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Article A Novel (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; [email protected] (C.H.); [email protected] (Z.L.) 2 College of Chemistry and Environmental Science, Qujing Normal University, Qujing 655011, China; [email protected] (S.Y.); [email protected] (L.Y.); [email protected] (P.L.) 3 Department of Medicine, Qujing Qilin Vocational and Technical School, Qujing 655000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0874-099-8658

 Academic Editors: Victor Mamane and John S. Fossey  Received: 26 September 2019; Accepted: 31 October 2019; Published: 7 November 2019

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 [1–4]. Among these metal ions, Fe3+ is the most abundant essential trace element in the organism. It is involved in several biological processes, such as transport, cellular respiration, enzymatic catalysis regulation of transcription, DNA repair [5–8], and so on. 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 [9–11]. Therefore, the detection and 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].

Molecules 2019, 24, 4032; doi:10.3390/molecules24224032 www.mdpi.com/journal/molecules Molecules 2019, 24, 4032 2 of 13

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 2+ ligand charge transfer (MLCT ) excited state of Ru(bpy)3 , which resulted in efficient quenching of the luminescence. The phenothiazine moiety of Rubp-Ptz can be oxidized to a stable entity by 2+ 2+ Cu . 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 [18–22], photocatalysis [23,24], water-oxidation catalyst [25,26], and luminescence sensing [27,28]. Similarly, they are applied in photodynamic therapy [29] and bio-probes [30–32] owing to their low cytotoxicity, redox stability, 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) Cl 2H O[39] were obtained according to the literature procedures. 2 2· 2 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 1 taken in the range of 4000–400 cm− 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 3 of 13

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 10 mL). The combined organic was washed with × 5% sodium bicarbonate (aq) (25 mL), dried over anhydrous Na2SO4, and concentrated in vacuo. 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. 1 H-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 3). The combined aqueous layers were extracted with × 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 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) Cl 2H O (207 mg, 0.40 mmol) and H L 2 2· 2 2 (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 = Molecules 2019, 24, x FOR PEER REVIEW 4 of 14

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 ℃ 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 powder. Yield: 117 mg (30%). 1H-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, Molecules 2019, 24, 4032 4 of 13 12H), 7.75 (d, J = 8.0 Hz, 2H), 7.80 (d, J = 4.0 Hz, 2H), 7.85 (d, J = 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 4.0(d, Hz, J 2H),= 8.0 7.90–7.94Hz, 2H). 13 (m,C-NMR 2H), 8.10–8.14 (400 MHz, (m, DMSO- 6H), 8.18–8.24d6) δ = 49.6, (m, 124.4, 6H), 8.31 124.9, (s, 2H),125.7,8.43 126.7, (d, J128.3,= 8.0 131.9, Hz, 2H) , 13 8.82–8.90132.9, 134.2, (m, 8H), 138.2, 9.11 138.4, (d, J = 138.8,8.0 Hz, 141.5, 2H). 147.2,C-NMR 148.2, (400 151.7, MHz, 151.9, DMSO- 152.0,d6 )152.5,δ = 49.6 157.0,, 124.4, 157.1, 124.9, 157.3, 125.7, 126.7,163.4. 128.3, ESI-HRMS: 131.9, 132.9, m/ 134.2,z = 343.0735 138.2, 138.4, [M − 138.8, 4PF6 141.5,]4+, 505.7527 147.2, 148.2, [M − 151.7, 3PF6] 151.9,3+, 831.1112 152.0,152.5, [M – 157.0,2PF6] 157.1,2+, + 4+ 3+ 4+ 2+ 157.3,1087.1864 163.4. ESI-HRMS:[M – PF6] . Theoreticalm/z = 343.0735 exact [M mass4PF for6 ]C74,H505.752756F24N16P [M4Ru2: 3PFm/z6 =] 343.0737, 831.1112 [M [M − 4PF – 2PF6] 6,] , + 3+ −2+ + − 4+ 1087.1864505.7535 [M [M – − PF 3PF6] 6.] Theoretical, 831.1132 [M exact − 2PF mass6] , 1086.1885 for C74H56 [MF24 −N PF16P6]4.Ru Found:2: m/z C,= 65.1;343.0737 H, 4.0; [M N, 4PF16.7.6] , 3+ 2+ + −1 − 505.7535Calculated [M for3PF C6]74H,56 831.1132F24N16P4Ru [M2: C,2PF 64.81;6] , 1086.1885H, 4.1; N, [M16.34%.PF6 ]IR. Found:(KBr, cm C,) 65.1; 3445br H, 4.0;1624m, N, 16.7. − − − 1 Calculated1528m, 1465m, for C74 H1434m,56F24N 1312w,16P4Ru 1241w,2: C, 64.81; 1169w, H, 4.1;1047w, N, 16.34%. 837s, 764m, IR (KBr, 728w, cm 558m.− ) 3445br 1624m, 1528m, 1465m, 1434m, 1312w, 1241w, 1169w, 1047w, 837s, 764m, 728w, 558m. 3. Result and Discussion 3. Result and Discussion 3.1. Synthesis 3.1. Synthesis A novel dinuclear Ru(II) polypyridyl complex containing 1,8-naphthyridine was first designedA novel and dinuclear synthesized. Ru(II) polypyridylThe target complex complex was containing prepared 1,8-naphthyridine in four steps (Scheme was first 1). designed The andstarting synthesized. material The 2,7-methyl-1,8-naphthyridine target complex was prepared was in foursynthesized steps (Scheme according1). The to startingthe literature material 2,7-methyl-1,8-naphthyridine[37,38]. In the first step, 2,7-methyl-1,8-naphthyridine was synthesized according towas the oxidized literature to [37 dialdehyde,38]. In the by first the step, 2,7-methyl-1,8-naphthyridineselenium dioxide. The Schiff was base oxidized was obtained to dialdehyde in good by theyield selenium with the dioxide. reaction The of Schidialdehydeff base was obtainedand 5-amino-1,10-phenanthroline. in good yield with the reaction The of Schiff dialdehyde base was and then 5-amino-1,10-phenanthroline. reduced by sodium borohydride The Schi ff basetowas give then the reducedligand H by2L. sodium All the borohydrideorganic compounds to give the were ligand purified H2L. by All column the organic chromatography. compounds were purified by column chromatography. cis-[Ru(bpy) Cl ] H O reacted with the ligand H L to give the cis-[Ru(bpy)2Cl2]·H2O reacted with the ligand H2L to2 ·give2 the complex [{Ru(bpy)2}2(μ2-H2L)]Cl4. µ µ complexThen, [{Ru(bpy)the Cl− ion2} 2of( 2[{Ru(bpy)-H2L)]Cl42}.2( Then,μ2-H2L)]Cl the Cl4 −wasion exchanged of [{Ru(bpy) by2} 2the( 2 PF-H62−L)]Cl of saturated4 was exchanged aqueous by µ 1 13 theNH PF64−PFof6 saturatedto obtain aqueous[{Ru(bpy) NH2}2(4μPF2-H6 to2L)](PF obtain6)4 [{Ru(bpy). The 1H-NMR,2}2( 2-H 13C-NMR,2L)](PF6) 4and. The MSH-NMR, spectra ofC-NMR, the andcomplex MS spectra were of consistent the complex with were the consistentexpected structure. 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 Figure1. 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]. Molecules 2019, 24, x FOR PEER REVIEW 5 of 14

