Interaction of soft donor sites with a hard metal ion: crystallographically characterized blue emitting fluorescent probe for Al(III) with cell staining studies

Abstract

A naphthalene based compound, 1-((E)-(2-(2-(phenylthio)ethylthio)phenylimino)methyl)naphthalen-2-ol (L₂) has been synthesized and characterized by FTIR, ¹H NMR, mass spectra and single crystal X-ray structure analysis. L₂ shows a blue shift with a large fluorescence enhancement in the presence of Al³⁺ which is attributed to a chelation-enhanced fluorescence (CHEF) effect with inhibition of an intramolecular charge transfer (ICT) process and cis/trans isomerization. L₂ binds to Al³⁺ with 2 : 1 stoichiometry (mole ratio) with an association constant (K_a) of 0.15 × 10⁴ M^−1/2. The detection limit for Al³⁺ is 5 × 10⁻⁸ M in DMSO/H₂O (1 : 2, v/v). L₂ can efficiently detect intracellular Al³⁺ under fluorescence microscope

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Introduction

Despite being a non-essential element, the detection of aluminum is of great interest due to its potential toxicity and widespread application in automobiles, computers, packaging materials, electrical equipment, machinery food additives, clinical drugs, water purification and building construction.^1,2 Furthermore, it is well known that 40% of soil acidity is due to aluminum toxicity.³ Aluminum leaching from soil by acid rain is deadly to growing plants.⁴ The World Health Organization (WHO) prescribed the average human intake of aluminum as around 3–10 mg day⁻¹ with a weekly dietary intake of 7 mg kg⁻¹ body weight.⁵ Aluminum toxicity damages the central nervous system, and it is suspected of playing a role in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. It is also responsible for intoxication in hemodialysis patients.⁶ A high content of aluminum in the body can do harm to the brains and kidneys.^7,8 Fluorescence methods have advantages over other sophisticated methods such as atomic absorption or inductively coupled plasma mass/atomic emission spectrometric methods due to its operational simplicity, high sensitivity, rapidity, nondestructive methodology, direct visual perception and inexpensive operational cost.⁹ Most of the reported Al³⁺ fluorescent sensors^10–20 have either limited selectivity/sensitivity or require tedious synthetic protocols. A short fluorescence lifetime,²¹ low fluorescence quantum yield²² and the dual donor–acceptor²³ properties of the naphthalene moiety make it an ideal fluorophore. Optimization of host–guest interactions by controlling the stereoelectronic factors enhances the selectivity over competing analytes. Moreover, the strong binding of a receptor to an analyte may lead to a loss of selectivity. Interaction of soft donor sites of a receptor with a hard metal ion enhances its selectivity over competing guests.²⁴ Recently, we have been engaged in the development of Al³⁺ selective “turn-on” fluorescent probes^18–20 with better and improved properties. Herein, we report a very simple turn-on fluorescent probe (L₂) having soft donor sites (sulphur) for hard metal ions (Al³⁺), the structure of which has been confirmed by the single crystal X-ray analysis.

Experimental

Materials

2-Aminothiophenol (Aldrich, USA), 2-hydroxynaphthaldehyde (Alfa Aesar, Germany), 2-chloroethyl phenyl sulfide (Aldrich, USA), MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide from Aldrich, USA) and Al(NO₃)₃·9H₂O (Merck, India) were used as received. All other chemicals and solvents were of analytical grade and used without further purification. Solvents used are of spectroscopic grade. Mili-Q Milipore® 18.2 MΩ cm⁻¹ water was used throughout all the experiments.

