A Reversible Bis(Salamo)-Based Fluorescence Sensor for Selective Detection of Cd2+ in Water-Containing Systems and Food Samples.

A novel, simple, highly selective, and sensitive fluorescence chemosensor for detecting Cd2+ that was constructed from a bis(salamo)-type compound (H₄L) with two N₂O₂ chelating moieties as ionophore was successfully developed. Sensor H₄L could show fluorescence turn-on response rapidly and significant selectivity to Cd2+ over many other metallic ions (Cu2+, Ba2+, Ca2+, K⁺, Cr3+, Mn2+, Sr2+, Co2+, Na⁺, Li⁺, Ni2+, Ag⁺, and Zn2+), and a clear change in color from colorless to yellow that can be very easily observed via the naked eyes in the existence of Cd2+, while other metallic ions do not induce such a change. Interestingly, its fluorescent intensity was increased sharply with the increased concentration of Cd2+. The detection limit of sensor H₄L towards Cd2+ was down to 8.61 × 10-7 M.

1. Introduction

Metallic ions play a key role in daily life [1,2,3,4]. Cadmium, which is an essential resource in the earth, is widely used in all kinds of agricultural processes and industry containing chemical industry, electronics industry, nuclear industry semiconducting, quantum dots, phosphate fertilizers, and other fields [5]. However, it should be vigilant that Cd2+ is a heavy metallic ion with highly toxic [6]. Some new studies reveal that a considerable amount of cadmium go into the environment from waste water, waste residue, and exhaust gas not only damages the environment, but also endangers human health [7]. Hence, with ever-increasing concern for environment and human health, there is now a much greater demand for the development of a rapid and convenient detection method for Cd2+ ion [8,9].

Up until now, with the development of optical sensors for recognizing heavy and transition metal ions in living organisms [10,11], intense efforts have been devoted to the design and synthesis of high sensitivity fluorescence sensors due to the lower cost and rapid response, as well as the easy operability of the fluorescent technique [12,13,14,15,16,17]. According to the relevant literatures, the metallic coordination compounds with salen-type N2O2 ligands and their corresponding analogues could be used to catalysis [18], nonlinear optical materials and magnetic materials [19,20,21,22,23,24,25,26,27,28], supramolecular architecture [29,30], ions recognition [31,32,33,34,35,36,37,38,39,40], and biological fields, and so forth [41,42,43,44,45,46,47]. Today, researches on the participation of bis(salamo)-type compounds in ion recognition are not to be explored [48,49,50,51,52,53,54,55,56,57,58]. Notably, when compared with most of known fluorescence probes for Zn2+ and Cu2+, there are relatively few reports on fluorescent probes for Cd2+.

As part of our ongoing interest in the development of new optical (both colorimetric and fluorescence) chemosensor, in the present paper, we report a bis(salamo)-type tetraoxime sensor H4L for detecting Cd2+ by turn-on response in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solution. The sensor H4L has higher sensitivity for Cd2+ than other metallic ions that are based upon change in color by naked eyes. The mechanism of fluorescence change has been well demonstrated. The sensor H4L as a reliable fluorescence probe displayed high sensitivity toward Cd2+ in water-containing systems, and was able to detect Cd2+ in food samples.

2. Experimental

2.1. Materials and Methods

All of the reactions were performed under an air atmosphere. Boron tribromide (99.9%), methyl trioctyl ammonium chloride (90%), 2-hydroxy-3-methoxybenzaldehyde (99%), and pyridinium chlorochromate (98%) were gained from Alfa Aesar. Hydrobromic acid 33 wt % acetic acid solution was gained from J & K Scientific Ltd. (Beijing, China). Other solvents and reagents that were used in this work were analytical grade from Tianjin Chemical Reagent Factory (Tianjin, China). Melting points were measured using a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company (Beijing, China) and were uncorrected. 1H NMR spectra were made via German Bruker AVANCEDRX-400 spectrophotometer (Karlsruhe, Germany). All of the UV–vis and fluorescence spectra tests were measured on a Shimadzu UV-2550 (Kyoto, Japan) and Perkin-Elmer LS-55 spectrometer (Waltham, MA, USA). The solvent also has a great influence on the fluorescence of the complex. Through a series of experiments, we finally obtained the best selectivity for cadmium ions in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) [59,60].

2.2. Preperation of Sensor H4L

The bis(salamo)-type tetraoxime sensor H4L was prepared on the basis of the reported methods [61,62,63,64,65,66,67,68]. The 1H NMR, IR, and UV-vis spectra of H4L are nearly consistent with the previous result (Figure 1). The major reaction steps of sensor H4L are demonstrated in Scheme 1.

Figure 1

1HNMR spectra of sensor H4L in DMSO.