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 2,2’-bipyridine and 1,10-phenanthroline unit and that the LUMO + 2 was localized primarily on the two 2,2’-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 Molecules 2019, 24, 4032 5 of 13 transfer (PET) process in the new complexes [42–44].

Figure 1. Electron distribution and energy diagram of HOMO and LUMO orbitals for the complex 4+. [{Ru(bpy)2}2(µ2-H2L)]

3.3. Selectivity of Complex to Metal Ions Figure 1. Electron distribution and energy diagram of HOMO and LUMO orbitals for the The selectivity of the complex to metal ions was evaluated with fluorescence experiments. complex [{Ru(bpy)2}2(μ2-H2L)]4+. 5 The fluorescence experiments were recorded with a concentration of 10− mol/L in the presence of tetrabutyl perchlorate amine (TBAP, 0.1 mol/L) in CH CN/H O (1:1, v/v) at room temperature, then 5.0 3.3. Selectivity of Complex to Metal Ions 3 2 equivalents of nitrate or perchlorate salts of metal ions (Ag+, Ca2+, Co2+, Ba2+, Cr3+, Cd2+, Ni2+, Li+, TheNa+ selectivity, Mg2+, Zn2 of+, Mnthe2 complex+, Fe3+, Fe to2+ ,metal Pb2+, ions Hg2+ was, and evaluated Cu2+) were with added fluor toescence the solution, experiments. respectively. The fluorescenceThe fluorescence experiments properties arewere recorded recorded and with compared a concentration in Figure2. As of shown10−5 mol/L in Figure in the2, the presence complex of tetrabutylshowed a strongperchlorate fluorescence amine at 612(TBAP, nm. However,0.1 mol/L) the fluorescencein CH3CN/H of the2O solution(1:1, v was/v) quenchedat room to 2+ 3+ temperature,48 a.u. and then 23 a.u.,5.0 equivalents respectively, whenof nitrate Cu orand perchlorate Fe were added.salts of The metal quenching ions (Ag of the+, Ca fluorescence2+, Co2+, intensities with Pb2+ and Hg2+ was small. All other metal ions tested did not cause any significant Ba2+, Cr3+, Cd2+, Ni2+, Li+, Na+, Mg2+, Zn2+, Mn2+, Fe3+, Fe2+, Pb2+, Hg2+, and Cu2+) were added to the change to the luminescence intensity of the solution. The phenomena of the fluorescence indicated solution, respectively. The properties are recorded and compared in Figure 2. As that the complex had a selectivity for Cu2+ and Fe3+ ions. Therefore, it can be used as a “turn-off” shown in Figure 2, the complex2 +showed3+ a strong fluorescence at 612 nm. However, the fluorescent chemosensor for Cu and Fe in the CH3CN/H2O media. This dramatic quenching of the 2+ fluorescenceinitial luminescence of the solution of RuL was induced quenched by Cu to2+ 48and a.u. Fe3 +andwas 23 due a.u., to therespectively, reverse photoinduced when Cu electronand 3+ 2+ 2+ Fe weretransfer added. (PET) The from quenching the 2,20-bipyridine of the fluorescence ruthenium (II) intensities moieties to thewith 1,8-naphthyridine Pb and Hg was nitrogen small. atom All otherbecause metal of theions decrease tested did in the not electron cause densityany significant upon Cu change2+ and Fe to3+ thecomplexation luminescence [45]. intensity In order to 2+ 3+ of thefurther solution. evaluate The phenomena the selectivity of of the [{Ru(bpy) fluorescence2}2(µ2-H 2indicatedL)](PF6)4 for that detecting the complex Cu and had Fe a selectivity, competitive for Cuexperiments2+ and Fe3+ ions. were Therefore, carried out atit can room be temperature. used as a “turn-off” Five equivalents fluorescent of Cu 2chemosensor+ and Fe3+ were for added Cu2+ to + and Fethe3+ in solution the CH of3CN/H the complex2O media. (CH 3ThisCN/ Hdramatic2O = 1:1, quenchingv/v), respectively. of the Then,initial the luminescence other metal ionsof RuL (Ag , 2+ 2+ 2+ 3+ 2+ 2+ + + 2+ 2+ 2+ 2+ 2+ 2+ inducedCa by, Co Cu2+, and Ba ,Fe Cr3+ was, Cd due, Ni to the, Li reverse, Na , Mgphotoinduced, Zn , Mn electron, Fe , transfer Pb , Hg (PET)) were from added the to the mixed solution, respectively (Figure3; Figure4). As shown in Figures3 and4, the fluorescence 2,2’-bipyridine ruthenium (II) moieties to the 1,8-naphthyridine nitrogen atom because of the of the complex with Cu2+ had no obvious influence when the other metal ions existed in the system. decrease in the electron density upon Cu2+ and Fe3+ complexation [45]. In order to further However, Ca2+, Cr3+, and Zn2+ had a small influence, and the other ions had no obvious influence in 2+ 3+ evaluatethe Fethe3+ ionselectivity experiments. of [{Ru(bpy) Therefore,2}2 the(μ2-H complex2L)](PF can6)4 befor used detecting as an excellent Cu and fluorescent Fe , competitive chemosensor for detecting Cu2+ and Fe3+ ions [14,46]. Molecules 2019, 24, x FOR PEER REVIEW 6 of 14

