Characterization of the aggregation-induced enhanced emission, sensing, and logic gate behavior of 2-(1-hydroxy-2-naphthyl)methylene hydrazone

Sensors and Actuators B 177 (2013) 493–499

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Characterization of the aggregation-induced enhanced emission, sensing, and logic gate behavior of 2-(1-hydroxy-2-naphthyl)methylene hydrazone Xia Cao a , Xi Zeng a , Lan Mu a,∗ , Yi Chen a , Rui-xiao Wang a , Yun-qian Zhang a , Jian-xin Zhang b , Gang Wei c a b c

Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, PR China The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences; Guiyang 550002, PR China CSIRO Materials Science and Engineeing, P.O. Box 218, Lindfeld, NSW 2070, Australia

a r t i c l e

i n f o

Article history: Received 31 July 2012 Received in revised form 26 October 2012 Accepted 5 November 2012 Available online 19 November 2012 Keywords: 2-(1-Hydroxy-2-naphthyl)methylene hydrazone Aggregation-induced emission enhancement Turn-on/off sensor Zn2+ /Co2+ Molecular logic gate

a b s t r a c t 2-(1-Hydroxy-2-naphthyl)methylene hydrazone (HMNH), with two naphthalene planar conjugated moieties linked by a rotatable N N single bond, has been feasibly synthesized. This molecule is characterized by aggregation-induced emission enhancement properties, exhibiting markedly enhanced fluorescence after aggregation in DMF (N,N-dimethylformamide)/H2 O mixed solvent. According to spectroscopic data, crystal structure determination, SEM, and fluorescence microscopy, rotation round the intramolecular single bond is hindered by the combined effects of intramolecular hydrogen-bond formation and ordered molecular stacking with aggregate formation, which inhibit non-radiative transitions in the aggregated state. HMNH has excellent metal ion recognition properties, isolated HMNH working as a fluorescence turn-on sensor for Zn2+ , but aggregated HMNH with AIEE as a turn-off sensor for Co2+ . Using OH− and Zn2+ /Co2+ as chemical inputs and the fluorescence signal as output, AND and NAND logic gates operating at the molecular level have been constructed. This single molecule sensor for multiple analytes by isolated or aggregated is promising and has been rarely exploited. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Many organic luminescent molecules in high concentrations or the solid state show aggregation-induced fluorescence quenching [1]; the non-radiative decay of intermolecular interaction reduces the luminescence efficiency, which significantly quenches the fluorescence and limits the application of such molecules. However, there are some unusual molecules for which the isolated species are faintly emissive while the aggregates are strongly luminescent [2,3]. Since the discovery of the phenomenon of aggregationinduced emission enhancement (AIEE) [4], some organic molecules that exhibit pronounced fluorescence enhancement in their aggregated or solid states have attracted increasing attention. Fluorophores with AIEE characteristics have been successfully utilized in sensors [5], electroluminescent materials [6], organic lightemitting diodes [7], and photoemitters [8]. Their mechanisms of action are usually explained in terms of restriction of intermolecular rotation [9], the formation of an intramolecular hydrogen bond [10], intramolecular charge-transfer [11], or excited-state protontransfer [12]. In terms of molecular structure, most of them have been found to be associated with aromatic groups through rotatable C C [13], C N [14], or N N [15] single bonds. Comparison

∗ Corresponding author. Tel.: +86 851 3624031; fax: +86 851 3620906. E-mail address: [email protected] (L. Mu). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.11.003

