Colorimetric cadmium ion detection in aqueous solutions by newly synthesized Schiff bases

Two newly synthesized Schiff bases DMCA and DMBA were used for selective detection of Cd2+ over a wide range of other metal ions in acetonitrile (ACN)/ Tris-HCl buffer (10 mM, pH 7.32, v/v 2:1). The sensors can detect Cd2+ ions by colour changes from colourless to orange for DMBA and yellow to reddish for DMCA. Response of the probes towards metal ions was investigated by using UV-vis spectroscopy. The complex stoichiometry between the sensors, DMBA and DMCA, and Cd2+ was found to be 2:1 and the binding constants were calculated to be 2.65 ×1012 M-2 and 4.95 ×1012 M-2, respectively. The absorbance-based detection limits of DMBA and DMCA were calculated as 0.438 μM and 0.102 μM, respectively. The sensors were also successfully applied to real samples.


Introduction
As one of the highly toxic heavy metal ions, cadmium is widely distributed in water, soil and crops, generated from its use sources such as fertilizers, the combustion of fossil fuels, paint pigments, Ni-Cd rechargeable batteries, causing serious problems for human health [1][2][3]. Due to the high affinity to sulphur, Cadmium ion (Cd 2+ ) can interfere with Ca 2+ and Zn 2+ in the binding sites of some enzymes containing sulphur [4,5]. It causes these enzymes to malfunction, causing severe organ damage. Cadmium and cadmium compounds are category I carcinogens [6], and are known to be associated with liver and kidney damage, cancer mortality and cardiovascular disease [7][8][9]. Thus, it is an essential point to develop detection methods for cadmium.
Several methods have been reported to detect Cd 2+ ; however, these methods are generally expensive, time consuming and have sophisticated synthetic procedures [10][11][12][13]. As alternative methods, fluorometric and colorimetric sensors require easier procedures. In recent years, fluorescent sensors have gained growing interest in detecting Cd 2+ ions [14][15][16][17]. Many of them, however, have some limitations such as having a poor detection limit [18] and complicated synthesis steps [19]. Moreover, most of the sensors for Cd 2+ also give response to Zn 2+ ions due to their similar properties [20]. Recently, several colorimetric sensors for Cd 2+ have been also reported [21][22][23][24]. These sensors also suffer from long response time, poor selectivity, complex organic synthesis and poor detection limits [25]. Nowadays, Schiff base derivatives have been increasingly used as colorimetric sensors due to having simple synthesis steps and high selectivity. Especially, cinnamaldehydebased and benzaldehyde-based Schiff base derivatives have been developed for the detection of metal ions such as Ni 2+ [26], Ag + [27], Al 3+ [28,29] Cu 2+ /Hg 2+ [30]. However, simple, rapid, highly selective, and sensitive colorimetric sensors for Cd 2+ are still rare and needed to be developed.
In this paper, we presented 2 newly synthesized Schiff base derivatives, one of them is a benzaldehydebased sensor and the other is a cinnamaldehyde-based sensor, which detected Cd 2+ ions by colour change in aqueous solutions. Response of the sensors was investigated using UV-vis spectroscopy in the presence of various metal ions. The detection limits of DMBA and DMCA were calculated to be 0.438 µM and 0.102 µM, respectively.

Materials and methods
N-phenyl-o-phenylenediamine, 4-(dimethylamino)cinnamaldehyde and 4-(dimethylamino)benzaldehyde were purchased from Sigma-Aldrich. The solvents and the other chemicals used in the experiments were commercially obtained. The solution of Fe 3+ and Fe 2+ was prepared separately by dissolving in 0.1 M HCl. Unless otherwise stated, the solutions of the metal ions tested were prepared from nitrate salts or chloride salts of them in deionized water. A NMR spectrometer (a Bruker NMR spectrometer (Bruker Ultrashield Plus Biospin Avance III 400 MHz NaNoBay FT-NMR)) was used to record 1 H and 13 C NMR spectra. An Agilent LC-MS/MS 6460Triple Quadrupole mass spectrometer was used to perform ESI-MS analyses. Shimadzu UV-1800 spectrophotometer was used to record UV-vis spectra.