Apparatus

¹HNMR spectra were recorded in DMSO-d₆ with a Bruker Advance 600 MHz and 300 MHz NMR spectrometer using TMS as an internal standard. Absorption and fluorescence spectra were recorded on a Shimadzu Multi Spec 1501 absorption spectrophotometer and Hitachi F-4500 fluorescence spectrophotometer, respectively. Mass spectra were recorded with a QTOF Micro YA 263 mass spectrometer in ESI positive mode. Fourier transform infrared (FTIR) spectra were recorded on a JASCO FTIR spectrophotometer (model: FTIR-H20). X-Ray crystal data were collected by using a STOE IPDS-II two-circle diffractometer. Intensities were corrected for Lorentz polarization and for absorption. The structure was solved by direct method. Hydrogen atoms bound to carbon were idealized. Structural refinements were obtained with full-matrix least-squares based on F² using the program SHELXL.²⁵ The fluorescence imaging system was comprised of an inverted fluorescence microscope (Leica DM 1000 LED), digital compact camera (Leica DFC 420C), and an image processor (Leica Application Suite v3.3.0). The microscope was equipped with a mercury 50 W lamp.

Synthesis of 2-(2-(phenylthio)ethylthio)benzenamine (L₁) (Scheme 1)

Na metal (200 mg, 8.6 mmol) was dissolved in 10 mL ice cold ethanol under stirring to generate sodium ethoxide. Then, 2-aminothiophenol (250.38 mg, 2 mmol) was added slowly to the solution of sodium ethoxide with continuous stirring at room temperature. After 30 min, 2-chloroethylphenylsulfide (345.36 mg, 2 mmol) was added slowly with continuous stirring for another 3 h. Removal of solvent was followed by solvent extraction (water/ethyl acetate). The organic layer was purified by column chromatography (ethyl acetate/hexane, 1 : 1, v/v) to obtain the target compound (L₁). Yield 82%. QTOF–MS ES⁺: [M + Na]⁺ = 284.10 (100%) (Fig. S1, ESI,†), ¹HNMR (300 MHz, DMSO-d₆) (Fig. S2 and Fig. S3, ESI†): δ (ppm); 7.210–7.194 (d, 2H, ArH), 7.173 (s, 1H, ArH), 7.128–7.123 (d, 2H, ArH), 7.113–7.082 (m, 1H, ArH), 7.039–7.010 (t, 1H, ArH), 6.737–6.706 (d, 1H, ArH), 6.514–6.461 (t, 1H, ArH), 5.330 (s, 1H, –NH₂) 3.030–2.980 (m, 2H, –CH₂–), 2.854–2.802 (m, 2H, –CH₂–); IR (cm⁻¹) (Fig. S4, ESI†): ν(NH, bending) 1606.12, ν(C=N), 1579.41. Elemental analysis as calculated for C₁₄H₁₅NS₂ (%): C, 64.33; H, 5.78; N, 5.36. Found (%): C,64.42; H,5.84; N, 5.32.

Scheme 1 Synthesis of L₁ and L₂.

Synthesis of 1-((E)-(2-(2-(phenylthio)ethylthio)phenylimino)methyl)naphthalen-2-ol (L₂) (Scheme 1)

Methanol solutions of 2-hydroxy naphthaldehyde (172.18 mg, 1 mmol) and L₁ (261.41 mg, 1 mmol) were mixed slowly followed by reflux for 4 h. The reaction mixture was kept overnight at room temperature to get pure crystalline L₂. Yield 86%; ¹HNMR (600 MHz, DMSO-d₆) (Fig. S5 and S6, ESI†): δ (ppm), 15.432 (s, 1H, OH), 9.666 (s, 1H, –CH=N), 8.546–8.518 (d, 1H, ArH), 7.995–7.965 (d, 1H, ArH), 7.876–7.823 (t, 2H, ArH), 7.596–7.545 (t, 1H, ArH), 7.430–7.361 (m, 4H, ArH), 7.287–7.274 (m, 5H, ArH), 7.210–7.183 (m, 1H, ArH), 3.172 (s, 4H, –CH₂–); QTOF–MS ES⁺ (Fig. S7, ESI†): [M + Na]⁺ = 438.10 (100%); IR (cm⁻¹) (Fig. S8, ESI†): ν(OH) 3426.89, ν(C=N) 1579.41. Elemental analysis as calculated for C₂₅H₂₁NOS₂ (%): C, 72.25; H, 5.09; N, 3.37. Found (%): C, 72.03; H, 5.13; N, 3.28.

