A cell compatible fluorescent chemosensor for Hg²⁺ based on a novel rhodamine derivative that works as a molecular keypad lock

Abstract

A novel fluorescent chemosensor based on rhodamine derivative (Rh1) has been designed and synthesized for detection of Hg²⁺ ions, which exhibits high sensitivity and selectivity over other metal ions in aqueous solution and living cells. Moreover, this “Off–On”-type fluorescent sensor could successfully mimic a molecular level keypad lock in the presence of Cu²⁺ ions. Stimulated by the two chemical inputs (Hg²⁺ and Cu²⁺), Rh1 undergoes transformation of the structure between spirocyclic and ring-opened spirocyclic, which results in the change of fluorescence. Significantly, the outputs of the system depend on not only the proper combination but also on the correct order of the input signals, which is the most important feature of the keypad lock system. Only a specific sequence of inputs, i.e. the correct password, results in strong fluorescence emission at 555 nm which can be used to “open” this molecular keypad lock. Therefore, this molecular keypad lock has the potential for application in security devices, which could be used to authorize a user, to verify authentication of a product, or to initiate a higher process.

---

Introduction

In recent years, various molecular switches, logic gates and logic circuits based on chemical systems capable of elaborating binary (Boolean) information have been proposed because of the wide variety of organic molecular designs, synthesis, and light-emitting properties that are available.¹ The combination of these molecular logic gates and logic circuits has been used for mimicking various electronic devices at the molecular level. The primary motivation for this is to develop novel paradigms for information processing that, by moving beyond silicon-based technology, could lead to electronic devices of extremely small size, low power consumption and unprecedented performance. Implementation of various mimicking electronic devices, such as memory units,² comparators,³ and demultiplexers,⁴ performing digital operations with numerous chemical systems is now possible. Recently, a keypad lock, which is an important electronic logic device, has been mimicked at the molecular level.⁵ This device can be used for numerous applications in which access to an object or data is to be restricted to a limited number of persons having the exact password to open the keypad lock. The most important advantage of a keypad lock system over simple logic gate is that its output signals are dependent not only on the proper combination of inputs but also on the correct order by which these inputs are introduced. Therefore, the development of such molecular devices, capable of authorizing password entries, is a particularly attractive research goal as it represents a new approach for protecting information on molecular scale.

To mimic various electronic devices, numerous chemical systems responding to a large variety of activating input signals (e.g., light, electrical, magnetic, and chemical) have been developed.^1k,6 Among these chemical systems, the most important one is fluorescent chemosensors with high selectivity and sensitivity for a specific analyte, such as heavy metal ions, anions, and pH. In particular, a fluorescent chemosensor for heavy metal ions not only can be used to detect the presence of the heavy metal ions (such as Hg²⁺ ions) in aqueous solution and living cells, but also to integrate these heavy metal ions as chemical-driven molecular machines in future molecular computing. Therefore, there is an urgent need to develop innovative and convenient fluorescent chemosensors for heavy metal ions (such as Hg²⁺ ions) because of the dual important application of a single molecule.

For fluorescent chemosensors, suitable fluorescence indicators that are sensitive to heavy metal ions (such as Hg²⁺ ions) concentrations and exhibit changes in fluorescence intensity have been used as molecular recognition materials. Among the numerous indicators, rhodamine derivatives are an excellent candidate due to their high fluorescence sensitivity to heavy metal ions, in addition to the excellent spectroscopic properties of large molar extinction coefficients, high fluorescence quantum yields and visible wavelength excitation. They can change their geometric structures among two states as a result of stimuli. Typically, in the absence of metal ions, the molecules prefer their spirolactam ring-closed geometry,⁷ which shows little absorption and fluorescence, whereas upon the addition of specific metal ions, the chelation or reaction of metal ions with sensor molecules will simultaneously open the spirolactam ring, which gives rise to a strong fluorescence emission.⁸ Simultaneously, they display a strong color development against the colorless blank during the sensing event, which is an important feature that would facilitate “naked-eye” detection. Therefore, many molecular switches and logic gates using rhodamine derivatives as indicator have been proposed.⁹ However, up to now, there has been no report on molecular keypad locks based on rhodamine derivatives as far as we know.