UV-vis absorption measurements
DMBA (3.15 mg, 0.01 mmol) and DMCA (3.42 mg, 0.01 mmol) were dissolved in ACN (10 mL) and 30 µ L of the sensors (1 mM) were diluted with 1.470 mL ACN/Tris-HCl buffer (10 mM, pH 7.32, v/v 2:1) to make final concentrations of 20 µ M. For each spectrum, 1.0 mL of a probe solution was added to a 1-cm quartz cell, to which different stock solutions of cations were gradually added by using a micro-pipette. All absorption spectra were collected from 220 to 800 nm. Upon addition of each metal ions tested to the sensors solutions, the spectral readings were immediately recorded.

Determination of Cd 2+ in real samples
To evaluate the analytical applicability of the sensors, DMBA and DMCA, they were used to detect Cd 2+ ions in tap water samples collected in Osmaniye, Turkey. The tap water samples were spiked with solutions of Cd 2+ and were diluted with ACN/Tris-HCl buffer (10 mM, pH 7.32, v/v 2:1) to obtain samples at concentrations of 0, 2, 5, and 10 mg. L −1 (ppm) Cd 2+ , respectively. All spectroscopic measurements were done under the same experimental conditions proposed for the selectivity experiments, and measurements were performed at least triplicate and resulting averages were reported.

Design and synthesis of the sensors, DMBA and DMCA
The molecular structures of the sensors were designed to contain an N-phenyl-o-phenylenediamine as a binding part for Cd 2+ , a cinnamaldehyde moiety (for DMCA), and a benzaldehyde moiety (for DMBA) as chromophore parts. The binding parts of the sensors consist of 2 nitrogen atoms to give 2 5-membered rings in 2:1 binding between the sensors and Cd 2+ . The binding part and the chromophore parts were linked via the formation of the C =N bonds in a 1-step procedure with 81% and 86% yields for DMBA and DMCA, respectively (Scheme 1). The structures of the sensors were verified by NMR ( 13 C NMR and 1 H NMR) and ESI-mass spectrometry. Scheme 1. Synthesis of the probes, DMBA and DMCA.

The absorption and colorimetric properties of DMBA and DMCA
We first evaluated the spectroscopic properties of DMBA and DMCA and their interactions with various metal ions. The colourless compound DMBA (20 µM) in ACN/Tris-HCl buffer (10 mM, pH 7.32, v/v 2:1) displays a maximum absorption at 349 nm ( ε = 4.75 ×10 4 M −1 cm −1 , only DMBA) that may be ascribed to nπ * transition [31]. However, the addition of Cd 2+ resulted in a decrease in the absorption intensity at 349 nm and formation of a new absorption peak at 488 nm ( Figure 1a) with a remarkable colour change from colourless to orange (Figure 1b inset). As depicted in Figure 1b, the tested metal ions including Cu 2+ , Cr 3+ , Cu + , Na + , Hg 2+ , Mg 2+ , Ca 2+ , Zn 2+ , Ag + , Pb 2+ , K + , Co 2+ , Fe 2+ , Mn 2+ , and Ni 2+ did not respond to DMBA while Fe 3+ caused a decreasing the absorption intensity at 349 nm without any increase at 488 nm. The detection of the target cation in the presence of other metal ions in real sample is an important assay. Competition experiments were performed to confirm the high selectivity of the detection system. First, the meal ions (200 µ M) such as Cu 2+ , Cr 3+ , Cu + , Na + , Hg 2+ , Mg 2+ , Ca 2+ , Zn 2+ , Ag + , Pb 2+ , K + , Co 2+ , Fe 2+ , Mn 2+ and Ni 2+ were preincubated with DMBA (20 µ M). As expected, no remarkable change was observed (red bars in Figure 2a). However, the addition of Cd 2+ (20 µ M) to each of them resulted in an increase in the absorption intensity at 488 nm (blue bars in Figure 2a). These results show that none of the metal ions tested affect the sensing properties of DMBA to Cd 2+ . Moreover, the effects of pH on the stability of the sensor and its Cd 2+ ?complex were investigated and monitored by absorption spectra in a pH range from 1 to 10. The pH of the solutions was adjusted by adding HCl (0.1 m) and NaOH (0.1M) into the solutions. As depicted in Figure 2b, the sensor, DMBA, is not stable at pH 1-2 and gives response to H + ions at pH 3. The absorbance intensity of DMBA remains constant at pH between 4 and 9, which indicates that Cd 2+ can be detected with DMBA in the environmental pH 4-9.
We repeated the same selectivity experiments for DMCA. The absorption spectral changes of DMCA after coordination with Cd 2+ in ACN/Tris-HCl buffer (10 mM, pH 7.32, v/v 2:1) were studied first. As seen in Figure 3a, the solution of DMCA alone (20 µM) exhibits an absorption maximum at 396 nm ( ε = 5.08 Cd 2+ ion can be detected with DMCA in the environmental pH range of 4-10.