Results and discussion

Fig. 1 shows the single-crystal X-ray structure of L₂. The crystal packing diagram of L₂ (Fig. S9, ESI†) indicates strong π–π stacking interactions between two adjacent L₂ molecules. Detailed structural parameters of L₂ are presented in Table 1 and Table 2.

Fig. 1 ORTEP diagram of L₂ (50% thermal ellipsoid).

Table 1 Refinement parameters for L₂

Crystal parameters	Ligand (L2)
Molecular formula	C25H21NOS2
Formula weight	415.57
Crystal system	Monoclinic
Space group	P21/c
a/Å	20.480(3)
b/Å	7.4814 (17)
c/Å	13.808 (3)
α/°	90
β/°	108.579 (3)
γ/°	90
Z	4
V/cm^3	2003.8(7)
T/K	100(2)
λ/Å	0.7107
ρ/g cm^−3	1.378
μ/mm^−1	0.283
Crystal size/mm	4 0.43 × 0.25 × 0.07
h/k/l	–25 → 24/0 → 9/0 → 17
F(000)	872
θ range/°	2.9–24.6
GOF	0.97
T min and Tmax	0.836 and 1
Reflection collected	2828
Independent reflection	2828
Data/restraints/parameters	4109/0/265
R factor (all data)	0.108
R factor (final)	0.046
Largest peak and hole/e Å^−3	0.34 and −0.33

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Table 2 Geometric parameters [BOND LENGTHS (Å) & BOND ANGLES (⁰)] of L₂

