A Naphthalimide-Based Fluorescence “Off-on-Off” Chemosensor for Relay Detection of Al3+ and ClO−

A novel Al³⁺ chemosensor NPA was designed and synthesized on basis of the mechanism of ICT and CHEF. Upon addition of Al³⁺, the probe NPA displayed a bright green fluorescence under UV radiation and visual color change from yellow to colorless. Spectrum titrations showed that NPA could be recognized as a fluorescent turn-on probe with 10⁻⁸ M detection level. The probe was successfully applied in real water sample and test paper. More important, NPA-Al³⁺ complex were used as a fluorescent turn-off probe for the detection of ClO⁻ with the detection as low as 2.34 × 10⁻⁸ M. The performance of NPA to Al³⁺ and NPA-Al³⁺ complex to ClO⁻ demonstrated that NPA could be served as a sensitive probe and exhibit INHIBIT logic gate behavior with Al³⁺ and ClO⁻ as inputs.

Introduction

Aluminum, the third most abundant metal element in the Earth's crust, is widely used in human's daily life including pharmaceuticals, textile, kitchen utensils, and paper industries (Zhang et al., 2016; Kaur and Kaur, 2017). Moreover, Al³⁺ ion widely exists in the environment, normally in natural waters and many plants, which can enter the human body through foods and water. However, excess aluminum can damage the human nervous system, and has tightly relation to many diseases such as Alzheimer's disease, Parkinson's disease, anemia, dementia, encephalopathy, and gastrointestinal diseases (Helal et al., 2013; Liu et al., 2014, 2017; Jiang et al., 2015; Zeng et al., 2018). In addition, hypochlorous acid (HOCl), known as an important reactive oxygen species (ROS) in many living organisms, plays a vital role in many biological processes (Zhu et al., 2014). Hypochlorite is a key microbicide that is used for natural defense because it behaves as a strong nucleophilic non-radical oxidant (Shi et al., 2010). However, it also results in many pathological diseases, especially is implicated in inflammation-associated injury including hepatic ischemia-reperfusion injury, lung injury, rheumatoid arthritis, and atherosclerosis. Moreover, excessive or misplaced of hypochlorite can also cause detrimental effect on tissues (Lei et al., 2017; Lin et al., 2017; Shen et al., 2017). Therefore, the determination of Al³⁺ and ClO⁻ in biological samples is of great importance.

In the last few decades, the designing a molecular system, which displayed significant changes in electronic, magnetic, or optical signals even at low concentration during the detection for a specific guest species, was a hot topic for many researchers (Kim et al., 2013; Liu et al., 2014; Kang Y. et al., 2016; Lim et al., 2017). Among them, small-molecule optical probes, which exhibited many merits including high selectivity and sensitivity, tunability, simple manipulation, and direct visualization, were employed as a powerful tool in the facet of trace analysis and rapid detection for various analytes (Qin and Yang, 2015; Wang et al., 2015; Simon et al., 2016; Liu et al., 2017; Murugan et al., 2017). Up to now, lots of papers that one probe only for the detection of one special analyte such as Al³⁺ ion (Tang et al., 2015; Gupta and Kumar, 2016; Xie et al., 2017) or Hypochlorous acid (Zhang et al., 2017) had been reported, but the design idea, one fluorescent probe for successively recognition of two different analytes, had gained increasing attention in considering its high efficiency and potential cost reduction (Wang et al., 2014; Ye et al., 2014; Zhao et al., 2015; Wen and Fan, 2016; Xie et al., 2016; Zhai et al., 2016; Zhu et al., 2016; Wu et al., 2017). Many excellent probes had been reported for relay recognition of two different ions through fluorescent “off-on-off” (Borasea et al., 2015; He et al., 2015; Zhao et al., 2015; Rai et al., 2016; Bhattacharyya et al., 2017; Das et al., 2017; Jo et al., 2017; Dwivedi et al., 2018; Feng et al., 2018; Lim et al., 2018) or “on-off-on” (Diao et al., 2016; Zhao et al., 2016; Sarkar et al., 2017) mode. Moreover, 1, 8-naphthalimide fluorophore, holding many excellent photophysics properties, such as high photostability, visible absorption and fluorescence emission, large Stokes' shift and high fluorescence quantum yield (Dimov et al., 2014; Yu and Zhang, 2014; Kang L. et al., 2016; Li et al., 2018; Fu et al., 2019), was successfully applied in designing fluorescent probes toward various analytes.