Herein, we intend to report the synthesis and characterization of a novel fluorescent chemosensor to detect Hg²⁺ both in aqueous solution and in living cells with rhodamine derivative 1 (Rh1) as fluorescent indicator. Moreover, Rh1 has been identified to respond as On and Off state of fluorescence depending upon the sequence of addition of Hg²⁺ and Cu²⁺ ions into the solution. Based on this property, a molecular keypad lock based on rhodamine derivative Rh1 was constructed whose fluorescence is in the On state in response to a specific sequence of chemical inputs (Hg²⁺ and Cu²⁺ ions).

Experimental section

Reagents and materials

Rhodamine 6G and hydrazine hydrate were supplied by Shanghai Chemical Reagents (Shanghai) and used as received. Lawesson's Reagent (2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphetane-2,4-disulfide) and 2-hydroxy-1-naphthaldehyde were purchased from Alfa Aesar. Toluene was treated by sodium chips and then distilled. Ethanol was treated by magnesium ribbon and then distilled. Other chemicals are of analytical reagent grade and used without further purification except when specified. All inorganic metal salts were analytical grade and used without further purification. The water used in our present work was deionized. NaAc-HAc buffer solution was prepared using proper amount of NaAc and HAc (analytical grade) under adjustment by a pH meter.

Characterization

Absorption spectra were recorded with a Shimadzu UV-3000 spectrophotometer. Fluorescence spectra were measured with a Hitachi F-4500 fluorescence spectrophotometer with a 10 mm quartz cuvette. The excitation and emission wavelength bandpasses were both set at 5 nm. ¹H and ¹³C NMR spectra were recorded using a mercury-300BB spectrometer (Varian, USA) operated at 300 MHz with tetramethylsilane (TMS) as internal standard. Mass spectra were performed on Agilent 1100 MS series and AXIMA CFR MALDI/TOF (Matrix assisted laser desorption ionization/Time-of-flight) MS (COMPACT). All of the measurements were done at room temperature, about 298 K.

Synthesis of the materials

Synthesis of rhodamine-6G thiohydrazide. Rhodamine-6G hydrazide was prepared according to a literature method.¹⁰ The method used for the synthesis of rhodamine-6G thiohydrazide was according to the procedure described in the literature.¹¹Rhodamine-6G hydrazide (0.6 g, 1.3 mmol) and Lawesson's Reagent (0.52 g, 1.3 mmol) were dissolved in dry toluene, and the resulting mixture was refluxed and stirred for 4 h under N₂ atmosphere. The solvent was removed under reduced pressure to give a pink solid. The crude product was purified by flash column chromatograph using CH₂Cl₂ as eluent to afford rhodamine-6G thiohydrazide (yield: 16%). ¹HNMR (CDCl₃), δ (ppm): 1.30–1.35 (t, 6H, NCH₂CH₃), 1.91 (s, 6H, xanthene-CH₃), 3.22–3.24 (q, 4H, NCH₂CH₃), 4.83 (s, N-NH₂), 5.30 (s, NHCH₂CH₃), 6.14 (s, 2H, xanthene-H), 6.44 (s, 2H, xanthene-H), 7.08 (dd, 1H, Ar-H), 7.49 (dd, 2H, Ar-H), 8.12 (dd, 1H, Ar-H). FAB-MSm/z: [1 + H]⁺ calc. for C₂₆H₂₈N₄OS, 444.2; found, 445.2.

