Achieving highly sensitive detection of Cu2+ based on AIE and FRET strategy in aqueous solution

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 211 (2019) 272–279

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Achieving highly sensitive detection of Cu2+ based on AIE and FRET strategy in aqueous solution Jianting Yang, Jie Chai, Binsheng Yang, Bin Liu ⁎ Key Laboratory of Chemical Biology, Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China

a r t i c l e

i n f o

Article history: Received 8 October 2018 Received in revised form 28 November 2018 Accepted 9 December 2018 Available online 11 December 2018 Keywords: Aggregation–induced emission Fluorescence resonance energy transfer Cu2+ Nile red

a b s t r a c t The aggregation-induced emission (AIE) luminogens are now showing strong potential in mimicking the energy donor of fluorescence resonance energy transfer (FRET) system. Herein, one highly efficient FRET system 1-NiR is successfully fabricated in aqueous solution based on an AIE active compound 1 and fluorescence dyes (Nile red (NiR)). 1 acts as the energy donor and NiR acts as the acceptor in the FRET system with the optimum concentrations ratio [1]/[NiR] = 100. Besides, the AIE(1) itself displays excellent selectivity for Cu2+ ions at 525 nm with the detection limit of 1.32 × 10−7 M. While through the FRET system of 1-NiR system, the detection limit of Cu2+ can be further decreased to 9.12 nM by monitoring the fluorescence at 630 nm. As a result, using an AIE probe to detect Cu2+ based on FRET mechanism is a promising strategy. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The phenomenon of aggregation-induced emission (AIE) for organic compounds was first discovered in 2001 [1–5]. Fluorescent compounds with AIE exhibit weakly emission in good solvent, however it shows strong emission with aggregate formation in poor solvent [6–10]. In principle, there are three proposed AIE mechanisms, including the restriction of intramolecular rotations (RIR), restriction of intramolecular vibrations (RIV) and restriction of intramolecular motions (RIM) [4,11]. AIE active molecules exhibit good properties with large Stokes shifts, high quantum yields, wavelengths red-shift in aggregated states and detection of analytes in aqueous solution. On the contrary, many conventional luminophores experience some effects of emission quenching, partially or completely in the aggregate state. This phenomenon is documented as aggregation-caused quenching (ACQ) [4]. Therefore, plenty of AIE molecules have been explored to overcome the ACQ effect. To date, AIE molecules have a wide range of potential applications in fluorescence sensors [12], organic lasers and optoelectronic devices [13–16]. Copper is third in abundance among the essential heavy metal ions in the human body and plays an important role in a variety of fundamental physiological processes in organisms ranging from bacteria to mammals [17]. Therefore, taking in suitable copper is essential for our

⁎ Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (B. Liu).

https://doi.org/10.1016/j.saa.2018.12.020 1386-1425/© 2018 Elsevier B.V. All rights reserved.

good health. According to the U.S. Environmental Protection Agency (EPA), the maximum acceptable level of Cu2+ is 20 μM in drinking water. Excessive copper is related to cellular damage, triggering in series of diseases such as Alzheimer's [18–20] and Wilson's [21,22] diseases. Thus, it is indispensable to develop analytical methods for detecting Cu2+, in which fluorescent chemosensors cause great attention because of its high resolution, selectivity and sensitivity. One of traditional method of detecting Cu2+ is using rhodamine dyes [23–26], in which Cu2+ can induce spirolactam ringopening and further achieved “turnon” sensing mechanism. However, some defects such as the instability and obvious cytotoxicity of rhodamine dyes limited its application [27]. Another method of detecting Cu2+ is using Schiff-base fluorescence probes with AIE activity [28–30]. However, detection limit of most of those probes is around micromole level, which cannot detect the copper in biological samples. Works related to this area still face great challenge. The aims of this work is to study the interaction of AIE molecular with Cu2+ ions and further decrease the detection limit of Cu2+ using the fluorescence resonance energy transfer (FRET) process. In this paper, one simple aggregation-induced emission (AIE) activity compound AIE(1) was synthesized based on the condensation reaction of 5-chlorosalicylaldehyde, 1-pyrenecarboxaldehyde with carbohydrazide and. AIE(1) displays excellent AIE activity with strong green emission in aqueous solution (525 nm), while the green emission is quenched in the presence of Cu2+ ions. To further decrease detection limit of Cu2+ ions, fluorescence dye (Nile red) is introduced to AIE(1) system. Obvious FRET process was found between AIE(1) and NiR. To our surprise, 1-

J. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 211 (2019) 272–279

Cu2+

AIE

Quenching

FRET

λem= 630 nm

λem= 525 nm

Cl

273

N OH

H H N N O

N

N

N

O

O

Nile red

1

Scheme 1. Schematic illustration of the detection of Cu2+ based on FRET strategy in aqueous solution.

