Naked-eye and fluorescence detection of basic pH and F⁻ with a 1,8-naphthalimide-based multifunctional probe

Abstract

Based on 1,8-naphthalimide, a novel multifunctional colorimetric and fluorescent probe was designed and synthesized for basic pH and F⁻ naked-eye and fluorescence recognition with high sensitivity and excellent selectivity. The probe can detect F⁻ quantitatively in a concentration range of 0–16.7 μM and 0–20 μM in different methods, and the detection limit could be as low as 25 nM for F⁻. The detection of F⁻ on chromatography plates and the preliminary application of the probe to detect fluoride contents in toothpaste have been successfully demonstrated.

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Introduction

Protons play an important role in many chemical and biological processes, and it is very important to develop methods that can monitor the change of pH.¹ For example, in cellular biology, pH homeostasis plays a pivotal role in various cellular events such as apoptosis, cell proliferation,² phagocytosis,³ drug resistance⁴ and endocytosis.⁵ Intracellular pH values are normally in the range of 6.8–7.4 and abnormal cellular pH values are known to be linked with inappropriate cellular functions, which are associated with many diseases including Alzheimer's disease,⁶ cancer⁷ and others. As a result, many methods have been applied to measure pH variations including nuclear magnetic resonance, microelectrodes, absorption and fluorescence spectroscopy.⁸ On the other hand, detection of fluoride ion has attracted more and more attention of researchers because the knowledge about the importance of such ion in biological and medical processes has grown in recent years. As is well known, this anion plays an important role in dental health⁹ and has potential use for the treatment of osteoporosis.¹⁰ Besides, overexposure to fluoride can lead to acute gastric and kidney problems.¹¹

For measuring pH values, fluorescent probes have become the most promising choice because of their simplicity, high sensitivity and instantaneous response.¹² As for the detection of F⁻, because of the same reason, the great achievement has been made in the fluorescent probes toward F⁻.¹³

In general, fluorescent probes depend on emission or position change and could be significantly influenced by excitation power and detector sensitivity. By comparison, based on absorption properties of the ground state, color changes are more suitable for direct observation with the naked eye.¹⁴ Although great progress has been achieved in the fluorescent detection of pH and F⁻, the research on the development of colorimetric probes toward pH, especially basic pH and F⁻ has been started in recent years and it is necessary to develop novel colorimetric probes for pH and F⁻.

As a widely used fluorophore, 1,8-naphthalimide-based derivatives have many excellent optical properties such as good photostability, high fluorescence quantum yields and large Stokes' shift.¹⁵ Besides, their photophysical properties can be easily tuned through judicious structural modifications.¹⁶ In fact, based on 1,8-naphthalimide, some pH (ref. 17) and F⁻-fluorescent probes¹⁸ have been reported. In contrast, there are not many 1,8-naphthalimide-based colorimetric probes for pH and F⁻ which have been reported.¹⁹ Therefore, the development of 1,8-naphthalimide-based colorimetric probes toward pH and F⁻ is still challenging.

Herein, a novel salicylaldehyde derivative of 1,8-naphthalimide (probe 1, Scheme S1†), as a colorimetric and fluorescent probe toward basic pH and F⁻ was easily synthesized. The probe can sense basic pH and F⁻ in colorimetric and fluorescent methods. Moreover, this probe can quantitatively detect F⁻ in a wide range with the dramatic color and fluorescence change. The detection limit on UV-vis and fluorescence response of the probe can be as low as 25 and 402 nM respectively.

Results and discussions

Synthesis and spectral characterization

In this work, synthetic route employed for the preparation of the novel 1,8-naphthalimide-based basic pH and F⁻ sensitive dual functional probe 1 is shown in Scheme S1.† The reaction of monoethanolamine and 4-bromo-1,8-naphthalic anhydride gives the product 3 in a 88.8% yield. Compound 2 was obtained with a yield of 89.7% by the reaction of compound 3 and hydrazinium hydroxide. The reaction of compound 2 with salicylaldehyde led to the probe 1 in a 84.2% yield. The chemical structure of 1 was confirmed by HRMS, ¹H NMR, and ¹³C NMR.

