A quinoline-based fluorescence “on-off-on” probe for relay identification of Cu2+ and Cd2+ ions

Accepted Manuscript A quinoline-based fluorescence “on-off-on” probe for relay identification of Cu2+ and Cd2+ ions

Juan Han, Xu Tang, Yun Wang, Renjie Liu, Lei Wang, Liang Ni PII: DOI: Reference:

S1386-1425(18)30745-5 doi:10.1016/j.saa.2018.07.081 SAA 16350

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

16 November 2017 25 July 2018 30 July 2018

Please cite this article as: Juan Han, Xu Tang, Yun Wang, Renjie Liu, Lei Wang, Liang Ni , A quinoline-based fluorescence “on-off-on” probe for relay identification of Cu2+ and Cd2+ ions. Saa (2018), doi:10.1016/j.saa.2018.07.081

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ACCEPTED MANUSCRIPT A quinoline-based fluorescence “on-off- on” probe for relay identification of Cu2+ and Cd2+ ions Juan Hana, Xu Tangb, Yun Wangb, Renjie Liub, Lei Wangb, Liang Nib,c a

School of Food and Biological Engineering, Jiangsu University, Zhenjiang, 212013,

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,

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b

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PR China

c

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212013, PR China

Key Laboratory of Preparation and Application of Environmental Friendly Materials

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(Jilin Normal University), Ministry of Education, Changchun, 130103, PR China

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Corresponding author:

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Juan Han, E-mail: [email protected]; Fax:+86-0511-88791800.

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Liang Ni, E-mail: [email protected]; Fax: +86-0511-88791800.

ACCEPTED MANUSCRIPT Abstract A novel fluorescent probe based 8-hydroxyquinoline (8-HQ) has been designed and synthesized. The probe 1 exhibits fast relay recognition performance for Cu2+ and Cd2+ via a fluorescence “on−off−on” response signal. The probe 1 itself has a strong

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emission peak at 471 nm in EtOH/H2O (v/v= 1:9) solution with a blue fluorescence. The addition of Cu2+ immediately results in the quenching of fluorescence and the

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limit of detection is 2.7 × 10-8 M. Moreover, the formation of [1-Cu2+] complexes can

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also serve as a new efficient probe system for the relay recognition of Cd2+ with the emergence of a new fluorescence signal under the same conditions. The study also

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found that the probe 1 has high selectivity for target ions in the presence of other competing ions. The probe 1 has been successfully applied to detect and analyze the

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trace amount of Cu2+ and Cd2+ in environmental water samples.

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Keywords: :Fluorescent probe, 8-hydroxyquinoline, Relay recognition, Cu2+ and

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Cd2+

ACCEPTED MANUSCRIPT 1. Introduction Copper, as the third most abundant essential trace elements in human body, plays very important roles in various physiological environmental processes1-7. With the extensive use of copper ions in industry and agriculture, a significant environmental

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pollutant of copper ions has attracted considerable attention. High concentration of copper is toxic and harmful seriously to environment, which will result in human’s

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health problem by food chain finally8. However, both its deficiency and excess from

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the normal permissible limit would lead to the physiological disorders and health

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issues such as Alzheimer's disease, Parkinson’s and coronary heart diseases9-14. Unlike copper, cadmium is not an essential element of the human body because

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of its toxicity to procreation, kidneys, nervous system. The widespread use of Cd2+ in many industrial processes not only brings considerable economic benefits, but also

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causes serious environmental pollution.The international agency for cancer research

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(IARC) classifies cadmium as a human carcinogen, causing serious health damage to humans. Meanwhile, the United States poisons and Disease Registry (ATSDR) lists

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cadmium as the seventh most harmful substance to human health15, 16. Cadmium can be enriched through the food chain in the human body for more than 10 years, causing

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chronic poisoning leading to renal dysfunction, calcium metabolism disorders and some forms of cancer incidence increased17. Itis well-known that “Itai Itai disease” is caused by chronic cadmium poisoning. At present, a series of analytical methods for the detection of cadmium and copper have been widely developed, including atomic absorption spectrometry (AAS)18, 19, inductively coupled plasma mass spectroscopy (ICP-MS)20, 21and cyclic voltammetry (CV)22. However, those classical methods are limited by tedious sample

ACCEPTED MANUSCRIPT pretreatment, time-consuming analysis and high-cost instrumentations, so they are very difficult to be used widely used in vivo and on-site detection. Comparatively, the fluorescent approaches hold significant advantages over those methods because of its high selectivity and sensitivity,low costs,short response time.Thus, the design and development of fluorescent probes that are capable of detecting important ion species

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inenvironmental and biological systems have attracted considerable attention and

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great efforts in recent years 23-25.

