Two squaramide-based fluorescent probes for Cu2+ and Cd2+

doi:10.21203/rs.3.rs-4856463/v1

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Two squaramide-based fluorescent probes for Cu2+ and Cd2+

https://doi.org/10.21203/rs.3.rs-4856463/v1

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The development of potential toxic metal ion probes is of great significance in the field of environmental detection. Herein, two squaramide ligands (2a, 2b) were constructed by combining the characteristics of squaric acid and imine groups. 2a and 2b can recognize Cu²⁺ and Cd²⁺, with LOD of 1.26 × 10^-8 M and 2.04 × 10^-8 M, respectively, and have the advantages of fast response and wide pH range. The binding ratio and binding mode of the probe and the target ion were determined by Job’s plot, ESI-MS and ¹H NMR.

Squaramide

Cu2+ detection

Cd2+ detection

Fluorescent probe

Copper and cadmium are common environmental pollutants. It will have a great harm to human health, if they accumulate in the human body [1]. Copper ion (Cu²⁺), as one of the indispensable trace elements in mammalian cells, plays an important role in organisms due to its involvement in central nervous system processes including neuropeptide activation and neuronal signal transduction [2, 3]. However, excessive Cu²⁺ may lead to cytotoxicity, vomiting, liver damage and degenerative diseases (such as andrology and Alzheimer 's disease) [4]. Long-term exposure to almost ubiquitous cadmium ion (Cd²⁺) has been found to be associated with many health risks, such as emphysema, asthma, bronchitis and hypertension [5]. Therefore, ultrasensitive detection of Cu²⁺ and Cd²⁺ is crucial. Compared with other analytical methods (such as atomic absorption/emission spectra, electron microprobe, inductive coupled plasma-mass microprobe and anodic stripping voltammetry) [6–11], small molecule fluorescent probes have the advantages of simple operation, high selectivity, high sensitivity, instantaneous response and low cost. It has become the most important and popular quick response tool [12–14]. In recent years, many reported organic Cu²⁺ and Cd²⁺ probes have the disadvantages of long response time and low sensitivity [15–19]. Therefore, it is of great significance to develop new fluorescent probes for Cu²⁺ and Cd²⁺ detection that can shorten the response time and improve the sensitivity.

The first report of squaric acid derivatives can be traced back to 1966. Cohen described the synthesis of a series of squaric acid derivatives substituted by alkoxy, hydroxyl and amino groups from 1,2-dihydroxycyclobutenedione [20]. As one of the squaric acid derivatives, squaramide has been widely used in organic catalysis [21], self-assembly [22], anion recognition [23] and other fields. However, the reports of squaramide fluorescent probes for the detection of metal ions are extremely rare. On the other hand, Schiff bases containing imine groups are used for the detection of various metal ions due to the presence of favorable binding sites [24–27]. Therefore, based on the characteristics of squaramide and imine groups, we designed and synthesized two squaramide ligands (2a, 2b) by a simple method (Scheme 1), and systematically studied their sensing properties for the first time.

These two probes have superior sensing performance, including short response time, high sensitivity, strong anti-interference ability and so on. Among them, 2a can specifically detect Cu²⁺, and 2b can detect Cd^{2 +} with high selectivity. Surprisingly, 2a has an aggregation-induced emission enhancement (AIEE) effect. The sensing properties of probes 2a/2b for Cu²⁺/Cd²⁺ were studied by UV-visible absorption spectroscopy and fluorescence emission spectroscopy. The binding mechanism of 2a/2b with Cu²⁺/Cd²⁺ was studied by ¹H NMR, ESI-MS and Job^’s plot. In addition, 2a/2b has a more feasible detection ability for Cu²⁺/Cd²⁺ in different actual water samples. This work provides a feasible strategy for the structural design of ion fluorescent probes and an effective tool for the detection of Cu²⁺ and Cd²⁺.