with previous studies of AIEE using for, exploiting multi-function sensing properties can be established of detection method for different species. The characterized of AIEE of organic luminescent molecule is used not only for gathering state light-emitting; it also acts as sensor for the identification of different species. Not just as selective fluorescence turn-on sensor, but turn-off sensor for identification of ion by utilizing the non-aggregation and aggregation fluorescence enhancement or quenching. Therefore, exploration in the area of new AIEE molecules, especially those that can be easily synthesized, with simple structures allowing for functional group diversification, is still of great interest. Recently, molecule-based logic systems and their potential applications have attracted considerable attention. In particular, many fluorescent molecules in logic gates showing AND [16], NAND [17], OR [18], NOR [19], INHIBIT [20], or YES and NOT [21] functions have been reported as molecular-level devices. Molecular systems that are capable of behaving as logic gates process chemical, electrical, or optical inputs and generate light output signals and can thus serve as chemically driven fluorescent switches [22]. Fluorescent versions of molecular switches are particularly attractive because of their visibility and high sensitivity of detection with inexpensive instrumentation [23]. Inspired by the above results, in the present study, 2(1-hydroxy-2-naphthyl)methylene hydrazone (HNMH) has been synthesized by the condensation of 1-hydroxy naphthaldehyde and hydrazine hydrate by a simple and straightforward method. The

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idea of the molecular design is that the two naphthalene moieties are connected by a rotatable N N single bond, while intramolecular hydrogen bonds between them ensure intramolecular rotation only about the N N bond. In a good solvent, HNMH displays very weak fluorescence, while strong emission is observed when it is placed in a poor solvent, which corresponds to typical AIEE behaviour. Although naphthalene derivatives have long been studied with regard to their chemical sensor properties in the solution state [24], the AIEE characteristics of HNMN and its smart sensing of Zn2+ and Co2+ in different media has not hitherto been demonstrated. Although, fluorescent sensors for Zn2+ or Co2+ have been studied [25–27], most of the Co2+ sensors are fluorescence quenching, even if the fluorescence enhancement, but poor selectivity [28]. Single sensor has the identification capability for Zn2+ and Co2+ at the same time with different states ways is very rare. Importantly, the combination of two cationic species with HNMN as a chemical input to trigger fluorescence emission or quenching as the output allowed the construction of molecular logic gates with binary logic functions. The requirements for an AND molecular logic gate based on HNMH in a good solvent are satisfied with Zn2+ ions and OH− as two inputs and fluorescence enhancement as output. On the contrary, HNMH, Co2+ , and OH− can serve to produce a NAND logic gate under aggregation conditions. Hence, HNMH may potentially be utilized in molecular-level devices.

Solutions of other metal ions were likewise prepared at 2.00 × 10−3 M in DMF. 2.1. Synthesis of HNMH and X-ray crystallography HNMH was synthesized as reported previously [24]. Its identity was confirmed by 1 H NMR, IR, and EI-MS (Supporting Information, Figs. S1–S3). The experiments of IR were made with solid slice coupled with KBr. Took 1–2 mg HNMH in an agate mortar and ground into fine powder, then mixed 100 mg KBr (A.R level) evenly, the mixture was loaded in the mould to press a thin sheet; The data was collected and manipulated by OPUS-IT, OPUS-JCAMP, OPUS-FitCurve and OPUS-IR software packages of Bruker Vertex 70 FTIR spectrometer, the spectrogram was processed by origin8.0 software. A single crystal of HNMH was grown by slow crystallization from a solution in CH3 Cl/MeOH (3:2, v/v), and its high-quality crystal structure was determined by X-ray diffraction analysis. X-ray data were collected on a Smart ApexII CCD diffractometer, the structural solution and full matrix least-squares refinement were performed with the SHELXS-97 and SHELXL-97 software packages; the crystal structure determination and analytical method were the same as our previous reported paper [29]. 2.2. Absorption and fluorescence measurements