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Among these works been reported, there are only a few single cadmium ion selective fluorescent probes have been reported26, 27. At present, the greatest challenge

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of constructing cadmium ion selective fluorescent probes is to overcome the

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interference of other transition metal ions, especially zinc ions. Due to the resemblance of the two metal ionsin electronic configuration, similar spectral changes are usually observed when Zn2+ and Cd2+ are coordinated with fluorescent probes28-31.

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Thus development of artificial probes which can effectively and selectively recognize Cd2+ without the interference from Zn2+ still remains a challenging field of research32.

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Herein, we have designed and synthesized a novel quinoline based fluorescence

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probe 1 for the relay recognition of Cu2+ and Cd2+ in EtOH/H2O (v/v= 1:9) solution. The optical properties were studied by the fluorescence emission and absorption spectra. The probe 1 shows “on-off” property for Cu2+ with high selectivity and sensitivity. Furthermore, the formation of [1-Cu2+] complexes can also be used as an efficient new probe system for Cd2+ with “off-on” fluorescence signal under the same conditions. 2. Experimental

ACCEPTED MANUSCRIPT 2.1. Materials and instruments 8-hydroxyquinoline, 5-Chloro-2-hydroxyaniline and ethyl bromoacetate were purchased from Aladdin Chemical Reagent Ltd., and used without further purification. All the metal chlorate salts (Na+, Zn2+, Hg2+, Fe2+, Fe3+, Cu2+, Mn2+, Al3+, Cr3+, K+,

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Ca2+, Sr2+, Cs2+, Pb2+, Co2+, Li+ and Cd2+) and all solvents were purchased from Sinopharm Chemical Reagent Ltd.,The 1H NMR and 13C NMR spectra were recorded

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on an AVANCEII 400 MHz spectrometer (Bruker BioSpin). Mass Spectrometry (MS)

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were measured by a Liquid Chromatgraphy-Ion Trap Mass Spectrometry (Thermo LXQ). Fluorescence spectra measurements were performed on a Cary Eclipse

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fluorescence spectrometer (CaryEclipse) and UV-Vis spectra were recorded on UV-

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Vis spectrophotometer (UV-2450) 2.2. Synthesis

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A mixture of 8-hydroxy-2-methylquinoline (1.6 g, 10.05 mmol), BrCH2CO2Et

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(1.7 g, 10.18 mmol) and K2CO3 (5 g, 36.18 mmol) in acetone (20 mL) was heated under reflux for 24 h. After cooling, the mixture was filtered and evaporated to

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generate the crude residue. The final product a was obtained by column chromatography (mesh chromatography silica, 10:1 dichloromethane/ethyl acetate)

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Then, 0.75 g SeO2 was added to a 1,4-dioxane solution (20 mL) of a (1.5 g, 6 mmol) at 65 oC and the temperature of the above mixture was increased to 80 ℃. After 2 h, the mixture was cooled to room temperature. The precipitate was filtered off. Organic phase was concentrated in vacuum. The pure product b was obtained by recrystallization from ethyl acetate/hexane (1.167 g, 75% yield).1H NMR (400 MHz, DMSO-d6) δ 10.14 (d, 1H), 8.55 (d, 1H), 8.01 (d, 1H), 7.70 (m, 2H), 7.25 (m, 1H), 5.13 (d, 2H), 4.26 (m, 2H), 1.22 (t, 3H). (Fig. S1)