2.1 AIEE performance of 2a

We studied the visual fluorescence response of 2a in DMF solution with different water fractions (ƒ_w, range 0%-90% (v/v)) (Fig. 1a). In DMF solution, 2a is yellow-green under 356 nm UV irradiation, and the maximum emission peak is at 520 nm. However, with the increase of water fraction (ƒ_w increased from 10–60%), the fluorescence emission intensity gradually decreased (Fig. 1c), the fluorescence color of the solution becomes yellow and the maximum emission peak is red-shifted (from 520nm to 542 nm ) (Fig. 1b). The decrease of fluorescence intensity can be attributed to the increase of the polarity of the mixed solvent with the increase of water fraction, which affects the conformation and charge distribution of the fluorescent molecules, resulting in the enhancement of intramolecular charge transfer (ICT) effect [28]. However, when the water fraction exceeds 70% (ƒ_w increases from 70–90%), the fluorescence intensity increases sharply. This phenomenon can be attributed to the increase of water fraction in the DMF-water mixed solvent, and the restriction of intramolecular motions (RIM) process limits the non-radiative decay of the aggregation state and promotes the transition of the excitation energy [29].

We also performed dynamic light scattering (DLS) studies on different DMF-water fractions (ƒ_w 60%, 70%, 80%, 90%). These studies have shown that the average particle size (Zav) decreases with the increase of water fraction, which provides strong evidence for the formation of aggregates (Zav at ƒ_w 60% = 693.7 nm ; at ƒ_w 70% = 588.5 nm ; at ƒ_w 80% = 406.7nm ; at ƒ_w 90% = 312.2nm (Fig. S13). Therefore, 2a can be considered as an AIEE material because it can emit brighter fluorescence in an aggregated state. Unfortunately, 2b does not have this phenomenon.

2.2 pH and Time Response

Figure S14a shows that probe 2a shows a stable fluorescence signal in the range of pH 5.0–12.0. However, after the addition of Cu²⁺, probe 2a has almost no fluorescence signal in this pH range. In addition, the pH range of probe 2b is 4.0–9.0 (Fig. S14b). Therefore, the subsequent experiments were carried out at pH = 7.3. Undoubtedly, 2a/2b is superior to most Cu²⁺/Cd²⁺ probes that can only obtain good sensing performance in a narrow pH range [30–35]. It is gratifying that when Cu²⁺/Cd²⁺ is rapidly added to the 2a/2b solution, the fluorescence is completely quenched/turned on within 5 s (Fig.S14c). Compared with the previously reported Cu²⁺/Cd²⁺ probes, [36–41] 2a and 2b showed shorter response time.

2.3 Research on sensing performance Sensing studies of 2a/2b to Cu²⁺/Cd²⁺

Due to the presence of imine (C = N) groups in 2a and 2b, it is often used as a binding site for various metal ions. [42–44] Therefore, we consider to develop it as a new type of square amide metal ion fluorescent probe, and test this idea through experiments. The selective analysis of nitrate solutions of different metal ions was carried out under 365 nm UV light, and the results were shown in Fig. 2a. Surprisingly, when Cu²⁺ was added, the original yellow-green fluorescence of 2a was quenched, while when other metal ions were added, the fluorescence did not change significantly. When Cd²⁺ was added to the DMF:H₂O (5:3, v/v) 2b solution, the probe showed a fluorescence turn-on phenomenon, identically, when other metal ions are added, the fluorescence did not change significantly (Fig. 2d). Based on this phenomenon, we further studied the fluorescence emission spectra after adding different metal ions. The fluorescence emission spectrum shows that the signal peak of 2a at 520 nm disappeared after adding Cu²⁺ (Fig. 2b), which confirms that the fluorescence quenching occurs only in the presence of Cu²⁺. After the addition of Cd²⁺, the emission peak of 2b at 521 nm increased sharply, while the intensity of the emission peak did not change significantly when other metal ions were added (Fig. 2e). The above results show that the probes 2a/2b have high selectivity for Cu²⁺/Cd²⁺.

