A new optical sensor for Al³⁺/Fe³⁺ based on PET and chelation-enhanced fluorescence

Abstract

A simple probe (compound 1) sensing Al³⁺ or Fe³⁺ by suppression of photoinduced electron/energy transfer (PET) process and chelation-enhanced fluorescence is described. Compound 1 was prepared from 2-(bromomethyl)benzo[d]thiazole and pyridine-2,6-diamine. Compound 1 showed a much stronger fluorescence response to Al(III) than to Fe(III). The best fluorescence response of 1 to Al³⁺ was found to be at pH near 7. The results indicated that 1 could be a promising fluorescent turn-on chemosensor for Al³⁺ or Fe³⁺. Ratiometric sensing of 1 to Al³⁺ or Fe³⁺ was accomplished by plotting the absorbance ratio at 338 to 310 nm versus Al³⁺ or Fe³⁺ concentration. The fluorescent turn-on response of 1 to Al³⁺ or Fe³⁺ can be attributed to suppression of PET process and chelation-enhanced fluorescence upon binding with Al³⁺ or Fe³⁺. The binding mode of 1-Al³⁺ or 1-Fe³⁺ was found to be a 1:1 complex based on Job’s plot, electrospray ionization (ESI) mass spectroscopy (MS) data, and density functional theory (DFT) calculation results.

Introduction

Aluminum, the third most abundant element after oxygen and silicon in the Earth’s crust [1], is distributed ubiquitously in our daily life. Aluminum-containing derivatives are widely used in food additives, water treatment, and medicine [2]. However, overloading with aluminum will lead to malfunction of the human central nervous system [3]. Ferric ion is the most familiar metal ion, being found as an essential element in myoglobin, hemoglobin, and cytochromes, and playing an indispensable role in many biological processes such as cellular metabolism, gene regulation, electron transfer, and regulation of cell growth and differentiation [4–6]. Nonetheless, high concentration of ferric ion in organisms or the environment can induce biological disorders such as anemia, liver and kidney damage, heart failure, and diabetes, and cause serious environmental pollution [7, 8]. Therefore, it is important to develop efficient methods for detection and quantification of Al³⁺ or Fe³⁺ for food safety and quality control, clinical, and environmental applications. Among the various methods, fluorescence spectroscopy has become a powerful tool for sensing trace amounts of samples because of its simplicity, sensitivity, and fast response time [9–14]. Therefore, it is crucial to develop suitable fluorescent probes for Al³⁺ or Fe³⁺.

Many kinds of signaling mechanisms have been proposed and applied in fluorescent detection of Al³⁺ or Fe³⁺, including photoinduced electron/energy transfer (PET) [15–19], intramolecular charge transfer (ICT) [20–22], fluorescence resonance energy transfer (FRET) [23–25], excited-state intramolecular proton transfer [26], spirolactam to ring-open equilibrium of rhodamine derivatives [27–29], chelation-enhanced fluorescence [30–32], inhibiting nonradiative process by complexation of metal cations to C=N group [19, 32], etc. It is generally recognized that one probe with a fluorescent turn-on signal for specific species is more efficient [33, 34] than one with fluorescent quenching due to high signal-to-noise ratio. As the paramagnetic nature of Fe³⁺ always leads to a quenching ability, fluorescence enhancement through chelation of Fe³⁺ with a probe is a challenging research topic in vitro as well as in vivo. Although several “turn-on” probes that work with selective response to Al³⁺ or Fe³⁺ have been reported [35–38], their poor solubility in aqueous solution and complicated synthetic processes inhibit practical applications. New fluorescent sensors with simple synthetic processes and improved solubility in water have been developed in our group [39, 40]. In this work, a simple probe for Al³⁺ or Fe³⁺ with fluorescent enhancement was designed and synthesized. The nitrogen and sulfur atoms in 1 are well-known acceptor sites for metal ions. If probe 1 is coordinated with a certain metal ion, its molecular skeleton will be fixed with rigidity. Chelation of Al³⁺ or Fe³⁺ with probe 1 will result in fluorescent enhancement and shifts in absorption bands.

Experimental

All chemicals of analytical grade for syntheses were purchased from commercial suppliers and used without further purification. Tetrahydrofuran (THF) of spectroscopic grade and deionized water were used throughout the spectroscopic experiments as solvents. Nuclear magnetic resonance (NMR) spectra were recorded with a 300-MHz Varian spectrometer. Electrospray ionization mass spectra (ESI–MS) were measured on an LC-MSD-Trap-SL instrument. Fluorescence spectra were measured with a Cary Eclipse fluorescence spectrometer. UV–Vis absorption spectra were obtained on a UV-5200PC UV/vis spectrophotometer.