ACCEPTED MANUSCRIPT Finally, probe 1was synthesized from the reaction of b(1.04 g, 4mmol) and 5Chloro-2-hydroxyanilin (0.58 g, 4mmol) in ethanol with catalytic amount of acetic acid. (Scheme 1) The product was obtained as yellow solid with a yield of 80.4% (1.23 g).1H NMR (400 MHz, DMSO-d6) δ 8.6 (dd, 1H), 8.45 (dd, 1H), 8.05 (dd, 1H), 8.0 (dd, 1H), 7.82 – 7.56 (m, 3H), 7.23 (dd, 1H), 5.18 (s, 2H), 4.22 (q, 2H), 1.24 (t,

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3H) (Fig. S2); 13C NMR (101 MHz, CDCl3) δ 169.15, 163.40, 154.35, 150.01, 143.01,

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139.62, 138.10, 130.21, 129.91, 129.30, 127.05, 121.31, 120.81, 120.62, 113.41, 111.31, 65.82, 61.21, 14.51(Fig. S3). C20H15O4N2Cl (M = 382.5), MS (M + Na+): m/z

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= 405.23. (M+H+): m/z = 383.24(Fig. S4).

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Scheme 1 Synthesis and molecular structure of probe 1.

2.3. UV–vis and fluorescence spectra measurements Stock solutions (10 mM) of various metal ions were prepared from NaCl, CsCl, PbCl2, CoCl2, ZnCl2, CuCl2, NiCl2, HgCl2, CdCl2, CrCl3, FeCl2, FeCl3, LiCl, MgCl2, CaCl2, AlCl3, SrCl2 and MnCl2. Stock solution of probe 1 (1 mM) was also prepared in EtOH and this stock solution was further diluted with a mixed solution of EtOH/H2O(v/v, 1:9) to be a final concentration of 10 µM. The target metal ions

ACCEPTED MANUSCRIPT selectivity experiments were carried out by addition of the same doses of a series of metal ions into the probe 1 solution system. Meanwhile, the optical changes also were measured when different doses of target metal ions are added. For all the measurement of fluorescence spectra, the excitation wavelength was set at 340 nm

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with both the excitation and emission slit widths were 5 nm. 2.4. Applications

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Three types of real-life samples (tap water, lake water and Yangtze River) were

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collected to investigate the practical application of Probe 1. Tap water was collected

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from our lab. Lake water was obtained from the YuDai River in our school. Yangtze River was firstly filtered through filter paper (Sinopharm Chemical Reagent Co., Ltd.,

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shanghai China,Φ=9 cm) to remove impurities. Stock solutions (10 mM) of Cu2+ and Cd2+ were prepared from CuCl2, CdCl2 with distilled water. Stock solution of probe 1

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(1 mM) was also prepared in EtOH and further diluted to the final concentration of 10

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μM. The levels of Cu2+ and Cd2+ in real water samples were obtained from the linear relationship between the fluorescence intensity and the metal ion concentration. A

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standard addition method, in which different concentrations (5 and 8 µM) of Cu2+ and Cd2+ were added into the samples, was used for further evaluation of the assay.

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3. Results and discussion 3.1. Spectroscopic studies of probe 1 with metal ions In order to investigate the response properties of probe 1 to metal ions, the interaction of 1 with various guest species was established by UV–vis absorption and fluorescence spectroscopic analysis. The corresponding UV–vis spectrum was shown in Fig.S5. The single probe 1 showed two main absorption peaks at 276 nm and 330

ACCEPTED MANUSCRIPT nm in ethanol solvent. Upon addition of Cu2+ (10 equiv.), the absorbance bands centered at 276 nm and 330 nm decreased obviously, whereas a new absorption peak appeared at 262 nm. Upon addition of Cd2+, a weak bathochromic shift from 276 to 298 nm and 330 to 350 nm was noticed. And other metal ions did not generate any significant absorption spectrum change under the same conditions. Additionally, the

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selectivity of probe 1 to various metal ions was examined in EtOH/H2O (v/v= 1:9)

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solution by fluorescence spectrophotometer. The fluorescence spectra of the probe 1

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system in the presence of different metal ions were obtained by excitation at 340 nm. As shown in Fig. 1, single probe 1 showed a strong fluorescence emission at 471 nm

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with a blue fluorescence emission. The presence of Cu2+ ions caused a great fluorescence quenching of the probe 1, displaying an obvious fluorescence ON-OFF

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behavior. Besides, the addition of Cd2+ caused a weak reduction of the fluorescence intensity with a red shift (40 nm) of fluorescence spectral. Meanwhile, the fluorescent

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color of solutions changed from blue to green. However, upon the addition of various

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other metal ions, almost negligible changes of fluorescence intensity were observed. The experimental results indicated that probe 1 has a good optical selectivity for Cu2+

1+other metal ions

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Cu2+ Cd2+ 2+ 1+Cu2+ 1+Cu

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Intensity (a.u.)