The limit of detection (LOD) is an important index to evaluate the sensitivity of fluorescent probes. Therefore, we further carried out the fluorescence titration experiment of 2a/2b by Cu²⁺ / Cd²⁺. As shown in Fig. 2c, with the addition of Cu²⁺, the fluorescence emission peak of 2a at 520 nm gradually weakened. According to the experimental results, we use 1/[Cu²⁺] as the x-axis and the fluorescence intensity [1/( I-I₀ )] at 520 nm as the y-axis, as shown in Fig. S15a. Then the binding constant between the fluorescent probe 2a and Cu²⁺ was calculated to be 2.26 × 10⁵ M^− 1 by Benesi-Hildebrand equation [45]. The slope of the fluorescence titration curve is -72.166, and the adjusted R² is 0.994 (which indicates that there is a good linear relationship between the fluorescence intensity and the Cu²⁺ concentration) (Fig. S15c). According to the formula LOD = 3σ/S, where σ is the standard deviation of the maximum fluorescence intensity measured by 10 2a blank samples, and S is the slope of the ratio of the fluorescence intensity of probe 2a at 520 nm to the Cu²⁺ concentration [46], The LOD of the fluorescent probe for Cu²⁺ was calculated to be 1.26 × 10^− 8 M, which is lower than the World Health Organization (WHO) standard [47]. On the contrary, with the addition of Cd²⁺, the fluorescence emission peak of 2b at 521 nm gradually increased (Fig. 2f and Fig. S15e). When the amount of Cd²⁺ reached 1 Eq. (10µM), the fluorescence intensity basically did not change. Similarly, the binding constant between 2b and Cd²⁺ is 2.31 × 10⁵ M^− 1 (Fig. S15b), and the LOD is 2.04 × 10^− 8 M, which is significantly lower than the (WHO) standard [48]. The above results indicate that 2a/2b are highly sensitive Cu²⁺/Cd²⁺ fluorescent probes.

In addition, in order to further understand the selectivity of probe 2a/2b to Cu²⁺/Cd²⁺, we investigated the effect of adding different metal ions of 5 eq. , and then adding Cu²⁺/Cd²⁺ of 5 eq. , about the emission intensity, as shown in Fig. 3. When there are other metal ions in the solution, it has little effect on the fluorescence intensity of probe 2a/2b. However, after the addition of Cu²⁺/Cd²⁺, the fluorescence signal of probe 2a was rapidly quenched, and the fluorescence signal of 2b was rapidly enhanced. The experimental results show that the probe 2a/2b exhibits high anti-interference and can detect Cu²⁺/Cd²⁺ with high selectivity.

2.4 Sensing mechanism of 2a/2b to Cu²⁺/Cd²⁺

We studied the UV-vis absorption spectra of probe 2a for Cu²⁺ in DMF solution and probe 2b for Cd²⁺ in DMF/H₂O solution (2:8, v/v), respectively. As shown in Fig. 4, with the increase of Cu²⁺ addition, the absorption peak at 410 nm gradually weakens, while the absorption peaks at 330 nm and 265 nm gradually increase. Two isoabsorptive point were observed at 334 nm and 446 nm. At the same time, with the increase of Cd²⁺ addition, the absorption peak at 369 nm gradually weakened, while the absorption peak at 239 nm gradually increased. Three isoabsorptive point were observed at 305 nm, 462 nm and 553 nm. These phenomena indicate that there is an interaction between the probes 2a/2b and Cu²⁺/Cd²⁺, which changes the original conjugated structure and forms a new product.