Stock solution of 1 (30 μM) in THF/H₂O (9/1, v/v) was prepared for spectroscopic investigations. Stock solutions of metal ions (1 mM) were prepared in deionized water from nitrates or chlorides of Na⁺ (alkaline metal ion), Mg²⁺, Ca²⁺ (alkaline earth metal ions), Al³⁺ (group IIIA metal ion), Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ (first-row transition-metal ions), and Cd²⁺, Hg²⁺, Pb²⁺ (toxic heavy-metal ions) of analytical grade. In titration experiments, 3 mL stock solution of 1 was put into a quartz optical cell with optical path of 1 cm. Stock solution of each metal ion was added into the quartz optical cell step by step via a microsyringe, and the solution was stirred for 5 min before recording spectra. For fluorescence measurements, the excitation wavelength was provided at 310 nm, and emission was collected from 320 to 550 nm. The excitation slit and emission slit were both set at 5 nm.

The synthetic process of 1 is shown in Scheme 1. The intermediate 2 was synthesized according to literature [41].

Scheme 1

Synthetic process of 1

Synthesis of 1

To a mixture of 2,6-diaminopyridine (109 mg, 1 mmol) and anhydrous K₂CO₃ (303 mg, 2.2 mmol) in CH₃CN (15 mL), a solution of 2-(bromomethyl)benzothiazole 2 (510 mg, 2.23 mmol) in 15 mL THF/CH₃CN (4/1, v/v) was added dropwise within 20 min. The mixture was stirred at room temperature for 72 h. Then, the solvents were removed under reduced pressure. The residue was dissolved in dichloromethane (40 mL) and washed three times with water. The organic phase was dried over anhydrous sodium sulfate. After removing the solvents, the crude product was subjected to column chromatography on silica gel with petroleum ether/ethyl acetate (1/1, v/v) to afford an off-white solid (110 mg, 43.2 %). ¹H NMR (CDCl₃) δ (ppm): 4.24 (s, 2H), 4.90 (d, J = 6.0 Hz, 2H), 5.16 (t, J = 6.0 Hz, 1H), 5.85–5.90 (m, 2H), 7.19–7.25 (m, 1H), 7.31–7.37 (m, 1H), 7.42–7.48 (m, 1H), 7.83 (t, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H) (Fig. S1, Electronic Supplementary Material, ESM). ¹³C NMR (CDCl₃), δ (ppm): 44.76, 96.54, 97.90, 121.72, 122.67, 124.83, 125.91, 135.14, 139.55, 153.29, 157.00, 157.54, 172.70 (Fig. S2, ESM). MS (ESI–MS): m/z calculated for C₁₃H₁₂N₄S, 256.33 [M]⁺. Found: 256.2, 257.2 [M + H]⁺ (Fig. S3, ESM).

Results and discussion

The response of fluorescence spectra to Al³⁺ or Fe³⁺

The response of 1 to various metal ions, such as Na⁺ (alkaline metal ion), Mg²⁺, Ca²⁺ (alkaline earth metal ions), Al³⁺ (group IIIA metal ion), Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ (first-row transition-metal ions), and Cd²⁺, Hg²⁺, Pb²⁺ (toxic heavy-metal ions) was investigated firstly by fluorescence spectra (Fig. 1). It could be seen that 1 (30 μmol/L) exhibited no fluorescence in THF/H₂O (9/1, v/v) solution. The intramolecular photoinduced electron transfer (PET) effect from the lone pair of electrons of the N atom on –NH– of pyridine group to the adjacent benzo[d]thiazole would result in no fluorescence of 1. However, addition of Al³⁺ (30 μmol/L) caused an outstanding fluorescence enhancement centered at 393 nm. Addition of Fe³⁺ (30 μmol/L) caused a moderate fluorescence enhancement centered at 393 nm. Addition of Cu²⁺ (30 μmol/L) increased the fluorescence intensity of 1 very slightly. The fluorescence intensity of 1 at 393 nm was not increased after addition of 30 μmol/L Na⁺, Ca²⁺, Mg²⁺, Co²⁺, Mn²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺ or Hg²⁺, respectively. This result indicates that compound 1 shows superior selectivity towards Al³⁺ over Fe³⁺ and that compound 1 could be used as a turn-on fluorescence sensor for Al³⁺ or Fe³⁺. The time dependence of the response of 1 to Al³⁺ or Fe³⁺ was investigated by recording the fluorescence intensity at 393 nm over time (Fig. S4, ESM). The results revealed that the reaction of compound 1 (30 μM) and Al³⁺ (100 μM) or Fe³⁺ (100 μM) was completed within 5 min. Therefore, all measurements were performed after mixing and stirring for 5 min.