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 Molecules 2019, 24, x FOR PEER REVIEW 6 of 14 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, experimentsthe fluorescence were of carriedthe complex out at with room Cu temperature.2+ had no obvious Five influence equivalents when of theCu 2+other and metalFe3+ were ions addedexisted to in the the solutionsystem. However,of the complex Ca2+, Cr(CH3+, 3andCN/H Zn2O2+ had= 1:1, a smallv/v), respectively. influence, and Then, the otherthe other ions metalhad no ions obvious (Ag+, influenceCa2+, Co2+ ,in Ba the2+, CrFe3+3+, ionCd 2+experiments., Ni2+, Li+, Na Therefore,+, Mg2+, Zn the2+, Mn complex2+, Fe2+ ,can Pb 2+be, Hg used2+) wereas an addedexcellent to thefluorescent mixed solution, chemosensor respectively for detecting ( Figure Cu 3;2+ and Figure Fe3+ ions4). As [14,46]. 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 3+ had no obvious influence in the Fe ion experiments.Ag+, Ca Therefore,2+, Co2+, Ba2+, Cr 3+the,Cd2+ ,complex can be used as an 1200 excellent fluorescent chemosensor for detecting Cu Ni2+2+, andLi+, Na +Fe, Mg3+2+ ,ions Zn2+, Mn [14,46].2+ [{Ru(bpy) } (μ -H L)]4+ Molecules 2019, 24, 4032 2 2 2 2 6 of 13 Fe2+ 1000

+ 2+ 2+ 2+ 3+ 2+ Ag , Ca2+ , Co , Ba , Cr ,Cd , 1200 Pd 800 Ni2+, Li+, Na+, Mg2+, Zn2+, Mn2+ [{Ru(bpy) } (μ -H L)]4+ 2 2 2 2 Fe2+ 1000 600

Pd2+ Intensity (a.u.) Intensity 800 2+ 400 Hg

Cu2+ 600 200 3+ Fe

Intensity (a.u.) Intensity 2+ 400 Hg 0 500 550 600 650 700 750 800 Cu2+

200 wavelength/nm 3+ Fe

0 Figure 2. Fluorescence500 spectra 550 of 600 the complex 650 [{Ru(bpy) 7002}2( 750μ2-H2L)]4+ 800(10−5 mol/L in

CH3CN/H2O = 1:1, v/v, excitation wavelengthwavelength/nm = 450 nm) upon the addition of 5.0 equivalents of various metal ions at room temperature. 4+ 5 Figure 2. Fluorescence spectra of the complex [{Ru(bpy)2}2(µ2-H2L)] (10− 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 roomFigure temperature. 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 Complex + other metal ions + Cu2+ of various metal ions at room temperature. Complex + other metal ions Complex + Cu2+ 1200

Complex + other metal ions + Cu2+ 1000 Complex + other metal ions Complex + Cu2+ 1200

) 800 a.u. ( 1000600 ) Intensity 800400 a.u. ( 600200

Intensity 4000 1 2 3 4 5 6 7 8 9 10111213141516

200 4+ 5 Figure 3. Changes in the fluorescence intensity of the complex [{Ru(bpy)2}2(µ2-H2L)] (10− mol/L in 2+ CH3CN/H2O = 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Cu in the presence of 4+ other metal ions. The0 red bars represent the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)] upon the addition of 5.0 equiv. of1 Cu2 2+ in 3 the4 presence 5 6 of: 71 Cd 82+, 2 9 Pb 101112131415162+, 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 4+ the changes in the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)] upon the addition of other metal 4+ ions, respectively. The blue bar represents the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)] when 5.0 equiv. of Cu2+ were added. Molecules 2019, 24, x FOR PEER REVIEW 7 of 14

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. Molecules 2019, 24, 4032 7 of 13

