A Single Fluorescent Sensor for Hg²⁺ and Discriminately Detection of Cr³⁺ and Cr(VI)

Abstract

An iminocrown ether was synthesized and its fluorescence properties were studied in the presence of a variety of cations and anions in 99 % aqueous medium. The results revealed the interesting ability of the iminocrown ether in discriminately detection of Cr(III) and Cr(VI) ions in addition to detection of Hg²⁺ ion. Among various environmentally relevant metal ions, Cr³⁺ and Hg²⁺ enhanced and quenched the fluorescence emission, respectively and among anions only dichromate ion, Cr(VI), quenched the emission while the rest of ions insignificantly influenced the fluorescence emission. Selectivity of the iminocrown ether was also investigated and proved in the presence of excess of common competing ions. Furthermore, the fluorescence intensity of the iminocrown ether was studied as a function of concentrations of the three ions by performing a titration experiment for each one of them. The detection limits of 5.36 × 10⁻⁸, 2.06 × 10⁻⁶, and 7.49 × 10⁻⁸ mol L⁻¹ were also calculated for Hg²⁺, Cr³⁺, and Cr(VI), respectively.

Introduction

Transition metals are significantly important mostly due to their either hazardous nature or crucial roles in environmental and biological systems [1, 2]. Mercury and chromium are two of these metal-ions which have attracted a great deal of attention. Mercury as Hg²⁺ inorganic salt is one of the most abundant toxic metals and causes environmental problems [3], in addition to a wide variety of health issues such as severe damage in central nervous system, heart, kidney, and genes even in low concentrations [4–6]. Furthermore, among the different valence states of chromium, Cr(III) and Cr(VI) are the most stable and hence the most prevalent ones in nature. Cr³⁺ as an essential trace element for biochemical processes, in addition to improving insulin sensitivity plays a vitally important role in metabolism of carbohydrates, proteins, and lipids [7]. Deficiency of chromium leads to a series of health problems such as diabetes and cardiovascular disease [8]. On the other hand, Cr(VI) easily finds its way into the environment as a pollutant because of its widespread utilization in surface industry, batteries, welding, corrosion control, catalysis, and etc. [9, 10]. Cr(VI) is more hazardous to the environment and humans compared to the other valence states of chromium and leads to severe damages in liver, kidney, and DNA [11, 12]. Hence, development of efficient methods for monitoring trace amounts of mercury and chromium ions has gained considerable attention.

A wide variety of methods such as atomic absorption [13, 14], coupled plasma-mass spectroscopy [15, 16], and voltammetry [17, 18] have been used for mercury and chromium detection in solutions. These methods are costly, time consuming and require precise pretreatments, trained operators and complicated instrumentation. Fluorescent sensors have proved to be convenient alternatives for these methods due to fast response times, low costs, easy detection, high sensitivity, and high selectivity [19, 20]. Thus, development of novel fluorescent sensors is of remarkable interest. Though, a series of fluorescent sensors have been reported for individually detection of Hg²⁺, Cr³⁺ and Cr(VI) ions [21–26], to the best of our knowledge, no fluorescent sensor has been reported capable of detecting these three ions simultaneously. Moreover, most of the reported sensors for these ions suffer from poor solubility and applicability in aqueous media [27–30]. Also some reports have been published for detection of both Hg²⁺ and Cr³⁺ ions, [31, 32] which fail in distinguishing between the two ions. On the other hand, most of the fluorescent sensors reported for Cr³⁺ are of the quenching type because of its paramagnetic nature. Thus, it is an impressive challenge to develop sensors of enhancing type for Cr³⁺. Considering the vitally important role of Cr³⁺ in biological processes and the toxic nature of Cr(VI), distinguishing between the two valence states is significantly important and demanded. As far as we are aware, some reports have been published for distinguishing between Cr³⁺ and Cr(VI) [33, 34], but no claim has been published reporting a fluorescent sensor capable of discriminating between Cr(III) and Cr(VI) ions, meanwhile differentiating them from rest of the ions. Designing a single sensor capable of discriminately detection of Cr(III) and Cr(VI) in addition to detection of Hg²⁺ with functionality in mostly aqueous medium can facilitate detection of the three ions and reduce the costs considerably beside saving the time and trouble of several sample preparation.