Scheme 1

Synthetic route to sensor H4L.

3. Results and Discussion

3.1. pH Effect of Sensor H4L

For the sake of avoid the disturbance by protons in the recognition process of heavy metallic ions and to get optimum conditions, we concentrated on the pH influence on the fluorescence intensity. As depicted in Figure 2, the sensor H4L have barely changed in the fluorescent intensity from pH = 3.0 to 11.0, indicating that sensor H4L was consistent. The weak fluorescent sensor H4L may be owing to intra-molecular photo-induced electron transfer. However, H4L-Cd2+ showed strong fluorescence, on account of the bonding of H4L with Cd2+ lead to the inhibition of intra-molecular photo-induced electron transfer process. The results indicated that sensor H4L can be used to detect Cd2+ and the process of detection was not affected greatly by pH values.

Figure 2

Changes in fluorescence intensity of H4L (c = 1 × 10−5 M) and Cd2+ (c = 1 × 10−3 M) performed at different pH values at room temperature. (DMF/H2O = 9:1, v/v, λex = 323 nm, λem = 495 nm).

3.2. General UV–vis Measurements

UV–vis spectra of sensor H4L in the existence of 14 metallic ions (Cd2+, Cu2+, Ba2+, Ca2+, K+, Cr3+, Mn2+, Sr2+, Co2+, Na+, Li+, Ni2+, Ag+, and Zn2+ with nitrate anions) were investigated. As shown in Figure 3a, sensor H4L have two intense absorption bands at 313 and 358 nm, which could be attributed to π → π* transition and reveals that sensor H4L includes a larger conjugation system. However, when 3.0 equiv. Cd2+ were added into the mixed solution, a new absorption peak emerged at 375 nm (Figure 3a). Meanwhile, the change in absorbance is almost same for Cd2+, Ni2+, Zn2+, and Mn2+ ions, indicating that the H4L were also involved in coordination to the Cd2+, Ni2+, Zn2+, and Mn2+ ions, respectively. However, when Cd2+ ion was added, sensor H4L could show a clear change in color from faint yellow to green that can be very easily observed via the naked eyes under UV light at 365 nm, while other metallic ions do not induce such a change. The colour change that is observed is mainly due to the charge transfer transition of the complex. The results revealed sensor H4L could be applied to selective identification of Cd2+ according to the color changes.

Figure 3

(a) UV-vis spectra of sensor H4L (1 × 10−5 M) recorded in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions after addition of 3.0 equiv. of metallic ions (1 × 10−3 M); (b) Absorption spectra of sensor H4L in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions upon gradual addition of Cd2+ (0.0 to 3.0 equiv.).

The responses of sensor H4L to Cd2+ (1 × 10−3 M) were studied further via UV–vis titration experiments, as depicted in Figure 3b. With the addition of Cd2+ from 0.0–3.0 equiv. in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions, two peaks that were observed at 342 and 358 nm were clearly declined and a new band at 375 nm emerged. Meanwhile, one definitive isosbestic point was also noted at 326 nm, indicating that a new species is produced. When 3.0 equiv. Cd2+ was added, the absorption no longer change, which showed 1:3 stoichiometry between Cd2+ and sensor H4L (Figure 3b).

3.3. General Fluorescence Measurements

For the sake of evaluating the selectivity of sensor H4L, original screening of H4L to the bonding ability of metallic ions was performed in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions. The fluorescent spectra of sensor H4L in the existence of a series of metallic ions (Cd2+, Cu2+, Ba2+, Ca2+, K+, Cr3+, Mn2+, Sr2+, Co2+, Na+, Li+, Ni2+, Ag+, and Zn2+ with nitrate anions) were gained, followed via excitation at 323 nm. As shown in Figure 4a, when Cd2+ ion was added, the position of the emission peak was red-shifted from 412 to 486 nm, the fluorescence intensity increased from 125 to 712, and the fluorescence quantum yield increased from 2.6 to 13.8%. Significantly, H4L showed excellent selectivity for Cd2+ in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions with a strong fluorescence response, with either a very weak or no fluorescence response to other metallic ions (Figure 4a). It is different from the previously reported sensors for Cd2+ showing a wide range of response to Zn2+, sensor H4L nearly non-fluorescent respond to Zn2+ could be owing to its rigid cavity with bigger size. Therefore, this rigid cavity could be definitely suitable for bonding with Cd2+, but not suitable for bonding with Zn2+ that possess smaller ionic radius. Furthermore, according to the corrected Benesi-Hildebrand formula, the bonding constant for the bonding of Cd2+ to sensor H4L was estimated as 4.98 × 104 M−1 and Zn2+ to sensor H4L was estimated as 3.89 × 104 M−1 [69]. The result suggested that sensor H4L displays outstanding selectivity for Cd2+ than other metallic ions.

