An efficient and sensitive chemosensor based on salicylhydrazide for naked-eye and fluorescent detection of Zn2+

We reported here the synthesis of a diarylethene with a 2,4-dihydroxybenzoyl hydrazine moiety (1O) for Zn2+ recognition. The compound is easy to prepare with a high yield up to 85%. Compound 1O can act as a highly selective and specific fluorescent sensor for Zn2+ without interference by other common metal ions. The LOD for Zn2+ detection was determined to be 1.28 × 10−6 mol L−1. Meanwhile, 1O can be used as a naked-eye detector for the Zn2+ ion with an obvious color change from colorless to olive. Based on the fluorescent properties of 1O, we constructed a logic circuit with four inputs of the combinational stimuli of UV/vis light and Zn2+/EDTA, and one output of fluorescence intensity.


Introduction
As we all know, zinc(II) is the second most abundant and essential element aer iron ions (Fe 2+ and Fe 3+ ) in the human body and performs a variety of functions. [1][2][3] It is an integral part of numerous enzymes and plays a critical role in various biological processes, such as protein metabolism, the immune system, gene transcription and regulation. [4][5][6][7] The research on Zn 2+ has drawn considerable attention among biologists, chemists, environmentalists and pharmacologists for its chemical and physical properties. The concentration of Zn 2+ in the human body ranges from nanomolar (nM) to millimolar (mM) 8 and it is indispensable to living organisms. Depletion of biological Zn 2+ leads to a decrease in wound health strength as a result of impaired collagen synthesis. 9 Zn 2+ is a relatively nontoxic element, while its high level is cytotoxic. Unbalanced metabolism of Zn 2+ may lead to a series of diseases, such as Alzheimer's disease, 10 Parkinson's disease, 11 diabetes, 12 prostate cancer 13 and immune dysfunction. 14 It is important for us to maintain the balance of Zn 2+ in human body. Therefore, monitoring the distribution and concentration of Zn 2+ in environmental or biologic samples becomes important.
The demand for chemosensors that are selective and sensitive for specic target ion is continuously increasing. Numerous sensitive detection methods for metal ion recognition have been widely used in the eld of analytical chemistry, biology and environmental processes, [15][16][17][18] such as mass spectrometry, atomic absorption spectroscopy and high performance liquid chromatography. However, these methods are laborious and require the use of complex equipment. Fluorescence is a powerful tool to detect target ions for its simplicity, easy implementation, high sensitivity and low detection limit. [19][20][21] When a specic uorescent chemosensor is added to a solution of target metal ion, a color change can be observed accompanied by the changes of uorescent characteristics. Based on this apparent phenomenon, we can design efficient chemosensors for the recognition of specic ions.
Over the years, many uorescent chemosensors have been reported for the detection of Zn 2+ . Several Zn 2+ sensors have been developed based on different uorophores, such as quinoline, 22-24 uorescein, 25-28 coumarin, 29-32 peptide, 33 and pyrene. 34 However, most of them lack the smartness in Zn 2+ selectivity, sensitivity and interference resistance from Cd 2+ ion, 35,36 or there is a low yield resulting from complex purication protocols. [37][38][39] As shown in Table 1, most of these sensors can not distinguish Zn 2+ from Cd 2+ . [40][41][42][43][44] Zn 2+ and Cd 2+ are located in the same group of the periodic table and show similar photo-physical changes in these sensors. 45 So, an active search of new analytic agents with higher sensitivity and selectivity for Zn 2+ continues at the present time.
Photo-stimuli responsive materials have attracted much attention due to their potential applications in optical devices, controlled release, clean energy, sensors, etc. Until now, plenty of materials based on different photoactive groups have been explored. Spiropyran, spirooxazine, diarylethene and fulgide derivatives are photochromic materials based on photo-induced isomerization involving ring-opening/closing reactions. 46,47 Photochromism refers to a reversible change in the properties of a molecule in response to light. Among the various photochromic compounds, diarylethenes are gaining increasing attention in the eld of photo-electronics, such as optical memory media and photo-switching devices, due to their high thermal stability, excellent fatigue resistance, and characteristic bistability. 48,49 In the current work, we reported a Zn 2+ chemosensor with a 2,4-dihydroxybenzoyl hydrazine unit. Salicylhydrazide is one of the important members in Schiff base family because it offers a number of possibilities for different modes of coordination with transition metal ions. 50,51 On the other side, considering the advantages of fast response and excellent thermal stability for diarylethene derivatives, we designed and synthesized the compound 1O. The chemosensor detected Zn 2+ with high selectivity and specicity accompanied by obvious color changes by stimuli of lights and metal ions. Besides, addition of Zn 2+ into the compound 1O resulted in a change in the absorbance spectra, making 1O a naked-eye detector for Zn 2+ . The photochromic process of the diarylethene derivate was shown in Scheme 1.