NiR system can be used to Cu2+ detection at 630 nm on nanomolar level. To best our knowledge, it is the first time that using this mechanism reduces the detection limit of Cu2+ (Scheme 1). 2. Experimental 2.1. Materials 5-Chlorosalicylaldehyde, 1-pyrenecarboxaldehyde, Nile red (NiR), carbohydrazide, metal salts and organic solvent were obtained commercially and used without further purification. Fluorescence spectra were measured using a Fluoromax-X spectrofluorometer (HORIBA Jobin Yvon, France) and the absorption spectra were recorded on a Varian 50 Bio spectrophotometer (Varian, USA). 1 H NMR spectra were recorded on Bruker-600 MHz spectrometers (Bruker, Germany), the chemical shifts (δ) were reported as ppm in DMSO. ESI-MS spectra were conducted by Agilent 6520 Accurate-Mass Q-TOF LC/MS mass spectrometer (Thermo Scientific, USA). Dynamic light scattering (DLS) analysis was measured with BECKMAN COULTER DelsaTM Nano C particle analyzer (Beckman, USA). Element analysis was performed with a Vario EL III analyzer (Vario, Germany). The time-resolved fluorescence measurements were performed using an FL920 fluorescence lifetime spectrometer (Edinburgh Instruments, Livingston, UK). Scanning electron microscopy was measured at JEOL-JSM6701F scanning electron microscope (SEM) (JEOL, Japan). TEM was measured by TecnaiG2 F20 S-TWIN TMP instrument (FEI, USA). The stock solution of 1 was prepared by dissolving 4.4 mg of 1 in 10.0 mL DMF. The fluorescence intensity at the wavelength of 415–700 nm was detected with a slit width of 5 nm for excitation and emission. The UV–Vis spectra were measured at the range of 200–800 nm.

filtration and the pure product was obtained as a yellow solid 1 (yield: 32%) (Scheme 2). ESI mass spectrometry: m/z = 441.1112 [M + H]+, [M + H]+ calculated 441.11 (Fig. S2). 1H NMR (DMSO, Fig. S3), δ (ppm, 600 MHz, TMS): 11.112–11.055 (d, 2H), 9.312 (s, 1H), 8.793 (s, 1H), 8.638 (s, 1H), 8.507 (s, 1H), 8.410–8.341 (m, 4H), 8.258–8.219 (m, 2H), 8.126–8.101 (t, 2H), 7.842 (s, H), 7.289–7.275 (d, 1H), 6.958–6.944 (d, 1H). Anal. Calcd for 1: C25H17ClN4O2 (%), C, 68.11; H, 3.89; N, 12.71. Found (%): C, 68.04; H, 4.06; N, 12.74. The scanning electronic microscopy (SEM) measurement for power of 1 was shown in Fig. S4. 2.3. Fluorescence Spectral Properties The Cu2+ and other metal ions were dissolved in water to afford 1.0 mM solution. The 10 μM 1 stock solution were prepared in DMF. The fluorescence spectral properties of probe 1 were investigated in 10.0 mM PBS buffer at pH 7.4 with the addition of various concentrates of Cu2+ in a 3 mL closed cuvette. And the fluorescence spectral properties of the fluorescence resonance energy transfer (FRET) process of 1 and Nile red and 1-NiR system with the addition of various concentrates of Cu2+ were also tested. The solution was fresh prepared and then the fluorescence spectra were recorded immediately. 3. Results and Discussion 3.1. Aggregation-induced Emission Activity 1 To verify the AIE behavior, the fluorescent and UV–Vis spectra in different content H2O/DMF were measured. The concentration of 1 was kept constant at 10 μM. The fluorescent spectroscopy was shown in

2.2. Synthesis Cl

O H2N OH

H N

H N O

NH2

Cl

N OH

EtOH

EtOH

A solution of 5-chlorosalicylaldehyde (0.7829 g, 5.0 mmol) in absolute ethanol (10 mL) was added dropwise to a vigorous stirring solution of carbohydrazide (0.9008 g, 10.0 mmol, 97%) in the same solvent (30 mL) at room temperature for 4 h (see Scheme 2). After that time, white precipitate was formed. The precipitate is collected and washed with distilled water to remove excess carbohydrazide and dried to give 2 (yield: 82%). 1H NMR (DMSO, Fig. S1), δ (ppm, 600 MHz, TMS): 10.445 (s, 2H), 8.209 (s, 1H), 8.110 (s, 1H), 7.970 (s, 1H), 7.184–7.170 (d, 1H), 6.864–6.850 (d, 1H), 4.081 (s, 2H). 2 (0.93 g, 4.0 mmol) and 1-pyrenecarboxaldehyde (0.9211 g, 4.0 mmol) was dissolved in 30 mL absolute ethanol and the mixture was refluxed for 3 h. After the reaction was completed, it was cooled to room temperature and filtered. The filtrate was allowed to stand for several days to remove the filtrate by

Cl

N OH

H N

H N

N

O

1 Scheme 2. The synthesis routing of 1.

H N

H N O

NH2

2 O

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Fig. 1. a) Fluorescence spectra of 10 μM 1 in DMF/water mixtures with different water fractions, pH 7.4. Inset: effect of water volume on the FI at 525 nm, (λex, 400 nm). b) The corresponding fluorescence images of 1 under UV lamp illumination. c) The UV–Vis spectra of 10 μM 1 in DMF/water mixtures with different water fractions. d) TEM image of 10 μM 1 in aqueous solutions.

