Journal of Chemistry
Volume 2015 (2015), Article ID 165295, 9 pages
http://dx.doi.org/10.1155/2015/165295

Research Article

Synthesis of Novel Fluorescent Sensors Based on Naphthalimide Fluorophores for the Highly Selective Hg²⁺-Sensing

Yordkhuan Tachapermpon,¹ Pornthip Piyanuch,¹ Narupon Prapawattanapol,² Kanjarat Sukrat,³ Kullatat Suwatpipat,¹ and Nantanit Wanichacheva¹

¹Department of Chemistry, Faculty of Science, Silpakorn University, Nakhon Pathom 73000, Thailand
²Department of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
³Chemistry Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand

Received 27 November 2014; Revised 15 January 2015; Accepted 26 January 2015

Academic Editor: Pedro Derosa

Copyright © 2015 Yordkhuan Tachapermpon et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

With an aim to develop the new sensors for optical detection of Hg²⁺ ions, two novel fluorometric sensors were designed and successfully prepared using 2-(3-(2-aminoethylsulfanyl)propylsulfanyl)ethanamine and one or two N-methylnaphthalimide moieties (1 and 2). Sensor 1 was obtained via N-alkylation, N-imidation and a one-pot nucleophilic aromatic substitution, and N-formylation of the amine, while sensor 2 was prepared via N-alkylation, N-imidation, and nucleophilic aromatic substitution. The characterization, including ¹H NMR, ¹³C NMR, and mass spectrometry, was then performed for 1 and 2. The Hg²⁺-binding behaviors of the sensors were investigated in terms of sensitivity and selectivity by fluorescence spectroscopy. Sensor 1 especially provided the reversible and highly Hg²⁺-selective ON-OFF fluorescence behavior by discriminating various interfering ions such as Pb²⁺, Co²⁺, Cd²⁺, Mn²⁺, Fe²⁺, K⁺, Na⁺, and in particular Cu²⁺ and Ag⁺ with a detection limit of 22 ppb toward Hg²⁺ ions.

1. Introduction

Mercury is recognized as a toxic heavy metal as well as the hazardous and poisonous pollutants, which can be accumulated and persisted in the environment: soil, water, and air [1–5]. Mercury is considered as a neurotoxin which can lead to many serious human health problems such as permanent central nervous system damage, DNA damage, and mitosis impairment [6–8]. In the marine system, inorganic mercury is changed to methylmercury via bacteria systems. This methylmercury can then go through biological membranes and finally enter into the food chain [2–4, 8]. Fluorescence spectroscopy is an alternative method for the detection of many analytes, which presents several promising approaches and can be very useful in the qualitative and quantitative detection, such as the determination of small molecules [9], proton [10], anions [11], or cations, including Hg²⁺ ions [8, 12–26]. The method is relatively inexpensive, nondestructive to samples, sensitive, and selective and allows prompt determination, which can be utilized as a tool for real time tracking of Hg²⁺ [8, 26].

Although there have been many fluorescence Hg²⁺-chemosensors designed by utilizing multidentate nitrogen, oxygen, and thioether-containing ionophores [8, 26], such as cyclen [12, 13], hydroxyquinoline [14], azine [17, 18], cyclams [19–21], calixarenes [22–24], and diazatetrathia crown ethers [25], many of these sensors still have some limitations in production and practical usages. This may be because of the relatively high cost of commercial ionophores, low selectivity, or cross sensitivity toward competitive cations such as silver ions, copper ions, and lead ions, as a result from their similarity in chemical behaviors to Hg²⁺ [14, 17, 18, 21, 22, 27–29]. Moreover, some sensors exhibit the absorptions and fluorescent emissions in the UV region, so their optical signals are in the same ranges of wavelength absorbed by matrix interference in biological samples and glass optical systems [26]. This can lead to the low sensor signals and more limitations on further uses of these sensors in portable and inexpensive testing devices [26].

In most fluorescent sensor systems, an ionophore is responsible for selective binding of a target ion, whereas the sensitivity is normally controlled by a selected fluorophore. Here, our strategy is to introduce an acyclic ionophore which contained nitrogen and sulfur atoms as a receptor site for Hg²⁺. Unlike preorganized ionophores such as cyclen, cyclam, and calixarenes, acyclic ionophore can offer a high degree of freedom for self-assembly to Hg²⁺. This is due to some electrostatic interactions between ions coordinated with donor atoms of acyclic ionophore. As a result, the selectivity of these sensor systems is fairly high. Moreover, since the naphthalimide fluorophore has strong fluorescence, high absorption coefficient, high quantum yield, and large Stokes shift, it was used as the signaling portion of the sensors here [8, 15, 30–32]. In addition, we expected that the modification of naphthalimide at the 4-position by an amine group could induce a bathochromic shift of absorption and fluorescence spectra into visible region, which will be beneficial for further fabrication of economical devices.