General methods
All solvents were of analytical purity and were puried by distillation before use. Other reagents were used without further purication. Mass spectra were measured with a Bruker amazon SL Ion Trap Mass spectrometer. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with tetramethylsilane as an internal standard.
Infrared spectra (IR) were recorded on a Bruker Vertex-70 spectrometer. Melting point was measured on a WRS-1B melting point apparatus. Fluorescence spectra were measured using a Hitachi F-4600 spectrophotometer. The uorescence quantum yield was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. Absorption spectra were measured using an Agilent 8453 UV/vis spectrophotometer with an MUL-165 UV lamp and a MVL-210 visible lamp as equipments of photoirradiation. The solutions of metal ions (0.1 mol L À1 ) were prepared by the dissolution of their respective metal nitrates in distilled water, except for K + , Ba 2+ , Mn 2+ , and Hg 2+ (all of their counter ions were chloride ions). Necessary dilutions were made according to each experimental set up. All of the measurements were conducted at room temperature unless otherwise stated.

Synthesis of 1O
Diarylethene 1O was synthesized as presented in Scheme 2. Compound 2 was synthesized according to the previous reported similar method. 58 Compound 2 (0.25 g, 0.5 mmol) and 2,4-dihydroxybenzoyl hydrazine (0.1 g, 0.6 mmol) were dissolved in a round-bottom ask with ethanol (20 mL). Aer reuxed for 4 h until no compound 2 was detected by the TLC silica gel plate. The mixture was cooled to room temperature and concentrated under vacuum. The crude product was puri-ed by recrystallization with ethanol to give compound 1O (0.29 g, 0.46 mmol) as a white solid in 83% yield. Mp 458-459 K;