2. Materials and methods 1 H NMR spectra were measured on Nova-400 NMR spectrometers (Varian) at 293 K using TMS as an internal standard. ESI mass spectra were obtained on an HPLC-MSD-Trap-VL spectrometer (Agilent). Single-crystal X-ray diffraction studies were conducted on a Smart CCD Apex2 diffractometer (Bruker). IR spectra were recorded on a Vertex 70 FTIR spectrometer (Bruker). Fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer (Varian) equipped with a xenon discharge lamp and a 1 cm quartz cell. Absorbance spectra were recorded on a TU1901 spectrophotometer (Beijing General Instrument Co., China). Scanning electron microscopy (SEM) images were recorded on a QuantaTM 450 FEG microscope (FEI) at 5.0 kV. Fluorescence images of the microcrystals were obtained by means of a C-SHGL fluorescence microscope (Nikon) with an inverted fluorescence micro-manipulation system. Suspensions were obtained using a TGL-16GA centrifuge (Shangdong Xing Ke Intelligent Technology Co., Ltd., China). All of the experiments were performed at room temperature. All reagents of analytical grade were purchased directly from chemical suppliers and used without further purification. Solutions of metal ions were prepared from nitrate or chloride salts of analytical grade (Aldrich and Alfa Aesar Chemical Co., Ltd.) that had previously been stored in a desiccator containing selfindicating silica under vacuum and were used without any further purification. Doubly-distilled water was used in all of the experiments. Tris–HCl buffer stock solution in DMF (N, N-dimethylformamide, 4.00 × 10−3 M, pH 8) was prepared from 0.04 M Tris and the appropriate amount of HCl. A stock solution of HNMH (1.00 × 10−4 M) was prepared in a 100 mL volumetric flask by dissolving 3.4 mg of the pure compound in absolute DMF and then diluting to the mark with further absolute DMF. A Zn2+ stock solution (2.00 × 10−3 M) was prepared in a 100 mL volumetric flask by dissolving 13.6 mg of ZnCl2 in absolute DMF and then diluting to the mark with further absolute DMF. A Co2+ stock solution (2.00 × 10−3 M) was prepared in a 100 mL volumetric flask by dissolving 58.2 mg of Co(NO3 )2 ×6H2 O in absolute DMF and then diluting to the mark with further absolute DMF.

Fluorescence and UV–vis spectroscopy measurements were made by adding 1 mL of a solution of HNMH and metal ions to 40 ␮M Tris–HCl buffer in DMF/H2 O (1 mL) and mixing well for 30 min before the test. All measurements were carried out at room temperature. 2.3. Preparation of HNMH for SEM and fluorescence images A 10 ␮M solution of HNMH in DMF/H2 O (40:60) was added to a 25 mL flask, and a suspension was obtained by centrifugation. The preparation of samples for SEM involved applying a few drops of a suspension to a glass slide, which was covered with a black film. After drying at room temperature, the prepared samples were sputter-coated with gold/palladium. Samples for fluorescence microscopy were prepared by applying a few drops of a suspension to a glass slide and then covering with a coverslip. 3. Results and discussion 3.1. Molecular structure Crystallographic data for HNMH have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC-871025. Copies of the data can be obtained on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 0 1223 762911 or [email protected]] Compound HNMH crystallizes in the monoclinic system and the crystallographic data are listed in Supporting Information Table S1. 3.2. AIEE characteristics of HNMH HNMH can be readily dispersed in DMF, but is insoluble in water. Fluorescence studies revealed that upon excitation of a dilute (10 ␮M) solution in DMF at 438 nm, a weak fluorescence emission was observed at 510 nm. The fluorescence intensity increased continuously when 30–60% volume fractions of water were added to the solution in DMF. The AIEE phenomenon can be seen directly from the spectral changes in Fig. 1a. Inset 1 in Fig. 1a shows that the fluorescence intensity at the emission maximum peak of HNMH increased with increasing water fraction, reaching a maximum at

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The bathochromic effect and the spectral change are clearly indicative of the formation of an aggregate, with the effective conjugation length of the HNMH molecule being extended from that in the isolated twisted form to that in the planar form upon aggregation [30]. Usually, the formation of aggregates can be observed in suspensions made by centrifugal concentration [31]. The fluorescence of HNMH aggregates in a poor solvent was supported by SEM images and fluorescence microscopy results. Fig. 2a shows a photograph of HNMH in different solutions and the solid state under 365 nm UV illuminations, when observed by the naked eye. No obvious fluorescence was observed in DMF solution (left), but strong fluorescence was observed in DMF/H2 O solution (middle), and a bright-yellow color was observed for a powder sample (right). The solution of HNMH in DMF/H2 O (40:60) was separated by centrifugation. An SEM image (Fig. 2b) and a fluorescence microscopy image (Fig. 2c) of the centrifuged suspension showed a nanoparticle morphology and the bright-yellow color of the nanoaggregate due to the restricted motion and the formation of aggregates in the mixed solvent. Furthermore, fluorescence spectra of HNMH were obtained from a suspension, the supernatant from centrifugation of the solution in DMF/H2 O, and the solid state (Supporting Information, Fig. S4), which validated that the measured spectra originated from the same luminophore whether in the solid state or dispersed in solution. 3.3. Investigation of the characteristic mechanism of AIEE

