, pH = 7.23), the metal
cation full scan fluorescent experiment was carried out (Fig. 3). From the analyses of
the experimental results, it can be seen that Lpov is most severely affected by Cu2+,
and the fluorescence intensity quenched the 89.38%. Subsequently, Cu2+ titration
experiments were completed in order to determine the optimal coordination ratio of
Lpov to Cu2+. After calculation and analysis of the results, the optimal coordination
ratio of Lpov to Cu2+ was determined to be 1:3. The mass spectral analysis of the
Lpov-3Cu2+ complex (Supporting information) also allowed the determination of the
coordination ratio of Lpov to the optimal coordination ratio of Cu2+ is 1:3.
Fig. 3 Fluorescence spectra of Lpov and metal ions (20 equiv.) in DMF/H2O buffer
solution (Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23)
3.3 Performance investigation of Cu2+ detection by Lpov
It can be seen from Fig. 3 that the fluorescence emission of Lpov was the
strongest at 382 nm. However, the addition of Cu2+ made the maximum emission peak
of Lpov disappear. In the titration experiment, it was found that with the addition of
Cu2+, the titration reached the end point until 3 eq. (Fig. 4). Meanwhile, the solution
in the cuvette changed from colorless to a dark green visible to the naked eye. Thus, it
was evident that Lpov has a very excellent ability to recognize Cu2+, and in order to
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further develop the performance of Lpov to detect Cu2+, the numerical values that
LOD (Limit of Detection) and LOQ (Limit of Quantitation) of Lpov for Cu2+ were
calculated based on the processing of titration spectrogram data and previous
literature [52-54].
(F  F )2