Taking above statements into consideration, in this paper, we prepared and characterized a 1, 8-naphthalimide-based fluorescent probe NPA for the detection of Al³⁺ embodied in colorimetric and fluorescent turn-on was ascribed to the co-contribution of intramolecular charge transfer (ICT) and chelation enhanced fluorescence (CHEF). The binding stoichiometry between the NPA and Al³⁺ had been clarified according to various spectroscopic measurements and data analysis. Moreover, the performance of in situ formed NPA-Al³⁺ complex for the detection of ClO⁻ was investigated. The NPA-Al³⁺ complex exhibited a fluorescent turn-off response in the detection of ClO⁻. Inspiringly, to the best of our knowledge, a single probe for sequential detection of Al³⁺ and ClO⁻ through fluorescence “off-on-off” mode is scarcely documented.

Materials and Methods

Reagents and Instrument

All the solvents and reagents (analytical or spectroscopic grade) were purchased commercially and used as received. Metal salts [NaClO₄, KClO₄, Mg(ClO₄)₂, Ba(ClO₄)₂, Zn(ClO₄)₂•6H₂O, Cu(ClO₄)₂•6H₂O, AgNO₃, Cd(NO₃)₂, Pb(NO₃)₂, Co(NO₃)₂•6H₂O, Ni(NO₃)₂•6H₂O, Ca(NO₃)₂•4H₂O, Al(NO₃)₃•9H₂O, MnSO₄•H₂O, HgCl₂, FeCl₂•4H₂O] were obtained from commercial suppliers, and used as received without further purification. ¹H NMR and ¹³C NMR spectra were recorded on a AV- 600 spectrometer in DMSO-d₆ solution. The chemical shifts (δ) are reported in ppm and coupling constants (J) in Hz relative to TMS (0.00) for ¹H NMR and ¹³C NMR. Mass spectra were measured on a Waters Xevo UPLC/G2-SQ Tof MS spectrometer. Absorption spectra were recorded using a Pgeneral TU-1901 UV-vis spectrophotometer. Fluorescence measurements were performed on a Perkin Elmer LS55 fluorescence spectrometer at room temperature.

Synthesis of the Probe NPA

NPA was prepared in four steps from naphthalimide as the starting material, as shown in Scheme 1. Compound 1–4 were synthesized using the literature method (Kang L. et al., 2016; Li et al., 2018).

SCHEME 1

Scheme 1. Synthesis of probe NPA.

The compound 4 (330 mg, 0.97 mmol) and 2-amino-propane-1, 3-diol (72 mg, 0.8 mmol) were dissolved in ethanol (45 mL) and stirred over anhydrous Et₃N (80 mg, 0.58 mmol) for 24 h at room temperature. The crude product was purified by column chromatography with CH₃OH/CHCl₃ (1:20, v/v) as the eluent and further purified by recrystallization from ethanol to get orange-yellow solid NPA (210 mg, yield: 46%); m.p: 195–196°C.¹HNMR (600 MHz, DMSO) (Supplementary Figure 1): δ (ppm) (d, J = 7.2 Hz, 1H), 8.43 (d, J = 8.5 Hz, 1H), 8.39 (d, J = 8.1 Hz, 1H), 7.83–7.75 (m, 1H), 7.50 (d, J = 8.5 Hz, 1H), 7.35 (d, J = 8.1 Hz, 1H), 4.71 (t, J = 5.4 Hz, 2H), 4.07–3.97 (m, 2H), 3.82–3.72 (m, 1H), 3.54–3.45 (m, 2H), 3.46–3.38 (m, 2H), 3.27 (s, 4H), 3.10 (s, 2H), 2.80 (s, 4H), 1.64–1.54 (m, 2H), 1.39–1.30 (m, 2H), 0.92 (t, J = 7.4 Hz, 3H). ¹³C NMR (151 MHz, DMSO) (Supplementary Figure 2): δ (ppm) 168.62, 163.32, 162.81, 155.32, 131.96, 130.42, 130.26, 128.89, 125.84, 125.10, 122.37, 115.42, 114.92, 60.86, 59.69, 52.53, 52.40, 51.91, 38.91, 29.51, 19.59, 13.52. MS (ESI) (Supplementary Figure 3): m/z [NPA+H⁺]⁺ calcd 469.2451, found 469.2458.