Crown ethers, first discovered by Pedersen [35], have been the subject of many studies in various fields such as chromatography [36, 37], phase transfer catalysis [38], ion extraction and transport [39, 40], membranes [41], and electrochemistry [42, 43]. Crown ethers are well-known for selectively formation of complexes with metal ions by electrostatic ion-dipole interaction between metal-ions and electron rich donor atoms of the crown ether. Complex formation of crown ethers is highly influenced by different parameters including the relative size of the crown ether cavity and the metal ion radius, number and type of electron donor atoms in the ring, basicity of the donor atoms, coplanarity of the crown ring, and the electrical charge on the metal ion [35]. These unique properties of crown ethers in complex formation make them promising compounds for selectively detection of metal ions. Nitrogen-containing crown ethers (imino/azacrown ethers) are particularly known for their high affinity toward transition metals [44] and have been the subject of a series of studies in metal recognition area [45]. To date a number of fluorescent sensors based on nitrogen-containing crown ethers have been published for detection of various metal-ions [46, 47] but, only few of them contained imino groups [48]. The interaction of these imino groups with-metal ions result in dramatic changes in the fluorescence properties of the fluorophore which is the key factor in the fluorescent sensors. Herein, we report application of an iminocrown ether as a fluorescent sensor in both cation and anion recognition in 99 % aqueous medium. The three benzo groups, in addition to the quite fixed cavity of the iminocrown ether result in a solid structure and hence a selective sensor. Besides, the two nitrogen atoms in the structure of iminocrown ether enhance its affinity toward transition metals.

Experimental

Materials and Reagents

Salicylaldehyde, O-phenylenediamine (Sigma Aldrich), potassium carbonate, 1,2-diaminobenzene, methanol, dimethylformamide (Merck), and dibromoethane (Acros Organics) were purchased and used without further purification. Stock solutions of all cations were prepared using their nitrate salts, and those of anions were prepared using their sodium or potassium salts.

Aparatus

FT-IR spectra were obtained in KBr disks on a WQF-510A FTIR spectrometer in 400–4000 cm⁻¹ region. Fluorescence measurements were collected on a Cary Eclipse Fluorescent Spectrophotometer. UV-vis absorption spectra were obtained using an Analytik Jena Specord S600 spectrophotometer.

Synthesis of BFE

1,2-Bis(2-formylphenoxy)ethane (BFE) was synthesized based on a previous report [49] with slight modification. Salicylic aldehyde (1.560 g, 12.77 mmol) and 1,2-dibromoethane (1.0 g, 5.32 mmol) were added to dimethylformamide (8.0 mL) containing potassium carbonate (1.765 g, 12.77 mmol), and the mixture solution was stirred for 24 h at 50 °C under nitrogen protection. The resulted dark brown solution was then poured onto 200 mL water while stirring, and 1,2-bis(2-formylphenoxy)ethane was collected by filtration as a brown powder. Finally, colorless crystals were obtained after recrystallization from ethanol and subsequently from ethyl acetate. Yield: 1.09 g (75 %). M.p.: 144–146 °C (lit. [49] 146–148 °C). IR (KBr, cm⁻¹): 3083, 2949, 2870, 1688, 1598, 1483, 1455, 1407, 1250, and 1067.

Synthesis of TBC

3,4:9,10:13,14-Tribenzo-1,12-diaza-5,8-dioxacyclotetradecane-1,11-diene (TBC) was synthesized similar to the procedure reported before [50]. To a solution of BFE (2.7 g, 10 mmol) in 150 mL methanol, (1.08 g, 10 mmol) ortho-phenylene diamine in 30 mL methanol was added and the mixture was refluxed for 3 h. The solvent was evaporated to 30 mL and cooled to room temperature and then 60 mL of water was added while stirring. The suspension was let stand at 0 °C to yield a precipitate which was filtered off and dried at ambient temperature. Yield: 2.4 g (70 %). M.p.: 177–179 °C (lit. [51] 179–180 °C). IR (KBr, cm⁻¹): 3064, 2925, 2880, 1641, 1600, 1585, 1483, 1455, 1407, 1231, and 1052. The overall synthetic procedure of TBC is depicted in Scheme 1.