In this study, the designed sensors, 1 and 2, composed of 2-(3-(2-aminoethylsulfanyl)propylsulfanyl)ethanamine and one or two -methylnaphthalimide moieties were prepared. Their Hg²⁺ sensing behaviors were examined under various conditions, including foreign ion interference. These sensors should function in visible region with high sensitivity and selectivity toward Hg²⁺.

2. Experimental Section

2.1. Materials

The reagents and solvents were purchased from Fluka Chemical Corporation while the metal salts (perchlorate salts) were purchased from Strem chemicals, Inc. All reagents, solvents, and metal salts were used as received.

2.2. Methods

NMR spectra in CDCl₃ solutions were recorded using Bruker Avance 300 spectrometer operating at 75 MHz for ¹³C and 300 MHz for ¹H. Fluorescence spectra were measured using Perkin Elmer Luminescence spectrometer LS 50B. The emission slit widths used in fluorescence measurements were 5.0 nm with a scan rate of 300 nm/min. ThermoElectron LCQ-DECA-XP, ESI-Ion trap mass spectrometer, was used to record mass spectra of the synthetic compounds. Discovery Studio 5.5 program package was employed to perform the molecular modeling studies.

2.3. Syntheses

2.3.1. 2-[3-(2-Aminoethylsulfanyl)propylsulfanyl]ethanamine

This compound was prepared using the same procedure as in reference [33]. Its synthetic pathway is presented in Scheme 1.

Scheme 1: Syntheses of fluorescence sensors 1 and 2.

2.3.2. 4-Bromo-N-methylnaphthalimide

0.20 g of 4-bromo-1,8-naphthalic anhydride (0.72 mmol) was dissolved in dried ethanol (8 mL), followed by the addition of methylamine (0.4 mL). After the solution was refluxed under an argon atmosphere for 18 h, the solution was filtered and its solvent was removed under rotary evaporator. 30 mL of dichloromethane (CH₂Cl₂) was added; then three times of extraction were done using 30 mL of deionized water each. The CH₂Cl₂ portion was dried over anhydrous sodium sulfate before it was removed by rotary evaporator. Finally, yellow powder of 4-bromo--methylnaphthalimide was obtained and purified by preparative thin layer chromatography (TLC) (CH₂Cl₂, ), 41%. ¹H NMR (300 MHz, CDCl₃): δ 3.57 (s, 3H), 7.85 (t,  Hz, 1H), 8.04 (d,  Hz, 1H), 8.42 (d,  Hz, 1H), 8.57 (d,  Hz, 1H), 8.66 (d,  Hz, 1H) ppm; ¹³C NMR (75 MHz, CDCl₃): δ 27.0 (CH₃), 121.8 (C), 122.7 (C), 127.9 (CH), 128.3 (C), 130.1 (C), 130.3 (C), 130.9 (CH), 131.5 (CH), 133.5 (2CH), 163.6 (2C=O) ppm. HRMS (ESI) calcd for C₁₃H₉BrN (M+H)⁺ 289.9817, found 289.9736.