Preparation of Solutions for Spectral Detection

All stock solutions (10 mM) including metal cations and the anions were prepared with distilled water, while the stock solution of compound NPA (0.1 mM) was prepared in CH₃OH (100 mL). One milliliter of NPA solution (0.1 mM) was diluted in 9 mL CH₃OH to make the test solutions (10 μM). For fluorescence measurements, excitation was set at 400 nm, and the excitation and emission slit widths were 10 and 10 nm, respectively. All spectroscopic measurements were performed in CH₃OH at room temperature.

Stock solution of NPA (0.1 mM) was prepared by NPA (0.01 mmoL) was dissolved in CH₃OH (100 mL), the test solutions of NPA (10 μM) was prepared by adding 1 mL of NPA stock solution (0.1 mM) was diluted in 9 mL CH₃OH in CH₃OH.

Stock solutions (10 mM) including the metal cations and the anions were prepared with ultrapure water, respectively. For spectrum measurement, the test solutions were prepared by adding certain amount of stock solution using a pipette into NPA stock solution. For fluorescence measurements, excitation was set at 400 nm, and the excitation and emission slit widths were 10 and 10 nm, respectively.

Determination of Binding Constant and Detection Limit

According to the fluorescence intensity data, the binding constant of NPA with Al³⁺ was calculated based on the modified Benesi–Hildebrand equation (Li et al., 2018) as followed. Where, F_max, F and F_min are the fluorescence intensities of NPA in the presence of Al³⁺ at saturation, at an intermediate Al³⁺ concentration, and absence of Al³⁺, respectively. K is the stability constant.

$\begin{array}{l}\frac{1}{F-F_{min}}=\frac{1}{K(F_{max}-F_{min})\left[A{l}^{3+}\right]}-\frac{1}{F_{max}-F_{min}}\end{array}$

The limit of detection (LOD) of Al³⁺ was calculated on the basis of 3δ/K according to the fluorescence changes, δ is the standard deviation of the blank solution, and K is slope of calibration curve (Borasea et al., 2015; Zeng et al., 2018).

Results and Discussion

The UV-Vis Spectra Responses of Probe NPA

Various metal ions: K⁺, Na⁺, Ca²⁺, Mg²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, Cd²⁺, Hg²⁺, Mn²⁺, Ag⁺, Zn²⁺, and Al³⁺ were used to observe the selectivity of probe NPA (10 μM) and their UV-vis spectra were measured in CH₃OH. As shown in Figure 1, NPA exhibited a characteristic absorbance band at 400 nm in the presence of tested metal ions (K⁺, Na⁺, Ag⁺, Ca²⁺, Mg²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, Cd²⁺, Hg²⁺, Mn²⁺, Zn²⁺) and there was almost no difference compared with that of NPA itself. However, upon the addition of Al³⁺, the maximum absorbance peak of NPA was blue-shifted with 21 nm from 399 to 378 nm, and the solution color was changed from yellow to colorless. This result might attribute to the decrease of conjugated degree resulting from the decrease of electron donating ability of piperazine ring to the naphthalimide after the complexation with Al³⁺ (De Silva et al., 1997; Urano et al., 2015; Kang L. et al., 2016).

FIGURE 1

Figure 1. UV-vis absorbance spectra of NPA (10 μM) in the absence and presence of various mental ions (50 μM) in CH₃OH. Inset: The color change of NPA (10 μM) in the presence of Al³⁺ ions (5 equiv.) in CH₃OH under natural light.