Fig. 1. (a) Fluorescence spectrum change of HNMH (10 ␮M) in DMF/H2 O. Inset (1) shows the variation in the integrated emission intensity of HNMH with increasing water fraction in DMF. Inset (2) shows the change in the emission maximum peak of HNMH. (b) UV absorption spectra changes of HNMH (10 ␮M) in DMF/H2 O. Inset (3) shows the variation in the integrated absorption of HNMH with increasing water fraction in DMF. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)

about 60% water. Inset 2 shows that the peak position of the emission maximum was continuously red-shifted with the increase of water volume fraction. The fluorescence intensity was nine-fold higher in DMF/H2 O (40:60) than that in pure DMF, which was directly related to the fact that more aggregates would form in the mixed solvent. The quantum yields for HNMH in a dilute solution (˚s = 0.057) and the aggregated state (˚a = 0.27), which were calculated using naphthalene (˚ = 0.23 in hexane) as a reference, indicated the emission enhancement phenomenon. UV–vis absorption spectra are shown in Fig. 1b. In DMF solution, HNMH displays a structured absorption spectrum. However, with increasing water fraction, the aggregate began to form, and the maximum absorption peak at 409 nm diminished. Meanwhile, a new shoulder band appeared at around 458 nm, the absorbance of which increased continuously. Inset 3 in Fig. 1b shows the variation of the absorption at 458 nm with increasing water fraction in DMF. The maximum absorption peak changed distinctly and an intense narrow absorption band appeared. A 60% volume fraction of water was found to induce the strongest AIEE effect of HNMH.

Crystal structures of HNMH (shown in Fig. 3a) can help to explain the mechanism of the AIEE phenomenon. The forma˚ tion of intramolecular hydrogen bonds in HNMH (H· · ·N1 8.2 A; O1 H· · ·N1 147◦ ) not only enlarged the conjugated ␲-system, but also formed a stable six-membered “ C11 N1· · ·H O1 C9 C10 ” ring, thereby ensuring intramolecular rotation was allowed only about the N N single bond. The two six-membered rings rotate freely about the N N bond in dispersion such that it is difficult for them to adopt a coplanar rigid structure. Hence, most of the excited-state energy loss, i.e., fluorescence emission, is very weak. It is well established that an increase in the rigidity of a molecule can lead to a decrease in its vibration, which is likely to be accompanied by a decrease in the internal conversion of excited molecules and an increase in the fluorescence quantum yield. A planar symmetric rigid molecule was determined from the crystal structure of HNMH. The angles C(11) N(1) N(1a), N(1) C(11) C(10), and N(1) C(11) H(11) were determined as 113.1(2)◦ , 121.9(3)◦ , and 119.0◦ , respectively. The torsion angles C(11) N(1) N(1a) C(11a), C(10) C(11) N(1) N(1a), and C(4) C(10) C(11) N(1) were determined as −180(2)◦ , −178.9(2)◦ , and −178.2(3)◦ , respectively. Meanwhile, due to the intermolecular C H· · ·␲ interactions, such as C(3) H(3)· · ·Cg(C1 C2 C3 C4 C5 C6), C(7) H(7)· · ·Cg(C4 C6 C7 C8 C9 C10), and C(8) H(8)· · · Cg(C1 C2 C3 C4 C5 C6), the molecules are unable to adopt a typical intermolecular ␲–␲ stacked arrangement, but arrange into a slanted stack (Fig. 3b). The packing modes of the structural units in the crystals show an ordered structure (Fig. 3c). The effect of the viscosity of the medium on the emission from HNMH was studied in a viscous solvent. The observed fluorescence enhancement of HNMH on increasing the volume fraction of glycerine from 10% to 90% in glycol/glycerine mixtures (Supporting Information, Fig. S5) may be attributed to the high viscosity inhibiting intramolecular free rotation [32]. Furthermore, the effect of luminophore concentration was studied in different volume fractions of water (Supporting Information, Fig. S6). When the concentration of HNMH was below 1 ␮M, there was no fluorescence enhancement, even if the volume of water reached 90%. However, clear variations were observed up to an appropriate concentration;