Results and discussion
Photochromic and uorescent behaviors The absorption spectrum and uorescence changes of 1O were measured in tetrahydrofuran (2.0 Â 10 À5 mol L À1 ) at room temperature. As shown in Fig. 1, the absorption maximum of 1O was observed at 312 nm (3 ¼ 4.30 Â 10 4 mol À1 L cm À1 ), which was resulted from p-p* transition. 52 On irradiation with 297 nm light, a new absorption band centered at 565 nm (3 ¼ 3.29 Â 10 3 mol À1 L cm À1 ) appeared and the color of 1O solution turned purple due to the formation of closed-ring isomer 1C. At the same time, the absorption band peaked at 312 nm decreased gradually. Reversely, the purple solution could be completely bleached upon irradiation with visible light (l > 500 nm) and its absorption spectrum recovered to that of the openring isomer 1O. Fig. S5 † showed the emission spectral changes of 1O by alternating irradiation with UV and visible light. When excited with 365 nm light, an emission peak of 1O was observed at 580 nm. The uorescence quantum yield of 1O was determined to be 0.006. On irradiation with 297 nm light, the photocyclization happened and its emission intensity decreased slightly due to the formation of non-uorescent closed-ring isomer 1C. The uorescence quantum yield of 1C was determined to be 0.004. The emission intensity of 1O was quenched to ca. 80% at the photostationary state. This phenomenon indicated that the diarylethene unit exhibited relatively low uorescent modulation efficiency in tetrahydrofuran and it would provide a low background when act as a Zn 2+ sensor. The residual uorescence in the photostationary state might be attributed to the incomplete cyclization reaction and existence of parallel conformations. 53,54 Back irradiation with appropriate wavelength visible light regenerated its opening isomer and recovered the original emission intensity.
Absorption changes induced by Zn 2+ /EDTA and UV/vis light The absorption property of 1O was investigated in tetrahydrofuran (2.0 Â 10 À5 mol L À1 ) at room temperature. As shown in Fig. 2A and Co 2+ . New absorption bands centered at 408 nm (3 ¼ 2.09 Â 10 4 mol À1 L cm À1 ), 438 nm (3 ¼ 1.25 Â 10 4 mol À1 L cm À1 ) and 420 nm (3 ¼ 1.60 Â 10 4 mol À1 L cm À1 ) were observed respectively. The colorless 1O solution turned olive on addition of Zn 2+ , Ni 2+ and Co 2+ over other metal ions (Fig. 2E). The above results indicated that 1O could be used as a detective colorimetric sensor for Zn 2+ . But the selectivity is not so good with the interference of Ni 2+ and Co 2+ . Under the same experiment conditions, we studied the optimal algorithm of 1O induced by Zn 2+ and UV/vis light. As shown in Fig. 2B, with the addition of 26 equiv. of Zn 2+ (0.1 mol L À1 ) to the solution of 1O, a new absorption band centered at 408 nm appeared with the concomitant color change from colorless to olive due to the formation of 1O-Zn 2+ complex (1O 0 ) with a much steadier rigid construction than 1O. When Zn 2+ was added to the solution of 1C, the absorbance at 410 nm (3 ¼ 1.88 Â 10 4 mol À1 L cm À1 ) increased, at the same time, the 565 nm absorbance was red-shied by 35 nm (Fig. 2C). The phenomena might be on account of formation of the 1C-Zn 2+ complex (1C 0 ). Then, by addition of excess EDTA (0.1 mol L À1 ) to the solution of 1O 0 or 1C 0 , the absorption band all recovered to 1O or 1C because EDTA possibly stripped Zn 2+ away from the cavity by the binding zone. Furthermore, as shown in Fig. 2D, when the maximum absorption band of 1O 0 was reached, upon irradiation with 297 nm UV light, a new  absorption band appeared clearly which centered at 600 nm (3 ¼ 1.44 Â 10 4 mol À1 L cm À1 ) and the color changed from olive to dark slate gray for the formation of the closed-ring isomer 1C 0 . Meanwhile, it could return back to the open-ring isomer 1O 0 on irradiation with visible light (l > 500 nm), indicating that the open-ring isomer 1O 0 and the closed-ring isomer 1C 0 reaction was reversible.

Fluorescence response to metal ions
Under the same experimental conditions, Fig. 3 showed the emission spectral and uorescence color changes of 1O induced by various metal ions (5 equiv. 0.1 mol L À1 ) such as Zn 2+ , Al 3+ , Fe 3+ , Cr 3+ , K + , Ba 2+ , Ca 2+ , Ni 2+ , Mg 2+ , Mn 2+ , Cd 2+ , Sr 2+ , Co 2+ , Pb 2+ and Ag + . We found that 1O can detect Zn 2+ in tetrahydrofuran (2.0 Â 10 À5 mol L À1 ). The uorescence of 1O was notably changed when Zn 2+ was added, while the addition of other cations caused no obvious changes (Fig. 3A). When Zn 2+ was added to the solution of 1O, the uorescence intensity was enhanced evidently and the emission peak was blue shied from 580 nm to 515 nm with a concomitant uorescent color change from dark to bright aliceblue caused by the formation of 1O 0 . The results showed that there was no interference of Cd 2+ or Mg 2+ , indicating good selectivity of Zn 2+ . Therefore, 1O can be used as an efficient uorescence chemosensor for Zn 2+ recognition.
To further evaluate the responsive nature of 1O induced by Zn 2+ , a series of uorescence titration tests were carried out in tetrahydrofuran (2.0 Â 10 À5 mol L À1 ) at room temperature. When Zn 2+ was added to the solution of 1O from 0 to 18 equiv., the uorescence intensity increased signicantly with a blue shi of 65 nm from 580 nm to 515 nm (Fig. 4A). The titration experiment was shown in Fig. S6. † Compared with 1O, the uorescence intensity was enhanced by 31 fold. The increased emission intensity and blue shi could be ascribed to the formation of 1O 0 . The uorescent quantum yield of 1O 0 was determined to be 0.044. The stable chelation of 1O with Zn 2+ inhibited the C]N isomerization and led to a rigid uorophore structure, causing enhanced uorescence intensity. 55,56 The uorescence spectrum of 1O 0 recovered to that of 1O by adding an aqueous solution of excess EDTA (0.1 mol L À1 ) which possibly strips Zn 2+ away from the cavity by the binding zone, indicating that the complexationdecomplexation reaction between 1O and Zn 2+ was reversible. Similar to 1O, the uorescence of 1C could also be effectively modulated by Zn 2+ in tetrahydrofuran. Aer adding 8.0 equiv. of Zn 2+ to the solution of 1C, its uorescence intensity was enhanced by 4 folds and the emission peak was blue-shied from 580 to 530 nm due to the formation of 1C 0 (Fig. 4B). We also investigated the photochromism of 1O 0 . As shown in Fig. 4C, when the maximum intensity was reached, upon irradiation with 297 nm UV light, the emission intensity of 1O 0 was quenched to ca. 15% due to the formation of 1C 0 , the uorescent quantum yield of 1C 0 was determined to be 0.028. It can come back to that of 1O 0 by irradiation with appropriate visible light and followed by a color change from bright aliceblue to powderblue. The irradiation with UV/vis light reaction between 1O 0 and 1C 0 was also reversible.
To investigate the coordination mode of 1O with Zn 2+ , Job's plot analysis was performed according to the reported method. 57 As shown in Fig. 5A, the maximum value was achieved when the molar fraction of [1O]/([1O] + [Zn 2+ ]) was about 0.5, demonstrating a 1 : 1 stoichiometry between 1O and Zn 2+ . Based on the 1 : 1 stoichiometry and uorescence titration data, the association constant (K a ) of 1O with Zn 2+ was calculated from the slope and intercept of the linear plot to be 7.17 Â 10 3 L mol À1 (R ¼ 0.984) (Fig. 5B). The detection limit was calculated to be 1.28 Â 10 À6 mol L À1 for Zn 2+ (Fig. 5C). To further conrm the coordination mode of 1O and Zn 2+ , ESI mass spectra were recorded. The ESI-MS peak at 699.1031 assigned to [1O + Zn 2+ À 3H] À (calcd 699.0241) was observed (Fig. S7 †), providing strong evidence for the formation of a 1 : 1 complex between 1O and Zn 2+ . The proposed binding mode between 1O and Zn 2+ was shown in Scheme 3.