2.3.3. Sensor 1

4-Bromo-N-methylnaphthalimide (0.102 g, 0.35 mmol) was reacted with 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine (0.017 g, 0.08 mmol) in N,N-dimethylformamide (7 mL), followed by the addition of 0.24 mL of triethylamine under an argon atmosphere. After the solution was refluxed for 18 h, its solvent was removed by rotary evaporator. 20 mL of CH₂Cl₂ was added; then three times of extraction were done using 20 mL of deionized water each. The CH₂Cl₂ portion was dried over anhydrous sodium sulfate before it was removed by rotary evaporator. Finally, to obtain yellow powder of sensor 1, the residue was purified by preparative TLC (CH₂Cl₂ : MeOH 97 : 3, ) to yield 18.3 mg of 1 as a yellow powder, 53%. ¹H NMR (300 MHz, CDCl₃): δ 1.74–1.96 (m, 2H), 2.63–2.72 (m, 6H), 2.98 (t,  Hz, 2H), 3.49 (t,  Hz, 2H), 3.53 (s, 3H), 3.61 (q,  Hz, 2H), 6.72 (d,  Hz, 1H), 7.64 (t,  Hz, 1H), 8.19 (d,  Hz, 1H), 8.20 (s, 1H), 8.46 (d,  Hz, 1H), 8.59 (d,  Hz, 1H) ppm; ¹³C NMR (75 MHz, CDCl₃): δ 26.8 (CH₂), 28.9 (CH₂), 30.1 (CH₂), 30.3 (CH₂), 30.9 (CH₃), 31.5 (CH₂), 37.2 (CH₂), 41.7 (CH₂), 104.5 (CH), 110.8 (C), 120.5 (C), 123.0 (C), 124.9 (CH), 126.1 (CH), 129.7 (C), 132.2 (CH), 134.3 (CH), 149.0 (C=O), 161.2 (C), 164.4 (C=O), 164.9 (C=O) ppm. The calculated HRMS (ESI) for C₂₁H₂₅N₃O₃S₂Na⁺ (M+Na)⁺ is 454.1230 while the obtained HRMS (ESI) is 454.1239.

2.3.4. Sensor 2

Using similar procedure to sensor 1, sensor 2 was obtained as yellow oil with the yield of 6% (2.9 mg), (CH₂Cl₂ : MeOH = 95 : 5, ). ¹H NMR (300 MHz, CDCl₃): δ 1.88–1.98 (m, 2H), 2.70 (t,  Hz, 4H), 2.95 (t,  Hz, 4H), 3.52 (s, 6H), 3.54–3.60 (m, 4H), 6.68 (d,  Hz, 2H), 7.62 (t,  Hz, 2H), 8.10 (d,  Hz, 2H), 8.44 (d,  Hz, 2H), 8.57 (d,  Hz, 2H) ppm; ¹³C NMR (75 MHz, CDCl₃): δ 26.4 (CH₂), 29.5 (2CH₂), 29.9 (2CH₂), 42.9 (2CH₂), 54.9 (2CH₃), 103.7 (2CH), 108.0 (2C), 120.1 (2C), 121.8 (2C), 124.3 (2CH), 128.3 (2CH), 129.2 (2C), 130.5 (2CH), 134.0 (2CH), 150.1 (2C), 163.1 (2C=O), 164.0 (2C=O) ppm. The calculated HRMS (ESI) for C₂₁H₂₅N₃O₃S₂Na⁺ (M+Na)⁺ is 635.1757 while the obtained HRMS (ESI) is 635.1692.

2.4. Binding Studies

For 1 and 2, their binding studies to metal ions were done in acetonitrile (CH₃CN) and CH₂Cl₂, respectively. By dissolving selected amount of salt in water, 1.0 × 10−2 M of perchlorate salt solutions was prepared. The fluorescence titration was carried out with solutions of 1.5 × 10⁻⁷ and 8.2 × 10−7 M for 1 and 2, respectively, and the excitation wavelengths () were 426 nm for 1 and 420 nm for 2. Fluorescence spectra were collected at a fixed wavelength range of 440–620 nm.

3. Results and Discussion

3.1. Design and Preparation of 1 and 2

2-(3-(2-Aminoethylsulfanyl)propylsulfanyl)ethanamine, which has two atoms of S and N for the selective binding sites of Hg²⁺, was utilized as an acyclic host [5, 8]. The selective ion binding was expected to occur via the self-assembly of the acyclic sensors and Hg²⁺ as there are favorable ion-dipole interactions of the ions coordinated with the donor atoms [5, 8]. As a result, there were emission changes of the -methylnaphthalimide fluorophores. Moreover, the acyclic host was expected to provide a high degree of freedom and consequently correct directional binding to Hg²⁺ compared to rigid ionophores, such as cyclens, cyclams, and diazatetrathia crown ethers.

Sensor 1 was synthesized by -alkylation, -imidation, one-pot nucleophilic aromatic substitution, and -formylation of the amine [34]. Sensor 2 was prepared via -alkylation, -imidation, and nucleophilic aromatic substitution.