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Fig. 2. (a) Photograph of HNMH (10 ␮M) in pure DMF (left), in DMF/H2 O (middle), and solid powder (right) under 365 nm UV illumination; (b) SEM image of HNMH; (c) fluorescence microscopy image of a centrifuged suspension of HNMH (DMF/H2 O, 40:60). (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)

Fig. 3. (a) Molecular structure of HNMH, (b) details of the C H· · ·␲ interaction, (c) crystal packing arrangement of HNMH in the a–c plane.

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Fig. 4. Fluorescence spectra of HNMH (10 ␮M, DMF/H2 O, Tris–HCl, pH 8) solution in the presence of different metal ions (200 ␮M). (a) In isolated, (b) in aggregated.

the ordered stacking of aggregates is effective in limiting the intramolecular rotation. The enhanced emission of HNMH is attributed to the combined effects of molecular planarization, ordered stacking, and inhibition of intramolecular rotation. All of the above results indicate that HNMH is an AIEE molecule, in which free intramolecular rotation of fluorophore can be inhibited on going from solution to the aggregated state. 3.4. Selective recognition of Zn2+ or Co2+ by HNMH As mentioned above, only weak fluorescence was observed when HNMH was dispersed in DMF (DMF/H2 O, 90:10, Tris–HCl, pH 8, isolated HNMH), but the emission was markedly enhanced with increasing volume fraction of water (DMF/H2 O, 40:60, Tris–HCl, pH 8, aggregated HNMH). So, one might apply the AIEE characteristic in a sensor for metal ion detection by fluorescence enhancement or quenching. Isolated HNMH displays weak fluorescence. After the addition of Zn2+ (200 ␮M, DMF), significant fluorescence enhancement (11-fold) at 516 nm could be observed, as shown in Fig. 4a. The fluorescence turn-on effect of HNMH upon the addition of Zn2+ ions could be readily distinguished by the naked eye under UV irradiation at 365 nm (inset in Fig. 4a). Under identical conditions, a solution of HNMH-Zn2+ showed almost no changes in the presence of other common metal ions, such as Na+ , K+ , Rb+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , Pb2+ , Bi2+ , Mn2+ , Ni2+ , Cu2+ , Cd2+ , Hg2+ , Al3+ , Fe3+ , Sc3+ , Ti3+ , Cr3+ , or Co2+ ; no fluorescence enhancements were observed (Supporting Information, Fig. S7). Clearly, Isolated HNMH serves as a selective fluorescence turn-on sensor for Zn2+ detection. On the contrary, the addition of Co2+ to aggregated HNMH solution