Basic pH sensing properties of 1

To study the optical responses of 1 to pH, the standard pH titrations were conducted. As shown in Fig. 1, upon increasing pH from 11 to 14, the absorption spectra of 1 exhibited a shift from 461 to 571 nm with a pronounced red shift of 110 nm. Meanwhile, a great decrease in the fluorescence spectra of 1 in mixture of ethanol and water upon increasing the pH from 11 to 14 (right of Fig. 1). Moreover, the solution color of 1 changed by degrees from yellow to blue and the fluorescence decreased with pH increasing (Fig. 2). Such spectra change whether in absorption or emission spectra of compound 1 can be ascribed to the formation of 1O from the reaction between 1 and OH⁻ (Scheme 1). Such great spectra change, color and fluorescence change of compound 1 with pH increasing imply that compound 1 can be a naked-eye and fluorescent ‘on–off’ probe for basic pH.

Fig. 1 Absorption (left) and emission (right) spectra change of compound 1 (2.0 × 10⁻⁵ M, V_ethanol : V_{HEPES buffer} = 1 : 1) in different pH conditions (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14). The excitation wavelength was 460 nm.

Fig. 2 Photographs of compound 1 (2.0 × 10⁻⁵ M, V_ethanol : V_{HEPES buffer} = 1 : 1) in different pH conditions (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 from left to right) in daylight (up) and under a UV lamp (365 nm, down).

Scheme 1 Proposed sensing mechanism toward basic pH and F⁻.

F⁻ sensing properties of 1

To study the sensing properties of 1 toward F⁻, UV-vis and fluorescence titration experiments (Fig. 3) were conducted with 0.01 M F⁻ water solution in CH₃CN solution of 1 (1.0 × 10⁻⁵ M) Upon addition of F⁻, the peak at 441 nm in the UV-vis spectra decreased gradually while a new band developed at 572 nm, and then, the band reached the maxima at 4 equiv of F⁻ (left of Fig. 3). Meanwhile, three clear isosbestic points were observed at 324, 400 and 483 nm, indicating that only one product was generated from 1 upon the reaction to F⁻. Moreover, the solution color of 1 changed by degrees from yellow to blue in the presence of different concentrations of F⁻ (up of Fig. 4), implied that compound 1 can serve as a highly sensitive ‘naked-eye’ probe for F⁻ in CH₃CN solution. Fluorescence titration experiment (right of Fig. 3) demonstrated that a great F⁻ and the fluorescence of the CH₃CN solution of compound 1 decreased little by little in the presence of different concentrations of F⁻ (down of Fig. 4). Such fluorescence change brought from addition of F⁻ indicates that compound 1 can be an ‘on–off’ fluorescence probe for F⁻. The absorption spectra, fluorescence spectra, color and fluorescence change of 1 could be ascribed to the intermolecular hydrogen bond between F⁻ and the probe 1 (Scheme 1).²⁰ As demonstrated in Fig. 5, in the concentration range of 0 and 16.7 μM, the absorption peaks of 1 are in good linear relationship with F⁻ concentration, while in the range between 0 and 20 μM, a good linear relationship between the fluorescence peak with F⁻ concentration was obtained, implying that F⁻ can be quantitatively detected in a wider concentration range both with UV-vis and fluorescence spectroscopy. From the UV-vis titration experiment, the LOD values were calculated as 25 and 467 nM. According to the IUPAC definition, the LOD was calculated using the relationship LOD = (3.3 × standard deviation)/slope. To calculate the relative standard deviation, the absorption measurements of the blank samples were taken. As shown in the left of Fig. 5, the absorption calibration values were normalized between the minimum and the maximum ratio of intensity at 441 nm to that at 572 nm, and then a linear regression curve was fitted to these normalized data to get the slope. With this approach, the LOD value was found to be 59 nM. From the linear calibration graph with the fluorescence titration experiment (right of Fig. 5), the detection limit of probe 1 for F⁻ was found to be about 402 nM based on signal-to-noise ratio (S/N) = 3,²¹ which was sufficiently low for the detection of F⁻. These results led us to conclude that 1 could be an effective colorimetric and fluorescent probe for F⁻. In general, the LOD is usually lower by emission method than that by absorption method. the higher LOD obtained by emission method may be ascribed to the influence of water on the emission spectra of compound 1 in CH₃CN and no influence of water was observed on the absorption spectra of compound 1 (Fig. S1 in the ESI†).

Fig. 3 Absorption (left) and emission (right) spectra change of compound 1 (1.0 × 10⁻⁵ M, acetonitrile) upon addition of F⁻ in water. The excitation wavelength was 460 nm.