0
0

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N 1
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LOD = K × δ / S；LOQ = 10 × δ / S；
(N = 20); K = 3.
N in the formula is measured number of times for Lpov blank solution and S
represents the slope of the fluorescence intensity versus sample concentration curve,
K is coefficient determined at a certain level of confidence (IUPAC recommends K =
3 for spectrochemical analysis), where F0 is the fluorescence intensity of the Lpov
blank solution (The blank solution is the LPOV solution without adding metal ions, c =
1×10-3 mol/L). The LOD = 7.22×10-6 M and LOQ = 2.41×10-5 M of Lpov were
calculated. Through the titration experiment data, referring to the previous literature
and combining the Benesi-Hildebrand equation (1), the value of the binding constant
Ka (Ka = 2.29×1013 M-1) was calculated [55,56]. The order of the binding constant is
as high as 1013, which indicates that the Lpov-3Cu2+ complex generated by the
reaction of Lpov with Cu2+ is extremely stable.
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1/(F - F0) = 1/(Fmax - F0){(Ka [Mm+]n) + 1}
(1)
F is the fluorescence intensity of the Lpov-3Cu2+ complex at 382 nm, F0 is the
fluorescence intensity at 382 nm when the Cu2+ ion concentration is zero, and Fmax is
the maximum fluorescence intensity at 382 nm (EX WL: 325 nm). [Mm+] is the
concentration of Cu2+ ions and n is the coordination ratio of Lpov to Cu2+ (n = 3). The
Benesi-Hildebrand fit curve of Lpov to Cu2+ is shown in Fig. 5 shown.
Fig. 4 Fluorescence titration of Cu2+ in DMF/H2O buffer solution with Lpov. (The
inset shows a scatter plot of the fluorescence intensity of Lpov at 382 nm with Cu2+
concentration; Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
Fig. 5 Benesi-Hildebrand fit curve of Cu2+ with Lpov
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One of the most important indicators for distinguishing the quality of
fluorescent chemical sensors is the ability to recognize specific ions without
interference from other ions. Therefore, it is very necessary to do anti-interference
experiments. Under the same conditions, the anti-interference experiment of Lpov to
identify Cu2+ was conducted. According to the anti-interference experimental data, it
can be clearly observed that other metal ions do not interfere with the detection of
Cu2+. As shown in Fig. 6, the interference of other metal ions on the recognition of
Cu2+ by Lpov can be seen directly from the histogram.
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Fig. 6 Lpov (1×10-3 mol/L) in DMF/H2O buffer solution for Cu2+ recognition anti-
interference experiment. (Emission peak at 382 nm. The black bar is the fluorescence
intensity of Lpov at 382 nm after adding common metal ion, the red bar is the
fluorescence intensity of the Lpov-3Cu2+ complex after adding common metal ion.
Lpov, 1×10-3 mol/L; cation, 1×10-2 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH =
7.23).
The wider pH range of the sensor, the wider the range of applications. As
shown in Fig. 7, the pH application range of Lpov was studied. The change of
fluorescence intensity of Lpov and Cu2+ was tested under the condition of pH = 2 ~
13. According to the experimental data, the change of fluorescence intensity of Lpov
and Cu2+ was relatively stable in the range of pH = 4 ~ 8, which indicated that the
recognition effect of Lpov on Cu2+ was very stable in a wide range of pH.
Fluorescent chemical sensors have the advantage of fast response times,
eliminating tedious testing times and allowing chemical sensors to dominate in
various applications. The response time of Lpov to Cu2+ was tested. After the change
of fluorescence intensity and time statistics, it was found that the response time of
Lpov to Cu2+ was very short. The response time is approximately 10 s, and the
fluorescence tends to stabilize after 10 s, as shown in Fig. 8.
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Fig. 7 The pH response of Lpov and Cu2+ in fluorescent chemical sensors (Lpov,
1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v)
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Fig. 8 Time response diagram of interaction between Lpov and Cu2+ in fluorescent
chemical sensor (Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
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To further verify the optimal coordination ratio of Lpov to Cu2+, Job working
curves were measured and plotted. As shown in Fig. 9, at the excitation wavelength of
325 nm, the fluorescence intensity of Lpov exhibits a turning point at the mass
fraction (Cu2+) of 0.75. The results indicated that the optimal coordination ratio of
Lpov to Cu2+ should be 1:3. The mass spectrogram of the Lpov-3Cu2+ complex can
also be used to infer the optimal coordination ratio of Lpov to Cu2+, as shown in Fig.
10. As can be seen in the mass spectra, m/z = 838.1234 is the molecular ion peak of
[Lpov+3Cu2+]4+. Compared with other references [57,58], it is found that the binding
constant of Lpov for Cu2+ recognition is larger, which indicates that the copper(II)
complex of Lpov is more stable. In addition, the LOD of the chemical sensor Lpov is
smaller than those of other Cu2+ probes in the literatures, indicating that the chemical
sensor is more sensitive to recognize Cu2+. Binding constants and detection lines
between the related chemosensors mentioned in literatures are compared, as shown in
Table 1.
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Fig. 9 Job working curve of Cu2+ on Lpov (Lpov, 1×10-3 mol/L; Tris-HCl buffer
solution, 9/1, v/v, pH = 7.23)
Fig. 10 ESI-MS spectrum (a) ESI-MS spectrum of Lpov; (b) ESI-MS spectrum of the
Lpov-3Cu2+ complex
Table 1 Comparison of binding constants and detection lines between the
chemosensors.
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3.4 Mechanism of recognition of Cu2+ by Lpov
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The stability order of complexes generated from divalent Mn2+ to Zn2+ with
ligands containing N coordination atoms can be observed in the following order:
Mn2+<Fe2+<Co2+<Ni2+<Zn2+<Cu2+, the sequence called the Irving-Williams sequence,
which roughly corresponds to the variation of the weak-field CFSE [59,60].
According to Jahn-Teller effect, there are three electrons in the two degenerate d
orbitals (Eg) of divalent copper (II) ions in octahedral complexes due to the splitting
of crystal field. The number of electrons occupied in these two orbitals is different,
and the complex is a nonlinear molecule, which leads to the splitting of Eg [61]. The
results of splitting lead to Jahn-Teller distortion, and the Irving-Williams (CFSE
increase) order obtained from the crystal field splitting energy is abnormal, which
leads to the Lpov-3Cu2+ complex being more stable than other divalent metal (II)
complexes in the same period.
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The effect of the complexes in different oxidation states on the charge density is
linear, indicating that there is a strong relationship between the charge density around
the metal and the oxidation state [62,63]. In the presence of Cu2+, the fluorescence
intensity of the system is quenched. This indicates that Cu2+ interacts with the system
effectively and has the photoinduced electron transfer (PET) effect, which reduces the
fluorescence intensity.
Based on the Irving-Williams sequence, the copper (II) complexes are more
stable in comparison to those of other same period divalent metal ions. Based on all
the above experimental results, a mechanism for Lpov to recognize Cu2+ was
proposed. Compared with oxygen atoms, the electronegativity of N atoms is relatively
small, the ability to attract electrons is relatively weak, and it is easy to give electrons,
so the coordination ability of N atoms is stronger than that of O atoms [64,65]. When
Lpov was combined with three Cu2+ ions, the two Cu2+ ions are respectively
combined with the N3O coordination sites, which makes Lpov present a spiral
structure. After the sensor molecular screwed, a larger coordination site was formed in
the middle, and the last Cu2+ entered the coordination site to combine with four N
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atoms, and finally formed a 1:3 type complex. Mechanism of recognition of Cu2+ by
Lpov is shown in Fig. 11.
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Fig. 11 Proposed recognition mechanism of Cu2+ by Lpov
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3.5 Computational procedure for Lpov-3Cu2+ complex
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For the structures of the ligand and its complexes, full optimization was
performed by Density Functional Theory (DFT) using Gaussian’09.
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The calculation results of LPOV showed that the molecular orbital No.171 is a
HOMO orbital, E171 = -5.85798 eV, the molecular orbital No.172 is a LUMO orbital,
E172 = -2.16327 eV, and the energy gap ΔE = E172 - E171 = 3.69471 eV, the energy gap
is large. In order to fully analyze the charge density distribution, choose to calculate
the electron cloud and distribution of LUMO+2 (174), LUMO+1 (173), LUMO (172),
HOMO (171), HOMO-1 (170), HOMO-2 (169) orbits respectively (Fig. 12a). It can
be seen from the figure that the HOMO, LUMO of LPOV and the electron clouds in
the adjacent orbits are mostly distributed locally. LUMO and LUMO+1 orbital
electron clouds were mainly distributed on the part of O-phenanthroline. HOMO and
HOMO-1 orbital electron clouds were mainly distributed on the half-salamo-like part.
HOMO-2 and LUMO+2 orbital electron clouds were mainly distributed on the half-
salamo-like part and the part of O-phenanthroline.
10
11
12
13
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16
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18
19
20
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22
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26
27
28
29
30
For the α-spin orbit of the Lpov-3Cu2+ complex, the molecular orbital No.199 is
the HOMO orbital, E199 = -3.05712 eV, the molecular orbital No.200 is the LUMO
orbital, E200 = -1.83946 eV and the energy gap ΔE = E200 - E199 = 1.21766 eV. In order
to fully analyze the charge density distribution, select the LUMO+2 (202), LUMO+1
(201), LUMO (200), HOMO (199), HOMO-1 (198), HOMO-2 (197) orbits to
calculate the electron cloud and distribution (Fig. 12b). It can be seen from the figure
that the HOMO, LUMO of the α-spin orbit of the Lpov-3Cu2+ complex and the
electron clouds in the adjacent orbit mostly show local distribution. HOMO and
LUMO orbital electron clouds were mainly distributed on the benzene ring part of the
O-phenanthroline. For the β-spin orbital of the Lpov-3Cu2+ complex, the molecular
12