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and Cd2+.

1+Cd

400

2+

1+Cu

0

400

2+

500

Wavelength (nm)

600

ACCEPTED MANUSCRIPT Fig.1 Fluorescence responses of probe 1 (10 μM) in EtOH-H2O (1:9, v/v) (λex = 340 nm). 3.2 Competition experiments To further check the practical applicability as a selective fluorescent probe,

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competition experiments were carried out by addition of 10 equiv of other competing cations to the solution of [1-Cu2+] system. As shown in Fig. 2, the addition of other

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competitive metal ions (except Cd2+) had no prominent interference with the

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determination of Cu2+, which meant that probe 1 has high selectivity for Cu2+.

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1+Metal ions 1.0

2+

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0.8

1+Metal ions +Cu

I/I0

0.6

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0.4

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0.2

0.0

2+

3+

2+

2+

3+

+

2+

2+

+

2+

2+

+

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2+

2+

2+

2+

3+

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Cu Al Ca Co Cr Cs Cd Fe Li Mg Mn Na Sr Zn Pb Hg Ni Fe

Fig.2 Competitive selectivity of probe 1 (10 μM) toward Cu2+ (10 equiv) in the

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presence of various MI(metal ions). Bars represent the intensity ratios of fluorescence. I0 represents the fluorescence of probe 1, I represents the fluorescence of 1+MI or 1+MI+Cu2+.

What's worth noting is another discovery,with the addition of 10 equiv of Cd2+ to the [1-Cu2+]complex system, the fluorescence has been restored to a certain extent. The corresponding emission spectra of [1-Cu2+] system after addition of other common metal ions were shown in Fig. 3a, It is obvious that the addition of Cd2+

ACCEPTED MANUSCRIPT caused the emergence of a strong emission peak at 510 nm, and the addition of other metal ions did not caused any change of spectra. The results indicated that the [1-Cu2+] system can response to Cd2+ by remarkable fluorescence enhancement with high specifity. The possible reason for this phenomenon is that the binding capacity of probe 1 to Cd2+ is stronger, and the Cd2+ will replace Cu2+ to coordinate with the

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probe 1. This also provides another way for the detection of Cd2+. According to the

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previous study, the probe 1 can be usually used for selective recognition of Cd2+ ion,

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but there are several problems in this experiment that the red-shift of emission spectrum is small range and there is an overlap between the emission spectrum of

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[1+Cd2+] system and the single probe 1 (Fig. 1). It is not sensitive for the detection of low concentration Cd2+ and the error is large. So in contrast to the single probe 1, the

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[1-Cu2+] complex as the detection system for Cd2+ seems to be more feasible. Subsequently, the competitive experiments were also performed to study the influence

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of other metal ions on the detection of [1-Cu2+] system towards Cd2+. As shown in Fig.

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3b, the presence of other metal ions did not interfered with the identification of Cd2+ ions by the [1-Cu2+] system. The result further demonstrated that the [1-Cu2+] system

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can be used as a practical ion selective fluorescent probe for Cd2+ detection in the presence of most competing metal ions. Based on the above research, we can

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successfully construct a relay identification system of Cu2+ and Cd2+ based on probe 1.

(a)

500 2+

[1-Cu ]+Cd

2+

[1+Cu ]+MI

2+

[1+Cu ]+MI+Cd

2+

2+

400

300

300

I/I0

Intensity (a.u.)

