Metal ion detection by naphthylthiourea derivatives through ‘turn-on’ excimer emission

Abstract

The detection of multiple heavy and transition metal (HTM) ions has been achieved through ‘turn-on’ excimer emission by three naphthylthiourea derivatives (L1, L2 and L3) in DMSO. Mechanistic studies suggest that the photoinduced electron transfer from the thiourea moiety to the naphthalene excimer is suppressed in the presence of Hg²⁺, Cu²⁺, Co²⁺ and Ag⁺, leading to enhanced emission intensity from the excimer. Interestingly, the ligands were able to detect all the above said HTM ions through ‘turn-on’ fluorescence, at micromolar concentrations of the analytes. While Hg²⁺ and Cu²⁺ ions were detected by all the ligands, Co²⁺ and Ag⁺ ions were detected only by L2 and L3, respectively. Furthermore, the detection of Hg²⁺ ions by L1 has been demonstrated in aqueous condition, wherein the fluorescence quantum yield showed the highest value (0.25 ± 0.01), compared to other metal–ligand complexes in DMSO. To the best of our knowledge, this is the first report regarding multi-ion detection by thiourea based ligands through ‘turn-on’ excimer emission.

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Introduction

Heavy and transition metal (HTM) ions play an important role in regulating various physiological processes in organisms.¹ However, an excess of HTM ions causes a number of diseases such as Minamata, Menkes, Wilson, Alzheimer's, Parkinson's, prion, wheezing and asthma.^2–4 In certain cases, the presence of HTM ions leads to the damage of lipids, nucleic acids and proteins through facilitation of the formation of reactive oxygen species in the system.⁵ HTM ions have also been widely used in industry, pharmaceutics as well as in agriculture. Hence, the detection and quantification of HTM ions are of paramount importance.⁶

In this context, the development of fluorescent molecules, which act as chemosensors and/or chemodosimeters having optical sensing ability for the determination of HTM ions, is highly desirable.^7–10 Thiourea based ligands have been utilized as chemosensors for the detection of numerous anions.¹¹ Also, thiourea based ligands bind metal ions and these metal complexes have biological activities such as antiviral, antibacterial, antifungal, anticancer, antitubercular, antithyroidal, herbicidal, and insecticidal activities.¹² Nevertheless, reports on the detection of HTM ions by thiourea based ligands through ‘turn-on’ fluorescence has been sporadic, at best. As an analytical tool, the ‘turn-on’ fluorescence method has added advantages over the ‘turn-off’ fluorescence method since the latter can be due to non-specific reasons and leads to erroneous inferences in sensor studies.^9b,13

Herein, we report the design, synthesis and photophysical properties of naphthylthiourea based ligands L1, L2 and L3 (Chart 1) for the detection of multiple HTM ions (Cu²⁺, Hg²⁺, Ag⁺ and Co²⁺) through chelation induced enhanced fluorescence (CHEF) in DMSO. Furthermore, ligand L1 has been utilized for sensing Hg²⁺ ions in aqueous medium (10 mM HEPES, 40% DMSO, pH = 7.2) through ‘turn on’ excimer emission.

Chart 1 Thiourea based naphthalene derivatives utilized in the present study.

The sensing ability of the receptors towards a variety of metal ions has been investigated by UV-vis, fluorescence, ¹H and ¹³C NMR techniques. The present study represents the first report of the detection of various metal ions via ‘excimer fluorescence enhancement’ of napthylthiourea based ligands in the presence of paramagnetic and heavy metal ions at the micromolar concentration range.

Results and discussion

Chart 1 depicts the structure of the thiourea based naphthyl ligands (L1, L2 and L3) utilized in the present study. According to the Hard and Soft Acids and Bases (HSAB) theory, sulfur atoms prefer binding with soft metal ions such as Hg²⁺ and Ag⁺.¹⁴ In addition to this, the presence of sulfur along with nitrogen atoms enhances the coordination propensity of the ligands to bind borderline metal ions such as Co²⁺ and Cu²⁺.