3.2. Sensitivity Studies

Absorption and fluorescence measurements were performed to explore the spectroscopic and photophysical properties of the designed sensors. As shown in Figure 1, the absorption and emission spectra of solution 1 were in the visible region ( nm,  nm) and there was a large Stokes shift approximately 86 nm in solution of 1. It should be noted that the modification of 2-(3-(2-aminoethylsulfanyl)propylsulfanyl)ethanamine at the 4-position of the naphthalimide fluorophore can induce the absorption and the emission spectra to the red region compared to the reported sensors [8, 35], modifying naphthalimide fluorophore at -position which exhibited absorption and fluorescent emissions in the UV region (,  nm).

Figure 1: Absorption and fluorescence spectra of 1 (0.92 μM) in acetonitrile.

The sensitivity studies of 1 and 2 were performed by adding Hg²⁺ with different concentrations to the solutions of the designed sensors; then observe their emission responses. The sensors showed high Hg²⁺-sensitivity justified from the emission of the modified -methylnaphthalimide fluorophores centered at about 512 nm and 492 nm for 1 and 2, respectively. The fluorescence spectra of sensors 1 and 2 in the absence and presence of various concentrations of Hg²⁺ were illustrated in Figures 2 and 3, respectively.

Figure 2: Fluorescence spectra ( 426 nm) of 1 (0.15 μM) in CH₃CN as a function of [Hg²⁺]. a: 0 M, b: 4.6 μM, c: 6.3 μM, d: 6.8 μM, e: 7.0 μM, f: 7.9 μM, g: 10 μM, and h: 19 μM. (The inset shows intensity of sensor 1 versus concentration of Hg²⁺.)

Figure 3: Fluorescence spectra ( 420 nm) of 2 (0.82 μM) in CH₂Cl₂ as a function of [Hg²⁺]. a: 0 M, b: 1.2 μM, c: 2.7 μM, d: 3.3 μM, e: 4.6 μM, f: 24.3 μM, and g: 79.6 μM. (The inset shows intensity of sensor 2 versus concentration of Hg²⁺.)

The fluorescence signal of the sensor reached the maximum when there was no Hg²⁺ ion. On the other hand, the signal was decreased (“turn off”) when there was an increase in Hg²⁺ concentration. As can be seen in Figure 2, when an ion complexation was operative, there was an “ON-OFF” switching mechanism, responding to Hg²⁺ ion complexation, observed in the fluorescence of 1. Moreover, nonlinear curve fitting of the changes in fluorescence titration results was used to calculate the association constant, [8, 33, 36–40]. In this case, was 3.7 × 10⁷ M⁻¹ for 1 : 1 complex formation of 1-Hg²⁺, which also agreed well with molecular modeling shown in Figure 4 as well as Job’s plot analysis shown in Figure 5. Using coumarin 6 as a reference with a fluorescence quantum yield () of 0.78 in ethanol [41], the of 1 was also observed with the value of 0.14.

(a)

(b)

Figure 4: Optimized structures of (a) 1-Hg²⁺ and (b) 2-Hg²⁺ complexes from MD with local density approximation (LDA) of local functional PWC. (The distances between Hg²⁺ and donor atoms are indicated in Å.)

(a)

(b)

Figure 5: Job’s plots for (a) 1 with Hg²⁺ in CH₃CN and (b) 2 with Hg²⁺ in CH₂Cl₂. The total concentrations of [Sensors] + [Hg²⁺] are 1 μM.

According to the plot of fluorescence intensity as a function of the Hg²⁺ concentrations, the detection limit of 1 for the determination of Hg²⁺ was calculated and found to be 1.1 × 10−7 M or 22 ppb for Hg²⁺. Compared with the previously reported Hg²⁺-fluorescence sensors, the value of detection limit of 1 here is lower or in the same range [8, 14, 19–22, 28, 29, 32, 42–45] and it is low enough to detect mercury levels in environmental sources and dietary [46, 47].

For sensor 2, quantitative binding affinities of 2 with Hg²⁺ were also investigated. As can be seen in Figure 3, sensor 2 provided fluorescence quenching upon Hg²⁺ binding. However, in comparison to sensor 1, sensor 2 was an inferior sensor in term of sensitivity. Moreover, the detection limit for the analysis of Hg²⁺ with the value of 2.94 × 10⁻⁷ M or 59 ppb toward Hg²⁺ was observed for sensor 2. The fluorescence quantum yield () of 2 was obtained with the value of 0.33. In addition, for 2 was 1.1 × 10¹¹ M⁻², suggesting that it should be 1 : 2 complex formation of sensor 2-Hg²⁺.