Investigation of complexation between the sensors and Cd 2+
In order to confirm the binding stoichiometry between the sensors, DMBA and DMCA, and Cd 2+ , Job's method and UV-vis titration values were used. As shown in Figure 5a    Moreover, the linear concentration range and the detection limit of DMBA were obtained. The absorption intensity (at 488 nm) was linearly dependent on the concentration of Cd 2+ in the range from 0 to 10 µM ( R 2 = 0.982). The detection limit was calculated to be 0.438 µM based on 3 σ /k ( Figure 6) via absorption-based measurement. Job's method and UV-vis titration were also used to understand binding mode between DMCA and Cd 2+ . As shown in Figure 7a (Job's plot), DMCA/Cd 2+ molar fractions represented a maximum absorption peak (at 545 nm) when it was close to 0.33, which indicated that the binding between DMCA and Cd 2+ was in 2:1 stoichiometry. As seen in Figure 7b, the absorption band at 545 nm increased gradually up to 0.5 Scheme 2. Proposed reversible binding mechanism between DMBA and Cd 2+ .  each sample was analysed 3 times and the recovery values were calculated (Table 1). These results showed the suitability and applicability of the sensors for the detection of Cd 2+ in real samples. ions. Compared to some selected sensors, the sensors, DBMA and DMCA, exhibit an excellent ability to detect Cd 2+ ions by changes in both colour and UV-vis absorption spectra with a low detection limit in the presence of various metal ions in aqueous media, as presented in Table 2.

Conclusion
We presented 2 new Schiff base derivatives, a benzaldehyde-based sensor (DMBA) and a cinnamaldehydebased sensor (DMCA), for colorimetric sensing of Cd 2+ ions in aqueous solutions. The sensors depicted visible absorption characteristics that range from colourless to orange for DMBA and yellow to reddish for DMCA.
The sensors had a 2-nitrogen Cd 2+ -receptor moiety and coordinates with Cd 2+ in a 2:1 binding mode with a reversible response. The binding constant of the complexes was calculated as 2.65 ×10 12 M −2 for DMBA and 4.95 ×10 12 M −2 for DMCA. The detection limits of DMBA and DMCA were calculated via absorption-based measurement and found to be 4.38 ×10 −7 M and 1.02 ×10 −7 , respectively, which gave a marked sensitivity towards Cd 2+ . For the practical application, the sensors were applied to real samples for identifying Cd 2+ in tap water. Therefore, the sensors, DMBA and DMCA, could serve as a colorimetric sensor for the detection of Cd 2+ in aqueous solutions.