It is envisaged that complex formation between the ligands and metal ions might increase the fluorescence intensity from the system via suppression of photoinduced electron transfer (PET) from the electron rich sulfur atoms to the naphthalene units.^8b,15 The absorption studies of L1, L2 and L3 (75 μM) were carried out in the presence of acetate and chloride salts of various metal ions in DMSO. L1, L2 and L3 exhibit an absorption band around 296 nm which is assigned to the naphthyl π–π* transition. From the pool of metal ions (Hg²⁺, Fe²⁺, Zn²⁺, Ca²⁺, Cd²⁺, Mn²⁺, Ag⁺, Ni²⁺, Pb²⁺, Cu²⁺, Sn²⁺, Co²⁺, Yb³⁺ and Eu³⁺), the presence of Hg²⁺, Cu²⁺, Co²⁺ and Ag⁺ ions results in absorption spectral shift (Fig. S1–S3†), associated with a ‘turn-on’ fluorescence signal.

Sensing of Hg²⁺ ions using L1, L2 or L3

We observed a black color precipitate upon mixing a 75 μM solution of L1 with Hg²⁺ ions (1–3 equivalents). After removing the precipitate, the supernatant liquid exhibits a new shoulder band absorption at 327 nm (Fig. S4†). Similarly, 1–3 equivalents of Hg²⁺ ions were added to L1 in DMSO and the fluorescence response was recorded. While L1 exhibits a weak fluorescence upon excitation at 345 nm, treatment with three equivalents of Hg²⁺ ions triggers ca. 17 fold enhancement in the emission intensity. Fluorescence emission spectra of L1 in the presence of different equivalents of Hg²⁺ ions are depicted in Fig. 1.

Fig. 1 Steady state fluorescence spectra of L1 (75 μM) upon addition of Hg²⁺ (0–3 equivalents) in DMSO. Inset shows a photograph of L1 (0.75 M) in the absence (left) and presence of 3 equiv. of Hg²⁺ ion (right). λ_ex was 345 nm.

The broad nature of the emission peak at 426 nm is assigned to the naphthalene excimer.¹⁶ Upon addition of metal ions to the ligands, non-covalent interactions between the ligands increase, leading to enhanced aggregation propensity in the system. The enhanced self-assembly of the ligands results in the formation of naphthalene excimer in the system. The observation of red shifted absorption spectra for L1, in the presence of Hg²⁺ ions, also substantiates the presence of self-assembly in the system. In order to confirm the formation of naphthalene excimer in the system, Time Correlated Single Photon Counting (TCSPC) studies were carried out. The solution was excited at 338 nm using a LED source and the excited state lifetime of the species was found to be 14.2 ns with a relative amplitude of 98% (Fig. S5 and S6†). Lifetime studies established that the broad emission is from naphthalene excimer.

A significant enhancement in the luminescence quantum yield was observed in the case of L1 upon treatment with Hg²⁺ ions and the emission quantum yield was 0.20 ± 0.01 (Φ_f = 0.36 in cyclohexane). The enhanced emission quantum yields for L2 and L3 are tabulated in Table 1.

Table 1 Photophysical properties, stoichiometric ratio and detection limit of L1, L2 and L3 in the presence of Hg²⁺ ions in DMSO

Ligands with Hg^2+	λabs, nm	λem, nm	Stoichiometric ratio (L : M)	Detection limit	Lifetime, ns (relative amplitude %)	Φ^a
a Error = ±5%.
L1–Hg^2+	327	428	1 : 3	15 μM	τ1 = 1.16 ns (1.61%), τ2 = 14.2 ns (98.39%) CHISQ = 1.21	0.20
L2–Hg^2+	297, 310, 326	428	1 : 2	10 μM	τ1 = 0.24 (2.07%), τ2 = 14.2 (97.93%) CHISQ = 1.21	0.15
L3–Hg^2+	312	427	1 : 2	14 μM	τ1 = 4.9 ns (1.83%), τ2 = 14.38 ns (93.66%), τ3 = 0.123 ns (4.51%) CHISQ = 1.00	0.16

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The stoichiometry between Hg²⁺ and the ligands was determined by Job's plot. We observed an anomalous Job's plot having two maxima which is quite uncommon. The fact that we observed a black coloured precipitate during the sensing process possibly indicates that L1 undergoes a desulfurization reaction in the presence of Hg²⁺ ion. The anomalous Job's plot is presumably due to the desulfurization reaction, which brings about dynamic rearrangements in the bonding modes of the system.¹⁷ The measured emission intensity reached a maximum when the mole fraction of ([Hg²⁺]/[Hg²⁺] + [L1]) was 0.75, suggesting a 1 : 3 stoichiometry between L1 and Hg²⁺ ions (Fig. S7†). However, Job's study of L2 and L3 systems indicates a stoichiometry of 1 : 2 (Fig. S8 and S9†).