BOND LENGTH (Å)	BOND ANGLES (°)	BOND ANGLES (°)
C1—O1 1.343 (3)	O1—C1—C10 122.8 (2)	C1—C10—C11 119.8 (2)
C15—C16 1.389 (3)	C16—C15—H15 119.9	C23—C24—H24 120.2
C1—C10 1.391 (3)	O1—C1—C2 115.8 (2)	C1—C10—C9 118.9 (2)
C15—H15 0.95	C17—C16—C15 120.6 (2)	C25—C24—H24 120.2
C1—C2 1.410 (3)	C10—C1—C2 121.4 (2)	C11—C10—C9 121.2 (2)
C16—C17 1.384 (3)	C17—C16—H16 119.7	C26—C25—C24 120.6 (2)
O1—H1W 0.87 (3)	C1—O1—H1W 105.6 (18)	O1—C1—C10 122.8 (2)
C16—H16 0.95	C15—C16—H16 119.7	C16—C15—H15 119.9
C2—C3 1.358 (3)	C3—C2—C1 119.9 (2)	O1—C1—C2 115.8 (2)
C17—C18 1.391 (3)	C16—C17—C18 119.6 (2)	C17—C16—C15 120.6 (2)
C2—H2 0.95	C3—C2—H2 120.1	C10—C1—C2 121.4 (2)
C17—H17 0.95	C16—C17—H17 120.2	C17—C16—H16 119.7
C3—C4 1.415 (3)	C1—C2—H2 120.1	C1—O1—H1W 105.6 (18)
C18—S19 1.767 (2)	C18—C17—H17 120.2	C15—C16—H16 119.7
	C2—C3—C4 121.5 (2)	C3—C2—C1 119.9 (2)
	C17—C18—C13 119.8 (2)	C16—C17—C18 119.6 (2)
C3—H3 0.95	C2—C3—H3 119.3	C1—C10—C11 119.8 (2)
S19—C20 1.803 (2)	C17—C18—S19 126.00 (19)	C23—C24—H24 120.2
C4—C5 1.414 (3)	C4—C3—H3 119.3	C1—C10—C9 118.9 (2)
C20—C21 1.518 (3)	C13—C18—S19 114.11 (18)	C25—C24—H24 120.2
C4—C9 1.418 (3)	C5—C4—C3 120.6 (2)	C11—C10—C9 121.2 (2)
C20—H20A 0.99	C18—S19—C20 104.75 (12)	C26—C25—C24 120.6 (2)
C5—C6 1.369 (4)	C5—C4—C9 119.8 (2)	O1—C1—C10 122.8 (2)
C20—H20B 0.99	C21—C20—S19 112.75 (18)	C16—C15—H15 119.9
C5—H5 0.95	C3—C4—C9 119.6 (2)	O1—C1—C2 115.8 (2)
C21—S22 1.807 (2)	C21—C20—H20A 109	C17—C16—C15 120.6 (2)
C6—C7 1.397 (4)	C6—C5—C4 120.9 (2)	C10—C1—C2 121.4 (2)
C21—H21A 0.99	S19—C20—H20A 109	C17—C16—H16 119.7
C6—H6 0.95	C6—C5—H5 119.6	C1—O1—H1W 105.6 (18)
C21—H21B 0.99	C21—C20—H20B 109	C15—C16—H16 119.7
C7—C8 1.361 (3)	C4—C5—H5 119.6	—
S22—C23 1.764 (2)	S19—C20—H20B 109
C7—H7 0.95	C5—C6—C7 119.1 (2)	—
C23—C24 1.387 (3)	H20A—C20—H20B 107.8
C8—C9 1.411 (3)	C5—C6—H6 120.5	—
C23—C28 1.396 (3)	C20—C21—S22 112.38 (17)
C8—H8 0.95	C7—C6—H6 120.5	—
C24—C25 1.395 (4)	C20—C21—H21A 109.1
C9—C10 1.444 (3)	C8—C7—C6 121.4 (2)	—
C24—H24 0.95	S22—C21—H21A 109.1
C10—C11 1.440 (3)	C8—C7—H7 119.3	—
C25—C26 1.374 (4)	C20—C21—H21B 109.1
C11—N12 1.291 (3)	C6—C7—H7 119.3	—
C25—H25 0.95	S22—C21—H21B 109.1
C11—H11 0.95	C7—C8—C9 121.4 (2)	—
C26—C27 1.381 (4)	H21A—C21—H21B 107.9
N12—C13 1.407 (3)	C7—C8—H8 119.3	—
C26—H26 0.95	C23—S22—C21 104.46 (12)
C13—C14 1.394 (3)	C9—C8—H8 119.3	—
C27—C28 1.378 (4)	C24—C23—C28 119.2 (2)
C13—C18 1.402 (3)	C8—C9—C4 117.4 (2)	—
C27—H27 0.95	C24—C23—S22 125.66 (19)
C14—C15 1.379 (3)	C8—C9—C10 123.8 (2)	—
C28—H28 0.95	C28—C23—S22 115.14 (19)
C14—H14 0.95	C4—C9—C10 118.8 (2)	—
	C23—C24—C25 119.7 (2)

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Fig. 2 illustrates the changes in the UV-Vis spectra of L₂ (100 μM in DMSO/H₂O, 1 : 2, v/v) in the presence of Al³⁺. Immediately after the addition of Al³⁺ into a greenish-yellow solution of L₂, the color changes to light yellow and gradually the intensity of color decreases with increasing time. The absorption spectrum of the [L₂–Al³⁺] system shows two close peaks at 440 and 465 nm in the visible region in addition to two other peaks in the UV region. Upon the gradual addition of Al³⁺, the absorbance at the visible region gradually decreases to a 1 : 2 molar ratio of Al³⁺ to L₂. The plot of absorbance (at 440 nm) with concentration of Al³⁺ is presented as the inset of Fig. 2.

Fig. 2 Changes of the UV-Vis spectra of L₂ (100 μM) in DMSO/H₂O (1 : 2, v/v) with externally added Al³⁺ (0, 10, 20, 30, 40, and 50 μM). (Inset) Plot of absorbance of L₂ as a function of [Al³⁺] at 450 nm.