In order to further explore the complexation ratio and complexation mechanism between probes 2a/2b and Cu²⁺/Cd²⁺, we first used Job^’s plot to reveal the chemical relationship between them (Fig. S16). Job^’s plot showed that the binding ratios of 2a/2b to Cu²⁺/Cd²⁺ were both 1:1. In addition, the characteristic peaks in ESI-MS can also confirm the above proposed stoichiometric relationship (Fig. S9-Fig. S12). Based on the results of Job^’s plot and ESI-MS, we proposed a possible coordination model (Scheme 2). In order to further confirm the complexation form of probes 2a/2b with Cu²⁺/Cd²⁺, we performed ¹H NMR titration in DMSO-d₆. As shown in Fig. 5, when 1 eq. Cu²⁺ is added, Only the proton signal of the O-H group of 2a disappeared, while the other proton signals were almost unchanged. In the presence of 1 eq. Cd²⁺, the imine proton (H₁) and aromatic proton (H₂) of 2b undergo significant low-field shifts (Fig. S17). These results indicate that Cu²⁺ can interact with the oxygen atom of the hydroxyl group of probe 2a, while Cd²⁺ binds to 2b through the nitrogen atom of the imine. It is basically in accordance with the binding mode proposed by Scheme. 2.

Since the O-H proton signal of 2a disappeared after binding to Cu²⁺, based on this result, we propose that the fluorescence emission of probe 2a is attributed to the excited-state intramolecular proton transfer (ESIPT) process. The introduction of Cu²⁺ leads to the inactivation of the ESIPT process, and therefore, the fluorescence is quenched [49]. In addition, in DMF:H₂O (5:3, v/v) solution, 2b has almost no fluorescence, which is due to the effect of photoinduced electron transfer (PET) effect. After combining Cd²⁺, the PET process of 2b is blocked, making its fluorescence turn on [50].

2.5 Real water sample testing for Cu²⁺/Cd²⁺

It is necessary to detect the level of Cu²⁺/Cd²⁺ in the natural environment. We selected tap water, Jialing River water and Qingxi River water as real water samples to test the detection ability of probe 2a/2b. Before the experiment, the water samples were filtered to remove suspended solids and precipitated impurities. Then, three different concentrations of Cu²⁺/Cd²⁺ (1, 1.5, 2 µM) were added to these water samples for recovery test, and each experiment was repeated three times under the same conditions. In order to recover Cu²⁺/Cd²⁺, we established a standard curve based on the fluorescence intensity between Cu²⁺/Cd²⁺ and probe 2a/2b. The standard curve has a good linear relationship (adj. R² > 0.99) in the target ion concentration range of 1–10 µM, and it was applied to the recovery test of Cu²⁺ in actual water samples. The results are shown in Table 1, for all samples, the recoveries of probe 2a for Cu²⁺ and probe 2b for Cd²⁺ were 97.0%-103.5% and 95.0%-109.5%, respectively, and the relative standard deviation (RSD) was less than 5%. The above results show that the probe 2a/2b has good feasibility for detecting the concentration of Cu²⁺/Cd²⁺ in actual samples.

Table 1

The detection results of Cu²⁺/Cd²⁺ in actual water samples by probe 2a/2b
Samples	Cu²⁺ added (µM)	Cu²⁺ found (µM)^a	Recovery (%)	RSD (%) (n = 3)
Tap water	1.00	0.97	97.0	1.52
Jialin Raver Water	1.50	1.48	98.7	0.73
Qinxi Raver Water	2.00	2.07	103.5	2.38
Samples	Cd²⁺ added (µM)	Cd²⁺ found (µM)^b	Recovery (%)	RSD (%) (n = 3)
Tap water	1.00	0.95	95.0	3.01
Jialin Raver Water	1.50	1.46	97.3	1.06
Qingxi Raver Water	2.00	2.19	109.5	4.26
^a Probe 2a detects Cu²⁺
^b Probe 2b detects Cd²⁺

2.6 The visual application of 2a

In order to develop a portable field sensing platform of probe 2a, we immersed the filter paper in dichloromethane solution containing 2a for 10 min [51]. After soaking, the filter paper was dried in a vacuum oven, then the prepared filter paper was used for Cu²⁺ detection. As shown in Fig. 6, under 365 nm UV light irradiation, 2a filter paper showed yellow-green fluorescence. After adding 10 µL 1 mM metal ion solution, obvious black spots appeared on the filter paper added with Cu²⁺, that is, fluorescence quenching, while the fluorescence intensity on the filter paper added with other metal ions remained basically unchanged. This result shows that the 2a filter paper has high selectivity and specificity for the detection of Cu²⁺ in solution. Subsequently, we used 2a filter paper to test the response of different Cu²⁺ concentrations. Fig. S18 showed that the fluorescence quenching phenomenon could still be observed when the concentration of Cu²⁺ was 10 µM. This indicates that the 2a filter paper can be used for the detection of trace Cu²⁺, which further expands the application range of probe 2a.