As mentioned before, the fluorescence enhancement observed from L1 upon addition of Hg²⁺ ions is presumably due to the reduced PET rate between excited naphthalene and sulfur atoms due to removal of sulfur by a desulfurization reaction. From the fluorescence titration, the detection limit for Hg²⁺ ions by L1 has been determined and the value was found to be 15 μM (Fig. S10†).^7a

To gain more insight into the mechanistic behaviour between the ligands and Hg²⁺, ¹³C NMR experiments were carried out. Fig. 2 shows the ¹³C NMR spectra of L1 in the presence and absence of 3 equivalents of Hg²⁺ ions. As is shown in the figure, the prominent peak at 181 ppm, which is assigned to C=S carbon, disappeared upon the addition of 3 equivalents of Hg²⁺ ions. Instead, a new peak was observed at 154 ppm, which can be assigned to C=O carbon, suggesting that L1 had undergone an atom replacement reaction upon treatment with Hg²⁺. The peak at 174 ppm corresponds to acetate carbonyl carbon from the counter ion of Hg²⁺. NMR spectral analysis confirmed that a desulfurization reaction had occurred in the presence of Hg²⁺ ions, yielding a new product and insoluble mercury sulfide which precipitated out (photograph in Fig. 1 inset).

Fig. 2 ¹³C NMR experiment of (a) L1 and (b) L1 in the presence of 3.0 equivalents of Hg²⁺ acetate in DMSO-d₆.

In order to support the above observation, the black colored precipitate was examined by the Energy Dispersive X-ray analysis (EDAX) technique and the result indicates the presence of both sulfur and mercury, confirming the formation of HgS (Fig. S11†). While conversion of thioamide into urea can be catalyzed by various metal ions,¹⁸ the process was effective exclusively for Hg²⁺ ions in the present case. This is, presumably, due to the selective interaction between Hg²⁺ ions and the ligands. This behaviour is quite analogous to Czarnik's Hg²⁺ ions selective chemodosimeter.^8b A proposed mechanism for the mercury ion induced conversion of L1 into the corresponding urea derivative (L1U) is shown in Scheme 1.

Scheme 1 Hg²⁺ ion induced desulfurization of L1.

Results taken together suggested that the initial coordination between Hg²⁺ and the ligands (L1, L2 and L3) is followed by a desulfurization reaction, resulting in the precipitation of black colored HgS. The resulting thiourea based system, LIU, has enhanced propensity to aggregates through hydrogen bonding interactions, leading to the formation of closely packed napthalene units where the excimer formation is more feasible. The photophysical properties, stoichiometric ratios and detection limits for the ligands upon treatment with Hg²⁺ in DMSO are summarized in Table 1. The UV-vis, steady state, time resolved fluorescence spectra and detection limits for L2 and L3 systems in the presence of Hg²⁺ ions are provided in the ESI (Fig. S12–S21†).

Sensing of Cu²⁺ ions using L1, L2 and L3

Next, photophysical studies of L1 were carried out in the presence of Cu²⁺ ions. Unlike the previous system, no precipitation was observed upon the formation of L1–Cu²⁺ complex. Conversely, the absorption spectra of L1 show isosbestic points, indicating the formation of L1–Cu²⁺ complex (Fig. S22†). Furthermore, the naphthalene excimer emission from L1 is enhanced upon the addition of Cu²⁺ ions. Here also, the metal–ligand binding prevents the electron transfer, resulting in enhanced emission intensity from the naphthalene excimer. However, the enhancement was non-linear, with an initial blue shift (15 nm), followed by red shifts to the original wavelength (λ_emiss = 426 nm) (Fig. S23†). The inflection points in the fluorescence titration experiment indicate the presence of stepwise and multiple equilibriums in the system (Fig. S24†).¹⁹ The usual Job's plot analysis was, thus, inadequate to confirm the number of Cu²⁺ ions binding to L1.