Complex + other metal ions + Fe3+ Complex + other metal ions Complex + Fe3+ 1200

1000

800

600

400 Intensity(a.u.)

200

0 1 2 3 4 5 6 7 8 9 10111213141516

4+ 5 Figure 4. Changes in the fluorescence intensity of the complex [{Ru(bpy)2}2(µ2-H2L)] (10− mol/L in 3+ CH3CN /H2O = 1:1, v/v, excitation wavelength = 450 nm) toward Fe in the presence of other metal 4+ ions. The red bars represent the fluorescence intensity of [{Ru(bpy)2}2(µ2-H2L)] when 5.0 equiv. of 3+ + 2+ 2+ 2+ 2+ 3+ 2+ 2+ + FeFigurewere 4. added Changes in the in presence the fluorescence of: 1 Ag , 2 Baintensity, 3 Ca of, 4 Cdthe complex, 5 Co , 6[{Ru(bpy) Cr , 7 Fe 2}2,( 8μ Hg2-H2L)], 9 Li4+ (10, −5 2+ 2+ + 2+ 2+ 2+ 2+ 10mol/L Mg ,in 11 CH Mn3CN, 12 /H Na2O ,= 13 1:1, Ni v/v,, 14 excitation Pb , 15 Zn wavelength, 16 Cu ;= the 450 green nm) barstoward represent Fe3+ in the the changes presence in fluorescence intensity of [{Ru(bpy) } (µ -H L)]4+ when other metal ions were added, respectively. of other metal ions. The red bars 2represen2 2 2 t the fluorescence intensity of [{Ru(bpy)2}2(μ2-H2L)]4+ The blue bar represents the fluorescence intensity of [{Ru(bpy) } (µ -H L)]4+ when 5.0 equiv. of Fe3+ when 5.0 equiv. of Fe3+ were added in the presence of: 1 Ag2 2 +, 22 Ba2 2+, 3 Ca2+, 4 Cd2+, 5 Co2+, 6 Cr3+, were added. 7 Fe2+, 8 Hg2+, 9 Li+, 10 Mg2+, 11 Mn2+, 12 Na+, 13 Ni2+, 14 Pb2+, 15 Zn2+, 16 Cu2+; the green bars 2+ 3+ 3.4. Sensitivityrepresent of the Complex changes to in Cu fluorescenceand Fe Ions intensity of [{Ru(bpy)2}2(μ2-H2L)]4+ when other metal ions Fluorescencewere added, titrationrespectively. experiments The blue forbar therepres complexents the with fluorescence the progressive intensity addition of [{Ru(bpy) of Cu22}+2(μand2- 4+ 3+ Fe3+ ionsH2L)] were when carried 5.0 equiv. out. As of shownFe were in added. Figures 5 and6, 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 3.4. Sensitivity of Complex to Cu2+ and Fe3+ Ions equivalents. The metal ions combined with the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4, which made the µ electronFluorescence transfer process titration easier experiments than in the complex for the [{Ru(bpy) complex2} 2with( 2-H the2L)](PF progressive6)4 [46–48 ].addition There was of aCu2+ good linear relationship between the fluorescence intensity and the concentration of Cu2+ ions with and Fe3+ ions were carried out. As shown in Figures 5 and 6, the fluorescence intensity of the the correlation coefficient of R = 0.9987 when the concentration of Cu2+ varied from 5 10 7 mol/L 2+ − 3+ complex5 gradually decreased at 612 nm with an increasing concentration of Cu× and Fe ions to 10− mol/L (Figure7). There was a linear relationship between the fluorescence intensity and the concentrationfrom 0 to 1.5 of Fe equivalents.3+ ions with the The correlation metal coeionsffi cientcombined of R = 0.9501 with whenthe thecomplex concentration [{Ru(bpy) of Fe23}+2(μ2- variedH2L)](PF from6)4 6.25, which10 6 molmade/L to 8.0the 10electron6 mol/ L (Figuretransfer8). Furthermore,process easier the equation:than in 3 σ/thek (where complexσ × − × − is[{Ru(bpy) the standard2}2(μ deviation2-H2L)](PF of blank6)4 [46–48]. measurements, There kwas is the slopea good between linear intensity relationship vs. the concentration between the offluorescence ions) was used intensity to obtain and the the detection concentration limit of Cu of2 Cu+ and2+ ions Fe3+ with[48,49 the]. The correlation detection coefficient limits of Cu of2+ R = and0.9987 Fe3 +whenwere 39.9the concentration nmol/L and 6.68 of nmol Cu2+/L, varied which from were 5 far × lower10−7 mol/L thanthe to 10 maximum−5 mol/L allowable(Figure 7). level There ofwas the a WHO linear for relationship drinking water. between The results the fluorescence suggested that intensity the complex and was the potentially concentration applicable of Fe for3+ ions 2+ 3+ quantitativewith the correlation analysis of coefficient Cu and Fe of Rions = 0.9501 in environmental when the concentration and biological systems.of Fe3+ varied from 6.25 × 10−6 mol/L to 8.0 × 10−6 mol/L (Figure 8). Furthermore, the equation: 3σ/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+ [48]. The detection Molecules 2019, 24, x FOR PEER REVIEW 8 of 14 limits of Cu2+ 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.

1200 Molecules 2019, 24, x FOR PEER REVIEW 8 of 14

1000 limits of Cu10002+ and Fe3+ were 39.9 nmol/L and 6.68 nmol/L, which were far lower than the 0 equiv. 800 maximum allowable level of the WHO for drinking water. The results suggested that the 600 2+ 2+ 3+ complex was800 potentiallyCu applicable for quantitative analysis of Cu and Fe ions in

environmental and biological systems. Intensity 400 Molecules 2019, 24, 4032 1.6 equiv. 8 of 13 200 600

1200 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Cu2+ equiv 1000 Intensity (a.u.) Intensity 400 1000 0 equiv. 800

600 200 2+ 800 Cu

Intensity 400 1.6 equiv. 200 0 600

550 600 6500 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Wavelength (nm) Cu2+ equiv Intensity (a.u.) Intensity 400

200 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/0v, excitation wavelength = 450 nm) upon the addition of Cu(ClO4)2 (0–1.0 equiv.) at room temperature;550 the plot 600 of emission 650 intensity 700 at 750 612 nm 800 vs. the added Cu2+ Wavelength (nm) equivalents is shown in µ 5 the inset. Figure 5. Emission spectra of the complex [{Ru(bpy)2}2( 2-H2L)](PF6)4 (10− 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 2+ Figuretemperature; 5. Emission the plot ofspectra emission of intensity the complex at 612 nm vs.[{Ru(bpy) the added2}2 Cu(μ2-Hequivalents2L)](PF6)4 is(10 shown−5 mol/L in in the inset. CH3CN/H2O = 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Cu(ClO4)2 (0–1.0 equiv.) at room1200 temperature; the plot of emission intensity at 612 nm vs. the added Cu2+

equivalents is shown in 1200 0 equiv 1000

the inset. 1000 800

600 Intensity 400 1 equiv 800 200

0 1200 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Fe3+ equiv. 1200

600 0 equiv 1000 1000 800

600 Intensity 400 1 equiv Intensity (a.u.) 400 800 200

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Fe3+ equiv.

200 600

Intensity (a.u.) 400 0 500 550 600 650 700 750 800 200 wavelenght (nm)

5 Figure 6. Emission spectra0 of the complex [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (10− mol/L in CH3CN/H2O 500 550 600 650 700 750 800 = 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Fe(ClO4)3 (0–1.6 equiv.) at room Figure 6. Emission spectra of the complex [{Ru(bpy)2}2(μ2-H2L)](PF6)4 (10−5 mol/L in temperature; the plot of emission intensity atwavelenght 612 nm vs. (nm) the added Fe3+ equivalents is shown in CH3CN/Hthe2O inset. = 1:1, v/v, excitation wavelength = 450 nm) upon the addition of Fe(ClO 4)3 (0–1.6 3+ equiv.)Figure at room 6. Emissiontemperature; spectra the of plot the ofcomplex emission [{Ru(bpy) intensity2}2(μ 2-Hat2 L)](PF612 nm6)4 (10vs.−5 themol/L added in Fe

equivalentsCH3CN/His shown2O = 1:1, in v / v, excitation wavelength = 450 nm) upon the addition of Fe(ClO4)3 (0–1.6 the inset.equiv.) at room temperature; the plot of emission intensity at 612 nm vs. the added Fe3+ equivalents is shown in the inset. Molecules 2019, 24, x FOR PEER REVIEW 9 of 14