1
2
orbital No.198 is the HOMO orbital, E198 = -3.77036 eV, the molecular orbital No.199
is the LUMO orbital, E199 = -1.83484 eV, and the energy gap ΔE = E199 - E198
=
3
1.93552 eV. In order to fully analyze the charge density distribution, choose to
calculate the electron cloud and distribution of LUMO+2 (201), LUMO+1 (200),
LUMO (199), HOMO (198), HOMO-1 (197), HOMO-2 (196) orbits respectively (Fig.
12b). It can be seen from the figure that the HOMO, LUMO of the β-spin orbit of the
Lpov-3Cu2+ complex and the electron clouds in the adjacent orbit mostly show local
distribution. The HOMO electron cloud was mainly distributed on the the terminal
group of the O-phenanthroline and Cu (II) atom. The LUMO electron cloud is mainly
distributed on the part of O-phenanthroline.
4
5
6
7
8
9
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12
13
14
15
16
17
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23
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25
26
27
28
29
30
Fig. 12 Molecular orbital energy diagrams of LPOV and the Lpov-3Cu2+ complex. (a)
Molecular orbital energy diagram of LPOV. (b) Molecular orbital energy diagram of
the Lpov-3Cu2+ complex
3.6 Detection of Cu2+ in tap water samples
Copper (II) ions in drinking water are often caused by water leaching from
copper (II) pipes, and high levels of dissolved oxygen can accelerate copper
corrosion. The tap water sample experiment was used to verify the practicability of
Lpov. A known amount of Cu2+ was added to the tap water as the tap water sample.
After adding Lpov, the recovery rate was calculated based on the measured
fluorescence intensity, as shown in Fig. 13. The recovery rate ranges from 98.3% to
101.25% and the relative standard deviation is controlled within 3%. The results of
this experiment proved that Lpov can be used for the determination of Cu2+ in tap
water.
Fig. 13 Determination of Cu2+ in tap water sample (n = 3; Lpov, 1×10-3 mol/L).
2-
-
3.7 Study on the performance of detecting HPO4 and H2PO4 with the Lpov-3Cu2+
complex
13