Further photophysical studies were carried out for L2/L3–Cu²⁺ systems in DMSO (Fig. S25–S27†). While there was no significant shift in the emission maximum for the L2–Cu²⁺ complex, the fluorescence spectra of the L3–Cu²⁺ complex shows a blue shift of 25 nm in the presence of increasing amounts (1–6 equivalents) of Cu²⁺ ion (Fig. S28†).²⁰ In order to gain a clear understanding of the structure of the L3–Cu²⁺ complex, ¹H NMR experiments were employed in DMSO-d₆ (Fig. S29†). Upon addition of 1 equivalent of Cu²⁺, the –NH_a proton underwent a downfield shift from 9.71 ppm to 14.8 ppm and the –NH_b proton at 7.62 ppm was shifted to 9.7 ppm. The NMR peak at 9.7 ppm corroborates the formation of –NH_b/SH. The downfield shift of NH_a and NH_b protons is presumably due to the metal binding event by the sulfur atom, resulting in an electron withdrawing effect from NH.^16c,21 The peak at 9.7 ppm disappeared upon addition of two equivalents of the metal ion, indicating a possible deprotonation process in the system.^16c However, we did not observe such a deprotonation process during the formation of the L2–Cu²⁺ complex.

Job's plot analysis indicates that L2 and L3 form 1 : 1 and 1 : 2 complexes, respectively, with Cu²⁺ ions (Fig. S30 and S31†). The binding constants for L2–Cu²⁺ and L3–Cu²⁺ were calculated and the values were 1.2 × 10³ M⁻¹ and 4 × 10¹² M⁻², respectively (Fig. S32 and S33†).²² The detection limits of Cu²⁺ in DMSO in the presence of L2 and L3 were also in the micromolar range (Fig. S34 and S35†).

The lifetimes of naphthalene excimer in L2 and L3 were measured in the presence of Cu²⁺ ions and the decay profile of L3 in the presence of Cu²⁺ ion is given in Fig. 3. The corresponding decay of L2–Cu²⁺ complex is given in the ESI (Fig. S36†). The multi exponential decay traces obtained for L2 and L3 copper complexes with two fast decays (∼0.5 ns and ∼2 ns) in addition to the excimer decay (∼13.1 ns) indicate that: (i) the photoinduced electron transfer process between the ligand and naphthalene is only partially stopped by the metal ion binding and (ii) the paramagnetic effect of copper ion also contributes to emission quenching in the system. This hypothesis is substantiated by the reduced fluorescence quantum yield (Table 2) of L3–Cu²⁺ compared with previous metal–ligand systems.

Fig. 3 Excited state decay of L3 (75 μM) in the presence of Cu²⁺ ions in DMSO. λ_ex was 345 nm and emission was collected at 400 nm.

Table 2 Photophysical properties, stoichiometric ratio, detection limit and binding constant (K_a) of L2 and L3 in the presence of Cu²⁺ ions in DMSO

Complex	λabs, nm	λem, nm	Stoichiometric ratio (L : M)	Detection limit	Lifetime, ns (relative amplitude %)	Φ^a	Ka
a Error±5%.
L2–Cu^2+	297, 312, 328 & 347	426	1 : 1	10 μM	τ1 = 1.56 ns (3.24), τ2 = 14.3 ns (92.61), τ3 = 0.23 ns (4.14) CHISQ = 1.07	0.14	1.2 × 10^3 M^−1
L3–Cu^2+	316 & 338	398	1 : 2	13 μM	τ1 = 2.77 ns (14.62%), τ2 = 0.13 ns (39.75%), τ3 = 0.64 ns (45.63%) CHISQ = 1.13	0.08	4 × 10^12 M^−2

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We carried out a study of metal ion binding reversibility in L2/L3–Cu²⁺ complex systems by adding EDTA (Fig. S37 and S38†). The fluorescence of the L3–Cu²⁺ complexes was slightly enhanced upon treatment with EDTA, which could be due to chelation between EDTA and Cu²⁺ ions, resulting in a decreased PET rate between the naphthalene excimer and the paramagnetic metal ion. The fluorescence intensity of the L2–Cu²⁺ system was not changed upon addition of EDTA. The results indicated that the binding is not reversible in the presence of EDTA. While the ligands L1, L2 and L3 bind both Cu²⁺ and Hg²⁺, the interference between the metal ions is nil since binding with Hg²⁺ ions alone leads to precipitation.