Molecules 2019, 24, x FOR PEER REVIEW 9 of 14 7 1000 Y = -8.432 x 10 X+965.1 2 Molecules 2019, 24, 4032 R = 0.9987 9 of 13

800

7 6001000 Y = -8.432 x 10 X+965.1 R2 = 0.9987

400800 Intensity (a.u.)

200600

0400 Intensity (a.u.) 0.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 1.0x10-5

200

Figure 7. Plot of linear0 fitting for emission intensity vs. the concentration of Cu2+ (mol/L). 0.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 1.0x10-5

2+ Figure 7. Plot of linear fitting for emission intensity vs. the concentration of Cu (mol/L).

Figure 7. Plot of linear fitting for emission intensityY=-5.036 vs. the x 10 concentration8X + 4222.9 of Cu2+ (mol/L). 1000 R2=0.9501

800

Y=-5.036 x 108X + 4222.9 1000 600 R2=0.9501

800 Intensity (a.u.) Intensity 400

600 200

-6 -6 -6 -6 -6 -6 -6 -6 -6 -6 Intensity (a.u.) Intensity 6.2x10400 6.4x10 6.6x10 6.8x10 7.0x10 7.2x10 7.4x10 7.6x10 7.8x10 8.0x10

3+ Figure 8. Plot of linear fitting for emission intensity vs. the concentration of Fe (mol/L). 200 3.5. Mode of Binding with Cu2+ and Fe3+ Figure 8. Plot of linear fitting for emission intensity vs. the concentration of Fe3+ (mol/L). 6.2x10-6 6.4x10-6 6.6x10-6 6.8x10-6 7.0x10-6 7.2x102+ -6 7.4x103-6+7.6x10-6 7.8x10-6 8.0x10-6 In order to explore the binding stoichiometry of Cu and Fe with [{Ru(bpy)2}2(µ2-H2L)](PF6)4, Job’s3.5. plot Mode analyses of Binding were usedwith Cu to2+ determine and Fe3+ binding stoichiometries. The analysis results suggested that [{Ru(bpy) } (µ -H L)](PF ) binds with Cu2+ and Fe3+ in 3:2 and 1:1 stoichiometry ratios, respectively In2 2order2 2to explore6 4 the binding stoichiometry of Cu2+ and Fe3+ with [{Ru(bpy)2}2(μ2- (Figures9 and 10). The ESI-MS experiments were further carried out by adding 2.0 equivalents of Cu 2+ H2L)](PF6Figure)4, Job’s 8. plotPlot ofanalyses linear fitting were for used emission to determ intensityine vs.binding the concentration stoichiometries. of Fe3+ (mol/L).The analysis and Fe3+ to the acetonitrile solution of [{Ru(bpy) } (µ -H L)](PF ) , respectively (Figures S9 and S10). results suggested that [{Ru(bpy)2}2(μ2-H2L)](PF2 2 62)4 binds2 with6 4 Cu2+ and Fe3+ in 3:2 and 1:1 10+ Thestoichiometry peaks3.5. Mode at m of/ z Bindingratios,= 424.33 respectivelywith andCu2+ 646.05and (FiguresFe3+were 9 assigned and 10). toThe [M ESI-MS12PF experiments6 4ClO4 were6H] furtherand 7+ − − − [M 12PF6 ClO4 6H] , respectively. The calculated2+ molecular3+ weights of [M 12PF6 4ClO4 −carried −out by− adding 2.0 equivalents of Cu and Fe to2+ the acetonitrile3+ − solution− of 6H]10+ andIn order [M to12PF exploreClO the binding6H]7+ stoichiometrywere 423.70 andof Cu 647.65,and respectively.Fe with [{Ru(bpy) The M2}2 was(μ2- [{Ru(bpy)2}2(μ2-H2L)](PF6 6)4, respectively4 (Figures S9 and S10). The peaks at m/z = 424.33 and − H2L)](PF6)4, Job’s− plot analyses− −were used to determine binding stoichiometries. The analysis [({Ru(bpy)2}2(µ2-H2L))3Cu2](PF6)12(ClO4)4 (Figure S11a).10+ The peaks at m/z = 254.407+ and 335.41 646.05 were assigned to [M − 12PF6 − 4ClO4 − 6H] and [M − 12PF6 − ClO2+ 4 − 6H]3+ , respectively. were attributedresults suggested to [M that4PF [{Ru(bpy)2ClO ]62}+2(andμ2-H [M2L)](PF3PF6)4 binds2ClO with]5 +Cu, respectively. and Fe in The 3:2 calculated and 1:1 The calculated molecular6 weights 4of [M − 12PF6 − 4ClO6 4 − 6H]10+4 and [M − 12PF6 − ClO4 − 6H]7+ stoichiometry ratios,− respectively− (Figures6+ 9 −and 10).− The ESI-MS5+ experiments were further molecular weights of [M 4PF6 2ClO4] and [M 3PF6 2ClO4] were 254.53 and 334.43. The M were 423.70 and 647.65,− respectively.− The M− was2+ [({Ru(bpy)− 3+ 2}2(μ2-H2L))3Cu2](PF6)12(ClO4)4 was [({Ru(bpy)carried out} ( µby-H addingL))Fe](PF 2.0 ) equivalents(ClO ) (Figure of S11b),Cu and respectively. Fe to the This acetonitrile indicated that solution the ratios of (Figure S11a2 2). The2 peaks2 at m6/z4 = 254.404 3 and 335.41 were attributed to [M − 4PF6 − 2ClO4]6+ and [{Ru(bpy)2}2(μ2-H2L)](PF6)4, respectively 2(Figures+ S93+ and S10). The peaks at m/z = 424.33 and of [{Ru(bpy)2}2(µ2-H2L)](PF6)4 bound with Cu and Fe were 3:2 and 1:1, respectively. The results 10+ 7+ were consistent646.05 were with assigned Job’s plot to [M analyses. − 12PF6 − 4ClO4 − 6H] and [M − 12PF6 − ClO4 − 6H] , respectively. The calculated molecular weights of [M − 12PF6 − 4ClO4 − 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 − 4PF6 − 2ClO4]6+ and Molecules 2019, 24, x FOR PEER REVIEW 10 of 14