Scheme 1

synthetic procedure of TBC

Results and Discussion

Cation Sensing

Sensing properties of TBC toward cations were studied in EtOH/H₂O (1:99, v/v) solution following excitation at 290 nm which caused an emission with a maximum at 373 nm. The fluorescence emission changes were monitored upon individually addition of 20 equivalents of different cations including Hg²⁺, Cr³⁺, Mn²⁺, Mg²⁺, Na⁺, K⁺, Ca²⁺, Cd²⁺, Pb²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺ to 3 mL 1 × 10⁻⁵ M solution of TBC. The results (Fig. 1) revealed that the fluorescence intensity of TBC quenched in the presence of Hg²⁺ ions, whereas the presence of Cr³⁺ ions had an opposite effect on the spectrum and enhanced the intensity. The rest of ions negligibly influenced the fluorescence spectrum of TBC. The quenching effect of Hg²⁺ ion can be attributed to the interaction of the ion with electron-donor oxygen and nitrogen atoms of TBC and heavy atom effect [52]. On the other hand, the enhancement of the fluorescence intensity in the presence of Cr³⁺ is probably due to the chelation of TBC with Cr³⁺ ion and inhibition of lone-pair electrons of the imine nitrogen atoms from taking part in photoinduced electron transfer (PET) process [53]. The strong affinity of Cr³⁺ to bind with receptors containing sp² nitrogen atoms is already well-known [54].

Fig. 1

Fluorescence emission of TBC in the presence of different metal ions including Hg²⁺, Cr³⁺, Mn²⁺, Mg²⁺, Na⁺, K⁺, Ca²⁺, Cd²⁺, Pb²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺ in EtOH/H₂O (1:99, v/v) solution, [TBC] = 1 × 10⁻⁵ M and [Mⁿ⁺] = 1 × 10⁻⁴ M (20 eq.) (λ_ex : 290 nm and λ_em : 373 nm)

The quenching mechanism of the sensor with Hg²⁺ was further investigated using the following Stern-Volmer equation (1):

$$ {\mathrm{F}}_0/\mathrm{F}\kern0.5em =\kern0.5em 1\kern0.5em +\kern0.5em {\mathrm{K}}_{sv}\left[\mathrm{Q}\right] $$

(1)

where F₀ and F are the fluorescence intensities in the absence and presence of the quencher, respectively. [Q] is the concentration of the quencher, i.e. Hg²⁺ and K_sv is the quenching constant. Plotting the F₀/F versus concentration of Hg²⁺ ion gave a linear plot (Fig. 2). The linearity of the plot indicates that the quenching mechanism is either purely static or purely dynamic. To determine the right mechanism, absorption spectra of TBC was recorded in absence and presence of Hg²⁺ ion. The absorption spectrum of TBC in presence of Hg²⁺ is distorted from that of TBC in absence of Hg²⁺ ion (Fig. 3) which results from a complex formation between TBC and Hg²⁺ ion before the excitation and suggests that a static quench has taken place. The quenching constant for Hg²⁺ was calculated to be 5.03 × 10⁴ mol⁻¹ by Stern-Volmer equation.

Fig. 2

Stern-Volmer plot for titration of TBC with different concentrations of Hg²⁺ ion

Fig. 3

The absorption spectra of TBC in absence and presence of Hg²⁺, Cr³⁺, and Cr(VI)

To determine the stoichiometry between TBC and the two metal-ions, the Job’s plot method was applied (Fig. 4). For both Cr³⁺ and Hg²⁺ a 1:1 complexation was obtained.

Fig. 4

Job’s plots for the complexation of TBC with Cr³⁺ (a) and Hg²⁺ (b), where I and I₀ are the fluorescence intensities in the presence and absence of the metal-ions

Binding Constant

The binding constants of TBC to both Hg²⁺ and Cr³⁺ were calculated by Benesi-Holdebrand equation (2):

$$ \begin{array}{l}1/\varDelta \mathrm{F}=\left(1/\varDelta {\mathrm{F}}_{\max}\right)+\left(1/\mathrm{K}{\left[\mathrm{C}\right]}^{\mathrm{n}}\right)\ \left(1/\varDelta {\mathrm{F}}_{\max}\right)\hfill \\ {}\varDelta \mathrm{F}=\left({\mathrm{F}}_{\mathrm{x}}-{\mathrm{F}}_0\right),\mathrm{and}\ \varDelta {\mathrm{F}}_{\max }=\left({\mathrm{F}}_{\infty }-{\mathrm{F}}_0\right).\hfill \end{array} $$

(2)

where F₀, F_x, and F_∞ are the emission intensities in absence, at an intermediate, and at a metal-ion concentration of a complete interaction, respectively. K is the binding constant, [C] is the concentration of the metals, and n is the number of binding metal-ions to TBC (here n = 1). The values of K obtained from the slopes were 1.16 × 10⁵ and 1.32 × 10⁴ mol⁻¹ for Hg²⁺ and Cr³⁺, respectively.