Fig. 1a. It showed three fluorescent peaks at 403, 430 and 471 nm when dissolved in 100% DMF. As the water content increased, these bands were simultaneous decreased and new band at 525 nm appeared when the water content reached to 70%. The FI reached the maximum at 100% water solution. The red-shifted emission is due to the excimer formation between pyrene rings with higher contents of water. Moreover, the amidic acid form was more stable in tautomerization due to its complete delocalization in the whole molecule at higher amounts of water (N70%) [1,31,32]. It indicated that 1 was a typical AIE active compound [33]. The photograph given in Fig. 1b clearly showed the colour change of 1 with the water content increased. The UV–Vis spectra were shown in Fig. 1c. As a result, three absorbance bands at 382, 365 and 286 nm was observed when dissolved in DMF. As the water content increased, these absorbance bands were decreased and new bands at 385 and 410 nm formed when water content reached 70%, which was in consistent with the FI change. Besides, a long absorption tail was observed when water content is N70%, which may be due to the absorption of the nano particles [34]. It showed that nano size aggregation was formed in water solution. This conclusion was verified by transmission electron microscopy (TEM) (Fig. 1d). The TEM images showed spherical morphology and the diameters about 75 nm. At the same time the fluorescence microscopic image indicated that these nano particles showed strong fluorescence (Fig. S5). In addition, the Tyndall effect of 1 in DMF and aqueous solutions were also

investigated 1 formed nano particles in aqueous solutions (Fig. S6). Besides, dynamic light scattering (DLS) in different content H2O/DMF (water content: 0%, 20%, 40%, 60%, 80% and 100%) were also measured (Fig. S7). As a result, the average size of 1 was estimated to be about 36 nm in DMF. As the water content increased, the average size gradually increased. Lastly, fluorescence decay profiles of 10 μM 1 were investigated (Fig. S8). As shown in Fig. S8 and Table S1, with the fraction of H2O increased, the fluorescence lifetime τ1 decreased from 1.9118 to 1.5658 ns, τ2 increased from 3.6312 to 9.1217 ns gradually. It suggested that the aggregated molecule increased with the water content. It also further indicates that the aggregates of 1 were formed with the increasement of water content. These data well confirmed that 1 was a significant AIE active compound in the aqueous medium. This AIE effect could be explained normally by the restriction of intramolecular rotation (RIR), which facilitate radiative release of the photo-excited energy and suppress the non-radiative decay. In addition, the FI of 1 at different concentration in aqueous solution was researched. As presented in Fig. 2a, the emission of 525 nm gradually increased with the concentration of 1. The FI gained the maximum at 0.1 mM. When the concentration reached 1.0 mM, FI began to decrease due to the bigger precipitate formation. Besides, 1 also showed a weak emission peak at 471 nm when the concentration was below 1 × 10−7 M, but a new peak at 525 nm appeared when the concentration reached 5 × 10−7 M (red curve) (Fig. 2a inset). The wavelength and

Fig. 2. a) Fluorescence spectra of 1 in aqueous solution with different concentrations from 0.001 μM to 1.0 mM. Inset: Fluorescence spectra of 1 from 0.001 μM to 0.5 μM. b) Fluorescence spectra of 1 (10 μM) in aqueous solution at different pH from 7.0 to 14.0. Inset: the plot of FI at 525 nm to the pH values (up) and fluorescence spectra of 1 in aqueous solution at pH 13 (down).

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Fig. 3. a) Fluorescence spectra of 1 (10 μM) with the addition of various metal ions (10 equiv.) in PBS buffer, pH 7.4, λex = 400 nm (metal ions: K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2 , Mn2+, Ag+, Al3+, Fe3+, Bi3+ and Cr3+). b) Fluorescent intensity of 10 μM 1 upon addition of 20 equiv. various metal ions containing 10 equiv. of Cu2+ in PBS buffer at pH 7.4. c) Fluorescence spectrum changes of 10 μM 1 upon incremental addition of Cu2+ (0–10 μM) in PBS buffer, pH 7.4. Inset: Changes of fluorescence intensity at 525 nm with addition of Cu2+. d) Job's plot between 1 and Cu2+.