Fig. 1

Fluorescence spectra of 1 (30 μmol/L) in the presence of different metal ions (30 μmol/L) in THF/H₂O (9:1, v/v)

The fluorescence spectra of 1 upon titration of Al³⁺ or Fe³⁺ are shown in Fig. 2a and b, respectively. A new emission band centered at 393 nm appeared, and the fluorescence was gradually enhanced upon gradual addition of Al³⁺ or Fe³⁺. The maximum fluorescent enhancement (67-fold and 28-fold for Al³⁺ and Fe³⁺, respectively) was observed at 393 nm when the amounts of metal ions and 1 were equal. The fluorescence intensity at 393 nm and the concentration of Al³⁺ or Fe³⁺ showed good linear relationship in the range from 2 to 40 μM (inset in Fig. 2a, b, respectively). The detection limit of 1 for Al³⁺ and Fe³⁺ was determined to be 29 and 46 nM (3σ/slope), respectively. The fluorescence enhancement for 1 in the presence of Al³⁺ or Fe³⁺ could be ascribed to the fixing in the molecular skeleton increasing the rigidity of 1 and decreasing the PET effect of the N atom on –NH– of pyridine group, after it was coordinated with Al³⁺ or Fe³⁺.

Fig. 2

a Fluorescence spectra of 1 (30 μmol/L) upon addition of Al³⁺ (inset fluorescence intensity at 393 nm of 1 as a function of Al³⁺ concentration). b Fluorescence spectra of 1 (30 μmol/L) upon addition of Fe³⁺ (inset fluorescence intensity at 393 nm of 1 as a function of Fe³⁺ concentration)

Job’s plots were used to determine the stoichiometries between 1 and Al³⁺ as well as 1 and Fe³⁺. It can be seen in Fig. 3 that the fluorescent maxima of 1 are both at molar fraction of 0.5 for Al³⁺ or Fe³⁺, indicating that the stoichiometries between 1 and Al³⁺ as well as 1 and Fe³⁺ are both 1:1.

Fig. 3

Job’s plot evaluated from the fluorescence spectra of 1 and Al³⁺ a or Fe³⁺ b at 393 nm in THF/H₂O (9:1, v/v)

As several kinds of metal ions often coexist in one system, coexisting metal ions may interfere with the detection of target ions. To determine the influence of other cations on the fluorescent detection of Al³⁺ or Fe³⁺ in THF/H₂O (9/1, v/v) solution, 300 μmol/L of metal ions such as Na⁺, Hg²⁺, Mg²⁺, Mn²⁺, Co²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Zn²⁺, and Ni²⁺ were added to the solution of 1-Al³⁺ or 1-Fe³⁺, respectively. As shown in Fig. 4a, b, the fluorescence intensity of 1-Al³⁺ or 1-Fe³⁺ was almost unaffected by the presence of the other metal ions. Although Cd²⁺ caused a slight decrease in the fluorescence intensity of 1-Al³⁺ or 1-Fe³⁺, this was negligible compared with the enhancement of the fluorescence upon addition of Al³⁺ or Fe³⁺. Based on these results, it was inferred that 1 could be a potential turn-on fluorescent probe for Al³⁺ or Fe³⁺ with high selectivity and high sensitivity in THF/H₂O (9/1, v/v) solution.

Fig. 4

Fluorescence responses of 1 (30 μmol/L) to Al³⁺ (30 μmol/L) a and Fe³⁺ (30 μmol/L) b in presence of other metal ions. F₀: fluorescence intensity of 1 containing Al³⁺ or Fe³⁺ without addition of competing ions, F: fluorescence intensity of 1 containing Al³⁺ or Fe³⁺ after addition of competing ions (300 μmol/L)

UV–Vis absorption responses of 1 to Al³⁺ or Fe³⁺

The absorption spectral properties of 1 upon titration of Al³⁺ or Fe³⁺ were studied in THF/H₂O (9/1, v/v). As shown in Fig. 5a, the absorption spectrum of 1 displayed two absorption bands at 225 and 310 nm. Upon addition of Al³⁺, the absorption intensity at 310 nm decreased, and at the same time a new absorption peak at 338 nm appeared and the absorbance increased with increasing concentration of Al³⁺. The 28-nm redshift could be explained by the fact that coordination between 1 and Al³⁺ inhibited rotation of the σ-bond between the pyridine and benzothiazole groups and increased the rigidity of the molecular skeleton. In addition, charge transfer between the electron-rich ligand and electron-deficient metal ions also increased the conjugation property of the molecule. The increase in the rigidity and the conjugation property of 1-Al³⁺ turned on its fluorescence. A concomitant isosbestic absorption point at 315 nm indicated that only one intermediate complex was formed during the titration process. In view of the fluorescent Job’s plot results, it could be concluded that only one 1:1 complex was formed in the 1-Al³⁺ system.