Fig. 4 Photographs of compound 1 (1.0 × 10⁻⁵ M, acetonitrile) upon addition of F⁻ at various concentrations (0, 0.33 × 10⁻⁵ M, 0.67 × 10⁻⁵ M, 1.0 × 10⁻⁵ M, 1.3 × 10⁻⁵ M, 1.7 × 10⁻⁵ M, 2.0 × 10⁻⁵, 2.3 × 10⁻⁵, 2.7 × 10⁻⁵, 3.0 × 10⁻⁵, 3.3 × 10⁻⁵, from left to right) in water in daylight (up) and under a UV lamp (365 nm, down).

Fig. 5 The linearity of absorbance (left) and emission intensity (right) of compound 1 (1.0 × 10⁻⁵ M, acetonitrile) with respect to F⁻ concentrations. The excitation wavelength was 460 nm. A₅₇₂ and A₄₄₁ represent the absorbance at 572 nm and 441 nm.

The sensing properties of 1 were also investigated in CH₃CN upon addition of various anions such as Cl⁻, Br⁻, I⁻, SO₄²⁻, NO₃⁻, HSO₄⁻, HSO₃⁻, SO₃²⁻, and F⁻. Upon addition of 10 equiv of each anion, only F⁻ induced a distinct spectrum change while other anions showed either no or slight change in the absorption and emission spectra relative to the free probe 1 (Fig. 6 and red bars of Fig. 7). Consistently with the changes of UV-vis and emission spectra, the solution color of 1 in the presence of F⁻ changed from yellow to blue (up of Fig. S2 in the ESI†) and fluorescence decreased greatly (down of Fig. S2 in the ESI†), indicating that compound 1 can serve as a highly selective ‘naked-eye’ and fluorescent ‘on–off’ probe for F⁻ in CH₃CN solution.

Fig. 6 Absorption (left) and emission (right) spectra change of compound 1 (1.0 × 10⁻⁵ M, acetonitrile) upon addition of different anions in water. The anion salts represent tetrabutyl ammonium chloride, t, Na₂SO₄, NaNO₃, NaHSO₄, NaHSO₃, and Na₂SO₃. The excitation wavelength was 460 nm.

Fig. 7 Absorption (left) and emission (right) responses of 1 (1.0 × 10⁻⁵ M, acetonitrile) upon addition of 10 equiv different anions in water (red bars), and absorption and emission change of the mixture of 1 and F⁻ after addition of other anions (the ratio of each other anion to 1 is 10 equiv, green bars) in water. A and A₀ represent the absorbance at 572 nm, I and I₀ represent the emission at 525 nm. The anion salts from 1 to 9 represent tetrabutyl ammonium chloride, t, Na₂SO₄, KNO₃, NaHSO₄, NaHSO₃, and Na₂SO₃. The excitation wavelength was 460 nm.

To further assess its utility as a F⁻-selective probe, its UV-vis and fluorescence spectra response to F⁻ in the presence of other anions Cl⁻, Br⁻, I⁻, SO₄²⁻, NO₃⁻, HSO₄⁻, HSO₃⁻, and SO₃²⁻ (green bars of Fig. 7) was also tested. The results demonstrated that all of the selected anions have no interference in the detection of F⁻ with the UV-vis spectroscopy method, while both I⁻ and HSO₄⁻ have some interference on the detection of F⁻ with the fluorescence spectroscopy method. This result strongly indicates that compound 1 could be an excellent colorimetric and fluorescent probe towards F⁻.

The reversibility of the change in optical spectra toward pH and F⁻ has been investigated by monitoring of absorption and emission spectral changes. The absorption and emission spectra of compound 1 can be recovered while controlling the pH applied to the solution (Fig. S3 in the ESI†). To examine the reversibility of the binding of compound 1 to F⁻, 80 μL water was added to the solution of 1 containing 4 equiv F⁻. As shown in the left of Fig. S4 in the ESI,† When 80 μL water was added to the solution of 1 containing 4 equiv F⁻, absorption signals identical to those of 1 were restored, demonstrating that F⁻ is removed from the hydrogen bonding of compound 1 and F⁻ by water. As for the emission spectra, addition of water made the emission intensity at 525 nm increase (green line in the right of Fig. S4 in the ESI†), but could not made it enhance the same as that of only compound 1 in CH₃CN. In order to study clearly the reversibility of the optical spectra change, emission spectra was also done upon addition of 80 μL water into the CH₃CN solution of only 1 (blue line in the right of Fig. S4 in the ESI†), which was the same as that of the CH₃CN solution of 1 containing 4 equiv F⁻ and 80 μL water. Such emission spectra results demonstrated that water can decrease the emission intensity at 525 nm of compound 1 in CH₃CN and the detection of 1 toward F⁻ is irreversible.