L₂ has a maximum emission at 520 nm while an excited emission occurs at 360 nm. The solution of [L₂–Al³⁺] complex emits a blue color under a hand held UV light. Fig. 3(A) shows the emission spectra of L₂ (10 μM) in the presence of different metal ions (200 μM) in DMSO/H₂O (1 : 2, v/v). While various cations such as Na⁺, Mg²⁺, K⁺, Ca²⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Hg²⁺ and Pb²⁺ do not affect the fluorescence intensity of L₂, Al³⁺ enhances its fluorescence intensity along with a remarkable blue shift from 520 to 455 nm. The selectivity of L₂ towards Al³⁺ has been examined in a ternary mixture containing L₂ (10 μM), Al³⁺ (200 μM) and foreign metal ions (200 μM). Fig. 3(B) reveals that the fluorescence intensity of the [L₂–Al³⁺] system decreases in the presence of Cu²⁺ and Fe³⁺. However, SCN⁻ could mask both without affecting the emission intensity of the [L₂–Al³⁺] system. The effect of pH on the fluorescence intensity of the [L₂–Al³⁺] system has been presented in Fig. 4, which indicates that the emission intensity of the [L₂–Al³⁺] system is at a maximum at pH 7.3. Fig. 5 illustrates the changes in the fluorescence intensity of the [L₂–Al³⁺] system with time.

Fig. 3 (A) Emission spectra of L₂ (10 μM) in DMSO/H₂O (1 : 2, v/v) solution in the presence of 100 μM of Na⁺, K⁺, Ca²⁺, Mg²⁺, Ag⁺, Mn²⁺, Hg²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Al³⁺ respectively (λ_ex, 360 nm). (B) Effect of foreign cations (200 μM) on the highest emission intensity of the [L₂–Al³⁺] system in DMSO/H₂O (1 : 2, v/v).

Fig. 4 Emission spectra of L₂ (10 μM) in the presence of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130,140, 150, 160, 170, 180, 190, and 200 μM Al³⁺ in DMSO/H₂O (1 : 2, v/v) solution at room temperature (λ_ex, 360 nm). Inset shows the plot of emission intensity (at 455 nm) vs. [Al³⁺].

Fig. 5 Benesi–Hildebrand plot for determination of the binding constant (λ_em = 455 nm).

The fluorescence quantum yield of L₂ at room temperature is 23.1 × 10⁻³ (at 455 nm). The emission intensity of L₂ (10 μM, λ_em, 455 nm) gradually increases with added Al³⁺ to a maximum (30-fold) at 200 μM of Al³⁺ (Fig. 6) whereas the fluorescence quantum yield increases to 45.4 × 10⁻³ (1.97-fold) (details in ESI†). The limit of detection has been found as 5 × 10⁻⁸ M (Fig. S10, ESI†).

Fig. 6 Job plot in DMSO/H₂O solution (λ_em, 455 nm).

The binding constant of L₂ for Al³⁺ has been estimated using the Benesi–Hildebrand equation,²⁶ (F_∞ − F₀)/(F_x − F₀) = 1 + 1/K_a[C]ⁿ, where F₀, F_x and F_∞ are the emission intensities of L₂ in the absence of Al³⁺, at an intermediate [Al³⁺], and at a concentration of complete interaction, respectively. While, K_a is the binding constant, C is the concentration of Al³⁺ and n is the number of Al³⁺ atoms bound per L₂ (here, n = 1/2). We have used 1 μM L₂ and 0.1–2 μM Al³⁺ (λ_em, 455 nm). The slope of the line of best fit (Fig. 7) gives the K_a as 0.15 × 10⁴ M^−1/2.

Fig. 7 Fluorescence intensity change for the solutions L₂ and (L₂ + Al³⁺) with respect to change in pH of the solutions (DMSO/H₂O, 1 : 2 ratio).

Fig. 8 shows that the highest emission intensity occurs at 1/3 mole fraction of Al³⁺ indicating 2 : 1 (L₂/Al³⁺, mole ratio) stoichiometry for the [L₂–Al³⁺] system. The mass spectrum also supports this stoichiometry (Fig. S11, ESI†).

Fig. 8 Fluorescence intensity of the [L₂–Al³⁺] system as a function of time.