Selectivity of the sensor toward Hg²⁺ and Cr³⁺ was also investigated in the presence of 5 equivalents of competing metal-ions (except for Hg²⁺ and Cr³⁺ competition which was performed in presence of 1 equivalent of each ion) in EtOH/H₂O (1:99, v/v) solution. The results (Fig. 5) demonstrated that even much higher concentrations of the competing ions barely affected the fluorescence intensity of Hg²⁺ + TBC systems and proved the high selectivity of the sensor toward Hg²⁺ ion. Furthermore, equivalent amounts of Cr³⁺ did not interfere in Hg²⁺ detection which means TBC has more affinity toward Hg²⁺ than Cr³⁺. The results of selectivity experiment for Cr³⁺ (Fig. 6) also revealed the selectivity of the sensor toward Cr³⁺ ion except in presence of Hg²⁺ ion.

Fig. 5

Selectivity of TBC for Hg²⁺ (1 eq.) in the presence of other metal ions (5 eq. except for Cr³⁺ (1 eq.)) in EtOH/H₂O (1:99, v/v) solution (λ_ex : 290 nm and λ_em : 373 nm)

Fig. 6

Selectivity of TBC for Cr³⁺ (1 eq.) in the presence of other metal ions (5 eq.) in EtOH/H₂O (1:99, v/v) solution (λ_ex : 290 nm and λ_em : 373 nm)

To evaluate the influence of Hg²⁺ and Cr³⁺ concentrations on the fluorescence intensity of the TBC, titration experiment was performed in EtOH/H₂O (1:99, v/v) solution for each ion. The sensor demonstrated higher sensitivity toward Hg²⁺ compared to Cr³⁺ and it was observed that relatively lower starting concentrations of Hg²⁺ ions were utilized for the experiment. With increasing the ions concentration, the emission intensity of TBC decreased gradually (Fig. 7) and a good linearity was obtained between Hg²⁺ concentrations and reduction in the fluorescence intensity of the sensor (Fig. 7, inset). As for Cr³⁺ ions, with the increase of the ion concentration, the fluorescence emission intensity increased (Fig. 8) and resulted in a linear plot of the Cr³⁺ concentrations versus the fluorescence intensity (Fig. 8, inset). Furthermore, detection limits (DL) were calculated for both Hg²⁺ and Cr³⁺ in EtOH/H₂O (1:99, v/v) solution on the basis of DL = 3σ/m, where σ is the standard deviation of blank solutions and m is the slope of fluorescence intensity against concentration of the target ion. The detection limit values obtained as 5.36 × 10⁻⁸ and 2.06 × 10⁻⁶ mol L⁻¹ were for Hg²⁺ and Cr³⁺, respectively which show the sensitivity of the sensor toward both ions particularly Hg²⁺.

Fig. 7

Fluorescence emission of TBC upon addition of Hg²⁺ ions in EtOH/H₂O (1:99, v/v) solution, [TBC] = 1 × 10⁻⁵ M, [Hg²⁺] = (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9 and10 eq.). Inset: fluorescence emission as a function of Hg²⁺ concentration (0–1 eq.) (λ_ex : 290 nm and λ_em : 373 nm)

Fig. 8

Fluorescence emission of TBC upon addition of Cr³⁺ ions in EtOH/H₂O (1:99, v/v) solution, [TBC] = 1 × 10⁻⁵ M, [Cr³⁺] = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and10 eq.). Inset: fluorescence emission as a function of Cr³⁺ concentration (0–10 eq.) (λ_ex : 290 nm and λ_em : 373 nm)

Anion Sensing

The sensing properties of TBC were also investigated toward anions under the same conditions as for cations. Fluorescence emission of TBC was studied in the presence of a variety of anions including Cr₂O₇ ²⁻, CN⁻, HCO₃ ⁻, CO₃ ²⁻, HPO₄ ²⁻, CH₃CO₂ ⁻, NO₃ ⁻, H₂PO₄ ⁻, S₂O₃ ²⁻, F⁻, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, SCN⁻, and NO₂ ⁻ (Fig. 9). It was observed that the fluorescence emission interestingly quenched in the presence of dichromate ions, Cr(VI), which is a stable form of chromium other than Cr(III). The rest of anions either slightly increased or negligibly decreased the intensity, meaning no interference in Cr(VI) detection. The quenching mechanism of TBC with Cr(VI) was further investigated by the same procedure previously applied for Hg²⁺ ion using the Stern-Volmer equation (1). Results (Fig. 10) show a linear plot of F₀/F against the concentration of Cr(VI) ion which indicates an either purely static or purely dynamic quench as the quenching mechanism of TBC emission by Cr(VI) ion. Since the absorption spectrum of TBC in the presence of Cr(VI) ion distorts from that of TBC in absence of the ion, a static quench occurs. The value of quenching constant was also calculated as 2.37 × 10⁵ mol⁻¹. The complexation between TBC and Cr(VI) was also determined by Job’s plot method and results showed a 1:1 stoichiometry (Fig. 11).