Sensing of Co²⁺ ions using L2 and Ag⁺ ions using L3

It has been found that L2 and L3 ligands can also be used for the detection of Co²⁺ and Ag⁺ ions, respectively, through a ‘turn-on’ excimer signal. Addition of Co²⁺ ions (75 μM) to a solution of L2 (75 μM) in DMSO result in an appreciable decrease of the initial peak absorbance at 296 nm and a gradual increase of absorbance around 329 nm, along with a clear isosbestic point at 311 nm. This suggests the formation of a host guest complex between Co²⁺ and L2 (Fig. S39†). In the case of Ag⁺ ions, slightly red shifted absorption bands were observed for L3 (Fig. S40†).

In both cases, the complex formation results in an enhancement of emission from the naphthalene excimer (Fig. S41 and S42†). It is worth mentioning here that this is the first report of the detection of Co²⁺ ions by a thiourea ligand system, based on ‘turn-on’ fluorescence. Job's plot analysis was carried out and the results indicate that the stoichiometry between L2–Co²⁺ is 1 : 1 (Fig. S43†). However, the Job's plot for the L3–Ag⁺ system does not show any change in emission upto 0.50 mole fraction and then shows an increase, followed by an inflection point at 2 equivalents of Ag⁺ ions (Fig. S44†). Since ligand L3 contains –NH groups, it is likely that silver ions are reduced to silver nanoparticles and this parallel process affects the Job's analysis.²³

The time resolved emission studies indicated that a single type of ligand–metal complex was formed in the above two cases and photoinduced electron transfer between the ligand and the fluorophore was almost nil (Fig. S45 and S46†). The binding constant for L2–Co²⁺ was 2.1 × 10⁴ M⁻¹ (Fig. S47†) and the binding constant for L3–Ag⁺ system was not calculated due to the presence of parallel processes. The detection limits for sensing Co²⁺ and Ag⁺ ions are given in Table 3 (Fig. S48 and S49†).

Table 3 Photophysical properties, stoichiometric ratio, detection limit and binding constant (K_a) of L2 and L3 in the presence of Co²⁺ and Ag⁺ ions in DMSO

Complex	λabs, nm (isosbestic point)	λem, nm	Stoichiometric ratio (L : M)	Detection limit	Lifetime, ns (relative amplitude %)	Φ^a	Ka
a Error = ±5%.
L2–Co^2+	296 & 329 (311)	426	1 : 1	15 μM	τ1 = 2.5 ns (1.20%), τ2 = 14.26 ns (96.45%), τ3 = 0.16 ns (2.35%) CHISQ = 1.14	0.21	2.1 × 10^4 M^−1
L3–Ag^+	309	427	1 : 2	23 μM	τ1 = 6.50 ns (2.62%), τ2 = 14.4 ns (93.75%), τ3 = 0.12 ns (3.63%) (CHISQ) = 1.13	0.15

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While the ¹H NMR spectrum of the L3–Ag⁺ complex shows shifts for both NH_a and NH_b protons and the mechanism was similar to that for the Cu²⁺–L3 complex (Fig. S50†), no peak shifts were observed for the L2–Co²⁺ complex formation (Fig. S51†) (vide infra). The photophysical properties of L2–Co²⁺ and L3–Ag⁺ complexes are summarized in Table 3.

We have analysed the reversibility effect on L2–Co²⁺ and L3–Ag⁺ by adding EDTA to the system (Fig. S52 and S53†). The results indicate that the metal ion binding events are largely reversible only for the L2–Co²⁺ system. The reversible nature of the binding in the L2–Co²⁺ system is reflected in the NMR analysis also where no shift peak was observed, indicating that the exchange rate between the free and complexed species was faster than the NMR time scale.

Interference studies for all the above metal ligand systems were assessed in the presence of various background metal ions and the results are summarized in Fig 4. Other metal ions did not cause significant changes in fluorescence intensity, except for Pb²⁺ and Fe²⁺ ions (Fig. 4 and S54–59†).