Basic pH and F⁻ sensing mechanisms of 1

To interpret basic pH sensing mechanism, calculations based on time-dependent density functional theory (TD-DFT) were performed on this system. For compound 1, the highest occupied molecular orbital (HOMO) is mainly located on the conjugated unit of compound 1 and the lowest unoccupied molecular orbital (LUMO) is on the 1,8-naphthalimide group (Fig. 8). As for 1O, the HOMO and LUMO hold their locations at benzene ring and 1,8-naphthalimide respectively (Fig. 8). The lowest energy transitions of 1 and 1O comes from HOMO to LUMO (Table 1). The HOMO and LUMO energies of 1O are higher than those of 1 and the energy difference between HOMO and LUMO of 1O (ΔE = 0.03029 eV) is smaller than that of 1 (ΔE = 0.1223 eV) (Fig. 9). The calculated absorption-peak positions are in good agreement with the experimental results (black, red, blue and cyan lines of Fig. S5 in the ESI†), implying that the sensing mechanism that we pointed out is reasonable. In addition, DFT calculations were carried out for the geometry optimizations of the probe. From the optimized structure (Fig. 10) and the bond lengths (C1–N1, 1.39 Å; N1–N2, 1.37 Å; N2–C2, 1.29 Å), we can conclude that the 2-hydroxy phenyl was conjugated to the fluorophore. As a result, the deprotonation of the phenol can alter the push–pull effect of the fluorophore greatly.

Fig. 8 Frontier molecular orbitals of 1, 1O, and 1F relevant to the lowest energy transition.

Table 1 The contribution of each orbital transition to the lower energy transition in visible range

Orbital transition	Oscillator strength (f) (1)	Oscillator strength (f) (1O)	Oscillator strength (f) (1F)
HOMO → LUMO	0.2620	0.3474	0.1617
HOMO − 1 → LUMO		0.1473	0.0844

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Fig. 9 Frontier molecular orbital energies and energy difference between HOMO and LUMO of 1, 1O, and 1F.

Fig. 10 Optimized structure of compound 1 at the B3LYP/6-31G(d) level.

The mechanism of the intermolecular hydrogen bond between F⁻ and the probe 1 was confirmed by the titration of the active proton NMR spectrum of 1 in 0.5 mL DMSO-d₆ (1.0 × 10⁻⁵ M) with different amounts of the F⁻ anion (0, 1, 2, and 3 equiv). As shown in Fig. 11, upon addition of F⁻, the peaks of two active protons at 10.25 and 11.45 shifted to low field in the range from 11 to 16.5, confirming that there are two intermolecular hydrogen bonds between F⁻ and two active hydrogen atoms of the probe 1.

Fig. 11 Quantitative titration of the active proton NMR spectrum of 1 in 0.5 mL DMSO-d₆ (1.0 × 10⁻⁵ M) with different amounts of F⁻ anion (from up to down: 0, 1, 2, and 3 equiv).

To further interpret F⁻ sensing mechanism, TD-DFT calculations were also performed on this system. As mentioned above, the HOMO and LUMO of 1 are located at the conjugated unit of compound 1 and the 1,8-naphthalimide group respectively (Fig. 8). As for 1F, the HOMO is mainly located on the conjugated unit and the LUMO holds its location at benzene ring (Fig. 8). The HOMO and LUMO energies of 1F are much lower than those of 1 and the energy difference between HOMO and LUMO of 1F (ΔE = 0.09123 eV) is much smaller than that of 1 (ΔE = 0.1223 eV) (Fig. 9). As shown in Table 1, the lowest energy transition of 1F comes from HOMO to LUMO, which is the same as that of 1, the calculated absorption-peak positions are in good agreement with the experimental results (green and magenta lines of Fig. S5 in the ESI†), implying that the sensing mechanism that we pointed out is reasonable.

The preliminary application of 1

As we have mentioned above, probe 1 can quantitatively detect F⁻ in a very wide concentration range. Herein, we have also selected several commercially available toothpastes as samples to detect the content of fluoride in them. The pretreatment procedure is the same for all the samples and the following is how to pretreatment one kind of toothpaste. Toothpaste 1 (328.7 mg) or 2 (353.2 mg) was immersed into 10 mL water for 12 h at 20 °C and centrifugated. 10 μL supernatant was added to 3 mL acetonitrile solution of 1 (1.0 × 10⁻⁵ M) to detect F⁻ concentration. The original spectra of the detection of F⁻ in toothpaste are shown in Fig. S6 in the ESI,† and the F⁻ contents of the samples are selected in Table 2.