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resulted in significant fluorescence quenching at 537 nm, as shown in Fig. 4b; the solution changed from colorless to aubergine under irradiation, and could be observed by the naked eye (inset in Fig. 4b). The addition of other metal ions to the HNMH solution did not lead to such fluorescence quenching, that is, no significant spectral changes of HNMH-Co2+ were observed upon addition of Na+ , K+ , Rb+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , Pb2+ , Bi2+ , Mn2+ , Ni2+ , Cu2+ , Cd2+ , Hg2+ , Al3+ , Fe3+ , Sc3+ , Ti3+ , Cr3+ , or Zn2+ under identical conditions (Supporting Information, Fig. S8). The findings suggest that the AIEE features of HNMH might be exploited in a fluorescence turn-off sensor for Co2+ detection. Fluorescence titration data for treating HNMH (10 ␮M, isolated HNMH) with Zn2+ were obtained (Supporting information, Fig. S9a). Upon addition of Zn2+ , the fluorescence intensity of the isolated HNMH solution gradually increased. The saturation behavior of the fluorescence intensity reveals that a 1:1 stoichiometry was best for the binding mode of Zn2+ and HNMH, which was also supported by Job plot data (Supporting Information, Fig. S9a, inset). According to the 1:1 model, the stability constant (Ka ) of HNMH with Zn2+ was calculated to be 1.11 × 105 . In the same way, fluorescence titration of HNMH (10 ␮M, aggregated HNMH) with Co2+ ions (Supporting Information, Fig. S9b) yielded a 1:1 stoichiometry (Supporting Information, Fig. S9b, inset) and a binding constant of 1.33 × 105 . Under the above determination conditions, calibration curves of the fluorescence intensity for the detection of Zn2+ and Co2+ were constructed. The fluorescence increment showed a good linear relationship with the concentration of Zn2+ in the range 8.0–200 ␮M (R = 0.995, n = 16) and a detection limit of 3.12 × 10−2 ␮M (Supporting Information, Fig. S10a). The fluorescence quenching was linearly related to the concentration of Co2+ in the range 0.2–17.0 ␮M (R = 0.996, n = 12), with a detection limit of 2.5 × 10−2 ␮M (Supporting Information, Fig. S10b). As other studies reported [25], the mechanism of fluorescence enhancement of hydrazone derivative by Zn2+ attributed to the blocking of photo-induced electron transfer process from nitrogen in hydrazone moiety to related conjugated groups; two nitrogens on hydrazone moiety as well as phenolic oxygen may participate in binding with Zn2+ . As a hydrazone derivative of structural similarity, the fluorescence enhancement of isolated HNMH by the Zn2+ was also responsible for selectively binding between HNMH and Zn2+ . Certainly, Co2+ and other ions can not binding with isolated HNMH, no behavior of fluorescence enhancement was observed. The fluorescence enhancement of aggregated HNMH is attributed to the combined effects of intramolecular planarization and intermolecular ordered aggregation in the aggregated state. To seek further more detailed information of the mechanism of fluorescence quenching of aggregated HNMH by Co2+ , 1 H NMR experiment was carried out (Supporting Information, Fig. 11). Upon addition of Co2+ to aggregated HNMH solution, the resonances corresponding to the protons of naphthalene ring split and shift toward higher field or lower field, no significant chemical shift change from protons of hydroxyl groups were observed. These suggested that only two nitrogens on hydrazone moiety participate in binding with Co2+ , but the phenolic oxygen did not involved in the complexation. As a result, the complexation between Co2+ and nitrogen atoms cannot restrict the intramolecular rotation of C C single bond as well as decrease the repulsive force between hydroxy groups. The rotation around of single bond of the naphthalene moiety and the repulsive force between hydroxy groups causes the distortion of molecule, so the degree of conjugation is dramatically reduced, and thus show fluorescence quenching by Co2+ . 3.5. Characteristics of AND and NAND logic gates Logic gates are binary switches for which the input conditions (0 or 1) determine the output state (0 or 1). Most of the chemical

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X. Cao et al. / Sensors and Actuators B 177 (2013) 493–499 Table 1 Fluorescence spectral behavior of HNMH (10 ␮M) alone and in the presence of Zn2+ (200 ␮M) and H+ (pH 8) in isolated (left), Co2+ (200 ␮M) and H+ (pH 8) in aggregated (right).

I II III IV

Input1

Input2

Output emission at 516 nm

Input1

Input2

Output emission at 537 nm

0 0 1 1

0 1 0 1

0 (low) 0 (low) 0 (low) 1 (high)