+

intensity vary with the different concentration. This phenomenon further indicated 1 was an AIE active compound. Moreover, AIE features of 1 at different pH (7.0–14.0) were further investigated by fluorescent spectroscopy. As shown in Fig. 2b, the FI gradually decreased with the increasement of pH, and enormous blue shift was observed from 525 nm to 471 nm at pH 13 (inset), the molecule is less fluorescent at high pH. According to literature, pKa of –OH on salicylic acid group was 13.40 [35]. At high pH solution, phenolic hydroxyl group will be deprotonated and the intramolecular N⋯H\\O hydrogen bonds must be destroyed. Therefore, it can be concluded that the formation of intramolecular N⋯H\\O hydrogen bonds was important for the AIE activity of 1. Moreover, as the whole molecule becomes charged at high pH, it may be more soluble in aqueous environment. This could contribute to the low emission here. 3.2. Cu2+ Sensing of 1 The structure of molecular 1 not only has typical π-conjugated system, but also has phenolic hydroxyl group and –CN bonds (Scheme 2), which is potential to build a high-coordination ability to metal ions [29,30]. Therefore, in order to explore the influence of AIE(1) on various

metal ions such as K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Ag+, Al3+, Fe3+, Bi3+ and Cr3+, a wide range of metal ions were evaluated in water solution (pH 7.4, 10.0 mM PBS), and the results were shown in Fig. 3a. Satisfactorily, upon addition of 10 equiv. various metal ions to 1, 1 (10 μM) showed the dramatic changes in the PL spectra to Cu2+, which exhibited severely fluorescence quenched, while no apparent fluorescence changes were observed upon addition of other metal ions. It demonstrated that fluorescence emission at 525 nm of AIE(1) was selectively quenched by Cu2+. In order to evaluate the possible interference on detection of Cu2+ in the presence of other interfering metal ions for AIE(1), the fluorescence spectra changes of AIE(1) (10 μM) towards Cu2+ (100 μM) in the presence of other metal ions (200 μM) were investigated and the results were given in Fig. 3b. As presented in Fig. 3b, there was no obvious influence on detection of Cu2+ in the presence of other interfering metal ions for AIE(1). As a result, AIE(1) could be a good fluorescence sensor, which was used in investigating Cu2+ in aqueous medium. Then the detail titration of AIE(1) to Cu2+ was also investigated in aqueous solution and the result was shown in Fig. 3c. Upon addition of Cu2+ (0–10 μM) to AIE(1) (10 μM), the fluorescence intensity at 525 nm decreased gradually. The result indicated that the fluorescence

Fig. 4. a) Particle size of 1 (10 μM) determined by dynamic light scattering (DLS) in aqueous solutions. b) The DLS of 1 (10 μM) containing 10 equiv. Cu2+ in aqueous solutions.

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intensity quenched almost 10-fold upon addition of 0.5 equiv. Cu2+ and the fluorescence intensity shown negligible change with continuous addition Cu2+. It shown little amount Cu2+ also has high quenching ability towards AIE(1). Fig. 3c inset showed the FI changes of AIE(1) with addition of Cu2+. Besides, the detection limit of AIE(1) to Cu2+ was calculated to be 1.32 × 10−7 M using the equation DL = K × Sb1/S, where K = 3, Sb1 is the standard deviation of the blank solution (10 times) and S is the slope of the calibration curve [36]. To further study the stoichiometric ratio between AIE(1) and Cu2+, the Job's plot of FI at 525 nm [Cu2+]/[1 + Cu2+] was obtained in Fig. 3d. As a result, an inflection points appeared at about [Cu2+]/[1 + Cu2+] = 0.2. 3.3. Mechanism Investigation of 1 and Cu2+

Fig. 5. 1H NMR spectra of 1 in the presence of Cu2+ in DMSO-d6.

To further explore the fluorescence change mechanism of AIE(1) with Cu2+, dynamic light scattering (DLS) and 1HNMR titrations of AIE (1) with Cu2+ in water was measured. The DLS changes of AIE(1) and AIE(1) containing 10 equiv. Cu2+ were shown in Fig. 4. As presented in Fig. 4a, the average size of aggregation for AIE(1) was estimated to be about 86 nm in water. Such a large particle size proved that 1 was formed aggregated in aqueous solutions, which was accordance with the FI change. And with the addition of 10 equiv. Cu2+, the average

Fig. 6. a) Normalized emission spectra of 1 (λex, 400 nm) and absorption spectra of NiR in aqueous solution. b) Fluorescence spectra of 1, NiR and 1-NiR. Inset: The corresponding fluorescence photographs. c) Fluorescence spectra changes of 1 with different concentrations of NiR. d) Particle size of 1-NiR determined by DLS in aqueous solutions. e) TEM image of 1NiR in aqueous solutions. f) Fluorescence decay profiles of 1-NiR in aqueous solutions. (1(10 μM), NiR(0.1 μM), 1-NiR([1] = 10 μM, [NiR] = 0.1 μM)).