Table 2 F⁻ Contents of Samples Detected with Probe 1

Samples	F^− content (annotated mg kg^−1)	F^− content (calculated mg kg^−1)
1	0.10%	0.13%
2	0.14%	0.12%

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F⁻ sensing of 1 as solid state

As shown in the above, probe 1 can detect F⁻ in CH₃CN solution, it inspired us to further investigate the possibility of detection as solid materials for point-of-care detection application. The detection of F⁻ on chromatography plates containing 0.023% (w/w) 1 was carried out successfully. Upon exposure to different concentrations of F⁻ DMSO solution, the concentration-dependent dramatic color changes from yellow to blue were observed, which could be ascribed to the hydrogen bonding interaction between probe 1 and F⁻ (Fig. S7 in the ESI†). Additionally, a blue color and a strong fluorescence of the chromatography plates were obtained only upon the addition of F⁻ (Fig. 12), demonstrating the good selectivity of probe 1 as solid materials toward F⁻.

Fig. 12 Images of chromatography plates for the selectivity of F⁻ upon addition of 0.1 M of various anions (from left to right, the anions used were blank, Cl⁻, Br⁻, I⁻, SO₄²⁻, NO³⁻, HSO₄⁻, HSO₃⁻, SO₃²⁻, and F⁻) in DMSO solution in daylight (up) and under a UV lamp (365 nm, down).

Experimental section

Instruments

Mass spectra were obtained on high resolution mass spectrometer (IonSpec4.7 Tesla FTMS-MALDI/DHB). FT-IR spectra were recorded on a NEXUS-470 spectrometer at frequencies ranging from 400 to 4000 cm⁻¹. Samples were thoroughly mixed with KBr and pressed into pellet form. ¹H and ¹³C NMR spectra were recorded on a Bruker 400 NMR spectrometer. Chemical shifts are reported in parts per million using tetramethylsilane (TMS) as the internal standard.

All spectral characterizations were carried out in HPLC-grade solvents at 20 °C within a 10 mm quartz cell. UV-vis absorption spectroscopy was measured with a TU-1901 double-beam UV-vis Spectrophotometer, and fluorescence spectroscopy was determined on a Hitachi F-4600 spectrometer.

DFT calculations

The energies and oscillator strengths with 80–120 lowest energy electronic transitions were obtained using time-dependent DFT (TD-DFT) with Becke's three parameterized Lee–Yang–Par (B3LYP) ex-change functional with 6-31G(d) and 6-31G**+ LanL2DZ basis sets.

Materials and reagents

All commercial grade chemicals and solvents were purchased and were used without further purification. The probe 1 was synthesized according to Scheme S1† and following reported method.²²

Synthesis of compound 3

56 mL 4-bromo-1,8-naphthalic anhydride (2.7600 g, 10.0 mmol) ethanol solution was heated to 80 °C under nitrogen atmosphere and 0.66 mL monoethanolamine was added to the above solution. After the mixture was refluxed for 4 h, the reaction liquid was cooled to room temperature and filtered. The filter cake was washed for 3 times with methanol and the product 3 was obtained (2.8326 g, 88.8%).

Characterization of 3. ¹H NMR (400 MHz, DMSO-d₆, TMS): δ_H 8.50 (m, 2H), 8.27 (m, 1H), 8.16 (m, 1H), 7.96 (m, 1H), 4.82 (s, 1H), 4.13 (t, 2H), 3.63 (t, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ_C 163.44, 163.39, 132.94, 131.94, 131.77, 130.19, 129.44, 129.21, 128.74, 123.31, 122.53, and 58.17.

Synthesis of compound 2

The mixture of 3 (1.1156 g, 3.35 mmol), hydrazinium hydroxide (0.85 mL, 13.98 mmol), and 2-methoxyethanol (20 mL) were refluxed for 5 h. The reaction liquid was filtered and the filter cake was obtained as the final product 2 (0.8500 g, 89.7%).

Characterization of 2. ¹H NMR (400 MHz, DMSO-d₆, TMS): δ_H 9.10 (s, 1H), 8.60 (d, 1H), 8.40 (d, 1H), 8.28 (d, 1H), 7.62 (m, 1H), 7.24 (d, 1H), 4.80 (t, 1H), 4.67 (d, 2H), 4.12 (t, 4H), 3.57 (t, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ_C 164.30, 163.50, 153.62, 134.64, 131.00, 129.82, 128.66, 124.55, 122.30, 118.89, 107.90, 104.44, 58.44, and 41.82.