Synthesis of rhodamine derivative Rh1. To a solution of rhodamine-6G thiohydrazide (0.047 g, 0.1 mmol) in absolute alcohol (30 mL) was added 2-hydroxy-1-naphthaldehyde (0.017 g, 0.1 mmol). After the addition, the mixture was refluxed with stir for 12 h. The solvent was evaporated under reduced pressure, and the crude product was purified by flash column chromatograph using CH₂Cl₂ as eluent to afford Rh1 (yield: 80%) as a yellow solid. ¹H NMR (CDCl₃), δ (ppm): 1.29–1.34 (t, 6H), 1.92 (s, 6H), 3.18–3.25 (q, 4H), 5.30 (s, 2H), 6.30 (s, 2H), 6.60 (s, 2H), 7.12 (dd, 1H), 7.20 (dd, 1H), 7.40 (dd, 1H), 7.49 (m, 2H), 7.58 (m, 1H), 7.81 (t, 2H), 8.22 (m, 2H), 9.69 (s, 1H), 12.79 (s, 1H). ¹³C NMR (CDCl₃), δ (ppm):14.74, 16.78, 38.38, 64.68, 96.33, 105.89, 108.53, 119.44, 119.88, 120.13, 120.29, 122.26, 123.46, 127.21, 127.33, 127.64, 128.00, 128.10, 129.08, 130.00, 132.45, 134.00, 135.71, 136.41, 158.10, 158.33, 161.09, 166.786. FAB-MSm/z: [1+ H]⁺ calc. for C₂₆H₂₈N₄OS, 598.24; found, 599.4.

Cell culture and imaging

The Rat Schwann cells (RSC 96) were provided by Norman Bethune College of Medicine Jilin University (China). Cells were grown in H-DMEM (Dulbecco's Modified Eagle's Medium, High Glucose) supplemented with 10% FBS (Fetal Bovine Serum) in an atmosphere of 5% CO₂, 95% air at 37 °C. Cells were plated on 6-well plate at 5 × 10⁶cells per well and allowed to adhere for 12 h. Fluorescence imaging was performed with an Olympus fluorescence microscope (BX51, Olympus, Japan). Immediately before the experiments, the cells were washed with phosphate-buffered saline (PBS) and then incubated with 10 μM of Rh1 (in the culture medium) for 10 min at 37 °C. Experiments to assess Hg²⁺ uptake were performed in the same media supplemented with 50 μM Hg(ClO₄)₂ for 0.5 h.

Results and discussion

Rhodamine derivatives are generally non-fluorescent and colorless, whereas when they coordinate with some specific metal ions, ring-opening of the corresponding spirolactam gives rise to a strong fluorescence emission and a pink color. Bearing this in mind, we envisaged that, if introducing proper binding sites, it would be possible to achieve a rhodamine spirolactam based chemosensor highly selective for heavy metal ions (such as Hg²⁺ ion) via color/fluorescence changes. Based on the theory of hard and soft acids and bases (HSAB theory), Hg²⁺ is a representative example of soft acid, and S²⁻ is a soft base. Therefore, a sulfur-based functional group must be a good candidate as the S is a strong binding site for Hg²⁺ ions. Based on the above reasons, we designed and synthesized a fluorescent chemosensor based on rhodamine derivative Rh1 for detecting Hg²⁺ ions that works as a molecular keypad lock. Rh1 contains an S atom, an N atom attached to the N-bearing spiro ring and an O atom from phenolic hydroxyl of 2-hydroxy-1-naphthaldehyde, which might be a highly selective chemosensor for detecting Hg²⁺ ions since the S, N, O binding sites are a good choice to be parts of a selective receptor. The synthetic procedure of this material is outlined in Scheme 1. Rhodamine-6G hydrazide was prepared according to a literature method,¹⁰ and treatment of it with Lawesson's reagent in refluxing toluene gave, after flash column chromatography on silica, the corresponding rhodamine-6G thiohydrazide in a yield of ca. 16%. Rh1 was successfully synthesized with 2-hydroxy-1-naphthaldehyde and rhodamine-6G thiohydrazide as an intermediate. Both Rh1 and rhodamine-6G thiohydrazide were characterized using various analytical spectroscopic techniques (see the Experimental Section), which agreed well with the proposed structures.

Scheme 1 Synthesis procedure of Rh1.