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size of AIE(1) changed to 53 nm (Fig. 4b), which was consistent with the fluorescence quenching. It indicated that the average size of aggregation AIE(1) had been affected by the addition of Cu2+, which may be due to the influence of the heavy metal ion effect and the coordination effect. The 1H NMR spectra titrations 1 with or without Cu2+ were shown in Fig. 5. As can be seen from Fig. 5, the chemical shift at 11.1 and 11.0 ppm was attributed to the hydrogen atom of the –CHN–, the peak at 9.3 ppm was considered to be the hydrogen atom of the phenolic hydroxyl. Upon addition toCu2+, the peaks at 11.1, 11.0 and 9.3 decreased gradually. The results indicated the phenolic hydroxyl and the nitrogen atom of –CHN– of AIE(1) participated in the binding with Cu2+. 3.4. FRET System Fabricating of AIE(1)-NiR The FRET process between aggregation state of AIE(1) molecular donor and fluorescence dye acceptor was investigated (Fig. 6). Nile red (NiR) was selected as an energy acceptor, since the absorption spectrum of NiR is largely overlapped with the emission band of AIE(1) (Fig. 6a). With the addition of NiR (0.1 μM) to the aqueous solution of 1 (10 μM), the fluorescence at 525 nm was decreased and a new emission appeared at 630 nm (Fig. 6b), which is identified to be the emission of NiR (λex, 400 nm). The photograph clearly showed the colour change of 1, NiR and 1-NiR in Fig. 6b inset. It indicated that FRET existed between 1 and NiR, which 1 acted as the energy donor and NiR acted as the energy acceptor in this system [29]. In order to investigate the detailed FRET process between 1 and NiR, fluorescence spectra of 1 upon addition of different concentrations of NiR were measured (Fig. 6c). Upon gradual addition of NiR from 0 to 1 × 10−7 M, FI of 1 decreased along with a slight blue shift (525 nm), while the FI of NiR increased accompanied with a slight red shift (630 nm). However, the FI of NiR began to decrease when the concentration ratio [1]/[NiR] is under 100:1. It indicated that the FRET efficiency between 1 and NiR decreased at high concentration of NiR. Probably in the presence of high concentration of NiR, the AIE of 1 was

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disturbed which induced the poor spectrum overlap between 1 and NiR. To further prove this deduction, dynamic light scattering (DLS) of [1]/[NiR] = 100:1 was monitored (Fig. 6d). The result suggested that the size distribution of 1 in aqueous solution was about 48 nm in the presence of NiR, but the average size of 1 in aqueous solution was about 86 nm (Fig. 4a). This indicated that new assembly 1-NiR had been formed. Subsequently, the morphology and size of the new assembly were investigated by transmission electron microscopy (TEM). The TEM images showed spherical morphology and the diameters around 50 nm (Fig. 6e), indicating that they formed spherical nanoparticles. To support the occurrence of FRET process, time-resolved fluorescence measurements were also performed. The fluorescence decay value of samples was fitted using a sum of exponentials: I(t) = ∑Bi exp(−τ/τi), where Bi and τi are the amplitude and lifetime, respectively, of the ith component. I(t) was convoluted with the measured instrumental response and then compared with the experimental data by nonlinear least-squares methods. Samples ([1]:[NiR] = 100:1) were excited at 405 nm and the monitored wavelength was selected at 525 nm. As shown in Fig. 6f, the decay curve of 1 (red line) was fitted with fluorescence lifetimes of τ1 = 1.5218 ns and τ2 = 9.3262 ns. For the 1-NiR assembly (green line), it decreased to τ1 = 1.2513 ns and τ2 = 6.4148 ns (Table S2), which further validates that energy transfer takes place from the 1 donor to NiR acceptor. The energy transfer efficiency (ΦET) and antenna effect (AE) of the 1NiR assembly were further investigated. Energy transfer efficiency was estimated by the widely used empirical parameters, which was from the fluorescence quenching rate of 1 in the 1-NiR assembly structure [37]. As a result, ΦET was calculated to be 60.6% when the mixing molar ratio of donor/acceptor was at 100:1 (Fig. S9, Table S4), Supporting Information). Notably, the antenna effect at this mixing ratio was calculated to be AE = 13.45 (Fig. S10, Table S4), indicating that the 1 was used as a good energy donor in aqueous environment. Herein, AIE(1) was regarded as the peripheral donor and NiR was the central acceptor. In addition to the molar ratio of donor/acceptor (100:1), the average distance (r) between the core domain and the

Fig. 7. a) Fluorescence spectra of 1-NiR ([1] = 10 μM, [NiR] = 1 × 10−7) with the addition of various metal ions (10 μM) in PBS buffer, pH 7.4, λex = 400 nm (metal ions: K+, Ca2+, Ni2+, Cu2 , Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Ag+, Al3+, Fe3+, Bi3+ and Cr3+). b) Fluorescent intensity of 1-NiR upon addition of 20 μM various metal ions containing 10 μM Cu2+ in PBS buffer. c) Fluorescence spectrum changes of 1-NiR upon incremental addition of Cu2+ (0–5 μM) in PBS buffer, pH 7.4. Inset: Changes of fluorescence intensity at 630 nm with addition of Cu2 + . d) Fluorescence decay profiles of NiR, 1-NiR, 1-NiR + Cu2+ in aqueous solutions. ([1] = 10 μM, [NiR[ = 0.1 μM, [Cu2+] = 10 μM).