Figure 4

(a) Fluorescent spectra and (b) fluorescent intensity at 323 nm of sensor H4L (1 × 10−5 M) in the existence of different metallic ions (c = 1 × 10−3 M, excess amounts Cu2+, Ba2+, Ca2+, K+, Cr3+, Mn2+, Sr2+, Co2+, Na+, Li+, Ni2+, Ag+, and Zn2+) in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0, λex = 323 nm) solution (the black bars delegate the addition of excess amounts of metallic ions to the solution of probe H4L and the pink bars delegate the subsequent addition of Cd2+ to the mixed solution).

For the sake of the practicability of sensor H4L as a receptor of Cd2+ selective probe, competitive experiments were performed with various metallic ions in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions. As shown in Figure 4b, there are no other ions can result any clear changes in the fluorescent spectrum of sensor. The selectivity of sensor H4L for Cd2+ and other metallic ions was measured. These results also demonstrated that other metallic ions could not interfere with the detection of Cd2+.

To further understand the coordination of sensor H4L with the Cd2+, fluorescent responses of sensor H4L to changing concentrations of Cd2+ in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions at room temperature were investigated in Figure 5. With the addition of increasing Cd2+ at an excitation wavelength of 323 nm, the fluorescent emission intensity at 485 nm gradually raised while the intensity at 407 nm reduced. Furthermore, after the addition of Cd2+, the absorbance at 485 nm exhibited a sharp increase when the ratio of [Cd2+]/[H4L] is below 3:1, and no longer change when the ratio reaches 3:1. In addition, the fitting curve of fluorescence emission intensity with Cd2+ concentrations was obtained by the data obtained of the fluorescence titration experiment (Figure 5). We have performed 1H NMR spectra of sensor H4L and in which the presence of 3.0 equiv. Cd2+ (Figure S1). Moreover, the detection limit is a considerably significant parameter in molecular recognition, the LOD and LOQ parameters were estimated to be 8.61 × 10−7 M and 2.87 × 10−6 M, respectively [61,69]. The LOD and LOQ were calculated based on the following equations:LOD = 3 × δ/S; LOQ = 10 × δ/S. where S represents the standard deviation of the blank measurements, and δ is the slope of the intensity versus sample concentration curve.

Figure 5

(a) Fluorescent emission spectra of sensor H4L (1 × 10−5 M) with subsequent addition of Cd2+ (0–3.0 equiv. λex = 323 nm) in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions. Inset: Naked-eyes visible colour changes of sensor H4L solution before and after addition of Cd2+. (b) Linear fitting of sensor H4L to Cd2+ bonding constant.

At the same time, the Hill equation is used in determining the binding constant of ions and H4L, the bonding constant for the bonding of Cd2+ to sensor H4L was estimated as 4.98 × 104 M−1 [48,59]. These results indicate that probe H4L displays satisfactory Cd2+ detecting ability. log(F-F where Fmin, Fmax, and F are the emission intensities in the absence, presence of saturated Cd2+, and the addition of a given amount of Cd2+ concentration, respectively. [Cd2+] is the concentration of free Cd2+.

To know the stoichiometry between the sensor H4L and Cd2+ in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions, job’s plot has been performed (Figure 6). When the molar fraction of Cd2+ was 0.75, the intensity at 495 nm reached an extreme value, indicating the formation of a 3:1 complex between Cd2+ and H4L.

Figure 6

The job’s plot examined between Cd2+ and H4L, indicating the 3:1 stoichiometry, which was carried out by fluorescence spectra (λex = 323 nm).

Furthermore, according to the reversible fluorescent switch of the sensor H4L to Cd2+ in a coordination compound solution, we regard it as a two-input molecular logic gate, while the emission at 373 nm serves as the output. As depicted in Figure 7 and Table 1, when the output was zero (both the Cd2+ and EDTA are absent), corresponding the gate being closed and this system shows weak fluorescence. When Cd2+ alone was existent, the output is one and the relevant to the gate being open, so intense fluorescence was observed. Thus, the sensor H4L is able to serve as a logic gate. This result demonstrates that the sensor H4L as a reversible fluorescence probe.

Figure 7

(a) Schematic presentation of “OFF–ON” system for sensor H4L (c = 1 × 10−5 M) in the existence of Cd2+ (c = 1 × 10−3 M) and EDTA (c = 1 × 10−3 M); (b) Performance of sensor H4L in the INHIBIT gate mode in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0, λex = 373 nm). The bars display the fluorescent output of sensor H4L at 373 nm in the existence of Cd2+ and EDTA serve as inputs.