[M − 3PF6 − 2ClO4]5+, respectively. The calculated molecular weights of [M − 4PF6 − 2ClO4]6+ and Molecules[M − 3PF 20196 −, 242ClO, x FOR4]5+ PEER were REVIEW 254.53 and 334.43. The M was [({Ru(bpy)2}2(μ2-H2L))Fe](PF6)4(ClO 10 of 144)3 (Figure S11b ), respectively. This indicated that the ratios of [{Ru(bpy)2}2(μ2-H2L)](PF6)4 bound 2+ 3+ [Mwith − 3PF Cu6 −and 2ClO Fe4]5+ were, respectively. 3:2 and 1:1,The respectively.calculated molecular The results weights were of consistent [M − 4PF 6with − 2ClO Job’s4]6+ andplot [Manalyses. − 3PF6 − 2ClO4]5+ were 254.53 and 334.43. The M 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 0.35 10 of 13

0.30

0.25 0.35

0.20 0.30

*X 0.15

0 0.25 /I 0 I 0.10 0.20

0.05

*X 0.15 0 /I 0 I 0.00 0.10

-0.05 0.05 0.00.10.20.30.40.50.60.70.80.91.0 2+ 2+ 0.00 X=[[Cu ]/{[Cu ]+[{Ru(bpy) } (µ -H L)](PF ) ]} 2 2 2 2 6 4 -0.05 0.00.10.20.30.40.50.60.70.80.91.0 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+ 2+ 2+ binding. X=[[Cu ]/{[Cu ]+[{Ru(bpy)2}2(µ2-H2L)](PF6)4]}

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.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

0.350.25

0.300.20 *X 0

/I 0.250.15 0 I

0.200.10 *X 0 0.05 /I 0.15 0 I

0.100.00

0.05-0.05 0.00.10.20.30.40.50.60.70.80.91.0 0.00 3+ 3+ X=[[Fe ]/{[Fe ]+[{Ru(bpy)2}2(µ2-H2L)](PF6)4]} -0.05 4+ 3+ Figure 10. Jobs plot in CH30.00.10.20.30.40.50.60.70.80.91.0CN/H2O(v/v, 1:1) of the complex [{Ru(bpy)2}2(µ2-H2L)] and Fe binding. 4+ 3+ Figure 10. Jobs plot in CH3CN/H3+2O (v/v3+, 1:1) of the complex [{Ru(bpy)2}2(μ2-H2L)] and Fe 4. Conclusions X=[[Fe ]/{[Fe ]+[{Ru(bpy)2}2(µ2-H2L)](PF6)4]} binding. In summary,a novel fluorescent chemosensor was synthesized from 2,7-dimethyl-1,8-naphthyridine 4. Conclusion 1 13 and 5-amino-1,10-phenanthrolineFigure 10. Jobs plot in CH.3 TheCN/HH-NMR,2O (v/v, 1:1) C-NMR,of the complex and ESI-HRMS [{Ru(bpy)2}2 results(μ2-H2L)] were4+ and consistent Fe3+ with the expectedbinding. 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+ 4.or Conclusion 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. Figure S1: 1H-NMR spectrum for 1 1,8-naphthyridine-2,7-dicarbaldehyde (in CDCl3), Figure S2: H-NMR spectrum for L (in DMSO-d6 and a Molecules 2019, 24, 4032 11 of 13

1 small amount of CDCl3), Figure S3: H-NMR spectrum for H2L (in DMSO-d6 and a small amount of CDCl3). 1 13 Figure S4: H-NMR spectrum for [{Ru(bpy)2}2(µ2-H2L)](PF6)4 (in DMSO-d6), Figure S5: C-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.

References

1. Frausto da Silva, J.; Williams, R.J.P. The Biological Chemistry of the Elements; Clarendon Press: Oxford, UK, 1993; p. 539. 2. Darwish, I.A.; Blake, D.A. Development and validation of a one-step immunoassay for determination of cadmium in human serum. Anal. Chem. 2011, 74, 52–58. [CrossRef][PubMed] 3. Fang, M.; Xia, S.; Bi, J.; Travis, P.; Wigstrom, T.P.; Valenzano, L.; Wang, J.; Tanasova, M.; Rudy, L.; Luck, R.L.; et al. Detecting Zn(II) ions in live cells with near-infrared fluorescent probes. Molecules 2019, 24, 1592. [CrossRef] 4. Bi, J.; Fang, M.; Wang, J.; Xia, S.; Zhang, Y.; Zhang, J.; Vegesna, G.; Zhang, S.; Marina Tanasova, M.; Luo, F.T.; et al. Near-infrared fluorescent probe for sensitive detection of Pb(II) ions in living cells. Inorg. Chim. Acta. 2017, 468, 140–145. [CrossRef][PubMed] 5. Liu, X.; Theil, E.C. Ferritins: Dynamic management of biological iron and oxygen chemistry. Acc. Chem. Res. 2005, 38, 167–175. [CrossRef][PubMed] 6. Da Silvaand, J.J.R.F.; Williams, R.J.P. In the Biological Chemistry of the Elements: The Inorganic Chemistry of Life; Clarendon: Oxford, UK, 1991. 7. De Silva, A.P.; Gunaratne, H.Q.N.; Gunlaugsson, T.; Huxley, A.J.M.; McCoy, C.P.; Rademacher, J.T.; Rice, T.E. Signaling recognition events with fluorescent sensors and switches. Chem. Rev. 1997, 97, 1515–1566. [CrossRef][PubMed] 8. Mense, S.M.; Zhang, L. Heme: A versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases. Cell Res. 2006, 16, 681–692. [CrossRef] 9. Domink, H.; Svetlana, L. Wilson disease: Not just a copper disorder. Analysis of a Wilson disease model demonstrates the link between copper and lipid metabolism. Mol. Biosyst. 2007, 3, 816–824. 10. Georgopoulos, P.G.; Roy, A.; Yonone-Lioy, M.J.; Opiekun, R.E.; Lioy, P.J. Environmental copper: Its dynamics and human exposure issues. J. Toxicol. Environ. Health B 2001, 4, 341–394. [CrossRef] 11. Praveen, L.; Reddy, M.L.P.; Varma, R.L. Dansyl-styrylquinoline conjugate as divalent iron sensor. Tetrahedron Lett. 2010, 51, 6626–6629. [CrossRef] 12. Wang, K.P.; Lei, Y.; Zhang, S.J.; Zheng, W.J.; Chen, J.P.; Chen, S.; Hu, Z.Q. Fluorescent probe for Fe(III) with high selectivity and its application in living cells. Sens. Actuator B-Chem. 2017, 252, 1140–1145. [CrossRef] 13. Schmitteland, M.; Lin, H. Quadruple-channel sensing: A molecular sensor with a single type of receptor site for selective and quantitative multi-ion analysis. Angew. Chem. Int. Ed. 2007, 46, 893–896. [CrossRef] [PubMed] 14. Wang, L.; Ye, D.; Li, W.; Liu, Y.; Li, L.; Zhang, W.; Ni, L. Fluorescent and colorimetric detection of Fe(III) and Cu(II) by a difunctional rhodamine-based probe. Spectrochim. Acta Part A 2017, 183, 291–297. [CrossRef] [PubMed] 2+ 15. Ajayakumar, G.; Sreenath, K.; Gopidas, K.R. Phenothiazine attached Ru(bpy)3 derivative as highly selective turn-ON luminescence chemodosimeter for Cu2+. Dalton Trans. 2009, 21, 1180–1186. [CrossRef][PubMed] Molecules 2019, 24, 4032 12 of 13