As mentioned above, we speculated the S, N, O binding sites might be sensitive towards the Hg²⁺ ion, thus we tested the effect of Hg²⁺ ions on the fluorescence behavior of Rh1. The fluorescence spectra were recorded upon excitation at 500 nm at room temperature with the gradual addition of small amounts of Hg²⁺ ions (0–10 equiv, 0–100 μM) into Rh1 solution, and shown in Fig. 1a. In the absence of Hg²⁺, as expected, the Rh1 solution exhibited very weak fluorescence in the range from 510 nm to 650 nm. Upon the addition of increasing concentrations of Hg²⁺, a new emission band peaking at 555 nm appeared and developed, which can be ascribed to the delocalized xanthene moiety of rhodamine group.¹² The fluorescence response of Rh1 to other metal ions under the same condition was also investigated. As illustrated in Fig. 1b, no significant spectral changes of Rh1 were observed in the presence of K⁺, Na⁺, Ca²⁺, Mg²⁺, Ba²⁺, Zn²⁺, Cd²⁺, Cu²⁺ and Pb²⁺ (1 mM), indicating that Rh1 could recognize Hg²⁺ from other metal ions even those that exist in high concentrations.

Fig. 1 (a) Emission spectra of Rh1 (10 μM) in the presence of increasing concentration of Hg²⁺ in buffered (NaAc-HAc, pH = 7) water/CH₃CN (1 : 1, v/v). λ_ex = 500 nm. (b) Normalized fluorescence response I/I₀ of Rh1 (10 μM) in the presence of various metal ions (1 × 10⁻³ M). λ_ex = 500 nm and λ_em = 555 nm. Inset shows fluorescence change upon addition of Hg²⁺, Zn²⁺, Cd²⁺, Cu²⁺ , Pb²⁺ and blank (5 equiv). (c) Job's plot for determining the stoichiometry of Rh1 and Hg²⁺ ions (the total concentration of Rh1 and Hg²⁺ ions was 10 μM).

To understand the recognition abilities of Rh1 towards Hg²⁺ ions, a Job's plot analysis was conducted to determine the binding stoichiometry of the Rh1-Hg²⁺ complex, by maintaining the total Rh1 and Hg²⁺ ions constant (10 μM) and changing the mole fraction of Hg²⁺ from 0 to 1. From the Job's plot shown in Fig. 1c, we can observe that the significant increase in fluorescence intensity of Rh1 resulted from the complexation with Hg²⁺. The fluorescence intensity went through a maximum at a molar fraction of about 0.5 of Hg²⁺, indicating that a 1 : 1 stoichiometry was the most possible one for the binding mode of Hg²⁺ and Rh1.

The association constant for Rh1 binding to Hg²⁺ was determined from the absorption titration data. Upon gradual addition of Hg²⁺ into the solution, a new absorption peaking around 527 nm emerged with increasing intensity which corresponds to the appearance of a pink color, suggesting the formation of the ring-opened tautomer of Rh1 upon Hg²⁺ binding (Fig. 2a and b). Since Rh1 binds with Hg²⁺ to form a complex with a complexing ratio of 1 : 1, the equilibrium can thus be described as follows:

Fig. 2 (a) Absorption spectra of Rh1 (10 μM) in the presence of increasing concentration of Hg²⁺ in buffered (NaAc-HAc, pH = 7) water/CH₃CN (1 : 1, v/v). (b) Color changes of Rh1 (10 μM) upon addition of different amounts of Hg²⁺. (c) Absorbance as a function of [Hg²⁺] calculated from eqn (3): ■, data points experimentally obtained.

Here, Ks denotes the association constant. The relative absorbance α is defined as the ratio of free Rh1 to the total amount of Rh1 in buffered water/CH₃CN. It can be experimentally determined by measuring the absorbance values in the presence of different concentrations of Hg²⁺:

where A₀ and A_t are the limiting absorbance values for α = 1 (in the absence of Hg²⁺) and α = 0 (Rh1 is completely complexed with Hg²⁺). According to the derivation following the mass law,¹³ the relationship between α and Hg²⁺ concentration can be represented as follows:

From the above discussion, it is apparent that the absorbance of Rh1 has a distinct functional relationship with the Hg²⁺ concentration, and the association constant Ks, which provides the basis for the detection of the Ks value. The fitted curve to incorporate experimental data for Hg²⁺ is presented in Fig. 2c, which gave an association constant Ks value of 8.42 × 10⁴ M⁻¹ for Rh1 binding to Hg²⁺. The strong binding ability of Rh1 towards Hg²⁺ could be ascribed to the introduction of the S atom.