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periphery were calculated to be 4.36 nm (Table S4). All of these data indicate that the 1-NiR assembly functions as an excellent energy transfer system in aqueous environment. 3.5. AIE(1)-NiR System for Cu2+ Sensing From above results, it can be seen that the energy transfer efficiency obtained the best when mixing molar ratio of donor/acceptor was at 100:1([1] = 10 μM, [NiR] = 10−7 M). Therefore, it can decrease detection limit of Cu2+ though monitoring the FI changes of NiR at 630 nm. As presented in Fig. 7a, the interaction of 1-NiR with various metal ions such as K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Ag+, Al3 + , Fe3+, Bi3+ and Cr3+ was studied. As a result, with addition 10 μM Cu2+, both emission of NiR (630 nm) and AIE(1) (525 nm) decreased gradually, while no apparent fluorescence changes were observed upon addition other metal ions (10 μM). It demonstrated that fluorescence emission of 1-NiR was selectively quenched by Cu2+. In order to investigate the possible interference on detection of Cu2+ in the presence of other interfering metal ions for 1-NiR, the fluorescence spectra changes of 1-NiR towards Cu2+ (10 μM) in the presence of other metal ions (20 μM) were investigated and the results were given in Fig. 7b. As presented in Fig. 7b, there was no obvious influence on detection of Cu2+ in the presence of other interfering metal ions for 1-NiR. Then, the detail titration of 1-NiR to Cu2+ was also investigated in aqueous solution and the result was shown in Fig. 7c. With addition microscale Cu2+ to 1-NiR, FI at 630 nm decreased gradually and Fig. 7c inset shown the FI changes (630 nm) with addition Cu2+. Besides, the detection limit of 1-NiR to Cu2+ was determined to be 9.12 nM. Compared to control experiment of fluorescence titrations of Cu2+ towards AIE(1) (Fig. 3c), the detection limit further decreased. Besides, the fluorescence lifetimes of 1-NiR + Cu2+ and 1-NiR was also investigated. As presented in Fig. 7d, the fluorescence lifetimes of 1-NiR + Cu2+ (red line) shapely decreased to τ1 = 0.7961 ns and τ2 = 2.7328 ns compared to 1-NiR τ1 = 1.0836 ns and τ2 = 5.3128 ns (green line) (Table S3), which was accordance with the FI changes of 1-NiR + Cu2+ and 1-NiR. 4. Conclusions In summary, we have provided a novel method to decrease the detection limit of Cu2+ through FRET and AIE strategy in aqueous environment. The Schiff base compound 1 exhibits excellent AIE properties in aqueous solution with green fluorescence. The green fluorescence at 525 nm of AIE(1) is selectively quenched in the presence of Cu2+. Importantly, AIE(1) is a good energy donor, which can transfer its energy to NiR dyes though FRET mechanism at a high molar ratio 100:1. The presence of small amount of Cu2+ can cause severe fluorescence quenching of AIE(1)–NiR systems at 630 nm, by which the detection limit of Cu2+ further decrease to nano level. Therefore, it is a well anticipated method that it can reduce the detection limit of Cu2+ through FRET mechanism for AIE system. Acknowledgements We are very grateful for financial support from the National Natural Science Foundation of China (No. 21271122, 21571117) and International Cooperation Research Project of Shanxi Province (2015081049). We thank the Scientific Instrument Center and Institute of Resources of Shanxi University and Environment Engineering and Scientific Instrument Center of Shanxi University. Appendix A. Supplementary Data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2018.12.020.