400

(b)

2+

[1+Cu ]+other MI

200

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0 400

450

500

550

Wavelength (nm)

600

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0 2+ 2+ 3+ 2+ 2+ 3+ + 2+ 2+ + 2+ 2+ + 2+ 2+ 2+ 2+ 2+ 3+ [1+Cu ] Cd Al Ca Co Cr Cs Cu Fe Li Mg Mn Na Sr Zn Pb Hg Ni Fe

ACCEPTED MANUSCRIPT Fig. 3. (a)Fluorescence spectra of [1-Cu2+] system (10 μM) in the presence of various metal ions(Ex = 340 nm); (b)Competitive selectivity of [1-Cu2+] system toward Cd2+ (10 equiv) in the presence of MI(metal ions) .Bars represent the

ratios of

fluorescence intensity. Ic represents the fluorescence intensity of [1-Cu2+]+Cd2+, I

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represents the fluorescence intensity of [1-Cu2+]+MI or [1-Cu2+]+MI+Cd2+. 3.3. Fluorescence titrations of Cu2+ and Cd2+

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To further investigate the sensitivity of probe 1, the fluorescence titration

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experiments of Cu2+ and Cd2+ were subsequently performed, respectively. Firstly, the

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fluorescence spectra of probe 1 containing different concentration of Cu2+ were explored in EtOH-H2O solution upon excitation at 340 nm. As shown in Fig. 4a, the

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fluorescence emission of probe 1 was gradually quenched with the incremental addition of Cu2+ and reached to the minimum when 1 equiv. of Cu2+was introduced.

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As the Fig. 4b shown, a good liner relationship (R = 0.998) was noted between (F0-F)

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and [Cu2+] at Cu2+ concentration from 1 µM to 10 µM. The detection limit was calculated to be 2.7 × 10-8 M (L = 3 σ / K, K = 6.41 × 107, σ = 0.58), much lower than

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most of the reported in literature[38,39]. The Job’s plot revealed 1:1 stoichiometry for the binding between 1 and Cu2+ (Fig. S6). And this conclusion is further proved in the

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mass spectrometric analysis process of probe 1 and [1-Cu2+] complex. As shown in the Fig.S4 (the MS of probe 1), there are two main peak at m/z 383.24 ([1+H]+) and m/z 405.23 ([1+Na]+). After the probe 1 was combined with Cu2+ (Fig.S8), a new peak was observed at m/z 482.14 ([1+Cu2++Cl-+H+]), which further indicated the stoichiometry of 1:1 between the probe 1 and Cu2+. Besides, the association constant of probe 1 with Cu2+ in EtOH-H2O solution was accordingly calculated to be 5.643 × 104 M −1 (Fig. S7).

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(a)

600

2+

Cu

Y=614.1X-1.481

600

2

R =0.998 400

600

400

F0-F

Intensity (a.u.)

(b)

800

Intensity (a.u.)

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200

0

0.5

1

1.5

2

Cu2+ / equiv

400

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200 0

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0.0

650

0.2

0.4

0.6 2+

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1.0

-5

Cu / 10 M

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Wavelength (nm)

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Fig.4 (a) Change in emission spectrum of1upon gradual addition of Cu2+ (10 µM)in

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EtOH-H2O (v/v =1:9). (b)The linear relationship between [Cu2+] concentration and

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[F0-F]

Then, the generated [1-Cu2+] complex system was directly applied to the

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fluorescence titration experiment of Cd2+ without separation and purification. As shown in Fig. 5a, the fluorescence intensity was dramatically increased with the

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gradual addition of Cd2+ to the solution, and the emission intensity reached a

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maximum when the concentration of Cd2+ was 1.0 equiv. There was a linear fluorescence response to Cd2+ concentration ranging from 1 µM to 10 µM (Fig. 5b),

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the corresponding detection limit was calculated to be 1.7 × 10-8 M (L = 3 σ / K, K = 2.28 × 107, σ = 0.13). The Job’s plot revealed 1:1 stoichiometry for the binding

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between [1-Cu2+] and Cd2+ (Fig. S9) and the MS spectrum (Fig. S11) of the [1+ Cd2+ + H+ + Cl-] complex also verify the combined ratio ([1 + Cu2+ + Cl- + H+]:m/z: 482.14; [1 + Cd2+ + H+ + Cl-]: m/z 531.13). The association constant of 1 with Cu2+ in aqueous solution was also accordingly calculated to be 1.374 × 104 M

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(Fig. S10).

The experimental results indicated that the in situ [1-Cu2+] complex system was sensitive for the quantitative determination of Cd2+.

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a Intensity (a.u.)