In addition, the quantitative sensing of Al³⁺ ion was elucidated by UV-vis titration of probe NPA in CH₃OH solution (Figure 2). NPA alone showed a major absorbance band at 399 nm, a new band at 378 nm appeared upon addition of Al³⁺ and the intensity increased gradually and then kept constant until the amount added of Al³⁺ was more than 20 μM. The good relationship was found between the ratio of absorbance (A₃₇₈/A₃₉₉) vs. the concentration of Al³⁺ (Supplementary Figure 4), and the limit of detection (LOD) for Al³⁺ was calculated as 3.61 × 10⁻⁸ M on the basis of 3 δ/K (where δ is deviation of the blank signal and K is slope of calibration curve; Borasea et al., 2015; Zeng et al., 2018). These results indicated that NPA could be used as an absorbance-ratiometric probe for the detection of Al³⁺.

FIGURE 2

Figure 2. UV-vis spectral of NPA (10 μM) upon addition of increasing concentration of Al³⁺ (0–5 equiv.) in CH₃OH. Insert: Plot of absorbance intensity ratio (A₃₇₈/A₃₉₉) as a function of the Al³⁺ concentration.

Fluorescence Spectral Responses of NPA

The fluorescence properties of NPA were investigated in the presence of a variety of metal ions in CH₃OH (Figure 3). NPA itself exhibited almost no fluorescence emission upon excitation at 400 nm. However, the addition of Al³⁺ (5 equiv.) into NPA induced an obvious fluorescence enhancement at 505 nm, which might be attributed to the CHEF effect through the formation of a rigid system after binding with Al³⁺ (Kang L. et al., 2016; Zeng et al., 2019). In contrast, few fluorescence changes were observed in the presence of other tested metal ions, demonstrating that NPA was highly selective for Al³⁺ over competing metal ions.

FIGURE 3

Figure 3. Fluorescence spectral changes (λ_ex = 400 nm) of NPA (10 μM) in the presence of various mental ions (50 μM) in CH₃OH. Inset: The color change of NPA (10 μM) in the presence of Al³⁺ ions (5 equiv.) in CH₃OH under 365 nm UV lamp.

To further validate the utility of probe NPA, the fluorescence titrations of NPA were performed by gradually increasing the concentration of Al³⁺ (Figure 4). The emission intensity of NPA at 505 nm progressively increased upon addition of Al³⁺, and the emission intensity remained constant when the quantity of Al³⁺ added was over 15 μM (Figure 4, Insert). Moreover, the fluorescence enhancement of sensor NPA depending on the concentration of Al³⁺ was in a linear manner (Supplementary Figure 5). The detection limit (LOD) for Al³⁺ ion according to fluorescence changes was measured to be 2.03 × 10⁻⁸ M on the basis of 3 δ/K (Borasea et al., 2015; Zeng et al., 2018). The result clearly demonstrated that the probe NPA was highly efficient in sensing Al³⁺ at nanomolar level.

FIGURE 4

Figure 4. Fluorescence emission spectral (λ_ex = 400 nm) of NPA (10 μM) with increasing concentration of Al³⁺ in CH₃OH. Inset: Plot of fluorescence intensity at 505 nm as a function of the Al³⁺ concentration.

Further, tolerance of fluorescence intensity of NPA-Al³⁺ complex in presence of other metal ions was tested (Figure 5). All competitive metal ions had no obvious interference on the Al³⁺ detection, indicated that NPA-Al³⁺ complex was hardly affected by these coexistent metal ions. Accordingly, NPA can be used as selective fluorescent probe for Al³⁺ determination without disturbance of other competing metal ions.

FIGURE 5

Figure 5. Fluorescence intensity of NPA (10 μM) at 505 nm upon addition of Al³⁺ (50 μM) in the presence of various mental ions (50 μM) (λ_ex = 400 nm). (Error bar was represented as mean ± standard deviation, n = 3).