Synthesis of compound 1

The mixture of compound 2 (0.1555 g, 0.57 mmol), salicylaldehyde (0.0843 g, 0.69 mmol), and 20 mL methanol was reacted for 6 h at 65 °C. The reaction liquid was filtered and the filter cake was obtained as the final product 1 (0.180 g, 84.2%).

Characterization of 1. HRMS (EI) m/z: calcd for C₂₁H₁₇N₃O₄ [M + Na]⁺, 398.1117; found, 398.1117. ¹H NMR (400 MHz, DMSO-d₆, TMS): δ_H 11.45 (s, 1H), 10.25 (s, 1H), 8.80 (d, 2H), 8.46 (d, 1H), 8.38 (m, 1H), 7.84 (d, 1H), 7.77 (m, 1H), 7.61 (m, 1H), 7.27 (m, 1H), 6.92 (m, 2H), 4.81 (s, 1H), 4.13 (t, 2H), and 3.60 (t, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ_C 164.25, 163.52, 156.72, 146.70, 142.43, 133.97, 131.30, 131.22, 129.66, 128.66, 126.89, 125.37, 122.52, 120.99, 120.01, 119.01, 116.61, 111.31, 106.84, 58.39, and 41.96.

Conclusions

In summary, based on 1,8-naphthalimide, a novel colorimetric and fluorescent probe 1 toward basic pH and F⁻ has been developed. In aqueous solution containing 50% ethanol, probe 1 exhibited the color change from yellow to blue and fluorescence decrease with pH changing from 11 to 14. In CH₃CN, addition of F⁻ can make the color of 1 change from yellow to blue with great fluorescence weakness. The LOD toward F⁻ can be as low as 25 nM. The color and fluorescence responses of 1 on chromatography plates toward F⁻ and the preliminary application of 1 to detect fluoride contents in toothpaste were applied.

Acknowledgements

We are grateful for the financial supports from National Natural Science Foundation of China (50903075 and J1210060), New Teachers' Joint Fund for Doctor Stations from Ministry of Education of the People's Republic of China (20114101120003) and Zhengzhou University.