A significant blue shift along with a large enhancement of emission intensity is attributed to the Al³⁺ induced CHEF process leading to the inhibition of both ICT and free rotation around the C=N bond of L₂ (Fig. 9).^10,27 Reduction of imine double bond of L₂ with NaBH₄/MeOH has enhanced the fluorescence intensity at 455 nm indicating reduced ICT due to the loss of conjugation. Moreover, addition of Al³⁺ to the reduced L₂ causes only a slight fluorescence enhancement, attributed only to the chelation-enhanced fluorescence (CHEF) process.

Fig. 9 Proposed binding mode for interaction of Al³⁺ with L₂.

¹H NMR studies

In order to support the binding of Al³⁺ with L₂, ¹H NMR titration was performed. Al³⁺ (as its nitrate salt) was added to the DMSO-d₆ solution of L₂. Significant spectral changes were observed (Fig. 10). Addition of Al³⁺ to L₂ causes the loss of the –OH proton (H_a) with subsequent binding to electron deficient Al³⁺ which results in the disappearance of the –OH peak (H_a) and downfield shifting of ortho proton to –OH group (H_b). The imine C–H peak (H_c) moves to the downfield region supporting the coordination of imine nitrogen (–CH=N–) to Al³⁺. Sulfur atom (S₁) binds to Al³⁺ resulting in the split of the singlet aliphatic C–H peak (H_d and H_e) into a multiplet with marginal shifting due to coupling with the surrounding –CH₂ protons (H_d and H_e).

Fig. 10 ¹H NMR spectra of L₂ with Al(NO₃)₃·9H₂O in DMSO-d₆: (I) L₂; (II) L₂ with 1 equivalent of Al³⁺.

Cell imaging studies

Fig. 11 shows the fluorescence imaging of intracellular Al³⁺ in contaminated living cells. Candida albicans cells (IMTECH No. 3018) from an exponentially grown culture in a yeast extract glucose broth medium (pH 6.0, incubation temperature, 37 °C) were centrifuged at 3000 rpm for 10 min and washed twice with 0.1 M HEPES buffer at pH 7.4. Then, the cells were treated with 30 μM Al³⁺ for 45 min in 0.1 M HEPES buffer (pH 7.4). After incubation, the cells were washed again with HEPES buffer at pH 7.4 and mounted on a grease free glass slide. Cells were observed under a fluorescence microscope equipped with a UV filter after adding L₂ (5 μM in DMSO). Cells incubated only with Al³⁺ were used as a control. L₂ can permeate easily through tested living cells without any harm (as the cells remain alive even after 30 min of exposure to L₂ at 5 μM). MTT cell toxicity assays (details in ESI†) were also performed to establish the non-toxic nature of the probe towards the tested living cells. A graphical presentation of the MTT assay is shown in Fig. 12.

Fig. 11 Fluorescence microscope images of Candida albicans cells. (a) IMTECH No. 3018. Candida albicans cells; (b) cells incubated with Al³⁺; (c) Al³⁺ incubated cells treated with L₂; (d) magnified view of (c).

Fig. 12 Graphical presentation of MTT assay studies for L₂ (MTT = 3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide). S = Al³⁺ salt, L = L₂.

Conclusion

We have successfully synthesized and crystallographically characterized a fluorescent probe for the selective detection of Al³⁺ in a DMSO/H₂O medium. It is inexpensive, easy to synthesize and highly crystalline. Upon interaction with Al³⁺, L₂ produced a blue fluorescence with a large enhancement of the emission intensity associated with blue shift. The binding constant of L₂ for Al³⁺ is 0.15 × 10⁴ M^−1/2. LOD of the method is 8 × 10⁻⁸ M. The probe can easily permeate the cell membrane and efficiently stain intracellular Al³⁺ ions. MTT cell toxicity assays indicate the non-toxic nature of the probe to living cells.

Acknowledgements

The authors are grateful to IUC-DAE, Kolkata Centre for funding. Sisir Lohar and Animesh Sahana are grateful to CSIR, New Delhi for providing fellowship.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 829497 contains the crystallographic information of the probe. See DOI: 10.1039/c2ra21911e

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