Table 1

The molecular logic gate and the each symbolic expression of the INHIBIT logic gate function.

Inputs		Outputs
Cd2+	EDTA	(INHIBIT Logic Gate) Intensity λmax
0	0	low flu.	0
1	0	high flu.	1
0	1	low flu.	0
1	1	low flu.	0

3.4. The Detecting Mechanism of H4L for Cd2+

According to the fluorescent spectra, the detecting mechanism of the sensor H4L for Cd2+ was suggested, as follows (Figure 8). The fluorescent intensity of the sensor H4L response to Cd2+ may be assigned to CHEF and PET. Before being coordinated with Cd2+, sensor H4L displayed a weaker fluorescence due to the lone pair of electrons of nitrogen atoms, which gives rise to an intra-molecular PET. Furthermore, the lone electron pairs of the nitrogen atoms give rise to a nonradiative process by the n-π* state, which also led to a wide degree of fluorescent quenching. Conversely, after H4L was coordinated to Cd2+, the radiation process was primarily via the π-π* state and the coordination compound was more rigid [48,59]. In addition, the PET process was restrained by the addition of Cd2+ analyte at the receptor site. It is obvious that the appearance of the ICT process influenced PET in this system, but fluorophore and receptor distances and the orientation between them can also contribute in the overall PET process.

Figure 8

The sensing mechanism of the sensor H4L for Cd2+.

3.5. Time and Temperature Effect of Probe H4L

The influences of time and temperature on the fluorescent intensity of sensor H4L toward Cd2+ were also studied in Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) solutions. Figure 9 illustrates that fluorescence intensity of sensor H4L did not vary with further prolong the reaction time. Besides, the fluorescent intensity of sensor H4L almost constant in the temperature range of 0–90 °C. Therefore, sensor H4L could be used for rapid response to Cd2+ in room temperature, which is of significant practicability for the detection of Cd2+.

Figure 9

(a) Fluorescence intensity changes of the solution contain sensor H4L and 3.0 equiv. Cd2+ with the delay of time; (b) Fluorescent intensity changes of the solution contain H4L and 3.0 equiv. Cd2+ at different temperatures.

3.6. Test Strips Measurements

Currently, test strips analytical equipment have gained momentous attention on account of their high sensitivity, low-cost, and quick response. It is perhaps the most convenient of modern detection tools, as a change in color could be seen via the naked eyes. Hence, in this study, filter papers were performed via soaking filter papers into Tris-phosphate buffer (c = 0.2 M, DMF/H2O = 9:1, v/v, pH = 7.0) mixed solutions of probe H4L (1 × 10−5 M), and then drying by exposure to air. The filter papers, including sensor H4L, were applied to detect Cd2+ and other metallic ions. As depicted in Figure 10, after Cd2+ and the other metallic ions were added on the test tools, respectively, the distinct color changes were seen merely with Cd2+ solution under UV light at 365 nm, and potentially competitive metallic ions have no work on the detection of Cd2+ via the filter papers (Figure S2). It is a really actual convenient method that quickly measures the Cd2+, and it could be able to be satisfactorily used in the fields of food security and environmental surveillance.

Figure 10

The result of colorimetric measured photographs with sensor H4L for detecting Cd2+ under irradiation at 365 nm.

3.7. Application in Food Samples

Furthermore, we investigated the applicability of sensor H4L in food samples. 500 mg of crushed naturally containing cadmium rice were put into the PTFE microwave digestion tank by the addition of 3 mL of HNO3 and 4 mL of H2O2, and the mixture solution containing cadmium(II) needs be digested after keeping 30–60 min. Then, the mixture solution was dried in vacuo and was added into the mixed solution of DMF and H2O (c = 1 × 10−3 M, DMF/H2O = 9:1, v/v, pH = 7). As shown in Figure 11, after added the sample solution to sensor H4L, the fluorescence intensity is on the increase.

Figure 11

Fluorescence spectral response of sensor H4L in cadmium contaminated rice.

4. Conclusions

In this paper, we presented a new bis(salamo)-type fluorescence sensor H4L, which could serve as a great promising analytical kit for measuring Cd2+ via different fluorescence changes and changes in color from light yellow to green that can be measured via naked eyes. The detection limit about fluorescent response of probe H4L to Cd2+ was down to 8.61 × 10−7 M. As designed, the filter papers could conveniently, efficiently, and simply detect Cd2+ in solution. Moreover, Cd2+ in the food samples was detected by the sensor H4L simply and effectively. We believe this study will inspire the development of bis(salamo)-based chemosensor by optimizing unsaturated metal coordinating sites for practicability to many other metallic ions detecting in analytical chemistry, medical treatment, biological, and environmental fields.