References [1] X. Chen, X.Y. Shen, E. Guan, Y. Liu, A. Qin, J.Z. Sun, B.Z. Tang, A pyridinylfunctionalized tetraphenylethylene fluorogen for specific sensing of trivalent cations, Chem. Commun. 49 (2013) 1503–1505. [2] Y. Hong, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission, Chem. Soc. Rev. 40 (2011) 5361–5388. [3] M.P. Aldred, C. Li, G.F. Zhang, W.L. Gong, A.D.Q. Li, Y. Dai, D. Ma, M.Q. Zhu, Fluorescence quenching and enhancement of vitrifiable oligofluorenes endcapped with tetraphenylethene, J. Mater. Chem. 22 (2012) 7515–7528. [4] J. Mei, N.L.C. Leung, R.T.K. Kwok, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission: together we shine, united we soar, Chem. Rev. 115 (2015) 11718–11940. [5] J. Mei, Y.N. Hong, J.W.Y. Lam, Y.H. Tang, B.Z. Tang, Aggregation-induced emission: the whole is more brilliant than the parts, Adv. Mater. 26 (2014) 5429–5479. [6] Z. Zhao, C. Deng, S. Chen, J.W.Y. Lam, W. Qin, P. Lu, Z. Wang, H.S. Kwok, Y. Ma, H. Qiu, B.Z. Tang, Full emission color tuning in luminogens constructed from tetraphenylethene benzo-2,1,3-thiadiazole and thiophene building blocks, Chem. Commun. 47 (2011) 8847–8849. [7] H. Li, Z. Chi, X. Zhang, B. Xu, S. Liu, Y. Zhang, J. Xu, New thermally stable aggregationinduced emission enhancement compounds for non-doped red organic lightemitting diodes, Chem. Commun. 47 (2011) 11273–11275. [8] J. Zhou, Z.F. Chang, Y.B. Jiang, B.N. He, M. Du, P. Lu, Y.N. Hong, H.S. Kwok, A.J. Qin, H.Y. Qiu, Z.J. Zhao, B.Z. Tang, From tetraphenylethene to tetranaphthylethene: structural evolution in AIE luminogen continues, Chem. Commun. 49 (2013) 2491–2493. [9] B.K. An, S.K. Kwon, S.D. Jung, S.Y. Park, Enhanced emission and its switching in fluorescent organic nanoparticles, J. Am. Chem. Soc. 124 (2002) 14410–14415. [10] L.L. Zhu, Y.L. Zhao, Cyanostilbene-based intelligent organic optoelectronic materials, J. Mater. Chem. C 1 (2013) 1059–1065. [11] H. Qian, M.E. Cousins, E.H. Horak, A. Wakefield, M.D. Liptak, I. Aprahamian, Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission, Nat. Chem. 9 (2017) 83–87. [12] D. Wu, A.C. Sedgwick, T. Gunnlaugsson, E.U. Akkaya, J. Yoon, T.D. James, Fluorescent chemosensors: the past, present and future, Chem. Soc. Rev. 46 (2017) 7105–7123. [13] G. Liang, L.T. Weng, J.W.Y. Lam, W. Qin, B.Z. Tang, Crystallization-induced hybrid nano-sheets of fluorescent polymers with aggregation-induced emission characteristics for sensitive explosive detection, ACS Macro Lett. 3 (2014) 21–25. [14] M.A. Baldo, M.E. Thompson, S.R. Forrest, High-efficiency fluorescent organic lightemitting devices using a phosphorescent sensitizer, Nature 403 (2000) 750–753. [15] T. Han, Y. Hong, N. Xie, S. Chen, N. Zhao, E. Zhao, J.W.Y. Lam, H.H.Y. Sung, Y. Dong, B. Tong, B.Z. Tang, Defect-sensitive crystals based on diaminomaleonitrilefunctionalized Schiff base with aggregation-enhanced emission, J. Mater. Chem. C 1 (2013) 7314–7320. [16] Q. Zhu, L. Huang, Z. Chen, S. Zheng, L. Lv, Z. Zhu, D. Cao, H. Jiang, S. Liu, A new series of C-6 unsubstituted tetrahydropyrimidines: convenient one-pot chemoselective synthesis, aggregation-induced and size-independent emission characteristics, Chem. Eur. J. 19 (2013) 1268–1280. [17] L. Huang, F. Hou, P. Xi, D. Bai, M. Xu, Z. Li, G. Xie, Y. Shi, H. Liu, Z. Zeng, A rhodaminebased “turn-on” fluorescent chemodosimeter for Cu2+ and its application in living cell imaging, J. Inorg. Biochem. 105 (2011) 800–805. [18] K.J. Barnham, C.L. Masters, A.I. Bush, Neurodegenerative diseases and oxidative stress, Nat. Rev. Drug Discov. 3 (2004) 205–214. [19] E. Gaggelli, H. Kozlowski, D. Valensin, G. Valensin, Copper homeostasis and neurodegenerative disorders (Alzheimer's, Prion, and Parkinson's diseases and amyotrophic lateral sclerosis), Chem. Rev. 106 (2006) 1995–2044. [20] G. Millhauser, Copper binding in the prion protein, Acc. Chem. Res. 37 (2004) 79–85. [21] S. Leary, P.A. Cobine, B.A. Kaufman, G. Guercin, A. Mattman, J. Palaty, G. Lockitch, D.R. Winge, P. Rustin, R. Horvath, E.A. Shoubridge, The human cytochrome c oxidase assembly factors SCO1 and SCO2 have regulatory roles in the maintenance of cellular copper homeostasis, Cell Metab. 5 (2007) 9–20. [22] S. Lutsenko, A. Gupta, J.L. Burkhead, V. Zuzel, Cellular multitasking: the dual role of human Cu-ATPases in cofactor delivery and intracellular copper balance, Arch. Biochem. Biophys. 476 (2008) 22–32. [23] V. Dujols, F. Ford, A.W. Czarnik, A long-wavelength fluorescent chemodosimeter selective for Cu(II) ion in water, J. Am. Chem. Soc. 119 (1997) 7386–7387. [24] M.H. Min, X.F. Wang, Y.M. Chen, L.M. Wang, H.L. Huang, J.G. Shi, Highly sensitive and selective Cu2+ sensor based on electrospun rhodamine dye doped poly(ether sulfones) nanofibers, Sensors Actuators B Chem. 188 (2013) 360–366. [25] J.F. Zhang, Y. Zhou, J. Yoon, Y. Kim, S.J. Kim, J.S. Kim, Naphthalimide modified rhodamine derivative: ratiometric and selective fluorescent sensor for Cu2+ based on two different approaches, Org. Lett. 12 (2010) 3852–3855. [26] S. Goswami, D. Sen, A.K. Das, N.K. Das, K. Aich, H.K. Fun, C.K. Quah, A.K. Maity, P. Saha, A new rhodamine-coumarin Cu2+-selective colorimetric and ‘off–on’ fluorescence probe for effective use in chemistry and bioimaging along with its bound Xray crystal structure, Sensors Actuators B Chem. 183 (2013) 518–525. [27] B. Liu, X. Hu, J. Chai, J. Zhu, B. Yang, Y. Li, Application of nanodiamonds in Cu(II)based rhodamine B probes for NO detection and cell imaging, J. Mater. Chem. B 4 (2016) 3358–3364. [28] Y.S. Wu, C.Y. Li, Y.F. Li, D. Li, Z. Li, Development of a simple pyrene-based ratiometric fluorescent chemosensor for copper ion in living cells, Sensors Actuators B Chem. 222 (2016) 1226–1232. [29] J. Chai, Y.B. Wu, B.S. Yang, B. Liu, The photochromism, light harvesting and selfassembly activity of a multi-function Schiff-base compound based on the AIE effect, J. Mater. Chem. C 6 (2018) 4057–4064. [30] B. Liu, H.L. Zhou, B.S. Yang, X.Q. Hu, Aggregation–induced emission activity and further Cu2+-induced self-assembly process of two Schiff compounds, Sensors Actuators B Chem. 246 (2017) 554–562.