3.3. Sensor Binding Modes

The Discovery Studio 5.5 program was used to model sensors 1 and 2 as well as the binding conformations of sensors 1 and 2 upon binding Hg²⁺. From the X-ray crystal structure of N,N ′-(3,7-diazanonylene)-bis-napthalimide (PDB 1CX3), the initial structures of 1 and 2 were modified and optimized using CHARMm force field. The low energy configurations of the sensors 1 and 2 were further investigated by molecular dynamics (MD) simulations in the implicit solvent model (in acetonitrile and dichloromethane) at the constant temperature at 300 K for 1000 ps with a time step of 1 fs under NVT ensemble. The distance-dependent dielectrics for acetonitrile and dichloromethane were 37.5 and 8.93, respectively. Moreover, using the density functional theory with local density approximation (LDA) of local functional PWC with implicit distance-dependent dielectrics and the energy of sensors, Hg²⁺ and complex were calculated. The complexation energies of the host-guest structure were obtained from the energy of complex minus energy of sensor minus energy of Hg²⁺ [8, 48].

The results of MD simulations show that ion recognitions of the sensors created the self-assembly of sensor 1 with Hg²⁺ ion and sensor 2 with two Hg²⁺ ions. The sensor-Hg²⁺ recognitions occurred because electrostatic interactions of Hg²⁺ coordinated with the S and N atoms of the sensor are favored, which resulted in the formation of helical wrapping-like structures. Figure 4 shows the distances between Hg²⁺ and donor atoms, indicating the binding sites of Hg²⁺ bound to the sensors. The lowest calculated complexation energies of 1-Hg²⁺and 2-2Hg²⁺ were −212.2 and −137.4 kcal/mol, respectively. These energies suggested the stability of the complexes for a 1 : 1 complex formation of 1 : Hg²⁺ and a 1 : 2 complex formation of 2 : 2Hg²⁺, respectively. These complex formations also agreed well with Job’s plot experiments shown in Figure 5. The Job’s plot analysis with respect to 512 nm (for 1) and 492 nm (for 2) showed maximum fluorescence signals at the mole fraction of 0.5 and 0.67 which can be attributed to the existence of 1 : 1 and 1 : 2 stoichiometry.

3.4. Selectivity Studies

The fluorescence spectra of the sensors before and after the addition of perchlorate salts of Hg²⁺, Co²⁺, Ag⁺, Cd²⁺, Mn²⁺, K⁺, Pb²⁺, Na⁺, Fe²⁺, Ca²⁺, Mg²⁺, and Cu²⁺ were observed and compared. The results show that a quenching in fluorescence appeared clearly after the addition of Hg²⁺ into the solution as shown in Figure 6. The selectivity of 1 to Hg²⁺ is also the highest compared to other cations. Moreover, the quenching of the fluorescence signal cannot be affected by the addition of water (0–6.3 μL) into the solution of sensor as illustrated in Figure 7.

(a)  

(b)

Figure 6: (a) Fluorescence spectra of 1 ( nm, 0.15 μM) with addition of perchlorate salts of Hg²⁺, Co²⁺, Ag⁺, Cd²⁺, Mn²⁺, K⁺, Pb²⁺, Na⁺, Fe²⁺, Ca²⁺, Mg²⁺, and Cu²⁺, (123 μM). (b) The luminescence under a UV of 1 (2.3 μM) in the presence and absence of Hg²⁺, Cu²⁺, Co²⁺, Ag⁺, Cd²⁺, Mn²⁺, K⁺, Pb²⁺, Na⁺, Fe²⁺, Ca²⁺, and Mg²⁺ (3.0 μM).

Figure 7: Fluorescence spectra ( 426 nm) of 1 (0.15 μM) in acetonitrile as a function of water. 0 μL, 0.6 μL, 1.38 μL, 1.5 μL, 2.7 μL, 3.4 μL, 5.1 μL, and 6.3 μL.