In this work, we designed two novel fluorescent probes 2a and 2b by combining the strategy of squaric acid and imino Schiff base. We accidentally found that the fluorescence intensity of 2a can be significantly enhanced in the aggregated state, with a significant AIEE effect. The subtle differences in the binding sites in 2a and 2b lead to significant differences in the fluorescence sensing properties of metal ions. Among them, probe 2a can detect Cu²⁺ with high selectivity, high sensitivity (LOD = 1.26 × 10^− 8 M), and wide pH range (5.0–12.0). Probe 2b can be used for specific sensing of Cd²⁺, and also has high sensitivity (LOD = 2.04 × 10^− 8 M), and the applicable pH range is (4.0–9.0). In addition, both probes 2a and 2b can achieve instantaneous response to the target ion within 5 s, and both have strong anti-interference. The binding ratio (1:1) and binding mode of the probe to the target ion were determined by job’s plot, ESI-MS and ¹H NMR. In terms of practical application, we further developed 2a into a fluorescent filter paper strip for the identification of trace Cu²⁺ in the environment, demonstrating the potential applicability of 2a in the field of Cu²⁺ detection. Therefore, this study provides new insights and tools for the detection of Cu²⁺ and Cd²⁺, which is of great significance for detecting and improving environmental pollution.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution

Y.Li.: Formal analysis, data collation, conception, synthesis experiment, purification process and material performance experiment, wrote a manuscript. H.W.: The synthesis experiment was carried out. Q.T.: Visualization, investigation. B.W.: conceive and modify the manuscript.

Acknowledgement

This work was supported by the China West Normal University Foundation and Sichuan University of Science & Engineering [no. 463127 and 471368], the Institute of Applied Chemistry of China West Normal University.

Data Availability

Data will be made available on request.

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No competing interests reported.

SupportingInformation.docx
Scheme1.png
Scheme 1 The Synthesis Route of 2a and 2b
Scheme2.png
Scheme 2 Possible coordination models for probes 2a/2b with Cu²⁺/Cd²⁺
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Editorial decision: Revision requested
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Reviewers agreed at journal
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06 Aug, 2024
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Two squaramide-based fluorescent probes for Cu2+ and Cd2+

Status:

Version 1

Abstract

Figures

1. Introduction

2. Results and discussion

2.1 AIEE performance of 2a

2.2 pH and Time Response

2.3 Research on sensing performance Sensing studies of 2a/2b to Cu²⁺/Cd²⁺

2.4 Sensing mechanism of 2a/2b to Cu²⁺/Cd²⁺

2.5 Real water sample testing for Cu²⁺/Cd²⁺

2.6 The visual application of 2a

3. Conclusion

Declarations

Declaration of Competing Interest

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

Two squaramide-based fluorescent probes for Cu2+ and Cd2+

Status:

Version 1

Abstract

Figures

1. Introduction

2. Results and discussion

2.1 AIEE performance of 2a

2.2 pH and Time Response

2.3 Research on sensing performance Sensing studies of 2a/2b to Cu2+/Cd2+

2.4 Sensing mechanism of 2a/2b to Cu2+/Cd2+

2.5 Real water sample testing for Cu2+/Cd2+

2.6 The visual application of 2a

3. Conclusion

Declarations

Declaration of Competing Interest

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.3 Research on sensing performance Sensing studies of 2a/2b to Cu²⁺/Cd²⁺

2.4 Sensing mechanism of 2a/2b to Cu²⁺/Cd²⁺

2.5 Real water sample testing for Cu²⁺/Cd²⁺