400

200 y=228.739x+5.168

200

0

300

Cd

2

4

2+

6

Cd

2+

8

R2=0.9964

10

/ eq.

F-F0

Intensity (a.u.)

b

400

100

200

100

0

400

450

500

550

600

0.0

650

0.2

Wavelength (nm)

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0.6

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0

2+

0.8

1.0

-5

Cd / 10 M.

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Fig.5 (a) Fluorescence spectra of [1-Cu2+] complex (10 µM) upon the progressive

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fluorescence intensity of [1-Cu2+] and [Cd2+].

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addition of Cd2+ in various concentration ranges. (b) Relationship between the relative

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3.4. Effect of pH on the fluorescence properties

pH as an important factor affecting the optical properties, usually have an

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important impact on the test performance of the probes. Thus, it is necessary to further

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investigate the effect of system pH on the detection of probe 1 to Cu2+ and Cd2+, the relay recognition experiments on Cu2+and Cd2+ in different pH systems (pH=4~10) were carried out. The pH was adjusted by adding 0.5 M hydrochloric acid and 0.5 M

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sodium hydroxide solution. As shown in Fig. 6, the emission intensity at 471 nm of

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probe 1 almost remained constant in the pH range 4~10. In this pH range, the changes of the fluorescence intensities of [1-Cu2+] + Cd2+ and [1 + Cu2+] were insignificant, which meant that the effect of pH on the detection is negligible. Those experimental results indicated that the probe 1 is suitable for the relay identification of Cu2+ and Cd2+ ions at a wide pH range.

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a

1 2+ 1+Cu 2+ 2+ 1+Cu +Cd

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b

400

c

200

3

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pH

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Intensity (a.u.)

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Fig.6 Fluorescence intensities of probe 1 (a), [1-Cu2+]+Cd2+(b) and 1+ Cu2+ (c) under

3.5. Application in real samples

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3.5.1 Real Water samples analysis

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different pH conditions (λex= 340 nm).

In order to evaluate the feasibility of this novel probe to real water samples, the

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levels of Cu2+ and Cd2+ in tap water, lake water and Yangtze River water were investigated. The concentration of metal ions in the sample solutions was also obtained from the linear relationship (Fig. 4b, Fig. 5b) between the fluorescence

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intensity (F-F0) and the metal ion concentration. The recovery(R) of the spiked ions

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were obtained by comparing the test values (C) with the added standard values(C0) of the concentration of metal ions. As shown in Table 1, the concentration of Cu2+ was 0.052, 0.152, 0.191 μM, and the concentration of Cd2+ is 0.032, 0.055, 0.085 μM, respectively. The percent recoveries of those samples are found to be in the range of 98.9%–102.02%. These experimental results demonstrated that probe 1 is capable for quantitative relay detection of Cu2+ and Cd2+ in real water samples. Table 1 Determination of Cu2+ and Cd2+ in real water samples by standard addition method

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Cu2+

Probe 1

Tap water

Cu2+

Lake water

Yangtze

Cu2+

C0 (µM)

F-F0

C (µM)

0

1.65±1.02

0.052±0.017

5

306.36±1.38

5.013±0.023

99.2

8

496.30±1.32

8.106±0.022

100.6

0

7.85±1.26

0.152±0.021

5

316.85±1.86

5.184±0.031

8

497.91±2.58

8.132±0.043

0

10.25±1.68

0.191±0.028

5

316.32±1.98

5.175±0.033

99.6

8

497.13±2.76

8.132±0.046

99.2

12.48±2.52

0.032±0.011

River

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water

Tap water

5

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99.7

100.9

190.15±14.49

8.087±0.063

100.6

0

17.75±2.76

0.055±0.012

5

119.63±10.35

5.004±0.045

98.9

8

183.70±10.12

8.031±0.044

99.7

0

24.61±2.99

0.085±0.013

5

123.79±13.34

5.186±0.058

102.02

8

191.54±10.81

8.148±0.047

100.7

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Lake water

100.6

5.081±0.036

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Cd2+

R(%)

121.39±8298

8

[1-Cu2+]