Fig. 4 Bar chart illustrating the fluorescence response for L3 (75 μM) upon addition of 2 equivalents of Hg²⁺ and interfering metal ions: 1, Hg²⁺; 2, Hg²⁺–Ni²⁺; 3, Hg²⁺–Ag⁺; 4, Hg²⁺–Co²⁺; 5, Hg²⁺–Yb²⁺; 6, Hg²⁺–Ca²⁺; 7, Hg²⁺–Cu²⁺; 8, Hg²⁺–Pb²⁺; 9, Hg²⁺–Sn²⁺; 10, Hg²⁺–Cd²⁺; 11, Hg²⁺–Mn²⁺; 12, Hg²⁺–Zn²⁺; 13, Hg²⁺–Eu³⁺; 14, Hg²⁺–Fe²⁺ in DMSO. λ_ex was 345 nm and λ_em was 426 nm.

Sensing Hg²⁺ ions in water using L1

It was found that increasing the number of C=S groups in the ligands leads to the detection of increased Hg²⁺ ions. Thus, ligand L1 was chosen for mercury ion detection in aqueous conditions.

We carried out a photophysical study of L1 in aqueous medium (10 mM, 4-(2-hydroxyethyl) piperazine 1-ethanesulfonic acid (HEPES) with 60% DMSO, pH 7.2), and an excellent selective emission response towards Hg²⁺ ions was shown. Even though the absorption spectra of L1 changed in the presence of Cu²⁺ and Hg²⁺ ions (1 equivalent) (Fig. S60†), emission enhancement was observed only in the presence of Hg²⁺ ions. The emission intensity at 426 nm was enhanced ∼4 times upon addition of 1 equivalent of Hg²⁺ ions (λ_ex = 345 nm), with a quantum yield value of 0.25 ± 0.01. Fig. 5 contains the emission characteristics for naphthalene excimer in the presence of Hg²⁺ and other metal ions examined.

Fig. 5 Steady state fluorescence spectra of L1 (75 μM) upon addition of Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Fe²⁺, Hg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Sn²⁺, Zn²⁺, Yb³⁺ and Eu³⁺ (1 equivalent) in aqueous medium (10 mM HEPES, 60% DMSO, pH = 7.2). λ_ex was 345 nm.

The excited state lifetime of L1 upon treatment with Hg²⁺ in aqueous medium (HEPES, 60% DMSO, pH 7.2) shows the characteristic features of naphthalene excimer emission with 18 ns lifetime and 99% relative amplitude (Fig. S61 and S62†). In the present case, we also observed a black colored precipitate upon addition of Hg²⁺ to L1. Since the observations were similar to those for the L1–Hg²⁺ system in DMSO, it is clear that a desulfurization reaction was also taken place in the aqueous system. The emission spectra of L1 (75 μM) upon addition of Hg²⁺ ions (0–8 equivalents) and the corresponding Job's plot are given in the ESI (Fig. S63 and S64†). The detection limit of Hg²⁺ ions in the presence of L1 was found to be 8 μM in the aqueous medium (Fig. S65†).

Interference studies were carried out for the L1–Hg²⁺ system in aqueous medium (10 mM HEPES, 60% DMSO, pH = 7.2) in the presence of background metal ions. L1 shows a remarkable selectivity towards Hg²⁺ ions as is shown in Fig. 6. This suggests that mercury ion detection by L1 is not affected by the presence of other metal ions listed (except for Sn²⁺, where a slight interference is observed).

Fig. 6 Bar chart illustrating the fluorescence response for L1 (75 μM) upon addition of 3 equivalents of Hg²⁺ and interfering metal ions: 1, Hg²⁺; 2, Hg²⁺–Cu²⁺; 3, Hg²⁺–Cd²⁺; 4, Hg²⁺–Pb²⁺; 5, Hg²⁺–Co²⁺; 6, Hg²⁺–Yb³⁺; 7, Hg²⁺–Eu³⁺; 8, Hg²⁺–Zn²⁺; 9, Hg²⁺–Mn²⁺; 10, Hg²⁺–Ni²⁺; 11, Hg²⁺–Ca²⁺; 12, Hg²⁺–Ag⁺; 13, Hg²⁺–Fe²⁺; 14, Hg²⁺–Sn²⁺; 15, L1 in aqueous medium (10 mM HEPES, 60% DMSO, pH = 7.2). λ_ex was 345 nm and λ_em was 426 nm.