Notes and references

1. (a) Z. Q. Hu, M. Li, M. D. Liu, W. M. Zhuang and G. K. Li, Dyes Pigm., 2013, 96, 71; (b) M. Chesler, Physiol. Rev., 2003, 83, 1183.
2. (a) D. Perez-Sala, D. Collado-Escobar and F. Mollinedo, J. Biol. Chem., 1995, 270, 6235; (b) A. Ishaque and M. Al-Rubeai, J. Immunol. Methods, 1998, 221, 43; (c) R. A. Gottlieb, J. Nordberg, E. Skowronski and B. M. Babior, Proc. Natl. Acad. Sci. U. S. A., 1996, 93, 654; (d) D. Lagadic-Gossmann, L. Huc and V. Lecureur, Cell Death Differ., 2004, 11, 953; (e) M. Cervelli, R. Amendola, F. Polticelli and P. Mariottini, Amino Acids, 2012, 42, 441.
3. (a) M. Miksa, H. Kornura, R. Wu, K. G. Shah and P. Wang, J. Immunol. Methods, 2009, 342, 71; (b) C.-L. Hsu, W. Lin, D. Seshasayee, Y.-H. Chen, X. Ding, Z. Lin, E. Suto, Z. Huang, W. P. Lee, H. Park, M. Xu, M. Sun, L. Rangell, J. L. Lutman, S. Ulufatu, E. Stefanich, C. Chalouni, M. Sagolla, L. Diehl, P. Fielder, B. Dean, M. Balazs and F. Martin, Science, 2012, 335, 89.
4. (a) H. Y. Li and Z. M. Qian, Med. Res. Rev., 2002, 22, 225; (b) J. B. Russell and J. L. Rychlik, Science, 2001, 292, 1119.
5. (a) M. Lakadamyali, M. J. Rust, H. P. Babcock and X. W. Zhuang, Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 9280; (b) E. J. Adie, S. Kalinka, L. Smith, M. J. Francis, A. Marenghi, M. E. Cooper, M. Briggs, N. P. Michael, G. Milligan and S. Game, Biotechniques, 2002, 33, 1152.
6. T. A. Davies, R. E. Fine, R. J. Johnson, C. A. Levesque, W. H. Rathbun, K. F. Seetoo, S. J. Smith, G. Strohmeier, L. Volicer and L. Delva, Biochem. Biophys. Res. Commun., 1993, 194, 537.
7. (a) M. Schindler, S. Grabski, E. Hoff and S. M. Simon, Biochemistry, 1996, 35, 2811; (b) H. Izumi, T. Torigoe, H. Ishiguchi, H. Uramoto, Y. Yoshida, M. Tanabe, T. Ise, T. Murakami, T. Yoshida, M. Nomoto and K. Kohno, Cancer Treat. Rev., 2003, 29, 541.
8. (a) D. Ellis and R. C. Thomas, Nature, 1976, 262, 224; (b) R. G. Zhang, S. G. Kelen and J. C. Lamanna, J. Appl. Phys., 1990, 68, 1101; (c) S. J. A. Hesse, G. J. G. Ruijter, C. Dijkema and J. Visser, J. Biotechnol., 2000, 77, 5.
9. (a) S. Ayoob and A. K. Gupta, Crit. Rev. Environ. Sci. Technol., 2006, 36, 433; (b) N. J. Cochrane, F. Cai, N. L. Huq, M. F. Burrow and E. C. Reynolds, J. Dent. Res., 2010, 89, 1187.
10. (a) D. Briancon, Rev. Rhum. Mal. Osteo-Articulaires, 1997, 64, 78; (b) M. Kleerekoper, Endocrinol. Metab. Clin. North Am., 1998, 27, 441.
11. G. M. Whitford, J. Dent. Res., 1990, 69, 539.
12. (a) W. Shi, X. Li and H. Ma, Angew. Chem., Int. Ed., 2012, 51, 6432; (b) S. Chen, Y. Hong, Y. Liu, J. Liu, C. W. T. Leung, M. Li, R. T. K. Kwok, E. Zhao, J. W. Y. Lam, Y. Yu and B. Z. Tang, J. Am. Chem. Soc., 2013, 135, 4926; (c) H. J. Kim, C. H. Heo and H. M. Kim, J. Am. Chem. Soc., 2013, 135, 17969; (d) M. H. Lee, J. H. Han, J. H. Lee, N. Park, R. Kumar, C. Kang and J. S. Kim, Angew. Chem., Int. Ed., 2013, 52, 6206; (e) D. Asanuma, Y. Takaoka, S. Namiki, K. Takikawa, M. Kamiya, T. Nagano, Y. Urano and K. Hirose, Angew. Chem., Int. Ed., 2014, 53, 6085; (f) D. Aigner, S. M. Borisov, P. Petritsch and I. Klimant, Chem. Commun., 2013, 49, 2139; (g) J. Madsen, I. Canton, N. J. Warren, E. Themistou, A. Blanazs, B. Ustbas, X. Tian, R. Pearson, G. Battaglia, A. L. Lewis and S. P. Armes, J. Am. Chem. Soc., 2013, 135, 14863; (h) Z. Yang, W. Qin, J. W. Y. Lam, S. Chen, H. H. Y. Sung, I. D. Williams and B. Z. Tang, Chem. Sci., 2013, 4, 3725; (i) S. Zhu, W. Lin and L. Yuan, Dyes Pigm., 2013, 99, 465; (j) J. Fan, C. Lin, H. Li, P. Zhan, J. Wang, S. Cui, M. Hu, G. Cheng and X. Peng, Dyes Pigm., 2013, 99, 620; (k) G. Li, D. Zhu, L. Xue and H. Jiang, Org. Lett., 2013, 15, 5020.