Response Time Studies

As for an excellent fluorescent probe, fluorescent stability and response time are two crucial factors. Hence, the changes of fluorescent intensity of NPA and the response time of NPA to Al³⁺ were investigated (Supplementary Figure 6). The results demonstrated that the fluorescence signal of NPA remained stable for a long time in the absence of Al³⁺, implying the NPA had good fluorescence stability. Moreover, after addition of Al³⁺, the fluorescence intensity of NPA at the 505 nm reached the maximum within 10 s and maintained constant more than 3 min, indicating the unique feature of high complexation ability of NPA with Al³⁺.

Binding Stoichiometry and Sensing Mechanism

The total concentration of Al³⁺ and ligand was 50 μM with the molar ratio of Al³⁺ changed from 0.1 to 0.9. The fluorescence mission was measured for each sample in CH₃OH with the excitation wavelength at 400 nm. The maximum point appeared at a mole fraction of 0.5 (Figure 6), indicating a 1:1 stoichiometry of the binding mode between NPA and Al³⁺, and which was further clarified by a peak at m/z 594.1575, which was assignable to [NPA – 2H⁺ + K⁺+ Al³⁺ + ${\text{NO}}_3^{-}$]⁺ (calcd. m/z 594.1547) in the ESI mass spectrum (Figure 7).

FIGURE 6

Figure 6. Job's plot of NPA with Al³⁺ in CH₃OH. {[Al³⁺]/([Al³⁺] + [NPA])} is the molar fraction of Al³⁺ ion.

FIGURE 7

Figure 7. ESI–MS spectrum of NPA (50 μM) upon addition of 5 equiv. of Al³⁺ in CH₃OH.

According to the above results, the association constant was calculated as 7.06 × 10⁴ M⁻¹ according to the fluorescence titration data (Supplementary Figure 7), basing on the Benesi-Hildebrand plot (Li et al., 2018).

To better evaluate the interaction of NPA with Al³⁺, ¹H NMR spectra of NPA were constructed in the absence and presence of different equivalent Al³⁺ in DMSO (Figure 8). The protons of H₁ and H₂ of NPA at around 2.80 and 3.27 ppm were shifted downfield to 3.42 upon the addition of Al³⁺. In addition, the peak of NPA at around 3.10 ppm was also shifted downfield to 3.42 ppm, indicated that the two nitrogen atoms of piperazine ring might coordinate the Al³⁺. Moreover, the proton of amide was disappeared supporting the occurrence of deprotonation upon the interaction of amide with the Al³⁺. According to the above results, the two nitrogen atoms of piperazine ring and the amide nitrogen atom might coordinate with Al³⁺ (Scheme 2).

FIGURE 8

Figure 8. ¹H NMR spectra of NPA with Al³⁺ in DMSO d₆.

SCHEME 2

Scheme 2. Proposed mechanism for the fluorescent sensing of NPA and its in situ complex with Al³⁺ and ClO⁻.

Reversibility of NPA Toward Al³⁺

The fluorescence intensity enhancement of NPA on interaction with Al³⁺ was found to be reversible by using EDTA as the recovering reagent. When the strong metal ion chelating agent EDTA was added gradually to a mixture of NPA-Al³⁺, the UV-vis absorbance spectral and fluorescence spectral of the solution of NPA-Al³⁺ almost recovered to the original condition in the absence of Al³⁺ (Supplementary Figures 8, 9). These results indicating that recognition process can be made reversible merely by treatment with EDTA. Besides, as for an excellent chemosensor, reversibility and regeneration are crucial for the fabrication of apparatus to sense Al³⁺.

Application of NPA for Al³⁺ Analysis in Test Paper

Interestingly, the noticeable colorimetric changes of the system and qualitative recognition of Al³⁺ in solution were confirmed by the simple test strips. The required test strips were immersed in CH₃OH (10 mL) including Al³⁺ and then dried in air. The color changes in various concentrations of Al³⁺ under sunlight and under 365 nm UV light were illustrated in Supplementary Figure 10. Visual color changed from yellow to colorless and bright-green fluorescence increased gradually with increasing amounts of Al³⁺ were clearly observed. This result showed that NPA might be used as a portable detector for Al³⁺.