As an attempt to improve the metal ion detection ability of the ligands, we carried out a sensing study at low temperatures since ‘turn-on’ fluorescence signals can be more intense at lower temperatures. Ligand L1 was utilized for the study with Hg²⁺ ions. The ‘turn-on’ emission from L1 (7.5 × 10⁻⁵ M) was measured at room temperature as well as at 5 °C upon addition of Hg²⁺ (7.5 × 10⁻⁵–7.5 × 10⁻⁸ M) in aqueous medium (10 mM HEPES, 60% DMSO, pH = 7.2) (Fig. S66†). However, we could only find a marginal increase in the emission intensity in the presence of Hg²⁺ ion at 7.5 × 10⁻⁸ M, which was below the detection limit of L1 at room temperature.

Conclusion

In summary, the detection of multiple HTM ions was successfully achieved using three different naphthylthiourea ligands in DMSO through ‘turn-on’ excimer fluorescence. Among a pool of various transition metal ions, ligand L1 responded only towards micromolar concentration of Hg²⁺ and Cu²⁺ ions. Exclusive detection of Hg²⁺ ions by L1 was also achieved in aqueous condition through a selective fluorescence ‘turn-on’ signal, which makes L1 a promising candidate for the biological detection of mercury ions. Incorporating more C=S bonds in the ligand system was found to be an effective strategy to increase the detection events of Hg²⁺ ions per molecule. In addition to Hg²⁺ and Cu²⁺ ions, ligands L2 and L3 were also utilized for the recognition of Co²⁺ and Ag⁺, respectively, in DMSO. Mechanistic studies indicated that differential binding ability can be imparted to the ligands through facilitating specific interactions between the ligands and metal ions.

Experimental section

Chemicals

All the chemicals were of analytical grade and were purchased from Sigma Aldrich, USA and Sd. Fine Chemicals, India. The metal ions used in the present study were either chloride or acetate salts of Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Fe²⁺, Sn²⁺, Yb³⁺ and Eu³⁺. 1-Naphthylisothiocyanate was synthesized following a reported procedure.²⁴ Naphthylthiourea based ligands L1 and L2 were synthesized according to previously published procedures.^11c Anthracene (Φ_f = 0.36 in cyclohexane) was used as the standard for the fluorescence quantum yield determination. All the spectra were recorded after 48 h of the sample preparation.

Instruments

¹H and ¹³C NMR data were collected on a Bruker 400 MHz spectrometer (¹H: 400 MHz; ¹³C: 100 MHz). Mass spectra were recorded using a Micromass Q-TOF mass spectrometer. Luminescence experiments were done using an Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. UV-vis experiments were performed on a JASCO UV-vis spectrophotometer (Model 660) with cuvettes having a path length of 1.00 cm. The lifetime studies were conducted using the Time Correlated Single Photon Counting Technique (TCSPC) with a micro channel plate photomultiplier tube (MCP–PMT) as the detector and a nanosecond LED as the excitation source (model Fluorocube, Horiba Jobin Yvon). Chemical analysis data was collected using an FEI Quanta FEG 200-High Resolution Scanning Electron Microscope equipped with an energy dispersive X-ray (SEM-EDX technique) spectrometer.

Synthesis of L3

Compound ethylenediamine (0.081 g, 1.35 mmol) and 1-naphthylisothiocyanate (0.5 g, 2.7 mmol) were added to dry dichloromethane (20 mL). The mixture was stirred at room temperature for 12 h. The volatiles were removed under reduced pressure. The residue was purified by column chromatography using silica gel as the stationary phase and 0.5% MeOH in dichloromethane as the eluent to get the pure product in 87% yield. ¹H NMR (400 MHz, DMSO-d₆): δ = 3.64 (4H, s, –CH₂), 7.43 (2H, d, J = 7.2 Hz, ArH), 7.49 (2H, d, J = 8 Hz, ArH), 7.59–7.53 (4H, m, –ArH), 7.62 (2H, bs, –NH_b), 7.91–7.84 (4H, m, ArH), 7.99–7.95 (2H, m, ArH), 9.71 (2H, s, –NH_a); ¹³C NMR (100 MHz, DMSO-d₆): δ = 43.71, 122.87, 125.14, 125.79, 126.19, 126.79, 128.08, 129.80, 133.99, 181.86; HRMS (ES⁺): m/z calcd for C₂₄H₂₂N₄S₂: 430.13; found: 431.13 [M + H]⁺.

Acknowledgements

We thank the Department of Chemistry, IIT Madras. We also thank Prof. A. K. Mishra, IIT Madras for time resolved experiments. C. A. thanks CSIR for a fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45510f

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