Fig. 9

Fluorescence emission of TBC in the presence of different anions including Cr₂O₇ ²⁻, CN⁻, HCO₃ ⁻, CO₃ ²⁻, HPO₄ ²⁻, CH₃CO₂ ⁻, NO₃ ⁻, H₂PO₄ ⁻, S₂O₃ ²⁻, F⁻, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, SCN⁻, and NO₂ ⁻ in EtOH/H₂O (1:99, v/v) solution, [TBC] = 1 × 10⁻⁵ M and [A^n-] = 1 × 10⁻⁴ M (20 eq.) (λ_ex : 290 nm and λ_em : 373 nm)

Fig. 10

Stern-Volmer plot for titration of TBC with different concentrations of Cr(VI) ion

Fig. 11

Job’s plots for the complexation of TBC with Cr(VI), where I and I₀ are the fluorescence intensities in the presence and absence of the metal-ions

Binding Constant

The binding constant of TBC to Cr(VI) was also estimated by the same procedure illustrated before for Hg²⁺ and Cr³⁺ ions, using the Benesi-Hidebrand equation (2). The value of the binding constant was obtained as 9.97 × 10⁴ mol⁻¹.

To ensure the lack of interference by common accompanying anions in detection of dichromate, selectivity test was also performed in which the fluorescence emission changes of Cr(VI) + TBC system were examined in the presence of various competing ions. The obtained results (Fig. 12) revealed that competing anions even in much higher concentrations (5 eq.) insignificantly affected the emission of the system and this fact proved high selectivity of the sensor toward Cr(VI) ion.

Fig. 12

Selectivity of TBC for Cr(VI) (1 eq.) in the presence of other anions (5 eq.) in EtOH/H₂O (1:99, v/v) solution (λ_ex : 290 nm and λ_em : 373 nm)

A titration experiment was also performed to evaluate the effect of Cr(VI) concentration on the fluorescence intensity. As presented (Fig. 13), upon addition of Cr(VI) concentrations to TBC solution, the fluorescence emission intensity decreased gradually. Also, plotting the concentrations of dichromate ions against the emission intensity (Fig. 13, inset), showed a linear relationship between the two factors. The detection limit was then calculated for Cr(VI) ion by a similar method applied for Hg²⁺ and Cr³⁺ before and 7.49 × 10⁻⁸ mol L⁻¹ was obtained which shows high sensitivity of the sensor toward Cr(VI) ion.

Fig. 13

Fluorescence emission of TBC upon addition of Cr(VI) ions in EtOH/H₂O (1:99, v/v) solution, [TBC] = 1 × 10⁻⁵ M, [Cr(VI)] = (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 eq.). Inset: fluorescence emission as a function of Cr(VI) concentration (0–1 eq.) (λ_ex : 290 nm and λ_em : 373 nm)

Conclusion

TBC was synthesized with a two-step procedure. The fluorescence properties of the synthesized iminocrown ether were investigated in the presence of a variety of cations and anions and results revealed its capability in selectively detection of Hg²⁺, Cr³⁺, and Cr(VI) in 99 % aqueous media. Quenching mechanisms were also determined to be of static type for Hg²⁺ and Cr(VI) ions with K_sv = 5.03 × 10⁴ and 2.37 × 10⁵ mol⁻¹, respectively. The binding constants were also estimated as 1.16 × 10⁵, 1.32 × 10⁴, and 9.97 × 10⁴ mol⁻¹ for Hg²⁺, Cr³⁺, and Cr(VI), respectively. Furthermore, in the selectivity experiment, even 5 equivalents of the common competing ions did not interfere in recognition of the target ions which proves the high selectivity of the sensor toward the ions. Titration experiments demonstrated a linear relationship between the concentration of the ions and the fluorescence intensity variation. Finally, the limits of detection were calculated as 5.36 × 10⁻⁸, 2.06 × 10⁻⁶ and 7.49 × 10⁻⁸ mol L⁻¹ for Hg²⁺, Cr³⁺, and Cr(VI), respectively. Quite low detection limits acquired, showed the high sensitivity of the sensor toward the three ions, particularly Hg²⁺ and Cr(VI).