Application of NPA for Al³⁺ Analysis in Water Samples

In order to verify the practical application of NPA, ultrapure water and tap water samples were analyzed by the proposed fluorimetric method. The results were summarized in Supplementary Table 1 with satisfactory recovery of Al³⁺, indicating that the present fluorescent probe seem to be applicable for the determination of Al³⁺ in environmental analysis.

Fluorescent “On-Off” Sensing of ClO⁻ by NPA-Al³⁺ Complex

In order to further explore the performance of NPA-Al³⁺ complex to different anionic, the fluorescence spectra of NPA-Al³⁺ complex were investigated in presence of various 100 μM anions (ClO⁻, ROO⁻, H₂O₂, OH⁻, NO⁻, Cl⁻, Br⁻, ${\text{NO}}_3^{-}$, ${\text{BF}}_4^{-}$, ${\text{ClO}}_4^{-}$) in CH₃OH (Figure 9). The result showed that fluorescence intensity of NPA-Al³⁺ complex was affected to tolerable degree by I⁻, H₂O₂, and ${\text{BF}}_4^{-}$ except for ClO⁻, which completely quenched the fluorescence of NPA-Al³⁺ complex. This result suggested that the NPA-Al³⁺ complex could distinguish ClO⁻ anion over other ROO⁻, H₂O₂, OH⁻, NO⁻, Cl⁻, Br⁻, ${\text{NO}}_3^{-}$, ${\text{BF}}_4^{-}$, ${\text{ClO}}_4^{-}$ by fluorescence “on-off” mode.

FIGURE 9

Figure 9. Fluorescence spectra (λ_ex = 400 nm) of NPA-Al³⁺ system in presence of various anions (100 μM) in CH₃OH.

Furthermore, competition experiments were carried out by addition of various anions (100 μM) to the solution of NPA-Al³⁺ in the presence of 100 μM of ClO⁻. As shown in Figure 10, competitive anion had no prominent interference with the determination of ClO⁻, which meant that NPA-Al³⁺ could perform as a highly selective probe for ClO⁻ via a fluorescence “turn-off” mechanism.

FIGURE 10

Figure 10. Changes in emission intensity of the NPA-Al³⁺ complex at 505 nm upon addition of 100 μM of various anions followed by addition of ClO⁻ (λ_ex = 400 nm). (Error bar was represented as mean ± standard deviation, n = 3).

To further investigate the binding ability and limit of detection of the NPA-Al³⁺ complex with ClO⁻. The complex sensing capability was studied in detail analysis with fluorescence titration (Figure 11). Upon addition of ClO⁻ to the solution containing NPA-Al³⁺ complex, fluorescent intensity at 505 nm gradually diminishes, and 40 μM of ClO⁻ could lead to complete fluorescent quenching (Figure 11, inset). Obviously, the fluorescence intensity of NPA-Al³⁺ complex depending on the concentration of ClO⁻ was in a linear manner (Supplementary Figure 11). According to the fluorescence titration data, the detection limit of NPA-Al³⁺ complex was determined and be found to 2.34 × 10⁻⁸ M (Supplementary Table 3). Furthermore, fluorescence quenching in situ also indicated that the occurrence of dissociation of NPA-Al³⁺ complex, and the NPA was regenerated upon the addition of ClO⁻, which further supported by the verified experiments as followed. Firstly, the peak at m/z 469.2438 appeared in ESI–MS spectrum of NPA-Al³⁺ complex upon addition of ClO⁻ in CH₃OH, which was assignable to [NPA+H⁺]⁺ (calcd m/z 469.2451; Supplementary Figure 12) compared with the ESI–MS spectrum of NPA itself (Supplementary Figure 13). Moreover, the UV-vis absorbance and fluorescence spectrum of NPA-Al³⁺ complex in the absence and presence of ClO⁻ were measured (Figure 12), respectively. The result showed that both of them were almost the same as that of NPA itself. Lastly, the titration of ¹H NMR (Supplementary Figure 14) and ¹³C NMR (Supplementary Figure 15) were also investigated to clarify the sensing mechanism. The result displayed that NPA was regenerated to some extent upon the addition of ClO⁻ to the NPA-Al³⁺ complex. The above result implies that the NPA-Al³⁺ complex could act as a fluorescent turn-off probe for ClO⁻ recognition. According to above results, the “off-on-off” mechanism was achieved with sequence specificity (Al³⁺ and ClO⁻) in CH₃OH (Scheme 2). In addition, the results of the comparison between NPA and those reported sensors (one fluorescent probe for successively recognition of two different analytes) were summarized in Supplementary Table 2.