J. Yang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 211 (2019) 272–279 [31] A. Singh, R. Singh, M. Shellaiah, E.C. Prakash, H. Chang, P. Raghunath, M. Lin, H. Lin, A new pyrene-based aggregation induced ratiometric emission probe for selective detections of trivalent metal ions and its living cell application, Sensors Actuators B Chem. 207 (2015) 338–345. [32] A.C. Gonc, J. Capelo, L. Capelo, C. Lodeiro, A.A. Santos, A seleno-pyrene selective probe for Hg2+ detection in either aqueous or aprotic systems, Sensors Actuators B Chem. 239 (2017) 311–318. [33] J. Luo, Z. Xie, J.W.Y. Lam, L. Cheng, B.Z. Tang, H. Chen, C. Qiu, H.S. Kwok, X. Zhan, Y. Liu, D. Zhu, Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole, Chem. Commun. 37 (2001) 1740–1741. [34] M.J. O'Connell, S.M. Bachilo, C.B. Huffman, V.C. Moore, M.S. Strano, E.H. Haroz, K.L. Rialon, P.J. Boul, W.H. Noon, C. Kittrell, J. Ma, R.H. Hauge, R.B. Weisman, R.E.

279

Smalley, Band gap fluorescence from individual single-walled carbon nanotubes, Science 297 (2002) 593–596. [35] B. Liu, Y.Q. Li, B.S. Yang, S.P. Huang, Synthesis, characterization and properties of chromium(III) complex [Cr(SA)(en)2]Cl·2H2O, J. Inorg. Biochem. 100 (2006) 1462–1469. [36] N. Kumari, N. Dey, K. Kumar, S. Bhattacharya, Exclusive detection of sub-nanomolar levels of palladium(II) in water: an excellent probe for multiple applications, Chem. Asian J. 9 (2014) 3174–3181. [37] S.W. Guo, Y.S. Song, Y.L. He, X.Y. Hu, L.Y. Wang, Highly efficient artificial lightharvesting systems constructed in aqueous solution based on supramolecular selfassembly, Angew. Chem. Int. Ed. 57 (2018) 3163–3167.