0 1 0 1

0 0 1 1

1 (high) 1 (high) 1 (high) 0 (low)

contrary, when both Co2+ ions and OH− were added to the HNMH solution, the fluorescence was completely quenched, resulting in output “0” (Fig. 5b). The corresponding truth table for a digital NAND logic gate is presented in Table 1 (right). Thus, the response of the naphthyl sensor to cations has been successfully applied in the construction of molecular switches with binary logic functions. 4. Conclusion A symmetrical fluorescent molecule, 2-(1-hydroxy-2naphthyl)methylene hydrazone (HNMH) has been easily synthesized, It shows characteristic AIEE fluorescence properties in DMF/H2 O solution, which may be attributed to the combined effects of intramolecular planarization and intermolecular ordered aggregation, reducing the non-radiative transition in the aggregated state. We have successfully developed a bifuctional sensor capable of detecting Zn2+ and Co2+ with characteristics of AIEE, it exhibit excellent selectivity and sensitivity. In addition, exploiting these sensing properties, AND and NAND logic gates based on the isolated HNMH and aggregated HNMH could be constructed. HNMH has proved to be capable of performing simple logic operations by employing OH− and Zn2+ /Co2+ ions as inputs and the fluorescence signal for the output. The respective fluorescence-based logic gates may represent potential candidates for use in molecular logic circuits. Fig. 5. (a) Fluorescence emission spectra of HNMH in isolated (pH 8) excited at 456 nm in the presence of chemical inputs to test its digital AND logic gate function. (b) HNMH in aggregated (pH 8) excited at 456 nm in the presence of chemical inputs to test its digital NAND logic gate function.

systems that are capable of performing simple logic operations employ small molecules/ions as inputs and a fluorescence signal as the output [33]. An AND gate has two inputs and one output, and is expressed as two series-wound switches in a circuit diagram [34]. On the contrary, a NAND gate is composed of an AND gate and a NOT gate [35]. Taking advantage of the fluorescence behavior of HNMH in response to Zn2+ and Co2+ , and the fact that both HNMHZn2+ and HNMH-Co2+ are sensitive to pH (Supporting Information, Fig. S12), AND and NAND logic gates were constructed for signal enhancement or quenching under appropriate conditions. The logic characteristics of the AND gate of HNMH in DMF/H2 O was determined by observing the fluorescence spectra under four possible input conditions (Fig. 5a). The isolated HNMH showed a low fluorescence emission (‘0’ state). When either of the two chemical inputs of OH− (pH 8, input1 ) and Zn2+ (200 ␮M, input2 ) ions was present alone, the fluorescence of HNMH was still low (‘0’ state). Only in the simultaneous presence of both OH− and Zn2+ , a considerable fluorescence enhancement at 516 nm was observed for the HNMH solution (‘1’ state, output). The fluorescence intensity patterns under the four input conditions and the corresponding truth table for the logic circuit of the AND function are given in Table 1 (left). Meanwhile, the NAND logic operation of HNMH with Co2+ and OH− as the inputs was demonstrated. When either OH− (pH 8, input1 ) or Co2+ ion (200 ␮M, input2 ) was present alone, the aggregated HNMH showed strong fluorescence emission. On the

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Biographies Xia Cao is currently working towards a M.S. degree in applied chemistry at Guizhou University. Her research interest is analytical chemistry. Xi Zeng received a B.S. degree in chemistry in 1982 from the Guizhou University, China. Currently, he is a professor in the school of Chemistry and Chemical Engineering, Guizhou University. His research interests are supramolecular chemistry and chemosensors. Lan Mu earned a Ph.D. degree in 2007 from the University of Guizhou, China. Currently, she is a professor in the School of Chemistry and Chemical Engineering, Guizhou University. Her research interest is analytical chemistry. Yi Chen is currently working towards a M.S. degree in applied chemistry at Guizhou University. His research interest is analytical chemistry. Rui-xiao Wang is currently working towards a M.S. degree in applied chemistry at Guizhou University. His research interest is applied chemistry. Yun-qian Zhang received a B.S. degree in chemistry in 1984 from the Guizhou University, China. Currently, he is a professor in the school of Chemistry and Chemical Engineering, Guizhou University. His research interests are crystal structure analysis. Jian-xin Zhang received a B.S. degree in chemistry in 1981 from Guizhou University. Currently, he is professor in the Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences. His research interest is supramolecular chemistry. Gang Wei earned a Ph.D. degree from the University of Newcastle, Australia. Currently, he is a senior research scientist of Industrial Physics Division, Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia. His research interest is materials chemistry.