1
2
After experimental research on Cu2+ detection, the Lpov-3Cu2+ complex was
2-
-
unexpectedly found to have good selectivity for HPO4 /H2PO4 in the anion anti-
2-
-
3
interference experiment. The anions of HPO4 and H2PO4 are important buffer pairs
in the living system, which can maintain a certain pH value, thus ensuring the stability
of the blood of the living body. First, the anion anti-interference experiment of Lpov
to Cu2+ was done in Tris-HCl buffer solution, as shown in Fig. 14. After adding
4
5
6
2-
7
HPO4 to the Lpov-3Cu2+ complex, the fluorescence intensity increased by 970.83%,
-
8
and after adding H2PO4 , the fluorescence intensity increased by 427.08%. However,
9
the fluorescence intensity after adding other anions was not much different from that
10
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12
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23
24
25
26
27
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29
of the Lpov-3Cu2+ complex. Therefore, using the Lpov-3Cu2+ complex as a new
2-
-
sensor, it can continue to recognize HPO4 /H2PO4 with remarkable recognition.
3.8 Titration of the Lpov-3Cu2+ complex to 2Lpov-Cu2+ and Job work curves
To determine the coordination ratio of the Lpov-3Cu2+ complex to HPO4
2-
-
2-
/H2PO4 , take HPO4 as an example, the Lpov-3Cu2+ complex was titrated against
2-
HPO4 (as shown in Fig. S6). The processing of the titration experimental data and
the summary of the mass spectrometry data revealed that the Lpov-3Cu2+ complex
2-
with HPO4 both with an optimal coordination ratio of 1:1. At the same time, the
results of the Job working curve experiment also proved that the optimal coordination
2-
ratio of [Lpov+3Cu2+]4+ to HPO4 is 1:1 (as shown in Fig. S7), which laid a
foundation for the later mechanism exploration. Reference to other literatures [66,67],
comparison of binding constants and detection lines between the chemosensors is
shown in Table 1.
Fig. 14 The Lpov-3Cu2+ complex (1×10-3 mol/L) in DMF/H2O buffer solution for
2-
-
HPO4 /H2PO4 recognition anti-interference experiment. (Emission peak at 382 nm.
The black bar is the fluorescence intensity of LPOV, the red bar is the fluorescence
intensity of LPOV with Cu2+ added, the blue bar is the fluorescence intensity of the
Lpov-3Cu2+ complex after adding common metal anions, the pink bar is the
14