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0 Cd2+

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Ions

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System

YangtzeRiver

Cd2+

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water

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For an on-site convenient use, we developed paper-based fluorescent sensors for detections. A series of 3.0 μL of 10 μM probe 1 solution in EtOH was dropped on to filter paper strips and then dried to get light blue fluorescence spots (Fig. 7). Each spot was dropped with 1 μL containing 1.0 nmol of various metal ions. Under the UV light, it can be clearly observed that the addition of Cu2+ caused a dark fluorescence quenching area and the addition of Cd2+ caused a yellow emission area within the blue spot, while other metal ions did not change the fluorescence emission of the spots in

ACCEPTED MANUSCRIPT the tape. In order to continue verifying the feasibility of relay detection in the tapes, with the addition of Cd2+ to the fluorescence emission quenched tape caused by Cu2+, the same yellow fluorescence emission area was observed again. This result showed that the sensing paper tape prepared on the basis of the probe 1 can effectively realize

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of probe 1 in the rapid on site detection of the target ion.

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the relay identification of the Cu2+ and Cd2+, it also provides a convenient application

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Fig.7 Photographic image of metal ions (1 nmol) detection by probe 1 on filter paper under the UV-lamp (λ=365) .

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Similarly, probe 1 was also used as a fluorescent sensor for the determination Cu2+ and Cd2+ in milk sample. Before testing, milk samples need to be deproteinized. The

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trichloroacetic acid (1.0 mL, 15%) was added to Approximately 10 g of homogenized milk and the solution was shaken and centrifuged, and filtered through microfilters with a pore size of 0.45 m to removed the denatured proteins and collected the extracts. Then, the detection tests of Cu2+ and Cd2+ in milk samples were determined by standard addition method. The results were displayed in Table 2. The recovery of Cu2+ ranged from 99.18% to 100.85% and recovery of Cd2+ ranged from 98.96% to 101.49%. Therefore, these results showed that the present method could serve a potential detection system for Cu2+and Cd2+ in real food samples.

ACCEPTED MANUSCRIPT Table 2 Determination of Cu2+ and Cd2+ in milk samples by standard addition method System

Ions

Probe 1

Cu2+

F-F0

C (µM)

R(%)

5.0

305.97±1.26

4.993±0.018

99.18

8.0

494.75±1.41

8.056±0.026

100.85

5.0

117.23±4.71

4.954±0.052

98.96

8.0

193.26±3.28

8.152±0.035

101.49

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Cd2+

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[1-Cu2+]

C0 (µM)

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4. Conclusion

In summary, we provided a new strategy for probe design that one molecular

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could capture different metal ions. A highly selective fluorescence probe 1 for relay

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recognition of Cu2+ and Cd2+ was designed and characterized. The single probe 1 has strong blue fluorescence emission and it exhibited a high selectivity to Cu2+ over

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other metal ions with a fluorescence ON–OFF behavior in EtOH-H2O solution. The

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generated [1-Cu2+] complex system can be directly applied to the relay recognition of Cd2+ without separation and purification. Upon the addition of Cd2+, the quenched [1Cu2+] complex exhibited a noticeable fluorescence enhancement with a red-shift. The

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detection limits for Cu2+ and Cd2+ are calculated to be 2.7 × 10-8 M and 1.7 × 10-8 M,

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respectively. In addition, the probe 1 was successfully applied to the relay detection of Cu2+ and Cd2+ in the real environmental water samples via the ‘‘on-off-on” mechanism with sequence specificity. And the test strips based on probe 1 also has a good qualitative recognition performance for target ions.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos.

ACCEPTED MANUSCRIPT 31470434, 21576124, 21507047 and 21676124), China Postdoctoral Science Foundation (2017M610308), Postdoctoral Science Foundation of Jiangsu Province (No. 1701107B), Zhenjiang Social development project (No. SH2016019) and the Science Foundation of State General Administration of the People’s Republic of

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ACCEPTED MANUSCRIPT A quinoline-based fluorescence “on-off- on” probe for relay identification of

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Cu2+ and Cd2+ ions

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The general procedure for the experiment

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights •A novel fluorescence probe with simple structure for relay recognition of Cu2+ and Cd2+ was synthesized. •The proposed method exhibited high selectivity and sensitivity, wide range of linear response and low detection limit.

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• The fluorescence probe can also be used for the quantitative relay detection of target ions in real water samples.