16. Qian, B.; Váradi, L.; Trinchi, A.; Suzie, M.; Reichman, S.M.; Bao, L.; Lan, M.; Wei, G.; Ivan, S.; Cole, I.S. The design and synthesis of fluorescent coumarin derivatives and their study for Cu2+ sensing with an application for aqueous Soil extracts. Molecules 2019, 24, 3569. [CrossRef] 17. Wang, J.; Wei, T.; Ma, F.; Li, T.; Niu, Q. A novel fluorescent and colorimetric dual-channel sensor for the fast, reversible and simultaneous detection of Fe3+ and Cu2+ based on terthiophene derivative with high sensitivity and selectivity. J. Photoch. Photobiol. A 2019, 383, 111982–111990. [CrossRef] 18. Kalyanasundaram, K. Photophysics, and solar energy conversion with tris(bipyridyl)ruthenium(II) and its analogs. Coord. Chem. Rev. 1982, 46, 159–244. [CrossRef] 19. Hara, M.; Waraksa, C.C.; Lean, J.T.; Lewis, B.A.; Mallouk, T.E. Photocatalytic water oxidation in a buffered

Tris(2,20-bipyridyl) ruthenium complex-colloidal IrO2 system. J. Phys. Chem. A 2000, 104, 5275–5280. [CrossRef] 20. Elmorsy, R.M.; Su, R.; Faddab, A.A.; Etmanb, H.A.; Tawfik, E.H.; El-Shafei, A. Effect of terthiophene spacer position in Ru(II) bipyridyl complexes on the photocurrent and photovoltage for high efficiency dye-sensitized solar cells. Dyes Pigments 2018, 156, 348–356. [CrossRef] 21. Medved’ko, A.V.; Ivanov, V.K.; Kiskin, M.A.; Sadovnikov, A.A.; Apostolova, E.S.; Grinberg, V.A.; Emets, V.V.; Chizhov, A.O.; Nikitin, O.M.; Magdesieva, T.V.; et al. The design and synthesis of thiophene-based ruthenium(II) complexes as promising sensitizers for dye-sensitized solar cells. Dyes Pigments 2017, 140, 169–178. [CrossRef] 22. Morita, K.; Sakai, K.; Ozawa, H. A new class of molecular-based photoelectrochemical cell for solar hydrogen

production consisting of two mesoporous TiO2 Electrodes. ACS Appl. Energy Mater. 2019, 2, 987–992. [CrossRef] 23. Santiago-Portillo, A.; Baldoví, H.G.; Carbonell, E.; Navalón, S.; Álvaro, M.; García, H.; Ferrer, B.

Ruthenium(II) tris(2,20-bipyridyl) complex incorporated in UiO-67 as photoredox catalyst. J. Phys. Chem. C 2018, 122, 29190–29199. [CrossRef] 24. Li, X.; Hao, Z.; Zhang, F.; Li, H. Reduced graphene oxide-immobilized tris(bipyridine) ruthenium(II) complex for efficient visible-light-driven reductive dehalogenation reaction. ACS Appl. Mater. Interfaces 2016, 8, 12141–12148. [CrossRef][PubMed] 25. Tong, L.; Thummel, R.P. Mononuclear ruthenium polypyridine complexes that catalyze water oxidation. Chem. Sci. 2016, 7, 6591–6603. [CrossRef][PubMed] 26. Rabten, W.; Kärkaäs, M.D.; Åkermark, T.; Chen, H.; Liao, R.; Tinnis, F.; Sun, J.; Siegbahn, P.E.M.; Andersson, P.G.; Åkermark, B. Catalytic water oxidation by a molecular ruthenium complex: Unexpected generation of a single-site water oxidation catalyst. Inorg. Chem. 2015, 54, 4611–4620. [CrossRef] 27. Meng, T.; Wang, H.; Zheng, Z.; Wang, K. pH-switchable Off-On-Off near-infrared luminescence based on a dinuclear ruthenium(II) complex. Inorg. Chem. 2017, 56, 4775–4779. [CrossRef] 28. Cheng, D.; Pan, Y.; Wang, L.; Zeng, Z.; Yuan, L.; Zhang, X. Selective visualization of the endogenous peroxynitrite in an inflamed mouse model by a mitochondria-targetable two-photon ratiometric fluorescent probe. J. Am. Chem. Soc. 2017, 139, 285–292. [CrossRef] 29. Cloonan, S.M.; Elmes, R.B.P.; Erby, M.L.; Bright, S.A.; Poynton, F.E.; Nolan, D.E.; Quinn, S.J.; Gunnlaugsson, T.; Williams, D.C. Detailed biological profiling of a photoactivated and apoptosis inducing pdppz ruthenium(II) polypyridyl complex in cancer cells. J. Med. Chem. 2015, 58, 4494–4505. [CrossRef] 30. Gao, Q.; Zhang, W.; Song, B.; Zhang, R.; Guo, W.; Yuan, J. Development of a novel lysosome-targeted ruthenium(II) complex for phosphorescence/time-Gated luminescence assay of biothiols. Anal. Chem. 2017, 89, 4517–4524. [CrossRef] 31. Hara, D.; Komatsu, H.; Son, A.; Nishimoto, S.; Tanabe, K. Water-soluble phosphorescent ruthenium complex with a fluorescent coumarin unit for ratiometric sensing of oxygen levels in living cells. Bioconjugate Chem. 2015, 26, 645–649. [CrossRef] 32. Sheet, S.K.; Sen, B.; Thounaojam, R.; Aguan, K.; Khatua, S. Ruthenium(II) complex-based luminescent bifunctional probe for Ag+ and phosphate ions: Ag+-assisted detection and imaging of rRNA. Inorg. Chem. 2017, 56, 1249–1263. [CrossRef] 33. Gavrilova, A.L.; Bosnich, B. Principles of mononucleating and binucleating Ligand Design. Chem. Rev. 2004, 104, 349–384. [CrossRef][PubMed] Molecules 2019, 24, 4032 13 of 13