1
2
fluorescence intensity of LPOV after adding common metal anions. LPOV, 1×10-3
mol/L; cation, 1×10-2 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
3
4
3.9 Computational procedure for 2Lpov-Cu2+ complex
5
For the α-spin orbit of the 2Lpov-Cu2+ complex, the molecular orbital No.351
is the HOMO orbital, E351 = -9.25337 eV, the molecular orbital No.352 is the LUMO
orbital, E352 = -6.62316 eV and the energy gap ΔE = E352 - E351 = 2.63021 eV. In order
to fully analyze the charge density distribution, select the LUMO+2(354), LUMO+1
(353), LUMO (352), HOMO (351), HOMO-1 (350), HOMO-2 (349) orbits to
calculate the electron cloud and distribution (Fig. 15). It can be seen from the figure
that the HOMO, LUMO of the α-spin orbit of the 2Lpov-Cu2+ complex and the
electron clouds in the adjacent orbit mostly show local distribution. The LUMO
orbital electron clouds were mainly distributed on the part of the O-phenanthroline.
The HOMO orbital electron clouds were mainly distributed on the half-salamo-like
part. For the β-spin orbital of the 2Lpov-Cu2+ complex, the molecular orbital No.350
is the HOMO orbital, E350 = -8.96004 eV, the molecular orbital No.351 is the LUMO
orbital, E351 = -8.72330 eV, and the energy gap ΔE = E351 - E350 = 0.23674 eV. In
order to fully analyze the charge density distribution, choose to calculate the electron
cloud and distribution of LUMO+2 (353), LUMO+1 (352), LUMO (351), HOMO
(350), HOMO-1 (349), HOMO-2 (348) orbits respectively (Fig. 15). It can be seen
from the figure that the HOMO, LUMO of the β-spin orbit of the Lpov-3Cu2+
complex and the electron clouds in the adjacent orbit mostly show local distribution.
The HOMO and LUMO orbital electron clouds were mainly distributed on the part of
the O-phenanthroline and Cu(II) atom.
6
7
8
9
10
11
12
13
14
15
16
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29
30
Fig. 15 Molecular orbital energy diagrams of the 2Lpov-Cu2+ complex.
2-
-
3.10 On the recognition mechanism of HPO4 /H2PO4 by sensor [Lpov+3Cu2+]4+
It can be inferred from the titration experiments (Fig. 16) and Job working
-
curves (Fig. 17) that the optimal coordination ratio of Lpov-3Cu2+ and H2PO4 is 1:1.
15