In view of the good sensing performances of Rh1 to Hg²⁺ in aqueous solution, fluorescence imaging experiments were carried out in living cells on a Olympus fluorescence microscope to further demonstrate the practical applicability of Rh1 in living cells. As shown in Fig. 3b, after staining Rat Schwann cells with Rh1 (10 μM) for 10 min, they displayed no detectable fluorescence signal in living cells. Upon addition of 50 μM Hg²⁺ ions, the fluorescence intensity increases dramatically to show a clear red intracellular fluorescence (Fig. 3d). The results indicate that Rh1 can provide a fluorescence enhancement with excellent cell-permeability and biocompatibility for tracing the Hg²⁺ ion in cells, specifically and rapidly. Therefore, Rh1 could be considered for application for in vitro imaging of Hg²⁺ ion in living cells and potentially in vivo.

Fig. 3 Fluorescence images of Hg²⁺ ions in Rat Schwann cells with Rh1 (10 μM). Bright-field transmission image (a, c) and fluorescence image (b, d) of Rat Schwann cells incubated with 0 μM and 50 μM of Hg²⁺ ions for 30 min, respectively (excited with green light).

In the experiment of testing the selectivity of Rh1, an interesting phenomenon attracted our attentions. There was no obvious color change in the presence of interfering effect metal ions, such as K⁺, Na⁺, Ca²⁺, Mg²⁺, Ba²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ as shown in Fig. 4. However, the solution of Rh1 exhibits an obvious pink color with the addition of Cu²⁺, thus almost no fluorescence change is observed (insert in Fig 1b). Therefore, we investigated the detailed response of Rh1 to Cu²⁺ ion.

Fig. 4 Change in color of Rh1 (10 μM) in buffered (NaAc-HAc, pH = 7) water/CH₃CN (1 : 1, v/v) with metal ions: blank, K⁺, Na⁺, Ca²⁺, Mg²⁺, Ba²⁺, Zn²⁺, Cd²⁺, Cu²⁺ and Pb²⁺ (1 × 10⁻³ M, from left to right).

Unlike the varying tendency of fluorescence emission, upon addition of small amounts of Cu²⁺, the absorbance was significantly enhanced with a new peak appearing at 527 nm (Fig. 5a), whereas the absorption spectra of free Rh1 exhibited only a very weak band above 500 nm, which was ascribed to the spirolactam form of Rh1. Upon the addition of 5 equiv of Cu²⁺, the new peak was enhanced up to log ε = 5.25, accompanied with an obvious color change from colorless to pink simultaneously (Fig. 5b). The Job's plot analysis in Fig. 5c was conducted to determine the binding stoichiometry of the Rh1-Cu²⁺ complex. It was observed that a maximum absorption was reached when the molar fraction of Cu²⁺ ions was about 0.5, confirming that a 1 : 1 stoichiometry is most possible for the binding mode of Cu²⁺ and Rh1. The association constant for Rh1 binding to Cu²⁺ was also determined from the absorption titration data. Since Rh1 bound with Cu²⁺ to form a complex with a complexing ratio of 1 : 1, the data still can be fitted by eqn (3). Fig. 6 shows the fitted curve to incorporate the experimental data for Cu²⁺, which gives an association constant Ks value of 1.98 × 10⁴ M⁻¹ for Rh1 binding to Cu²⁺. Upon addition of small amounts Cu²⁺, the enhancement in absorbance clearly suggested the formation of the delocalized xanthane moiety of the rhodamine group, along with a distinct color change from colorless to pink. However, the fluorescence of the open-ring form of Rh1 was quenched by Cu²⁺, which could be explained based on the well-known paramagnetic effect of the Cu(II) d⁹ system.¹⁴

Fig. 5 (a) Absorption spectra of Rh1 (10 μM) in the presence of increasing concentration of Cu²⁺ in buffered (NaAc-HAc, pH = 7) water/CH₃CN (1 : 1, v/v). (b) Color changes of Rh1 (10 μM) upon addition of different amounts of Cu²⁺. (c) Job's plot for determining the stoichiometry of Rh1 and Cu²⁺ ions (the total concentration of Rh1 and Cu²⁺ ions was 10 μM).