13. (a) L. Gai, H. Chen, B. Zou, H. Lu, G. Lai, Z. Li and Z. Shen, Chem. Commun., 2012, 48, 10721; (b) I.-S. Ke, M. Myahkostupov, F. N. Castellano and F. P. Gabbai, J. Am. Chem. Soc., 2012, 134, 15309; (c) X.-M. Liu, Q. Zhao, W.-C. Song and X.-H. Bu, Chem.–Eur. J., 2012, 18, 2806; (d) L. Xiong, J. Feng, R. Hu, S. Wang, S. Li, Y. Li and G. Yang, Anal. Chem., 2013, 85, 4113; (e) Y. Yang, Q. Zhao, W. Feng and F. Li, Chem. Rev., 2013, 113, 192; (f) P. Hou, S. Chen, H. Wang, J. Wang, K. Voitchovsky and X. Song, Chem. Commun., 2014, 50, 320; (g) J. Du, M. Hu, J. Fan and X. Peng, Chem. Soc. Rev., 2012, 41, 4511; (h) B. Ke, W. Chen, N. Ni, Y. Cheng, C. Dai, H. Dinh and B. Wang, Chem. Commun., 2013, 49, 2494; (i) C. Saravanan, S. Easwaramoorthi, C.-Y. Hsiow, K. Wang, M. Hayashi and L. Wang, Org. Lett., 2014, 16, 354; (j) D. Saravanakumar, S. Devaraj, S. Iyyampillai, K. Mohandoss and M. Kandaswamy, Tetrahedron Lett., 2008, 49, 127.
14. Z. Li, X. Liu, W. Zhao, S. Wang, W. Zhou, L. Wei and M. Yu, Anal. Chem., 2014, 86, 2521.
15. (a) M. Kumar, N. Kumar and V. Bhalla, Chem. Commun., 2013, 49, 877; (b) M. H. Lee, B. Yoon, J. S. Kim and J. L. Sessler, Chem. Sci., 2013, 4, 4121; (c) T. Liu, Z. Xu, D. R. Spring and J. Cui, Org. Lett., 2013, 15, 2310.
16. (a) P. Mahato, S. Saha, E. Suresh, R. Di Liddo, P. P. Parnigotto, M. T. Conconi, M. K. Kesharwani, B. Ganguly and A. Das, Inorg. Chem., 2012, 51, 1769; (b) M. H. Lee, J. H. Han, P.-S. Kwon, S. Bhuniya, J. Y. Kim, J. L. Sessler, C. Kang and J. S. Kim, J. Am. Chem. Soc., 2012, 134, 1316; (c) P. Wang, J. Liu, X. Lv, Y. Liu, Y. Zhao and W. Guo, Org. Lett., 2012, 14, 520; (d) J. Shi, Y. Wang, X. Tang, W. Liu, H. Jiang, W. Dou and W. Liu, Dyes Pigm., 2014, 100, 255.
17. (a) X. Zhou, F. Su, H. Lu, P. Senechal-Willis, Y. Tian, R. H. Johnson and D. R. Meldrum, Biomaterials, 2012, 33, 171; (b) M. H. Lee, N. Park, C. Yi, J. H. Han, J. H. Hong, K. P. Kim, D. H. Kang, J. L. Sessler, C. Kang and J. S. Kim, J. Am. Chem. Soc., 2014, 136, 14136.
18. (a) X. Zheng, W. Zhu, D. Liu, H. Ai, Y. Huang and Z. Lu, ACS Appl. Mater. Interfaces, 2014, 6, 7996; (b) R. Jun, W. Zhen, Z. Ying, L. Yan and X. Zuxun, Dyes Pigm., 2011, 91, 442; (c) A. Bamesberger, C. Schwartz, Q. Song, W. Han, Z. Wang and H. Cao, New J. Chem., 2014, 38, 884; (d) J. F. Zhang, C. S. Lim, S. Bhuniya, B. R. Cho and J. S. Kim, Org. Lett., 2011, 13, 1190; (e) Q. Song, A. Bamesberger, L. Yang, H. Houtwed and H. Cao, Analyst, 2014, 139, 3588.
19. (a) R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger and T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936; (b) H. S. Jung, X. Chen, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2013, 42, 6019; (c) M. H. Lee, B. Yoon, J. S. Kim and J. L. Sessler, Chem. Sci., 2013, 4, 4121; (d) B. H. Shankar, D. T. Jayaram and D. Ramaiah, Chem.–Eur. J., 2014, 9, 1636.
20. Y. Zhou, J. F. Zhang and J. Yoon, Chem. Rev., 2014, 114, 5511.
21. Y. Liu, H. Li, M. Pei, G. Zhang, L. Hu and J. Han, Talanta, 2013, 115, 190; D. Yu, F. Huang, S. Ding and G. Feng, Anal. Chem., 2014, 86, 8835.
22. (a) Z. Li, M. Yu, L. Zhang, M. Yu, J. Liu, L. Wei and H. Zhang, Chem. Commun., 2010, 46, 7169; (b) H. Tian, J. Gan, K. C. Chen, J. He, Q. L. Song and X. Y. Hou, J. Mater. Chem., 2002, 12, 1262; (c) J. Zhou, H. Liu, B. Jin, X. Liu, H. Fu and D. Shangguan, J. Mater. Chem. C, 2013, 1, 4427.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00596e

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