Fig. 5

a Absorption spectra of 1 (30 μmol/L) upon addition of Al³⁺ in THF/H₂O (9:1, v/v) and b Benesi–Hildebrand plot (absorbance at 338 nm) of 1 between 1/(A − A ₀) and 1/[Al³⁺]

The association constant of the complex 1-Al³⁺ was calculated by the Benesi–Hildebrand expression (Eq. 1) [42].

$$\frac{1}{{A - A_{0} }} = \frac{1}{{K_{\text{a}} \left( {A_{\hbox{max} } - A_{0} } \right)[\text{Al}^{3 + } ]}} + \frac{1}{{A_{\hbox{max} } - A_{0} }},$$

(1)

where A ₀ is the absorbance of 1 at 338 nm in the absence of Al³⁺, A is the absorbance of 1 at 338 nm at a certain concentration of Al³⁺, A _max is the absorbance of 1 at a complete-interaction concentration of Al³⁺, K _a is the association constant, and [Al³⁺] is the concentration of Al³⁺. On the basis of the plot of 1/(A − A ₀) versus 1/[Al³⁺], the association constant was determined from the slope to be 2.99 × 10⁴ M⁻¹ (Fig. 5b).

The ratio between the absorbance at 338 nm and that at 310 nm of 1 increased linearly with increasing Al³⁺ concentration in the range of 6.67 × 10⁻⁶–1.60 × 10⁻⁴ mol/L (Fig. 6). This result indicated that 1 showed high colorimetric selectivity to Al³⁺ and might be a promising colorimetric sensor for Al³⁺.

Fig. 6

Ratio between absorbance at 310 and at 338 nm of 1 upon titration of Al³⁺

A similar experimental result was obtained for the titration of Fe³⁺ (Fig. S5, ESM). Upon titration of Fe³⁺, the new absorption peak and the concomitant isosbestic absorption point also appeared at 338 and 315 nm, respectively, indicating that only one intermediate complex was formed during the titration process. In view of the results of Job’s plot, it could be concluded that only one 1:1 complex was formed during the titration of Fe³⁺ into the solution of 1. The association constant of the complex between 1 and Fe³⁺ was also calculated by the Benesi–Hildebrand expression (Eq. 1) (as shown in Fig. S6, ESM) to be 6.86 × 10³ M⁻¹. The ratio between the absorbance at 338 and 310 nm of 1 increased linearly with increasing Fe³⁺ concentration in the range of 0–1.60 × 10⁻⁴ mol/L (Fig. S7, ESM). This result indicated that 1 also showed high colorimetric selectivity to Fe³⁺ and might be a promising colorimetric sensor for Fe³⁺.

The positions of the response wavelengths were the same for 1 toward Al³⁺ and Fe³⁺. Under 365-nm lamp irradiation, the solutions of 1-Al³⁺ and 1-Fe³⁺ turned blue, while the solutions of 1 in the presence of other ions did not change at all (Fig. 7). It could still be found that the solution of 1 displayed a light-yellow color upon addition of Fe³⁺ because of the intrinsic color of Fe³⁺ (Fig. S8, ESM), and the solution of 1 exhibited no color upon addition of Al³⁺ as well as the other ions. Therefore, it is easy to distinguish the solution of Fe³⁺ from Al³⁺ in combination with the fluorescent spectra measurement. The paramagnetic nature of Fe³⁺ might quench the fluorescence of 1-Fe³⁺ to some degree. The lower association constant of 1-Fe³⁺ than that of 1-Al³⁺ could also contribute to the superior Al³⁺ compared with Fe³⁺ sensitivity.