Application in logic circuit
On the basis of the fact that the absorption and uorescent intensity of the target diarylethene could be effectively  modulated by Zn 2+ /EDTA and light, a type of logic circuits was constructed by using light irradiation and Zn 2+ /EDTA as the input signals. The uorescence at 515 nm was used as an output signal. As shown in Fig. 6, the photochromic behaviors of 1O could be effectively modulated by Zn 2+ /EDTA and UV/vis light. Thus, one logic circuit was constructed by using the combination of four input signals (In1: 297 nm UV light, In2: l > 500 nm visible light, In3: Zn 2+ , and In4: EDTA) and an output (uorescence emission at 515 nm). The four inputs and one output could be either "on" or "off" state with different Boolean values. When 297 nm light was employed, In1 was switched to "on" state with a Boolean value of "1". Similarly, In2 was "1" corresponding to irradiation with appropriate visible light (l > 500 nm), In3 was "1" corresponding to the addition of Zn 2+ , and In4 was "1" corresponding to the addition of EDTA. The emission intensity of 1O at 515 nm was regarded as the initial value and when the change of uorescence intensity at 515 nm was 31 fold larger than the initial value, it was regarded as "on" state with a Boolean value of "1". Otherwise, it was regarded as "off" state with a Boolean value of "0". Upon the stimuli of different inputs, the diarylethene exhibited an on-off-on photochromic switching behavior. As a result, 1O could read a string of four inputs and write one output (Table 2).

Conclusions
In summary, a highly sensitive uorescent "turn-on" sensor based on a photochromic diarylethene derivative with a salicylhydrazide unit was developed. It exhibited excellent photochromic properties, high selectivity and specicity toward Zn 2+ over other metal ions. It could also be used as a naked-eye detector for Zn 2+ . Furthermore, the diarylethene showed excellent uorescent switching behaviors with distinctive color changes in response to the combinational inputs of light and Zn 2+ . Based on these characteristics, a logic circuit was designed by the uorescence intensity as the output signal with the inputs of UV/vis lights and Zn 2+ /EDTA. All these results will be helpful for the design and construction of new diarylethene derivatives with multi-addressable states and potential applications in uorescent sensors for special ions.