Fig. 6 Absorbance as a function of [Cu²⁺] calculated from eqn (3): ■, data points experimentally obtained.

Since the Ks values of Rh1-Hg²⁺ and Rh1-Cu²⁺ are comparable, Hg²⁺ and Cu²⁺ would compete against each other to coordinate with Rh1 when they were coexist in a system, resulting in a transformation between Rh1-Hg²⁺ and Rh1-Cu²⁺ with fluorescence. Based on this, we investigated the different fluorescence state “ON and OFF” of Rh1 with changing the addition sequence, using Hg²⁺ (2 equiv.) and Cu²⁺ (10 equiv.) as inputs (Fig. 7). When the first input is Hg²⁺, the fluorescence emission at 555 nm significantly enhanced. Although the sequential addition of Cu²⁺ led to a relative decline of the fluorescence intensity, it was still in the “On” state. On reversal of the input sequence, i.e. for Cu²⁺ as the first input and Hg²⁺ as the second one, it caused the fluorescence intensity far below its initial maximum, i.e. “Off” state. The two input signals of Hg²⁺ and Cu²⁺ were defined as In 1 and In 2, respectively. These inputs can be encoded with binary digits applying positive logic conventions (off = 0, on = 1). If we regard fluorescence intensity of 75 as the threshold values, Output = 0 when its corresponding spectral value was lower than 75; Output = 1 when its corresponding spectral value was higher than 75.

Fig. 7 Fluorescence emission output of Rh1 with different input sequences: (1) Hg²⁺ as the first input followed by Cu²⁺ as the second one; (2) Cu²⁺ as the first input followed by Hg²⁺ as the second one.

As discussed above, Fig. 8 presents the outputs in the fluorescence channel, obtained for different addition sequences. Only in input sequence of Hg²⁺ and Cu²⁺, the fluorescent intensity was higher than the threshold value (Output = 1), resulting in strong fluorescence. However, changing the sequence of the inputs, it could not initiate a strong fluorescence at 555 nm (Output = 0). Therefore, according to this sequence-dependent phenomenon, a molecular keypad lock was constructed.

Fig. 8 Fluorescence output resulting from different input sequences: (a) In 1 (Hg²⁺ first) and In 2 (Cu²⁺ second); b) In 2 (Cu²⁺ first) and In 1 (Hg²⁺ second).

To simplify the input sequence as password of the molecular keypad lock, inputs Hg²⁺ and Cu²⁺ were designated as “K” and “E”, respectively. When the input signal “K” was added first and followed by “E”, the emission at 555 nm was in the “On” state, and it created a secret password “KEY” (Y defines On state). Inverting the addition sequence of inputs, i.e. the first input is “E” and the second input is “K”, gave an obvious fluorescence quenching (Off, designated as the character “N”). Thus, the wrong password “EKN” failed to open the lock. Therefore, only the authorized user who knows the exact password “KEY” can open the lock, which is a new approach for protecting information at the molecular scale (Fig. 9). Due to the fact that the use of numerical digits (0–9) and letters (A–Z) as PIN numbers in a two-digit password allows a total of more than 700 different combinations,^5j it added to the complexity of cracking the keypad lock, and improved remarkably the security of the molecular device.

Fig. 9 Fluorescent keypad to access a secret password at 555 nm with different input sequences.

Conclusions

In summary, a novel fluorescent chemosensor based on rhodamine derivative Rh1 was designed and synthesized for the detection of Hg²⁺ in aqueous solution and living cells. The fluorescent indicator of Rh1 can efficiently recognize Hg²⁺ over other metal ions by the fluorescence intensity increase, which could be ascribed to the delocalized xanthene moiety of the rhodamine group. Moreover, this “Off–On”-type fluorescent sensor could successfully mimic a molecular level keypad lock in the presence of Cu²⁺ ions. Only the correct password resulted in strong fluorescence emission at 555 nm which can be used to “open” this molecular keypad lock. Wrong passwords failed to open the lock, which can result in the “alarm” signal indicating the wrong password. Therefore, this molecular keypad lock has the potential for protecting information at the molecular scale, which would be used to authorize a user, to verify authentication of a product, or to initiate a higher process.