Fig. 7

Photograph of 1 (50 μmol/L) in THF/H₂O (v/v, 9:1) solution with 10 equiv metal ions under irradiation by 365-nm lamp

pH effect on fluorescence of 1 and 1-Al³⁺ complex

To determine a suitable pH range for 1 as a chemosensor for Al³⁺, the fluorescence spectra of 1 with and without Al³⁺ in different pH solutions were investigated. As shown in Fig. 8, 1 revealed no fluorescence at 393 nm between pH 2.6 and 11.1. After addition of Al³⁺, the fluorescence of 1-Al³⁺ solution was far stronger than that of 1 with pH value less than 9. It is well known that Al³⁺ will precipitate as Al(OH)₃ in weak basic solution and change to AlO ⁻₂ in strong basic solution. These results indicate that 1 can be applied for monitoring Al³⁺ fluorimetrically over a wide pH range.

Fig. 8

Fluorescence intensities (393 nm) of 1 in THF/H₂O (30 μM) (9:1, v/v) with and without Al³⁺ (2 equiv) as a function of pH. 1-Al³⁺ (circle) and 1 (square)

Sensing mechanism

According to the absorption and emission properties of 1 with and without Al³⁺ or Fe³⁺, the binding mechanism of 1 with Al³⁺ or Fe³⁺ was assumed. The fluorescence of the benzo[d]thiazole moiety was quenched by the intramolecular PET effect from the lone pair of electrons of the N atom on 2′ position of pyridine group to the adjacent benzo[d]thiazole. This PET effect would result in no fluorescence of 1. After it was bound with Al³⁺ or Fe³⁺, the lone pair of electrons on the nitrogen of 1 was stabilized, and intramolecular PET to the adjacent benzo[d]thiazole group upon photoexcitation of 1 was thermodynamically unfavorable, resulting in the significant fluorescence enhancement (Scheme 2). Evidence was obtained by carrying out ESI mass analysis of 1-Al³⁺. 1 exhibited a peak at m/z = 256.2 (calcd. 256.3 for C₁₃H₁₂N₄S), which corresponded to [1]⁺ as shown in Fig. S3 (ESM). Several new peaks were clearly observed after AlCl₃ (60 μmol) was added to the solution of 1 (30 μmol), as shown in Fig. S9 (ESM), in which m/z = 318.8 corresponds to [1 + Al + Cl]⁺ with chemical formula C₁₃H₁₂AlClN₄S (calcd. 318.7) and m/z = 301.2 corresponds to [1 + Al + H₂O] with formula C₁₃H₁₄AlN₄OS (calcd. 301.1).

Scheme 2

Sensing mechanism of 1 towards metal ions

To further understand the recognition process of 1 toward Al³⁺, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions of 1 and 1 + Al³⁺ were determined by density functional theory (DFT) calculations, in which 6–31 g(d) basic set was used for C, H, N, S, and Cl, and lanl2dz was used for Al [43]. As shown in Fig. 9, the HOMO distribution of 1 was concentrated exclusively on the amino pyridine moieties without contribution from benzothiazole group. The LUMO distribution of 1 was concentrated on the benzothiazole group without contribution from pyridine group. The dihedral angle between pyridine and benzothiazole groups was about 88.2°. Although the HOMO distribution of 1 + Al³⁺ was still concentrated mainly on the amino pyridine moieties, the contribution of benzothiazole group and Al³⁺ was very obvious. The LUMO distribution of 1 + Al³⁺ was also mainly concentrated on the benzothiazole group, whereas the contribution of pyridine group and Al³⁺ was also very obvious. The dihedral angle between pyridine and benzothiazole groups changed to about 70.9°, indicating that the structural rigidity was promoted after 1 was bound with Al³⁺. This effect made the excited state of 1 + Al³⁺ radiating. The calculation results were coincident with our experimental results.

Fig. 9

Distributions of HOMO and LUMO of 1 and 1 + Al³⁺ determined by DFT calculations

Conclusions

A simple, sensitive “turn–on” chemosensor 1 for detection of Al³⁺ or Fe³⁺ was synthesized and characterized. The new sensor displayed ratiometric and colorimetric response toward Al³⁺ or Fe³⁺. It exhibited fluorescence enhancement with good sensitivity in the presence of Al³⁺ or Fe³⁺ without significant interference from other metal ions such as Na⁺, Hg²⁺, Mg²⁺, Mn²⁺, Co²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Zn²⁺, and Ni²⁺. The sensor could be applied for monitoring Al³⁺ or Fe³⁺ within a wide pH range. The coordination between 1 and Al³⁺ or Fe³⁺ increased the rigidity of the molecular skeleton and suppressed the PET process, resulting in fluorescence enhancement. The results indicated 1 to be a promising “turn-on” optical sensor for Al³⁺ or Fe³⁺.