FIGURE 11

Figure 11. Fluorescence spectral (λ_ex = 400 nm) of NPA-Al³⁺ complex at different added concentration of ClO⁻ in CH₃OH. Insert: Plot of fluorescence intensity of verse ClO⁻ concentration in in CH₃OH.

FIGURE 12

Figure 12. Fluorescence spectrum (A) and UV-vis absorbance spectrum (B) of NPA in CH₃OH while adding Al³⁺ (3 eq.) and ClO⁻ (10 eq.) to the solution.

Application as Logic Gate Function

NPA alone displayed very weak fluorescence emission. Maximum emission at 505 nm appeared after coordination of NPA with the Al³⁺. Moreover, when ClO⁻ was added to the above solution, the emission intensity at 505 nm was quenched. In addition, the fluorescence “off-on-off” response for Al³⁺ and ClO⁻ were carried out (Supplementary Figure 16), and the result showed that cycle times was more than 5 according to NPA fluorescent signal upon the alternate addition of Al³⁺ and ClO⁻. Due to the remarkable fluorescence changes of probe NPA in the presence Al³⁺ and ClO⁻, it would be able to construct a two output combinatorial logic circuit with two signal inputs that are input 1 (Al³⁺) and input 2 (ClO⁻; Figure 13A). Input 1 caused fluorescence enhancement, equivalent to a YES operation, while input 2 resulted in fluorescence quenching, thereby enforcement of the NOT gate. In the presence of both inputs, the quenching (by Input 2) has precedence over the fluorescence enhancement by Input 1, which was in accordance with the truth table illustrated in Figure 13B. Hence, by monitoring the emission maxima at 505 nm by two inputs (Al³⁺ and ClO⁻), a monomolecular circuit displaying an INHIBIT logic function could be achieved.

FIGURE 13

Figure 13. (A) The logic circuit displaying memory unit with two inputs (Al³⁺ and ClO⁻) and one output; (B) corresponding truth table.

Conclusion

In conclusion, we had developed a naphthalimide-based sensor NPA which exhibited a turn-on fluorescence response toward Al³⁺ with a bright green fluorescence under UV radiation and visual color change from yellow to colorless detected by naked-eye. The binding stoichiometry of NPA with Al³⁺ was 1:1 confirmed by job's plot, HRMS and ¹H NMR and ¹³C NMR titration. The application of NPA for detection and assessing the existence of Al³⁺ in real sample was also successfully achieved. Moreover, NPA-Al³⁺ complex were further used as a fluorescent turn-off probe for the detection of ClO⁻ with the detection as low as 2.34 × 10⁻⁸ M. The INHIBIT molecular logic gate was effectively constructed by the performance of NPA to Al³⁺ and NPA-Al³⁺ complex to ClO⁻. The development of multi-functional chemosensor for the detection of biological-related analyst in pure water is our future work.

Data Availability

The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.

Author Contributions

Z-YX constructed the workflow and completed the paper. N-NL synthesized and purified the compound. X-JS and T-TL measured and interpreted the data the UV/Vis and fluorescence experiments. HF and FY performed the real sample analysis.

Funding

This work was supported by Heilongjiang Postdoctoral Funds for scientific research initiation (No. LBH-Q14023) and the Research Science Foundation in Technology Innovation of Harbin (No. 2017RAQXJ022).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank our team leader Professor Ying Fu (Northeast Agricultural University) for financial supporting in paper publishing.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2019.00549/full#supplementary-material

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