Acknowledgements

The authors gratefully thank the financial supports of the National Natural Science Foundations of China (Grant No. 50872130) and the Science and Technology Developing Project of Jilin Province (Grant No. 20100533).

References

1. (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and E. T. Rice, Chem. Rev., 1997, 97, 1515; (b) B. L. Feringa, Molecular witches, Wiley-VCH, Weinheim 2001; (c) F. M. Raymo, Adv. Mater., 2002, 14, 401; (d) V. Balzini, A. Credi and M. Venturi, ChemPhysChem, 2003, 4, 49; (e) X. Guo, D. Zhang, T. Wang and D. Zhu, Chem. Commun., 2003, 914; (f) A. P. de Silva and N. D. McClenaghan, Chem.–Eur. J., 2004, 10, 574; (g) D. H. Qu, Q. C. Wang and H. Tian, Angew. Chem., 2005, 117, 5430; (h) D. H. Qu, F. Y. Ji, Q. C. Wang and H. Tian, Adv. Mater., 2006, 18, 2035; (i) J. Andreasson, S. D. Straight, G. Kodis, C. D. Park, M. Hambourger, M. Grevaldo, B. Albinson, T. A. Moore, A. L. Moore and D. Gust, J. Am. Chem. Soc., 2006, 128, 16259; (j) A. Credi, Angew. Chem., Int. Ed., 2007, 46, 5472; (k) U. Pischel, Angew. Chem., Int. Ed., 2007, 46, 4026; (l) K. Szacilowski, Chem. Rev., 2008, 108, 3481; (m) S. D. Straight, P. A. Liddell, Y. Terazono, T. A. Moore, A. L. Moore and D. Gust, Adv. Funct. Mater., 2007, 17, 777; (n) R. Ballardini, P. Ceroni, A. Credi, M. T. Gandolfi, M. Maestri, M. Semararo, M. Venturi and V. Balzani, Adv. Funct. Mater., 2007, 17, 740.
2. R. Baron, A. Onopriyenko, E. Katz, O. Lioubashevski, I. Willner, S. Wang and H. Tian, Chem. Commun., 2006, 2147.
3. Y. Liu, W. Jiang, H. Y. Zhang and C. J. Li, J. Phys. Chem. B, 2006, 110, 14231.
4. J. Andreasson, S. D. Straight, S. Bandyopadhyay, R. H. Mitchell, T. A. Moore, A. L. Moore and D. Gust, J. Phys. Chem. C, 2007, 111, 14274.
5. (a) D. Margulies, C. E. Felder, G. Melman and A. Shanzer, J. Am. Chem. Soc., 2007, 129, 347; (b) G. Strack, M. Ornatska, M. Pita and E. Katz, J. Am. Chem. Soc., 2008, 130, 4234; (c) M. Kumar, R. Kumar and V. Bhalla, Chem. Commun., 2009, 7384; (d) M. Zhou, X. Zheng, J. Wang and S. Dong, Chem.–Eur. J., 2010, 16, 7719; (e) J. Andrasson, S. D. Straight, T. A. Moore, A. L. Moore and D. Gust, Chem.–Eur. J., 2009, 15, 3936; (f) J. Halamek, T. K. Tam, S. Chinnapareddy, V. Bocharova and E. Katz, J. Phys. Chem. Lett., 2010, 1, 973; (g) J. Wang and C. S. Ha, Analyst, 2010, 135, 1214; (h) J. Halamek, T. K. Tam, G. Strack, V. Bocharova, M. Pita and E. Katz, Chem. Commun., 2010, 2405; (i) S. Kumar, V. Luxami, R. Saini and D. Kaur, Chem. Commun., 2009, 3044; (j) M. Suresh, A. Ghosh and A. Das, Chem. Commun., 2008, 3906; (k) Z. Guo, W. Zhu, L. Shen and H. Tian, Angew. Chem., Int. Ed., 2007, 46, 5549; (l) M. Kumar, A. Dhir and V. Bhalla, Org. Lett., 2009, 11, 2567; (m) P. Singha, J. Kaura and W. Holzer, Sens. Actuators, B, 2010, 150, 50.