-
1
2
The LOD and LOQ of H2PO4 were calculated as per IUPAC guidelines and it was
found to be 5.83 ×10-8 and 1.96 ×10-7 M, respectively. H2PO4 releases two molecules
-
3
of H+ through two-step ionization. H+ combines with the complex Lpov-3Cu2+, and
then releases Cu2+. The results are consistent with the ESI-MS spectra of complex
4
-
5
[Lpov+3Cu2+]4++H2PO4 (Fig. 18). This may be due to the relatively small Ksp of the
3-
6
product that is generated from PO4 with Cu2+ [68]. Mechanism of recognition of
2-
-
7
HPO4 /H2PO4 by the Lpov-3Cu2+ complex is shown in Fig. 19.
8
-
9
Fig. 16 Fluorescence titration of H2PO4 in DMF/H2O buffer solution with the Lpov-
10
11
12
13
14
15
16
17
18
19
20
3Cu2+ complex. (The Lpov-3Cu2+ complex, 1×10-3 mol/L; cation, 1×10-2 mol/L; Tris-
HCl buffer solution, 9/1, v/v, pH = 7.23).
-
Fig. 17 Job working curve of H2PO4 on the Lpov-3Cu2+ complex
2-
Fig. 18 ESI-MS spectrum (a) ESI-MS spectrum of the 2Lpov-Cu2+ complex (HPO4
-
); (b) ESI-MS spectrum of the 2Lpov-Cu2+ complex (H2PO4 )
2-
-
Fig. 19 Proposed recognition mechanism of HPO4 /H2PO4 by the Lpov-3Cu2+
complex
21 4. Conclusions
22
23
24
25
26
27
28
29
An O-phenanthroline-based bis(half-salamo)-like chemical sensor Lpov was
synthesized with 2,9-dimethyl-1,10-phenanthroline and 2-[O-(1-
ethyloxyamide)]oxime-6-methoxyphenol. Lpov can detect Cu2+ by the phenomenon
2-
of fluorescence quenching and the Lpov-3Cu2+ complex can detect HPO4 and
-
H2PO4 within the detection range. Half-salamo-like structure was introduced into O-
phenanthroline, which makes Lpov have two N3O coordination sites. Different from
the traditional N2O2 coordination sites of salamo-like compounds, the introduction of
N atom of O-phenanthroline with stronger coordination ability makes the coordination
16

1
2
3
4
5
mode more novel and provides more possibilities for the coordination modes. By
calculation, LOD = 7.2215×10-6 M and binding constant Ka = 2.29×1013 M-1. The
proposed recognition mechanism of the sensor is verified by the results of mass
spectrometry and DFT calculations.
6
Declarations of interest:
7
8
The authors declare no conflict of interest.
9
Acknowledgements
10
11
12
13
This work was supported by the National Natural Science Foundation of China
(21761018) and the Program for Excellent Team of Scientific Research in Lanzhou
Jiaotong University (201706), both of which are gratefully acknowledged.
17

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21

1
Table 1. Comparison of binding constants and detection lines between chemosensors.
Binding
Detection limit Identificatio
No.
sensor
constant
Reference
(M)
n substance
(M-1)
1
6.42 × 104
4.7 × 10-6
Cu2+
[57]
2
2.4 × 102
1.7 × 10-6
Cu2+
[58]
-
3
3.84 × 107
5.5 × 10−8
H2PO4
[66]
-
4
5
2.1 × 105
None
H2PO4
[67]
2.29 × 1013
3.26× 109
7.22 × 10-6
5.83 × 10-8
Cu2+
This work
This work
-
H2PO4
2
3
4
22

1
2
3
Conflicts of Interest: The authors declare no competing financial interests.
4
Research Highlights
5
6
1. An novel O-phenanthroline-based bis(half-salamo)-like fluorescent chemical
sensor LPOV with two N3O cavities was synthesized.
7
8
9
2. The sensor Lpov can detect quickly and efficiently Cu2+ and the Lpov-Cu2+
-
10
11
12
13
14
complex can continuously detect HPO42- and H2PO4 .
3. The order of the binding constant is as high as 1013 (Ka = 2.29×1013 M-1).
15
16
17
18
19
20
21
22
No conflict of interest exits in the submission of this manuscript, and manuscript is
approved by all authors for publication. I would like to declare on behalf of my co-
authors that the work described was original research that has not been published
previously, and not under consideration for publication elsewhere, in whole or in part.
All the authors listed have approved the manuscript that is enclosed. In addition, we
solemnly declare that the article is original and unpublished and is not being
considered for publication elsewhere.
23
24
LEGENDS
25
26
27
28
29
Scheme 1 Synthesis route to Lpov
Fig. 1 Effect of the solvent system on the fluorescence intensity of the Lpov
Fig. 2 The effect of water content in solvent (DMF:H2O, v/v) on the fluorescence
intensity of Lpov
30
31
32
Fig. 3 Fluorescence spectra of Lpov and metal ions (20 equiv.) in DMF/H2O buffer
solution (Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23)
33
34
23