34. Nakatani, K.; Sando, S.; Saito, I. Recognition of a single guanine bulge by 2-acylamino-1,8-naphthyridine. J. Am. Chem. Soc. 2000, 122, 2172–2177. [CrossRef] 35. Hoock, C.; Reichert, J.; Schmidtke, M. Fluorescent 1,7-dialkylamino-[1,8]-naphthyridines: Preparation and spectroscopic properties. Molecules 1999, 4, 264–271. [CrossRef] 36. Nakataniz, K.; Sando, S.; Kumasawa, H.; Kikuchi, J.; Saito, I. Recognition of guanine-guanine mismatches by the dimeric form of 2-amino-1,8-naphthyridine. J. Am. Chem. Soc. 2001, 123, 12650–12657. [CrossRef] [PubMed] 37. Vu, C.; Walker, D.D.; Wells, J.; Fox, S. Elaboration of 1,8-naphthyridine-2,7-dicarboxaldehyde into novel 2,7-dimethylimine Derivatives. J. Heterocycl. Chem. 2002, 829, 829–832. [CrossRef] 38. Cavanaugh, M.A.; Cappo, V.M.; Alexander, C.J.; Good, M.L. Substituted 1,8-naphthyridine complexes of iron(II) and iron(III). Inorg. Chem. 1976, 11, 2615–2621. [CrossRef] 39. Chandler, C.J.; Deady, L.W.; Reiss, J.A.; Tzimos, V. The synthesis of macrocyclic polyether-diesters incorporating 1,10-phenanthrolino and 1,8-naphthyridino subunits. J. Heterocycl. Chem. 1982, 19, 1017–1019. [CrossRef] 40. Becke, A.D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 1372–1377. [CrossRef] 41. Sullivan, B.P.; Salmon, D.J.; Meyer, T.J. Monomeric and dimeric pyrazole and pyrazolyl complexes of ruthenium. Inorg. Chem. 1978, 17, 3334–3341. [CrossRef] 42. Best, Q.A.; Xu, R.S.; McCarroll, M.E.; Wang, L.C.; Dyer, D.J. Design and investigation of a series of rhodamine-based fluorescent probes for optical measurements of pH. Org. Lett. 2010, 12, 3219–3221. [CrossRef] 43. Zhang, S.T.; Li, P.; Liao, C.; Luo, T.; Kou, X.; Xiao, D. A highly sensitive luminescent probe based on Ru(II)-bipyridine complex for Cu2+, l-histidine detection and cellular imaging. Spectrochim. Acta Part A 2018, 201, 161–169. [CrossRef][PubMed] 44. Jiang, W.; Yang, S.; Lu, W.; Gao, B.; Xu, L.; Sun, X.; Jiang, D.; Xu, H.J.; Ma, M.; Cao, F. A novel fluorescence turn off-on nano-sensor for detecting Cu2+ and Cysteine in living cells. J. Photoch. Photobiol. A 2018, 362, 14–20. [CrossRef] 45. Anbu, S.; Ravishankaran, R.; Guedes da Silva, M.F.C.; Karande, A.A.; Pombeiro, A.J. Differentially selective chemosensor with fluorescence off on responses on Cu2+ and Zn2+ ions in aqueous media and applications − in pyrophosphate sensing, live cell imaging, and cytotoxicity. Inorg. Chem. 2014, 53, 6655–6664. [CrossRef] [PubMed] 46. Cheng, F.; He, C.; Ren, M.; Wang, F.; Yang, Y. Two dinuclear Ru(II) polypyridyl complexes with different photophysical and cation recognition properties. Spectrochim. Acta Part A 2015, 136, 845–851. [CrossRef] 47. Liao, Q.; Li, A.; Li, Z.; Jiang, Y. 2-Amino-5-(p-dimethylamino) phenyl-1, 3, 4-thiadiazoleasa selective fluorescent chemosensor for Hg2+. Chem. J. Chin. U. 2008, 29, 2531–2534. 48. Xu, J.; Liu, N.; Hao, C.; Han, Q.; Duan, Y.; Wu, J. A novel fluorescence on-off-on peptide-based chemosensor forsimultaneous detection of Cu2+, Ag+ and S2-. Sens. Actuator B 2019, 280, 129–137. [CrossRef] 49. Wang, L.; Qin, W.; Tang, X.; Dou, W.; Liu, W.; Teng, Q.; Yao, X. A selective, cell-permeable fluorescent probe for Al3+ in living cells. Org. Biomol. Chem. 2010, 8, 3751–3757. [CrossRef]

Sample Availability: Samples of the compounds are available from the authors.

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