6. (a) A. P. de Silva and S. Uchiyama, Nat. Nanotechnol., 2007, 2, 399; (b) D. Margulies, G. Melman and A. Shanzer, J. Am. Chem. Soc., 2006, 128, 4865.
7. T. Nguyen and M. B. Francis, Org. Lett., 2003, 5, 3245.
8. B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-VCH, Verlag GmbH: New York, 2001.
9. (a) Y. Xiang and A. Tong, Org. Lett., 2006, 8, 1549; (b) Y. Xiang, A. Tong, P. Jin and Y. Ju, Org. Lett., 2006, 8, 2863; (c) Y. K. Yang, K. J. Yook and J. Tae, J. Am. Chem. Soc., 2005, 127, 16760; (d) J. Y. Kwon, Y. J. Jang, Y. J. Lee, K. M. Kim, M. S. Seo, W. Nam and J. Yoon, J. Am. Chem. Soc., 2005, 127, 10107; (e) H. Zheng, Z. H. Qian, L. Xu, F. F. Yuan, L. D. Lan and J. G. Xu, Org. Lett., 2006, 8, 859; (f) J. S. Wu, I. C. Hwang, K. S. Kim and J. S. Kim, Org. Lett., 2007, 9, 907; (g) D. Wu, W. Huang, C. Duan, Z. Lin and Q. Meng, Inorg. Chem., 2007, 46, 1538; (h) J. Chen, A. F. Zheng, A. H. Chen, Y. Gao, C. He, X. Kai, G. Wu and Y. Chen, Anal. Chim. Acta, 2007, 599, 134; (i) K. P. Prathish, D. James, J. Jaisy and T. Prasada Rao, Anal. Chim. Acta, 2009, 647, 84; (j) Y. Xiang, Z. Li, X. Chen and A. Tong, Talanta, 2008, 74, 1148; (k) M. Suresh, A. Shrivastav, S. Mishra, E. Suresh and A. Das, Org. Lett., 2008, 10, 3013; (l) Y. Zhao, X. B. Zhang, Z. X. Han, L. Qiao, C. Y. Li, L. X. Jian, G. L. Shen and R. Q. Yu, Anal. Chem., 2009, 81, 7022; (m) V. Bhalla, R. Tejpal and M. Kumar, Sens. Actuators, B, 2010, 151, 180.
10. (a) Y. K. Yang, K. J. Yook and J. Tae, J. Am. Chem. Soc., 2005, 127, 16760; (b) X. F. Yang, X. Q. Guo and Y. B. Zhao, Talanta, 2002, 57, 883.
11. (a) Z. X. Han, X. B. Zhang, Z. Li, Y. J. Gong, X. Y. Wu, Z. Jin, C. M. He, L. X. Jian, J. Zhang, G. L. Shen and R. Q. Yu, Anal. Chem., 2010, 82, 3108; (b) H. Zheng, Z. H. Qian, L. Xu, F. F. Yuan, L. D. Lan and J. G. Xu, Org. Lett., 2006, 8, 859; (c) W. Liu, L. Xu, R. Sheng, P. Wang, H. Li and S. Wu, Org. Lett., 2007, 9, 3829.
12. D. Wu, W. Huang, C. Duan, Z. Lin and Q. Meng, Inorg. Chem., 2007, 46, 1538.
13. (a) X. B. Zhang, C. C. Guo, Z. Z. Li, G. L. Shen and R. Q. Yu, Anal. Chem., 2002, 74, 821; (b) Y. Zhao, X. B. Zhang, Z. X. Han, L. Qiao, C. Y. Li, L. X. Jian, G. L. Shen and R. Q. Yu, Anal. Chem., 2009, 81, 7022.
14. (a) J. S. Kim and D. T. Quang, Chem. Rev., 2007, 107, 3780; (b) M. Suresh, A. Shrivastav, S. Mishra, E. Suresh and A. Das, Org. Lett., 2008, 10, 3013.

---