1
2
Fig. 4 Fluorescence titration of Cu2+ in DMF/H2O buffer solution with Lpov. (The
inset shows a scatter plot of the fluorescence intensity of Lpov at 382 nm with Cu2+
concentration; Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
3
4
5
6
7
Fig. 5 Benesi-Hildebrand fit curve of Cu2+ with Lpov
Fig. 6 Lpov (1×10-3 mol/L) in DMF/H2O buffer solution for Cu2+ recognition anti-
interference experiment. (Emission peak at 382 nm. The black bar is the fluorescence
intensity of Lpov at 382 nm after adding common metal ion, the red bar is the
fluorescence intensity of the Lpov-3Cu2+ complex after adding common metal ion.
Lpov, 1×10-3 mol/L; cation, 1×10-2 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH =
7.23).
8
9
10
11
12
13
14
Fig. 7 The pH response of Lpov and Cu2+ in fluorescent chemical sensors (Lpov,
1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v)
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16
17
18
19
20
21
22
23
Fig. 8 Time response diagram of interaction between Lpov and Cu2+ in fluorescent
chemical sensor (Lpov, 1×10-3 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
Fig. 9 Job working curve of Cu2+ on Lpov (Lpov, 1×10-3 mol/L; Tris-HCl buffer
solution, 9/1, v/v, pH = 7.23)
Fig. 10 ESI-MS spectrum (a) ESI-MS spectrum of Lpov; (b) ESI-MS spectrum of the
Lpov-3Cu2+ complex
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25
26
27
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Fig. 11 Proposed recognition mechanism of Cu2+ by Lpov
Fig. 12 Molecular orbital energy diagrams of LPOV and the Lpov-3Cu2+ complex. (a)
Molecular orbital energy diagram of LPOV. (b) Molecular orbital energy diagram of
the Lpov-3Cu2+ complex
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30
31
24

1
2
Fig. 13 Determination of Cu2+ in tap water sample (n = 3; Lpov, 1×10-3 mol/L).
Fig. 14 The Lpov-3Cu2+ complex (1×10-3 mol/L) in DMF/H2O buffer solution for
3
2-
-
4
HPO4 /H2PO4 recognition anti-interference experiment. (Emission peak at 382 nm.
The black bar is the fluorescence intensity of LPOV, the red bar is the fluorescence
intensity of LPOV with Cu2+ added, the blue bar is the fluorescence intensity of the
Lpov-3Cu2+ complex after adding common metal anions, the pink bar is the
fluorescence intensity of LPOV after adding common metal anions. LPOV, 1×10-3
mol/L; cation, 1×10-2 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.23).
5
6
7
8
9
10
11
12
13
14
15
16
Fig. 15 Molecular orbital energy diagrams of the 2Lpov-Cu2+ complex.
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Fig. 16 Fluorescence titration of H2PO4 in DMF/H2O buffer solution with the Lpov-
3Cu2+ complex. (The Lpov-3Cu2+ complex, 1×10-3 mol/L; cation, 1×10-2 mol/L; Tris-
HCl buffer solution, 9/1, v/v, pH = 7.23).
-
17
18
19
Fig. 17 Job working curve of H2PO4 on the Lpov-3Cu2+ complex
2-
Fig. 18 ESI-MS spectrum (a) ESI-MS spectrum of the 2Lpov-Cu2+ complex (HPO4
-
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22
); (b) ESI-MS spectrum of the 2Lpov-Cu2+ complex (H2PO4 )
2-
-
Fig. 19 Proposed recognition mechanism of HPO4 /H2PO4 by the Lpov-3Cu2+
23
complex
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25

1
2
3
4
Scheme 1
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3
4
5
Fig. 1
27

1
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3
4
5
6
Fig. 2
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1
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7
Fig. 3
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