For the competitive studies of 1, the competitive fluorescence signaling behaviors of 1 with Hg²⁺ in the presence of 10 equivalents of the other cations as background were investigated at 512 nm and the results were shown in Figure 8. was the initial fluorescence emission signal, while was the fluorescence emission 0.15 μM of 1 observed when there were also competitive background cations in the solution (90 μM each of Cu²⁺, Co²⁺, Ag⁺, Cd²⁺, Mn²⁺, K⁺, Pb²⁺, Na⁺, Fe²⁺, Ca²⁺, and Mg²⁺) and Hg²⁺ (9 μM). When was the fluorescence intensity of 1 in the presence of Hg²⁺ ions only, was equal to 0.21 and was used as a reference. In these competitive studies, values of 1 were 0.20–0.35 which indicated that Hg²⁺-induced fluorescence quenching in the background interfering ions was relatively consistent. In addition, the sensing ability of 1 represented the selectivity for Hg²⁺ in the presence of Ag⁺, Cu²⁺, and Pb²⁺, which were important cation competitors.

Figure 8: (where was the initial fluorescence emission signal and was the fluorescence emission) in the 1-Hg²⁺ system containing [1] = 0.15 μM, [Hg²⁺] = 9 μM and [Mn+] = 90 μM in CH₃CN.

In the selectivity studies of 2, there are some fluorescence intensity changes at 492 nm promoted by some cations including Cu²⁺ and Ag⁺; hence, in terms of the selectivity to Hg²⁺, 2 was a poorer Hg²⁺-fluorescence sensor compared to 1. The lower selectivity of 2 might be due to inappropriate ions-binding sites and the steric effect from two bulky -methylnaphthalimide groups upon Hg²⁺-binding. Moreover, the decrease in Hg²⁺-sensing sensitivity and selectivity may be due to the increase of another unit of the -methylnaphthalimide moiety in the chemical structure of 2, as compared to 1.

3.5. Reversibility Studies

For sensing applications, the fluorescence sensor should have the reversibility features [49]. Herein, the reversibility studies of sensor 1 were carried out. Figure 9 represented the fluorescence intensity of 1 when Hg²⁺ and triethylamine (Et₃N) were added alternately into the solution of 1.

Figure 9: Fluorescence intensity before and after adding alternately Hg²⁺/Et₃N into the solution of 1 (4.5 μM), with the different concentrations: 0/0, 0.7/0, 0.7/1.4, 1.4/1.4, and 1.4/4.4.

In previous sensor applications, triethylamine (Et₃N), tetraethylammonium iodide (TEAI), and ethylenediaminetetraacetic acid (EDTA) had been utilized for reversibility studies [2, 49, 50]. In this study, Et₃N was chosen due to its high complex formation constant with Hg²⁺ (6.31 × 10⁷ M⁻²) [51] and its solubility in acetonitrile. Et₃N was expected to be able to dissociate the 1-Hg²⁺ complexes via the ligand exchange mechanism, as a result sensor 1 can be free again.

Herein, the fluorescence signal of sensor 1 was “turned off” at 512 nm when the Hg²⁺ concentration was added. By adding Et₃N into the solution containing 1 and Hg²⁺, the fluorescence emission of 1 was “turned on,” suggesting the dissociation of the 1-Hg²⁺ complex. On further alternate addition of Hg²⁺ ions and Et₃N, the fluorescence intensities of sensor 1 were “turned off” and “turned on” repeatedly. This result illustrates the reversible behavior of sensor 1 for sensing of Hg²⁺ ions.

4. Conclusion

Novel fluorescent sensors, 1 and 2, for Hg²⁺ detection were successfully developed based on the modified -methylnaphthalimide fluorophores using a conventional three-step synthesis. The modification of the amino group of the ionophore at 4-position of -methylnaphthalimide could induce a bathochromic shift of absorption and the fluorescence spectra to the visible region. Moreover, sensor 1 has high sensitivity and selectivity toward Hg²⁺ over other interfering ions including Cu²⁺, Ag⁺, and Pb²⁺. Sensor 1 also shows reversible behavior which is important feature for applications. Hence, because of the synthetic simplicity, economic cost of material, and the good properties of these new sensors, the new sensor reported here offers great potential for the further fabrication of economical on-site tool for the detection of Hg²⁺ and commercial uses.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the Thailand Research Fund (TRF) and Faculty of Science, Silpakorn University, Thailand (Grant RSA 5680034) for funding. Thanks are due to Faculty of Science, Silpakorn University (Grant SRF-JRG-2557-04) and the Integrated Innovation Academic Center: IIAC Chulalongkorn University Centenary Academic Development Project (Grant CU56-EN14) for partial funding. The authors also acknowledge the Computational Chemistry Unit Cell (CCUC), Faculty of Science, Chulalongkorn University, for software packages and research facilities.

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